Optical heterodyne interferometer and optical heterodyne interferometer method

Abstract

To provide an optical heterodyne interference measuring apparatus capable of reducing influence of noise.SOLUTION: An optical heterodyne interference measuring apparatus 1 includes a beam generation part 10, a beam splitter 21, a mirror 31, a first optical detector 41, a linear polarizing plate 42, a second photodetector 51, a linear polarizing plate 52, an AD conversion part 60 and a computer 70. The beam generation part 10 generates and outputs a first light beam and a second light beam where polarized lights are orthogonal to each other. The first light beam and the second light beam differ in optical frequency just for heterodyne frequency. The computer 70 sets heterodyne frequency ωh to a rational multiple value of peak frequency ωp when any peak frequency that is constant temporally in a frequency spectrum of intensity noise included in the first light beam or the second light beam outputted by the beam generation part 10 is ωp.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a measurement device and a measurement method that utilize optical heterodyne interference. [Background technology]

[0002] The detection band of a photodetector that detects the intensity of light is much lower than the frequency of the light to be detected, so the photodetector can only measure the time-averaged light intensity and cannot directly measure the phase of light.

[0003] If the first or second light beam carries information about the object to be measured as phase information, and the intensity of the light produced by combining and interfering with the first and second light beams is detected by a photodetector, the value of the detection signal output from the photodetector will correspond to the phase information, from which information about the object to be measured can be obtained. However, in this case, the intensities of the first and second light beams must each be known.

[0004] Moreover, accurate phase information cannot be obtained if the intensity of the first light beam or the second light beam contains noise, such as fluctuations in the intensity of the light output from the light source due to temperature fluctuations, intensity fluctuations in the frequency (50 Hz or 60 Hz) due to the commercial power supply that supplies power to the light source, and noise from the photodetector.

[0005] Optical heterodyne interferometry (see Patent Documents 1 and 2) is said to be able to reduce the effect of noise and determine phase information based on the detection signal output from the photodetector, even if noise is contained in the intensity of the first or second light beam.

[0006] In the optical heterodyne interferometry, the optical frequency ω1 of the first optical beam and the optical frequency ω2 of the second optical beam are made slightly different from each other. h(=ω2-ω1) is called the heterodyne frequency, and is set to a value that is much lower than the optical frequencies of the first and second optical beams and sufficiently lower than the band of the photodetector. When the intensity of the light resulting from the interference of the first and second optical beams is detected by the photodetector, the detection signal I(t) output from the photodetector is equal to the heterodyne frequency ω h has a sinusoidally time-varying component.

[0007] The electric field E1(t) of the first light beam is expressed by the following equation (1), and the electric field E2(t) of the second light beam is expressed by the following equation (2). t is time, and φ1 and φ2 are phases. When the intensity of the light obtained by heterodyne interference of the first and second light beams is detected by a photodetector, the detection signal I(t) output from the photodetector is expressed by the following equation (3).

[0008]

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[0009]

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[0010]

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[0011] Here, 1 / T corresponds to the bandwidth of the photodetector. The third term on the right side of the above equation (3) is the heterodyne frequency ω h Even if the electric field amplitude e1 of the first light beam or the electric field amplitude e2 of the second light beam is unknown, the heterodyne frequency ω hIt is possible to obtain the phase of the component (third term on the right-hand side) that changes sinusoidally over time by: Furthermore, if noise is included in the electric field amplitude e1 of the first light beam or the electric field amplitude e2 of the second light beam, it is believed that the effect of noise can be reduced and phase information can be obtained by setting the heterodyne frequency to a frequency where the noise intensity is small in the noise frequency spectrum. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Publication No. 2-83428 [Patent Document 2] JP 2010-210541 A Summary of the Invention [Problem to be solved by the invention]

[0013] In the process of research and development of a measurement technique using optical heterodyne interferometry, the inventors have found that even if the heterodyne frequency is set to a frequency with low noise intensity in the noise frequency spectrum, the effect of reducing the influence of noise is not sufficient.In addition, the inventors have found that, conversely, even if the heterodyne frequency is set to a frequency with high noise intensity in the noise frequency spectrum, the effect of reducing the influence of noise can be sufficiently obtained when certain conditions are satisfied.

[0014] The present invention has been made based on the inventors' findings as described above, and aims to provide an optical heterodyne interferometry device and an optical heterodyne interferometry method that can reduce the effects of noise. [Means for solving the problem]

[0015] The optical heterodyne interference measurement device of the first aspect of the present invention includes: (1) a beam generating unit that generates and outputs a first optical beam and a second optical beam having optical frequencies different from each other by a heterodyne frequency and having mutually orthogonal polarizations; (2) a first optical detector that detects a temporal change in intensity due to heterodyne interference between the first optical beam and the second optical beam after both or either one of the first optical beam and the second optical beam is reflected by or transmitted through a measurement object, and outputs a first detection signal; and (3) a peak frequency that is constant over time in a frequency spectrum of intensity noise contained in the first optical beam or the second optical beam output from the beam generating unit is defined as ω. p When the peak frequency ω is ω 1 , the heterodyne frequency, which is the difference in optical frequency between the first optical beam and the second optical beam output from the beam generating unit, is p and (4) a calculation unit that acquires information about the object to be measured based on the phase of a component of the first detection signal that varies over time in a sinusoidal manner at the heterodyne frequency.

[0016] The optical heterodyne interference measurement device of the second aspect of the present invention includes: (1) a beam generating unit that generates and outputs a first optical beam and a second optical beam, the optical frequencies of which differ from each other by a heterodyne frequency and which are orthogonal to each other, based on light reflected by or transmitted through a measurement object; (2) a first optical detector that detects a temporal change in intensity due to heterodyne interference between the first optical beam and the second optical beam, and outputs a first detection signal; and (3) a frequency spectrum detector that determines, as ω, any peak frequency that is constant over time in the frequency spectrum of the intensity noise contained in the first optical beam or the second optical beam output from the beam generating unit. p When the peak frequency ω is ω 1 , the heterodyne frequency, which is the difference in optical frequency between the first optical beam and the second optical beam output from the beam generating unit, is p and (4) a calculation unit that acquires information about the object to be measured based on the phase of a component of the first detection signal that varies over time in a sinusoidal manner at the heterodyne frequency.

