MIRROR SCAN VELOCITY CONTROL

Abstract: An improved mirror scan control for driving a movable mirror (14) in an interferometer with a constant scan velocity comprises a closed loop servo control which provides constant velocity mirror scan in response to a phase comparison of a signal derived from the beat frequency of a two frequency laser beam (16) passing through the interferometer and a selected frequency reference signal, wherein the difference in frequencies or beat frequency of the two frequency laser beam is stabilized at a specified difference in frequencies. The mirror scan control employs a phase lock control loop which locks the frequency of the referenced signal with the frequency of the signal derived from the laser beam to provide precise mirror velocity control.

Description: MIRROR SCAN VELOCITY CONTROL Field of the Invention The present invention pertains to spectropho- to etric instrumentation, and in particular to Fourier transform infrared spectrophoto eters utilizing a inter¬ ferometer and a laser to obtain spectral data. Background of the Invention The FT-IR spectrophotometer consists of two basic parts: (1) an optical system which includes the interferometer and (2) a dedicated computer used to ana¬ lyze the information contained in the light beam pro¬ duced. The advantages and improved performance of a Fourier transform infrared (FT-IR) spectrophotometer result from the use of an interferometer, rather than a grating or prism, to obtain spectral data (FT-IR). An interferometer permits measurement of the entire spectral range of a sample in a fraction of the time previously required. The operation of a Michelson interferometer to analyze infrared light which passes through a sample as applid to FT-IR spectrophotometry is well known. The interferometer consists of a pair of perpendicularly arranged optical paths, each having a reflector or mirror positioned at its end to reflect light traversing the path. One mirror is fixed. The other mirror is longitu¬ dinally movable to increase or decrease the length of the light path. A light beam entering the interferometer is split into two components by a beam splitter sσ that a ^epar^te componeαt of the beam will traverse §a * ch optical path. After* ref ection the components are reeδmbined at • * _. the beam splitter to constructively and destructively interfere. The reconstructed beam is thereafter directed through a sample and focused onto a photodetector for measurement of intensity. The intensity of the reconstructed wave depends on the difference in length of the optical paths over which the component beams travel. Generally, when the movable mirror is scanned at a constant velocity, the intensity of an emerging light beam will modulate in a regular sinusoidal manner for any selected wavelength of light passing through the interferometer. A typical infrared light beam leaving the in¬ terferometer is a complex mixture of modulation frequen¬ cies due to its polychromatic nature. After the infrared light beam has passed through a sample material, it can be detected to determine wavelengths of light which have been absorbed by the sample. This is accomplished by measuring change in the sinusoidal pattern expected when the light beam exits the interferometer. Measurement of the differences in the characteristic sinusoidal pattern for each light wavelength indicates those wavelengths of light which are absorbed by the sample. Infrared light absorbance characteristics provide a spectrum from which the matter comprising the sample can be determined. The output of a detector measuring the inten¬ sity modulation of the emerging beam can be recorded at very precise intervals during a mirror scan, to produce a plot known as an interferogram. The interferogram is a record of the output signal produced by the infrared detector as a function of the different length optical paths traversed by the components of the infrared beam in the interferometer. Successive scans of the sample are obtained and co-added to obtain an average interferogram » having improved signal-to-noise characteristics. The average interferogram provides information and data re- lating to the spectral characteristics of the sample material. After mathematical preparation, a Fourier transform calculation is performed on the interferogram to obtain a spectral fingerprint of the sample composi¬ tion. The results are compared against known reference data to determine the composition of the sample. Most Fourier transform techniques require aver¬ aging of a large number of interferograms in order to obtain accurate results. As many as 32 to 50 scans dur¬ ing which measurements are taken may be averaged. It is important for an interferogram to be precisely reproduci¬ ble in order to maintain accuracy in its averaging. * Since an interferogram is created as a function of mirror position, more accuracy in the interferogram and resul¬ tant Fourier transformation will result if more accuracy is obtained in the measurement of mirror position at the time when data points are measured which define the in¬ terferogram then more accuracy in the interferogram. To accomplish accuracy and reproducibility for an interferogram, both the rate of sampling and mirror velocity must be very precisely controlled. Alternative¬ ly, the exact position of the mirror may be measured when a data sample is taken. Most modern systems accomplish sampling rate and mirror velocity control and/or mirror position measurement by passing a laser beam concurrently through the interferometer with the infrared light. The laser beam is used to directly measure the movement and/or position of the movable mirror. Since the laser * beam undergoes the same splitting;-and traverse of chang-. τ.ng optical paths as the infrared light, the recombined wavelength with an interference pattern containing information of mirror scan velocity. The -interference signal also serves to indicate the position of the mirror during a scan and to initiate and correlate the collection of data points at uniform intervals of mirror displacement. In a conventional system, when the movable mirror is moving at a constant velocity, a Doppler shift is generated in the component of the laser beam travers¬ ing the changing optical path. When the Doppler shifted beam is recombined with the component traversing the fixed length path, a modulated frequency beam exhibiting a measurable beat frequency is produced, yielding a series of varying intensity or fringe patterns which may be analyzed to determine mirror position or velocity. The frequency of the laser beam is much too high for measurement by common detectors. Conventional systems generally drive the moving mirror at a velocity which produces a 5 KHz amplitude modulation in the exiting beam whose frequency is equal to the magnitude of the Doppler shift. At faster mirror velocities, the modulation fre¬ quency will increase providing increased resolution while at slower mirror velocities the modulation frequency will decrease. Precision with this technique can be main¬ tained to approximately one cycle in 5,000. In a conventional system, however, a movable mirror must be scanning to obtain a Doppler shift in the light beam traversing its path, and thus a measurable modulation frequency. When the movable 'mirror is sta¬ tionary, the light beams traveling along adjacent paths of the interferometer are combined to form an identical frequency light beam without modulation. Thus when the mirror is not moving, there is no information obtained in the recombined beam which can be used to determine mirror position or mirror velocity. This occurs at every in¬ stance that the movable mirror reaches the end of its scan and stops to proceed in the other direction. O PI Furthermore, in a conventional system the Dop¬ pler shift generated by a scanning mirror produces the same modulation effect in the recombined beam independent of the direction of mirror travel. For instance, a 5 KHz modulated beam may be obtained for travel of the mirror in either a forward or reverse direction. Thus it is impossible to determine the direction of mirror travel from the recombined emerging light beam, even though the difference between optical paths may be increasing or decreasing. This shortcoming requires additional cir¬ cuitry to obtain an indication of direction of mirror travel so that the exact position of the mirror may be determined at any given time. Lastly, with conventional systems modulation of the recombined beam becomes very difficult to measure as the velocity of a mirror scan becomes very slow. For instance, for a 0.3 centimeter per second scan velocity, a modulation frequency of 5 KHz is obtained in the recom¬ bined light beam. However, if the mirror is driven at a scan velocity of 0.03 centimeters per second, the modula¬ tion frequency is reduced to .5 KHz. Thus, as the scan velocity is decreased, the modulation frequency in the recombined light beam is decreased to a level which is difficult to measure with modern electronic detectors, providing less accuracy in resolution. An FT-IR spectrophotometer has limited resolu¬ tion for frequency measurement determined by its ability to produce an interferogram. The only moving part funda¬ mental to the optical system is a movable mirror of the interferometer. This part determines the accuracy with wh^ch a spectrophotometer lean measure, frequencies. The accuracy with which the spectrophotometer can analyze a ' - *■ sample is directly related -to the ability of the instru- ent to control and measure the movement of the movable mirror. Conventional the use of a laser reference con¬ tinues to suffer limited precision and control ability. Improvements in the precision with which mirror position can be measured and mirror velocity controlled will necessarily produce significant improvement in the accur¬ acy which an FT-IR spectrophotometer can analyze a sample substance. Summary of the Invention The present invention comprises improved mirror scan control for driving the movable mirror of an inter¬ ferometer with a constant scan velocity, and for deter¬ mining the position of the movable mirror with greater accuracy, in an FT-IR spectrophotometer. The invention utilizes a laser which generates a laser beam having two component frequencies, a standard Michelson interferom¬ eter and a reference signal source. A closed loop servo control provides constant velocity mirror scan in re¬ sponse to a comparison of a signal derived from the laser beam passing through the interferometer and a shifted frequency reference clock signal. A second closed loop servo control is used to stabilize the frequency differ¬ ence between the two component frequencies of the laser beam. The scan servo control and the laser servo control employ phase lock control loops, which are locked to a common reference signal source, to accomplish precise and stable mirror control through analysis of heterodyne frequency ' modulation in the laser beam. _ ^A laser beam having two components of slightly . differing frequency is obtained by applying a magnetic field to an helium-neon gas laser. This phenomenon is well known and described as the Zeeman effect. The dif- ference in frequencies between the components of the laser beam is stabilized s at a desired difference or beat frequency by the laser servo control. Stabilization is accomplished by phase locking a heterodyne signal exhib¬ iting the beat frequency of the beam components to a first reference signal having a frequency equal to the frequency difference desired between the components of the laser beam. The laser beam having differing frequency com¬ ponents is directed through the interferometer. Each component of the beam is combined with its opposing com¬ ponent after traversing the optical path of the interfer¬ ometer. The combined heterodyne beam exhibits a continu¬ al modulation or beat frequency equal- to the frequency difference of the components, plus a Doppler affected change in frequency caused by scanning of the movable mirror. The continually displayed modulation frequency of the heterodyne beam provides continuous information indicating scan velocity and position of the movable mirror in the interferometer. The reference signal source generates a second reference signal having a frequency which is based upon the first reference signal frequency applied to the laser servo control. The second reference signal is increased or decreased in frequency by a value corresponding to the known Doppler affected change in frequency caused when the movable mirror is scanned at a selected constant velocity. The second reference signal is applied to the scan servo control and compared in phase with the hetero¬ dyne beam leaving the interferometer to generate r a con¬ trol signal ξαr obtaining, constant gcan velocity^of ^ie • mirror. The scan velocity of the mirror is corrected until a phase lock is obtained between the second refer¬ ence signal and the heterodyne beam signal. A phase lock OMPI between the signals will maintain a constant velocity scan of the mirror while providing a reference signal having information corresponding to scan velocity and mirror displacement. A direction control is also provided to in¬ struct the reference signal source when to adjust the second reference signal upwardly or downwardly in fre¬ quency. Adjustment of the reference signal upwardly or downwardly in frequency determines the direction and velocity in which the movable mirror will be driven by the scan servo control. With the movable mirror at a standstill, a modulation or beat frequency equal to the frequency difference between components of the laser beam will be exhibited in the exiting heterodyne beam. A scan of the movable mirror in one direction will generate an increase in the modulation frequency due to the Doppler effect, while a scan of the movable mirror in the other direction will generate a decrease in the modulation frequency due to the same Doppler effect. Therefore, by adjusting the reference signal upwardly or downwardly in frequency from the base modulation frequency of the heterodyne signal, the movable mirror can be directed to scan in a forward or rearward direction. Due to the advantageous use of a two-frequency laser which yields a continually modulated beam emerging from the interferometer, the direction of mirror scan can be easily determined and directed by an upward or down¬ ward frequency shift in the modulation frequency of the beam. The scan servo control can maintain -a constant velocity mirror scan in either direction along the entire range of scan. This is accomplished by the phase lock control since a stable frequency reference signal is applied to the scan control for each direction of mirror movement. Advantageously this may allow samples to be accurately obtained in both forward and rearward scans of the mirror, substantially reducing the time required to take sufficient data for accurate averaging techniques. Furthermore, accurate determination of the position of the mirror may be easily obtained through either forward or rearward scans by simple phase shift counting techniques known in the art. As the movable mirror is scanned, the number of cycles which the phasing of the heterodyne beam advances or retards relative to the reference signals provides a measure of the distal position of the movable mirror within the scan range. Thus by keeping track of the accumulated number of shifted cycles, the exact difference in length of the optical path in the interferometer and the position of the movable mirror in its scan range, can be easily determined for every sampling taken. The improved mirror scan control overcomes the deficiencies in prior art systems by providing continuous and precise determination of the position of the mirror and control of mirror scan velocity. The scan control eliminates the need for a separate optical system to re- * establish position of the mirror after each scan revers¬ al. The increased resolution in obtaining interferogram data with this system yields more accurate analysis of a sample material by a spectrophotometer. Due to the con¬ tinuous information signal provided by the system, it is only necessary to calibrate the spectrophotometer one time for each scan series, thus reducing the complexity and arjjounts j of daj_a neςessary to ojptain .an acijurate ^ Fourier transformation. WPO Description of the Drawings Figure 1 is a schematic drawing of an interfer- ometric portion of a Fourier transform infrared spectro¬ photometer and the control circuit which comprises the present invention; Figure 2 is a schematic representation of the interferometric portion of the spectrophotometer depict¬ ing the polarization relationship of the individual com¬ ponents of the two-frequency laser as the laser beam passes through the interferometer; Figure 3 is a schematic of the electrical cir¬ cuit of a first mode of the scan servo control. Best Mode of the Invention The interferometric portion of the Fourier transform infrared (FT-IR) spectrophotometer is described with reference to Figure 1. A Michelson interferometer is depicted which comprises a beam splitter 10 positioned to distribute a portion of an incident light beam along each of two perpendicular optical paths 11 and 13. The beam splitter 10 receives a laser beam 16 from a magneti¬ cally influenced laser 18, and an infrared light beam, shown bounded by lines 20, generated by an infrared light source 22. Generally, the infrared beam 20 is reflected and collimated by a non-planar mirror 2 for entry into - the interferometer, while the laser beam 16 is directly applied to the beam splitter 10 through an opening 26 centrally located in the mirror 24. The beam splitter 10.reflects a first portion of each beam 16 and 20, along the first fixed length optical path 11, which is bounded by an adjustable mirror. 12. The light beams 16 and 20 are reflected by the mir¬ ror 12 to return along the optical path 11 to the beam splitter 10. A second portion of each of the light beams 16 and 20 is passed through the beam splitter 10 along the second optical path 13 which is bounded by a movable mirror 14. The movable mirror 14 is longitudinally mova¬ ble with respect to the optical path 13, to change the length of the optical path within the selected scan range, indicated by arrow 15. The movable mirror 14 is driven by a linear motor 28 which is a commercially available element manufactured by Systems Magnetic Co. and available under Part No. ES-11269. The second portions of each of the light beams 16 and 20 are reflected from the movable mirror to return along optical path 13 to the beam splitter 10, where they are recombined with the first portions of the light beams 16 and 20 returning along the first optical path 11. The recombined portions of the laser beams 16 form a hetero¬ dyne beam 30. The light beam 30 contains information of the velocity and position of the movable mirror 14 through intensity modulation caused by interference phe¬ nomena. The recombined portions of infrared beams 20 form a heterodyned beam 32 which has each individual frequency modulated at a characteristic rate to provide a range of modulated frequencies of infrared light which can be applied to the sample material under analysis. The recombined laser and infrared beams, 30 and 32, respectively, are directed along an exit path 33 of the interferometer in which a reflector 34 similar to reflector 24, is positioned. The reflector 34 receives the collimated infrared heterodyne beam 32 and reflects . " and focuses the beam on a sample chamber 36. The infra- red >eam 32 passes through ,the sample yhamb^r 36 and reflects from a third mirror 38 to focus on a an infra- ** red photodetector 40. The "photodetector 40 receives the amplitude modulated infrared beam which is modified by the sample material through which it passes to produce an electrical information signal proportional to the modified modulation of the beam which is used to generate an interferogram. The modulated laser beam 30 passes from the interferometer through an opening 42 in the mirror 34. Beam 30 is directed to a detector 44. Preferably the detector 44 comprises an array of photodetectors for measuring the intensity of the modulated laser beam 30 at varying points in its cross section. Electrical signals 45 produced by detector 44 are used to obtain an average measure of. the intensity modulation, or beat frequency, which the beam 30 exhibits. The detector 44 may, how¬ ever, simply comprise a single photodetector centrally positioned in the modulated laser beam 30. The signals 45 are applied to a scan servo control 50 to produce a scan drive signal 52. The scan drive signal 52 is applied to the linear motor 28 to control the velocity and direction of movement of the movable mirror 14. The He-Ne laser 10 is magnetically influenced to produce a laser beam with two different frequency components, each having opposing circular polarization. The differing frequencies and polarizations are used to obtain a continuous flow of information in the heterodyne beam 30 leaving the interferometer. Referring to Figure 2, the laser beam 16 having two component frequencies is passed through a quarter waveplate 15 before entering the interferometer. The quarter waveplate 15 converts each of the circularly polarized components into a linearly polarized component. " One linearly polarized component exists in a plane parallel -with the drawing, as shown by the bars 17, and has a frequency f- j . The other linear component exists in a plane perpendicular to the drawing, as shown by the dots 19, and has a frequency t^ . This is due to the opposing circular polarization the incident components exhibit. Thus, the light beam directed into the interferometer consists of two components each having an individual frequency and polarization making them clearly distinguishable from one another. The first portion 21 of the laser beam 16 re¬ flected along the fixed length optical path 11, passes through a second quarter waveplate 23, reflects from the adjustable mirror 12, and again passes through the quar¬ ter waveplate 23 in returning to the beam splitter 10. Passing the first portion 21 of the beam 16 twice through the quarter waveplate 23 acts to rotate the polarization of each component through a 90° angle about the axis of the beam. Thus, the first component 17 of frequency f- j which was vertically polarized upon entering the fixed optical path 11 returns to the beam splitter 10 with a horizontal polarization as shown by bars 17'. Similarly, the second component of frequency f2 having a horizontal polarization entering the fixed length optical path 11 returns to the beam splitter 10 with a vertical polariza¬ tion, shown by dots 19'. The second portion 25 of the laser beam 16 which passes through the beam splitter 10 and along opti¬ cal path 13, is reflected from the moving mirror 14 with¬ out change in polarization. Each of the components of the second portion 25 of the laser beam may, however, be changed in frequency by a alue Δf. This is caused by a. Doppler effect produced on the wave by movement of the movable mirror 14. -Thus the component having frequency f-| changes in frequency to \ ± Δf, and the component having frequency f2 changes ' in frequency to f2 * Δ ^» Since only like polarized waves will combine upon returning the to the beam splitter 10, the component of the laser beam having frequency f- j which has traversed the first optical path 11 and which has been rotated in polarization by 90", will recombine with the component of the laser beam having frequency f 2 •__ Δ f which has tra¬ versed the second optical path 13. A resulting recom¬ bined wave of one polarization 27 will thus exhibit a frequency of fi - (f2 + Δf) . The resulting recombined wave of the other polarization 29 exhibits a frequency of (fi ± Δf) - f2» τhe recombined waves 27 and 29 having orthogonal polarization are then directed from the inter¬ ferometer through a polarizer plate 31 which filters out one of the two polarized waves. Thus, the detector 44 will receive a light beam having linear polarization in one plane and having a frequency which is intensity mo * du- lated by the combination of the differing frequency com¬ ponents of the laser beam, one of which may have a Dop¬ pler shift in frequency Δf introduced. It should be noted that a Doppler shift Δf is introduced to the frequency of each of the components only when the movable mirror 14 is moving. When the movable mirror 14 is held stationary, no Doppler effect is generated. Thus, when the mirror 14 is stationary the component having frequency f-j traversing the first opti¬ cal path 11 will recombine in the beam splitter 10 with the opposing component having frequncy f2 traversing the second optical path 13 to yield a heterodyne beam which exhibits a modulation, or beat frequency equal to the difference between the component frequencies, i.e., f- j - f 2 . *-; The detector ? 44-will receive a wave having a continuous measurable beat frequency. When the frequency equals f-j - f 2r it can be determined that the mirror is stationary. Due to the continual intensity modulation or beat frequency exhibited by the exiting light beam, even when the mirror 14 is stationary, an information signal will be produced which permits control of the mirror 14 throughout the range of its scan. The continual inten¬ sity modulation also permits control of the mirror scan velocity and duration. This is easily accomplished by monitoring the increase or decrease in the beat frequency of the exiting light beam intensity, which incremental value is indicative of mirror velocity. By determining whether the beat frequency is increased or decreased from the frequency f^ - f 2 display at mirror standstill, the direction of mirror travel is indicated. When the mirror 14 is moving at a constant velocity scan, the Doppler shift Δf introduced to the heterodyne wave will be con¬ stant, and thus exhibit a constant shift in beat frequen¬ cy which the presented mirror scan control system uses to advantage to control mirror scan velocity. The magnetically influenced laser 18 is stabil¬ ized to provide a constant difference between the compo¬ nent frequencies, i.e., f-| - f2r and a constant beat frequency of intensity change in the laser beam 16. The stabilized beat frequency increases the resolution of analysis of the mirror velocity and position. An exactly predictable beat frequency of intensity change permits accurate measurement of a Doppler shift frequency Δf to determine constant velocity scan. Referring to Figure 1 , a basic element of the mirror scan control system is a reference frequency syn¬ thesizer 54. The frequency synthesizer 54 generates a reference signal 5.6, which is supplied to a laser servo f W - . __. • 5 . ' control 58 to stabilizeJthe difference in frequencies between the components of laser beam 16 at a constant value. The stabilized difference in frequency between - jRE OMPI " IPO components of the laser beam 16 provides a constant in¬ tensity modulation signal when the components of beam 16 are heterodyned. Due to the slightly differing frequen¬ cies of the laser components, a beat frequency is exhib¬ ited by the intensity modulation of the heterodyned beam 30. This beat frequency is used as a measurement tool for determining the velocity and position of the movable mirror 14 at any point throughout a scan, as discussed. The reference signal 56 may also be applied to a dedicated computer (not shown) as a reference indica¬ tive of the laser beam modulation or beat frequency as required for data analysis. The frequency synthesizer 54 also generates a second reference signal 60 which has a frequency equal to the first reference signal 56 with a selected upward or downward frequency shift ΔR. The upward or downward direction of frequency shift is determined by a forward/reverse input 62 applied to the frequency synthe¬ sizer by a directional control 64. The absolute value of the frequency shift ΔR is determined by a programmable scan speed input 66. The second reference signal 60 is applied to the scan servo control 50 to provide a refer¬ ence for control of the scan velocity of the mirror 14. The reference frequency synthesizer 54 com¬ prises digital electronic circuitry generally known in the art for generating shifted frequency signals of vari¬ able frequency. For example, the frequency synthesizer 54 may include a crystal oscillator which generates a frequency stabilized signal having a uniform periodic wa^forπ.. The output signal of the crystal, oscillator may'be varied in frequency, for example, by"a divider or -. multiplier circuit to provide reference signals 56 and 60 which are used to stabilize the laser beam and to synch- ronize performance of the other elements of the mirror scan control system. The frequency of the reference signal 56 may be selected within a range of frequencies. The range is determined by the differences in frequency that may be exhibited between the components of laser beam 16 through adjustment to laser operating parameters. For a magneti¬ cally influenced He-Ne laser the range is generally 100 to 1500 KHz. It is preferable to select a frequency at a lower portion of this range to provide a more useful signal adapted for use with currently available digital electronic components. For example, the reference signal 56 may be controlled at a frequency of 250 KHz. As pre¬ viously noted, this may be obtained by selecting a crys¬ tal oscillator of the correct frequency or by using a number of known frequency generators, such as one having a multiplier/divider circuit applied to a higher frequen¬ cy crystal oscillator. The 250 KHz frequency selected for reference signal 56 is equal to the exact frequency difference, f-j - f , desired between the components of the laser beam 16 when the laser 18 is stabilized. This advantageously permits the use of a simple phase lock closed loop servo to stabilize the frequency difference. A stabilized frequency difference between components of the beam 16, - results in a stable intensity modulation, or beat fre¬ quency equal to the 250 KHz frequency of the reference signal when the laser components are heterodyned. This stable 250 KHz beat frequency is easily measurable in the heterodyned las ' er beam. It provides a useful information signal which can -be used to determine the velocity and position of the movable mirror 14. The frequency of the second reference signal 60 is determined by the selected frequency of the first reference signal 56. The reference signal 60 is thus a 250 KHz signal which is shifted in frequency upwardly or downwardly by a selected value ΔR. The absolute value of the frequency shift ΔR is programmable into the frequency synthesizer. Various circuits and techniques for obtain¬ ing a frequency shift in a signal are known and obvious to those skilled in the art pertaining to frequency syn¬ thesizers, and such is not considered a part of the in¬ vention presented in this application. The reference signal 60 is preferably shifted in frequency by 5 KHz, upwardly or downwardly, to provide a 245 KHz signal and a 255 KHz signal as instructed by the directional control 64. The frequency shift ΔR which is added to or subtracted from the 250 KHz frequency of the first reference signal 56, will accurately determine the scan velocity of the movable mirror 14 through the scan servo control 50. The 5 KHz frequency shift se¬ lected will drive the movable mirror 14 at a constant velocity of approximately 0.3 centimeters per second throughout the mirror scan, in either direction. The directional control 64 comprises an up/down counter array responsive to the number of shifts in 'phase between the beat frequency of the laser beam intensity (250 KHz) and the average frequency of the intensity modulation of the heterodyned beam emerging from the interferometer (250 KHz * Doppler shift). The direction¬ al control 64 maintains a count of the number of phase 1 shifts occurring between these signals to determine the distance that the mirror has traversed in a scan. For example, " the δp-down counter array may have an up-count input responsive to the signal 68 to generate an in-_ creasing count for each cycle of the entering laser beam frequency. Also, the up-down counter array may have a down-count input responsive to the signal 74 to decrease a count for each cycle of the intensity modulation of the emerging laser beam. Consequently, the up-down counter array counts the number of phase shifts occurring between the signals. The number of phase shifts is compared by a comparator to a programmed value equal to a selected number of phase shifts traversed before the end of the total scan length. A typical scan range of 2 centimeters exhibits approximately 6.3 x 10 4 shifts in phase as the mirror 14 moves through a scan. By determining the number of phase shifts the mirror has traversed within the known scan range, the directional control determines when a selected position at each end of the scan is reached by the mirror 14, by comparing the number of phase shifts traversed with the number programmed in the comparator. When mirror scan has reached the selected position, i.e., a selected number of phase shifts, the comparator produces a forward/reverse signal 62 which is supplied to the reference frequency synthesizer 54. The direction control 64 instructs the synthesizer to change the frequency shift ΔR of the second reference signal 60 from an increased value to a decreased value or from the decreased value to an increased value. Change of the second reference signal 60 instructs the scan servo control to change the scan direction of the movable ' mirror. The design of the up/down counter array which maintains an accurate count of phase shifts is generally known to those skilled in the art. The up/down counter cascade is a standard digital technique and may be constructed with Motorola CMOS 4029 counters as described in the related data sheet. The specific design of the dire ic -tional counter 64 is n»ot con>s_idere*d part of the invention presented herein. OMPI A signal 68 having a beat frequency of the laser beam, is obtained for the directional control 64 from the output of a photodetector 70 receiving a rear- wardly directed portion 72 of the laser beam. Signal 68 is identical to the input signal obtained from the laser for the laser servo control 58, which is described in copending application of Hersher and Wyntes,.and can be obtained in the manner described therein. A signal 74 having an average frequency of the heterodyned beam 30 is obtained from a summation of the electrical signals pro¬ duced by a detector 44 which detects the intensity change in the heterodyned beam 30 leaving an interferometer. The laser stabilization control 58 stabilizes the frequency difference between the differing frequency components of the laser beam 16. The laser servo control 58 accomplishes stabilization by phase locking a hetero¬ dyned beat signal 76 obtained from mixing the components of laser beam 72, with the first reference signal 56 supplied by the reference frequency synthesizer 54. The phase lock control loop is closed by supplying a correc¬ tion signal to length adjusting elements of the laser 18, which accurately control the difference in frequency exhibited in the laser beam 16 and 72. By phase locking the heterodyned beat signal of beam 76 with the stable 250 KHz reference signal 56, the beat frequency of the laser beam is stabilized 250 KHz. Since the beat fre¬ quency is exactly equal to the difference in frequency between the components combined to obtain it, this as¬ sures the frequency difference between components of the laser beam is also stabilized at 250 KHz. The circuit and operation of the laser servo control 58 is described in copending patent application for A Laser Stabilization Control Means, invented by Wyntjes and Hersher, and filed concurrently with this application. Thus, the beat frequency of the laser beam 16 entering jthe interferometer is stabilized at a known value of 250 KHz. Therefore, the beam 30 exiting the interferometer will also exhibit the known beat frequency of 250 KHz when the movable mirror 14 is stationary and no Doppler shift is introduced. Accurate control of the beat frequency of intensity exhibited by the laser beam traversing the interferometer, provides a high resolution gauge for determining the position of the movable mirror and for control velocity of the scan. Advantageously, the beat frequency in the laser beam is present in the heterodyne beam 30 continuously, independent of mirror movement to provide for continuous management of the movable mirror 14. A measurement of the number of phase shifts occurring between the beat frequency of the heterodyned beam leaving the interferometer and the beat frequency exhibited by the laser beam entering the interferometer provides accurate determination of mirror displacement along the scan range. This can be accomplished by stan¬ dard digital counting techniques, as described and used in the directional control 64. A measurement of the difference in frequency between the beat frequency exhib¬ ited by the heterodyned beam leaving the interferometer and the beat frequency exhibited by the laser beam Enter¬ ing the interferometer, provides accurate indication of velocity of the scanning mirror. By maintaining this frequency difference constant, scan velocity of the mir¬ ror can be maintained constant. The scan servo control 50 comprising a phase lock servo control loop supplies a scan drive signal 52 for controlling the movement of mirror 14. The electri- .- cal circuit of the scan servo control is shown in the upper portion of the circuit in Figure 3. Referring to Figure 3, the scan servo control 50 receives three sig¬ nals 80, 81 and 82 produced by the detector 44 in re¬ sponse to intensity fluctuations exhibited by the laser beam 30 which the detector 44 receives. The signals 80, 81 and 82 provide information of the frequency and phase of the heterodyne beam 30 emerging from the interferom¬ eter. The signals 80, 81 and 82 are electrical signals whose voltage oscillates with a frequency equal to the intensity variation of that portion of the heterodyne beam 30 which they measure. Although the three signals have identical frequencies, generally they may differ in phase according to phase differences occurring across the cross section of the beam 30. The signals 80, 81 and 82 are passed through resistors 84 and summed at summing node 86. The summed signal from node 86 is applied to an operational ampli¬ fier 88 which produces a signal proportional to the aver¬ age phase, of the three signals 80-82. This signal shall be termed the average phase signal 90. The average phase signal 90 possesses a phase equal to the average of the three phases of the individual signals 81-83. The average phase signal is " applied to a phase comparator 92. The phase comparator 92 also receives the second reference signal 60 produced by the reference frequency synthesizer 54. The phase comparator compares the phases of the signals 90 and 60 and produces an out¬ put signal 93 which is proportional to the difference in phase between them. The phase detector 92 is a commer¬ cially available device manufactured by Motorola, Inc. and available as Part No. MC14046B. Further information regarding the .phase^detecjor may be obtained from the Motorola CMOS Data Book on page 7-124. OMPI The output signal 93 of the phase comparator 92 is a voltage signal whose duration is proportional to the difference in phase between the average phase signal 90, characteristic of the average intensity modulation of the heterodyne beam 30 leaving the interferometer, and the reference signal 60, whose frequency is characteristic of the velocity at which the mirror is desired to scan. Since the intensity modulation of the heterodyne beam 30 includes a frequency difference Δf caused by the Doppler shift related to mirror movement, and the beat frequency of the beam 30 without the Doppler shift Δf is known to be stabilized at 250 KHz, the comparison of the phase signal 90 obtained from beam 30, with the 250 KHz based reference signal 60 having a frequency selected to in¬ clude the Doppler shift frequency at the scan velocity desired, yields a clear indication of the correction required to mirror scan to obtain the desired constant velocity. This correction is indicated by signal 93. By bringing -the signals 90 and 60 into a locked phase rela¬ tionship, the output signal 93 is stabilized at a high impedance value causing no further correction to the velocity at which the mirror is being driven. The output signal 93 of phase detector 92 is supplied to an integrator circuit 95 to obtain a control signal 94 which is proportional to the time integral' of the phase difference between the signals 90 and 60, i.e., of the voltage changes in output signal 93. Thus, the control signal 94 is proportional to the time integral of the position error of mirror 14 from an ideal scan at constant velocity. * . This position error is corrected by increasing or decreasing the voltage of the the control signal 94, as dete mined, by an increase or decrease of the voltage of output signal 93 caused by phase compara- - tor 92 in response to lead -or lag phase error between the compared signals 60 and 90. The voltage control signal OMPI WIPO 94 will be adjusted in increasing or decreasing incre¬ ments corresponding to greater and lesser phase differ¬ ence between the signals 60 and 90. The integrator circuit 95 may be comprised, for example, of a low pass filter network selected to match the frequency response characteristics of the other ele¬ ments in the servo loop, as known in standard phase lock loop technology. Further information on integrator cir¬ cuit designs applicable to this invention is discussed in the Motorola CMOS Data Book, previously cited. The control signal 94 is applied to operational amplifier 96 with a supply source voltage signal. Control signal 94 directs operational amplifier 96 to proportionally increase or decrease the supply voltage of scan drive signal 52 applied to the linear motor. An increase in voltage of drive signal 52 increases mirror scan velocity, while a decrease in voltage of signal 52 decreases mirror scan velocity. The operational amplifier is ground referenced to obtain a single ended mode of operation as described in many reference materials, for example Analog and Digi¬ tal Electronics written by Vassos and Ewing, and pub¬ lished by John Wiley and Sons, N.Y., N.Y. When the interferometer is in operation and the mirror 14 is scanned, the scan control servo circuit continually compares the phase relationship between the intensity modulation of heterodyne wave 30 and the refer¬ ence signal 60; The control signal 94 generated will be incrementally adjusted to drive operational amplifier 96 to incrementally adjust*Scan " drive signal 52, the mirror 14 at a scan velocity which will bring the average phase signal 90 into phase with the reference signal 60. Scan velocity is adjusted until a lock in phase is obtained between average phase signal 90 and reference signal 60. When . a lock in phase is obtained between the reference signal 60 and the average phase signal 90, the control signal 94 will stabilize, as will the scan drive signal 52 which it directs, to provide a constant voltage drive signal 52 to the linear motor 28. The constant voltage signal obtains a constant velocity scan of the moving mirror 14 at the selected scan velocity. The selected scan velocity can be easily changed by programming the reference signal synthesizer 54 to generate a reference signal 56 having a different absolute value frequency shift ΔR. The different fre¬ quency reference signal will be phase locked to the meas¬ ured intensity modulation of the beam 30 to drive the mirror 14 at a different scan velocity producing a Doppler shift equivalent to the frequency shift in the reference signal. As the mirror 14 reaches an end of scan, the direction control 64 instructs the reference signal syn¬ thesizer 54 to change the frequency shift ΔR in the ref¬ erence signal 56 from an increase to a decrease, or vice versa. The change in sign of the reference signal frequency shift ΔR requires the doppler shift produced by the moving mirror to change in sign, i.e., from a +5 KHz to a -5KHz. This requires the mirror to scan in the opposing direction. Therefore, phase comparison in the scan servo control will produce a scan drive signal 52 which * ! instructs the linear motor 28 to scan the mirror 14 in the^opposing direction. Bidirectional scan velocity control is .accomplished without additional circuitry to the scan circuit described. OMPI_ - b- Since the reference signal 56 and the beat frequency measurable in the heterodyned beam 30 are con¬ tinuously displayed, scan velocity and position control are maintaned throughout the directional change of mirror scan. This affords greater assurance of accuracy and reduces the number of scans required since the usable portion of each scan of the mirror 14 during which con¬ trol is maintained, is increased. OMPI _

Claims: What is claimed is: 1. A closed loop servo control for controlling the movement of a movable mirror in an interferometer used for spectroscopic measurement of a sample material, comprising: first means for producing a heterodyne laser beam having a continuous modulation frequency from which movement of said movable mirror may be determined; second means for producing a reference signal having a frequency characteristic of a desired modu¬ lation frequency of said heterodyne laser beam, which indicates a constant rate of movement of said movable mirror; third means for detecting said modulation fre¬ quency of said laser beam to obtain an electrical signal characteristic of said modulation frequency; fourth means receiving said reference signal from said second means and said electrical signal from said third means, for comparing the phase be¬ tween said reference signal and said electrical signal and producing an error signal responsive to the difference in phase between said signals with respect to time; and control means responsive to said error sig'nal for controlling movement of said movable mirror, to obtain a heterodyne laser beam from said interferom¬ eter having a modulation frequency which, when de¬ tected by said third means, produces an electrical signal having a phase which is locked in phase with said reference signal, to determine movement of said „ movable mirror. 2. The servo control of claim 1, wherein said first means includes a laser influenced by a magnetic field to obtain a laser beam having a plurality of component modes of differing frequency, which compo¬ nent modes produce a laser beam having a continual modulation frequency when heterodyned. 3. The servo control of claim 2, wherein said first means additionallly comprises means for mixing a frequency of one component of said laser beam with a frequency of another component of said laser beam to produce a continual modulation frequency in said heterodyne laser beam. 4. The servo control of claim 2, wherein said first means additionally comprises means for mixing a frequency of one component of said laser beam with a frequency of another component of said laser beam to produce an amplitude modulation frequency in said heterodyne laser beam. 5. The servo control of claim 1, wherein said third means comprises at least one photodetector respon¬ sive to the modulation frequency of said laser beam. 6. The servo control of claim 1, wherein said second means comprises a crystal oscillator which generates a stable output signal of a known frequency, arid means for electrically processing said signal from said crystal oscillator to obtain a base signal having a selected frequency characteristic of modu¬ lation frequency of said laser beam when said mova¬ ble mirror is stationary, and means for shifting the _. 5 frequency of said base signal by a selected value in an upward or downward manner to obtain said refer- " ÷ 2 ' ence signal. 7. The servo control of claim 1, wherein said fourth means comprises a phase detector and an integrator, which generates an error signal proportional to the integral of the differences in phase between said reference signal from said second means and said electrical signal from said third means. 8. The servo control of claim 1, wherein said control means responsive to said error signal comprises an operational amplifier responsive to said error sig¬ nal for producing a movement signal, and a drive means for driving said movable mirror responsive to said movement signal. 9. The servo control of claim 1, wherein said third means for detecting said modulation frequency com¬ prises a photodetector array for measuring the in¬ tensity variance of said laser beam at different port-ions of its cross section, and means for averag¬ ing a plurality of signals obtained from said photo- detectors to produce said electrical signal. 10. A closed loop servo control for controlling the movement of a movable mirror in an interferometer used for spectroscopic measurement of a sample ma¬ terial comprising: first means for producing a heterodyne laser beam having a continuous amplitude modulation fre¬ quency from which the rate of movement of the mov¬ able mirror may be determined; second means for producing a reference signal having a frequency characteristic of a desired modu¬ lation frequency of.said heterodyne laser beam, which indicates a constant rate of movement of said movable mirror; τ CMΓI_ • A'-. VVIPO third means responsive to said first means, for detecting said modulation frequency of said laser beam to obtain an electrical signal characteristic of said modulation frequency; output means receiving said reference signal from said second means and said electrical signal from said third means, responsive to the difference in frequency between said signals and producing an output signal proportional to said difference; and drive means for driving the movable mirror responsive to said output signal to increase or decrease the rate at which said movable mirror is moved. 11. A closed loop servo control for controlling the movement of a movable mirror in an interferometer used for spectroscopic measurement of a sammple material comprising: - first means for producing a heterodyne laser beam having a continuous amplitude modulation fre¬ quency from which the rate of movement of the mov¬ able mirror may be determined; second means for producing a reference signal having a frequency characteristic of a desired modu¬ lation frequency of said heterodyne laser beam, which indicates a constant rate of movement of said movable mirror; third means responsive to said first means, for detecting said modulation frequency of said laser beam to obtain an electrical signal characteristic of said modulation frequency; output means receiving said reference signal from said second means and said electrical signal from said third means, responsive to the difference in phase between said -signals and producing an out¬ put signal proportional to said difference; and drive means for driving the movable mirror responsive to said output signal to increase or decrease the rate at which said movable mirror is move . 12. A mirror scan servo control for bidirectionally driving a movable mirror of an interferometer used for spectroscopic measurement, at constant velocity scan comprising: means for generating a laser beam having a characteristic modulation frequency and directing said beam through said interferometer to receive a change in modulation frequency in response to a doppler effect generated by movement of the movable mirror and exiting said beam from said interferom¬ eter with a modulation frequency characteristic of the rate of scan of said movable mirror; a reference clock producing a pair of constant frequency reference signals, a fast signal having a frequency greater than said modulation frequency and a second signal having a frequency less than said modulation frequency; means for detecting said laser beam to produce an electrical signal having a frequency proportional to the modulation frequency displayed by said laser beam; ' a phase detector receiving said reference sig¬ nal and said electrical signal, the detector produc¬ ing an output signal having a voltage proportional to the phase difference between said reference sig¬ nal and said electrical signal, an integrator re¬ ceiving said phase detector output signal, for sum¬ ming said output signal of said phase detector and producing a drive signal proportional to the inte¬ grated phase difference of said reference signal and said electrical signal; and drive means for bidirectionally driving said movable mirror responsive to said drive signal, said drive means responding to said drive signal to drive said mirror at a velocity which generates a fre¬ quency.change in said laser beam as generated by the doppler effect of the moving mirror, to bring the phase of the modulation frequency into phase with the phase of said reference signal. 13. The closed loop servo control of claim 1 wherein said reference frequency produced by said second means is equal to a desired modulation frequency of said heterodyne laser beam, which indicates a con¬ stant rate of movement of said movable mirror. 14. A closed loop servo control for controlling the movement of a movable mirror in an interferometer used for spectroscopic measurement of a sample materia , comprising: first means for producing a heterodyne laser beam having a continuous modulation frequency from which movement of said movable mirror may be determined; second means for producing a reference signal having a frequency characteristic of a desired modu¬ lation frequency of said heterodyne laser beam," which indicates a constant rate of movement of said movable mirror; third means for detecting said modulation fre¬ quency of said laser beam to obtain an electrical signal characteristic of said modulation frequency; fourth means receiving said reference signal #from said second means and said electrical signal from said third means, for comparing the phase be¬ tween said reference signal and said electrical signal and producing an error signal responsive to .U BSTlTUTE SHEET the difference in frequency between said signals with respect to time; and control means responsive to said error signal for controlling the rate of movement of said movable mirror, and to incrementally adjust said rate to • obtain a phase lock between said electrical signal characteristic of said heterodyne beam modulation frequency from which movement of said mirror may be determined. 15. The servo control of claim 1 wherein said second means is responsive to a direction signal to produce an increase or decrease in frequency of said refer¬ ence signal and additionally comprising means for determining the position of said movable mirror within a range of movement, and for producing a direction signal. 16. The -servo control of claim 15 wherein said second means additionally comprises means for changing the frequency of said reference signal a selected value, to increase or decrease said reference signal rela¬ tive to said continuous modulation frequency of said laser beam to obtain bidirectional control of said movable mirror. SUBSTITUTE SHEET OMPI

Inventors: HERCHER MICHAEL, WIJNTJES GEERT, HERCHER, MICHAEL, WIJNTJES, GEERT
Applicants: BECKMAN INSTRUMENTS INC, BECKMAN INSTRUMENTS, INC.
Application year: 1984
Main classification: G01B9
IPC codes: G01J3, G01J9