Abstract:
An optical component especially suited for common path heterodyne interferometry comprises a symmetric dual-periscope configuration. Each periscope is substantially identical to the other with regard to certain design aspects. The resulting design is an optical component that is highly stable with variations in temperature and angular deviations.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   The present invention claims priority to U.S. Provisional Application No. 60/449,993, filed Feb. 25, 2003 and is herein incorporated by reference for all purposes. 
   The present invention is related to the following commonly owned, co-pending applications: U.S. application Ser. No. 10/180,086, filed Jun. 27, 2002, U.S. application Ser. No. 10/349,758, filed Jan. 22, 2003), U.S. application Ser. No. 10/293,209, filed Nov. 12, 2002, and U.S. application Ser. No. 10/788,166, filed Feb. 25, 2004 concurrently filed herewith, entitled “APODIZATION OF BEAMS IN AN OPTICAL INTERFEROMETER”, each of which are herein incorporated by reference for all purposes. 

   STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of NASA SIM Prime Contact No. NAS7-1407 awarded by the National Aeronautics and Space Administration (NASA). 

   BACKGROUND OF THE INVENTION 
   The present invention relates generally to interferometry and in particular to a highly stable optical component for use in concentric beam heterodyne interferometry. 
   Precision laser interferometry is used to precisely determine the distance to one or more fiducial points, such as a flat mirror, rooftop mirror, or corner-cube retro-reflector (“retro”), or between such fiducial points. An interferometer generally is composed of three components or subsystems: (1) a radiation source (e.g., a laser), (2) an optics component for producing beams of light for reference, measurement, and so on (herein referred to as a “beam launcher”), and (3) a signal processor (e.g., an observer or a photo-detector and associated electronic circuits) or other processing component to perform the interferometric determinations. In some configurations, the photo-detector is included in the beam launcher component, while much of the supporting electronics (e.g., the phase meter(s) and computer) remain with the signal processor. As can be appreciated other subsystem configurations are possible. 
   Interferometers can be configured to operate in a number of ways. The present invention is applicable to optical interferometers in general, operating in the regions of the electromagnetic spectrum commonly referred to as the infra-red (IR) light region, visible light region, and ultra-violet (UV) light region. Since there are many configurations of optical interferometers, only a small sampling of interferometer configurations will be discussed for background purposes. It will therefore be understood that a “beam” in the context of the present invention can be IR, visible light, or UV. 
   Some existing beam launchers for interferometers do not produce collinear antiparallel beams. Alternatively, if the launchers do produce collinear antiparallel beams, the launchers suffer from problems including thermal drift, cross talk, beam-walk, and/or non-common-path optics, among others. 
   Some existing launchers that do not produce collinear antiparallel beams sometimes function by directing a single beam towards a first one of the retros. The single beam hits the first retro at a point offset from a vertex of the retro. The retro-reflected beam emerges from the first retro at a symmetrically located offset point, and the beam then is directed to a second retro. The beam and retros are positioned and aligned such the reflected beam hits the second retro also offset from the vertex, with the emerging beam doubly reflected back to an entrance point on the launcher. Such a circuitous configuration is sometimes referred to as a “racetrack” configuration. Any imperfection in the construction of a retro can affect the orientation of the individual facets of the retro, which can cause the retro-reflected beam to emerge at a deflected angle, giving a “dihedral” error that affects the measured distance. If, in addition, the retro or launcher moves in such a manner as to cause a lateral beam displacement, this displacement times the deflection angle results in an error in the measured distance (an example of a “beam-walk” error). 
   Precision laser interferometry can be carried out in at least two modes, namely, the “homodyne” mode or the “heterodyne” mode. Either mode can be used for the racetrack configuration. 
   In the homodyne mode, a beam launcher splits a laser beam of a single frequency into two beams. One beam is directed out to the retro(s) to measure the distance. Upon returning to the beam launcher, the beam is aligned and collocated (and the polarization aligned, if needed) with the other portion of the original beam, and the resulting combined beam is directed onto a photo-detector. If the extra distance traveled by the measurement beam is an integer multiple of half the laser wavelength, then, when recombined, the two beams are in phase and add constructively, resulting in an increased signal from the photo-detector. If the measurement beam is an odd multiple of a quarter of the wavelength longer, the beams add destructively, resulting in a reduced signal from the photo-detector. If the distance between the retros changes, the signal fluctuates, and the fluctuations in the signal give a measure of the relative motion of the retros. A signal processor (e.g., an observer or a photo-detector and electronic circuit) “counts fringes” to determine the change in distance between the retros relative to an initial distance. The resolution of a homodyne interferometer is limited, as it is difficult to measure changes in distance significantly smaller than the laser wavelength (typically a half to several micrometers) due to intensity fluctuations of the laser. 
   A heterodyne interferometer configuration uses two beams that are offset in frequency to slightly different frequencies. Typically, the beams originate from a single laser. The difference between the frequencies is chosen to be convenient for detectors and electronics. Typically, the frequency difference is in the range of about 10 kHz to about 100 MHz. Typically, one frequency-offset laser beam (the “measurement beam”) emanates from the beam launcher to interrogate the distance to the retro(s) while the second frequency-offset laser beam (the “local oscillator” or LO) beam remains internal to the beam launcher. When the measurement beam and the LO beam are aligned, collocated, and with aligned polarizations, and are directed onto the photo-detector, the photo-detector produces a “beat” signal. By comparing this beat signal to the known difference of frequency offsets between the laser beams, it is possible to track changes in the relative phase of the signal to find the change in retro distance relative to the initial value. With precision phase meters, it is possible to resolve distances to small fractions of the laser wavelength, resulting in measurements with sub-nanometer precision. 
   When measuring distances with fine precision, various error sources can affect the results. The laser intensity can fluctuate. The laser radiation is often routed to the beam launcher by means of optical fibers, where small effects such as a temperature variation or a strain on the fiber can affect the apparent optical length of the fiber and can result in a phase change that erroneously appears to be a measured displacement of the fiducial points. These errors can be reduced by replacing the “known difference” of the laser frequency offsets with a “reference signal” that measures the frequency difference directly. This reference signal is created by mixing a portion of the LO beam with the “reference beam”, which is a portion of the first laser beam that does not interrogate the distance between retros, and directing the combined beam onto a second photo-detector. The use of a reference beam significantly reduces the errors introduced by any common element (e.g., laser or fiber), but it cannot correct for elements that are unique to the measurement path or the reference path. Other errors can be reduced by sharing elements between the measurement and LO beams. The measurements are not affected by elements in the beam-path “downstream” from the point where the two laser beams are first combined (the point where they become aligned, like-polarized, and collocated), as the elements are common to both beams. 
   High precision interferometry requires improvements in the areas of thermal drift and angular deviations of the beam. It is difficult to create an operating environment that eliminates or otherwise reduces thermal drift in the interferometric equipment to acceptable levels. Therefore, it is desirable to decrease the sensitivity of one or more components of an interferometer to variations in ambient temperature. Similarly, beam alignment can not be absolutely maintained. Therefore, it is desirable to provide a design that can compensate for angular deviations when they occur. 
   BRIEF SUMMARY OF THE INVENTION 
   An optical component for use in an interferometer comprises a symmetric periscope arrangement. Two periscopes, each having certain substantially identical design parameters are aligned for common path interferometry, which includes collocated beam and spatially separated beam configurations. A source beam is received, from which a measurement beam and a reference beam are produced. The reference beam is reassembled with a returning measurement beam that has interrogated a target. The symmetric periscope arrangement cancels common distortions along the reference beam path and the returning measurement beam path so that the beams are always properly reassembled, despite distorting optical effects as might arise due to temperature fluctuations, minor angular deviations, and so on. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is described in further detail by way of illustrative embodiments as shown in the following figures, wherein: 
       FIG. 1  is a simplified schematic showing an illustrative embodiment of an interferometer that incorporates aspects of the present invention; 
       FIG. 2  is an enlarged view of the optical element shown in  FIG. 1 , highlighting the reflective and semi-reflective surfaces; 
       FIG. 2A  illustrates the separation distances among the reflective surfaces shown in  FIG. 2 ; 
       FIG. 2B  illustrates the beam paths within the optical component; 
       FIG. 2C  illustrates the various beams that propagate along the beam paths shown in  FIG. 2B ; 
       FIGS. 3A–3D  show an assembly sequence a particular embodiment of the optical component of the present invention; 
       FIG. 3E  is an isometric cutaway view of the optical component; 
       FIGS. 4A–4D  show the generation of the various beams in the optical component; 
       FIG. 5  is an isometric view of the device of  FIG. 1 , showing beam propagation; 
       FIG. 6  shows an alternate embodiment of an interferometer, incorporating the optical component of the present invention; 
       FIG. 7  shows an alternate embodiment of the optical component of the present invention; and 
       FIGS. 8A and 8B  show beam intensity profiles along various points in the optical paths. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows an embodiment of the present invention as incorporated in an illustrative example of an interferometer. The figure shows the basic components of an optical interferometer that operates in a common path heterodyne mode. The embodiment shown performs measurements in a “racetrack” (RT) configuration to monitor changes in separation distance between fiducial points. A collimator  102   a  provides a first source beam. In the specific embodiment shown, the collimator  102   a  is an off-axis parabolic collimator. The particular collimator shown is described in more detail in U.S. application Ser. No. 10/349,758. Briefly, a fiber optic  122   a  provides a beam of light along light path  113   a . The beam reflects off a parabolic mirror  115   a  and travels along light path  111   a . It can be appreciated of course that other suitable light sources (including UV, visible, and IR light) can be used. It is noted that light sources that do not produce collimated beams can also be used. 
   For heterodyne mode operation, a second collimator  102   b  is provided. In the specific embodiment shown, the second collimator is also an off-axis parabolic collimator, though any other suitable light source can be used. A fiber optic  122   b  is arranged to direct a beam along axis  113   b  to parabolic mirror  115   b . The reflected and collimated light travels along axis  111   b . The resulting beam is referred to as a local oscillator (LO) beam. 
   The optical component  104 , also referred to as a beam launcher, receives and directs the source beams produced by the collimators  102   a  and  102   b . The beam launcher  104  comprises various optic surfaces and openings to define optical paths within the component. The openings are identified by the reference numerals  181 – 186 . The surfaces of the beam launcher  104  include reflective surfaces  172 ,  174  of a double-sided mirror  142 . A channel formed through the double-sided mirror  142  terminates at one end with an opening  184 . A reflective surface  172   a  having an opening through it is aligned with the opening  184 . A mirror  144  is provided with an opening  182 . A reflective surface  176  of the mirror  144  is provided with an opening that is aligned with the opening  182 . The mirror  144  may include a beam combining element  144   a  that includes a reflecting surface  177  that is partially reflective and partially transmissive (beam splitter). The beam launcher  104  further comprises a fold mirror  146  having a reflective surface  178 . In the embodiment described herein, the reflective surfaces comprise layers of Au (gold) deposited by known thin film deposition methods. It can be appreciated that the reflective surfaces can be produced with other materials and using other fabrication techniques. Disposed on the reflective surface  172  is a first apodization mask  162 . A second apodization mask  164  is disposed relative to the reflective surface  174 . It is noted that the apodization masks are not necessary to practice the present invention. As will be discussed below, the apodization masks are provided to enhance the performance of the interferometer. 
