Laser interferometer and control method for laser interferometer

Abstract

Provided are a laser interferometer and a control method for the laser interferometer, which can measure information originating from a measurement target with high precision regardless of the type of laser light source. The laser interferometer is characterized by including: a laser light source that emits outgoing light; a light splitter that splits the outgoing light into first split light and second split light that is incident on a measurement target; a light modulator that is disposed in an optical path in which the first split light travels, and that modulates the first split light into reference light having different frequencies; an optical path length changing portion that is provided between the light splitter and the light modulator, and that changes a first optical path length that is an optical path length between the light splitter and the light modulator; a light receiving element that receives interference light between object light that is generated by the outgoing light being reflected by the measurement target and the reference light, and that outputs a light receiving signal; and a control portion that controls an operation of the optical path length changing portion in accordance with a second optical path length that is an optical path length between the light splitter and the measurement target.

Description

Technical Field

[0001] This invention relates to a laser interferometer and a control method for a laser interferometer. Background Technology

[0002] Patent Document 1 discloses a laser vibrator that irradiates an object with a laser and measures the vibration velocity based on the scattered laser light subjected to Doppler displacement. This laser vibrator extracts the vibration velocity of the object from the Doppler signal included in the scattered laser light.

[0003] Furthermore, the laser vibrator described in Patent Document 1 includes an acousto-optic modulator (AOM) that shifts the frequency of a laser by changing the frequency of the supplied ultrasonic wave. By using this acousto-optic modulator to shift the frequency of the laser, the frequency-shifted laser can be used as a reference light, thereby enabling the extraction of the vibration velocity of an object from the Doppler signal.

[0004] Patent Document 1: Japanese Patent Application Publication No. 2007-285898

[0005] In the method described in Patent Document 1, the measurement results are affected by the characteristics of the laser. That is to say, there is a problem that the accuracy of the measured speed of the object decreases depending on the type of laser source. Summary of the Invention

[0006] The laser interferometer described in the application example of the present invention is characterized by comprising: a laser source that emits outgoing light; a light splitter that splits the outgoing light into a first split light and a second split light incident on a measurement object; a light modulator disposed in the optical path of the first split light and modulating the first split light into reference light of a different frequency; an optical path length changing unit disposed between the light splitter and the light modulator, which changes a first optical path length, which is the optical path length between the light splitter and the light modulator; a light receiving element that receives the interference light between the object light generated by the outgoing light reflected by the measurement object and the reference light, and outputs a light receiving signal; and a control unit that controls the operation of the optical path length changing unit according to the second optical path length, which is the optical path length between the light splitter and the measurement object.

[0007] The control method for a laser interferometer according to an application example of the present invention is characterized in that the laser interferometer comprises: a laser source emitting outgoing light; a light splitter dividing the outgoing light into a first split light and a second split light incident on a measurement object; a light modulator disposed in the optical path of the first split light, modulating the first split light into reference light of different frequencies; an optical path length changing unit disposed between the light splitter and the light modulator, changing the optical path length between the light splitter and the light modulator; and a light receiving element receiving the interference light between the object light generated by the reflection of the outgoing light from the measurement object and the reference light, and outputting a light receiving signal. The control method for the laser interferometer includes the following steps: preparing a sample with known displacement or velocity as the measurement object; the optical path between the light splitter and the light modulator... When the length is a first length, the received light signal is acquired, and a first measured value as the displacement or velocity of the sample is calculated based on the acquired light signal; the operation of the optical path length changing unit is controlled so that the optical path length between the optical splitter and the optical modulator reaches a second length; when the optical path length between the optical splitter and the optical modulator is the second length, the received light signal is acquired, and a second measured value as the displacement or velocity of the sample is calculated based on the acquired light signal; and when the first measured value is close to the known displacement or velocity of the sample, the optical path length between the optical splitter and the optical modulator is set to the first length, and when the second measured value is close to the known displacement or velocity of the sample, the optical path length between the optical splitter and the optical modulator is set to the second length. Attached Figure Description

[0008] Figure 1 This is a functional block diagram illustrating the laser interferometer according to the first embodiment.

[0009] Figure 2 It is shown Figure 1 The diagram shows a schematic representation of the sensor head.

[0010] Figure 3 yes Figure 2 The enlarged view is a diagram showing the optical path length change section involved in the first configuration example.

[0011] Figure 4 This is a diagram showing the optical path length change section involved in the second configuration example.

[0012] Figure 5 This is a diagram showing the optical path length change section involved in the third configuration example.

[0013] Figure 6 It is shown Figure 1 The diagram shows an example of the hardware configuration of the control unit.

[0014] Figure 7 This is a flowchart illustrating the control method of the laser interferometer according to the second embodiment.

[0015] Figure 8 This is a functional block diagram illustrating the laser interferometer according to the third embodiment.

[0016] Figure 9 yes Figure 8 The diagram shows a schematic representation of the sensor head.

[0017] Figure 10 It is shown Figure 9 A perspective view of a first example of a light modulator.

[0018] Figure 11 It is shown Figure 9 A top view of a portion of a second configuration example of the optical modulator shown.

[0019] Figure 12 It is shown Figure 9 A top view of a third configuration example of the optical modulator shown.

[0020] Figure 13 For incident light K i A conceptual diagram illustrating the generation of multiple diffracted beams when incident from a direction perpendicular to the surface of the vibrating element.

[0021] Figure 14 It is the composition of incident light K i A conceptual diagram illustrating a light modulator whose direction of travel forms an angle of 180° with the direction of travel of reference light L2.

[0022] Figure 15 It is the composition of incident light K i A conceptual diagram illustrating a light modulator whose direction of travel forms an angle of 180° with the direction of travel of reference light L2.

[0023] Figure 16 It is the composition of incident light K i A conceptual diagram illustrating a light modulator whose direction of travel forms an angle of 180° with the direction of travel of reference light L2.

[0024] Figure 17 This is a cross-sectional view showing an optical modulator with a packaged structure.

[0025] Figure 18 The circuit diagram shown is an example of the configuration of a single-stage inverter oscillator circuit.

[0026] Figure 19This is an example of the LCR equivalent circuit of a vibrating element.

[0027] Figure 20 This is a schematic diagram of the sensor head of the laser interferometer according to the fourth embodiment.

[0028] Figure 21 yes Figure 20 A magnified view of a portion of the image.

