Laser interferometer and method for controlling a laser interferometer

Abstract

To provide a laser interferometer and its control method capable of accurately measuring information resulting from an object to be measured without depending on a type of laser light source.SOLUTION: A laser interferometer comprises: a laser light source that emits emission light; a light splitter that splits the emission light into first split light, and second split light incident on an object to be measured; a light modulator that is disposed on an optical path on which the first split light advances, and modulates the first split light into reference light having a different frequency; an optical path length change section that is provided between the light splitter and the light modulator, and changes a first optical path length which is an optical path length between the light splitter and the light modulator; a light receiving element that receives interference light of the reference light and object light generated by reflecting the emission light at the object to be measured, and outputs a light reception signal; and a control section that controls an operation of the optical path length change section in accordance with a second optical path length which is an optical path length between the light splitter and the object to be measured.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a laser interferometer and a method for controlling a laser interferometer.

Background Art

[0002] Patent Document 1 discloses a laser vibrometer that measures the vibration speed of an object by irradiating the object with laser light and measuring the vibration speed based on the scattered laser light that has received a Doppler shift. In this laser vibrometer, the vibration speed of the object is extracted from the Doppler signal included in the scattered laser light.

[0003] Further, the laser vibrometer described in Patent Document 1 includes an acousto-optic modulator (AOM) that shifts the frequency of the laser light by changing the supplied ultrasonic frequency. By using this acousto-optic modulator to shift the frequency of the laser light and using the laser light with the shifted frequency as the reference light, the vibration speed of the object can be extracted from the Doppler signal.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the method described in Patent Document 1, the measurement results are affected by the characteristics of the laser light. That is, depending on the type of the laser light source, there is a problem that the accuracy of the speed measured for the measurement target becomes low.

Means for Solving the Problems

[0006] The laser interferometer according to an application example of the present invention is A laser light source that emits light, An optical splitter that splits the emitted light into a first split beam and a second split beam that is incident on the object to be measured, Arranged in the optical path through which the first divided light travels, A vibrating element is provided, and the first divided light is incident on the vibrating vibrating element, An optical modulator that modulates the first divided light into reference light of a different frequency, An optical path length changing unit is provided between the optical splitter and the optical modulator to change the first optical path length, which is the optical path length between the optical splitter and the optical modulator. A light-receiving element that receives the interference light between the object light generated by the reflection of the emitted light from the object to be measured and the reference light, and outputs a received signal. A control unit controls the operation of the optical path length changing unit according to the optical path difference between the second optical path length, which is the optical path length between the optical splitter and the object to be measured, and the first optical path length. An oscillator circuit that outputs a reference signal using the aforementioned vibrating element as a signal source, A demodulator that receives the reference signal output from the oscillation circuit and demodulates a sample signal originating from the object to be measured from the received light signal based on the reference signal, It is characterized by being equipped with [the following features].

[0007] A control method for a laser interferometer according to an application example of the present invention is: Examples of applications of the present invention Laser interferometer of control do It is a method, A step of preparing a sample whose displacement or velocity is known as the object to be measured, When the optical path length between the optical splitter and the optical modulator is a first length, the process includes acquiring the received light signal and calculating a first measured value, which is the displacement or velocity of the sample, based on the acquired received light signal. A step of controlling the operation of the optical path length changing unit so that the optical path length between the optical splitter and the optical modulator becomes a second length, When the optical path length between the optical splitter and the optical modulator is the second length, the process includes acquiring the received light signal and calculating a second measured value, which is the displacement or velocity of the sample, based on the acquired received light signal. The process involves setting the optical path length between the optical splitter and the optical modulator to the first length when the first measured value is close to the known displacement or velocity of the sample, and setting the optical path length between the optical splitter and the optical modulator to the second length when the second measured value is close to the known displacement or velocity of the sample. It is characterized by having the following features. [Brief explanation of the drawing]

[0008] [Figure 1] This is a functional block diagram showing a laser interferometer according to the first embodiment. [Figure 2] Figure 1 is a schematic diagram showing the sensor head section. [Figure 3] This is a partially enlarged view of Figure 2, showing the optical path length changing section according to the first configuration example. [Figure 4] This figure shows the optical path length changing section according to the second configuration example. [Figure 5] This figure shows the optical path length changing section according to the third configuration example. [Figure 6] Figure 1 is a block diagram showing an example of the hardware configuration of the control unit. [Figure 7] This is a flowchart illustrating the control method for the laser interferometer according to the second embodiment. [Figure 8] This is a functional block diagram showing a laser interferometer according to the third embodiment. [Figure 9] Figure 8 is a schematic diagram of the sensor head section. [Figure 10] Figure 9 is a perspective view showing a first configuration example of the optical modulator. [Figure 11] This is a plan view showing a part of the second configuration example of the optical modulator shown in Figure 9. [Figure 12] Figure 9 is a plan view showing a third configuration example of the optical modulator. [Figure 13] This is a conceptual diagram illustrating the generation of multiple diffracted light beams when incident light Ki is incident on the surface of a vibrating element from a direction perpendicular to the surface. [Figure 14]This is a conceptual diagram illustrating an optical modulator configured such that the angle between the direction of propagation of the incident light Ki and the direction of propagation of the reference light L2 is 180°. [Figure 15] This is a conceptual diagram illustrating an optical modulator configured such that the angle between the direction of propagation of the incident light Ki and the direction of propagation of the reference light L2 is 180°. [Figure 16] This is a conceptual diagram illustrating an optical modulator configured such that the angle between the direction of propagation of the incident light Ki and the direction of propagation of the reference light L2 is 180°. [Figure 17] This is a cross-sectional view showing an optical modulator having a package structure. [Figure 18] This circuit diagram shows an example of an oscillator circuit configuration, specifically a single-stage inverter oscillator circuit. [Figure 19] This is an example of an LCR equivalent circuit for a vibrating element. [Figure 20] This is a schematic diagram of the sensor head portion of the laser interferometer according to the fourth embodiment. [Figure 21] This is a magnified view of a portion of Figure 20. [Modes for carrying out the invention]

[0009] The laser interferometer of the present invention will be described in detail below based on the embodiments shown in the accompanying drawings. 1. First Embodiment First, the laser interferometer according to the first embodiment will be described. Figure 1 is a functional block diagram showing a laser interferometer according to the first embodiment.

[0010] The laser interferometer 1 shown in Figure 1 comprises a sensor head unit 51 equipped with an optical system 50 and a signal generator 59, a demodulation circuit 52 that receives the received light signal from the optical system 50, and a control unit 57. The laser interferometer 1 measures the displacement and velocity of an object to be measured by utilizing the interference of laser light.

[0011] 1.1. Sensor head section Figure 2 is a schematic diagram showing the sensor head unit 51 shown in Figure 1.

[0012] As described above, the sensor head unit 51 includes an optical system 50. As shown in Figure 2, the optical system 50 includes a light source 2 (laser light source), a collimating lens 3, a polarizing beam splitter 4, a half-wave plate 5, a quarter-wave plate 6, a quarter-wave plate 8, an analyzer 9, a photodetector 10, a frequency shifter type optical modulator 12, and an optical path length changing unit 15.

[0013] Light source 2 emits light L1 (first laser light) of a predetermined wavelength. Photodetector 10 converts the received light into an electrical signal. Optical modulator 12 is equipped with AOM 60 and modulates the emitted light L1 to generate reference light L2 (second laser light) containing the modulated signal. The emitted light L1 incident on the moving object to be measured 14 is reflected as object light L3 (third laser light) containing a sample signal originating from the object to be measured 14.

[0014] The optical path of the emitted light L1 from the light source 2 is defined as optical path 18. Along optical path 18, the half-wave plate 5 and the collimating lens 3 are arranged in that order from the polarizing beam splitter 4 side. Optical path 18 is coupled to optical path 20 by transmission through the polarizing beam splitter 4. Along optical path 20, the quarter-wave plate 8, the optical path length changing unit 15, and the optical modulator 12 are arranged in that order from the polarizing beam splitter 4 side. Optical path 18 is coupled to optical path 22 by reflection through the polarizing beam splitter 4. Along optical path 22, the quarter-wave plate 6 and the object to be measured 14 are arranged in that order from the polarizing beam splitter 4 side. Along optical path 24, the analyzer 9 and the photodetector 10 are arranged in that order from the polarizing beam splitter 4 side. Furthermore, optical paths 20 and 22 are each coupled to optical path 24.

[0015] The emitted light L1 from the light source 2 passes through optical paths 18 and 20 and enters the optical modulator 12. The emitted light L1 also passes through optical paths 18 and 22 and enters the object to be measured 14. The reference light L2 generated by the optical modulator 12 passes through optical paths 20 and 24 and enters the photodetector 10. The object light L3 generated by reflection from the object to be measured 14 passes through optical paths 22 and 24 and enters the photodetector 10.

[0016] The following provides a further explanation of each part of the optical system 50. 1.1.1.Light source Light source 2 is a laser light source that emits coherent emitted light L1. Examples of light source 2 include gas lasers such as He-Ne lasers, DFB-LDs (Distributed feedback laser diodes), FBG-LDs (Fiber Bragg grating laser diodes), VCSELs (Vertical Cavity Surface Emitting Lasers), and semiconductor laser elements such as FP-LDs (Fabry-Perot Laser Diodes).

[0017] The light source 2 preferably includes a semiconductor laser element. This makes it possible to miniaturize the light source 2. As a result, the laser interferometer 1 can be miniaturized. In particular, the sensor head section 51, which houses the optical system 50, can be miniaturized and lightened, which is also useful in improving the operability of the laser interferometer 1.

[0018] The emitted light L1 passes sequentially through the collimating lens 3 and the half-wave plate 5 before entering the polarized beam splitter 4. The collimating lens 3 is a lens that makes the transmitted emitted light L1 into parallel light. The half-wave plate 5 is an optical element that rotates the polarization direction of linearly polarized light.

[0019] 1.1.2. Polarizing Beam Splitter The polarizing beam splitter 4 is an optical element that splits the outgoing light L1 into transmitted light L1a (first split light) and reflected light L1b (second split 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 linearly polarized outgoing light L1, with a ratio of P-polarized to S-polarized light of, for example, 50:50, is incident on the polarizing beam splitter 4.

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

[0021] The transmitted light L1a, which is P-polarized after passing through the polarizing beam splitter 4, is converted to circular polarization by the quarter-wave plate 8 and incident on the optical modulator 12. The circular polarization of the transmitted light L1a incident on the optical modulator 12 is f m It undergoes a frequency shift of [Hz] and is reflected as reference light L2. Therefore, reference light L2 has a frequency f m The modulated signal is in [Hz]. The reference light L2 is converted to S-polarized light when it passes through the quarter-wave plate 8 again via the optical path length changing section 15. The S-polarized light of the reference light L2 is reflected by the polarizing beam splitter 4, passes through the analyzer 9, and is incident on the photodetector 10.

[0022] The reflected light L1b, which is S-polarized and reflected by the polarizing beam splitter 4, is converted to circular polarization by the quarter-wave plate 6 and incident on the moving object to be measured 14. The circular polarization of the reflected light L1b incident on the object to be measured 14 is f d It undergoes a Doppler shift of [Hz] and is reflected as object light L3. Therefore, object light L3 has a frequency f d The sample signal is in [Hz]. The object light L3 is converted to P-polarized light when it passes through the quarter-wave plate 6 again. The P-polarized object light L3 passes through the polarizing beam splitter 4, passes through the analyzer 9, and is incident on the photodetector 10.

[0023] As mentioned above, since the emitted light L1 is coherent, the reference light L2 and object light L3 are incident on the photodetector 10 as interfering light.

[0024] Alternatively, a non-polarizing beam splitter may be used instead of a polarizing beam splitter. In this case, the quarter-wave plates 6 and 8 become unnecessary, allowing for a reduction in the number of components and thus miniaturization of the laser interferometer 1.

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

[0026] 1.1.4. Photodetector The reference light L2 and object light L3 are incident on the photodetector 10 via the polarizing beam splitter 4 and the analyzer 9. As a result, the reference light L2 and object light L3 undergo optical heterodyne interference, |f m -f d Interfering light with a frequency of |[Hz] is incident on the photodetector 10. The photodetector 10 outputs the received signal of the interfering light to the demodulation circuit 52 via the current-voltage converter 531. The demodulation circuit 52 demodulates the sample signal from this received signal using a method described later, thereby ultimately determining the movement of the object to be measured 14, i.e., the vibration velocity and displacement. Examples of photodetectors 10 include photodiodes.

[0027] The current-voltage converter 531 is a transimpedance amplifier (TIA) that converts the current output from the photodetector 10 into a voltage signal.

[0028] 1.1.5. Optical Modulator The optical modulator 12 shown in Figures 1 and 2 is equipped with an AOM60. The AOM60 is an Acousto-Optic Modulator. An Acousto-Optic Modulator is a frequency modulator that uses a piezoelectric element to vibrate a crystal, creating a standing wave of compression and rarefaction within the crystal, which is then used as a diffraction grating. The AOM60 shown in Figures 1 and 2 is equipped with the aforementioned crystal and mirror, although not shown in the diagram. Light that has passed through the crystal via the optical path 20 is reflected by the mirror and then transmitted through the crystal again. In this way, the AOM60 generates a reference light L2 from the incident transmitted light L1a.

[0029] The AOM60 receives a high-frequency signal from the signal generator 59, which will be described later. This signal drives the piezoelectric element, causing the crystal to vibrate.

[0030] The optical modulator 12 may also be equipped with various optical modulators such as an electro-optic modulator (EOM) instead of the AOM 60.

[0031] 1.1.6. Signal generator The signal generator 59 generates the drive signal Sd, which is input to the AOM 60. The signal generator 59 also generates the reference signal Ss, which is input to the demodulation circuit 52.

[0032] The signal generator 59 can be anything as long as it is capable of generating a signal with good characteristics such as frequency stability and low jitter. Specific examples of the signal generator 59 include function generators, signal generators, crystal oscillators, and PLL (Phase Locked Loop) circuits.

[0033] 1.1.7. Optical Path Length Change Section The optical path length changing unit 15 shown in Figure 2 has the function of changing the optical path length of the optical path 20 through which the transmitted light L1a (first split light) travels. The optical path 20 is the optical path connecting the polarizing beam splitter 4 and the optical modulator 12.

[0034] 1.1.7.1. First Configuration Example of Optical Path Length Changing Section The optical path length changing unit 15 shown in Figure 2 comprises a first reflecting element 151 and a second reflecting element 152, and a drive unit 153 that drives the first reflecting element 151. The first reflecting element 151 and the second reflecting element 152 are optical elements that switch the optical path 20 through which transmitted light L1a travels. The drive unit 153 changes the distance between the first reflecting element 151 and the second reflecting element 152 by moving the first reflecting element 151.

[0035] The drive unit 153 generates a driving force to move the first reflecting element 151, for example, parallel to the optical path 20. This allows the optical path length of the optical path 20 to be changed. The drive unit 153 also holds the first reflecting element 151 in the desired position. The drive unit 153 may be configured to move the second reflecting element 152 instead of the first reflecting element 151, or it may be configured to move both the first reflecting element 151 and the second reflecting element 152. Furthermore, the direction of movement does not matter as long as the optical path length of the optical path 20 can be changed.

[0036] The drive unit 153 is a device that moves the first reflecting element 151 along a straight line, and examples include a linear stage, an electric actuator, a piezo actuator, and the like.

[0037] When the drive unit 153 moves the first reflecting element 151, the physical distance between the first reflecting element 151 and the second reflecting element 152 changes. This also changes the optical path length (optical distance) between the polarizing beam splitter 4 and the optical modulator 12 (optical path 20). By changing the optical path length of optical path 20 in this way, the optical path length between the polarizing beam splitter 4 and the object to be measured 14 (optical path 22) can be made equal to the optical path length of optical path 20. When these optical path lengths are equal to each other, the measurement accuracy of the displacement of the object to be measured 14 can be improved according to the following principle.

[0038] In the demodulation circuit 52, a sample signal derived from the measurement object 14 can be demodulated from the received light signal. Then, the displacement of the measurement object 14 can be obtained from the sample signal. When the measurement accuracy of this displacement is Δd, the measurement accuracy Δd is expressed by the following formula (1).

[0039] [Number]

[0040] In the above formula (1), when the optical path difference d is 0, the second and third terms on the right side also become 0. In this case, theoretically, the measurement accuracy Δd of the displacement is not affected by the line width Δf (phase fluctuation) of the emitted light L1 or the fluctuation Δn of the refractive index of the atmosphere. On the other hand, when the optical path difference d is not 0, the line width Δf and the fluctuation Δn of the refractive index of the atmosphere affect the measurement accuracy Δd of the displacement. In particular, depending on the type of the light source 2, the line width Δf of the emitted light L1 may be a relatively large value. Then, depending on the type of the light source 2, there is a concern that the measurement accuracy Δd of the displacement of the measurement object 14 deteriorates.

[0041] Therefore, in the present embodiment, the operation of the optical path length changing unit 15 is controlled by the control unit 57 so that the optical path difference d approaches 0, that is, the optical path length of the optical path 22 and the optical path length of the optical path 20 approach each other. As a result, theoretically, the measurement accuracy Δd of the displacement is less likely to be affected by the line width Δf [Hz] of the emitted light L1. As a result, regardless of the type of the light source 2, the displacement of the measurement object 14 can be accurately measured. Also, the speed of the measurement object 14 can be calculated from the displacement.