[0017] In the optical heterodyne interference measurement device according to the first or second aspect of the present invention, the setting unit sets the heterodyne frequency ω h and peak frequency ω p Relative to ω h / ω p When expressed as a rational number n / m (where m and n are positive integers with only one common divisor), the heterodyne frequency ω where n is 10 or less is h It is preferable to set the heterodyne frequency ω h It is also preferable to set the heterodyne frequency ω h It is also preferable to set the heterodyne frequency ω h It is also preferable to set the heterodyne frequency ω h The peak frequency ω p It is also preferable to set it to the same value as

[0018] The optical heterodyne interference measurement device of the first aspect of the present invention preferably further includes a beam splitter that collectively splits the first and second light beams output from the beam generating unit into two beams and outputs them as the first and second split beams, and a second photodetector that detects a temporal change in intensity due to heterodyne interference between the first and second light beams included in the second split beam and outputs a second detection signal. In this case, the first photodetector detects a temporal change in intensity due to heterodyne interference between the first and second light beams after both or either one of the first and second light beams included in the first split beam output from the beam splitter is reflected or transmitted by the measurement object, and outputs a first detection signal. The calculation unit acquires information about the measurement object based on the phase of the component of the first detection signal that changes over time at the heterodyne frequency and the phase of the component of the second detection signal that changes over time at the heterodyne frequency.

[0019] In the optical heterodyne interference measurement device of the first or second aspect of the present invention, the calculation unit preferably acquires information about the measurement object based on the phase of the component of the first detection signal that changes over time at the heterodyne frequency and the phase of the electrical signal generated by the beam generation unit that changes over time at the heterodyne frequency. The beam generation unit preferably includes a frequency shifter that generates and outputs the first light beam and the second light beam based on the laser light output from the laser light source. The calculation unit preferably acquires at least one of information about the measurement object, including retardation of the measurement object, displacement of the measurement object, change in refractive index of the measurement object, surface shape of the measurement object, and surface roughness of the measurement object, as information about the measurement object.

[0020] The optical heterodyne interference measurement method according to the first aspect of the present invention includes: (1) a beam generating step of generating and outputting a first optical beam and a second optical beam having optical frequencies different from each other by a heterodyne frequency and having mutually orthogonal polarizations; (2) a first optical detection step of detecting a temporal change in intensity due to heterodyne interference between the first optical beam and the second optical beam by a first optical detector after both or either one of the first optical beam and the second optical beam is reflected by or transmitted through a measurement object, and outputting a first detection signal; and (3) calculating a peak frequency that is constant over time in a frequency spectrum of intensity noise contained in the first optical beam or the second optical beam outputted in the beam generating step as ω. p When the peak frequency ω is a heterodyne frequency that is a difference in optical frequency between the first optical beam and the second optical beam output in the beam generating step, p and (4) a calculation step of acquiring information about the object to be measured based on the phase of a component of the first detection signal that varies over time in a sinusoidal manner at the heterodyne frequency.

[0021] The optical heterodyne interference measurement method according to the second aspect of the present invention includes: (1) a beam generation step of generating and outputting a first optical beam and a second optical beam, the optical frequencies of which differ by a heterodyne frequency and have mutually orthogonal polarizations, based on light reflected by or transmitted through a measurement object; (2) a first optical detection step of detecting a temporal change in intensity due to the heterodyne interference between the first optical beam and the second optical beam with a first optical detector and outputting a first detection signal; and (3) calculating a peak frequency that is constant over time in a frequency spectrum of intensity noise contained in the first optical beam or the second optical beam output in the beam generation step as ω. p When the peak frequency ω is a heterodyne frequency that is a difference in optical frequency between the first optical beam and the second optical beam output in the beam generating step, p and (4) a calculation step of acquiring information about the object to be measured based on the phase of a component of the first detection signal that varies over time in a sinusoidal manner at the heterodyne frequency.

[0022] In the optical heterodyne interference measurement method according to the first or second aspect of the present invention, in the setting step, a heterodyne frequency ω h and peak frequency ω p Relative to ω h / ω p When expressed as a rational number n / m (where m and n are positive integers with only one common divisor), the heterodyne frequency ω where n is 10 or less is h It is preferable to set the heterodyne frequency ω h It is also preferable to set the heterodyne frequency ω h It is also preferable to set the heterodyne frequency ω h It is also preferable to set the heterodyne frequency ω h The peak frequency ω p It is also preferable to set it to the same value as

[0023] The optical heterodyne interference measurement method according to the first aspect of the present invention preferably further includes a bifurcating step in which the first and second light beams output from the beam generating step are bifurcated into first and second bifurcated light by a beam splitter, and a second optical detection step in which a second optical detector detects a temporal change in intensity due to heterodyne interference between the first and second light beams included in the second bifurcated light and outputs a second detection signal. In this case, in the first optical detection step, after both or either of the first and second light beams included in the first bifurcated light output from the beam splitter are reflected or transmitted by the measurement object, the first optical detector detects a temporal change in intensity due to heterodyne interference between the first and second light beams and outputs a first detection signal. In the calculation step, information about the measurement object is obtained based on the phase of the component of the first detection signal that changes over time at the heterodyne frequency and the phase of the component of the second detection signal that changes over time at the heterodyne frequency.

[0024] In the optical heterodyne interference measurement method according to the first or second aspect of the present invention, in the calculation step, it is preferable to obtain information about the measurement object based on the phase of a component of the first detection signal that changes over time at the heterodyne frequency and the phase of the electrical signal that changes over time at the heterodyne frequency generated in the beam generation step. In the beam generation step, it is preferable to generate and output the first and second light beams by a frequency shifter based on the laser light output from the laser light source. In the calculation step, it is preferable to obtain at least one of information about the measurement object, including retardation of the measurement object, displacement of the measurement object, change in refractive index of the measurement object, surface shape of the measurement object, and surface roughness of the measurement object, as the information about the measurement object. Effect of the Invention

[0025] According to the present invention, it is possible to reduce the influence of noise in measurements using optical heterodyne interferometry. [Brief description of the drawings]

