SYSTEMS AND METHODS FOR CHARACTERIZING AND CORRECTING CYCLIC ERRORS IN DISTANCE MEASURING AND DISPERSION INTERFEROMETRY

Abstract: The invention features interferometry systems that characterize cyclic errors. In many embodiments, the systems include a phase-shifting component (81), such as a phase shifter or frequency shifter, for introducing a variable and controlled phase between the reference and measurement beams of an interferometer (69). By analyzing the distance measurements of the interferometer (69) as a function of the introduced phase, an analyzer (27, 29) in the interferometry system can characterize the cyclic errors in the interferometer. Once the cyclic errors are characterized, the analyzer (27, 29) can directly correct the distance measurements to remove contributions from cyclic errors.

Description: SYSTEMS AND METHODS FOR CHARACTERIZING AND CORRECTING CYCLIC ERRORS IN DISTANCE MEASURING AND DISPERSION INTERFEROMETRY Background ofthe Invention This invention relates to interferometers, e.g., displacement measuring and dispersion interferometers that measure displacements of a measurement object such as a mask stage or a wafer stage in a lithography scanner or stepper system. Displacement measuring interferometers monitor changes in the position of a measurement object relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object. In many applications, the measurement and reference beams have orthogonal polarizations and different frequencies. The different frequencies can be produced, for example, by laser Zeeman splitting, by acousto-optical modulation, or internal to the laser using birefringent elements or the like. The orthogonal polarizations allow a polarizing beam splitter to direct the measurement and reference beams to the measurement and reference objects, respectively, and combine the reflected measurement and reference beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam, which subsequently passes through a polarizer. The polarizer mixes polarizations ofthe exit measurement and reference beams to form a mixed beam. Components ofthe exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams. A detector measures the time-dependent intensity ofthe mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a "heterodyne" signal having a beat frequency equal to the difference between the frequencies ofthe exit measurement and reference beams. If the lengths ofthe measurement and reference paths are changing relative to one another, e.g., by translating a stage that includes the measurement object, the measured beat frequency includes a Doppler shift equal to 2vp / λ , where v is the relative velocity ofthe measurement and reference objects, λ is the wavelength of the measurement and reference beams, and p is the number of passes to the reference and measurement objects. Changes in the relative position ofthe measurement object correspond to changes in the phase ofthe measured interference signal, with a 2π phase change substantially equal to a distance change L of λ l(np) , where n is the refractive index ofthe medium through which the light beams travel, e.g., air or vacuum, and where L is a round-trip distance change, e.g., the change in distance to and from a stage that includes the measurement object. Unfortunately, this equality is not always exact. Many interferometers include what are known as "cyclic errors," which are contributions to the phase of the measured interference signal and have a sinusoidal dependence on the change in optical path length pnL . In particular, the first order cyclic error has a sinusoidal dependence on (ZπpnL) I λ and the second order cyclic error has a sinusoidal dependence on 2 (ZπpnL) I λ . Higher order cyclic errors can also be present. Cyclic errors can be produced by "beam mixing," in which a portion of an input beam that nominally forms the reference beam propagates along the measurement path and/or a portion of an input beam that nominally forms the measurement beam propagates along the reference path. Such beam mixing can be caused by ellipticity in the polarizations ofthe input beams and imperfections in the interferometer components, e.g., imperfections in a polarizing beam splitter used to direct orthogonally polarized input beams along respective reference and measurement paths. Because of beam mixing and the resulting cyclic errors, there is not a strictly linear relation between changes in the phase ofthe measured interference signal and the relative optical path length pnL between the reference and measurement paths. If not compensated, cyclic errors caused by beam mixing can limit the accuracy of distance changes measured by an interferometer. Cyclic errors can also be produced by imperfections in transmissive surfaces that produce undesired multiple reflections within the interferometer. For a general reference on the theoretical cause of cyclic error, see, for example, C.W. Wu and R.D. Deslattes, "Analytical modelling ofthe periodic nonlinearity in heterodyne interferometry," Applied Optics, 37, 6696-6700, 1998. In dispersion measuring applications, optical path length measurements are made at multiple wavelengths, e.g., 532 nm and 1064 nm, and are used to measure dispersion of the gas in the measurement path of the distance measuring interferometer. The dispersion measurement can be used to convert the optical path length measured by a distance measuring interferometer into a physical length. Such a conversion can be important since changes in the measured optical path length can be caused by gas turbulence in the measurement arm even though the physical distance to the measurement object is unchanged. In addition to the extrinsic dispersion measurement, the conversion ofthe optical path length to a physical density requires knowledge of an intrinsic value for the refractivity ofthe gas. The factor r is a suitable intrinsic value and is the reciprocal dispersive power ofthe gas for the wavelengths used in the dispersion interferometry. The factor T can be measured separately or based on literature values. Cyclic errors in the interferometer also contribute to dispersion measurements and measurements ofthe factor . Summary ofthe Invention The invention features interferometry systems that characterize, and compensate for, cyclic errors. In some embodiments, the system includes an interferometer, a phase-shifting component that produces a variable and controlled phase shift between the reference and measurement beams ofthe interferometer, and an analyzer that controls the phase-shifting component and records the phase measured by the interferometer. Based on the measured phases and their dependence on the phase shift produced by the phase-shifting component, the analyzer estimates coefficients that characterize cyclic errors in the interferometry system. In other embodiments, the system includes an interferometer configured to measure a dispersion value in an optical path to a measurement object for each of multiple positions ofthe measurement object and an analyzer that estimates coefficients that characterize cyclic errors in the dispersion measurement based on the multiple, measured dispersion values. In additional embodiments, features ofthe phase-shifting and dispersion measuring embodiments are combined. In general, in one aspect, the invention features an interferometry system including an interferometer, a detection system, and an analyzer. During operation, the interferometer directs a reference beam along a reference path and a measurement beam along a measurement path contacting a measurement object and combines the reference and measurement beams to produce overlapping exit beams. The overlapping exit beams are indicative of changes in a relative optical path length to a measurement object. The interferometer also includes a variable phase shifter, which produces a phase shift in one, or both, ofthe reference and measurement beams. During operation, the detection system mixes polarizations ofthe overlapping exit beams to produce a mixed beam and measures a time-varying intensity ofthe mixed beam. The analyzer is coupled to the phase shifter and the detection system. During operation, the analyzer controls the value ofthe phase shift produced by the phase shifter, measures a phase corresponding to the time- varying intensity ofthe mixed beam, and determines a spectral representation, such as a Fourier sine and cosine series, of cyclic errors in the interferometry system based on the phase ofthe mixed beam for each of multiple values ofthe phase shift. The interferometry system can include any ofthe following features. The analyzer can determine the spectral representation based on the measured phase ofthe mixed beam for each of multiple values ofthe phase shift for each of multiple positions ofthe measurement object. The analyzer can include a memory and during operation can store the spectral representation in the memory. The interferometer can include a beam splitter that combines the reference beam and measurement beam to produce the overlapping exit beams, and wherein the measurement path contacts the phase shifter between the measurement object and the beam splitter. The interferometer can include a polarizing beam splitter that directs the reference beam along the reference path and the measurement beam along the measurement path, and wherein the measurement path contacts the phase shifter between the polarizing beam splitter and the measurement object. The interferometer can includes a polarizing beam splitter that receives the reference beam from the reference path and the measurement beam from the measurement path, and wherein the measurement path contacts the phase shifter between the measurement object and the polarizing beam splitter. The phase shifter can be an electro-optical modulator. The phase shifter can include an optical delay line and a translator for adjusting the length ofthe delay line, wherein the analyzer controls the translator. The phase shifter can include a pair of prisms and a translator for varying the relative positions ofthe prisms, wherein the analyzer controls the translator. The phase shifter can include a gas cell defining an optical path and a gas handling system for varying the gas pressure in the cell, wherein the analyzer controls the gas handling system. During operation, the interferometer can direct a second reference beam along a second reference path and a second measurement beam along a second measurement path contacting the measurement object and combine the second reference beam and the second measurement beam to produce a second pair of overlapping exit beams. The second pair of overlapping exit beams is indicative of changes in the relative optical path length to the measurement object. During operation, the detection system mixes polarizations of the second pair of overlapping exit beams to produce a second mixed beam and measures a time-varying intensity ofthe second mixed beams. The analyzer measures a phase corresponding to the time- varying intensity ofthe second mixed beam and determines the spectral representation based on the measured phases of each ofthe mixed beams for each of multiple values ofthe phase shift. In one embodiment, the analyzer can provide an initial value for the phase shift, measure the phase corresponding to the time-varying intensity of each ofthe mixed beams for each of multiple positions ofthe measurement object and the initial value ofthe phase shift, and then repeat the providing and measuring steps for additional values ofthe phase shift. In general, in another aspect, the invention features an interferometry system including a source, an interferometer, a detection system, and an analyzer. During operation, the source provides reference and measurement beams having different frequencies, and includes a frequency shifter, such as an acouso-optical modulator, that shifts the frequencies ofthe reference and measurement beams by equal amounts. During operation, the interferometer directs the reference beam along a reference path and the measurement beam along a measurement path contacting a measurement object and combines the reference and measurement beams to produce overlapping exit beams. The overlapping exit beams are indicative of changes in a relative optical path length to the measurement object. During operation, the detection system mixes polarizations ofthe overlapping exit beams to produce a mixed beam and measures a time-varying intensity ofthe mixed beam. The analyzer is coupled to the frequency shifter and the detection system. During operation, the analyzer causes the frequency shifter to shift the frequencies ofthe reference and measurement beams and produce a corresponding phase shift between the overlapping exit beams, measures a phase corresponding to the time- varying intensity of the mixed beam, and determines a spectral representation of cyclic errors in the interferometry system based on the measured phase ofthe mixed beam for each of multiple values of the phase shift. The interferometry system can include any of the following features. The analyzer can determine the spectral representation based on the measured phase ofthe mixed beam for each of multiple values of the phase shift for each of multiple positions ofthe measurement object. The source can provide a second reference beam and a second measurement beam having frequencies that are not shifted by the frequency shifter. During operation the interferometer directs the second reference beam along a second reference path and the second measurement beam along a second measurement path contacting the measurement object and combines the second reference beam and the second measurement beam to produce a second pair of overlapping exit beams. The second pair of overlapping exit beams is indicative of changes in the relative optical path length to the measurement object. During operation the detection system mixes polarizations ofthe second pair of overlapping exit beams to produce a second mixed beam and measures a time- varying intensity of the second mixed beams. The analyzer then measures a phase corresponding to the time-varying intensity ofthe second mixed beam. The analyzer can cause the frequency shifter to produce an initial value for the phase shift and measure the phase corresponding to the time-varying intensity of each ofthe mixed beams for each of multiple positions ofthe measurement object and the initial value ofthe phase shift. The analyzer then repeat the causing and measuring steps for additional values of the phase shift and determines the spectral representation based on the measured phases. In general, in another aspect, the invention features an interferometry system including a dispersion measuring interferometer, a detection system, and an analyzer. During operation, the interferometer produces first and second pairs of overlapping exit beams, the first pair of exit beams having a first wavelength and the second pair of exit beams having a second wavelength different from the first wavelength. For example, the wavelength can differ by at least 1 nm. The first and second pairs of exit beams are each indicative of changes in a relative optical path length to a measurement object. During operation, the detection system mixes polarizations of the first pair of overlapping exit beams to produce a first mixed beam, mixes polarizations ofthe second pair of overlapping exit beams to produce a second mixed beam, and measures a time- varying intensity of each of the mixed beams. The analyzer is coupled to the detection system. During operation, the analyzer measures a phase corresponding to the time-varying intensity of each ofthe mixed beams, at each of multiple positions ofthe measurement object. The analyzer calculates a dispersion value for each of the multiple positions, wherein the dispersion value for a particular position is equal to a function ofthe measured phases at that particular position. Then, the analyzer determines a spectral representation of cyclic error contributions to dispersion measurements by the interferometer based on the calculated dispersion values. The interferometry system can include any ofthe following features. The analyzer can further include a memory and during operation it can store the spectral representation in the memory. The analyzer can determine the spectral representation by expressing the calculated dispersion values as a function that includes a Fourier series in at least one ofthe measured phases and inverting the Fourier series. The dispersion value can be equal to a function of a weighted difference between the measured phases. For example, if the first and second wavelengths λ x and λ 2 satisfy the equation λ : /λ 2 = l jl 2 , where l λ and l 2 are integers, then weighted difference can be equal to / 1 φ 1 - / 2 φ 2 , where φ α and φ 2 are the measured phases. In such a case, l γ and l 2 can each be less than 50. The interferometer can also combine a first reference beam and a first measurement beam to produce the first pair of overlapping exit beams, and include a variable phase shifter that produces a phase shift in at least one ofthe first reference beam and the first measurement beam. In such a case, the analyzer is coupled to the phase shifter and during operation controls the value ofthe phase shift produced by the phase shifter. Alternatively, the interferometry system can include a source that provides a first reference beam and a first measurement beam, the source including a frequency shifter for shifting the frequencies ofthe first reference and measurement beams by equal amounts. In such a case, the interferometer combines the first reference and measurement beams to produce the first pair of overlapping exit beams. The analyzer is coupled to the frequency shifter and causes the frequency shifter to shift the frequencies of the first reference and measurement beams, and produces a corresponding phase shift between the first pair of overlapping exit beams. In general, in another aspect, the invention features an interferometry system including a dispersion measuring interferometer, a detection system, and an analyzer. The interferometer produces first and second pairs of overlapping exit beams, the first pair of exit beams having a first wavelength and the second pair of exit beams having a second wavelength different from the first wavelength. The first and second pairs of exit beams are each indicative of changes in a relative optical path length to a measurement object. The detection system mixes polarizations ofthe first pair of overlapping exit beams to produce a first mixed beam, mixes polarizations of the second pair of overlapping exit beams to produce a second mixed beam, and measures a time- varying intensity of each ofthe mixed beams. The analyzer is coupled to the detection system. During operation the analyzer measures a phase coπesponding to the time- varying intensity of each of the mixed beams at each of multiple positions ofthe measurement object, calculates a dispersion value for each of the multiple positions, and filters the dispersion values to determine an averaged dispersion value having reduced cyclic error. The dispersion values for a particular position being equal to a function ofthe measured phases at that particular position. In some embodiments, the analyzer averages over the dispersion values by summing the dispersion values corresponding to equally-spaced measured phases from one ofthe mixed beams, the equally-spaced measured phases spanning an interval that is a multiple of 2π . For example, if the first and second wavelengths λ j and λ 2 satisfy the equation λ x /λ 2 = t jl 2 , l Υ and l 2 being integers, and the dispersion values are equal to l^ x - / 2 φ 2 , φ : and φ 2 being the measured phases, then the analyzer averages over the dispersion values by summing the dispersion values corresponding to equally-spaced values of φ 1 spanning a 2π/ 2 interval. In a related aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes: a stage for supporting the wafer; an illumination system for imaging spatially patterned radiation onto the wafer; a positioning system for adjusting the position ofthe stage relative to the imaged radiation; and any of the interferometry systems described above for measuring the position ofthe stage. In another related aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes: a stage for supporting the wafer; and an illumination system including a radiation source, a mask, a positioning system, a lens assembly, and any ofthe interferometry systems described above. During operation, the source directs radiation through the mask to produce spatially patterned radiation, the positioning system adjusts the position ofthe mask relative to the radiation from the source, the lens assembly images the spatially patterned radiation onto the wafer, and the interferometry system measures the position ofthe mask relative to the radiation from the source. In another related aspect, the invention features a lithography system for use in fabricating integrated circuits including first and second components and any ofthe interferometry systems described above. The first and second components are movable relative to each other, and the first component includes the measurement object. During operation the interferometry system measures the position ofthe first component relative to the second component. In another related aspect, the invention features a lithography system for fabricating integrated circuits including first and second components, and any of the interferometry systems described above. The first component includes the measurement object, which is contacted by the measurement path, and the second component is contacted by the reference path. The interferometry system measures the relative position ofthe first and second components. In another related aspect, the invention features a beam writing system for use in fabricating a lithography mask. The beam writing system includes: a source providing a write beam to pattern a substrate; a stage supporting the substrate; a beam directing assembly for delivering the write beam to the substrate; a positioning system for positioning the stage and beam directing assembly relative one another; and any of the interferometry systems described above for measuring the position ofthe stage relative to the beam directing assembly. In general, in another aspect, the invention features a method for characterizing cyclic eπors in an interferometer. The method includes: directing a reference beam along a reference path and a measurement beam along a measurement path contacting a measurement object; combining the reference beam and the measurement beam to produce overlapping exit beams that are indicative of changes in a relative optical path length to the measurement object; introducing at least three phase-shifts to at least one ofthe reference and measurement beams for each of multiple positions ofthe measurement object; mixing polarizations ofthe reference and measurement beams to produce mixed beam; measuring a phase coπesponding to a time-varying intensity ofthe mixed beam for each ofthe phase shifts for each ofthe multiple positions ofthe measurement object; and determining a spectral representation of cyclic eπors in the interferometer based on the measured phases. The method can include any ofthe following features. The introducing step can includes introducing at least five phase-shifts to at least one ofthe reference and measurement beams for each of multiple positions ofthe measurement object. The phase shifts can be introduced to one ofthe reference beam and the measurement beam and not the other ofthe reference beam and the measurement beam. The phase shifts can be introduced to both the reference beam and the measurement beam. The method can include the following additional steps: combining a second reference beam and a second measurement beam to produce a second pair of overlapping exit beams that are indicative of changes in a coπesponding relative optical path length to the measurement object; mixing polarizations ofthe second reference and measurement beams to produce a second mixed beam; measuring a phase coπesponding to a time-varying intensity of the second mixed beam for each of the multiple positions ofthe measurement object; and determining the spectral representation based on the measured phases for first and second mixed beams. In a related aspect, the invention features an interferometry method including: determining a spectral representation of cyclic eπors in an interferometer using the method above; measuring an optical path length using the interferometer; and coπecting the measured optical path length for cyclic eπors using the spectral representation. In general, in another aspect, the invention features a method for characterizing cyclic eπors in a dispersion measuring interferometer. The method includes: providing a value for dispersion in an optical path length to a measurement object measured by the interferometer for each of multiple positions ofthe measurement object; and determining a spectral representation of cyclic eπor contributions to dispersion measurement in the interferometer based on the dispersion values. The determining step can include expressing the dispersion values as a function that includes a Fourier series that depends on the position ofthe measurement object, and inverting the Fourier series to determine the spectral representation. In a related aspect, the invention features an interferometry method including: determining a spectral representation of cyclic eπor contributions to dispersion measurements in an interferometer using the method above; measuring an optical path length using the interferometer; and coπecting the measured optical path length using the spectral representation. In further aspect, the invention features a lithography method for use in fabricating integrated circuits on a wafer. The lithography method includes supporting the wafer on a moveable stage; imaging spatially patterned radiation onto the wafer; adjusting the position ofthe stage; and measuring the position ofthe stage using any ofthe interferometry methods described above. In another related aspect, the invention features a lithography method for use in the fabrication of integrated circuits. The lithography method includes: directing input radiation through a mask to produce spatially patterned radiation; positioning the mask relative to the input radiation; measuring the position of the mask relative to the input radiation using any ofthe interferometry methods described above; and imaging the spatially patterned radiation onto a wafer. Either a stage supporting the mask or an illumination system providing the input radiation includes the measurement object. In another related aspect, the invention features a lithography method for fabricating integrated circuits on a wafer including: positioning a first component of a lithography system relative to a second component of a lithography system to expose the wafer to spatially patterned radiation; and measuring the position ofthe first component relative to the second component using any ofthe interferometry methods described above, wherein the first component includes the measurement object. In another related aspect, the invention features a beam writing method for use in fabricating a lithography mask. The beam writing method includes: directing a write beam to a substrate to pattern the susbtrate; positioning the substrate relative to the write beam; and measuring the position ofthe substrate relative to the write beam using any ofthe interferometry methods described above. Embodiments ofthe invention can include many advantages. For example, they can provide complete and accurate characterization of first, second, and higher order cyclic eπors, which can be used to increase the accuracy of interferometric measurements. In addition, they can characterize cyclic eπors caused by beam mixing, multiple reflections within the interferometer, and other sources. By characterizing the cyclic eπors, embodiments ofthe invention can permit rapid coπection ofthe phase measured by the interferometer, such as is usually necessary during online applications when the measurement object can be rapidly scanned or stepped. Moreover, two-channel and dispersion measuring embodiments ofthe invention can permit ongoing characterization of cyclic eπors as regular online operation ofthe interferometer occurs. The cyclic eπor characterization can be applied to optical distance measurements, dispersion measurements, and measurements of intrinsic optical properties ofthe gas in the measurement arm ofthe interferometer such as the reciprocal dispersive power T . In addition, the interferometry systems can be used in lithography and mask writing applications. Other features and advantages will be apparent from the following detailed description and from the claims. Brief Description of the Drawings FIG. la is a schematic diagram of an interferometry system that characterizes and coπects cyclic eπors by using a phase shifter. FIGS, lb-le are schematics of phase shifters suitable for use with systems described herein. FIG. If is a schematic of an electronic circuit for use with the system of FIG. la. FIGS. 2a and 2c are schematic diagrams of additional embodiments of an interferometry system that characterizes and coπects cyclic eπors by using a phase shifter. FIG. 2b is a schematic of an electronic circuit for use with the systems of FIGS. 2a and 2c. FIG. 3a is a schematic diagram of an additional embodiment of an interferometry system that characterizes and coπects cyclic eπors by using a phase shifter. FIG. 3b is a schematic of an electronic circuit for use with the system of FIG. 3a. FIG. 4 is a schematic diagram of an additional embodiment of an interferometry system that characterizes and coπects cyclic eπors by using a phase shifter. FIG. 5a is a schematic diagram of an interferometry system that characterizes and coπects cyclic eπors by using a frequency modulator to produce variable and controlled phase shifts between reference and measurement beams. FIG. 5b is a schematic of an electronic circuit for use with the system of FIG. 5a. FIG. 6a is a schematic diagram of an embodiment of an interferometry system that characterizes and coπects cyclic eπors by analyzing dispersion measurements. FIG. 6b is a schematic of an electronic circuit for use with the system of FIG. 6a. FIG. 7 is a schematic diagram of an embodiment of an interferometry system that characterizes and coπects cyclic eπors by using a frequency modulator and analyzing dispersion measurements. FIG. 8 is a schematic diagram of another embodiment of an interferometry system that characterizes and coπects cyclic eπors by using a frequency modulator and analyzing dispersion measurements. FIG. 9a is a schematic diagram of another embodiment of an interferometry system that characterizes and coπects cyclic eπors by using a frequency modulator and analyzing dispersion measurements. FIG. 9b is a schematic of an electronic circuit for use with the system of FIG. 9a. FIG. 10 is a general schematic diagram of an interferometry system that characterizes and coπects cyclic eπors by using a phase-shifting component such as a phase shifter or a frequency modulator. FIG. 11a is schematic diagram of a lithography system that includes an interferometry system described herein and is used to make integrated circuits. FIGS. llb-c are flow charts that described steps for making integrated circuits. FIG. 12 is a schematic of a beam writing system that includes an interferometry system described herein. Detailed Description The invention features interferometry systems that characterize cyclic eπors. In many embodiments, the systems include a phase-shifting component for introducing a variable and controlled phase between the reference and measurement beams of an interferometer. By analyzing the distance measurements of the interferometer as a function ofthe introduced phase, an analyzer in the interferometry system can characterize the cyclic eπors in the interferometer. Once the cyclic eπors are characterized, the analyzer can directly coπect the distance measurements to remove contributions from cyclic eπors. Alternatively, or in addition, to including the phase shifting component, the interferometry systems can perform dispersion measurements of the gas in the measurement arm at multiple positions of the measurement object and the analyzer can determine cyclic eπors in the system based on the dispersion measurements. First, we shall present a general description ofthe interferometry systems. Thereafter, we will describe particular embodiments in greater detail. A general schematic of an interferometry system 1000 is shown in FIG. 10. An interferometer 1002 directs one or more measurement beams 1004 to a measurement object 1006. In distance measuring applications, the measurement object translates, as indicated by aπow 1010, and the interferometry system measures changes in the optical path length pnL to the measurement object, where L is the round trip distance, n is the refractive index, and p is the number of passes. In dispersion interferometry applications, the interferometer uses multiple measurement beams having different wavelengths to additionally characterize changes in the refractive index n ofthe gas 1012 in the path toward measurement object 1006. Such characterization permits the system to convert the optical path length change pnL into a physical path length change L . Alternatively, in some other applications, the position of measurement object 1006 is fixed and the interferometer determines the reciprocal dispersive power T of gas 1012 at the multiple wavelengths used in the dispersion interferometry applications. Interferometer 1002 mixes the measurement beam with a reference beam to form a mixed beam 1014, whose phase φ is measured by a detector 1016. In the absence of any non-linear contributions such as cyclic eπors, the measured phase φ equals the ideal phase φ , which equals φ = npkL . In this expression, k is the wavenumber 2π/λ coπesponding to the wavelength λ of the measurement beam (the reference and measurement beams are typically frequency-shifted with respect to one another to produce a heterodyne mixing frequency). However, most interferometers have cyclic eπor contributions to the measured phase φ caused by beam mixing, multiple reflections, imperfections in optics, nonlinearities in analog signal handling and processing, and aliasing in associated digital signal processing. Including the cyclic eπor contribution, an expression for φ is given by Eq. (1): φ = φ + T A r cosrφ + T B r sinrφ (1) r=l r=l where A r and R r are the coefficients for coπesponding cyclic eπors of order r . The Fourier sine and cosine series in Eq. (1) is an example of a spectral represention ofthe cyclic eπor nonlinearity. In other embodiments, different spectral representations can be used. For example, the cyclic eπor nonlinearity can be expressed as a series of Chebyshev polynomial functions, or other sets of orthogonal functions that form a complete set. For the remainder ofthe application, however, the Fourier sine and cosine series will be used as the spectral representation ofthe cyclic eπor nonlinearity. To characterize the cyclic eπor coefficients, interferometry system 1000 includes a phase-shifting component 1020 that produces a variable and controlled phase shift φ between the reference and measurement beams at some point prior to the formation of mixed beam 1014. The phase φ is in addition to any optical delay introduced between the reference and measurement beams by changes in the position of measurement object 1006. The phase-shifting component can be an optoelectronic or optomechanical element that imparts a variable retardation to the reference and/or measurement beam(s). Alternatively, the phase-shifting component can be an optoelectronic or acousto-optical frequency modulator that changes the frequency of at least one ofthe reference and measurement beams to produce a coπesponding change in the phase shift associated with propagation distances in the interferometer. Specific embodiments ofthe phase-shifting component and placements ofthe phase- shifting component within the interferometer will be described in greater detail below. In general, the phase shift φ affects the cyclic eπor contribution to the measured phase φ and the cyclic eπor coefficients A r and B r depend on the phase shift φ , i.e., A r {§) and R (φ) . Since the effect of phase shift φ on the cyclic eπor contribution to measured phase φ can also be expressed by a spectral representation analogous to the spectral representation for φ , Eq. (1) can be rewritten as: φ(φ,φ) = φ + φ + Θ(φ,φ) (2) where Θ(φ,φ) = ∑cos q${c qr cosrφ + b qr sinrφ) )++ ^^ssin^φ(a^ r cosrφ + b^ r sinrφj g=0,r=l q=0,r=l (3) Thus, the cyclic eπor contribution Θ(φ, φ) to the measured phase φ(φ, φ) has been completely characterized by coefficients a qr , b qr , a q ' r , and b q ' r . Note also that in Eq. (2) the phase shift φ directly adds to the phase φ , which is true in most embodiments. However, in some embodiments, which will be described in greater detail below, the phase shift φ does not directly add to phase φ . Whether or not the phase shift φ adds to phase φ depends on the placement ofthe phase-shifting component within interferometer 1002. An analyzer 1022 controls phase-shifting component 1020 by sending a signal 1024 to the phase-shifting component that indicates the desired phase shift φ , and receives a signal 1026 from detector 1016 that is indicative of the measured phase φ(φ, φ) . Analyzer 1022 records values for the measured phase φ(φ, φ) for different values of phase shift φ and for different positions of measurement object 1006 (which changes the value of φ = npkL ). Analyzer 1022 uses the recorded values to determine values for at least some of the cyclic eπor coefficients a qr , b qr , a q ' r , and b q ' r , as described further below. Whether all ofthe cyclic eπor coefficients can be accurately determined using the recorded values depends on the position of phase-shifting component 1020 within the interferometry system. In particular, the size ofthe coefficients a 0r and b 0r relative to the size of the remaining portion of Θ(φ, φ) depends on the position of phase-shifting component 1020 within the interferometry system. However, the effect of coefficients a 0r and b 0r on the measured phase φ(φ,φ) is independent of the phase shift φ and as a consequence can not be determined from an analysis of values of measured phase φ(φ, φ) . In any case, analyzer 1022 can, in principle, determine all of the remaining cyclic eπor coefficients based on the recorded values of measured phase φ(φ,φ) as a function of phase shift φ . As a consequence, phase-shifting component 1020 can be positioned in certain ofthe embodiments to characterize cyclic eπors that arise at least in part from a particular physical phenomena such as multiple reflections between particular surfaces within the interferometer. After analyzer 1022 determines values for coefficients a qr , b qr , a' , and b q ' r , it can use the values to rapidly coπect the measured phase φ(φ,φ) for cyclic eπor contributions to thereby determine a more accurate value for the optical path length pnL . For example, since the cyclic eπor contribution θ(φ,φ) to the measured phase φ(φ,φ) is typically small relative to φ = npkL , analyzer 1022 can convert the measured phase φ into φ = npkL by performing the following iteration until a value for φ converges, to the required level of accuracy, as shown in Eq. (4) below: (4) φ '+1 ) = φ -φ -Θ(φW,φ) the superscript denoting the order of iteration with the total number of iterations being j . In the iterative procedure, the analyzer uses the determined cyclic eπor coefficients and any phase shift φ value in the expression for Θ(φ, φ) . Once the analyzer determines a value for cp = npkL , the optical path length, coπected for cyclic eπors excluding the effect of cyclic eπor contributions represented by coefficients 0r and b 0r , is determined simply by npL = φlk . As illustrated by the example above, after the analyzer determines accurate values for the cyclic eπor coefficients, it needs only a single measurement of phase φ to determine a value for the optical path length to the measurement object having a cyclic eπor coπection. In particular, it is not necessary for the analyzer to measure multiple values of φ coπesponding to multiple values the phase shift φ to determine the value for the optical path length having a cyclic eπor coπection. This feature is crucial for applications in which the measurement object is being rapidly scanned or stepped and there is insufficient time for multiple measurements of phase φ at each position ofthe measurement object. For example, in lithography applications, measurement stages are scanned at speeds on the order of meters per second and the interferometry system is required to monitor the position ofthe stage with an accuracy on the order of nanometers. However, to determine the values for the cyclic eπor coefficients a qr , b qr , a q ' r , and b q ' r analyzer 1022 does record values for the measured phase φ(φ,φ) for multiple values of phase-shift φ at each of multiple positions ofthe measurement object (coπesponding to values of cp ). Analyzer 1022 then uses the recorded values to Fourier invert Eq. (2), which gives the following expressions for the cyclic eπor coefficients: = ~ Jj[( ( ( P ' Φ) _ Φ)-( φ ( ( ' Φo)-Φo)] cos (' *φ ) cos (^Φ φ ^Φ π 0 to 2π a V = ~ JJT( ( > Φ) - Φ) - (φ(φ> Φo ) - Φo )]sin(rφ)cosføφ /φ φ π 0 to 2π (5) V = ~ Jjϊ φ(φ*» Φ) _ Φ) ~ (φ(φ. Φo ) - Φo )]cos(rφ)sin(^φ ]dφd 71 0 to 2π 6 V = " J " JK (Φ' Φ) ~ Φ) ~ (φ(φ' Φo ) - Φo )]sin(πρ)sinføφ)rfφ Jφ 0 to 2π for q ≥ 1 , r ≥ 1 where the phase shift φ 0 is a reference phase shift chosen by the analyzer, e.g., φ 0 = 0. The two dimensional integrations in Eq. (5) may be executed as two dimensional Fast Fourier transform (FFT) algorithms. To integrate the expressions in Eq. (5), the analyzer determines the terms in square bracket ofthe integrands from measured values of φ , which are indexed by the phase shift φ and φ 0 , both of which are known, and the ideal phase φ , which is only known approximately and coπesponds to the position ofthe measurement object. In particular, only the approximate value φ of the ideal phase φ is known, i.e., φ « φ ≡ φ - φ , which ignores the cyclic eπor contribution. To address the uncertainty with regard to ideal phase φ , the analyzer performs one ofthe following two techniques. According to the first technique, analyzer 1022 directly determines the value ofthe ideal phase φ for each position of the measurement object based on the measured values of φ(φ,φ) for multiple values ofthe phase shift φ . In particular, and based on Eq. (2), analyzer 1022 sums the measured values of φ(φ, φ) to determine phase cp according to the following integral shown in Eq. (6): (6) Thereafter, using exact values for φ , analyzer 1022 estimates the values for the cyclic coefficients based on Eqs. (5). According to the second technique, analyzer 1022 estimates the cyclic eπor coefficients iteratively by using the approximate value φ for an initial value of φ in Eqs. (5). In particular, analyzer 1022 determines first order values for the cyclic eπor coefficients according to Eqs. (5) by replacing φ with φ in Eqs. (5). Thereafter, analyzer 1022 determines first order values for φ accordmg to the iteration in Eq. (4), based on the first order values for the cyclic eπor coefficients. Then, analyzer 1022 determines second order values for the cyclic eπor coefficients according to Eqs. (5) by replacing φ with the first order values for φ in Eq. (5). Analyzer 1022 repeats the procedure until values for the cyclic eπor coefficients converge. As described previously, in applications where rapid coπection ofthe cyclic eπor contribution during is necessary, initially determining the cyclic eπor coefficients as described above is especially useful. Using the determined cyclic eπor coefficients, the analyzer can coπect each optical path length measurement for the cyclic eπor contribution based only on that one measurement. However, determining the cyclic eπor coefficients themselves can be time-consuming, because the analyzer requires multiple measurements, and in particular, multiple measurements in which the phase shift φ is varied at each of multiple positions ofthe measurement object. As a result, during determination ofthe cyclic eπor coefficients, the scanning or stepping rate ofthe measurement object needs to be slow enough to permit the multiple measurements at each position ofthe measurement object. To overcome this limitation, interferometer 1002 can mix a second pair of reference and measurement beams in which the measurement beam reflects from the measurement object to produce another mixed beam 1054 that is not affected by phase-shifting component 1020. Another detector 1056 measures the phase φ' of mixed beam 1054 and sends a signal 1064 indicative of that phase to analyzer 1022. Alternatively, the interferometer can include only the first pair of reference and measurement beams, which produces both mixed beams 1014 and 1054, provided that phase-shifting component 1020 only introduces the phase shift φ between the first pair of reference and measurement beams after mixed beam 1054 has been formed. Mixed beam 1014 and detector 1016 define a first channel for interferometry system 1000 and mixed beam 1054 and detector 1056 define a second channel for interferometry system 1000. Including the cyclic eπor contribution, an expression for φ' is given by Eq. (7) φ' = φ + A r ' cosrφ +^ B r ' sinrφ (7) r=l ι-=l where ^ and R> are cyclic eπor coefficients that have values that are in general different from, but in many cases similar to, the values of coπesponding cyclic eπor coefficients A r and B r . In addition, although we have ignored it here, the optical path length npL implicit in the phase φ in Eq. (7) can include an offset from the coπesponding term in Eq. (1). During operation, analyzer 1022 simultaneously records values for the measured phases φ and φ' , as an ordered pair. The value for φ depends both on the value of phase shift φ introduced by the phase-shifting component and on the position of measurement object 1006, whereas the value for φ' depends only on the position of measurement object 1006. Because of this property, analyzer 1022 indexes each value of measured phase φ to a position ofthe measurement object (and thereby to the value of φ ) according to the value of φ' measured simultaneously with that value of measured phase φ . This permits analyzer 1022 to collect data for characterizing cyclic eπors in a flexible manner. In particular, values for the measured phase φ(φ, φ) at multiple values of phase shift φ and a single value for the position of the measurement object need not be measured as the measurement object rests at that specific position. Instead, analyzer 1022 can sort values for the measured phase φ(φ, φ) recorded over multiple scans ofthe measurement object and at multiple phase shifts φ into groups in which each member has a coπesponding value of φ' that is identical to that of every other member in the group. For example, analyzer 1022 records values for phases φ and φ' as ordered pairs [φ ,φ' ] as interferometry system 1000 is operating under normal conditions, e.g., scanning measurement object 1006. During this time and in addition to recording the ordered pairs, analyzer 1022 may be coπecting the cyclic eπor contribution to φ and/or φ' based on some guess or prior calibration ofthe cyclic eπor coefficients. After each scan ofthe measurement object, analyzer 1022 causes phase-shifting component 1020 to change the value of phase shift φ by some increment until data is collected at phase shifts φ spanning a requisite set of values. At this point, analyzer 1022 has sufficient data to determine and/or update the values for the cyclic eπor coefficients used in the coπection ofthe cyclic eπor contribution to the measured phase(s). Analyzer 1022 sorts the ordered pairs [ φ , φ' ] into groups in which the φ' entry is the same. Although the measured phase φ' includes a cyclic eπor contribution, each value of φ' coπesponds to a unique position ofthe measurement object (at least under typical conditions in which the cyclic eπor coefficients a qr , b qr , a q ' r , and b q ' r do not significantly change during the period used to acquire a new set of measurements). Thus, the sorting provides groups of φ(φ, φ) that have different values of phase shift φ and identical, albeit unknown, values for the phase φ . Analyzer 1022 determines the value of the phase φ for each group by performing the integration prescribed by Eq. (6) and thereafter determines the values for the cyclic coefficients a qr , b qr , ά qr , and b q ' r based on Eq. (5) and the data in all ofthe groups. Alternatively, analyzer 1022 can determine the cyclic eπor coefficients using the iterative technique described above. If desired, analyzer 1022 can also determine the cyclic eπor coefficients A r ' and B r ' coπesponding to the second channel by Fourier inversion of Eq. (7) for the measured values of φ'(φ) and coπected values ofthe phase φ determined by using the analysis from the first channel. Using the two-channel system described above, the interferometry system can operate continuously to measure changes in the position ofthe measurement object and simultaneously record data sufficient to completely characterize cyclic eπors within the system as a function of time and/or other parameters ofthe system. The analysis ofthe recorded data can be performed without interrupting scanning or stepping ofthe measurement object. In particular, the system can characterize the cyclic eπors multiple times during the day as the system operates. As a result, changes in the cyclic eπor coefficients due to for example environmental lactors or changes in tilt and/or yaw of a lithographic stage minor serving as part of interferometer system that degrade optics or affect the alignment ofthe interferometer of optics can be monitored frequently or as required throughout daily operation. As will be seen below, the analysis performed by analyzer 1022 can be changed in a variety of ways. For example, a measured observable different from the one used in Eqs. (5), such as an observable that includes the difference between the phases measured by the two channels, can be used to isolate the cyclic eπor coefficient based on the measurements of interferometry system 1000. It is important to note that in applications in which a rapid scanning or stepping rate for the measurement object is not critical, interferometry system 1000 can operate by simply coπecting the measured phase φ(φ, φ) for the cyclic eπor contribution without actually characterizing the cyclic eπor coefficients. In such applications only a single channel is necessary and analyzer 1022 records values for measured phase φ(φ,φ) at multiple values of phase-shift φ for each position of the measurement object during a scan. At each position, analyzer 1022 determines ideal phase φ and thereby optical path length npL - φ / k , by integrating over φ(φ, φ) according to Eq. (6). In other embodiments, described in greater detail below, the phase-shifting component imparts the phase-shift φ to both the reference and measurement beams, rather than between the reference and measurement beams. Such embodiments can be useful for example in characterizing cyclic eπors caused by multiple reflections in the interferometer. The analysis performed by analyzer 1022 is similar to that described above except that there is no addition or subtraction of phase-shift φ from the terms in Eqs. (2)-(5). In another set of embodiments, the phase-shifting component is not used. Instead, the system includes a dispersion interferometer that measures the optical path length pn(λ)L to the measurement object at two wavelengths λ λ and λ 2 by using two pairs of reference and measurements beams having wavelengths λ 1 and λ 2 respectively. The wavelengths λ ± and λ 2 satisfy the relation λ 1 /λ 2 =Jι/t * 2 where /•-_ and l 2 are integers. A pair of detectors measures the intensities of mixed beams derived from the two pairs of reference and measurement beams and an analyzer measures phases φ λl and φ λ2 coπesponding to the intensities ofthe mixed beams. The analyzer records phases φ λl and φ λ2 at each of multiple positions of the measurement objects and also calculates the dispersion parameter Φ = ( Ψ λ i/p) ~ ( λ 2 /p) f° r eacn ofthe multiple positions. In the absence of cyclic eπors in each ofthe measured phases φ λl and φ λ2 , the dispersion parameter Φ is directly proportional to the dispersion n(λ λ ) - n(λ 2 ) in the measurement arm. However, each of the phases φ λl and φ λ2 coπesponding to the optical path length measurements include cyclic eπors. The cyclic eπors can be expressed as shown in Eq. (1). Moreover, the dispersion parameter Φ also includes contributions from the cyclic eπors in the phases φ λl and φ λ2 . In fact, as will be shown in greater detail below, the contribution ofthe cyclic eπors to the coπection for dispersion effects, TΦ , in φ λl and φ λ2 is greater than the contribution of cyclic eπors to either of the phases φ λl and φ λ2 by about the factor T . The factor F T which is the reciprocal dispersive power equal to T = «(λ 2 )), is typically 1 to 2 orders of magnitude greater than 1. Thus, the dispersion coπection rΦ is more sensitive to cyclic eπors than are the measured phases φ λl and φ λ2 . To characterize the cyclic eπor coefficients that contribute to the dispersion parameter, the analyzer measures the dispersion parameter Φ as a function of one or both of the measured phases φ λl and φ λ2 by changing the position of the measurement object. Thereafter, the analyzer filters the dispersion parameter Φ to eliminate the effect ofthe cyclic eπor coefficients or Fourier analyzes the dispersion parameter Φ to determine the values ofthe cyclic eπor coefficients that contribute to the dispersion parameter in a manner similar to that shown in Eqs. (2) and (5). The theoretical expression includes Fourier series expansions in one or both of the ideal phases φ λl and φ λ2 coπesponding to the measured phases φ λl and φ λ2 . In the Fourier analysis, the analyzer substitutes the measured phases φ λl and φ λ2 for the ideal phases φ λl and φ λ2 . The substitution is used effectively because the cyclic eπor contributions to the measured phases φ λl and φ λ2 relative to respective ideal phases φ λl and φ λ2 are much less than cyclic eπor contribution to the dispersion parameter Φ in relation to the dispersion parameter Φ . As a result, the values for the cyclic eπor coefficients are determined with a relative precision substantially the same as the cyclic eπor contributions in the measured phases φ λl and φ λ2 relative to the respective ideal phases φ λl and φ λ2 . In further embodiments, the dispersion interferometry system can also include a phase-shifting component as described previously to further refine the determined values for the cyclic eπors. In such systems the analyzer records optical distance measurements as a function ofthe phase-shift φ and dispersion measurements as a function of multiple positions ofthe measurement object to characterize cyclic eπors in the system. As with the two-channel system described previously, the dispersion measuring embodiment permits cyclic eπors to be characterized in the background during normal distance measuring operations in which the measurement object is scanned or stepped. In general, for the embodiments ofthe interferometry system described herein, the analyzer includes one or more computer processors for performing the appropriate analysis steps. The numerical and symbolic steps described herein can be converted into a digital program executed, e.g., on a digital signal processor (DSP) according to methods well known in the art. The digital program can be stored on a computer readable medium such as a hard disk and can be executable by the computer processors in the analyzer. Alternatively, the appropriate analysis steps can be converted into a digital program that is hardwired into dedicated electronic circuits within the analyzer that executes the steps. Methods for generating such dedicated electronic circuits based on a given numerical or symbolic analysis procedure are also well known in the art. Detailed descriptions of specific embodiments follow below. While they differ in some details, the disclosed embodiments otherwise share many common elements and naturally fall into several groups depending on the type of end use application and on whether the cyclic eπors are being measured and the measured cyclic eπors used to coπect for effects of nonlinearities due to the cyclic eπors or whether the cyclic eπors are being filtered out with respect to optical path length. As will be seen, the disclosed embodiments within each group also differ in the details of how their interferometric optical paths are implemented and/or how certain information signals are handled electronically. A first group of embodiments ofthe several groups comprise distance measuring interferometers operating with one wavelength. A second group of embodiments ofthe several groups comprise interferometers operating with a primary wavelength for the measuring purpose of the interferometer and a second wavelength which can be scanned over a given wavelength interval for the purpose of measuring cyclic eπors in the interferometer at either or both the first and the second wavelengths. A third group of embodiments ofthe several groups comprise both an apparatus and method for measuring and coπecting for cyclic eπors in dispersion related signals of distance measuring interferometry wherein the effects of gas in the measuring path of a distance measuring interferometer are coπected by a dispersion interferometry based procedure. Embodiments of a fourth group ofthe several groups comprise both an apparatus and method for measuring and coπecting for cyclic eπors in both the dispersion related signals and the distance measuring related signals of distance measuring interferometry using dispersion interferometry to determine the effects of a gas on the measured optical path of the distance measuring interferometry and an apparatus and method for filtering out the cyclic eπors in the dispersion related signals. Embodiments in a fifth group ofthe several groups comprise both an apparatus and method for measuring and coπecting for cyclic eπors in both a dispersion related signal and a refractivity related signal or refractivity related signals used to measure intrinsic optical properties of a gas. Embodiments in the fifth group ofthe several groups further comprise an apparatus and method for filtering out the cyclic eπors in both a dispersion related signal and a refractivity related signal or refractivity related signals used to measure intrinsic optical properties of a gas. FIG. la depicts in schematic form in accordance with the prefeπed apparatus and method ofthe first embodiment ofthe present invention. The first embodiment is from the first group of embodiments. The interferometer depicted in FIG. la is a polarizing, heterodyne, single pass interferometer. Although the described embodiment is a heterodyne system, the instant invention is readily adapted for use in a homodyne system in which the reference and measurement beams have the same frequencies. While the apparatus has application for a wide range of radiation sources, the following description is taken by way of example with respect to an optical measuring system. Referring to FIG. la, a light beam 7 emitted from source 1 passes through a modulator 3 becoming light beam 9. Modulator 3 is excited by a driver 5. Source 1 is preferably a laser or like source of coherent radiation, preferably polarized, and having a wavelength λ**_ . Modulator 3 may, for example, be an acousto-optical device or a combination of acousto-optical devices with additional optics for selectively modulating polarization components of beam 7. Modulator 3 preferably shifts the oscillation frequency of one linearly polarized component of beam 7 an amount f with respect to an orthogonally linearly polarized component, the directions of polarizations ofthe components, denoted herein as x and y which are parallel and orthogonal, respectively, to the plane of FIG. la. The oscillation frequency f is determined by the driver 5. Light source 1 such as a laser can be any of a variety oi frequency modulation apparatus and/or lasers. For example, the laser can be a gas laser, e.g., a HeNe laser, stabilized in any of a variety of conventional techniques known to those skilled in the art, see for example, T. Baer et al, "Frequency Stabilization of a 0.633 μm He-Ne-longitudinal Zeeman Laser," Applied Optics, 19, 3173-3177 (1980); Burgwald et al, U.S. Pat. No. 3,889,207, issued June 10, 1975; and Sandstrom et al, U.S. Pat. No. 3,662,279, issued May 9, 1972. Alternatively, the laser can be a diode laser frequency stabilized in one of a variety of conventional techniques known to those skilled in the art, see for example, T. Okoshi and K. Kikuchi, "Frequency Stabilization of Semiconductor Lasers for Heterodyne-type Optical Communication Systems," Electronic Letters, 16, 179-181 (1980) and S. Yamaqguchi and M. Suzuki, "Simultaneous Stabilization ofthe Frequency and Power of an AlGaAs Semiconductor Laser by Use ofthe Optogalvanic Effect of Krypton," IEEE J. Quantum Electronics, QE-19, 1514-1519 (1983). Two optical frequencies may be produced by one ofthe following techniques: (1) use of a Zeeman split laser, see for example, Bagley et al, U.S. Patent No. 3,458,259, issued July 29, 1969; G. Bouwhuis, "Interferometrie Mit Gaslasers," Ned. T. Natuurk, 34, 225-232 (Aug. 1968); Bagley et al, U.S. Patent No. 3,656,853, issued April 18, 1972; and H. Matsumoto, "Recent interferometric measurements using stabilized lasers," Precision Engineering, 6(2), 87-94 (1984); (2) use of a pair of acousto-optical Bragg cells, see for example, Y. Ohtsuka and K. Itoh, "Two-frequency Laser Interferometer for Small Displacement Measurements in a Low Frequency Range," Applied Optics, 18(2), 219-224 (1979); N. Massie et al, "Measuring Laser Flow Fields With a 64-Channel Heterodyne Interferometer," Applied Optics, 22(14), 2141-2151 (1983); Y. Ohtsuka and M. Tsubokawa, "Dynamic Two-frequency Interferometry for Small Displacement Measurements," Optics and Laser Technology, 16, 25-29 (1984); H. Matsumoto, ibid.; P. Dirksen, et al, U.S. Patent No. 5,485,272, issued Jan. 16, 1996; N. A. Riza and M. M. K. Howlader, "Acousto-optic system for the generation and control of tunable low-frequency signals," Opt. Eng., 35(4), 920- 925 (1996); (3) use of a single acousto-optic Bragg cell, see for example, G. E. Sommargren, commonly owned U.S. Patent No. 4,684,828, issued Aug. 4, 1987; G. E. Sommargren, commonly owned U.S. Patent No. 4,687,958, issued Aug. 18, 1987; P. Dirksen, et al., ibid.; (4) use of two longitudinal modes of a randomly polarized HeNe laser, see for example, J. B. Ferguson and R. H. Morris, "Single Mode Collapse in 6328 A HeNe Lasers," Applied Optics, 17(18), 2924-2929 (1978); (5) use of birefringent elements or the like internal to the laser, see for example, V. Evtuhov and A. E. Siegman, "A "Twisted-Mode" Technique for Obtaining Axially Uniform Energy Density in a Laser Cavity," Applied Optics, 4(1), 142-143 (1965); or the use ofthe systems described in U.S.S.N 09/061,928 filed 4/17/98 entitled "Apparatus to Transform Two Non-Parallel Propagating Optical Beam Components into Two Orthogonally Polarized Beam Components" by Henry A. Hill et al., the contents of which are incorporated herein by reference. The specific device used for the source of beam 9 will determine the diameter and divergence of beam 9. For some sources, e.g., a diode laser, it will likely be necessary to use conventional beam shaping optics, e.g., a conventional microscope objective, to provide beam 9 with a suitable diameter and divergence for elements that follow. When the source is a HeNe laser, for example, beam shaping optics may not be required. As shown in FIG. la, interferometer 69 comprises a reference retroreflector 91, object retroreflector 92, quarter wave phase retardation plates 77 and 78, and a polarizing beam splitter 71. This configuration is known in the art as a polarized Michelson interferometer. The position of object retroreflector 92 is controlled by translator 67. Beam 9 incident on interferometer 69 results in beams 33 and 34 as illustrated in FIG. la. Beams 33 and 34 contain information at wavelength λ τ about the optical path length through the gas in measuring path 98 and about the optical path length through the reference path, respectively. Beams 33 and 34 exit interferometer 69 and enter detector system 89 illustrated in diagrammatic form in FIG. la. In detector system 89, a first portion of beam 33 is reflected by nonpolarizing beam splitter 63A, reflected by minor 63B, and incident on beam splitter 63C. A portion of the first portion of beam 33 reflected by nonpolarizing beam splitter 63 A is reflected by polarizing beam splitter 63C to become a first component of beam 41. A first portion of beam 34 is reflected by nonpolarizing beam splitter 63A and incident of polarizing beam splitter 63C. The first portion of beam 34 reflected by nonpolarizing beam splitter 63A is transmitted by polarizing beam splitter 63C to become a second component of beam 41. A second portion of beam 33 is transmitted by nonpolarizing beam splitter 63A, transmitted by variable phase shifter 81, reflected by minor 63D, and incident on polarizing beam splitter 63E. The second portion of beam 33 transmitted by nonpolarizing beam splitter 63A is reflected by polarizing beam splitter 63E to become a first component of beam 43. A second portion of beam 34 is transmitted by nonpolarizing beam splitter 63A and incident of polarizing beam splitter 63E. The second portion of beam 34 transmitted by nonpolarizing beam splitter 63A is transmitted by polarizing beam splitter 63E to become a second component of beam 43. Interferometer 69 introduces phase shifts φ x and φ 2 between the first and second components of beam 41 and the first and second components of beam 43, respectively. The magnitude of phase shifts φ± and φ 2 are related to round-trip physical length L i of measurement path 98 according to the formulae φ,* = Lipk±n j , i = 1 and 2 , (8) where p is the number of passes through the respective reference and measurement legs, n t is the refractive index of a gas in measurement path 98 coπesponding to the optical path introducing the phase shift φ ( * and to wavenumber k λ = 2π/λ 1 . The interferometer shown in FIG. la is for p = 1 so as to illustrate in the simplest manner the function ofthe apparatus ofthe first embodiment. To those skilled in the art, the generalization to the case when p ≠ 1 is a straight forward procedure. The nominal value for L t , i = 1 and 2 , coπesponds to twice the difference between the physical length of measuring path i and an associated reference path. The lengths L and L 2 are equal to a high accuracy, the beams associated with phase shifts φ x and φ 2 being derived from the same beam 33. Except for higher order effects such as turbulence in the gas and other sources of localized density gradients in the gas, the refractive indices n x and n 2 are equal to a high accuracy, the beams associated with phase shifts φ ± and φ 2 being derived from the same beam 33. Variable phase shifter 81 shown in FIG. la introduces a phase shift φ into the second portion of beam 33 transmitted by nonpolarizing beam splitter 63 A, the magnitude of the phase shift φ being adjustable and controlled by signal 30. A series of different embodiments of variable phase shifter 81 is depicted in diagrammatic form in FIGS. Ib-le. A first embodiment of variable phase shifter 81 not shown in FIGS. Ib-le comprises a set of phase shifter plates, of either the nonbirefringent or birefringent type, of differing optical thicknesses wherein the phase shift is changed by sequentially inserting in the path of a respective beam different phase shift plates from the set of phase shift plates. A second embodiment of variable phase shifter 81 shown in FIG. lb comprises an electro-optical modulator 72, the phase shift being controlled, e.g., by a signal 30 from computer 29. A third embodiment of variable phase shifter 81 shown in FIG. lc comprises two substantially identical prisms 74A and 74B oriented so as to not change the direction of propagation of a beam transmitted by the pair of prisms, the phase shift introduced by the pair of prisms being altered by changing the degree of overlap of the pair of prisms by translator 67B controlled, e.g., by a signal 30 from computer 29. A fourth embodiment of variable phase shifter 81 shown in FIG. Id comprises a cell formed with windows 90 A and 90B separated by a right circular cylinder 91 filled with a predetermined gas, the phase shift introduced by the fourth embodiment being altered by changing the density ofthe predetermined gas in the cell. The gas handling system for changing the density ofthe predetermined gas in the cell formed by windows 90A and 90B and cylinder 91 is not shown in FIG. Id. A fifth embodiment of variable phase shifter 81 shown in FIG. le comprises two minors 65A and 65B and retroreflector 93, the phase shift introduced by the fifth embodiment being altered by changing the separation ofthe retroreflector 93 and the pair of minors 65A and 65B by translator 67C controlled, e.g., by a signal 30 from computer 30. In a next step as shown in FIG. la, phase-shifted beams 41 and 43 pass through polarizers 79A and 79B, respectively, impinge upon photodetectors 85 and 87, respectively, and generate electrical interference signals, heterodyne signals s λ and s 2 , respectively, preferably by photoelectric detection. Polarizers 79A and 79B are oriented so as to mix polarization components of beam 41 and the polarization components of beam 43, respectively. The signals s x and s 2 have the form Si ; = Λ,*(t)cos[ ; *(t)] i = 1 and 2 (9) The signal s t is the real part, s i R , of a complex number s t where s t comprises a causal, stable, i.e., absolutely summable, real sequence. Thus, the Fourier transform S j R (jω) of s,- completely defines S j ^jω) [see Chapter 10 "Discrete Hubert Transforms" in Discrete-Time Signal Processing, (Prentice Hall, 1989) by A. V. Oppenheim and R. W. Schafer] where Si(jω) = S itR (jω) + iS u (j(o) , i = l and 2 , (10) S j (j(d) is the imaginary component of S, (./ω) , ω is an angular frequency, and j is the imaginary number (-l) • The imaginary component s itI of s t is obtained from the inverse Fourier transform of S,* / (yω) with s / * !/ = (t)sin[α ; *(t)] , t = l and 2 . (11) The phase α, (t) can be obtained from s, :>Λ and s i f according to the formulae α, (t) = arctan V i = 1 and 2 (12) V Λ ,Λ/ The time-dependent arguments α,(t) are expressed in terms of other quantities according to the formulae α 1 (t) = 2π 1 t + φ 1 +ζ 1 + Λ 1 (13) α 2 (t) = 2π i t + φ 2 + ζ 2 + Λ 2 + φ where A x and Λ 2 comprise the nonlinear eπors including in particular cyclic enor terms, φ is the phase shift produced by phase shifter 81 without substantially altering the nonlinearity Λ 2 , and phase offsets ζ λ and ζ 2 comprise all contributions to the phases α ± and α 2 , respectively, that are not related or associated with the optical path of the measurement path 98 or reference path and not related to nonlinear enors. Heterodyne signals s λ and s 2 are transmitted to electronic processor 27 for analysis as electronic signals 23 and 25, respectively, in either digital or analog format, preferably in digital format. Referring now to FIG. If, electronic processor 27 further comprises electronic processors 1274A and 1274B to determine measured phases φ x and φ 2 , respectively, by either digital or analog signal processes, preferably digital processes, using time-based phase detection such as a digital Hubert transform phase detector [see section 4.1.1 of "Phase-locked loops: theory, design, and applications" 2nd ed. McGraw-Hill (New York) 1993, by R. E. Best] or the like and the phase of driver 5. The phase of driver 5 is transmitted by electrical signal, reference signal 21, in either digital or analog format, preferably in digital format, to electronic processor 27. A reference signal, an alternative reference signal to reference signal 21, may also be generated by an optical pick off means and detector (not shown in figures) by splitting off a portion of beam 9 with a nonpolarizing beam splitter, mixing the portion of the beam 9 that is split off, and detecting the mixed portion to produce an alternative heterodyne reference signal. Referring again to FIG. If, the measured phases φ 1 and φ 2 are next subtracted one from the other in electronic processor 1277, preferably by digital processes, to create the phase Φ . Formally, Φ = (φ - Φι) , (15) or Φ = j pA: 1 (« 2 Z 2 - « 1 E 1 ) + (ς 2 -ζ 1 ) + [Λ 2 (φ 2 ) φ) - Λ 1 (φ 1 )] + φ (16) Note that the effects of turbulence on the refractive indices ofthe gas in the measuring paths substantially cancel out in Φ as well as the effects of Doppler shifts produced by translations of retroreflector 92. The turbulence effects on the refractive index ofthe gas in the measuring paths cancel out in Φ because the measurement beam components of beams 41 and 43 are both derived from the same beam 33. The nonlinearity A, can in general be expressed in terms of cyclic nonlinearity ψ ( * and a noncyclic nonlinearity r\, , i.e. Λ ; * = ψ ; + η* i l = 1 anci 2 . (17) For the remainder ofthe application the non-cyclic nonlinearity will be ignored. A spectral representation of the cyclic nonlinearity ψ, , in terms of c j and φ 2 , can be based on different families of orthogonal polynomials and functions. Two examples are a series comprising Fourier sine and cosine functions and a series comprising Chebyshev polynomial functions. Without departing from the spirit and scope ofthe present invention, the Fourier sine and cosine series spectral representation of ψ ; will be used in the subsequent embodiments and in the first embodiment is expressed as: Ψl(φl) = Cl,θ(Φl) + Ψl,l(φl) 2 (<P 2 ) = C 2β (φ 2 ) + ∑C 2q (φ 2 )cosqty + ∑S 2q (φ 2 )smq§ , 9=1 9 =1 (18) with Ψι,ι( φ .) = ∑ .*( φ |) 9=1 - sinrφ, / = 1 and 2 (19) [9 = 0,1,2,... , "?( φ <)= ∑ < cosr < + Σ b ιqr sin «P, (20) f = 1 and 2 , r=l r=l (21) It is of interest to note that ψ 21 (φ 2 ) = ψ 2 (φ 2 ,θ) - C 2>0 (φ 2 ) . The C iq and S iq are written in Eqs. (20) and (21) in terms of cosine and sine series terms with arguments ofthe series terms being harmonics of φ,* . For some configurations of interferometers, in particular multiple pass interferometers, it is possible for a system comprising a source, interferometer, and detector to generate cyclic nonlinearities that are subharmonics of φ,* . Should subharmonic cyclic eπors be present in a system, Eqs. (20) and (21) are augmented so as to include cosine and sine series terms with arguments that are subharmonics φ,* . The subsequent description of procedures to determine the coefficients in the cosine and sine series will be in terms ofthe series representations given by Eqs. (18), (19), (20), and (21) without departing from the spirit and scope ofthe present invention. It is possible for a system comprising a source, interferometer, detector, and digital signal processing to generate cyclic nonlinearities that are neither subharmonics or harmonics of φ, . The non-subharmonic, nonharmonic cyclic eπors are produced for example by aliasing in the digital signal processing and have frequencies which are aliases of harmonics and subharmonics of φ ; . Should non- subharmonic, nonharmonic cyclic eπors be present in a system, Eqs. (20) and (21) are augmented so as to include cosine and sine series terms with arguments that are the appropriate aliases of harmonics and/or subharmonics of φ ; * . The subsequent description of procedures to determine the coefficients in the cosine and sine series will be in terms of the series representations given by Eqs. ( 18), ( 19), (20), and (21 ) without departing from the spirit and scope ofthe present invention. The spectral representation given by Eqs. (20) and (21) is in general valid for an interferometer where the phase ofthe heterodyne signal is changing at substantially a constant rate. The coefficients ofthe spectral representation given by Eqs. (20) and (21) will in general depend on the rate of change ofthe phase as a result, for example, ofthe properties of group delay experienced by the heterodyne signal. Group delay, often called envelope delay, describes the delay of a packet of frequencies and the group delay at a particular frequency is defined as the negative of the slope ofthe phase curve at the particular frequency [see H. J. Blinchikoff and A. I. Zverev, Filtering in the Time and Frequency Domains, Section 2.6, ι976 (Wiley, New York)]. To describe the effect of cyclic enors for the interferometer wherein the phase ofthe heterodyne signal is changing at two or more different substantially constant rates, coefficients ofthe spectral representation are obtained for each ofthe different substantially constant rates. Coefficients ofthe spectral representation conesponding to the set of two or more different substantially constant rates of change ofthe heterodyne phase may alternatively be effectively represented in terms of a function ofthe rate of change of phase of heterodyne signal. The function may need include only the first few terms of a simple power series in the rate of change or may comprise orthogonal functions or polynomials. The cyclic eπor terms C 1>0 and C 2 0 are principally the result of cyclic eπor generation by an interferometer, e.g., spurious internal multiple reflections of beams in the measurement and/or reference legs and/or departures of quarter wave phase retardation plates and polarizing beam splitters from desired properties. The remaining cyclic terms C iq and S iq for / = 1 and 2 q ≥ 1 are generally effected by polarization and frequency mixing from a number of different sources operating independently or in combination, e.g., polarization and frequency mixing in the interferometer source, misalignment ofthe interferometer source with respect to the interferometer, departures from desired properties of a polarizing beam splitter used to separate reference and measurement beams based on relative states of polarization, and alignment properties ofthe detector units including mixers and analyzers with respect to the interferometer. The magnitude of cyclic eπors C 1 0 and C 2 0 can be substantially the same and the magnitude of cyclic eπors ψ l x and ψ 2 1 defined by Eqs. (19) can be substantially the same. However, the magnitude of cyclic eπors C 1 0 and C 2 0 are generally not equal and the magnitude of cyclic enors x ι and ψ 2 1 are generally not equal because of departures of reflection and transmission properties ofthe beam splitters 63A, 63B, and 63E from desired properties. It will be evident to those skilled in the art that the noncyclic nonlinearity η,* can be omitted in subsequent descriptions of embodiments ofthe present invention without departing from the scope and spirit ofthe invention. In a next step, Φ is measured as a function φ ± and φ 2 for a set of values for φ , the required number of different values of φ being dependent on the complexity of ψ x and ψ 2 and the relative precision required for the measured values of ψ x and ψ 2 . The set of values of φ introduced by variable phase shifter 81 is controlled by electronic signal 30 from computer 29. From the measured values of Φ , measured values ofthe quantity Ψ2(φ2>Φ) - ψ2(φ2>Φo) + (φ - Φo) ( 22 ) are obtained from [φ(φχ , φ) - Φ(φι , Φo )] where φ 0 is an initial value used for φ . An expression for ψ 2 (φ 2 ,φ) - ψ 2 (φ 2 ,φ 0 ) can be written, according to Eqs. (18), as Ψ2( 2 >Φ) - Ψ2(Φ2 > ΦO) = ^ C 2> <?(φ 2 )(c θS 9φ - COS 9φ 0 ) 9=1 (23) + ^ S 2,g (φ 2 )( sin ≠ ~ sin ≠ 0 ) 9=1 A procedure is described for the determination of certain of the Fourier coefficients listed in Eqs. (20) and (21) for an end use application requiring compensation for cyclic eπors in φ x but not in φ 2 . A second procedure is described for the determination of certain other ofthe Fourier coefficients listed in Eqs. (20) and (21) for an end use application requiring compensation for cyclic eπors in both ψ 1 and φ 2 . These two procedures are presented as examples of two possible procedures and the particular descriptions of two procedures do not limit by way of examples the scope of the present invention. The second ofthe two procedures is described first wherein compensation for cyclic enors in part in both φ x and φ 2 is accomplished. For the second procedure, the Fourier coefficients a 2qr , b 2qr , a 2qr , and b 2 ' qr , q ≥ 1 and r > 1 , can be determined as a function of q and r by iterative procedures for the magnitude of cyclic enors typically found in interferometers. For conditions where the cyclic nonlinearity term ψ 2 ≤. 1/10 coπesponding to an eπor in position of ≤ 5 nm in a single pass interferometer with λ = 633 nm, the Fourier coefficients representing [ψ 2 (φ 2 **Φ) _ Ψ 2 (φ 2 > Φo)] can be obtained from the measured quantity Φ by an efficient iterative process. For a situation where the cyclic nonlinearity term ψ 2 ~ 1 , the Fourier coefficients in [ψ 2 (φ 2 'Φ) _ Ψ 2 (φ 2 'Φo)l can be obtained by generation of a series of simultaneous transcendental equations ofthe Fourier coefficients. The iterative procedure for determination ofthe Fourier coefficients will be described herein, the condition ψ 2 -≤ 1/10 generally being met in interferometry systems and the iterative procedure being a simpler procedure. The first step in the iterative process is to assume that the cyclic nonlinearities in φ 2 are negligible and compute the Fourier coefficients in [ψ 2 (φ 2 ,φ) - ψ 2 (φ 2 ,φ 0 )l , using φ 2 as the variable of integration. The second step of the iterative process is to use the Fourier coefficients in [ψ 2 (φ 2 ,φ) - ψ 2 (φ 2 ,φ 0 )l obtained in the first step of the iterative process to generate a first iterated q> 2 conected for the first iterated cyclic nonlinearities in [ψ 2 (φ 2 ,φ) - ψ 2 (φ 2 > Φo)l anc ** then compute the second iteration values for the Fourier coefficients in [ψ 2 ( 2 'Φ) _ Ψ 2 ( 2 »Φo)l using the first iterated φ 2 as the variable of integration. The iterative process is repeated until the sequence of iterated values for the Fourier coefficients in fψ 2 (φ 2 ,φ) - ψ 2 (φ 2 'Φo)] m Φ 2 nave converged to required relative precisions. With ψ 2 ≤ l/l 0 , the number of steps in the iterative process may not require more than one or two. The relative precision to which the cyclic enors [ Ψ 2 (φ 2 , φ) - C 2)0 (φ 2 )] can be determined by the first embodiment will have a magnitude ofthe order of magnitude ofthe cyclic eπor term C 2 0 (φ 2 )/2 expressed in radians, cyclic eπor term C 2 0 (φ 2 ) not being determined in the procedure used to evaluated |Ψ 2 (φ 2 ,φ) - C 2 0 (φ 2 )| combined with the absolute precision, expressed in radians, to which φ is known. The residual cyclic eπors from |Ψ 2 (φ ,ψ) - C 2 0 (φ 2 )| in φ 2 after coπection for [ψ 2 (φ 2 ,φ) - C 2 0 (φ 2 )| may be described as entering as a series of second order effects, a second order effect being derived from a product of two first order effects, the first order effects being |Ψ 2 (φ 2 ,φ) - 2 , 0 ^ 2 )] » £ 2 , 0 (φ 2 ) > and the absolute precision to which φ is known. The cyclic eπors represented by Ψι ) ι(φι) are next determined to a relative precision substantially the same as the relative precision to which [ Ψ 2 (φ 2 , φ) - C 2 0 (φ 2 )| is determined in previously described steps of the first embodiment. The next step in the cyclic eπor evaluation procedure for addressing cyclic eπors in (p x uses φ 2 - φ conected for cyclic enors [Ψ 2 (φ 2 ,φ) - C 2 0 (φ 2 )| , i.e., |φ 2 - [ψ 2 (φ 2 » Φ) - C 0 (φ 2 )] - φ| , as a vailable of integration in a Fourier analysis of {-Φ + [ψ 2 (φ2 >Φ) - C2,o(φ2)] + φ} • The quantity j-Φ + [ψ2(φ2>Φ) ~ C2,o( 2)] + φ} is to a good approximation Ψι ( ι(φι) within a constant as evident from the following expression based on Eqs. (16) and (18); {-Φ + [ψ 2 (φ 2 ,φ) - C 2) o(φ 2 )] + φ} = ψ 1)1 (φ 1 ) + (ζ 1 -ζ 2 ) (24) + [Cι,o(φι) - C 2) o(φ 2 )] , where the term pk 1 (n 1 L l - n 2 L 2 ) and the η,* terms have been omitted for reasons previously described. The offset term (ζ x -ζ 2 ) is a constant and generally, |Cι, 0 (φι)- C 2) o(φ 2 )| <l |Cι,o(φι)|/lO , ( 2 5) C"ι , o(φι) being approximately equal to C 2 0 (φ 2 ) and for the beam splitting properties of beam splitter 63A being substantially the same for transmitted and reflected beams, e.g., 50/50. The Fourier coefficients *l,gr 0,r *2,0 t—ig=\ X and + [ b l,0,r - b 2,0,r) of cosrψ ! and sinrφ- L , respectively, in j -Φ + [ψ 2 (φ 2 , ψ) - C 2 o (φ 2 )] + Φ I can be determined as a function of r by one of several procedures. For the conditions where the cyclic nonlinearity terms ψ x ≤ 1/10 and ψ 2 ≤ l/l 0 , the Fourier coefficients for r > 1 in Ψι,ι(φι) + (ζι -ζ∑) + [ c ι,o(φι) - C 2 ,o(φ2,o)]} . the ri g ht han d sid e of Eq. (24), can be obtained from the quantity {- Φ + [Ψ 2 (φ 2 , φ) - C 2 0 (φ 2 )J + φ by an iterative process the same as described for the determination ofthe Fourier coefficients in [ψ 2 (φ 2 ,φ)-ψ 2 (φ 2 ,φ 0 )] . The relative precision to which the cyclic eπors in | ψι,ι(φι) + C 1; o(φι) - C 2 0 φ 2;0 J I can be determined by the first embodiment will have a magnitude ofthe order of magnitude ofthe cyclic enor term C 2 0 (φ 2 )/2 expressed in radians, cyclic eπor term C 2 0 (φ 2 ) not being determined in the procedure used to evaluate either [Ψ 2 (φ 2 > Φ) ~ C 2> o(φ 2 )l or I Ψ ι,ι (φ l ) + C 1>0 (φ i ) - C 2i0 (φ 2 ,o ) I » combined with the absolute precision, expressed in radians, to which φ is known. The residual cyclic enors from Ψι,ι(φι) + Cι ) o(φι) - C , 2j0 (φ 2) o) \ in Φi after conection for Ψι,ι(φι) + Cι ) o(φι) - C , 2 ,o( 2 ,o) [ rnay be described as entering as a series of second order effects, a second order effect being derived from a product of two first order effects, the first order effects being j ψ 1;1 (φ x ) + C 1 0 (φ 1 ) - C 2 0 (φ 2, o ) 11 > C 2;0 (φ 2 ) , and the absolute precision to which φ is known. The apparatus and method ofthe first embodiment permits measurement and subsequent coπection for certain ofthe cyclic eπors in φ x and φ 2 to a relative precision determined by the magnitude of C 2 0 (φ 2 ) and the absolute precision to which φ is known. As previously noted, the C 2 0 (φ 2 ) term arises principally from effects associated with the interferometer so that cyclic eπors generated in the source and/or beam transport to the interferometer, misalignment of the source with respect to the interferometer, and departures from desired properties of a polarizing beam splitter used to separate reference and measurement beams, the cyclic eπors so generated generally being modified by alignment properties ofthe detector units including mixers and analyzers with respect to the interferometer, can be conected to a high level of precision, i.e., conected up to second order effects, the second order effects being products of two first order effects, the first order effects being certain other cyclic eπors such as C 2 0 (φ 2 ) and/or the absolute precision to which φ is known. The next procedure to be described is the first ofthe two procedures wherein compensation in part for cyclic eπors in φ x is accomplished without the determination in part ofthe cyclic eπors in φ 2 . The first step in the first procedure is to measure φ 2 as a function of φ 2 and a set of values for φ , the required number of different values of φ being dependent on the complexity of ψ x and ψ 2 and the relative precision required for the determined values of ψ x . The set of values of φ introduced by variable phase shifter 81 is controlled by electronic signal 30 from computer 29. The next step is to filter φ 2 by an integral transform so as to eliminate terms with C 2 q (φ 2 ) and S 2 q (φ 2 ) , q ≥ 1 , as factors in the representation of ψ 2 and therefore in the representation of φ 2 . The integral transform φ 2 of φ 2 for stationary values for φ 2 is given by the formula where the range of integration over φ is modulo 2π . For an application where the series representation for ψ 2 given by Eq. (18) has been augmented to include terms with arguments that are subharmonics of φ 2 , the integration range over φ in Eq. (26) will be modified from modulo 2π to a range such that integration of the subharmonic terms is zero. A similar procedure would be used for an application where the series representation for ψ 2 given by Eq. (18) has been augmented to include terms with arguments that coπespond to non-subharmonics and/or nonharmonics of φ 2 generated for example by aliasing. The integral transform in Eq. (26) can be generalized to include non-stationary values for φ 2 such as φ 2 changing a constant rate, constant to a relative precision sufficient to meet requirements of an end use application where for example air turbulence in the measurement path does not present a limitation. The integral filter in Eq. (26) will, in practice, generally be implemented as a digital filter by a digital signal processor [see, e.g., J. G. Proakis and D. G. Manolakis, DIGITAL SIGNAL PROCESSING: Principles, Algorithms, and Applications, Second Edition, (Macmillan, New York) 1992]. The filtered φ 2 , φ 2 , is equivalent to the iterated value obtained for φ 2 in the second procedure described for determination ofthe cyclic eπors in part in φ x and φ 2 . The remaining description ofthe properties of φ 2 is the same as coπesponding portions ofthe description given for the iterated φ 2 . The remaining steps in the determination ofthe cyclic eπors in part in φ x is the same as coπesponding steps in the determination ofthe cyclic enors in part in φ 1 ofthe second procedure, φ 2 being used in place of iterated value of φ 2 . The remaining description of the first procedure for the determination of the cyclic enors in part in φ x is the same as coπesponding portions of the description given for the determination ofthe cyclic eπors in part in φ x and φ 2 ofthe second procedure. An important advantage ofthe first embodiment is with respect to required absolute precision to which φ must be known to meet requirements of an end use application, a given known absolute precision for φ introducing eπors as a factor in a second order effect in phase measurements conected for cyclic enors by the first embodiment. Nonlinearity in generation and in electronic processing ofthe heterodyne signals can also be a source of cyclic enors. Certain ofthe cyclic eπors due to nonlinearity in the generation and in the electronic processing of a heterodyne signal will have heterodyne phases the same as harmonics ofthe heterodyne phase of a desired heterodyne signal and thus contribute to cyclic eπors in the measured phase ofthe desired heterodyne signal. The certain ofthe cyclic eπors due to nonlinearity in the generation and in the signal processing will be measured and monitored by the first embodiment the same as described for cyclic eπors generated by other cited mechanisms. An important advantage ofthe first embodiment is the determination of cyclic eπors in part in situ without modifying the measurement path coπesponding to φ λ . A further advantage ofthe first embodiment, when using the second procedure for determination of cyclic eπors in part in φ x and φ 2 , is that the cyclic enors can be measured during normal operation of a distance measuring interferometer without interfering with the normal operation of distance measuring. The further advantage may be stated accordingly as the measurement and monitoring ofthe cyclic enors in part in φ 1 in situ without modifying the measurement path conesponding to φ x or without interfering with normal operation of a distance measuring interferometer. Another advantage of the first embodiment is that the function of measuring and/or monitoring cyclic eπors in part in ψ λ and φ 2 remains operative even when cyclic enors in φ x and φ 2 are slowly varying functions of time and/or of either φ x or φ 2 , e.g., the cyclic enors depend upon the location ofthe measurement object 92. It will be evident to those skilled in the art that variable phase shifter 81 can alternatively be relocated in FIG. la so as to introduce a phase shift φ into the first portion of beam 33 reflected by beam splitter 63 A instead of into the second portion of beam 33 transmitted by beam splitter 63A without departing from the scope and spirit ofthe present invention. The apparatus and method ofthe first group of embodiments would permit for the alternative location of variable phase shifter 81, the measurement and subsequent coπection for cyclic eπors, excluding C 1 ,o(φι) and the effects of Ci o(φι) > m Φi an d φ 2 to a relative precision determined by the magnitude of Oι ) o(φι) - The cyclic eπor terms Oι ! o(φι) nd C 2; o(φ 2 ) can as previously noted be substantially the same so that the function of the first embodiment is substantially the same for either ofthe two cited locations of variable phase shifter 81. The remaining description ofthe first embodiment with the alternative location of variable phase shifter 81 is the same as conesponding portions ofthe description given for the first embodiment. To those skilled in the art, it will be apparent that the measured quantity Φ is insensitive to effects of gas in the measuring path ofthe interferometer, particularly turbulent effects, the paths ofthe two beams used to generate Φ being substantially coextensive. This latter feature ofthe invention makes it possible to obtain the information about cyclic eπors with reduced systematic eπors that are potentially present as a result of systematic gas flow patterns in a measuring path, i.e., it is not necessary to assume that averaged gas flow patterns in the measurement path produces no systematic eπors. This latter feature ofthe present invention also makes it possible to obtain information about certain ofthe cyclic enors with reduced statistical enors without having to average over a large number of independent measurements of Φ . FIG. 2a depicts in schematic form in accordance with the prefeπed apparatus and method ofthe second embodiment ofthe present invention. The second embodiment is from the first group of embodiments. Interferometer 169 depicted in FIG. 2a comprises a polarizing, heterodyne, single pass interferometer, light beam 109, and source of light beam 109. The description of light beam 109 and source of light beam 109 is the same as coπesponding portions ofthe description given for light beam 9 and source of light beam 9 ofthe first embodiment. Referring to FIG. 2a, a first portion of beam 109 is transmitted by polarizing beam splitter 158A as input beam 109A and a second portion of beam 109 is reflected by polarizing beamsplitter 158A and by minor 158B to form input beam 109B. Input beams 109A and 109B enter interferometer 169. Retroreflectors 191 and 192 and polarizing beam splitter 171 ofthe second embodiment perform like operations as retroreflectors 91 and 92 and polarizing beam splitter 71, respectively, ofthe first embodiment. The description of the propagation of beam 109A through interferometer 169 to form output measurement and reference beams 133A and 134A, respectively, is the same as conesponding portions of the description given for the propagation of beam 9 through interferometer 69 ofthe first embodiment to form output measurement and reference beams 33 and 34, respectively. The description ofthe propagation of beam 109B through interferometer 169 to form output measurement and reference beams 133B and 134B, respectively, is the same as coπesponding portions ofthe description given for the propagation of beam 9 through interferometer 69 ofthe first embodiment to form output measurement and reference beams 33 and 34, respectively, except with respect to the variable phase shifter 181. The description of variable phase shifter 181 ofthe second embodiment is the same as coπesponding portions of the description given for variable phase shifter 81 of the first embodiment. In the second embodiment, variable phase shifter 181 introduces a phase shift φ in the progenitor of output measurement beam 133B. Interferometer 169 introduces relative phase shift φ 3 between beams 133A and 134A and relative phase shift φ 4 + φ between beams 133B and 134B. In a next step as shown in FIG. 2a, beams 133A, 134A, 133B, and 134B exit interferometer 169 and enter detector system 189. In detector system 189, beam 133 A is reflected by minor 163 A and a portion thereof transmitted by polarizing beam splitter 163B to form a first component of phase shifted beam 141 and a portion of beam 134A is reflected by polarizing beam splitter 163B to form a second component of phase shifted beam 141. In addition, beam 133B is reflected by minor 163C and a portion thereof transmitted by polarizing beam splitter 163D to form a first component of phase shifted beam 143 and a portion of beam 134B is reflected by polarizing beam splitter 163D to form a second component of phase shifted beam 143. Phase shifted beams 141 and 143 pass through polarizers 179A and 179B, respectively, and impinge upon photodetectors 185 and 187, respectively, resulting in electrical interference signals, heterodyne signals s 3 and s 4 , respectively, preferably by photoelectric detection. Polarizers 179A and 179B are oriented so as to mix polarization components of phase shifted beam 141 and the polarization components of phase shifted beam 143, respectively. The signals s 3 and ,s 4 have the form 4(t)cos[α ; (t)] i = 3 and 4 (27) where the time-dependent arguments α,*(t) are given by 3 (t) = 2π 1 t + φ 3 + ζ 3 + Λ 3 , (28) α 4 (f) = 2π/if + φ 4 + ζ 4 + Λ 4 + φ , with phase offsets ζ 3 and ζ 4 comprising all contributions to α 3 and α 4 , respectively, that are not related or associated with the optical paths of measurement path 198 or reference paths and excluding nonlinear effects that are represented by Λ 3 and Λ 4 , respectively, and φ representing the phase shift produced by phase shifter 181. The description ofthe representations of s 3 and s 4 by Eq. (27) is the same as the description given ofthe conesponding representations of s x and s 2 ofthe first embodiment by Eq. (9). Heterodyne signals s 3 and s 4 are transmitted to electronic processor 127 for analysis as electronic signals 123 and 125, respectively, in either digital or analog format, preferably in digital format. Referring now to FIG. 2b, electronic processor 127 comprises electronic processors 2274A, 2274B, and 2277 which perform like functions as electronic processors 1274A, 1274B, and 1277 ofthe first embodiment. The description ofthe steps in processing heterodyne signals s 3 and s 4 by electronic processor 127 for phase Φ [subsequently defined by Eq. (30)] is the same as conesponding portions of description given for steps in processing heterodyne signals λ and s 2 , respectively, of the first embodiment by electronic processor 27 for phase Φ [defined by Eq. (15)]. Electronic processors 2274A and 2274B determine the measured phases Φ3 = φ3 +ζ3 + Λ 3(φ3) / (29) Φ 4 = Φ 4 +ζ 4 + Λ 4 (φ 4 ,φ) + φ • Electronic processor 1277 subtracts φ 3 from φ 4 to form Φ . Formally, Φ = (φ 4 - Φ3) (30) or Φ = pk(n 4 L 4 - n 3 L 3 ) + ( . 4 • ζ3) + [Λ4(φ 4 > Φ) - 3(φ3)] + Φ (31) Note that the effects of Doppler shifts produced by the translation of retroreflector 192 cancel out in Φ . The nonlinearity A, is expressed in terms of cyclic nonlinearity ψ,* and a non-cyclic nonlinearity η, , i.e. Λ y = ψ,- + η ; * / = 3 and 4 (32) where ψ, for i = 3 and 4 can to a high order of accuracy be written as Ψ3(φ3) = ∑C 3 , g (φ 3 ) 9 =1 (33) ψ 4 (φ 4 » Φ) = ∑ C 4>9 (<p 4 )cos ?φ + ∑ S 4 , 9 (φ 4 )sin ?φ 9 =1 9 =1 with 9 =1 (34) = Σ sinrφ, , ι = 3 and 4 r=l V [9 = 1, 2 ,..., '?