laser interferometer

Abstract

Provided is a laser interferometer capable of suppressing the incidence of return light on a laser light source and capable of suppressing a decrease in the precision of demodulation of a sampling signal originating from a measurement object based on a light reception signal. The laser interferometer includes a laser light source; a collimator that generates collimated light; a light modulator that modulates the collimated light into reference light having different frequencies; and a light reception element that receives object light and the reference light and outputs a light reception signal, the light axis of the collimated light is set as a first light axis, the light axis of the return light is set as a second light axis when the return light is generated, the position at which the collimated light is generated is set as a reference position, when the offset width of the first light axis and the second light axis in the reference position is set as Δy, the effective diameter of the collimator is set as κ, the optical path length of the collimated light is set as R, the distance from the reference position to the light modulator is set as L, and the wavelength of the collimated light is set as λ, the following expression (A) is satisfied

Description

TECHNICAL FIELD

[0001] The present application relates to a laser interferometer. BACKGROUND

[0002] A laser vibrometer as a device for measuring a vibration velocity of an object is disclosed in Patent Literature 1. The laser vibrometer irradiates laser light to a measured object and measures a vibration velocity based on scattered light subjected to Doppler shift.

[0003] Specifically, the laser vibrometer described in Patent Literature 1 is provided with an Acousto-Optic Modulator (AOM). The Acousto-Optic Modulator shifts the frequency of laser light by changing the frequency of supplied ultrasonic waves. The laser vibrometer uses the laser light whose frequency is shifted as reference light. Then, scattered light from the measured object and the reference light from the Acousto-Optic Modulator are made to coincide with each other and beat frequency is extracted. The vibration velocity of the measured object is calculated from the beat frequency thus extracted.

[0004] Patent Literature 1: Japanese Patent Application Laid-Open No. 2007-285898 SUMMARY

[0005] In a laser light source, laser oscillation sometimes becomes unstable due to return light intrusion. In the laser vibrometer described in Patent Literature 1, when laser oscillation becomes unstable, the quality of laser light decreases. Thus, in the laser vibrometer, the S / N ratio decreases or the phase of oscillating laser light is discontinuous. As a result, there is a problem that the measurement accuracy of the vibration velocity of the object decreases.

[0006] The laser interferometer according to an application example of the present application is characterized by comprising:

[0007] a laser light source that emits laser light;

[0008] a collimating section that collimates the laser light to generate collimated light;

[0009] a light modulator that modulates the collimated light into reference light having a different frequency; and

[0010] a light receiving element that receives object light generated by reflection of the collimated light from a measured object and the reference light, and outputs a light receiving signal,

[0011] the optical axis of the collimated light is set as a first optical axis,

[0012] when return light of the reference light or the object light generated by the collimated section goes to the laser light source, the optical axis of the return light is set as a second optical axis,

[0013] a position at which the collimated light is generated in the collimating section is set as a reference position,

[0014] When the offset width of the first optical axis and the second optical axis in the reference position is set as Δy,

[0015] The effective diameter of the collimating portion is set as κ,

[0016] The optical path of the collimated light in the collimating portion is set as R,

[0017] The distance of the reference position from the light modulator is set as L,

[0018] When the wavelength of the collimated light is set as λ,

[0019] The first optical axis and the second optical axis are offset in a manner satisfying the following expression (A)

[0020] [Expression 1]

[0021] BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a functional block diagram illustrating a laser interferometer according to a first embodiment.

[0023] Figure 2 is a functional block diagram illustrating Figure 1 is a schematic configuration diagram of the sensor head.

[0024] Figure 3 is a functional block diagram illustrating Figure 2 is a perspective view illustrating a first configuration example of the light modulator.

[0025] Figure 4 is a plan view illustrating a part of a second configuration example of the light modulator.

[0026] Figure 5 is a plan view illustrating a third configuration example of the light modulator.

[0027] Figure 6 is a conceptual diagram illustrating that a plurality of diffracted lights are generated when incident light K i is a conceptual diagram illustrating that a plurality of diffracted lights are generated when incident light K

[0028] Figure 7 is a conceptual diagram illustrating that the light modulator is configured such that the angle formed by the traveling direction of the incident light K i is a conceptual diagram illustrating that the light modulator is configured such that the angle formed by the traveling direction of the incident light K

[0029] Figure 8 is a conceptual diagram illustrating that the light modulator is configured such that the angle formed by the traveling direction of the incident light K i is a conceptual diagram illustrating that the light modulator is configured such that the angle formed by the traveling direction of the incident light K

[0030] Figure 9 is a conceptual diagram illustrating a light modulator configured so that the traveling direction of incident light K i is a conceptual diagram illustrating a light modulator in which the angle formed by the traveling direction of the reference light L2 and the traveling direction of the incident light K

[0031] Figure 10 is a cross-sectional view illustrating a light modulator having a package structure.

[0032] Figure 11 is a circuit diagram illustrating the configuration of a first-order inverter oscillation circuit.

[0033] Figure 12 is an example of an LCR equivalent circuit of a vibration element.

[0034] Figure 13 is a schematic diagram illustrating the optical path of the outgoing light L1 emitted from the laser light source shown in Figure 2 , and the optical path in the case where the reference light L2 generated by the reflection of the outgoing light L1 by the light modulator goes to the laser light source as the return light L5.

[0035] Figure 14 is a schematic configuration diagram illustrating the sensor head of the laser interferometer according to the second embodiment.

[0036] Figure 15 is a schematic diagram illustrating the optical path of the outgoing light L1 emitted from the laser light source shown in Figure 14 , and the optical path in the case where the reference light L2 generated by the reflection of the outgoing light L1 by the light modulator goes to the laser light source as the return light L5.

[0037] Figure 16 is a schematic configuration diagram illustrating the mounting structure of the optical system provided in the laser interferometer according to the first modification.

[0038] Figure 17 is a schematic configuration diagram illustrating the mounting structure of the optical system provided in the laser interferometer according to the second modification.

[0039] Figure 18 is a schematic configuration diagram illustrating the mounting structure of the optical system provided in the laser interferometer according to the third modification.

[0040] Figure 19 is a schematic configuration diagram illustrating the mounting structure of the optical system provided in the laser interferometer according to the fourth modification.

[0041] Figure 20 is a schematic configuration diagram illustrating the mounting structure of the optical system provided in the laser interferometer according to the fifth modification.

[0042] BRIEF DESCRIPTION OF THE DRAWINGS

[0043] 1 … laser interferometer, 2 … laser light source, 3 … collimator lens, 4 … polarization beam splitter, 4a … polarization beam splitter, 4b … polarization beam splitter, 4c … polarization beam splitter, 6 … 1 / 4 wave plate, 7 … 1 / 2 wave plate, 8 … 1 / 4 wave plate, 9 … polarizer, 10 … light receiving element, 12 … light modulator, 12H … light modulator, 14 … measurement object, 16 … seating portion, 17 … shielding element, 18 … optical path, 20 … optical path, 22 … optical path, 24 … optical path, 30 … vibration element, 30A … vibration element, 30B … vibration element, 31 … substrate, 32 … groove, 33 … pad, 34 … diffraction grating, 35 … pad, 36 … vibration direction, 37 … mirror, 39 … substrate, 40 … prism, 41 … mirror, 42 … prism, 44 … collimator lens, 45 … circuit element, 50 … optical system, 50D … optical system, 50E … optical system, 50F … optical system, 50G … optical system, 50H … optical system, 51 … sensor head, 52 … demodulation circuit, 53 … preprocessing portion, 54 … oscillation circuit, 55 … demodulation processing portion, 70 … container, 72 … container main body, 74 … lid, 76 … wire, 120 … light modulation vibrator, 172 … opening, 301 … first electrode, 302 … second electrode, 303 … diffraction grating seating portion, 305 … piezoelectric substrate, 306 … comb-shaped electrode, 307 … ground electrode, 311 … front surface, 312 … back surface, 531 … current-voltage converter, 532 … ADC, 533 … ADC, 534 … first band-pass filter, 535 … second band-pass filter, 536 … first delay adjuster, 537 … second delay adjuster, 538 … multiplier, 539 … third band-pass filter, 540 … first AGC, 541 … second AGC, 542 … adder, 551 … multiplier, 552 … multiplier, 553 … phase shifter, 555 … first low-pass filter, 556 … second low-pass filter, 557 … divider, 558 … arctangent operator, 559 … output circuit, 721 … first recess, 722 … second recess, A1 … first optical axis, A2 … second optical axis, C0 … parallel capacitor, C1 … series capacitor, C3 … third capacitor, Cd … second capacitor, Cg … first capacitor, GND … GND terminal, K -2s … diffracted light, K -1s … diffracted light, K 0s … diffracted light, K 1s … diffracted light, K 2s … diffracted light, K i…incident light, L1…series inductance, L1…exit light, L1a…reflected light, L1b…transmitted light, L2…reference light, L3…object light, L4…object reference light, L5…return light, N…normal line, P…pitch, R1…equivalent series resistance, Rd…limiting resistance, Rf…feedback resistance, S1…first signal, S2…second signal, Sd…drive signal, Ss…reference signal, Vcc…terminal, X1…terminal, X2…terminal, Y…terminal, jp1…branch portion, jp2…branch portion, ps1…first signal path, ps2…second signal path, x…signal, y…signal, Δy…deviation width, β…incident angle, θ'…deviation angle, θ B …blaze angle, θ S …tilt angle. DETAILED DESCRIPTION

[0044] Hereinafter, the laser interferometer of the present application will be described in detail based on the embodiment shown in the drawings.

[0045] 1. First Embodiment

[0046] First, the laser interferometer related to the first embodiment will be described.

[0047] Figure 1 is a functional block diagram showing the laser interferometer related to the first embodiment.

[0048] Figure 1 The laser interferometer 1 shown in the drawing has a sensor head 51 provided with an optical system 50 and an oscillation circuit 54, and a demodulation circuit 52 to which a light-receiving signal from the optical system 50 is input.

[0049] 1.1. Sensor Head

[0050] Figure 2 is a schematic configuration diagram of the sensor head 51 shown in Figure 1 The sensor head 51 shown in the drawing has the optical system 50, the oscillation circuit 54, and the demodulation circuit 52.

[0051] 1.1.1. Optical System

[0052] As described above, the sensor head 51 is provided with the optical system 50.

[0053] As shown in Figure 2 , the optical system 50 is provided with a laser light source 2, a collimator lens 3 (collimating portion), a polarizing beam splitter 4 (light divider), a 1 / 4 wavelength plate 6, a 1 / 4 wavelength plate 8, an analyzer 9, a light-receiving element 10, a frequency shift type light modulator 12, and a seating portion 16 in which a measurement target object 14 is disposed.

[0054] The laser light source 2 emits outgoing light Ll of a prescribed wavelength (laser light). The light receiving element 10 converts received light into an electric signal. The light modulator 12 has the vibrating element 30, modulates the outgoing light Ll, and generates reference light L2 containing a modulation signal. The placement section 16 can be provided as needed, and the measurement target object 14 can be disposed. The outgoing light Ll incident on the measurement target object 14 is reflected as object light L3 containing a Doppler signal, i.e., a sample signal, originating from the measurement target object 14.

[0055] The optical path of the outgoing light Ll emitted from the laser light source 2 is set as optical path 18. The optical path 18 is combined with optical path 20 by reflection at the polarizing beam splitter 4. The 1 / 4 wave plate 8 and the light modulator 12 are disposed in this order from the polarizing beam splitter 4 side on the optical path 20. In addition, the optical path 18 is combined with optical path 22 by transmission at the polarizing beam splitter 4. The 1 / 4 wave plate 6 and the placement section 16 are disposed in this order from the polarizing beam splitter 4 side on the optical path 22.

[0056] The optical path 20 is combined with optical path 24 by transmission at the polarizing beam splitter 4. The polarizer 9 and the light receiving element 10 are disposed in this order from the polarizing beam splitter 4 side on the optical path 24.

[0057] The outgoing light Ll emitted from the laser light source 2 is incident on the light modulator 12 via the optical path 18 and the optical path 20. In addition, the outgoing light Ll is incident on the measurement target object 14 via the optical path 18 and the optical path 22. The reference light L2 generated by the light modulator 12 is incident on the light receiving element 10 via the optical path 20 and the optical path 24. The object light L3 generated by reflection at the measurement target object 14 is incident on the light receiving element 10 via the optical path 22 and the optical path 24.

[0058] Note that the "optical path" in the present specification refers to a path set between optical members through which light travels. In addition, the "optical axis" described later refers to a central axis of a light beam passing through the optical path.

[0059] Hereinafter, each section of the optical system 50 will be further described.

[0060] 1.1.1.1. Laser light source

[0061] The laser light source 2 is a laser light source that emits outgoing light Ll having interferability. The laser light source 2 preferably uses a light source having a line width of MHz band or less. Specifically, a gas laser such as a He-Ne laser, a semiconductor laser element such as a DFB-LD (Distributed feedback-laser diode), an FBG-LD (Fiber Bragg Grating-laser diode), a VCSEL (Vertical Cavity Surface Emitting Laser), and an FP-LD (Fabry-Perot Laser Diode) can be cited.

[0062] The laser light source 2 is particularly preferably a semiconductor laser element. Thereby, the laser light source 2 can be particularly downsized. Thus, downsizing of the laser interferometer 1 can be achieved. In particular, downsizing and weight reduction of the sensor head 51 in which the optical system 50 is accommodated in the laser interferometer 1 can be achieved, and thus, the operability of the laser interferometer 1 can be improved.

[0063] 1.1.1.2. Collimator lens

[0064] The collimator lens 3 is a convex lens disposed between the laser light source 2 and the polarization beam splitter 4. The collimator lens 3 collimates the outgoing light Ll emitted from the laser light source 2.

[0065] Note that, in a case where the outgoing light Ll emitted from the laser light source 2 is sufficiently collimated, for example, in a case where a gas laser such as a He-Ne laser is used as the laser light source 2, the collimator lens 3 can be omitted. In this case, it can be considered that the gas laser has both the laser light source 2 and the collimating portion.

[0066] On the other hand, in a case where the laser light source 2 is a semiconductor laser element, as shown in the present embodiment, the laser interferometer 1 preferably has the collimator lens 3 as the collimating portion. Thereby, the outgoing light Ll emitted from the semiconductor laser element can be collimated. As a result, both collimated light and downsizing of the laser interferometer 1 can be achieved.

[0067] 1.1.1.3. Polarization beam splitter

[0068] The polarizing beam splitter 4 is an optical splitter disposed between the laser light source 2 and the light modulator 12 and between the laser light source 2 and the measurement object 14. The polarizing beam splitter 4 splits the emergent light LI into reflected light Lla (first split light) and transmitted light Lib (second split light). In addition, the polarizing beam splitter 4 has a function of transmitting P-polarized light and reflecting S-polarized light. Hereinafter, a case where the emergent light LI, in which the ratio of P-polarized light to S-polarized light in linearly polarized light is set to, for example, 50:50, is incident on the polarizing beam splitter 4 is considered.

[0069] The reflected light Lla, which is S-polarized light reflected by the polarizing beam splitter 4, is converted into circularly polarized light by the ¼ wave plate 8, and is incident on the light modulator 12. The circularly polarized light of the reflected light Lla incident on the light modulator 12 receives a frequency shift of f m [Hz], and is reflected as reference light L2. Thus, the reference light L2 contains a modulation signal of frequency f m [Hz]. The P-polarized light of the reference light L2 is transmitted through the polarizing beam splitter 4 and the polarizer 9, and is incident on the light-receiving element 10.

[0070] The transmitted light Lib, which is P-polarized light transmitted through the polarizing beam splitter 4, is converted into circularly polarized light by the ¼ wave plate 6, and is incident on the measurement object 14 in a moving state. The circularly polarized light of the transmitted light Lib incident on the measurement object 14 receives a Doppler frequency shift of f d [Hz], and is reflected as object light L3. Thus, the object light L3 contains a sampling signal of frequency f d [Hz]. The S-polarized light of the object light L3 is reflected by the polarizing beam splitter 4, is transmitted through the polarizer 9, and is incident on the light-receiving element 10.

[0071] As described above, since the emergent light LI has interferability, the reference light L2 and the object light L3 are incident on the light-receiving element 10 as interference light.

[0072] Note that, instead of the polarizing beam splitter, a non-polarizing beam splitter can be used. In this case, the ¼ wave plate 6 and the ¼ wave plate 8, and the like are no longer needed, and thus miniaturization of the laser interferometer 1 by reduction in the number of components can be achieved. In addition, an optical splitter other than the polarizing beam splitter 4 can be used.

[0073] 1.1.1.4. Polarizer

[0074] The S-polarized light and the P-polarized light, which are orthogonal to each other, are independent of each other, and thus beat caused by interference does not occur by simply superimposing them. Therefore, the light wave obtained by superimposing the S-polarized light and the P-polarized light is passed through a polarizer 9 inclined by 45° with respect to both the S-polarized light and the P-polarized light. By using the polarizer 9, components common to each other are transmitted and interference occurs. As a result, in the polarizer 9, the reference light L2 and the object light L3 interfere with each other to generate interference light having a frequency of |f m -f d |[Hz].

[0075] 1.1.1.5. Light-receiving element

[0076] The reference light L2 and the object light L3 are incident on a light-receiving element 10 via the polarization beam splitter 4 and the polarizer 9. The reference light L2 and the object light L3 undergo optical heterodyne interference to be incident on the light-receiving element 10 as interference light having a frequency of |f m -f d |[Hz]. The sampling signal is demodulated from the interference light by the method described later, and thus the motion of the measurement object 14, that is, the vibration velocity or the displacement, can be finally obtained. As the light-receiving element 10, for example, a photodiode or the like can be given.

[0077] 1.1.1.6. Light modulator

[0078] Figure 3 is a perspective view showing a first configuration example of the light modulator 12 shown in Figure 2

[0079] 1.1.1.6.1. Outline of the first configuration example of the light modulator

[0080] The frequency shift type light modulator 12 has a light modulating vibrator 120. Figure 3 The light modulating vibrator 120 shown in the drawing has a plate-shaped vibration element 30 and a substrate 31 that supports the vibration element 30.

[0081] The vibration element 30 is composed of a material that repeatedly performs a mode of vibrating by applying an electric potential to deform in a direction along a face. In the present configuration example, the vibration element 30 is a crystal AT vibrator that performs thickness shear vibration in a high frequency region of the MHz band along a vibration direction 36. A diffraction grating 34 is formed on the surface of the vibration element 30. The diffraction grating 34 has a structure in which grooves 32 having a component intersecting the vibration direction 36, that is, a plurality of grooves 32 extending in a straight line in a direction intersecting the vibration direction 36 are periodically arranged.

[0082] ​The substrate 31 has a surface 311 and a back surface 312 which have a front-back relationship with each other. The vibration element 30 is disposed on the surface 311. In addition, a pad 33 for applying a potential to the vibration element 30 is provided on the surface 311. On the other hand, a pad 35 for applying a potential to the vibration element 30 is also provided on the back surface 312.

[0083] The size of the substrate 31 is, for example, set to a range of 0.5 mm or more and 10.0 mm or less in the long side. In addition, the thickness of the substrate 31 is, for example, set to a range of 0.10 mm or more and 2.0 mm or less. As one example, the shape of the substrate 31 is set to a square of 1.6 mm on a side, and the thickness thereof is set to 0.35 mm.

[0084] The size of the vibration element 30 is, for example, set to a range of 0.2 mm or more and 3.0 mm or less in the long side. In addition, the thickness of the vibration element 30 is, for example, set to a range of 0.003 mm or more and 0.5 mm or less.

[0085] As one example, the shape of the vibration element 30 is set to a square of 1.0 mm on a side, and the thickness thereof is set to 0.07 mm. In this case, the vibration element 30 oscillates at a fundamental oscillation frequency of 24 MHz. Note that the oscillation frequency can be adjusted in a range of 1 MHz to 1 GHz by changing the thickness of the vibration element 30 or even taking harmonics into account.

[0086] Note that although the diffraction grating 34 is formed on the entire surface of the vibration element 30 in the above-described embodiment, it can be formed on only a part thereof. Figure 3

[0087] The size of the light modulation by the light modulator 12 is given by the inner product of the difference wave vector between the wave vector of the outgoing light L1 incident to the light modulator 12 and the wave vector of the reference light L2 outgoing from the light modulator 12, and the vector of the vibration direction 36 of the vibration element 30. Although the vibration element 30 performs thickness-shear vibration in the present embodiment, since the vibration is in-plane vibration, light modulation cannot be performed even if light is incident perpendicularly to the surface of the vibration element 30 alone. Therefore, in the present embodiment, the diffraction grating 34 is provided on the vibration element 30, so that light modulation can be performed according to the principle described later.

[0088] Figure 3 The diffraction grating 34 shown in the drawing is a blazed grating. The blazed grating refers to a grating in which the cross-sectional shape of the diffraction grating is stepped. The linear grooves 32 of the diffraction grating 34 are disposed so that the extension direction thereof is orthogonal to the vibration direction 36.

[0089] When the oscillation circuit 54 shown in Figs. 7 and 8 applies a potential to the vibration element 30 via the pad 33, the vibration element 30 performs thickness-shear vibration in the vibration direction 36. Figure 1 Figure 2 The oscillation circuit 54 shown in Figs. 7 and 8 is a circuit for applying a potential to the vibration element 30.​​Figure 3 When the vibration element 30 is supplied with a drive signal Sd (an alternating voltage is applied), the vibration element 30 oscillates. Although the power (drive power) required for the oscillation of the vibration element 30 is not particularly limited, it is as small as 0.1 μW to 100 mW. Therefore, the drive signal Sd output from the oscillation circuit 54 can be used to oscillate the vibration element 30 without being amplified.

[0090] Further, the existing optical modulator also requires a structure for maintaining the temperature of the optical modulator, and thus it is difficult to reduce the size. Further, since the existing optical modulator has a large power consumption, it is difficult to miniaturize and save power of the laser interferometer. In contrast, in the present configuration example, since the size of the vibration element 30 is very small and the power required for oscillation is also small, it is easy to miniaturize and save power of the laser interferometer 1.

[0091] 1.1.1.6.2. Formation method of diffraction grating

[0092] Although the formation method of the diffraction grating 34 is not particularly limited, as one example, a method in which a model is produced by using a mechanical ruling method (ruling machine), and a groove 32 is formed on an electrode formed on the surface of the vibration element 30 of the crystal AT resonator by a nanoimprint method can be given. Here, the reason for being on the electrode is that, in the case of the crystal AT resonator, a high-quality thickness shear vibration can be generated on the electrode in principle. Note that the formation of the groove 32 is not limited to the electrode, and can be on the surface of a material of a non-electrode portion. Further, instead of the nanoimprint method, a processing method realized by exposure and etching, an electron beam lithography method, a focused ion beam processing method (FIB), or the like can be used.

[0093] Further, it can be configured that the diffraction grating is formed on the chip of the crystal AT resonator using a resist material, and a metal film or a mirror film formed of a dielectric multilayer film is provided thereon. By providing the metal film or the mirror film, the reflectance of the diffraction grating 34 can be improved.

[0094] Further, it can be configured that a resist film is formed on the chip or wafer of the crystal AT resonator, and after processing is performed by 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 influence due to moisture absorption or the like of the resist material, and the chemical stability of the diffraction grating 34 can be improved. Further, by providing a metal film of Au, Al, or the like having high conductivity, it can also be used as an electrode for driving the vibration element 30.

[0095] Note that the diffraction grating 34 can also be formed using a technique such as anodized aluminum (porous aluminum oxide).

[0096] 1.1.1.6.3. Other configuration examples of the light modulator

[0097] The vibration element 30 is not limited to a crystal oscillator, and can be, for example, a Si oscillator, an elastic surface wave (SAW) device, a ceramic oscillator, or the like.

[0098] Figure 4 is a plan view showing a part of a second configuration example of the light modulator 12. Figure 5 is a plan view showing a third configuration example of the light modulator 12.

[0099] Figure 4 The vibration element 30A shown is a Si oscillator manufactured from a Si substrate using MEMS technology. MEMS (Micro Electro Mechanical Systems) is a micro mechanical electronic system.

[0100] The vibration element 30A has a first electrode 301 and a second electrode 302 adjacent on the same plane with a gap therebetween, a diffraction grating mount portion 303 provided on the first electrode 301, and a diffraction grating 34 provided on the diffraction grating mount portion 303. The first electrode 301 and the second electrode 302, for example, use electrostatic attraction as a driving force, and vibrate in a direction in which the first electrode 301 and the second electrode 302 approach and depart from each other, i.e., in a direction along the left-right direction of Figure 4 The vibration element 30A shown vibrates in a manner in which the axis connecting the first electrode 301 and the second electrode 302 repeatedly approaches and departs from each other. Thereby, in-plane vibration can be provided to the diffraction grating 34. The oscillation frequency of the Si oscillator is, for example, from 1 kHz to several hundred MHz. Figure 4 The vibration element 30A shown vibrates in a manner in which the axis connecting the first electrode 301 and the second electrode 302 repeatedly approaches and departs from each other. Thereby, in-plane vibration can be provided to the diffraction grating 34. The oscillation frequency of the Si oscillator is, for example, from 1 kHz to several hundred MHz.

[0101] Figure 5 The vibration element 30B shown is a SAW device that utilizes a surface wave. SAW (Surface Acoustic Wave) is an elastic surface wave.

[0102] The vibration element 30B has a piezoelectric substrate 305, a comb-shaped electrode 306 provided on the piezoelectric substrate 305, a ground electrode 307, a diffraction grating mount portion 303, and a diffraction grating 34. When an alternating voltage is applied to the comb-shaped electrode 306, an elastic surface wave is excited by the inverse piezoelectric effect. Thereby, in-plane vibration can be provided to the diffraction grating 34. The oscillation frequency of the SAW device is, for example, from several hundred MHz to several GHz.

[0103] As with the crystal AT oscillator, for the above such device, by providing the diffraction grating 34, light modulation can be performed according to the principle described later.

[0104] On the other hand, in the case where the vibration element 30 is a crystal oscillator, a modulation signal of high precision can be generated using the extremely high Q value possessed by the crystal. The Q value is an index indicating the sharpness of the peak of resonance. In addition, the crystal oscillator also has the advantage of being less susceptible to external disturbances. Thus, by using a modulation signal modulated by the optical modulator 12 provided with the crystal oscillator, a sampling signal originating from the measurement object 14 can be acquired with high precision.

[0105] 1.1.1.6.4. Light modulation by the vibration element

[0106] Next, the principle of modulating light using the vibration element 30 will be described.

[0107] Figure 6 is a conceptual diagram illustrating that a plurality of diffracted lights K i are generated when the incident light K i is incident from a direction perpendicular to the surface of the vibration element 30.

[0108] When the incident light K i is incident to the diffraction grating 34 that is undergoing thickness-shear vibration in the vibration direction 36, as shown in Figure 6 , a plurality of diffracted lights K ns are generated by the diffraction phenomenon. ns n is the order of the diffracted light K 0s , and n = 0, ±1, ±2,.... Note that, in the diffraction grating 34 shown in Figure 6 , a blazed grating shown in Figure 3 is not illustrated, but a diffraction grating formed by the repetition of a concave-convex is illustrated as an example of another diffraction grating. In addition, the illustration of the diffracted light K 0s is omitted in Figure 6 .

[0109] Although the incident light K i is incident from a direction perpendicular to the surface of the vibration element 30 in Figure 6 , the incident angle is not particularly limited, and the incident angle can be set so as to be obliquely incident with respect to the surface of the vibration element 30. In the case of oblique incidence, the traveling direction of the diffracted light K ns also changes in correspondence therewith.

[0110] Note that, depending on the design of the diffraction grating 34, light of high order of |n| ≥ 2 can not appear. Therefore, in order to stably obtain a modulation signal, it is desirable to set |n| = 1. That is, in the laser interferometer 1 of Figure 2 , it is preferable that the frequency-shift type optical modulator 12 be configured so that ±1-order diffracted light is used as the reference light L2. By this configuration, stabilization of the measurement by the laser interferometer 1 can be achieved.

[0111] On the other hand, when higher-order light with |n|≥2 appears from the diffraction grating 34, the optical modulator 12 can be configured such that any diffracted light of order ±2 or higher, not ±1st order diffracted light, is used as the reference light L2. Thus, since higher-order diffracted light can be utilized, the laser interferometer 1 can be made more frequent and smaller.

[0112] In this embodiment, as an example, the optical modulator 12 is configured such that the incident light K incident on the optical modulator 12... i The angle formed by the direction of entry of the light and the direction of travel of the reference light L2 emitted from the light modulator 12 is 180°. Three examples will be given below.

[0113] Figure 7 to Figure 9 These are respectively the incident light K i A conceptual diagram illustrating the optical modulator 12, where the angle formed by the direction of travel of the light L1 and the direction of travel of the reference light L2 is 180°.

[0114] Figure 7 The optical modulator 12 shown includes a reflector 37 in addition to the vibrating element 30. The reflector 37 is configured to diffract a pair of beams K. 1s The light is reflected and returned to the diffraction grating 34. At this time, the diffracted light K... 1s The angle formed by the incident angle relative to mirror 37 and the reflection angle in mirror 37 is 180°. As a result, the diffracted light K exiting from mirror 37 and returning to diffraction grating 34... 1s The light is diffracted again using diffraction grating 34, and directed towards the incident light K that is incident on the light modulator 12. i It travels in the opposite direction to the direction of travel. Therefore, by adding a reflector 37, the aforementioned incident light K can be satisfied. i The condition is that the angle formed by the direction of entry of the light and the direction of travel of the reference light L2 is 180°.

[0115] Furthermore, by using the reflector 37, the reference light L2 generated by the light modulator 12 becomes light that has undergone two frequency modulations. Therefore, compared to using a single vibrating element 30, higher frequency modulation can be achieved by using the reflector 37.

[0116] exist Figure 8 In the middle, the vibrating element 30 is relative to Figure 6 The configuration is tilted. The tilt angle θ at this point... S Set to satisfy the aforementioned incident light K i The condition is that the angle formed by the direction of entry of the light and the direction of travel of the reference light L2 is 180°.

[0117] Figure 9 The diffraction grating 34 shown has a blaze angle θB the Bragg grating. Also, when the incident light K i that travels at the incident angle β with respect to the normal N of the surface of the vibrating element 30 B is incident on the diffraction grating 34, the reference light L2 returns at the same angle with respect to the normal N as the blaze angle θ B Thus, by making the incident angle β equal to the blaze angle θ i , the condition that the angle formed by the direction of entry of the incident light K Figure 7 and the direction of travel of the reference light L2 is 180° can be satisfied. In this case, the mirror 37 shown in Figure 8 , or tilting the vibrating element 30 itself as shown in , can not be used to satisfy the condition, and thus further miniaturization and high frequency of the laser interferometer 1 can be achieved. In particular, in the case of the blaze grating, the configuration that satisfies the condition is called "Littrow configuration", and has the advantage that the diffraction efficiency of the diffracted light can be particularly improved.

[0118] Figure 9 Note that the pitch P of the Bragg grating represents the pitch of the blaze grating, and as one example, the pitch P is set to 1 μm. Also, the blaze angle θ B is set to 25°, for example. In this case, in order to satisfy the condition, the incident angle β of the incident light K i with respect to the normal N is also set to 25°.

[0119] 1.1.1.6.5. Package structure

[0120] Figure 10 is a cross-sectional view showing the optical modulator 12 having a package structure.

[0121] Figure 10 The optical modulator 12 shown in Figure 10 is provided with a container 70 that is a case, an optical modulating vibrator 120 housed in the container 70, and a circuit element 45 that constitutes an oscillation circuit 54. Note that the container 70 is, for example, hermetically sealed in a reduced-pressure atmosphere such as a vacuum, or an inert gas atmosphere such as nitrogen or argon.

[0122] As shown in Figure 10 , the container 70 has a container main body 72 and a lid 74. The container main body 72 has a first recessed portion 721 provided inside thereof, and a second recessed portion 722 provided inside the first recessed portion 721 and deeper than the first recessed portion 721. The container main body 72 is composed of, for example, a ceramic material, a resin material, or the like. Also, although not shown, the container main body 72 is provided with internal terminals provided on an inner surface, external terminals provided on an outer surface, a wiring that connects the internal terminals and the external terminals, and the like.

[0123] Further, the opening portion of the container body 72 is plugged by the lid 74 via a sealing member such as a sealing ring or low-melting glass, which is not shown. The material of the lid 74 uses a material that transmits laser light, such as a glass material.

[0124] The light modulation vibrator 120 is arranged on the bottom surface of the first recess portion 721. The light modulation vibrator 120 is supported on the bottom surface of the first recess portion 721 via a joining member, which is not shown. Further, the internal terminal of the container body 72 and the light modulation vibrator 120 are electrically connected via a conductive material such as a wire, a joining metal, or the like, which is not shown.

[0125] The circuit element 45 is arranged on the bottom surface of the second recess portion 722. The circuit element 45 is electrically connected to the internal terminal of the container body 72 via the wire 76. Thus, the light modulation vibrator 120 and the circuit element 45 are also electrically connected via the wiring provided in the container body 72. Note that a circuit other than the oscillation circuit 54 described later can be provided in the circuit element 45.

[0126] By adopting such a packaging structure, since the light modulation vibrator 120 and the circuit element 45 can be overlapped, the physical distance between them can be made short, and the length of the wiring between the light modulation vibrator 120 and the circuit element 45 can be shortened. Thus, it is possible to suppress the entry of noise from the outside into the drive signal Sd, or conversely, the drive signal Sd from becoming a noise source. Further, it is possible to protect both the light modulation vibrator 120 and the circuit element 45 from the outside environment with one container 70. Thus, not only the miniaturization of the sensor head 51 can be achieved, but also the reliability of the laser interferometer 1 can be improved.

[0127] Note that the structure of the container 70 is not limited to the illustrated structure, and for example, the light modulation vibrator 120 and the circuit element 45 can have independent packaging structures. Further, although not shown, other circuit elements that constitute the oscillation circuit 54 can be housed in the container 70. Note that the container 70 can be omitted as needed.

[0128] Further, the light modulator 12 is not limited to the light modulator described above that has the vibration element 30, and for example, can be an acousto-optic modulator (AOM), an electro-optic modulator (EOM), or the like. Note that in the case where the AOM or the EOM is applied to the light modulator 12, the AOM or the EOM can be additionally provided with a light reflection function.

[0129] 1.1.2. Oscillation Circuit

[0130] As Figure 1As shown, the oscillation circuit 54 outputs a drive signal Sd that is input to the light modulator 12 of the optical system 50. In addition, the oscillation circuit 54 outputs a reference signal Ss that is input to the demodulation circuit 52.

[0131] In the oscillation circuit 54, as long as it is a circuit that enables the oscillation element 30 to oscillate, it is not particularly limited, and various circuits can be used. Figure 11 is a circuit diagram showing a configuration of a first-order inverter oscillation circuit as one example of a circuit configuration.

[0132] Figure 11 The oscillation circuit 54 shown has a circuit element 45, a feedback resistor Rf, a limiting resistor Rd, a first capacitor Cg, a second capacitor Cd, and a third capacitor C3.

[0133] The circuit element 45 is an inverter IC. A terminal X1 and a terminal X2 of the circuit element 45 are terminals that are connected to an inverter inside the circuit element 45, respectively. A terminal GND is connected to a ground potential, and a terminal Vcc is connected to a power supply potential. A terminal Y is a terminal for oscillation output.

[0134] The first capacitor Cg is connected between the terminal X1 and the ground potential. In addition, the limiting resistor Rd and the second capacitor Cd that are connected in series with each other are connected in order from the terminal X2 side between the terminal X2 and the ground potential. Further, one end of the feedback resistor Rf is connected between the terminal X1 and the first capacitor Cg, and the other end of the feedback resistor Rf is connected between the terminal X2 and the limiting resistor Rd.

[0135] In addition, one end of the oscillation element 30 is connected between the first capacitor Cg and the feedback resistor Rf, and the other end of the oscillation element 30 is connected between the second capacitor Cd and the limiting resistor Rd. Thus, the oscillation element 30 becomes a signal source of the oscillation circuit 54.

[0136] Figure 12 is an example of an LCR equivalent circuit of the oscillation element 30.

[0137] As shown, Figure 12 the LCR equivalent circuit of the oscillation element 30 is configured by a series capacitor C1, a series inductor L1, an equivalent series resistor R1, and a parallel capacitor C0.

[0138] In the oscillation circuit 54 shown, Figure 11 when the capacitance of the first capacitor Cg is set to C g and the capacitance of the second capacitor Cd is set to C d , the load capacitance C L is given by the following formula (a).

[0139] [Formula 2]

[0140]

[0141] Thus, the oscillation frequency f osc is given by the following expression (b).

[0142] [Expression 3]

[0143]

[0144] f Q is the natural vibration number of the vibration element 30.

[0145] According to the above expression (b), it is known that the oscillation frequency f L of the signal output from the terminal Y can be finely adjusted by appropriately changing the load capacitance C osc .

[0146] In addition, the difference Δf between the natural vibration number f Q of the vibration element 30 and the oscillation frequency f osc of the oscillation circuit 54 is given by the following expression (c).

[0147] [Expression 4]

[0148]

[0149] Here, since C1 « C0, C1 « C L , Δf is approximately given by the following expression (d).

[0150] [Expression 5]

[0151]

[0152] Thus, the oscillation frequency f osc of the oscillation circuit 54 becomes a value corresponding to the natural vibration number f Q of the vibration element 30.

[0153] Here, when the vibration element 30 is fixed to the container 70, for example, when being subjected to an expansion stress due to temperature via the fixing portion, the natural vibration number f Q varies. In addition, when the vibration element 30 is inclined, the natural vibration number f Q varies due to the influence of gravity and the like by the dead weight.

[0154] In the oscillation circuit 54, even if the natural vibration number f Q varies due to such a reason, based on the above expression (d), the oscillation frequency f osc varies in conjunction with the variation. That is, the oscillation frequency f oscbecomes the natural vibration number f Q from the value of Δf. Thus, the vibration of the vibration element 30 is stabilized, and the displacement amplitude is stabilized. Since the displacement amplitude is stabilized, the modulation characteristics of the optical modulator 12 are stabilized, and thus the demodulation accuracy of the sample signal in the demodulation circuit 52 can be improved.

[0155] As one example, it is preferable that Δf = |f osc -f Q ≤ 3000 [Hz], and further preferably 600 [Hz].

[0156] As described above, in the laser interferometer 1 according to the present embodiment, the optical modulator 12 includes the vibration element 30. The optical modulator 12 modulates the reflected light Lla (first divided light) as the collimated light using the vibration element 30.

[0157] According to this configuration, the optical modulator 12 can be miniaturized and lightened. Thus, the laser interferometer 1 can be miniaturized and lightened.

[0158] In addition, the laser interferometer 1 according to the present embodiment includes the demodulation circuit 52 and the oscillation circuit 54. The oscillation circuit 54 includes the vibration element 30 as a signal source thereof, and outputs a reference signal Ss to the demodulation circuit 52 as shown in FIG. 2. Figure 1 The demodulation circuit 52 demodulates the sample signal originating from the measurement target object 14 from the light-receiving signal based on the reference signal Ss.

[0159] According to this configuration, even if the natural vibration number f Q varies, the oscillation frequency f osc of the oscillation circuit 54 can be changed to a value corresponding to the natural vibration number f Q of the vibration element 30, and thus the vibration of the vibration element 30 can be easily stabilized. Thus, the temperature characteristics of the modulation signal can be made to correspond to the temperature characteristics of the vibration element 30, and the modulation characteristics of the optical modulator 12 can be stabilized. As a result, the demodulation accuracy of the sample signal in the demodulation circuit 52 can be improved.

[0160] In addition, in the above-described configuration, the temperature characteristics of the reference signal Ss output from the oscillation circuit 54 to the demodulation circuit 52 can also be made to correspond to the temperature characteristics of the vibration element 30. In this way, both the temperature characteristics of the modulation signal and the temperature characteristics of the reference signal correspond to the temperature characteristics of the vibration element 30, and thus the behavior of the variation of the modulation signal accompanying the temperature change coincides with or approximates the behavior of the variation of the reference signal Ss. Thus, even if the temperature of the vibration element 30 changes, the influence on the demodulation accuracy can be suppressed, and the demodulation accuracy of the sample signal originating from the measurement target object 14 can be improved.

[0161] Further, in the present embodiment, since the power consumption of the oscillation circuit 54 is low, it is possible to easily realize the power saving of the laser interferometer 1.

[0162] Note that, instead of the oscillation circuit 54, a signal generator such as a function generator or a signal generator can be used.

[0163] 1.2. Demodulation circuit

[0164] The demodulation circuit 52 performs a demodulation process of demodulating a sample signal originating from the measurement object 14 from a light-receiving signal output from the light-receiving element 10. The sample signal contains, for example, phase information and frequency information. Further, it is possible to acquire the displacement of the measurement object 14 from the phase information and the velocity of the measurement object 14 from the frequency information. If it is possible to acquire different information in this way, since it has a function as a displacement meter or a velocity meter, it is possible to realize the high functionality of the laser interferometer 1.

[0165] The demodulation circuit 52 sets its circuit configuration according to the manner of the modulation process. In the laser interferometer 1 related to the present embodiment, the light modulator 12 provided with the vibration element 30 is used. Since the vibration element 30 is an element that performs single vibration, the vibration velocity changes every moment within a cycle. Therefore, the modulation frequency also changes with time, and the existing demodulation circuit cannot be used as it is.

[0166] The existing demodulation circuit refers to, for example, a circuit that demodulates a sample signal from a light-receiving signal containing a modulation signal modulated using an acousto-optic modulator (AOM). In the acousto-optic modulator, the modulation frequency does not change. Therefore, although the existing demodulation circuit can demodulate a sample signal from a light-receiving signal containing a modulation signal in which the modulation frequency does not change, in the case of a light modulator 12 that modulates a modulation signal in which the modulation frequency changes, it cannot demodulate as it is.

[0167] Therefore, Figure 1 The demodulation circuit 52 illustrated in the drawing is provided with a preprocessing section 53 and a demodulation processing section 55. The light-receiving signal output from the light-receiving element 10 is first guided to the demodulation processing section 55 after passing through the preprocessing section 53. The preprocessing section 53 performs preprocessing on the light-receiving signal. By this preprocessing, a signal that can be demodulated by the existing demodulation circuit is obtained. Thus, in the demodulation processing section 55, the sample signal originating from the measurement object 14 is demodulated by a known demodulation manner.

[0168] 1.2.1. Configuration of preprocessing section

[0169] Figure 1The illustrated preprocessing section 53 includes a first band-pass filter 534, a second band-pass filter 535, a first delay adjuster 536, a second delay adjuster 537, a multiplier 538, a third band-pass filter 539, a first AGC 540, a second AGC 541, and an adder 542. Note that AGC stands for Auto Gain Control.

[0170] Between the light-receiving element 10 and the preprocessing section 53, a current-voltage converter 531 and an ADC 532 are connected in this order from the side of the light-receiving element 10. The current-voltage converter 531 is a trans-impedance amplifier that converts a current output from the light-receiving element 10 into a voltage signal. The ADC 532 is an analog-digital converter that converts an analog signal into a digital signal at a prescribed number of sampling bits.

[0171] The current output from the light-receiving element 10 is converted into a voltage signal by the current-voltage converter 531. The voltage signal is converted into a digital signal by the ADC 532 and is divided into two signals, a first signal S1 and a second signal S2, by the branching section jp1. In Figure 1 The path of the first signal S1 is set as a first signal path ps1, and the path of the second signal S2 is set as a second signal path ps2.

[0172] Between the oscillation circuit 54 and the second delay adjuster 537, an ADC 533 is connected. The ADC 533 is an analog-digital converter that converts an analog signal into a digital signal at a prescribed number of sampling bits.

[0173] The first band-pass filter 534, the second band-pass filter 535, and the third band-pass filter 539 are filters that selectively transmit signals of specific frequency bands.

[0174] The first delay adjuster 536 and the second delay adjuster 537 are circuits that adjust the delay of signals. The multiplier 538 is a circuit that generates an output signal that is proportional to the product of two input signals. The adder 542 is a circuit that generates an output signal that is proportional to the sum of two input signals.

[0175] Next, the operation of the preprocessing section 53 is described along the flow of the first signal S1, the second signal S2, and the reference signal Ss.

[0176] The first signal S1, after passing through the first band-pass filter 534 arranged on the first signal path ps1, is adjusted in group delay by the first delay adjuster 536. The group delay adjusted by the first delay adjuster 536 corresponds to the group delay of the second signal S2 caused by the second band-pass filter 535 to be described later. By this delay adjustment, the delay time accompanying the passing of the filter circuit can be made uniform between the first band-pass filter 534 through which the first signal S1 passes and the second band-pass filter 535 and the third band-pass filter 539 through which the second signal S2 passes. The first signal S1 that has passed through the first delay adjuster 536 is input to the adder 542 via the first AGC 540.

[0177] The second signal S2, after passing through the second band-pass filter 535 arranged on the second signal path ps2, is input to the multiplier 538. In the multiplier 538, the second signal S2 is multiplied by the reference signal Ss output from the second delay adjuster 537. Specifically, the reference signal Ss represented by cos(ω0t + B sin ωt + φ) output from the oscillation circuit 54 is digitally converted by the ADC 533 and adjusted in phase by the second delay adjuster 537, and is input to the multiplier 538. ω0 m t) represents the reference signal Ss output from the oscillation circuit 54. ω0 m is the angular frequency of the modulation signal by the optical modulator 12, and t is time. Thereafter, the second signal S2, after passing through the third band-pass filter 539, is input to the adder 542 via the second AGC 541.

[0178] In the adder 542, an output signal proportional to the sum of the first signal S1 and the second signal S2 is output.

[0179] 1.2.2. Basic principle of pre-processing

[0180] Next, the basic principle of the pre-processing in the pre-processing section 53 will be described. Note that the basic principle referred to here means the principle described in Japanese Patent Application Laid-Open No. 2-38889. In the following description, as an example, a system in which the frequency as the modulation signal varies in a sinusoidal manner and the displacement of the measurement object 14 also varies in the optical axis direction by single vibration will be considered. Here, E m , Ed、 are set as

[0181] [Equation 6]

[0182] E m = a m {cos(ω0t + B sin ω m t + φ m ) + i sin(ω0t + B sin ω m t + φ m)} (1)

[0183] E d = a d {cos(ω0t + A sinω d t + φ d ) + i sin(ω0t + A sinω d t + φ d )} (2)

[0184] φ = φ m - φ d (3)

[0185] The light-receiving signal I PD output from the light-receiving element 10 is logically represented by the following equation.

[0186] [Equation 7]

[0187] I PD = <|E m + E d | 2 > = <|E m 2 + E d 2 + 2E m E d |> = a m 2 + a d 2 + 2a m a d cos(B sinω m t - A sinω d t + φ) (4)

[0188] Note that E m , E d , ω m , ω d , ω0, a m , a d are as shown below.

[0189] [Equation 8]

[0190] E m : electric field component of the modulation signal from the light modulator

[0191] E d : electric field component of the sampling signal from the measurement target object

[0192] initial phase of the modulation signal from the light modulator

[0193] Initial phase of a sampling signal originating from a measurement object

[0194] Phase difference of an optical path of a laser interferometer

[0195] ω m : angular frequency of a modulation signal originating from an optical modulator

[0196] ω d : angular frequency of a sampling signal originating from a measurement object

[0197] ω0: angular frequency of an exit light exiting from a light source

[0198] a m : coefficient

[0199] a d : coefficient

[0200] In addition, < > in the formula (4) indicates a time average.

[0201] The first and second terms of the above formula (4) indicate direct current components, and the third term indicates an alternating current component. When the alternating current component is set as I PD·AC , I PD·AC is as shown in the following formula.

[0202] [Formula 9]

[0203] I PD·AC = 2a m a d cos(B sinω m t - A sinω d t + φ) = 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)

[0204]

[0205]

[0206] A: phase shift of a sampling signal

[0207] f dmax : Doppler frequency shift of a sampling signal

[0208] f d : frequency of a sampling signal

[0209] B: phase shift of the modulation signal

[0210] f mmax : Doppler frequency shift of the modulation signal

[0211] f m : frequency of the modulation signal

[0212] Here, the ν-th Bessel function of the following formula (8) and formula (9) is known.

[0213] [Formula 10]

[0214] cos{ζsin(2πf v t)} = J0(ζ) + 2J2(ζ)cos(2 · 2πf v t) + 2J4(ζ)cos(4 · 2πf v t) +... (8)

[0215] sin{ζsin(2πf ν t)} = 2J1(ζ)sin(1 · 2πf v t) + 2J3(ζ)sin(3 · 2πf v t) +... (9)

[0216] When the Bessel function of the above formula (8) and formula (9) is used to expand the above formula (5) series, it can be transformed as the following formula (10).

[0217] [Formula 11]

[0218] I PD.AC 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)

[0219] where J0(B), J1(B), J2(B),... are Bessel coefficients, respectively.

[0220] When transformed as above, it can be said that the waveband corresponding to a specific order can be extracted logically by a bandpass filter.

[0221] ​Therefore, in the aforementioned pre-processing section 53, the light-receiving signal is pre-processed based on the logic with the following flow.

[0222] First, the light-receiving signal output from the aforementioned ADC 532 is divided into two signals, a first signal S1 and a second signal S2, by the branching section jp1. The first signal S1 passes through the first band-pass filter 534. The center angular frequency of the first band-pass filter 534 is set to ω m . Thus, the first signal S1 after passing through the first band-pass filter 534 is represented by the following equation.

[0223] [Equation 12]

[0224] I pass1 = J1(B) { -cos(ω m t + A sinω d t - φ) + cos(ω m t - A sinω d t + φ)} = -2J1(B) sin(ω m t) sin(A sinω d t - φ) (11)

[0225] On the other hand, the second signal S2 passes through the second band-pass filter 535. The center angular frequency of the second band-pass filter 535 is set to a value different from that of the first band-pass filter 534. Here, as one example, the center angular frequency of the second band-pass filter 535 is set to 2ω m . Thus, the second signal S2 after passing through the second band-pass filter 535 is represented by the following equation.

[0226] [Equation 13]

[0227]

[0228] The second signal S2 after passing through the second band-pass filter 535 is multiplied by the reference signal Ss with the multiplier 538. The second signal S2 after passing through the multiplier 538 is represented by the following equation.

[0229] [Equation 14]

[0230]

[0231] The second signal S2 after passing through the multiplier 538 passes through the third band-pass filter 539. The center angular frequency of the third band-pass filter 539 is set to the same value as that of the first band-pass filter 534. Here, as one example, the center angular frequency of the third band-pass filter 539 is set to ω m . Thus, the second signal S2 after passing through the third band-pass filter 539 is represented by the following equation.

[0232] [Formula 15]

[0233]

[0234] Thereafter, the first signal S1 represented by the above Formula (11) is adjusted in phase and amplitude by the first delay adjuster 536 and the first AGC 540.

[0235] In addition, the second signal S2 represented by the above Formula (14) is also adjusted in amplitude by the second AGC 541 so that the amplitude of the second signal S2 coincides with the amplitude of the first signal S1.

[0236] Then, the first signal S1 and the second signal S2 are added by the adder 542. The addition result is represented by the following Formula (15).

[0237] [Formula 16]

[0238] I 53 = cos(ω m t + A sin ω d t - φ) (15)

[0239] As in the above Formula (15), as a result of the addition, unnecessary terms can be eliminated and necessary terms can be extracted. The result is input to the demodulation processing section 55.

[0240] 1.2.3. Configuration of Demodulation Processing Section

[0241] The demodulation processing section 55 performs demodulation processing of the sampling signal originating from the measurement target object 14 based on the signal output from the preprocessing section 53. Although not particularly limited as the demodulation processing, a known quadrature detection method can be cited. The quadrature detection method is a method of performing demodulation processing by operating an input signal to mix mutually orthogonal signals from the outside.

[0242] Figure 1 The demodulation processing section 55 illustrated is a digital circuit provided with a multiplier 551, a multiplier 552, a phase shifter 553, a first low-pass filter 555, a second low-pass filter 556, a divider 557, an arctangent calculator 558, and an output circuit 559.

[0243] 1.2.4. Demodulation Processing by Demodulation Processing Section

[0244] In the demodulation processing, first, the signal output from the preprocessing section 53 is divided into two signals by a branching section jp2. In the multiplier 551, one of the divided signals is multiplied by the signal output from the oscillation circuit 54, which is cos(ω mt) a reference signal Ss. In the multiplier 552, the divided other signal is multiplied by a signal obtained by shifting the phase of the reference signal Ss output from the oscillation circuit 54 by -90° with the phase shifter 553, and the signal is represented by -sin(ω m t) a signal represented. The reference signal Ss and the signal obtained by shifting the phase of the reference signal Ss are signals whose phases are deviated from each other by 90°.

[0245] The signal passed through the multiplier 551 passes through the first low-pass filter 555, and then is input as a signal x to the divider 557. The signal passed through the multiplier 552 passes through the second low-pass filter 556, and then is input as a signal y to the divider 557. In the divider 557, a division operation of the signal y by the signal x is performed, and the output y / x is passed through the arctangent operator 558 to thereby calculate an output atan(y / x).

[0246] Then, the phase φ is calculated by passing the output atan(y / x) from the output circuit 559 to thereby calculate the phase as information originating from the measurement target object 14. In the output circuit 559, by the phase unwrapping process, the phase connection in the case where the phase is jumped by 2π at adjacent points is performed. The phase information outputtable from the demodulation processing section 55 enables the displacement of the measurement target object 14 to be calculated. Thus, a displacement meter is realized. In addition, the velocity can be calculated from the displacement. Thus, a velocity meter is realized.

[0247] Although the circuit configuration of the demodulation processing section 55 has been described above, the circuit configuration of the digital circuit described above is one example, and is not limited thereto. For example, the configuration of the pre-processing section 53 is not limited to the configuration described above. In addition, the demodulation processing section 55 is not limited to the digital circuit, and can be an analog circuit. In the analog circuit, the F / V converter circuit or the Δ∑ counter circuit can be included.

[0248] In addition, the circuit configuration of the demodulation processing section 55 described above can be configured to calculate the frequency information originating from the measurement target object 14. The velocity of the measurement target object 14 can be calculated on the basis of the frequency information.

[0249] 1.3. Suppression of return light achieved by optical axis deviation

[0250] Next, the suppression of return light achieved by the optical axis deviation will be described.

[0251] The outgoing light L1 emitted from the laser light source 2 is diffused due to the diffraction phenomenon of light. Specifically, the outgoing light L1 emitted from the laser light source 2 at the optical path R LD The optical path R of the outgoing light L1 emitted from the laser light source 2 at the position at which the distance x LD is separated is represented using the diffusion emission angle θ and the following equation (16).

[0252] [Equation 17]

[0253] R = R LD +θx LD (16)

[0254] In the above equation (16), the second term is the diffraction term. The diffraction phenomenon of light is generally represented by the following equation (16-1).

[0255] [Formula 18]

[0256] R LD sin θ=mλ (16-1)

[0257] Since the diffraction terms of higher orders after the second order are sufficiently small, they are not considered. In this case, m can be set to 1. In addition, since the diffusion emission angle θ is generally small, it is θ << 1. Therefore, the above equation (16-1) can be transformed into the following equation (16-2).

[0258] [Formula 19]

[0259]

[0260] Thus, equation (16) can be transformed into equation (16-3).

[0261] [Formula 20]

[0262]

[0263] According to the above formula (16-3), the optical path R of the outgoing light L1 emitted from the laser source 2 when it enters the collimating lens 3 can be calculated.

[0264] Figure 13 It shows from Figure 2 The diagram shows the light trail of the emitted light L1 from the laser source 2, and the light trail of the reference light L2 generated by the reflection of the emitted light L1 by the optical modulator 12 as the return light L5 to the laser source 2. It should be noted that... Figure 13 For ease of illustration, the diagram will show the extended light path after refraction occurs midway. Additionally, the return light L5 includes... Figure 13 In addition to the light originating from the reference light L2, which is not shown in the diagram, the light originating from the object light L3 may also be included. Furthermore, the return light L5 may consist solely of the light originating from the object light L3.

[0265] exist Figure 13In the present embodiment, the center of the collimator lens 3 is set as the origin O (reference position), and the distance from the origin O to the light modulator 12 is set as L. Note that, in the case where the collimator lens 3 is omitted, the position at which collimated light is generated is set as the origin O (reference position). For example, in a gas laser such as a He-Ne laser, since collimated light is emitted from an emission point, the emission point becomes the origin. In addition, the optical path length when the return light L5 reaches the collimator lens 3 is set as R'.

[0266] Thus, the optical path length R' of the return light L5 is represented by the following expression (16-4).

[0267] [Expression 21]

[0268]

[0269] In the present embodiment, the return light L5 is suppressed from being incident on the laser light source 2 by causing the first optical axis Al to deviate from the second optical axis A2. The first optical axis Al is the optical axis of the emission light LI which is collimated light, and the second optical axis A2 is the optical axis of the reference light L2 and the optical axis of the return light L5 originating from the reference light L2. In the present specification, causing the first optical axis Al to deviate from the second optical axis A2 is referred to as "optical axis deviation". Note that, as shown in FIG. 6, the direction in which the optical axis is deviated can be a direction intersecting the plane in which the optical system 50 extends, in addition to a direction in the plane. Figure 2

[0270] The return light L5 becomes a cause of instability of laser oscillation in the laser light source 2. Therefore, if the optical intensity of the return light L5 can be reduced by optical axis deviation, stabilization of laser oscillation can be achieved.

[0271] In the instability of laser oscillation caused by the return light L5, self-mixing combination in the laser light source 2 is involved. The self-mixing combination is quantified by an index M which is the product of a "combination coefficient" and the "optical amount of the return light L5". Here, the combination coefficient is proportional to the inverse of the resonator length of the laser light source 2. Therefore, the self-mixing combination is easily significant in the case where the optical amount of the return light L5 increases in a semiconductor laser element in which the resonator length is short. If the self-mixing combination is suppressed in the semiconductor laser element, it is considered that the self-mixing combination is suppressed in most kinds of light sources.

[0272] Here, the present inventors have found that, in order to suppress the self-mixing combination in the laser light source 2, it is necessary that the index M satisfies M < 10 -6 This corresponds to satisfying OD6 in the case where the blocking ability of the optical intensity in the optical axis deviation is expressed by the optical density (OD value). Thus, if the blocking ability of the optical axis deviation satisfies OD6, stabilization of laser oscillation can be achieved.

[0273] ​Based on this, the light intensity P0 of the outgoing light L1 and the light intensity P of the return light L5 that is blocked without deviating from the optical axis r As long as the following relation of Expression (17) is satisfied.

[0274] [Expression 22]

[0275]

[0276] Therefore, the condition that the return light L5 does not enter the collimator lens 3 is investigated. The return light L5 becomes a light beam having an optical path R' when it reaches the collimator lens 3, but if this light beam does not overlap with the effective diameter of the collimator lens 3, the light intensity P r is actually zero. That is, the above relation of Expression (17) is satisfied.

[0277] As a result of the deviation of the optical axis, a deviation occurs between the first optical axis Al and the second optical axis A2 at the position of the collimator lens 3. In Figure 13 , the deviation width of the first optical axis Al and the second optical axis A2 in the collimator lens 3 is set to Δy. In addition, the effective diameter of the collimator lens 3 is set to κ. In this way, in order to satisfy the above relation of Expression (17), the required condition of the deviation width Δy is expressed by the following Expression (17-1).

[0278] [Expression 23]

[0279]

[0280] When the above Expression (16-4) is substituted into the above Expression (17-1), the required condition of the deviation width Δy is expressed by the following Expression (A).

[0281] [Expression 24]

[0282]

[0283] Thus, in order to satisfy the above Expression (A), only the deviation width Δy of the deviation of the optical axis needs to be selected.

[0284] Here, as shown in Figure 13 , the deviation width Δy is adjusted by tilting the light modulator 12. Specifically, in the optical system 50 shown in Figure 13 , the light modulator 12 is tilted so that the optical axis (second optical axis A2) of the reference light L2 generated by the light modulator 12 deviates from the optical axis (first optical axis Al) of the reflected light Lla (outgoing light Ll) that enters the light modulator 12. The angle formed by the first optical axis Al and the second optical axis A2 is set to a deviation angle θ'.

[0285] The deviation width Δy is expressed by the following Expression (17-2) using the deviation angle θ'.

[0286] [Num 25]

[0287] Δy = L tan θ' (17-2)

[0288] The deviation angle θ' is generally small, and thus θ' « 1. Accordingly, the above-described equation (17-2) can be transformed into the following equation (17-3).

[0289] [Num 26]

[0290]

[0291] Accordingly, in order to realize the deviation width Δy satisfying the above-described equation (A), it is only necessary to adjust the distance L and the deviation angle θ' based on the above-described equation (17-3).

[0292] 1.3.1. First Calculation Example

[0293] The first calculation example is shown as an example in a case where the above-described equation (A) is satisfied.

[0294] The parameters and the calculation results of the first calculation example are shown in Table 1.

[0295] Table 1

[0296]

[0297] In the first calculation example, the deviation width Δy that is necessary in order to satisfy the above-described equation (A) becomes 0.7 mm as shown in Table 1. This deviation width Δy is a value that can be realized in the small optical system 50 in which the distance x corresponding to the focal length of the collimator lens 3 is on the order of 1 mm and the distance L of the collimator lens 3 from the light modulator 12 is on the order of 10 mm. LD

[0298] Accordingly, in the first calculation example, it is proved that the optical system 50 satisfying the above-described equation (A) is realized. Thereby, it is possible to suppress the incidence of the return light L5 against the laser light source 2 to the extent that the laser oscillation does not become unstable.

[0299] 1.3.2. Second Calculation Example

[0300] The second calculation example is shown as an example in a case where the above-described equation (A) is satisfied.

[0301] The parameters and the calculation results of the second calculation example are shown in Table 2.

[0302] Table 2

[0303]

[0304] ​In the second calculation example, the deviation width Δy necessary to satisfy the formula (A) becomes 5.7 mm as shown in Table 2. This deviation width Δy is a value that can be sufficiently achieved in the optical system 50 in which the distance x of the collimator lens 3 is on the order of 1 mm and the distance L of the collimator lens 3 from the light modulator 12 is on the order of 100 mm. LD

[0305] Thus, in the second calculation example, by setting the deviation angle θ' to be equal to or greater than the above value, the formula (A) can be satisfied. Thereby, it is possible to suppress the incidence of the return light L5 against the laser light source 2 to the extent that the laser oscillation does not become unstable.

[0306] 1.4. Effects of the First Embodiment

[0307] As described above, the laser interferometer 1 according to the present embodiment includes the laser light source 2, the collimator lens 3 (collimating section), the light modulator 12, and the light receiving element 10. The laser light source 2 emits the outgoing light L1 (laser light). The collimator lens 3 collimates the outgoing light L1 and generates collimated light. The light modulator 12 modulates the outgoing light L1 as the collimated light into the reference light L2 having a different frequency. The light receiving element 10 receives the object light L3 generated by the reflection of the outgoing light L1 as the collimated light by the measurement target object 14 and the reference light L2, and outputs a light receiving signal.

[0308] Here, when the optical axis of the outgoing light L1 as the collimated light is set as a first optical axis Al, and the return light L5 that the generated reference light L2 or object light L3 goes to the laser light source 2 via the polarization beam splitter 4 is generated, the optical axis of the return light L5 is set as a second optical axis A2. In addition, the center of the collimator lens 3 (position at which the collimated light is generated) is set as a reference position, the deviation width of the first optical axis Al and the second optical axis A2 in the reference position is set as Δy, and the effective diameter of the collimator lens 3 is set as κ. Furthermore, the optical path of the outgoing light L1 in the collimator lens 3 is set as R, the distance of the reference position from the light modulator 12 is set as L, and the wavelength of the outgoing light L1 is set as λ.

[0309] In the laser interferometer 1, the first optical axis Al and the second optical axis A2 are deviated in a manner that satisfies the following formula (A).

[0310] [Equation 27]

[0311]

[0312] According to this configuration, it is possible to sufficiently suppress the light intensity of the return light L5 incident to the laser light source 2. Thereby, it is possible to achieve the stabilization of the laser oscillation in the laser light source 2. As a result, since the quality of the outgoing light L1 is stabilized, it is possible to suppress the decrease in the accuracy of the demodulation of the sample signal from the light receiving signal in the demodulation circuit 52.​

[0313] It should be noted that when the deviation in width Δy exceeds the aforementioned range, the probability of the returned light L5 incident on the collimating lens 3 increases. Therefore, the intensity of the returned light L5 incident on the laser source 2 increases, potentially causing laser oscillation to become unstable.

[0314] Furthermore, although the deviation width Δy can be appropriately adjusted according to other parameters as described above, it is preferable, as an example, to satisfy 0.10 ≤ Δy ≤ 10.0, more preferably 0.50 ≤ Δy ≤ 10.0, even more preferably 2.10 ≤ Δy ≤ 10.0, and particularly preferably 2.30 ≤ Δy ≤ 10.0. Additionally, when considering the size of the sensor head 51, the upper limit of the deviation width Δy is more preferably 6.00 or less, and even more preferably 3.00 or less.

[0315] It should be noted that when the deviation width Δy is below the lower limit, the deviation angle θ′ becomes too small, and the manufacturability of the optical system 50 may decrease. On the other hand, when the deviation width Δy is above the upper limit, it may be difficult to miniaturize the sensor head 51 because the polarization beam splitter 4 needs to be enlarged.

[0316] Furthermore, the distance L between the collimating lens 3 and the light modulator 12 is preferably set to 5.0 mm or more and 200 mm or less, more preferably 10.0 mm or more and 100 mm or less. This allows for both minimizing the size of the optical system 50 and ensuring a sufficient working distance.

[0317] 2. Second Implementation Method

[0318] Next, the laser interferometer involved in the second embodiment will be described.

[0319] Figure 14 This is a schematic diagram showing the sensor head of the laser interferometer 1 according to the second embodiment. Figure 15 It shows from Figure 14 The diagram shows the light trail of the emitted light L1 from the laser source 2 and the light trail of the reference light L2 generated by the reflection of the emitted light L1 by the optical modulator 12 as the return light L5 to the laser source 2.

[0320] The following description focuses on the second embodiment, but will concentrate on the differences from the first embodiment, omitting details of the same aspects. It should be noted that in the figures, the same reference numerals are used to denote the same components as in the first embodiment.

[0321] 2.1. Suppression of Returning Light Achieved by Optical Axis Offset and Blocking Elements

[0322] The laser interferometer 1 according to the second embodiment includes the blocking element 17.

[0323] The blocking element 17 is an iris (aperture) disposed between the collimator lens 3 and the polarization beam splitter 4. The blocking element 17 has an opening 172 disposed in correspondence with the optical path 18. The blocking element 17 suppresses the return light L5 generated by the light modulator 12 or the measurement object 14 and the like from being incident on the laser light source 2.

[0324] That is, the laser interferometer 1 according to the present embodiment includes the polarization beam splitter 4 (light splitter) and the blocking element 17. The polarization beam splitter 4 splits the emergent light L1 as collimated light into the reflected light L1a (first split light) incident on the light modulator 12 and the transmitted light L1b (second split light) incident on the measurement object 14. The blocking element 17 is disposed between the collimator lens 3 (collimating section) and the polarization beam splitter 4 (light splitter) and has the opening 172 through which the emergent light L1 as collimated light, i.e., the emergent light L1 parallelized by the collimator lens 3, passes.

[0325] The blocking element 17 only needs to have a function of blocking the return light L5, and thus the structure is very simple. Therefore, the laser interferometer 1 is facilitated in terms of structure simplification. Note that the blocking element 17 can also be a slit or a pinhole, and the structure is not particularly limited.

[0326] In the optical system 50 shown in FIG. 8, the blocking element 17 is disposed in addition to the optical axis deviation, and thus the light intensity of the return light L5 reaching the laser light source 2 is suppressed. The principle thereof will be described below. Figure 14 and Figure 15 In the optical system 50 shown in FIG. 8, the blocking element 17 is disposed in addition to the optical axis deviation, and thus the light intensity of the return light L5 reaching the laser light source 2 is suppressed. The principle thereof will be described below.

[0327] Since the blocking element 17 is disposed between the collimator lens 3 and the polarization beam splitter 4, if the diameter D of the opening 172 of the blocking element 17 is made smaller than the optical path length R' of the return light L5, the light intensity of the return light L5 can be reduced. Note that in the case where the opening 172 is not a perfect circle, the diameter D of the opening 172 is the diameter of a perfect circle inscribed in the opening 172.

[0328] In the present embodiment, it is also sufficient that the light intensity P0 of the emergent light L1 and the light intensity P r satisfy the following expression (17).

[0329] [Expression 28]

[0330]

[0331] ​Here, the light intensity of the return light L5 about to be incident on the shielding element 17 is set as P' r. The light intensity P' r of the return light L5 is represented by the following equation (18). Note that the following equation (18) is an example in the case where the light intensity distribution of the outgoing light Ll is a Gaussian distribution or a distribution based on a Gaussian distribution.

[0332] [Equation 29]

[0333] P' r = Pl x P2 x P3 (18)

[0334]

[0335]

[0336]

[0337]

[0338] P3 = 1 - cos 2 (2Δψ) (18-3)

[0339] In the above equation (18-1), P is the peak light intensity of the outgoing light Ll, r is the position from the center axis of the outgoing light Ll to the radial direction, and w is the distance from the center axis of the outgoing light Ll in the radial direction at the position where the light intensity of the outgoing light Ll is 1 / e 2 of the peak light intensity. Note that in the case of the outgoing light Ll, since θ « 1, θ can be set as θ = λ / r. Thus, w is represented as the above equation (18-1-1). In this way, the above equation (18-1) can be transformed as the above equation (18-1-2). Note that in the above equation (18-2), Refl M is the light reflectance in the light modulator 12, and Refl S is the light reflectance in the measurement target object 14. In addition, in the above equation (18-3), Δψ is the error in the setting angle of the 1 / 2 wave plate composed of the two 1 / 4 wave plates of the 1 / 4 wave plate 6 and the 1 / 4 wave plate 8.

[0340] Pl represented in the above equation (18-1) represents the light intensity of the outgoing light Ll passing through the opening 172 of the shielding element 17. P2 represented in the above equation (18-2) represents the reflectance of the light modulator 12 and the reflectance of the measurement target object 14. P3 represented in the above equation (18-3) represents the intensity of the light returned to the laser light source 2 side via the polarizing beam splitter 4 when unexpected polarized light is generated accompanying the allowable angle error of the 1 / 2 wave plate.

[0341] Based on the above-described formula (18), the light intensity Pr of the return light L5 that is not shielded by the optical axis deviation or the shielding element 17 and that passes through the opening 172 is represented by the following formula (19).

[0342] [Formula 30]

[0343]

[0344]

[0345] In the above-described formula (19), x and y are positions in a cross section of the return light L5 along two axes that are orthogonal to each other. In addition, L pin is the distance of the collimator lens 3 from the shielding element 17. Also, in the case where the light intensity distribution of the exit light Ll is a Gaussian distribution, when the standard deviation of the light intensity distribution is σ, w = 2σ.

[0346] Thus, in order for the light intensity Pr calculated by the above-described formula (19) to satisfy the formula (17), it is sufficient to select the diameter of the opening 172 of the shielding element 17 Thus, it is possible to realize the laser interferometer 1 in which the stabilization of the laser oscillation is realized.

[0347] In addition, in the present embodiment, the diameter of the opening 172 is set to The angle formed by the first optical axis Al and the second optical axis A2 is set to the deviation angle θ'. Also, the distance of the shielding element 17 from the light modulator 12 is set to ΔL. At this time, the laser interferometer 1 preferably satisfies the following formula (B).

[0348] [Formula 31]

[0349]

[0350] [Formula 32]

[0351] Δy = Lθ' (B-1)

[0352] ΔL = L - L pin (B-2)

[0353] According to this configuration, by using both the optical axis deviation and the shielding element 17, it is possible to both suppress the light intensity of the return light L5 incident on the laser light source 2 and improve the S / N ratio in the light-receiving signal. Since both the optical axis deviation and the shielding element 17 can be realized with a simple structure, it is possible to easily realize the miniaturization and the lightening of the laser interferometer 1.

[0354] Note that, when the diameter of the opening 172 When the lower limit value is exceeded, the light intensity of the outgoing light Ll passing through the opening 172 becomes small. As a result, the S / N ratio in the light-receiving signal decreases, and the measurement accuracy of the displacement or velocity of the measurement object 14 can decrease. On the other hand, when the diameter of the opening 172 is When the upper limit value is exceeded, the light intensity that can be reduced by the shielding member 17 becomes small. As a result, it can be impossible to sufficiently reduce the light intensity of the return light L5 incident on the laser light source 2.

[0355] Here, the derivation process of the above-described equation (B) is described.

[0356] Since the return light L5 is not made incident on the opening 172, the deviation width Δy of the first optical axis Al from the second optical axis A2 in the shielding member 17 pin The required condition is expressed based on the equation (17-1) and the following equation (19-2).

[0357] [Equation 33]

[0358]

[0359] In the above-described equation (19-2), R pin is the optical path of the return light L5 in the shielding member 17. The optical path R pin is expressed by the following equation (19-3).

[0360] [Equation 34]

[0361]

[0362] The deviation width Δy pin is expressed by the following equation (19-4).

[0363] [Equation 35]

[0364] Δy pin = (L - L pin ) tan θ' (19-4)

[0365] The deviation angle θ' is generally small, and thus θ' « 1. As a result, the above-described equation (19-4) can be transformed into the following equation (19-5).

[0366] [Equation 36]

[0367]

[0368] As a result, in order to realize the deviation width Δy pin that satisfies the above-described equation (A), it is only necessary to adjust the distance ΔL and the deviation angle θ' based on the above-described equation (19-5).

[0369] Here, when the formula (19-3) and the above formula (19-5) are substituted into the formula (19-2), the following formula (B-3) is derived.

[0370] [Formula 37]

[0371]

[0372] And, the following formula (B-4) of a quadratic inequality is derived from the above formula (B-3).

[0373] [Formula 38]

[0374] φ pin 2 -ΔLθ'φ pin + λ≤0 (B-4)

[0375] Note that ΔL in the above formula (B-4) is defined by the following formula (B-5).

[0376] [Formula 39]

[0377] ΔL = (L - L pin ) (B-5)

[0378] When the quadratic inequality of the above formula (B-4) is solved, the following formula (B-6) is derived.

[0379] [Formula 40]

[0380]

[0381] Here, the term represented by the following formula (B-7) in the above formula (B-6) can be approximated as shown in the following formula (B-8).

[0382] [Formula 41]

[0383]

[0384]

[0385] When the above formula (B-8) is used, the formula (B-6) can be deformed as the formula (B) which is a derivation object.

[0386] [Formula 42]

[0387]

[0388] The formula (B) is derived as above.

[0389] In addition, the diameter of the opening 172 is As a specific numerical range, the relationship of the following formula (20) is preferably satisfied.

[0390] [Formula 43]

[0391] 0.10 < φ pin ≤ 10.0 (20)

[0392] When the diameter of the opening 172 is located within this range, the probability of the return light L5 being shielded in the shielding member 17 becomes high. Thereby, the light intensity of the return light L5 reaching the laser light source 2 can be suppressed, and the laser oscillation can be suppressed from becoming unstable.

[0393] Note that the diameter of the opening 172 is preferably satisfied is more preferably satisfied is further preferably satisfied

[0394] In addition, the distance of the light modulator 12 from the light receiving member 10 is set to L pD At this time, the laser interferometer 1 preferably satisfies the following formula (21).

[0395] [Formula 44]

[0396] ΔL = L - L pin ≤ L PD (21)

[0397] In the above formula (21), L is the distance of the collimator lens 3 from the light modulator 12. In addition, L pin is the distance of the collimator lens 3 from the shielding member 17.

[0398] According to this configuration, since the distance L PD can be ensured to be relatively long, the distance of the laser interferometer 1 from the measurement target object 14 that can be measured, i.e., the working distance, can be further expanded.

[0399] Note that the distance L pin of the collimator lens 3 from the shielding member 17 is preferably set to 0.5 mm or more and 15.0 mm or less, and more preferably 1.0 mm or more and 10.0 mm or less. Thereby, both the large size of the optical system 50 can be avoided, and the shielding member 17 can effectively function.

[0400] 2.1.1. Third Calculation Example

[0401] A third calculation example is shown as an example of calculating the range of the diameter of the opening 172 specified in the above formula (B). The parameters and the calculation results of the third calculation example are shown in Table 3.

[0402] Table 3

[0403]

[0404] In the third calculation example, as shown in Table 3, when the deviation width Δy of the returned light L5 in the collimating lens 3 is set to 0.80 mm, and the distance x between the collimating lens 3 and the laser source 2 is set... LD When set to 4.0mm, the diameter of the opening 172 of the blocking element 17 is... The setting is 0.096 to 0.633 mm, which can sufficiently block the return light L5. Therefore, in the third calculation example, the incident light L5 on the laser source 2 can be suppressed to a level where the laser oscillation will not become unstable.

[0405] 2.1.2. Fourth Calculation Example

[0406] As the diameter of the opening 172 specified in the above formula (B) is calculated The fourth calculation example is shown as an example of the range. The parameters and calculation results of the fourth calculation example are shown in Table 4.

[0407] Table 4

[0408]

[0409] In the fourth calculation example, as shown in Table 4, when the deviation width Δy of the returned light L5 in the collimating lens 3 is set to 0.80 mm, and the distance x between the collimating lens 3 and the laser source 2 is set... LD When set to 3.0mm, the diameter of the opening 172 of the blocking element 17 is... The setting is 0.032 to 0.581 mm, which can sufficiently block the return light L5. Therefore, in the fourth calculation example, the incident light L5 on the laser source 2 can be suppressed to a level where the laser oscillation will not become unstable.

[0410] 2.1.3. Fifth Calculation Example

[0411] The fifth calculation example is shown as an example of the case where the light intensity Pr calculated using the above equation (19) satisfies the above equation (17). The parameters and calculation results of the fifth calculation example are shown in Table 5.

[0412] Table 5

[0413]

[0414] In the fifth calculation example, as shown in Table 5, the diameter of the opening 172 of the blocking element 17 is... The deviation width Δy of the returned light L5 in the collimating lens 3 is set to 1.80 mm, and the distance x between the collimating lens 3 and the laser source 2 is set to 2.10 mm (≈3.0σ). LDSetting it to 4.0mm, the error Δψ of the half-wavelength plate's setting angle is suppressed to 0.2deg, thus P r The result of calculating / P0 is less than 1×10 -6 In other words, it has been proven that by satisfying the relationship in equation (A) with a deviation width Δy, the intensity of the reflected light L5 incident on the laser source 2 can be suppressed. Therefore, in the fifth calculation example, the incident light L5 on the laser source 2 can be suppressed to a degree that prevents laser oscillation from becoming unstable.

[0415] 2.1.4. Calculation Examples Six through Nine

[0416] The sixth to ninth calculation examples are shown as examples of cases where the light intensity Pr calculated using the above equation (19) satisfies the above equation (17). The parameters and calculation results of the sixth to ninth calculation examples are shown in Table 6.

[0417] Table 6

[0418]

[0419] In the sixth calculation example, as shown in Table 6, the diameter of the opening 172 of the blocking element 17 is... The deviation width Δy of the returned light L5 in the collimating lens 3 is set to 2.10mm, and the distance x between the collimating lens 3 and the laser source 2 is set to 2.30mm. LD Setting it to 4.0mm suppresses the error Δψ of the half-wavelength plate's setting angle to 1.0deg, thus P r The result of calculating / P0 is less than 1×10 -6 .

[0420] In the seventh calculation example, as shown in Table 6, the diameter of the opening 172 of the blocking element 17 is... The deviation width Δy of the returned light L5 in the collimating lens 3 is set to 1.60mm, and the distance x between the collimating lens 3 and the laser source 2 is set to 1.40mm. LD Setting it to 3.0mm suppresses the error Δψ of the half-wavelength plate's setting angle to 0.2deg, thus P r The result of calculating / P0 is less than 1×10 -6 .

[0421] In the eighth calculation example, as shown in Table 6, the diameter of the opening 172 of the blocking element 17 is... The deviation width Δy of the returned light L5 in the collimating lens 3 is set to 1.90mm, and the distance x between the collimating lens 3 and the laser source 2 is set to 2.10mm. LD Setting it to 3.0mm suppresses the error Δψ of the half-wavelength plate's setting angle to 1.0deg, thus Pr The result of calculating / P0 is less than 1×10 -6 .

[0422] In the ninth calculation example, as shown in Table 6, the diameter of the opening 172 of the blocking element 17 is... The deviation width Δy of the returned light L5 in the collimating lens 3 is set to 6.00mm, and the distance x between the collimating lens 3 and the laser source 2 is set to 2.00mm. LD Setting it to 40mm, the error Δψ of the half-wavelength plate's setting angle is suppressed to 0.2deg, thus P r The result of calculating / P0 is less than 1×10 -6 .

[0423] Therefore, in the fifth to ninth calculation examples, it was proven that by satisfying the relationship of the above equation (A) through the deviation width Δy, the incident light L5 on the laser source 2 can be suppressed to a degree that the laser oscillation will not become unstable.

[0424] 3. First to fifth variations of the laser interferometer

[0425] Next, the laser interferometers involved in the first to fifth variations will be explained.

[0426] Figure 16 This is a schematic diagram showing the mounting structure of the optical system of the laser interferometer involved in the first modified example. Figure 17 This is a schematic diagram showing the mounting structure of the optical system of the laser interferometer involved in the second variation. Figure 18 This is a schematic diagram showing the mounting structure of the optical system of the laser interferometer involved in the third variation. Figure 19 This is a schematic diagram showing the mounting structure of the optical system of the laser interferometer involved in the fourth variation. It should be noted that... Figure 16 to Figure 19 In order to avoid complicating the accompanying drawings, the optical axis deviation is not shown.

[0427] The following description, while outlining the first to fourth variations, focuses on the differences from the described embodiments, omitting details of the same points. It should be noted that... Figure 16 to Figure 19 In the figures, the same reference numerals are used to indicate the same components as in the embodiments described above.

[0428] Figure 16 The optical system 50D of the laser interferometer 1 shown includes a substrate 39. A laser source 2, an optical modulator 12, and a light-receiving element 10 are respectively mounted on this substrate 39. Furthermore, in... Figure 16In the illustrated substrate 39, the light-receiving element 10, the laser light source 2, and the light modulator 12 are arranged in this order in a direction orthogonal to the optical path 22.

[0429] In addition, Figure 16 The illustrated optical system 50D is provided with the prisms 40 and 42. The prism 40 is disposed on the optical path 24 between the light-receiving element 10 and the polarizer 9. The prism 42 is disposed on the optical path 20 between the light modulator 12 and the 1 / 4 wavelength plate 8.

[0430] Furthermore, Figure 16 The illustrated optical system 50D is provided with the collimator lens 44. The collimator lens 44 is disposed on the optical path 18 between the laser light source 2 and the polarization beam splitter 4.

[0431] In the first modification example as above, the same effects as those of the embodiment can be obtained.

[0432] Figure 17 The illustrated optical system 50E of the laser interferometer 1 is the same as that of the optical system 50D illustrated in Fig. 1 except for the arrangement of the elements and the like. Figure 16 The illustrated optical system 50D is provided with the prisms 40 and 42. The prism 40 is disposed on the optical path 24 between the light-receiving element 10 and the polarizer 9. The prism 42 is disposed on the optical path 20 between the light modulator 12 and the 1 / 4 wavelength plate 8.

[0433] In the second modification example as above, the same effects as those of the embodiment can be obtained. Figure 17 In the second modification example as above, the same effects as those of the embodiment can be obtained.

[0434]

[0435] The illustrated optical system 50F of the laser interferometer 1 is the same as that of the optical system 50E illustrated in Fig. 2 except for the arrangement of the elements and the like and the laser light received by the light-receiving element 10. Figure 18 The illustrated optical system 50E of the laser interferometer 1 is the same as that of the optical system 50D illustrated in Fig. 1 except for the arrangement of the elements and the like. Figure 17 In the second modification example as above, the same effects as those of the embodiment can be obtained.

[0436] Figure 18 In the second modification example as above, the same effects as those of the embodiment can be obtained.

[0437] ​The emergent light L1 emitted from the laser light source 2 is split into two emergent lights L1 by the polarizing beam splitter 4 via the prism 40. The emergent light L1 reflected by the polarizing beam splitter 4 is incident on the measurement target object 14 in a moving state via the 1 / 4 wave plate 6. The emergent light L1 receives a Doppler shift by the measurement target object 14, and is reflected as the object light L3. The object light L3 is incident on the light modulator 12 via the 1 / 4 wave plate 6, the polarizing beam splitter 4, and the 1 / 4 wave plate 8. The object light L3 receives a frequency shift by the light modulator 12, and is reflected as the object reference light L4. The object reference light L4 is incident on the light-receiving element 10 via the 1 / 4 wave plate 8, the polarizing beam splitter 4, the prism 42, and the polarizer 9.

[0438] On the other hand, the emergent light L1 transmitted through the polarizing beam splitter 4 is incident on the light-receiving element 10 via the prism 42 and the polarizer 9.

[0439] Then, the object reference light L4 and the emergent light L1 are incident on the light-receiving element 10 as interference light. The object reference light L4 is laser light containing a modulation signal and a sample signal.

[0440] In addition, in the present modification, the light-receiving element 10 receives interference light of the object reference light L4 and the emergent light L1, and the demodulation circuit 52 demodulates the sample signal contained in the object reference light L4 on the basis of the reference signal Ss and the modulation signal contained in the object reference light L4.

[0441] In the third modification as described above as well, the same effects as those of the embodiment can be obtained.

[0442] Figure 19 The optical system 50G of the laser interferometer 1 illustrated is the same as the optical system 50F illustrated except that the direction of the light reflecting surface possessed by the polarizing beam splitter 4 is different. Figure 18 The optical system 50G of the laser interferometer 1 illustrated is the same as the optical system 50F illustrated except that the direction of the light reflecting surface possessed by the polarizing beam splitter 4 is different.

[0443] The emergent light L1 emitted from the laser light source 2 is split into two emergent lights L1 by the polarizing beam splitter 4 via the prism 40.

[0444] The emergent light L1 reflected by the polarizing beam splitter 4 is incident on the light modulator 12 via the 1 / 4 wave plate 8. The emergent light L1 receives a frequency shift by the light modulator 12, and is reflected as the reference light L2. The reference light L2 is incident on the measurement target object 14 in a moving state via the 1 / 4 wave plate 8, the polarizing beam splitter 4, and the 1 / 4 wave plate 6. The reference light L2 receives a Doppler shift by the measurement target object 14, and is reflected as the object reference light L4. The object reference light L4 is incident on the light-receiving element 10 via the 1 / 4 wave plate 6, the polarizing beam splitter 4, the prism 42, and the polarizer 9.

[0445] On the other hand, the outgoing light L1 transmitted through the polarization beam splitter 4 is incident on the light receiving element 10 via the prism 42 and the analyzer 9.

[0446] Then, the object reference light L4 and the emitted light L1 are incident on the light-receiving element 10 as interference beams. The object reference light L4 is a laser beam containing a modulation signal and a sampling signal.

[0447] In addition, in this modified example, the light receiving element 10 receives the interference light between the object reference light L4 and the emitted light L1, and the demodulation circuit 52 demodulates the sampled signal contained in the object reference light L4 based on the reference signal Ss and the modulation signal contained in the object reference light L4.

[0448] In this fourth variation, the same effect as the described embodiment can also be obtained.

[0449] Based on the above... Figure 16 to Figure 19 The mounting structure shown facilitates the miniaturization of the laser interferometer 1. It should be noted that the component configuration is not limited to the configuration illustrated.

[0450] In addition, Figure 16 to Figure 19 In the mounting structure shown, the size of the light-receiving element 10 is, for example, 0.1 mm square, the size of the laser light source 2 is, for example, 0.1 mm square, and the size of the light modulator 12 is, for example, 0.5 to 10 mm square. Furthermore, the size of the substrate 39 on which the above components are mounted is, for example, 1 to 10 mm square. Thus, the optical system can be miniaturized down to the size of the substrate 39.

[0451] The above-described embodiments and variations all possess what is known as a Michelson-type interference optical system. In contrast, Figure 20 The optical system 50H of the laser interferometer 1 shown differs in that it is a Mach-Zehnder type interferometric optical system. This invention can also be applied to laser interferometers with Mach-Zehnder type interferometric optical systems.

[0452] Figure 20 This is a schematic structural diagram showing the optical system 50H of the laser interferometer 1 involved in the fifth modification. It should be noted that... Figure 20 The diagram only shows the main optical elements and omits some of them.

[0453] Figure 20The illustrated optical system 50H is provided with the laser light source 2, the polarization beam splitters 4a, 4b, 4c (optical splitter), the 1 / 4 wave plate 6, the 1 / 2 wave plate 7, the polarizer 9, the optical modulator 12H, the mirror 41, the light receiving element 10, and the blocking element 17. In the optical system 50H, the optical modulator 12H has optical transmissivity. Thus, in the optical modulator 12H, an acousto-optic modulator (AOM), an electro-optic modulator (EOM), or the like can be cited.

[0454] In the fifth modification as above, by tilting the optical modulator 12H, it is also possible to suppress the return light L5 from being incident on the laser light source 2. Thus, in the fifth modification, the same effects as the embodiment can be obtained.

[0455] Although the laser interferometer of the present application has been described based on the illustrated embodiment above, the laser interferometer of the present application is not limited to the embodiment, and the configuration of each part can be replaced with any configuration having the same function. In addition, in the laser interferometer related to the embodiment, any other configuration can be added. In addition, the laser interferometer of the present application can be a laser interferometer in which any two or more of the embodiment and the modifications are combined.

[0456] The laser interferometer of the present application can be applied to, for example, a vibrometer, an inclinometer, a distance meter (length meter), and the like, in addition to the displacement meter or the velocity meter described above. In addition, as the use of the laser interferometer of the present application, a fiber-optic gyroscope that enables optical comb interferometry such as distance measurement, 3D imaging, and spectroscopy, and that realizes an angular velocity sensor, an angular acceleration sensor, and the like can be cited.

Claims

1. A laser interferometer, characterized by, Possessing: a laser light source that emits laser light; a collimator that collimates the laser light to generate collimated light; a light modulator that modulates the collimated light into reference light of different frequencies; and a light receiving element that receives object light generated by reflection of the collimated light from a measurement target and the reference light and outputs a light receiving signal, the optical axis of the collimated light is set as a first optical axis, when return light of the reference light or the object light goes to the laser light source, the optical axis of the return light is set as a second optical axis, the position at which the collimated light is generated in the collimator is set as a reference position, when the offset width of the first optical axis and the second optical axis in the reference position is set as Δy, the effective diameter of the collimator is set as κ, the optical path length of the collimated light in the collimator is set as R, the distance from the reference position to the light modulator is set as L, when the wavelength of the collimated light is set as λ, the first optical axis and the second optical axis are offset in a manner that satisfies the following formula (A) 2. The laser interferometer according to claim 1, wherein the laser light source is a semiconductor laser element, the collimator is a collimator lens. Possessing:

3. The laser interferometer of claim 1, wherein, a light splitter that splits the collimated light into first split light that is incident on the light modulator and second split light that is incident on the measurement target; and a shielding element that is disposed between the collimator and the light splitter and has an opening through which the collimated light passes.

4. The laser interferometer according to claim 3, wherein an angle formed by the optical axis of the first split light that is incident on the light modulator and the optical axis of the reference light that is generated by the light modulator is set as an offset angle θ', when the distance from the shielding element to the light modulator is set as ΔL, the following formula (B) is satisfied When the diameter of the opening is set to φ pin , 5. The laser interferometer according to any one of claims 1 to 4, wherein the light modulator has a vibration element, the light modulator modulates the collimated light using the vibration element. Possessing: a demodulation circuit that demodulates a sample signal originating from the measurement target from the light receiving signal based on a reference signal; and 6. The laser interferometer of claim 5, wherein, an oscillation circuit that outputs the reference signal to the demodulation circuit, the vibration element is a signal source of the oscillation circuit. ​ ​

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