[0026] [Figure 1] FIG. 1 is a diagram showing the configuration of an optical heterodyne interferometer 1. As shown in FIG. [Diagram 2] FIG. 2 is a diagram showing the configuration of the optical heterodyne interferometer 2. As shown in FIG. [Diagram 3] FIG. 3 is a diagram showing the configuration of the optical heterodyne interference measurement apparatus 3. As shown in FIG. [Figure 4] FIG. 4 is a diagram showing an example of a frequency spectrum of intensity noise contained in the laser light output from a He—Ne laser light source. [Diagram 5] FIG. 5 is a schematic diagram showing the temporal change in intensity of each of the reference signal and measurement signal obtained by heterodyne interference in the ideal case in which the output light from the laser light source does not contain noise, as well as the temporal change in the output light intensity when the output light from the laser light source contains noise. [Figure 6] FIG. 6 is a diagram showing a schematic diagram of changes over time in the intensities of a reference signal and a measurement signal obtained by heterodyne interference when the output light from a laser light source contains noise. [Figure 7] FIG. 7 is a diagram showing a schematic diagram of the change over time in the intensity of a heterodyne signal (reference signal or measurement signal) with a heterodyne frequency of 80 Hz, and the change over time in the intensity of noise with a peak frequency of 320 Hz. [Figure 8] FIG. 8 is a diagram showing a sine wave representing the change over time in intensity of a heterodyne signal (reference signal or measurement signal) with a heterodyne frequency of 80 Hz, and a sine wave representing the change over time in intensity of noise with a peak frequency of 60 Hz. [Figure 9] FIG. 9 is a diagram showing the relationship between the sampling number per period of an 80 Hz heterodyne signal and the determined phase. [Figure 10] FIG. 10 is a flowchart of an optical heterodyne interferometry method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Hereinafter, the embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are given the same reference numerals, and duplicated descriptions are omitted. The present invention is not limited to these examples, but is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0028] First, the overall configuration of the heterodyne interference measuring device and the heterodyne interference measuring method will be described, and then the essential parts of the configuration will be described.

[0029] 1 is a diagram showing the configuration of an optical heterodyne interferometer 1. The optical heterodyne interferometer 1 includes a beam generating unit 10, a beam splitter 21, a mirror 31, a first photodetector 41, a linear polarizing plate 42, a second photodetector 51, a linear polarizing plate 52, an AD conversion unit 60, and a computer 70. The optical heterodyne interferometer 1 can measure the retardation of a measurement object 90 placed on the optical path between the beam splitter 21 and the mirror 31.

[0030] The beam generating unit 10 generates and outputs a first light beam and a second light beam whose polarizations are orthogonal to each other (beam generating step). The first light beam and the second light beam have optical frequencies different from each other by the heterodyne frequency. The beam generating unit 10 includes a laser light source 11, a half-wave plate 12, a quarter-wave plate 13, a laser driver 14, a motor 15, and a motor driver 16. Note that the polarizations being orthogonal to each other means that when each polarization state is expressed by a Jones vector, they are orthogonal as complex vectors. Examples of polarization states that are orthogonal to each other include a combination of x-polarized light and y-polarized light, a combination of right-handed circularly polarized light and left-handed circularly polarized light, and more generally, a combination of two elliptically polarized lights.

[0031] As shown in the figure, for convenience of explanation, an xyz Cartesian coordinate system is set. The z-axis is parallel to the direction of light propagation. The x-axis is parallel to the paper surface and perpendicular to the direction of light propagation. The y-axis is perpendicular to the paper surface.

[0032] The laser light source 11 is driven by the laser driver 14 to output a laser light. The laser light output from the laser light source 11 is linearly polarized light. For example, the laser light output from the laser light source 11 is linearly polarized light parallel to the x-axis direction. Any laser light source may be used as the laser light source 11, and an example of the laser light source is a He-Ne laser light source. The laser driver 14 is electrically connected to the laser light source 11 and causes the laser light source 11 to perform laser oscillation. The laser driver 14 is supplied with power of 100 V AC at a frequency of 60 Hz from a commercial power source.

[0033] The half-wave plate 12 is optically connected to the laser light source 11. The half-wave plate 12 has a flat plate shape and is fixed perpendicular to the rotation shaft of the motor 15. The half-wave plate 12 rotates at a constant speed w as the rotation shaft of the motor 15 rotates. The motor driver 16 is electrically connected to the motor 15, and receives instructions from the computer 70 to drive the motor 15 to rotate the half-wave plate 12 at the constant speed w.

[0034] The laser light output from the laser light source 11 passes through the half-wave plate 12 rotating at a constant speed w, and is converted into a superposition of two light components (a right-handed circularly polarized component and a left-handed circularly polarized component) with different optical frequencies. The intensities of the right-handed circularly polarized component and the left-handed circularly polarized component output from the half-wave plate 12 may or may not be equal to each other. The optical frequencies of the right-handed circularly polarized component and the left-handed circularly polarized component are changed by ±2w with respect to the optical frequency of the laser light output from the laser light source 11. In other words, the difference in optical frequency between the right-handed circularly polarized component and the left-handed circularly polarized component is four times the rotation speed w of the half-wave plate 12.

[0035] The quarter-wave plate 13 is optically connected to the half-wave plate 12. The fast axis of the quarter-wave plate 13 is on the xy plane and inclined at a predetermined angle (for example, 45°) with respect to the x-axis direction. However, the right-handed circularly polarized component and the left-handed circularly polarized component output from the half-wave plate 12 become an x-polarized component and a y-polarized component by passing through the quarter-wave plate 13. The intensities of the x-polarized component and the y-polarized component output from the quarter-wave plate 13 may or may not be equal to each other. The optical frequencies of the x-polarized component and the y-polarized component are changed by ±2w with respect to the optical frequency of the laser light output from the laser light source 11. That is, the difference in optical frequency between the x-polarized component and the y-polarized component (heterodyne frequency) is 4w.

[0036] The beam generating unit 10 outputs one of the x-polarized light component and the y-polarized light component output from the quarter-wave plate 13 as a first light beam, and outputs the other as a second light beam. The first light beam and the second light beam have mutually orthogonal polarizations and differ in optical frequency by the heterodyne frequency 4w. The half-wave plate 12 and the quarter-wave plate 13, which rotate at a constant speed, constitute a frequency shifter that generates and outputs the first light beam and the second light beam, which have mutually different optical frequencies and mutually orthogonal polarizations, based on the laser light output from the laser light source 11.

[0037] The beam splitter 21 is optically connected to the beam generating unit 10. The beam splitter 21 splits the first and second light beams output from the beam generating unit 10 into two at a predetermined intensity ratio and outputs them as the first and second split lights (two-splitting step). The beam splitter 21 is, for example, a half mirror that splits at an intensity ratio of 1:1. The beam splitter 21 may also have polarization dependency. Therefore, each of the first and second split lights output from the beam splitter 21 includes the first and second light beams whose polarizations are orthogonal to each other at a predetermined intensity ratio (for example, an intensity ratio of 1:1). The beam splitter 21 outputs the reflected first split light to the mirror 31 and outputs the transmitted second split light to the linear polarizing plate 52.

[0038] The mirror 31 is optically connected to the beam splitter 21. The mirror 31 receives the first branched light that is output from the beam splitter 21 and passes through the object to be measured 90, and vertically reflects the first branched light. The first branched light reflected by the mirror 31 passes through the object to be measured 90 again, and then the first branched light that is transmitted through the beam splitter 21 is input to the linear polarizing plate 42.

[0039] The first split light having passed through the measurement object 90 twice has retardation information of the measurement object 90 as phase information. The first and second light beams included in the first split light are x-polarized and y-polarized components whose optical frequencies differ from each other by the heterodyne frequency 4w. By aligning the fast axis of the measurement object 90 with the x-axis or y-axis direction, the first and second light beams included in the first split light undergo different phase changes due to the birefringence of the measurement object 90 (the property that the refractive index differs depending on the polarization direction). In addition, in cases where the retardation information of the measurement object 90 is unknown, the retardation may be measured while rotating the measurement object 90, and the fast axis of the measurement object 90 may be aligned with the x-axis or y-axis direction based on the angle at which the retardation becomes an extreme value.

[0040] The linear polarizer 42 is optically connected to the beam splitter 21. The transmission axis of the linear polarizer 42 is on the xy plane and is inclined at a predetermined angle (for example, 45°) with respect to the x-axis direction. The linear polarizer 42 receives the first branched light arriving from the beam splitter 21, mixes the first and second light beams (x-polarized component and y-polarized component) contained in the first branched light at a predetermined intensity ratio (for example, intensity ratio 1:1), converts the light into a single linearly polarized component, and outputs the light. At this time, it is preferable to mix the first and second light beams so that the amplitude of the sine wave contained in the heterodyne signal is maximized.

[0041] The first photodetector 41 is optically coupled to the linear polarizer 42. The first photodetector 41 receives the light output from the linear polarizer 42, detects a temporal change in the light intensity, and outputs a first detection signal (first photodetection step). This first detection signal represents a temporal change in intensity due to heterodyne interference of the first light beam and the second light beam (x-polarized component and y-polarized component) contained in the first branched light, and includes a component that changes sinusoidally over time at a heterodyne frequency 4w. The first detection signal is a measurement signal that has retardation information of the measurement object 90 as phase information.

[0042] The linear polarizer 52 is optically connected to the beam splitter 21. The transmission axis of the linear polarizer 52 is on the xy plane and is inclined at a predetermined angle (for example, 45°) with respect to the x-axis direction. The linear polarizer 52 receives the second split light arriving from the beam splitter 21, mixes the first and second light beams (x-polarized component and y-polarized component) contained in the second split light at a predetermined intensity ratio (for example, intensity ratio 1:1), converts the light into a single linearly polarized component, and outputs the light. At this time, it is preferable to mix the first and second light beams so that the amplitude of the sine wave contained in the heterodyne signal is maximized.

[0043] The second photodetector 51 is optically coupled to the linear polarizer 52. The second photodetector 51 receives the light output from the linear polarizer 52, detects a temporal change in the light intensity, and outputs a second detection signal (second photodetection step). This second detection signal represents a temporal change in intensity due to heterodyne interference of the first light beam and the second light beam (x-polarized component and y-polarized component) contained in the second branched light, and includes a component that changes sinusoidally over time at a heterodyne frequency 4w. The second detection signal is a reference signal that does not have retardation information of the measurement object 90 as phase information.

[0044] The AD conversion unit 60 is electrically connected to the first photodetector 41 and the second photodetector 51. The AD conversion unit 60 can sample analog signal data at a rate sufficiently faster than the heterodyne frequency 4w and convert it into digital signal data. For example, the sampling rate of the AD conversion unit 60 may be at least twice the rate of the heterodyne frequency 4w, and may be at least 10 times the rate. The AD conversion unit 60 inputs the first detection signal (measurement signal) output from the first photodetector 41 as an analog signal, converts it into a digital signal, and outputs it. The AD conversion unit 60 also inputs the second detection signal (reference signal) output from the second photodetector 51 as an analog signal, converts it into a digital signal, and outputs it.

[0045] The computer 70 is electrically connected to the AD conversion unit 60. The computer 70 has the functions of both a setting unit and a calculation unit, as described below. The computer 70 as a setting unit sets the heterodyne frequency 4w to a desired value, and instructs the monitor driver 16 to rotate the half-wave plate 12 at a constant speed w by the motor 15 (setting step). The setting of the heterodyne frequency 4w will be described in detail later.

[0046] The computer 70 as a calculation unit inputs the first detection signal (measurement signal) and the second detection signal (reference signal) output as digital signals from the AD conversion unit 60, calculates the phase of the component of these signals that changes over time in a sinusoidal manner at a heterodyne frequency 4w, and calculates the phase difference (retardation) Δφ of the measurement object 90 based on these phases (calculation step). Furthermore, the computer 70 can calculate Δn (the difference between the refractive index for x-polarized light and the refractive index for y-polarized light), which is an index of birefringence, from the following formula (4) based on this phase difference Δφ. d is the thickness of the measurement object 90 in the light transmission direction, and λ is the wavelength of light. This formula takes into consideration that the first branched light passes through the measurement object 90 twice.

[0047]

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[0048] 2 is a diagram showing the configuration of the optical heterodyne interferometer 2. The optical heterodyne interferometer 2 includes a beam generating unit 10, a first photodetector 41, a linear polarizing plate 42, an AD conversion unit 60, and a computer 70. The optical heterodyne interferometer 2 can measure the retardation of a measurement object 90 placed on the optical path between the beam generating unit 10 and the linear polarizing plate 42.

[0049] In this configuration, the first and second light beams (x-polarized and y-polarized components) output from the beam generating unit 10 pass through the measurement object 90 and are input to the linear polarizing plate 42. The transmission axis of the linear polarizing plate 42 is on the xy plane and is inclined at a predetermined angle (e.g., 45°) with respect to the x-axis direction. The linear polarizing plate 42 inputs the first and second light beams that have passed through the measurement object 90, mixes these first and second light beams (x-polarized and y-polarized components) at a predetermined intensity ratio (e.g., intensity ratio 1:1), converts them into light having a single linearly polarized component, and outputs it. At this time, it is preferable to mix the first and second light beams so that the amplitude of the sine wave included in the heterodyne signal is maximized.

[0050] The first photodetector 41 receives the light output from the linear polarizer 42, detects the temporal change in the light intensity, and outputs a first detection signal. This first detection signal represents the temporal change in intensity due to heterodyne interference of the first and second light beams (x-polarized and y-polarized components), and includes a component that changes sinusoidally over time at a heterodyne frequency 4w. The first detection signal is a measurement signal that has retardation information of the measurement object 90 as phase information.

[0051] The AD conversion unit 60 receives the first detection signal (measurement signal) output from the first photodetector 41 as an analog signal, converts it into a digital signal, and outputs it. The AD conversion unit 60 also receives an electrical signal (reference signal) that changes over time at the heterodyne frequency 4w output from the motor driver 16 of the beam generating unit 10 as an analog signal, converts it into a digital signal, and outputs it. The reference signal output from the motor driver 16 does not have to be a sine wave.

[0052] The signal output from the motor driver 16 serves as an auxiliary for outputting the timing of the frequency 4w. A waveform (not limited to a sine wave) oscillating at a frequency 2w is output from the motor driver 16, and thresholds are set for the rising and falling edges of this waveform, and the timing at which the thresholds are exceeded can be used as a phase reference. Alternatively, a waveform oscillating at a frequency w can be output from the motor driver 16, and the timing of the frequency 4w can be determined based on this during analysis by a computer.

[0053] The computer 70 as a calculation unit inputs the first detection signal (measurement signal) and the second detection signal (reference signal) output as digital signals from the AD conversion unit 60, obtains the phase of the component of these signals that changes over time sinusoidally at the heterodyne frequency 4w, and obtains a phase difference (retardation) Δφ of the measurement object 90 based on these phases (calculation step). Furthermore, the computer 70 can obtain Δn (the difference between the refractive index for x-polarized light and the refractive index for y-polarized light), which is an index of birefringence, based on this phase difference Δφ.

[0054] Compared to the configuration shown in Fig. 1, the configuration shown in Fig. 2 does not require the beam splitter 21, the mirror 31, the second photodetector 51, and the linear polarizing plate 52, and the optical system is simplified, making it easy to adjust the optical system. Note that in both the configurations of Fig. 1 and Fig. 2, a reference signal is not required, and the phase difference Δφ of the measurement object 90 can be obtained only from the measurement signal. In this case, for example, it is preferable to perform calibration without placing the measurement object 90.

[0055] 3 is a diagram showing the configuration of the optical heterodyne interferometer 3. The optical heterodyne interferometer 3 includes a beam generator 10, a polarizing beam splitter 22, a mirror 31, a quarter-wave plate 33, a first photodetector 41, a linear polarizer 42, a quarter-wave plate 53, an AD converter 60, and a computer 70. The optical heterodyne interferometer 3 can measure the displacement of a measurement object 90.

[0056] In this configuration, the polarizing beam splitter 22 is optically connected to the beam generating unit 10. Of the first light beam and second light beam (x-polarized component and y-polarized component) output from the beam generating unit 10, the polarizing beam splitter 22 transmits the first light beam (x-polarized component) and outputs it to the quarter-wave plate 53, and reflects the second light beam (y-polarized component) and outputs it to the quarter-wave plate 33.

[0057] The first light beam transmitted through the polarizing beam splitter 22 is reflected by the object to be measured 90 after passing through the quarter-wave plate 53, and passes through the quarter-wave plate 53 again to be input to the polarizing beam splitter 22. The fast axis of the quarter-wave plate 53 is on the xy plane and is inclined by 45° with respect to the x-axis direction. The first light beam is converted from an x-polarized component to a y-polarized component by passing through the quarter-wave plate 53 twice, and is therefore reflected by the polarizing beam splitter 22 and output to the linear polarizer 42.

[0058] The second light beam reflected and output by the polarizing beam splitter 22 passes through the quarter-wave plate 33, is reflected by the mirror 31, passes through the quarter-wave plate 33 again, and is input to the polarizing beam splitter 22. The fast axis of the quarter-wave plate 33 is on the xy plane and is inclined by 45° with respect to the x-axis direction. The second light beam is converted from a y-polarized component to an x-polarized component by passing through the quarter-wave plate 33 twice, and is then transmitted through the polarizing beam splitter 22 and output to the linear polarizer 42.

[0059] The linear polarizer 42 is optically connected to the polarizing beam splitter 22. The transmission axis of the linear polarizer 42 is on the xy plane and is inclined at a predetermined angle (for example, 45°) with respect to the x-axis direction. The linear polarizer 42 inputs the first light beam (y-polarized component) and the second light beam (x-polarized component) arriving from the polarizing beam splitter 22, mixes the first light beam and the second light beam (y-polarized component and x-polarized component) at a predetermined intensity ratio (for example, an intensity ratio of 1:1), converts them into light having a single linearly polarized component, and outputs it. At this time, it is preferable to mix the first light beam and the second light beam so that the amplitude of the sine wave included in the heterodyne signal becomes the largest.

[0060] The first photodetector 41 receives the light output from the linear polarizer 42, detects the temporal change in the light intensity, and outputs a first detection signal. This first detection signal represents the temporal change in intensity due to heterodyne interference of the first and second light beams (y-polarized and x-polarized components) contained in the first branched light, and includes a component that changes sinusoidally over time at a heterodyne frequency 4w. The first detection signal is a measurement signal having the difference in the optical path lengths of the first and second light beams (i.e., displacement information of the measurement object 90) as phase information.

[0061] The AD conversion unit 60 receives the first detection signal (measurement signal) output from the first photodetector 41 as an analog signal, converts it to a digital signal, and outputs it. The AD conversion unit 60 also receives an electrical signal (reference signal) that changes over time at the heterodyne frequency 4w output from the motor driver 16 of the beam generating unit 10 as an analog signal, converts it to a digital signal, and outputs it. The reference signal output from the motor driver 16 does not have to be a sine wave. Also, the reference signal is not necessary, and the displacement of the measurement object 90 can be obtained from the measurement signal alone.

[0062] The computer 70, acting as a calculation unit, inputs the first detection signal (measurement signal) and the second detection signal (reference signal) output as digital signals from the AD conversion unit 60, determines the phase of the component of these signals that varies sinusoidally over time at the heterodyne frequency 4w, and determines the displacement of the object to be measured 90 based on these phases (calculation step).

[0063] The optical heterodyne interferometer can be configured in various ways in addition to the configurations of the optical heterodyne interferometers 1 to 3 shown in FIGS.

[0064] For example, a frequency shifter that generates and outputs a first light beam and a second light beam having different optical frequencies based on a laser beam output from a light source may have a configuration including the half-wave plate 12 and the quarter-wave plate 13 shown in Figs. 1 to 3, or may have a configuration including only the half-wave plate 12 and not the quarter-wave plate 13. A frequency shifter having the latter configuration can generate and output light in which a right-handed circularly polarized component and a left-handed circularly polarized component having different optical frequencies are superimposed as the first light beam and the second light beam. The polarization states of the right-handed circularly polarized light and the left-handed circularly polarized light are orthogonal to each other.

[0065] Other configurations of frequency shifters include a configuration using an electro-optical modulation element or an acousto-optical modulation element, a configuration in which light is split into two and one of the split beams is reflected by a mirror that moves at a constant speed and then the two split beams are combined, and a configuration in which light is split into two and one of the split beams is diffracted by a radial diffraction grating that rotates at a constant speed and then the two split beams are combined.

[0066] The beam generating unit that generates and outputs a first light beam and a second light beam having mutually orthogonal polarizations can be configured using a light source and a frequency shifter, as well as a configuration using a transverse Zeeman laser light source and a configuration that combines light having mutually orthogonal polarizations output from two external cavity laser diodes.

[0067] By using the optical heterodyne interferometry, it is possible to measure the retardation when light passes through a measurement object (e.g., a transmissive spatial light modulator, a crystal such as quartz, an oriented liquid crystal, a photoelastic body such as glass) (Fig. 1, Fig. 2), the retardation when light is reflected by a measurement object (e.g., a reflective spatial light modulator), the displacement of the measurement object (the reflective surface) using the reflection of light on the measurement object (Fig. 3), the change in the optical path length along the path that light passes through the measurement object, the change in the refractive index of the measurement object, the surface shape of the measurement object, the surface roughness of the measurement object, etc. The surface roughness or surface shape of the measurement object can be obtained by measuring the displacement of the measurement object on a surface using the reflection of light from the measurement object. The change in the refractive index of the measurement object corresponds to the change in the optical path length when the physical distance is constant in the displacement of the measurement object using the reflection of light from the measurement object. In addition, in the case of retardation measurement, the measurement object may be placed on the optical path between the light source and the frequency shifter.

[0068] Fig. 4 is a diagram showing an example of the frequency spectrum of intensity noise contained in laser light output from a He-Ne laser light source. As shown in this diagram, while there is a tendency that the intensity noise decreases as the frequency increases overall, there are peak frequencies at which the intensity noise is prominently large compared to nearby intensity noise levels. For example, the commercial power frequency of 60 Hz, frequencies that are integer multiples of this 60 Hz, frequencies that are integer multiples of this 320 Hz, and frequencies that are integer multiples of this 320 Hz are peak frequencies at which the intensity noise is prominently large compared to nearby intensity noise levels.

[0069] In the configurations of the optical heterodyne interferometers 1 to 3 shown in Figures 1 to 3, the peak frequency in the frequency spectrum of the intensity noise contained in the first light beam or the second light beam output from the beam generating unit 10 coincides with the peak frequency in the frequency spectrum of the intensity noise contained in the laser light output from the laser light source 11.

[0070] 5 is a diagram showing the time variation of the intensity of each of the reference signal and the measurement signal obtained by heterodyne interference in an ideal case where the output light from the laser light source does not contain noise, and the time variation of the output light intensity in a case where the output light from the laser light source contains noise. The time variation of the intensity of the output light from the laser light source is exaggerated. In the ideal case, the time variation of the intensity of each of the reference signal and the measurement signal has an ideal sinusoidal shape.

[0071] FIG. 6 is a diagram showing a schematic diagram of the time change in the intensity of each of the reference signal and the measurement signal obtained by heterodyne interference when the output light from the laser light source contains noise as shown in FIG. 5. As shown in this figure, when the output light from the laser light source contains noise, the time change in the intensity of each of the reference signal and the measurement signal generally does not have an ideal sine wave shape, but has a distorted shape that differs depending on the peak. Specifically, the peak intensity of each of the reference signal and the measurement signal differs for each peak, or the shape around the peak of each of the reference signal and the measurement signal is asymmetric. In such a case, the phase cannot be accurately obtained based on the measurement signal and the reference signal.

[0072] Next, in the optical heterodyne interference measurement device and the optical heterodyne interference measurement method, a heterodyne frequency ω 2 that can solve the above problems described with reference to FIG. 5 and FIG. 6 and reduce the influence of noise is h This section explains how to set this up.

[0073] 10 is a flowchart of an optical heterodyne interferometry method. The optical heterodyne interferometry method includes a beam generation step S1, a first optical detection step S2, and a calculation step S3. In the beam generation step S1, a first optical beam and a second optical beam whose optical frequencies differ from each other by the heterodyne frequency and whose polarizations are orthogonal to each other are generated and output. Also, in the beam generation step S1, any peak frequency that is constant over time in the frequency spectrum of the intensity noise contained in the first optical beam or the second optical beam is determined as ω p Then, the heterodyne frequency, which is the difference in optical frequency between the first optical beam and the second optical beam, is the peak frequency ω p In the first light detection step S2, the first light detector detects the change in intensity over time due to heterodyne interference between the first light beam and the second light beam, and outputs a first detection signal. In the calculation step S3, information about the object to be measured is obtained based on the phase of a component of the first detection signal that changes over time sinusoidally at the heterodyne frequency. The setting step will be described in detail below.

[0074] In the setting step, the computer 70 as a setting unit sets the heterodyne frequency ω h is set to the desired value, and the set heterodyne frequency ω h The optical fiber 10 instructs the beam generating unit 10 to generate and output a first optical beam and a second optical beam having optical frequencies different from each other by only 100 .mu.m and polarized lights perpendicular to each other.

[0075] Specifically, the setting unit (computer 70) sets any peak frequency that is constant over time in the frequency spectrum of the intensity noise included in the first light beam or the second light beam output from the beam generating unit 10 as ω p Then, the heterodyne frequency ω h The peak frequency ω p In the frequency spectrum of the intensity noise shown in Fig. 4, the commercial power frequency of 60 Hz, the frequency of integer multiples of this 60 Hz, the frequency of 320 Hz, and the frequency of integer multiples of this 320 Hz are peak frequencies that are constant over time.

[0076] Preferably, the setting unit is a heterodyne frequency ω h and peak frequency ω p Relative to ω h / ω p When expressed as a rational number n / m (where m and n are positive integers with only one common divisor), the heterodyne frequency ω where n is 10 or less is h More preferably, the setting unit sets the heterodyne frequency ω h Most preferably, the setting unit sets the heterodyne frequency ω h The peak frequency ω p Set it to the same value as

[0077] Preferably, the setting unit is h and peak frequency ω p Relative to ω h / ω pWhen expressed as a rational number n / m (where m and n are positive integers with the only common divisor being 1), the heterodyne frequency ω where m is 10 or less is h More preferably, the setting unit sets the heterodyne frequency ω h Most preferably, the setting unit sets the heterodyne frequency ω h The peak frequency ω p Set it to the same value as

[0078] For example, the setting unit focuses on the intensity noise of a peak frequency of 320 Hz, and sets the heterodyne frequency to 80 Hz, which is 1 / 4 of this peak frequency, with n=1 and m=4. FIG. 7 is a diagram showing a schematic diagram of the time change in the intensity of a heterodyne signal (reference signal or measurement signal) with a heterodyne frequency of 80 Hz, and the time change in the intensity of noise with a peak frequency of 320 Hz. The time change in the intensity of the noise is exaggerated. As shown in this diagram, the peak intensity of the heterodyne signal is constant at each peak, and the shape around the peak of the heterodyne signal is also constant at each peak. This means that the influence of noise is reduced, and the phase can be accurately determined based on the measurement signal and the reference signal.

[0079] Also, for example, the setting unit focuses on the intensity noise with a peak frequency of 60 Hz, and sets the heterodyne frequency to 80 Hz, which is 4 / 3 times the peak frequency, with n=4 and m=3. Figure 8 is a diagram showing a sine wave representing the time change in the intensity of a heterodyne signal (reference signal or measurement signal) with a heterodyne frequency of 80 Hz, and a sine wave representing the time change in the intensity of noise with a peak frequency of 60 Hz. As shown in this figure, the time for n periods of the 80 Hz heterodyne signal is equal to the time for m periods of the 60 Hz intensity noise.

[0080] Therefore, if n periods of an 80 Hz heterodyne signal are considered as one set, the influence of noise in one set does not change over time. The next set is also affected by noise in the same way. Figure 9 is a diagram showing the relationship between the sampling number for each period of an 80 Hz heterodyne signal and the phase to be obtained. As shown in this figure, the phase to be obtained in each period of a set of an 80 Hz heterodyne signal is different, but it is not necessary to take the average of the phase over n periods of a set of an 80 Hz heterodyne signal, and it is sufficient to find the phase in periods that are affected by noise in the same way in each of the multiple sets. However, the average may be taken.

[0081] That is, among the sampling numbers 1, 2, 3, 4,... shown for each period of the 80 Hz heterodyne signal in Figure 9, the phases obtained for sampling numbers p, p+n, p+2n,...,(p+qn),... are similarly affected by noise, so by obtaining the phases of these sampling numbers, the effects of noise can be reduced. p is an integer between 1 and 4, and q is a positive integer. On the other hand, if the phases of sampling numbers other than these are obtained, they will be significantly affected by noise.

[0082] It is to be noted that a large value of n is undesirable because it increases the change in the effect of noise due to slight fluctuations in the heterodyne frequency. From this viewpoint, n is preferably 10 or less, and more preferably 1. From the same viewpoint, m is preferably 10 or less, and more preferably 1.

[0083] It is also preferable to set the heterodyne frequency to a value that is a rational multiple of the peak frequency, and then, for fine adjustment, change the heterodyne frequency little by little to search for the heterodyne frequency that minimizes the fluctuations in retardation and displacement. [Explanation of symbols]

[0084] 1 to 3...optical heterodyne interference measuring device, 10...beam generating unit, 11...laser light source, 12...half wavelength plate, 13...quarter wavelength plate, 14...laser driver, 15...motor, 16...motor driver, 21...beam splitter, 22...polarizing beam splitter, 31...mirror, 33...quarter wavelength plate, 41...first photodetector, 42...linear polarizing plate, 51...second photodetector, 52...linear polarizing plate, 53...quarter wavelength plate, 60...AD conversion unit, 70...computer, 90...measurement object.

Claims

1. a beam generating unit that generates and outputs a first light beam and a second light beam having optical frequencies different from each other by a heterodyne frequency and having polarizations perpendicular to each other; a first photodetector that detects a temporal change in intensity due to heterodyne interference between the first light beam and the second light beam after both or either one of the first light beam and the second light beam are reflected by or transmitted through a measurement object, and outputs a first detection signal; Any peak frequency that is constant over time in the frequency spectrum of the intensity noise included in the first light beam or the second light beam output from the beam generating unit is defined as ω ｐ When the heterodyne frequency, which is the difference in optical frequency between the first optical beam and the second optical beam output from the beam generating unit, is set to the peak frequency ω ｐ A setting unit that sets the value to a rational multiple of a calculation unit that acquires information about the object to be measured based on a phase of a component of the first detection signal that varies over time in a sinusoidal manner at the heterodyne frequency; An optical heterodyne interferometer comprising:

2. a beam generating unit that generates and outputs a first light beam and a second light beam, the first light beam and the second light beam having optical frequencies different from each other by a heterodyne frequency and having polarizations perpendicular to each other, based on light reflected by or transmitted through the object to be measured; a first photodetector that detects a temporal change in intensity due to heterodyne interference between the first light beam and the second light beam and outputs a first detection signal; Any peak frequency that is constant over time in the frequency spectrum of the intensity noise included in the first light beam or the second light beam output from the beam generating unit is defined as ω ｐ When the heterodyne frequency, which is the difference in optical frequency between the first optical beam and the second optical beam output from the beam generating unit, is set to the peak frequency ω ｐ A setting unit that sets the value to a rational multiple of a calculation unit that acquires information about the object to be measured based on a phase of a component of the first detection signal that varies over time in a sinusoidal manner at the heterodyne frequency; An optical heterodyne interferometer comprising:

3. The setting unit is configured to set the heterodyne frequency ω ｈ and the peak frequency ω ｐ Relative to ω ｈ / ω ｐ is expressed as a rational number n / m (where m and n are positive integers whose common divisor is 1), n ​​is 10 or less, ｈ Set 3. The optical heterodyne interference measuring apparatus according to claim 1.

4. The setting unit is configured to set the heterodyne frequency ω ｈ Set 4. The optical heterodyne interferometer according to claim 3.

5. The setting unit is configured to set the heterodyne frequency ω ｈ and the peak frequency ω ｐ Relative to ω ｈ / ω ｐ is expressed as a rational number n / m (where m and n are positive integers whose common divisor is 1), m is 10 or less, ｈ Set 5. The optical heterodyne interference measuring device according to claim 1.

6. The setting unit is configured to set the heterodyne frequency ω ｈ Set 6. The optical heterodyne interferometer according to claim 5.

7. The setting unit is configured to set the heterodyne frequency ω ｈ The peak frequency ω ｐ Set it to the same value as 7. An optical heterodyne interference measuring device according to claim 1.

8. a beam splitter that collectively splits the first light beam and the second light beam output from the beam generating unit into two beams and outputs the first split light and the second split light; a second photodetector that detects a temporal change in intensity due to heterodyne interference between the first light beam and the second light beam included in the second branched light, and outputs a second detection signal; Further comprising: the first photodetector detects a temporal change in intensity due to heterodyne interference between the first light beam and the second light beam after both or either one of the first light beam and the second light beam included in the first split light output from the beam splitter is reflected by or transmitted through the measurement object, and outputs a first detection signal; The calculation unit acquires information about the object to be measured based on a phase of a component of the first detection signal that changes over time at the heterodyne frequency and a phase of a component of the second detection signal that changes over time at the heterodyne frequency.

8. An optical heterodyne interference measuring device according to claim 1, 3 or 7.

9. The calculation unit acquires information about the measurement object based on a phase of a component of the first detection signal that changes over time at the heterodyne frequency and a phase of an electrical signal that changes over time at the heterodyne frequency generated by the beam generation unit.

9. An optical heterodyne interference measuring device according to claim 1.

10. the beam generating unit includes a frequency shifter that generates and outputs the first light beam and the second light beam based on a laser beam output from a laser light source; 10. The optical heterodyne interference measuring device according to claim 1.

11. The calculation unit acquires, as the information on the measurement object, at least one of information on the retardation of the measurement object, the displacement of the measurement object, the change in refractive index of the measurement object, the surface shape of the measurement object, and the surface roughness of the measurement object.

11. The optical heterodyne interference measuring device according to claim 1.

12. a beam generating step of generating and outputting a first light beam and a second light beam having optical frequencies different from each other by a heterodyne frequency and having polarizations perpendicular to each other; a first optical detection step of detecting a temporal change in intensity due to heterodyne interference between the first optical beam and the second optical beam after both or either one of the first optical beam and the second optical beam are reflected by or transmitted through a measurement object, using a first optical detector, and outputting a first detection signal; Any peak frequency that is constant over time in the frequency spectrum of the intensity noise contained in the first light beam or the second light beam output in the beam generating step is defined as ω ｐ When the heterodyne frequency, which is the difference in optical frequency between the first optical beam and the second optical beam output in the beam generating step, is set to the peak frequency ω ｐ A setting step of setting the value to a rational multiple of a calculation step of acquiring information about the object to be measured based on a phase of a component of the first detection signal that varies over time in a sinusoidal manner at the heterodyne frequency; An optical heterodyne interferometry method comprising:

13. a beam generating step of generating and outputting a first light beam and a second light beam, the first light beam and the second light beam having optical frequencies different from each other by a heterodyne frequency and having polarizations perpendicular to each other, based on light reflected by or transmitted through the object to be measured; a first light detection step of detecting a temporal change in intensity due to heterodyne interference between the first light beam and the second light beam by a first light detector and outputting a first detection signal; Any peak frequency that is constant over time in the frequency spectrum of the intensity noise contained in the first light beam or the second light beam output in the beam generating step is defined as ω ｐ When the heterodyne frequency, which is the difference in optical frequency between the first optical beam and the second optical beam output in the beam generating step, is set to the peak frequency ω ｐ A setting step of setting the value to a rational multiple of a calculation step of acquiring information about the object to be measured based on a phase of a component of the first detection signal that varies over time in a sinusoidal manner at the heterodyne frequency; An optical heterodyne interferometry method comprising:

14. In the setting step, the heterodyne frequency ω ｈ and the peak frequency ω ｐ Relative to ω ｈ / ω ｐ is expressed as a rational number n / m (where m and n are positive integers whose common divisor is 1), n ​​is 10 or less, ｈ Set 14. The optical heterodyne interferometry method according to claim 12 or 13.

15. In the setting step, the heterodyne frequency ω ｈ Set The optical heterodyne interferometry method according to claim 14.

16. In the setting step, the heterodyne frequency ω ｈ and the peak frequency ω ｐ Relative to ω ｈ / ω ｐ is expressed as a rational number n / m (where m and n are positive integers whose common divisor is 1), m is 10 or less, ｈ Set 16. The optical heterodyne interference measuring device according to claim 12, wherein the optical heterodyne interference measuring device is a cascade type optical heterodyne measuring device.

17. In the setting step, the heterodyne frequency ω ｈ Set 17. The optical heterodyne interferometer according to claim 16.

18. In the setting step, the heterodyne frequency ω ｈ The peak frequency ω ｐ Set it to the same value as The optical heterodyne interference measuring method according to any one of claims 12 to 17.

19. a bi-branching step of collectively bi-branching the first light beam and the second light beam output from the beam generating step by a beam splitter and outputting the first bi-branched light and the second bi-branched light; a second optical detection step of detecting a temporal change in intensity due to heterodyne interference between the first optical beam and the second optical beam contained in the second split light by a second optical detector and outputting a second detection signal; Further comprising: In the first light detection step, after both or either one of the first light beam and the second light beam included in the first split light output from the beam splitter are reflected by or transmitted through the measurement object, a temporal change in intensity caused by heterodyne interference between the first light beam and the second light beam is detected by the first light detector, and a first detection signal is output; In the calculation step, information about the object is acquired based on a phase of a component of the first detection signal that varies over time at the heterodyne frequency and a phase of a component of the second detection signal that varies over time at the heterodyne frequency.

19. The optical heterodyne interference measuring method according to claim 12, or any one of claims 14 to 18.

20. In the calculation step, information about the measurement object is acquired based on a phase of a component of the first detection signal that varies over time at the heterodyne frequency and a phase of an electrical signal that varies over time at the heterodyne frequency generated in the beam generation step. The optical heterodyne interference measuring method according to any one of claims 12 to 19.

21. In the beam generating step, the first light beam and the second light beam are generated and output by a frequency shifter based on a laser beam output from a laser light source. The optical heterodyne interference measuring method according to any one of claims 12 to 20.

22. In the calculation step, at least one of information on the retardation of the measurement object, information on the displacement of the measurement object, information on a change in refractive index of the measurement object, information on a surface shape of the measurement object, and information on a surface roughness of the measurement object is acquired as information on the measurement object. The optical heterodyne interference measuring method according to any one of claims 12 to 21.

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