   To complete the discussion of  FIG. 1 , additional optics include fold mirrors  112   a  and  112   b  to direct the LO beam produced by the collimator  102   b  to the beam combining element  144   a . A pair of shallow wedge prisms form a “Risley pair” (Risley prism)  112   c  for increased precision alignment of the LO beam with respect to the beam combining element. The opening  186  of the beam launcher  104  is aligned with the other photodetector. The photodetector  106  comprises known processing electronics such as detectors, pre-amps, phase meters, and so on to perform interferometric measurements. The photodetector shown is a dual unit detector for redundancy. A single unit detector can be used. A mask  108  is provided at the entrances to each of the photodetectors of the dual detector units  106 . The mask is to help reduce signal cross-talk, a cause of cyclic error. Additional detail of the mask  108  is provided in U.S. application Ser. No. 10/293,209, filed Nov. 12, 2002. 
     FIG. 2  shows an enlarged view of the optical component  104 . The optical element can be viewed as a pair of “symmetric periscopes.” A first “periscope” is defined by the reflective surfaces  172   a  and  176 . A second “periscope” is defined by the reflective surfaces  174  and  178 . In accordance with an aspect of the present invention, opposed reflective surfaces of the first periscope are arranged in substantially parallel relation. Reflective surface  172   a  is formed on a first surface of the double-sided mirror  142 . Reflective surface  172   a  is in opposed relation to and in parallel relation to reflective surface  176 . Likewise, opposed reflective surfaces of the second “periscope” are in parallel relation, namely, reflective surface  174  is in opposed relation to and in parallel relation to reflective surface  178 . In a particular embodiment of the optical component  104 , the reflective surface  172   a  is parallel to the reflective surface  176  to within to 10 μR (micro-radians), or 2 arcsec. Similarly, the reflective surface  174  is parallel to the reflective surface  176  to within to 10 μR, or 2 arcsec. 
   The embodiment of the optical component  104  shown in  FIG. 2  includes spacers  202  and  204 . The spacer  202  serves to provide mechanical support to maintain the parallel relation between the reflective surface  172   a  formed on one side of the double-sided mirror  142  and the reflective surface  176  formed on the mirror  144 . Likewise, the spacer  204  serves to provide mechanical support to maintain the parallel relation between the reflective surface  174  formed on the other side of the double-sided mirror  142  and the reflective surface  178  formed on the fold mirror  146 . A suitable material used for the spacers is a ceramic called ZERODUR®, a transparent glass ceramic known for its extremely low coefficient of thermal expansion, 0±0.10×10−6/° K from 0 to 50° C. For high precision interferometry, ZERODUR® is a preferred material. However, other spacer materials can be used depending on the design parameters of the interferometer. 
   In accordance with another aspect of the present invention, the spacing between the spaced apart reflective surfaces of the first “periscope” is substantially equal to the spacing of the spaced apart reflective surfaces of the second “periscope.” The spacers  202  and  204 , therefore, also serve to establish and maintain equal spacing between the reflective surfaces of each “periscope.” Therefore, referring to  FIG. 2A , the spacer  202  establishes the separation distance between reflective surfaces  172   a  and  176  is D 1 . It is noted that “separation distance” refers to the length of the perpendicular between the plane containing reflective surface  172   a  and the plane containing the reflective surface  176 . Similarly, for the second “periscope” the spacer  204  establishes the separation distance between reflective surfaces  174  and  178  is D 2 , where D 1  is substantially equal to D 2 . In a particular embodiment of the optical component  104 , the distance D 1  (and D 2 ), is in the range of 10–20 mm±25 μm. It is worth noting that the magnitude of the separation distance is not important, only that D 1  and D 2  are substantially equal. 
   In accordance with still another aspect of the present invention, is that the first “periscope” is substantially in parallel relation to the second “periscope.” The particular embodiment of the optical component  104  shown in  FIGS. 2 and 2A  achieves this arrangement via the use of the double-sided mirror  142 . The two major surfaces  212  and  214  of the double-sided can be machined to be parallel to within any desirable margin of error. In this way, the two “periscopes” are constrained to be in parallel relation to each other. In a particular embodiment of the optical component  104 , the two “periscopes” are parallel to within 150 μR, or 30 arcsec. 