[0029] Explanation of reference numerals in the attached figures

[0030] 1…Laser interferometer, 1A…Laser interferometer, 1B…Laser interferometer, 2…Light source, 3…Collimating lens, 4…Polarization beam splitter, 5…1 / 2 wavelength plate, 6…1 / 4 wavelength plate, 8…1 / 4 wavelength plate, 9…Analyzer, 10…Light receiving element, 12…Optical modulator, 14…Object to be measured, 15…Optical path length changing unit, 15A…Optical path length changing unit, 18…Optical path, 20…Optical path, 22…Optical path, 24…Optical path, 30…Vibrating element, 30A…Vibrating element, 30 B…Vibration element, 31…Substrate, 32…Groove, 33…Pad, 34…Diffraction grating, 35…Pad, 36…Vibration direction, 37…Mirror, 45…Circuit element, 50…Optical system, 51…Sensor head, 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…Connection line, 120…Optical modulation oscillator, 150…Optical reflecting surface, 151…First 152…Second reflective element, 153…Driver unit, 154a…Right-angle prism reflector, 154b…Right-angle prism reflector, 155a…Right-angle prism reflector, 155b…Right-angle prism reflector, 156…Substrate, 157…Substrate, 158…Roof prism reflector, 159…Roof prism reflector, 160…Integrated prism reflector, 161…Integrated prism reflector, 171…Refractive index variable element, 172…Input unit, 301…First electrode, 302…Second electrode Two electrodes, 303…diffraction grating mounting portion, 305…piezoelectric substrate, 306…comb-shaped electrode, 307…ground electrode, 311…surface, 312…inside, 531…current-to-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 capacitor, C1…series capacitor, GND…terminal, K 0s …diffraction light, K 1s …diffraction light, K i …incident light, K ns…Diffracted light, L1…Series inductance, L1…Outgoing light, L1a…Transmitted light, L1b…Reflected light, L2…Reference light, L3…Object light, N…Normal, P…Spacing, R1…Equivalent series resistance, Rd…Limiting resistance, Rf…Feedback resistance, S102…Preparation process, S104…Measurement process, S106…End judgment process, S108…Optical path length change process, S110…Measurement 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 …Shining corner. Detailed Implementation

[0031] The laser interferometer of the present invention will now be described in detail based on the embodiments shown in the accompanying drawings.

[0032] 1. First Implementation Method

[0033] First, the laser interferometer involved in the first embodiment will be described.

[0034] Figure 1 This is a functional block diagram illustrating the laser interferometer according to the first embodiment.

[0035] Figure 1 The laser interferometer 1 shown includes: a sensor head 51, which has an optical system 50 and a signal generator 59; a demodulation circuit 52, which receives the light signal from the optical system 50; and a control unit 57. The laser interferometer 1 uses the interference of laser light to measure the displacement or velocity of an object.

[0036] 1.1. Sensor Head

[0037] Figure 2 It is shown Figure 1 The diagram shows a schematic configuration of the sensor head 51.

[0038] As described above, the sensor head 51 has an optical system 50.

[0039] like Figure 2 As shown, the optical system 50 includes a light source 2 (laser light source), a collimating lens 3, a polarization beam splitter 4, a half-wave plate 5, a quarter-wave plate 6, a quarter-wave plate 8, an analyzer 9, a light receiving element 10, a frequency-shifting optical modulator 12, and an optical path length changing unit 15.

[0040] Light source 2 emits outgoing light L1 (first laser) of a specified wavelength. Light receiving element 10 converts the received light into an electrical signal. Optical modulator 12, equipped with AOM 60, modulates the outgoing light L1 to generate reference light L2 (second laser) including the modulated signal. The outgoing light L1 incident on the moving measurement object 14 is reflected into object light L3 (third laser) including the sample signal originating from the measurement object 14.

[0041] The optical path of the emitted light L1 from the light source 2 is designated as optical path 18. A half-wave plate 5 and a collimating lens 3 are sequentially arranged on optical path 18, starting from the polarization beam splitter 4 side. Optical path 18 is combined with optical path 20 through transmission from the polarization beam splitter 4. On optical path 20, a quarter-wave plate 8, an optical path length changing unit 15, and an optical modulator 12 are sequentially arranged, starting from the polarization beam splitter 4 side. Optical path 18 is combined with optical path 22 through reflection from the polarization beam splitter 4. On optical path 22, a quarter-wave plate 6 and the object to be measured 14 are sequentially arranged, starting from the polarization beam splitter 4 side. On optical path 24, an analyzer 9 and a light-receiving element 10 are sequentially arranged, starting from the polarization beam splitter 4 side.

[0042] In addition, optical path 20 and optical path 22 are combined with optical path 24 respectively.

[0043] The emitted light L1 from the light source 2 is incident on the light modulator 12 via optical paths 18 and 20. Additionally, the emitted light L1 is incident on the object to be measured 14 via optical paths 18 and 22. The reference light L2 generated by the light modulator 12 is incident on the light receiving element 10 via optical paths 20 and 24. The object light L3 generated by reflection at the object to be measured 14 is incident on the light receiving element 10 via optical paths 22 and 24.

[0044] The following is a further explanation of each part of the optical system 50.

[0045] 1.1.1. Light source

[0046] Light source 2 is a laser source that emits an interferometric outgoing beam L1. Examples of light sources 2 include gas lasers such as He-Ne lasers, semiconductor laser elements such as DFB-LD (Distributed feedback-laser diode), FBG-LD (Fiber bragg grating with laser diode), VCSEL (Vertical Cavity Surface Emitting Laser), and FP-LD (Fabry-Perot Laser Diode).

[0047] The light source 2, which includes a semiconductor laser element, is particularly preferred. This allows for the miniaturization of the light source 2. Consequently, miniaturization of the laser interferometer 1 becomes possible. In particular, miniaturization and weight reduction of the sensor head 51 housing the optical system 50 in the laser interferometer 1 are possible, thus improving the operability of the laser interferometer 1.

[0048] The outgoing light L1 passes sequentially through the collimating lens 3 and the half-wave plate 5 before entering the polarization beam splitter 4. The collimating lens 3 is a lens that sets the transmitted outgoing light L1 as parallel light. The half-wave plate 5 is an optical element that rotates the polarization direction of linearly polarized light.

[0049] 1.1.2. Polarization beam splitter

[0050] The polarization beam splitter 4 is an optical element that splits the outgoing light L1 into transmitted light L1a (first split beam) and reflected light L1b (second split beam). Furthermore, the polarization beam splitter 4 has the function of transmitting P-polarized light and reflecting S-polarized light. Hereinafter, we consider the case where the outgoing light L1, which is linearly polarized and has a P-polarized light to S-polarized light ratio of, for example, 50:50, is incident on the polarization beam splitter 4.

[0051] As described above, the polarization beam splitter 4 reflects the S-polarized light of the outgoing light L1, allowing the P-polarized light to pass through.

[0052] The transmitted light L1a, which is P-polarized light transmitted through the polarization beam splitter 4, is converted into circularly polarized light by the quarter-wave plate 8 and incident on the optical modulator 12. The circularly polarized light L1a incident on the optical modulator 12 receives f m A frequency shift of [Hz] is reflected as reference light L2. Therefore, reference light L2 includes a frequency f. mThe modulation signal is [Hz]. The reference light L2 is converted into S-polarized light when it passes through the 1 / 4 wavelength plate 8 again via the optical path length changing section 15. The S-polarized light of the reference light L2 is reflected by the polarization beam splitter 4, passes through the analyzer 9, and is incident on the light receiving element 10.

[0053] The reflected light L1b, which is S-polarized light reflected by the polarization beam splitter 4, is converted into circularly polarized light by the quarter-wave plate 6 and incident on the moving measurement object 14. The circularly polarized light L1b incident on the measurement object 14 is received by f. d The Doppler shift of [Hz] is reflected as object light L3. Therefore, object light L3 includes a frequency f. d The sample signal is [Hz]. The object light L3 is converted into P-polarized light when it passes through the 1 / 4 wavelength plate 6 again. The P-polarized light of the object light L3 passes through the polarization beam splitter 4, passes through the analyzer 9, and is incident on the light receiving element 10.

[0054] As described above, the emitted light L1 is interferometric, therefore, the reference light L2 and the object light L3 are incident on the light-receiving element 10 as interference light.

[0055] It should be noted that a non-polarized beam splitter can also be used instead of a polarized beam splitter. In this case, the quarter-wave plate 6 and the quarter-wave plate 8 are not needed, thus enabling miniaturization of the laser interferometer 1 by reducing the number of components.

[0056] 1.1.3. Polarizer

[0057] Since the S-polarized and P-polarized beams are orthogonal and independent, they do not exhibit interference-induced beats simply by coinciding. Therefore, the light wave formed by coinciding the S-polarized and P-polarized beams passes through an analyzer 9 tilted at 45° relative to both beams. By using the analyzer 9, shared components of light can be transmitted through each other, resulting in interference. Consequently, within the analyzer 9, the reference beam L2 interferes with the object beam L3, generating a beam with |f|. m -f d Interference light with a frequency of [Hz].

[0058] 1.1.4. Light-receiving element

[0059] Reference light L2 and object light L3 are incident on the light-receiving element 10 via polarization beam splitter 4 and analyzer 9. Thus, reference light L2 and object light L3 undergo heterodyne interference, exhibiting |f... m -f dInterference light with a frequency of [Hz] is incident on the light-receiving element 10. The light-receiving element 10 outputs the received signal of the interference light to the demodulation circuit 52 via a current-to-voltage converter 531. The demodulation circuit 52 demodulates the sample signal from the received signal using a method described later, thereby ultimately determining the motion, i.e., vibration velocity or displacement, of the object to be measured 14. Examples of light-receiving elements 10 include, for example, a photodiode.

[0060] It should be noted that the current-to-voltage converter 531 is a transimpedance amplifier (TIA) that converts the current output from the light-receiving element 10 into a voltage signal.

[0061] 1.1.5. Optical Modulator

[0062] Figure 1 and Figure 2 The optical modulator 12 shown includes an AOM60. The AOM60 is an acousto-optic modulator. An acousto-optic modulator uses a piezoelectric element to vibrate a crystal, generating a standing wave with varying density within the crystal, which is then used as a frequency modulator for a diffraction grating. Figure 1 and Figure 2 Although not illustrated, the AOM60 described above includes the crystal and mirror. Light that has passed through the crystal via optical path 20 is reflected by the mirror and then passes through the crystal again. Thus, the AOM60 generates the reference light L2 from the incident transmitted light L1a.

[0063] A high-frequency signal is input to AOM60 from signal generator 59 (described later). This signal drives the piezoelectric element, causing the crystal to vibrate.

[0064] It should be noted that the optical modulator 12 can also replace the AOM60 and have various optical modulators such as an electro-optic modulator (EOM).

[0065] 1.1.6. Signal Generator

[0066] Signal generator 59 generates a drive signal Sd that is input to AOM60. Additionally, signal generator 59 generates a reference signal Ss that is input to demodulation circuit 52.

[0067] The signal generator 59 only needs to be able to generate signals with good characteristics such as frequency stability and low jitter, and can be any signal generator. Specific examples of signal generators 59 include function generators, signal generators, crystal oscillators, and PLL (Phase Locked Loop) circuits.

[0068] 1.1.7. Optical Path Length Change Section

[0069] Figure 2The optical path length changing unit 15 shown has the function of changing the optical path length of the optical path 20 in which the transmitted light L1a (first split light) travels. The optical path 20 is the optical path that connects the polarization beam splitter 4 and the optical modulator 12.

[0070] 1.1.7.1. First configuration example of the optical path length changing unit

[0071] Figure 2 The optical path length changing unit 15 shown includes a first reflective element 151 and a second reflective element 152, and a driving unit 153 for driving the first reflective element 151. The first reflective element 151 and the second reflective element 152 are optical elements that respectively switch the optical path 20 traveled by the transmitted light L1a. The driving unit 153 changes the distance between the first reflective element 151 and the second reflective element 152 by moving the first reflective element 151.

[0072] The driving unit 153 generates a driving force, for example, to move the first reflecting element 151 parallel to the optical path 20. This allows for a change in the optical path length of the optical path 20. Furthermore, the driving unit 153 holds the first reflecting element 151 at a target position. It should be noted that the driving unit 153 can also be configured to move the second reflecting element 152 without moving the first reflecting element 151, or it can be configured to move both the first reflecting element 151 and the second reflecting element 152. Moreover, the direction of movement is not limited as long as the optical path length of the optical path 20 can be changed.

[0073] As the drive unit 153, a device that moves the first reflective element 151 along a straight line is used, such as a linear stage, an electric actuator, a piezoelectric actuator, etc.

[0074] 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. Consequently, the optical path length (optical distance) between the polarization beam splitter 4 and the optical modulator 12 (optical path 20) also changes. By changing the optical path length of optical path 20 in this way, the optical path length between the polarization 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 become equal, the measurement accuracy of the displacement of the object to be measured 14 can be improved according to the following principle.

[0075] The demodulation circuit 52 can demodulate the sample signal originating from the object to be measured 14 from the received light signal. Furthermore, it can determine the displacement of the object to be measured 14 from the sample signal. When the measurement accuracy of this displacement is set as Δd, the measurement accuracy Δd is expressed by the following formula (1).

