Laser interferometer

Abstract

To provide a laser interferometer that can obtain a light reception signal with high S / N ratio by differential amplification processing, and improve accuracy of demodulating a sample signal derived from a measuring object from the light reception signal by inhibiting incidence of return light to a laser light source.SOLUTION: The laser interferometer comprises: a laser light source; a first light splitter splitting laser light into first split light and second split light; an optical modulator modulating the first split light into reference light; a second light splitter splitting object light generated by the second split light reflected on a measuring object and reference light into third split light and fourth split light; a first light receiving element receiving the third split light; and a second light receiving element receiving the fourth split light. An optical axis of the first split light travelling from the first light splitter to the optical modulator is displaced with respect to an optical axis of the reference light travelling from the optical modulator to the first light splitter. A difference between an optical path length from the second light splitter to the first light receiving element and an optical path length from the second light splitter to the second light receiving element is 20 mm or less.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a laser interferometer.

Background Art

[0002] Patent Document 1 discloses a laser vibrometer as a device for measuring the vibration speed of an object. In this laser vibrometer, a laser beam is irradiated onto the object to be measured, and the vibration speed is measured based on the scattered laser beam that has received the Doppler shift.

[0003] Specifically, the laser vibrometer described in Patent Document 1 includes an acousto-optic modulator (AOM). The acousto-optic modulator shifts the frequency of the laser beam by changing the supplied ultrasonic frequency. In the laser vibrometer, the laser beam with the shifted frequency is used as the reference light. Then, the scattered laser beam from the object to be measured and the reference light from the acousto-optic modulator are overlapped to extract the beat frequency. The vibration speed of the object to be measured is obtained from the beat frequency thus extracted.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In a laser light source, the return light may enter, causing the laser oscillation to become unstable. In the laser vibrometer described in Patent Document 1, when the laser oscillation becomes unstable, the quality of the laser light deteriorates. As a result, in the laser vibrometer, the S / N ratio (signal-to-noise ratio) of the received light signal obtained by receiving the scattered laser light decreases, or the phase of the laser light emitted from the laser light source becomes discontinuous. As a result, there is a problem that the measurement accuracy of the vibration speed of the object decreases.

Means for Solving the Problems

[0006] The laser interferometer according to an application example of the present invention includes a laser light source that emits laser light, a first optical splitter that splits the laser light into a first split light and a second split light, an optical modulator that modulates the first split light into reference lights having different frequencies, a second optical splitter that splits the object light generated by reflecting the second split light by the measurement object and the reference light into a third split light and a fourth split light, a first light receiving element that receives the third split light, a second light receiving element that is disposed at a position different from the first light receiving element and receives the fourth split light, and is characterized in that the optical axis of the first split light from the first optical splitter toward the optical modulator and the optical axis of the reference light from the optical modulator toward the first optical splitter are displaced, the difference between the optical path length from the second optical splitter to the first light receiving element and the optical path length from the second optical splitter to the second light receiving element is 20 mm or less.

Brief Description of the Drawings

[0007]

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Embodiments for Carrying Out the Invention

[0008] Hereinafter, the laser interferometer of the present invention will be described in detail based on the embodiments shown in the accompanying drawings. 1. First Embodiment First, the laser interferometer according to the first embodiment will be described. Fig. 1 is a functional block diagram showing the laser interferometer 1 according to the first embodiment.

[0009] The laser interferometer 1 shown in Fig. 1 includes a sensor head unit 51 having an optical system 50, a differential amplifier circuit 530, a current-voltage converter 531, and an oscillation circuit 54, and a demodulation circuit 52 into which a light reception signal from the optical system 50 is input.

[0010] 1.1. Sensor head unit Fig. 2 is a schematic configuration diagram showing the sensor head unit 51 shown in Fig. 1. In Fig. 2, the X-axis, Y-axis, and Z-axis are set as three mutually orthogonal axes and are indicated by arrows. The tip side of the arrow is defined as "plus", and the base end side of the arrow is defined as "minus". Fig. 2 is a view (plan view) seen from above the Z-axis.

[0011] 1.1.1. Optical system As described above, the sensor head unit 51 includes an optical system 50.

[0012] As shown in Fig. 2, the optical system 50 includes a laser light source 2, a collimating lens 3, a first optical splitter 4, a second optical splitter 5, a half-wave plate 6, a quarter-wave plate 7, a quarter-wave plate 8, a polarimeter 9, a first light receiving element 10, a second light receiving element 11, a frequency shifter type optical modulator 12, and a set unit 16 in which a measurement object 14 is arranged.

[0013] The laser light source 2 emits emitted light L1 (laser light). The first light receiving element 10 and the second light receiving element 11 convert the received light into an electrical signal. The optical modulator 12 includes a vibrating element 30, changes the frequency of the emitted light L1, and generates reference light L2 including a modulation signal. The set unit 16 may be provided as necessary and is configured to be able to arrange the measurement object 14. The emitted light L1 incident on the measurement object 14 is reflected as object light L3 including a sample signal that is a Doppler signal derived from the measurement object 14.

[0014] Let the optical path of the emitted light L1 emitted from the laser light source 2 be the optical path 18. On the optical path 18, a collimating lens 3 and a half-wave plate 6 are arranged in this order from the laser light source 2 side. The optical path 18 is coupled to the optical path 20 by reflection at the first optical splitter 4. On the optical path 20, a quarter-wave plate 8 and an optical modulator 12 are arranged in this order from the first optical splitter 4 side. Also, the optical path 18 is coupled to the optical path 22 by transmission through the first optical splitter 4. On the optical path 22, a quarter-wave plate 7 and a set unit 16 are arranged in this order from the first optical splitter 4 side.

[0015] The optical path 20 is coupled to the optical path 24 by transmission through the first optical splitter 4. On the optical path 24, an analyzer 9 and a second optical splitter 5 are arranged in this order from the first optical splitter 4 side. Then, the optical path 24 is split by the second optical splitter 5 into an optical path 26 and an optical path 28. The optical path 26 extends between the second optical splitter 5 and the first light receiving element 10. The optical path 28 extends between the second optical splitter 5 and the second light receiving element 11.

[0016] The emitted light L1 emitted from the laser light source 2 is split into two by the first optical splitter 4. One passes through the optical path 20 and enters the optical modulator 12. The other passes through the optical path 22 and enters the measurement object 14. The reference light L2 generated by the optical modulator 12 enters the second optical splitter 5 through the optical path 20 and the optical path 24. The object light L3 generated by reflection at the measurement object 14 enters the second optical splitter 5 through the optical path 22 and the optical path 24.

[0017] The reference light L2 and the object light L3 are split into two by the second optical splitter 5. One enters the first light receiving element 10 through the optical path 26, and the other enters the second light receiving element 11 through the optical path 28.

[0018] Note that the "optical path" in this specification refers to the path along which light travels, set between optical components. Also, the "optical axis" described later refers to the central axis of the light beam passing through the optical path.

[0019] Hereinafter, each part of the optical system 50 will be further described. 1.1.1.1. Laser light source The laser light source 2 is a laser light source that emits output light L1 having coherence. As the laser light source 2, a light source with a line width of MHz band or less is preferably used. Specifically, gas lasers such as He-Ne lasers, DFB-LD (Distributed feedback - laser diode), FBG-LD (Fiber bragg Grating attached laser diode), VCSEL (Vertical Cavity Surface Emitting Laser), and semiconductor laser elements such as FP-LD (Fabry-Perot Laser Diode) can be mentioned.

[0020] The laser light source 2 is particularly preferably a semiconductor laser element. This makes it possible to particularly miniaturize the laser light source 2. Therefore, the laser interferometer 1 can be miniaturized. In particular, in the laser interferometer 1, since the sensor head portion 51 in which the optical system 50 is accommodated can be miniaturized and lightened, it is also useful in terms of improving the operability of the laser interferometer 1.

[0021] 1.1.1.2. Collimating lens The collimating lens 3 is a convex lens disposed between the laser light source 2 and the first optical splitter 4. The collimating lens 3 collimates the output light L1 emitted from the laser light source 2.

[0022] When the output light L1 emitted from the laser light source 2 is sufficiently collimated, for example, when a gas laser such as a He-Ne laser is used as the laser light source 2, the collimating lens 3 may be omitted.

[0023] On the one hand, when the laser light source 2 is a semiconductor laser element, the laser interferometer 1 preferably includes a collimating lens 3 disposed between the laser light source 2 and the first optical splitter 4. Thereby, the emitted light L1 emitted from the semiconductor laser element can be collimated. As a result, since the emitted light L1 becomes collimated light, it is possible to suppress the enlargement of various optical components that receive the emitted light L1, and the laser interferometer 1 can be miniaturized.

[0024] The emitted light L1 that has become collimated light passes through the half-wave plate 6 and is converted into linearly polarized light with an intensity ratio of P-polarized light to S-polarized light of, for example, 50:50, and enters the first optical splitter 4.

[0025] 1.1.1.3. Optical Splitter The first optical splitter 4 is a polarization beam splitter disposed between the laser light source 2 and the optical modulator 12 and between the laser light source 2 and the object to be measured 14. The first optical splitter 4 has a function of transmitting P-polarized light and reflecting S-polarized light. By this function, the first optical splitter 4 splits the emitted light L1 into a first split light L1a that is the reflected light at the first optical splitter 4 and a second split light L1b that is the transmitted light of the first optical splitter 4.

[0026] The first split light L1a, which is S-polarized light reflected by the first optical splitter 4, is converted into circularly polarized light by the quarter-wave plate 8 and enters the optical modulator 12. The circularly polarized light of the first split light L1a incident on the optical modulator 12 undergoes a frequency shift of f m [Hz] and is reflected as the reference light L2. Therefore, the reference light L2 includes a modulation signal of frequency f m [Hz]. The reference light L2 is converted into P-polarized light when passing through the quarter-wave plate 8 again. The P-polarized light of the reference light L2 passes through the first optical splitter 4 and the analyzer 9 and enters the second optical splitter 5.

[0027] The second split light L1b, which is P-polarized light transmitted through the first optical splitter 4, is converted into circularly polarized light by the quarter-wave plate 7 and enters the object to be measured 14 in a moving state. The circularly polarized light of the second split light L1b incident on the object to be measured 14 has a frequency of f dReceives the Doppler shift of [[Hz]] and reflects it as object light L3. Therefore, the object light L3 includes a sample signal of frequency f d [[Hz]]. When the object light L3 passes through the quarter-wave plate 7 again, it is converted to S-polarized light. The S-polarized light of the object light L3 is reflected by the first optical splitter 4, passes through the analyzer 9, and is incident on the second optical splitter 5.

[0028] As described above, since the emitted light L1 has coherence, the reference light L2 and the object light L3 are incident on the first light receiving element 10 and the second light receiving element 11 as interference light.

[0029] The second optical splitter 5 is a polarization beam splitter disposed between the analyzer 9, the first light receiving element 10, and the second light receiving element 11. The second optical splitter 5 splits the interference light (reference light L2 and object light L3) into a third split light L6a and a fourth split light L6b. Specifically, the S-polarized light of the interference light is reflected by the second optical splitter 5 and is incident on the first light receiving element 10 as the third split light L6a. The P-polarized light of the interference light passes through the second optical splitter 5 and is incident on the second light receiving element 11 as the fourth split light L6b.

[0030] Note that an unpolarized beam splitter may be used instead of the polarization beam splitter. In this case, components such as the half-wave plate 6, the quarter-wave plate 7, and the quarter-wave plate 8 become unnecessary, so the size of the laser interferometer 1 can be reduced by reducing the number of components. Also, an optical splitter other than the beam splitter may be used.

[0031] 1.1.1.4. Analyzer Since the S-polarized light and the P-polarized light that are orthogonal to each other are independent of each other, no beat due to interference appears simply by overlapping them. Therefore, the light wave obtained by overlapping the S-polarized light and the P-polarized light is passed through an analyzer 9 that is inclined by 45° with respect to both the S-polarized light and the P-polarized light. By using the analyzer 9, light of components common to each other can be transmitted to cause interference. As a result, in the analyzer 9, the reference light L2 and the object light L3 interfere with each other, and |f m -f dInterference light having a frequency of [Hz] is generated.

[0032] 1.1.1.5. Light receiving element The third split light L6a is incident on the first light receiving element 10. The fourth split light L6b is incident on the second light receiving element 11 arranged at a position different from that of the first light receiving element 10. The third split light L6a and the fourth split light L6b are the interference lights described above. The first light receiving element 10 and the second light receiving element 11 receive this interference light and output a received signal. By demodulating the sample signal from this received signal by a method described later, finally, the movement of the measurement object 14, that is, the vibration speed and displacement can be obtained. Examples of the first light receiving element 10 and the second light receiving element 11 include a photodiode and the like.

[0033] The first light receiving element 10 receives the third split light L6a and outputs a photocurrent. The second light receiving element 11 receives the fourth split light L6b and outputs a photocurrent. In a differential amplifier circuit 530 described later, the difference between the photocurrent from the first light receiving element 10 and the photocurrent from the second light receiving element 11 is taken and output as a differential current. In a current-voltage converter 531 described later, this differential current is converted into a voltage signal.

[0034] 1.1.1.6. Optical modulator FIG. 3 is a perspective view showing a first configuration example of the optical modulator 12 shown in FIG. 2.

[0035] 1.1.1.6.1. Outline of the first configuration example of the optical modulator The frequency shifter type optical modulator 12 has an optical modulation oscillator 120. The optical modulation oscillator 120 shown in FIG. 3 includes a plate-shaped vibration element 30 and a substrate 31 that supports the vibration element 30.

[0036] The vibrating element 30 is composed of a material that repeats a mode of vibrating so as to be distorted in a direction along the surface when a potential is applied. In this configuration example, the vibrating element 30 is a crystal AT oscillator that performs thickness-shear vibration along the vibration direction 36 in a high-frequency region in the MHz band. A diffraction grating 34 is formed on the surface of the vibrating element 30. The diffraction grating 34 has grooves 32 having a component intersecting the vibration direction 36, that is, a plurality of linear grooves 32 extending in a direction intersecting the vibration direction 36.

[0037] The substrate 31 has a front surface 311 and a back surface 312 that are in a front-back relationship with each other. The vibrating element 30 is disposed on the front surface 311. Further, pads 33 for applying a potential to the vibrating element 30 are provided on the front surface 311. On the other hand, pads 35 for applying a potential to the vibrating element 30 are also provided on the back surface 312.

[0038] The size of the substrate 31 is, for example, such that the long side is about 0.5 mm or more and 10.0 mm or less. Also, the thickness of the substrate 31 is, for example, about 0.10 mm or more and 2.0 mm or less. As an example, the shape of the substrate 31 is a square with one side being 1.6 mm, and its thickness is 0.35 mm.

[0039] The size of the vibrating element 30 is, for example, such that the long side is about 0.2 mm or more and 3.0 mm or less. Also, the thickness of the vibrating element 30 is, for example, about 0.003 mm or more and 0.5 mm or less.

[0040] As an example, the shape of the vibrating element 30 is a square with one side being 1.0 mm, and its thickness is 0.07 mm. In this case, the vibrating element 30 oscillates at a fundamental oscillation frequency of 24 MHz. Note that the oscillation frequency can be adjusted in the range of 1 MHz to 1 GHz by changing the thickness of the vibrating element 30 or considering overtones.

[0041] Note that in FIG. 3, the diffraction grating 34 is formed over the entire surface of the vibrating element 30, but it may be formed only partially.

[0042] The magnitude of the optical modulation by the optical modulator 12 is given by the inner product of the difference wave vector between the wave vector of the emitted light L1 incident on the optical modulator 12 and the wave vector of the reference light L2 emitted from the optical modulator 12, and the vector of the vibration direction 36 of the vibrating element 30. In this configuration example, the vibrating element 30 vibrates in thickness shear, but since this vibration is in-plane vibration, even if light is incident perpendicularly to the surface of the vibrating element 30 alone, optical modulation cannot be performed. Therefore, in this configuration example, by providing a diffraction grating 34 on the vibrating element 30, optical modulation is enabled by the principle described later.

[0043] The diffraction grating 34 shown in FIG. 3 is a blazed diffraction grating. A blazed diffraction grating refers to one in which the cross-sectional shape of the diffraction grating is stepped. The linear groove 32 of the diffraction grating 34 is provided such that its extending direction is orthogonal to the vibration direction 36.

[0044] When a drive signal Sd (applying an alternating voltage) is supplied from the oscillation circuit 54 shown in FIGS. 1 and 2 to the vibrating element 30 shown in FIG. 3, the vibrating element 30 oscillates. The power (drive power) required for the oscillation of the vibrating element 30 is not particularly limited, but is as small as about 0.1 μW to 100 mW. Therefore, it can be used to oscillate the vibrating element 30 without amplifying the drive signal Sd output from the oscillation circuit 54.

[0045] In addition, since a conventional optical modulator sometimes requires a structure for maintaining the temperature of the optical modulator, it has been difficult to reduce the volume. Also, a conventional optical modulator has a problem that it is difficult to miniaturize and power-save a laser interferometer because of its large power consumption. In contrast, in this configuration example, since the volume of the vibrating element 30 is very small and the power required for oscillation is also small, it is easy to miniaturize and power-save the laser interferometer 1.

[0046] 1.1.1.6.2. Method of forming diffraction grating The method for forming the diffraction grating 34 is not particularly limited. As an example, a method of creating a mold using a mechanical scribing method (ruling engine) and forming grooves 32 by nanoimprint lithography on an electrode formed on the surface of the vibrating element 30 of the crystal AT oscillator can be mentioned. Here, the reason for using the electrode is that, in the case of a crystal AT oscillator, in principle, high-quality thickness-shear vibration can be generated on the electrode. Note that the formation of the grooves 32 is not limited to the electrode, and it may be on the surface of the material of the non-electrode portion. Also, instead of the nanoimprint lithography method, a processing method by exposure and etching, an electron beam lithography method, a focused ion beam processing method (FIB), etc. may be used.

[0047] Also, a diffraction grating may be formed of a resist material on the chip of the crystal AT oscillator, and a mirror film made of a metal film or a dielectric multilayer film may be provided there. By providing the metal film or the mirror film, the reflectivity of the diffraction grating 34 can be increased.

[0048] Furthermore, a resist film may be formed on the chip or wafer of the crystal AT oscillator, processed by etching, then the resist film may be removed, and thereafter, a metal film or a mirror film may be formed on the processed surface. In this case, since the resist material is removed, the influence due to moisture absorption of the resist material is eliminated, and the chemical stability of the diffraction grating 34 can be enhanced. Also, by providing a highly conductive metal film such as Au or Al, it can also be used as an electrode for driving the vibrating element 30.

[0049] Note that the diffraction grating 34 may be formed using a technique such as anodic aluminum oxide (porous alumina).

[0050] 1.1.1.6.3. Other Configuration Examples of the Optical Modulator The vibrating element 30 is not limited to a crystal oscillator, and for example, it may be a Si oscillator, a surface acoustic wave (SAW) device, a ceramic oscillator, etc.

[0051] FIG. 4 is a plan view showing a part of a second configuration example of the optical modulator 12. FIG. 5 is a plan view showing a third configuration example of the optical modulator 12.

[0052] The vibrating element 30A shown in FIG. 4 is a Si vibrator manufactured from a Si substrate using MEMS technology. MEMS (Micro Electro Mechanical Systems) is a micro electro mechanical system.

[0053] The vibrating element 30A includes a first electrode 301 and a second electrode 302 adjacent to each other on the same plane with a gap therebetween, a diffraction grating mounting portion 303 provided on the first electrode 301, and a diffraction grating 34 provided on the diffraction grating mounting portion 303. The first electrode 301 and the second electrode 302 vibrate so as to repeatedly approach and separate from each other in the left - right direction of FIG. 4, that is, along the axis connecting the first electrode 301 and the second electrode 302 shown in FIG. 4, using, for example, electrostatic attraction as a driving force. Thereby, in - plane vibration can be imparted to the diffraction grating 34. The oscillation frequency of the Si vibrator is, for example, about 1 kHz to several 100 MHz.

[0054] The vibrating element 30B shown in FIG. 5 is a SAW device that utilizes surface acoustic waves. SAW (Surface Acoustic Wave) is an elastic surface wave.

[0055] The vibrating element 30B includes a piezoelectric substrate 305, a comb - shaped electrode 306 provided on the piezoelectric substrate 305, a ground electrode 307, a diffraction grating mounting portion 303, and a diffraction grating 34. When an alternating voltage is applied to the comb - shaped electrode 306, an elastic surface wave is excited by the inverse piezoelectric effect. Thereby, in - plane vibration can be imparted to the diffraction grating 34. The oscillation frequency of the SAW device is, for example, about several 100 MHz to several GHz.

[0056] For the devices as described above, by providing the diffraction grating 34, similar to the case of a crystal AT oscillator, optical modulation can be achieved by the principle described later.

[0057] On the one hand, when the vibration element 30 is a crystal oscillator, a highly accurate modulation signal can be generated by utilizing the extremely high Q value of the crystal. The Q value is an index indicating the sharpness of the resonance peak. In addition, the crystal oscillator has the characteristic of being less susceptible to external disturbances. Therefore, by using the modulation signal modulated by the optical modulator 12 provided with the crystal oscillator, the sample signal derived from the measurement object 14 can be acquired with high accuracy.

[0058] 1.1.1.6.4. Optical Modulation by Vibration Element Next, the principle of modulating light using the vibration element 30 will be described.

[0059] FIG. 6 is a conceptual diagram for explaining that when incident light K is incident from a direction perpendicular to the surface of the vibration element 30, a plurality of diffracted lights are generated. i When the incident light K is incident on the diffraction grating 34 that is undergoing thickness shear vibration along the vibration direction 36, due to the diffraction phenomenon, as shown in FIG. 6, a plurality of diffracted lights K are generated. n is the order of the diffracted light K, and n = 0, ±1, ±2, ···. Note that the diffraction grating 34 shown in FIG. 6 is not the blazed diffraction grating shown in FIG. 3, but as an example of another diffraction grating, a diffraction grating formed by repeating unevenness is illustrated. Also, in FIG. 6, the illustration of the diffracted light K is omitted.

[0060] When the incident light K is incident on the diffraction grating 34 that is undergoing thickness shear vibration along the vibration direction 36, i due to the diffraction phenomenon, as shown in FIG. 6, a plurality of diffracted lights K ns are generated. n is the order of the diffracted light K ns and n = 0, ±1, ±2, ···. Note that the diffraction grating 34 shown in FIG. 6 is not the blazed diffraction grating shown in FIG. 3, but as an example of another diffraction grating, a diffraction grating formed by repeating unevenness is illustrated. Also, in FIG. 6, the illustration of the diffracted light K 0s is omitted.

[0061] In FIG. 6, the incident light K i is incident from a direction perpendicular to the surface of the vibration element 30, but this incident angle is not particularly limited, and the incident angle may be set so that the incident light is incident obliquely with respect to the surface of the vibration element 30. When incident obliquely, the traveling direction of the diffracted light K ns also changes accordingly.

[0062] Depending on the design of the diffraction grating 34, higher-order light with |n| ≥ 2 may not appear. Therefore, in order to stably obtain a modulation signal, it is desirable to set it to |n| = 1. That is, in the laser interferometer 1 of FIG. 2, the frequency shifter type optical modulator 12 is preferably arranged such that the ±1st order diffracted light is used as the reference light L2. With this arrangement, the stabilization of the measurement by the laser interferometer 1 can be realized.

[0063] On the other hand, when higher-order light with |n| ≥ 2 appears from the diffraction grating 34, the optical modulator 12 may be arranged such that any diffracted light of ±2nd order or higher, rather than the ±1st order diffracted light, is used as the reference light L2. As a result, since higher-order diffracted light can be used, the high-frequency operation and miniaturization of the laser interferometer 1 can be realized.

[0064] In this embodiment, as an example, the incident light K incident on the optical modulator 12 i is configured such that the angle formed by the incident direction of the light and the traveling direction of the reference light L2 emitted from the optical modulator 12 is 180°. Hereinafter, three examples will be described while showing FIGS. 7 to 9.

[0065] FIGS. 7 to 9 are conceptual diagrams for explaining the optical modulator 12 configured such that the angle formed by the traveling direction of the incident light K i and the traveling direction of the reference light L2 is 180°.

[0066] The optical modulator 12 shown in FIG. 7 includes a mirror 37 in addition to the vibrating element 30. The mirror 37 is arranged to reflect the diffracted light K 1s and return it to the diffraction grating 34. At this time, the angle formed by the incident angle of the diffracted light K 1s with respect to the mirror 37 and the reflection angle at the mirror 37 is 180°. As a result, the diffracted light K 1s emitted from the mirror 37 and returned to the diffraction grating 34 is diffracted again by the diffraction grating 34 and becomes the incident light K incident on the optical modulator 12 iIt will travel in the direction opposite to the traveling direction. Therefore, by adding the mirror 37, the condition that the angle formed by the incident direction of the incident light K i and the traveling direction of the reference light L2 is 180° can be satisfied.

[0067] Also, by passing through the mirror 37 in this way, the reference light L2 generated by the optical modulator 12 will undergo frequency modulation twice. Therefore, by using the mirror 37 in combination, frequency modulation with a higher frequency becomes possible compared to the case where only the vibration element 30 is used.

[0068] In FIG. 8, the vibration element 30 is tilted with respect to the arrangement in FIG. 6. The tilt angle θ S at this time is set so as to satisfy the condition that the angle formed by the incident direction of the incident light K i and the traveling direction of the reference light L2 is 180°.

[0069] The diffraction grating 34 shown in FIG. 9 is a blazed diffraction grating having a blaze angle θ B . When the incident light K i traveling at an incident angle β with respect to the normal N on the surface of the vibration element 30 is incident on the diffraction grating 34, the reference light L2 returns at the same angle as the blaze angle θ B with respect to the normal N. Therefore, by making the incident angle β equal to the blaze angle θ B , the condition that the angle formed by the incident direction of the incident light K i and the traveling direction of the reference light L2 is 180° can be satisfied. In this case, without using the mirror 37 shown in FIG. 7 and without tilting the vibration element 30 itself as shown in FIG. 8, the above condition can be satisfied, so that the laser interferometer 1 can be further miniaturized and made to operate at a higher frequency. In particular, in the case of a blazed diffraction grating, the arrangement that satisfies the above condition is called a "retro arrangement", and there is also an advantage that the diffraction efficiency of the diffracted light can be particularly increased.

[0070] Note that the pitch P in FIG. 9 represents the pitch of the blazed diffraction grating. As an example, the pitch P is set to 1 μm. Also, the blaze angle θB is, for example, 25°. In this case, in order to satisfy the above conditions, the incident angle β of the incident light K i with respect to the normal N may also be set to 25°.

[0071] 1.1.1.6.5. Package Structure FIG. 10 is a cross-sectional view showing an optical modulator 12 having a package structure.

[0072] The optical modulator 12 shown in FIG. 10 includes a container 70 that is a housing, an optical modulation oscillator 120 housed in the container 70, and a circuit element 45 that constitutes an oscillation circuit 54. The container 70 is hermetically sealed in a reduced-pressure atmosphere such as a vacuum or an inert gas atmosphere such as nitrogen or argon.

[0073] As shown in FIG. 10, the container 70 has a container body 72 and a lid 74. Among these, the container body 72 has a first recess 721 provided inside thereof and a second recess 722 provided inside the first recess 721 and deeper than the first recess 721. The container body 72 is made of, for example, a ceramic material, a resin material, or the like. Although not shown, the container body 72 includes internal terminals provided on the inner surface, external terminals provided on the outer surface, wiring for connecting the internal terminals and the external terminals, and the like.

[0074] The opening of the container body 72 is closed by a lid 74 via a sealing member such as a seal ring or a low-melting-point glass (not shown). As the constituent material of the lid 74, a material that can transmit laser light, such as a glass material, is used.

[0075] The optical modulation oscillator 120 is disposed on the bottom surface of the first recess 721. The optical modulation oscillator 120 is supported on the bottom surface of the first recess 721 by a bonding member (not shown). Further, the internal terminals of the container body 72 and the optical modulation oscillator 120 are electrically connected via a conductive material (not shown) such as a bonding wire or a bonding metal.

[0076] On the bottom surface of the second recess 722, a circuit element 45 is disposed. The circuit element 45 is electrically connected to the internal terminal of the container body 72 via a bonding wire 76. Thereby, the optical modulator oscillator 120 and the circuit element 45 are also electrically connected via the wiring provided in the container body 72. Note that, in addition to the oscillation circuit 54 described later, other circuits may be provided in the circuit element 45.

[0077] By adopting such a package structure, the optical modulator oscillator 120 and the circuit element 45 can be stacked, so that the physical distance between the two can be reduced, and the wiring length between the optical modulator oscillator 120 and the circuit element 45 can be shortened. For this reason, it is possible to suppress external noise from entering the drive signal Sd or, conversely, the drive signal Sd from becoming a noise source. In addition, both the optical modulator oscillator 120 and the circuit element 45 can be protected from the external environment by one container 70. Therefore, while reducing the size of the sensor head unit 51, the reliability of the laser interferometer 1 can be enhanced.

[0078] Note that the structure of the container 70 is not limited to the illustrated structure. For example, the optical modulator oscillator 120 and the circuit element 45 may have individual package structures. Also, although not shown, other circuit elements constituting the oscillation circuit 54 may be accommodated in the container 70. Note that the container 70 may be provided as necessary or may be omitted.

[0079] In addition, the optical modulator 12 is not limited to one including the vibration element 30 as described above. For example, it may be an acousto-optic modulator (AOM), an electro-optic modulator (EOM), or the like. When applying an AOM or an EOM to the optical modulator 12, an optical reflection function may be added to the AOM or the EOM.

[0080] 1.1.2. Differential Amplifier Circuit and Current-Voltage Converter FIG. 11 is a circuit diagram showing a configuration example of the differential amplifier circuit 530 and the current-voltage converter 531 shown in FIG. 1.

[0081] As described above, the first light receiving element 10 and the second light receiving element 11 are, for example, photodiodes, and each has a cathode and an anode.

[0082] The differential amplifier circuit 530 shown in FIG. 11 includes a connection wiring 530a that connects the anode of the first light receiving element 10 and the cathode of the second light receiving element 11, and a connection wiring 530b that connects the cathode of the first light receiving element 10 and the anode of the second light receiving element 11. The differential amplifier circuit 530 performs a differential amplification process of taking the difference between the photocurrent I1 from the first light receiving element 10 and the photocurrent I2 from the second light receiving element 11, and outputs it as a differential current Id.

[0083] The current-voltage converter 531 shown in FIG. 11 includes a first input terminal 531a, a second input terminal 531b, an operational amplifier 531c, a feedback resistor 531d, and an output terminal 531e.

[0084] The connection wiring 530a is connected to the first input terminal 531a, and the connection wiring 530b is connected to the second input terminal 531b.

[0085] The first input terminal 531a is connected to the inverting input terminal of the operational amplifier 531c. The second input terminal 531b is connected to the non-inverting input terminal of the operational amplifier 531c. Also, the second input terminal 531b and the non-inverting input terminal of the operational amplifier 531c are connected to a reference potential such as a ground potential.

[0086] The feedback resistor 531d is connected between the output terminal and the inverting input terminal of the operational amplifier 531c. The feedback resistor 531d performs negative feedback and converts current into voltage. By applying negative feedback by the feedback resistor 531d, the potential difference between the inverting input terminal and the non-inverting input terminal of the operational amplifier 531c is almost zero. As a result, a so-called virtual short circuit is established.

[0087] The output terminal 531e is connected to the output terminal of the operational amplifier 531c. The current-voltage converter 531, also called a transimpedance amplifier (TIA), converts the differential current Id input to the first input terminal 531a into a voltage signal and outputs it from the output terminal 531e as a received light signal.

[0088] By including the differential amplifier circuit 530 described above, the signal-to-noise ratio (S / N ratio) can be increased in the received light signal output from the current-voltage converter 531 or in the calculation result of an operation using the received light signal.

[0089] Specifically, the differential amplifier circuit 530 can perform a differential process that takes the difference between the photocurrent I1 from the first light-receiving element 10 and the photocurrent I2 from the second light-receiving element 11, and outputs it as the differential current Id. According to the differential process, the DC components of the photocurrents I1 and I2 are canceled or suppressed. For this reason, the differential current Id contains almost no DC component. As a result, the conversion gain can be increased in the current-voltage converter 531, and the S / N ratio of the received light signal can be increased. Also, when performing an operation to amplify the AC signal included in the received light signal in the demodulation circuit 52 described later, there is no need to consider the DC component. As a result, the accuracy of the operation is improved, and the S / N ratio of the operation result can be increased.

[0090] Furthermore, the amplitude of the differential current Id becomes larger than the amplitudes of the photocurrents I1 and I2. That is, in the differential amplifier circuit 530, amplification processing is performed on the photocurrents I1 and I2, and the amplified differential current Id is output. Therefore, by using this differential current Id, a received light signal with a high S / N ratio can be obtained.

[0091] Also, noise (common-mode noise) corresponding to the wiring length is mixed into the connection wirings 530a and 530b. This noise often mixes in the same phase with respect to both of the connection wirings 530a and 530b. For this reason, by performing differential processing, the noise is canceled or suppressed. As a result, the S / N ratio of the received light signal can be increased.

[0092] Note that the configuration of the differential amplifier circuit 530 is not limited to the above configuration. For example, after independently converting the photocurrents I1 and I2 into voltage signals, the phase of one of the voltage signals may be inverted, and then the differential voltage between the two voltage signals may be calculated, and the calculation result may be used as the received optical signal.

[0093] An ADC 532 shown in FIG. 1 is arranged between the current-voltage converter 531 and the demodulation circuit 52. The ADC 532 is an analog-to-digital converter that converts an analog signal into a digital signal with a predetermined number of sampling bits.

[0094] 1.1.3. Oscillation circuit As shown in FIG. 1, the oscillation circuit 54 outputs a drive signal Sd input to the optical modulator 12 of the optical system 50. Further, the oscillation circuit 54 outputs a reference signal Ss input to the demodulation circuit 52.

[0095] The oscillation circuit 54 is not particularly limited as long as it is a circuit capable of oscillating the vibration element 30, and circuits with various configurations are used. FIG. 12 is a circuit diagram showing the configuration of a single-stage inverter oscillation circuit as an example of the circuit configuration.

[0096] The oscillation circuit 54 shown in FIG. 12 includes 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.

[0097] The circuit element 45 is an inverter IC. Terminals X1 and X2 of the circuit element 45 are terminals connected to the inverters inside the circuit element 45, respectively. The terminal GND is connected to the ground potential, and the terminal Vcc is connected to the power supply potential. The terminal Y is a terminal for oscillation output.

[0098] A first capacitor Cg is connected between the terminal X1 and the ground potential. Also, a limiting resistor Rd and a second capacitor Cd, which are connected in series with each other, are connected in this order from the terminal X2 side between the terminal X2 and the ground potential. Further, one end of a 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.

[0099] Also, one end of the vibrating element 30 is connected between the first capacitor Cg and the feedback resistor Rf, and the other end of the vibrating element 30 is connected between the second capacitor Cd and the limiting resistor Rd. Thereby, the vibrating element 30 serves as a signal source of the oscillation circuit 54.

[0100] FIG. 13 is an example of the LCR equivalent circuit of the vibrating element 30. As shown in FIG. 13, the LCR equivalent circuit of the vibrating element 30 is composed of a series capacitor C1, a series inductance L1, an equivalent series resistance R1, and a parallel capacitor C0.

[0101] In the oscillation circuit 54 shown in FIG. 12, when the capacitance of the first capacitor Cg is C g and the capacitance of the second capacitor Cd is C d , the load capacitance C L is given by the following formula (a).

[0102]

Equation

[0103] Then, the oscillation frequency f osc output from the terminal Y of the oscillation circuit 54 is given by the following formula (b).

[0104]

Equation

[0105] f Q is the natural oscillation frequency of the vibrating element 30. According to the above formula (b), by appropriately changing the load capacitance C L it can be seen that the oscillation frequency f of the signal output from terminal Y osc can be finely adjusted.

[0106] Also, the natural frequency f of the vibration element 30 Q and the oscillation frequency f of the oscillation circuit 54 osc The difference Δf between them is given by the following formula (c).

[0107]

Equation

[0108] Here, since C1 << C0 and C1 << C L Δf is approximately given by the following formula (d).

[0109]

Equation

[0110] Therefore, the oscillation frequency f of the oscillation circuit 54 osc becomes a value corresponding to the natural frequency f of the vibration element 30 Q .

[0111] Here, when the vibration element 30 is fixed to, for example, the container 70 and receives thermal expansion stress through the fixing portion, the natural frequency f Q varies. Also, when the vibration element 30 is tilted, it is affected by gravity due to its own weight, etc., and the natural frequency f Q varies.

[0112] In the oscillation circuit 54, even if the natural frequency f Q varies for such reasons, based on the above formula (d), the oscillation frequency f osc will change in conjunction with the variation. That is, the oscillation frequency f osc is always Δf different from the natural frequency f QIt becomes a value deviated therefrom. Thereby, the vibration of the vibration element 30 is stabilized, and the displacement amplitude is stabilized. Since the modulation characteristics of the optical modulator 12 are stabilized by stabilizing the displacement amplitude, the demodulation accuracy of the sample signal in the demodulation circuit 52 can be improved.

[0113] As an example, Δf = |f osc - f Q | ≦ 3000 [Hz] is preferable, and 600 [Hz] is more preferable.

[0114] As described above, in the laser interferometer 1 according to the present embodiment, the optical modulator 12 includes the vibration element 30, and the optical modulator 12 modulates the first split light L1a using the vibration element 30.

[0115] According to such a configuration, miniaturization and weight reduction of the optical modulator 12 can be achieved. Thereby, miniaturization and weight reduction of the laser interferometer 1 can be achieved.

[0116] The laser interferometer 1 also includes a demodulation circuit 52 and an oscillation circuit 54. The oscillation circuit 54 uses the vibration element 30 as its signal source and outputs a reference signal Ss toward the demodulation circuit 52 as shown in FIG. 1. The demodulation circuit 52 demodulates a sample signal derived from the measurement object 14 from the received light signal based on the reference signal Ss.

[0117] According to such a configuration, even if the natural frequency f Q of the vibration element 30 fluctuates, the oscillation frequency f osc of the oscillation circuit 54 can be changed to a value corresponding to the natural frequency f Q of the vibration element 30, so that the vibration of the vibration element 30 can be easily stabilized. Thereby, 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.

[0118] In addition, in the above 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. By doing so, since 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, the behavior of the fluctuation of the modulation signal accompanying the temperature change and the behavior of the fluctuation of the reference signal Ss coincide or are approximated. Therefore, 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 derived from the measurement object 14 can be improved.

[0119] Furthermore, since the power consumption of the oscillation circuit 54 is low, power saving of the laser interferometer 1 can be easily achieved.

[0120] Note that, instead of the oscillation circuit 54, a signal generator such as a function generator or a signal generator may be used.

[0121] 1.2. Demodulation Circuit The demodulation circuit 52 performs a demodulation process of demodulating a sample signal derived from the measurement object 14 from the received light signal output from the current-voltage converter 531. The sample signal includes, for example, phase information and frequency information. And from the phase information, the displacement of the measurement object 14 can be obtained, and from the frequency information, the speed of the measurement object 14 can be obtained. If different physical quantities can be obtained in this way, functions as a displacement meter or a speed meter can be provided, so that the high functionality of the laser interferometer 1 can be achieved.

[0122] The circuit configuration of the demodulation circuit 52 is set according to the modulation process method. In the laser interferometer 1 according to the present embodiment, the optical modulator 12 including the vibration element 30 is used. Since the vibration element 30 is a single-vibrating element, the vibration speed changes moment by moment within the period. Therefore, the modulation frequency also changes with time, and a conventional demodulation circuit cannot be used as it is.

[0123] A conventional demodulation circuit refers to, for example, a circuit that demodulates a sample signal from a received optical signal containing a modulation signal modulated using an acousto-optic modulator (AOM). In an acousto-optic modulator, the modulation frequency does not change. Therefore, a conventional demodulation circuit can demodulate a sample signal from a received optical signal containing a modulation signal with a constant modulation frequency, but it cannot directly demodulate a received optical signal containing a modulation signal modulated by the optical modulator 12 with a changing modulation frequency.

[0124] Therefore, the demodulation circuit 52 shown in FIG. 1 includes a preprocessing unit 53 and a demodulation processing unit 55. The received optical signal output from the current-voltage converter 531 is first passed through the preprocessing unit 53 and then led to the demodulation processing unit 55. The preprocessing unit 53 performs preprocessing on the received optical signal. Through this preprocessing, a signal that can be demodulated by a conventional demodulation circuit is obtained. Therefore, the demodulation processing unit 55 demodulates the sample signal derived from the measurement object 14 by a known demodulation method.

[0125] 1.2.1. Configuration of the preprocessing unit The preprocessing unit 53 shown in FIG. 1 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.

[0126] The received optical signal output from the current-voltage converter 531 is split into two signals, a first signal S1 and a second signal S2, at the branching unit jp1. In FIG. 1, the path of the first signal S1 is defined as the first signal path ps1, and the path of the second signal S2 is defined as the second signal path ps2.

[0127] An ADC 533 is connected between the oscillation circuit 54 and the second delay adjuster 537. The ADC 533 is an analog-to-digital converter that converts an analog signal into a digital signal with a predetermined number of sampling bits.

[0128] 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 in specific frequency bands, respectively.

[0129] The first delay adjuster 536 and the second delay adjuster 537 are circuits that adjust the delay of signals, respectively. The multiplier 538 is a circuit that generates an output signal proportional to the product of two input signals. The adder 542 is a circuit that generates an output signal proportional to the sum of two input signals.

[0130] Next, the operation of the pre - processing unit 53 will be described along the flow of the first signal S1, the second signal S2, and the reference signal Ss.

[0131] The first signal S1 is passed through the first band - pass filter 534 arranged on the first signal path ps1, and then the group delay is adjusted 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 by the second band - pass filter 535 described later. By this delay adjustment, the delay times associated with the passage 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 enters the adder 542 via the first AGC 540.

[0132] The second signal S2 is passed through the second band - pass filter 535 arranged on the second signal path ps2 and then enters the multiplier 538. In the multiplier 538, the reference signal Ss output from the second delay adjuster 537 is multiplied by the second signal S2. Specifically, the reference signal Ss represented by cos(ω m t) output from the oscillation circuit 54 is digitally converted by the ADC 533, phase - adjusted by the second delay adjuster 537, and input to the multiplier 538. ω mis the angular frequency of the modulation signal by the optical modulator 12, and t is the time. Thereafter, the second signal S2 is passed through the third band-pass filter 539, then passes through the second AGC 541, and is input to the adder 542. The adder 542 outputs an output signal proportional to the sum of the first signal S1 and the second signal S2.

[0133] 1.2.2. Basic Principle of Pretreatment Next, the basic principle of pretreatment in the pretreatment unit 53 will be described. In the following description, as an example, a system will be considered in which the frequency of the modulation signal changes in a sinusoidal shape and the displacement of the measurement object 14 also changes in a single vibration in the optical axis direction. Here, E m 、E d 、φ are

[0134]

Number

[0135] When it is set as PD , the received light signal I output from the current-voltage converter 531 is theoretically expressed by the following equation.

[0136]

Number

[0137] Note that E m 、E d 、φ m 、φ d 、φ, ω m 、ω d 、ω0, a m 、a d are as follows respectively.

[0138]

Number

[0139] Also, <> in Equation (4) represents the time average. The first and second terms of the above formula (4) represent the DC components, and the third term represents the AC component. Let this AC component be I PD·AC Then, I PD·AC becomes as follows.

[0140]

Equation

[0141] Here, ν-order Bessel functions as shown in the following formulas (8) and (9) are known.

[0142]

Equation

[0143] When the above formula (5) is expanded into a series using the Bessel functions of the above formulas (8) and (9), it can be transformed as shown in the following formula (10).

[0144]

Equation

[0145] However, J0(B), J1(B), J2(B), ··· are Bessel coefficients respectively.

[0146] When transformed as above, theoretically, it can be said that the band corresponding to a specific order can be extracted by a band-pass filter.

[0147] Therefore, in the above-described preprocessing unit 53, based on this theory, the received signal is preprocessed in the following flow.

[0148] First, the received signal output from the current-voltage converter 531 is split into two signals, a first signal S1 and a second signal S2, by the branching unit jp1. The first signal S1 is passed through the first band-pass filter 534. The first band-pass filter 534 has a central angular frequency of ω mIt is set to this. As a result, the first signal S1 after passing through the first band-pass filter 534 is represented by the following equation.

[0149]

Equation

[0150] On the other hand, the second signal S2 is passed 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 the center angular frequency of the first band-pass filter 534. Here, as an example, the center angular frequency of the second band-pass filter 535 is set to 2ω m As a result, the second signal S2 after passing through the second band-pass filter 535 is represented by the following equation.

[0151]

Equation

[0152] The reference signal Ss is multiplied by the second signal S2 after passing through the second band-pass filter 535 by the multiplier 538. The second signal S2 after passing through the multiplier 538 is represented by the following equation.

[0153]

Equation

[0154] The second signal S2 after passing through the multiplier 538 is passed 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 the center angular frequency of the first band-pass filter 534. Here, as an example, the center angular frequency of the third band-pass filter 539 is set to ω m As a result, the second signal S2 after passing through the third band-pass filter 539 is represented by the following equation.

[0155]

Equation

[0156] Thereafter, the phase of the first signal S1 represented by the above formula (11) is adjusted by the first delay adjuster 536, and the amplitude is adjusted by the first AGC 540.

[0157] Also, the amplitude of the second signal S2 represented by the above formula (14) is adjusted by the second AGC 541 so that the amplitude of the second signal S2 is aligned with the amplitude of the first signal S1.

[0158] 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).

[0159]

Equation

[0160] As shown in the above formula (15), as a result of the addition, unnecessary terms disappear and necessary terms can be extracted. This result is input to the demodulation processing unit 55.

[0161] 1.2.3. Configuration of the Demodulation Processing Unit The demodulation processing unit 55 performs demodulation processing for demodulating a sample signal derived from the measurement object 14 from the signal output from the preprocessing unit 53. The demodulation processing is not particularly limited, but examples include known quadrature detection methods. The quadrature detection method is a method of performing demodulation processing by performing an operation of externally mixing signals orthogonal to each other with respect to an input signal.

[0162] The demodulation processing unit 55 shown in FIG. 1 is a digital circuit including 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.

[0163] 1.2.4. Demodulation Processing by the Demodulation Processing Unit In the demodulation process, first, the signal output from the preprocessing unit 53 is split into two by the branching unit jp2. For one of the split signals, in the multiplier 551, it is multiplied by the reference signal Ss represented by cos(ω m t) output from the oscillation circuit 54. For the other split signal, in the multiplier 552, it is multiplied by the signal represented by -sin(ω m t) whose phase of the reference signal Ss output from the oscillation circuit 54 is shifted by -90° by the phase shifter 553. The reference signal Ss and the signal with the phase of the reference signal Ss shifted are signals with a 90° phase shift from each other.

[0164] The signal passing through the multiplier 551 is passed through the first low-pass filter 555 and then input to the divider 557 as the signal x. The signal passing through the multiplier 552 is passed through the second low-pass filter 556 and then input to the divider 557 as the signal y. In the divider 557, division of the signal y by the signal x is performed, and the output y / x is passed through the arctangent calculator 558 to obtain the output atan(y / x).

[0165] After that, by passing the output atan(y / x) through the output circuit 559, the phase φ d as information from the measurement object 14 is obtained. In the output circuit 559, phase unwrapping processing is performed to perform phase connection when there is a 2π phase jump at adjacent points. And from the phase information, the displacement of the measurement object 14 can be calculated. Thereby, a displacement meter is realized. Also, the speed can be obtained from the displacement. Thereby, a speed meter is realized.

[0166] The circuit configuration of the demodulation processing unit 55 has been described above, but the circuit configuration of the above digital circuit is an example and is not limited thereto. For example, the configuration of the preprocessing unit 53 is not limited to the above configuration. Also, the demodulation processing unit 55 is not limited to a digital circuit and may be an analog circuit. The analog circuit may include an F / V converter circuit or a ΔΣ counter circuit.

[0167] Further, the circuit configuration of the demodulation processing unit 55 described above may be such that frequency information derived from the measurement object 14 can be obtained. Based on the frequency information, the speed of the measurement object 14 can be calculated.

[0168] 1.3. Suppression of Return Light by Optical Axis Shift Next, suppression of return light by shifting the optical axis will be described. Note that "distance" in the following description all refers to the optical distance (optical path length).

[0169] The emitted light L1 emitted from the laser light source 2 diffuses due to the diffraction phenomenon of light. Specifically, the light diameter R LD of the emitted light L1 emitted from the laser light source 2 with a light diameter R is expressed by the following formula (16) at a position separated by a distance x LD away.

[0170]

Equation

[0171] Among the above formula (16), the second term is the diffraction term. The diffraction phenomenon of light is generally expressed by the following formula (16-1).

[0172]

Equation

[0173] Since the higher-order diffraction terms of the second order and later are sufficiently small, they are not considered. In this case, m = 1 can be set. Also, since the diffraction emission angle θ is generally small, θ ≪ 1. Therefore, the above formula (16-1) can be transformed into the following formula (16-2).

[0174]

Equation

[0175] Then, the above formula (16) can be transformed into the following formula (16-3).

[0176]

Number

[0177] From the above formula (16-3), the optical path radius R when the emitted light L1 emitted from the laser light source 2 enters the collimating lens 3 can be obtained.

[0178] FIG. 14 is a schematic diagram showing the optical path of the emitted light L1 (first split light L1a) emitted from the laser light source 2 shown in FIG. 2 and the optical path when the reference light L2 generated by reflecting the emitted light L1 by the optical modulator 12 travels back to the laser light source 2 as the return light L5. In FIG. 14, for the sake of illustration, the optical path refracted by the first optical splitter 4 is extended and shown. Also, the return light L5 may include light derived from the object light L3 in addition to the light derived from the reference light L2 shown in FIG. 14. Further, the return light L5 may be composed only of light derived from the object light L3.

[0179] In FIG. 14, the center of the collimating lens 3 is set as the origin O (reference position), and the distance between the origin O and the optical modulator 12 is set as L. When the collimating lens 3 is omitted, the position where the collimated emitted light L1 is generated is set as the origin O (reference position). For example, in the case of a gas laser such as a He-Ne laser, since the collimated emitted light L1 is emitted from the emission point, the emission point becomes the origin. Also, let the optical path radius when the return light L5 reaches the collimating lens 3 be R'. Then, the optical path radius R' of the return light L5 is represented by the following formula (16-4).

[0180]

Number

[0181] In the present embodiment, the optical axis A of the emitted light L1 (first split light L1a) shown in FIG. 14 L1a and the optical axis A of the reference light L2 L2By shifting [the relevant axes], the return light L5 is suppressed from entering the laser light source 2. In this specification, the optical axis A L1a and the optical axis A L2 Shifting [the relevant axes] is referred to as "optical axis shift". In FIGS. 2 and 14, as an example, the optical axis A is shifted in the Z-axis direction with respect to the optical axis of specular reflection. L2

[0182] The return light L5 causes the laser oscillation in the laser light source 2 to become unstable. Therefore, if the light intensity of the return light L5 can be reduced by the optical axis shift, the stabilization of the laser oscillation can be achieved.

[0183] Self-mixing coupling in the laser light source 2 is involved in the destabilization of the laser oscillation by the return light L5. The self-mixing coupling is quantified by an index M which is the product of the "coupling coefficient" and the "light quantity of the return light L5". Among these, the coupling coefficient is proportional to the reciprocal of the resonator length of the laser light source 2. For this reason, the self-mixing coupling is likely to become apparent when the light quantity of the return light L5 increases in a semiconductor laser element with a short resonator length. If the self-mixing coupling is suppressed in the semiconductor laser element, it is considered that the self-mixing coupling is suppressed in most types of light sources.

[0184] In order to suppress the self-mixing coupling in the laser light source 2, it is preferable that this index M satisfies M < 10 -6 . This corresponds to satisfying OD6 when the shielding ability of the light intensity in the optical axis shift is expressed by the optical density (OD value). If the optical axis shift has a shielding ability that satisfies OD6, the stabilization of the laser oscillation can be achieved.

[0185] Based on this, the light intensity P0 of the emitted light L1 and the light intensity P of the return light L5 not shielded by the optical axis shift r only need to satisfy the relationship of the following formula (17).

[0186]

Equation

[0187] Therefore, the condition under which the return light L5 does not enter the collimating lens 3 is examined. When the return light L5 reaches the collimating lens 3, it becomes a beam having an optical diameter R'. If this beam does not overlap with the effective diameter of the collimating lens 3, the light intensity P r can be made substantially zero. That is, the relationship of the above formula (17) is satisfied.

[0188] As a result of the optical axis shift, at the position of the collimating lens 3, the optical axis A of the outgoing light L1 L1 and the optical axis A of the return light L5 L5 shift from each other. In FIG. 14, the shift width between the optical axis A L1 and the optical axis A L5 at the collimating lens 3 is defined as Δy [mm]. Also, the effective diameter of the collimating lens 3 is defined as κ [mm]. Then, the condition required for the shift width Δy to satisfy the relationship of the above formula (17) is expressed by the following formula (17-1).

[0189]

Equation

[0190] Substituting the above formula (16-4) into the above formula (17-1), the condition required for the shift width Δy is expressed by the following formula (A).

[0191]

Equation

[0192] Therefore, the shift width Δy of the optical axis shift may be selected so as to satisfy the above formula (A).

[0193] Here, as shown in FIG. 14, the shift width Δy is adjusted by tilting the optical modulator 12. Specifically, in the optical system 50 shown in FIG. 14, the optical axis A of the reference light L2 generated by the optical modulator 12 L2 and the optical axis A of the first divided light L1a incident on the optical modulator 12 L1aThe optical modulator 12 is tilted so as to be displaced. The optical axis A of the first divided light L1a incident on the optical modulator 12 L1a and the optical axis A of the reference light L2 generated by the optical modulator 12 L2 Let the angle formed therewith be the displacement angle θ'. The displacement width Δy is expressed by the following formula (17-2) using the displacement angle θ'.

[0194]

Equation

[0195] Since the displacement angle θ' is generally small, θ'≪1. Therefore, the above formula (17-2) can be transformed into the following formula (17-3).

[0196]

Equation

[0197] Therefore, in order to realize the displacement width Δy that satisfies the above formula (A), the distance L and the displacement angle θ' may be adjusted based on the above formula (17-3).

[0198] And if the displacement width Δy satisfies the above formula (A), the light intensity of the return light L5 incident on the laser light source 2 can be sufficiently suppressed. Thereby, the stabilization of laser oscillation in the laser light source 2 can be achieved. As a result, since the quality of the emitted light L1 is stabilized, it is possible to suppress a decrease in the accuracy of demodulating the sample signal from the received signal in the demodulation circuit 52.

[0199] Note that when the displacement width Δy deviates from the range of the above formula (A), the probability that the return light L5 enters the collimating lens 3 increases. For this reason, the light intensity of the return light L5 incident on the laser light source 2 becomes high, and there is a possibility that the laser oscillation becomes unstable.

[0200] Also, the deviation width Δy is appropriately adjusted according to other parameters. As an example, it is preferably 0.10 ≦ Δy ≦ 10.0, more preferably 0.50 ≦ Δy ≦ 10.0, still more preferably 2.10 ≦ Δy ≦ 10.0, and particularly preferably 2.30 ≦ Δy ≦ 10.0. Also, considering the size of the sensor head unit 51, the upper limit value of the deviation width Δy is more preferably 6.00 or less, and still more preferably 3.00 or less.

[0201] Note that if the deviation width Δy is less than the lower limit value, the deviation angle θ' becomes too small, and the manufacturability of the optical system 50 may decrease. On the other hand, if the deviation width Δy exceeds the upper limit value, it is necessary to increase the size of the first optical splitter 4 and the second optical splitter 5, so the miniaturization of the sensor head unit 51 may become difficult.

[0202] Also, the distance L between the collimating lens 3 and the optical modulator 12 is preferably 5.0 mm or more and 200 mm or less, and more preferably 10.0 mm or more and 100 mm or less. Thereby, an increase in the size of the optical system 50 can be suppressed.

[0203] 1.3.1. First calculation example As an example of the case where the formula (A) is satisfied, a first calculation example is shown. Each parameter and calculation result of the first calculation example are as shown in Table 1.

[0204]

Table 1

[0205] In the first calculation example, as shown in Table 1, the deviation width Δy required to satisfy the formula (A) is 0.7 mm. This deviation width Δy is a feasible value even in a small optical system 50 where the distance x LD corresponding to the focal length of the collimating lens 3 is about 1.0 mm and the distance L between the collimating lens 3 and the optical modulator 12 is about 10.0 mm.

[0206] Therefore, in the first calculation example, the realization of the optical system 50 satisfying the formula (A) is verified. Thereby, the incidence of the return light L5 on the laser light source 2 can be suppressed to such an extent that the laser oscillation does not become unstable.

[0207] 1.3.2. Second calculation example As an example of the case where the formula (A) is satisfied, a second calculation example is shown. Each parameter and calculation result of the second calculation example are as shown in Table 2.

[0208] [Table 2]

[0209] In the second calculation example, as shown in Table 2, the deviation width Δy required to satisfy the formula (A) is 5.7 mm. This deviation width Δy is a value that can be sufficiently realized even in an optical system 50 of such a size that the distance x corresponding to the focal length of the collimating lens 3 is about 30 mm and the distance L between the collimating lens 3 and the optical modulator 12 is about 100 mm. LD Therefore, also in the second calculation example, the realization of the optical system 50 satisfying the formula (A) is verified. Thereby, the incidence of the return light L5 on the laser light source 2 can be suppressed to such an extent that the laser oscillation does not become unstable.

[0210] Therefore, also in the second calculation example, the realization of the optical system 50 satisfying the formula (A) is verified. Thereby, the incidence of the return light L5 on the laser light source 2 can be suppressed to such an extent that the laser oscillation does not become unstable.

[0211] 1.4. Suppression of return light by optical axis shift and shielding element Next, a first modification of the optical system 50 shown in FIGS. 2 and 14 will be described. In this first modification, the return light is suppressed by not only the optical axis shift but also the combined use of the optical axis shift and the shielding element 17.

[0212] FIG. 15 is a schematic configuration diagram showing the sensor head portion 51 of the laser interferometer 1 according to the first modification. FIG. 16 is a schematic diagram showing the optical paths of the emitted light L1 and the first split light L1a emitted from the laser light source 2 shown in FIG. 15, and the optical path when the reference light L2 generated by reflecting the first split light L1a by the optical modulator 12 travels back to the laser light source 2 as the return light L5.

[0213] The sensor head portion 51 according to FIG. 15 includes a shielding element 17 in addition to the configuration shown in FIG. 2.

[0214] The shielding element 17 is an iris (aperture) disposed between the collimating lens 3 and the first optical splitter 4. The shielding element 17 has an aperture 172 provided corresponding to the optical path 18. As shown in FIG. 16, the shielding element 17 suppresses the return light L5 derived from the reference light L2 from entering the laser light source 2.

[0215] The shielding element 17 only needs to have a function of shielding the return light L5, so its structure is very simple. Therefore, it contributes to the simplification of the structure of the laser interferometer 1. Note that the shielding element 17 may be a slit, a pinhole, or the like, and its structure is not particularly limited.

[0216] Since the shielding element 17 is provided between the collimating lens 3 and the first optical splitter 4, if the diameter φ of the aperture 172 of the shielding element 17 is made smaller than the light diameter R' of the return light L5, the light intensity of the return light L5 can be reduced. When the aperture 172 is not a perfect circle, the diameter φ of the aperture 172 is the diameter of the perfect circle inscribed in the aperture 172. pin In this modification as well, the light intensity P0 of the emitted light L1 and the light intensity P of the return light L5 that is not shielded by the optical axis shift and the shielding element 17 only need to satisfy the relationship of the following formula (17). pin is the diameter of the perfect circle inscribed in the aperture 172.

[0217] In this modification as well, the light intensity P0 of the emitted light L1 and the light intensity P of the return light L5 that is not shielded by the optical axis shift and the shielding element 17 r only need to satisfy the relationship of the following formula (17).

[0218]

Equation

[0219] Here, let the light intensity of the return light L5 immediately before it enters the shielding element 17 be P’r. The light intensity P’r of the return light L5 is represented by the following formula (18). Note that the following formula (18) is an example when the light intensity distribution of the emitted light L1 is a Gaussian distribution or a distribution similar thereto.

[0220]

Equation

[0221] In the above formula (18-1), P is the peak light intensity of the emitted light L1, r is the radial position from the central axis of the emitted light L1, and w is the distance from the central axis of the emitted light L1 in the radial direction at the position where the light intensity of the emitted light L1 becomes 1 / e 2 of the peak light intensity. In the case of the emitted light L1, since θ≪1, θ = λ / r can be set. Therefore, w is represented as in the above formula (18-1-1). Then, the above formula (18-1) can be transformed as in the above formula (18-1-2). Note that in the above formula (18-2), Refl M is the light reflectivity in the optical modulator 12, and Refl S is the light reflectivity in the measurement object 14. Also, in the above formula (18-3), Δψ is the installation angle error of the half-wave plate that is pseudo-constituted by two quarter-wave plates 6 and 8.

[0222] P1 represented by the above formula (18-1) represents the light intensity of the emitted light L1 that passes through the aperture 172 of the shielding element 17. P2 represented by the above formula (18-2) represents the reflectivity of the optical modulator 12 and the reflectivity of the measurement object 14. P3 represented by the above formula (18-3) represents the intensity of the light that returns to the laser light source 2 side through the first optical splitter 4 when unintended polarization occurs due to the allowable angle error of the half-wave plate.

[0223] Based on the above formula (18), the light intensity Pr of the return light L5 that is not shielded by the optical axis shift or the shielding element 17 and passes through the aperture 172 is represented by the following formula (19).

[0224]

Number

[0225] In the above formula (19), x and y are positions along two axes orthogonal to each other in the cross section of the return light L5. Also, in the above formula (19-1), L pin is the distance between the collimating lens 3 and the shielding element 17. Further, when the light intensity distribution of the emitted light L1 is a Gaussian distribution, if the standard deviation of the light intensity distribution is σ, then w = 2σ.

[0226] Therefore, the diameter φ of the aperture 172 of the shielding element 17 is selected so that the light intensity Pr calculated by the above formula (19) satisfies the above formula (17). pin By doing so, the laser interferometer 1 with stabilized laser oscillation can be realized.

[0227] Also, in the present embodiment, the diameter of the aperture 172 is φ pin and the optical axis A of the first split light L1a incident on the optical modulator 12 L1a and the optical axis A of the reference light L2 generated by the optical modulator 12 L2 The angle formed with is the deviation angle θ'. Further, the distance between the shielding element 17 and the optical modulator 12 is ΔL [mm]. At this time, the laser interferometer 1 preferably satisfies the following formula (B).

[0228]

Number

[0229]

Number

[0230] According to such a configuration, by using the optical axis shift and the shielding element 17 in combination, it is possible to increase the S / N ratio in the received light signal while suppressing the light intensity of the return light L5 incident on the laser light source 2. Since both the optical axis shift and the shielding element 17 can be realized with a simple structure, it is possible to easily reduce the size and weight of the laser interferometer 1.

[0231] Note that when the diameter φ of the aperture 172 pin is less than the lower limit value, the light intensity of the emitted light L1 passing through the aperture 172 becomes small. As a result, the S / N ratio in the received light signal may decrease, and the measurement accuracy of the displacement and speed of the measurement object 14 may decrease. On the other hand, when the diameter φ of the aperture 172 pin exceeds the upper limit value, the light intensity that can be reduced by the shielding element 17 becomes small. Therefore, there is a possibility that the light intensity of the return light L5 incident on the laser light source 2 cannot be sufficiently reduced.

[0232] Here, the derivation process of the above formula (B) will be described. In order not to make the return light L5 incident on the aperture 172, the deviation width Δy L1 between the optical axis A of the emitted light L1 and the optical axis A of the return light L5 L5 in the shielding element 17 pin [mm] is obtained based on the above formula (17-1) and is represented by the following formula (19-2).

[0233]

Equation

[0234] In the above formula (19-2), R pin [mm] is the optical diameter of the return light L5 in the shielding element 17. The optical diameter R pin is represented by the following formula (19-3).

[0235]

Equation

[0236] Deviation width Δy pin is expressed by the following formula (19-4).

[0237]

Equation

[0238] Since the deviation angle θ' is generally small, θ'≪1. Therefore, the above formula (19-4) can be transformed into the following formula (19-5).

[0239]

Equation

[0240] Therefore, in order to realize the deviation width Δy pin that satisfies the above formula (A), the distance ΔL and the deviation angle θ' may be adjusted based on the above formula (19-5).

[0241] Here, when the above formula (19-3) and the above formula (19-5) are substituted into the above formula (19-2), the following formula (B-3) is derived.

[0242]

Equation

[0243] And from the above formula (B-3), the following quadratic inequality of formula (B-4) is derived.

[0244]

Equation

[0245] Note that ΔL in the above formula (B-4) is defined by the following formula (B-5).

[0246]

Equation

[0247]

Number

[0248] Here, in the above formula (B-6), the term represented by the following formula (B-7) can be approximated as shown in the following formula (B-8).

[0249]

Number

[0250] Using the above formula (B-8), the above formula (B-6) can be transformed into the above formula (B) which is the object to be derived.

[0251]

Number

[0252] The formula (B) is derived as described above. Also, the diameter φ of this opening 172 pin [mm] preferably satisfies the relationship of the following formula (20) as a specific numerical range.

[0253]

Number

[0254] When the diameter φ of the opening 172 pin is within this range, the probability that the return light L5 is blocked by the shielding element 17 increases. Thereby, the light intensity of the return light L5 reaching the laser light source 2 can be suppressed, and the instability of the laser oscillation can be suppressed.

[0255] Also, the distance between the optical modulator 12 and the first light receiving element 10 and the second light receiving element 11 is L PDLet's assume. Here, it is assumed that the distance between the optical modulator 12 and the first light receiving element 10 and the distance between the optical modulator 12 and the second light receiving element 11 are equal. At this time, the laser interferometer 1 preferably satisfies the following formula (21).

[0256] [Number]

[0257] In the above formula (21), L is the distance between the collimating lens 3 and the optical modulator 12. Also, L pin is the distance between the collimating lens 3 and the shielding element 17.

[0258] According to such a configuration, since the distance L PD can be relatively long ensured, the distance between the laser interferometer 1 and the measurable measurement object 14, that is, the working distance can be made wider.

[0259] Note that the distance L pin between the collimating lens 3 and the shielding element 17 is preferably 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, while avoiding the enlargement of the optical system 50, the shielding element 17 can function effectively.

[0260] 1.4.1. Third calculation example As an example of calculating the range of the diameter φ pin of the aperture 172 defined by the above formula (B), a third calculation example is shown. Each parameter and calculation result of the third calculation example are as shown in Table 3.

[0261] [Table 3]

[0262] In the third calculation example, as shown in Table 3, the deviation width Δy of the return light L5 in the collimating lens 3 is 0.80 mm, and the distance x LDWhen set to 4.0 mm, the diameter φ of the opening 172 of the shielding element 17 pin By setting it to 0.096 to 0.633 mm, the return light L5 can be sufficiently shielded. Therefore, in the third calculation example, the incidence of the return light L5 on the laser light source 2 can be suppressed to such an extent that the laser oscillation does not become unstable.

[0263] 1.4.2. The Fourth Calculation Example As an example of calculating the range of the diameter φ of the opening 172 defined by the formula (B), the fourth calculation example is shown. Each parameter and calculation result of the fourth calculation example are as shown in Table 4. pin

[0264]

Table 4

[0265] In the fourth calculation example, as shown in Table 4, when the deviation width Δy of the return light L5 in the collimating lens 3 is 0.80 mm and the distance x between the collimating lens 3 and the laser light source 2 LD is set to 3.0 mm, the diameter φ of the opening 172 of the shielding element 17 pin By setting it to 0.032 to 0.581 mm, the return light L5 can be sufficiently shielded. Therefore, in the fourth calculation example, the incidence of the return light L5 on the laser light source 2 can be suppressed to such an extent that the laser oscillation does not become unstable.

[0266] 1.4.3. The Fifth Calculation Example Next, as an example of the case where the light intensity Pr calculated by the above formula (19) satisfies the formula (17), the fifth calculation example is shown. Each parameter and calculation result of the fifth calculation example are as shown in Table 5.

[0267]

Table 5

[0268] In the fifth calculation example, as shown in Table 5, the diameter φ of the opening 172 of the shielding element 17 pinis 2.10 mm (≈3.0σ), the deviation width Δy of the return light L5 in the collimating lens 3 is 1.80 mm, and the distance x between the collimating lens 3 and the laser light source 2 LD is set to 4.0 mm, and by suppressing the error Δψ of the installation angle of the half-wave plate to 0.2 deg, P r / P0 calculation result is less than 1×10 -6 less than. That is, it is verified that by making the deviation width Δy satisfy the relationship of the above formula (A), the light intensity of the return light L5 incident on the laser light source 2 can be suppressed. Therefore, in the fifth calculation example, the incidence of the return light L5 on the laser light source 2 can be suppressed to such an extent that laser oscillation does not become unstable.

[0269] 1.4.4. Sixth to Ninth Calculation Examples As an example of the case where the light intensity Pr calculated by the above formula (19) satisfies the above formula (17), the sixth to ninth calculation examples are shown. The parameters and calculation results of the sixth to ninth calculation examples are as shown in Table 6.

[0270]

Table 6

[0271] In the sixth calculation example, as shown in Table 6, the diameter φ of the opening 172 of the shielding element 17 pin is 2.10 mm, the deviation width Δy of the return light L5 in the collimating lens 3 is 2.30 mm, and the distance x between the collimating lens 3 and the laser light source 2 LD is set to 4.0 mm, and by suppressing the error Δψ of the installation angle of the half-wave plate to 1.0 deg, P r / P0 calculation result is less than 1×10 -6 less than.

[0272] In the seventh calculation example, as shown in Table 6, the diameter φ of the opening 172 of the shielding element 17 pin is 1.60 mm, the deviation width Δy of the return light L5 in the collimating lens 3 is 1.40 mm, and the distance x between the collimating lens 3 and the laser light source 2 LD is set to 3.0 mm, and by suppressing the error Δψ of the installation angle of the half-wave plate to 0.2 deg, Pr The calculation result of / P0 is less than 1×10 -6 .

[0273] In the eighth calculation example, as shown in Table 6, the diameter φ of the aperture 172 of the shielding element 17 pin is set to 1.90 mm, the deviation width Δy of the return light L5 in the collimating lens 3 is set to 2.10 mm, and the distance x between the collimating lens 3 and the laser light source 2 LD is set to 3.0 mm. By suppressing the error Δψ of the installation angle of the half-wave plate to 1.0 deg, P r The calculation result of / P0 is less than 1×10 -6 .

[0274] In the ninth calculation example, as shown in Table 6, the diameter φ of the aperture 172 of the shielding element 17 pin is set to 6.00 mm, the deviation width Δy of the return light L5 in the collimating lens 3 is set to 2.00 mm, and the distance x between the collimating lens 3 and the laser light source 2 LD is set to 40 mm. By suppressing the error Δψ of the installation angle of the half-wave plate to 0.2 deg, P r The calculation result of / P0 is less than 1×10 -6 .

[0275] Therefore, in the fifth to ninth calculation examples, it is verified that by satisfying the relationship of the deviation width Δy in the above formula (A), the incidence of the return light L5 on the laser light source 2 is suppressed to such an extent that the laser oscillation does not become unstable.

[0276] 1.5. Influence of optical axis shift on differential amplification processing Next, the influence of the optical axis shift on the differential amplification processing will be described.

[0277] FIGS. 17, 18, 19, and 20 are schematic diagrams for explaining the influence of the optical axis shift on the differential amplification processing. FIGS. 17 and 18 are diagrams showing the configuration of the comparative example, and FIGS. 19 and 20 are diagrams showing the configuration of the present embodiment. In FIGS. 17 to 20, for the sake of illustration, the optical paths refracted by the first optical splitter 4 and the second optical splitter 5 are extended and shown.

[0278] In FIGS. 17 to 20, the optical path 18 between the laser light source 2 and the first optical splitter 4, the optical path 20 between the first optical splitter 4 and the optical modulator 12, the optical path 22 between the first optical splitter 4 and the object to be measured 14, the optical path 24 between the first optical splitter 4 and the second optical splitter 5, the optical path 26 between the second optical splitter 5 and the first light receiving element 10, and the optical path 28 between the second optical splitter 5 and the second light receiving element 11 are schematically illustrated.

[0279] Also, in each figure, the distance between the first optical splitter 4 and the optical modulator 12 is defined as L BS and the distance between the first optical splitter 4 and the object to be measured 14 is defined as WDs. Further, in each figure, the distance between the second optical splitter 5 and the first light receiving element 10 is defined as ΔL PD1 and the distance between the second optical splitter 5 and the second light receiving element 11 is defined as ΔL PD2 as well.

[0280] First, FIGS. 17 and 18, which are comparative examples, will be described. In FIGS. 17 and 18, the distance ΔL PD1 and the distance ΔL PD2 are different. In this case, depending on the relationship between the distance L BS and the distance WDs, the interference light (reference light L2 and object light L3) may not be incident on both the first light receiving element 10 and the second light receiving element 11.

[0281] Specifically, FIG. 17 illustrates a special case where the distance L BS is equal to the distance WDs. The angle formed by the optical axis A L1a of the first split light L1a traveling from the first optical splitter 4 toward the optical modulator 12 and the optical axis A L2 of the reference light L2 traveling from the optical modulator 12 toward the first optical splitter 4 is defined as the "deviation angle θ ref ". On the other hand, the angle formed by the optical axis A L1b of the second split light L1b traveling from the first optical splitter 4 toward the object to be measured 14 and the optical axis A L3 of the object light L3 traveling from the object to be measured 14 toward the first optical splitter 4 is defined as the "deviation angle θ sam ".

[0282] The distance L BSIn the special case where it is equal to the distance WDs, the deviation angle θ ref and the deviation angle θ sam become equal to each other as shown in FIG. 17. In this case, both the reference light L2 and the object light L3 can be made incident on both the first light receiving element 10 and the second light receiving element 11.

[0283] However, in the above-described special case, since the distance WDs is fixed, it is not realistic. When measuring the displacement and velocity of the measurement object 14, it is realistic to adjust the distance WDs according to the measurement scene. Therefore, next, the case where the distance L BS is different from the distance WDs will be considered.

[0284] FIG. 18 illustrates the case where the distance L BS is different from the distance WDs. When the distance L BS is different from the distance WDs, the deviation angle θ ref and the deviation angle θ sam are also different from each other. Then, both the reference light L2 and the object light L3 cannot be made incident on both the first light receiving element 10 and the second light receiving element 11. This reason can be explained as follows. As shown in FIG. 18, when the distance ΔL PD1 and the distance ΔL PD2 are different, even if the three points of the emission point of the reference light L2, the first light receiving element 10, and the second light receiving element 11 are adjusted to be located on the same straight line, the three points of the emission point of the object light L3, the first light receiving element 10, and the second light receiving element 11 cannot be arranged on the same straight line. Therefore, light cannot be passed through so as to pass through these three points.

[0285] Next, FIGS. 19 and 20 which are the present embodiment will be described. In FIGS. 19 and 20, the distance ΔL PD1 and the distance ΔL PD2 are equal. In this case, regardless of the relationship between the distance L BS and the distance WDs, interference light (reference light L2 and object light L3) can be made incident on both the first light receiving element 10 and the second light receiving element 11.

[0286] Specifically, FIG. 19 illustrates a special case where the distance L BS and the distance WDs are equal. In this case, the deviation angles θ ref and the deviation angle θ sam are equal to each other as shown in FIG. 19. Therefore, both the reference light L2 and the object light L3 can be made to enter both the first light receiving element 10 and the second light receiving element 11.

[0287] FIG. 20 illustrates a case where the distance L BS and the distance WDs are different. When the distance L BS and the distance WDs are different, the deviation angles θ ref and the deviation angle θ sam are also different from each other. However, since the distance ΔL PD1 and the distance ΔL PD2 are equal, both the reference light L2 and the object light L3 can be made to enter both the first light receiving element 10 and the second light receiving element 11. This reason can be explained as follows. As shown in FIG. 20, if the distance ΔL PD1 and the distance ΔL PD2 are equal, when the emission point of the reference light L2, the first light receiving element 10, and the second light receiving element 11 are adjusted to be located on the same straight line, the emission point of the object light L3, the first light receiving element 10, and the second light receiving element 11 can be arranged on another same straight line. Therefore, light can be made to pass through these three points.

[0288] From the above, in this embodiment, the distance ΔL PD1 and the distance ΔL PD2 are set to be equal. Thereby, even when the distance L BS and the distance WDs are different, both the reference light L2 and the object light L3 can be made to enter both the first light receiving element 10 and the second light receiving element 11. As a result, an optical system 50 that can perform differential amplification processing while adopting optical axis deviation can be realized.

[0289] Note that theoretically, the distance ΔL PD1 and the distance ΔL PD2is required to be equal, but depending on the distance WDs, the size of the optical system 50, etc., to some extent, the distance ΔL PD1 and the distance ΔL PD2 can allow a deviation therebetween. Specifically, for the distance ΔL PD1 and the distance ΔL PD2 when the difference therebetween is ΔL dif the difference ΔL dif satisfies the following formula (C).

[0290]

Equation

[0291] If the difference ΔL dif is within this range, it is possible to realize an optical system 50 that can perform differential amplification processing while adopting an optical axis shift.

[0292] Hereinafter, the process by which the above formula (C) is derived will be described. FIG. 21 is a schematic diagram for explaining the influence of the optical axis shift on the differential amplification processing. With respect to the schematic diagram shown in FIG. 20, a distance difference ΔL dif is provided between the first light receiving element 10 and the second light receiving element 11. FIG. 22 is a partially enlarged view of FIG. 21.

[0293] In FIG. 21, the first light receiving element 10 and the second light receiving element 11 are arranged so as to align with the optical axis A L2 of the reference light L2. Then, because there is a distance difference ΔL dif between the first light receiving element 10 and the second light receiving element 11, for example, when the optical axis A L3 of the object light L3 is aligned with the first light receiving element 10, theoretically, the optical axis A L3 cannot be aligned with the second light receiving element 11.

[0294] The angle formed by the optical axis A L2 and the optical axis A L3 is represented by |θ ref -θ sam | as shown in FIG. 22. Then, at the position of the second light receiving element 11, the optical axis AL2 and the optical axis A L3 The deviation width d from is expressed by the following formula (22).

[0295] [Number]

[0296] Here, the optical path diameter of the reference light L2 at the position of the second light receiving element 11 is R qom and the optical path diameter of the object light L3 is R sam is used.

[0297] In FIG. 21, the optical path diameter of the emitted light L1 is restricted by the shielding element 17. Therefore, R of the reference light L2 qom is calculated based on the above formula (16-3) and is expressed as the following formula (23).

[0298] [Number]

[0299] On the other hand, the optical path diameter R of the object light L3 sam is also calculated based on the above formula (16-3) and is expressed as the following formula (24).

[0300] [Number]

[0301] Here, as an example, L = L PD is used. Then, the optical axis A of the reference light L2 at the position of the first light receiving element 10 shown in FIG. 21 L2 The deviation width y ref is equal to the deviation width Δy of the optical axis A at the position of the collimating lens 3 shown in FIG. 16 L5 . Therefore, tanθ ref = Δy / L PD ≈ θ ref Thus, θ in the above formula (23) ref is obtained. Also, tanθ sam = Δy / (L PD + ΔWDref )≒θ sam Accordingly, θ in the above formula (24) sam is obtained.

[0302] The parameters used in the above formula (23) and the above formula (24) can take values such as the following, for example.

[0303] · Wavelength λ = 400 to 1600 nm · Distance L between the collimating lens 3 and the optical modulator 12 = 90 mm · Distance L between the collimating lens 3 and the shielding element 17 pin = 8 mm · Deviation width Δy of the optical axis A at the position of the collimating lens 3 L5 is 1.6 mm · Diameter φ of the aperture 172 of the shielding element 17 pin = 1.50 mm · Distance L between the optical modulator 12 and the first light receiving element 10 PD = 90 mm · Distance L BS and the difference ΔWD between the distance WDs ref = 30 to 60 mm

[0304] According to the above formula (23) and the above formula (24), the optical diameter R of the reference light L2 at the position of the second light receiving element 11 qom , and the optical diameter R of the object light L3 at the position of the second light receiving element 11 sam are obtained.

[0305] FIG. 23 is a diagram schematically showing the optical diameters of the reference light L2 and the object light L3 that have reached the position of the second light receiving element 11 shown in FIG. 22.

[0306] The optical axis A at the position of the second light receiving element 11 L2 and the optical axis A L3If the deviation width d from [something] is equal to or less than a predetermined value, as shown in Fig. 23, the object light L3 and the reference light L2 will overlap. The interference light received by the first light-receiving element 10 and the second light-receiving element 11 corresponds to the overlapping portion OL of the object light L3 and the reference light L2 shown in Fig. 23. Therefore, in order to output a light-receiving signal with a high S / N ratio from the first light-receiving element 10 and the second light-receiving element 11, it is necessary to consider the overlapping manner of the object light L3 and the reference light L2.

[0307] The light quantity distributions of the reference light L2 and the object light L3 can be regarded as Gaussian. When the deviation width d increases, light quantity attenuation occurs even in the overlapping portion OL. Considering the allowable S / N ratio, it is necessary to suppress the light quantity attenuation in the overlapping portion OL to, for example, within 1%. Then, the allowable deviation width d calculated using the above parameters becomes, for example, 0.12 mm or less. Therefore, the distance ΔL PD1 and the distance ΔL PD2 and the difference ΔL dif of are calculated for the allowable value. First, the above formula (22) is transformed as shown in the following formula (25).

[0308]

Number

[0309] Here, as described above, L = L PD and, when y ref = Δy, the above formula (25) can be transformed as shown in the following formula (26).

[0310]

Number

[0311] By inputting the above-described parameters into the above formula (26), the above formula (C) is derived.

[0312] 1.5.1. The 10th calculation example As an example of the case where the formula (C) is satisfied, the 10th calculation example is shown. Each parameter and calculation result of the 10th calculation example are as shown in Table 7. In this calculation example, due to the above-described optical axis shift, the incidence of the return light L5 on the laser light source 2 is suppressed to such an extent that the laser oscillation does not become unstable, and the attenuation of the light amount in the overlapping portion OL is suppressed. Therefore, in order to obtain the effect, the difference ΔL PD1 and the distance ΔL PD2 and the difference ΔL dif The example of the allowable value is calculated using the following parameters.

[0313]

Table 7

[0314] In the 10th calculation example, as shown in Table 7, the diameter φ pin of the aperture 172 of the shielding element 17 is set to 1.50 mm. In this case, if the difference ΔWD ref is 1000 mm or less, the optical system volume V 50 is 10 cc or more, and the wavelength λ of the emitted light L1 is in the realistic range of 400 to 1600 mm, the allowable value of the difference ΔL dif can be set to 20 mm or less. That is, in this calculation example, if the allowable value of the difference ΔL dif is set to 20 mm or less, it is proved that while suppressing the return light L5 due to the optical axis shift, the above-described deviation width d can be suppressed to a predetermined value or less, and the differential amplification process can be performed.

[0315] In addition, when the diameter φ pin is set to 1.50 mm, even if the shielding element 17 is omitted, the return light L5 incident on the laser light source 2 is shielded to the same extent as when the shielding element 17 is installed. Therefore, the 10th calculation example can also be regarded as a calculation example when the shielding element 17 is omitted.

[0316] In addition, the optical system diameter φ 50 is the optical system volume V 50, and it affects the overall size of the laser interferometer 1. Considering the compatibility between the handleability and ease of manufacture of the laser interferometer 1, the optical system diameter φ 50 is preferably 15 mm or more and 200 mm or less, and more preferably 20 mm or more and 150 mm or less. Note that the optical system diameter φ 50 refers to the diameter of the perfect circle in which the optical system 50 is inscribed when the optical system 50 is viewed from the Z-axis direction.

[0317] 1.5.2. 11th to 16th calculation examples As examples of the case where the formula (C) is satisfied, the 11th to 16th calculation examples are shown. The parameters and calculation results of the 11th to 16th calculation examples are as shown in Table 8. In these calculation examples, in order to obtain the effect that the incidence of the return light L5 to the laser light source 2 is suppressed to such an extent that the laser oscillation does not become unstable and the attenuation of the light amount in the overlapping portion OL is suppressed due to the above-described optical axis shift, the distance ΔL PD1 and the distance ΔL PD2 The difference ΔL dif The allowable value examples are calculated using the following parameters.

[0318]

Table 8

[0319] As shown in Table 8, also in these calculation examples, it is confirmed that if the difference ΔL dif is set to 20 mm or less, it is possible to perform differential amplification processing while suppressing the return light L5 due to the optical axis shift.

[0320] As described above, the laser interferometer 1 according to the present embodiment includes a laser light source 2, a first optical splitter 4, an optical modulator 12, a second optical splitter 5, a first light receiving element 10, and a second light receiving element 11. The laser light source 2 emits emitted light L1 (laser light). The first optical splitter 4 splits the emitted light L1 into a first split light L1a and a second split light L1b. The optical modulator 12 modulates the first split light L1a into reference light L2 having different frequencies. The second optical splitter 5 splits the interference light between the object light L3 generated by reflecting the second split light L1b from the measurement object 14 and the reference light L2 into a third split light L6a and a fourth split light L6b. The first light receiving element 10 receives the third split light L6a, and the second light receiving element 11 is disposed at a position different from the first light receiving element 10 and receives the fourth split light L6b.

[0321] And in the laser interferometer 1 according to the present embodiment, the optical axis A of the first split light L1a from the first optical splitter 4 toward the optical modulator 12 L1a and the optical axis A of the reference light L2 from the optical modulator 12 toward the first optical splitter 4 L2 are deviated. Also, the distance ΔL PD1 (optical path length) from the second optical splitter 5 to the first light receiving element 10 and the distance ΔL PD2 (optical path length) from the second optical splitter 5 to the second light receiving element 11, and the difference ΔL dif is 20 mm or less.

[0322] According to such a configuration, differential amplification processing using the first light receiving element 10 and the second light receiving element 11 becomes possible. Also, by shifting the optical axis, the incidence of the return light L5 to the laser light source 2 can be suppressed. And by suppressing the difference ΔL dif within a predetermined range, the adverse effect of the optical axis shift on the differential amplification processing can be suppressed. As a result, the S / N ratio of the received signal can be effectively increased, and the accuracy of demodulating the sample signal derived from the measurement object 14 from the received signal can be increased.

[0323] Note that when the difference ΔL dif exceeds the upper limit value, the deviation width between the reference light L2 and the object light L3 increases, and differential amplification processing becomes difficult.

[0324] Also, the distance ΔL PD1 (optical path length) from the second optical splitter 5 to the first light receiving element 10, and the distance ΔL PD2 (optical path length) from the second optical splitter 5 to the second light receiving element 11, and the difference ΔL dif is preferably 10 mm or less. That is, the difference ΔL dif preferably satisfies the following formula (C-1).

[0325]

Equation

[0326] Thereby, compared with the case where the difference ΔL dif satisfies the above formula (C), the difference ΔWD BS between the distance L ref and the distance WDs can be extended. This difference ΔWD ref is a parameter that affects the working distance of the laser interferometer 1. Therefore, when the difference ΔL dif satisfies the above formula (C), the handling performance of the laser interferometer 1 is improved.

[0327] Also, as described above, the laser interferometer 1 may include a shielding element 17. The shielding element 17 is disposed between the laser light source 2 and the first optical splitter 4 and has an opening 172 through which the emitted light L1 (laser light) passes.

[0328] According to such a configuration, the shielding element 17 shields the return light L5 without hindering the passage of the emitted light L1. Therefore, by using the shielding element 17 while adopting the optical axis shift, the incidence of the return light L5 on the laser light source 2 can be particularly suppressed.

[0329] Note that the diameter φ pin [mm] of the opening 172 of the shielding element 17 preferably satisfies the relationship of the above formula (20), but more preferably satisfies 0.10 ≦ φ pin ≦ 6.00, and even more preferably satisfies 0.30 ≦ φ pin ≦ 3.00.

[0330] The diameter φ of the aperture 172 of the shielding element 17 pin [mm] satisfies 0.50 ≦ φ pin ≦ 1.50, which is particularly preferable.

[0331] As a result, the shielding element 17 has sufficient shielding ability for the return light L5 without inhibiting the passage of the emitted light L1. As a result, the S / N ratio of the light reception signals output from the first light receiving element 10 and the second light receiving element 11 can be particularly increased.

[0332] Incidentally, when the diameter φ pin is less than the lower limit value, depending on the optical path diameter of the emitted light L1, the shielding element 17 may inhibit the passage of the emitted light L1, and the intensity of the interference light incident on the first light receiving element 10 and the second light receiving element 11 may decrease. On the other hand, when the diameter φ pin exceeds the upper limit value, the return light L5 is likely to pass through the aperture 172, so that the amount of return light L5 incident on the laser light source 2 may increase.

[0333] In addition, the difference ΔL PD1 (optical path length) from the second optical splitter 5 to the first light receiving element 10 and the distance ΔL PD2 (optical path length) from the second optical splitter 5 to the second light receiving element 11 is preferably 8 mm or less. That is, the difference ΔL dif preferably satisfies the following formula (C-2). dif

[0334]

Equation

[0335] As a result, even when using the shielding element 17 with a relatively small diameter φ of 0.50 mm for the aperture 172, differential amplification processing can be performed. As a result, while particularly suppressing the return light L5 due to the optical axis shift, a high S / N ratio of the light reception signal by the differential amplification processing can be realized. pin

[0336] ​​Furthermore, the distance ΔL PD1 (optical path length) from the second optical splitter 5 to the first light receiving element 10, and the distance ΔL PD2 (optical path length) from the second optical splitter 5 to the second light receiving element 11, and the difference ΔL dif are preferably 1 mm or less. That is, the difference ΔL dif preferably satisfies the following formula (C-3).

[0337]

Equation

[0338] As a result, regardless of the presence or absence of the shielding element 17, the optical system volume V 50 can be reduced to about 10 cc. As a result, the laser interferometer 1 with particularly reduced size can be realized.

[0339] 1.6. Arrangement of optical modulator and light receiving elements Next, the arrangement of the optical modulator 12, the first light receiving element 10, and the second light receiving element 11 will be described.

[0340] In the laser interferometer 1, the working distance may vary depending on the measurement scene. Then, regardless of the value of the distance WDs, it is necessary to suppress the incidence of the return light L5 to the laser light source 2 by the shielding element 17. Since the arrangements of the first light receiving element 10 and the second light receiving element 11 are set according to the arrangement of the optical modulator 12, in the following description, the relationship between the arrangement of the optical modulator 12 and the arrangements of the first light receiving element 10 and the second light receiving element 11 will be described.

[0341] Fig. 24 is a diagram showing the change in the optical axis A BS of the object light L3 when the distance WDs differs in three patterns with respect to the distance L L3 . The distance L BS is the distance between the first optical splitter 4 and the optical modulator 12, and the distance WDs is the distance between the first optical splitter 4 and the measurement object 14. In Fig. 24, the measurement object 14 is at positions P 14-1 , P 14-2 , P14-3 illustrates the case where the object to be measured 14 is at position P 14-1 When it is at, L BS =WDs holds. When the object to be measured 14 is at P 14-2 When it is at, L BS >WDs holds. When the object to be measured 14 is at P 14-3 When it is at, L BS <WDs holds.

[0342] Also, in FIG. 24, when the distance L PD with respect to the distance L - L pin differs in three patterns, the positional relationship between the shielding element 17, the first light receiving element 10, and the second light receiving element 11 is illustrated. The distance L PD is the distance between the optical modulator 12, the first light receiving element 10, and the second light receiving element 11. In FIG. 24, the shielding element 17 is at positions P 17-1 , P 17-2 , P 17-3 are illustrated.

[0343] Also, the distance L is the distance between the collimating lens 3 and the optical modulator 12. The distance L pin is the distance between the collimating lens 3 and the shielding element 17.

[0344] In the following description, the magnitude relationship between the distance L PD and the distance L - L pin will be described by dividing it into three patterns. In any case, the inclination of the optical modulator 12 is adjusted so that the reference light L2 is incident on the first light receiving element 10 and the second light receiving element 11.

[0345] 1.6.1. L - L pin =L PD When The shielding element 17 is provided for the purpose of shielding the return light L5 from the reference light L2. Therefore, the distance L - L pin , which is the distance between the optical modulator 12 and the shielding element 17, is set to a distance that can shield the return light L5 from the reference light L2 and the return light L5 from the object light L3. Also, the diameter φ of the aperture 172 of the shielding element 17 pinIt is also set to a diameter capable of shielding the return light L5.

[0346] L BS When L = WDs, both the return light L5 from the reference light L2 and the return light L5 from the object light L3 are shielded by the shielding element 17 at the position P. 17-1 will be shielded by the shielding element 17 at the position P.

[0347] The inclination of the measurement object 14 is also adjusted so that the object light L3 is incident on the first light receiving element 10 and the second light receiving element 11. For this reason, the return light L5 from the object light L3 reaches outside the opening 172 of the shielding element 17 and will be shielded.

[0348] Also, L - L pin = L PD Since this is the case, the arrival position of the return light L5 from the object light L3 at the shielding element 17 is constant regardless of the relationship between the distance WDs and the distance L. That is, L BS > WDs and L BS < WDs, in either case, the return light L5 from the object light L3 is shielded by the shielding element 17. Therefore, when L - L BS pin = L PD is satisfied, theoretically, the return light L5 does not pass through the shielding element 17.

[0349] 1.6.2. L - L pin < L PD When L - L pin < L PD When, at the position P 17-2 the shielding element 17 at the position P approaches the first optical splitter 4 compared to when L - L pin = L PD BS is satisfied. Then, the arrival position of the return light L5 from the object light L3 at the shielding element 17 changes according to the relationship between the distance WDs and the distance L.

[0350] L BS = WDs, the return light L5 from the object light L3 is at the position P 17-2It is shielded by the shielding element 17 located therein.

[0351] L BS <When WDs, the arrival position of the return light L5 from the object light L3 is L BS = When WDs, it moves farther away from the opening 172 of the shielding element 17. For this reason, the return light L5 from the object light L3 is shielded by the shielding element 17.

[0352] L BS > When WDs, the arrival position of the return light L5 from the object light L3 is L BS = When WDs, it approaches the opening 172 of the shielding element 17. For this reason, there is a possibility that the return light L5 from the object light L3 passes through the opening 172. However, even if the return light L5 from the object light L3 passes through the opening 172, the return light L5 after passing through the opening 172 travels in a direction away from the laser light source 2. For this reason, the probability that the return light L5 is incident on the laser light source 2 becomes very low.

[0353] 1.6.3. L-L pin >L PD When L-L pin >L PD When, the position P 17-3 The shielding element 17 located therein is farther away from the first optical splitter 4 than when pin =L PD When. Then, the arrival position of the return light L5 from the object light L3 at the shielding element 17 changes according to the relationship between the distance WDs and the distance L BS and.

[0354] L BS = When WDs, the return light L5 from the object light L3 is shielded by the shielding element 17 located at the position P 17-3 Therein.

[0355] L BS > When WDs, the arrival position of the return light L5 from the object light L3 is L BS=WDs, the light beam L5 is farther away from the opening 172 of the shielding element 17. Therefore, the return light L5 originating from the object light L3 is shielded by the shielding element 17.

[0356] L BS <WDsのとき、物体光L3由来の戻り光L5の到達位置は、L BS WDs, the light L5 is closer to the aperture 172 of the shielding element 17. Therefore, there is a possibility that the return light L5 originating from the object light L3 passes through the aperture 172. BS <WDsのとき、ずれ角度θ sam Since the return light L5 originating from the object light L3 is relatively small, the probability that the return light L5 originating from the object light L3 will be incident on the laser light source 2 after passing through the opening 172 may be increased. Therefore, the positional relationship between the shielding element 17 and the first and second light receiving elements 10 and 11 is set to LL pin >L PD In other words, it is preferable that the optical system 50 of the laser interferometer 1 does not satisfy LL pin ≦L PD It is preferable that the following is satisfied.

[0357] As described above, the distance LL from the optical modulator 12 to the shielding element 17 pin (Optical path length) is the distance L from the optical modulator 12 to the first light receiving element 10 PD It is preferable that the optical path length is less than or equal to the optical path length.

[0358] According to this configuration, regardless of the distance WDs corresponding to the working distance, it is possible to sufficiently reduce the probability that the return light L5 is incident on the laser light source 2. This makes it possible to prevent the laser oscillation from becoming unstable and to prevent the S / N ratio of the received light signal from decreasing.

[0359] 3. Second to fourth modified examples of laser interferometer Next, laser interferometers according to second to fourth modified examples will be described.

[0360] FIG. 25 is a schematic configuration diagram showing an optical system included in the laser interferometer 1 according to the second modification. FIG. 26 is a schematic configuration diagram showing an optical system included in the laser interferometer 1 according to the third modification. FIG. 27 is a schematic configuration diagram showing an optical system included in the laser interferometer 1 according to the fourth modification.

[0361] Hereinafter, the second to fourth modifications will be described. In the following description, the differences from the above-described embodiment will be mainly described, and the description of the same matters will be omitted. In FIGS. 25 to 27, the same components as those in the above-described embodiment are denoted by the same reference numerals.

[0362] 3.1. Second Modification In the above-described embodiment, the optical axes A L2 and A L3 are shifted in the Z-axis direction. In contrast, in the second modification, as shown in FIG. 25, the optical axes A L2 and A L3 are shifted in the X-Y plane. Even if the directions in which the optical axes A L2 and A L3 are shifted are different, the effects of the optical axis shift are the same. Therefore, in the second modification as well, the same effects as those in the above-described embodiment can be obtained.

[0363] 3.2. Third Modification In the above-described embodiment, the second optical splitter 5 has a polarization beam splitter. In contrast, in the third modification, as shown in FIG. 26, the second optical splitter 5 has a triangular prism. The triangular prism can split the interference light into two regardless of the polarization state of the interference light. Therefore, in the third modification as well, the same effects as those in the above-described embodiment can be obtained.

[0364] 3.3. Fourth Modification The fourth modification further includes a reflection element 15. The extending direction of the optical path 28 is changed by the reflection element 15. And the physical length between the second optical splitter 5 and the second light receiving element 11 is longer than the physical length between the second optical splitter 5 and the first light receiving element 10.

[0365] As shown in Fig. 27, the laser interferometer 1 according to the fourth modification further includes an optical path length changing unit 56. The optical path length changing unit 56 is arranged on the optical path 26, that is, between the second optical splitter 5 and the first light receiving element 10, and changes the optical path length between the second optical splitter 5 and the first light receiving element 10.

[0366] According to such a configuration, by increasing the optical path length of the optical path 26 passing through the optical path length changing unit 56, the optical path lengths between the second optical splitter 5 and the first light receiving element 10 and between the second optical splitter 5 and the second light receiving element 11 can be made closer to each other. As a result, even when the physical lengths cannot be made close to each other due to various obstacles, the optical path lengths can be made close to each other. Consequently, an optical system 50 that satisfies the above formula (C) can be realized even in the presence of various obstacles.

[0367] Further, the optical path length changing unit 56 is arranged at the position where the third split light L6a is incident and has a refractive index variable whose refractive index changes.

[0368] According to such a configuration, the optical path length of the optical path 26 passing through the optical path length changing unit 56 can be easily changed only by changing the refractive index.

[0369] Examples of the refractive index variable include an optical multilayer film filter and the like. By using a high refractive index material for the multilayer film provided in the optical multilayer film filter, the optical path length can be increased even with a thin film thickness. Since the material and thickness of the film can be easily changed for a multilayer film, the target optical path length can be easily realized.

[0370] Let the physical length of the optical path length changing unit 56 having the refractive index variable be ΔPL n2 and the refractive index of the refractive index variable be n2. Also, let the physical length between the second optical splitter 5 and the first light receiving element 10 be ΔPL PD1 and the refractive index of the air surrounding the optical path length changing unit 56 be n1. Then, the distance ΔL PD1 which is the optical path length between the second optical splitter 5 and the first light receiving element 10 is represented by the following formula (27).

[0371] [Number]

[0372] Even in the fourth modification example as described above, the same effects as those of the above-described embodiment can be obtained. As described above, the laser interferometer of the present invention has been described based on the illustrated embodiments. However, the laser interferometer of the present invention is not limited to the above-described embodiments, and the configuration of each part can be replaced with any configuration having the same function. Further, any other arbitrary components may be added to the laser interferometer according to the above-described embodiment. Further, the laser interferometer of the present invention may be a combination including any two or more of the above-described embodiment and the respective modification examples.

[0373] In addition to the displacement meter and speed meter described above, the laser interferometer of the present invention is also applicable to, for example, a vibration meter, an inclinometer, a distance meter (length measuring instrument), etc. Further, as the applications of the laser interferometer of the present invention, there may be mentioned distance measurement, 3D imaging, an optical communication interference measurement technique enabling spectroscopy, etc., and an optical fiber gyro realizing an angular velocity sensor, an angular acceleration sensor, etc.

[0374] Further, two or more of the laser light source, the optical modulator, the first light receiving element, and the second light receiving element may be mounted on the same substrate. Thereby, miniaturization and weight reduction of the optical system can be easily achieved, and the ease of assembly can be enhanced.

[0375] Further, although the above-described embodiments and the modification examples have a so-called Michelson type interference optical system, the laser interferometer of the present invention is also applicable to those having other types of interference optical systems, for example, a Mach-Zehnder type interference optical system.

Explanation of Reference Numerals

[0376] 1…Laser interferometer, 2…Laser light source, 3…Collimating lens, 4…First optical splitter, 5…Second optical splitter, 6…Half-wave plate, 7…Quarter-wave plate, 8…Quarter-wave plate, 9…Photodetector, 10…First light receiving element, 11…Second light receiving element, 12…Optical modulator, 14…Object to be measured, 15…Reflection element, 16…Setting part, 17…Shielding element, 18…Optical path, 20…Optical path, 22…Optical path, 24…Optical path, 26…Optical path, 28…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, 45…Circuit element, 50…Optical system, 51…Sensor head part, 52…Demodulation circuit, 53…Preprocessing part, 54…Oscillation circuit, 55…Demodulation processing part, 56…Optical path length change part, 70…Container, 72…Container body, 74…Lid, 76…Bonding wire, 120…Optical modulation vibrator, 172…Opening, 301…First electrode, 302…Second electrode, 303…Diffraction grating placement part, 305…Piezoelectric substrate, 306…Comb-shaped electrode, 307…Ground electrode, 311…Surface, 312…Back surface, 530…Differential amplifier circuit, 530a…Connection wiring, 530b…Connection wiring, 531…Current-voltage converter, 531a…First input terminal, 531b…Second input terminal, 531c…Operational amplifier, 531d…Feedback resistor, 531e…Output terminal, 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…Inverse tangent calculator, 559…Output circuit, 721…First recess, 722…Second recess, A L1 …Optical axis, A L1a …Optical axis, A L1b …Optical axis, A L2 …Optical axis, A L3 …Optical axis, A L5 …Optical axis, C0…Parallel capacitance, C1…Series capacitance, C3…Third capacitor, Cd…Second capacitor, Cg…First capacitor, GND…GND terminal, I1…Photoelectric current, I2…Photoelectric current, Id…Differential current, K -2s …Diffracted light, K -1s…Diffracted light, K 0s …Diffracted light, K 1s …Diffracted light, K 2s …Diffracted light, K i …Incident light, L... Distance, L1... Series inductance, L1... Emergent light, L1a... First split light, L1b... Second split light, L2... Reference light, L3... Object light, L5... Return light, L6a... Third split light, L6b... Fourth split light, L BS …Distance, L PD …Distance, L pin …Distance, N... Normal, n1... Refractive index, n2... Refractive index, O... Origin, OL... Overlap part, P... Pitch, R... Optical path, R’... Optical path, R1... Equivalent series resistance, Rd... Limiting resistance, Rf... Feedback resistance, R pin …Optical path, R qom …Optical path, R sam …Optical path, S1... First signal, S2... Second signal, Sd... Drive signal, Ss... Reference signal, Vcc... Terminal, WDs... Distance, X1... Terminal, X2... Terminal, Y... Terminal, d... Deviation width, jp1... Branch part, jp2... Branch part, ps1... First signal path, ps2... Second signal path, x... Signal x, x LD …Distance, y... Signal y, y ref …Deviation width, ΔL PD1 …Distance, ΔL PD2 …Distance, ΔL dif …Difference in distance, ΔWD ref …Difference in distance, Δy... Deviation width, Δy pin …Deviation width, ΔPL n2 …Physical length, ΔPL PD1 …Physical length, β... Angle of incidence, θ'... Deviation angle, θ B …Blaze angle, θ S …Tilt angle, θ ref …Deviation angle, θ sam …Deviation angle, φ pin …Diameter

Claims

1. A laser light source that emits laser light, A first optical splitter that splits the laser light into a first split light and a second split light, An optical modulator that modulates the first split light into reference lights with different frequencies, A second optical splitter that splits the object light generated by reflecting the second split light from a measurement object and the reference light into a third split light and a fourth split light, A first light receiving element that receives the third split light, A second light receiving element that is disposed at a position different from the first light receiving element and receives the fourth split light, comprising The optical axis of the first split light from the first optical splitter toward the optical modulator and the optical axis of the reference light from the optical modulator toward the first optical splitter are displaced, A laser interferometer, wherein a difference between an optical path length from the second optical splitter to the first light receiving element and an optical path length from the second optical splitter to the second light receiving element is 20 mm or less.

2. The laser interferometer according to claim 1, wherein a difference between an optical path length from the second optical splitter to the first light receiving element and an optical path length from the second optical splitter to the second light receiving element is 10 mm or less.

3. The laser interferometer according to claim 2, wherein a difference between an optical path length from the second optical splitter to the first light receiving element and an optical path length from the second optical splitter to the second light receiving element is 1 mm or less.

4. The laser interferometer according to claim 1 or 2, further comprising a shielding element disposed between the laser light source and the first optical splitter and having an opening through which the laser light passes.

5. The laser interferometer according to claim 4, wherein a diameter of the opening is 0.50 mm or more and 1.50 mm or less.

6. The laser interferometer according to claim 4 or 5, wherein a difference between an optical path length from the second optical splitter to the first light receiving element and an optical path length from the second optical splitter to the second light receiving element is 8 mm or less.

7. The laser interferometer according to any one of claims 4 to 6, wherein an optical path length from the optical modulator to the shielding element is equal to or less than an optical path length from the optical modulator to the first light receiving element.

8. The laser light source is a semiconductor laser element, The laser interferometer according to any one of claims 1 to 7, further comprising a collimating lens disposed between the laser light source and the first optical splitter.

9. The laser interferometer according to any one of claims 1 to 8, further comprising an optical path length changing unit disposed between the second optical splitter and the first light receiving element and configured to change an optical path length between the second optical splitter and the first light receiving element.

10. The laser interferometer according to claim 9, wherein the optical path length changing unit is disposed at a position where the third split light is incident and has a refractive index variable body whose refractive index changes.

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