( φ ) = ∑ fl ' cosr Φ' + ∑ sin Φ/ I i = 3 and 4 V /-=l r=l (35) . (36) The C iq and S /g are written in Eqs. (35) and (36) in terms of cosine and sine series terms with arguments ofthe series terms being harmonics of cp ; . For some configurations of interferometers, in particular multiple pass interferometers, it is possible for a system comprising a source, interferometer, and detector to generate cyclic nonlinearities that are subharmonics of φ, or aliases of harmonics and/or subharmonics of φ,* . Should subharmonic, aliases of harmonics, and/or aliases of subharmonics type cyclic enors be present in the system, Eqs. (35) and (36) are augmented so as to include cosine and sine series terms with arguments that are subharmonics, aliases of harmonics, and/or aliases of subharmonics of φ ( * as well as subharmonics, aliases of harmonics, and/or aliases of subharmonics of φ . The subsequent description of procedures the determine the coefficients in the cosine and sine series will be in terms ofthe series representations given by Eqs. (33), (34), (35), and (36) without departing from the spirit and scope ofthe present invention. The difference in the structure ofthe series representations of Eqs. (18) , (19), (20), and (21) ofthe first embodiment and the structure ofthe series representations of Eqs. (33), (34), (35), and (36) ofthe second embodiment is significant. The difference in structures is significant because as will be shown, substantially all ofthe cyclic eπors present in the second embodiment can be measured and monitored in contrast to the situation present in the first embodiment wherein the cyclic enor terms C 1>0 and C 2 0 can not be measured and monitored as described herein. The difference in structure arises because variable phase shifter 181 in the second embodiment is located in the measurement path 198 in between beam splitter 171 and retroreflector 192 whereas in the first embodiment, variable phase shifter 81 is located external to interferometer 69. As an example ofthe effect ofthe relative locations of variable phase shifters 181 and 81, consider the impact ofthe respective locations ofthe variable phase shifters on spurious beams passing along the measurement paths of interferometers 169 and 69, spurious beams being generated by spurious internal multiple reflections. Those spurious beams leading to components ofthe output measurement beams that have made multiple passes to the retroreflectors will experience multiple phase shifts through variable phase shifter 181 ofthe second embodiment in comparison to only a single pass through the variable phase shifter 81 ofthe first embodiment. The magnitude of cyclic eπors C 3 q and C 4 q , q ≥ 1 , can be substantially the same. However, the magnitude of cyclic enors C 3 q and C 4>q , q ≥ l , aτe generally not equal because of departures of reflection and transmission properties ofthe beam splitters 158A, 158B, 163B, 163D, and 173 from desired properties. Noncyclic nonlinearity η,* will be omitted in subsequent description of the second embodiment as so indicated in the description ofthe first embodiment. In a next step, Φ is measured as a function φ 3 and φ 4 for a set of values for φ , the required number of different values of φ being dependent on the complexity of ψ 3 and ψ 4 and the precision required for the measured values of ψ 3 and ψ 4 . The set of values of φ introduced by variable phase shifter 181 is controlled by electronic signal 130 from computer 129. From the measured values of Φ , measured values ofthe quantity ψ 4 (φ 4 ,φ) - ψ 4 (φ 4 ,Φo) + (φ - Φo) (37) are obtained from [φ(φ 3 ,φ)- φ(φ 3 ,φ 0 )] where φ 0 is an initial value used for φ . An expression for ψ 4 (φ 4 , φ) - ψ 4 (φ 4 , φ 0 ) , according to Eq. (33), can be written as ψ 4 (φ 4 ,φ) - ψ 4 (φ 4 ,Φo) = ∑ C 4;9 (φ 4 )(cos9φ - cos?φ 0 ) 9=1 (38) + ∑ S 4,g (φ 4 )( sin ≠ ~ sin ≠ 0 ) 9=1 A procedure is described for the determination of certain ofthe Fourier coefficients listed in Eqs. (35) and (36) for an end use application requiring compensation for cyclic eπors in φ 3 but not in φ 4 . A second procedure is described for the determination ofthe Fourier coefficients listed in Eqs. (35) and (36) for an end use application requiring compensation for cyclic enors in both φ 3 and φ 4 . These two procedures are presented as examples of two possible procedures and the presentation of particular two procedures by way of examples does not limit the scope and spirit ofthe present invention. The second ofthe two procedures is described first wherein compensation for cyclic eπors in both φ 3 and φ 4 is accomplished. For the second procedure, the Fourier coefficients a 4qr , b 4qr , a 4 ' qr , and b 4 ' qr , q ≥ l and r ≥ l, can be determined as a function of q and r by one of several procedures for the magnitude of cyclic eπors typically found in interferometers. For conditions where the cyclic nonlinearity term ψ 4 ≤ 1/10 , conesponding to an eπor in position of • ≤ 5 nm in a single pass interferometer with λ = 633 nm, the Fourier coefficients representing [ψ 4 (φ 4 ,φ) - ψ 4 (φ 4 ,φ 0 )l can be obtained from the measured quantity Φ by an efficient iterative process. For a situation where the cyclic nonlinearity term ψ 4 ~ 1 , the Fourier coefficients in [ψ 4 (φ 4 ,φ) - ψ 4 (φ 4 ,φ 0 )l can be obtained by generation of a series of simultaneous transcendental equations ofthe Fourier coefficients. The iterative procedure for determination ofthe Fourier coefficients will be described herein, the iterative process the same as the conesponding iterative process ofthe second procedure described for the first embodiment, the condition ψ 4 ≤ 1/10 generally being met in interferometry systems and the iterative procedure being a simpler procedure. The next step in the cyclic eπor evaluation procedure addresses the cyclic enor ψ 3 in φ 3 using (φ 4 - ψ 4 - φ) as the variable of integration in a Fourier analysis of [(φ 4 - ψ 4 ) - φ - Φ] , the difference between φ 3 and (φ 4 - ψ 4 - φ) being small other than a constant (ζ 4 - ζ 3 ) which can be determined. The Fourier coefficients of cos rφ 3 and sinrφ 3 , Σ fl 3,9, , Σ< qr r = 1, 2, 3, (39) 9=1 respectively, can be determined as a function of r by one of several procedures. For the conditions where the cyclic nonlinearity term ψ 3 ≤, 1/10 , a condition generally being met in interferometry systems, the Fourier coefficients in ψ 3 can obtained from the measured quantity [(φ 4 - ψ 4 ) - φ - φ| by an iterative process the same as the coπesponding iterative process described for the second procedure ofthe first embodiment. The next procedure to be described is the first of the two procedures wherein compensation for cyclic enors in φ 3 is accomplished with the effects of cyclic eπors in φ 4 being eliminated by a signal filtering process although the cyclic eπors in φ 4 are not determined. The first step in the first procedure is to measure φ 4 as a function of φ 4 and a set of values for φ , the required number of different values of φ being dependent on the complexity of ψ 3 and ψ 4 and the relative precision required for the measured values of ψ 3 . The set of values of φ introduced by variable phase shifter 181 is controlled by electronic signal 130 from computer 129. The next step is to filter φ 4 by an integral transform so as to eliminate terms with C 4 ? (φ 4 ) and S j9 (φ ) , q ≥ 1 , as factors in the representation of ψ 4 and therefore in the representation of φ 4 . The integral transform φ 4 of φ 4 for stationary values for φ 4 is given by the formula where the range of integration over φ is modulo 2π . For an application where the series representation for ψ 4 given by Eq. (33) has been augmented to include terms with arguments that are subharmonics of φ 4 , the integration range over φ in Eq. (40) will be modified from modulo 2π to a range such that integration of the subharmonic terms is zero. A similar procedure would be used for an application where the series representation for ψ 4 given by Eq. (33) has been augmented to include terms with arguments that conespond to nonharmonics and/or non- subharmonics of φ generated for example by aliasing. The integral transform in Eq. (40) can be generalized to include non-stationary values for φ 4 such as φ 4 changing a constant rate, constant to a relative precision sufficient to meet requirements of an end use application where for example air turbulence in the measurement path does not present a limitation. The integral filter in Eq. (40) will, in practice, generally be implemented as a digital filter by a digital signal processor (see, e.g., J. G. Proakis and D. G. Manolakis, ibid.). The filtered φ 4 , Φ » is equivalent to the iterated value obtained for φ 4 in the second procedure described for determination ofthe cyclic enors in φ 3 and φ 4 . The remaining description ofthe properties of φ 4 is the same as coπesponding portions ofthe description given for the iterated φ 4 . The remaining steps in the determination ofthe cyclic enors in φ 3 is the same as conesponding steps in the determination ofthe cyclic eπors in φ 3 ofthe second procedure, φ 4 being used in place of iterated value of φ 4 . The remaining description ofthe first procedure for the determination ofthe cyclic enors in part in φ 3 is the same as conesponding portions ofthe description given for the determination ofthe cyclic enors in part in φ 3 and φ 4 ofthe second procedure. The remaining description ofthe second embodiment ofthe present invention is the same as conesponding portions ofthe description given for the first embodiment. The principal advantage ofthe second embodiment ofthe present invention in relation to the first embodiment of the present invention is in the substantially complete determination of the cyclic eπors in an interferometer system, the first embodiment ofthe present invention permitting a determination of a subset of the cyclic eπors. However, the second embodiment of the present invention can require significantly more data to obtain a given level of statistical accuracy for the determined cyclic enors as compared to the amount of data required with the first embodiment for a cyclic enor determination made with gas in the measurement path. In accordance with a variant of the second embodiment ofthe present invention, an apparatus and method is described from the first group of embodiments. The variant ofthe second embodiment is depicted in diagrammatic form in FIG. 2c. The difference between the variant ofthe second embodiment and the second embodiment is in the location of the variable phase shifter 181. In the variant ofthe second embodiment, interferometer 169A is the same as interferometer 169 of second embodiment except for variable phase shifter 181 that is located external to interferometer 169A. The properties ofthe variant ofthe second embodiment with respect to cyclic enors in the measured relative phase of measurement and reference output beams are formally equivalent to the properties ofthe first embodiment with respect to the cyclic eπors. The difference in the properties ofthe second embodiment and the variant ofthe second embodiment with respect to cyclic eπors in the measured relative phase of measurement and reference output beams is a consequence ofthe respective locations ofthe variable phase shifter 181 in the second embodiment and the variant ofthe second embodiment, the location being internal to an interferometer in one case and external to an interferometer in a second case. The remaining description of the variant ofthe second embodiment is the same as coπesponding portions ofthe description given for the second embodiment ofthe present invention. FIG. 3a depicts in schematic form in accordance with the prefeπed apparatus and method ofthe third embodiment ofthe present invention. The third embodiment is from the first group of embodiments. Interferometer 269 depicted in FIG. 3a comprises a differential plane minor interferometer, object minor 292, light beam 209, and source of light beam 209. The description of light beam 209 and source of light beam 209 is the same as conesponding portions ofthe description given for light beam 9 and source of light beam 9 ofthe first embodiment. Referring to FIG. 3a, beam 209 enters interferometer 269. A first portion of beam 209 is transmitted by polarizing beam splitter 258A as input measurement beam 211 polarized in the plane of FIG. 3a. A second portion of beam 209 is reflected by polarizing beamsplitter 258A and reflected by minor 258B to form input beam 212 polarized orthogonal to the plane of FIG. 3a. Beam 212 is transmitted by half wave phase retardation plate 279A as input reference beam 214 polarized in the plane of FIG. 3a, half wave phase retardation plate 279A being oriented so as to rotate the plane of polarization of beam 212 by 90°. Measurement beam 211 is transmitted by polarizing interface 273 of polarizing beam splitter 271 and reflected back by object minor 292 to retrace its path to polarizing beam splitter 271 as beam 213. The measurement beam double passes a quarter wave phase retardation plate 277 so that measurement beam 213 is polarized orthogonal to the plane of FIG. 3a, quarter wave phase retardation plate 277 being located between object minor 292 and polarizing beam splitter 271 and oriented so that a double pass rotates the plane of polarization ofthe measurement beam by 90°. Measurement beam 213 is reflected by polarizing interface 273 and retroreflected by retroreflector 291 as beam 215 polarized perpendicular to the plane of FIG. 3a. Measurement beam 215 is reflected by polarizing interface 273 and reflected back by object minor 292 to retrace its path to polarizing beam splitter 271 as beam 217. The measurement beam double passes quarter wave phase retardation plate 277 so that measurement beam 217 is polarized in the plane of FIG. 3a. Measurement beam 217 is transmitted by polarizing mterface 273 as exit measurement beam 233 polarized in the plane of FIG. 3a. Reference beam 214 is transmitted by polarizing interface 273 and reflected back by reference minor 291 to retrace its path to polarizing beam splitter 271 as beam 216. The reference beam double passes quarter wave phase retardation plate 277 so that reference beam 216 is polarized orthogonal the plane of FIG. 3a. Reference beam 216 is reflected by polarizing interface 273 and retroreflected by retroreflector 291 as beam 218 polarized perpendicular to the plane of FIG. 3a. Reference beam 218 is reflected by polarizing interface 273 and reflected back by reference minor 291 to retrace its path to polarizing beam splitter 271 as beam 220. The reference beam double passes quarter wave phase retardation plate 277 so that reference beam 220 is polarized in the plane of FIG. 3a. Reference beam 220 is transmitted by polarizing interface 273 as exit reference beam 234 polarized in the plane of FIG. 3a. Exit reference beam 234 is transmitted by polarizing beam splitter 263A as one component of phase-shifted output beam 241. Exit measurement beam 233 is transmitted by half wave phase retardation plate 279B, reflected by minor 263B, and reflected by polarizing beam splitter 263A as a second component of phase-shifted output beam 241. Half wave phase retardation plate 279B is oriented so as to rotate the plane of polarization of exit measurement beam 233 by 90°. In a next step as shown in FIG. 3a, phase-shifted output beam 241 passes through polarizer 279C and impinges upon photodetector 285 resulting in a electrical interference signal, heterodyne signal s 5 , preferably by photoelectric detection. Polarizer 279C is oriented so as to mix polarization components of phase- shifted output beam 241. Signal ^ 5 has the form where time-dependent argument α 5 (t) is given by α 5 (t) = 2π/ 1 t + φ 5 + ζ 5 + Λ 5 (φ 5 ,φ) (42) and φ is the phase shift introduced into both the measurement and reference beams by translation of retroreflector 291 by translation transducer 267B. The description ofthe representation of s 5 by Eq. (41) is the same as the description given ofthe conesponding representations of s x and s 2 ofthe first embodiment by Eq. (9). Heterodyne signal s 5 is transmitted to electronic processor 227 for analysis as electronic signal 223 in either digital or analog format, preferably in digital format. Note that φ does not appear directly in Eq. (42) for 5 except through Λ 5 (φ 5 ,φ) in contrast to measured phases φ 2 and φ 4 of the first and second embodiments, respectively. The reason that φ does not appear directly in Eq. (42) is because translation of retroreflector 291 introduces a phase shift φ in both measurement and reference beams simultaneously. However, φ does appear in Λ 5 (φ 5 ,φ) though terms which represent cyclic eπors arising for example from spurious beams making multiple passes to the object minor 292, the spurious beams being generated for example by spurious reflections and/or departures of quarter wave phase retardation plates from desired properties. Referring to FIG. 3b, electronic processor 227 comprises electronic processor 2274. Electronic processor 2274 performs like function as electronic processor 1274A ofthe first embodiment. The description ofthe step in processing heterodyne signal s 5 by electronic processor 227 for phase φ 5 is the same as coπesponding portions ofthe description given of steps in the processing of heterodyne signal s x of first embodiment by electronic processor 27. The measured phase φ 5 is expressed in terms of other quantities according to the equation where phase offset ζ 5 comprises all contributions to φ 5 that are not related or associated with the optical paths of the measurement path 298 or reference paths and are not related or associated with nonlinear effects and Λ 5 comprises the nonlinear effects. The nonlinearity Λ 5 is expressed in terms of cyclic nonlinearity ψ 5 and a non-cyclic nonlinearity η 5 , /. e. Λ 5 = ψ 5 + η5 (44) The cyclic nonlinearity ψ 5 can to a high order of accuracy be written as Ψ 5 (φ 5 .Φ) = C 5 ,o(φ 5 ) + ∑ C 5;9 (φ 5 ) cos ?Φ + ∑^ 5,9 ( 5 )sin?φ 9=1 9=1 (45) with C 5*M = b 5 , 9 >- sinr s 9 = 0, 1, 2 ,... (46) •^(φs) = ∑ fl 5 Λ r ∞s φ 5 + ∑b5, 9 r sinrφ 5 , 9 = 0, 1, 2 ,... . (47) r =l r=l The 5>9 and S 5 q are written in Eqs. (46) and (47) in terms of cosine and sine series terms of harmonics of φ 5 . For some configurations of interferometers, in particular multiple pass interferometers, it is possible for a system comprising a source, interferometer, and detector to generate cyclic nonlinearities that are subharmonics of φ 5 . Should subharmonic cyclic eπors be present in the system, Eqs. (46) and (47) are augmented so as to include cosine and sine series terms with arguments that are subharmonics of φ 5 as well as subharmonics φ . The subsequent description of procedures to determine the coefficients in the cosine and sine series will be in terms ofthe series representations given by Eqs. (45), (46), and (47) without departing from the spirit and scope ofthe present invention. It is possible for a system comprising a source, interferometer, detector, and digital signal processing to generate cyclic nonlinearities that are neither subharmonics or harmonics of φ, . The non-subharmonic, nonharmonic cyclic eπors are produced for example by aliasing in the digital signal processing and have frequencies which are aliases of harmonics and subharmonics of φ,* . Should non- subharmonic, nonharmonic cyclic enors be present in a system, Eqs. (46) and (47) are augmented so as to include cosine and sine series terms with arguments that are the appropriate aliases of harmonics and/or subharmonics of φ ; * . The subsequent description of procedures to determine the coefficients in the cosine and sine series will be in terms ofthe series representations given by Eqs. (45), (46), and (47) without departing from the spirit and scope of the present invention. The cyclic enor terms C 5 q and S 5 q , q ≥ 1 , are principally the result of cyclic eπor generation by interferometer 269 and object minor 292, e.g., spurious internal multiple reflections of beams in the measurement and/or reference legs and/or departures of quarter wave phase retardation plates and polarizing beam splitters from desired properties. The remaining cyclic term C 5 0 is generally effected by polarization and frequency mixing from a number of different sources operating independently or in combination, e.g., polarization and frequency mixing in the interferometer source, misalignment of the interferometer source with respect to the interferometer, departures from desired properties of a polarizing beam splitter used to separate reference and measurement beams based on relative states of polarization, and alignment properties ofthe detector units including mixers and analyzers with respect to the interferometer. The non-cyclic nonlinearity η,* will be omitted in subsequent description ofthe third embodiment as so indicated in the description ofthe first embodiment. In a next step, φ 5 is measured as a function φ for a set of values for φ 5 , the required number of different values of φ and φ 5 being dependent on the complexity of ψ 5 and the precision required for the measured values of ψ 5 . The set of values of φ introduced by translation of retroreflector 291 by translation transducer 267B is controlled by electronic signal 230 from computer 229. From the measured values of φ 5 , measured values ofthe quantity Φ5(Φ5>Φ)-Φ5( 5>ΦO) ( 48 ) are obtained where φ 0 is an initial value used for φ . An expression for φ 5 (φ 5 ,φ)-φ 5 (φ 5 ,φ 0 ), according to Eq. (45), can be written as Φ5( 5 > Φ)-Φ5(Φ5>ΦO) = ∑C 5)g (φ 5 )(cθS9φ-COS9φ 0 ) 9=1 Σ S 5,q ( φ 5 )( sin ≠ - sin ≠ 0 ) 9=1 The Fourier coefficients C 5? (φ 5 ) and S 5 ^(φ 5 ), q ≥ 1 , are obtained in a next step by a Fourier analysis of φ 5 (φ 5 , φ) - φ 5 (φ 5 , φ 0 ) for the set of values of Φ5- The Fourier coefficients a 5 qr , b bqr , a' qr , and b' qr , q≥l and r > 1 , can be determined as a function of q and r by iterative procedures for the magnitude of cyclic enors typically found in interferometers. For conditions where the cyclic nonlinearity term ψ 5 ≤ 1/10, the Fourier coefficients a 5qr , b 5qr , α^.and b 5 ' qr , q≥l can be obtained from the Fourier coefficients C 5q (φ 5 ) and S 5 „ (φ 5 ) , q≥l, the Fourier coefficients obtained in the last step, by an efficient iterative process. A cyclic nonlinearity term ψ 5 ≤, 1/10 conesponds to an eπor in measurement object position of ≤ 5 nm in a single pass interferometer with λ = 633 nm. For a situation where the cyclic nonlinearity term ψ 5 ~ 1 , the Fourier coefficients a 5 qr , b 5qr , a 5 ' qr , and b' qr , q≥l can be obtained from the Fourier coefficients C 5g (φ 5 ) and S 5 ^(φ 5 ) , q ≥ 1 , by generation of a series of simultaneous transcendental equations ofthe Fourier coefficients. The description ofthe iterative procedure for determination of the Fourier coefficients a qr , b qr , « 5gr ,and b b ' qr , q≥l and r > 1 , is the same as conesponding portions ofthe description ofthe iterative process described for second procedure of the first embodiment, the condition ψ 5 ≤ 1/10 generally being met in interferometry systems. The relative precision to which the cyclic enors [ψ 5 (φ 5 ,φ) - C 5)0 (φ 5 )| can be determined by the third embodiment will have a magnitude ofthe order of magnitude of C 50 (φ 5 )/2 , the cyclic eπor term C 50 (φ 5 ) being expressed in radians, combined with an absolute precision, expressed in radians, to which φ is known. The magnitude of the cyclic enor term C 50 (φ 5 ) effects the relative precision to which the cyclic eπors |Ψ 5 (φ 5 ,φ) - Cs^Φs)] can be determined because C 50 (φ 5 ) is not determined in the third embodiment. The residual cyclic eπors from [ψ 5 (φ 5 ,φ)-C 5;0 (φ 5 )] in φ 5 after conection for [ψ 5 (φ 5 ,φ)- C 5>0 (φ 5 )] may be described as entering as a second order effects comprising products of first order effects, first order effects such as [ψ 5 (φ 5 , φ) - C 5>0 (φ 5 )] , C 5 0 (φ 5 ) , and the absolute precision to which φ is known. The apparatus and method ofthe third embodiment permits measurement and subsequent conection for cyclic eπors in φ 5 , excluding C 5 0 (φ 5 ) which is not determined in the third embodiment, to a relative precision determined by the magnitude of [Ψ 5 (φ 5 ,φ) - C 5 0 (φ 5 )1 and C 5 0 (φ 5 ) and the absolute precision to which φ is known. As previously noted, the C 5 0 (φ 5 ) term is generally effected by polarization and frequency mixing from a number of different sources operating independently or in combination, e.g., polarization and frequency mixing in the interferometer source, misalignment ofthe interferometer source with respect to the interferometer, departures from desired properties of a polarizing beam splitter used to separate reference and measurement beams based on relative states of polarization, and alignment properties ofthe detector units including mixers and analyzers with respect to the interferometer. Thus, the third embodiment can measure and compensate for cyclic eπors principally the result of cyclic eπor generation by interferometer 269 and object minor 292, e.g., spurious internal multiple reflections of beams in the measurement and/or reference legs and/or departures of quarter wave phase retardation plates and polarizing beam splitters from desired properties. The remaining description ofthe third embodiment is the same as conesponding portions ofthe description given for the first embodiment ofthe present invention. It will be evident to those skilled in the art that phase shift φ introduced by translation of retroreflector 291 can be introduced by an additional phase shifter, the same type of phase shifter as phase shifter 81 ofthe first embodiment, located between retroreflector 291 and beam splitter 271 position to intercept conesponding measurement and reference beams, e.g., beams 215 and 218, without departing from the scope and spirit ofthe present invention. A variant ofthe third embodiment is described wherein the phase shift φ is modulated at a frequency large compared to the heterodyne frequency ofthe heterodyne signal and with an amplitude of modulation modulo π and the detected heterodyne phase is filtered by a low pass filter to effectively filter out the cyclic enor I Ψ 5 (cp 5 , φ) - C 5j0 (φ 5 )| • For an application where the series representation for ψ 5 given by Eq. (45) has been augmented to include terms with arguments that are subharmonics of φ 5 , the amplitude of modulation of φ will be modified from modulo π to a amplitude such that integration ofthe subharmonic terms by the low pass filtering is zero. The description of the first prefeπed embodiment noted that the configuration of interferometer illustrated in FIGS, la-lf is known in the art as polarized Michelson interferometer. The description ofthe third embodiment noted that the configuration ofthe interferometer illustrated in FIGS. 3a and 3b is known in the art as a differential plane minor interferometer. Other forms ofthe Michelson interferometer and forms of other interferometers such as the high stability plane minor interferometer, or the angle-compensating interferometer or similar device such as is described in an article entitled "Differential interferometer anangements for distance and angle measurements: Principles, advantages and applications" by C. Zanoni, VDI Berichte Nr. 749, 93-106 (1989), may be incorporated into the apparatus of the present invention, the foregoing article being herein incorporated by reference, as when working with stages commonly encountered in the lithographic fabrication of integrated circuits without departing from the spirit or scope of the present invention. It will be evident to those skilled in the art that the apparatus and method ofthe first embodiment and the apparatus and method ofthe third embodiment can be combined into a combined apparatus and method wherein substantially complete determination of the cyclic eπors for the interferometer system ofthe combined apparatus and method. The description ofthe first embodiment noted that the apparatus and method ofthe first embodiment permits measurement and subsequent conection for cyclic eπors generated principally in the source and/or beam transport to the interferometer, misalignment ofthe source with respect to the interferometer, and departures from desired properties of a polarizing beam splitter used to separate reference and measurement beams, the cyclic enors to generated generally being modified by alignment properties of the detector units including mixers and analyzers with respect to the interferometer. The description ofthe third embodiment noted that the apparatus and method ofthe third embodiment permits measurement and subsequent coπection for cyclic eπors principally the result of cyclic enor generation by interferometer 269 and object minor 292, e.g., spurious internal multiple reflections of beams in the measurement and/or reference legs and/or departures of quarter wave phase retardation plates and polarizing beam splitters from desired properties. The principal advantage ofthe combined apparatus and method ofthe first and third embodiments in relation to either the first embodiment or the third embodiment is the same as the principal advantage ofthe second embodiment, i.e. the substantially complete determination ofthe cyclic eπors for an interferometer system, the first embodiment and the third embodiment permitting determinations of substantially mutually exclusive subsets ofthe cyclic eπors. FIG. 4 depicts in schematic form in accordance with the prefened apparatus and method ofthe fourth embodiment of the present invention. The fourth embodiment is from the first group of embodiments. The apparatus ofthe fourth embodiment comprises an interferometer 310 having a dynamic beam steering assembly which redirects the measurement and reference beams to minimize undesired consequences of changes in the angular orientation of the measurement object minor 334. As shown in FIG. 4, polarizing beam splitter 312 in an interferometer 310 receives an input beam 314 from a laser source (not shown) and separates input beam 314 into reference beam 316 (dotted line) and measurement beam 318 (solid line), which are linearly polarized orthogonal to one another. The polarizing beam splitter 312 includes a polarizing interface 362 for reflecting beams having a polarization orthogonal to the plane of FIG. 4 and a back reflective surface 364 for reflecting beams transmitted through polarizing interface 362. The description of input beam 314 is the same as the coπesponding description given for light beam 9 ofthe first embodiment. Polarizing beam splitter 312 directs the measurement beam 318 to a beam steering assembly 320, which includes a beam steering minor 322 and a pair of piezoelectric transducers 324 and 326. The transducers are coupled to the beam steering minor 322 by flexures to orient the beam steering minor in response to a signal 328 from a servo controller 330. The beam steering assembly may include capacitance gauges to measure changes in orientation and/or position ofthe beam steering minor 322. The capacitance gauges may also be used to measure and/or monitor the properties ofthe piezoelectric transducers 324 and 326. Beam steering assembly 320 directs the measurement beam through reference retroreflector 332, reference retroreflector 332 being truncated so that a beam passing centrally through retroreflector 332 is not retroreflected, to contact a stage minor 334, i.e., the measurement object, at substantially normal incidence. Stage minor 334 then reflects the measurement beam back to retrace its path to beam steering assembly 320 and polarizing beam splitter 312. The measurement beam double passes a quarter wave plate 336, which is positioned between the beam steering assembly 320 and polarizing beam splitter 312 and which rotates the linear polarization ofthe measurement beam by 90°. Polarizing beam splitter 312 directs the reference beam 316 to the beam steering assembly 320, which in turn directs the reference beam to reference retroreflector 332. The reference retroreflector then directs the reference beam back to beam steering assembly 320 and onto polarizing beam splitter 312. The reference beam also double passes quarter wave plate 336, which rotates the linear polarization ofthe reference beam by 90°. Polarizing beam splitter 312 then recombines the polarization-rotated reference and measurement beams to form overlapping exit reference and measurement beams, which together form an output beam 340. A beam splitter 342 sends a portion of output beam 340 to a detector system 344, which measures the difference in directions of propagation ofthe exit reference and measurement beams. The detector system sends an eπor signal 350 indicative of any such difference in directions of propagation to servo controller 330, which sends signal 328 to beam steering assembly 320 in response to the eπor signal. The beam steering assembly 320 changes, in response to signal 328, the orientation of beam steering minor 322 preferably about a nodal point of reference retroreflector 332, changes in orientation of beam steering minor 322 about a nodal point of reference retroreflector 332 producing substantially reduced lateral shear effects experienced by the reference beam. Alternatively when the direction ofthe input beam 314 is constant, detector system 344 can measure the difference in position ofthe exit measurement beam from a reference position in the detector system and generate eπor signal 350 indicative of that difference in position, a difference in position ofthe exit measurement beam being the consequence of a change in direction of propagation ofthe exit measurement beam component of output beam 340. For example, the reference position can be the position ofthe exit measurement beam in the detector system conesponding to the measurement beam retroreflecting from stage minor 334, i.e., contacting the stage minor at normal incidence, and the stage minor 334 being at a nominal null orientation. In other embodiments, the detector system can include multiple detectors for determining the direction and position ofthe exit reference and measurement beams and can generate an enor signal based on such information. Changes in the angular orientation of stage minor 334 change the direction ofthe measurement beam and the direction of subsequent exit measurement beam. This causes detector system 344 to generate eπor signal 350. Servo-controller 330 responds to the enor signal by directing beam steering assembly 320 to reorient beam steering minor 322 so as to minimize the eπor signal, e.g., by directing the measurement beam to the stage minor at normal incidence. As a result, the exit reference and measurement beams remain substantially parallel to one another and the position ofthe exit measurement beam remains substantially constant over a range of angular orientations ofthe stage minor. Furthermore, since beam steering assembly 1520 also redirects twice both the reference and measurement beams and the measurement beam is directed to the stage minor at normal incidence, there is no change in the optical path length difference ofthe reference and measurement beams to first order in any angular orientation change of the stage minor and beam steering assembly when the paths ofthe measurement and reference beams have the substantially the same centroids and to first order in translation of beam steering minor 322 in a direction normal to the reflecting surface of beam steering minor 322. The remainder of output beam 340, after beam splitter 342, passes through a polarizer 345, which mixes polarizations ofthe exit reference and measurement beams to form a mixed beam 346. A signal processing system 348 measures the intensity of the mixed beam preferably by photoelectric detection to produce an electrical interference signal or electrical heterodyne signal, s 7 , and extracts the phase α 7 of the electrical heterodyne signal ,s 7 . Signal s η has the form = A n {t) cos α '(<)] (50) the time-dependent argument α 7 (t) being given by α 7 (t) = 2π 1 t + φ 7 + ζ 7 + Λ 7 (φ 7 ,φ) (51) and φ is the phase shift introduced into both the measurement and reference beams by translation of beam steering minor 322 by piezoelectric transducers 324 and 326. The description ofthe representation of .s 7 by Eq. (50) is the same as the description given ofthe conesponding representations of s λ and s 2 ofthe first embodiment by Eq. (9). Note that φ does not appear directly in Eq. (51) for α 7 except through Λ 7 (φ 7 ,φ) in contrast to measured phases φ 2 and φ 4 ofthe first and second embodiments, respectively. The reason that φ does not appear directly in Eq. (51) is because translation of beam steering minor 322 introduces a phase shift φ in both measurement and reference beams simultaneously. However, φ does appear in Λ 7 (c 7 ,φ) though terms which represent cyclic eπors arising for example from spurious beams making multiple passes to the stage minor 334, the spurious beams being generated for example by spurious reflections and/or departures of quarter wave phase retardation plates from desired properties. Referring to FIG. 4, signal processor 348 comprises an electronic processor that performs like function as electronic processor 1274A ofthe first embodiment. The description ofthe step in processing heterodyne signal ,s 7 by signal processor 348 for phase φ 7 is the same as conesponding portions ofthe description given of steps in the processing of heterodyne signal s λ of first embodiment by electronic processor 27. The measured phase φ 7 is expressed in terms of other quantities according to the equation φ 7 = φ 7 + ζ 7 + Λ(φ 7 ,φ) (52) where phase offset ζ 7 comprises all contributions to φ 7 that are not related or associated with the optical paths of measurement or reference paths and does not include nonlinear effects and Λ 7 comprises the nonlinear effects. The nonlinearity Λ 7 is expressed in terms of cyclic nonlinearity ψ 7 and a noncyclic nonlinearity η 7 , i.e. A η =ψ 7 +η 7 (53) Cyclic nonlinearity ψ can to a high order of accuracy be written as (54) with , (φv) = ∑ fl 7, 9 r cosrφ 7 + ^ r sinrφ 7 9 = 0,1,2,.. (55) ■r=l r=l S ι,q (Φv) = ∑ α V C0Sr Φ7 +∑* sinrφ 7 , 9 = 0,1,2,... . (56) Vr=l r=l The coefficients C lq and S 7g are written in Eqs. (55) and (56) in terms of cosine and sine series terms of harmonics of φ 7 . For some configurations of interferometers, in particular multiple pass interferometers, it is possible for a system comprising a source, interferometer, and detector to generate cyclic nonlinearities that are subharmonics of φ 7 . Should subharmonic cyclic enors be present in the system, Eqs. (55) and (56) are augmented so as to include cosine and sine series terms with arguments that are subharmonics of φ 7 as well as subharmonics φ . The subsequent description of procedures to determine the coefficients in the cosine and sine series will be in terms ofthe series representations given by Eqs. (54), (55), and (56) without departing from the spirit and scope ofthe present invention. It is possible for a system comprising a source, interferometer, detector, and digital signal processing to generate cyclic nonlinearities that are neither subharmonics or harmonics of φ, . The non-subharmonic, nonharmonic cyclic eπors are produced for example by aliasing in the digital signal processing and have frequencies which are aliases of harmonics and subharmonics of φ,* . Should non- subharmonic, nonharmonic cyclic eπors be present in a system, Eqs. (55) and (56) are augmented so as to include cosine and sine series terms with arguments that are the appropriate aliases of harmonics and/or subharmonics of φ ; * . The subsequent description of procedures to determine the coefficients in the cosine and sine series will be in terms ofthe series representations given by Eqs. (54), (55), and (56) without departing from the spirit and scope ofthe present invention. The cyclic eπor terms C l q and S l q , q ≥ 1 , are principally the result of cyclic eπor generation by interferometer 310 and stage minor 334, e.g., spurious internal multiple reflections of beams in the measurement and/or reference legs and/or departures of quarter wave phase retardation plates and polarizing beam splitters from desired properties. The remaining cyclic term C 7 0 is generally effected by polarization and frequency mixing from a number of different sources operating independently or in combination, e.g., polarization and frequency mixing in the interferometer source, misalignment of the interferometer source with respect to the interferometer, departures from desired properties of a polarizing beam splitter used to separate reference and measurement beams based on relative states of polarization, and alignment properties ofthe detector units including mixers and analyzers with respect to the interferometer. The description ofthe processing for the evaluation ofthe Fourier coefficients a lqr , b lqr , a l ' qr , and b l ' qr , q ≥ l and r ≥ 1 , is the same as coπesponding portions ofthe description ofthe iterative process described for the third embodiment. The description of the properties of [ψ 7 (φ 7 ,φ) - C 7 0 (φ 7 )| and C 7 0 (φ 7 ) is the same as coπesponding portions given for the descriptions ofthe properties of |Ψ 5 (φ 5 ,φ) - C 5 o(φ 5 )j and C 5 0 (φ 5 ), respectively, ofthe third embodiment. The remaining description ofthe fourth embodiment is the same as conesponding portions ofthe description given for the third embodiment. The description ofthe fourth embodiment noted that the configuration ofthe interferometer illustrated in FIG. 4 is an interferometer having a dynamic beam steering assembly. Other forms of interferometer having a dynamic beam steering assembly, such as described in commonly owned, U.S. Patent Application 09/157,131, filed Sept. 18, 1998, and entitled "Interferometer Having A Dynamic Beam Steering Assembly" by Henry A. Hill and Peter de Groot, may be incorporated into the apparatus of the present invention, the foregoing U.S. Patent Application being herein incoφorated in its entirety by reference, as when working with stages commonly encountered in the lithographic fabrication of integrated circuits without departing from the spirit or scope ofthe present invention. FIG. 5a depicts in schematic form in accordance with the prefened apparatus and method of a fifth embodiment ofthe present invention from the second group of embodiments. The interferometer depicted in FIG. 5a is a double pass differential plane minor interferometer. In accordance with the prefened apparatus and method of the fifth embodiment, the description of sources 401 and 402 is the same as coπesponding portions ofthe description given for source 1 ofthe first embodiment except with respect to the control ofthe wavelength of source 402. Sources 401 and 402 generate beams 407 and 408, respectively, with wavelengths λ and λ 10 , respectively. The wavelength λ 10 of the beam from source 402 is controlled by eπor signal 444 from computer and controller 429. As shown in FIG. 5a, a first portion of beam 407 is reflected by nonpolarizing beam splitter 451 A and a portion thereof reflected by nonpolarizing beam splitter 651B to form a first component of beam 440. In a next step, a first portion of beam 408 is reflected by nonpolarizing beam splitter 451C, reflected by minor 451D, and a portion thereof transmitted by nonpolarizing beam splitter 651B to form a second component of beam 440. Beam 440 impinges on wavelength monitor 484 of a well known type configured to monitor the ratio (λ 9 /λ 10 ) . The measured value ofthe ratio is transmitted to computer and controller 429 as electronic signal 420. Wavelength monitor 484 may comprise, e.g., interferometers with or without a vacuum in a measurement leg and or nonlinear elements such as β -BaBO 3 to double the frequency of a beam by second harmonic generation, SHG. Computer and controller 429 generates eπor signal 444 related to the difference between the wavelength ratio (λ 9 /λ 10 ) , the ratio as received from signal 420, and a ratio specified by computer and controller 429. Eπor signal 444 may control the wavelength, for example, of a laser by controlling the length ofthe laser cavity with a piezoelectric transducer or of a diode laser by controlling the injection cuπent ofthe diode laser. Continuing with FIG. 5a, a second portion of beam 407 is transmitted by nonpolarizing beam splitter 451A and passes through modulator 403 to form beam 409, the description thereof being the same as coπesponding portions given in the first embodiment respect to the generation of beam 9 from beam 7. The frequency shifted component of beam 409 is frequency shifted by frequency f x , the frequency of driver 405. In a next step, a second portion of beam 408 is transmitted by nonpolarizing beam splitter 451C and passes through a modulator 404 becoming light beam 410. Modulator 404 is excited by an electronic driver 405, similar to the excitation of modulator 403 and electronic driver 405. The frequency shifted component of beam 410 is frequency shifted by frequency f . Continuing with FIG. 5a, a first portion of beam 409 is transmitted by polarizing beam splitter 453A and a portion thereof transmitted by nonpolarizing beam splitter 453B to form a first component of beam 415. A first portion of beam 410 is transmitted by polarizing beam splitter 453E, is reflected by minor 453G, and a portion thereof reflected by nonpolarizing beam splitter 453B to form a second component of beam 415. The wavelengths ofthe first and second components of beam 415 are λ 9 and λ 10 , respectively, and both components are polarized in the plane of FIG. 5a. In a next step, a second portion of beam 409 is reflected by polarizing beam splitter 453A, is reflected by minor 453D, and passes through half wave phase retardation plate 479A to form beam 412. Polarizer 479A is oriented so as to rotate the polarization ofthe beam passing through polarizer 479A by 90 degrees. Beam 412 is polarized in the plane of FIG. 5a. The frequency of beam 412 is f(c/λ 9 ) + fΛ where c is the speed of light in a vacuum. A second portion of beam 410 is reflected by polarizing beam splitter 453E, is reflected by minor 453F, and passes through half wave phase retardation plate 479B to form beam 414. Polarizer 479B is oriented so as to rotate the polarization ofthe beam passing through polarizer 479B by 90 degrees. Beam 414 is polarized in the plane of FIG. 5a. The frequency of beam 414 is [(e/λ 10 ) + f x 1 . Beam 415 enters differential plane minor interferometer 469 and makes a double pass through the measuring path 498. As depicted in FIG. 5a, beams 412 and 414 enter differential plane minor interferometer 469 and make double passes through respective reference paths. Beams 415, 412, and 414 exit differential plane minor interferometer 469 as beams 431, 432, and 434, respectively. Differential plane minor interferometer 469 and external minors 491 and 492 comprise optical means to introduce phase shift φ 9 between the λ 9 wavelength component of beam 415 and beam 412 and a phase shift φ 10 between the λ 10 wavelength component of beam 415 and beam 414. The magnitude of phase shifts φ 9 and φ 10 are related to round-trip physical length L 9 and L 10 of measurement path 498 according to the formula φ ; * - L j pk j n j , t = 9 and 10 , (57) where p is the number of passes through the respective reference and measurement legs and «,* is the refractive index of gas in measurement path 498 coπesponding to wavenumber k t = 2π/λ,* . The nominal value for L t coπesponds to twice the difference ofthe physical length between the reflecting surfaces of the external minors 491 and 492. The position of external minor 492 is controlled by translator 467. The interferometer shown in FIG. 5a is for p = 2 so as to illustrate in the simplest manner the function of the apparatus of the fifth prefeπed embodiment of the present invention. To those skilled in the art, the generalization to the case when p ≠ 2 is a straight forward procedure. In a next step as shown in FIG. 5a, a first portion of beam 431 is reflected by nonpolarizing beam splitter 461 A and a portion thereof transmitted by nonpolarizing beam splitter 461B to form a measurement beam component of a first output beam 441. A first portion of beam 432 is reflected by minors 461C and 461D and a portion thereof reflected by nonpolarizing beam splitter 46 IB to form a reference beam component ofthe first output beam 441. A second portion of beam 431 is transmitted by nonpolarizing beam splitter 461A, reflected by minor 461E, and a portion thereof reflected by nonpolarizing beam splitter 461F to form a measurement beam component of a second output beam 442. Beam 434 is reflected by minors 461G and 461H and a portion thereof transmitted by nonpolarizing beam splitter 461E to form a reference beam component ofthe second output beam 442. Output beams 441 and 442 are mixed beams and impinge on detectors 485 and 486, respectively, to produce electrical interference signals, preferably by photoelectric detection. The electrical interference signals comprise heterodyne signals s 9 , s 10 , and two other heterodyne signals. The heterodyne signals s 9 and ^ 10 each have a heterodyne frequency equal to frequency f λ . The heterodyne frequencies for the two other heterodyne signals are |Δ/ " | ± f λ , where c is the speed of light in a vacuum. The apparatus and method ofthe fifth embodiment are operated such that |Δ | » Λ . (59) With the condition of Eq. (59) operative, the two other heterodyne signals, although they could be processed for additional information, are easily separated from heterodyne signals s 9 and .s 10 and eliminated in detectors 485 and 486 and/or in electronic processor 427 by electronic filtering. The heterodyne signals s 9 and s 10 generated in detectors 485 and 486, respectively, have the form Sj = Λ,*(t)cos[α;(t)] / = 9 and 10 (60) The time-dependent arguments α ; *(t) are given by α 9 (t) = 2π/ 1 t + φ 9 + ζ 9 + Λ 9 , (61) 10 (t) = 2π 1 t + φ 10 + ζ 10 + Λ 10 , where phase offsets ζ 9 and ζ 10 comprise all contributions to α 9 and α 10 , respectively, that are not related or associated with the optical path of the measurement path 498 or reference path and not related or associated with nonlinear enors, and Λ g and Λ 10 comprise the nonlinear enors including cyclic enor effects. The description ofthe representations of s 9 and s 10 by Eqs. (60) is the same as the description given ofthe conesponding representations of s ± and s 2 ofthe first embodiment by Eqs. (9). Heterodyne signals s and s 10 are transmitted to electronic processor 427 for analysis as electronic signals 423 and 424, respectively, in either digital or analog format, preferably in digital format. Referring now to FIG. 5b, electronic processor 427 comprises electronic processors 4274A, 4274B, and 4277 which perform like functions as electronic processors 1274A, 1274B, and 1277 ofthe first embodiment. The description ofthe steps in processing heterodyne signals s 9 and s 10 by electronic processor 427 for a phase Φ [Φ as subsequently defined by Eq. (63)] is the same as conesponding portions ofthe description given of steps in the processing ofthe heterodyne signals s x and s 2 , respectively, of the first embodiment by electronic processor 27 for a conesponding phase. Electronic processors 4274A and 4274B determine the measured phases φ 9 and φ 10 , φ 9 = φ 9 +ζ 9 + Λ 9 (φ 9 ) , (62) Φιo = Φιo + ζιo + Λ ιo(φιo) using the phase of driver 405 transmitted by signal 421. Electronic processor 4277 subtracts φ 9 from φ 10 to form Φ , i.e. Φ = (Φ10 - Φ9) (63) Phase Φ may be expressed in terms of other quantities as Φ = pk 9 (n x 0 0 - « 9 L 9 ) + pn x 0 L XQ (2πAf/c) (64) + (ζιo -ζ9) + [ψιo(φιo) - ψ9(φ9)] ' where the nonlinearity terms η 9 and η 10 have been omitted as per description given with respect to the first embodiment. The effects of turbulence on the refractive index ofthe gas in the measuring path cancel out in Φ as well the effects of Doppler shifts produced by the translation of minor 492 by translator 467. The turbulence effects on the refractive index ofthe gas in the measuring path cancel out in Φ because the measurement beam components of beams 441 and 442, the beams used to generate heterodyne signals s 9 and s 10 , respectively, are derived from different frequency components of beam 415 which are substantially coextensive in measurement path 498. In addition, L 9 and L 10 can be made equal to a high level of accuracy. Cyclic nonlinearities ψ 9 and ψ 10 can to a high order of accuracy be wπtten as ψ 9 = 2_ . a 9,r COSrφg + > 9> smrφ 9 , r=l r=l (65) ΨlO = ∑ α 10,r cos > ' φi0 + ∑ r sm > * φi0 r=l r=l Cyclic nonlinearities ψ 9 and ψ 10 are written in Eqs. (65) in terms of cosine and sine series terms of harmonics of φ 9 and φ 10 , respectively. For some configurations of interferometers, in particular multiple pass interferometers, it is possible for a system comprising a source, interferometer, and detector to generate cyclic nonlinearities that are subharmonics of φ 9 and/or φ 10 . Should subharmonic cyclic eπors be present in the system, Eqs. (65) are augmented so as to include cosine and sine series terms with arguments that are subharmonics of φ 9 and/or φ 10 . The subsequent description of procedures to determine the coefficients in the cosine and sine series will be in terms ofthe series representations given by Eqs. (65) without departing from the spirit and scope ofthe present invention. It is possible for a system comprising a source, interferometer, detector, and digital signal processing to generate cyclic nonlinearities that are neither subharmonics or harmonics of φ ; . The non-subharmonic, nonharmonic cyclic eπors are produced for example by aliasing in the digital signal processing and have frequencies which are aliases of harmonics and subharmonics of φ, . Should non- subharmonic, nonharmonic cyclic eπors be present in a system, Eqs. (65) are augmented so as to include cosine and sine series terms with arguments that are the appropriate aliases of harmonics and/or subharmonics of φ, . The subsequent description of procedures to determine the coefficients in the cosine and sine series will be in terms ofthe series representations given by Eqs. (65) without departing from the spirit and scope ofthe present invention. The equation for cyclic nonlinearity ψ 10 from Eqs. (65) may be rewritten in the form Ψio = ∑ α ιo ,r cosr[φ 9 + pn 10 (2πΔf/c)L X0 ] r=\ (66) ∑bi 0 ,r sinr[φ 9 + pn X0 (2πAf/c)L XQ ] / • =! noting that to a good approximation, Φio = Φ 9 + Pnιo{2nAf/c)L χ(: (67) A term pk 9 (L 10 n XQ ~~ L $ n $ ) n been omitted in Eq. (67), this term being of the order of 10 ~4 radians for a Af = 500 Mh z , L x 0 = 1 m , and the gas comprised of air at room temperature and at atmospheric pressure. The terms in Eq. (66) may be expanded using trigonometric identities and reananged according to the formula In a next step, Φ is measured as a function φ 9 and for a set of values for Af [defined in Eq. (58)], the required number of different values of Af being dependent on the complexity of ψ,* and the precision required for the measured values of ψ ; * . From the measured values of [φ - 7« 10 10 (2πΔ ' /c)l , measured values of the quantity Ψιo(φιo>Δ/)-Ψιo(φιo>4/o) (69) are generated where Af is an initial value of Af . The difference of cyclic eπors ψ 0 (φ x 0 , Af) - ψ x 0 (φ 10 , Af 0 ) of Eq. (69) is written in terms of other quantities using Eq. (68) as ΨIO(ΦIO>Δ/)-ΨIO(ΦIO>4/O) (70) -α 10r {sinr[n 10 (2πΔ//c) o] - sinr[« 10 (2πΔ 0 /c)E 10 ]} + sinrφg r=l b 10)r {cosr[n 10 (2πΔ//c)L 10 ]-cosr[« 10 (2πΔ 0 /c) 10 ]} The Fourier coefficients a 9r , b 9r , a 10 r , and b XOr can be determined by a procedure comprising a sequence of iterative procedures. The first step in the procedure is to obtain a first solution for a 10r and b 10r , r≥l, from an analysis of ψ 10 (φ 10 ,Δ )-ψ 10 (φ 10 ,ΔO) . The analysis comprises Fourier analyses of ψ 10 (φ 10 ,Δ/ ' )-ψ 10 (φ 10 ,Δ ' 0 ) wherein φ 9 is used as the variable of integration to yield values ofthe coefficients of cosrφ 9 and sinrφ 9 in Eq. (70) as functions of Af . The values of coefficients of cosrφ 9 and sinrφ 9 in Eq. (70) generate a set of simultaneous equations in a xo and b 10 r , r ≤ 1 , and the set of simultaneous equations are solved for the first solution for a 10r and b 1Qr , r < 1. The absolute precision to which the first solution for a 1Qr and b XOr , r < 1 , can be determined will have a magnitude of the order of magnitude ofthe product ofthe cyclic eπor term |ψ 9 |/2 expressed in radians and the cyclic eπor term |ψ 10 | expressed in radians. The combined effect of cyclic eπor terms |ψ 9 | and |ψ 10 | enters as a second order effect in the absolute precision to which ψ 10 can be determined. The second step in the procedure is to generate a first iterated solution for a 9 r and b 9 r , r > 1. The second step comprises iterated Fourier analyses of [φ - pn X0 L X0 (2πAf/c)] minus a ψ 10 based on the first solution for a XO r and b XQ r , r ≤ 1 , wherein φ 10 , conected for a ψ 10 based on the first solution for a XO r and b XQ r , r ≤ 1 , is used as the variable of integration in the iterated Fourier analyses. The description ofthe iterated Fourier analyses is the same as coπesponding portions of the description ofthe iterative procedure of the first embodiment. The absolute precision to which the first solution iterated solution for a 9 r and b 9 r can be determined will have a magnitude ofthe order of magnitude ofthe product ofthe cyclic enor term |ψ 9 | expressed in radians and the absolute precision to which cyclic eπor term |ψιo| expressed in radians is determined by the first solution for α 10 r and b 10 r , r ≤ l . The combined effect of cyclic eπor terms |ψ 9 j and |ψι 0 | enters as a third order effect in the absolute precision to which ψ 9 can be determined, second order in the magnitude of |ψ 9 | expressed in radians and first order in the magnitude of jvμ 10 1 expressed in radians. The third step in the procedure is obtain a second solution for a 1Q r and b 1Q r , r ≥ l , from an analysis of ψ 10 (φ 10 ,Δ/ " ) - Ψιo(φιo > 4/o) • The third step is the same as the first step except that the variable of integration used in the Fourier analyses of the first step is replaced in the third step by φ 9 conected for ψ 9 based on the first iterated solution for ψ 9 . The absolute precision to which the second solution for a 1Q r and b 10 r , r ≤ l , can be determined will have a magnitude of the order of magnitude ofthe product the cyclic enor term |ψ 10 | expressed in radians and the absolute precision to which cyclic eπor term |ψ 9 | expressed-in radians is determined by the first iterated solution for a 9 r and b 9 r , r ≤ 1. The combined effect of cyclic enor terms |ψ 9 | and |ψ 10 | enters as a fourth order effect in the absolute precision to which ψ 9 can be determined, second order in the magnitude of |ψ 9 | expressed in radians and second order in the magnitude of |ψ 10 | expressed in radians. The fourth step in the procedure is obtain a second iterated solution for a 9 r and b 9 r , r ≥ l. The fourth step comprises iterated Fourier analyses of φ - pn x0 L X0 (2πΔf /cj\ minus a ψ 10 based on the second solution for a xo r and b 10 r » r ≤ l , wherein φ 10 , conected for a ψ 10 based on the second solution for a x0 ,. and b 1Q r , r ≤ 1 , is used as the variable of integration in the iterated Fourier analyses. The fourth step is the same as the second step except for the variable of integration used in the respective Fourier analyses. The absolute precision to which the second iterated solution for a 9 r and b 9 >r , r ≥ l, can be determined will have a magnitude of the order of magnitude ofthe product the cyclic eπor term |ψ 9 | expressed in radians and the absolute precision to which cyclic enor term |ψ 10 | expressed in radians is determined by the second solution for 10>r and b 1Q r , r ≤ l . The combined effect of cyclic enor terms |ψ 9 [ and [vμ 10 1 enters as a fifth order effect in the absolute precision to which ψ 9 can be determined, third order in the magnitude of |ψ 9 | expressed in radians and second order in the magnitude of jψ 10 1 expressed in radians. The iterative process in the sequence of iterative procedures as described is continued until the Fourier coefficients a 9 r , b 9 r , a XQ r , and b XQ r , r ≥ l , are determined to requisite precision for an end use application. The iterative procedure ofthe iterative process should converge in several cycles to desired precision for |ψ 9 | ≤. 1/3 and |ψ 10 | ≤. 1/3. The remaining description of the fifth embodiment ofthe present invention is the same as conesponding portions ofthe description given for the first and second embodiments. There are two beams, one from the nonpolarizing beam splitter 461B and one from the nonpolarizing beam splitter 461 F, which can also be used in the second embodiment either to improve the statistical enor for example or in a manner ofthe first embodiment. It will be evident to those skilled in the art that means other than the acousto-optical modulator 403 can be used to produce a set of beam components frequency shifted by f 0 and f 0 + f x without departing from the scope and spirit of the invention as disclosed in the second embodiment. For example, a second laser source such as diode laser can be used as a source of a frequency shifted beam components. For a diode laser, the change in the frequency shift f 0 could be effected by changing the injection cuπent and/or the temperature ofthe diode laser. It will be evident to those skilled in the art that certain features ofthe first, second, third, and fifth embodiments can beneficially be used together in an apparatus for determination of effects of cyclic enors without departing from the scope and spirit ofthe present invention. FIGS. 6a and 6b depict in schematic form a sixth embodiment of the present invention. The sixth embodiment is from the third group of embodiments. Embodiments of the third group of embodiments comprise both apparatus and methods for measuring and coπecting for cyclic eπors in optical dispersion related signals such as used to measure and coπect for effects of a gas in a measuring path of a distance measuring interferometer. However, for embodiments ofthe third group of embodiments, a measured phase coπesponding to a phase used for determination of changes in optical path length of a measurement path in a distance measuring interferometer is not conected for cyclic enors. The effects of cyclic eπors in coπections for effects of a gas in a measuring path, conections generated from optical dispersion related signals, are greater than the efiects ot cyclic eπors in the measured phase by one and a half or more orders of magnitude. The sixth embodiment comprises apparatus and method for measuring and monitoring the dispersion of a gas in a measurement path and/or the change in the optical path length ofthe measurement path due to the gas. The refractive index of the gas and/or the physical length ofthe measurement path may be changing. In addition, the ratio ofthe wavelengths of light beams generated by adopted light sources is matched with a certain relative precision to a known ratio value comprised of low order nonzero integers. The sixth embodiment may also be considered, in part, as an extension ofthe apparatus and method ofthe fifth embodiment, the ratio of adopted wavelengths for the fifth embodiment being ofthe order of 1 and the ratio ofthe adopted wavelengths of sixth embodiment being ofthe order of 2. Referring to FIG. 6a and in accordance with the prefeπed apparatus and method of the sixth embodiment, the description of light beam 509 and the source of light beam 509 is the same as coπesponding portions ofthe description given for light beam 9 and the source of light beam 9 ofthe first embodiment. The wavelength of source 501 is λ n . In a next step, a light beam 508 emitted from a source 502 passes through a modulator 504 becoming light beam 510. Modulator 504 is excited by an electronic driver 506 similar to the excitation of modulator 503 by electronic driver 505, respectively. Source 502, similar to source 501, is preferably a laser or like source of polarized, coherent radiation, but preferably at a different wavelength, λ 12 - The ratio ofthe wavelengths (λ 11 /λ 12 ) has a known approximate ratio value l xl /l 12 , i-e- (λn/λ 12 ) ≡ (/u// 12 ) , (71) where l xx and l X2 comprise low order nonzero integer values. The x polarized components of beams 509 and 510 have oscillation frequencies shifted by amounts f and f 2 , respectively, with respect to the y polarized components of beams 509 and 510, respectively. The oscillation frequency f 2 is determined by electronic driver 506. In addition, the directions ofthe frequency shifts ofthe x components of beams 509 and 510 are the same. It will be appreciated by those skilled in the art that beams 507 and 508 may be provided alternatively be a single laser source emitting more than one wavelength, by a single laser source combined with optical frequency doubling means to achieve frequency doubling, a laser source with a nonlinear element internal to the laser cavity, etc., two laser sources of differing wavelengths combined with sum- frequency generation or difference-frequency generation, or any equivalent source configuration capable of generating light beams of two or more wavelengths. It will also be appreciated by those skilled in the art that one or both ofthe frequency shifts f and f 2 may be the result of Zeeman splitting, birefringent elements internal to a laser cavity, or like phenomena characteristic ofthe laser sources themselves. The generation of beams by a single laser with two widely separated wavelengths and for each beam, a pair of orthogonally polarized components, one component of each pair frequency shifted with respect to the second component ofthe coπesponding pair, is described in U. S. Patent 5,732,095 entitled "Dual Harmonic-Wavelength Split- Frequency Laser" and issued March 1998 to P. Zorabedian. It will be further appreciated by those skilled in the art that both the x and y polarization components of beam 509 and/or of beam 510 may be frequency shifted without departing from the scope and spirit ofthe invention, f x remaining the difference in frequencies ofthe x and y polarization components of beam 509 and f 2 remaining the difference in frequencies ofthe x and y polarization components of beam 510. Improved isolation of an interferometer and a laser source is generally possible by frequency shifting both x and y polarization components of a beam, t e degree of improved isolation depending on the means used for generating the frequency shifts. In a next step, beam 509 is reflected by minor 553A and a portion thereof reflected by dichroic nonpolarizing beamsplitter 553B to become a component of beam 513, the λ xx component. A portion of beam 510 is transmitted by dichroic nonpolarizing beamsplitter 553B to become a second component of beam 513, the λ 12 component, wherein the λ 12 component is preferably parallel and coextensive with the λ n component. In a further step, beam 513 propagates to an interferometer 569 comprised of optical means for introducing a phase shift φ xx between the x andy polarization components ofthe λ xx component of beam 513 and a phase shift φ 12 between the x and y polarization components of the λ 12 component of beam 513. The magnitude of phase shifts φ lx and φ 12 are related to round-trip physical length L of measurement path 598 according to the formulae φ,* = Lpk j n j , = 11 and 12 , (72) where p is the number of passes through the respective reference and measurement legs for a multiple pass interferometer, and n t is the refractive index of gas in measurement path 598 coπesponding to wavenumber k t = (2π)/λ ; * . As shown in FIG. 6a, interferometer 569 is comprised of a reference retroreflector 591, object retroreflector 592 with a position controlled by translator 567, quarter-wave phase retardation plates 577 and 578, and a polarizing beam splitter 573. This configuration is known in the art as a polarized Michelson interferometer and is shown as a simple illustration with p = 1 . Eqs. (72) are valid for the case where the paths for beams with one wavelength and the paths for beams with the second wavelength are substantially coextensive, a case chosen to illustrate in the simplest manner the function ofthe invention in the sixth embodiment. To those skilled in the art, the generalization to the case where the respective paths for beams with the two different wavelengths are not substantially coextensive is a straight forward procedure. After passing through interferometer 569, the portion of beam 513 passing through the measurement path becomes a phase-shifted beam 533 and the portion of beam 513 passing through the reference path containing retroreflector 591 becomes phase-shifted beam 534. Phase-shifted beams 533 and 534 are polarized orthogonal to the plane and in the plane of FIG. 6a, respectively. A conventional dichroic beam splitter 561 separates those portions of beam 533 coπesponding to wavelengths λ and λ 12 into beams 535 and 537, respectively, and those portions of beam 534 coπesponding to wavelengths λ lx and λ 12 into beams 536 and 538, respectively. Beams 535 and 536 enter detector system 589 and beams 537 and 538 enter detector system 590. In detector system 589 as shown in FIG. 6a, beam 535 is first reflected by minor 563A and then reflected by polarizing beam splitter 563B to form one component of beam 541. Beam 536 is transmitted by polarizing beam splitter 563B to become a second component of beam 541. In detector system 590, beam 537 is first reflected by minor 564A and then reflected by polarizing beam splitter 564B to form one component of beam 542. Beam 538 is transmitted by polarizing beam splitter 564B to become a second component of beam 542. Beams 541 and 542 pass through polarizers 579 and 580, respectively, impinge upon photodetectors 585 and 586, respectively, and generate preferably by photoelectric detection two electrical interference signals. The two electrical interference signals comprise two heterodyne signals s xx and s x2 , respectively. Polarizers 579 and 580 are preferably oriented so as to mix x and y polarization components of beams 541 and 542, respectively. The heterodyne signals s x x and s 12 coπespond to wavelengths λ x x and λ 12 , respectively. Signals s i = 1 1 and 12, have the form s, = 4*(t)cos[α,(t)] t = l l and 12 (73) Time-dependent arguments oc,(t) are given by 11 (t) = 2π/ 1 t + φ 11 + ζ n + Λ 11 , (74) a X2 (t) = 2πf 2 t + φ 12 + ζ 12 + A 12 , where phase offsets ζ,* comprise all contributions to the arguments ; that are not related or associated with measurement path 798 or reference paths and not related or associated with nonlinear enors and the Λ ; * comprise the nonlinear enors including cyclic enor terms. The description ofthe representations of s xx and s X2 by Eqs. (73) is the same as conesponding portions ofthe description given ofthe representations of s x and s 2 of the first embodiment by Eq. (9). Heterodyne signals s x x and s 12 are transmitted to electronic processor 527 for analysis as electronic signals 523 and 524, respectively, in either digital or analog format, preferably in digital format. Referring now to FIG. 6b, electronic processor 527 comprises electronic processors 5274A and 5274B to determine measured phases φ n and φ 12 , respectively, φ ; - = φ,* + ζ ; * + A,* , / = 11 and 12 , (75) by either digital or analog signal processes, preferably digital processes, using time-based phase detection such as a digital Hubert transform phase detector (R. E. Best, ibid.) or the like and the phase of electronic drivers 505 and 506, respectively. The phases of electronic drivers 505 and 506 are transmitted by electrical signals, reference signals 521 and 522, respectively, in either digital or analog format, preferably in digital format, to electronic processor 527. Reference signals, alternatives to reference signals 521 and 522, may also be generated by an optical pick off means and detectors (not shown in figures) by splitting off portions of beams 509 and 510 with beam splitters, preferably nonpolarizing beam splitters, mixing the respective portions of beam 509 and beam 510 that are split off, and detecting the mixed portions to produce alternative heterodyne reference signals. Referring again to FIG. 6b, phase φ n and phase φ 12 are next multiplied by l x /p and l X2 /p , respectively, in electronic processors 5275A and 5275B, respectively, preferably by digital processing, to produce phases (l /p)φ x and (/ 12 /p)φ 12 , respectively. Phases (l x /p)φ and ( ι 2 / )φi 2 are next added together in electronic processor 5276 and subtracted one from the other in electronic processor 5277, preferably by digital processes, to create the phases θ and Φ , respectively. Formally, Φ = '12 '11 9l2 Φll (77) p P Using the definitions given by Eqs. (75), phases θ and Φ can also be written as θ = l Φ = [χ(n X2 -n xl ) + K(n X2 + " 1 ι)] + -J( ι 2 ζi2 -'iiζn) where X = (/l2*12 + 'll*ll)/2 , (80) K = {l x2 k x2 - l xx k xx )/2 . (81) The phases φ 11 , θ , and Φ are transmitted to computer 529 as signal 525, in either digital or analog format, preferably in digital format. The noncyclic nonlinearity η, is omitted in subsequent description of the sixth embodiment, the basis for the omission being the same as the basis given for omission of the coπesponding noncyclic nonlinearity in the latter portion of the description ofthe first embodiment. The dispersion (n 2 - n l ) ofthe gas can be determined from θ and Φ using the formula + ρ} , (82) where β ψ = ξ ψ (R/χ)-Z ψ , (83) Λ ■ "12 ^11 ξ V = — Ψl2 + — Ψll (84) '12 '11 ■ ^ψ — ^Ψ 12 - — Ψn , (85) V > p Q = ξ(K/χ) - Z , (86) / / / ? -- iϋ 4.12 + + iiirll (87) V p p Z = • ^12 *• • 'll >- 12 ll (88) P P J For those applications related to distance measuring interferometry, the heterodyne phase φ n and phases θ and Φ may be used to determine the distance L as a quantity independent ofthe effects ofthe refractive index ofthe gas in the measuring path of the distance measuring interferometer using the formula where Y , the reciprocal dispersive power ofthe gas, is defined as ("ii - l) T = (90) '12 l) It is evident from the definition of K given by Eq. (81) that (R/χ) = 0 coπesponds to the wavelengths λ n and λ 12 being strictly harmonically related. For an application where |R/χ| > 0 and the value of (R/χ) must be known to a certain precision in the use of Eqs. (82) and/or (89) to meet an end use requirement, (R/χ) is measured by wavelength monitors (not shown in the figures). The wavelength monitors may comprise interferometers with or without vacuum cells and/or frequency doubling of light beams by SHG. For an application where the value of χ must be known to another certain precision in the use of Eqs. (82) and/or (89), χ is measured by a wavelength monitor. In addition, when values for χ and (R/χ) are both required, they may both be obtained from the same apparatus. A value for the reciprocal dispersive power Y can be obtained to a certain relative precision from known refractive properties of known constituents of a gas in the measuring path. For those applications where the gas composition is not known to a requisite accuracy and/or the refractive properties ofthe gas constituents is not known to a coπesponding requisite accuracy, Y can be measured by apparatus such as described in commonly owned copending United States Application No. 08/942,848 filed Oct. 2, 1997 and entitled "Apparatus And Methods tor Measuring Intrinsic Optical Properties Of A Gas," United States Application filed Oct. 21, 1998 and entitled "Interferometric Method And Apparatus For Measuring Intrinsic Optical Properties Of A Gas," and United States Provisional Application No. 60/075,595 filed Feb. 23, 1998 and entitled "Apparatus And Methods For Measuring Intrinsic Optical Properties Of A Gas," all three applications by Henry A. Hill, the foregoing applications being incorporated herein in their entirety by reference. The relative precision to which the dispersion (n X2 - n xl ) can be determined is limited in part by the effect of cyclic eπors, the magnitude of the effect, according to Eq. (82), being ofthe order of - n lx ) i = l l and 12 (91) Consider for example, an application where λ xx = 0.633 μm , λ xx = 2λ 12 , /? = 1 , Z = 0.5 m, and the gas is comprised of air at 25° C and a pressure of one atmosphere. For the example conditions, the magnitude ofthe contribution of x l to the relative precision as expressed by Eq. (91) is * 0.019| Ψll | , (92) being expressed in radians and |ψn| indicating the absolute value of ψ xl . Continuing with the example, for a specific cyclic eπor of |ψ xl | = 0.1 radians, a cyclic eπor in phase coπesponding in the example to a cyclic enor in a distance measurement of 5 nm, the specific cyclic enor limits the relative precision to which the dispersion (n x2 - n xx ) can be measured to » 0.2%. If a source for the λ beam is a NbYAG laser with λ ιx = 1.06μm , the coπesponding limits on the relative precision to which the dispersion (n 12 - n xx ) can be measured is «0.6%. The limitations of effects of cyclic eπors on the relative precision to which the dispersion (n X2 - « n ) can be determined propagate directly to limitations ofthe effects of cyclic enors on coπections for refractivity effects of gas in a measurement path of a distance measuring interferometer using dispersion interferometry. From inspection of Eq. (89), it is evident that the magnitude ofthe cyclic eπor contribution of ψ,* entering through ζ , is ≡ r|ψ,*| relative to the magnitude of the cyclic eπor contribution |ψ xι | entering through φ xx . For the two cases of λ xl = 0.633 μm with λ xx = 2λ 12 and λ xx = 1.06 μm also with λ n = 2λ 12 , the values for Y are 24 and 75, respectively. Thus the effects of cyclic eπor contributions to the conection term in Eq. (89) for the refractivity of a gas in a measuring path must be reduced by approximately one and a half or more orders of magnitude if the effects ofthe cyclic enor contributions resulting from the coπection term are to be ofthe order of or less than the effects ofthe cyclic enor contributions resulting directly from φ n . The cyclic enor terms Z^ given by Eq. (85) can be expressed as where Ψll(φll) = C ll(φll) ' (94) Ψi 2 (φi2) = Cι 2 (φι 2 ) , and M - i r sinrφ,* ι = ll and 12 (95) The C j are written in Eqs. (95) in terms of cosine and sine series terms with arguments ofthe series terms being harmonics of φ,* . For some configurations of interferometers, in particular multiple pass interferometers, it is possible for a system comprising a source, interferometer, and detector to generate cyclic nonlinearities that are subharmonics of φ,* . Should subharmonic cyclic enors be present in a system, Eqs. (95) are augmented so as to include cosine and sine series terms with arguments that are subharmonics φ,* . The subsequent description of procedures for the determination ofthe coefficients in the cosine and sine series will be in terms ofthe series representations given by Eqs. (94) and (95) without departing from the spirit and scope ofthe present invention. It is possible for a system comprising a source, interferometer, detector, and digital signal processing to generate cyclic nonlinearities that are neither subharmonics or harmonics of φ ; . The non-subharmonic, nonharmonic cyclic enors are produced for example by aliasing in the digital signal processing and have frequencies which are aliases of harmonics and subharmonics of φ,* . Should non- subharmonic, nonharmonic cyclic enors be present in a system, Eqs. (95) are augmented so as to include cosine and sine series terms with arguments that are the appropriate aliases of harmonics and/or subharmonics of φ ; * . The subsequent description of procedures to determine the coefficients in the cosine and sine series will be in terms ofthe series representations given by Eqs. (94) and (95) without departing from the spirit and scope ofthe present invention. In a next step, Φ is measured as a function of φ n and φ 12 over a certain range of values of φ xl and φ 12 . The measured values of Φ can be written, according to Eq. (79), as Φ = Lχ(n X2 - n xx ) + LK(n X2 + n xx ) + Z + Z . (96) For changes in L ofthe order of 10λ n to 100 λ xx , it is evident from Eq. (96) that, for the condition K/χ < - n xx )/(n l2 + n u ) , the Z ψ term is the dominant term with respect generating changes in Φ , typically by several orders of magnitude, other terms K , χ , and Z being constants and [( π i 2 _ n ιι)l( i 2 + w ιι)j ^ ■*■/• ( -2 x 10 5 J for air at 25° C and at a pressure of one atmosphere, λ lx ≥ 0.6 μm , and λ lx ≡ 2λ 12 . As a consequence, the measured values of Φ can be used directly in an effective procedure for determination of Z ψ . The ratio ofthe wavelengths λ /λ 12 in the sixth embodiment can be expressed as the ratio of low order nonzero integers, l xx /l X2 as per Eq. (71), with a certain relative precision. The ratio of φ 11 /φ 12 can therefore be expressed as with the same certain relative precision. The two parameter representation of Z ψ , the two parameters being φ n and φ 12 according to Eqs. (85), (94), and (95), can be reduced to a one parameter representation by using Eq. (97) to eliminate either φ xx or φ 12 m me iwo parameter representation of Z ψ . The subsequent description ofthe sixth embodiment will be in terms ofthe elimination of φ 12 , although the elimination of φ could have been chosen as the parameter to eliminate yielding substantially the same end results in terms ofthe determination of Z ψ . The resulting one parameter representation for Z ψ is With the ratio (l xl /l X2 ) being comprised ofthe ratio of low order nonzero integers, the terms in the one parameter representation for Z ψ can be rewritten as a cosine and sine series with arguments that are harmonics of φ xx , the ratio (l xx /l X2 ) being an integer, e.g., 2, 3,..., or as a cosine and sine series with arguments that include subharmonics of φ ιx , the ratio (l /l 2 ) being a noninteger, e.g., 3/2 , 4/ '3 , etc.. The one parameter representation for Z ψ as rewritten will be referenced as the reduced representation for Z ψ . The procedure for evaluation of the Fourier coefficients ofthe reduced representation for Z ψ uses φ xx as the variable of integration in the Fourier analysis such as described for the first step in the procedure used to obtain a first solution for a 10 r and b 1Q r , r ≥ l , ofthe fifth embodiment. The relative precision to which the cyclic eπor term Z ψ can be determined by the sixth embodiment will have a magnitude ofthe order of magnitude of 1/2 the cyclic eπor, expressed in radians, in φ 1:L , or if larger, the effect ofthe certain relative precision that the ratio of wavelengths (λ 11 /λ 12 ) is expressed as the ratio of l xx /l 2 • Thus the residual contribution of cyclic enors to Φ post coπection will enter as a second order effect, first order in the cyclic eπors in Φ and either first order in the cyclic eπors in either Φn or Φ 12 depending upon which of φ n and φ 12 are used as the variable of integration in the Fourier analysis or the certain relative precision that the ratio of wavelengths (λ 11 /λ 12 ) is expressed as the ratio of l x /l x2 ■ The remaining description ofthe sixth embodiment is the same as coπesponding portions ofthe descriptions given for the first and fifth embodiments of the present invention. A first variant of the sixth embodiment ofthe present invention is described wherein the first variant of the sixth embodiment is from the third group of embodiments. The description ofthe first variant ofthe sixth embodiment is the same as that given for the description ofthe sixth embodiment except with respect to the treatment of cyclic eπors. In the first variant ofthe sixth embodiment, the phase Φ given by Eq. (96) is filtered by either an integral transform of Φ with respect to φ u over an interval of 2π/ 12 or multiples thereof or the integral transform of Φ with respect to φ 12 over an interval of 2π/ n or multiples thereof. The description of the integral transform is similar to the description given for the integral transform used in the first procedure of the first embodiment ofthe present invention. The design ofthe integral transform algorithm is based on the properties ofthe reduced representation of Z ψ . The effectiveness of an integral transform in reducing or eliminating the effect of cyclic enors will depend on the magnitude ofthe cyclic enors in φ xx and φ 12 . The residual contribution of cyclic enors to Φ post filtering by an integral transform will enter as a second order effect, first order in the cyclic enors in Φ and either first order in the cyclic enors in either Φn or Φ 12 depending upon which of < n and <p 12 are used in execution ofthe integral transform or the effect ofthe certain relative precision that the ratio of wavelengths (λ 11 /λ 12 ) is expressed as the ratio of l x /l x2 ■ For an optimal integral transform, the residual second order effect will have an order of magnitude of Z ψ multiplied by either 1/2 the magnitude ofthe cyclic enors, expressed in radians, in either c lx or φ 12 whichever one is used in the integral transform or the effect ofthe certain relative precision that the ratio of wavelengths (λ 11 /λ 12 ) is expressed as the ratio of l x /l X2 ■ The first variant of the sixth embodiment does not place any restrictions on the motion ofthe minor 592 in order to be effective other than that there be some motion initially conesponding to either phase space 2π/ 12 , 2πl xx , or multiples thereof used in the integral transform procedure ofthe filter and that the cyclic enors do not change significantly between the periods when minor 592 is moved over a distance conesponding to the phase space 2π/ 12 , 2π/ 11 , or multiples thereof used in the integral transform procedure ofthe filter. The filtering out of cyclic enors by the first variant ofthe sixth embodiment effectively eliminates problems encountered in prior art based on filtering methods wherein there is an integration over fixed periods of time. A group of other variants ofthe sixth embodiment ofthe present invention are described, the other group of variants ofthe sixth embodiment comprising the apparatus and method of the sixth embodiment and at least one of the apparatus and methods ofthe first, second, third, and fifth embodiments ofthe present invention. The group of other variants ofthe sixth embodiment are from the fourth group of embodiments, embodiments of the fourth group of embodiments comprising apparatus and methods for measuring and conecting for cyclic enors in both the measured phase used for determination of changes in optical path length of a measurement path in a distance measuring interferometer and the associated optical dispersion related signal used to coπect the changes in optical path lengtn for effects of gas in the measuring path ofthe distance measuring interferometer. A second variant ofthe sixth embodiment from the other group of variants ofthe sixth embodiment comprises the apparatus and method ofthe sixth embodiment and the apparatus and method of the first embodiment of the present invention. For the second variant ofthe sixth embodiment, the cyclic eπors present in phases coπesponding to φ 11 and φ 12 ofthe sixth embodiment and generated in the source and/or beam transport to the interferometer, misalignment ofthe source with respect to the interferometer, and departures from desired properties of a polarizing beam splitter used to separate reference and measurement beams, the cyclic eπors so generated generally being modified by alignment properties of the detector units including mixers and analyzers with respect to the interferometer, can be conected to a high level of precision, i.e., conected up to second order effects in combination of cyclic enors and/or the absolute precision to which φ is known. The conesponding contributions of cyclic eπors to a Φ ofthe second variant ofthe sixth embodiment, the Φ coπesponding to the Φ ofthe sixth embodiment, are thus conected to a level of precision higher than for the coπesponding Φ in the sixth embodiment, i.e., conected up to third order effects in combination of cyclic enors and/or the absolute precision to which φ is known. The remaining description of the second variant of the sixth embodiment is the same as coπesponding portions of the descriptions given for the first and sixth embodiments. The advantage ofthe second variant ofthe sixth embodiment is realized when the cyclic eπors in phases of the second variant of the sixth embodiment conesponding to φ n and φ 12 of the sixth embodiment are primarily generated in the source and/or beam transport to the interferometer, misalignment of the source with respect to the interferometer, and departures from desired properties of a polarizing beam splitter used to separate reference and measurement beams. A third variant ofthe sixth embodiment from the other group of variants ofthe sixth embodiment comprises the apparatus and method ofthe sixth embodiment and the apparatus and method ofthe third embodiment ofthe present invention. For the third variant ofthe sixth embodiment, the cyclic eπors present in phases conesponding to φ n and φ 12 ofthe sixth embodiment and generated by the interferometer and object minor, e.g., spurious internal multiple reflections of beams in the measurement and/or reference legs and/or departures of phase retardation plates and polarizing beam splitters from desired properties, can be conected to a high level of precision, i.e., conected up to second order effects in the product of cyclic eπors and/or the absolute precision to which φ is known. The coπesponding contributions of cyclic eπors to a Φ ofthe third variant ofthe sixth embodiment, the Φ conesponding to the Φ ofthe sixth embodiment, are thus conected to a level of precision higher than in the sixth embodiment, i.e., conected up to third order effects in products of cyclic eπors and/or the absolute precision to which φ is known. The remaining description of the third variant of the sixth embodiment is the same as coπesponding portions ofthe descriptions given for the third and sixth embodiments. The advantage ofthe third variant ofthe sixth embodiment is realized when the cyclic eπors in phases ofthe third variant ofthe sixth embodiment coπesponding to φ n and φ 1 ofthe sixth embodiment are primarily generated by the interferometer and object minor, e.g., spurious internal multiple reflections of beams in the measurement and/or reference legs and/or departures of phase retardation plates and polarizing beam splitters from desired properties. A fourth variant ofthe sixth embodiment from the other group of variants of the sixth embodiment comprises the apparatus and method ofthe sixth embodiment and the apparatus and method ofthe second embodiment of the present invention. For the fourth variant of the sixth embodiment, substantially all of the cyclic eπors present in phases of the fourth variant ofthe sixth embodiment coπesponding to φ and φ 12 ofthe sixth embodiment can be conected to a high level of precision, that precision typically being at least second order in products of cyclic eπors and/or an absolute precision to which φ is known. The coπesponding contributions of cyclic eπors to a Φ ofthe fourth variant ofthe sixth embodiment, the Φ conesponding to the Φ ofthe sixth embodiment, are thus conected to a level of precision higher than for the coπesponding Φ in the sixth embodiment, that precision typically being at least third order in products of cyclic eπors and/or an absolute precision to which φ is known. The remaining description ofthe fourth variant ofthe sixth embodiment is the same as conesponding portions ofthe descriptions given for the second and sixth embodiments. The principal advantage ofthe fourth variant ofthe sixth embodiment in relation to the second and third variants ofthe sixth embodiment is in the substantially complete determination ofthe cyclic enors coπesponding to φ u and φ 12 ofthe sixth embodiment, cyclic eπors arising from the interferometer system including the source and detectors present, the second and third variants ofthe sixth embodiment permitting a determination of a subset of cyclic eπors. A fifth variant ofthe sixth embodiment from the other group of variants of the sixth embodiment comprises the apparatus and method of the sixth embodiment and the apparatus and method ofthe fifth embodiment of the present invention. In the fifth variant ofthe sixth embodiment, the apparatus and methods of the fifth embodiment are operative for only certain beams and/or beam components, those beams and/or beam components having one ofthe two different wavelengths, λ. or rs, X . For the purpose of discussion, let the beams and/or beam components with wavelength λ ιx be the certain beams and/or beam components. In a first step ofthe fifth variant of the sixth embodiment, the cyclic enors present in the certain beams and/or beam components and in particular in a phase coπesponding to φ n ofthe sixth embodiment are determined by application ofthe procedures of the fifth embodiment. In a second step ofthe fifth vaπant ot the sixth embodiment, cyclic eπors present in certain other ofthe beams and/or beam components, those beams and/or beam components having wavelength λ 12 , are determined in a Fourier analysis of a phase coπesponding to φ 12 ofthe sixth embodiment using for the variable of integration in the Fourier analysis φ u conected for effects of cyclic eπors. Either φ xx or φ 12 conected for effects of cyclic eπors is then used to determine effects of cyclic enors in a Φ ofthe fifth variant ofthe sixth embodiment, the Φ coπesponding to the Φ ofthe sixth embodiment. For the fifth variant ofthe sixth embodiment, substantially all ofthe cyclic eπors present in phases coπesponding to φ and φ 12 ofthe sixth embodiment can be conected to a high level of precision, a level of precision the same as the level of precision achieved in the fifth embodiment. The conesponding contributions of cyclic eπors to Φ ofthe fifth variant ofthe sixth embodiment are thus conected to a level of precision higher than for the conesponding Φ in the sixth embodiment. The remaining description ofthe fifth variant ofthe sixth embodiment is the same as coπesponding portions ofthe descriptions given for the fifth and sixth embodiments. The principal advantage of the fifth variant ofthe sixth embodiment in relation to the second and third variants of the sixth embodiment is substantially the same as the advantage described for the fourth variant of the sixth embodiment in relation to the second and third variants ofthe sixth embodiment. FIG. 7 depicts, in schematic form, a seventh prefeπed embodiment of the present invention from the fourth group of embodiments. Embodiments of the fourth group of embodiments comprise apparatus and methods for measuring and coπecting for cyclic eπors in both the measured phase used for determination of changes in optical path length of a measurement path in a distance measuring interferometer and the associated optical dispersion related signal used to conect the changes in optical path length for effects of gas in the measuring path ofthe distance measuring interferometer. The distance measuring interferometry of the seventh embodiment comprises apparatus and method for measuring and monitoring the dispersion of a gas in a measurement path and/or the change in the optical path length ofthe measurement path due to the gas wherein the refractive index ofthe gas and/or the physical length ofthe measurement path may be changing and where the ratio ofthe wavelengths of the light beams generated by the adopted light sources is matched to a known ratio value comprised ofthe ratio of low order nonzero integers and/or nonintegers. Many elements ofthe seventh embodiment perform like functions as elements ofthe sixth embodiment, the number of an element ofthe seventh embodiment performing a like function as an element ofthe sixth embodiment being equal to the number ofthe element ofthe sixth embodiment incremented by 100. The descriptions of sources 601 and 602 are the same as conesponding portions ofthe descriptions given for sources 501 and 502 ofthe sixth embodiment except with respect to the control of the wavelength of source 602. Sources 601 and 602 generate beams 607 and 608, respectively, with wavelengths λ 13 and λ 14 , respectively. The wavelength λ 14 ofthe beam from source 602 is controlled by eπor signal 644 from computer and controller 629. As shown in FIG. 7, a first portion of beam 607 is reflected by nonpolarizing beam splitter 651 A and a portion thereof reflected by dichroic beam splitter 651B to form a first component of beam 640. In a next step, a first portion of beam 608 is reflected by nonpolarizing beam splitter 651C, reflected by minor 651D, and a portion thereof transmitted by dichroic beam splitter 651B to form a second component of beam 640. Beam 640 impinges on wavelength monitor 684 of a well known type configured to monitor the ratio (λ 13 /λ 14 ) . The measured value of the ratio (λ 13 /λ 14 ) is transmitted to computer and controller 629 as electronic signal 620. Wavelength monitor 684 may comprise, e.g., interferometers with or without a vacuum in a measurement leg and/or nonlinear elements such as β -BaB0 3 to double the frequency of a beam by second harmonic generation, SHG. Computer and controller 629 generates eπor signal 644 related to the difference between the measured value of wavelength ratio (λ 13 /λ 14 ) , the ratio as received from signal 620, and a ratio specified by computer and controller 629. The wavelength of source 602 is controlled by eπor signal 644. Eπor signal 644 may control the wavelength, for example, of a laser by controlling the length ofthe laser cavity with a piezoelectric transducer or of a diode laser by controlling the injection cuπent ofthe diode laser. Continuing with FIG. 7, a second portion of beam 607 is transmitted by nonpolarizing beam splitter 651 A to form the λ 13 component of beam 613 by steps the description thereof being the same as coπesponding portions given in the sixth embodiment respect to the λ xx component of beam 513 being derived beam 507. A second portion of beam 608 is transmitted by nonpolarizing beam splitter 651C to form the λ 1 component of beam 613 by steps the description thereof being the same as coπesponding portions given in the sixth embodiment respect to the λ 12 component of beam 513 being derived beam 508. The description ofthe propagation of beam 613 through interferometer 669 and the generation of electrical interference signals, heterodyne signals s X3 and s X4 , transmitted as signals 623 and 624, respectively, is the same as coπesponding portions ofthe description given for the sixth embodiment with respect to the propagation of beam 513 through interferometer 569 and the generation of electrical interference signals s xx and s X2 transmitted as signals 523 and 524, respectively. The descriptions ofthe properties of heterodyne signals s X 3 and s l4 and respective phases φ 13 and φ 14 are the same as the coπesponding portions ofthe descriptions given for the properties of heterodyne signals s lx and s 12 and respective phases φ lx and φ 12 of the sixth embodiment. Further, equations conesponding to Eqs. (71) through (90) for the seventh embodiment are obtained from Eqs. (71) through (90) by the replacement of all subscripts 11 with subscripts 13 and the replacement of a subscripts 12 with subscripts 14. A procedure is described for the determination ofthe cyclic eπors in φ 13 , φ 14 , and Φ . It will be evident to those skilled in the art that variations ofthe described procedure can be employed without departing from the spirit and scope of the present invention. In a first step ofthe procedure, Φ is measured as a function φ 13 and φ 14 for a given range in φ 14 and for a set of values for λ 13 /λ 14 , the required number of different values of λ 13 /λ 14 being dependent on the complexity of ψ 13 and ψ 14 and the precision required for the measured values of ψ 13 and ψ 14 . The change in values of λ 13 /λ 14 is effected by computer and controller 629 by making changes in λ 14 through eπor signal 644. From the measured values of Φ , measured values ofthe quantity ψ 14 [φ 14 + « 14 (2πΔv 14 /c)z]- ψ 14 (φ 14 ) (99) are obtained where Δv 14 is defined as and λ 14 0 is an initial value of λ 14 . The expression for {ψ 14 [φι 4 + /z 14 (2πΔv 14 /c)z]- ψ 14 (φ 14 )} given by Eq. (99) can be written as {ψι 4 [φι +«ι 4 (2πΔv 14 /c) ]- ψ 14 (φ 14 )} = ∑βi ,r{ cos r [φi 4 + «i 4 (2πΔv 14 /c)z]- cosr(φ 14 )} r=X + ∑bi 4 , r {si n >{ i 4 + «ι 4 (2πΔv 14 /c)l]- sinr(φ 14 )} r=l (101) The Fourier coefficients a X4 r and b 14 r can be determined by the same type of iterative procedure described for the first embodiment. The result of that analysis is a determination of ψ 14 for the given range in φ 14 . In a next step, the Fourier coefficients a X3 r and b 13 r are determined by a Fourier analysis of Φ for a given range in φ 13 conesponding to the given range in φ 14 using («ι 3 /«ι 4 )(φι 4 - ψι 4 ) as the variable of integration. The result of this step is the determination of ψ 13 for the range of φ 13 used in the Fourier analysis. In a next step, the ψ 13 and ψ 14 obtained in the first two steps are used to compute Q ψ for the given range in φ 14 . With the determination of ψ 13 , ψ 14 , and g ψ , the dispersion (n 14 - n X3 ) and changes in path length conected for effects of gas in the measurement path can be computed that are conected for cyclic enors over the given range in φ 14 . The procedure is repeated for other range values of φ 14 as required for the end use application. Note that the values for / 13 and l 14 for the seventh embodiment may comprise both integer and noninteger nonzero values in comparison to the sixth embodiment wherein l xx and l x2 comprise nonzero integer values. The remaining description ofthe seventh embodiment is the same as conesponding portions ofthe descriptions given for the fifth and sixth embodiments. An advantage ofthe seventh embodiment is the capability to measure and monitor on line the effects of cyclic enors at the same time that compensation for refractivity effects of gas in the measuring path of a distance measuring interferometer is being performed. Another advantage ofthe seventh embodiment is that the adopted wavelengths may be either harmonically related or nonharmonically related. Another advantage ofthe seventh embodiment is that the determined values for the cyclic enors have a reduced sensitivity to the effects of turbulence in the gas present in a measuring path, that is the statistical accuracy to which the cyclic eπors are determined is only weakly dependent on the effects of turbulence in the gas in the measuring path. Another advantage ofthe seventh embodiment is that the cyclic eπors can change as the physical length ofthe measuring path is changed without substantially altering the effectiveness ofthe seventh embodiment with regard to conecting for the effects of cyclic enors. The values for the logarithmic derivative of the cyclic eπors with respect to the measurement path length that will not substantially alter the effectiveness ofthe seventh embodiment with regard to coπecting for the effects of cyclic eπors will depend in part on the precision required in conecting for the cyclic enors. FIGS. 8a and 8b depict, in schematic form, an eighth prefeπed embodiment of the present invention from the fourth group of embodiments. Embodiments ofthe fourth group of embodiments comprise apparatus and methods for measuring and coπecting for effects of cyclic eπors in both the measured phase used for determination of changes in optical path length of a measurement path in a distance measuring interferometer and the associated optical dispersion related signal used to coπect the changes in optical path length for effects of gas in the measuring path ofthe distance measuring interferometer. The distance measuring interferometry of the eighth embodiment comprises apparatus and method for measuring and monitoring the dispersion of a gas in a measurement path and/or the change in the optical path length ofthe measurement path due to the gas wherein the refractive index ofthe gas and the physical length of the measurement path may be changing and where the ratio ofthe wavelengths ofthe light beams generated by the adopted light sources is matched to a certain relative precision to a known ratio value comprised of nonzero low order integers and non integers. Many elements ofthe eighth embodiment perform like functions as elements ofthe seventh embodiment and unless indicated otherwise, the number of an element of the eighth embodiment performing a like function as an element ofthe seventh embodiment being equal to the number ofthe element ofthe seventh embodiment incremented by 100. The descriptions of sources 701 and 702B are the same as coπesponding portions ofthe descriptions given for sources 601 and 602, respectively, ofthe seventh embodiment. The description of source 702A is the same as conesponding portions ofthe description given for source 602 ofthe seventh embodiment except that the wavelength of source 702A is fixed. Sources 701, 702A, and 702B generate beams 707, 708A, and 708B, respectively, with wavelengths λ 15 , λ 16A , and, λ 16B , respectively. Beams 707, 708A, and 708B are polarized in the plane of FIG. 8. Wavelength λ 16B ofthe beam from source 702B is controlled by eπor signal 744 from computer and controller 729. For the eighth embodiment, m order to illustrate in a simple manner the present invention. The eighth embodiment may be configured to operative for negative values for (λ 16A - λ 16B ) and/or for |λ 16A - λ 16B | < λ 16A without departing from the scope and spirit ofthe present invention. For configurations ofthe apparatus of the eighth embodiment where the condition |λ 16A - λ 16B | «: λ 16B is not applicable, it may be desirable to change certain ofthe nonpolarizing beam splitters described for the eighth embodiment to dichroic beam splitters to improve overall efficiency ofthe source and interferometer system. As shown in FIG. 8, a first portion of beam 708A is reflected by nonpolarizing beam splitter 751 A and a portion thereof reflected by a nonpolarizing beam splitter 751B to form a first component of beam 740. In a next step, a first portion of beam 708B is reflected by nonpolarizing beam splitter 751C, reflected by minor 751D, and a portion thereof transmitted by nonpolarizing beam splitter 751B to form a second component of beam 740. Beam 740 impinges on wavelength monitor 784 of a well known type configured to monitor the ratio (λ 16A /λ 16B ) . The measured value ofthe ratio (λ 16A /λ 16B ) is transmitted to computer and controller 729 as electronic signal 720. Wavelength monitor 784 may comprise, for example, interferometers with or without a vacuum in a measurement leg and/or nonlinear elements such as β -BaB0 3 to double the frequency of a beam by second harmonic generation, SHG. Computer and controller 729 generates enor signal 744 related to the difference between the measured value ofthe wavelength ratio (λ 16A /λ 16B ) , the ratio received by signal 720, and a ratio specified by computer and controller 729. The wavelength of source 702B is controlled by enor signal 744. Enor signal 744 may control, for example, the wavelength of a laser by controlling the length ofthe laser cavity with a piezoelectric transducer or the wavelength of a diode laser by controlling the injection cuπent of a diode laser. Continuing with FIG. 8, a second portion of beam 708A is transmitted by nonpolarizing beam splitter 751A, enters modulator 704A, and exits modulator 704A as beam 710A comprised of two coextensive frequency components. Beam 710A is reflected by minor 753A, a portion thereof reflected by nonpolarizing beam splitter 753B, and a portion thereof transmitted by dichroic beam splitter 753D to form the λ 16A component and frequency shifted λ 16A component of beam 713. A second portion of beam 708B is transmitted by nonpolarizing beam splitter 751C, enters modulator 704B, and exits modulator 704B as beam 710B comprised of two coextensive frequency components. A portion of beam 710B is transmitted by nonpolarizing beam splitter 753B and a portion thereof transmitted by dichroic beam splitter 753D to form the λ 16B component and frequency shifted λ 16B component of beam 713. Beam 707 enters and exits modulator 703 as beam 709 comprised of two coextensive frequency components. Beam 709 is reflected by minor 753C and a portion thereof reflected by dichroic beam splitter 753D to form the λ 15 component and frequency shifted λ 15 component of beam 713. The descriptions ofthe modulators 703, 704A, and 704B and associated drivers 705, 706A, and 706B are the same as conesponding portions of descriptions given for drivers 603 and 604 and associated drivers 605 and 606 ofthe seventh embodiment. The frequency shifts introduced by modulators 703, 704A, and 704B are f x , f 2A , and f 2B , respectively. The nonfrequency shifted components of beam 713 are polarized in the plane of FIG. 8 and the frequency shifted components of beam 713 are polarized orthogonal to the plane of FIG. 8. The description ofthe propagation of beam 713 through interferometer 769 and the generation of electrical interference signals, comprising heterodyne signal s x 5 transmitted as signal 723 and heterodyne signals s X6A and s xβB transmitted as signal 724, is the same as coπesponding portions ofthe description given for the seventh embodiment with respect to the propagation of beam 613 through interferometer 669 and the generation of electrical interference signals s X3 and s X4 transmitted as signals 523 and 524, respectively. The descriptions of properties of heterodyne signals ,y 15 , s X 6A , and s X6B and respective phases φ 15 , Φι 6A and φ 16B are the same as the conesponding portions of the descriptions given for the properties of heterodyne signals s x3 and s 14 and respective phases φ 13 and φ 14 ofthe seventh embodiment and the same as coπesponding portions of the descriptions given for the properties of heterodyne signals s 9 and s 10 and respective phases φ 9 and φ 10 ofthe fifth embodiment. The system comprised of sources 702A and 702B, interferometer 769, detector systems 789 and 790, electronic processor 727, and computer and controller 729 is functionally equivalent to the conesponding system ofthe fifth embodiment depicted in FIG. 5a. In addition, the system comprised of sources 701 and 702B, interferometer 769, detector systems 789 and 790, electronic processor 727, and computer and controller 729 is functionally equivalent to the coπesponding system of the seventh embodiment depicted in FIG. 7. Thus, the cyclic eπors present in each of the measured phases φ 15 , φ 16A and ΦI 6B can be determined by procedures described in the fifth and seventh embodiments. The remaining description ofthe eighth embodiment is the same as conesponding portions ofthe descriptions given for the fifth and seventh embodiments. The advantages ofthe eighth embodiment are the same as listed for the seventh embodiment with the following additional advantage. With the eighth embodiment, the distance measuring function and the dispersion based system for compensating for gas in the measuring path, including turbulence effects in the gas, can be executed with two fixed wavelengths, λ 15 and λ 16A , simultaneous with and independent ofthe cyclic enor compensating procedure based on the variable wavelength λ 16B and one of the fixed wavelengths, either λ 15 or λ 15A . The distance measuring and dispersion interferometry ofthe apparatus and method of sixth, seventh, and eight embodiments of the present invention are of two types of distance measuring and dispersion interferometry apparatus and method described in commonly owned United States Patent Application Serial No. 09/078,254 filed May 13, 1998 and entitled "Interferometric Apparatus And Methods Using Electronic Frequency Processing For Measuring And Compensating For Refractive Index Effects In An Optical Path" and in United States Provisional Patent Application No. 60/075,586 filed Feb. 23, 1998 and entitled "Interferometer And Method For Measuring The Refractive Index And Optical Path Length Effects Of Air," both applications by Peter de Groot, Henry A. Hill, and Frank C. Demarest. Other forms of distance measuring and dispersion interferometers and methods such as described in commonly owned United States Provisional Patent Application No. 60/075,586, ibid., United States Patent Application Serial No. 09/078,254, ibid., United States Patent Application No. 09/078,163 filed May 13, 1998 and entitled "Apparatus And Methods Using Multiple Pass Interferometry For Measuring And Compensating For Refractive Index Effects In An Optical Path" by Henry Allen Hill, Peter J. de Groot, and Franklin C. Demarest, and United States Provisional Patent Application No. 60/075,566 filed Feb. 2, 1998 and entitled "Apparatus And Method For Measuring The Refractive Index And Optical Path Effects Of Air Using Interferometry" by Henry A. Hill, Peter de Groot, and Franklin C. Demarest may be incorporated into the apparatus and method ofthe present invention without departing from the spirit or scope ofthe present invention, the foregoing applications being incorporated herein in their entirety by reference. FIGS. 9 depicts, in schematic form, a ninth prefened embodiment of the present invention from the fourth group of embodiments. Many elements ofthe ninth embodiment perform like functions as elements ofthe eighth embodiment and unless indicated otherwise, the number of an element ofthe ninth embodiment performing a like function as an element ofthe eighth embodiment being equal to the number ofthe element ofthe conesponding eighth embodiment incremented by 100. The primary difference between the ninth and eithth embodiment is the use of nonlinear elements, e.g., β-BaBO 3 , to double the frequency of certain beams and/or beam components by SHG in source systems and in detector systems ofthe ninth embodiment. For the ninth embodiment as depicted in FIG. 9a, the descriptions of sources 802A and 802B and conesponding beams 808A and 808B are the same as conesponding portions ofthe descriptions given for sources 702A and 702B and conesponding beams 708A and 708B ofthe eighth embodiment. The description of the generation of beam 840 in the eighth embodiment from a portion of beam 808A transmitted by nonpolarizing beam splitter 851E and of beam 808B is the same as the portion ofthe description given for the generation of beam 740 in the eighth embodiment from beams 708A and 708B. The description ofthe generation of : a; λ 18A component, a frequency shifted λ 18A component, a λ 18B component, and i a frequency shifted λ 18B component of beam 813 in the ninth embodiment from a portion of beam 808A transmitted by nonpolarizing beam splitter 851E and of beam 808B is the same as the portion ofthe description given for the generation ofthe λ 16A component, a frequency shifted λ 16A component, a λ 16B component, and a frequency shifted λ 16B component of beam 713 in the eighth embodiment from beam 708A and of beam 708B. The frequencies of drivers 806A and 806B are f 2 and f 3 , respectively, so that the frequency shifts ofthe frequency shifted λ 18A component and the frequency shifted λ 18B component of beam 813 are f 2 and f 3 , respectively. Beam 807 is generated from a second portion of beam 808A (see FIG. 9a), the second portion of beam 808A being reflected by nonpolarizing beam splitter 851E, reflected by minor 851F, and transmitted by nonlinear element 893. Beam 807 is comprised of two frequency components, one component with wavelength λ 18A and a frequency doubled component with wavelength λ 18A /2. The frequency doubled component is generated by SHG in nonlinear element 893. The description ofthe generation of beam 809 from beam 807 is the same as coπesponding portions ofthe description given for the generation of beam 709 in the eighth embodiment from beam 707. The frequency of driver 805 is f x . Beam 809 is comprised of three spatially coextensive frequency components, the components being a λ 18A component, a λ 18A /2 component, and a frequency shifted λ 18A /2 component. The frequency shift of the frequency shifted λ 18A /2 component is f x . The λ 18A /2 and frequency shifted λ 18A /2 components of beam 809 are polarized orthogonal to and in the plane of FIG. 9a, respectively. The frequency shifted λ 18A /2 component of beam 809 is transmitted by optical filter 879A, reflected by minor 853C, transmitted by polarizer 879B, transmitted by half wave plate 879C, and a portion thereof reflected by dichroic beam splitter 853D to form a frequency shifted λ 18A /2 component of beam 813. Half wave phase retardation plate 879C is oriented so as to rotate the polarization of incident frequency shifted λ 18A /2 beam component by 45 degrees. Polarizer 879B is oriented to transmit the frequency shifted λ 18A /2 component of beam 809 and to block the λ 18A /2 component of beam 809. The optical filter 879A blocks the λ 18A component of beam 809. The descriptions ofthe modulator 803 and driver 805 are the same as coπesponding portions of descriptions given for driver 703 and driver 705 ofthe eighth embodiment. The frequency shift introduced by modulator 803 is f x . Beam splitter 873 depicted in FIG. 9a comprises a polarizing beam splitter interface for the two groups of frequency components of beam 813. One group ofthe two groups of frequency components comprise a λ 18A component, a frequency shifted λ 18A component, a λ 18B component, and a frequency shifted λ 18B component. A second group ofthe two groups of frequency components comprise a frequency shifted λ 18A /2 component. In addition, phase retardation plates 877 and 878 are quarter wave phase retardation plates for the first and second groups ofthe frequency components of beam 813. Beam 813 enters interferometer 869 (see FIG. 9a) and exits as two spatially separated exit beams 833 and 834. The λ 18A component and frequency shifted λ 18A component of beam 813 are measurement and reference beams, respectively, and exit interferometer 869 as components of beams 833 and 834, respectively. The λ 18B component and frequency shifted λ 18B component of beam 813 are measurement and reference beams, respectively, and exit interferometer 869 as other components of beams 833 and 834, respectively. A first portion ofthe frequency shifted λ 18A /2 component of beam 813 is transmitted by polarizing beam splitter 873 and exits interferometer 869 as a certain other component of exit beam 833. A second portion ofthe frequency shifted λ 18A /2 component of beam 813 are reflected by polarizing beam splitter 873 and exits interferometer 869 as a certain other component of exit beam 834. Exit beams 833 and 834 contain information at λ 18A and λ 18B about the optical path length in the measuring path including the path through the gas 898 and about the optical path length through the reference path, respectively. A first portion of each of beams 833 and 834 are reflected by nonpolarizing beam splitter 861 as beams 835 and 836, respectively. The description ofthe propagation of beams 835 and 836 through detector 889 to generate electrical interference signals, heterodyne signals s 18A and s X8B , is the same, except for optical filter 879D, as coπesponding portions ofthe description given for the eighth embodiment with respect to the propagation of beams 735 and 736 through detector 789 to generate heterodyne signals s X6A and s xβB , respectively. Optical filter 879D transmits the first group of frequency components of beam 841 and blocks the second group of frequency components of beam 841. A second portion of beam 833 is transmitted by nonpolarizing beam splitter 861, enters nonlinear element 894M, and exits nonlinear element 894M as beam 837 comprised of five coextensive components, a λ 18A component, a λ 18B component, a second λ 18A /2 component, a second λ 18B /2 component, and a frequency shifted λ 18A /2. The second λ 18A /2 component and second λ 18B /2 component are generated by SHG in nonlinear element 894M from the λ 18A component and the λ 18B component, respectively. Beam 837 is incident on optical filter and polarizer 882M with portions thereof transmitted and incident on detector 886M. A second portion of beam 834 is transmitted by nonpolarizing beam splitter 861, enters nonlinear element 894R, and exits nonlinear element 894R as beam 838 comprised of five spatially coextensive components, a frequency shifted λ 18A component, a frequency shifted λ 18B component, a frequency doubled, frequency shifted λ 18A component, a frequency doubled, frequency shifted λ 18B component, and a frequency shifted λ 18A /2. The frequency doubled, frequency shifted λ 18A component and frequency doubled, frequency shifted λ 18B component are generated by SHG in nonlinear element 894R from the frequency shifted λ 18A component and the frequency shifted λ 18B component, respectively. Beam 838 is incident on optical filter and polarizer 882R with portions thereof transmitted and incident on detector 886R. Optical filter and polarizer 882M transmits the second λ 18A /2 component, the second λ 18B /2 component, and the frequency shifted λ 18A /2 component of beam 837 and optical filter and polarizer 882R transmits the frequency doubled, frequency shifted λ 18B component, the frequency doubled, frequency shifted λ 18B component, and the frequency shifted λ 18A /2 component of beam 838. Optical filter and polarizer 882M and optical filter and polarizer 882R further mix the polarization components of the beams 837 and 838, respectively. Optical filter and polarizer 882M further blocks a λ 18A component and a λ 18B component and optical filter and polarizer 882R further blocks a frequency shifted λ 18A component and a frequency shifted λ 18B component. Interferometer 869 introduces phase shift φ 18A between the λ 18A and the frequency shifted λ 18A components of beam 841 and φ 18B between the λ 18B and the frequency shifted λ 18B components of beam 841. Signal 824M comprises two heterodyne signals with heterodyne frequencies f x and {2[(l/λ 18B )-(l/λ 18A )]c- 1 } . Signal 824R comprises two heterodyne signals with heterodyne frequencies (2f 2 -f x ) and {2[(l/λ 18B )- (l/λ 18A )]c + 2{f 3 -fι)}. The heterodyne signals with frequencies {2[(l/λ 18B )-(l/λ 18A )]c-Λ} and {2[(l/λ 18B )-(l/λ 18A )]e + 2(/ 3 -f x )} are rejected in electronic processor 827 by electronic filtering and therefore not included in subsequent description ofthe ninth embodiment. Interferometer 869 further introduces phase shifts φ 18M and φ 18R between certain beam components of beams 837 and 838, respectively, the certain beam components that form heterodyne signals s lm and 5 18R , respectively, with heterodyne frequencies f x and (2 2 -f x ) , respectively. The magnitude of phase shifts φ 18M and φ 18R are related to optical path lengths ofrespective paths according to the formulae where * 18A / 2 .Λ = 2π[(2/λ 18A ) + (Λ/c)] , (103) = 2π[( λιβA) + ( 2/c)] , lengths L M and R represent the equivalent physical path lengths from respective sources to respective detectors for the measurement and reference legs, respectively. The descriptions ofthe properties of heterodyne signals s 1QA , s lβB , 5 18M , and s R in terms of respective phases φ 18A , Φ I8B > 9i 8M » an ^ Φ 18R an ^ respective measured phases φ 18A , φ 18B , φ 18M , and φ 18R are the same as the conesponding portions ofthe description given for the properties of heterodyne signal s t and respective phase φ,- and respective measured phase φ,- of the first and subsequent embodiments. As shown in FIG. 9b, measured phases φ 18A and φ 18B are generated by phase detectors 8274A and 8274B, respectively, from signals heterodyne signals s 1Qh and s 1BB , respectively. The description ofthe phase detection is the same as the conesponding portions ofthe description given for the phase detection in electronic processor 27 ofthe first embodiment. It is evident that the system comprised of sources 802A and 802B, interferometer 869, detector system 889, and processors 8274A and 8274B of electronic processor 827 ofthe ninth embodiment is functionally equivalent with respect to generation and processing heterodyne signals s 1BA and s 1BB for respective measured phases φ 18A and φ 18B to the system comprised of sources 702A and 702B, interferometer 769, detector system 789 and conesponding phase detectors of electronic processor 727 ofthe eighth embodiment with respect to generation and processing heterodyne signals s xβA and s 16B for respective measured phases φ 16A and φ 16B . It is further evident that the system comprised of sources 802A and 802B, interferometer 869, detector system 889 and processors 8274A and 8274B of electronic processor 827 ofthe ninth embodiment is functionally equivalent with respect to generation and processing heterodyne signals s 1BA and s 1QB for respective measured phases Φι 8A and φ 18B to the system comprised of sources 601 and 602, interferometer 669, detector system 689 and conesponding phase detectors of electronic processor 627 ofthe seventh embodiment with respect to generation and processing heterodyne signals s x3 and s X4 for respective measured phases φ 13 and φ 14 .. Thus, cyclic enors present in each ofthe measured phases Φι 8A and φ 18B can be determined by procedures described in the seventh and eighth embodiments. As shown in FIG. 9b, measured phases Φι 8M and φ 18R are generated by phase detectors 8274C and 8274D, respectively, from signals heterodyne signals s 18M and s βR , respectively. The description ofthe phase detection is the same as the conesponding portions ofthe description given for the phase detection in electronic processor 27 ofthe first embodiment. In a next step, a phase Φ is obtained by electronic processor 8277 as the difference between the measured phases φ 18M and φ 18R , i.e., Φ = Φl8M - φl8R • 04) Phase Φ may be expressed in terms of other quantities according to the formula Φ = Lp2k lβA [n(k 1BΑ/2 )-n(k lBΑ )} + Z ψ +Z (105) where Z = R p πfzyή ^ ) + ζ 18M -ζ j . 8R phase offsets ζ 18M and ζ 18R comprising all contributions to the arguments α i 8M an£ * cii βR > respectively, of s 1BM and 5 18R , respectively, that are not related to the measurement and reference paths, respectively, and ψ 18M and ψ 18R comprise all cyclic eπors in Φι 8M and φ 18R , respectively. The non-cyclic nonlinearities η 18M and η 18R have been omitted for reasons indicated in the description ofthe first embodiment. The cyclic enor term Z ψ may be expressed to a good approximation as -∑^ cosr{zψfc 18A n(£ 18A )]} r=X (107) + ∑ έ i 8M , r sinr{lp[ 18A 2)Λ n(A: 18A / 2( l ) } r=X " ∑ 6 l8M , r smr{Zp[2£ 18A n(£ 18A )]} r=X Eq. (107) for Z ψ can be rewritten in the form ■Z ψ = ∑αi 8M , r Cθsr{Z j p[2£ 18A «(& 18A )]) r=l (108) + ∑* s r { L p[ 2k XBA n ( k xs?s)]} r=X where α 18M,r = α 18M,r cosr Φ ~ "lSM.r + bl8M,r sin ^ (109) b XBM f = b Xm,r cos - bl8M,r + α 18M,r sin Φ > and (110) The effective wavelength associated with φ is ofthe order of for a gas comprised of air at 25° C and at one atmosphere of pressure. Therefore, only the reduced set of coefficients ^XB M - anc * bi 8M , r are a function of L . The reduced set of coefficients α 18Mjr and b 1Bliιr is determined by a Fourier analysis of Φ using φ 18A as the variable of integration, the range of integration in φ XBR /k 1BA being «: 10 5 λ 18A and ≥* λ 18A . The phase φ 18A is the phase obtained from φ 18A with the compensation for cyclic enors described as an earlier step in the ninth embodiment. The cyclic enors in Φ may also be reduced or eliminated by filtering with an integral transform such as described for the sixth embodiment. The dispersion and changes in optical path length, respectively, compensated for cyclic enors are computed in the ninth embodiment using Eqs. (82), (83), (86), and (89), respectively, with the substitutions of φ 18A , Φ , Z ψ , Z , ζ 18A , and ψ 18A of he ninth embodiment for φ , Φ , Z ψ , Z , ζ xx , and ψ ofthe sixth embodiment with θ = 0 , (R/χ) = 0 , χ = 2& 18A , l xx = 2 , and l 12 = 1. Calculation ofthe dispersion conected for the effects of cyclic enors can also be calculated using Eq. (105). The remaining description ofthe ninth embodiment is the same as conesponding portions ofthe description given for the eighth embodiment. The advantages ofthe ninth embodiment are the same as those described for the eighth embodiment. It will be evident to those skilled in the art that beams used to generate signals for information to determine Φ may be pulsed beams without departing from the spirit and scope ofthe present invention. Pulsed beams can be used because ofthe slow rate of change in Φ as I is changed. The advantage of using pulsed beams is increased efficiency for generating a second harmonic of a beam using SHG, a pulsed mode of operation allowing increased beam power density during a pulse for the same average power density and the SHG efficiency being proportional to the square ofthe instantaneous power density of a beam in a nonlinear element. A tenth embodiment ofthe present invention (not shown in the figures) is described from the third group of embodiments ofthe present invention. The tenth embodiment coπesponds to the sixth embodiment the same as the ninth embodiment coπesponds to the seventh embodiment. For certain applications, only the effect of changes in the refractivity ofthe gas in the measuring path such as the result of turbulence in the gas need be compensated in a distance measuring interferometer. Under the condition that there are no other changes in φ 18M ofthe gas turbulence type, for example, that need be compensated, a variant ofthe ninth embodiment can beneficially be used wherein φ 18R is not measured. A similar variant ofthe tenth embodiment can beneficially be used for similar circumstances. The description ofthe sixth embodiment noted the enhanced sensitivity to the effects of cyclic enors of measurements of dispersion of a gas relative to the sensitivity to effects of cyclic eπors of measurements of refractivity of a gas and enhanced sensitivity to the effects of cyclic eπors of measurements of refractivity of a gas relative to the sensitivity to effects of cyclic eπors of measurements of index of refraction of a gas. As a consequence, measurement of intrinsic optical properties of a gas, e.g., the reciprocal dispersive power of a gas Y , exhibits enhanced sensitivity to the effects of cyclic eπors relative to the sensitivity to effects of cyclic eπors of measurements of refractivity of a gas and enhanced sensitivity to the effects of cyclic eπors relative to the sensitivity to effects of cyclic eπors of measurements of index of refraction of a gas. It will be evident to those skilled in the art that the embodiments ofthe present invention can beneficially be incoφorated into apparatus and methods to measure intrinsic optical properties of a gas such as Y compensated for the effects of cyclic eπors without departing from the spirit and scope ofthe present invention. Examples of apparatus and methods for measurement of intrinsic optical properties of a gas are described in United States Application No. 08/942,848, United States Application entitled "Interferometric Method and Apparatus for Measuring Intrinsic Opitcal Properties of a Gas" filed Oct. 21 , 1998, and United States Provisional Application No. 60/075,595, ibid. The interferometry systems described above characterize cyclic eπors and use the characterized cyclic enors to conect distance measurements, dispersion measurements, and intrinsic optical property measurements for cyclic nonlinearity. As a result, such interferometry systems provide highly accurate measurements. Such systems can be especially useful in lithography applications used in fabricating large scale integrated circuits such as computer chips and the like. Lithography is the key technology driver for the semiconductor manufacturing industry. Overlay improvement is one of the five most difficult challenges down to and below 100 nm line widths (design rules), see for example the Semiconductor Industry Roadmap, p82 (1997). Overlay depends directly on the performance, i.e. accuracy and precision, of the distance measuring interferometers used to position the wafer and reticle (or mask) stages. Since a lithography tool may produce $50-100M/year of product, the economic value from improved performance distance measuring interferometers is substantial. Each 1% increase in yield ofthe lithography tool results in approximately $lM/year economic benefit to the integrated circuit manufacturer and substantial competitive advantage to the lithography tool vendor. The function of a lithography tool is to direct spatially patterned radiation onto a photoresist-coated wafer. The process involves determining which location ofthe wafer is to receive the radiation (alignment) and applying the radiation to the photoresist at that location (exposure). To properly position the wafer, the wafer includes alignment marks on the wafer that can be measured by dedicated sensors. The measured positions ofthe alignment marks define the location ofthe wafer within the tool. This information, along with a specification ofthe desired patterning of the wafer surface, guides the alignment ofthe wafer relative to the spatially patterned radiation. Based on such information, a translatable stage supporting the photoresist-coated wafer moves the wafer such that the radiation will expose the conect location ofthe wafer. During exposure, a radiation source illuminates a patterned reticle, which scatters the radiation to produce the spatially patterned radiation. The reticle is also refened to as a mask, and these terms are used interchangeably below. In the case of reduction lithography, a reduction lens collects the scattered radiation and forms a reduced image of the reticle pattern. Alternatively, in the case of proximity printing, the scattered radiation propagates a small distance (typically on the order of microns) before contacting the wafer to produce a 1 : 1 image of the reticle pattern. The radiation initiates photo-chemical processes in the resist that convert the radiation pattern into a latent image within the resist. Interferometry systems are important components ofthe positioning mechanisms that control the position of the wafer and reticle, and register the reticle image on the wafer. If such interferometry systems include the phase measurement portion described above, the accuracy of distances measured by the systems increases as cyclic enor contributions to the distance measurement are minimized. In general, the lithography system, also refened to as an exposure system, typically includes an illumination system and a wafer positioning system. The illumination system includes a radiation source for providing radiation such as ultraviolet, visible, x-ray, electron, or ion radiation, and a reticle or mask for imparting the pattern to the radiation, thereby generating the spatially patterned radiation. In addition, for the case of reduction lithography, the illumination system can include a lens assembly for imaging the spatially patterned radiation onto the wafer. The imaged radiation exposes resist coated onto the wafer. The illumination system also includes a mask stage for supporting the mask and a positioning system for adjusting the position ofthe mask stage relative to the radiation directed through the mask. The wafer positioning system includes a wafer stage for supporting the wafer and a positioning system for adjusting the position ofthe wafer stage relative to the imaged radiation. Fabrication of integrated circuits can include multiple exposing steps. For a general reference on lithography, see, for example, J. R. Sheats and B. W. Smith, in Microlithography: Science and Technology (Marcel Dekker, Inc., New York, 1998), the contents of which is incorporated herein by reference. Interferometry systems described above can be used to precisely measure the positions of each ofthe wafer stage and mask stage relative to other components ofthe exposure system, such as the lens assembly, radiation source, or support structure. In such cases, the interferometry system can be attached to a stationary structure and the measurement object attached to a movable element such as one ofthe mask and wafer stages. Alternatively, the situation can be reversed, with the interferometry system attached to a movable object and the measurement object attached to a stationary object. More generally, such interferometry systems can be used to measure the position of any one component ofthe exposure system relative to any other component ofthe exposure system, in which the interferometry system is attached to, or supported by, one ofthe components and the measurement object is attached, or is supported by the other of the components. An example of a lithography scanner 1100 using an interferometry system 1126 is shown in Fig. 11a. The interferometry system is used to precisely measure the position of a wafer (not shown) within an exposure system. Here, stage 1122 is used to position and support the wafer relative to an exposure station. Scanner 1100 includes a frame 1102, which carries other support structures and various components carried on those structures. An exposure base 1104 has mounted on top of it a lens housing 1106 atop of which is mounted a reticle or mask stage 1116, which is used to support a reticle or mask. A positioning system for positioning the mask relative to the exposure station is indicated schematically by element 1117. Positioning system 1117 can include, e.g. , piezoelectric transducer elements and conesponding control electronics. Although, it is not included in this described embodiment, one or more of the interferometry systems described above can also be used to precisely measure the position ofthe mask stage as well as other moveable elements whose position must be accurately monitored in processes for fabricating lithographic structures (see supra Sheats and Smith Microlithography: Science and Technology). Suspended below exposure base 1104 is a support base 1113 that carries wafer stage 1122. Stage 1122 includes a plane minor 1128 for reflecting a measurement beam 1154 directed to the stage by interferometry system 1126. A positioning system for positioning stage 1122 relative to interferometry system 1126 is indicated schematically by element 1119. Positioning system 1119 can include, e.g., piezoelectric transducer elements and coπesponding control electronics. The measurement beam reflects back to the interferometry system, which is mounted on exposure base 1104. The interferometry system can be any of the embodiments described previously. During operation, a radiation beam 1110, e.g., an ultraviolet (UV) beam from a UV laser (not shown), passes through a beam shaping optics assembly 1112 and travels downward after reflecting from minor 1114. Thereafter, the radiation beam passes through a mask (not shown) carried by mask stage 1116. The mask (not shown) is imaged onto a wafer (not shown) on wafer stage 1122 via a lens assembly 1108 caπied in a lens housing 1106. Base 1104 and the various components supported by it are isolated from environmental vibrations by a damping system depicted by spring 1120. In other embodiments ofthe lithographic scanner, one or more ofthe interferometry systems described previously can be used to measure distance along multiple axes and angles associated for example with, but not limited to, the wafer and reticle (or mask) stages. Also, rather than a UV laser beam, other beams can be used to expose the wafer including, e.g. , x-ray beams, electron beams, ion beams, and visible optical beams. In some embodiments, the lithographic scanner can include what is known in the art as a column reference. In such embodiments, the interferometry system 1126 directs the reference beam (not shown) along an external reference path that contacts a reference minor (not shown) mounted on some structure that directs the radiation beam, e.g., lens housing 1106. The reference minor reflects the reference beam back to the interferometry system. The interference signal produce by interferometry system 1126 when combining measurement beam 1154 reflected from stage 1122 and the reference beam reflected from a reference minor mounted on the lens housing 1106 indicates changes in the position ofthe stage relative to the radiation beam. Furthermore, in other embodiments the interferometry system 1126 can be positioned to measure changes in the position of reticle (or mask) stage 1116 or other movable components of the scanner system. Finally, the interferometry systems can be used in a similar fashion with lithography systems involving steppers, in addition to, or rather than, scanners. As is well known in the art, lithography is a critical part of manufacturing methods for making semiconducting devices. For example, U.S. Patent 5,483,343 outlines steps for such manufacturing methods. These steps are described below with reference to FIGS, lib and lie. FIG. lib is a flow chart ofthe sequence of manufacturing a semiconductor device such as a semiconductor chip (e.g. IC or LSI), a liquid crystal panel or a CCD. Step 1151 is a design process for designing the circuit of a semiconductor device. Step 1152 is a process for manufacturing a mask on the basis ofthe circuit pattern design. Step 1153 is a process for manufacturing a wafer by using a material such as silicon. Step 1154 is a wafer process which is called a pre-process wherein, by using the so prepared mask and wafer, circuits are formed on the wafer through lithography. To form circuits on the wafer that coπespond with sufficient spatial resolution those patterns on the mask, interferometric positioning ofthe lithography tool relative the wafer is necessary. The interferometry methods and systems described herein can be especially useful to improve the effectiveness ofthe lithography used in the wafer process. Step 1155 is an assembling step, which is called a post-process wherein the wafer processed by step 1154 is formed into semiconductor chips. This step includes assembling (dicing and bonding) and packaging (chip sealing). Step 1156 is an inspection step wherein operability check, durability check and so on ofthe semiconductor devices produced by step 1155 are carried out. With these processes, semiconductor devices are finished and they are shipped (step 1157). FIG. lie is a flow chart showing details ofthe wafer process. Step 1161 is an oxidation process for oxidizing the surface of a wafer. Step 1162 is a CVD process for forming an insulating film on the wafer surface. Step 1163 is an electrode forming process for forming electrodes on the wafer by vapor deposition. Step 1164 is an ion implanting process for implanting ions to the wafer. Step 1165 is a resist process for applying a resist (photosensitive material) to the wafer. Step 1166 is an exposure process for printing, by exposure (i.e., lithography), the circuit pattern of the mask on the wafer through the exposure apparatus described above. Once again, as described above, the use ofthe interferometry systems and methods described herein improve the accuracy and resolution of such lithography steps. Step 1167 is a developing process for developing the exposed wafer. Step 1168 is an etching process for removing portions other than the developed resist image. Step 1169 is a resist separation process for separating the resist material remaining on the wafer after being subjected to the etching process. By repeating these processes, circuit patterns are formed and superimposed on the wafer. The interferometry systems described above can also be used in other applications in which the relative position of an object needs to be measured precisely. For example, in applications in which a write beam such as a laser, x-ray, ion, or electron beam, marks a pattern onto a substrate as either the substrate or beam moves, the interferometry systems can be used to measure the relative movement between the substrate and write beam. As an example, a schematic of a beam writing system 1200 is shown in FIG. 12. A source 1210 generates a write beam 1212, and a beam focusing assembly 1214 directs the radiation beam to a substrate 1216 supported by a movable stage 1218. To determine the relative position ofthe stage, an interferometry system 1220 directs a reference beam 1222 to a minor 1224 mounted on beam focusing assembly 1214 and a measurement beam 1226 to a minor 1228 mounted on stage 1218. Since the reference beam contacts a minor mounted on the beam focusing assembly, the beam writing system is an example of a system that uses a column reference. Interferometry system 1220 can be any ofthe interferometry systems described previously. Changes in the position measured by the interferometry system conespond to changes in the relative position of write beam 1212 on substrate 1216. Interferometry system 1220 sends a measurement signal 1232 to controller 1230 that is indicative ofthe relative position of write beam 1212 on substrate 1216. Controller 1230 sends an output signal 1234 to a base 1236 that supports and positions stage 1218. In addition, controller 1230 sends a signal 1238 to source 1210 to vary the intensity of, or block, write beam 1212 so that the write beam contacts the substrate with an intensity sufficient to cause photophysical or photochemical change only at selected positions of the substrate. Furthermore, in some embodiments, controller 1230 can cause beam focusing assembly 1214 to scan the write beam over a region ofthe substrate, e.g., using signal 1244. As a result, controller 1230 directs the other components ofthe system to pattern the substrate. The patterning is typically based on an electronic design pattern stored in the controller. In some applications the write beam patterns a resist coated on the substrate and in other applications the write beam directly patterns, e.g., etches, the substrate. An important application of such a system is the fabrication of masks and reticles used in the lithography methods described previously. For example, to fabricate a lithography mask an electron beam can be used to pattern a chromium- coated glass substrate. In such cases where the write beam is an electron beam, the beam writing system encloses the electron beam path in a vacuum. Also, in cases where the write beam is, e.g., an electron or ion beam, the beam focusing assembly includes electric field generators such as quadrapole lenses for focusing and directing the charged particles onto the substrate under vacuum. In other cases where the write beam is a radiation beam, e.g., x-ray, UV, or visible radiation, the beam focusing assembly includes coπesponding optics and for focusing and directing the radiation to the substrate. Other aspects, advantages, and modifications are within the scope ofthe following claims.

Claims: What is claimed is: 1. An interferometry system comprising: an interferometer which during operation directs a reference beam along a reference path and a measurement beam along a measurement path contacting a measurement object and combines the reference and measurement beams to produce overlapping exit beams, the overlapping exit beams indicative of changes in a relative optical path length to a measurement object, the interferometer comprising a variable phase shifter which during operation produces a phase shift in at least one ofthe reference and measurement beams; a detection system which during operation mixes polarizations ofthe overlapping exit beams to produce a mixed beam and measures a time-varying intensity ofthe mixed beam; and an analyzer coupled to the phase shifter and the detection system and which during operation controls the value ofthe phase shift produced by the phase shifter, measures a phase coπesponding to the time- varying intensity ofthe mixed beam, and determines a spectral representation of cyclic eπors in the interferometry system based on the phase ofthe mixed beam for each of multiple values of the phase shift. 2. The interferometry system of claim 1 , wherein the spectral representation is a Fourier sine and cosine series. 3. The interferometry system of claim 1, wherein the phase-shifter produces the phase-shift in one of the reference beam and measurement beam and not in the other ofthe reference beam and the measurement beam. 4. The interferometry system of claim 1, wherein the phase-shifter produces the phase-shift in the reference beam and the measurement beam. 5. The interferometry system of claim 1, wherein during operation the analyzer determines the spectral representation based on the measured phase ofthe mixed beam for each of multiple values ofthe phase shift for each of multiple positions ofthe measurement object. 6. The interferometry system of claim 1, wherein the analyzer comprises a memory and during operation stores the spectral representation in the memory. 7. The interferometry system of claim 1, wherein the interferometer comprises a beam splitter that combines the reference beam and measurement beam to produce the overlapping exit beams, and wherein the measurement path contacts the phase shifter between the measurement object and the beam splitter. 8. The interferometry system of claim 1, wherein the interferometer comprises a polarizing beam splitter that directs the reference beam along the reference path and the measurement beam along the measurement path and wherein the measurement path contacts the phase shifter between the polarizing beam splitter and the measurement object. 9. The interferometry system of claim 1, wherein the interferometer comprises a polarizing beam splitter that receives the reference beam from the reference path and the measurement beam from the measurement path and wherein the measurement path contacts the phase shifter between the measurement object and the polarizing beam splitter. 10. The interferometry system of claim 1, wherein the phase shifter is an electro-optical modulator. 11. The interferometry system of claim 1, wherein the phase shifter comprises an optical delay line and a translator for adjusting the length ofthe delay line, wherein the analyzer controls the translator. 12. The interferometry system of claim 1, wherein the phase shifter comprises a pair of prisms and a translator for varying the relative positions ofthe prisms, wherein the analyzer controls the translator. 13. The interferometry system of claim 1, wherein the phase shifter comprises a gas cell defining an optical path and a gas handling system for varying the gas pressure in the cell, wherein the analyzer controls the gas handling system. 14. The interferometry system of claim 1, wherein during operation the interferometer directs a second reference beam along a second reference path and a second measurement beam along a second measurement path contacting the measurement object and combines the second reference beam and the second measurement beam to produce a second pair of overlapping exit beams, the second pair of overlapping exit beams indicative of changes in the relative optical path length to the measurement object, wherein during operation the detection system mixes polarizations of the second pair of overlapping exit beams to produce a second mixed beam and measures a time-varying intensity ofthe second mixed beams, and wherein during operation the analyzer measures a phase coπesponding to the time-varying intensity ofthe second mixed beam and determines the spectral representation based on the measured phases of each ofthe mixed beams for each of multiple values ofthe phase shift. 15. The interferometry system of claim 14, wherein during operation the analyzer provides an initial value for the phase shift, measures the phase coπesponding to the time- varying intensity of each ofthe mixed beams for each of multiple positions ofthe measurement object and the initial value ofthe phase shift, and then repeats the providing and measuring steps for additional values ofthe phase shift. 16. The interferometry system of claim 14, wherein the phase-shifter produces the phase-shift in one ofthe first-mentioned reference beam and the first-mentioned measurement beam and not in the other ofthe first-mentioned reference beam and the first-mentioned measurement beam. 17. An interferometry system comprising: a source which during operation provides reference and measurement beams having different frequencies, the source comprising a frequency shifter which during operation shifts the frequencies of the reference and measurement beams by equal amounts; an interferometer which during operation directs the reference beam along a reference path and the measurement beam along a measurement path contacting a measurement object and combines the reference and measurement beams to produce overlapping exit beams, the overlapping exit beams indicative of changes in a relative optical path length to the measurement object; a detection system which during operation mixes polarizations ofthe overlapping exit beams to produce a mixed beam and measures a time-varying intensity ofthe mixed beam; and an analyzer coupled to the frequency shifter and the detection system, and which during operation causes the frequency shifter to shift the frequencies ofthe reference and measurement beams and produce a coπesponding phase shift between the overlapping exit beams, measures a phase coπesponding to the time-varying intensity ofthe mixed beam, and determines a spectral representation of cyclic eπors in the interferometry system based on the measured phase ofthe mixed beam for each of multiple values of the phase shift. 18. The interferometry system of claim 17, wherein during operation the analyzer determines the spectral representation based on the measured phase ofthe mixed beam for each of multiple values ofthe phase shift for each of multiple positions of the measurement obj ect. 19. The interferometry system of claim 17, wherein during operation the source provides a second reference beam and a second measurement beam having frequencies that are not shifted by the frequency shifter, wherein during operation the interferometer directs the second reference beam along a second reference path and the second measurement beam along a second measurement path contacting the measurement object and combines the second reference beam and the second measurement beam to produce a second pair of overlapping exit beams, the second pair of overlapping exit beams indicative of changes in the relative optical path length to the measurement object, wherein during operation the detection system mixes polarizations ofthe second pair of overlapping exit beams to produce a second mixed beam and measures a time-varying intensity ofthe second mixed beams, and wherein during operation the analyzer measures a phase coπesponding to the time-varying intensity ofthe second mixed beam. 20. The interferometry system of claim 19, wherein during operation the analyzer causes the frequency shifter to produce an initial value for the phase shift, measures the phase coπesponding to the time-varying intensity of each ofthe mixed beams for each of multiple positions ofthe measurement object and the initial value of the phase shift, and then repeats the causing and measuring steps for additional values of the phase shift, and wherein the analyzer determines the spectral representation based on the measured phases. 21. The interferometry system of claim 17, wherein the frequency shifter is an acousto-optical modulator. 22. An interferometry system comprising: a dispersion measuring interferometer which during operation produces first and second pairs of overlapping exit beams, the first pair of exit beams having a first wavelength and the second pair of exit beams having a second wavelength different from the first wavelength, the first and second pairs of exit beams each indicative of changes in a relative optical path length to a measurement object; a detection system which during operation mixes polarizations ofthe first pair of overlapping exit beams to produce a first mixed beam, mixes polarizations ofthe second pair of overlapping exit beams to produce a second mixed beam, and measures a time-varying intensity of each ofthe mixed beams; and an analyzer coupled to the detection system and which during operation measures a phase conesponding to the time- varying intensity of each ofthe mixed beams at each of multiple positions ofthe measurement object, calculates a dispersion value for each ofthe multiple positions, the dispersion value for a particular position equal to a function ofthe measured phases at that particular position, and determines a spectral representation of cyclic eπor contributions to dispersion measurements by the interferometer based on the calculated dispersion values. 23. The interferometry system of claim 22, wherein the analyzer further comprises a memory and during operation stores the spectral representation in the memory. 24. The interferometry system of claim 22, wherein during operation the analyzer determines the spectral representation by expressing the calculated dispersion values as a function that comprises a Fourier series in at least one ofthe measured phases and inverting the Fourier series. 25. The interferometry system of claim 22, wherein the dispersion value is equal to a function of a weighted difference between the measured phases. 26. The interferometry system of claim 25, wherein the first and second wavelengths λ 1 and λ 2 satisfy the equation λ x /λ 2 = l x // 2 , where l γ and l 2 are integers, and the weighted difference is equal to / 1 φ 1 - / 2 φ 2 , where φ 2 and φ 2 are the measured phases. 27. The interferometry system of claim 26, wherein l λ and l 2 are each less than 50. 28. The interferometry system of claim 22, wherein the first and second wavelengths differ by at least 1 nm. 29. The interferometry system of claim 22, wherein during operation the interferometer combines a first reference beam and a first measurement beam to produce the first pair of overlapping exit beams and wherein the interferometer comprises a variable phase shifter which during operation produces a phase shift in at least one ofthe first reference beam and the first measurement beam, and wherein the analyzer is coupled to the phase shifter and during operation controls the value ofthe phase shift produced by the phase shifter. 30. The interferometry system of claim 22 further comprising a source which during operation provides a first reference beam and a first measurement beam, the source comprising a frequency shifter for shifting the frequencies ofthe first reference and measurement beams by equal amounts, and wherein during operation the interferometer combines the first reference and measurement beams to produce the first pair of overlapping exit beams, and wherein the analyzer is coupled to the frequency shifter and during operation causes the frequency shifter to shift the frequencies ofthe first reference and measurement beams and produce a conesponding phase shift between the first pair of overlapping exit beams. 31. An interferometry system comprising: a dispersion measuring interferometer which during operation produces first and second pairs of overlapping exit beams, the first pair of exit beams having a first wavelength and the second pair of exit beams having a second wavelength different from the first wavelength, the first and second pairs of exit beams each indicative of changes in a relative optical path length to a measurement object; a detection system which during operation mixes polarizations ofthe first pair of overlapping exit beams to produce a first mixed beam, mixes polarizations ofthe second pair of overlapping exit beams to produce a second mixed beam, and measures a time-varying intensity of each ofthe mixed beams; and an analyzer coupled to the detection system and which during operation measures a phase conesponding to the time- varying intensity of each ofthe mixed beams at each of multiple positions ofthe measurement object, calculates a dispersion value for each ofthe multiple positions, the dispersion value for a particular position equal to a function ofthe measured phases at that particular position, and filters the dispersion values to determine an averaged dispersion value having reduced cyclic eπor. 32. The interferometry system of claim 31, wherein the analyzer averages over the dispersion values by summing the dispersion values coπesponding to equally-spaced measured phases from one ofthe mixed beams, the equally-spaced measured phases spanning an interval that is a multiple of 2π . 33. The interferometry system of claim 32, wherein the first and second wavelengths λ 1 and λ 2 satisfy the equation λ 1 /λ 2 = l x jl 2 , l and l 2 being integers, wherein the dispersion values are equal to l 1 φ 1 - Z 2 φ 2 , φ x and φ 2 being the measured phases, and wherein the analyzer averages over the dispersion values by summing the dispersion values conesponding to equally-spaced values of ψ 1 spanning a 2π/ 2 interval. 34. A method for characterizing cyclic enors in an interferometer, the method comprising: directing a reference beam along a reference path and a measurement beam along a measurement path contacting a measurement object; combining the reference beam and the measurement beam to produce overlapping exit beams that are indicative of changes in a relative optical path length to the measurement object; introducing at least three phase-shifts to at least one ofthe reference and measurement beams for each of multiple positions ofthe measurement object; mixing polarizations ofthe reference and measurement beams to produce mixed beam; measuring a phase coπesponding to a time-varying intensity ofthe mixed beam for each ofthe phase shifts for each ofthe multiple positions ofthe measurement object; and determining a spectral representation of cyclic enors in the interferometer based on the measured phases. 35. The method of claim 34, wherein the introducing step comprises introducing at least five phase-shifts to at least one ofthe reference and measurement beams for each of multiple positions ofthe measurement object 36. The method of claim 34, wherein the phase shifts are introduced to one ofthe reference beam and the measurement beam and not the other ofthe reference beam and the measurement beam. 37. The method of claim 34, wherein the phase shifts are introduced to the reference beam and the measurement beam. 38. The method of claim 34 further comprising: combining a second reference beam and a second measurement beam to produce a second pair of overlapping exit beams that are indicative of changes in a conesponding relative optical path length to the measurement object; mixing polarizations ofthe second reference and measurement beams to produce a second mixed beam; measuring a phase conesponding to a time- varying intensity ofthe second mixed beam for each ofthe multiple positions ofthe measurement object; and determining the spectral representation based on the measured phases for first and second mixed beams. 39. A method for characterizing cyclic enors in an interferometer comprising: providing a value for dispersion in an optical path length to a measurement object measured by the interferometer for each of multiple positions of the measurement object; and determining a spectral representation of cyclic eπor contributions to dispersion measurement in the interferometer based on the dispersion values. 40. The method of claim 39, wherein the determining step comprises expressing the dispersion values as a function that comprises a Fourier series that depends on the position ofthe measurement object and inverting the Fourier series to determine the spectral representation. 41. An interferometry method comprising: determining a spectral representation of cyclic enors in an interferometer using the method of claim 34; measuring an optical path length using the interferometer; and conecting the measured optical path length for cyclic enors using the spectral representation. 42. An interferometry method comprising: determining a spectral representation of cyclic eπor contributions to dispersion measurements in an interferometer using the method of claim 39; measuring an optical path length using the interferometer; and coπecting the measured optical path length using the spectral representation. 43. A lithography system for use in fabricating integrated circuits on a wafer, the system comprising: a stage for supporting the wafer; an illumination system for imaging spatially patterned radiation onto the wafer; a positioning system for adjusting the position ofthe stage relative to the imaged radiation; and the interferometry system of claims 1, 17, 22, or 31 for measuring the position ofthe stage. 44. A lithography system for use in fabπcating integrated circuits on a wafer, the system comprising: a stage for supporting the wafer; and an illumination system including a radiation source, a mask, a positioning system, a lens assembly, and the interferometry system of claim 1, 17, 22, or 31, wherein during operation the source directs radiation through the mask to produce spatially patterned radiation, the positioning system adjusts the position ofthe mask relative to the radiation from the source, the lens assembly images the spatially patterned radiation onto the wafer, and the interferometry system measures the position ofthe mask relative to the radiation from the source. 45. A lithography system for fabricating integrated circuits comprising first and second components, the first and second components being movable relative to each other, and the interferometry system of claim 1, 17, 22, or 31, wherein the first component comprises the measurement object and the interferometry system measures the position ofthe first component relative to the second component. 46. A lithography system for fabricating integrated circuits comprising first and second components, the first and second components being movable relative to each other, and the interferometry system of claim 1 or 17, wherein the first component comprises the measurement object contacted by the measurement path and the second component is contacted by the reference path, and wherein the interferometry system measures the relative position ofthe first and second components. 47. A lithography system for fabricating integrated circuits comprising first and second components, the first and second components being movable relative to each other, and the interferometry system of claim 22 or 31, wherein the interferometer directs a pair of measurement beams to contact the first component and a pair of reference beams to contact the second component, and wherein the interferometry system measures the relative position ofthe first and second components. 48. A beam writing system for use in fabricating a lithography mask, the system comprising: a source providing a write beam to pattern a substrate; a stage supporting the substrate; a beam directing assembly for delivering the write beam to the substrate; a positioning system for positioning the stage and beam directing assembly relative one another; and the interferometry system of claim 1 , 17, 22, or 31 for measuring the position ofthe stage relative to the beam directing assembly. 49. A lithography method for use in fabricating integrated circuits on a wafer comprising: supporting the wafer on a moveable stage; imaging spatially patterned radiation onto the wafer; adjusting the position ofthe stage; and measuring the position ofthe stage using the interferometry method of claim 41 or 42. 50. A lithography method for use in the fabrication of integrated circuits comprising: directing input radiation through a mask to produce spatially patterned radiation; positioning the mask relative to the input radiation; measuring the position ofthe mask relative to the input radiation using the interferometry method of claim 41 or 42, wherein one of a stage supporting the mask and a illumination system providing the input radiation include the measurement object; and imaging the spatially patterned radiation onto a wafer. 51. A lithography method for fabricating integrated circuits on a wafer comprising: positioning a first component of a lithography system relative to a second component of a lithography system to expose the wafer to spatially patterned radiation; and measuring the position ofthe first component relative to the second component using the method of claim 41 or 42 wherein the first component includes the measurement object. 52. A beam writing method for use in fabricating a lithography mask, the method comprising: directing a write beam to a substrate to pattern the susbtrate; positioning the substrate relative to the write beam; and measuring the position ofthe substrate relative to the write beam using the interferometry method of claim 41 or 42.

Inventors: HILL HENRY A, HILL, HENRY, A.
Applicants: HILL HENRY A, ZYGO CORP, HILL, HENRY, A., ZYGO CORPORATION
Application year: 2000
Main classification: G01B9
IPC codes: G01B9, G03F7, H01L21