[0042] Note that in a general laser light source, the frequency of the laser light is several 100 THz. This frequency does not change by more than one digit even if the type of the laser light source is changed. Also, in the above formula (1), the second term is often more dominant than the third term. Taking this into account, it is considered that the value of dΔf in the above formula (1) has a great influence on the measurement accuracy Δd. As a result of the study by the present inventor, even when the optical path difference d [m] is not 0, dΔf ≦ 1 × 10 8It was found that sufficient measurement accuracy Δd can be obtained if the following conditions are met.

[0043] Table 1 below shows four types of laser light sources, the typical linewidth Δf [Hz] of the laser light emitted from each laser light source, and dΔf ≤ 1 × 10⁻¹⁰. 8 This table shows reference values ​​for the range of allowable optical path difference d[m] to satisfy the condition.

[0044] [Table 1]

[0045] As shown in Table 1 above, the line width Δf differs depending on the type of laser light source, and consequently, the range of the allowable optical path difference d during adjustment also changes.

[0046] Figure 3 is a partially enlarged view of Figure 2, showing the optical path length changing section 15 according to the first configuration example.

[0047] The first reflective element 151 shown in Figure 3 comprises right-angle prism mirrors 154a and 154b, and a base material 156 that supports them. The right-angle prism mirrors 154a and 154b are optical elements each having a light-reflecting surface 150 that intersects the optical path 20 at a 45° angle. The right-angle prism mirrors 154a and 154b are arranged such that the angle between their light-reflecting surfaces 150 is 90°. As a result, the optical path 20 extending from the polarizing beam splitter 4 is folded back by a unit consisting of a pair of right-angle prism mirrors 154a and 154b, and heads toward the second reflective element 152. The base material 156 supports multiple pairs of the right-angle prism mirrors 154a and 154b together.

[0048] The second reflective element 152 shown in Figure 3 comprises right-angle prism mirrors 155a and 155b, and a base material 157 that supports them. The right-angle prism mirrors 155a and 155b are optical elements each having a light-reflecting surface 150 that intersects the optical path 20 at a 45° angle. The right-angle prism mirrors 155a and 155b are arranged such that the angle between their light-reflecting surfaces 150 is 90°. As a result, the optical path 20 extending from the first reflective element 151 is folded back by a unit consisting of a pair of right-angle prism mirrors 155a and 155b, and returns to the first reflective element 151 again. The base material 157 supports multiple pairs of the right-angle prism mirrors 155a and 155b together.

[0049] The right-angle prism mirrors 154a, 154b, 155a, and 155b are highly accurate and readily available. For this reason, they are useful as optical elements for use in the first reflecting element 151.

[0050] By folding the optical path 20 using the first reflector 151 and the second reflector 152, the optical path 20 can be connected between the first reflector 151 and the second reflector 152. When the drive unit 153 moves the first reflector 151, the physical distance between the first reflector 151 and the second reflector 152 changes. This also changes the optical path length of the optical path 20. Therefore, the optical path length changing unit 15 can bring the optical path length of the optical path 20 closer to the optical path length of the optical path 22, and preferably make them equal. As a result, the optical path difference d in equation (1) can be brought closer to 0, and preferably made 0.

[0051] As described above, the laser interferometer 1 according to this embodiment comprises a light source 2 (laser light source), a polarizing beam splitter 4 (optical splitter), an optical modulator 12, an optical path length changing unit 15, a light receiving element 10, and a control unit 57. The light source 2 emits outgoing light L1. The polarizing beam splitter 4 splits the outgoing light L1 into transmitted light L1a (first splitting light) and reflected light L1b (second splitting light) incident on the object to be measured 14. The optical modulator 12 is positioned in the optical path 20 through which the transmitted light L1a travels and modulates the transmitted light L1a into a reference light L2 of a different frequency. The optical path length changing unit 15 is provided between the polarizing beam splitter 4 and the optical modulator 12 and changes the first optical path length (optical path length of the optical path 20), which is the optical path length between the polarizing beam splitter 4 and the optical modulator 12. The light-receiving element 10 receives the interference light of object light L3, which is generated when the emitted light L1 is reflected by the object to be measured 14, and the reference light L2, and outputs a received light signal. The control unit 57 controls the operation of the optical path length changing unit 15 according to the second optical path length (optical path length of optical path 22), which is the optical path length between the polarizing beam splitter 4 and the object to be measured 14.

[0052] With this configuration, the optical path length of optical path 20 can be changed, making it possible to adjust the optical path length of optical path 20 to be close to the optical path length of optical path 22. This allows the optical path difference d in equation (1) above to be brought close to 0, and theoretically, the displacement measurement accuracy Δd becomes less susceptible to the influence of the line width Δf of the emitted light L1. As a result, the displacement of the object to be measured 14 can be measured accurately regardless of the type of light source 2.

[0053] Furthermore, even when using a light source 2 with a relatively large linewidth Δf of emitted light L1, the reduction in displacement measurement accuracy Δd can be suppressed by changing the optical path length of the optical path 20 according to the above principle. Therefore, the range of light sources 2 can be expanded without sacrificing displacement measurement accuracy Δd.

[0054] An example of the operation of the optical path length changing unit 15 controlled by the control unit 57 is to control the operation of the optical path length changing unit 15 so as to reduce the optical path difference d between the first optical path length (the optical path length of optical path 20) and the second optical path length (the optical path length of optical path 22).

[0055] As a result, the optical path difference d in equation (1) above approaches 0, and the displacement measurement accuracy Δd in the laser interferometer 1 becomes less susceptible to the influence of the line width Δf of the emitted light L1.

[0056] More specifically, the control unit 57 determines that the optical path difference d[m] between the first optical path length and the second optical path length, and the line width Δf of the emitted light L1, are such that 0≦dΔf≦1×10 8 The operation of the optical path length changing unit 15 is controlled to satisfy the following conditions.

[0057] By controlling the optical path length changing unit 15 to satisfy these conditions, it is possible to sufficiently improve the displacement measurement accuracy Δd in the laser interferometer 1, even when the line width Δf of the emitted light L1 is relatively large.

[0058] Of the aforementioned light sources 2, VCSELs (vertical-cavity surface-emitting laser diodes) or FP-LDs (Fabry-Perot type semiconductor laser diodes) have relatively large linewidths Δf of emitted light L1, but are inexpensive. Therefore, by using these as light source 2, the cost of the laser interferometer 1 can be reduced.

[0059] On the other hand, among the aforementioned light sources 2, the He-Ne laser or FBG-LD (fiber Bragg grating laser diode) has a relatively small linewidth Δf of the emitted light L1. Therefore, by using these as light sources 2, the accuracy of the laser interferometer 1 can be further improved.

[0060] Furthermore, the optical path length changing unit 15 shown in Figure 3 includes a first reflective element 151 and a second reflective element 152 as multiple optical elements, and a drive unit 153 that changes the distance between the optical elements. The first reflective element 151 and the second reflective element 152 are optical elements that switch the optical path 20 through which the transmitted light L1a (first split light) travels. The drive unit 153 has the function of moving at least one of the multiple optical elements.

[0061] With this optical path length changing unit 15, the distance the optical element moves by the drive unit 153 and the amount of change in the optical path length of the optical path 20 can be easily correlated. Therefore, an optical path length changing unit 15 that allows for easy adjustment of the optical path length can be realized. In addition, actuators that can be used as the drive unit 153 are readily available and offer high precision in adjusting the amount of movement. Thus, with this configuration of optical path length changing unit 15, the optical path length can be adjusted with high precision.

[0062] Furthermore, as mentioned above, the right-angle prism mirrors 154a, 154b, 155a, and 155b each have a light-reflecting surface 150 and are optical elements that reflect transmitted light L1a. By using such optical elements, the structure of the optical path length changing section 15 can be simplified, and losses associated with changing the optical path length can be suppressed.

[0063] Furthermore, the first reflective element 151 and the second reflective element 152 are provided with at least one pair, preferably two or more pairs, of the above-mentioned units. For example, the first reflective element 151 shown in Figure 3 is provided with three pairs of the above-mentioned units, and the second reflective element 152 is provided with two pairs of the above-mentioned units.

[0064] Because the first reflector 151 and the second reflector 152 comprise multiple units, the transmitted light L1a and the reference light L2 each make multiple round trips between the first reflector 151 and the second reflector 152. As a result, the optical path length can be changed to a length greater than the distance the first reflector 151 is moved. In other words, to change the same optical path length, the distance the first reflector 151 needs to be moved is smaller. For this reason, the optical path length changing unit 15 shown in Figure 3 can be easily miniaturized.

[0065] The configuration of the optical path length changing unit 15 shown in Figure 2 has been described above, but the configuration of the optical path length changing unit 15 is not limited to the above, as long as it can change the optical path length of the optical path 20.

[0066] 1.1.7.2. Second Configuration Example of Optical Path Length Changing Section Next, the optical path length changing unit 15 according to the second configuration example will be described. Figure 4 is a diagram showing the optical path length changing unit 15 according to the second configuration example.

[0067] The optical path length changing unit 15 shown in Figure 4 is the same as the optical path length changing unit 15 shown in Figure 3, except for the following differences. In the following explanation, the differences from the optical path length changing unit 15 shown in Figure 3 will not be explained.

[0068] The first reflecting element 151 shown in Figure 4 is equipped with a roof prism mirror 158. The second reflecting element 152 shown in Figure 4 is equipped with a roof prism mirror 159. The roof prism mirrors 158 and 159 are also called hollow retroreflectors and correspond to elements that integrate the pair of right-angle prism mirrors 154a and 154b provided in the first configuration example described above. By using such roof prism mirrors 158 and 159, the number of parts constituting the first reflecting element 151 and the second reflecting element 152 can be reduced compared to the first configuration example. For this reason, the optical path length changing unit 15 shown in Figure 4 is easy to assemble and miniaturize.

[0069] 1.1.7.3. Third Configuration Example of Optical Path Length Changing Section Next, the optical path length changing unit 15 according to the third configuration example will be described. Figure 5 is a diagram showing the optical path length changing unit 15 according to the third configuration example.

[0070] The optical path length changing unit 15 shown in Figure 5 is the same as the optical path length changing unit 15 shown in Figure 3, except for the following differences. In the following explanation, the differences from the optical path length changing unit 15 shown in Figure 3 will not be explained.

[0071] The first reflecting element 151 shown in Figure 5 includes an integrated prism mirror 160. The second reflecting element 152 shown in Figure 5 also includes an integrated prism mirror 161. The integrated prism mirror 160 corresponds to an element that integrates the multiple right-angle prism mirrors 154a and 154b provided in the first reflecting element 151 according to the first configuration example described above. Similarly, the integrated prism mirror 161 corresponds to an element that integrates the multiple right-angle prism mirrors 155a and 155b provided in the second reflecting element 152 according to the first configuration example described above. By using such integrated prism mirrors 160 and 161, the number of parts constituting the first reflecting element 151 and the second reflecting element 152 can be reduced compared to the first and second configuration examples. For this reason, the optical path length changing unit 15 shown in Figure 5 is particularly easy to assemble and miniaturize.

[0072] The integrated prism mirrors 160 and 161 are manufactured by forming a structure from, for example, glass or resin, and then depositing a high-reflectivity mirror film on the surface of the structure. Examples of high-reflectivity mirrors include dielectric multilayer films and metal films.

[0073] 1.2. Control Unit The control unit 57 controls the operation of the sensor head unit 51 and the demodulation circuit 52.

[0074] Specifically, the control unit 57 operates in at least two operating modes, an optical system adjustment mode and a measurement mode, which will be described later. In the optical system adjustment mode, the control unit 57 adjusts the optical path length of the optical path 20 using the optical path length changing unit 15 and sets it to the desired optical path length. In the measurement mode, it performs measurements on the object to be measured 14 while maintaining the set optical path length.

[0075] Figure 6 is a block diagram showing an example of the hardware configuration of the control unit 57 shown in Figure 1. The hardware configuration of the control unit 57 shown in Figure 6 includes a processor 571, a memory 572, and an external interface 573, all connected to each other by an internal bus 570. The processor 571 reads and executes a program stored in the memory 572, thereby enabling various controls by the control unit 57, such as switching between optical system adjustment mode and measurement mode, and controlling the operation of the sensor head 51 and demodulation circuit 52 in each mode.

[0076] Examples of processors 571 include CPU (Central Processing Unit) and DSP (Digital Signal Processor). Examples of memory include volatile memory such as RAM (Random Access Memory), non-volatile memory such as ROM (Read Only Memory), and removable external storage devices. Examples of external interfaces include USB (Universal Serial Bus), RS-232C, Ethernet (registered trademark), and wireless LAN (Local Area Network).

[0077] Furthermore, part or all of the control unit 57 may be implemented by hardware such as an LSI (Large Scale Integration), ASIC (Application Specific Integrated Circuit), or FPGA (Field-Programmable Gate Array).

[0078] Furthermore, the laser interferometer 1 shown in Figure 1 includes a display unit 58 connected to a control unit 57. The control unit 57 controls the display operation of the display unit 58. This allows the demodulation processing results of the demodulation circuit 52, error details, notification details, etc., to be displayed on the display unit 58 and communicated to the user. Examples of the display unit 58 include liquid crystal displays.

[0079] 1.3. Demodulation Circuit The demodulation circuit 52 performs demodulation processing to demodulate the sample signal originating from the object to be measured 14 from the light-receiving signal output from the light-receiving element 10. The sample signal includes, for example, phase information and frequency information. From the phase information, displacement information of the object to be measured 14 can be obtained, and from the frequency information, velocity information of the object to be measured 14 can be obtained. By obtaining different information in this way, the laser interferometer 1 can be given functions as a displacement meter and a velocity meter, thereby enhancing its functionality.

[0080] The demodulation process by the demodulation circuit 52 can be performed using a known frequency-modulated wave demodulator. The reference signal Ss generated by the signal generator 59 is used for the demodulation process.

[0081] 2. Second Embodiment Next, a control method for the laser interferometer according to the second embodiment will be described.

[0082] Figure 7 is a flowchart illustrating the control method of the laser interferometer according to the second embodiment.

[0083] The control method shown in Figure 7 is, for example, a method for controlling the operation of the laser interferometer 1 according to the first embodiment, and includes a preparation step S102, a measurement step S104, a termination determination step S106, an optical path length change step S108, a measured value comparison step S110, and an optical path length setting step S112. In this control method, as described above, the operation of the optical path length change unit 15 is controlled so that the optical path length of the optical path 22 of the laser interferometer 1 and the optical path length of the optical path 20 become equal. Each step will be described below.

[0084] 2.1. Preparation process In preparation step S102, a sample with a known displacement or velocity is prepared as the object to be measured 14. Examples of such samples include piezo actuators and oscillators. The sample is placed at the same location where the object to be measured 14 is positioned in the measurement mode.

[0085] 2.2. Measurement Process In measurement step S104, the displacement or velocity of the sample is measured using the laser interferometer 1. The optical path length of the optical path 20 during measurement is defined as the "first length". The measured value when the optical path length is the first length is defined as the "first measured value".

[0086] 2.3. Termination Decision Process In the termination decision step S106, a decision is made on whether or not to terminate the measurement based on whether the measurement values ​​to be compared in the measurement value comparison step S110, described later, are all available. Specifically, if the measurement values ​​to be compared with the first measurement value are all available, YES is selected in the termination decision step S106 and the measurement is terminated. On the other hand, if the measurement values ​​to be compared with the first measurement value are not all available, NO is selected in the termination decision step S106 and the process proceeds to the optical path length change step S108.

[0087] 2.4. Optical path length change process In the optical path length changing step S108, the optical path length of optical path 20 is changed. The change in the optical path length of optical path 20 can be performed in any pattern, but it is preferable to repeat the change with a constant change width so as to scan a specific range. This allows for efficient searching of the optical path length to be set.

[0088] Here, the optical path length of optical path 20 is referred to as the "second length". Then, the process returns to measurement step S104.

[0089] 2.5. Measurement Process In the second measurement step S104, the displacement or velocity of the sample is measured again using the laser interferometer 1. The measurement value when the optical path length is the second length is then defined as the "second measurement value". After that, the process proceeds to the termination determination step S106.

[0090] 2.6. Termination Decision Process In the second termination decision step S106, the decision to terminate the measurement is again made based on whether or not the measurement values ​​to be compared are available. Here, since the first and second measurement values ​​have already been acquired, the measurement is terminated, assuming that the measurement values ​​are available. The number of measurement values ​​is set appropriately according to the range and magnitude of the change in the optical path length to be searched.

[0091] 2.7. Measurement Value Comparison Process In the measurement value comparison step S110, the acquired first and second measurement values ​​are compared, and the optical path length to be set in the optical path 20 is determined based on the comparison result. For example, when the first measurement value is close to the known displacement or velocity of the sample, the first length is determined as the optical path length to be set. Also, when the second measurement value is close to the known displacement or velocity of the sample, the second length is determined as the optical path length to be set. If there are many measurement values, the optical path length obtained when the measurement value closest to the displacement or velocity of the sample is obtained is determined as the optical path length to be set.

[0092] 2.8. Optical path length setting process In the optical path length setting step S112, the optical path length of optical path 20 is set to the length identified in the measurement value comparison step S110. This makes it possible to bring the optical path difference d in equation (1) above closer to 0. As a result, the measurement accuracy Δd of the displacement becomes less susceptible to the influence of the line width Δf of the emitted light L1 and the fluctuations Δn of the refractive index of the atmosphere.

[0093] Each of the above steps is the operation of the control unit 57 in the optical system adjustment mode described above. In measurement mode, measurements are taken on the object to be measured 14 while maintaining the optical path length of the optical path 20 set in the optical system adjustment mode. This makes it possible to accurately measure the displacement of the object to be measured 14 regardless of the type of light source 2.

[0094] As described above, the control method for the laser interferometer according to this embodiment is a method for controlling a laser interferometer 1 which includes the aforementioned light source 2 (laser light source), polarizing beam splitter 4 (optical divider), optical modulator 12, optical path length changing unit 15, and light receiving element 10, and comprises a preparation step S102, a measurement step S104, an optical path length changing step S108, a measurement value comparison step S110, and an optical path length setting step S112.

[0095] In preparation step S102, a sample with a known displacement or velocity is prepared as the object to be measured 14.

[0096] In the first measurement step S104, when the optical path length between the polarizing beam splitter 4 (optical divider) and the optical modulator 12 (optical path length of optical path 20) is a first length, a light-receiving signal from the photodetector 10 is acquired, and a first measurement value, which is the displacement or velocity of the sample, is calculated based on the acquired light-receiving signal.

[0097] In the optical path length changing process S108, the operation of the optical path length changing unit 15 is controlled so that the optical path length of the optical path 20 becomes the second length.

[0098] In the second measurement step S104, when the optical path length of the optical path 20 is the second length, a light-receiving signal is acquired from the light-receiving element 10, and a second measurement value, which is the displacement or velocity of the sample, is calculated based on the acquired light-receiving signal.

[0099] In the measurement value comparison step S110 and the optical path length setting step S112, when the first measurement value is close to the known displacement or velocity of the sample, the optical path length of the optical path 20 is set to the first length, and when the second measurement value is close to the known displacement or velocity of the sample, the optical path length of the optical path 20 is set to the second length. Note that when setting the optical path length of the optical path 20 to the "first length" or "second length," it is not necessary to set it to the exact same length as the optical path length of the optical path 20 when the first measurement value or second measurement value was obtained in the measurement step S104; it may be slightly different. In other words, the "first length" and "second length" in the optical path length setting step S112 may each be concepts with a predetermined width, as long as the optical path length of the optical path 20 at the time of measurement is included within the range and there is no overlap between the ranges.

[0100] According to this control method, the optical path difference d in equation (1) above can be brought close to 0, and it is possible to easily set up an optical system 50 in which the displacement measurement accuracy Δd is less affected by the line width Δf of the emitted light L1. Furthermore, by using the optical system 50 adjusted in this way, the displacement of the object to be measured 14 can be measured with high accuracy regardless of the type of light source 2.

[0101] Furthermore, by adjusting the optical system 50 as described above, the measurement accuracy Δd of the displacement is less likely to decrease even when using a light source 2 with a relatively large linewidth Δf of the emitted light L1. This allows for a wider range of choices for the light source 2 without sacrificing the measurement accuracy Δd of the displacement.

[0102] 3. Third Embodiment Next, a laser interferometer according to the third embodiment will be described.

[0103] Figure 8 is a functional block diagram showing a laser interferometer according to the third embodiment. Figure 9 is a schematic configuration diagram of the sensor head unit 51 shown in Figure 8.

[0104] The third embodiment will be described below, focusing on the differences from the first embodiment, and similar matters will be omitted from the description. In each figure, components similar to those in the first embodiment are denoted by the same reference numerals.

[0105] The third embodiment is the same as the first embodiment, except that the configuration of the optical modulator is different. In the laser interferometer 1 according to the first embodiment described above, the optical modulator 12 is equipped with an AOM 60. In contrast, in the laser interferometer 1A according to the third embodiment, the optical modulator 12 is equipped with a vibrating element 30. Even with such an optical modulator 12, it is possible to shift the frequency of the transmitted light L1a and generate a reference light L2, similar to the optical modulator 12.

[0106] 3.1. Overview of the First Configuration Example of an Optical Modulator Figure 10 is a perspective view showing a first configuration example of the optical modulator 12 shown in Figure 9.

[0107] The frequency shifter type optical modulator 12 has an optical modulation oscillator 120. The optical modulation oscillator 120 shown in Figure 10 comprises a plate-shaped vibrating element 30 and a substrate 31 that supports the vibrating element 30.

[0108] The vibrating element 30 is made of a material that repeatedly vibrates in a mode that deforms in the direction along its surface when an electric potential is applied. In this example configuration, the vibrating element 30 is a quartz AT resonator that vibrates in thickness shear along the vibration direction 36 in the high-frequency region of the MHz band. A diffraction grating 34 is formed on the surface of the vibrating element 30. The diffraction grating 34 has a structure in which a plurality of linear grooves 32 are arranged periodically.

[0109] The substrate 31 has a front surface 311 and a back surface 312, which are in a front-back relationship with each other. A vibrating element 30 is placed on the front surface 311. The front surface 311 is also provided with a pad 33 for applying an electric potential to the vibrating element 30. On the other hand, the back surface 312 is also provided with a pad 35 for applying an electric potential to the vibrating element 30.

[0110] The size of the substrate 31 is, for example, approximately 0.5 mm to 10.0 mm on its longest side. The thickness of the substrate 31 is, for example, approximately 0.10 mm to 2.0 mm. As an example, the shape of the substrate 31 is a square with sides of 1.6 mm and a thickness of 0.35 mm.

[0111] The size of the vibrating element 30 is, for example, approximately 0.2 mm to 3.0 mm on its longest side. The thickness of the vibrating element 30 is, for example, approximately 0.003 mm to 0.5 mm.

[0112] For 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. By changing the thickness of the vibrating element 30 or by considering overtones, it is possible to adjust the oscillation frequency within the range of 1 MHz to 1 GHz.

[0113] In Figure 10, the diffraction grating 34 is formed on the entire surface of the vibrating element 30, but it may be formed only on a portion of the surface.

[0114] The intensity of optical modulation by the optical modulator 12 is given by the dot product of the difference wave vector between the wave vector of the transmitted light L1a incident on the optical modulator 12 and the wave vector of the reference light L2 emitted from the optical modulator 12, and the vector of the vibration direction 36 of the vibrating element 30. In this configuration example, the vibrating element 30 vibrates with thickness shear, but since this vibration is in-plane vibration, optical modulation cannot be achieved even if light is incident perpendicularly to the surface of the vibrating element 30 alone. Therefore, in this configuration example, by providing a diffraction grating 34 on the vibrating element 30, optical modulation is made possible by the principle described later.

[0115] The diffraction grating 34 shown in Figure 10 is a blazed diffraction grating. A blazed diffraction grating is a diffraction grating whose cross-sectional shape is stepped. The linear grooves 32 of the diffraction grating 34 are provided so that their extension direction is perpendicular to the vibration direction 36.

[0116] When a drive signal Sd is supplied (an AC voltage is applied) from the oscillation circuit 54 shown in Figure 9 to the vibrating element 30 shown in Figure 10, the vibrating element 30 oscillates. The power required for the oscillation of the vibrating element 30 (drive power) is not particularly limited, but is small, ranging from about 0.1 μW to 100 mW. Therefore, the drive signal Sd output from the oscillation circuit 54 can be used to make the vibrating element 30 oscillate without amplification.

[0117] Furthermore, the vibrating element 30 has a very small volume and requires little power to oscillate. Therefore, by using an optical modulator 12 equipped with the vibrating element 30, it is easy to miniaturize and reduce the power consumption of the laser interferometer 1.

[0118] 3.2. Method for forming diffraction gratings The method for forming the diffraction grating 34 is not particularly limited, but one example is to create a mold using a mechanical engraving method (ruling engine) and then form grooves 32 on the electrodes deposited on the surface of the vibrating element 30 of the quartz AT resonator using a nanoimprint method. The reason for using the electrodes is that, in the case of a quartz AT resonator, high-quality thickness-sliding vibrations can be generated on the electrodes in principle. Note that the grooves 32 are not limited to being formed on the electrodes, but may also be formed on the surface of the non-electrode material. In addition, instead of the nanoimprint method, processing methods such as exposure and etching, electron beam lithography, and focused ion beam (FIB) processing may be used.

[0119] Alternatively, a diffraction grating may be formed on the quartz AT oscillator chip using a resist material, and a mirror film made of a metal film or dielectric multilayer film may be provided thereon. By providing a metal film or mirror film, the reflectivity of the diffraction grating 34 can be increased.

[0120] Furthermore, a resist film may be formed on the chip or wafer of the quartz AT resonator, processed by etching, the resist film may be removed, and then a metal film or mirror film may be formed on the processed surface. In this case, since the resist material is removed, the effects of moisture absorption by the resist material are eliminated, and the chemical stability of the diffraction grating 34 can be improved. In addition, by providing a highly conductive metal film such as Au or Al, it can also be used as an electrode to drive the vibrating element 30.

[0121] The diffraction grating 34 may also be formed using techniques such as anodized alumina (porous alumina).

[0122] 3.3. Other Configuration Examples of Optical Modulators The vibrating element 30 is not limited to a quartz crystal resonator, but may also be a silicon resonator, a ceramic resonator, a surface acoustic wave (SAW) device, or the like.

[0123] Figure 11 is a plan view showing a part of the second configuration example of the optical modulator 12 shown in Figure 9. Figure 12 is a plan view showing the third configuration example of the optical modulator 12 shown in Figure 9.

[0124] The vibrating element 30A shown in Figure 11 is a Si resonator manufactured using MEMS technology. MEMS (Micro Electro Mechanical Systems) refers to micro-electromechanical systems.

[0125] The vibrating element 30A comprises a first electrode 301 and a second electrode 302 adjacent to each other on the same plane with a gap between them, a diffraction grating mounting section 303 provided on the first electrode 301, and a diffraction grating 34 provided on the diffraction grating mounting section 303. The first electrode 301 and the second electrode 302 vibrate, for example, using electrostatic attraction as a driving force, repeatedly approaching and separating from each other in the left-right direction of Figure 11, that is, along the axis connecting the first electrode 301 and the second electrode 302 as shown in Figure 6. This allows in-plane vibration to be imparted to the diffraction grating 34. The oscillation frequency of the Si vibrator is, for example, from 1 kHz to several hundred MHz.

[0126] The vibrating element 30B shown in Figure 12 is a SAW device that utilizes surface waves. SAW (Surface Acoustic Wave) refers to an elastic surface wave.

[0127] The vibration element 30B comprises a piezoelectric substrate 305, a comb-shaped electrode 306 provided on the piezoelectric substrate 305, a ground electrode 307, a diffraction grating mounting section 303, and a diffraction grating 34. When an AC voltage is applied to the comb-shaped electrode 306, surface acoustic waves are excited by the inverse piezoelectric effect. This allows in-plane vibration to be imparted to the diffraction grating 34. The oscillation frequency of the SAW device is, for example, several hundred MHz to several GHz.

[0128] For the devices described above, by providing a diffraction grating 34, optical modulation becomes possible using the principle described later, similar to the case of the quartz AT oscillator.

[0129] On the other hand, if the vibrating element 30 has a quartz crystal oscillator, it is possible to generate a highly accurate modulated signal by utilizing the extremely high Q factor of the quartz crystal. The Q factor is an indicator of the sharpness of the resonance peak. Furthermore, quartz crystal oscillators have the advantage of being less susceptible to external disturbances. Therefore, by using the modulated signal modulated by the optical modulator 12 equipped with a quartz crystal oscillator, it is possible to acquire a sample signal originating from the object to be measured 14 with high accuracy.

[0130] 3.4 Optical Modulation by Vibrating Elements Next, we will explain the principle of modulating light using the vibrating element 30.

[0131] Figure 13 shows the incident light K from a direction perpendicular to the surface of the vibrating element 30. i This is a conceptual diagram illustrating the generation of multiple diffracted light beams when an incident beam is incident on a light source.

[0132] Incident light K is directed to the diffraction grating 34, which is undergoing thickness-sliding vibration along the vibration direction 36. i When incident, due to the diffraction phenomenon, multiple diffracted light K is produced, as shown in Figure 13. ns This occurs. n is the diffracted light Kns The order is such that n = 0, ±1, ±2, ... Note that the diffraction grating 34 shown in Figure 13 is not the blazed diffraction grating shown in Figure 10, but rather an example of a diffraction grating made by repeating concaves and convexities. Also, in Figure 13, the diffracted light K 0s The illustration is omitted.

[0133] In Figure 13, the incident light K i The light is incident on the surface of the vibrating element 30 from a direction perpendicular to it, but this angle of incidence is not particularly limited, and the angle of incidence may be set so that it is incident on the surface of the vibrating element 30 at an angle. When incident at an angle, the diffracted light K ns The direction of movement also changes accordingly.

[0134] Depending on the design of the diffraction grating 34, higher-order light with |n|≧2 may not appear. Therefore, in order to obtain a stable modulation signal, it is desirable to set |n|=1. That is, in the laser interferometer 1A of Figure 9, it is preferable to arrange the frequency shifter type optical modulator 12 so that ±1st order diffracted light is used as the reference light L2. This arrangement makes it possible to stabilize the measurement by the laser interferometer 1A.

[0135] On the other hand, if higher-order light of |n|≧2 appears from the diffraction grating 34, the optical modulator 12 may be arranged so that any of the diffracted light of order ±2 or higher, rather than ±1st-order diffracted light, is used as the reference light L2. This allows the use of higher-order diffracted light, thereby enabling higher frequency and miniaturization of the laser interferometer 1A.

[0136] In this embodiment, as an example, the incident light K incident on the optical modulator 12 i The optical modulator 12 is configured such that the angle between the direction of entry of the light and the direction of propagation of the reference light L2 emitted from the optical modulator 12 is 180°. Three examples are described below.

[0137] Figures 14 to 16 show the incident light K iThis is a conceptual diagram illustrating an optical modulator 12 configured such that the angle between the direction of propagation of the light source and the direction of propagation of the reference light L2 is 180°.

[0138] In Figure 14, the optical modulator 12 is equipped with a mirror 37 in addition to the vibrating element 30. The mirror 37 is diffracted by the light K 1s It is positioned to reflect the light back to the diffraction grating 34. At this time, the diffracted light K directed to the mirror 37 1s The angle between the incident angle and the reflection angle at mirror 37 is 180°. As a result, the diffracted light K emitted from mirror 37 and returned to the diffraction grating 34 1s The incident light K is diffracted again by the diffraction grating 34 and incident into the optical modulator 12. i It will travel in the opposite direction to the direction of travel. Therefore, by adding mirror 37, the incident light K i The condition that the angle between the entry direction of the light and the direction of propagation of the reference light L2 is 180° can be satisfied.

[0139] Furthermore, by passing the light through the mirror 37 in this manner, the reference light L2 generated by the optical modulator 12 undergoes two frequency modulations. Therefore, by using the mirror 37 in combination, higher frequency modulation becomes possible compared to when the vibrating element 30 is used alone.

[0140] In Figure 15, the vibrating element 30 is tilted relative to the arrangement in Figure 13. The tilt angle θ at this time is the same as the incident light K mentioned above. i The setting is such that the angle between the entry direction of the light source and the direction of propagation of the reference light L2 is 180°.

[0141] The diffraction grating 34 shown in Figure 16 has a blaze angle θ. B This is a blazed diffraction grating having the following characteristics: The incident light K propagates at an incident angle β with respect to the normal N of the surface of the vibrating element 30. i When the light is incident on the diffraction grating 34, the blaze angle θ is relative to the normal N. B The reference light L2 will return at the same angle. Therefore, the incident angle β is equal to the blaze angle θ. B By making it equal to the aforementioned incident light Ki The condition that the angle between the entry direction of the light and the propagation direction of the reference light L2 is 180° can be satisfied. In this case, the above condition can be satisfied without using the mirror 37 shown in Figure 14, and without tilting the vibrating element 30 itself as shown in Figure 15, so that the laser interferometer 1 can be further miniaturized and its frequency can be increased. In particular, in the case of a blazed diffraction grating, the arrangement that satisfies the above condition is called the "Littrow arrangement," and it has the advantage of being able to particularly increase the diffraction efficiency of the diffracted light.

[0142] Note that the pitch P in Figure 16 represents the pitch of the blazed diffraction grating, and as an example, the pitch P is assumed to be 1 μm. Also, the blaze angle θ B It is set to 25°. In this case, in order to satisfy the above condition, the incident light K i The angle of incidence β relative to the normal N should also be set to 25°.

[0143] 3.5. Package Structure Figure 17 is a cross-sectional view showing an optical modulator 12 having a package structure.

[0144] The optical modulator 12 shown in Figure 17 comprises a housing container 70, an optical modulation oscillator 120 housed in the container 70, and circuit elements 45 constituting an oscillation circuit 54. The inside of the container 70 is hermetically sealed in a reduced-pressure atmosphere such as a vacuum, or inert gas atmosphere such as nitrogen or argon.

[0145] As shown in Figure 17, the container 70 has a container body 72 and a lid 74. The container body 72 has a first recess 721 provided inside it, 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, etc. Although not shown, the container body 72 is also equipped with internal terminals provided on the inner surface, external terminals provided on the outer surface, wiring connecting the internal terminals and the external terminals, etc.

[0146] Furthermore, the opening of the container body 72 is sealed by a lid 74 via a sealing member such as a sealing ring or low-melting-point glass (not shown). The lid 74 is made of a material that can transmit laser light, such as glass.

[0147] An optical modulation transducer 120 is positioned on the bottom surface of the first recess 721. The optical modulation transducer 120 is supported on the bottom surface of the first recess 721 by a bonding member (not shown). Furthermore, the internal terminals of the container body 72 and the optical modulation transducer 120 are electrically connected via a conductive material (not shown), such as a bonding wire or bonding metal.

[0148] A circuit element 45 is positioned on the bottom surface of the second recess 722. The circuit element 45 is electrically connected to the internal terminals of the container body 72 via bonding wires 76. This also electrically connects the optical modulation transducer 120 and the circuit element 45 via the wiring provided in the container body 72. Note that the circuit element 45 may be provided with circuits other than the oscillation circuit 54 described later.

[0149] By adopting this package structure, the optical modulation transducer 120 and the circuit element 45 can be stacked, reducing their physical distance and shortening the wiring length between them. This suppresses external noise from entering the drive signal Sd, or conversely, prevents the drive signal Sd from becoming a noise source. Furthermore, both the optical modulation transducer 120 and the circuit element 45 can be protected from the external environment within a single container 70. This allows for miniaturization of the sensor head 51 while improving the reliability of the laser interferometer 1.

[0150] The structure of the container 70 is not limited to the illustrated structure; for example, the optical modulation oscillator 120 and the circuit element 45 may have separate package structures. Furthermore, although not shown, the container 70 may house other circuit elements constituting the oscillation circuit 54. The container 70 may be provided as needed and may be omitted.

[0151] 3.6. Oscillator Circuit As shown in Figure 18, the oscillator circuit 54 outputs a drive signal Sd that is input to the optical modulator 12 of the optical system 50. Also, as shown in Figure 8, the oscillator circuit 54 outputs a reference signal Ss that is input to the demodulation circuit 52.

[0152] The oscillator circuit 54 is not particularly limited as long as it is a circuit capable of oscillating the vibration element 30, and various circuit configurations can be used. Figure 18 is a circuit diagram showing the configuration of a single-stage inverter oscillator circuit as an example of the circuit configuration of the oscillator circuit 54.

[0153] The oscillator circuit 54 shown in Figure 18 comprises 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.

[0154] Circuit element 45 is an inverter IC. Terminals X1 and X2 of circuit element 45 are connected to the inverter inside circuit element 45, respectively. Terminal GND is connected to ground potential, and terminal Vcc is connected to power supply potential. Terminal Y is the terminal for oscillation output.

[0155] 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 terminal X2. 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 the limiting resistor Rd.

[0156] Furthermore, one end of the vibrating element 30 is connected between the first capacitor Cg and the feedback resistor Rf, and the other end of the vibrating element 30 is connected between the second capacitor Cd and the limiting resistor Rd. As a result, the vibrating element 30 becomes the signal source for the oscillation circuit 54.

[0157] Figure 19 shows an example of the LCR equivalent circuit of the vibration element 30. As shown in Figure 19, the LCR equivalent circuit of the vibration element 30 consists of a series capacitance C1, a series inductance L1, an equivalent series resistance R1, and a parallel capacitance C0.

[0158] In the oscillator circuit 54 shown in Figure 18, the capacitance of the first capacitor Cg is set to C g And the capacitance of the second capacitor Cd is set to C d In this case, the load capacity C L This is given by equation (a) below.

[0159]

number

[0160] Then, the oscillation frequency f output from terminal Y of the oscillation circuit 54 will be generated. osc This is given by equation (b) below.

[0161]

number

[0162] f Q This is the natural frequency of the vibrating element 30. According to equation (b) above, the load capacity C L By appropriately changing the value, the oscillation frequency f of the signal output from terminal Y can be changed. osc It can be seen that it is possible to fine-tune it.

[0163] Furthermore, the natural frequency f of the vibrating element 30 Q The oscillation frequency f of the oscillation circuit 54 osc The difference Δf between and is given by equation (c) below.

[0164]

number

[0165] Here, C1< <C0、C1<<C L Therefore, Δf is approximately given by the following equation (d).

[0166]

number

[0167] Therefore, the oscillation frequency f of the oscillator circuit 54 osc The natural frequency f of the vibrating element 30 is Q The value will be determined accordingly.

[0168] Here, when the vibrating element 30 is fixed to, for example, the container 70, it is subjected to expansion stress due to temperature through the fixing part, and the natural frequency f Q The frequency f f changes. Also, when the vibrating element 30 is tilted, the natural frequency f changes due to the influence of gravity and other factors. Q It fluctuates.

[0169] For this reason, the oscillator circuit 54 has a natural frequency f Q Even if f f changes, the oscillation frequency f will be adjusted in conjunction with that change based on equation (d) above. osc This means that the oscillation frequency f will change. osc It is always Δf, with natural frequency f Q This results in a value that deviates from the original value. As a result, the vibration of the vibrating element 30 is stabilized, and the modulation characteristics of the optical modulator 12 can be stabilized. Consequently, the demodulation accuracy of the sampled signal in the demodulation circuit 52 can be improved.

[0170] For example, Δf = |f osc -f Q It is preferable that the frequency is ≤ 3000 [Hz], and more preferably 600 [Hz].

[0171] In the laser interferometer 1 according to this embodiment, the optical modulator 12 is equipped with a vibrating element 30. The optical modulator 12 modulates the transmitted light L1a (first split light) using the vibrating element 30.

[0172] With this configuration, the optical modulator 12 can be significantly miniaturized and lightened compared to the first embodiment. This allows for miniaturization and weight reduction of the laser interferometer 1.

[0173] Furthermore, the laser interferometer 1 according to this embodiment includes a demodulation circuit 52 and an oscillation circuit 54. As shown in Figure 8, the oscillation circuit 54 outputs a reference signal Ss to the demodulation circuit 52. Based on the reference signal Ss, the demodulation circuit 52 demodulates the sample signal originating from the object to be measured 14 from the received signal. The vibration element 30 is the signal source for the oscillation circuit 54.

[0174] With this configuration, the natural frequency f of the vibrating element 30 Q Even if it fluctuates, the oscillation frequency f of the oscillation circuit 54 osc The natural frequency f of the vibrating element 30 Q Since the value can be changed accordingly, the vibration of the vibrating element 30 can be easily stabilized. This makes it possible to match the temperature characteristics of the modulated signal to the temperature characteristics of the vibrating element 30, and stabilize the modulation characteristics of the optical modulator 12. As a result, the demodulation accuracy of the sampled signal in the demodulation circuit 52 can be improved.

[0175] Furthermore, in the above configuration, the temperature characteristics of the reference signal Ss output from the oscillation circuit 54 to the demodulation circuit 52 can also be made to correspond to the temperature characteristics of the vibrating element 30. In this case, both the temperature characteristics of the modulated signal and the temperature characteristics of the reference signal correspond to the temperature characteristics of the vibrating element 30, so the behavior of the fluctuations of the modulated signal and the behavior of the fluctuations of the reference signal Ss due to temperature changes will match or approximate. For this reason, even if the temperature of the vibrating element 30 changes, the impact on demodulation accuracy can be suppressed, and the demodulation accuracy of the sample signal originating from the object to be measured 14 can be improved.

[0176] Furthermore, in the first embodiment described above, the power consumption of the laser interferometer 1 tends to be relatively high because it is necessary to maintain the crystal temperature in AOM60. In contrast, in this embodiment, the power consumption of the oscillation circuit 54 is low, making it easy to reduce the power consumption of the laser interferometer 1.

[0177] As described above, the third embodiment also provides the same effects as the first embodiment.

[0178] 4. Fourth Embodiment Next, a laser interferometer according to the fourth embodiment will be described.

[0179] Figure 20 is a schematic diagram of the sensor head 51 of the laser interferometer according to the fourth embodiment. Figure 21 is a partially enlarged view of Figure 20.

[0180] The fourth embodiment will now be described, focusing on the differences from the third embodiment, and similar aspects will be omitted. In each figure, components similar to those in the third embodiment are denoted by the same reference numerals.

[0181] The fourth embodiment is the same as the third embodiment, except that the configuration of the optical path length changing section is different.

[0182] In the laser interferometer 1A according to the third embodiment described above, the optical path length changing unit 15 includes a first reflecting element 151 and a second reflecting element 152. In contrast, in the laser interferometer 1B according to the fourth embodiment, as shown in Figure 20, the optical path length changing unit 15A includes a refractive index variable element 171 and an input unit 172. The refractive index variable element 171 is placed in the optical path 20 through which the transmitted light L1a (first split light) travels, and its refractive index changes according to the input signal. The input unit 172 inputs a signal to the refractive index variable element 171.

[0183] In such an optical path length changing unit 15A, the optical path length of the optical path 20 can be changed by changing the refractive index. Furthermore, since the optical path length changing unit 15A does not have any movable parts, it has higher durability and reliability.

[0184] The refractive index variable material 171 is a light-transmitting medium placed on the optical path 20, and its refractive index changes when controlled by inputs such as electric fields, magnetic fields, heat, or light as control parameters (signals). For example, polymer-dispersed liquid crystals are known as a medium that uses an electric field as a control parameter. The refractive index of polymer-dispersed liquid crystals changes depending on the magnitude of the applied electric field.

