Laser interferometers and spectrometers

Abstract

To provide a laser interferometer that can reduce the frequency of the signal processed by the demodulation circuit, thereby reducing the cost of the demodulation circuit, and that can accurately measure even small displacements of the object being measured, and a spectrometer equipped with such a laser interferometer. [Solution] A laser interferometer comprising a laser light source, an optical modulator, a photodetector, a signal oscillator, and a demodulation circuit, wherein the demodulation circuit includes a DC offset removal unit for extracting the AC component of the laser light received signal, a first multiplier for multiplying the AC component by a reference signal, a first filter for outputting a first low-frequency signal, a second multiplier for multiplying the first multiplier signal by a reference signal, a second filter for outputting a second low-frequency signal, a Lissajous corrector for performing elliptic approximation processing on a Lissajous figure drawn using the first and second low-frequency signals, calculating a correction value, and adjusting the amplitude, and a phase calculator for calculating phase information.

Description

[Technical Field]

[0001] This invention relates to a laser interferometer and a spectrometer. [Background technology]

[0002] Patent Document 1 discloses a laser Doppler measurement device for determining the motion of a moving object. In this laser Doppler measurement device, a laser beam is irradiated onto the object to be measured, and the motion of the object is measured based on the scattered laser beam that has undergone a Doppler shift. Specifically, the amount of frequency shift of the laser beam is obtained by optical heterodyne interferometry, and the velocity and displacement of the moving object are determined from this amount of shift.

[0003] The laser Doppler measurement device described in Patent Document 1 includes a frequency shifter type optical modulator. This optical modulator comprises a thickness-sliding quartz AT resonator and a diffraction grating including a plurality of grooves arranged in parallel in the displacement direction of the resonator. The diffraction grating has grooves in a direction intersecting the vibration direction of the quartz AT resonator. When laser light is irradiated onto the diffraction grating, the laser light is diffracted and the frequency of the laser light is shifted. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-165700 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, thickness shear vibrations have a high resonant frequency. Therefore, the frequency of the modulation signal superimposed on the laser light by the optical modulator described in Patent Document 1 also becomes high. Consequently, in the laser Doppler measurement device described in Patent Document 1, it becomes necessary to adapt the circuits that perform calculations on the modulation signal and the circuits that convert analog signals to digital signals to handle high-frequency signals. As a result, these circuits become more expensive.

[0006] Therefore, the challenge is to realize a laser interferometer that can reduce the frequency of the signal processed by the demodulation circuit, thereby reducing the cost of the demodulation circuit.

[0007] On the other hand, when the frequency of the signal processed by the demodulation circuit is reduced as described above, limitations arise in the measurable displacement of the object being measured. For example, when measuring the vibration of an object, the measurement accuracy decreases if the amplitude falls below a predetermined value.

[0008] Therefore, there is a need to realize a laser interferometer that can accurately measure even small displacements of the object being measured, while simultaneously reducing costs as described above. [Means for solving the problem]

[0009] A laser interferometer according to an application example of the present invention is, A laser interferometer that irradiates an object with laser light, receives the laser light that has passed through the object, and obtains the displacement of the object, A laser light source that emits the aforementioned laser light, An optical modulator equipped with a vibrating element, which uses the vibrating element to add a modulation signal to the laser light, A photodetector that detects changes in the intensity of the laser light, which includes the modulated signal and the sample signal added by the object, and outputs a laser light reception signal, A signal oscillator that generates a reference signal of a reference frequency using the aforementioned vibration element as the source vibration, A demodulation circuit that demodulates the sample signal from the laser light reception signal based on the reference signal and obtains the displacement of the object, Equipped with, The demodulation circuit described above is A DC offset removal unit removes the DC component from the laser light received signal and extracts the AC component, A first multiplier that multiplies the AC component and the reference signal and outputs a first multiplier signal, A first filter that removes high-frequency components contained in the first multiply signal and outputs a first low-frequency signal, A second multiplier that multiplies the first multiplication signal and the reference signal and outputs a second multiplication signal, A second filter removes the high-frequency components contained in the second multiply signal and outputs a second low-frequency signal, A Lissajous corrector performs elliptic approximation on a Lissajous figure drawn using the first low-frequency signal and the second low-frequency signal, and adjusts at least one of the amplitudes of the first low-frequency signal and the second low-frequency signal based on the correction value obtained by the elliptic approximation. A phase calculator that demodulates the sample signal and calculates phase information originating from the object based on the corrected first low-frequency signal and the corrected second low-frequency signal output from the Lissajous corrector, It holds.

[0010] The spectroscopic apparatus according to an application example of the present invention is: A laser interferometer according to an application example of the present invention, A spectroscopic analysis unit having a spectroscopic optical system including a moving mirror and generating spectral information derived from the sample, Equipped with, The laser interferometer measures the displacement of the moving mirror, The spectroscopic analysis unit generates the spectroscopic spectral information based on the measurement results of the displacement of the moving mirror by the laser interferometer. [Brief explanation of the drawing]

[0011] [Figure 1] This is a functional block diagram showing a laser interferometer according to the first embodiment. [Figure 2] Figure 1 is a schematic diagram showing the interferometric optical system. [Figure 3] Figure 2 is a perspective view showing an example configuration of an optical modulator. [Figure 4] Figure 2 is a perspective view showing another example configuration of the optical modulator. [Figure 5] This is a table showing the coefficients included in the DC term and the terms representing the harmonic components in the series representation of a laser light reception signal. [Figure 6] This is a conceptual diagram to explain the challenges that arise in demodulation processing due to the small displacement of the object. [Figure 7] Figure 1 is a functional block diagram showing the configuration of the Lissajous compass. [Figure 8] Figure 7 is a schematic diagram illustrating the Lissajous correction performed by the Lissajous corrector. [Figure 9] This is a conceptual diagram illustrating the influence of a perfect circle with an aspect ratio of 1 and an ellipse with an aspect ratio of 0.4 on the calculation of phase information. [Figure 10] Figure 1 is a graph showing an example of the frequency characteristics of the phase delay amount of the AC component passing through the high-pass filter. [Figure 11] Figure 10 shows a graph illustrating the simulation results of the effect of the phase delay difference ψ1 on the measurement accuracy of the displacement in the design example shown. [Figure 12] Figure 10 shows a graph illustrating the simulation results of the effect of the phase delay difference ψ1 on the accuracy of the displacement in the design example shown. [Figure 13] This graph shows the frequency characteristics (bandwidth characteristics) of the gain and the frequency characteristics (phase characteristics) of the phase delay when the number of stages in a high-pass filter composed of an LCR circuit is changed to 1, 2, and 3 stages. [Figure 14] This is a functional block diagram showing a laser interferometer according to the second embodiment. [Figure 15] Figure 14 is a functional block diagram showing the configuration of the Lissajous compensator. [Figure 16] Figure 15 is a schematic diagram illustrating the Lissajous correction performed by the Lissajous corrector. [Figure 17]This is a functional block diagram showing a laser interferometer according to the third embodiment. [Figure 18] This is a functional block diagram showing a laser interferometer according to the fourth embodiment. [Figure 19] This is a functional block diagram showing a spectrometer according to the fifth embodiment. [Modes for carrying out the invention]

[0012] The laser interferometer and spectrometer according to the present invention will be described in detail below based on the embodiments shown in the accompanying drawings.

[0013] 1. First Embodiment First, the laser interferometer according to the first embodiment will be described.

[0014] Figure 1 is a functional block diagram showing the laser interferometer 1 according to the first embodiment. Figure 2 is a schematic configuration diagram showing the interference optical system 50 of Figure 1.

[0015] The laser interferometer 1 shown in Figure 1 comprises an interference optical system 50, a signal oscillation unit 51, and a demodulation circuit 52.

[0016] The interference optical system 50 shown in Figure 2 splits the laser light emitted from the laser light source 2 and directs it into the object 14 and the optical modulator 12, respectively. The laser light returning from the object 14 and the optical modulator 12 is then mixed and received by the photodetector 10. The photodetector 10 detects the intensity change of the laser light, which includes the sample signal added by the object 14 (phase information added to the laser light, etc.) and the modulation signal added by the optical modulator 12 (frequency information added to the laser light, etc.), and outputs a laser light reception signal.

[0017] The optical modulator 12 shown in Figure 1 includes a vibrating element 30. The optical modulator 12 adds a modulation signal to the laser light using the vibrating element 30. The signal oscillator 51 shown in Figure 1 generates a reference signal using the vibrating element 30 as the source vibration.

[0018] The demodulation circuit 52 shown in Figure 1 performs demodulation processing to demodulate the sample signal from the laser light reception signal based on the reference signal. This allows for the acquisition of the displacement of the object 14, etc.

[0019] 1.1 Interferometric Optics The interference optical system 50 shown in Figure 2 is a Michelson-type interference optical system. As shown in Figure 2, the interference optical system 50 comprises a laser light source 2, a collimating lens 3, an optical divider 4, a half-wave plate 6, a quarter-wave plate 7, a quarter-wave plate 8, an analyzer 9, and a photodetector 10.

[0020] The laser light source 2 emits laser light L1 with frequency f0. The photodetector 10 converts the intensity of the received light into an electrical signal. The optical modulator 12 uses the vibrating element 30 to change the frequency of the laser light L1 and generates a reference light L2 (laser light containing the modulation signal) that includes the modulation signal. Meanwhile, the laser light L1 incident on the object 14 is reflected as object light L3 (laser light containing the sample signal) that includes a sample signal originating from the object 14.

[0021] The optical path connecting the optical divider 4 and the laser light source 2 is defined as optical path 18. The optical path connecting the optical divider 4 and the optical modulator 12 is defined as optical path 20. The optical path connecting the optical divider 4 and the object 14 is defined as optical path 22. The optical path connecting the optical divider 4 and the light-receiving element 10 is defined as optical path 24. In this specification, "optical path" refers to the path through which light travels, set between optical elements.

[0022] On optical path 18, the half-wave plate 6 and the collimating lens 3 are arranged in that order from the optical splitter 4 side. On optical path 20, the quarter-wave plate 8 is arranged. On optical path 22, the quarter-wave plate 7 is arranged. On optical path 24, the analyzer 9 is arranged.

[0023] The laser light L1 emitted from the laser light source 2 travels through the optical path 18 and is split into two beams, the first split beam L1a and the second split beam L1b, by the optical splitter 4. The first split beam L1a travels through the optical path 20 and is incident on the optical modulator 12. The second split beam L1b travels through the optical path 22 and is incident on the object 14. The reference beam L2 generated by the frequency shift in the optical modulator 12 travels through the optical paths 20 and 24 and is incident on the photodetector 10. The object beam L3 generated by reflection from the object 14 travels through the optical paths 22 and 24 and is incident on the photodetector 10.

[0024] In the laser interferometer 1 equipped with the interference optical system 50 described above, phase information of the object 14 is obtained by optical heterodyne interferometry. Specifically, two beams of light with slightly different frequencies (reference light L2 and object light L3) are interfered with. Then, a sample signal is extracted from the intensity of the interference light in the demodulation circuit 52, and the displacement of the object 14 is determined from the sample signal. Optical heterodyne interferometry is less susceptible to the influence of disturbances, especially stray light at noise frequencies, when extracting the sample signal from the interference light, thus providing high robustness.