   It can be appreciated that the two “periscopes” can be physically separate components, rather than sharing a common component such as the double-sided mirror. Of course, a configuration which uses separate “periscopes” may be more difficult to align to attain a desired parallel relation. Nonetheless, such a configuration might be appropriate for a given design, and falls within the scope of the present invention. 
     FIG. 2B  shows the various beam paths taken by the various beams (identified in  FIG. 2C ) produced by the optical component  104 . A source beam enters the optical component along a beam path  222 , comprising incident segment A and reflected segment B. A portion of the beam that propagates along segment B passes through the opening  182  and exits the optical component as a measurement beam propagating along a segment A of a beam path  224 . Another portion of the beam along segment B of the beam path  222  is reflected by the reflective surface  176  as a reference beam along beam path  226 . The reflective surface  172   a  has an opening that corresponds to the opening of the reflective surface  176 , and is aligned with the reflective surface  176  so that the reference beam propagating along segment A of the beam path  226  is incident to the reflective surface  172   a . The reference beam reflects off of the reflective surface  172   a  and then propagates along segment B of the beam path  226 . 
   As for the measurement beam that is propagating along segment A of the beam path  224 , it is directed to a target (not shown). The target, for example, can be a pair of fiducial points, where a cornercube retro-reflector is provided at each fiducial point and arranged in a racetrack or other circuitous configuration. The measurement beam is reflected by the target and returns to the optical component  104  along segment B of the beam path  224 . The reflective surface  174  reflects the returning measurement beam along a segment A of a beam path  228  toward the reflective surface  178  which directs the returning measurement beam along segment B of the beam path  228 . The reflective surface  178  is aligned such that segment B of the beam path  228  passes through the opening  184 . Moreover, segment B of the beam path  228  lies along segment B of the beam path  226 . Consequently, the returning measurement beam that is propagating along segment B of the beam path  228  is reassembled (“reconstructed”) with the reference beam that is propagating along segment B of the beam path  226 . 
   A further note worth mentioning is that the reflective surface  172  does not constitute part of the “symmetric” periscope arrangement. The beam that is reflected by this surface has not yet been separated into the reference beam and the measurement beam. Consequently, the reflective surface  172  can be positioned anywhere that is suitable, for a particular design, to direct the source beam toward the reflective surface  176 . 
     FIGS. 3A–3D  show an assembly sequence illustrating the elements which comprise the optical component  104 .  FIG. 3E  is an isometric cutaway view of the optical component  104 .  FIG. 3A  shows the fold mirror  146  and the spacer  204 . A right angle channel  302  (shown more clearly in  FIG. 3E ) is formed through the spacer  204 , one end of which is the opening  183  shown in  FIG. 1 . As can be seen in  FIG. 1 , the right angle channel  302  opens to the reflective surface  178 . Another channel  304  is formed through the spacer  204 . One end of the channel  304  opens to the reflective surface  178  (occluded in this figure by the spacer  204 ) of the fold mirror  146 . As will be seen, the other end of the channel  304  faces the opening  184 . 
     FIG. 3B  shows the placement of the double-sided mirror  142 . One major surface of the double-sided mirror  142  is coated with a reflective coating such as gold to produce the reflective surface  174 , which is occluded in this figure. As can be seen in  FIGS. 1 and 3E , the reflective surface  174  is formed in alignment with an opening in the channel  302  formed through the spacer  204 . Reflective surfaces  172  and  172   a  are formed on major surface  312  of the double-sided mirror  142 . A channel  306  is formed through the double-sided mirror  142  with the opening  184  shown in  FIG. 1  and with an opening that is formed through the reflective surface  172   a.    
     FIGS. 3C and 3E  show the placement of the spacer  202 . Numerous channels are formed through the spacer, as shown in  FIG. 3E . An opening of one channel  308   a  constitutes the opening  181 . Another opening of another channel  308   b  in the spacer  202  aligns with the opening  182 . Yet another channel  308   d  has an opening in alignment with the reflective surface  172   a.    
     FIG. 3D  shows a completed assembly. The reflective surface  176  of the mirror  144  is aligned with the opening  182  of a channel formed through the spacer  202 . A channel formed through the reflective surface  176  provides an opening through which the measurement beam ( FIG. 2C ) can exit. The beam combining element  144   a  is in alignment with the opening of the channel  308  formed in the spacer  202 . It can be appreciated that other spacer designs can be used. In fact, the reflective surfaces can be formed on free-standing substrates. Such and embodiment is discussed below. 
   In one embodiment, the mirror  144  and the beam combining element  144   a  are fabricated on a single substrate. Alternatively, the beam combining element  144   a  can be fabricated as a component separate from the mirror  144 . This allows for placement of the beam combining element  144   a  elsewhere in the system. The present invention does not impose any requirement with respect to the positioning of the beam combining element. 
     FIGS. 4A–4D  show a beam propagation sequence in the optical component  104  during operation of the interferometer.  FIG. 5  is an isometric view of a solid-model rendering of the interferometer shown in  FIG. 1 , illustrating the beam paths of the interferometer. In  FIG. 4A , a source beam  502  is directed to the reflective surface  172 , where it is reflected toward the reflective surface  176  of the mirror  144  as a reflected beam  502 ′.  FIG. 4B  shows a portion of the reflected beam  502 ′ (more specifically, a core portion) passes through the opening  182  as a measurement beam  504  that is directed toward a target. As indicated in  FIG. 5 , the target comprises a pair of cornercube retro-reflectors CC 1 , CC 2  as the fiducial points of the interferometer. 
     FIG. 4B  further shows that another portion of the reflected beam  502 ′ is reflected off of the reflective surface  176 . Due to the opening formed through the reflective surface  176 , the reflected portion of the beam  502 ′ has an annular shape. This beam is referred to as the reference beam  402 . The reference beam  402  is directed toward the reflective surface  172   a.    
     FIGS. 4C and 5  indicate that the measurement beam  504  is reflected back by the target. More specifically, in the particular embodiment shown, the measurement beam is reflected by CC 1  to CC 2 , and from CC 2  back to the optical component  104  as a returning measurement beam  506 . The reflective surface  174  is aligned such that the returning measurement beam is incident on the reflective surface  174  and reflects the beam toward the reflective surface  178 . The beam is then reflected along segment B of the beam path  228  ( FIG. 2B ). Meanwhile, the reference beam  402  is directed by the reflective surface  172   a  along segment B of the beam path  226  ( FIG. 2B ). As discussed in connection with  FIG. 2B , segment B of the beam path  226  coincides with segment B of the beam path  228 , and so the reference beam  402  and the reflected returning measurement beam are combined along segment B of the beam path  226  to produce a reconstructed (reassembled) beam  404 . The beam  404  is “reconstructed” in the sense that the reference beam  402  and the exiting measurement beam  504  were originally produced from the source beam  502 , and so the beam  404  is can be loosely viewed as a reconstruction of the source beam  502 . 
   The distance traversed by the measurement beam  504  between the fiducial points may vary as the distance between the fiducial points vary. This will manifest itself in changes in the phase difference between the reference beam  404  and the reflected returning measurement beam when they are reassembled as beam  404 . These variations in the phase difference are the basis for the interferometric measurements. 
     FIG. 4D  shows a local oscillator (LO) beam  512  directed toward the beam combining element  144   a . The beam combining element comprises the beam splitter  177 . The LO beam  512  and the “reconstructed” (reassembled) beam  404  are mixed by the beam splitter  177  to produce a beam called the heterodyne signal. The beam splitter splits the heterodyne signal in two signals,  406   a ,  406   b . In the particular configuration of the interferometer shown in  FIG. 1 , the heterodyne signal  406   a  feeds into one photodetector of the photodetector pair  106 . The heterodyne signal  406   b  feeds into the other photodetector of the photodetector pair  106 , via the fold mirror  112   b . Each of the two photodetectors is a “dual” photodetector, one for the reference beam and the other for the measurement beam. Two such photodetectors are provided for redundancy, because the particular design shown in  FIG. 1  was intended for a system that requires a certain level of reliable operation. 
   Some very desirable optical properties result from the foregoing described constraints of the “symmetric periscope” design. The parallel configuration of the reflective surfaces of each “periscope” and the parallel relation between “periscopes” ensures colinearity of the axis of propagation of the reference beam  402  with respect to the axis of propagation of the returning measurement beam  506 . The equal separation distances (D 1 , D 2 ) ensures that the reference beam  402  and the measurement beam when they are reassembled are concentric. The “periscope” configuration reduces sensitivity to angular displacement of the incident source beam with respect to the optical component. 
   As ambient conditions vary, the optical component  104  is subject to thermal expansion and contraction. This translates to variations in the optical paths ( FIG. 2B ) within the optical component  104 . However, the physical relationships among the design elements remain unchanged, though there may be dimensional changes. Thus, for example, suppose an increase in temperature occurs. The spacers  202  and  204  expand (uniformly) to some degree. However, since both spacers are of the same material, both spacers will expand by the same amount. Consequently, the separations (D 1 , D 2 ) remain substantially equal, though the separation distance is different. Similarly, the parallel relation constraints remain satisfied. For example, expansion of the double-sided mirror  142  will not affect the parallel relationship between its major surfaces  212 ,  214  ( FIG. 2A ), and thus the parallelism between the “periscopes” remains unchanged. Since the physical relationships do not change with temperature, colinearity and concentricity of the reference beam  402  and the returning measurement beam  506  in the reconstructed beam  404  remain within design margins as temperature varies. The result is an optical component for manipulating beams in an interferometer that is greatly insensitive to temperature variation. 
   Furthermore, as indicated above, the tolerances of the design parameters of the “symmetric periscope” will depend on the desired performance, or allowed “error budget” in the interferometer. The disclosed design parameters were contemplated for high precision interferometric measurements on the order of less than ten picometers. 
     FIG. 6  shows another embodiment of the present invention. Here, the foregoing described elements of the optical component that produce the reconstructed beam  404  are shown in this figure. In this particular embodiment, the mirror  144 ′ includes an extended portion  144   a ′ that includes an opening  189 . The figure shows a solid-model view of the spacer  602  used in the embodiment of the invention. 
   A reference beam detector  622  and a measurement beam detector  624  are provided. The LO beam generator  612  produces a LO beam that is mixed, via the beam splitter  626 , with the reconstructed beam  404  to produce the heterodyne signal. The opening  189  allows a core portion of the heterodyne signal to pass into the measurement detector  624 . The core portion comprises a mixing of the returning measurement beam with the LO beam. A reflective surface  179  directs an annular portion of the heterodyne signal to the reference beam detector  622 , which constitutes the reference beam mixed with the LO beam. 
     FIG. 7  shows an alternate embodiment of the optical component  104 . In this embodiment, the spacers  202 ,  204  are omitted. Instead, the mirror  146 , the double-sided mirror  142 , mirror  144  are mounted on a base plate  702 . The beam combiner  144   a  is also provided. The mirror  144  and the mirror  146  each is aligned with respect to the double-sided mirror  142  to ensure adequate parallelism between the respective reflective surfaces. The separation distances D 1 , D 2  are substantially equal. The “periscope” defined by reflective surface  178  ( FIG. 2 ) of the mirror  146  and the reflective surface  176  ( FIG. 2 ) of the double-sided mirror  142 ) is parallel to the “periscope” defined by the reflective surface  172   a  ( FIG. 2 ) and the reflective surface  176  ( FIG. 2 ) of the mirror  144  by virtue of the common substrate ( 142 ) shared by both “periscopes”. When the base plate  702  expands and contracts with variations in the temperature, the change is uniform across the base plate. The elements can be permanently mounted to the base plate  702  such that each of the mirrors  142  and  144  will move away or toward the double-sided mirror  142  by the same amount, thus preserving the required spatial relationships. Therefore, the embodiment shown in  FIG. 7  will be optically stable with variations in temperature. 