[0185] By using a polymer-dispersed liquid crystal as the refractive index variable element 171, the refractive index can be changed using an easily controllable electric field as the control parameter. This allows for a simpler configuration of the optical path length changing section 15A.

[0186] Examples of media other than polymer-dispersed liquid crystals include media with temperature dependence of refractive index. Such media include, for example, inorganic materials such as quartz glass and organic materials such as acrylic resin. In this case, the input unit 172 is a temperature control unit that inputs heat as a control parameter, and for example, a heat exchange element such as a Peltier element. The optical path length L of the optical path 20 in the fourth embodiment 20 This can be calculated using the following formula (2).

[0187]

number

[0188] l in equation (2) above q1 ,l q2 and l q3 This corresponds to the physical distance schematically shown in Figure 21.

[0189] When adjusting the refractive index of the refractive index variable element 171, the optical path length L of the optical path 20 is adjusted based on the above formula (2). 20 Calculate the optical path length L based on that. 20 This can be set to the desired value.

[0190] As described above, the fourth embodiment also provides the same effects as the third embodiment.

[0191] Although the laser interferometer and the control method for the laser interferometer of the present invention have been described above based on the illustrated embodiments, the laser interferometer of the present invention is not limited to the above embodiments, and the configuration of each part can be replaced with any configuration having a similar function. Furthermore, the laser interferometer according to the above embodiment may have other arbitrary components added to it.

[0192] Furthermore, the laser interferometer of the present invention may include any two or more of the embodiments and configuration examples described above.

[0193] Furthermore, the control method for the laser interferometer of the present invention may be modified by adding any desired steps to the above embodiment. [Explanation of Symbols]

[0194] 1…Laser interferometer, 1A…Laser interferometer, 1B…Laser interferometer, 2…Light source, 3…Collimating lens, 4…Polarizing beam splitter, 5…Half wave plate, 6…Quarter wave plate, 8…Quarter wave plate, 9…Analyzer, 10…Photodetector, 12…Optical modulator, 14…Object to be measured, 15…Optical path length changer, 15A…Optical path length changer, 18…Optical path, 20…Optical path, 22…Optical path, 24…Optical path, 30…Vibrating element, 30A…Vibrating element, 30B…Vibrating element Child, 31...Substrate, 32...Groove, 33...Pad, 34...Diffraction grating, 35...Pad, 36...Vibration direction, 37...Mirror, 45...Circuit element, 50...Optical system, 51...Sensor head, 52...Demodulation circuit, 54...Oscillation circuit, 57...Control unit, 58...Display unit, 59...Signal generator, 60...AOM, 70...Container, 72...Container body, 74...Lid, 76...Bonding wire, 120...Optical modulation transducer, 150...Optical reflective surface, 151...First reflective element, 152...Second reflecting element, 153...Drive unit, 154a...Right-angle prism mirror, 154b...Right-angle prism mirror, 155a...Right-angle prism mirror, 155b...Right-angle prism mirror, 156...Substrate, 157...Substrate, 158...Roof prism mirror, 159...Roof prism mirror, 160...Integrated prism mirror, 161...Integrated prism mirror, 171...Refractive index variable element, 172...Input unit, 301...First electrode, 302...Second electrode ,303...Diffraction grating mounting section,305...Piezoelectric substrate,306...Comb-shaped electrode,307...Ground electrode,311...Front surface,312...Back surface,531...Current-voltage converter,570...Internal bus,571...Processor,572...Memory,573...External interface,721...First recess,722...Second recess,C3...Third capacitor,Cd...Second capacitor,Cg...First capacitor,C0...Parallel capacitance,C1...Series capacitance,GND...Terminal,K 0s ...diffracted light, K 1s ...diffracted light, K i ...incident light, K ns...diffracted light, L1...series inductance, L1...emitted light, L1a...transmitted light, L1b...reflected light, L2...reference light, L3...object light, N...normal, P...pitch, R1...equivalent series resistance, Rd...limiting resistance, Rf...feedback resistance, S102...preparation process, S104...measurement process, S106...termination decision process, S108...optical path length change process, S110...measured value comparison process, S112...optical path length setting process, Sd...drive signal, Ss...reference signal, Vcc...terminal, X1...terminal, X2...terminal, Y...terminal, β...incident angle, θ...tilt angle, θ B ...blaze angle

Claims

1. A laser light source that emits light, An optical splitter that splits the emitted light into a first split beam and a second split beam that is incident on the object to be measured, An optical modulator is positioned in the optical path through which the first divided light travels, and includes a vibrating element, which modulates the first divided light into a reference light of a different frequency by causing the first divided light to be incident on the vibrating vibrating element, An optical path length changing unit is provided between the optical splitter and the optical modulator to change the first optical path length, which is the optical path length between the optical splitter and the optical modulator. A light-receiving element that receives the interference light between the object light generated by the reflection of the emitted light from the object to be measured and the reference light, and outputs a received signal. A control unit controls the operation of the optical path length changing unit according to the optical path difference between the second optical path length, which is the optical path length between the optical splitter and the object to be measured, and the first optical path length. An oscillator circuit that outputs a reference signal using the aforementioned vibrating element as a signal source, A demodulator that receives the reference signal output from the oscillation circuit and demodulates a sample signal originating from the object to be measured from the received light signal based on the reference signal, A laser interferometer characterized by being equipped with the following features.

2. The laser interferometer according to claim 1, wherein the control unit controls the operation of the optical path length changing unit to reduce the optical path difference between the first optical path length and the second optical path length.

3. When the optical path difference between the first optical path length and the second optical path length is d [m] and the line width of the emitted light is Δf [Hz], the control unit sets 0 ≤ dΔf ≤ 1 × 10 ８ The laser interferometer according to claim 1 or 2, wherein the operation of the optical path length changing unit is controlled to satisfy the requirements.

4. The optical path length changing section is, Multiple optical elements that switch the optical path through which the first divided light travels, A drive unit that changes the distance between optical elements by moving at least one of the multiple optical elements, A laser interferometer according to any one of claims 1 to 3.

5. The laser interferometer according to claim 4, wherein the optical element is an element having a light-reflecting surface that reflects the first divided light.

6. The laser interferometer according to claim 5, wherein the optical element is a right-angle prism mirror or a roof prism mirror.

7. The laser interferometer according to claim 5 or 6, wherein the light-reflecting surface reflects the first split light multiple times.

8. The laser interferometer according to any one of claims 1 to 7, wherein the vibrating element is a quartz crystal resonator, a Si resonator, a ceramic resonator, or a surface acoustic wave device.

9. The oscillator circuit comprises circuit elements, The aforementioned optical modulator is Container and The vibrating element housed in the aforementioned container, The circuit element housed in the container, A laser interferometer according to any one of claims 1 to 8, comprising:

10. The laser interferometer according to any one of claims 1 to 9, wherein the demodulator acquires frequency information of the object to be measured from the sample signal and acquires velocity information of the object to be measured from the frequency information of the object to be measured.

11. The laser interferometer according to any one of claims 1 to 9, wherein the demodulator acquires phase information of the object to be measured from the sample signal and acquires displacement information of the object to be measured from the phase information of the object to be measured.

12. The optical path length changing section is, A refractive index variable body is placed in the optical path through which the first divided light travels, and whose refractive index changes according to the input signal, An input unit for inputting the signal to the variable refractive index body, A laser interferometer according to claim 1, having the following features.

13. The laser interferometer according to claim 12, wherein the refractive index variable body is composed of a polymer-dispersed liquid crystal.

14. The laser interferometer according to any one of claims 1 to 13, wherein the laser light source is a vertical-cavity surface-emitting laser diode or a Fabry-Perot type semiconductor laser diode.

15. The laser interferometer according to any one of claims 1 to 13, wherein the laser light source is a He-Ne laser or a laser diode with a fiber Bragg grating.

16. A method for controlling a laser interferometer according to any one of claims 1 to 15, A step of preparing a sample whose displacement or velocity is known as the object to be measured, When the optical path length between the optical splitter and the optical modulator is a first length, the process includes acquiring the received light signal and calculating a first measured value, which is the displacement or velocity of the sample, based on the acquired received light signal. A step of controlling the operation of the optical path length changing unit so that the optical path length between the optical splitter and the optical modulator becomes a second length, When the optical path length between the optical splitter and the optical modulator is the second length, the process includes acquiring the received light signal and calculating a second measured value, which is the displacement or velocity of the sample, based on the acquired received light signal. The process involves setting the optical path length between the optical splitter and the optical modulator to a first length when the first measured value is close to the known displacement or velocity of the sample, and setting the optical path length between the optical splitter and the optical modulator to a second length when the second measured value is close to the known displacement or velocity of the sample. A method for controlling a laser interferometer, characterized by having the following features.

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