[0025] The following provides a further explanation of each part of the interference optical system 50. 1.1.1. Laser light source Laser light source 2 is a laser light source that emits coherent laser light L1. Preferably, laser light source 2 uses a light source with a linewidth of MHz or less. Specifically, examples include gas lasers such as He-Ne lasers, DFB-LDs (Distributed Feedback Laser Diodes), FBG-LDs (Fiber Bragg Grating Laser Diodes), VCSELs (Vertical Cavity Surface Emitting Lasers), and semiconductor laser elements such as FP-LDs (Fabry-Perot Laser Diodes).

[0026] The laser light source 2 is preferably a semiconductor laser element. This makes it possible to miniaturize the laser light source 2. As a result, the laser interferometer 1 can be miniaturized.

[0027] Furthermore, the laser light source 2 may be a wavelength-swept laser element, or a constant-wavelength laser element that continuously emits laser light L1 with a constant wavelength. Of these, using the latter constant-wavelength laser element provides advantages such as simplification of the configuration of the laser light source 2, miniaturization, and reduction of load.

[0028] 1.1.2. Collimating Lenses The collimating lens 3 is an optical element placed between the laser light source 2 and the optical divider 4, and an aspherical lens is one example. The collimating lens 3 parallelizes the laser light L1 emitted from the laser light source 2 to create collimated light. However, if the laser light L1 emitted from the laser light source 2 is sufficiently parallelized, for example, if a gas laser such as a He-Ne laser is used as the laser light source 2, the collimating lens 3 may be omitted.

[0029] The collimated laser light L1 passes through the half-wave plate 6, where it is converted into linearly polarized light with a P-polarized to S-polarized intensity ratio of, for example, 50:50, and is then incident on the optical divider 4.

[0030] 1.1.3.Light splitter The optical splitter 4 is a polarization beam splitter placed between the laser light source 2 and the optical modulator 12, and between the laser light source 2 and the object 14. The optical splitter 4 has the function of transmitting P-polarized light and reflecting S-polarized light. Due to this function, the optical splitter 4 splits the laser light L1 into a first split beam L1a and a second split beam L1b.

[0031] The first split beam L1a, which is S-polarized, is converted to circular polarization by the quarter-wave plate 8 and incident on the optical modulator 12. The first split beam L1a incident on the optical modulator 12 is f MIt undergoes a frequency shift of [Hz] and is reflected as reference light L2 containing the modulated signal. As a result, the frequency of reference light L2 becomes f0+f M The reference light L2 is converted to P-polarized light when it passes through the quarter-wave plate 8 again. The P-polarized light of the reference light L2 passes through the light divider 4 and the analyzer 9 and is incident on the photodetector 10.

[0032] The second split beam L1b, which is P-polarized, is converted to circular polarization by the quarter-wave plate 7 and incident on the moving object 14. The second split beam L1b incident on the object 14 is f D It undergoes a Doppler shift of [Hz] and is reflected as object light L3 containing the sample signal. As a result, the frequency of object light L3 is f0-f D The object light L3 is converted to S-polarized light when it passes through the quarter-wave plate 7 again. The object light L3 is reflected by the light divider 4, passes through the analyzer 9, and enters the photodetector 10. The reference light L2 and the object light L3 enter the photodetector 10 as interference light.

[0033] 1.1.4. Analyzer Since S-polarized and P-polarized light, which are orthogonal to each other, are independent of each other, simply superimposing them may not produce beats due to interference. Therefore, the light wave obtained by superimposing S-polarized and P-polarized light is passed through an analyzer 9 tilted at 45° with respect to both the S-polarized and P-polarized light. By using analyzer 9, the reference light L2 and the object light L3 interfere well, and |f M -f D Interfering light with a beat frequency of | is generated.

[0034] 1.1.5. Photodetector When the interference light is incident on the light receiving element 10, the light receiving element 10 outputs a photocurrent (laser light reception signal) corresponding to the intensity of the interference light. A sample signal is demodulated from this laser light reception signal by a method described later. Examples of the light receiving element 10 include a photodiode. Note that what is received by the light receiving element 10 only needs to be laser light in which a modulation signal and a sample signal are superimposed as a result of passing through the optical modulator 12 and the object 14, and is not limited to the interference light generated by the above-described path. Further, "demodulating a sample signal from a laser light reception signal" in this specification refers to extracting a sample signal by performing various operations on the laser light reception signal. In the following description, not only the above-described photocurrent but also a voltage signal converted from the photocurrent are collectively referred to as a "laser light reception signal".

[0035] 1.2. Optical Modulator The optical modulator 12 shown in FIG. 1 includes a vibrating element 30. In the optical modulator 12, the frequency of the first divided light L1a is modulated using the vibration of the vibrating element 30. According to such a configuration, miniaturization, weight reduction, and low power consumption of the optical modulator 12 can be achieved. Further, the vibration of the vibrating element 30 serves as a source vibration when the signal oscillation unit 51 generates a reference signal I S Therefore, the modulation signal added to the reference light L2 using the vibrating element 30 and the reference signal I S output from the signal oscillation unit 51 with the vibrating element 30 as the source vibration both originate from the vibration energy of the vibrating element 30. For this reason, for example, even if a disturbance such as an impact or noise is applied to the optical modulator 12 and the vibration of the vibrating element 30 changes, both the modulation signal and the reference signal I S will change in the same manner. Then, in the process of arithmetic processing in the demodulation circuit 52, the influence of both disturbances can be canceled or reduced. As a result, a decrease in the S / N ratio (signal-to-noise ratio) of the sample signal demodulated by the demodulation circuit 52 can be suppressed.

[0036] The vibrating element 30 is, for example, a vibrator that utilizes a mechanical resonance phenomenon such as a crystal vibrator, a ceramic vibrator, or a silicon vibrator. The mechanical resonance of these vibrators has a high Q value and excellent frequency stability.

[0037] Examples of quartz crystal oscillators include quartz AT oscillators, SC-cut quartz crystal oscillators, tuning fork type quartz crystal oscillators, and quartz surface acoustic wave elements. The oscillation frequency of a quartz crystal oscillator is, for example, from 1 kHz to several hundred MHz.

[0038] A silicon resonator is a resonator comprising a single-crystal silicon piece manufactured from a single-crystal silicon substrate using MEMS technology, and a piezoelectric film. MEMS (Micro Electro Mechanical Systems) refers to micro-electromechanical systems. Examples of the shape of the single-crystal silicon piece include cantilever shapes such as two-legged tuning fork type and three-legged tuning fork type, as well as double-supported beam shapes. The oscillation frequency of a silicon resonator is, for example, from 1 kHz to several hundred MHz.

[0039] A ceramic resonator is a resonator comprising a piezoelectric ceramic piece manufactured by firing piezoelectric ceramics, and electrodes. Examples of piezoelectric ceramics include lead zirconate titanate (PZT), barium titanate (BTO), and potassium sodium niobate (KNN). The oscillation frequency of a ceramic resonator is, for example, several hundred kHz to several tens of MHz.

[0040] Of these, a quartz crystal oscillator is preferably used for the vibrating element 30. Because quartz itself is a piezoelectric material, quartz crystal oscillators have particularly high frequency stability.

[0041] The oscillation frequency of the vibrating element 30 is not particularly limited, but is preferably between 1 MHz and 100 MHz. In the frequency band within the above range, there are many vibrators with high mechanical resonance Q values. Therefore, by setting the oscillation frequency within the above range, the reference signal I output from the signal oscillation unit 51 is S This allows for the stabilization of the reference frequency.

[0042] Figure 3 is a perspective view showing an example configuration of the optical modulator 12 shown in Figure 2. An example of the optical modulator 12 shown in Figure 3 is the optical modulator disclosed in Japanese Patent Application Publication No. 2022-38156. Specifically, the optical modulator 12 shown in Figure 3 comprises a vibrating element 30 and a diffraction grating 434 provided on the vibrating element 30 for diffracting the first divided light L1a (divided laser light).

[0043] The vibrating element 30 shown in Figure 3 is a quartz AT resonator that vibrates in thickness-slip direction along vibration direction 436 in the high-frequency region of the MHz band. The vibrating element 30 is also provided with a diffraction grating 434. The diffraction grating 434 has a plurality of linear grooves 432 that extend in a direction intersecting the vibration direction 436. When a first splitting light L1a is irradiated onto such a diffraction grating 434, the frequency of the first splitting light L1a can be modulated and a reference light L2 can be generated, even when the vibrating element 30 is vibrating in thickness-slip direction.

[0044] The vibrating element 30 has a front surface 4311 and a back surface 4312 that are in a front-back relationship with each other. A diffraction grating 434 is arranged on the front surface 4311. The front surface 4311 is also provided with a first electrode 437 for applying a voltage to the vibrating element 30, and a pad 433 that is electrically connected to the first electrode 437. On the other hand, the back surface 4312 is provided with a second electrode 438 for applying a voltage to the vibrating element 30, and a pad 435 that is electrically connected to the second electrode 438. The first electrode 437 and the second electrode 438 are arranged so that they overlap each other via the vibrating element 30 when the front surface 4311 is viewed from above. When a voltage is applied between the first electrode 437 and the second electrode 438, thickness-sliding vibration is induced in the portion where the first electrode 437 and the second electrode 438 overlap.

[0045] The diffraction grating 434 shown in Figure 3 is positioned on the first electrode 437. In other words, in Figure 3, the diffraction grating 434 is formed by a plurality of grooves 432 formed on the surface of the first electrode 437, and when the first splitting light L1a is irradiated onto it, a reference light L2 is emitted as diffracted light.

[0046] The diffraction grating 434 shown in Figure 3 is a blazed diffraction grating as an example. A blazed diffraction grating is one in which the cross-sectional shape of the diffraction grating is stepped. However, the shape of the diffraction grating 434 is not limited to this.

[0047] Figure 4 is a perspective view showing another configuration example of the optical modulator 12 shown in Figure 2. In Figure 4, three mutually orthogonal axes, A-axis, B-axis, and C-axis, are defined and indicated by arrows. The tip of the arrow is considered "positive," and the base of the arrow is considered "negative."

[0048] The vibrating element 30 shown in Figure 4 is a tuning fork type quartz crystal oscillator. The vibrating element 30 shown in Figure 4 has a base 401 and a vibrating substrate having a first vibrating arm 402 and a second vibrating arm 403. Such tuning fork type quartz crystal oscillators are readily available because their manufacturing technology is established, and their oscillation is also stable. For this reason, the tuning fork type quartz crystal oscillator is suitable as the vibrating element 30. The optical modulator 12 shown in Figure 4 also comprises the vibrating element 30 and electrodes 404, 405 and an optical reflector 406 provided on the vibrating element 30.

[0049] The base 401 is the portion that extends along the A-axis. The first vibrating arm 402 is the portion that extends from the A-axis negative end of the base 401 toward the B-axis positive side. The second vibrating arm 403 is the portion that extends from the A-axis positive end of the base 401 toward the B-axis positive side.

[0050] Electrode 404 is a conductive film provided on the side of the first vibrating arm 402 and the second vibrating arm 403 that is parallel to the AB plane. Although not shown in Figure 4, electrodes 404 are provided on opposite sides of each other, and the first vibrating arm 402 and the second vibrating arm 403 are driven by applying voltages with opposite polarities to each electrode.

[0051] Electrode 405 is a conductive film provided on the side surfaces of the first vibrating arm 402 and the second vibrating arm 403 that intersect with the AB plane. Although not shown in Figure 4, electrodes 405 are also provided on opposite sides of each other, and the first vibrating arm 402 and the second vibrating arm 403 are driven by applying voltages with opposite polarities to each electrode.

[0052] The light reflecting section 406 is set on, for example, the side of the first vibrating arm 402 and the second vibrating arm 403 that intersects with the AB surface, and has the function of reflecting the first divided light L1a. Due to this function, the light reflecting section 406 has a vibration component with a large amplitude in the incident direction of the incident first divided light L1a, so that the frequency of the first divided light L1a can be efficiently modulated and the reference light L2 can be generated.

[0053] A tuning fork-type quartz oscillator uses a quartz crystal cut from a quartz substrate. Examples of quartz substrates used in the manufacture of tuning fork-type quartz oscillators include quartz Z-cut plates. In Figure 4, the X-axis is set parallel to the A-axis, the Y'-axis is parallel to the B-axis, and the Z'-axis is parallel to the C-axis. A quartz Z-cut plate is a substrate cut from a single crystal of quartz such that, for example, the X-axis is the electrical axis, the Y'-axis is the mechanical axis, and the Z'-axis is the optical axis. Specifically, in a Cartesian coordinate system consisting of the X-axis, Y'-axis, and Z'-axis, a substrate having a main surface in which the X-Y' plane, consisting of the X-axis and Y'-axis, is tilted counterclockwise by approximately 1° to 5° around the X-axis is cut from a single crystal of quartz and is preferably used as a quartz substrate. Then, by etching such a quartz substrate, a quartz crystal used in the vibrating element 30 shown in Figure 4 is obtained.

[0054] 1.3. Signal Oscillation Section The signal oscillation unit 51 shown in Figure 1 uses the vibration element 30 as the source vibration to generate a reference signal I S This will cause it to happen.

[0055] Examples of the signal oscillation unit 51 include an oscillator circuit using an inverter, a Colpitts oscillator circuit, etc. These oscillator circuits operate using the fundamental wave oscillation of the vibration element 30 as the source oscillation. Therefore, by using a vibration element 30 with a high Q value of mechanical resonance, a reference signal I with high frequency stability can be generated. S It can generate [this].

[0056] Furthermore, the optical modulator 12 and the signal oscillator 51 may be housed in a single package. This reduces the physical distance between them, thereby suppressing the effects of noise and other factors.

[0057] 1.4. Demodulation Circuit First, let's explain the configuration of the demodulation circuit 52.

[0058] 1.4.1. Configuration of the demodulation circuit The demodulation circuit 52 shown in Figure 1 includes a current-voltage converter 520, a high-pass filter 522 (DC offset removal unit), a band-pass filter 524 (third filter), a first phase adjuster 526, a second phase adjuster 528, a first multiplier 530, a second multiplier 532, a low-pass filter 534 (first filter), a low-pass filter 536 (second filter), an A / D converter 538, an A / D converter 540, a Lissajous corrector 542, a divider 546, an inverse tangent arithmetic unit 548 (phase calculator), and a signal output unit 550.

[0059] The current-voltage converter 520, also known as a transimpedance amplifier (TIA), converts the photocurrent output from the photodetector 10 into a voltage signal and outputs it as a laser light reception signal.

[0060] The high-pass filter 522 removes the DC offset of the DC component of the laser light received signal. This removes the AC component I of the laser light received signal. PD.AC It can be extracted.

[0061] The bandpass filter 524 receives the reference signal I output from the signal oscillator 51. SThis allows only the reference frequency component to pass through. As a result, the reference signal I, from which unwanted frequency components (noise components) have been removed, is obtained. S This is obtained. Furthermore, the reference signal I output from the signal oscillation unit 51 is obtained. S In cases where there are few unwanted frequency components, the bandpass filter 524 may be omitted.

[0062] The first phase adjuster 526 receives the reference signal I output from the bandpass filter 524. S The phase is adjusted. Specifically, the first phase adjuster 526 adjusts the reference signal I S The phase is adjusted to be in phase with the fundamental frequency component of the modulation signal contained in the laser light reception signal. This suppresses the effect of phase delay in the high-pass filter 522, and reduces the decrease in demodulation accuracy and precision of the phase information X derived from the object 14 that is ultimately calculated.

[0063] Reference signal I output from the first phase adjuster 526 S The signal is split into two at branching point 527. Note that the placement of the first phase adjuster 526 is not limited to the position shown in Figure 1, and it may be placed, for example, between branching point 527 and the first multiplier 530. In this case, the phase adjustment amount of the second phase adjuster 528 should be changed according to the change in placement compared to this embodiment.

[0064] The second phase adjuster 528 is located between the branch point 527 and the second multiplier 532. In other words, the second phase adjuster 528 adjusts the phase of the reference signal I, which has been adjusted by the first phase adjuster 526. S The phase is further adjusted. Specifically, the second phase adjuster 528 adjusts the one reference signal I that is output from the first phase adjuster 526 and branched at the branching point 527. S The phase is adjusted to cancel out the phase delay in the high-pass filter 522. This makes it possible to cancel out or reduce the effect of differences in the phase delay in the high-pass filter 522, even if the phase delay differs due to differences in the frequency of the AC components passing through the high-pass filter 522.

[0065] The first multiplier 530 takes the laser light received signal output from the high-pass filter 522 and the reference signal I output from the first phase adjuster 526. S We multiply by and . This gives us the first multiply signal I X1 This is obtained. First multiplication signal I X1 At branch point 531, it is divided into a first arithmetic path PS1 and a second arithmetic path PS2.

[0066] The second multiplier 532 receives the first multiplication signal I, which has been divided into the second arithmetic path PS2. X1 And the reference signal I output from the first phase adjuster 526 S Multiply by and . This gives the second multiply signal I X2 This can be obtained.

[0067] The low-pass filter 534 divides the first multiply signal I into the first operation path PS1. X1 The high-frequency components are removed. As a result, the low-pass filter 534 filters out the first low-frequency signal I which contains low-frequency components. LF1 The output is displayed. Note that the low-pass filter 534 may also be a band-pass filter.

[0068] The low-pass filter 536 receives the second multiplied signal I output from the second multiplier 532. X2 The high-frequency components are removed. As a result, the low-pass filter 536 filters out the second low-frequency signal I, which contains low-frequency components. LF2 The output is displayed. Note that the low-pass filter 536 may also be a band-pass filter.

[0069] The high-pass filter 522 (DC offset removal section), band-pass filter 524, first phase adjuster 526, first multiplier 530, second multiplier 532, low-pass filter 534, and low-pass filter 536 described above are components of an analog circuit.

[0070] The A / D converter 538 receives the first analog low-frequency signal I output from the low-pass filter 534. LF1This converts the signal to digital. This results in a digital first low-frequency signal I LF1 This can be obtained.

[0071] The A / D converter 540 receives the second analog low-frequency signal I output from the low-pass filter 536. LF2 This converts the signal to digital. This results in a digital second low-frequency signal I LF2 This can be obtained.

[0072] The Lissajous corrector 542 adjusts the amplitude of two input signals using a Lissajous figure so that their amplitudes are equal. Specifically, it first draws a Lissajous figure, then performs an ellipse approximation process on the obtained Lissajous figure to obtain an approximate ellipse. Next, it calculates a correction value to make the approximate ellipse closer to a perfect circle. Then, based on the calculated correction value, it adjusts the first low-frequency signal I LF1 The amplitude and the second low-frequency signal I LF2 The amplitude is adjusted. The correction by the Lissajous corrector 542 (Lissajous correction) will be described in detail later.

[0073] Furthermore, the Lissajous corrector 542 processes the first low-frequency signal I LF1 The amplitude and the second low-frequency signal I LF2 You may adjust either one of the amplitudes, or both.

[0074] The divider 546 processes the corrected first low-frequency signal C output from the Lissajous corrector 542 to the first arithmetic path PS1. P1 ·I LF1 The corrected second low-frequency signal C is output from the Lissajous corrector 542 to the second calculation path PS2. P2 ·I LF2 A division operation is performed by dividing by . This yields a division signal.

[0075] The inverse tangent unit 548 performs an inverse tangent operation on the division signal output from the divider 546. This calculates the phase information X originating from the object 14.

[0076] The signal output unit 550 performs phase connection, such as unwrapping, on the phase information X originating from the object 14. It also calculates the displacement and velocity of the object 14 as needed.

[0077] The Lissajous corrector 542, divider 546, inverse tangent arithmetic unit 548, and signal output unit 550 described above may be components of an analog circuit, but are preferably components of a digital circuit. Such a digital circuit is implemented in an electronic device such as an FPGA (programmable logic device), ASIC (application-specific integrated circuit), or microcomputer.

[0078] 1.4.2. Demodulation Process Next, the operation of the demodulation circuit 52 (demodulation process) will be explained. In the following explanation, as an example, a signal whose frequency changes sinusoidally will be used as the modulation signal, and the case in which the displacement of the object 14 is undergoing simple harmonic motion in the direction of incidence of light will be explained.

[0079] The laser light received signal output from the current-voltage converter 520 is input to the high-pass filter 522. The high-pass filter 522 removes the DC offset of the laser light received signal. This removes the AC component I of the laser light received signal. PD.AC It can be extracted. AC component I PD.AC This is expressed by the following formula (1).

[0080]

number

[0081] In equation (1) above, A is the product of the amplitude of the reference light L2 and the amplitude of the object light L3. Also, Φ M This is the phase originating from the optical modulator 12 (the phase of the reference light L2). Furthermore, X is the phase information given by the following equation (1a).

[0082]

number

[0083] In the above formula (1a), Φ S Φ is the phase originating from the object 14 (phase of object light L3), and Φ0 is the initial phase difference due to the optical path difference within the interference optical system 50.

[0084] The optical modulator 12 shown in Figure 2 has a modulation frequency f M A reference light L2 containing the modulated signal is generated. Therefore, Φ M This is given by equation (1b) below.

[0085]

number

[0086] In the above equation (1b), ω M The modulation frequency f M The angular frequency (modulation angular frequency) is ω M =2πf M Here, t is time. B is the modulation index in frequency modulation at the optical modulator 12. If the vibration of the vibrating element 30 is stable, B will be a constant value and will be expressed by the following equation (1b-1).

[0087]

number

[0088] In equation (1b-1) above, L0 is the amplitude of the vibrating element 30 in the direction from which the reference light L2 is emitted. λ is the wavelength of the laser light L1. Using equation (1b) above, equation (1) above can be expressed as equation (1c) below.

[0089]

number

[0090] Expanding the right-hand side of equation (1c) above using the series representation of Bessel functions yields equation (1d) below.

[0091]

number

[0092] Equation (1d) above represents the modulation angular frequency ω M Terms that do not include (DC terms) and sinω M Terms containing t (sinω M t term) and cos2ω M The term containing t (cos2ω M The terms can be divided into terms representing harmonic components, such as the t-term. Figure 5 shows each of these terms and the coefficients included in each term.

[0093] Figure 5 is a table showing the coefficients included in the DC term and the terms representing the harmonic components in the series representation of a laser light reception signal. Note that Figure 5 shows only a portion of the terms representing the harmonic components.

[0094] As shown in Figure 5, the coefficients of each term include cosX or sinX. In the demodulation circuit 52, these are extracted by calculation, ultimately resulting in the phase Φ originating from the object 14. S We calculate sinω. In particular, in this embodiment, M The coefficients of the t-term include sinX and cos2ω M We extract the cosX contained in the coefficient of the t term, and finally obtain the phase information X, from which we obtain the phase Φ. S Calculate.

[0095] Furthermore, the reference signal I output from the first phase adjuster 526 S This is expressed by the following equation (2).

[0096]

number

[0097] In equation (2) above, V q This is the amplitude.

[0098] In the first multiplier 530, the AC component I of the laser light received signal output from the high-pass filter 522 is processed. PD.AC And the reference signal I output from the first phase adjuster 526 S Multiply and by. This multiplication (first multiplication) is the operation of multiplying the above equation (1d) by the above equation (2). As a result, the first multiplied signal I X1 This is obtained. First multiplication signal I X1 This is expressed by the following equation (3).

[0099]

number

[0100] Equation (3) above gives the modulation angular frequency ω M Terms that do not include (DC terms) and sinω M Terms containing t (sinω M t term) and cos2ω M The term containing t (cos2ω M The terms can be divided into terms representing harmonic components, such as term t, and terms representing harmonic components. Each of these terms and the coefficients contained within each term are shown in "Result of the First Multiplication" in Figure 5. Note that in Figure 5, the coefficients of each term are shown in a simplified form.

[0101] First multiplying signal I X1 The signal is divided at branch point 531 shown in Figure 1. The first multiplication signal I is divided into the first calculation path PS1. X1 When it passes through the low-pass filter 534, the modulation angular frequency ω M High-frequency components including are cut off. The cutoff frequency of the low-pass filter 534 is, for example, ω M It is set to / 2. The first low-frequency signal I is output from the low-pass filter 534. LF1 This is expressed by the following formula (3a).

[0102]

number

[0103] Through the first multiplication as described above, the first low-frequency signal I divided into the first arithmetic path PS1 LF1 is down-converted in frequency.

[0104] Also, the concept of the first multiplication as described above is shown in FIG. 5. In FIG. 5, the transitions due to the first multiplication of sinX and cosX included in the coefficient and the second multiplication described later are represented using arrows. When the first multiplication is performed, sinX that was included in the coefficient of the sinω M t term shifts to the coefficients of the DC term and the cos2ω M t term, respectively. Also, cosX that was included in the coefficient of the cos2ω M t term shifts to the coefficient of the sinω M t term.

[0105] Next, when the high-frequency components are removed in the low-pass filter 534, only the DC term is output. That is, among the results of the first multiplication shown in FIG. 5, the component of the DC term including the coefficient surrounded by the thick solid line frame is output from the low-pass filter 534. In this way, the first low-frequency signal I LF1 in which the frequency has been down-converted is obtained.

[0106] On the other hand, in the second multiplier 532, the first multiplication signal I X1 divided into the second arithmetic path PS2 is multiplied by the reference signal I S output from the second phase adjuster 528. This multiplication (second multiplication) is an operation of multiplying the above formula (2) by the above formula (3). As a result, the second multiplication signal I X2 is obtained. The second multiplication signal I X2 is represented by the following formula (4).

[0107]

Equation

[0108] The above formula (4) includes a term (DC term) that does not include the modulation angular frequency ω M , a term that includes sinω M t (sinω M t term), and a cos2ω Mterms containing t (cos2ω M terms of harmonic components such as the t term) and are divided into. The coefficients contained in each of these terms are shown in the "Result of Second Multiplication" in FIG. 5. In FIG. 5, the coefficients of each term are shown with some simplification.

[0109] Second multiplication signal I X2 When passing through the low-pass filter 536, the high-frequency components including the modulation angular frequency ω M are cut off. The cut-off frequency of the low-pass filter 536 is set to, for example, ω M / 2. The second low-frequency signal I LF2 output from the low-pass filter 536 is represented by the following formula (4a).

[0110]

Equation

[0111] Note that a minus sign is originally attached to the right side of the second low-frequency signal I LF2 , but in the above formula (4a), it is in the form of the above formula (4a) by multiplying -1 on the right side. Note that when finally obtaining the displacement derived from the object 14, it is not affected by this multiplication.

[0112] Through the second multiplication as described above, the second low-frequency signal I LF2 divided into the second arithmetic path PS2 is made low-frequency.

[0113] Also, the concept of the second multiplication as described above is shown in FIG. 5. When the second multiplication is performed, among the results of the first multiplication, cosX contained in the coefficient of the sinω M t term (the coefficient surrounded by the thick dashed line) shifts to the coefficients of the DC term and the cos2ω M t term, respectively. Similarly for other terms, sinX shifts from the base end to the tip of the arrow.

[0114] Next, when the high-frequency components are removed by the low-pass filter 536, only the DC term is output. In other words, the DC term component, which includes the coefficient enclosed by the thick solid line in the result of the second multiplication shown in Figure 5, is output from the low-pass filter 536. In this way, the second low-frequency signal I is reduced in frequency. LF2 This can be obtained.

[0115] Here, in the results of the first multiplication shown in Figure 5, for the coefficients enclosed in the thick dashed border, the transition of cosX represented by the dashed arrow and the transition of cosX represented by the thick solid arrow are superimposed. In this case, the transitioned cosX represented by the dashed arrow can become a noise component for the transitioned cosX represented by the solid arrow, and therefore needs to be removed.

[0116] Therefore, in this embodiment, the aforementioned high-pass filter 522 is provided in the demodulation circuit 52. As described above, this high-pass filter 522 removes the DC offset of the laser light reception signal. By removing the DC offset, the coefficient (the coefficient of the DC term) shown underlined in Figure 5 is removed. As a result, the transition of cosX, represented by the dashed arrow, can be stopped. Consequently, the superposition of noise components can be prevented.

[0117] First low-frequency signal I containing sinX LF1 , and a second low-frequency signal I including cosX LF2 This is input to A / D converters 538 and 540 shown in Figure 1.

[0118] The first low-frequency signal I, digitally converted by A / D converter 538. LF1 and the second low-frequency signal I, which is digitally converted by the A / D converter 540. LF2 These are inputs to the Lissajous corrector 542, respectively.

[0119] In the Lissajous corrector 542, the first low-frequency signal I, which has been digitally converted, LF1 The amplitude and the digitally converted second low-frequency signal I LF2The amplitudes are adjusted to be the same. Specifically, the gain coefficient C is adjusted so that equation (5) below holds true. P1 and gain coefficient C P2 Determine and multiply these together. In this specification, such gain coefficient C P1 , C P2 The determination and multiplication of these factors is called "Lissajous correction." Lissajous correction will be explained in detail later.

[0120]

number

[0121] The corrected first low-frequency signal C is output from the Lissajous corrector 542 to the first calculation path PS1. P1 ·I LF1 , and the corrected second low-frequency signal C output from the Lissajous corrector 542 to the second calculation path PS2. P2 ·I LF2 These are given by equations (6) and (7) below, respectively.

[0122]

number

[0123]

number

[0124] The divider 546 processes the corrected first low-frequency signal C. P1 ·I LF1 Corrected second low-frequency signal C P2 ·I LF2 A division operation is performed by dividing by . This yields a division signal.

[0125] The inverse tangent unit 548 performs an inverse tangent operation on the division signal output from the divider 546. Inverse tangent calculation result I atan This is expressed by the following equation (8).

[0126]

number

[0127] Then, the result of the arctangent operation I, expressed by the above formula (8), is atan From this, phase information X can be obtained.

[0128] In the demodulation process described above, the first low-frequency signal I input to the A / D converters 538 and 540 LF1 and second low-frequency signal I LF2 Each frequency is the modulation frequency f M It is kept lower than that. In other words, the first low-frequency signal I input to the digital circuit LF1 and second low-frequency signal I LF2 The frequency has been lowered (down-converted). This allows the operating frequency (sampling frequency) of the A / D converters 538 and 540 to be reduced. As a result, the cost of the A / D converters 538 and 540 can be reduced.

[0129] Furthermore, the aforementioned digital circuits are implemented in devices such as FPGAs, and the down-conversion described above allows for a reduction in the operating frequency of FPGAs and other components. Specifically, even when using a vibration element 30 with an oscillation frequency in the MHz range, it becomes possible to use A / D converters or FPGAs with operating frequencies in the kHz range. Therefore, the down-conversion described above can also contribute to reducing the cost of electronic components such as FPGAs.

[0130] Furthermore, the modulation frequency f of the optical modulator 12, whose corresponding frequency was the rate-limiting frequency, was also affected by the above-mentioned modulation frequency. M This allows us to remove constraints. For example, the modulation frequency f, which was previously difficult to adopt due to the above-mentioned relationship with corresponding frequencies, can now be used. M This also makes it easier to use a very high vibration element 30 in the optical modulator 12. This expands the range of vibrating elements 30 that can be used.

[0131] Furthermore, the aforementioned analog circuit uses only two multipliers, keeping the number of multipliers to a minimum. This reduces noise interference associated with multiplication, enabling the calculation of highly accurate phase information X.

[0132] 1.4.3. Lissajous Correction Next, we will explain the operation of the Lissajous corrector 542 (Lissajous correction).

[0133] In the demodulation process described above, in order to obtain highly accurate phase information X, the corrected first low-frequency signal C used in the arctangent calculation is P1 ·I LF1 and the corrected second low-frequency signal C P2 ·I LF2 To achieve this, the Lissajous corrector 542 needs to process the digitally converted first low-frequency signal I LF1 The amplitude and the digitally converted second low-frequency signal I LF2 It is necessary to precisely match the amplitudes of the two elements, that is, to ensure that equation (5) above holds true with accuracy.

[0134] The coefficients A and V included in the above equation (5) q , J 1(B) , J 2(B) All of these are known. Therefore, the first low-frequency signal I LF1 The amplitude and the second low-frequency signal I LF2 If we can detect the amplitudes of each, then theoretically, the gain coefficient C P1 , C P2 It should be possible to uniquely determine the gain coefficient C. P1 and gain coefficient C P2 The ratio of gain coefficients should remain constant regardless of the measurement conditions. However, the problem is that this ratio of gain coefficients changes due to the influence of environmental factors, including disturbance noise. For example, if the distance (working distance) between the laser interferometer 1 and the object 14 changes, the ratio of gain coefficients is likely to change.

[0135] To address this challenge, for example, before using the laser interferometer 1 or at a predetermined timing, an appropriate ratio of gain coefficients should be found, and based on that, the gain coefficient C P1 , C P2 The appropriate value should be adjusted. According to the Lissajous corrector 542, by determining the ratio of the gain coefficients appropriately and optimally, even when the working distance changes, the appropriate gain coefficient C can be obtained. P1 , C P2 It is possible to determine this.

[0136] Furthermore, in the demodulation process described above, the phase information X is included in the coefficient of the DC term. Then, Lissajous correction is performed based on the amplitude, which includes the effects of external noise. For this reason, the demodulation process described above is highly robust against external noise.

[0137] On the other hand, the demodulation process described above presents another challenge. In the above demodulation process, the phase information X is included in the coefficient of the DC term. Therefore, if the displacement of the object 14 is small, the accuracy of calculating the phase information X may decrease. Below, we will explain this challenge and then detail the Lissajous correction that solves this problem.

[0138] First, the phase information due to the displacement of object 14 is X (t) Let's assume that. Figure 6 is a conceptual diagram illustrating the challenges that arise in demodulation processing due to the small displacement of object 14. Figure 6 shows the phase information X (t) The first low-frequency signal I when the change is less than one revolution of the unit circle. LF1 Waveform examples and second low-frequency signal I LF2 Examples of waveforms, and phase information X (t) This is the result of plotting the values ​​on a unit circle. Note that in Figure 6, the first low-frequency signal I LF1 The amplitude and the second low-frequency signal I LF2 The amplitudes of each are normalized to 1. Furthermore, Figure 6 assumes that the displacement of object 14 is simple harmonic motion.

[0139] Phase information X (t) To determine this with high accuracy, the phase information X associated with the displacement of the object 14 is required.(t) The change must be greater than or equal to one full rotation of the unit circle (a change greater than or equal to one full rotation of the unit circle as shown in Figure 6). However, if the displacement of the object 14 is small or if the initial phase φ0 shown in Figure 6 is not appropriate, the phase information X (t) The change may be less than one full rotation of the unit circle. In that case, sinX (t) and cosX (t) The maximum values ​​of each function may be less than 1, and the minimum values ​​may be greater than -1. This is because sinX is assumed in the principles of demodulation. (t) and cosX (t) These are not the maximum and minimum values ​​that it should take. First low-frequency signal I LF1 and second low-frequency signal I LF2 The gain coefficient C is calculated by comparing the maximum values ​​of each amplitude. P1 , C P2 When determining this, such "deviations" reduce the accuracy of calculating the ratio of gain coefficients in the computational process.

[0140] As shown in Figure 6, the first and second low-frequency signals assumed in the demodulation principle should oscillate with true amplitudes Am1 and Am2, respectively. However, the phase information X (t) If the change is less than one revolution of the unit circle, the first and second low-frequency signals will oscillate with apparent amplitudes Am1' and Am2' that are smaller than the true amplitudes Am1 and Am2.

[0141] In this case, the apparent vibration amplitudes Am1' and Am2' shown in Figure 6 are considered to be the true vibration amplitudes Am1 and Am2, and demodulation is performed accordingly, causing the ratio of the gain coefficients to deviate from the true value. As a result, the phase information X obtained through demodulation also deviates from the true value, which presents a problem.

[0142] However, phase information X (t) It is not easy to detect when the change is less than one revolution of the unit circle. In other words, considering the usage of the laser interferometer 1, it is likely that the displacement of the object 14 is often unknown, so the phase information X (t)It is not easy to identify the change.

[0143] Here, the phase information X (t) and the displacement of the object 14 will be described in relation to each other. The phase information X (t) and the phase information X (t) The demodulated displacement L of the object 14 demodulated from (t) In principle, the following formula (11) holds between and.

[0144]

Number

[0145] When the displacement of the object 14 is a simple harmonic vibration, the phase information X (t) is represented by the following formula (12).

[0146]

Number

[0147] In the above formula (12), φ max is the phase amplitude due to the displacement of the object 14. Also, for simplicity of explanation, the initial phase φ0 is set to 0. Then, the demodulated displacement L (t) is represented by the following formula (13).

[0148]

Number

[0149] In the above formula (13), L max is the demodulated displacement amplitude. Then, from the above formula (11) to the above formula (13), the following formula (15) is derived.

[0150]

Number

[0151] In the above equation (15), L max This is the demodulation displacement amplitude. Here, sinX is independent of the initial phase φ0 (t) and cosX (t) For each of the values ​​to be 1 and each of the values ​​to be -1, the following equation (16) must hold.

[0152]

number

[0153] Thus, from equations (15) and (16) above, equation (17) below can be derived.

[0154]

number

[0155] From equation (17) above, if the displacement amplitude of the object 14 is 1 / 4 or more of the wavelength, then sinX is independent of the initial phase φ0. (t) and cosX (t) It can be seen that the maximum value of each becomes 1 and the minimum value of each becomes -1. At this time, the phase information X (t) The change in this variable completes one revolution around the unit circle in Figure 6.

[0156] However, such constraints on the displacement amplitude of the measurable object 14 reduce the usability of the laser interferometer 1.

[0157] Therefore, the Lissajous corrector 542 determines an appropriate gain coefficient using a Lissajous figure so that the calculation accuracy of the phase information X does not decrease even when the displacement amplitude of the object 14 is small. (t) Even if the change is less than one full rotation of the unit circle, it is possible to suppress a decrease in the calculation accuracy of the phase information X.

[0158] FIG. 7 is a functional block diagram showing the configuration of the Lissajous corrector 542 shown in FIG. 1. FIG. 8 is a schematic diagram for explaining the Lissajous correction by the Lissajous corrector 542 shown in FIG. 7.

[0159] The Lissajous corrector 542 shown in FIG. 7 includes an elliptical approximation unit 562 and amplitude adjusters 564 and 566.

[0160] The elliptical approximation unit 562 has a function of drawing a Lissajous figure using the first low-frequency signal I LF1 and the second low-frequency signal I LF2 Since the phases of the first low-frequency signal I LF1 and the second low-frequency signal I LF2 are shifted from each other by approximately 90°, the Lissajous figure drawn is usually a closed ellipse or an open ellipse. In FIG. 8, as an example, the Lissajous figure (the Lissajous figure forming an open ellipse) when the change in the phase information X (t) is less than one cycle of the unit circle is illustrated. In such a case, as shown in FIG. 8, an elliptical arc AR1 having a major axis along the vertical axis and a minor axis along the horizontal axis is drawn as the Lissajous figure.

[0161] The elliptical approximation unit 562 has a function of performing elliptical approximation processing on the Lissajous figure to obtain an approximated ellipse EL1. In the elliptical approximation processing, the approximated ellipse EL1 is fitted to the elliptical arc AR1. As an example, a method of applying the following formula (21), which is the equation of an ellipse, can be mentioned.

[0162]

Equation

[0163] Note that the equation of the ellipse is not limited to the above formula (21). Also, in the elliptical approximation processing, by determining A to F and f0 in the above formula (21), an approximated ellipse EL1 that best fits the elliptical arc AR1 is determined. Examples of this fitting method include the maximum likelihood estimation method, the least squares method, the weighted iteration method, and the like.

[0164] A reference explaining elliptic approximation processing is "Kenta Yokota et al., Comparison of Accuracy of Elliptic Fitting: From Least Squares Method to Ultra-Precise Renormalization Method, IEICE Technical Report, 111(378): 2012.1.19·20, pp.75-82." The method described in this reference may also be used.

[0165] Next, the ellipse approximation section 562 is configured to calculate necessary correction values ​​through ellipse approximation processing. A specific example of ellipse approximation processing is the process of finding an approximate ellipse EL1 that fits the Lissajous figure and calculating correction values ​​to bring the approximate ellipse EL1 closer to a perfect circle CI. These correction values ​​could be, for example, parameters for transforming the approximate ellipse EL1 into a perfect circle CI. By using such correction values, appropriate Lissajous correction can be performed. In the example shown in Figure 8, a correction value (ratio of gain coefficients) is calculated to align the minor axis of the approximate ellipse EL1 with the major axis, and the gain coefficient C is calculated based on this. P1 ,C P2 Determine the gain coefficient C. P1 ,C P2 This is input to amplitude adjusters 564 and 566. This allows the elliptical arc AR1 to be corrected to a circular arc AR2. As a result, the phase information X corresponding to the circular arc AR2 is obtained, as shown in Figure 8. (t) The amplitude W of the vibration is closer to the true value than the amplitude of variation corresponding to the elliptical arc AR1. Therefore, the phase information X corresponding to the circular arc AR2 obtained by the correction is obtained. (t) By using this method, the displacement of object 14 can be determined with high accuracy.

[0166] The amplitude adjuster 564 has a gain coefficient C P1 to the first low-frequency signal I LF1 It has the function of multiplying by . This results in the corrected first low-frequency signal C P1 ·I LF1 The second low-frequency signal C after correcting the amplitude P2 ·I LF2 It can be adjusted to match the amplitude.

[0167] The amplitude adjuster 566 has a gain coefficient C P2 to the second low-frequency signal I LF2 It has the function of multiplying by . This corrects the second low-frequency signal C P2 ·I LF2 The amplitude of the first low-frequency signal C after correction. P1 ·I LF1 It can be adjusted to match the amplitude.

[0168] Note that either amplitude adjuster 564 or amplitude adjuster 566 may be omitted. In that case, it is preferable to omit amplitude adjuster 564. That is, the gain coefficient C P1 Let be set to 1, and the gain coefficient C be the relative value. P2 The ratio is calculated, and the gain coefficient C P2 This may be used to determine the second low-frequency signal I LF2 Only correct the first low-frequency signal I LF1 Use it as is without correction (gain coefficient C) P1 (This will be set to 1). The reason for doing this is that the first low-frequency signal I LF1 By minimizing the calculations for the first low-frequency signal I LF1 One advantage is that it is less likely to impair the real-time nature of the output, thus ensuring the temporal reliability of the output displacement.

[0169] The process of making the approximate ellipse EL1 closer to a perfect circle CI involves calculating a correction value that brings the flattening ratio of the approximate ellipse EL1 closer to 1. In this case, the closer the value is to 1, the greater the effect of the correction. Therefore, an acceptable range of flattening ratios was found through simulation.

[0170] First, in this specification, the flattening ratio H refers to the second low-frequency signal I in the approximate ellipse EL1. LF2 The diameter derived from the amplitude of the first low-frequency signal I LF1 This refers to the value obtained by dividing the amplitude by the diameter derived from it.

[0171] Figure 9 is a conceptual diagram illustrating the influence of a perfect circle with an aspect ratio H of 1 and an ellipse with an aspect ratio H of 0.4 on the calculation of phase information X.

[0172] When the flattening ratio H is 1, the demodulated phase is the true phase X. C The first low-frequency signal I has a flattening ratio H of 0.4. LF1 and second low-frequency signal I LF2 The demodulated phase without Lissajous correction is measured as phase X. E In this case, an error called the angular deviation D exists between these values. This angular deviation D affects the error in the measured displacement.

[0173] Therefore, in the above simulation, we calculated how close the aspect ratio H needs to be to 1 (how close the deflection angle D needs to be to 0) in order to keep the measured displacement within an acceptable range. As a result, if the aspect ratio H can be kept within the range of 0.92 to 1.08, then the true phase X C It was found that the probability of keeping the error in the measured displacement within 3.0 nm increases, regardless of the angle. This error is sufficiently good for, for example, when using the laser interferometer 1 as a displacement meter. Therefore, in the process of bringing the approximate ellipse EL1 closer to a perfect circle CI, it is preferable to keep the flattening ratio H within the range of 1.00 ± 0.08. Furthermore, in this process, it is even more preferable to keep the flattening ratio H within 1.00 ± 0.05. This increases the probability of keeping the error in the measured displacement within 2.0 nm.

[0174] 1.4.4. Example of High-Pass Filter Design In the high-pass filter 522 shown in Figure 1, the DC offset of the laser light received signal is removed, and the AC component passes through. However, depending on the frequency characteristics of the phase delay amount of the AC component passing through the high-pass filter 522, sinω M t term and cos2ω M The difference in phase delay between term t and term t becomes large, which may reduce the demodulation accuracy and precision of the phase information X.

[0175] Figure 10 is a graph showing an example of the frequency characteristics of the phase delay of the AC component passing through the high-pass filter 522 shown in Figure 1. The horizontal axis represents frequency. The left vertical axis represents gain. The right vertical axis represents the phase delay relative to the case where the gain is zero. The passband shown in Figure 10 is the bandwidth designed according to the frequency of the AC component to be passed through the high-pass filter 522. In the example shown in Figure 10, the modulation frequency f M Assuming that is 5MHz, we set 5MHz as the lower limit and twice that frequency, 2f M A passband is set with an upper limit of 10 MHz, corresponding to the specified frequency. By minimizing the difference in phase delay within this passband, the reduction in demodulation accuracy and precision of phase information X can be suppressed.

[0176] Here, we consider why the difference in phase delay within the passband affects the demodulation accuracy of phase information X.

[0177] In Figure 10, the phase delay when a 5MHz component passes through is denoted as Φ1, and the phase delay when a 10MHz component (twice the 5MHz component) passes through is denoted as Φ2. Then, the difference in phase delays ψ1 within the passband is given by |Φ1-Φ2|. This difference in phase delays ψ1 is the result of the arctangent calculation I in the demodulation process. atan The effect on this is represented by the following equation (31).

[0178]

number

[0179] The middle side of equation (31) above contains cosψ1 in the denominator, which differs from the right-hand side of equation (8) above. This difference indicates that it is an error factor that reduces the demodulation accuracy of the phase information X.

[0180] Therefore, in this embodiment, the high-pass filter 522 is set such that the difference in phase delay amount ψ1 satisfies the following equation (32).

[0181]

number

[0182] Furthermore, preferably, the high-pass filter 522 is set to satisfy the following equation (33).

[0183]

number

[0184] This configuration allows for improved demodulation accuracy and precision of the phase information X. As a result, the measurement accuracy of the displacement of the object 14 and the precision of the measured displacement can be improved.

[0185] Figure 11 is a graph showing the results of a simulation of the effect of the difference in phase delay ψ1 on the measurement accuracy of the displacement in the design example shown in Figure 10.

[0186] As shown in Figure 11, when the difference in phase delay ψ1 is 10 degrees or less, the measurement accuracy of the displacement is kept below 1 nm. Therefore, by setting the difference in phase delay ψ1 within the above range, sufficient measurement accuracy can be obtained.

[0187] Figure 12 is a graph showing the results of simulating the effect of the difference in phase delay ψ1 on the accuracy of the displacement in the design example shown in Figure 10.

[0188] As shown in Figure 12, when the difference in phase delay ψ1 is 1 [deg] or less, the accuracy of the measured displacement is approximately 100%, and strictly speaking, it is kept within 100 ± 0.01%. Therefore, by setting the difference in phase delay ψ1 within the above range, sufficient measurement accuracy can be obtained.

[0189] Methods for keeping the phase delay difference ψ1 within a predetermined range include, for example, changing the constants of the LCR circuit elements, increasing the number of stages, and lowering the cutoff frequency in the design of the high-pass filter 522.

[0190] Figure 13 is a graph created by calculating the frequency characteristics of the gain (bandwidth characteristics) and the frequency characteristics of the phase delay (phase characteristics) when the number of stages in a high-pass filter composed of an LCR circuit is changed to 1, 2, and 3 stages. When the number of stages is changed, the cutoff frequency of the bandwidth characteristics shifts, and the phase characteristics also shift.

[0191] In the design example shown in Figure 13, when the number of LCR circuit stages is set to three, the phase delay difference ψ1 is kept below 10 degrees. By adjusting the design of the LCR circuit in this way, the phase delay difference ψ1 can be kept below 10 degrees or even below 1 degree.

[0192] On the other hand, in this embodiment, by providing a second phase adjuster 528, the influence of the phase characteristics in the high-pass filter 522 can be suppressed. As a result, the demodulation accuracy and precision of the phase information X can be further improved while reducing the design burden of the high-pass filter 522.

[0193] Let ψ2 be the phase adjustment amount in the second phase adjuster 528. Ideally, the phase adjustment amount ψ2 should be equal to the difference in phase delay amounts ψ1 in the high-pass filter 522, but setting errors may occur. Therefore, we will examine the effect of the setting error δ (=|ψ2-ψ1|).

[0194] The effect of the setting error δ is the same as the effect of the difference in phase delay amount ψ1 described above. Therefore, by keeping the setting error δ to 10 [deg] or less, the measurement accuracy of the displacement can be kept to 1 nm or less. This provides sufficient measurement accuracy. Furthermore, by keeping the setting error δ to 1 [deg] or less, the accuracy of the measured displacement can be kept within 100 ± 0.01%. This provides sufficient measurement accuracy.

[0195] 2. Second Embodiment Next, a laser interferometer according to the second embodiment will be described. Figure 14 is a functional block diagram showing the laser interferometer 1 according to the second embodiment.

[0196] The second embodiment will be described below, focusing on the differences from the first embodiment, and similar matters will be omitted from the description. In Figure 14, components similar to those in the first embodiment are denoted by the same reference numerals.

[0197] In the first embodiment described above, the first phase adjuster 526 and the second phase adjuster 528 reduce the effect of phase delay in the high-pass filter 522 (DC offset removal section). In contrast, in the second embodiment, the first phase adjuster 526 and the second phase adjuster 528 are omitted from the demodulation circuit 52, while the Lissajous corrector 542 takes on the role they previously performed.

[0198] Figure 15 is a functional block diagram showing the configuration of the Lissajous corrector 542 shown in Figure 14. Figure 16 is a schematic diagram illustrating the Lissajous correction performed by the Lissajous corrector 542 shown in Figure 15.

[0199] The Lissajous corrector 542 shown in Figure 15 includes an ellipse approximation section 562, amplitude adjusters 564 and 566, and a phase adjuster 568.

[0200] The ellipse approximation section 562 shown in Figure 15 has the function of determining the angle difference d of the approximate ellipse EL1 with respect to the reference figure ST.

[0201] Furthermore, the elliptical approximation section 562 shown in Figure 15 is based on the angle difference d, and the first low-frequency signal I LF1 Second low-frequency signal I LF2 The second low-frequency signal I is set so that the phase difference between them is 90°. LF2 A corrected phase delay amount Δφ is determined to compensate for the phase delay amount.

[0202] The phase adjuster 568 shown in Figure 15 adjusts the corrected phase delay amount Δφ to the second low-frequency signal I LF2 It has the function of adding to the first low-frequency signal I LF1Second low-frequency signal I LF2 The phase difference can be brought closer to 90°. As a result, it becomes possible to calculate phase information X with high accuracy.

[0203] Furthermore, in the first embodiment described above, the first phase adjuster 526 and the second phase adjuster 528 are components of the analog circuit, but in this embodiment, they can be omitted. This provides the advantage of reducing the number of components in the analog circuit.

[0204] Next, we will explain the operation (Lissajous correction) of the Lissajous corrector 542 shown in Figure 15.

[0205] Figure 16 shows, as an example, the phase information X (t) This illustrates a Lissajous figure when the change is less than one revolution of the unit circle. In such cases, as shown in Figure 16, an elliptical arc AR0 is drawn as a Lissajous figure. Note that in the example shown in Figure 16, the first low-frequency signal I LF1 and the second low-frequency signal I LF2 This shows the elliptic arc AR0 when the phase difference with respect to is deviated from 90°.

[0206] Next, the ellipse approximation unit 562 has the function of performing an ellipse approximation process on the Lissajous figure to obtain an approximate ellipse EL0. In the ellipse approximation process, the approximate ellipse EL0 is fitted to the elliptical arc AR0.

[0207] Next, the ellipse approximation unit 562 calculates the angle difference d of the approximate ellipse EL0 with respect to the reference figure ST. The reference figure ST is the first low-frequency signal I used to draw the approximate ellipse EL0. LF1 and the second low-frequency signal I LF2 This refers to the ellipse drawn when the phase difference with respect to is 90°. Since this ellipse is a figure based on the phase difference that we want to achieve through correction, we should correct the approximate ellipse EL0 toward this figure (reference figure ST).

[0208] Therefore, the elliptic approximation unit 562 determines the corrected phase delay amount Δφ based on the angle difference d. The determined corrected phase delay amount Δφ is input to the phase adjuster 568. As a result, the phase adjuster 568 processes the first low-frequency signal I LF1 Second low-frequency signal I LF2 The phase difference is corrected to approach 90°. In other words, the role that the first phase adjuster 526 and the second phase adjuster 528 of the first embodiment played can be assigned to the Lissajous corrector 542. In this embodiment, the Lissajous corrector 542 performs the same function as in the first embodiment, namely, it calculates a correction value (ratio of gain coefficients) and adjusts the gain coefficient C based on this. P1 ,C P2 In addition to the function of determining the angle difference d, it also generates a first low-frequency signal I LF1 and the second low-frequency signal I LF2 It also has a function to determine a corrected phase delay amount Δφ to correct the phase difference. This makes it possible to reduce the number of analog circuit components (first phase adjuster 526 and second phase adjuster 528) while suppressing the influence of the phase characteristics in the high-pass filter 522, thereby improving the demodulation accuracy and precision of the phase information X.

[0209] Furthermore, the elliptic approximation section 562 deforms the approximate ellipse EL0 based on the angle difference d to obtain the approximate ellipse EL1.

[0210] Next, the elliptic approximation unit 562 calculates a correction value to bring the approximate ellipse EL1 closer to a perfect circle CI, and based on this, the gain coefficient C P1 , C P2 Determine the gain coefficient C. P1 , C P2 This is input to amplitude adjusters 564 and 566. As a result, the elliptical arc AR0 shown in Figure 16 can be corrected to a circular arc AR2. Then, the phase information X corresponding to the circular arc AR2 shown in Figure 16 is input. (t) The amplitude W of the oscillation can be brought closer to the true value than the amplitude of variation corresponding to the elliptical arc AR0. In the second embodiment described above, the same effects as in the first embodiment can be obtained.

[0211] 3. Third Embodiment Next, a laser interferometer according to the third embodiment will be described. Figure 17 is a functional block diagram showing the laser interferometer 1 according to the third embodiment.

[0212] The third embodiment will now be described, focusing on the differences from the first embodiment, and omitting explanations of similar matters. In Figure 17, components similar to those in the first embodiment are denoted by the same reference numerals.

[0213] The laser interferometer 1 shown in Figure 17 comprises a sensor head unit 55, a signal processing unit 56, and a display unit 57.

[0214] The sensor head unit 55 includes an interference optical system 50 and a signal oscillator 51. The sensor head unit 55 is positioned, for example, near the object 14.

[0215] The signal processing unit 56 has a demodulation circuit 52. The signal processing unit 56 may be installed, for example, at a location away from the sensor head unit 55. The Lissajous corrector 542 in the demodulation circuit 52 may have multiple operating modes. Examples of these operating modes include an automatic mode that repeatedly performs Lissajous correction and a manual mode that performs Lissajous correction at any desired timing. In the automatic mode, Lissajous figures that change over time are successively drawn based on the displacement of the object 14. Then, correction values ​​that change over time are generated (correction values ​​are updated) based on the changes in the Lissajous figures. This makes it possible to maintain an optimal correction value. As a result, the laser interferometer 1 can achieve higher measurement accuracy and improved robustness against disturbances.

[0216] The display unit 57 has the function of displaying the measurement results of the object 14. In addition, the display unit 57 may also have the function of displaying the operating mode of the Lissajous corrector 542. This allows the user of the laser interferometer 1 to easily recognize the current operating mode, thereby improving the usability of the laser interferometer 1.

[0217] The display unit 57 may display the operating mode by any means. Therefore, the display unit 57 may include, for example, a liquid crystal display element that displays the operating mode via images or characters, an organic EL display element, a light-emitting element such as a light-emitting diode that displays the operating mode via a light-emitting pattern, or a sound-producing element such as a speaker that displays the operating mode via sound or tone.

[0218] Furthermore, the display unit 57 may also be equipped with functions to display the operating mode, Lissajous figures, correction values, demodulated phase, measured displacement, and the like. In the third embodiment described above, the same effects as in the first embodiment can be obtained.

[0219] 4. Fourth Embodiment Next, a laser interferometer according to the fourth embodiment will be described. Figure 18 is a functional block diagram showing the laser interferometer 1 according to the fourth embodiment.

[0220] The fourth embodiment will now be described, focusing on the differences from the first and third embodiments, and omitting explanations of similar matters. In Figure 18, components similar to those in the first or third embodiment are denoted by the same reference numerals.

[0221] The laser interferometer 1 shown in Figure 18 comprises a sensor head unit 55, a signal processing unit 56, a display unit 57, and an input unit 58.

[0222] The input unit 58 receives input operations from the user of the laser interferometer 1 and transmits the input information to the signal processing unit 56. The signal processing unit 56 switches the operating mode of the Lissajous corrector 542 based on the input information. Examples of the input unit 58 include a keyboard, touch panel, microphone, etc.

[0223] The signal processing unit 56 shown in Figure 18 has an input receiving unit 560. The input receiving unit 560 receives input information and causes the Lissajous corrector 542 to change its operating mode. This allows the user to perform correction at any time. As a result, even when using the laser interferometer 1 in a very noisy environment, for example, Lissajous correction can be performed at an appropriate time and the correction value updated. This enables stable measurement based on the correction value updated at the appropriate time. In addition, in manual mode, the frequency of Lissajous correction can be reduced, thereby reducing the processing load on the Lissajous corrector 542.

[0224] In the fourth embodiment described above, the same effects as those of the first and third embodiments can be obtained.

[0225] 5. Fifth Embodiment Next, a spectroscopic apparatus according to the fifth embodiment will be described. Figure 19 is a functional block diagram showing the spectroscopic apparatus 900 according to the fifth embodiment.

[0226] The fifth embodiment will be described below, focusing on the differences from the first embodiment, and similar matters will be omitted from the description. In Figure 19, components similar to those in Figure 1 are denoted by the same reference numerals.

[0227] The spectroscopic apparatus 900 shown in Figure 19 comprises a laser interferometer 1 (a laser interferometer according to each embodiment described above) and a spectroscopic analysis unit 910.

[0228] The spectroscopic analysis unit 910 receives analytical light containing a sample-derived signal generated by interaction with the sample, and generates spectral information originating from the sample. The spectroscopic analysis unit 910 shown in Figure 19 comprises a spectroscopic optical system 920 and a calculation unit 930. The spectroscopic optical system 920 has an analytical light source 922, a movable mirror 924, and an analytical light receiving unit 926. In the spectroscopic optical system 920, analytical light emitted from the analytical light source 922 is irradiated onto the sample and then incident on an analytical light interferometer (not shown). In the analytical light interferometer, the analytical light that has passed through the sample and the analytical light that has passed through the movable mirror 924 are interfered with by moving the movable mirror 924 to change the optical path length. The interfered light is then received by the analytical light receiving unit 926, and an analytical light reception signal is obtained.

[0229] Meanwhile, the laser interferometer 1 measures the displacement of the moving mirror 924 and outputs a mirror position signal. Because the laser interferometer 1 can accurately measure the displacement of the moving mirror 924, it can generate a highly accurate mirror position signal.

[0230] The calculation unit 930 generates a waveform (interferogram) representing the intensity of interference light with respect to the optical path length in the spectroscopic optical system 920 based on the analyzed light reception signal and the mirror position signal, and performs a Fourier transform on this to generate spectral information.

[0231] Therefore, the spectroscopic analysis unit 910 can generate highly accurate spectral information based on the measurement results of the displacement of the moving mirror 924 by the laser interferometer 1. Furthermore, since the laser interferometer 1 can be easily reduced in cost, it also contributes to reducing the cost of the spectroscopic device 900.

[0232] Furthermore, the spectrometer 900 can be applied to FT-IR (Fourier infrared spectroscopy), FT-NIR (Fourier near-infrared spectroscopy), FT-VIS (Fourier visible light spectroscopy), FT-UV (Fourier ultraviolet spectroscopy), FT-THz (Fourier terahertz spectroscopy), etc., by appropriately changing the type of analytical light, etc.

[0233] Furthermore, by using an element capable of acquiring a two-dimensional light intensity distribution as the analytical light receiving unit 926, the spectrometer 900 can be applied to, for example, a white light interference shape analyzer, an optical tomography (OCT) imaging device, and the like.

[0234] 6. Effects achieved by each of the above embodiments The laser interferometer 1 according to each of the above embodiments irradiates the object 14 with a second split beam L1b (laser light), receives the object light L3 (laser light) that has passed through the object 14, and obtains the displacement of the object 14. This laser interferometer 1 comprises a laser light source 2, an optical modulator 12, a photodetector 10, a signal oscillator 51, and a demodulation circuit 52. The laser light source 2 emits laser light L1. The optical modulator 12 includes a vibrating element 30, and uses the vibrating element 30 to add a modulation signal to the first split beam L1a (laser light). The photodetector 10 detects changes in the intensity of the reference light L2 and the object light L3 (laser light including the modulation signal and the sample signal added by the object 14), and outputs a laser light reception signal. The signal oscillator 51 uses the vibrating element 30 as the source to generate a reference signal I of a reference frequency. S The demodulation circuit 52 generates the reference signal I S Based on this, the sample signal is demodulated from the laser light reception signal to obtain the displacement of the object 14.

[0235] The demodulation circuit 52 includes a high-pass filter 522 (DC offset removal unit), a first multiplier 530, a low-pass filter 534 (first filter), a second multiplier 532, a low-pass filter 536 (second filter), a Lissajous corrector 542, and an arctangent calculator 548 (phase calculator).

[0236] The high-pass filter 522 removes the DC component of the laser light received signal, and removes the AC component I PD.AC Extract the AC component I. The first multiplier 530 processes the AC component I. PD.AC and reference signal I S Multiply by and to get the first multiply signal I X1 It outputs the first multiplying signal I. The low-pass filter 534 outputs the first multiplying signal I. X1 The high-frequency components contained in the signal are removed, and the first low-frequency signal I LF1It outputs the first multiplication signal I. X1 and reference signal I S Multiply by and to get the second multiply signal I X2 It outputs the second multiplying signal I. The low-pass filter 536 outputs the second multiplying signal I. X2 Removes the high-frequency components contained in the second low-frequency signal I LF2 Outputs.

[0237] The Lissajous corrector 542 processes the first low-frequency signal I LF1 and second low-frequency signal I LF2 An elliptic approximation process is performed on the Lissajous figure drawn using the elliptic approximation process, and based on the correction value obtained from the elliptic approximation process, the first low-frequency signal I LF1 The amplitude and the second low-frequency signal I LF2 Adjust at least one of the amplitudes.

[0238] The arctangent unit 548 processes the corrected first low-frequency signal C output from the Lissajous corrector 542. P1 ·I LF1 and the corrected second low-frequency signal C P2 ·I LF2 Based on this, the sample signal is demodulated and phase information X originating from the object 14 is calculated.

[0239] With this configuration, the first low-frequency signal I generated in the demodulation circuit 52 LF1 and second low-frequency signal I LF2 The frequency will be reduced. This will allow for lowering the operating frequency of components such as FPGAs on which components like A / D converters 538 and 540 and part of the demodulation circuit 52 are implemented, thereby reducing costs.

[0240] Furthermore, with the above configuration, by performing Lissajous correction in the Lissajous corrector 542, a laser interferometer 1 that can accurately measure even small displacements of the object 14 can be realized.

[0241] In the laser interferometer 1 according to each of the above embodiments, it is preferable that the Lissajous corrector 542, in the ellipse approximation process, finds an approximate ellipse EL1 that fits the Lissajous figure and calculates a correction value that brings the approximate ellipse EL1 closer to a perfect circle CI. With this configuration, appropriate Lissajous correction can be performed.

[0242] In the laser interferometer 1 according to each of the above embodiments, the Lissajous corrector 542 receives the first low-frequency signal I LF1 and the second low-frequency signal I LF2 The Lissajous figure with a phase difference of 90° is defined as the reference figure ST, the angle difference d of the approximate ellipse EL0 relative to the reference figure ST is determined, and the first low-frequency signal I is calculated based on the angle difference d. LF1 and the second low-frequency signal I LF2 The phase difference may be corrected.

[0243] With this configuration, the first low-frequency signal I LF1 and the second low-frequency signal I LF2 The phase difference can be brought closer to 90°. This suppresses the influence of the phase characteristics in the high-pass filter 522, thereby improving the demodulation accuracy and precision of the phase information X.

[0244] In the laser interferometer 1 according to each of the above embodiments, the demodulation circuit 52 may have a bandpass filter 524 (third filter). In this case, the bandpass filter 524 is provided between the signal oscillation unit 51 and the first multiplier 530, and the reference signal I S Extract the reference frequency component contained within.

[0245] With this configuration, the reference signal I is obtained by removing unwanted frequency components (noise components). S This can be obtained.

[0246] In the laser interferometer 1 according to each of the above embodiments, the demodulation circuit 52 may have a first phase adjuster 526. In this case, the first phase adjuster 526 is provided between the bandpass filter 524 (third filter) and the first multiplier 530, and the reference signal I S Adjust the phase.

[0247] With this configuration, the effect of phase delay in the high-pass filter 522 can be suppressed, and the decrease in demodulation accuracy and precision of the final calculated phase information X can be suppressed.

[0248] In the laser interferometer 1 according to each of the above embodiments, the first phase adjuster 526 sets the reference signal I to be in phase with the fundamental frequency component of the modulated signal. S It is preferable to adjust the phase.

[0249] With this configuration, the phase of the fundamental frequency component of the modulation signal contained in the laser light reception signal and the reference signal I S The phases of and can be aligned. This suppresses the decrease in demodulation accuracy and precision of phase information X.

[0250] In the laser interferometer 1 according to each of the above embodiments, the demodulation circuit 52 may have a second phase adjuster 528. In this case, the second phase adjuster 528 is provided between the first phase adjuster 526 and the second multiplier 532, and adjusts the reference signal I such that the phase adjustment amount ψ2 is the same as the difference in phase delay amount ψ1 in the passband of the high-pass filter 522 (DC offset removal section) from the phase that is in phase with the fundamental frequency component of the modulation signal included in the laser light reception signal. S Adjust the phase.

[0251] This configuration allows for the cancellation or reduction of the phase characteristics in the high-pass filter 522. Furthermore, it reduces the design complexity of the high-pass filter 522.

[0252] In the laser interferometer 1 according to each of the above embodiments, it is preferable that the high-pass filter 522 (DC offset removal unit) is set such that ψ1 satisfies ψ1 ≤ 10 [deg], where ψ1 is the difference between the phase delay amount Φ1 when the fundamental frequency component of the modulated signal passes through and the phase delay amount Φ2 when a frequency component twice the fundamental frequency component of the modulated signal passes through.

[0253] This configuration allows for improved demodulation accuracy and precision of the phase information X. This, in turn, improves the measurement accuracy of the displacement of the object 14 and the precision of the measured displacement.

[0254] In the laser interferometer 1 according to each of the above embodiments, it is preferable that the high-pass filter 522 (DC offset removal unit) is set such that the difference in phase delay amount ψ1 satisfies ψ1 ≤ 1 [deg].

[0255] This configuration allows for improved demodulation accuracy and precision of the phase information X. This, in turn, improves the measurement accuracy of the displacement of the object 14 and the precision of the measured displacement.

[0256] In the laser interferometer 1 according to each of the above embodiments, the Lissajous corrector 542 may update the correction value based on the Lissajous figure that changes over time.

[0257] This configuration allows for the maintenance of optimal correction values. As a result, the laser interferometer 1 can achieve higher measurement accuracy and improved robustness against disturbances.

[0258] The laser interferometer 1 according to each of the above embodiments may include an input receiving unit 560 that receives user input information. In this case, the Lissajous corrector 542 updates the correction value based on the input information received by the input receiving unit 560.

[0259] With this configuration, a laser interferometer 1 capable of stable measurement based on correction values ​​updated at the appropriate time can be realized.

[0260] In the laser interferometer 1 according to each of the above embodiments, it is preferable that the wavelength of the laser light L1 emitted by the laser light source 2 is constant.

[0261] This configuration allows for simplification, miniaturization, and reduction of the load on the laser light source 2.

[0262] Furthermore, the spectroscopic apparatus 900 according to the above embodiment comprises a laser interferometer 1 according to each of the above embodiments and a spectroscopic analysis unit 910. The spectroscopic analysis unit 910 has a spectroscopic optical system 920 including a movable mirror 924 and generates spectroscopic spectral information derived from the sample. The laser interferometer 1 measures the displacement of the movable mirror 924. The spectroscopic analysis unit 910 then generates spectroscopic spectral information based on the measurement result of the displacement of the movable mirror 924 by the laser interferometer 1.

[0263] This configuration allows for the creation of a spectrometer 900 capable of generating highly accurate spectral information. Furthermore, it enables the creation of a lower-cost spectrometer 900.

[0264] Although the laser interferometer and spectrometer of the present invention have been described above based on the illustrated embodiments, the laser interferometer and spectrometer of the present invention are not limited to the above embodiments, and the configuration of each part may be replaced with any other component, or any other component may be added.

[0265] Furthermore, although a Michelson interferometry optical system is used in each of the above embodiments, other types of interferometry optical systems may also be used. [Explanation of Symbols]

[0266] 1…Laser interferometer, 2…Laser light source, 3…Collimating lens, 4…Optical divider, 6…Half wave plate, 7…Quarter wave plate, 8…Quarter wave plate, 9…Analyzer, 10…Photodetector, 12…Optical modulator, 14…Object, 18…Optical path, 20…Optical path, 22…Optical path, 24…Optical path, 30…Vibrating element, 50…Interference optical system, 51…Signal oscillator, 52…Demodulation circuit, 55…Sensor head, 56…Signal processing unit, 5 7…Display unit, 58…Input unit, 401…Base unit, 402…First vibration arm, 403…Second vibration arm, 404…Electrode, 405…Electrode, 406…Light reflection unit, 432…Groove, 433…Pad, 434…Diffraction grating, 435…Pad, 436…Vibration direction, 437…First electrode, 438…Second electrode, 520…Current-voltage converter, 522…High-pass filter, 524…Band-pass filter, 526…First phase adjuster 527...Branch point, 528...Second phase adjuster, 530...First multiplier, 531...Branch point, 532...Second multiplier, 534...Low-pass filter, 536...Low-pass filter, 538...A / D converter, 540...A / D converter, 542...Lissajous corrector, 546...Divider, 548...Incident tangent calculator, 550...Signal output section, 560...Input reception section, 562...Elliptic approximation section, 564...Amplitude Adjuster, 566…Amplitude adjuster, 568…Phase adjuster, 900…Spectrometer, 910…Spectroscopic analysis unit, 920…Spectroscopic optical system, 922…Analytical light source, 924…Moving mirror, 926…Analytical light receiving unit, 930…Calculation unit, 4311…Front surface, 4312…Back surface, AR0…Elliptical arc, AR1…Elliptical arc, AR2…Circular arc, Am1…Vibration amplitude, Am1'…Vibration amplitude, Am2…Vibration amplitude, Am2'…Vibration amplitude, CI…Circular circle, C P1 ...gain coefficient, C P2 ...gain coefficient, C P1 ·I LF1 ...Corrected first low-frequency signal, C P2 ...gain coefficient, C P2 ·I LF2 ...Corrected second low-frequency signal, D...declination, EL0...approximate ellipse, EL1...approximate ellipse, H...flatness, I PD.AC ...AC component, I LF1 ...first low frequency signal, I LF2 …Second low frequency signal, I S ...reference signal, I X1 ...First multiplying signal, I X2...Second multiplication signal, L1...Laser light, L1a...First division light, L1b...Second division light, L2...Reference light, L3...Object light, PS1...First calculation path, PS2...Second calculation path, ST...Reference shape, W...Vibration amplitude, X...Phase information, X C ...true phase, X E ...Measurement phase, d...Angle difference, Δφ...Correction phase delay amount, Φ1...Phase delay amount, Φ2...Phase delay amount

Claims

1. A laser interferometer that irradiates an object with laser light, receives the laser light that has passed through the object, and obtains the displacement of the object, A laser light source that emits the aforementioned laser light, An optical modulator equipped with a vibrating element, which uses the vibrating element to add a modulation signal to the laser light, A photodetector that detects changes in the intensity of the laser light, including the modulated signal and the sample signal added by the object, and outputs a laser light reception signal, A signal oscillator that generates a reference signal of a reference frequency using the aforementioned vibration element as the source vibration, A demodulation circuit that demodulates the sample signal from the laser light reception signal based on the reference signal and obtains the displacement of the object, Equipped with, The demodulation circuit described above is A DC offset removal unit removes the DC component from the laser light received signal and extracts the AC component, A first multiplier that multiplies the AC component and the reference signal and outputs a first multiplier signal, A first filter that removes high-frequency components contained in the first multiply signal and outputs a first low-frequency signal, A second multiplier that multiplies the first multiplication signal and the reference signal and outputs a second multiplication signal, A second filter removes the high-frequency components contained in the second multiplying signal and outputs a second low-frequency signal, A Lissajous corrector performs elliptic approximation on a Lissajous figure drawn using the first low-frequency signal and the second low-frequency signal, and adjusts at least one of the amplitudes of the first low-frequency signal and the second low-frequency signal based on the correction value obtained by the elliptic approximation. A phase calculator that demodulates the sample signal and calculates phase information originating from the object based on the corrected first low-frequency signal and the corrected second low-frequency signal output from the Lissajous corrector, A laser interferometer characterized by having the following features.

2. The laser interferometer according to claim 1, wherein the Lissajous corrector, in the ellipse approximation process, finds an approximate ellipse that fits the Lissajous figure and calculates the correction value that brings the approximate ellipse closer to a perfect circle.

3. The laser interferometer according to claim 2, wherein the Lissajous corrector uses the Lissajous figure when the phase difference between the first low-frequency signal and the second low-frequency signal is 90° as a reference figure, determines the angle difference of the approximate ellipse with respect to the reference figure, and corrects the phase difference between the first low-frequency signal and the second low-frequency signal based on the angle difference.

4. The laser interferometer according to claim 1, wherein the demodulation circuit is provided between the signal oscillator and the first multiplier and has a third filter that extracts the reference frequency component included in the reference signal.

5. The laser interferometer according to claim 4, wherein the demodulation circuit is provided between the third filter and the first multiplier and has a first phase adjuster for adjusting the phase of the reference signal.

6. The laser interferometer according to claim 5, wherein the first phase adjuster adjusts the phase of the reference signal so that it is in phase with the fundamental frequency component of the modulated signal.

7. The laser interferometer according to claim 5, wherein the demodulation circuit is provided between the first phase adjuster and the second multiplier, and the second phase adjuster adjusts the phase of the reference signal from a phase that is in phase with the fundamental frequency component of the modulation signal included in the laser light receiving signal, such that the component of the reference frequency produces a phase adjustment amount equal to the difference in the phase delay amount in the passband of the DC offset removal unit.

8. The DC offset removal unit calculates the difference between the phase delay amount when the fundamental frequency component of the modulated signal passes through and the phase delay amount when a frequency component twice the fundamental frequency component of the modulated signal passes through. １ When this is the case, ψ １ The laser interferometer according to claim 1, which is set to satisfy ≤ 10 [deg].

9. The DC offset removal unit removes the difference ψ of the phase delay amount. １ However, ψ １ The laser interferometer according to claim 8, which is set to satisfy ≤ 1 [deg].

10. The laser interferometer according to claim 1, wherein the Lissajous corrector updates the correction value based on the Lissajous figure which changes over time.

11. It is equipped with an input receiving unit that accepts user input information, The laser interferometer according to claim 1, wherein the Lissajous corrector updates the correction value based on the input information received by the input receiving unit.

12. The laser interferometer according to claim 1, wherein the wavelength of the laser light emitted by the laser light source is constant.

13. A laser interferometer according to claim 1, A spectroscopic analysis unit having a spectroscopic optical system including a moving mirror and generating spectral information derived from the sample, Equipped with, The laser interferometer measures the displacement of the moving mirror, The spectroscopic device is characterized in that the spectroscopic analysis unit generates the spectroscopic spectral information based on the measurement results of the displacement of the moving mirror by the laser interferometer.

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