[0076]

[0077] λ: Wavelength of the emitted light L1

[0078] Δφ: Phase of the sample signal

[0079] n: Refractive index of the atmosphere

[0080] d: The difference between the optical path length of optical path 22 and the optical path length of optical path 20 (optical path difference)

[0081] Δf: Linewidth of the emitted light L1 (phase fluctuation)

[0082] f: Frequency of emitted light L1

[0083] c: speed of light

[0084] Δn: Fluctuation in the refractive index of the atmosphere

[0085] In equation (1) above, when the optical path difference d is 0, the second and third terms on the right-hand side are also 0. In this case, the displacement measurement accuracy Δd is theoretically unaffected by the linewidth Δf (phase fluctuation) of the emitted light L1 or the atmospheric refractive index fluctuation Δn. On the other hand, when the optical path difference d is not 0, the linewidth Δf or the atmospheric refractive index fluctuation Δn affects the displacement measurement accuracy Δd. In particular, the linewidth Δf of the emitted light L1 can sometimes be a relatively large value depending on the type of light source 2. In this case, the measurement accuracy Δd of the displacement of the object 14 may deteriorate depending on the type of light source 2.

[0086] Therefore, in this 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 optical path 22 and the optical path length of optical path 20 are close to each other. As a result, the displacement measurement accuracy Δd is theoretically less affected by the linewidth Δf [Hz] of the emitted light L1. Consequently, the displacement of the object to be measured 14 can be measured with good accuracy regardless of the type of light source 2. Furthermore, the velocity of the object to be measured 14 can also be calculated from the displacement.

[0087] It should be noted that in typical laser sources, the laser frequency is several hundred THz. This frequency will not change by more than one digit even if the type of laser source is changed. Furthermore, in the above equation (1), the second term is more dominant than the third term. Based on this, it is believed that the value of dΔf in the above equation (1) has a significant impact on the measurement accuracy Δd. The inventors of this invention have found that even when the optical path difference d[m] is not 0, when dΔf ≤ 1 × 10⁻⁶... 8 At that time, sufficient measurement accuracy Δd can also be obtained.

[0088] Table 1 below shows four laser sources, the general linewidth Δf [Hz] of the laser emitted from each source, and the parameters for ensuring dΔf ≤ 1 × 10⁻⁶. 8 A table of reference values ​​for the range of allowed optical path difference d[m].

[0089] Table 1

[0090]

[0091] As shown in Table 1 above, the linewidth Δf varies depending on the type of laser source, and therefore the range of the allowable optical path difference d will also change with this adjustment.

[0092] Figure 3 yes Figure 2 The diagram is a partial enlarged view and shows the optical path length changing section 15 involved in the first configuration example.

[0093] Figure 3 The first reflecting element 151 shown includes right-angle prism mirrors 154a and 154b and a substrate 156 supporting them. The right-angle prism mirrors 154a and 154b are optical elements having light-reflecting surfaces 150 that intersect the light path 20 at an angle of 45°. Furthermore, the right-angle prism mirrors 154a and 154b are arranged such that the angle between their light-reflecting surfaces 150 is 90°. Thus, the light path 20 extending from the polarization beam splitter 4 is refracted by a pair of right-angle prism mirrors 154a and 154b, and faces the second reflecting element 152. The substrate 156 supports multiple pairs of right-angle prism mirrors 154a and 154b.

[0094] Figure 3 The second reflective element 152 shown includes right-angle prism reflectors 155a and 155b and a substrate 157 supporting them. The right-angle prism reflectors 155a and 155b are optical elements having light-reflecting surfaces 150 that intersect the light path 20 at an angle of 45°. Furthermore, the right-angle prism reflectors 155a and 155b are arranged such that the angle between their light-reflecting surfaces 150 is 90°. Thus, the light path 20 extending from the first reflective element 151 is refracted by the units consisting of a pair of right-angle prism reflectors 155a and 155b, and then returns to the first reflective element 151. The substrate 157 supports multiple pairs of right-angle prism reflectors 155a and 155b.

[0095] The right-angle prism reflectors 154a, 154b, 155a, and 155b have high precision and are readily available. Therefore, they are useful as optical elements for the first reflecting element 151.

[0096] By using the first reflective element 151 and the second reflective element 152 to refract the optical path 20, the first reflective element 151 and the second reflective element 152 can be connected by the optical path 20. Furthermore, when the driving unit 153 moves the first reflective element 151, the physical distance between the first reflective element 151 and the second reflective element 152 changes. This also allows the optical path length of the optical path 20 to change. Therefore, according to the optical path length changing unit 15, the optical path length of the optical path 20 can be made close to the optical path length of the optical path 22, preferably equal to it. As a result, the optical path difference d in the above equation (1) can be made close to 0, preferably 0.

[0097] As shown above, the laser interferometer 1 according to this embodiment includes a light source 2 (laser source), a polarization 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 an outgoing light L1. The polarization beam splitter 4 splits the outgoing light L1 into a transmitted light L1a (first split light) and a reflected light L1b (second split light) incident on the object to be measured 14. The optical modulator 12 is disposed in the optical path 20 along which the transmitted light L1a travels, modulating the transmitted light L1a into a reference light L2 with a different frequency. The optical path length changing unit 15 is disposed between the polarization beam splitter 4 and the optical modulator 12, changing the first optical path length (optical path length of optical path 20) between the polarization beam splitter 4 and the optical modulator 12. The light receiving element 10 receives the interference light between the object light L3 generated by the reflection of the outgoing light L1 from 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 based on the second optical path length (optical path length of optical path 22), which is the optical path length between the polarization beam splitter 4 and the object to be measured 14.

[0098] Based on this configuration, the optical path length of optical path 20 can be changed, thus allowing the optical path length of optical path 20 to be adjusted to be close to that of optical path 22. Consequently, the optical path difference d in equation (1) can be made close to 0, and theoretically, the displacement measurement accuracy Δd is less affected by the linewidth Δf of the emitted light L1. As a result, the displacement of the object 14 can be measured with good accuracy regardless of the type of light source 2.

[0099] Furthermore, when using a light source 2 with a relatively large linewidth Δf of emitted light L1, the decrease in displacement measurement accuracy Δd can be suppressed by changing the optical path length of optical path 20, based on the aforementioned principle. Therefore, the options for light source 2 can be expanded without sacrificing displacement measurement accuracy Δd.

[0100] As an example of the operation of the optical path length changing unit 15 controlled by the control unit 57, the operation of the optical path length changing unit 15 can be used to reduce the optical path difference d between the first optical path length (optical path length of optical path 20) and the second optical path length (optical path length of optical path 22).

[0101] Therefore, since the optical path difference d in the above equation (1) is close to 0, the measurement accuracy Δd of the displacement in the laser interferometer 1 is difficult to be affected by the linewidth Δf of the outgoing light L1.

[0102] More specifically, the control unit 57 controls the operation of the optical path length changing unit 15 so that the optical path difference d[m] between the first optical path length and the second optical path length and the linewidth Δf of the emitted light L1 satisfy 0≤dΔf≤1×10 8 .

[0103] By controlling the optical path length changing unit 15 in a way that satisfies this condition, the displacement measurement accuracy Δd in the laser interferometer 1 can be significantly improved even when the linewidth Δf of the emitted light L1 is relatively large.

[0104] Although the VCSEL (Vertical-Cavity Surface-Emitting Laser Diode) or FP-LD (Fabry-Perot Laser Diode) in the aforementioned light source 2 have a relatively large linewidth Δf of the emitted light L1, they are inexpensive. Therefore, by using them as light source 2, the cost of laser interferometer 1 can be reduced.

[0105] On the other hand, the linewidth Δf of the emitted light L1 from the He-Ne laser or FBG-LD (laser diode with fiber Bragg grating) in the aforementioned light source 2 is relatively small. Therefore, by using them as light source 2, further improvements in the precision of the laser interferometer 1 can be achieved.

[0106] in addition, Figure 3 The optical path length changing unit 15 shown includes a first reflective element 151 and a second reflective element 152, which are 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 of the transmitted light L1a (first split light). The drive unit 153 has the function of moving at least one of the multiple optical elements.

[0107] According to this optical path length changing unit 15, the moving distance of the optical element based on the drive unit 153 can be easily correlated with the change in the optical path length of the optical path 20. Therefore, the optical path length changing unit 15, which allows for easy adjustment of the optical path length, can be realized. In addition, the actuator used as the drive unit 153 is readily available, and the accuracy of adjusting the moving amount is high. Therefore, according to this configuration of the optical path length changing unit 15, the optical path length can be adjusted with high precision.

[0108] Furthermore, as described above, the right-angle prism reflectors 154a, 154b, 155a, and 155b are optical elements that each have a light-reflecting surface 150 and reflect transmitted light L1a. By using such optical elements, the structure of the optical path length changing section 15 can be simplified, and losses due to changes in the optical path length can be suppressed.

[0109] Furthermore, the first reflective element 151 and the second reflective element 152 each have at least one pair of the aforementioned units, and preferably two or more pairs. For example, Figure 3 The first reflective element 151 shown has three pairs of the above-described units, and the second reflective element 152 has two pairs of the above-described units.

[0110] By having multiple units in the first reflective element 151 and the second reflective element 152, the transmitted light L1a and the reference light L2 respectively travel back and forth multiple times between the first reflective element 151 and the second reflective element 152. As a result, the optical path length can be changed to be longer than the distance required to move the first reflective element 151. That is, with the same change in optical path length, the distance required to move the first reflective element 151 can be less. Therefore, Figure 3 Miniaturization of the optical path length changing unit 15 shown is easy.

[0111] The above is about Figure 2 The configuration of the optical path length changing unit 15 shown has been described, but the configuration of the optical path length changing unit 15 is not limited to the above content as long as it can change the optical path length of the optical path 20.

[0112] 1.1.7.2. Second configuration example of the optical path length changing unit

[0113] Next, the optical path length changing unit 15 involved in the second configuration example will be explained. Figure 4 This is a diagram showing the optical path length changing unit 15 involved in the second configuration example.

[0114] Figure 4 The optical path length changing unit 15 shown differs from the following in all aspects. Figure 3 The optical path length changing unit 15 shown is the same. In the following description, it refers to the optical path length changing unit 15. Figure 3 The similarities of the optical path length changing unit 15 shown are omitted from the description.

[0115] Figure 4 The first reflective element 151 shown includes a roof prism reflector 158. Figure 4The second reflective element 152 shown includes a roof prism reflector 159. Roof prism reflectors 158 and 159, also referred to as hollow rear-facing reflectors, are respectively equivalent to elements formed by integrating the pair of right-angle prism reflectors 154a and 154b included in the first configuration example. By using these roof prism reflectors 158 and 159, the number of components constituting the first reflective element 151 and the second reflective element 152 can be reduced compared to the first configuration example. Therefore, Figure 4 The assembly and miniaturization of the optical path length changing unit 15 shown is easy.

[0116] 1.1.7.3. Third configuration example of the optical path length changing unit

[0117] Next, the optical path length changing unit 15 involved in the third configuration example will be explained. Figure 5 This is a diagram showing the optical path length changing unit 15 involved in the third configuration example.

[0118] Figure 5 The optical path length changing unit 15 shown differs from the following in that it is similar in other aspects. Figure 3 The optical path length changing unit 15 shown is the same. In the following description, [the following will refer to] the optical path length changing unit 15. Figure 3 The similarities of the optical path length changing unit 15 shown are omitted from the description.

[0119] Figure 5 The first reflective element 151 shown has an integrated prism reflector 160. Figure 5 The second reflective element 152 shown includes an integrated prism reflector 161. The integrated prism reflector 161 is equivalent to an element formed by integrating the plurality of right-angle prism reflectors 154a and 154b included in the first reflective element 151 according to the first configuration example. Similarly, the integrated prism reflector 161 is equivalent to an element formed by integrating the plurality of right-angle prism reflectors 155a and 155b included in the second reflective element 152 according to the first configuration example. By using such integrated prism reflectors 160 and 161, the number of components constituting the first reflective element 151 and the second reflective element 152 can be reduced compared to the first or second configuration example. Therefore, Figure 5 The assembly and miniaturization of the optical path length changing unit 15 shown is particularly easy.

[0120] The integrated prism reflectors 160 and 161 are manufactured, for example, by forming a high-reflectivity reflector on the surface of a molded structure such as glass or resin. Examples of high-reflectivity reflectors include dielectric multilayer films and metal films.

[0121] 1.2. Control Unit

[0122] The control unit 57 controls the operation of the sensor head 51 and the demodulation circuit 52.

[0123] Specifically, the control unit 57 operates in at least two modes: the optical system adjustment mode and the measurement mode, as described later. In the optical system adjustment mode, the control unit 57 adjusts the optical path length of the optical path 20 via the optical path length changing unit 15, setting it to the desired optical path length. In the measurement mode, not only is the set optical path length maintained, but measurements are also performed on the object 14 to be measured.

[0124] Figure 6 It is shown Figure 1 A block diagram showing an example of the hardware configuration of the control unit 57.

[0125] Figure 6 The hardware configuration of the control unit 57 shown includes a processor 571, a memory 572, and an external interface 573 interconnected by an internal bus 570. The processor 571 reads the program stored in the memory 572 and executes it, thereby enabling various controls performed by the control unit 57, such as switching control between optical system adjustment mode and measurement mode, and controlling the operation of sensor head 51 or demodulation circuit 52 in each mode.

[0126] Examples of processors 571 include CPUs (Central Processing Units) and DSPs (Digital Signal Processors). 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).

[0127] Alternatively, part or all of the control unit 57 may be implemented using hardware such as LSI (Large Scale Integration), ASIC (Application Specific Integrated Circuit), or FPGA (Field-Programmable Gate Array).

[0128] in addition, Figure 1The laser interferometer 1 shown includes a display unit 58 connected to a control unit 57. The control unit 57 controls the display operation of the display unit 58. As a result of the demodulation processing of the demodulation circuit 52, error messages, notification messages, etc., can be displayed on the display unit 58, and the user can be notified. For example, a liquid crystal display device can be used as the display unit 58.

[0129] 1.3. Demodulation Circuit

[0130] The demodulation circuit 52 performs demodulation processing on the sample signal originating from the object to be measured 14, which is the light-receiving signal output from the light-receiving element 10. The sample signal contains, for example, phase information and frequency information. Furthermore, displacement information of the object to be measured 14 can be obtained from the phase information, and velocity information of the object to be measured 14 can be obtained from the frequency information. If different information can be obtained in this way, it has the function of a displacement meter or a velocity meter, and thus the laser interferometer 1 can be made highly functional.

[0131] The demodulation process based on demodulation circuit 52 can be performed using a known frequency modulation demodulator. The reference signal Ss generated by signal generator 59 is used in the demodulation process.

[0132] 2. Second Implementation Method

[0133] Next, the control method of the laser interferometer according to the second embodiment will be described.

[0134] Figure 7 This is a flowchart illustrating the control method of the laser interferometer according to the second embodiment.

[0135] Figure 7 The control method shown 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, an end judgment step S106, an optical path length change step S108, a measurement value comparison step S110, and an optical path length setting step S112. In this control method, the operation of the optical path length change unit 15 is controlled as described above, so that the optical path length of the optical path 22 of the laser interferometer 1 becomes equal to the optical path length of the optical path 20. Each step will be described below.

[0136] 2.1. Preparation process

[0137] In preparation step S102, a sample with known displacement or velocity is prepared as the object to be measured 14. Examples of samples include piezoelectric actuators and oscillators. It should be noted that the sample is positioned at the same location as the object to be measured 14 is positioned in the measurement mode.

[0138] 2.2. Measurement Procedure

[0139] In measurement step S104, the displacement or velocity of the sample is measured using a laser interferometer 1. The optical path length of the optical path 20 during measurement is set as the "first length". Furthermore, the measured value when the optical path length is the first length is set as the "first measured value".

[0140] 2.3. End of judgment process

[0141] In the termination judgment step S106, the determination of whether to terminate the measurement is based on whether the measured values ​​that were compared in the measurement value comparison step S110 (described later) are consistent. Specifically, if the measured value of the object being compared with the first measured value is sufficiently consistent with the first measured value, "Yes" is selected in the termination judgment step S106, and the measurement is terminated. On the other hand, if the measured value of the object being compared with the first measured value is not sufficiently consistent with the first measured value, "No" is selected in the termination judgment step S106, and the process proceeds to the optical path length change step S108.

[0142] 2.4. Optical path length change procedure

[0143] In the optical path length changing process S108, the optical path length of optical path 20 is changed. The change of the optical path length of optical path 20 can be performed in any mode, but it is preferable to repeat the change with a constant width by scanning a specific range. This allows for efficient retrieval of the desired optical path length.

[0144] Here, the optical path length of optical path 20 is set to "second length". Then, return to measurement step S104.

[0145] 2.5. Measurement Procedure

[0146] In the second measurement step S104, the displacement or velocity of the sample is measured again using the laser interferometer 1. Then, the measurement value when the optical path length is the second length is set as the "second measurement value". After that, the process moves to the end judgment step S106.

[0147] 2.6. End the judgment process

[0148] In the second termination judgment process S106, the determination of whether to terminate the measurement is based again on whether the measured values ​​used as comparison objects are consistent. Here, since the first and second measured values ​​have been obtained, the measurement is considered to be consistent and the measurement is terminated. It should be noted that the number of measured values ​​can be appropriately set according to the range of the retrieval optical path length or the change width.

[0149] 2.7. Measurement Value Comparison Process

[0150] In the measurement 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, a first length is determined as the optical path length to be set. Similarly, when the second measurement value is close to the known displacement or velocity of the sample, a second length is determined as the optical path length to be set. In the case of multiple measurement values, the optical path length at which the measurement value closest to the displacement or velocity of the sample is obtained is determined as the optical path length to be set.

[0151] 2.8. Optical path length setting procedure

[0152] In the optical path length setting process S112, the optical path length of optical path 20 is set to the length determined in the measurement value comparison process S110. As a result, the optical path difference d in the above equation (1) can be made close to 0. As a result, the measurement accuracy Δd of the displacement is less affected by the linewidth Δf of the emitted light L1 or the fluctuation Δn of the refractive index of the atmosphere.

[0153] The above-described steps constitute the operation of the control unit 57 in the aforementioned optical system adjustment mode. In measurement mode, the optical path length of the optical path 20 set in the optical system adjustment mode is maintained while measuring the object to be measured 14. Therefore, the displacement of the object to be measured 14 can be measured with high accuracy regardless of the type of light source 2.

[0154] As shown above, the control method of the laser interferometer involved in this embodiment is a method for controlling the laser interferometer 1 that includes a light source 2 (laser light source), a polarization beam splitter 4 (optical splitter), an optical modulator 12, an optical path length changing unit 15, and a light receiving element 10. The method includes 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.

[0155] In the preparation step S102, a sample with known displacement or velocity is prepared as the object of measurement 14.

[0156] In the first measurement process S104, when the optical path length (optical path length of optical path 20) between the polarization beam splitter 4 (optical splitter) and the optical modulator 12 is a first length, the light-receiving signal from the light-receiving element 10 is acquired, and the first measured value of displacement or velocity as a sample is calculated based on the acquired light-receiving signal.

[0157] 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 reaches the second length.

[0158] In the second measurement process S104, when the optical path length of the optical path 20 is the second length, the light-receiving signal from the light-receiving element 10 is acquired, and based on the acquired light-receiving signal, a second measurement value of displacement or velocity as a sample is calculated.

[0159] 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 a first length; 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 a second length. It should be noted that when setting the optical path length of the optical path 20 to the "first length" or "second length," it does not need to be exactly the same length as the optical path length of the optical path 20 when the first or second measurement value is obtained in the measurement step S104; they can be slightly different. That is, the "first length" and "second length" in the optical path length setting step S112 include the optical path length of the optical path 20 during measurement, and if the ranges do not overlap, they can each be a concept with a specified width.

[0160] According to this control method, the optical path difference d in the above equation (1) can be made close to 0, and the setting of the optical system 50 can be easily achieved so that the displacement measurement accuracy Δd is not affected by the linewidth Δf of the emitted light L1. Furthermore, by using the optical system 50 adjusted in this way, the displacement of the object 14 can be measured with good accuracy regardless of the type of light source 2.

[0161] Furthermore, by adjusting the optical system 50 as described above, the displacement measurement accuracy Δd is not easily reduced even when using a light source 2 with a relatively large linewidth Δf of the emitted light L1. Therefore, the options for the light source 2 can be expanded without sacrificing the displacement measurement accuracy Δd.

[0162] 3. Third Implementation Method

[0163] Next, the laser interferometer according to the third embodiment will be described.

[0164] Figure 8 This is a functional block diagram illustrating the laser interferometer according to the third embodiment. Figure 9 yes Figure 8 The diagram shows a schematic configuration of the sensor head 51.

[0165] The third embodiment will now be described, but the description will focus on the differences from the first embodiment, and the same items will be omitted. It should be noted that in the figures, the same reference numerals are used to label the same components as in the first embodiment.

[0166] In the third embodiment, the configuration of the optical modulator is the same as that in the first embodiment.

[0167] In the laser interferometer 1 according to the first embodiment described above, the optical modulator 12 includes an AOM 60. In contrast, in the laser interferometer 1A according to the third embodiment, the optical modulator 12 includes a resonant element 30. This optical modulator 12 can also shift the frequency of the transmitted light L1a to generate a reference light L2, just like the optical modulator 12.

[0168] 3.1. Summary of the First Configuration Example of an Optical Modulator

[0169] Figure 10 It is shown Figure 9 A perspective view of a first configuration example of the optical modulator 12 shown.

[0170] The frequency-shift optical modulator 12 has an optical modulation oscillator 120. Figure 10 The optical modulation oscillator 120 shown has a plate-shaped vibrating element 30 and a substrate 31 supporting the vibrating element 30.

[0171] The vibrating element 30 is made of a material that repeatedly vibrates in a pattern of deformation along a plane by applying a potential. In this configuration example, the vibrating element 30 is a crystal AT oscillator that vibrates in the high-frequency domain of the MHz band along the vibration direction 36 with thickness shearing. A diffraction grating 34 is formed on the surface of the vibrating element 30. The diffraction grating 34 has a structure in which multiple straight grooves 32 are periodically arranged.

[0172] The substrate 31 has a surface 311 and a inner surface 312 that are mutually related. A vibrating element 30 is disposed on the surface 311. In addition, a pad 33 for applying a potential to the vibrating element 30 is provided on the surface 311. On the other hand, a pad 35 for applying a potential to the vibrating element 30 is also provided on the inner surface 312.

[0173] The size of the substrate 31 is, for example, set to a length of 0.5 mm or more and 10.0 mm or less. Additionally, the thickness of the substrate 31 is, for example, set to a length of 0.10 mm or more and 2.0 mm or less. As an example, the substrate 31 is a square with one side of 1.6 mm and a thickness of 0.35 mm.

[0174] The size of the vibrating element 30 is, for example, set to a length of 0.2 mm or more and 3.0 mm or less. Additionally, the thickness of the vibrating element 30 is, for example, set to a length of 0.003 mm or more and 0.5 mm or less.

[0175] As an example, the shape of the vibrating element 30 is set as a square with one side of 1.0 mm and its thickness is set as 0.07 mm. In this case, the vibrating element 30 oscillates at a fundamental oscillation frequency of 24 MHz. It should be noted that by changing the thickness of the vibrating element 30 or considering overtones, the oscillation frequency can be adjusted in the range of 1 MHz to 1 GHz.

[0176] It should be noted that, in Figure 10 In this process, although the diffraction grating 34 is formed on the entire surface of the vibrating element 30, it may also be formed on only a portion of it.

[0177] The intensity of light modulation performed by the light modulator 12 is provided by the inner product of the difference wavenumber vector of the transmitted light L1a incident on the light modulator 12 and the wavenumber vector of the reference light L2 emitted from the light modulator 12, and the vector of the vibration direction 36 of the vibrating element 30. In this configuration example, the vibrating element 30 performs thickness shear vibration, but since this vibration is in-plane vibration, light modulation cannot be performed even if light is incident perpendicularly to the surface of the vibrating element 30. Therefore, in this configuration example, by providing the diffraction grating 34 on the vibrating element 30, light modulation can be performed according to the principle described later.

[0178] Figure 10 The diffraction grating 34 shown is a blazed diffraction grating. A blazed diffraction grating is a diffraction grating whose cross-sectional shape is stepped. The straight grooves 32 of the diffraction grating 34 are arranged such that their extension direction is orthogonal to the vibration direction 36.

[0179] From Figure 9 The oscillator circuit shown has 54 pairs. Figure 10 When the vibrating element 30 shown is supplied with a drive signal Sd (an AC voltage is applied), the vibrating element 30 oscillates. Although the power (drive power) required for the oscillation of the vibrating element 30 is not particularly limited, it is as small as 0.1 μW to 100 mW. Therefore, the drive signal Sd output from the oscillation circuit 54 can be used to make the vibrating element 30 oscillate without amplification.

[0180] Furthermore, the vibrating element 30 is very small in size, and the power required for oscillation is also small. Therefore, by using the optical modulator 12 equipped with the vibrating element 30, miniaturization and power saving of the laser interferometer 1 are easily achieved.

[0181] 3.2. Methods for forming diffraction gratings

[0182] While there is no particular limitation on the method for forming the diffraction grating 34, one example is the method of forming the groove 32 on the electrode, which has been filmed on the surface of the resonating element 30 of the crystal AT oscillator, using a nanoimprinting method to create a mold by mechanical scribing (scrubbing machine). The reason for placing it on the electrode is that, in the case of a crystal AT oscillator, high-quality thickness shear vibration can theoretically be generated on the electrode. It should be noted that the groove 32 is not limited to the electrode; it can also be formed on the surface of a material other than the electrode. Furthermore, alternatives to nanoimprinting include exposure and etching-based processing methods, electron beam lithography, and focused ion beam (FIB) processing.

