laser interferometer

Abstract

The present application provides a laser interferometer that can improve the accuracy of demodulating a sample signal from a measurement object from a light-receiving signal even when a signal in which a phase output from an oscillation circuit has changed is used as a reference signal. The laser interferometer includes a laser light source that emits first laser light; a light modulator that includes a vibration element and generates second laser light including a modulation signal; a light-receiving element that receives the second laser light and third laser light including a sample signal; and a calculation unit that calculates a displacement of the measurement object from the light-receiving signal based on a reference signal. The calculation unit includes a preprocessing unit that performs preprocessing of extracting a frequency modulation component from the light-receiving signal and outputs a preprocessed signal; a demodulation processing unit that performs demodulation processing of extracting the sample signal from a mixed signal obtained by mixing a quadrature signal into the preprocessed signal; and a quadrature signal generation unit that generates the quadrature signal based on a phase of the reference signal and an amplitude of the preprocessed signal.

Description

Technical Field

[0001] This invention relates to laser interferometers. Background Technology

[0002] Patent Document 1 discloses an optical Doppler velocimeter that measures the speed of a measured object by utilizing the Doppler effect generated in the reflected beam of light from the object when a light beam is irradiated onto the object by a light source.

[0003] Specifically, in the optical Doppler velocimeter described in Patent Document 1, firstly, a light beam emitted from a light source is irradiated onto the object to be measured to obtain a measurement beam. Then, a reference beam is obtained by modulating the light beam using an optical modulator. A phase modulation signal is supplied to the optical modulator. The light beam is then phase-modulated using this phase modulation signal within the optical modulator.

[0004] Next, a photodetector is used to detect the composite light, including the measurement beam and the reference beam, to obtain a photodetector signal with a frequency modulation component corresponding to the phase modulation signal. Then, the obtained photodetector signal is demodulated using AM and FM signals. From this, the velocity of the object being measured is determined.

[0005] In such an optical Doppler velocimeter, the phase modulation signal and two pilot signals are synchronized with the sinusoidal signal output from the crystal oscillator. This ensures that the phase modulation signal and the optical detection signal obtained from the photodetector are in phase. Consequently, the distortion components in the demodulated signal after FM demodulation are minimized, enabling high-precision speed measurement.

[0006] Patent Document 1: Japanese Patent Application Publication No. 07-151772

[0007] In the optical Doppler velocimeter described in Patent Document 1, a crystal oscillator is used as the source of the reference signal. The crystal oscillator includes a crystal element and a circuit for driving the crystal element. For example, an oscillator circuit including an inverter can be used as the circuit for driving the crystal oscillator. Such an oscillator circuit has the advantage of being easy to miniaturize due to its relatively simple circuit structure. On the other hand, when using an oscillator circuit, the phase of the voltage signal extracted from the oscillator circuit is affected by the electrostatic capacitance of the capacitor included in the oscillator circuit, the parasitic capacitance of the circuit, etc. The electrostatic capacitance of the capacitor and the parasitic capacitance of the circuit change according to the ambient temperature and other environmental factors, therefore, the phase of the voltage signal extracted from the oscillator circuit also changes. Consequently, the phase of the sinusoidal wave signal, which serves as the reference for synchronizing the aforementioned phase modulation signal and the two pilot signals, also changes. As a result, there is a technical problem that it is difficult to accurately match the phase of the optical detection signal with the phase modulation signal, leading to a decrease in the accuracy of the velocity measurement of the object being measured. Summary of the Invention

[0008] The laser interferometer described in the application examples of the present invention is characterized by comprising:

[0009] The laser source emits the first laser beam;

[0010] An optical modulator has a vibrating element and uses the vibrating element to modulate the first laser to generate a second laser including a modulated signal;

[0011] A light-receiving element receives a third laser and a second laser, and outputs a light-receiving signal, wherein the third laser includes a sampling signal generated by the reflection of the first laser from the object being measured; and

[0012] The computing unit calculates the displacement of the object being measured from the received light signal based on the reference signal.

[0013] The arithmetic unit has:

[0014] The preprocessing unit performs preprocessing to extract the frequency modulation component from the received light signal and outputs the preprocessed signal;

[0015] The demodulation processing unit mixes the quadrature signals with the preprocessed signal to obtain a mixed signal, and then performs demodulation processing to extract the sampled signal from the mixed signal; and

[0016] The quadrature signal generation unit generates the quadrature signal based on the phase of the reference signal and the amplitude of the preprocessed signal, or based on the phase of the reference signal and the amplitude of the mixed signal. Attached Figure Description

[0017] Figure 1This is a functional block diagram illustrating the laser interferometer according to the first embodiment.

[0018] Figure 2 It is shown Figure 1 The diagram shows a simplified configuration of the sensor head.

[0019] Figure 3 It is shown Figure 2 A perspective view of a first example of a light modulator.

[0020] Figure 4 This is a top view showing a portion of a second configuration example of an optical modulator.

[0021] Figure 5 This is a top view showing a third configuration example of an optical modulator.

[0022] Figure 6 This describes the incident light K being incident from a direction perpendicular to the surface of the vibrating element. i A conceptual diagram showing the generation of multiple diffracted lights.

[0023] Figure 7 This explains the composition of the incident light K. i A conceptual diagram of an optical modulator whose forward direction forms an angle of 180° with the forward direction of the reference light L2.

[0024] Figure 8 This explains the composition of the incident light K. i A conceptual diagram of an optical modulator whose forward direction forms an angle of 180° with the forward direction of the reference light L2.

[0025] Figure 9 This explains the composition of the incident light K. i A conceptual diagram of an optical modulator whose forward direction forms an angle of 180° with the forward direction of the reference light L2.

[0026] Figure 10 This is a cross-sectional view showing an optical modulator with a packaged structure.

[0027] Figure 11 This is a circuit diagram showing the configuration of a unipolar inverter oscillation circuit.

[0028] Figure 12 This is an example of the LCR equivalent circuit of a vibrating element.

[0029] Figure 13 It is shown Figure 1 The flowchart shows an example of a method for setting the phase quantity using the phase quantity setting unit.

[0030] Figure 14 It shows that based on Figure 13The example shown illustrates the waveforms of the preprocessed signal S(t) before calibration of the laser interferometer, the phase information after demodulation by the demodulation processing unit (phase information before expansion), and the phase information after phase expansion processing by the signal output unit (phase information after expansion).

[0031] Figure 15 It shows that based on Figure 13 The following are examples of waveforms for the laser interferometer after calibration: the pre-processed signal S(t), the phase information after demodulation by the demodulation processing unit (phase information before expansion), and the phase information after phase expansion processing by the signal output unit (phase information after expansion).

[0032] Figure 16 This is a functional block diagram illustrating the laser interferometer according to the second embodiment.

[0033] Explanation of reference numerals in the attached figures

[0034] 1…Laser interferometer, 1A…Laser interferometer, 2…Laser source, 3…Collimating lens, 4…Optical splitter, 6…1 / 2 wavelength plate, 7…1 / 4 wavelength plate, 8…1 / 4 wavelength plate, 9…Polarizer, 10…Light receiving element, 12…Optical modulator, 14…Object to be measured, 16…Setting unit, 18…Optical path, 20…Optical path, 22…Optical path, 24…Optical path, 30…Vibrating element, 30A…Vibrating element, 30B…Vibrating element, 31…Substrate, 32…Groove, 33…Gap, 34…Diffraction grating, 35…Gap, 36…Vibration direction, 37…Reflector 45…Circuit element, 50…Optical system, 51…Sensor head, 52…Arithmetic unit, 53…Preprocessing unit, 54…Oscillator circuit, 55…Demodulation processing unit, 55A…Demodulation processing unit, 57…Orthogonal signal generation unit, 57A…Orthogonal signal generation unit, 70…Container, 72…Container body, 74…Lid, 76…Connecting wire, 120…Optical modulation oscillator, 301…First electrode, 302…Second electrode, 303…Diffraction grating mounting part, 305…Piezoelectric substrate, 306…Comb-shaped electrode, 307…Ground electrode, 311…Front side, 312…Back side 531… Current-to-voltage converter, 532… ADC, 533… ADC, 534… First bandpass filter, 535… Second bandpass filter, 536… First delay adjuster, 538… Multiplier, 539… Third bandpass filter, 540… First AGC section, 541… Second AGC section, 542… Adder, 551… Multiplier, 552… Multiplier, 553… Inverting amplifier, 555… First low-pass filter, 556… Second low-pass filter, 557… Divider, 558… Arctangent operator, 559… Signal output section, 560… Mixer Signal amplitude operator, 571… fourth bandpass filter, 572… Hilbert conversion filter, 573… second delay adjuster, 574… reference signal phase operator, 577… absolute value operator, 578… third low-pass filter, 579… phase setting unit, 579A… phase setting unit, 580… adder, 581… cosine operator, 582… sine operator, 721… first recess, 722… second recess, C0… parallel capacitor, C1… series capacitor, C3… third capacitor, Cd… second capacitor, Cg… first capacitor, cos(θ) m (t))…cosine wave signal, GND…GND terminal, K -2s …diffraction light, K -1s …diffraction light, K 0s …diffraction light, K 1s …diffraction light, K 2s …diffraction light, K i…Incident light, L1…Outgoing light, L1…Series inductance, L1a…First branch light, L1b…Second branch light, L2…Reference light, L3…Object light, N…Normal, P…Spacing, R1…Equivalent series resistance, Rd…Limiting resistance, Rf…Feedback resistance, S(t)…Preprocessing signal, S1…First signal, S2…Second signal, S102…Process, S104…Process, S106…Process, S108…Process, S110…Process, S112…Process, S114…Process, S116…Process, S118…Process, S120…Process, sin(θ) m (t))…sine wave signal, Sa…amplitude signal, Sd…drive signal, Ss…reference signal, Vcc…terminal, X1…terminal, X2…terminal, Y…terminal, i…signal i, jp1…branch, jp2…branch, ps1…first signal path, ps2…second signal path, r…signal r, x…signal x, y…signal y, β…incidence angle, θ B …flare angle, θ S …tilt angle. Detailed Implementation

[0035] The laser interferometer of the present invention will now be described in detail based on the embodiments shown in the accompanying drawings.

[0036] 1. First Implementation Method

[0037] First, the laser interferometer involved in the first embodiment will be described.

[0038] Figure 1 This is a functional block diagram illustrating the laser interferometer according to the first embodiment.

[0039] Figure 1 The laser interferometer 1 shown has an optical system 50, a sensor head 51 equipped with a current-to-voltage converter 531 and an oscillation circuit 54, and an arithmetic unit 52 that receives light detection signals from the optical system 50.

[0040] 1.1. Sensor Head

[0041] Figure 2 It is shown Figure 1 A simplified structural diagram of the sensor head 51 is shown.

[0042] 1.1.1. Optical System

[0043] As previously mentioned, the sensor head 51 has an optical system 50.

[0044] like Figure 2As shown, the optical system 50 includes a laser light source 2, a collimating lens 3, an optical splitter 4, a half-wave plate 6, a quarter-wave plate 7, a quarter-wave plate 8, a polarizer 9, a light receiving element 10, a frequency shifter type optical modulator 12, and a mounting section 16 on which the object to be measured 14 is disposed.

[0045] Laser source 2 emits outgoing light L1 (first laser). Light receiving element 10 converts the received light into an electrical signal. Optical modulator 12 includes a vibrating element 30, which changes the frequency of the outgoing light L1 to generate reference light L2 (second laser) including a modulation signal. Setting unit 16 can be configured to accommodate the object to be measured 14 as needed. Outgoing light L1 incident on the object to be measured 14 is reflected as object light L3 (third laser), which includes a sampling signal as a Doppler signal originating from the object to be measured 14.

[0046] The optical path of the emitted light L1 from the laser source 2 is designated as optical path 18. On optical path 18, starting from the laser source 2 side, a collimating lens 3 and a half-wave plate 6 are sequentially arranged. Optical path 18 is combined with optical path 20 through reflection from optical splitter 4. On optical path 20, starting from the optical splitter 4 side, a quarter-wave plate 8 and an optical modulator 12 are sequentially arranged. Furthermore, optical path 18 is combined with optical path 22 through transmission through optical splitter 4. On optical path 22, starting from the optical splitter 4 side, a quarter-wave plate 7 and a mounting section 16 are sequentially arranged.

[0047] Optical path 20 is combined with optical path 24 through optical splitter 4. On optical path 24, polarizer 9 and light receiving element 10 are arranged sequentially starting from the side of optical splitter 4.

[0048] The emitted light L1 from the laser source 2 is split into two by the optical splitter 4. One beam passes through optical path 20 and enters the optical modulator 12. The other beam passes through optical path 22 and enters the object to be measured 14. The reference light L2 generated by the optical modulator 12 passes through optical paths 20 and 24 and enters the light-receiving element 10. The object light L3 generated by reflection from the object to be measured 14 passes through optical paths 22 and 24 and enters the light-receiving element 10.

[0049] It should be noted that the "optical path" in this specification refers to the path through which light travels between optical components.

[0050] The following is a further explanation of the various parts of the optical system 50.

[0051] 1.1.1.1. Laser source

[0052] Laser source 2 is a laser source that emits interferometric outgoing light L1. Laser source 2 preferably uses a light source with a linewidth below MHz. Specifically, examples include gas lasers such as He-Ne lasers, semiconductor laser elements such as DFB-LD (Distributed feedback-laser diode), FBG-LD (laser diode with fiber bragg grating), VCSEL (Vertical Cavity Surface Emitting Laser), and FP-LD (Fabry-Perot Laser Diode).

[0053] The laser source 2 is particularly preferably a semiconductor laser element. This allows for the miniaturization of the laser source 2. Consequently, the laser interferometer 1 can be miniaturized. In particular, it enables the miniaturization and weight reduction of the sensor head 51 housing the optical system 50 in the laser interferometer 1, thus also improving the operability of the laser interferometer 1.

[0054] 1.1.1.2. Collimating Lens

[0055] Collimating lens 3 is a convex lens disposed between laser source 2 and optical splitter 4. Collimating lens 3 parallelizes the outgoing light L1 emitted from laser source 2.

[0056] It should be noted that when the outgoing light L1 emitted from the laser source 2 is sufficiently parallelized, 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.

[0057] On the other hand, when the laser source 2 is a semiconductor laser element, it is preferable that the laser interferometer 1 includes a collimating lens 3 disposed between the laser source 2 and the optical splitter 4. This allows the outgoing light L1 emitted from the semiconductor laser element to be parallelized. As a result, the outgoing light L1 becomes collimated, thus suppressing the enlargement of various optical components receiving the outgoing light L1 and enabling the miniaturization of the laser interferometer 1.

[0058] The collimated outgoing light L1 passes through the half-wave plate 6 and is thus converted into linearly polarized light with an intensity ratio of P-polarized light to S-polarized light of, for example, 50:50, and enters the optical splitter 4.

[0059] 1.1.1.3. Optical splitter

[0060] Optical splitter 4 is a polarization beam splitter disposed between laser source 2 and optical modulator 12, and between laser source 2 and the object to be measured 14. Optical splitter 4 has the function of allowing P-polarized light to pass through and reflecting S-polarized light. Through this function, optical splitter 4 splits the outgoing light L1 into a first split beam L1a, which is the reflected light on optical splitter 4, and a second split beam L1b, which is the transmitted light on optical splitter 4.

[0061] The first split beam L1a, which is S-polarized light reflected by optical splitter 4, is converted into circularly polarized light by quarter-wave plate 8 and then enters optical modulator 12. The circularly polarized light of the first split beam L1a entering optical modulator 12 is received by f. m A frequency shift of [Hz] is applied, and this light is reflected as a reference light L2. Therefore, the reference light L2 includes a frequency f. m The modulation signal is [Hz]. 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 optical splitter 4 and the polarizer 9 and enters the light receiving element 10.

[0062] The second split beam L1b, which is the P-polarized light transmitted through the optical splitter 4, is converted into circularly polarized light by the quarter-wave plate 7 and then incident on the moving measurement object 14. The circularly polarized light of the second split beam L1b incident on the measurement object 14 is received by f. d The Doppler frequency shift of [Hz] is reflected as object light L3. Therefore, object light L3 includes frequency f. d The sampling signal is [Hz]. The object light L3 is converted into S-polarized light when it passes through the 1 / 4 wavelength plate 7 again. The S-polarized light of the object light L3 is reflected by the optical splitter 4 and passes through the polarizer 9 into the light receiving element 10.

[0063] As mentioned earlier, the emitted light L1 is interferometric; therefore, the reference light L2 and the object light L3 are used as interference light and incident on the light-receiving element 10.

[0064] It should be noted that a non-polarized beam splitter can also be used instead of a polarized beam splitter. In this case, there is no need for the 1 / 2 wavelength plate 6, 1 / 4 wavelength plate 7, and 1 / 4 wavelength plate 8, thus enabling miniaturization of the laser interferometer 1 through a reduction in the number of components. Furthermore, an optical splitter other than a beam splitter can also be used.

[0065] 1.1.1.4. Analyzer

[0066] The S-polarized and P-polarized beams, being orthogonal to each other, are independent of each other; therefore, simple overlap will not result in interference-induced beat frequencies. Thus, the light wave causing the S-polarized and P-polarized beams to overlap passes through a polarizer 9 tilted at 45° relative to both the S-polarized and P-polarized beams. By using the polarizer 9, mutually common components of light can pass through and interfere. As a result, in the polarizer 9, the reference light L2 and the object light L3 interfere, generating a beam with |f|. m -f d Interference light with a frequency of [Hz].

[0067] 1.1.1.5. Light-receiving element

[0068] When interference light enters the light-receiving element 10, the light-receiving element 10 outputs a light-receiving signal. The sampling signal is demodulated from this light-receiving signal using a method described later, thereby ultimately determining the motion, i.e., vibration velocity and displacement, of the object 14 being measured. Examples of light-receiving elements 10 include photodiodes. The light-receiving element 10 receives interference light and outputs a photocurrent.

[0069] 1.1.1.6. Optical Modulator

[0070] Figure 3 It is shown Figure 2 A perspective view of a first configuration example of the optical modulator 12 shown.

[0071] 1.1.1.6.1. Summary of the First Configuration Example of an Optical Modulator

[0072] The frequency shifter type optical modulator 12 has an optical modulation oscillator 120. Figure 3 The optical modulation oscillator 120 shown has a plate-shaped vibrating element 30 and a substrate 31 supporting the vibrating element 30.

[0073] The vibrating element 30 is made of a material that vibrates repeatedly in a skewed manner along a plane by applying a potential. In this configuration example, the vibrating element 30 is a crystal AT oscillator that undergoes thickness shear vibration along the vibration direction 36 in the high-frequency region of the MHz band. A diffraction grating 34 is formed on the surface of the vibrating element 30. The diffraction grating 34 includes grooves 32 having components that intersect the vibration direction 36, i.e., a plurality of straight grooves 32 extending in a direction intersecting the vibration direction 36.

[0074] The substrate 31 includes a front side 311 and a back side 312 that are opposite to each other. A vibrating element 30 is disposed on the front side 311. In addition, a pad 33 for applying a potential to the vibrating element 30 is provided on the front side 311. On the other hand, a pad 35 for applying a potential to the vibrating element 30 is also provided on the back side 312.

[0075] The size of the substrate 31 is, for example, set to approximately 0.5 mm or more and 10.0 mm or less on the long side. Furthermore, the thickness of the substrate 31 is, for example, set to approximately 0.10 mm or more and 2.0 mm or less. As an example, the shape of the substrate 31 is set to a square with one side of 1.6 mm and its thickness is set to 0.35 mm.

[0076] The size of the vibrating element 30 is, for example, set to be approximately 0.2 mm or more and 3.0 mm or less on the long side. In addition, the thickness of the vibrating element 30 is, for example, set to be approximately 0.003 mm or more and 0.5 mm or less.

[0077] As an example, the vibrating element 30 is shaped as a square with one side of 1.0 mm and a thickness of 0.07 mm. In this case, the vibrating element 30 oscillates at a fundamental oscillation frequency of 24 MHz. It should be noted that the oscillation frequency can be adjusted in the range of 1 MHz to 1 GHz by changing the thickness of the vibrating element 30 or taking overtones into account.

[0078] It should be pointed out that, in Figure 3 In this process, the diffraction grating 34 is formed entirely on the surface of the vibrating element 30, but it may also be formed only on a part of its surface.

[0079] The magnitude of the light modulation in the light modulator 12 is assigned the following value: the dot product of the difference between the wavenumber vector of the emitted light L1 incident on the light modulator 12 and the wavenumber vector of the reference light L2 emitted from the light modulator 12, and the vector of the vibration direction 36 of the vibrating element 30. In this configuration example, the vibrating element 30 undergoes thickness shear vibration; however, since this vibration is in-plane vibration, light modulation cannot be achieved even if light is incident perpendicularly onto the surface of the vibrating element 30. Therefore, in this configuration example, by providing a diffraction grating 34 on the vibrating element 30, light modulation can be achieved using the principle described later.

[0080] Figure 3 The diffraction grating 34 shown is a blazed diffraction grating. A blazed diffraction grating is a diffraction grating with a stepped cross-sectional shape. The straight grooves 32 of the diffraction grating 34 are configured such that their extension direction is orthogonal to the vibration direction 36.

[0081] If from Figure 1 and Figure 2 The oscillation circuit 54 shown is directed towards Figure 3 When the vibrating element 30 shown is supplied with a drive signal Sd (an applied AC voltage), the vibrating element 30 oscillates. The power (drive power) required for the oscillation of the vibrating element 30 is not particularly limited, but it is as low as approximately 0.1 μW to 100 mW. Therefore, it is possible to make the vibrating element 30 oscillate without amplifying the drive signal Sd output from the oscillation circuit 54.

[0082] Furthermore, conventional optical modulators often require structures to maintain their temperature, making it difficult to reduce their size. Additionally, conventional optical modulators consume a lot of power, hindering the miniaturization and power-saving of laser interferometers. To address this, in this embodiment, the oscillation element 30 is very small, and the power required for oscillation is also low, thus facilitating the miniaturization and power saving of the laser interferometer 1.

[0083] 1.1.1.6.2. Methods for forming diffraction gratings

[0084] The method for forming the diffraction grating 34 is not particularly limited. However, as an example, the following method can be described: a mold is fabricated using a mechanical scribing method, and a groove 32 is formed on the electrode of the vibrating element 30 of the crystal AT oscillator by nanoimprinting. Here, it is set on the electrode because, in the case of the crystal AT oscillator, in principle, high-quality thickness shear vibration can be generated on the electrode. It should be noted that the formation of the groove 32 is not limited to the electrode and can also be on the surface of the material in the non-electrode portion. In addition, exposure and etching-based processing methods, electron beam lithography, focused ion beam processing (FIB), etc., can be used instead of nanoimprinting.

[0085] Furthermore, a diffraction grating can be formed on the chip of the crystal AT oscillator using a photoresist material. A metal film or a mirror film composed of multiple dielectric films can also be incorporated here. By incorporating the metal film and mirror film, the reflectivity of the diffraction grating 34 can be improved.

[0086] Furthermore, a resist film can be formed on the chip or wafer of the crystal AT oscillator. After etching, the resist film is removed, and then a metal film or a mirror film is formed on the processed surface. In this case, since the resist material is removed, there is no effect from the moisture absorption of the resist material, which improves the chemical stability of the diffraction grating 34. In addition, by setting a highly conductive metal film such as Au or Al, it can also be used as an electrode to drive the oscillation element 30.

[0087] It should be noted that the diffraction grating 34 can also be formed using techniques such as anodic aluminum oxide (porous aluminum oxide).

[0088] 1.1.1.6.3. Other configuration examples of optical modulators

[0089] The vibrating element 30 is not limited to a crystal oscillator; for example, it can also be a Si oscillator, a surface acoustic wave (SAW) device, a ceramic oscillator, etc.

[0090] Figure 4This is a top view showing a portion of a second configuration example of the optical modulator 12. Figure 5 This is a top view showing a third configuration example of the optical modulator 12.

[0091] Figure 4 The vibrating element 30A shown is a Si oscillator manufactured from a Si substrate using MEMS technology. MEMS (Micro-Electro-Mechanical Systems) refers to micro-electro-mechanical systems.

[0092] The vibrating element 30A includes a first electrode 301 and a second electrode 302 adjacent to each other on the same plane with a gap between them, a diffraction grating mounting portion 303 disposed on the first electrode 301, and a diffraction grating 34 disposed on the diffraction grating mounting portion 303. The first electrode 301 and the second electrode 302 are driven, for example, by electrostatic attraction, along... Figure 4 The left and right directions, that is, along the connection Figure 4 The first electrode 301 and the second electrode 302 shown are vibrated by repeatedly approaching and separating from each other along their axes. This allows in-plane vibration to be applied to the diffraction grating 34. The oscillation frequency of the Si oscillator is, for example, around 1 kHz to several hundred MHz.

[0093] Figure 5 The vibrating element 30B shown is a SAW device that utilizes surface waves. SAW (Surface Acoustic Wave) refers to surface acoustic waves.

[0094] The vibrating element 30B includes a piezoelectric substrate 305, a comb-shaped electrode 306 disposed on the piezoelectric substrate 305, a ground electrode 307, a diffraction grating mounting portion 303, and a diffraction grating 34. When an alternating voltage is applied to the comb-shaped electrode 306, surface acoustic waves are excited due to the inverse piezoelectric effect. This allows in-plane vibration to be applied to the diffraction grating 34. The oscillation frequency of the SAW device is, for example, around several hundred MHz to several GHz.

[0095] Regarding the device described above, by setting the diffraction grating 34, optical modulation can also be performed using the same principle as the crystal AT oscillator, as described later.

[0096] On the other hand, when the vibrating element 30 is a crystal oscillator, the extremely high Q value of the crystal can be used to generate a high-precision modulation signal. The Q value is an indicator of the sharpness of the resonant peak. In addition, crystal oscillators have the advantage of being less susceptible to external interference. Therefore, by using the modulation signal modulated by the optical modulator 12 equipped with a crystal oscillator, the sampling signal originating from the object to be measured 14 can be acquired with high precision.

[0097] 1.1.1.6.4. Optical Modulation of Vibrating Elements

[0098] Next, the principle of using the vibrating element 30 to modulate light will be explained.

[0099] Figure 6 The incident light K was incident from a direction perpendicular to the surface of the vibrating element 30. i A conceptual diagram illustrating the situation where multiple diffracted lights are generated simultaneously.

[0100] If the incident light K i If the light is incident on a diffraction grating 34 that is undergoing thickness shear vibration along the vibration direction 36, then through diffraction phenomena, such as... Figure 6 As shown, multiple diffracted beams K are generated. ns n is the diffracted light K ns The order of n, n = 0, ±1, ±2, ... It should be noted that in Figure 6 The diffraction grating 34 shown is not... Figure 3 The blazed diffraction grating shown is not an example of a diffraction grating, but rather an example of a diffraction grating obtained by repeating concave and convex shapes. Furthermore, in Figure 6 In the text, the diffraction light K is omitted. 0s The illustration.

[0101] exist Figure 6 In the middle, the incident light K i The light enters from a direction perpendicular to the surface of the vibrating element 30; however, this angle is not particularly limited, and the angle can also be set by allowing the light to enter at an angle relative to the surface of the vibrating element 30. In the case of angled entry, the diffracted light K... ns The direction of its movement also changes accordingly.

[0102] It should be noted that, according to the design of the diffraction grating 34, higher-order light with |n|≥2 may not sometimes appear. Therefore, in order to obtain a stable modulation signal, it is desirable to set |n|=1. That is, in Figure 2 In the laser interferometer 1, a frequency shifter-type optical modulator 12 is preferably configured such that ±1st order diffracted light is used as reference light L2. This configuration enables the stabilization of measurements in the laser interferometer 1.

[0103] On the other hand, when higher-order light with |n|≥2 appears from the diffraction grating 34, the optical modulator 12 can be configured to use any diffraction light of ±2 or higher, instead of ±1st-order diffraction light, as the reference light L2. Thus, since higher-order diffraction light can be utilized, the laser interferometer 1 can be made more frequent and smaller.

[0104] In this embodiment, as an example, the incident light K of the incident light modulator 12 is... iThe optical modulator 12 is configured such that the angle between its entry direction and the direction of travel of the reference light L2 emitted from the optical modulator 12 is 180°. Below, the reference light... Figures 7 to 9 Three examples will be used to illustrate this.

[0105] Figures 7 to 9 These are respectively the incident light K i A conceptual diagram illustrating a light modulator 12 whose forward direction forms an angle of 180° with the forward direction of reference light L2.

[0106] Figure 7 The optical modulator 12 shown includes a resonant element 30 and a reflector 37. The reflector 37 is configured to reflect the diffracted light K. 1s The light is reflected back to diffraction grating 34. At this point, the diffracted light K... 1s The angle between the incident angle and the reflection angle in mirror 37 is 180°. As a result, the diffracted light K emitted from mirror 37 and returning to diffraction grating 34... 1s The light is diffracted again by the diffraction grating 34, and directed towards the incident light K that is incident on the modulator 12. i It moves in the opposite direction to the direction of travel. Therefore, by adding reflector 37, the incident light K can be satisfied as described above. i The condition that the angle between the direction of entry of the light and the direction of travel of the reference light L2 is 180°.

[0107] Furthermore, the reference light L2 generated by the optical modulator 12 is frequency-modulated twice via the reflector 37. Therefore, compared to using a single vibrating element 30, higher frequency modulation can be achieved by using the reflector 37.

[0108] exist Figure 8 In, relative to Figure 6 The configuration causes the vibrating element 30 to tilt. The tilt angle θ at this time... S Set to satisfy the previously stated condition, incident light K i The condition that the angle between the direction of entry of the light and the direction of travel of the reference light L2 is 180°.

[0109] Figure 9 The diffraction grating 34 shown has a blaze angle θ B A blazed diffraction grating. Additionally, incident light K, with an incident angle β, propagates relative to the normal N of the surface of the vibrating element 30. i When incident on the diffraction grating 34, the reference light L2 is relative to the normal N with a blaze angle θ. B The same angle returns. Therefore, by making the incident angle β equal to the blaze angle θ B It can satisfy the previously mentioned condition that the incident light K iThe angle between the direction of entry of the light beam and the direction of travel of the reference light L2 is 180°. In this case, it is possible to achieve [something] without using [something]. Figure 7 The mirror 37 shown will not be as... Figure 8 When the vibrating element 30 is tilted as shown, the aforementioned conditions are met, thus enabling further miniaturization and high-frequency operation of the laser interferometer 1. In particular, in the case of a blazed diffraction grating, the configuration that satisfies the aforementioned conditions is called the "Littrow configuration," which also has the advantage of significantly improving the diffraction efficiency of the diffracted light.

[0110] It should be pointed out that, Figure 9 The pitch P represents the spacing of the blazed diffraction grating; as an example, the pitch P is set to 1 μm. Furthermore, the blaze angle θ... B For example, it is set to 25°. In this case, in order to satisfy the condition, relative to the incident light K... i The angle of incidence β of the normal N can also be 25°.

[0111] 1.1.1.6.5. Packaging Structure

[0112] Figure 10 This is a cross-sectional view showing an optical modulator 12 with an encapsulation structure.

[0113] Figure 10 The optical modulator 12 shown includes a container 70 as a frame, an optical modulation oscillator 120 housed in the container 70, and circuit elements 45 constituting an oscillation circuit 54. It should be noted that the container 70 is, for example, hermetically sealed in a reduced pressure atmosphere such as a vacuum, or in an inert gas atmosphere such as nitrogen or argon.

[0114] like Figure 10 As shown, the container 70 has a container body 72 and a lid 74. The container body 72 has a first recess 721 disposed inside it and a second recess 722 disposed inside the first recess 721 and deeper than the first recess 721. The container body 72 is made of, for example, a ceramic material or a resin material. Furthermore, although not shown, the container body 72 includes internal terminals disposed on its inner surface, external terminals disposed on its outer surface, and wiring connecting the internal and external terminals.

[0115] Furthermore, the opening of the container body 72 is blocked by the cap 74 through a sealing ring (not shown), low-melting-point glass, or other sealing components. The cap 74 can be made of a material that is transparent to laser light, such as glass.

[0116] An optical modulation oscillator 120 is disposed on the bottom surface of the first recess 721. The optical modulation oscillator 120 is supported on the bottom surface of the first recess 721 by a connecting member (not shown). Furthermore, the internal terminals of the container body 72 are electrically connected to the optical modulation oscillator 120, for example, by a conductive material (not shown) such as a connecting wire or a connecting metal.

[0117] A circuit element 45 is disposed on the bottom surface of the second recess 722. The circuit element 45 is electrically connected to the internal terminals of the container body 72 via a connecting wire 76. Thus, the optical modulation oscillator 120 and the circuit element 45 are also electrically connected via wiring provided in the container body 72. It should be noted that circuitry other than the oscillation circuit 54 described later may also be disposed in the circuit element 45.

[0118] By employing this packaging structure, the optical modulation oscillator 120 and the circuit element 45 can be overlapped, thus reducing their physical distance and shortening the wiring length between them. This helps suppress noise from the outside entering the drive signal Sd, or conversely, prevents the drive signal Sd from becoming a noise source. Furthermore, a container 70 protects both the optical modulation oscillator 120 and the circuit element 45 from external environmental influences. Therefore, miniaturization of the sensor head 51 is achieved while improving the reliability of the laser interferometer 1.

[0119] It should be noted that the structure of container 70 is not limited to the structure shown in the figure. For example, the optical modulation oscillator 120 and circuit element 45 can also have separate packaging structures. Furthermore, although not shown in the figure, other circuit elements constituting the oscillation circuit 54 can also be housed in container 70. It should be noted that container 70 can be provided as needed or omitted.

[0120] 1.1.2. Current-to-voltage converter

[0121] The current-to-voltage converter 531, also known as a transimpedance amplifier (TIA), converts the photocurrent (photodetector signal) output from the photodetector 10 into a voltage signal and outputs it as a photodetector signal.

[0122] It should be noted that the optical system 50 may also have multiple light-receiving elements 10. In this case, by providing a differential amplifier circuit between the multiple light-receiving elements 10 and the current-to-voltage converter 531, differential amplification processing of the photocurrent can be performed, thereby improving the S / N ratio (signal-to-noise ratio) of the photodetector signal. It should also be noted that differential amplification processing can be performed on voltage signals.

[0123] A current-to-voltage converter 531 and an arithmetic unit 52 are disposed between them. Figure 1The ADC532 shown is an analog-to-digital converter that converts analog signals into digital signals at a predetermined sampling bit rate.

[0124] 1.1.3. Oscillator Circuit

[0125] like Figure 1 As shown, the oscillation circuit 54 outputs a drive signal Sd, which is input to the light modulator 12 of the optical system 50. Furthermore, the oscillation circuit 54 outputs a reference signal Ss, which is input to the arithmetic unit 52.

[0126] The oscillation circuit 54 is not particularly limited as long as it can cause the oscillating element 30 to oscillate; various circuit configurations can be used. As an example of circuit configuration, in... Figure 11 The example shown is a circuit diagram illustrating the configuration of a unipolar inverter oscillation circuit.

[0127] Figure 11 The oscillation circuit 54 shown includes circuit element 45, feedback resistor Rf, limiting resistor Rd, first capacitor Cg, second capacitor Cd, and third capacitor C3.

[0128] Circuit element 45 is an inverter IC. Terminals X1 and X2 of circuit element 45 are respectively connected to the internal inverter of circuit element 45. Terminal GND is connected to the ground potential, and terminal Vcc is connected to the power supply potential. Terminal Y is the terminal used for oscillation output.

[0129] A first capacitor Cg is connected between terminal X1 and the ground potential. Furthermore, a limiting resistor Rd and a second capacitor Cd, connected in series between terminal X2 and the ground potential, are connected sequentially from the terminal X2 side. Also, one end of a feedback resistor Rf is connected between terminal X1 and the first capacitor Cg, and the other end of the feedback resistor Rf is connected between terminal X2 and the limiting resistor Rd.

[0130] Furthermore, one end of the vibrating element 30 is connected between the first capacitor Cg and the feedback resistor Rf, and the other end of the vibrating element 30 is connected between the second capacitor Cd and the limiting resistor Rd. Thus, the vibrating element 30 becomes the signal source of the oscillation circuit 54.

[0131] Figure 12 This is an example of the LCR equivalent circuit of the vibrating element 30.

[0132] like Figure 12 As shown, the LCR equivalent circuit of the vibration element 30 consists of a series capacitor C1, a series inductor L1, an equivalent series resistance R1, and a parallel capacitor C0.

[0133] exist Figure 11 In the oscillation circuit 54 shown, when the capacitance of the first capacitor Cg is set to C... g Let the capacitance of the second capacitor Cd be C. d The load capacitance C is given by the following formula (a). L .

[0134] Mathematical Formula 1

[0135]

[0136] Therefore, the oscillation frequency f output from terminal Y of the oscillation circuit 54 is given by the following formula (b). osc .

[0137] Mathematical formula 2

[0138]

[0139] f Q It is the natural vibration number of the vibrating element 30.

[0140] According to formula (b) above, it can be seen that by appropriately changing the load capacitance C... L The oscillation frequency f of the signal output from terminal Y is osc Make minor adjustments.

[0141] Furthermore, the natural vibration coefficient f of the vibrating element 30 is given by the following formula (c). Q The oscillation frequency f of the oscillation circuit 54 osc The difference Δf.

[0142] Mathematical Formula 3

[0143]

[0144] Here, because C1 < <C0、C1<<C L Therefore, Δf can be approximated by the following formula (d).

[0145] Mathematical expression 4

[0146]

[0147] Therefore, the oscillation frequency f of the oscillation circuit 54 osc Becoming the natural vibration number f of the vibrating element 30 Q The corresponding value.

[0148] Here, when the vibrating element 30 is fixed to the container 70, for example, if it receives expansion stress caused by temperature through the fixing part, the natural vibration coefficient f QChanges occur. Furthermore, if the vibrating element 30 is tilted, it will be affected by gravity and other factors due to its own weight, thus affecting the natural vibration coefficient f. Q Changes have occurred.

[0149] In the oscillating circuit 54, even for such reasons, the inherent oscillation number f Q The oscillation frequency f has changed. osc This also changes based on the above formula (d), causing it to move in tandem with this change. In other words, the oscillation frequency f osc Always from the natural vibration number f Q The value of the offset Δf. As a result, the vibration of the vibrating element 30 is stable, and the displacement amplitude is stable. Since the displacement amplitude is stable, the modulation characteristics of the optical modulator 12 are stable, thus improving the demodulation accuracy of the sampled signal in the arithmetic unit 52.

[0150] As an example, the preferred value is Δf = |f osc -f Q |≤3000[Hz], more preferably 600[Hz].

[0151] As described above, in the laser interferometer 1 of this embodiment, the optical modulator 12 includes a vibration element 30, and the optical modulator 12 uses the vibration element 30 to modulate the first split beam L1a.

[0152] Based on this configuration, the optical modulator 12 can be miniaturized and made lighter. Therefore, the laser interferometer 1 can be miniaturized and made lighter.

[0153] Furthermore, the laser interferometer 1 includes a calculation unit 52 and an oscillation circuit 54. The oscillation circuit 54 uses the vibrating element 30 as its signal source, such as... Figure 1 As shown, a reference signal Ss is output to the arithmetic unit 52. The arithmetic unit 52 demodulates the sampling signal originating from the object to be measured 14 from the light detection signal based on the reference signal Ss.

[0154] Based on this configuration, even the natural vibration coefficient f of the vibrating element 30... Q The change can also affect the oscillation frequency f of the oscillation circuit 54. osc The change is related to the natural vibration number f of the vibrating element 30. Q The corresponding values ​​allow for easy stabilization of the vibration of the vibrating element 30. This enables the temperature characteristics of the modulation signal to correspond to the temperature characteristics of the vibrating element 30, stabilizing the modulation characteristics of the optical modulator 12. Consequently, the demodulation accuracy of the sampled signal in the arithmetic unit 52 is improved.

[0155] Furthermore, in the above configuration, the temperature characteristics of the reference signal Ss output from the oscillation circuit 54 to the arithmetic unit 52 can also correspond to the temperature characteristics of the vibrating element 30. Therefore, since both the temperature characteristics of the modulation signal and the temperature characteristics of the reference signal correspond to the temperature characteristics of the vibrating element 30, the behavior of the modulation signal variation caused by temperature changes is consistent with or approximately similar to the behavior of the reference signal Ss variation. Thus, even if the temperature of the vibrating element 30 changes, the impact on demodulation accuracy can be suppressed, and the demodulation accuracy of the sampled signal originating from the measurement object 14 can be improved.

[0156] Moreover, since the oscillation circuit 54 consumes little power, it is easy to reduce the power consumption of the laser interferometer 1.

[0157] Furthermore, as mentioned above, the preferred vibrating element 30 is a crystal oscillator. This allows for the generation of a high-precision modulation signal by utilizing the extremely high Q value of the crystal. As a result, the sampling signal originating from the object to be measured 14 can be acquired with high precision.

[0158] It should be noted that signal generators such as function generators and signal generators can also be used instead of oscillator circuit 54.

[0159] 1.2. Arithmetic Unit

[0160] The arithmetic unit 52 performs demodulation processing to demodulate the sampling signal originating from the object to be measured 14 from the optical detection signal output from the current-to-voltage converter 531. The sampling signal includes, for example, phase information and frequency information. Furthermore, the displacement of the object to be measured 14 can be obtained from the phase information, and the velocity of the object to be measured 14 can be obtained from the frequency information. If different physical quantities can be obtained in this way, it can function as a displacement meter and a velocimeter, thus enabling high functionality of the laser interferometer 1.

[0161] The arithmetic unit 52 sets its circuit configuration according to the modulation processing method. In the laser interferometer 1 according to this embodiment, an optical modulator 12 equipped with a vibrating element 30 is used. The vibrating element 30 is a single vibrating element, so its vibration speed changes constantly within a period. Therefore, the modulation frequency also changes over time, so conventional demodulation circuits cannot be used directly.

[0162] Conventional demodulation circuits, for example, refer to circuits that demodulate a sampled signal from a light detection signal, which includes a modulated signal modulated using an acousto-optic modulator (AOM). In an AOM, the modulation frequency does not change. Therefore, conventional demodulation circuits can demodulate a sampled signal from a light detection signal that includes a modulated signal with a fixed modulation frequency. However, in the case of a modulated signal modulated by a light modulator 12 with a changing modulation frequency, direct demodulation is not possible.

[0163] to this end, Figure 1 The arithmetic unit 52 shown includes a pre-processing unit 53, a demodulation processing unit 55, and a quadrature signal generation unit 57. The light detection signal output from the current-voltage converter 531 is first passed through the pre-processing unit 53 and then guided to the demodulation processing unit 55. The pre-processing unit 53 performs pre-processing on the light detection signal. This pre-processing extracts the frequency modulation component from the received light signal, resulting in a signal that can be demodulated using a conventional demodulation circuit. Therefore, in the demodulation processing unit 55, the sampling signal originating from the object being measured 14 is demodulated using a known demodulation method. Furthermore, in the quadrature signal generation unit 57, a cosine wave signal cos(θ) as a quadrature signal is generated based on the reference signal Ss output from the oscillation circuit 54 and the pre-processed signal S(t) output from the pre-processing unit 53. m (t) and the sinusoidal signal sin(θ) m (t)).

[0164] The aforementioned functions of the arithmetic unit 52 are implemented, for example, by having hardware such as a processor, memory, external interface, input unit, and display unit. These components can communicate with each other via an internal bus.

[0165] Examples of processors include FPGA (Field-Programmable Gate Array), CPU (Central Processing Unit), and DSP (Digital Signal Processor).

[0166] Examples of storage devices include HDD (Hard Disk Drive), SSD (Solid State Drive), EEPROM (Electrically Erasable Programmable Read-Only Memory), ROM (Read-Only Memory), and RAM (Random Access Memory).

[0167] External interfaces include, for example, digital input / output ports such as USB (Universal Serial Bus) and Ethernet (registered trademark) ports.

[0168] As an input unit, examples include various input devices such as keyboards, mice, touch panels, and touchpads. As a display unit, examples include liquid crystal display panels and organic EL (Electroluminescence) display panels.

[0169] 1.2.1. Composition of the pretreatment unit

[0170] Figure 1 The preprocessing unit 53 shown includes a first bandpass filter 534, a second bandpass filter 535, a first delay adjuster 536, a multiplier 538, a third bandpass filter 539, a first AGC unit 540, a second AGC unit 541, and an adder 542. It should be noted that AGC stands for Automatic Gain Control.

[0171] The photodetector signal output by the current-to-voltage converter 531 is split into two signals, a first signal S1 and a second signal S2, by the branch JP1. Figure 1 In this process, the path of the first signal S1 is set as the first signal path ps1, and the path of the second signal S2 is set as the second signal path ps2.

[0172] The first bandpass filter 534, the second bandpass filter 535, and the third bandpass filter 539 are filters that selectively allow signals in a specified frequency band to pass through.

[0173] The first delay adjuster 536 is a circuit that uses a memory to temporarily store the signal to adjust the signal delay. The multiplier 538 is a circuit that generates an output signal proportional to the product of the two input signals. The adder 542 is a circuit that generates an output signal proportional to the sum of the two input signals.

[0174] Next, the operation of the preprocessing unit 53 will be explained according to the flow of the first signal S1 and the second signal S2.

[0175] After passing through the first bandpass filter 534 configured on the first signal path ps1, the group delay of the first signal S1 is adjusted by the first delay adjuster 536. The group delay adjusted by the first delay adjuster 536 is equivalent to the group delay of the second signal S2 based on the second bandpass filter 535 (described later). Through this delay adjustment, the delay time caused by the filter circuits passing through the first bandpass filter 534 through which the first signal S1 passes is made consistent between the second bandpass filter 535 and the third bandpass filter 539 through which the second signal S2 passes. The first signal S1 after passing through the first delay adjuster 536 is input to the adder 542 via the first AGC unit 540.

[0176] After passing through the second bandpass filter 535 configured on the second signal path ps2, the second signal S2 is input to the multiplier 538. In the multiplier 538, the second signal S2 is multiplied by the cosine wave signal cos(θ) output by the quadrature signal generation unit 57. m (t)). After that, the second signal S2 passes through the third bandpass filter 539 and is then input to the adder 542 via the second AGC unit 541.

[0177] Adder 542 outputs a signal that is proportional to the sum of the first signal S1 and the second signal S2.

[0178] 1.2.2. Preprocessing

[0179] Next, the preprocessing in the preprocessing unit 53 will be explained. It should be noted that, in the following explanation, as an example, a system in which the modulation signal frequency varies in a sinusoidal pattern and the displacement of the object 14 being measured also varies in a single vibration along the optical axis is considered. Here, E... m E d , Set it as follows.

[0180] Mathematical formula 5

[0181] E m =a m {cos(ω0t+B sinω m t+φ m )+i sin(ω0t+B sinω m t+φ m )} (1)

[0182] E d =a d {cos(ω0t+A sinω d t+φ d )+i sin(ω0t+A sinω d t+φ d (2)

[0183] φ=φ m -φ d (3)

[0184] At this time, the light detection signal I output from the current-to-voltage converter 531 PD Theoretically, it can be expressed by the following formula.

[0185] Mathematical formula 6

[0186] I PD =<|E m +E d | 2 >

[0187] =<|E m 2 +E d 2 +2E m E d |>

[0188] =a m 2 +a d 2 +2a m a d cos(B sinω m tA sinω d t+φ) (4)

[0189] It should be pointed out that E m E d , ω m ω d , ω0, a m a d As shown below.

[0190] Mathematical Formula 7

[0191] E m The electric field component originating from the modulation signal of the optical modulator.

[0192] E d The electric field component originates from the sampling signal of the object being measured.

[0193] The initial phase of the modulation signal originating from the optical modulator

[0194] Originating from the initial phase of the sampling signal of the object being measured

[0195] Optical path phase difference of laser interferometer

[0196] ω m The angular frequency of the modulation signal originating from the optical modulator.

[0197] ω d : Originates from the angular frequency of the sampled signal of the object being measured.

[0198] ω0: Angular frequency of the emitted light from the light source

[0199] a m :coefficient

[0200] a d :coefficient

[0201] In addition, <> in formula (4) represents time average.

[0202] The first and second terms of equation (4) above represent the DC component, and the third term represents the AC component. When this AC component is set as I... PD·AC At that time, I PD·AC As shown in the following formula.

[0203] Mathematical formula 8

[0204] I PD·AC =2a m a d cos(B sinω m tA sinω d t+φ)

[0205] =2a m a d {cos(B sinω m t)cos(A sinω d t-φ)+sin(B sinω m t)sin(A sinω d t-φ)} (5)

[0206]

[0207]

[0208] A: Phase shift of the sampled signal

[0209] f dmax Doppler frequency shift of the sampled signal

[0210] f d Frequency of the sampled signal

[0211] B: Phase shift of the modulated signal

[0212] f mmax Doppler frequency shift of the modulated signal

[0213] f m Frequency of the modulating signal

[0214] Here, we know that there are ν-order Bessel functions as shown in Equations (8) and (9).

[0215] Mathematical formula 9

[0216] cos{ζsin(2πf v t)}=J0(ζ)+2J2(ζ)cos(2·2πf v t)+2J4(ζ)cos(4·2πf νt)+… (8)

[0217] sin{ζsin(2πf ν t)}=2J1(ζ)sin(1·2πf v t)+2J3(ζ)sin(3·2πf ν t)+… (9)

[0218] When the Bessel functions of the above formulas (8) and (9) are used to perform a series expansion of the above formula (5), it can be transformed as shown in the following formula (10).

[0219] Mathematical formula 10

[0220] I PD·AC =2a m a d [{J0(B)+2J2(B)cos(2·ω m t)+2J4(B)cos(4·ω m t)+…}cos(A sinω d t-φ)-{2J1(B)sin(1·ω m t)+2J3(B)sin(3·ω m t)+…}sin(A sinω d t-φ)] (10)

[0221] Where J0(B), J1(B), J2(B), ... are Bessel coefficients.

[0222] If the transformation is performed as described above, it can be theoretically said that the frequency band corresponding to a specified order can be extracted through a bandpass filter.

[0223] Therefore, in the aforementioned preprocessing unit 53, based on this theory, the AC component of the optical detection signal is preprocessed according to the following process.

[0224] First, the AC component of the photodetector signal output from the current-to-voltage converter 531 is normalized in amplitude by the ADC 532. The signal after passing through the ADC 532 is represented by the following formula (10-1).

[0225] Mathematical formula 11

[0226] I ADC_beat ={J0(B)+2J2(B)cos(2ω m t)+2J4(B)cos(4ω m t)+…}cos(A sinω d t-φ)-2{J0(B)+J1(B)sin(ω mt)+J3(B)sin(3ω m t)+…}sin(A sinω d t-φ) (10-1)

[0227] Then, the signal after passing through ADC532 is split into two signals, a first signal S1 and a second signal S2, by branch JP1. The first signal S1 passes through a first bandpass filter 534. In the first bandpass filter 534, the center angular frequency is set to ω. m Therefore, the first signal S1 after passing through the first bandpass filter 534 is represented by the following formula.

[0228] Mathematical expression 12

[0229] I BPF1 =J1(B){-cos(ω m t+A sinω d t-φ)+cos(ω m tA sinω d t+φ)}

[0230] =-2J1(B)sin(ω) m t)·sin(A sinω d t-φ) (11)

[0231] On the other hand, the second signal S2 passes through the second bandpass filter 535. The center angular frequency of the second bandpass filter 535 is set to a value different from the center angular frequency of the first bandpass filter 534. Here, as an example, the center angular frequency of the second bandpass filter 535 is set to 2ω. m Therefore, the second signal S2 after passing through the second bandpass filter 535 is represented by the following formula.

[0232] Mathematical formula 13

[0233] I BPF2 =2J2(B)cos(2ω) m t)·cos(A sinω d t-φ) (12)

[0234] The second signal S2 after passing through the second bandpass filter 535 is multiplied in the multiplier 538 by the cosine wave signal cos(θ) output by the quadrature signal generation unit 57 (described later). m (t)). The second signal S2 after passing through multiplier 538 is represented by the following formula.

[0235] Mathematical formula 14

[0236] I 538 =I BPF2 *cos(θm (t))

[0237] =2J2(B)cos(2ω) m t)·cos(A sinω d t-φ)·cos(ω m t-α)

[0238] ={J2(B)cos(A sinω)} d t-φ)}·{cos(3ω m t-α)+cos(ω m t+α)} (13)

[0239] In the above formula (13), α is the offset magnitude when the phase of the reference signal Ss deviates from its original phase. The original phase refers to the phase when the preprocessed signal S(t) output from the preprocessing unit 53 is a signal that has only undergone frequency modulation, or when it becomes a signal equivalent to that.

[0240] The second signal S2, after passing through multiplier 538, passes through third bandpass filter 539. The center angular frequency of third bandpass filter 539 is set to the same value as the center angular frequency of first bandpass filter 534. Here, as an example, the center angular frequency of third bandpass filter 539 is set to ω. m Therefore, the second signal S2 after passing through the third bandpass filter 539 is represented by the following formula.

[0241] Mathematical formula 15

[0242] I BPF3 =J2(B)cos(ω) m t+α)cos(A sinω d t-φ) (14)

[0243] Then, for the first signal S1 represented by the above formula (11), the phase is adjusted by the first delay adjuster 536 and the amplitude is adjusted by the first AGC unit 540.

[0244] Furthermore, the amplitude of the second signal S2 represented by the above formula (14) is also adjusted by the second AGC unit 541 so that the amplitude of the second signal S2 is consistent with the amplitude of the first signal S1. The first signal S1 after amplitude adjustment is represented by the following formula (14-1), and the second signal S2 after amplitude adjustment is represented by the following formula (14-2).

[0245] Mathematical formula 16

[0246] I AGC1 =-sin(ω) m t)·sin(A sinω dt-φ) (14-1)

[0247] I AGC2 =cos(ω m t+α)cos(A sinω d t-φ) (14-2)

[0248] In addition, the first signal S1 and the second signal S2 are added in adder 542. The addition result is used as the preprocessing signal S(t). The preprocessing signal S(t) is represented by the following formula (15).

[0249] Mathematical formula 17

[0250] S(t)=I AGC1 +I AGC2

[0251] =-sin(ω) m t)·sin(A sinω d t-φ)+cos(ω m t+α)cos(A sinω d t-φ) (15)

[0252] As shown in formula (15) above, the preprocessed signal S(t) is represented by a formula including the phase offset magnitude α. Therefore, when the phase offset magnitude α is an integer multiple of π, the above formula (15) is represented by the following formula (15-1).

[0253] Mathematical formula 18

[0254] S(t) = -sin(ω) m t)·sin(A sinω d t-φ)+cos(ω m t)cos(A sinω d t-φ)

[0255] =cos(ω m t+A sinω d t-φ) (15-1)

[0256] In this specification, when the current processed signal S(t) can be represented by the above formula (15-1), it is called "phase consistent". On the other hand, when the current processed signal S(t) can be represented by the above formula (15) and the phase offset α is a value other than an integer multiple of π, it is called "phase inconsistent".

[0257] When the phases are consistent, the preprocessed signal S(t) can be represented by the above formula (15-1), and therefore can be said to be a signal that has only undergone frequency modulation. In such a preprocessed signal S(t), the frequency modulation component is extracted, and therefore, the demodulation accuracy of the sampled signal is improved in the demodulation processing unit 55. On the other hand, when the phases are inconsistent, the preprocessed signal S(t) can be said to be a signal in which frequency modulation and amplitude modulation overlap. In such a preprocessed signal S(t), it is difficult to improve the demodulation accuracy of the sampled signal in the demodulation processing unit 55.

[0258] 1.2.3. Structure of the Orthogonal Signal Generation Unit

[0259] Figure 1 The quadrature signal generation unit 57 shown includes a fourth bandpass filter 571, a Hilbert conversion filter 572, a second delay adjuster 573 (reference signal delayer), a reference signal phase operator 574, an absolute value operator 577, a third low-pass filter 578, a phase setting unit 579, an adder 580, a cosine operator 581, and a sine operator 582.

[0260] In this embodiment, based on the phase of the reference signal Ss and the amplitude of the preprocessing signal S(t), the quadrature signal generation unit 57 generates a cosine wave signal cos(θ) as an orthogonal signal. m (t) and the sinusoidal signal sin(θ) m (t)). In this specification, the process of generating such orthogonal waveforms is referred to as "orthogonal waveform generation process".

[0261] An ADC 533 is connected between the oscillator circuit 54 and the fourth bandpass filter 571. The ADC 533 is an analog-to-digital converter that converts analog signals into digital signals with a predetermined number of sampling bits. The fourth bandpass filter 571 is a filter that selectively allows signals in a specified frequency band to pass through.

[0262] Hilbert transform filter 572 performs Hilbert transform on the reference signal Ss to obtain signal i. The reference signal Ss output by oscillator circuit 54 is composed of cos(ω m The signal represented by t). ω m t is the angular frequency of the modulation signal of the optical modulator 12, and t is time. The Hilbert transform process is used to shift the phase of the reference signal Ss by π / 2.

[0263] The second delay adjuster 573 is a circuit that uses a memory that temporarily stores the signal to adjust the delay of the signal, so that the reference signal Ss has the same delay as the delay generated by the Hilbert conversion process. This yields the signal r.

[0264] The reference signal phase operator 574 calculates the phase of the reference signal Ss based on the signal i output by the Hilbert conversion filter 572 and the signal r output by the second delay adjuster 573. Specifically, it performs an arctangent operation on the ratio of signal i to signal r, i.e., an atan(i / r) operation.

[0265] The absolute value calculator 577 calculates the absolute value of the preprocessed signal S(t) output by the preprocessing unit 53. The third low-pass filter 578 is a filter that filters out high-frequency signals based on the absolute value of the preprocessed signal S(t) output by the absolute value calculator 577.

[0266] The phase setting unit 579 has the functions of acquiring the envelope of the signal output by the third low-pass filter 578, acquiring the maximum and minimum values ​​(amplitude of the envelope) of the envelope, and outputting the phase quantity a.

[0267] The output of adder 580 and the output of reference signal phase operator 574 are proportional to the sum of the output of phase quantity setting unit 579. Cosine operator 581 generates a cosine wave signal cos(θ) based on the signal output from adder 580. m (t)). The sine wave arithmetic unit 582 generates a sine wave signal sin(θ) based on the signal output by the adder 580. m (t)).

[0268] 1.2.4. Orthogonal Waveform Generation Processing

[0269] In the quadrature waveform generation process, firstly, the reference signal Ss output by the ADC533 is input to the fourth bandpass filter 571. In the fourth bandpass filter 571, the center angular frequency is set to ω. m The reference signal Ss output by the fourth bandpass filter 571 is split into two, one of which is input to the Hilbert conversion filter 572, and the other is input to the second delay adjuster 573.

[0270] Hilbert conversion filter 572 shifts the phase of the reference signal Ss by π / 2 to generate signal i. Second delay adjuster 573 delays the reference signal Ss to generate signal r. Signals i and r are input to reference signal phase operator 574.

[0271] The reference signal phase operator 574 performs an operation on atan(i / r) to obtain the phase of the reference signal Ss. The result atan(i / r) is input to the adder 580.

[0272] On the other hand, the absolute value operator 577 acquires the absolute value of the preprocessed signal S(t). This allows the waveform of the negative side of the preprocessed signal S(t) to be converted to the positive side for synthesis. The signal from the absolute value operator 577 is input to the third low-pass filter 578.

[0273] The third low-pass filter 578 filters out signals in the high-frequency band. This allows for easy and high-precision acquisition of the envelope in the phase setting unit 579. The signal from the third low-pass filter 578 is input to the phase setting unit 579.

[0274] The phase setting unit 579 sets the phase amount 'a' to be added to the operation result atan(i / r) in the adder 580 based on the signal from the third low-pass filter 578. In other words, the quadrature signal generation unit 57 adjusts the phase of the reference signal Ss. The setting method will be described later.

[0275] In adder 580, the sum of the output from reference signal phase operator 574 and the output from phase quantity setting unit 579 is calculated. Here, the sum is set as β. β is a + atan(i / r). Additionally, cosine operator 581 generates a cosine wave signal cos(θ). m (t)), the sine wave operator 582 generates a sine wave signal sin(θ). m (t)). Cosine wave signal cos(θ) m (t) is input to multiplier 538 and demodulation processing unit 55 (described later), the sine wave signal sin(θ) m (t) is input to the demodulation processing unit 55. It should be noted that θ m (t) is ω m t-β.

[0276] 1.2.5. Phase Quantity Setting Method

[0277] In the phase setting unit 579, when the phases are inconsistent, the phase amount 'a' added by the adder 580 is set to minimize the effect of the aforementioned amplitude modulation. Consequently, in the cosine operator 581 and the sine operator 582, a cosine wave signal cos(θ) based on the phase amount 'a' is generated. m (t) and the sinusoidal signal sin(θ) m (t)). Therefore, the cosine wave signal cos(θ) m The amplitude modulation (θ) is reflected in the preprocessed signal S(t) via multiplier 538, reducing the effect of amplitude modulation in the preprocessed signal S(t). Furthermore, the final result is that the phase is consistent. Additionally, when the phase is consistent, the cosine signal cos(θ) m (t) and the sinusoidal signal sin(θ) mWhen the preprocessing signal S(t) is input to the demodulation processing unit 55, it can be demodulated with high precision.

[0278] Figure 13 It is shown Figure 1 The flowchart shows an example of a method for setting the phase quantity using the phase quantity setting unit 579. Figure 13 The phase quantity setting shown is preferably performed using a standard sample vibrating at a single frequency as the measurement object 14. Therefore, the phase quantity setting unit 579 can more accurately determine the phase quantity 'a' to be added in the adder 580. Figure 13 In the phase quantity setting method shown, the amplitude of the preprocessed signal S(t) is repeatedly evaluated while the value of the phase quantity a is gradually changed. Furthermore, the phase quantity a at which the amplitude falls below a predetermined value is stored in memory as the optimal value. Moreover, after determining the optimal phase quantity a, by fixing the phase quantity a, high-precision measurements can be performed on various measurement objects 14.

[0279] Therefore, for example, before measuring the object 14 using the laser interferometer 1, the aforementioned standard sample is used. Figure 13 The phase setting method is shown. This allows for automatic calibration of the laser interferometer 1. Examples of standard samples include piezoelectric elements and crystal oscillators.

[0280] exist Figure 13 In the illustrated process S102, the sign function sgn and the phase quantity a are first initialized. Specifically, the value 1 is input into the sign function sgn, and the value a0 is input into the phase quantity a. The value a0 can be any value.

[0281] In step S104, the envelope of the preprocessed signal S(t) is obtained via the absolute value arithmetic unit 577 and the third low-pass filter 578. Methods for obtaining the envelope include, for example, using a low-pass filter or a Hilbert transform. Furthermore, in step S104, the maximum and minimum values ​​of the envelope are obtained, and the difference dS0 between the maximum and minimum values ​​is stored in memory. This difference dS0 corresponds to the amplitude of the preprocessed signal S(t).

[0282] In process S106, the phase quantity 'a' is updated and set as the output value in the phase quantity setting unit 579 using the formula a + sgn * Δa → a. This formula means that, based on the two possible values ​​of the sign function sgn (1 or -1), a small amount Δa is added or subtracted from the current value of the phase quantity 'a' to obtain a new phase quantity 'a'. The small amount Δa is only required to be less than the phase quantity 'a' and is not particularly limited. The updated phase quantity 'a' is output to the adder 580. Therefore, in the quadrature signal generation unit 57, the phase of the reference signal Ss is adjusted based on the new phase quantity 'a' set by the phase quantity setting unit 579, generating two orthogonal signals, namely the cosine wave signal cos(θ). m (t) and the sinusoidal signal sin(θ) m (t)). Then, in the preprocessing unit 53, based on the cosine wave signal cos(θ) m (t) generates a new preprocessing signal S(t).

[0283] In step S108, for the new preprocessing signal S(t), the difference between the maximum and minimum values ​​of the envelope is obtained and stored in memory, similar to step S104. In step S108, this difference is set as dS1.

[0284] In step S110, it is determined whether the difference dS1 is below a predetermined value. The predetermined value refers to, for example, the difference between the maximum and minimum values ​​of the envelope when the currently processed signal S(t) is considered as a signal only modulated by frequency. Therefore, if the difference dS1 is below the predetermined value, it can be determined that the current phase quantity a is optimal. Therefore, the process ends. On the other hand, when the difference dS1 is greater than the predetermined value, it can be determined that the current phase quantity a is not optimal, and therefore, the process proceeds to step S112.

[0285] In step S112, the number of times the judgment in step S110 was performed is obtained, i.e., the number of times it was obtained. Furthermore, it is determined whether the obtained number is greater than or equal to a predetermined number. The predetermined number refers to, for example, the actual number of repetitions required to determine the optimal phase value a when the amplitude of the preprocessed signal S(t) is repeatedly evaluated while gradually changing the value of the phase quantity a, as described above; specifically, the maximum value among these actual values ​​can be used. Specific examples of the number of repetitions include the number of times the phase quantity a is updated.

[0286] When the number of acquisitions exceeds a predetermined number, it is determined that it is difficult to determine the optimal phase value 'a', and therefore, the process proceeds to step S114. In step S114, the phase value setting unit 579 issues an error. In this case, an error is displayed on the aforementioned display unit, and the user can be prompted to take appropriate action, such as changing the standard sample. After the error is issued, the process ends.

[0287] On the other hand, when the number of times obtained is less than the predetermined number, the process is transferred to step S116.

[0288] In step S116, it is determined whether the difference dS0 > difference dS1 holds true. If it does, it can be determined that the amplitude modulation effect has been reduced in the new preprocessing signal S(t). Furthermore, it can be determined that the value of the sign function sgn after initialization in step S102 is appropriate. In this case, the process proceeds to step S118. In step S118, the current value of difference dS1 is input into difference dS0. Then, the process returns to step S106. In the second step S106, the phase quantity a is updated again using the formula a + sgn * Δa → a. Regarding the sign function sgn in this formula, since its appropriate value has already been determined, it remains unchanged. Then, in the preprocessing unit 53, a new preprocessing signal S(t) reflecting the updated phase quantity a is generated.

[0289] In the second step S108, for the new preprocessed signal S(t), the difference between the maximum and minimum values ​​of the envelope is obtained and stored in memory. The difference dS1 obtained in the second step S108 is smaller than the difference dS1 obtained in the first step S108. Therefore, for the new preprocessed signal S(t), the effect of amplitude modulation is further reduced, approaching that of a signal that has only undergone frequency modulation.

[0290] On the other hand, in step S116, when the difference dS0 > difference dS1 is not true, that is, when difference dS0 ≤ difference dS1, it can be determined that the effect of reduced amplitude modulation in the new preprocessed signal S(t) has not been confirmed. Furthermore, it can be determined that the value of the symbol function sgn initialized in step S102 is inappropriate. In this case, the process proceeds to step S120.

[0291] In step S120, the sign function sgn is set to be the opposite of the current value. That is, the value obtained by multiplying the current value by -1 is input into the new sign function sgn. After that, the process moves to step S118.

[0292] according to Figure 13 The process shown involves repeatedly updating the phase quantity a, the preprocessing signal S(t), and the difference dS0, as long as no error occurs, until the difference dS1 falls below a predetermined value. As a result, the phase quantity a is adjusted in the phase quantity setting unit 579, and a cosine wave signal cos(θ) is generated as an orthogonal signal based on this adjustment. m (t) and the sinusoidal signal sin(θ) m (t)) until the preprocessed signal S(t) is considered as a signal that has only been frequency modulated.

[0293] By generating the cosine wave signal cos(θ) in this way m The preprocessed signal S(t) is input to the preprocessing unit 53, thereby making the preprocessed signal S(t) closer to the original phase. As a result, the laser interferometer 1 can be calibrated. Furthermore, after calibration, the measured object 14 can be measured using the obtained optimal phase value a. Thus, the accuracy of demodulating the sampling signal from the optical detection signal can be improved, and the measurement of the measured object 14 can be performed with high precision.

[0294] It should be noted that the calibration of laser interferometer 1 can be performed at any timing and frequency.

[0295] 1.2.6. Composition of the demodulation processing unit

[0296] The demodulation processing unit 55 performs demodulation processing to extract the sampling signal originating from the object to be measured 14 from the preprocessed signal S(t). There are no particular limitations on the demodulation processing method, but the well-known quadrature detection method can be cited as an example. The quadrature detection method is a method of demodulation processing that involves externally mixing mutually orthogonal signals from the input signal.

[0297] Figure 1 The demodulation processing unit 55 shown is a digital circuit that includes a multiplier 551, a multiplier 552, an inverting amplifier 553, a first low-pass filter 555, a second low-pass filter 556, a divider 557, an arctangent operator 558, and a signal output unit 559.

[0298] Multipliers 551 and 552 are circuits that generate an output signal proportional to the product of the two input signals. Inverting amplifier 553 is a circuit that generates an output signal with a gain of -1, no change in amplitude, and an inverted phase of the input signals. The first low-pass filter 555 and the second low-pass filter 556 are filters that remove signals from the high-frequency band.

[0299] Divider 557 is a circuit that generates an output signal proportional to the quotient of the two input signals. Arctangent amplifier 558 is a circuit that outputs the arctangent of the input signals. Signal output unit 559 calculates the phase value obtained by arctangent amplifier 558. Calculate the phase The information originates from the object being measured 14. Furthermore, the signal output unit 559 performs phase connection when there is a 2π phase jump between two adjacent points through phase expansion processing. Additionally, the displacement of the object being measured 14 is calculated based on the obtained phase information. Thus, a displacement meter is realized. Furthermore, the velocity of the object being measured 14 can be determined from the displacement. Thus, a speedometer is realized.

[0300] It should be noted that the demodulation processing unit 55 is not limited to digital circuits, but can also be an analog circuit. The analog circuit may also include an F / V converter circuit and a ΔΣ counter circuit.

[0301] Furthermore, in the demodulation processing unit 55 described above, frequency information originating from the object being measured 14 can also be calculated. Based on the frequency information, the velocity of the object being measured 14 can be calculated.

[0302] 1.2.7. Demodulation Processing

[0303] In the demodulation process, the preprocessed signal S(t) is first split into two by branch jp2. For one of the split signals, the sine wave signal sin(θ) output by the sine arithmetic unit 582 is processed by the inverting amplifier 553 in multiplier 551. m Multiplication is performed on the preprocessed signal S(t). That is, in multiplier 551, the sinusoidal signal sin(θ) is mixed into the preprocessed signal S(t). m The signal obtained by inverting the phase of (t) is -sin(θ) m (t)). For the other segmented signal, in multiplier 552, it is multiplied by the cosine wave signal cos(θ) output by cosine operator 581. m (t)). That is, in multiplier 552, the cosine signal cos(θ) is mixed into the preprocessed signal S(t). m (t)).

[0304] The signal passing through multiplier 551 passes through first low-pass filter 555, and is then input as signal y into divider 557. The signal passing through multiplier 552 passes through second low-pass filter 556, and is then input as signal x into divider 557. It should be noted that in this specification, signals x and y are referred to together as a "mixed signal". In divider 557, a division operation is performed between signal y and signal x, and the output y / x is passed through arctangent operator 558 to obtain the result atan(y / x). Thus, the phase information of the sampled signal is obtained.

[0305] Then, by inputting the calculation result atan(y / x) into the signal output unit 559, the displacement and velocity of the object to be measured 14 are output.

[0306] Figure 14 It means to perform based on Figure 13 The following are examples of waveforms of the laser interferometer 1 before calibration: the preprocessed signal S(t), the phase information after demodulation by the demodulation processing unit 55 (phase information before expansion), and the phase information after phase expansion processing by the signal output unit 559 (phase information after expansion).

[0307] like Figure 14 As shown, the amplitude of the preprocessed signal S(t) before calibration changes significantly, becoming a signal with overlapping frequency and amplitude modulation. Therefore, the amplitude of the envelope of the preprocessed signal S(t) also increases. Thus, in Figure 14 In the unwound phase information shown, irregular waveforms are generated at the locations indicated by the arrows, resulting in discontinuous waveforms in the unwound phase information. Under these circumstances, it is difficult to accurately determine the displacement of the object 14 based on the unwound phase information.

[0308] Figure 15 It means that a process based on Figure 13 The following are examples of waveforms of the laser interferometer 1 after calibration: the pre-processed signal S(t), the phase information after demodulation by the demodulation processing unit 55 (phase information before expansion), and the phase information after phase expansion processing by the signal output unit 559 (phase information after expansion).

[0309] like Figure 15 As shown, the amplitude change of the preprocessed signal S(t) after calibration is small, almost becoming a signal modulated only by frequency. Therefore, the amplitude of the envelope of the preprocessed signal S(t) also decreases. Therefore, in Figure 15 The phase information before expansion, as shown, did not produce irregular waveforms. Furthermore, Figure 15 The waveform of the unfolded phase information shown becomes a continuous waveform. In this case, the displacement of the object 14 can be determined with excellent accuracy based on the unfolded phase information.

[0310] 1.2.8. Effects of the First Implementation Method

[0311] As described above, the laser interferometer 1 according to this embodiment includes a laser source 2, an optical modulator 12, a light-receiving element 10, and a calculation unit 52. The laser source 2 emits an outgoing light L1 (first laser). The optical modulator 12 includes a vibration element 30, which modulates the outgoing light L1 to generate a reference light L2 (second laser) including a modulation signal. The light-receiving element 10 receives an object light L3 (third laser) including a sampling signal generated by the reflection of the outgoing light L1 by the object to be measured 14, and the reference light L2, and outputs a received light signal. The calculation unit 52 calculates the displacement of the object to be measured 14 based on the reference signal Ss and the received light signal.

[0312] Furthermore, the arithmetic unit 52 includes a preprocessing unit 53, a demodulation processing unit 55, and an orthogonal signal generation unit 57. The preprocessing unit 53 performs preprocessing to extract the frequency modulation component from the received light signal and outputs a preprocessed signal S(t). The demodulation processing unit 55 mixes the orthogonal signal in the preprocessed signal S(t) to obtain signals x and y (mixed signals), and then performs demodulation processing to extract the sampled signal from signals x and y. In this embodiment, the orthogonal signal generation unit 57 generates the aforementioned orthogonal signal based on the phase of the reference signal Ss and the amplitude of the preprocessed signal S(t).

[0313] In this configuration, the orthogonal signal generation unit 57 adjusts the phase of the reference signal Ss based on the amplitude of the preprocessed signal S(t). Furthermore, the phase of the reference signal Ss is adjusted by using the cosine wave signal cos(θ) as the adjusted signal. m The preprocessing signal S(t) is input to the preprocessing unit 53, thereby making the preprocessing signal S(t) close to the signal that has only undergone frequency modulation, which improves the accuracy of demodulating the sampling signal originating from the object to be measured 14 from the received light signal. In other words, the laser interferometer 1 can be calibrated in order to perform high-precision demodulation processing. As a result, even when using the reference signal Ss output by the oscillation circuit 54, the laser interferometer 1 can be realized to measure the displacement and velocity of the object to be measured 14 with high precision. As a result, the laser interferometer 1 can be easily miniaturized, lightened, and reduced in cost.

[0314] Furthermore, the quadrature signal generation unit 57 has a phase amount setting unit 579 that sets the phase of the quadrature signal based on the amplitude of the preprocessed signal S(t).

[0315] As mentioned above, the phase setting unit 579 has the function of setting the phase amount 'a' to be added in the adder 580. Furthermore, the quadrature signal generation unit 57 adjusts the phase of the reference signal Ss based on the phase amount 'a' to generate an orthogonal signal, namely a cosine wave signal cos(θ). m (t) and the sinusoidal signal sin(θ) m The quadrature signal is mixed with the preprocessed signal S(t) in the demodulation processing unit 55. By appropriately setting the phase amount a, the phases of the preprocessed signal S(t) and the quadrature signal can be made consistent. As a result, the demodulation processing unit 55 can perform demodulation processing on the preprocessed signal S(t) with high precision.

[0316] The phase setting unit 579 sets the phase of the quadrature signal so that the difference between the maximum value of the amplitude of the preprocessed signal S(t) and the minimum value of the amplitude of the preprocessed signal S(t) is below a predetermined value.

[0317] Therefore, in order to make the preprocessed signal S(t) a signal that is only frequency modulated or equivalent, the optimal phase amount a can be efficiently found in the phase amount setting unit 579.

[0318] The quadrature signal generation unit 57 includes a Hilbert conversion filter 572, a second delay adjuster 573 (reference signal delayer), and a reference signal phase operator 574. The Hilbert conversion filter 572 performs a Hilbert conversion on the reference signal Ss to obtain signal i. The second delay adjuster 573 delays the reference signal Ss to obtain signal r. The reference signal phase operator 574 performs an arctangent operation on the ratio of signal i to signal r to obtain the phase of the reference signal Ss.

[0319] With this configuration, the phase of the reference signal Ss can be obtained instantaneously without sampling. Therefore, in the quadrature signal generation unit 57 having such a phase setting unit 579, the phase of the reference signal Ss can be reflected in the quadrature signal in a short time. As a result, the optimal phase amount a can be quickly found in the phase setting unit 579.

[0320] 2. Second Implementation Method

[0321] Next, the laser interferometer involved in the second embodiment will be described.

[0322] Figure 16 This is a functional block diagram illustrating the laser interferometer according to the second embodiment.

[0323] The second embodiment will now be described; however, the description will focus on the differences from the first embodiment, and details concerning the same aspects will be omitted. It should be noted that... Figure 16 In this drawing, the same reference numerals are used to indicate the same configuration as in the first embodiment.

[0324] In the first embodiment described above, the quadrature signal generation unit 57 generates quadrature signals based on the phase of the reference signal Ss and the amplitude of the preprocessed signal S(t) obtained from the envelope of the preprocessed signal S(t). In this embodiment, the quadrature signal generation unit 57A generates quadrature signals based on the phase of the reference signal Ss and the amplitudes of signals x and y (mixed signals).

[0325] 2.1. Demodulation Processing Department

[0326] Figure 16 The demodulation processing unit 55A shown here, except for the addition of a mixed-signal amplitude arithmetic unit 560, is all related to... Figure 1 The demodulation processing unit 55 shown is the same.

[0327] The mixed-signal amplitude operator 560 performs a 2(x) operation on two input signals, x and y. 2 +y 2 ) 1 / 2 This circuit performs calculations and outputs the results. The result is equivalent to the "amplitude of the mixed signal". In this specification, the signal output from the mixed signal amplitude calculator 560 is referred to as the "amplitude signal Sa".

[0328] 2.2. Orthogonal signal generation unit

[0329] Figure 16 The quadrature signal generation unit 57A shown here, except for omitting the absolute value calculator 577 and the third low-pass filter 578, and inputting the amplitude signal Sa output by the mixed signal amplitude calculator 560 to the phase setting unit 579A, is otherwise identical to... Figure 1 The orthogonal signal generation unit 57 shown is the same.

[0330] The phase quantity setting unit 579A has the function of acquiring the maximum and minimum values ​​of the amplitude signal Sa and the function of outputting the phase quantity a. Therefore, the phase quantity setting method of the phase quantity setting unit 579A, in addition to... Figure 13 Except for replacing "envelope of preprocessing signal S(t)" with "amplitude signal Sa", all other values ​​are consistent with... Figure 13 The method shown is the same.

[0331] 2.3. Effects of the Second Implementation Method

[0332] The laser interferometer 1A according to this embodiment, like the one in the first embodiment, includes a laser source 2, an optical modulator 12, a light-receiving element 10, and a computing unit 52. Among them, as shown... Figure 16 As shown, the arithmetic unit 52 includes a preprocessing unit 53, a demodulation processing unit 55A, and an orthogonal signal generation unit 57A. The preprocessing unit 53 performs preprocessing to extract the frequency modulation component from the received light signal and outputs a preprocessed signal S(t). The demodulation processing unit 55A, after mixing the orthogonal signal with the preprocessed signal S(t) to obtain signals x and y (mixed signals), performs demodulation processing to extract sampled signals from signals x and y. The orthogonal signal generation unit 57A generates orthogonal signals based on the phase of the reference signal Ss and the amplitudes of signals x and y.

[0333] In this configuration, the orthogonal signal generation unit 57A adjusts the phase of the reference signal Ss based on the amplitude signal Sa. Furthermore, it uses the cosine wave signal cos(θ) as the adjusted signal... mBy inputting the preprocessing signal S(t) into the preprocessing unit 53, the preprocessing signal S(t) can be made close to a signal that has only undergone frequency modulation, thereby improving the accuracy of demodulating the sampling signal originating from the object to be measured 14 from the received light signal. This allows for the calibration of the laser interferometer 1A. As a result, even when using the reference signal Ss output by the oscillation circuit 54, a laser interferometer 1A capable of measuring the displacement and velocity of the object to be measured 14 with high precision can be achieved. This facilitates the miniaturization, weight reduction, and cost reduction of the laser interferometer 1A.

[0334] In addition, the quadrature signal generation unit 57A has a phase amount setting unit 579A that sets the phase of the quadrature signal based on the amplitude of the signals x and y (mixed signals).

[0335] Like the phase setting unit 579, the phase setting unit 579A also has the function of setting the phase amount 'a' to be added in the adder 580. Furthermore, like the quadrature signal generation unit 57A and the quadrature signal generation unit 57, the quadrature signal generation unit 57A adjusts the phase of the reference signal Ss based on the phase amount 'a' to generate a quadrature signal, i.e., a cosine wave signal cos(θ). m (t) and the sinusoidal signal sin(θ) m The quadrature signal is mixed with the preprocessed signal S(t) in the demodulation processing unit 55. By appropriately setting the phase amount a, the phases of the preprocessed signal S(t) and the quadrature signal can be made consistent. As a result, the demodulation processing unit 55 can perform demodulation processing on the preprocessed signal S(t) with high precision.

[0336] The phase setting unit 579A sets the phase of the quadrature signal so that the difference between the maximum value of the amplitude of the signal x and y (mixed signal) and the minimum value of the amplitude of the signal x and y (mixed signal) is below a predetermined value.

[0337] Therefore, in order to make the preprocessed signal S(t) a signal that is only frequency modulated or equivalent, the optimal phase amount a can be efficiently found in the phase amount setting unit 579A.

[0338] also, Figure 16 The demodulation processing unit 55A shown has a mixed signal amplitude calculator 560 that obtains the amplitude of signals x and y (mixed signals).

[0339] With this configuration, the amplitude of the preprocessed signal S(t) can be obtained without needing to obtain the envelope of the preprocessed signal S(t). In other words, the absolute value operator 577 and the third low-pass filter 578 in the first embodiment can be omitted. This simplifies the configuration of the quadrature signal generation unit 57A.

[0340] In the second embodiment described above, the same effect as in the first embodiment can also be obtained.

[0341] The laser interferometer of the present invention has been described above based on the illustrated embodiments. However, the laser interferometer of the present invention is not limited to the above embodiments, and the configuration of each part can be replaced with any configuration having the same function. Furthermore, other arbitrary components can be added to the laser interferometer involved in the above embodiments. Moreover, the laser interferometer of the present invention can also combine two laser interferometers including the above embodiments. Furthermore, the functional parts of the laser interferometer of the present invention can be divided into multiple elements, or multiple functional parts can be combined into one functional part.

[0342] In addition to the aforementioned displacement gauges and velocimeters, the laser interferometer of the present invention can also be applied to, for example, vibration meters, inclinometers, and distance meters (length measuring devices). Furthermore, as applications of the laser interferometer of the present invention, examples include optical comb interferometry technology that enables distance measurement, 3D imaging, and beam splitting, as well as fiber optic gyroscopes that enable angular velocity sensors and angular acceleration sensors.

[0343] Furthermore, two or more of the laser source, optical modulator, and light-receiving element can be mounted on the same substrate. This allows for easy miniaturization and weight reduction of the optical system while improving assembly ease.

[0344] Furthermore, while the above embodiments have a so-called Michelson-type interferometric optical system, the laser interferometer of the present invention can also be applied to embodiments having other types of interferometric optical systems, such as Mach-Zehnder-type interferometric optical systems.

Claims

1. A laser interferometer, characterized by, Possessing: a laser light source that emits first laser light; a light modulator that possesses a vibration element and modulates the first laser light using the vibration element to generate second laser light that includes a modulation signal; a light-receiving element that receives third laser light that includes a sample signal generated by the first laser light being reflected by a measurement target object and the second laser light, and outputs a light-receiving signal; and an arithmetic unit that calculates a displacement of the measurement target object from the light-receiving signal based on a reference signal, the arithmetic unit has: a pre-processing unit that performs pre-processing that extracts a frequency-modulated component from the light-receiving signal, and outputs a pre-processed signal; a demodulation processing unit that performs demodulation processing that extracts the sample signal from a mixed signal obtained by mixing a quadrature signal into the pre-processed signal, after the mixed signal is obtained; and a quadrature signal generation unit that generates the quadrature signal based on a phase of the reference signal and an amplitude of the pre-processed signal or based on the phase of the reference signal and an amplitude of the mixed signal.

2. The laser interferometer according to claim 1, wherein the quadrature signal generation unit has a phase amount setting unit that sets a phase of the quadrature signal in accordance with the amplitude of the pre-processed signal or the amplitude of the mixed signal.

3. The laser interferometer according to claim 2, wherein the phase amount setting unit sets the phase of the quadrature signal so that a difference between a maximum value of the amplitude of the pre-processed signal and a minimum value of the amplitude of the pre-processed signal or a difference between a maximum value of the amplitude of the mixed signal and a minimum value of the amplitude of the mixed signal is equal to or less than a predetermined value.

4. The laser interferometer according to any one of claims 1 to 3, wherein the quadrature signal generation unit has: a Hilbert transform filter that performs Hilbert transform processing on the reference signal to obtain a signal i; a reference signal delay unit that causes the reference signal to be delayed to obtain a signal r; and a reference signal phase arithmetic unit that performs an inverse tangent operation on a ratio of the signal i / the signal r to obtain a phase of the reference signal.

5. The laser interferometer according to any one of claims 1 to 3, wherein the demodulation processing unit has a mixed signal amplitude arithmetic unit that obtains the amplitude of the mixed signal.

6. The laser interferometer according to any one of claims 1 to 3, wherein the laser interferometer possesses: an oscillation circuit that outputs the reference signal, the vibration element is a signal source of the oscillation circuit.

7. The laser interferometer according to claim 6, wherein the vibration element is a crystal oscillator. ​ ​ ​

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