Laser interferometer and light splitting device

Abstract

Provided are a laser interferometer capable of reducing the frequency of a signal calculated and processed in a demodulation circuit and reducing the cost of the demodulation circuit, and a spectroscopic device provided with the laser interferometer. A laser interferometer is provided with: an optical modulator that adds a modulation signal to laser light using a resonator element; a light receiving element that detects a change in the intensity of the laser light including the sampled signal and the modulated signal, and outputs a laser reception signal; a signal oscillation unit that generates a reference signal of a first frequency with the vibration element as a source vibration; and a demodulation circuit that demodulates the sampled signal from the laser reception signal on the basis of the reference signal, the demodulation circuit comprising: a DC offset removal unit that removes the offset; a first phase regulator that regulates the phase of the reference signal; a first multiplier that multiplies the laser reception signal by a reference signal; a first filter; a second multiplier multiplying the first multiplication signal by a reference signal; a second filter; and a phase calculator that calculates a phase.

Description

Technical Field

[0001] The invention relates to a laser interferometer and a spectrometer. Background Art

[0002] Patent Document 1 discloses a laser Doppler measurement device for detecting the motion of a moving object. This device irradiates the object with laser light and measures its motion based on the scattered laser light, which has undergone a Doppler frequency shift. Specifically, the frequency shift of the laser light is determined using optical heterodyne interferometry, and the velocity and displacement of the moving object are determined from this shift.

[0003] The laser Doppler measurement device described in Patent Document 1 includes a frequency-shifting optical modulator. This optical modulator comprises a quartz AT-type oscillator that vibrates in thickness-shear fashion, and a diffraction grating comprising a plurality of grooves arranged in parallel in the quartz AT-type oscillator's displacement direction. The diffraction grating has grooves in a direction intersecting the vibration direction of the quartz AT-type oscillator. When laser light is irradiated onto the diffraction grating, the laser light is diffracted, shifting its frequency.

[0004] Patent Document 1: Japanese Patent Application Laid-Open No. 2020-165700 Summary of the Invention

[0005] However, the resonant frequency of thickness shear vibration is high. Therefore, the frequency of the modulation signal superimposed on the laser light by the optical modulator described in Patent Document 1 also increases. Consequently, the laser Doppler measurement device described in Patent Document 1 requires circuits that process the modulation signal and convert analog signals into digital signals to handle high-frequency signals. This results in increased costs for these circuits.

[0006] Therefore, a technical problem is to realize a laser interferometer capable of reducing the frequency of a signal to be arithmetic-processed by a demodulation circuit and reducing the cost of the demodulation circuit.

[0007] Technical means for solving technical problems

[0008] The laser interferometer involved in the application example of the present invention irradiates a laser onto a target object and receives the laser light that has passed through the target object to obtain the displacement of the target object. The laser interferometer comprises: a laser source that emits the laser light; an optical modulator that has a vibration element and uses the vibration element to add a modulation signal to the laser light; a light receiving element that detects the intensity change of the laser light including the sampling signal and the modulation signal added by the target object and outputs a laser reception signal; a signal oscillator that uses the vibration element as a source oscillator to generate a reference signal of a first frequency; and a demodulation circuit that demodulates the sampling signal from the laser reception signal based on the reference signal to obtain the displacement of the target object. The demodulation circuit comprises: a DC offset removal unit that removes the DC offset. a first phase adjuster for adjusting the phase of the reference signal; a first multiplier for multiplying the laser receiving signal output from the DC offset removal unit by the reference signal output from the first phase adjuster and outputting a first multiplication signal; a first filter for removing high-frequency components included in the first multiplication signal; a second multiplier for multiplying the first multiplication signal by the reference signal output from the first phase adjuster and outputting a second multiplication signal; a second filter for removing high-frequency components included in the second multiplication signal; and a phase calculator for calculating the phase originating from the target as the sampling signal based on the signal output from the first filter and the signal output from the second filter.

[0009] The spectroscopic device involved in the application example of the present invention comprises: the laser interferometer involved in the application example of the present invention; and a spectroscopic analysis unit, which has a spectroscopic optical system including a movable mirror, and generates spectroscopic spectrum information from a sample, wherein the laser interferometer measures the displacement of the movable mirror, and the spectroscopic analysis unit generates the spectroscopic spectrum information based on the measurement result of the displacement of the movable mirror in the laser interferometer. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 This is a functional block diagram showing the laser interferometer according to the first embodiment.

[0011] Figure 2 It shows Figure 1 Schematic diagram of the interference optical system.

[0012] Figure 3 It shows Figure 2 A perspective view of a structural example of a light modulator shown.

[0013] Figure 4 It shows Figure 2 A perspective view of another structural example of the light modulator shown.

[0014] Figure 5 This is a table showing coefficients included in the DC term and the term representing the harmonic component in the series representation of the laser reception signal.

[0015] Figure 6 Is shown by Figure 1 The graph shown is an example of the frequency characteristics of the phase delay amount of the AC component of the high-pass filter.

[0016] Figure 7 It is shown in Figure 6 The graph is a result of simulating the effect of the difference ψ1 in the phase delay amount on the displacement measurement accuracy in the design example shown.

[0017] Figure 8 It is shown in Figure 6 The graph is a simulation result of the effect of the difference ψ1 in the phase delay amount on the accuracy of the displacement in the design example shown.

[0018] Figure 9 This graph shows the frequency characteristics of gain (bandwidth characteristics) and phase delay (phase characteristics) calculated for a high-pass filter constructed using LCR circuits and varying the number of stages to one, two, and three.

[0019] Figure 10 This is a functional block diagram showing a laser interferometer according to the third embodiment.

[0020] Figure 11 This is a functional block diagram showing a laser interferometer according to the fourth embodiment.

[0021] Figure 12 This is a functional block diagram showing a spectroscopic device according to the fifth embodiment.

[0022] Description of Reference Numerals

[0023] 1: Laser interferometer; 2: Laser source; 3: Collimating lens; 4: Light splitter; 6: Half-wavelength plate; 7: Quarter-wavelength plate; 8: Quarter-wavelength plate; 9: Analyzer; 10: Light receiving element; 12: Light modulator; 14: Target; 18: Optical path; 20: Optical path; 22: Optical path; 24: Optical path; 30: Vibration element; 50: Interference optical system; 51: Signal oscillator; 52: Demodulation circuit; 401: Base; 402: First vibrating arm; 403: Second vibrating arm; 404: Electrode; 405: Electrode; 406: Light reflecting portion; 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: first amplitude adjuster; 544: second amplitude adjuster; 546: divider; 548: inverse tangent operator; 550: signal output unit; 900: spectrometer; 910: spectroscopic analysis unit; 920: spectroscopic optical system; 922: analysis light source; 924: movable mirror; 926: analysis light receiving unit; 930: operation unit; 4311: surface; 4312: inner surface; f M : modulation frequency; L1: outgoing light; L1a: first split light; L1b: second split light; L2: reference light; L3: object light; PS1: first calculation path; PS2: second calculation path; Φ1: phase delay amount; Φ2: phase delay amount. DETAILED DESCRIPTION

[0024] Hereinafter, the laser interferometer and the spectroscopic device of the present invention will be described in detail based on the embodiments shown in the drawings.

[0025] 1. First Implementation

[0026] First, a laser interferometer according to a first embodiment will be described.

[0027] Figure 1 1 is a functional block diagram showing the laser interferometer 1 according to the first embodiment. Figure 2 It shows Figure 1 Schematic structural diagram of the interference optical system 50.

[0028] Figure 1 The laser interferometer 1 shown includes an interference optical system 50 , a signal oscillator 51 , and a demodulation circuit 52 .

[0029] Figure 2 The interferometric optical system 50 shown splits the laser light emitted from the laser source 2, and the splits are incident on the target 14 and the optical modulator 12, respectively. The laser light returning from the target 14 and the optical modulator 12 are then mixed and received by the light receiving element 10. The light receiving element 10 detects changes in the intensity of the laser light, which includes the sampling signal (phase information, etc., added to the laser light) added by the target 14 and the modulation signal (frequency information, etc., added to the laser light) added by the optical modulator 12, and outputs a laser reception signal.

[0030] Figure 1 The optical modulator 12 shown includes a vibration element 30. The optical modulator 12 uses the vibration element 30 to add a modulation signal to laser light.

[0031] in addition, Figure 1 The signal oscillating unit 51 shown uses the vibration element 30 as a source vibration to generate a reference signal.

[0032] Figure 1 The demodulation circuit 52 shown demodulates the laser reception signal into a sampling signal based on the reference signal, thereby acquiring the displacement of the target object 14 and the like.

[0033] 1.1. Interference optical system

[0034] Figure 2 The interference optical system 50 shown is a Michelson type interference optical system. Figure 2 As shown, the interference optical system 50 includes a laser light source 2 , a collimating lens 3 , a beam splitter 4 , a half-wave plate 6 , a quarter-wave plate 7 , a quarter-wave plate 8 , an analyzer 9 , and a light receiving element 10 .

[0035] Laser source 2 emits outgoing light L1 at frequency f0. Light receiving element 10 converts the intensity of the received light into an electrical signal. Optical modulator 12 uses oscillator 30 to change the frequency of outgoing light L1, generating reference light L2 (laser light containing the modulated signal) containing the modulation signal. Meanwhile, outgoing light L1 incident on target 14 is reflected as object light L3 (laser light containing the sampling signal) containing the sampling signal originating from target 14.

[0036] The optical path connecting the optical splitter 4 and the laser source 2 is optical path 18. The optical path connecting the optical splitter 4 and the optical modulator 12 is optical path 20. The optical path connecting the optical splitter 4 and the target 14 is optical path 22. The optical path connecting the optical splitter 4 and the light receiving element 10 is optical path 24. It should be noted that the term "optical path" in this specification refers to a path for light propagation established between optical elements.

[0037] On optical path 18, a half-wave plate 6 and a collimator lens 3 are arranged in order from the beam splitter 4 side. On optical path 20, a quarter-wave plate 8 is arranged. On optical path 22, a quarter-wave plate 7 is arranged. On optical path 24, an analyzer 9 is arranged.

[0038] Light L1 emitted from laser source 2 is split into two by optical splitter 4 via optical path 18. One of the split light beams L1, first split light L1a, enters optical modulator 12 via optical path 20. Furthermore, the other of the split light beams L1, second split light L1b, enters target object 14 via optical path 22. Reference light L2, generated by frequency shifting by optical modulator 12, enters light receiving element 10 via optical paths 20 and 24. Object light L3, generated by reflection from target object 14, enters light receiving element 10 via optical paths 22 and 24.

[0039] In the laser interferometer 1 equipped with the interference optical system 50 described above, phase information of the target object 14 is determined using optical heterodyne interferometry. Specifically, two light beams (reference light L2 and object light L3) with slightly different frequencies are caused to interfere with each other. Phase information is then extracted in the demodulation circuit 52 based on the intensity of the interfering light, and the displacement of the target object 14 is determined based on this phase information. Optical heterodyne interferometry makes extracting phase information from the interfering light less susceptible to interference, particularly stray light at frequencies that can cause noise, resulting in high robustness.

[0040] Hereinafter, each component of the interference optical system 50 will be further described.

[0041] Laser source

[0042] The laser source 2 is a laser source that emits coherent light L1. A light source with a linewidth in the MHz band or below is preferably used as the laser source 2. Specifically, examples include gas lasers such as He-Ne lasers, semiconductor laser elements such as DFB-LDs (Distributed Feedback Laser Diodes), FBG-LDs (Fiber Bragg Grating Laser Diodes), VCSELs (Vertical Cavity Surface Emitting Lasers), and FP-LDs (Fabry-Perot Laser Diodes).

[0043] The laser light source 2 is particularly preferably a semiconductor laser element. This allows the laser light source 2 to be particularly miniaturized. Consequently, the laser interferometer 1 can be miniaturized.

[0044] 1.1.2. Collimating lens

[0045] The collimating lens 3 is an optical element disposed between the laser source 2 and the light splitter 4. An aspherical lens can be cited as an example. The collimating lens 3 collimates the light L1 emitted from the laser source 2. It should be noted that, if the light L1 emitted from the laser source 2 is sufficiently collimated, for example, when a gas laser such as a He-Ne laser is used as the laser source 2, the collimating lens 3 can be omitted.

[0046] The collimated outgoing light L1 passes through the half-wave plate 6 , is converted into linearly polarized light having an intensity ratio of P-polarized light to S-polarized light of, for example, 50:50, and enters the light splitter 4 .

[0047] 1.1.3. Optical Splitter

[0048] The light splitter 4 is a polarization beam splitter disposed between the laser source 2 and the optical modulator 12, and between the laser source 2 and the target 14. The light splitter 4 transmits P-polarized light and reflects S-polarized light. This function allows the light splitter 4 to split the emitted light L1 into first split light L1a, which is reflected light from the light splitter 4, and second split light L1b, which is transmitted light from the light splitter 4.

[0049] The first split light L1a, which is the S-polarized light reflected by the light splitter 4, is converted into circularly polarized light by the quarter-wave plate 8 and enters the light modulator 12. The first split light L1a entering the light modulator 12 is subjected to the f M [Hz], and is reflected as reference light L2. Therefore, the reference light L2 includes the modulation frequency f M That is, the frequency of the reference light L2 is f0 + f M The reference light L2 is converted into 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 splitter 4 and the analyzer 9 and enters the light receiving element 10.

[0050] The second split light L1b, which is the P-polarized light transmitted through the light splitter 4, is converted into circularly polarized light by the quarter-wave plate 7 and enters the moving target 14. The second split light L1b entering the target 14 is affected by f D [Hz] Doppler frequency shift, and is reflected as object light L3. Therefore, object light L3 includes Doppler frequency f D [Hz] sampling signal. That is, the frequency of the object light L3 is f0-f DThe object light L3 is converted into S-polarized light when it passes through the quarter-wave plate 7 again. The S-polarized light of the object light L3 is reflected by the light splitter 4 , passes through the analyzer 9 , and is incident on the light receiving element 10 .

[0051] Since the outgoing light L1 has coherence, the reference light L2 and the object light L3 are incident on the light receiving element 10 as interfering light.

[0052] 1.1.4. Polarization Analyzer

[0053] Since the orthogonal S-polarized light and P-polarized light are independent of each other, simply superimposing them will not cause interference-induced beats. Therefore, the light waves obtained by superimposing the S-polarized light and the P-polarized light pass through the analyzer 9, which is tilted at 45° relative to both the S-polarized light and the P-polarized light. By using the analyzer 9, the common components of light are transmitted, causing interference. As a result, the reference light L2 and the object light L3 interfere with each other in the analyzer 9, generating a light having |f M -f D |The interference light of the beat frequency.

[0054] 1.1.5. Light-receiving element

[0055] If the interference light is incident on the light receiving element 10, the light receiving element 10 outputs a photocurrent (laser reception signal) corresponding to the intensity of the interference light. By demodulating the sampling signal from the laser reception signal using the method described below, the movement of the target object 14, that is, the displacement and speed, can be finally determined. As the light receiving element 10, for example, a light emitting diode can be listed. It should be noted that what is received by the light receiving element 10 is the laser light emitted from the laser source 2, and the laser light is superimposed with the modulation signal and the sampling signal because its frequency and phase are modulated by the optical modulator 12 and the target object 14, and is not limited to the above-mentioned interference light. In addition, "demodulating the sampling signal from the laser reception signal" in this specification means performing various operations on the laser reception signal to extract the sampling signal.

[0056] 1.2. Optical Modulator

[0057] Figure 1 The light modulator 12 shown in FIG. 1 includes a vibration element 30. Figure 2As shown, the optical modulator 12 uses a vibrating element 30 to modulate the frequency of the first segmented light L1a. This configuration enables miniaturization, weight reduction, and low power consumption of the optical modulator 12. Furthermore, the oscillation of the vibrating element 30 serves as the source oscillation for the signal oscillator 51 to generate the reference signal. Therefore, both the modulated signal added to the reference light L2 by the vibrating element 30 and the reference signal output from the signal oscillator 51, using the vibrating element 30 as the source oscillation, originate from the vibration energy of the vibrating element 30. Therefore, even if disturbances such as shock or noise are applied to the optical modulator 12, causing the vibration of the vibrating element 30 to change, both the modulated signal and the reference signal change in the same manner. This cancels out or reduces the effects of interference on both sides during the computational processing in the demodulation circuit 52. Consequently, a decrease in the signal-to-noise ratio (S / N) of the sampled signal demodulated by the demodulation circuit 52 can be suppressed.

[0058] The vibration element 30 is a resonator that generates a periodic signal, such as a crystal resonator, a ceramic resonator, or a Si resonator. These resonators utilize a mechanical resonance phenomenon, and therefore have a high Q value and excellent stability in vibration frequency.

[0059] Examples of the crystal oscillator include a crystal AT oscillator, an SC-cut crystal oscillator, a tuning fork-type crystal oscillator, and a crystal surface acoustic wave device. The oscillation frequency of the crystal oscillator is, for example, about 1 kHz to several hundred MHz.

[0060] A silicon resonator is a resonator composed of a single-crystal silicon wafer and a piezoelectric film, manufactured using MEMS technology from a single-crystal silicon substrate. MEMS (Micro Electro Mechanical Systems) refers to microelectromechanical systems. Examples of single-crystal silicon wafer shapes include cantilever beams (two-legged tuning fork, three-legged tuning fork, and double cantilever beams). The oscillation frequency of a silicon resonator ranges from 1 kHz to several hundred MHz.

[0061] A ceramic resonator is made of piezoelectric ceramic sheets and electrodes, manufactured by sintering and hardening piezoelectric ceramics. Examples of piezoelectric ceramics include lead zirconate titanate (PZT) and barium titanate (BTO). The oscillation frequency of a ceramic resonator ranges from several hundred kHz to several tens of MHz.

[0062] Among them, a crystal oscillator is preferably used as the vibration element 30. Since the crystal of the crystal oscillator itself is a piezoelectric material, the frequency stability is particularly high.

[0063] The oscillation frequency of the vibration element 30 is not particularly limited, but is preferably between 1 MHz and 100 MHz. Within the frequency range, there are many vibrators with high mechanical resonance Q values. Therefore, by setting the oscillation frequency within the range, the reference signal I output from the signal oscillator 51 can be realized. S1 The first frequency f M stabilization.

[0064] Figure 3 It shows Figure 2 A perspective view of a structural example of the light modulator 12 is shown.

[0065] As Figure 3 The optical modulator 12 shown in the figure can be, for example, the optical modulator disclosed in Japanese Patent Application Laid-Open No. 2022-38156. Specifically, Figure 3 The illustrated optical modulator 12 includes a vibration element 30 and a diffraction grating 434 provided on the vibration element 30 and diffracting the first divided light L1 a (divided laser light).

[0066] Figure 3 The illustrated resonator element 30 is a quartz crystal AT resonator that undergoes thickness-shear vibration along a vibration direction 436 in the high-frequency region of the MHz band. Furthermore, a diffraction grating 434 is provided in the resonator element 30. The diffraction grating 434 has a plurality of linear grooves 432 extending in a direction intersecting the vibration direction 436. When the diffraction grating 434 is irradiated with the first segmented light L1a, even when the resonator element 30 undergoes thickness-shear vibration, the frequency of the first segmented light L1a can be modulated to generate reference light L2.

[0067] The vibration element 30 includes a surface 4311 and an inner surface 4312 having an inner-outer relationship with each other. A diffraction grating 434 is arranged on the surface 4311. In addition, a first electrode 437 for applying a voltage to the vibration element 30 and a pad 433 electrically connected to the first electrode 437 are provided on the surface 4311. On the other hand, a second electrode 438 for applying a voltage to the vibration element 30 and a pad 435 electrically connected to the second electrode 438 are provided on the inner surface 4312. The first electrode 437 and the second electrode 438 are arranged so as to overlap with each other across the vibration element 30 when looking down at the surface 4311. In addition, the pad 433 and the pad 435 are arranged so as not to overlap with each other across the vibration element 30. If a voltage is applied between the first electrode 437 and the second electrode 438, thickness shear vibration is induced at the overlapping portion of the first electrode 437 and the second electrode 438.

[0068] Figure 3 The diffraction grating 434 shown is arranged on the first electrode 437. That is, Figure 3The diffraction grating 434 is formed by a plurality of grooves 432 formed on the surface of the first electrode 437. When the diffraction grating 434 is irradiated with the first split light L1a, the reference light L2 as diffracted light is emitted.

[0069] As an example, Figure 3 The diffraction grating 434 shown is a blazed diffraction grating. A blazed diffraction grating is a grating having a stepped cross-sectional shape. It should be noted that the shape of the diffraction grating 434 is not limited to this.

[0070] Figure 4 It shows Figure 2 The perspective view of another structural example of the light modulator 12 shown in FIG. Figure 4 In the diagram, the A-axis, B-axis, and C-axis are set as three mutually orthogonal axes and are indicated by arrows. The tip of the arrow is set as "positive" and the base of the arrow is set as "negative."

[0071] Figure 4 The vibration element 30 shown is a tuning fork type crystal resonator. Figure 4 The vibration element 30 shown has a vibration substrate having a base 401, a first vibration arm 402, and a second vibration arm 403. Such a tuning fork type crystal vibrator can be easily obtained and has stable oscillation because the manufacturing technology is already established. Therefore, the tuning fork type crystal vibrator is preferably used as the vibration element 30. In addition, Figure 4 The illustrated optical modulator 12 includes a vibration element 30 , electrodes 404 and 405 provided on the vibration element 30 , and a light reflecting portion 406 .

[0072] The base 401 extends along the A-axis. The first vibrating arm 402 extends from the A-axis negative end of the base 401 toward the B-axis positive end. The second vibrating arm 403 extends from the A-axis positive end of the base 401 toward the B-axis positive end.

[0073] The electrode 404 is a conductive film provided on the side surface parallel to the AB plane in the first vibration arm 402 and the second vibration arm 403. Figure 4 Although not shown in the figure, the electrodes 404 are provided on the side surfaces facing each other, and voltages are applied with different polarities to drive the first vibrating arm 402 and the second vibrating arm 403 .

[0074] The electrode 405 is a conductive film provided on the side surface intersecting the AB plane in the first vibration arm 402 and the second vibration arm 403. Figure 4 Although not shown in the figure, the electrodes 405 are also provided on the side surfaces facing each other, and voltages are applied with different polarities to drive the first vibrating arm 402 and the second vibrating arm 403 .

[0075] The light reflecting portion 406 is provided on a side surface of the first vibrating arm 402 and the second vibrating arm 403, for example, intersecting the AB plane, and has the function of reflecting the first segmented light L1a. Due to this function, the light reflecting portion 406 has a vibration component with a large amplitude in the incident direction of the incident first segmented light L1a, thereby efficiently modulating the frequency of the first segmented light L1a to generate reference light L2.

[0076] In a tuning fork type crystal oscillator, a crystal piece cut from a crystal substrate is used. Examples of the crystal substrate used in the manufacture of a tuning fork type crystal oscillator include a crystal Z-cut plate. Figure 4 In the embodiment, an X-axis parallel to the A-axis, a Y'-axis parallel to the B-axis, and a Z'-axis parallel to the C-axis are set. A crystal Z-cutting plate is, for example, a substrate cut from a single crystal of crystal in such a manner that the X-axis is an electrical axis, the Y'-axis is a mechanical axis, and the Z'-axis is an optical axis. Specifically, it is preferred that a substrate having a principal surface with an X-Y' plane formed by the X-axis and the Y'-axis tilted counterclockwise by about 1° to 5° around the X-axis is cut from a single crystal of crystal in an orthogonal coordinate system formed by the X-axis, the Y'-axis, and the Z'-axis, and used as a crystal substrate. Then, by etching such a crystal substrate, a substrate for crystal cutting is obtained. Figure 4 The crystal piece of the vibration element 30 is shown.

[0077] 1.3.Signal Oscillation Unit

[0078] Figure 1 The signal oscillator 51 shown uses the vibration element 30 as a source vibration to generate a first frequency f M The reference signal I S1 .

[0079] As the signal oscillator 51, for example, an oscillation circuit using an inverter, a Colpitts oscillation circuit, etc., can be cited. These oscillation circuits operate using the fundamental wave oscillation of the vibration element 30 as a source oscillation. Therefore, by using the vibration element 30 with a high Q value of mechanical resonance, a reference signal I with high frequency stability can be generated. S1 .

[0080] It should be noted that the optical modulator 12 and the signal oscillator 51 can be housed in a single package, thereby reducing the physical distance between them and suppressing the influence of noise and the like.

[0081] 1.4. Demodulation Circuit

[0082] First, the configuration of the demodulation circuit 52 will be described.

[0083] 1.4.1. Structure of the demodulation circuit

[0084] Figure 1The demodulation circuit 52 shown 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 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 first amplitude adjuster 542 and a second amplitude adjuster 544 (amplitude adjustment unit), a divider 546, an inverse tangent operator 548 (phase calculator), and a signal output unit 550.

[0085] The current-to-voltage converter 520 , also called a transimpedance amplifier (TIA), converts the photocurrent output from the light-receiving element 10 into a voltage signal and outputs the voltage signal as a laser reception signal.

[0086] The high-pass filter 522 removes the offset of the direct current component (DC offset) of the laser reception signal, thereby obtaining a laser reception signal composed of an alternating current component.

[0087] The bandpass filter 524 allows only the first frequency f to be transmitted to the reference signal output from the signal oscillator 51. M Thus, a reference signal is obtained from which unnecessary frequency components (noise components) are removed. It should be noted that, when the unnecessary frequency components in the reference signal output from the signal oscillator 51 are small, the bandpass filter 524 can be omitted.

[0088] The first phase adjuster 526 adjusts the phase of the reference signal output from the bandpass filter 524. Specifically, the first phase adjuster 526 adjusts the phase of the reference signal to be in phase with the phase of the fundamental frequency component of the modulation signal included in the laser reception signal. The reference signal output from the first phase adjuster 526 is split into two at a branch point 527.

[0089] The first multiplier 530 multiplies the laser reception signal output from the high-pass filter 522 by the reference signal output from the first phase adjuster 526. This generates a first multiplied signal. The first multiplied signal is split at a branch point 531 into a first operation path PS1 and a second operation path PS2.

[0090] The second multiplier 532 multiplies the first multiplication signal divided into the second operation path PS2 by the reference signal output from the first phase adjuster 526. Thus, a second multiplication signal is obtained.

[0091] The low-pass filter 534 cuts off the high-frequency components of the first multiplication signal divided into the first operation path PS1, thereby obtaining a first multiplication signal including low-frequency components. It should be noted that the low-pass filter 534 may also be a band-pass filter.

[0092] The low-pass filter 536 cuts off the high-frequency component of the second multiplication signal output from the second multiplier 532, thereby obtaining a second multiplication signal including low-frequency components. It should be noted that the low-pass filter 536 may also be a band-pass filter.

[0093] It should be noted that the high-pass filter 522 (DC offset removal unit), band-pass filter 524 , first phase adjuster 526 , first multiplier 530 , second multiplier 532 , low-pass filter 534 , and low-pass filter 536 are components of analog circuits.

[0094] The A / D converter 538 digitally converts the analog first multiplication signal output from the low-pass filter 534 to obtain a digital first multiplication signal.

[0095] The A / D converter 540 digitally converts the analog second multiplication signal output from the low-pass filter 536 to obtain a digital second multiplication signal.

[0096] The first amplitude adjuster 542 and the second amplitude adjuster 544 adjust the amplitudes of the first multiplication signal and the second multiplication signal so that the amplitudes of the input signals match. This optimizes the demodulation accuracy of phase X. It should be noted that these can be set as needed and, for example, can be omitted when the amplitude deviation of the input signals is small. Furthermore, when adjustment can be performed using only one of the first amplitude adjuster 542 or the second amplitude adjuster 544, the other can be omitted.

[0097] The divider 546 divides the first multiplication signal output from the first amplitude adjuster 542 by the second multiplication signal output from the second amplitude adjuster 544. This obtains a division signal.

[0098] The arctangent operator 548 performs an arctangent operation on the divided signal output from the divider 546 , thereby calculating a phase as sampling information from the target object 14 .

[0099] The signal output unit 550 performs phase connection such as expansion processing on the phases from the target object 14. In addition, the displacement and velocity of the target object 14 are calculated as needed.

[0100] It should be noted that the first amplitude adjuster 542, the second amplitude adjuster 544, the divider 546, the arctangent calculator 548, and the signal output unit 550 described above may also constitute analog circuits, but are preferably components of digital circuits. Such digital circuits are implemented in electronic devices such as FPGAs (programmable logic devices), ASICs (application-specific integrated circuits), and microcomputers.

[0101] 1.4.2. Demodulation Processing

[0102] Next, the operation (demodulation processing) of demodulation circuit 52 will be described. The following description uses as an example a case where a signal with a sinusoidal frequency variation is used as the modulation signal and the displacement of target object 14 is a single oscillation in the optical axis direction.

[0103] The laser reception 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 reception signal. Thus, a laser reception signal consisting of an AC component is obtained. The AC component is taken as I PD.AC It should be noted that in the following description, the AC component I PD.AC The laser receiving signal I output from the high-pass filter 522 is called PD.AC The laser receiving signal I output from the high-pass filter 522 PD.AC It is represented by the following formula (1).

[0104] [Formula 1]

[0105]

[0106] In the above formula (1), A is the amplitude. In addition, Φ M is the phase from the optical modulator 12. Furthermore, X is the phase given by the following equation (1a).

[0107] [Formula 2]

[0108]

[0109] In the above formula (1a), Φ S is the phase originating from the target 14 , and Φ0 is the initial phase difference based on the optical path difference within the interference optical system 50 .

[0110] Figure 2 The optical modulator 12 shown generates a signal including a modulation frequency f M The reference light L2 of the modulated signal. Therefore, Φ M It is given by the following formula (1b).

[0111] [Formula 3]

[0112]

[0113] In the above equation (1b), B is a modulation phase shift in frequency modulation by the optical modulator 12. Also, t is time.

[0114] When the above formula (1b) is used, the above formula (1) is expressed by the following formula (1c).

[0115] [Formula 4]

[0116]

[0117] The right side of equation (1c) includes a trigonometric function that changes the angle portion with time. In this case, if expressed using a Bessel function series, the right side of equation (1c) can be expanded as shown in equation (1d).

[0118] [Formula 5]

[0119]

[0120] The above formula (1d) is divided into two parts, excluding the modulation angular frequency ω M The term (DC term) and the term including sinω M t term (sinω M t term), including cos2ω M t term (cos2ω M t term) such harmonic components. Figure 5 A table summarizing these terms and the coefficients included in each term is shown.

[0121] Figure 5 This is a list showing coefficients included in the terms representing the DC term and harmonic components in the series representation of the laser reception signal.

[0122] like Figure 5 As shown, the coefficients of each term include cosX or sinX. In the demodulation circuit 52, these are extracted by operation, thereby finally calculating the phase Φ originating from the target object 14. S In particular, in this embodiment, sinω is extracted M The coefficients of the t term include sinX and cos2ω M The coefficient of the t term includes cosX, and the final phase X is obtained.

[0123] Here, the reference signal I output from the first phase adjuster 526 is S1 It is represented by the following formula (2).

[0124] [Formula 6]

[0125]

[0126] In the above formula (2), V q is the amplitude. In addition, ω M is the modulation frequency f M The angular frequency (modulation angular frequency) is 2πf M .

[0127] In the first multiplier 530, the laser reception signal I output from the high-pass filter 522 is multiplied by PD.AC and the reference signal I output from the first phase adjuster 526 S1 Multiplication. This multiplication (first multiplication) is the operation of multiplying the above equation (1d) by the above equation (2). Thus, the first multiplication signal is obtained. The first multiplication signal is Figure 1 The high frequency component of the first multiplication signal divided into the first operation path PS1 is cut off by the low pass filter 534. The cut-off angular frequency of the low pass filter 534 is set to, for example, ω M / 2. Thus, the first multiplication signal output from the low-pass filter 534 becomes Figure 5 The signal is composed of the DC component shown. This component includes sinX but does not include the modulation angular frequency ω M Therefore, through the above processing, the first multiplication signal is lowered in frequency.

[0128] The first multiplication is shown in Figure 5 .exist Figure 5 In FIG, arrows are used to schematically show how the coefficients sinX and cosX change through the first multiplication and the second multiplication described later. If the first multiplication is performed, sinω M The sinX included in the coefficient of the t term is transferred to the coefficient of the DC term and cos2ω M The coefficient of the t term. In addition, cos2ω M The cosX included in the coefficient of the t term is converted to sinω M The coefficient of the t term and sin3ω M The coefficient of the t term. Similarly, for other terms, sinX or cosX shifts from the base end to the end of the arrow.

[0129] Next, if the harmonic components are removed by the low-pass filter 534, only the DC term is output. That is, Figure 5 The components of the DC term of the coefficients surrounded by the thick frame of the solid line in the result of the first multiplication shown are output from the low-pass filter 534. The first multiplication signal I output from the low-pass filter 534 is het1It is represented by the following formula (3).

[0130] [Formula 7]

[0131]

[0132] In the second multiplier 532, the first multiplication signal I divided into the second operation path PS2 is multiplied by het1 and the reference signal I output from the first phase adjuster 526 S1 Multiplication. This multiplication (second multiplication) is an operation of multiplying the first multiplication signal by the above formula (2). Thus, a second multiplication signal is obtained. The high-frequency component of the second multiplication signal is cut off by the low-pass filter 536. The cutoff angular frequency of the low-pass filter 536 is set to, for example, ω M / 2. Thus, the second multiplication signal output from the low-pass filter 536 becomes Figure 5 The signal is composed of the DC component shown. This component includes cosX but does not include the modulation angular frequency ω M Therefore, through the above processing, the second multiplication signal is lowered in frequency.

[0133] The second multiplication is shown in Figure 5 If the second multiplication is performed, then the result of the first multiplication, sinω M The coefficients of the t term (the coefficients surrounded by the thick dashed box) include cosX, which is transferred to the coefficients of the DC term and cos2ω. M The coefficient of the t term. Similarly, for other terms, sinX or cosX shifts from the base end to the end of the arrow.

[0134] Next, if the harmonic components are removed by the low-pass filter 536, only the DC term is output. That is, Figure 5 The components of the DC term of the coefficients enclosed by the thick frame of the solid line in the result of the second multiplication shown are output from the low-pass filter 536. The second multiplication signal I output from the low-pass filter 536 is het2 It is represented by the following formula (4).

[0135] [Formula 8]

[0136]

[0137] By the above method, the first multiplication signal and the second multiplication signal can be respectively lowered in frequency, and the first multiplication signal I including the component of sinX can be converted to het1 and a second multiplication signal I consisting of components including cosX het2 The signal is output to the A / D converter 538 and the A / D converter 540 .

[0138] It should be noted that in Figure 5 In the coefficients enclosed by a thick dashed frame in the result of the first multiplication shown, the shift of cosX indicated by the dashed arrows and the shift of cosX indicated by the thick solid arrows overlap. In this case, the shifted cosX indicated by the dashed arrows becomes a noise component relative to the shifted cosX indicated by the solid arrows, and therefore needs to be removed.

[0139] Therefore, in this embodiment, the high-pass filter 522 is provided in the demodulation circuit 52. As mentioned above, the high-pass filter 522 removes the DC offset of the laser reception signal. Figure 5 The underlined coefficients (DC term coefficients) in [C] are shown. Therefore, the shift of cosX indicated by the dotted arrows can be prevented. As a result, the superposition of noise components can be prevented.

[0140] The first multiplication signal I is digitally converted by the A / D converter 538. het1 The second multiplication signal I is input to the first amplitude regulator 542. The second multiplication signal I is digitally converted by the A / D converter 540. het2 The first amplitude adjuster 542 and the second amplitude adjuster 544 adjust the amplitude of the input signals so that the amplitudes match each other. Specifically, in the first amplitude adjuster 542, J2(b) V q Multiplication is performed, and 2J1(b) is multiplied in the second amplitude regulator 544. Thus, the first multiplication signal I outputted from the first amplitude regulator 542 is het1 and the second multiplication signal I output from the second amplitude regulator 544 het2 They are given by the following formulas (5) and (6) respectively.

[0141] [Formula 9]

[0142]

[0143] [Formula 10]

[0144]

[0145] The divider 546 performs multiplication on the first multiplication signal I output from the first amplitude regulator 542. het1 divided by the second multiplication signal I output from the second amplitude adjuster 544 het2 Thus, a division signal is obtained.

[0146] The arc tangent operator 548 performs an arc tangent operation on the divided signal output from the divider 546. The arc tangent operation result I atan It is represented by the following formula (7).

[0147] [Formula 11]

[0148]

[0149] According to the arc tangent operation result I expressed by the above formula (7) atan Find the phase X.

[0150] In the demodulation process described above, the first multiplication signal I input to the A / D converter 538 and the A / D converter 540 is het1 and the second multiplication signal I het2 The frequencies of the M That is, frequency down-conversion is performed. This can reduce the corresponding frequency of the A / D converter 538 and the A / D converter 540, and reduce the cost of the A / D converter 538 and the A / D converter 540.

[0151] Furthermore, if the digital circuit is implemented in, for example, an FPGA, the down-conversion can also reduce the corresponding frequency of the FPGA, etc. Therefore, the down-conversion can also contribute to the cost reduction of electronic components such as FPGA.

[0152] Furthermore, the modulation frequency f of the optical modulator 12 corresponding to the frequency limitation can be controlled. M For example, it is easy to use the modulation frequency f that is difficult to adopt in the prior art with the above-mentioned corresponding frequency relationship. M A very high-frequency vibration element, for example, a vibration element having an oscillation frequency in the MHz band, is used for the optical modulator 12. This can broaden the selection of applicable vibration elements.

[0153] Furthermore, the number of multipliers in the analog circuit is kept to a minimum of two, thereby suppressing the influx of noise associated with multiplication and enabling the calculation of the phase X with high accuracy.

[0154] 2. Second Implementation

[0155] Next, a laser interferometer according to a second embodiment will be described.

[0156] Hereinafter, the second embodiment will be described. However, in the following description, the differences from the above-described embodiment will be mainly described, and descriptions of the same matters will be omitted.

[0157] The second embodiment is the same as the first embodiment except that the structure of the high-pass filter 522 is different.

[0158] The DC offset of the laser reception signal is removed in the high-pass filter 522, and the AC component is passed. However, depending on the frequency characteristics of the phase delay of the AC component passing through the high-pass filter 522, the phase of the AC component may be delayed by the sinω M t term and cos2ω M As the difference in the phase delay amount in the t term increases, the demodulation precision and accuracy of the phase X deteriorate.

[0159] Figure 6 Is shown by Figure 1 The graph shows an example of the frequency characteristics of the phase delay of the AC component of the high-pass filter 522. The horizontal axis is frequency. The left vertical axis is gain. The right vertical axis is the phase delay when the gain is zero as a reference. Figure 6 The passband shown is a band designed according to the frequency of the AC component passing through the high-pass filter 522. Figure 6 In the example shown, it is assumed that the modulation frequency f M The passband is set to 5 MHz, with 5 MHz as the lower limit and 10 MHz (twice the upper limit) as the upper limit. By minimizing the difference in phase delay within this passband, it is possible to suppress the degradation of the demodulation precision and accuracy of the phase X.

[0160] Here, the reason why the difference in the phase delay amount within the band affects the demodulation accuracy of the phase X will be considered.

[0161] exist Figure 6 In the example, the phase delay when the 5MHz component passes is Φ1, and the phase delay when the 10MHz component (twice the 5MHz) passes is Φ2. In this way, the difference in phase delay ψ1 within the passband is given by |Φ1-Φ2|. atan The difference in phase delay ψ1 has an influence expressed by the following equation (8).

[0162] [Formula 12]

[0163]

[0164] The right side of the above equation (8) includes cosψ1, which is different from the right side of the above equation (7). Therefore, it can be seen that this difference becomes an error factor that deteriorates the demodulation accuracy of the phase X.

[0165] Therefore, in this embodiment, the high-pass filter 522 is set so as to satisfy the following equation (9) with respect to the difference ψ1 between the phase delay amounts.

[0166] [Formula 13]

[0167]

[0168] Furthermore, it is preferable to set the high-pass filter 522 so as to satisfy the following formula (10).

[0169] [Formula 14]

[0170]

[0171] According to such a configuration, the demodulation precision and accuracy of the phase X are optimized. As a result, the measurement precision of the displacement of the target object 14 and the accuracy of the measured displacement are optimized.

[0172] Figure 7 It is shown in Figure 6 The graph is a result of simulating the effect of the phase delay difference ψ1 on the displacement measurement accuracy in the design example shown.

[0173] like Figure 7 As shown in the figure, when the phase delay difference ψ1 is 10 degrees or less, the displacement measurement accuracy is suppressed to less than 1 nm. Therefore, by setting the phase delay difference ψ1 within the above range, sufficient measurement accuracy can be achieved.

[0174] Figure 8 It is shown in Figure 6 The graph is a result of simulating the effect of the phase delay difference ψ1 on the displacement accuracy in the design example shown.

[0175] like Figure 8 As shown, when the phase delay difference ψ1 is 1 degree or less, the accuracy of the measured displacement is suppressed to within 100±0.01%. Therefore, by setting the phase delay difference ψ1 within this range, sufficient measurement accuracy can be achieved.

[0176] As described above, methods for suppressing the phase delay difference ψ1 within a predetermined range include, for example, changing the constants of the elements of the LCR circuit, increasing the number of stages, and lowering the cutoff frequency in the design of the high-pass filter 522.

[0177] Figure 9 This graph calculates the gain frequency characteristics (bandwidth characteristics) and phase delay frequency characteristics (phase characteristics) when the number of stages in a highpass filter composed of LCR circuits is changed to one, two, and three. Changing the number of stages shifts the cutoff frequency of the bandwidth characteristics and the phase characteristics.

[0178] exist Figure 9In the design example shown, when the number of LCR circuit stages is set to three, the phase delay difference ψ1 is suppressed to less than 10 degrees. By adjusting the LCR circuit design, the phase delay difference ψ1 can be suppressed to less than 10 degrees and even less than 1 degree.

[0179] In the second embodiment described above, the same effects as those of the first embodiment are obtained.

[0180] 3. Modification of the Second Embodiment

[0181] Next, a laser interferometer 1 according to a modified example of the second embodiment will be described.

[0182] Hereinafter, a modified example of the second embodiment will be described. However, in the following description, the differences from the second embodiment will be mainly described, and descriptions of the same matters will be omitted.

[0183] In this variation of the second embodiment, the signal input to the second amplitude adjuster 544 is additionally divided by cos ψ1. This division by cos ψ1 is equivalent to an adjustment that cancels the amplitude change reflecting the phase delay difference ψ1. This cancels or reduces the error factor represented by equation (8). As a result, the demodulation accuracy and precision of phase X are further optimized compared to the second embodiment.

[0184] Furthermore, by adding the above-mentioned calculation, the burden of designing the high-pass filter 522 can be reduced.

[0185] In the above-described modified example, the same effects as those of the second embodiment can be obtained.

[0186] 4. Third Implementation

[0187] Next, a laser interferometer 1 according to a third embodiment will be described.

[0188] Figure 10 This is a functional block diagram showing a laser interferometer 1 according to the third embodiment.

[0189] Hereinafter, a third embodiment will be described. However, in the following description, the differences from the above-described embodiment will be mainly described, and descriptions of the same matters will be omitted.

[0190] The third embodiment is the same as the first embodiment except for the configuration of the demodulation circuit 52. Specifically, in the third embodiment, a second phase adjuster 528 is added to the demodulation circuit 52.

[0191] Figure 10The second phase adjuster 528 is provided between the branch point 527 and the second multiplier 532. That is, the second phase adjuster 528 further adjusts the phase of the reference signal whose phase has been adjusted by the first phase adjuster 526.

[0192] As in the first embodiment, the first phase adjuster 526 adjusts the phase of the reference signal so that it is in phase with the fundamental frequency component of the modulated signal included in the laser reception signal. In contrast, the second phase adjuster 528 adjusts the phase of the reference signal output from the first phase adjuster 526 and branched off at the branch point 527 to cancel out the phase delay difference ψ1 in the high-pass filter 522. Thus, even if the phase delay difference ψ1 exists in the high-pass filter 522, its effect can be canceled or reduced in the second multiplication performed by the second multiplier 532.

[0193] The above effect is verified by calculation below.

[0194] With the passage of the high-pass filter 522, the laser receiving signal I PD.AC The influence of the generated phase delay difference ψ1 is expressed by the following equation (1e).

[0195] [Formula 15]

[0196]

[0197] In the first multiplier 530, the laser reception signal I output from the high-pass filter 522 is multiplied by PD.AC and the reference signal I output from the first phase adjuster 526 S1 Thus, a first multiplication signal is obtained. The first multiplication signal I het1 It is represented by the following formula (11).

[0198] [Formula 16]

[0199]

[0200] Then, if the harmonic components are removed by the low-pass filter 534, only the DC term is output. In addition, the amplitude is adjusted by the first amplitude adjuster 542. As a result, the first multiplication signal I output from the first amplitude adjuster 542 is het1 It is represented by the following formula (12).

[0201] [Formula 17]

[0202]

[0203] On the other hand, as a result of the phase adjustment by the second phase adjuster 528, the reference signal I output from the second phase adjuster 528 is S2 It is represented by the following formula (13).

[0204] [Formula 18]

[0205]

[0206] In the second multiplier 532, the first multiplication signal I divided into the second operation path PS2 is multiplied by het1 The second multiplication signal I is multiplied by the reference signal IS2 output from the second phase adjuster 528. het2 The second multiplication signal I het2 It is represented by the following formula (14).

[0207] [Formula 19]

[0208]

[0209] Then, if the harmonic components are removed by the low-pass filter 536, only the DC term is output. In addition, the amplitude is adjusted by the second amplitude adjuster 544. As a result, the second multiplication signal I output from the second amplitude adjuster 544 is het2 It is represented by the following formula (15).

[0210] [Formula 20]

[0211]

[0212] As described above, the influence of the phase delay difference ψ1 is eliminated in both equations (12) and (15). Therefore, by providing second phase adjuster 528, the influence of the phase characteristics of high-pass filter 522 can be suppressed. As a result, the demodulation accuracy and precision of phase X are further optimized. Furthermore, the design burden of high-pass filter 522 can be reduced.

[0213] Here, the phase adjustment amount in the second phase adjuster 528 is set to ψ2. Ideally, the phase adjustment amount ψ2 is equal to the difference ψ1 in the phase delay amount in the high-pass filter 522. However, this amount can sometimes be subject to setting errors. Therefore, the influence of the setting error δ (= |ψ2 - ψ1|) is examined.

[0214] The influence of the setting error δ is the same as the influence of the phase delay difference ψ1 in the second embodiment.

[0215] Specifically, by suppressing the setting error δ to 10 degrees or less, the displacement measurement accuracy can be suppressed to 1 nm or less, thereby achieving sufficient measurement accuracy.

[0216] Furthermore, by suppressing the setting error δ to less than 1 degree, the accuracy of the measured displacement can be suppressed to within 100 ± 0.01%, thereby achieving sufficient measurement accuracy.

[0217] In the third embodiment described above, the same effects as those of the first embodiment can be obtained.

[0218] 5. Fourth Implementation

[0219] Next, a laser interferometer 1 according to a fourth embodiment will be described.

[0220] Figure 11 1 is a functional block diagram showing a laser interferometer 1 according to a fourth embodiment.

[0221] Hereinafter, a fourth embodiment will be described. However, in the following description, the differences from the above-described embodiment will be mainly described, and descriptions of the same matters will be omitted.

[0222] The fourth embodiment is the same as the third embodiment except that the structure of the demodulation circuit 52 is different.

[0223] In the aforementioned third embodiment, the first phase adjuster 526 is provided between the bandpass filter 524 and the branch point 527 , and the second phase adjuster 528 is provided between the branch point 527 and the second multiplier 532 .

[0224] In contrast, in the fourth embodiment, the first phase adjuster 526 is provided between the branch point 527 and the first multiplier 530. In addition, the phase adjustment amount ψ2 in the second phase adjuster 528 is changed accordingly.

[0225] Therefore, the fourth embodiment is the same as the third embodiment except for the arrangement and setting of the first phase adjuster 526 .

[0226] As in the first embodiment, the first phase adjuster 526 adjusts the phase of the reference signal to be in phase with the phase of the fundamental frequency component of the modulation signal included in the laser reception signal.

[0227] On the other hand, the second phase adjuster 528 adjusts the phase of the reference signal output from the bandpass filter 524 and branched at the branch point 527 so that it is in phase with the phase of the fundamental frequency component of the modulated signal included in the laser reception signal, and adjusts the phase adjuster so that the difference ψ1 in the phase delay amount in the high-pass filter 522 is offset. This produces the same effect as the third embodiment.

[0228] The above effects are examined below.

[0229] When the first phase adjuster 526 adjusts the phase of the reference signal to be in phase with the phase of the fundamental frequency component of the modulation signal included in the laser reception signal, the phase adjustment amount is set to ψ0.

[0230] The second phase adjuster 528 adjusts the phase of the reference signal branched from the branch point 527 to be in phase with the fundamental frequency component of the modulation signal included in the laser reception signal. Therefore, the phase adjustment amount ψ2 in the second phase adjuster 528 is set to ψ2 = ψ0.

[0231] In this embodiment, the phase adjustment amount ψ2 in the second phase adjuster 528 is supplemented with an adjustment amount that cancels the phase delay difference ψ1 in the high-pass filter 522. Therefore, the phase adjustment amount ψ2 in the second phase adjuster 528 is set to ψ2 = ψ0 + ψ1.

[0232] In the fourth embodiment, the displacement measurement accuracy can be reduced to 1 nm or less by suppressing the setting error δ (= |ψ2 - ψ1|) of the phase adjustment amount ψ2 to 10 degrees or less. This achieves sufficient measurement accuracy.

[0233] Furthermore, by suppressing the setting error δ to less than 1 degree, the accuracy of the measured displacement can be suppressed to within 100 ± 0.01%, thereby achieving sufficient measurement accuracy.

[0234] In the fourth embodiment described above, the same effects as those of the first embodiment can be obtained.

[0235] 6. Fifth Implementation

[0236] Next, a spectroscopic device according to a fifth embodiment will be described.

[0237] Figure 12 This is a functional block diagram showing a spectroscopic device 900 according to the fifth embodiment.

[0238] Hereinafter, the fifth embodiment will be described. However, in the following description, the differences from the first embodiment will be mainly described, and the description of the same matters will be omitted. Figure 12 In, with Figure 1 The same structure is marked with the same symbol.

[0239] Figure 12 The spectroscopic device 900 shown includes the laser interferometer 1 according to each of the above-described embodiments and a spectroscopic analysis unit 910 .

[0240] The spectroscopic analysis unit 910 receives analysis light including a sample-derived signal generated by the interaction with the sample, and generates spectroscopic spectrum information derived from the sample. Figure 12 The spectroscopic analysis unit 910 shown includes a spectroscopic optical system 920 and a computing unit 930. The spectroscopic optical system 920 includes 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 a sample and then enters an analytical light interferometer. In the analytical light interferometer, while the movable mirror 924 is moved to change the optical path length, interference is generated between the analytical light passing through the sample and the analytical light passing through the movable mirror 924. The analytical light receiving unit 926 then receives the interfering light to obtain an analytical light reception signal.

[0241] On the other hand, the laser interferometer 1 measures the displacement of the movable mirror 924 and outputs a mirror position signal. The laser interferometer 1 can measure the displacement of the movable mirror 924 with high precision, and thus can generate a mirror position signal with high precision.

[0242] The calculation unit 930 generates a waveform (interference pattern) representing the intensity of interference light relative to the optical path length in the spectroscopic optical system 920 based on the analysis of the light reception signal and the mirror position signal, and performs Fourier transform on the waveform to generate spectroscopic spectrum information.

[0243] Therefore, the spectroscopic analysis unit 910 can generate highly accurate spectroscopic spectrum information based on the measurement result of the displacement of the movable mirror 924 by the laser interferometer 1 .

[0244] Furthermore, the laser interferometer 1 can be easily reduced in cost as described above. Therefore, with the above-described configuration, it is possible to realize the spectroscopic device 900 which can be easily reduced in cost and has excellent wave number resolution.

[0245] It should be noted that the spectrometer 900 can be applied to FT-IR (Fourier infrared spectroscopy), FT-NIR (Fourier near infrared spectroscopy), FT-VIS (Fourier visible spectroscopy), FT-UV (Fourier ultraviolet spectroscopy), FT-THz (Fourier terahertz spectroscopy), etc. by appropriately changing the type of analysis light.

[0246] Furthermore, the spectroscopic device 900 can be applied to, for example, a white interference profile measurement device, an optical tomography (OCT) imaging device, and the like by using an element capable of acquiring a two-dimensional light intensity distribution as the analysis light receiving unit 926 .

[0247] 7. Effects of the above-mentioned embodiments

[0248] The laser interferometer 1 according to each of the aforementioned embodiments is a laser interferometer that irradiates a target object 14 with second segmented light L1b (laser light), receives object light L3 (laser light) that has passed through the target object 14, and detects the displacement of the target object 14. The laser interferometer 1 includes a laser source 2, an optical modulator 12, a light receiving element 10, a signal oscillator 51, and a demodulation circuit 52. The laser source 2 emits output light L1 (laser light). The optical modulator 12 includes a vibration element 30 and uses the vibration element 30 to add a modulation signal to the first segmented light L1a (laser light). The light receiving element 10 detects intensity changes in reference light L2 and object light L3 (laser light), which include a sampling signal and the modulation signal applied by the target object 14, and outputs a received laser signal. The signal oscillator 51 generates a reference signal of a first frequency using the vibration element 30 as a source oscillator. The demodulation circuit 52 demodulates the received laser signal into a sampling signal based on the reference signal to detect the displacement of the target object 14.

[0249] The demodulation circuit 52 includes a high-pass filter 522 (DC offset removal unit), a first phase adjuster 526 , a first multiplier 530 , a low-pass filter 534 (first filter), a second multiplier 532 , a low-pass filter 536 (second filter), and an inverse tangent operator 548 (phase calculator).

[0250] High-pass filter 522 removes the offset of the DC component of the laser reception signal. First phase adjuster 526 adjusts the phase of the reference signal. First multiplier 530 multiplies the laser reception signal output from high-pass filter 522 by the reference signal output from first phase adjuster 526 and outputs a first multiplied signal. Low-pass filter 534 removes high-frequency components included in the first multiplied signal. Second multiplier 532 multiplies the first multiplied signal by the reference signal output from first phase adjuster 526 and outputs a second multiplied signal. Low-pass filter 536 removes high-frequency components included in the second multiplied signal. Inverse tangent operator 548 calculates the phase originating from target object 14 as a sampling signal based on the signals output from low-pass filter 534 and low-pass filter 536.

[0251] With this configuration, by performing two multiplications in demodulation circuit 52, the frequencies of the first and second multiplication signals can be lowered. In other words, the frequency of the signal provided for computation can be reduced. This reduces the operating frequencies of A / D converter 538, A / D converter 540, and the FPGA incorporated into a portion of demodulation circuit 52, thereby achieving cost reduction.

[0252] In the laser interferometer 1 according to each of the above embodiments, the demodulation circuit 52 includes a bandpass filter 524 (third filter). The bandpass filter 524 is provided between the signal oscillator 51 and the first phase adjuster 526 and extracts the first frequency component included in the reference signal.

[0253] According to such a configuration, a reference signal is obtained from which unnecessary frequency components (noise components) are removed.

[0254] In the laser interferometer 1 according to each of the above embodiments, the demodulation circuit 52 includes a first amplitude adjuster 542 and a second amplitude adjuster 544 (amplitude adjustment unit). The first amplitude adjuster 542 and the second amplitude adjuster 544 match the amplitude of the first multiplication signal output from the low-pass filter 534 (first filter) and the amplitude of the second multiplication signal output from the low-pass filter 536 (second filter).

[0255] According to this configuration, the first multiplication signal and the second multiplication signal having mutually matched amplitudes are obtained, thereby optimizing the demodulation accuracy of the phase X.

[0256] In the laser interferometer 1 involved in each of the above embodiments, the high-pass filter 522 (DC offset removal unit) is set so that when the difference between the phase delay amount Φ1 when the component of the frequency of the modulation signal passes through and the phase delay amount Φ2 when the component of twice the frequency of the modulation signal passes through is set to ψ1, ψ1≤10[deg].

[0257] According to such a configuration, the demodulation precision and accuracy of the phase X are optimized. As a result, the measurement precision of the displacement of the target object 14 and the accuracy of the measured displacement are optimized.

[0258] In the laser interferometer 1 according to each of the above-described embodiments, the high-pass filter 522 (DC offset removal unit) is set so that the difference ψ1 in the phase delay amount satisfies ψ1≦1[deg].

[0259] According to such a configuration, the demodulation precision and accuracy of the phase X are optimized. As a result, the measurement precision of the displacement of the target object 14 and the accuracy of the measured displacement are optimized.

[0260] In the laser interferometer 1 according to each of the above embodiments, the demodulation circuit 52 includes a first amplitude adjuster 542 and a second amplitude adjuster 544 (amplitude adjustment unit). The first amplitude adjuster 542 and the second amplitude adjuster 544 adjust at least one of the amplitude of the first multiplication signal output from the low-pass filter 534 (first filter) and the amplitude of the second multiplication signal output from the low-pass filter 536 (second filter). Furthermore, the first amplitude adjuster 542 and the second amplitude adjuster 544 have the functions of adjusting the amplitudes of the first multiplication signal and the second multiplication signal to match each other and adjusting the amplitude of the second multiplication signal to cancel the amplitude change reflecting the phase delay difference ψ1.

[0261] According to such a configuration, it is possible to cancel or reduce the error factor that deteriorates the demodulation accuracy of the phase X. As a result, the demodulation accuracy and precision of the phase X are further optimized.

[0262] In the laser interferometer 1 according to each of the above embodiments, the first phase adjuster 526 adjusts the phase of the reference signal so that the phase of the reference signal is in phase with the fundamental frequency component of the modulation signal included in the laser reception signal.

[0263] With this configuration, the phase of the fundamental frequency component of the modulated signal included in the laser reception signal can be matched with the phase of the reference signal, thereby suppressing degradation in the demodulation precision and accuracy of the phase X.

[0264] In the laser interferometer 1 according to each of the above embodiments, the demodulation circuit 52 includes a second phase adjuster 528. The second phase adjuster 528 is provided between the signal oscillator 51 and the second multiplier 532, and is configured to adjust the phase of the reference signal so that it is in phase with the fundamental frequency component of the modulated signal included in the laser reception signal. Based on this phase in phase, the second phase adjuster 528 generates a phase adjustment amount ψ2 that is equal to the difference ψ1 between the phase delay amount of the first frequency component in the passband of the high-pass filter 522 (DC offset removal unit).

[0265] With this configuration, the influence of the phase delay difference ψ1 in the high-pass filter 522 can be canceled or reduced during multiplication by the second multiplier 532. Furthermore, the burden of designing the high-pass filter 522 can be reduced.

[0266] In the laser interferometer 1 according to each of the above embodiments, the demodulation circuit 52 includes a second phase adjuster 528. The second phase adjuster 528 is provided between the first phase adjuster 526 and the second multiplier 532, and is configured to adjust the phase of the reference signal so as to generate a phase adjustment amount ψ2 that is equal to the difference ψ1 between the phase delay amount of the first frequency component in the passband of the high-pass filter 522 (DC offset removal unit) based on the phase of the reference signal output by the first phase adjuster 526.

[0267] With this configuration, the influence of the phase delay difference ψ1 in the high-pass filter 522 can be canceled or reduced during multiplication by the second multiplier 532. Furthermore, the burden of designing the high-pass filter 522 can be reduced.

[0268] Furthermore, the spectroscopic device 900 according to the above embodiments includes the laser interferometer 1 according to each of the above embodiments and a spectroscopic analysis unit 910. The spectroscopic analysis unit 910 includes a spectroscopic optical system 920 including a movable mirror 924, and generates spectroscopic spectrum 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 spectrum information based on the measurement result of the displacement of the movable mirror 924 by the laser interferometer 1.

[0269] With such a configuration, it is possible to easily reduce the cost of the spectroscopic device 900 .

[0270] The laser interferometer and spectrometer of the present invention have been described above based on the illustrated embodiments. However, the laser interferometer and spectrometer of the present invention are not limited to the aforementioned embodiments. The structure of each part may be replaced by any component, or any other component may be added.

[0271] Furthermore, although a Michelson interference optical system is used in each of the above-described embodiments, another interference optical system may be used.

Claims

1. A laser interferometer, characterized in that: The laser interferometer irradiates a target with laser light and receives the laser light that has passed through the target to obtain the displacement of the target. The laser interferometer has: a laser source, emitting the laser; an optical modulator including a vibration element and adding a modulation signal to the laser light using the vibration element; a light receiving element for detecting a change in the intensity of the laser light including the sampling signal and the modulation signal added by the target object, and outputting a laser reception signal; a signal oscillating unit, using the vibration element as a source vibration, to generate a reference signal of a first frequency; as well as A demodulation circuit demodulates the sampling signal from the laser reception signal based on the reference signal to obtain the displacement of the target object. The demodulation circuit has: A DC offset removal unit, configured to remove an offset of a DC component of the laser reception signal; a first phase adjuster, configured to adjust the phase of the reference signal; a first multiplier for multiplying the laser reception signal output from the DC offset removal unit by the reference signal output from the first phase adjuster and outputting a first multiplied signal; a first filter, for removing high-frequency components included in the first multiplication signal; a second multiplier, configured to multiply the first multiplication signal by the reference signal output from the first phase adjuster and output a second multiplication signal; a second filter, for removing high-frequency components included in the second multiplication signal; as well as A phase calculator calculates a phase originating from the target object as the sampling signal based on a signal output from the first filter and a signal output from the second filter.

2. The laser interferometer according to claim 1, wherein: The demodulation circuit has a third filter, The third filter is provided between the signal oscillating unit and the first phase adjuster, and is used to extract the component of the first frequency included in the reference signal.

3. The laser interferometer according to claim 1 or 2, characterized in that: The demodulation circuit includes an amplitude adjustment unit. The amplitude adjustment unit matches the amplitude of the first multiplication signal output from the first filter and the amplitude of the second multiplication signal output from the second filter to each other.

4. The laser interferometer according to claim 1 or 2, characterized in that: The DC offset removal unit is configured to: When the difference between the phase delay amount when the component of the frequency of the modulation signal passes and the phase delay amount when the component of the frequency twice of the modulation signal passes is ψ1, ψ1≤10[deg] is satisfied.

5. The laser interferometer according to claim 4, characterized in that The DC offset removal unit is set so that the difference ψ1 in the phase delay amount satisfies ψ1≤1[deg].

6. The laser interferometer according to claim 4, characterized in that The demodulation circuit includes an amplitude adjustment unit. The amplitude adjustment unit adjusts at least one of the amplitude of the first multiplication signal output from the first filter and the amplitude of the second multiplication signal output from the second filter. The amplitude adjustment unit has the following functions: adjusting the amplitude of the first multiplication signal and the amplitude of the second multiplication signal to match each other; and The amplitude of the second multiplication signal is adjusted to offset the amplitude variation reflecting the phase delay difference ψ1.

7. The laser interferometer according to claim 1, characterized in that The first phase adjuster adjusts the phase of the reference signal so that the phase of the reference signal is in phase with a fundamental frequency component of the modulation signal included in the laser reception signal.

8. The laser interferometer according to claim 7, characterized in that The demodulation circuit has a second phase adjuster, The second phase adjuster is arranged between the signal oscillation unit and the second multiplier, and is used to adjust the phase of the reference signal to match the phase that is in phase with the modulation signal included in the laser receiving signal, and generate a phase adjustment amount that is the same as the difference between the phase delay amount of the first frequency component in the pass band of the DC offset removal unit based on the phase that is in phase.

9. The laser interferometer according to claim 7 or 8, characterized in that: The demodulation circuit has a second phase adjuster, The second phase adjuster is provided between the first phase adjuster and the second multiplier, and is configured to adjust the phase of the reference signal so as to generate a phase adjustment amount equal to a difference between a phase delay amount of the component of the first frequency in a passband of the DC offset removal section, based on the phase of the reference signal output by the first phase adjuster.

10. A spectroscopic device, characterized in that: have: The laser interferometer according to claim 1 or 2; and The spectroscopic analysis unit has a spectroscopic optical system including a movable mirror and generates spectroscopic spectrum information from the sample. The laser interferometer measures the displacement of the moving mirror, The spectroscopic analysis unit generates the spectroscopic spectrum information based on a measurement result of the displacement of the movable mirror by the laser interferometer.

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