   A beneficial aspect of the invention is that the measurement beam is the unobstructed core of the source beam, which exhibits good propagation properties. An enhancement of this aspect of the invention includes the use of apodization masks. The figure shows apodization masks  162 ,  164 . Though not necessary for the practice of the present invention, these masks can further improve the performance of the interferometer by reducing the effects of diffraction. Further detail of the use of apodization masks is discussed in a concurrently filed application entitled “APODIZATION OF BEAMS IN AN OPTICAL INTERFEROMETER” identified by Ser. No. 10/788,166 filed Feb. 25, 2004. Briefly, for the case of the apodization mask  162 , it can be configured to apodize the core portion of the source beam to reduce diffraction of the core as it passes through the opening  182 . This greatly enhances the propagation properties of the measurement beam. 
     FIGS. 8A and 8B  show cross-sectional views of the various beams produced in the configuration of  FIG. 1 . A top view solid-model rendering of the interferometer is shown in  FIG. 8A . The various beams of interest are identified by reference letters “A” through “H”. The cross-sectional views of the identified beams are shown in  FIG. 8B . At location “A”, the collimated source beam exhibits a standard Gaussian intensity cross section  802 . At location “B”, the collimated source beam has been reflected off the patterned mirror surface ( 162 ,  172 ) to become the patterned (apodized) beam seen at “B”. The cross section at “B” shows a central core beam portion having an intensity cross section  802 . The central core beam portion constitutes the measurement beam  504 . The patterned beam at “B” also shows an annular portion having an intensity cross section  804 . This annular portion constitutes the reference beam  402  ( FIG. 4C ). At location “C”, it can be seen that the intensity cross section  802  of the measurement beam  504 , when it exits the optical component  104 , exhibits an apodized edge due to the beam shaping effect of the apodization mask  162  ( FIG. 1 ). The gear-teeth graphics in  FIG. 8B  are notational conventions used to represent an apodized beam edges. The intensity cross section  804  of the reference beam  402  at location “D” also shows apodized edges. 
   The measurement beam  504  exits the optic component, interrogates the two corner cube retro-reflector (retros) to be measured, and returns at location “E” as the returning measurement beam with an intensity cross section  806 . The cross section  806  represents diffraction resulting from propagation of the interrogation beam over a large distance and the reflection of the interrogation beam by the retros. However, by providing a suitable apodization mask  164  near the entrance for the measurement beam, the measurement beam can be shaped and thus cleaned up considerably to become the beam at location “F”. The beam cross section  808  at location “F” represents the apodized measurement beam. At location “G”, the reference beam  402  is combined with the apodized measurement beam to produce concentric beam pattern  810  at “G”. The beams are combined with the LO beam  512  at the beam splitter  177  ( FIGS. 1 and 4D ) produce the heterodyne signal. The resulting bundle of beams is cleaned up with a mask  108  to reduce signal cross-talk, resulting in beam pattern  812  at location “H”. 
   The disclosed embodiments show a reference beam that is annular in shape. However, it can be appreciated that other configurations are possible as well. For example, the reference beam need not be a complete annular ring. For various considerations (e.g., to avoid ghost reflections in some interferometric application or due to physical size constraints), it may be desirable to use only portions of the annular ring: the reference beam could look like a pair of crescent moons, one on either side of the measurement beam. Such beams are symmetric (both have same weighted centroid), and they preserve the spatially-separated common-path configuration. 
   The above-described arrangements of apparatus and methods are merely illustrative of applications of the principles of this invention and many other embodiments and modifications may be made without departing from the spirit and scope of the invention as defined in the claims. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalents.