Heterodyne Interferometer System

This application is related to the commonly owned United States patent applications entitled "Apparatus To Transform A Single Frequency, Linearly Polarized Laser Beam Into A Beam With Two, Orthogonally Polarized Frequencies", and "Apparatus To Transform A Single Frequency, Linearly Polarized Laser Beam Into A High Efficiency Beam With Two Orthogonally Polarized Frequencies" both of which name Gary E. Sommargren as sole inventor thereto and both of which are contemporaneously filed herewith on Mar. 12, 1985 and the contents of which are specifically incorporated by reference herein and which bear U.S. Ser. Nos. 710,859, and 710,927, respectively.

The present invention relates to gauging apparatus of the optical interferometer type. More particularly, the invention relates to apparatus which uses two optical frequencies to measure accurately either changes in length or changes in optical length.

The use of optical interferometry to measure changes in either length, distance, or optical length has grown significantly due not only to technological advances in lasers, photosensors, and microelectronics but also to an ever increasing demand for high precision, high accuracy measurements. The prior art interferometers can be generally categorized into two types based on the signal processing technique used, i.e., either homodyne or heterodyne, see for example, H. Matsumoto, "Recent Interferometric Measurements Using Stabilized Lasers", Precision Engineering, Vol. 6, pp. 87-94 (April 1984). The interferometers based on the heterodyne technique are generally preferred because (1) they are insensitive to low frequency drift and noise and (2) they can more readily have their resolution extended. Within the heterodyne type of interferometers, of particular interest are the ones based on the use of two optical frequencies, see for example, G. Bouwhuis, "Interferometrie Met Gaslasers", Ned. T. Natuurk, Vol. 34, pp. 225-232 (August 1968); Bagley et al, U.S. Pat. No. 3,458,259, issued July 29, 1969; Hewlett Packard Journal (August 1970); Bagley et al, U.S. Pat. No. 3,656,853, issued Apr. 18, 1972; Hewlett Packard Journal (April 1983).

In accordance with the instant invention, we provide a heterodyne interferometer system capable of measuring accurately either changes in length or changes in optical length comprising: (1) a source of a stabilized single frequency, linearly polarized input beam, most preferably a laser; (2) means, most preferably a frequency stabilized electronic oscillator to provide an electrical reference signal of frequency f.sub.o ; (3) means, most preferably an acousto-optic device, for transforming said input beam into a beam with two optical frequencies, with the two different frequencies orthogonally polarized, and with a frequency difference between the two frequencies equal to f.sub.o ; (4) means, most preferably an interferometer, having a fixed length optical path traversed by one of said two frequencies and a variable length optical path traversed by the other of said two frequencies; (5) means, most preferably a polarizer, for mixing said beams after they traverse said interferometer; (6) means, most preferably a photoelectric detector to produce an electrical measurement signal; (7) means, most preferably a phase meter/accumulator, to indicate the difference in phase between said reference signal and said measurement signal; ( 8) means for producing an output signal from said phase difference which is the change in length of said variable length optical path of said interferometer.

In the drawings:

FIG. 1 depicts in schematic form an embodiment of the instant invention;

FIG. 2 depicts in schematic form the block diagram of the processing electronics of the instant invention;

FIG. 3 illustrates the signals used to implement the measurement in the instant invention;

FIG. 4 depicts in schematic form an embodiment of the invention described in U.S. Ser. No. 710,859, filed Mar. 12, 1985; and

FIG. 5 depicts in schematic form the detailed propagation of the light beams through the birefrigent prism of FIG. 4.

FIG. 1 depicts in schematic form an embodiment of the instant invention. While the apparatus has application for a wide range of radiation sources, the following description is taken by way of example with respect to an optical measuring system. The term "radiant energy" as used herein includes, but is not limited to, electromagnetic energy of all frequency ranges. A light source (10), most preferably a laser, provides a beam (12) of optical energy which has a single, stabilized frequency and is linearly polarized. Light source (10) can be any of a variety of lasers. For example, it can be a gas laser, e.g., a helium-neon, stabilized in any of a variety of techniques known to those skilled in the art to produce beam (12), see for example, T. Baer et al, "Frequency stabilization of a 0.633 .mu.m He-Ne longitudinal Zeeman laser", Applied Optics, Vol. 19, pp. 3173-3177 (1980); Burgwald et al, U.S. Pat. No. 3,889,207 issued June 10, 1975; and Sandstrom et al, U.S. Pat. No. 3,662,279 issued May 9, 1972. Alternatively, light source (10) can be a diode laser frequency stabilized in one of a variety of techniques known to those skilled in the art to produce beam (12), see for example, T. Okoshi and K. Kikuchi, "Frequency Stabilization of Semiconductor Lasers for Heterodyne-type Optical Communication Systems, Electronic Letters, Vol. 16, pp. 179-181 (1980) and S. Yamaguchi and M. Suzuki, "Simultaneous Stabilization of the Frequency and Power of an AlGaAs Semiconductor Laser by Use of the Optogalvanic Effect of Krypton", IEEE J. Quantum Electronics, Vol. QE-19, pp. 1514-1519 (1983). The specific device used for source (10) will determine the diameter and divergence of beam (12). For some sources, e.g., a diode laser, it is necessary to use beam forming optics (14), e.g., a microscope objective, to provide an input beam (18) with a suitable diameter and divergence for the elements that follow. When source (10) is a helium-neon laser, for example, beam forming optics (14) may be required. The elements (10) and (14) are shown in a dashed box (16) which is the source of the input beam (18) which has one stabilized frequency f.sub.L and is linearly polarized. Electronic oscillator (30) provides a frequency stabilized reference electrical signal (32) of frequency f.sub.o to the power amplifier (34). The electrical output (36) of power amplifier (34) is used to drive the acousto-optical (A-O) device (20). A-O device (20), such as one comprising a Bragg cell, is preferably the apparatus disclosed in the commonly owned co-pending application, contemporaneously filed U.S. patent application entitled "Apparatus To Transform a Single Frequency, Linearly Polarized Laser Beam Into a Beam Into a Beam with Two Orthogonally Polarized Frequencies", bearing U.S. Ser. No. 710,859, filed Mar. 12, 1985, the contents of which are specifically incorporated by reference herein.

For ease of understanding, the pertinent portions of the description of the A-O device (20) described in U.S. Ser. No. 710,859 shall be repeated herein with reference to FIGS. 4 and 5 which correspond to FIGS. 1 and 2, respectively, of U.S. Ser. No. 710,859, with the exception that the reference numerals have been changed to avoid confusion where necessary.

As shown and preferred in FIG. 4, light source (10) is the same as described above with reference to FIG. 1, and provides a beam (12) of optical energy which has a single, stabilized frequency and is linearly polarized and can be any of a variety of lasers. For example, as previously mention, it can be a gas laser, e.g. a helium-neon gas laser, stabilized in any of a variety of conventional techniques known to those skilled in the art to produce beam (12). Alternatively, light source (10), as previously mentioned, can be diode laser frequency stabilized in one of a variety of techniques known to those skilled in the art to produce beam (12). As was true with respect to FIG. 1, the specific device used for source (10) will determine the diameter and divergence of beam (12). For some sources, e.g., a diode laser, it is necessary to use the aforementioned beam forming optics (14), e.g. a microscope objective, to provide an input beam (18) with a suitable diameter and divergence for the elements that follow. When source (10) is a helium-neon laser, for example, beam forming optics (14) may not be required. The elements (10) and (14) are shown in a dashed box (16) which is the source of the input beam (18) which has one stabilized frequency f.sub.L and is linearly polarized. The polarization orientation, by way of example, is preferably 45.degree. to the plane of the figure. As in FIG. 1, electrical oscillator (30) provides a frequency stabilized electrical signal (32) comprised of frequencies f.sub.1 and f.sub.2 to the power amplifier (34). The electrical output (36) of power amplifier (34) is used to drive a piezoelectric transducer (138) affixed to the acousto-optical Bragg cell (120). FIG. 5 depicts in schematic form the detailed propagation of input beam (18) through the acousto-optical Bragg cell (120) and the birefringent prism (144). Bragg cell (120) transforms the input beam (18) into two intermediate beams (140) and (142). While the linear polarization of beams (140) and (142) are the same as input beam (18), their directions of propagation and frequencies differ from those of input beam (18). In particular, the direction of propagation of beam (140) is deviated by a small angle, .delta., from that of (142). In addition, the frequency f.sub.L +f.sub.2 of beam (140) differs from the frequency f.sub.L +f.sub.l of beam (142) by an amount equal to f.sub.2 -f.sub.1, i.e., the difference of drive frequencies of the Bragg cell (120). The electrical output (36) of power amplifier (34) is adjusted so that beams (140) and (142) have approximately the same intensity which is approximately one-half that of input beam (18).

Beams (140) and (142) are incident on birefringent prism (144) which is made of a crystal (e.g. quartz) with the optical Axis parallel to the apex edge. The apex angle, .alpha. is chosen to satisfy the equation ##EQU1## where n.sub.o and n.sub.e are the ordinary and extraordinary indices of refraction, respectively and .theta. is the exit angle of the output beam (146). Birefringent prism (144) acts on intermediate beam (140) and (142) to split each into two orthogonally polarized beams, one parallel and one perpendicular to the plane of the figure, to produce an output beam (146) and two spurious beams (148) and (150). The output beam (146) has two orthogonal polarization components, corresponding to beams (40) and (50) in FIG. 1, and has a frequency difference f.sub.2 -f.sub.1 between the two orthogonal polarization components. The spatial separation of the polarization components is minute, only a small fraction of the beam diameter. It is illustratively shown greatly exaggerated in the figure. Spurious beams (148) and (150) have directions of propagation different from output beam (146) so that they can be eliminated by opaque stops (152) and (154). The function of blocking spurious beams (148) and (150) could, alternatively, be performed using a conventional pin hole spatial filter arrangement.

In some instances, where the desired frequency shift is 20 MHz or greater, the above device may be simplified by eliminating f.sub.1 in electrical signal (32) (i.e., f.sub.1 =0) causing beam (142) to have the same direction of propagation and frequency as input beam (18) but otherwise not altering the operation of the device. Beams (40) and (50) are collinear but are shown displaced in FIG. 1 for clarity, have nearly equal intensity, are orthogonally polarized, and differ in frequency by the frequency of the reference electrical signal, i.e., f.sub.o. Beams (40) and (50) are applied to the interferometer (70) which is comprised of a pair of retroreflectors (74) and (76) and polarization beamsplitter (72), although it should be noted that it is not limited to this particular interferometer configuration. The instant invention can be used with a wide variety of polarization type interferometers. Polarization beamsplitter (72) transmits completely the p polarized light, i.e., light with its polarization vector in the plane of incidence, and reflects completely the s polarized light, i.e., light with its polarization vector perpendicular to the plane of incidence.

The diagonal (73) is the beam splitter surface of polarization beamsplitter (72). Thus, beam (40) is transmitted by polarization beamsplitter (72) to produce beam (42), and beam (50) is reflected by polarization beamsplitter (72) to produce beam (52). Beam (52) is retroreflected by retroreflector (74) to produce beam (54) which is in turn again reflected by polarization beamsplitter (72) to produce beam (56). Retroreflector (74) is fixed relative to polarization beamsplitter (72). Retroreflectors (74) and (76) maintain the state of polarization between their output and input beams. Thus, beam (54) is pure s polarization, and beam (44) is pure p polarization. Polarization beamsplitter (72) then reflects beam (54) and transmits beam (44) to produce beams (56) and (46) which are collinear and orthogonally polarized. Retroreflector (76) is movable in a direction parallel with beam (42), see the arrow in FIG. 1. The mixing polarizer (60) mixes the parallel and overlapping portions of beams (46) and (56) to provide each of these beams with a component of similar polarization, i.e., beams (62) and (64). These similarly polarized beams (62) and (64) are mixed by a photoelectric detector (65) to produce an electrical signal (66) having a frequency of f.sub.c. Photoelectric detector (65) has a square law detection characteristic. As explained in the other figures, the phase meter/accumulator (68) uses signal (32) and signal (66) to produce an output signal (80) which corresponds to the change in length of the variable length optical path of the interferometer (70).

The frequency of the measurement signal (66), is given by equation:

where

f.sub.o is the frequency of the reference signal (32);

v is the velocity of the moving retroreflector (76). v is positive or negative when the motion is toward or away from beamsplitter (72), respectively;

.lambda. is the wavelength of the measurement beam (42).

The output (80), which is proportional to the displacement of the moving retroreflector (76), is derived by integrating the frequency difference between the measurement signal (66) and the reference signal (32).

Integrating the subtracted frequencies above is equivalent to adding the phase differences between them on a cycle-by-cycle basis.

FIG. 2 depicts in schematic form the block diagram of the phase meter/accumulator (68). Sinusoidal measurement signal (66) is converted by square wave generator (78) into square wave measurement signal (67), shown in detail in FIG. 3c. Sinusoidal reference signal (32) is converted by square wave generator (79) into square wave reference signal (33), shown in detail in FIG. 3a. Signal (33) is then converted by triangular wave generator (81) into triangular wave reference signal (82), shown in detail in FIG. 3b. Signal (82) is sampled by a high speed analog-to-digital converter (83) every cycle of the measurement signal (67) (FIGS. 3b and 3c).

The digital output (84) of the analog-to-digital converter (83) is proportional to the phase of the reference signal at the sampling instant.

The phase difference output (87) is derived by the phase difference calculator (86) from the digital output (84) of the analog-to-digital converter (83) and the corresponding previous cycle output (85) which was kept in a memory register (92). The phase difference output (87) is the measure of phase difference between the reference signal (32) and the measurement signal (66) at each sampling instant. For example, in FIG. 3 the phase difference output (87) at the instant of sample(i) will be derived by:

The phase differences (87) that are calculated at each cycle of the measurement signal are added to the content of the accumulator (88). The output (80) of the accumulator (88), which is the sum of phase differences is proportional to the displacement of the moving retroreflector (76). The measured displacement d is given by the equation: ##EQU2## where N is the output (80) of the accumulator (88);

.lambda. is the wavelength of the measurement beam (42); and

m is the number of bits of the analog-to-digital converter (83).

As indicated by the equation above, the measurement resolution is given by: ##EQU3##

With a 7-bit analog-to-digital converter (83), for instance, the measurement resolution is .lambda./508, which is substantially higher than the intrinsic interferometer resolution of .lambda./2.

In the measurement system described in Bagley et al, U.S. Pat. No. 3,656,853 issued Apr. 18, 1972, to increase the measurement resolution beyond .lambda./4 both the reference and measurement frequencies are multiplied by the desired factor, which results in a reduction of the maximum velocity allowed for the moving retroreflector. Thus, the higher the resolution being chosen, the lower is the maximum allowable velocity.

By contrast, in the instant invention where the measurement resolution is determined by the number of bits of the analog-to-digital converter (83), a high measurement resolution can be maintained without requiring a reduction in the maximum allowable velocity of the moving retroreflector.

Furthermore, the use of a relatively high frequency difference between beams (40) and (50) as opposed to the limited frequency difference attained by using a Zeeman split laser source, enhances the instant invention's measurement capability at substantially higher velocities of the moving retroreflector (76).

In addition, using an oscillator (30) to generate the frequency difference between beams (40) and (50) in the instant invention, enables the generation of the reference signal (32) from that frequency and eliminates the need for a second optical mixer which is required in the system described in the aforementioned Bagley et al, U.S. patents.

The principal advantages of the instant invention are: (1) increased measurement resolution, (2) the capability to measure rapidly changing lengths or optical lengths, (3) requires only one optical mixer, and (4) the ability to use a broad range of laser sources.

While the preferred embodiment of the invention has been disclosed, obviously modification can be made therein, without departing from the scope of the invention as defined in the following claims: