laser interferometer

Abstract

To provide a laser interferometer capable of more properly demodulating a Doppler signal derived from a measuring object irrespective of vibration conditions of a vibration element.SOLUTION: The laser interferometer comprises: a light source emitting a laser beam; an optical splitter splitting the laser beam emitted from the light source into a first optical path and a second optical path; an optical modulator including a vibration element provided on the first optical path or the second optical path and vibrating by current flowing, and modulating the laser beam using the vibration element; a light receiving element receiving a laser beam reflected by a measuring object provided on the first optical path or the second optical path to output a light receiving signal; and a demodulator circuit demodulating a Doppler signal derived from the measuring object from the light receiving signal on the basis of a reference signal and a modulation signal. Iq / f≤1×10-7 is satisfied when an amplitude value of the current flowing through the vibrating vibration element is denoted by Iq[A] and a vibration frequency of the vibration element is denoted by f[Hz].SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Patent Document 1 discloses a laser Doppler velocimeter (laser interferometer) that irradiates a vibrating object with a laser beam and measures the object's velocity by utilizing the frequency of the laser beam changed by the Doppler effect. The laser Doppler velocimeter requires a structure that modulates the light emitted from a laser light source to detect the directionality of the object's vibration phenomenon. For this reason, Patent Document 1 discloses the use of an acousto-optic modulator or an electro-optic modulator.

[0003] Furthermore, Patent Document 2 discloses a laser vibrometer (laser interferometer) that uses a vibration element such as a piezoelectric element or a quartz crystal oscillator instead of an expensive AOM (acousto-optic modulator). By irradiating these vibration elements with laser light, the frequency of the laser light is shifted. By using the frequency-shifted laser light as a reference light, a Doppler signal is demodulated from scattered laser light that has been Doppler-shifted by a vibrating object. The vibration velocity of the object can be measured from this Doppler signal. This laser vibrometer allows the use of inexpensive vibration elements, thereby reducing the cost of the laser vibrometer.

[0004] On the other hand, Patent Document 3 discloses a laser Doppler velocimeter configured to apply a sinusoidal signal to an optical modulator, receive a reference light beam obtained by frequency-shifting a light beam from a laser light source and a reflected light beam obtained by irradiating a measured object with the light beam using a photodetector, perform predetermined arithmetic processing on the received light signal, and then perform FM demodulation processing.In this laser Doppler velocimeter, by performing predetermined arithmetic processing before FM demodulation processing, it is possible to obtain a signal corresponding to the velocity of the measured object from the received light signal even when the frequency of the reference light beam is sinusoidally shifted. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 9-54293 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-285898 [Patent Document 3] Japanese Patent Application Publication No. 2-38889 Summary of the Invention [Problem to be solved by the invention]

[0006] Laser interferometers using vibration elements sometimes fail to accurately measure the velocity of an object. Specifically, depending on the vibration conditions of the vibration element, the intensity of the modulated signal in the reference light can be significantly reduced. In such cases, the Doppler signal from the object cannot be accurately demodulated. [Means for solving the problem]

[0007] A laser interferometer according to an application example of the present invention includes: a light source that emits laser light; a beam splitter that splits the laser beam emitted from the light source into a first optical path and a second optical path; a vibration element provided in the first optical path or the second optical path and vibrating when a current is applied thereto; By irradiating the laser light onto an optical modulator that modulates the laser light; a light receiving element that receives the laser light reflected by a measurement object provided in the first optical path or the second optical path and outputs a light receiving signal; an oscillation circuit that outputs a reference signal using the vibration element as a signal source; a demodulation circuit that demodulates a Doppler signal originating from the object to be measured from the received light signal based on the reference signal output from the oscillation circuit and the modulated signal originating from the optical modulator; Equipped with the oscillation circuit is a circuit that selectively amplifies a signal of a specific resonance frequency by inputting a drive signal to the vibration element and returning an output from the vibration element to its input, the oscillation frequency of the oscillation circuit changes according to the natural frequency of the vibration element; When the amplitude value of the current flowing through the vibrating vibration element is Iq [A] and the vibration frequency of the vibration element is f [Hz], Iq / f≦1×10 -7 The present invention is characterized in that: [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a functional block diagram showing a laser interferometer according to an embodiment. [Figure 2] FIG. 2 is a schematic diagram showing the configuration of the sensor head unit shown in FIG. [Figure 3] FIG. 3 is a perspective view showing a first configuration example of the optical modulator shown in FIG. [Figure 4] FIG. 10 is a plan view showing a part of a second configuration example of the optical modulator. [Figure 5] FIG. 10 is a plan view showing a third configuration example of the optical modulator. [Figure 6] 1 is a conceptual diagram illustrating the generation of multiple diffracted beams when incident light Ki is incident on the surface of a vibration element in a direction perpendicular to the surface. [Figure 7] FIG. 10 is a conceptual diagram illustrating an optical modulator configured so that the angle formed between the traveling direction of incident light Ki and the traveling direction of reference light L2 is 180°. [Figure 8] FIG. 10 is a conceptual diagram illustrating an optical modulator configured so that the angle formed between the traveling direction of incident light Ki and the traveling direction of reference light L2 is 180°. [Figure 9] FIG. 10 is a conceptual diagram illustrating an optical modulator configured so that the angle formed between the traveling direction of incident light Ki and the traveling direction of reference light L2 is 180°. [Figure 10] FIG. 1 is a cross-sectional view showing an optical modulator having a package structure. [Figure 11] FIG. 1 is a circuit diagram showing the configuration of a single-stage inverter oscillator circuit. [Figure 12] 1 is an example of an LCR equivalent circuit of a vibration element. [Figure 13] FIG. 1 is a conceptual diagram showing a change over time in the phase ψm of a modulated signal originating from an optical modulator. [Figure 14] 10 is a diagram illustrating the influence of each parameter on the waveform of the AC component IPD·AC of the received light signal IPD. [Figure 15] 10 is a graph showing the relationship between the equivalent series resistance and the drive level of a crystal resonator having an oscillation frequency f of 5 MHz. [Figure 16] 16 is a graph showing the relationship between the drive level and the current value flowing through the crystal resonator, which is obtained from the relationship shown in FIG. 15. [Figure 17] 10 is a graph showing the phase change of a sample signal demodulated by a demodulation circuit when a piezoelectric actuator vibrating at a displacement amplitude of 150 nm and a vibration frequency of 10 kHz is used as the measurement object. [Figure 18] 1 is a graph showing the relationship between the B value and the amplitude value Iq of the current flowing through a vibrating quartz crystal unit. [Figure 19] 10 is a graph showing the relationship between the B value and the coefficient of determination (R2 value) and standard deviation of the measured displacement when a sample signal derived from the object to be measured is demodulated by a demodulation circuit to measure the displacement. [Figure 20] 10 is a diagram showing an example of a waveform of a received light signal simulated under a predetermined initial condition. [Figure 21] 10 is a graph showing the relationship between the load capacitance CL of the oscillation circuit and Δf. [Figure 22] 10 is a graph showing the relationship between the load capacitance CL and the B value when the applied voltage Vq to the vibration element is 10 V and when it is 5 V. [Figure 23] 22. This is a diagram in which the optimum range of the load capacitance CL obtained from FIG. 22 is superimposed on the graph of FIG. [Figure 24] 10 is a graph showing the relationship between the capacitance of the second capacitor Cd and the cutoff frequency fc of the first-order CR low-pass filter when the resistance value of the limiting resistor Rd of the oscillator circuit is set to three levels: 50Ω, 100Ω, and 200Ω. [Figure 25] 10 is a graph showing the relationship between the resistance value of the limiting resistor Rd of the oscillation circuit and the B value when the load capacitance CL of the oscillation circuit is set to four levels: 80 pF, 100 pF, 120 pF, and 150 pF. [Figure 26] 10 is a graph showing the relationship between the resistance value of the limiting resistor Rd of the oscillation circuit and Δf when the load capacitance CL of the oscillation circuit is set to four levels: 80 pF, 100 pF, 120 pF, and 150 pF. [Figure 27] FIG. 10 is a schematic diagram showing a mounting structure of an optical system included in a laser interferometer according to a first modified example. [Figure 28] FIG. 10 is a schematic diagram showing a mounting structure of an optical system provided in a laser interferometer according to a second modified example. [Figure 29] FIG. 11 is a schematic diagram showing a mounting structure of an optical system provided in a laser interferometer according to a third modified example. [Figure 30] FIG. 11 is a schematic diagram showing a mounting structure of an optical system provided in a laser interferometer according to a fourth modified example. DETAILED DESCRIPTION OF THE INVENTION

[0009] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A laser interferometer according to the present invention will now be described in detail with reference to the embodiments shown in the accompanying drawings. FIG. 1 is a functional block diagram showing a laser interferometer according to an embodiment.

[0010] The laser interferometer 1 shown in FIG. 1 has a sensor head unit 51 equipped with an optical system 50 and an oscillation circuit 54, and a demodulation circuit 52 to which a light reception signal from the optical system 50 is input.

[0011] 1. Sensor head FIG. 2 is a schematic diagram showing the configuration of the sensor head unit 51 shown in FIG.

[0012] 1.1.Optical system As described above, the sensor head unit 51 includes the optical system 50 . As shown in FIG. 2, the optical system 50 includes a light source 2, a polarizing beam splitter 4 (optical divider), a quarter-wave plate 6, a quarter-wave plate 8, an analyzer 9, a light receiving element 10, a frequency shifter-type optical modulator 12, and a set section 16 in which an object to be measured 14 is placed.

[0013] The light source 2 emits output light L1 (first laser light) of a predetermined wavelength. The light receiving element 10 converts the received light into an electrical signal. The optical modulator 12 includes a vibration element 30 and modulates the output light L1 to generate reference light L2 (second laser light) containing a modulated signal. The setting unit 16 may be provided as needed, but is designed to allow the measurement object 14 to be placed thereon. The output light L1 incident on the measurement object 14 is reflected as object light L3 (third laser light) containing a sample signal, which is a Doppler signal derived from the measurement object 14.

[0014] The optical path of output light L1 emitted from the light source 2 is referred to as optical path 18. Optical path 18 is coupled to optical path 20 by reflection from the polarizing beam splitter 4. On optical path 20, a quarter-wave plate 8 and an optical modulator 12 are arranged in this order from the polarizing beam splitter 4 side. Furthermore, optical path 18 is coupled to optical path 22 by transmission through the polarizing beam splitter 4. On optical path 22, a quarter-wave plate 6 and an assembly unit 16 are arranged in this order from the polarizing beam splitter 4 side.

[0015] The optical path 20 is coupled to an optical path 24 by passing through the polarizing beam splitter 4. On the optical path 24, an analyzer 9 and a light receiving element 10 are arranged in this order from the polarizing beam splitter 4 side.

[0016] Emitted light L1 emitted from the light source 2 travels along optical paths 18 and 20 and enters the optical modulator 12. The emitted light L1 also travels along optical paths 18 and 22 and enters the measurement object 14. Reference light L2 generated by the optical modulator 12 travels along optical paths 20 and 24 and enters the light receiving element 10. Object light L3 generated by reflection from the measurement object 14 travels along optical paths 22 and 24 and enters the light receiving element 10.

[0017] Each part of the optical system 50 will be further described below. 1.1.1.Light source The light source 2 is a laser light source that emits coherent output light L1 with a narrow linewidth. When the linewidth is expressed as a frequency difference, a laser light source with a linewidth in the MHz range or less is preferably used. Specific examples include gas lasers such as HeNe lasers, and semiconductor laser elements such as DFB-LD (Distributed Feedback - Laser Diode), FBG-LD (Fiber Bragg Grating laser diode), and VCSEL (Vertical Cavity Surface Emitting Laser).

[0018] It is particularly preferable that the light source 2 includes a semiconductor laser element. This allows the light source 2 to be particularly compact, which in turn allows the laser interferometer 1 to be made smaller. In particular, the sensor head 51 of the laser interferometer 1, which houses the optical system 50, can be made smaller and lighter, which is also useful in terms of improving the operability of the laser interferometer 1.

[0019] 1.1.2.Polarizing Beam Splitter The polarizing beam splitter 4 is an optical element that splits incident light into transmitted light and reflected light. The polarizing beam splitter 4 also has the function of transmitting P-polarized light and reflecting S-polarized light. Below, we consider the case where output light L1, which is linearly polarized and has a P-polarized to S-polarized light ratio of, for example, 50:50, is incident on the polarizing beam splitter 4.

[0020] As described above, the polarizing beam splitter 4 reflects the S-polarized light of the output light L1 and transmits the P-polarized light.

[0021] The S-polarized light of the output light L1 reflected by the polarizing beam splitter 4 is converted into circularly polarized light by the quarter-wave plate 8 and enters the optical modulator 12. The first circularly polarized light of the output light L1 that enters the optical modulator 12 is f m [Hz] and is reflected as reference light L2. Therefore, the reference light L2 has a frequency f mThe reference light L2 is converted to P-polarized light when it passes through the quarter-wave plate 8 again. The P-polarized light of the reference light L2 passes through the polarizing beam splitter 4 and the analyzer 9 and enters the light-receiving element 10.

[0022] The P-polarized light L1 transmitted through the polarizing beam splitter 4 is converted into circularly polarized light by the quarter-wave plate 6 and is incident on the moving object 14. The second circularly polarized light of the output light L1 incident on the object 14 is f d The object light L3 is subjected to a Doppler shift of frequency f d The object light L3 is converted to S-polarized light when it passes through the quarter-wave plate 6 again. The S-polarized light of the object light L3 is reflected by the polarizing beam splitter 4, passes through the analyzer 9, and enters the light-receiving element 10.

[0023] As described above, the output light L1 has coherence, so the reference light L2 and the object light L3 enter the light receiving element 10 as interference light.

[0024] Note that a non-polarizing beam splitter may be used instead of the polarizing beam splitter. In this case, the quarter-wave plate 6 and the quarter-wave plate 8 are not required, which reduces the number of parts and allows for a more compact laser interferometer 1. Also, a light splitter other than the polarizing beam splitter 4 may be used.

[0025] 1.1.3.Analyzer Since the S-polarized light and the P-polarized light are orthogonal to each other and independent of each other, simply superimposing them does not produce beat noise due to interference. Therefore, the light waves of the superimposed S-polarized light and P-polarized light are passed through an analyzer 9 tilted at 45 degrees to both the S-polarized light and the P-polarized light. By using the analyzer 9, light with common components can be transmitted, causing interference. As a result, the reference light L2 and the object light L3 interfere with each other in the analyzer 9, and |f m -f d Interference light with a frequency of |[Hz] is generated.

[0026] 1.1.4.Photodetector The reference light L2 and the object light L3 are incident on the light receiving element 10 via the polarizing beam splitter 4 and the analyzer 9. As a result, the reference light L2 and the object light L3 undergo optical heterodyne interference, and |f m -f d Interference light having a frequency of | [Hz] is incident on the light receiving element 10. By demodulating a sample signal from this interference light using a method described below, it is possible to ultimately determine the movement of the measurement object 14, i.e., the vibration velocity and displacement. The light receiving element 10 may be, for example, a photodiode.

[0027] Optical Modulator FIG. 3 is a perspective view showing a first configuration example of the optical modulator 12 shown in FIG.

[0028] 1.1.5.1. Overview of the first example of optical modulator configuration The frequency shifter type optical modulator 12 has an optical modulation oscillator 120. The optical modulation oscillator 120 shown in FIG.

[0029] The vibration element 30 is made of a material that, when an electric potential is applied, repeats a vibration mode in which the vibration is distorted in a direction along the surface. In this configuration example, the vibration element 30 is a quartz crystal AT resonator that vibrates in a thickness-shear mode along a vibration direction 36 in a high frequency region in the MHz band. A diffraction grating 34 is formed on the surface of the vibration element 30. The diffraction grating 34 has a structure in which grooves 32 having a component intersecting the vibration direction 36, i.e., a plurality of linear grooves 32 extending in a direction intersecting the vibration direction 36, are periodically arranged.

[0030] The substrate 31 has a front surface 311 and a back surface 312 which are opposite surfaces. The vibration elements 30 are arranged on the front surface 311. The front surface 311 is also provided with pads 33 for applying a potential to the vibration elements 30. Meanwhile, the back surface 312 is also provided with pads 35 for applying a potential to the vibration elements 30.

[0031] The size of the substrate 31 is, for example, about 0.5 mm or more and 10.0 mm or less in terms of the long side. 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 each side measuring 1.6 mm, and the thickness is 0.35 mm.

[0032] The size of the vibration element 30 is, for example, about 0.2 mm or more and 3.0 mm or less in terms of the long side, and the thickness of the vibration element 30 is, for example, about 0.003 mm or more and 0.5 mm or less.

[0033] As an example, the shape of the vibrating element 30 is a square with sides of 1.0 mm and a thickness of 0.07 mm. In this case, the vibrating element 30 oscillates at a fundamental oscillation frequency of 24 MHz. Note that by changing the thickness of the vibrating element 30 and taking into account overtones, the oscillation frequency can be adjusted within a range from 1 MHz to 1 GHz.

[0034] In FIG. 3, the diffraction grating 34 is formed on the entire surface of the vibration element 30, but it may be formed on only a part of the surface.

[0035] The magnitude of the optical modulation by the optical modulator 12 is given by the dot product of the differential wave vector between the wave vector of the output light L1 entering the optical modulator 12 and the wave vector of the reference light L2 exiting from the optical modulator 12, and the vector in the vibration direction 36 of the vibration element 30. In this configuration example, the vibration element 30 vibrates in a thickness-shear manner, but because this vibration is an in-plane vibration, optical modulation cannot be achieved even if light is incident perpendicularly to the surface of the vibration element 30 alone. Therefore, in this configuration example, a diffraction grating 34 is provided in the vibration element 30, making optical modulation possible based on a principle described below.

[0036] The diffraction grating 34 shown in Fig. 3 is a blazed diffraction grating. A blazed diffraction grating is a diffraction grating whose cross section has a stepped shape. The linear grooves 32 of the diffraction grating 34 are arranged so that their extension direction is perpendicular to the vibration direction 36.

[0037] 1 and 2 to the vibration element 30 shown in Fig. 3, the vibration element 30 oscillates. The electric power (driving power) required for oscillation of the vibration element 30 is not particularly limited, but is small, about 0.1 μW to 100 mW. Therefore, the drive signal Sd output from the oscillation circuit 54 can be used to oscillate the vibration element 30 without amplifying it.

[0038] Furthermore, conventional optical modulators sometimes require a structure to maintain the temperature of the optical modulator, making it difficult to reduce their volume. Furthermore, conventional optical modulators have a problem in that they consume a lot of power, making it difficult to reduce the size and power consumption of the laser interferometer. In contrast, in this configuration example, the volume of the vibration element 30 is very small and the power required for oscillation is also small, making it easy to reduce the size and power consumption of the laser interferometer 1.

[0039] 1.1.5.2. Method of forming diffraction grating The method for forming the diffraction grating 34 is not particularly limited. One example is to create a mold using a mechanical ruling engine, and then form the grooves 32 on an electrode formed on the surface of the vibration element 30 of the quartz crystal AT resonator using a nanoimprinting method. The reason for forming the grooves on the electrode is that, in principle, a quartz crystal AT resonator can generate high-quality thickness-shear vibration on the electrode. The grooves 32 do not necessarily have to be formed on the electrode, but may also be formed on the surface of a material in the non-electrode portion. Alternatively, instead of nanoimprinting, other processing methods such as exposure and etching, electron beam lithography, and focused ion beam (FIB) processing may be used.

[0040] Alternatively, a diffraction grating may be formed on the chip of a quartz crystal AT resonator using a resist material, and a metal film or a dielectric multilayer mirror film may be provided on top of the grating. By providing a metal film or a mirror film, the reflectance of the diffraction grating 34 can be increased.

[0041] Furthermore, a resist film may be formed on a chip or wafer of a quartz crystal AT resonator, processed by etching, and then removed. A metal film or mirror film may then be formed on the processed surface. In this case, the resist material is removed, eliminating the effects of moisture absorption by the resist material and improving the chemical stability of the diffraction grating 34. Furthermore, by providing a highly conductive metal film such as Au or Al, it can also be used as an electrode for driving the resonator element 30.

[0042] The diffraction grating 34 may be formed using techniques such as anodic alumina (porous alumina).

[0043] 1.1.5.3.Other Configuration Examples of Optical Modulators The vibrating element 30 is not limited to a quartz vibrator, but may be, for example, a Si vibrator, a surface acoustic wave (SAW) device, a ceramic vibrator, or the like.

[0044] 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.

[0045] 4 is a Si vibrator manufactured from a Si substrate using MEMS technology. MEMS (Micro Electro Mechanical Systems) stands for micro-electromechanical systems.

[0046] The vibration 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, for example, by electrostatic attraction as a driving force, in the left-right direction in FIG. 4, i.e., along an axis connecting the first electrode 301 and the second electrode 302 shown in FIG. 4, so as to repeatedly approach and move away from each other. This allows in-plane vibration to be imparted to the diffraction grating 34. The oscillation frequency of the Si vibrator is, for example, from about 1 kHz to several hundred MHz.

[0047] The vibrating element 30B shown in Fig. 5 is a SAW device that uses surface waves. SAW (Surface Acoustic Wave) stands for surface acoustic wave.

[0048] The vibration element 30B includes a piezoelectric substrate 305, an interdigital 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 AC voltage is applied to the interdigital electrode 306, a surface acoustic wave is excited due to the inverse piezoelectric effect. This allows the diffraction grating 34 to vibrate in-plane. The oscillation frequency of the SAW device is, for example, from several hundred MHz to several GHz.

[0049] In the above-described device, by providing a diffraction grating 34, optical modulation becomes possible according to the principle described below, as in the case of a quartz crystal AT oscillator.

[0050] On the other hand, if the vibration element 30 is a quartz crystal oscillator, a highly accurate modulation signal can be generated by utilizing the extremely high Q value of the quartz crystal. The Q value is an index that indicates the sharpness of the resonance peak. Furthermore, a quartz crystal oscillator has the advantage of being less susceptible to external disturbances. Therefore, by using a modulation signal modulated by the optical modulator 12 equipped with a quartz crystal oscillator, a sample signal derived from the measurement object 14 can be acquired with high accuracy.

[0051] 1.1.5.4.Light modulation by vibration elements Next, the principle of modulating light using the vibration element 30 will be described.

[0052] FIG. 6 shows the incident light K i 1 is a conceptual diagram illustrating the generation of multiple diffracted beams when incident light.

[0053] Light K is incident on a diffraction grating 34 that is vibrating in thickness shear along a vibration direction 36. i When the incident light is incident on the surface, the diffraction phenomenon causes multiple diffracted light beams K ns is generated. n is the diffracted light Kns where n=0, ±1, ±2, .... The diffraction grating 34 shown in FIG. 6 is not the blazed diffraction grating shown in FIG. 3, but is a diffraction grating with repeated concaves and convexes as an example of another diffraction grating. Also, in FIG. 6, diffracted light K 0s is omitted from the illustration.

[0054] In Figure 6, the incident light K i is incident on the surface of the vibration element 30 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 light is incident obliquely on the surface of the vibration element 30. When the light is incident obliquely, the diffracted light K ns The direction of travel also changes accordingly.

[0055] Depending on the design of the diffraction grating 34, higher-order light of |n| ≥ 2 may not appear. Therefore, to obtain a stable modulated signal, it is desirable to set |n| = 1. That is, in the laser interferometer 1 of FIG. 2, the frequency shifter-type optical modulator 12 is preferably arranged so that the ±1st-order diffracted light is used as the reference light L2. This arrangement enables stable measurement by the laser interferometer 1.

[0056] On the other hand, when high-order light of |n|≧2 emerges from the diffraction grating 34, the optical modulator 12 may be arranged so that any of the ±2nd-order or higher-order diffracted light is used as the reference light L2, rather than the ±1st-order diffracted light. This allows the use of high-order diffracted light, thereby achieving higher frequency and miniaturization of the laser interferometer 1.

[0057] In this embodiment, as an example, incident light K i The optical modulator 12 is configured so that the angle formed between the direction of incidence of the reference light L2 and the direction of travel of the reference light L2 emitted from the optical modulator 12 is 180°. Three examples will be described below.

[0058] 7 to 9 show the incident light K i1 is a conceptual diagram illustrating an optical modulator 12 configured so that the angle formed between the traveling direction of the reference light L1 and the traveling direction of the reference light L2 is 180°.

[0059] The optical modulator 12 shown in FIG. 7 includes a mirror 37 in addition to the vibration element 30. The mirror 37 reflects the diffracted light K 1s The mirror 37 is arranged to reflect the diffracted light K 1s The angle between the incident angle of the diffracted light K and the reflection angle of the mirror 37 is 180°. 1s is diffracted again by the diffraction grating 34 and incident on the optical modulator 12. i Therefore, by adding the mirror 37, the incident light K i This satisfies the condition that the angle formed between the incident direction of the reference beam L1 and the traveling direction of the reference beam L2 is 180°.

[0060] Furthermore, by passing the reference light L2 through the mirror 37 in this way, the reference light L2 generated by the optical modulator 12 is subjected to frequency modulation twice. Therefore, by using the mirror 37 in combination, higher frequency modulation is possible compared to when the vibration element 30 is used alone.

[0061] In Fig. 8, the vibration element 30 is tilted with respect to the arrangement in Fig. 6. The tilt angle θ at this time is i The angle formed by the incident direction of the reference light L1 and the traveling direction of the reference light L2 is set to 180°.

[0062] The diffraction grating 34 shown in FIG. B The blazed diffraction grating has the following structure: incident light K propagating at an incident angle β with respect to the normal N of the surface of the vibration element 30. i is incident on the diffraction grating 34, the blaze angle θ B Therefore, the reference light L2 returns at the same angle as the incident angle β. B By making it equal to the incident light K i7 and 8, the condition that the angle between the direction of incidence of the reference beam L1 and the direction of travel of the reference beam L2 is 180° can be satisfied. In this case, the condition can be satisfied without using the mirror 37 shown in Fig. 7 or without tilting the vibration element 30 itself as shown in Fig. 8, which allows the laser interferometer 1 to be further miniaturized and operate at a higher frequency. In particular, in the case of a blazed diffraction grating, an arrangement that satisfies the condition is called a "Littrow arrangement," which has the advantage of being able to particularly increase the diffraction efficiency of the diffracted light.

[0063] 9 indicates the pitch of the blazed diffraction grating, and as an example, the pitch P is set to 1 μm. B In this case, in order to satisfy the above condition, the incident light K i The angle of incidence β with respect to the normal N should also be set to 25°.

[0064] Package Structure FIG. 10 is a cross-sectional view showing an optical modulator 12 having a package structure.

[0065] 10 includes a container 70 serving as a housing, an optical modulation oscillator 120 housed in the container 70, and a circuit element 45 constituting an oscillation circuit 54. The container 70 is hermetically sealed in a reduced pressure atmosphere such as a vacuum, or in an inert gas atmosphere such as nitrogen or argon.

[0066] 10, the container 70 has a container body 72 and a lid 74. Of these, the container body 72 has a first recess 721 provided therein 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 also has internal terminals provided on the inner surface, external terminals provided on the outer surface, wiring connecting the internal terminals and the external terminals, and the like.

[0067] The opening of the container body 72 is closed by a lid 74 via a sealing member such as a seal ring or low-melting-point glass (not shown). The lid 74 is made of a material that is transparent to laser light, such as a glass material.

[0068] 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). Furthermore, the internal terminal 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.

[0069] The circuit element 45 is disposed on the bottom surface of the second recess 722. The circuit element 45 is electrically connected to an internal terminal of the container body 72 via a bonding wire 76. As a result, the optical modulation oscillator 120 and the circuit element 45 are also electrically connected via the wiring provided in the container body 72. Note that the circuit element 45 may be provided with a circuit other than the oscillation circuit 54 described below.

[0070] By adopting such a package structure, the optical modulation oscillator 120 and the circuit element 45 can be stacked, thereby reducing the physical distance between them and shortening the wiring length between them. This makes it possible to prevent external noise from entering the drive signal Sd, or conversely, to prevent the drive signal Sd from becoming a noise source. Furthermore, a single container 70 can protect both the optical modulation oscillator 120 and the circuit element 45 from the external environment. This makes it possible to reduce the size of the sensor head unit 51 while improving the reliability of the laser interferometer 1.

[0071] The structure of the container 70 is not limited to the structure shown in the figure, and for example, the optical modulation oscillator 120 and the circuit element 45 may have separate package structures. Although not shown, the container 70 may also house other circuit elements that make up the oscillation circuit 54. The container 70 may be provided as needed, and may be omitted.

[0072] 1.2.Oscillator Circuit 1, the oscillation circuit 54 outputs a drive signal Sd to be input to the optical modulator 12 of the optical system 50. The oscillation circuit 54 also outputs a reference signal Ss to be input to the demodulation circuit 52.

[0073] The oscillator circuit 54 is not particularly limited and may have various configurations as long as it is a circuit that can oscillate the vibration element 30. Fig. 11 is a circuit diagram showing the configuration of a single-stage inverter oscillator circuit as an example of the circuit configuration.

[0074] The oscillation circuit 54 shown in FIG. 11 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.

[0075] The circuit element 45 is an inverter IC. The terminals X1 and X2 of the circuit element 45 are terminals connected to the inverters inside the circuit element 45. 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.

[0076] A first capacitor Cg is connected between terminal X1 and ground potential. A limiting resistor Rd and a second capacitor Cd are connected in series between terminal X2 and ground potential, in that order from the terminal X2 side. Furthermore, one end of a feedback resistor Rf is connected between terminal X1 and the first capacitor Cg, and the other end of the feedback resistor Rf is connected between terminal X2 and limiting resistor Rd.

[0077] One end of the vibration element 30 is connected between the first capacitor Cg and the feedback resistor Rf, and the other end of the vibration element 30 is connected between the second capacitor Cd and the limiting resistor Rd. This makes the vibration element 30 a signal source for the oscillation circuit 54.

[0078] FIG. 12 is an example of an LCR equivalent circuit of the vibration element 30. As shown in FIG. 12, the LCR equivalent circuit of the vibrating element 30 is made up of a series capacitance C1, a series inductance L1, an equivalent series resistance R1, and a parallel capacitance C0.

[0079] In the oscillation circuit 54 shown in FIG. 11, the capacitance of the first capacitor Cg is set to C g The capacitance of the second capacitor Cd is C d Then, the load capacitance C L is given by the following equation (a):

[0080]

number

[0081] Then, the oscillation frequency f output from the terminal Y of the oscillation circuit 54 is osc is given by the following equation (b):

[0082]

number

[0083] f Q is the natural frequency of the vibration element 30. According to the above formula (b), the load capacitance C L By appropriately changing the oscillation frequency f of the signal output from terminal Y, osc It can be seen that the

[0084] In addition, the natural frequency f of the vibration element 30 Q and the oscillation frequency f of the oscillator circuit 54 osc The difference Δf between and is given by the following equation (c):

[0085]

number

[0086] where C1< <C0、C1<<C L Therefore, Δf is approximately given by the following equation (d):

[0087]

number

[0088] Therefore, the oscillation frequency f of the oscillator circuit 54 osc is the natural frequency f of the vibration element 30 Q The value depends on the

[0089] Here, when the vibration element 30 is fixed to, for example, the container 70, if it is subjected to expansion stress due to temperature via the fixing part, the natural frequency f Q Furthermore, when the vibration element 30 is tilted, the natural frequency f Q fluctuates.

[0090] For this reason, the oscillator circuit 54 has a natural frequency f Q Even if fluctuates, the oscillation frequency f osc In other words, the oscillation frequency f osc is always increased by Δf, and the natural frequency f Q This stabilizes the vibration of the vibration element 30, making it possible to obtain a stable displacement amplitude L0. If the displacement amplitude L0 can be stabilized, the modulation characteristics of the optical modulator 12 can be stabilized, and the demodulation accuracy of the sample signal in the demodulation circuit 52 can be improved.

[0091] As an example, Δf = |f osc -f Q It is preferable that the frequency is |≦3000 [Hz], and more preferably 600 [Hz].

[0092] Instead of the oscillation circuit 54, a signal generator such as a function generator or a signal generator may be used.

[0093] 2. Demodulation circuit The demodulation circuit 52 performs a demodulation process to demodulate the light receiving signal output from the light receiving element 10 into a sample signal derived from the measurement object 14. The sample signal includes, for example, phase information and frequency information. Displacement information of the measurement object 14 can be obtained from the phase information, and velocity information of the measurement object 14 can be obtained from the frequency information. If different pieces of information can be obtained in this way, the laser interferometer 1 can be given the functions of a displacement meter and a velocity meter, thereby improving the functionality of the laser interferometer 1.

[0094] The circuit configuration of the demodulation circuit 52 is set according to the modulation processing method. The laser interferometer 1 according to this embodiment uses an optical modulator 12 equipped with a vibration element 30. The vibration element 30 is an element that vibrates simply, and therefore the vibration speed changes from moment to moment within the period. For this reason, the modulation frequency also changes over time, and a conventional demodulation circuit cannot be used as is.

[0095] A conventional demodulation circuit refers to a circuit that demodulates a sample signal from a received optical signal that includes a modulated signal modulated using, for example, an acousto-optic modulator (AOM). The modulation frequency of an acousto-optic modulator does not change. Therefore, while a conventional demodulation circuit can demodulate a sample signal from a received optical signal that includes a modulated signal whose modulation frequency does not change, it cannot directly demodulate a sample signal that includes a modulated signal modulated by an optical modulator 12 whose modulation frequency changes.

[0096] 1 includes a preprocessing unit 53 and a demodulation processing unit 55. The received light signal output from the light receiving element 10 first passes through the preprocessing unit 53 and is then guided to the demodulation processing unit 55. The preprocessing unit 53 performs preprocessing on the received light signal. This preprocessing results in a signal that can be demodulated by a conventional demodulation circuit. Therefore, the demodulation processing unit 55 demodulates the sample signal derived from the measurement object 14 using a known demodulation method.

[0097] 2.1. Configuration of the preprocessing unit 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 is Auto Gain Control.

[0098] Furthermore, a current-voltage converter 531 and an ADC 532 are connected in this order from the light-receiving element 10 side between the light-receiving element 10 and the pre-processing unit 53. The current-voltage converter 531 is a transimpedance amplifier that converts the current output from the light-receiving element 10 into a voltage signal. 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.

[0099] The current output from the light receiving element 10 is converted into a voltage signal by a current-voltage converter 531. The voltage signal is converted into a digital signal by an ADC 532 and split into two signals, a first signal S1 and a second signal S2, by a branching point jp1. In FIG. 1, the path of the first signal S1 is referred to as a first signal path ps1, and the path of the second signal S2 is referred to as a second signal path ps2.

[0100] Furthermore, an ADC 533 is connected between the oscillator circuit 54 and the second delay adjuster 537. The ADC 533 is an analog-to-digital converter, and converts an analog signal into a digital signal with a predetermined number of sampling bits.

[0101] First band-pass filter 534, second band-pass filter 535, and third band-pass filter 539 are filters that selectively transmit signals in specific frequency bands.

[0102] First delay adjuster 536 and second delay adjuster 537 are circuits that adjust the delay of a signal, respectively. Multiplier 538 is a circuit that generates an output signal proportional to the product of two input signals. Summer 542 is a circuit that generates an output signal proportional to the sum of two input signals.

[0103] 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.

[0104] The first signal S1 passes through a first band-pass filter 534 arranged on the first signal path ps1, and then the group delay is adjusted by a first delay adjuster 536. The group delay adjusted by the first delay adjuster 536 corresponds to the group delay of the second signal S2 caused by a second band-pass filter 535, which will be described later. This delay adjustment makes it possible to align the delay times associated with passing through the filter circuits between the first band-pass filter 534, through which the first signal S1 passes, and the second band-pass filter 535 and 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 passes through a first AGC 540 and is input to an adder 542.

[0105] The second signal S2 is passed through a second band-pass filter 535 arranged on the second signal path ps2, and then input to a multiplier 538. In the multiplier 538, the second signal S2 is multiplied by a reference signal Ss output from a second delay adjuster 537. Specifically, the cos(ω m The reference signal Ss represented by ωt is converted into a digital signal by the ADC 533, and its phase is adjusted by the second delay adjuster 537, and then input to the multiplier 538. m is the angular frequency of the modulated signal by the optical modulator 12, and t is time. After that, the second signal S2 passes through a third bandpass filter 539, then passes through a second AGC 541, and is input to a summer 542.

[0106] The adder 542 outputs an output signal proportional to the sum of the first signal S1 and the second signal S2 to the demodulation processing unit 55.

[0107] 2.2. Basic principles of pretreatment Next, the basic principle of preprocessing in the preprocessing unit 53 will be explained. The basic principle here refers to the principle described in Japanese Patent Application Laid-Open No. 2-38889. In this basic principle, a system is considered in which the frequency of the modulation signal changes sinusoidally and the displacement of the measurement object 14 also changes in simple harmonic motion in the optical axis direction. Here, E m , E d , φ,

[0108]

number

[0109] When this is the case, the light receiving signal I output from the light receiving element 10 is PD is theoretically expressed by the following equation:

[0110]

number

[0111] In addition, E m , E d , φ m , φ d , φ, ω m , ω d ,ω0,a m , a d are as follows:

[0112]

number

[0113] In addition, < > in equation (4) represents the time average. The first and second terms in the above equation (4) represent the DC component, and the third term represents the AC component. This AC component is called I PD·AC Then, I PD·AC is expressed as follows:

[0114]

number

[0115] Here, the v-order Bessel function as shown in the following equation is known.

[0116]

number

[0117] When the above formula (5) is expanded in a series using the Bessel functions of the above formulas (8) and (9), it can be transformed as follows:

[0118]

number

[0119] where J0(B), J1(B), J2(B), ... are Bessel coefficients.

[0120] Expanding on the above, it can be said that theoretically it is possible to extract a band corresponding to a specific order using a bandpass filter.

[0121] Based on this theory, the pre-processing unit 53 performs pre-processing on the received light signal in the following manner.

[0122] First, the received light signal output from the ADC 532 is split into two signals, a first signal S1 and a second signal S2, by the splitter jp1. The first signal S1 is passed through a first band-pass filter 534. The first band-pass filter 534 has a center angular frequency of ω m As a result, the first signal S1 after passing through the first band-pass filter 534 is expressed by the following equation.

[0123]

number

[0124] On the other hand, the second signal S2 is passed through a 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 expressed by the following equation.

[0125]

number

[0126] The second signal S2 after passing through the second band-pass filter 535 is multiplied by the reference signal Ss in the multiplier 538. The second signal S2 after passing through the multiplier 538 is expressed by the following equation.

[0127]

number

[0128] The second signal S2 after passing through the multiplier 538 is passed through a third band-pass filter 539. The central angular frequency of the third band-pass filter 539 is set to the same value as the central angular frequency of the first band-pass filter 534. Here, as an example, the central 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 expressed by the following equation.

[0129]

number

[0130] Thereafter, the first signal S1 expressed by the above equation (11) is adjusted in phase by the first delay adjuster 536 and in amplitude by the first AGC 540.

[0131] The amplitude of the second signal S2 expressed by the above equation (14) is also adjusted by the second AGC 541, so that the amplitude of the second signal S2 is made equal to the amplitude of the first signal S1.

[0132] The first signal S1 and the second signal S2 are then added together in the adder 542. The result of the addition is expressed by the following equation.

[0133]

number

[0134] As shown in the above equation (15), as a result of the addition, unnecessary terms are eliminated and necessary terms can be extracted. This result is input to the demodulation processing unit 55.

[0135] 2.3. Demodulation Processing Unit Configuration The demodulation processing unit 55 performs demodulation processing to demodulate the signal output from the preprocessing unit 53 into a sample signal originating from the measurement object 14. The demodulation processing is not particularly limited, but may be a well-known quadrature detection method. The quadrature detection method is a method of performing demodulation processing by externally mixing mutually orthogonal signals with respect to the input signal.

[0136] 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.

[0137] 2.4. Demodulation processing by the demodulation processing unit In the demodulation process, first, the signal output from the pre-processing unit 53 is split into two by the branching unit jp2. One of the split signals is multiplied by the multiplier 551 using cos(ω m The other signal after division is multiplied by a reference signal Ss expressed as -sin(ω mThe reference signal Ss and the signal obtained by shifting the phase of the reference signal Ss are signals whose phases are shifted by 90° from each other.

[0138] The signal that has passed through multiplier 551 passes through first low-pass filter 555 and is then input as signal x to divider 557. The signal that has passed through multiplier 552 passes through second low-pass filter 556 and is then input as signal y to divider 557. Divider 557 divides signal y by signal x, and passes the output y / x through arctangent calculator 558 to obtain output atan(y / x).

[0139] Thereafter, the output atan(y / x) is passed through the output circuit 559 to output the phase φ as information derived from the object 14 to be measured. d is obtained. In the output circuit 559, phase unwrapping is performed when there is a phase jump of 2π between adjacent points by phase unwrapping. Displacement information of the measurement object 14 can be calculated from the phase information output from the demodulation processing unit 55. This realizes a displacement meter that measures the displacement of the measurement object 14. Furthermore, velocity information can be obtained from the displacement information. This realizes a speed meter that measures the velocity of the measurement object 14.

[0140] The circuit configuration of the demodulation processing unit 55 has been described above, but the above digital circuit configuration is an example and is not limited to this. Furthermore, 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 and a ΔΣ counter circuit.

[0141] Furthermore, the circuit configuration of the demodulation processing unit 55 described above may be configured to obtain frequency information derived from the measurement object 14. Based on the frequency information, velocity information of the measurement object 14 can be calculated.

[0142] 2.5.Relationship between demodulation process and vibration conditions of vibration element The vibration conditions of the vibration element 30 affect the demodulation accuracy in the demodulation process of the sample signal. The B value included in the above formula (7) is the phase shift of the modulated signal. When this B value is small, the initial optical path phase difference in the laser interferometer 1 can cause the received light signal I output from the light receiving element 10 to PD AC component I PD·AC This problem will be explained in more detail below.

[0143] The light receiving signal I output from the light receiving element 10 PD AC component I PD·AC The intensity of is given by cos(ψ m -ψ d +φ0), where ψ m is the phase of the modulated signal from the optical modulator 12, and ψ d is the phase of the sample signal from the measurement object 14, and φ0 is the difference between the initial phase of the optical path 20 and the initial phase of the optical path 22, i.e., the initial optical path phase difference in the laser interferometer 1. m is calculated based on the above formula (5), m =Bsin(ω m t). Here, the phase ψ d =0.

[0144] FIG. 13 shows the phase ψ of the modulated signal coming from the optical modulator 12. m As mentioned above, the phase ψ m is calculated by using the B value mentioned above, m =Bsin(ω m t). Therefore, the phase of the modulated signal ψ m The change over time is represented by a sine wave waveform as shown in FIG.

[0145] Figure 14 shows the received light signal I PD AC component I PD·AC 10 is a diagram illustrating the influence of each parameter on the waveform of FIG.

[0146] In FIG. 14, the upper graph shows the relationship between the B value and the received light signal I when the B value is relatively small, specifically, when the B value is smaller than π. PD AC component I PD·AC is an example of a waveform shown in FIG.

[0147] When the B value is smaller than π, if the initial optical path phase difference φ0 takes a specific value such as 0 or π, the received light signal I PD AC component I PD·AC In this case, the demodulation accuracy in the demodulation process decreases, and the accuracy of the finally obtained displacement information and velocity information also decreases. On the other hand, when the initial optical path phase difference φ0 takes a specific value of ±π / 2, even if the B value is smaller than π, the intensity of the received light signal I PD AC component I PD·AC Therefore, when the B value is relatively small, the demodulation accuracy is susceptible to the influence of the initial optical path phase difference φ0 in the laser interferometer 1.

[0148] The lower graph in Figure 14 shows the received light signal I when the initial optical path phase difference φ0 is equal when the B value is relatively small and when the B value is relatively large. PD AC component I PD·AC 10A and 10B are comparison examples of waveforms shown in FIG.

[0149] As can be seen from this waveform, for example, when the B value is greater than π, the received light signal I PD AC component I PD·AC Therefore, by increasing the B value, the intensity of the received light signal I PD AC component I PD·AC The intensity of the beam is less susceptible to the initial optical path phase difference φ0 in the laser interferometer 1.

[0150] The B value is the frequency f of the modulating signal. m Doppler frequency shift of the modulated signal with respect to f mmaxWhen the vibration element 30 is used in a position tilted at an inclination angle θ as shown in FIG. 8, for example, the B value for the modulation signal generated by this vibration element 30 is expressed by the following formula (16).

[0151]

number

[0152] In the above formula (16), λ is the wavelength of the emitted light L1, and Lq is the amplitude value of the displacement of the vibration element 30. As can be seen from the above formula (16), the B value depends on the amplitude value Lq of the displacement of the vibration element 30. Therefore, by increasing the amplitude value Lq of the displacement of the vibration element 30, the B value can be increased.

[0153] In particular, when the diffraction grating 34 provided on the vibration element 30 is provided in the Littrow configuration described above, the following formula (17) holds true.

[0154]

number

[0155] In the above formula (17), n is the diffraction order, and P is the pitch of the diffraction grating 34. From the above formulas (16) and (17), the following formula (18) holds.

[0156]

number

[0157] As can be seen from the above formula (18), even in the case of the Littrow arrangement, the B value depends on the amplitude value Lq of the displacement of the vibration element 30. Therefore, by increasing the amplitude value Lq of the displacement of the vibration element 30, the B value can be increased.

[0158] 8, the vibration element 30 that vibrates in-plane is used in a tilted position, but the vibration element 30 may be an element that vibrates out-of-plane. In that case, the B value is expressed by the following formula (19).

[0159]

number

[0160] As can be seen from the above formula (19), even in the case of out-of-plane vibration, the B value depends on the amplitude value Lq of the displacement of the vibration element 30. Therefore, by increasing the amplitude value Lq of the displacement of the vibration element 30, the B value can be increased.

[0161] In light of the above-described relationship between the B value and the displacement amplitude value Lq, the present inventors have conducted extensive research into methods for increasing the displacement amplitude value Lq. As a result, the present inventors have found that there is a correlation between the amplitude value of the current flowing through the vibration element 30 and the displacement amplitude value Lq when the vibration element 30 is vibrating. The inventors have also found that the displacement amplitude value Lq can be increased efficiently and stably by optimizing the amplitude value of the current, which has led to the completion of the present invention.

[0162] Specifically, the laser interferometer 1 according to the embodiment includes a light source 2, a polarizing beam splitter 4 (optical divider), an optical modulator 12, a light receiving element 10, and a demodulation circuit 52. The light source 2 emits laser light. The polarizing beam splitter 4 splits the laser light emitted from the light source 2 into an optical path 20 (first optical path) and an optical path 22 (second optical path). The optical modulator 12 is provided on the optical path 20 and includes a vibration element 30 that vibrates when a current is passed through it, and modulates the laser light using the vibration element 30. The light receiving element 10 receives the laser light reflected by a measurement object 14 provided on the optical path 22 and outputs a light receiving signal. The demodulation circuit 52 demodulates a sample signal (Doppler signal) derived from the measurement object 14 from the light receiving signal based on a reference signal Ss and a modulation signal derived from the optical modulator 12.

[0163] In this embodiment, in particular, the laser light emitted by the light source 2 is defined as output light L1, the laser light modulated by the optical modulator 12 and containing a modulation signal is defined as reference light L2, and the laser light reflected by the measurement object 13 and containing a sample signal (Doppler signal) is defined as object light L3. The light receiving element 10 receives interference light between the object light L3 and the reference light L2, and the demodulation circuit 52 demodulates the sample signal contained in the object light L3 based on the standard signal Ss and the modulation signal contained in the reference light L2.

[0164] In such a laser interferometer 1, when the amplitude value of the current flowing through the vibrating vibration element 30 is Iq [A] and the vibration frequency of the vibration element 30 is f [Hz], Iq / f≦1×10 -7 Set Iq and f so that

[0165] This increases the probability that the laser interferometer 1 can demodulate the sample signal with the minimum required accuracy, regardless of the vibration conditions of the vibration element 30. In other words, by keeping Iq / f within the above range, the excitation power (drive level) of the vibration element 30 falls within an appropriate range, preventing abnormal oscillation in the vibration element 30 and preventing a decrease in the B value. Note that if Iq / f exceeds the above upper limit, the drive level of the vibration element 30 becomes excessive, causing a nonlinear phenomenon known as an increase in equivalent series resistance. The increase in equivalent series resistance reduces the current amplitude value Iq, which in turn reduces the B value.

[0166] Furthermore, the oscillator circuit 54 can oscillate not only a quartz oscillator but also a Si oscillator, a ceramic oscillator, or the like.

[0167] When the vibration element 30 is a quartz crystal vibrator, the amplitude value Iq [A] of the current flowing through the quartz crystal vibrator and the vibration frequency f [Hz] of the quartz crystal vibrator are 2×10 -10 ≦Iq / f≦1×10 -7 It is preferable that the value satisfies 2×10 -9 ≦Iq / f≦3×10 -8 It is more preferable that the -9 ≦Iq / f≦3×10 -8It is more preferable that the following is satisfied.

[0168] When Iq / f satisfies the above range, the sample signal derived from the measurement object 14 can be demodulated with higher accuracy regardless of the vibration conditions of the quartz crystal oscillator. In other words, the problem of demodulation accuracy decreasing depending on the vibration conditions is less likely to occur. This makes it possible to realize a laser interferometer 1 that can demodulate the sample signal more accurately than conventional laser interferometers.

[0169] If Iq / f is below the lower limit, depending on the vibration conditions, the amplitude value Lq of the crystal unit's displacement may be insufficient, resulting in a small B value. This may result in insufficient demodulation accuracy. On the other hand, if Iq / f is above the upper limit, the drive level of the crystal unit may become excessive, reducing the amplitude value Iq of the current and potentially resulting in a decrease in the B value. In this case, too, there is a risk that insufficient demodulation accuracy may be obtained.

[0170] The amplitude value Iq of the current flowing through the vibrating vibration element 30 can be obtained by attaching a current probe immediately after the vibration element 30 shown in Figure 11, displaying the excitation current waveform on an oscilloscope, and calculating the effective current value from that waveform.

[0171] Here, the relationship between the amplitude value Iq of the current and the B value can be theoretically explained as follows.

[0172] The differential equation theoretically representing the LCR equivalent circuit of the vibration element 30 corresponds to the differential equation theoretically representing the spring mechanical vibration system. Based on this correspondence, the current flowing through the vibration element 30 corresponds to the velocity of the mechanical vibration system, and the charge applied to the vibration element 30 corresponds to the displacement of the mechanical vibration system.

[0173] The charge is given by the time integral of the current. Therefore, the relationship between the B value, the amplitude value Lq of the displacement of the vibration element 30, and the current flowing through the vibration element 30 is expressed by the following formula (20).

[0174]

number

[0175] When the change in current over time in the oscillation circuit 54 is sinusoidal, the relationship between the B value, the amplitude value Lq of the displacement of the vibration element 30, and the current flowing through the vibration element 30 is expressed by the following formula (21).

[0176]

number

[0177] Load capacity C L When the frequency is changed, the angular frequency ω of the modulated signal from the optical modulator 12 m The change in is at the level of several hundred Hz and can be considered almost negligible. For the above reasons, it is explained that the B value and the amplitude value Lq of the displacement are proportional to the amplitude value Iq of the current.

[0178] In this case, the proportionality constant differs depending on the type of vibration element 30, specifically, the constituent material of the vibration element 30, such as quartz crystal, Si, ceramics, etc. This proportionality constant can be estimated based on a coefficient calculated by "piezoelectric efficiency ÷ Young's modulus of constituent material × element structural factor."

[0179] Here, we will use a quartz crystal unit and a silicon (Si) unit, each with a vibration frequency f of 32 kHz, as examples. The piezoelectric element in a quartz crystal unit is quartz, while the silicon (Si) unit uses materials such as lithium niobate or aluminum nitride. The piezoelectric coefficient d33 of quartz crystal is approximately 2.0 pm / V, while the piezoelectric constant d33 of the silicon (Si) unit is approximately 5.5 pm / V. The piezoelectric constant d33 represents the ratio of strains generated when an electric field of unit strength is applied. Based on Ohm's law, these values ​​and the equivalent series resistance of each unit are converted to a constant representing the ratio of strains generated when a unit current is applied: 60 nm / A for quartz crystal and 440 nm / A for the silicon (Si) unit. This means that the piezoelectric coefficient of the silicon (Si) unit is approximately 7.3 times higher than that of quartz crystal.

[0180] Multiplying these values ​​by the aforementioned "Iq / f" gives the displacement conversion efficiency of each piezoelectric element, i.e., the aforementioned piezoelectric efficiency. When comparing Iq / f when the same voltage is applied to a quartz crystal unit with a vibration frequency f of 32 kHz and a Si crystal unit, the value for the Si crystal unit is 0.375 times that of the quartz crystal unit, based on the ratio of equivalent series resistance values. Comparing the calculated piezoelectric efficiency results, it can be seen that the piezoelectric efficiency of the Si crystal unit is approximately 2.75 times higher than that of the quartz crystal unit.

[0181] On the other hand, when comparing Young's moduli, quartz is about 76 GPa and Si is about 185 GPa. Therefore, it can be said that Si is less likely to convert stress generated in a piezoelectric material into displacement than quartz.

[0182] From the above calculation results, when the coefficient expressed as "piezoelectric efficiency ÷ Young's modulus of constituent material" is calculated, the coefficient for a Si resonator is approximately 1.1 times that of a quartz crystal resonator.

[0183] The element structure factor is a parameter derived from the structure that converts the expansion and contraction of the piezoelectric material into element displacement. For example, in kHz-range resonators, a cantilever structure is one that makes it easy to obtain element displacement. In a cantilever structure, bending vibration is the primary vibration, and the displacement of the element's arm tip is proportional to the arm length. Therefore, the element structure factor in this case is the arm length. With a quartz crystal resonator, the arm length can be 1 mm or more, for example, 1.2 mm. On the other hand, a silicon resonator has a higher equivalent series resistance than a quartz crystal resonator, and its arm length is shorter than that of a quartz crystal resonator, for example, 0.55 mm. In other words, the arm length of a silicon resonator is 0.45 times that of a quartz crystal resonator. Calculating the coefficient expressed as "piezoelectric efficiency ÷ Young's modulus of constituent materials × element structure factor (= element arm length)" above, the coefficient of the silicon resonator is approximately 0.5 times that of the quartz crystal resonator.

[0184] This coefficient corresponds to the ratio of the amount of displacement when the same current value flows. Therefore, to obtain the same amount of displacement as a quartz crystal unit, a Si unit requires approximately twice the current value.

[0185] From the above explanation, it can be theoretically explained that in deriving the current amplitude value Iq, Iq can be calculated by making a correction taking into account the type of vibration element 30 within the range of Iq / f mentioned above, and the calculated value can be set.

[0186] Hereinafter, a further explanation will be given based on actual measured values, etc., using an example in which the vibration element 30 is a quartz crystal vibrator.

[0187] 2.6. Crystal Oscillator Vibration Conditions Next, vibration conditions when the vibration element 30 is a quartz crystal vibrator will be described in more detail.

[0188] 2.6.1. Relationship between current amplitude Iq and drive level First, the relationship between the current amplitude value Iq and the drive level will be described. Generally, in a quartz crystal unit, when the drive level exceeds a certain threshold, a nonlinear phenomenon occurs in which the equivalent series resistance increases. This nonlinear phenomenon reduces the current value flowing through the quartz crystal unit, and the B value decreases accordingly. For example, in the case of a quartz crystal unit with an oscillation frequency f of 1 MHz and a minimum equivalent series resistance of 1 Ω, the maximum current value that can flow is estimated to be about 100 mA. In addition, in the case of a quartz crystal unit with an oscillation frequency f of 5 MHz and an equivalent series resistance of 5 Ω, the maximum current value that can flow is estimated to be about 500 mA. Therefore, in a quartz crystal unit, the upper limit of Iq / f is 1 x 10 -7 This becomes:

[0189] Fig. 15 is a graph showing the relationship between the equivalent series resistance and the drive level of a crystal unit having an oscillation frequency f of 5 MHz. Fig. 16 is a graph showing the relationship between the drive level and the value of the current flowing through the crystal unit, which is calculated from the relationship shown in Fig. 15.

[0190] As shown in Figure 15, the equivalent series resistance of a crystal unit increases rapidly when the drive level of the crystal unit exceeds a certain threshold. This nonlinear phenomenon reduces the current flowing through the crystal unit. For a crystal unit with an oscillation frequency (f) of 5 MHz and an equivalent series resistance of 5 Ω, the threshold is estimated to exist at a drive level of approximately 100 mW.

[0191] FIG. 16 shows the relationship between the current value flowing through the crystal resonator and the drive level when the equivalent series resistance CI is 1Ω, 3Ω, 10Ω, 50Ω, and 100Ω.

[0192] For example, when a crystal unit with an equivalent series resistance CI of 1 Ω is operated at a drive level of 100 mW, it is estimated that a maximum current of about 300 mA will flow. An actual element can handle about 100 mA, and by using this value, the upper limit of Iq / f for this crystal unit is 1 x 10 -7 This becomes:

[0193] For the above reasons, the upper limit of Iq / f is basically 1×10 -7 Preferably, 3 × 10 -8 It is said that.

[0194] 2.6.2. Relationship between current amplitude Iq and B value Next, the relationship between the current amplitude value Iq and the B value will be described. When Iq / f is equal to or greater than the lower limit, the minimum necessary demodulation accuracy can be achieved in the laser interferometer 1, regardless of the vibration conditions of the quartz crystal oscillator, as described above. In other words, the lower limit of Iq / f is determined based on the B value that can be demodulated when the ADCs 532 and 533 shown in FIG. 1 have sufficiently high performance (high sampling bit rate). Specifically, the highest practical value for the sampling bit rate of the ADCs 532 and 533 is 14 bits. Therefore, the lower limit of Iq / f can be derived by determining the lowest B value that can be demodulated by the demodulation circuit 52 using the ADCs 532 and 533 with a sampling bit rate of 14 bits.

[0195] Fig. 17 is a graph showing the phase change of the sample signal demodulated by the demodulation circuit 52 when the measurement object is a piezoelectric actuator that vibrates at a displacement amplitude of 150 nm and a vibration frequency of 10 kHz. Fig. 17 compares how the demodulation accuracy changes when the B value is set to 0.05 and the SN ratio (signal-to-noise ratio) of the voltage of the received light signal is changed to 80 dB and 60 dB.

[0196] When the signal-to-noise ratio of the received light signal is set to 80 dB, sampling using high-precision ADCs 532 and 533 with a sampling bit count of 14 bits makes it possible to demodulate the sinusoidal phase change with minimum precision, as shown in Figure 17.

[0197] In contrast, when the signal-to-noise ratio of the received light signal is set to 60 dB, even if sampling is performed using high-precision ADCs 532 and 533 with a sampling bit rate of 14 bits, as shown in Figure 17, only an irregular waveform appears in the demodulation result, and the sinusoidal phase change cannot be demodulated.

[0198] From the above results, it can be seen that if the B value is 0.05 or more, demodulation processing can be performed with the minimum necessary accuracy in the demodulation circuit 52 by sampling with ADCs 532 and 533 having a sampling bit rate of 14 bits.

[0199] Therefore, the lower limit of the B value for achieving the minimum necessary demodulation accuracy is determined to be 0.05.

[0200] FIG. 18 is a graph showing the relationship between the B value and the amplitude value Iq of the current flowing through the vibrating quartz crystal unit.

[0201] As shown in Figure 18, a correlation expressed by a regression line is observed between the B value and the current amplitude value Iq. The formula shown in Figure 18 is an example of a formula that holds between y and x when the B value is y and the current amplitude value Iq is x. In addition, R 2is an example of a coefficient of determination found by regression analysis of experimental values. In the example shown in Fig. 18, the B value and the current amplitude value Iq fit the regression line model with a relatively high coefficient of determination.

[0202] Using the formula shown in Figure 18, the current amplitude Iq when the B value is 0.05 is calculated to be approximately 0.4 mA. Considering that the oscillation frequency f of the crystal unit can be selected in various ways, the lower limit of the current amplitude Iq can be set to 1 mA. When the oscillation frequency f of the crystal unit shown in Figure 18 is 5 MHz, the lower limit of Iq / f is 2 x 10 -10 This becomes:

[0203] FIG. 19 shows the relationship between the B value and the coefficient of determination (R 2 19 is a graph showing the relationship between the standard deviation and the sampling bit rate of the ADC532. Note that Fig. 19 shows the results of measurements taken while changing the sampling bit rate of the ADC532 to five levels: 4, 8, 11, 12, and 16 bits.

[0204] As an example, a sample vibrating at a vibration frequency of 10 kHz is used as the measurement object 14. The B value of the vibration element varies from 0.265 to 2.000. Furthermore, the laser light used is a laser light with a wavelength of 850 nm emitted from a VCSEL.

[0205] As shown in Figure 19, even when using an ADC532 with a sampling bit rate of 8 bits, which is a typical performance, the target accuracy can be achieved regardless of the B value. The target accuracy is as follows:

[0206] Regardless of the number of bits, the B value is greater than 0.5 and the coefficient of determination R 2 is 99.9% or more If the sampling bit rate is 8 bits or more, the standard deviation must be 1 nm or less regardless of the B value. When the sampling bit count is 4 bits, the standard deviation when the B value is greater than 1.0 is 1 nm or less

[0207] Furthermore, when looking at the transition of the standard deviation when the sampling bit count is 8 bits, it is recognized that if B ≥ 0.5, the target accuracy is achieved with a sufficient margin. Therefore, when the sampling bit count is 8 bits or more, if B ≥ 0.5, robustness during measurement can be sufficiently improved. Also, from the viewpoint of demodulation accuracy, it is recognized that the larger the B value, the more advantageous it is. When B ≥ 0.5 is satisfied using the formula shown in Figure 18 above, the amplitude value Iq of the current flowing through the crystal unit is calculated to be 10.5 mA or more. In this case, if the oscillation frequency f of the crystal unit is 5 to 6 MHz, the lower limit of Iq / f is 2 × 10 -9 This becomes:

[0208] In the upper diagram of Figure 19, markers for all data except 4-bit data overlap near a coefficient of determination of 1.000, making them indistinguishable. In the lower diagram of Figure 19, 12-bit data and 16-bit data almost overlap near a standard deviation of 0, making them indistinguishable.

[0209] FIG. 20 shows an example of a waveform of a light receiving signal simulated under given initial conditions. As mentioned above, the light receiving signal output from the light receiving element 10 is divided into a DC component and an AC component. In FIG. 20, in order to reproduce the various states that the initial optical path phase difference φ0 in the laser interferometer 1 can take, a gentle periodic fluctuation is given to φ0. Therefore, in FIG. 20, the DC component cos(φ0) corresponds to the long wave period, and the AC component cos(ψ m -ψ d +φ0) corresponds to the short wave period. m is the phase of the modulated signal from the optical modulator 12, and ψ m =Bsin(ω m t). d is the phase of the sample signal derived from the measurement object 14. In FIG. 20, the phase ψ d = 0. Also, as an example, B = 0.27.

[0210] The long wave period and short wave period will vary depending on the measurement conditions. Therefore, in order to perform stable measurements regardless of the movement of the measurement object 14, it is required that the line representing the waveform of the received light signal falls within the two areas marked "optimal zones" in Figure 20. The optimal zones are those where the AC component of the received light signal contains the fundamental wave with an angular frequency of 1·ω m and the harmonic component of angular frequency 2 ω m In other words, if it is outside the optimum zone, as shown in Figure 20, m The signal component of 2·ω m The signal component of 1·ω will be lost. m The signal component of the center angular frequency ω of the first band-pass filter 534 in the above formula (10) is m The component corresponding to 2·ω m The signal component is the signal component having the central angular frequency 2ω of the second band-pass filter 535 in the above formula (10). m It is a component corresponding to.

[0211] In order for the line representing the waveform of the received signal to fall within the "optimum zone," the AC component ψ m The amplitude of +φ0 should be greater than π / 3, and preferably greater than π / 2.

[0212] Based on this, the phase of the modulated signal ψ m The amplitude of change in Δψ m and the phase of the sample signal is ψ d The amplitude of change in Δψ d Then, Δψ m +Δψ d >π / 3 is sufficient. m It is preferable that is as large as possible toward π / 3, which allows for stable measurements.

[0213] In addition, the aforementioned ψ m =Bsin(ω m From the equation for t, the phase of the modulated signal ψ m The amplitude of change in Δψm corresponds to the B value. Therefore, it is preferable that the B value is π / 3 or more. This allows stable measurement even if the displacement of the measurement object 14 is even smaller. When the amplitude value Iq of the current flowing through the quartz crystal unit satisfies B≧π / 3 using the formula shown in FIG. 18, it is calculated to be 22.9 mA or more. In this case, if the oscillation frequency f of the quartz crystal unit is 5 to 6 MHz, the lower limit of Iq / f is 4×10 -9 This becomes:

[0214] For the above reasons, the lower limit of Iq / f is preferably 2×10 -10 More preferably, 2×10 -9 and more preferably 4 × 10 -9 It is said that.

[0215] Therefore, when the vibration element 30 is a quartz crystal vibrator, the amplitude value Iq [A] of the current flowing through the quartz crystal vibrator and the vibration frequency f [Hz] of the quartz crystal vibrator are 2×10 -10 ≦Iq / f≦1×10 -7 It is preferable that the value satisfies 2×10 -9 ≦Iq / f≦3×10 -8 It is more preferable that the -9 ≦Iq / f≦3×10 -8 It is more preferable that the following is satisfied.

[0216] By setting Iq / f within the above range, the sample signal derived from the measurement object 14 can be demodulated with higher accuracy regardless of the vibration conditions of the quartz crystal oscillator. Furthermore, the sample signal can be stably demodulated regardless of the movement of the measurement object 14.

[0217] The vibration conditions of the crystal resonator have been explained above, but the crystal resonator may be an AT resonator, a tuning fork resonator, or any other resonator.

[0218] Table 1 below compares the calculation results of Iq / f mentioned above for a crystal unit with a vibration frequency f in the MHz range and a crystal unit with a vibration frequency f in the kHz range. Note that each value shown in Table 1 is an example.

[0219] [Table 1]

[0220] As shown in Table 1 above, when Iq / f is calculated for a crystal unit with a vibration frequency f in the MHz band and a crystal unit with a vibration frequency f in the kHz band, the Iq / f is almost the same for both crystal units. A typical example of a crystal unit in the MHz band is an AT resonator, and a typical example of a crystal unit in the kHz band is a tuning fork resonator. This explains why, for any crystal unit, Iq / f only needs to be set to satisfy the above range, regardless of the vibration mode of the resonator.

[0221] 2.7.Vibration conditions of Si oscillator The oscillator circuit 54 can oscillate not only a quartz crystal oscillator but also a Si oscillator, a ceramic oscillator, etc. Here, the oscillation conditions when the oscillator element 30 is a Si oscillator will be explained in more detail. Note that in the following explanation, only the differences from the case of a quartz crystal oscillator will be explained, and explanations of similar points will be omitted.

[0222] 2.7.1. Relationship between current amplitude Iq and drive level First, the relationship between the current amplitude value Iq and the drive level will be described. Even with Si resonators, when the drive level exceeds a certain threshold, a nonlinear phenomenon occurs in which the equivalent series resistance increases. Therefore, even with Si resonators, the upper limit of Iq / f is basically 1×10 -7 However, silicon is easier to miniaturize than quartz, and nonlinearity tends to occur, reducing the amount of current that can be passed. -8 It is said that.

[0223] 2.7.2. Relationship between current amplitude Iq and B value Next, the relationship between the current amplitude value Iq and the B value will be described. In Si resonators and ceramic resonators, the series equivalent resistance is higher than that of quartz crystal resonators, so current tends to flow more slowly. Therefore, the lower limit of Iq / f is preferably 4×10 -10 More preferably, 4×10 -9 and more preferably 8 × 10 -9 It is said that.

[0224] Therefore, when the vibration element 30 is a Si vibrator, the amplitude value Iq [A] of the current flowing through the Si vibrator and the vibration frequency f [Hz] of the Si vibrator are 4×10 -10 ≦Iq / f≦5×10 -8 It is preferable that the value satisfies 4×10 -9 ≦Iq / f≦5×10 -8 It is more preferable that the -9 ≦Iq / f≦5×10 -8 It is more preferable that the following is satisfied.

[0225] When Iq / f satisfies the above range, the sample signal derived from the measurement object 14 can be demodulated with higher accuracy regardless of the vibration conditions of the Si resonator. Moreover, the sample signal can be stably demodulated regardless of the movement of the measurement object 14.

[0226] 2.8. Relationship between oscillator circuit settings and B value The laser interferometer 1 according to the embodiment includes the oscillator circuit 54 as described above. While this oscillator circuit 54 may be replaced with the signal generator described above, it is preferable that the oscillator circuit 54 is a circuit that selectively amplifies a signal of a specific resonant frequency by returning the output from the vibrating element 30 to the input, like the single-stage inverter oscillator circuit described above. Such a circuit utilizes the output from the vibrating element 30 to maintain highly stable vibration of the vibrating element 30. In other words, the vibrating element 30 serves as the signal source for the oscillator circuit 54. Therefore, such an oscillator circuit 54 facilitates simplification and miniaturization of the circuit configuration.

[0227] Furthermore, the oscillator circuit 54 outputs a highly accurate reference signal Ss, which is input to the demodulation circuit 52. Since the reference signal Ss output from the oscillator circuit 54 is input to the demodulation circuit 52, there is no need to separately provide a signal generator or the like for generating the reference signal Ss. Therefore, from this perspective as well, the oscillator circuit 54 contributes to making the laser interferometer 1 smaller and lighter.

[0228] Furthermore, when the oscillator circuit 54 is used, as described above, the natural frequency f of the vibration element 30 may be varied due to various factors. Q Even if the natural frequency f Q The oscillation frequency f osc This allows us to change the aforementioned Δf(=f osc -f Q ) is prevented from increasing significantly, and accordingly, the B value associated with Δf is prevented from becoming smaller. As a result, by using the oscillator circuit 54 described above, a larger B value can be ensured.

[0229] However, as given by the above equation (b), the oscillation frequency f osc is the natural frequency f of the vibration element 30 Q Therefore, Δf is always greater than 0. Therefore, in order to increase the B value, Δf needs to be optimized within a predetermined range.

[0230] 2.8.1.Load capacity C L and the relationship with B value Load capacitance C of the oscillator circuit 54 L is given by the above formula (a), but Δf is calculated by the above formula (d) using this load capacitance C L is inversely proportional to .

[0231] FIG. 21 shows the load capacitance C L 21 is a graph showing the relationship between the load capacitance C and Δf. The graph shown in FIG. 21 and the above formula (d) are consistent. Therefore, L By appropriately changing the value, Δf can be adjusted.

[0232] FIG. 22 shows the load capacitance C when the applied voltage Vq to the vibration element 30 is 10 V and when it is 5 V. L 10 is a graph showing the relationship between the B value and the color temperature.

[0233] When the applied voltage Vq is 10 V or 5 V, the load capacitance C L The change in the B value with respect to the load capacitance C L It is recognized that the maximum value of the B value is within the range of 50 to 150 pF.

[0234] Figure 23 shows the load capacitance C obtained from Figure 22. L 22 is a graph showing the optimum range of the saturation voltage Vcc, superimposed on the graph of FIG. 21.

[0235] The "optimum zone ZS" in Figure 23 is the load capacity C L This corresponds to the region where Δf is between 50 pF and 150 pF. From Fig. 23, it can be seen that a large B value can be obtained when Δf is between 240 [Hz] and 600 [Hz].

[0236] In FIG. 23, a zone with a larger Δf than the optimum zone ZS is called a "high Δf zone ZH," and a zone with a smaller Δf than the optimum zone ZS is called a "low Δf zone ZL."

[0237] In the high Δf zone ZH, Δf exceeds 600 Hz, and the impedance is large in that frequency band. This makes it difficult to obtain displacement of the vibration element 30.

[0238] In the low Δf zone ZL, Δf is less than 240 Hz and the load capacitance C L The limiting resistor Rd and the second capacitor Cd of the oscillation circuit 54 shown in FIG. 11 are equivalent to a first-order CR low-pass filter. For example, the load capacitance C LTo achieve a capacitance of 100 pF, the capacitance of the second capacitor Cd must be 160 to 250 pF. In this case, the cutoff frequency of the first-order CR low-pass filter is approximately 10 MHz. Typically, a first-order CR low-pass filter begins to attenuate at frequencies approximately 1 / 10 to 1 / 5 of the cutoff frequency. Therefore, if the vibration element 30 is oscillated at an oscillation frequency of, for example, 5 MHz, the first-order CR low-pass filter affects the oscillation behavior. In other words, the first-order CR low-pass filter passes the voltage signal of the oscillation circuit 54, attenuating the voltage signal. As a result, it becomes difficult to obtain displacement of the vibration element 30 in the low Δf zone ZL.

[0239] 2.8.2.Relationship between the resistance value of limiting resistor Rd and the B value The resistance value of the limiting resistor Rd of the oscillator circuit 54 is thought to have a large effect on ensuring the amplitude value Iq of the current flowing through the vibrating element 30. There are two main reasons for this. First, the limiting resistor Rd limits the value of the current flowing through the oscillator circuit 54, so the smaller the resistance value of the limiting resistor Rd, the larger the amplitude value Iq of the current. Second, the cutoff frequency of the above-mentioned first-order CR low-pass filter is thought to depend on the resistance value of the limiting resistor Rd.

[0240] FIG. 24 is a graph showing the relationship between the capacitance of the second capacitor Cd and the cutoff frequency fc of the first-order CR low-pass filter when the resistance value of the limiting resistor Rd of the oscillation circuit 54 is set to three levels: 50Ω, 100Ω, and 200Ω.

[0241] 24, the smaller the resistance value of the limiting resistor Rd, the higher the cutoff frequency fc. Therefore, from the viewpoint of the cutoff frequency fc, a smaller resistance value of the limiting resistor Rd is more advantageous for increasing the current amplitude value Iq.

[0242] FIG. 25 shows the load capacitance C L26 is a graph showing the relationship between the resistance value of the limiting resistor Rd of the oscillation circuit 54 and the B value when the load capacitance C of the oscillation circuit 54 is set to four levels of 80 pF, 100 pF, 120 pF, and 150 pF. L 10 is a graph showing the relationship between the resistance value of the limiting resistor Rd of the oscillation circuit 54 and Δf when the resistance is set to four levels of 80 pF, 100 pF, 120 pF, and 150 pF.

[0243] In Figure 25, the load capacitance C L Regardless of the resistance value, when the resistance value of the limiting resistor Rd becomes smaller than 50 Ω, the B value becomes smaller. The reason why the B value becomes smaller in this way is that, as shown in FIG. 26, when the resistance value of the limiting resistor Rd becomes smaller than 50 Ω, Δf becomes larger. When Δf becomes larger, as described above, the high Δf zone ZH shown in FIG. 23 is entered, making it difficult to obtain displacement of the vibration element 30. This is thought to be why the B value becomes smaller.

[0244] On the other hand, in Figure 25, the load capacitance C L Regardless of the resistance of the limiting resistor Rd, the change in the B value relative to the resistance of the limiting resistor Rd is accompanied by a maximum value. Therefore, based on the relationship between the resistance of the limiting resistor Rd and the B value shown in Figure 25, the resistance of the limiting resistor Rd is preferably 30 Ω or more and 200 Ω or less, and more preferably 40 Ω or more and 120 Ω or less. This allows a large B value to be obtained efficiently and stably.

[0245] In addition, the load capacitance C L When the load capacitance C is 150 pF, the B value is high, but the oscillation stability may decrease, especially when the resistance of the limiting resistor Rd is 100 Ω or less. L is preferably 50 pF or more and 150 pF or less, and more preferably 50 pF or more and less than 150 pF.

[0246] As described above, the oscillation circuit 54 is a circuit including the circuit element 45, which is an inverter IC, the feedback resistor Rf, the limiting resistor Rd, the first capacitor Cg, and the second capacitor Cd. gThe capacitance of the second capacitor Cd is C d Then, the load capacitance C L is given by the following equation (a):

[0247]

number

[0248] At this time, the load capacitance C L [pF] is preferably 50 pF or more and 150 pF or less, and more preferably 50 pF or more and less than 150 pF.

[0249] Load capacity C L By setting in such a range, a larger B value can be obtained. As a result, the sample signal derived from the measurement object 14 can be demodulated with higher accuracy regardless of the vibration conditions of the vibration element 30.

[0250] 3. First to fourth modified examples of laser interferometer Next, laser interferometers according to first to fourth modified examples will be described.

[0251] Fig. 27 is a schematic diagram showing the mounting structure of an optical system provided in a laser interferometer according to a first modified example. Fig. 28 is a schematic diagram showing the mounting structure of an optical system provided in a laser interferometer according to a second modified example. Fig. 29 is a schematic diagram showing the mounting structure of an optical system provided in a laser interferometer according to a third modified example. Fig. 30 is a schematic diagram showing the mounting structure of an optical system provided in a laser interferometer according to a fourth modified example.

[0252] The first to fourth modifications will be described below, focusing on the differences from the above embodiment, and omitting a description of similar points. Note that in Figures 27 to 30, the same reference numerals are used to designate the same components as those in the above embodiment.

[0253] The optical system 50D of the laser interferometer 1 shown in Fig. 27 includes a substrate 39. The light source 2, the optical modulator 12, and the light receiving element 10 are each mounted on this substrate 39. The light receiving element 10, the light source 2, and the optical modulator 12 are arranged in this order on the substrate 39 shown in Fig. 27 along a direction perpendicular to the optical path 22.

[0254] 27 also includes prisms 40 and 42. The prism 40 is provided on the optical path 24 between the light receiving element 10 and the analyzer 9. The prism 42 is provided on the optical path 20 between the optical modulator 12 and the quarter-wave plate 8.

[0255] 27 further includes a convex lens 44. The convex lens 44 is provided on the optical path 18 between the light source 2 and the polarizing beam splitter 4. By providing the convex lens 44, the output light L1 from the light source 2 can be focused and effectively utilized.

[0256] In the first modified example described above, similarly to the embodiment, the optical modulator 12 is provided in the optical path 20 (first optical path), and the measurement object 14 is provided in the optical path 22 (second optical path).

[0257] An optical system 50E of a laser interferometer 1 shown in FIG. 28 is similar to the optical system 50D shown in FIG. 27, except that the arrangement of elements and the like is different.

[0258] 28, a light source 2, a light receiving element 10, and an optical modulator 12 are arranged in this order along a direction perpendicular to the optical path 22. A prism 40 is provided on the optical path 18, and a prism 42 is provided on the optical path 20.

[0259] In the second modified example described above, similarly to the above embodiment, the optical modulator 12 is provided in the optical path 20 (first optical path), and the measurement object 14 is provided in the optical path 22 (second optical path).

[0260] An optical system 50F of a laser interferometer 1 shown in FIG. 29 is similar to the optical system 50E shown in FIG. 28, except that the arrangement of elements and the like is different and the laser light received by the light receiving element 10 is different.

[0261] 29, a light source 2, an optical modulator 12, and a light receiving element 10 are arranged in this order along a direction perpendicular to the optical path 22. A prism 42 is provided on the optical path 24.

[0262] The light L1 emitted from the light source 2 passes through the prism 40 and is split into a first optical path and a second optical path by the polarizing beam splitter 4. In the third modified example shown in Fig. 29, the combined optical path of the optical path 22 and the optical path 20 corresponds to the first optical path, and the optical path 24 corresponds to the second optical path.

[0263] The outgoing light L1 reflected by the polarizing beam splitter 4 passes through the quarter-wave plate 6 and is incident on the moving measurement object 14. The outgoing light L1 undergoes a Doppler shift at the measurement object 14 and is reflected as object light L3. The object light L3 passes through the quarter-wave plate 6, the polarizing beam splitter 4, and the quarter-wave plate 8 and is incident on the optical modulator 12. The object light L3 undergoes a frequency shift at the optical modulator 12 and is reflected as object reference light L4. The object reference light L4 passes through the quarter-wave plate 8, the polarizing beam splitter 4, the prism 42, and the analyzer 9 and is incident on the light-receiving element 10.

[0264] On the other hand, the outgoing light L1 transmitted through the polarizing beam splitter 4 passes through the prism 42 and the analyzer 9 and enters the light receiving element 10.

[0265] Then, the object reference light L4 and the output light L1 are incident as interference light on the light receiving element 10. The object reference light L4 is a laser light including a modulation signal and a sample signal.

[0266] In the third modified example described above, the measurement object 14 and the optical modulator 12 are each provided in the first optical path.

[0267] In addition, in this modified example, the light receiving element 10 receives the interference light between the object reference light L4 and the emitted light L1, and the demodulation circuit 52 demodulates the sample signal contained in the object reference light L4 based on the reference signal Ss and the modulated signal contained in the object reference light L4.

[0268] An optical system 50G of a laser interferometer 1 shown in FIG. 30 is similar to the optical system 50F shown in FIG. 29, except that the orientation of the light reflecting surface of the polarizing beam splitter 4 is different.

[0269] The light L1 emitted from the light source 2 passes through the prism 40 and is split into a first optical path and a second optical path by the polarizing beam splitter 4. In the fourth modified example shown in Fig. 30, the combined optical path of the optical path 20 and the optical path 22 corresponds to the first optical path, and the optical path 24 corresponds to the second optical path.

[0270] The outgoing light L1 reflected by the polarizing beam splitter 4 passes through the quarter-wave plate 8 and enters the optical modulator 12. The outgoing light L1 undergoes a frequency shift in the optical modulator 12 and is reflected as reference light L2. The reference light L2 passes through the quarter-wave plate 8, the polarizing beam splitter 4, and the quarter-wave plate 6 and enters the moving measurement object 14. The reference light L2 undergoes a Doppler shift in the measurement object 14 and is reflected as object reference light L4. The object reference light L4 passes through the quarter-wave plate 6, the polarizing beam splitter 4, the prism 42, and the analyzer 9 and enters the light-receiving element 10.

[0271] On the other hand, the outgoing light L1 transmitted through the polarizing beam splitter 4 passes through the prism 42 and the analyzer 9 and enters the light receiving element 10.

[0272] Then, the object reference light L4 and the output light L1 are incident as interference light on the light receiving element 10. The object reference light L4 is a laser light including a modulation signal and a sample signal.

[0273] In the fourth modified example described above, the measurement object 14 and the optical modulator 12 are each provided in the first optical path.

[0274] Also in this modified example, the light receiving element 10 receives the interference light between the object reference light L4 and the emitted light L1, and the demodulation circuit 52 demodulates the sample signal contained in the object reference light L4 based on the reference signal Ss and the modulated signal contained in the object reference light L4.

[0275] 27 to 30, the laser interferometer 1 can be easily miniaturized. The arrangement of the elements is not limited to that shown in the drawings. The "first optical path" and "second optical path" described above may be interchanged. For example, in the case of the fourth modified example, the optical path combining optical path 20 and optical path 22 may be the second optical path, and optical path 24 may be the first optical path. The same applies to the above embodiment and other modified examples.

[0276] 27 to 30, the size of the light receiving element 10 is, for example, 0.1 mm square, the size of the light source 2 is, for example, 0.1 mm square, and the size of the optical modulator 12 is, for example, 0.5 to 10 mm square. The size of the substrate 39 on which these are mounted is, for example, 1 to 10 mm square. This allows the optical system to be miniaturized to approximately the size of this substrate 39. The first to fourth modifications as described above also provide the same effects as those of the above embodiment.

[0277] The laser interferometer of the present invention has been described above based on the illustrated embodiment, but the laser interferometer of the present invention is not limited to the embodiment, and the configuration of each part can be replaced with any configuration having a similar function. Furthermore, any other components may be added to the laser interferometer according to the embodiment. Furthermore, an embodiment of the present invention may include any two or more of the embodiment and the modified examples. [Explanation of symbols]

[0278] 1...laser interferometer, 2...light source, 4...polarizing beam splitter, 6...quarter wave plate, 8...quarter wave plate, 9...analyzer, 10...light receiving element, 12...optical modulator, 14...measurement object, 16...setting unit, 18...optical path, 20...optical path, 22...optical path, 24...optical path, 30...vibration element, 30A...vibration element, 30B...vibration element, 31...substrate, 32...groove, 33...pad, 34...diffraction grating, 35...pad, 36...vibration direction, 3 7...mirror, 39...substrate, 40...prism, 42...prism, 44...convex lens, 45...circuit element, 50...optical system, 50D...optical system, 50E...optical system, 51...sensor head unit, 52...demodulation circuit, 53...preprocessing unit, 54...oscillation circuit, 55...demodulation processing unit, 70...container, 72...container body, 74...lid, 76...bonding wire, 120...optical modulation oscillator, 301...first electrode, 302...second electrode, 30 3...diffraction grating mounting portion, 305...piezoelectric substrate, 306...comb-shaped electrode, 307...ground electrode, 311...front surface, 312...rear surface, 531...current-voltage converter, 532...ADC, 533...ADC, 534...first band-pass filter, 535...second band-pass filter, 536...first delay adjuster, 537...second delay adjuster, 538...multiplier, 539...third band-pass filter, 542...adder, 551...multiplier , 552... multiplier, 553... phase shifter, 555... first low-pass filter, 556... second low-pass filter, 557... divider, 558... arctangent calculator, 559... output circuit, 721... first recess, 722... second recess, 540... first AGC, 541... second AGC, C0... parallel capacitance, C1... series capacitance, C3... third capacitor, Cd... second capacitor, Cg... first capacitor, GND... GND terminal, K -2s ...diffracted light, K -1s ...diffracted light, K 0s ...diffracted light, K 1s ...diffracted light, K 2s ...diffracted light, K i ...incident light, L1...series inductance, L1...outgoing light, L2...reference light, L3...object light, L4...object reference light, N...normal, P...pitch, R1...equivalent series resistance, R 2...coefficient of determination, Rd...limiting resistor, Rf...feedback resistor, S1...first signal, S2...second signal, Sd...drive signal, Ss...reference signal, Vcc...terminal, X1...terminal, X2...terminal, Y...terminal, ZH...high Δf zone, ZL...low Δf zone, ZS...optimum zone, jp1...branch, jp2...branch, ps1...first signal path, ps2...second signal path, x...signal, y...signal, β...angle of incidence, θ...tilt angle, θ B …blaze angle

Claims

1. a light source that emits laser light; a beam splitter that splits the laser beam emitted from the light source into a first optical path and a second optical path; an optical modulator provided in the first optical path or the second optical path, the optical modulator comprising a vibration element that vibrates when a current is passed therethrough, the optical modulator modulating the laser light by irradiating the vibration element with the laser light; a light receiving element that receives the laser light reflected by a measurement object provided in the first optical path or the second optical path and outputs a light receiving signal; an oscillation circuit that outputs a reference signal using the vibration element as a signal source; a demodulation circuit that demodulates a Doppler signal originating from the object to be measured from the received light signal based on the reference signal output from the oscillation circuit and the modulated signal originating from the optical modulator; Equipped with the oscillation circuit is a circuit that selectively amplifies a signal of a specific resonance frequency by inputting a drive signal to the vibration element and returning an output from the vibration element to its input, the oscillation frequency of the oscillation circuit changes according to the natural frequency of the vibration element; When the amplitude value of the current flowing through the vibrating vibration element is Iq [A] and the vibration frequency of the vibration element is f [Hz], Iq / f≦1×10 －７ A laser interferometer characterized by satisfying the above.

2. the vibration element is a quartz crystal vibrator, The amplitude value Iq [A] of the current flowing through the vibration element and the vibration frequency f [Hz] of the vibration element are expressed as follows: 2 x 10 －１０ ≦Iq / f≦1×10 －７ 2. The laser interferometer according to claim 1, wherein the above formula (1) is satisfied.

3. The amplitude value Iq [A] of the current flowing through the vibration element and the vibration frequency f [Hz] of the vibration element are expressed as follows: 2 x 10 －９ ≦Iq / f≦1×10 －７ 3. The laser interferometer according to claim 2, wherein the above formula (1) is satisfied.

4. the oscillator circuit is a circuit including an inverter, a feedback resistor, a limiting resistor, a first capacitor, and a second capacitor; The capacitance of the first capacitor is C ｇ [pF], and the capacitance of the second capacitor is C ｄ When [pF], The load capacitance C of the oscillator circuit Ｌ [pF] is given by the following formula (a): [Equation 1] The load capacitance C Ｌ 4. The laser interferometer according to claim 3, wherein [pF] is equal to or greater than 50 pF and equal to or less than 150 pF.

5. 5. The laser interferometer according to claim 4, wherein the resistance value of the limiting resistor is 30 Ω or more and 200 Ω or less.

6. the vibration element is a Si vibrator, The amplitude value Iq [A] of the current flowing through the vibration element and the vibration frequency f [Hz] of the vibration element are expressed as follows: 4 x 10 －１０ ≦Iq / f≦5×10 －８ 2. The laser interferometer according to claim 1, wherein the above formula (1) is satisfied.

7. 2. The laser interferometer according to claim 1, wherein the vibration element is a ceramic vibrator.

8. 2. The laser interferometer according to claim 1, wherein the vibration element is a surface acoustic wave device.

9. 9. The laser interferometer according to claim 1, wherein the vibration element is formed with a reflective diffraction grating.

10. the light modulator further comprises a housing; the oscillator circuit has a circuit element, 9. A laser interferometer according to claim 1, wherein the vibration element and the circuit element are housed in the container and are electrically connected inside the container.

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