[0183] Alternatively, a diffraction grating can be formed on the chip of the crystal AT oscillator using a photoresist material, and a reflective film based on a metal film or a dielectric multilayer film can be placed there. By placing a metal film or a reflective film, the reflectivity of the diffraction grating 34 can be improved.

[0184] Alternatively, a resist film can be formed on the chip or wafer of the crystal AT oscillator. After etching, the resist film is removed, and then a metal film or a mirror film is formed on the processed surface. In this case, since the resist material is removed, the effects caused by the moisture absorption of the resist material disappear, thus improving the chemical stability of the diffraction grating 34. Furthermore, by using highly conductive metal films such as Au or Al, they can also be used as electrodes to drive the oscillation element 30.

[0185] It should be noted that the diffraction grating 34 can also be formed using techniques such as anodic alumina (porous alumina).

[0186] 3.3. Other configuration examples of optical modulators

[0187] The vibrating element 30 is not limited to a crystal oscillator; for example, it can also be a Si oscillator, a ceramic oscillator, a surface wave (SAW) device, etc.

[0188] Figure 11 It is shown Figure 9 A top view of a portion of a second configuration example of the optical modulator 12 shown. Figure 12 It is shown Figure 9 A top view of a third configuration example of the optical modulator 12 shown.

[0189] Figure 11 The vibrating element 30A shown is a Si oscillator manufactured using MEMS technology. MEMS (Micro Electromechanical Systems) are tiny electromechanical systems.

[0190] The vibrating element 30A includes: a first electrode 301 and a second electrode 302, adjacent to each other on the same plane with a gap between them; a diffraction grating mounting portion 303 disposed on the first electrode 301; and a diffraction grating 34 disposed on the diffraction grating mounting portion 303. The first electrode 301 and the second electrode 302 are driven, for example, by electrostatic attraction, to vibrate along... Figure 11 The left and right directions, about to Figure 6 The shaft connecting the first electrode 301 and the second electrode 302 vibrates in a manner that repeatedly approaches and separates from each other. This allows in-plane vibration to be provided to the diffraction grating 34. The oscillation frequency of the Si oscillator is, for example, in the range of 1 kHz to several hundred MHz.

[0191] Figure 12 The vibrating element 30B shown is a SAW device that utilizes surface waves. SAW (Surface Acoustic Wave) is an elastic surface wave.

[0192] The vibrating element 30B includes a piezoelectric substrate 305, a comb-shaped electrode 306 disposed on the piezoelectric substrate 305, a ground electrode 307, a diffraction grating mounting portion 303, and a diffraction grating 34. When an alternating voltage is applied to the comb-shaped electrode 306, an elastic surface wave is excited through the inverse piezoelectric effect. This enables in-plane vibration of the diffraction grating 34. The oscillation frequency of the SAW device is, for example, in the range of several hundred MHz to several GHz.

[0193] Regarding such devices, by setting up a diffraction grating 34, similar to the case of a crystal AT oscillator, optical modulation can also be performed according to the principle described later.

[0194] On the other hand, when the vibrating element 30 has a crystal oscillator, the extremely high Q value of the crystal can be used to generate a high-precision modulation signal. The Q value is an indicator of the sharpness of the peak value of resonance. In addition, the crystal oscillator has the advantage of being difficult to be affected by external interference. Therefore, by using the modulation signal modulated by the optical modulator 12 equipped with the crystal oscillator, the sample signal from the object to be measured 14 can be acquired with high precision.

[0195] 3.4. Optical Modulation Based on Vibrating Elements

[0196] Next, the principle of using the vibrating element 30 to modulate light will be explained.

[0197] Figure 13 Incident light K was incident from a direction perpendicular to the surface of the vibrating element 30. i A conceptual diagram illustrating the generation of multiple diffracted lights.

[0198] Incident light K is incident on a diffraction grating 34 undergoing thickness shear vibration along vibration direction 36. iAt that time, due to diffraction phenomena, such as Figure 13 As shown, multiple diffracted beams K are generated. ns n is the diffracted light K ns The number of times, n = 0, ±1, ±2, ... . It should be noted that in Figure 13 The diffraction grating 34 shown is not illustrated. Figure 10 The diagram shows a blazed diffraction grating, while an example of another diffraction grating is illustrated with a repeating diffraction grating based on convexity and concavity. Additionally, in Figure 13 The diffraction light K is omitted in the text. 0s The illustration.

[0199] exist Figure 13 In the middle, the incident light K i The light is incident from a direction perpendicular to the surface of the vibrating element 30, but this incident angle is not particularly limited; it can also be set to be incident at an angle relative to the surface of the vibrating element 30. In the case of oblique incident light, the diffracted light K... ns Its direction of travel also changes accordingly.

[0200] It should be noted that, due to the design of the diffraction grating 34, higher-order light with |n|≥2 may not sometimes appear. Therefore, to obtain a stable modulation signal, it is preferable to set |n|=1. That is, in Figure 9 In the laser interferometer 1A, the frequency-shifting optical modulator 12 is preferably configured such that ±1st order diffracted light is used as reference light L2. This configuration enables stabilization of measurements based on the laser interferometer 1A.

[0201] On the other hand, when higher-order light of |n|≥2 appears from the diffraction grating 34, the optical modulator 12 can be configured to use any diffracted light of ±2 orders or higher as the reference light L2, instead of ±1 order diffracted light. Thus, higher-order diffracted light can be utilized, thereby enabling the laser interferometer 1A to be higher frequency and smaller in size.

[0202] In this embodiment, as an example, the optical modulator 12 is configured such that the incident light K incident on the optical modulator 12 i The angle between the direction of entry of the light and the direction of travel of the reference light L2 emitted from the light modulator 12 is 180°. Three examples will be explained below.

[0203] Figures 14 to 16 These are respectively the incident light K i A conceptual diagram illustrating the optical modulator 12, where the angle between the direction of travel of the light L1 and the direction of travel of the reference light L2 is 180°.

[0204] exist Figure 14In addition to the vibrating element 30, the optical modulator 12 also includes a reflector 37. The reflector 37 is configured to reflect the diffracted light K 1s After reflection, it returns to the diffraction grating 34. At this point, the diffracted light K... 1s The angle between the incident angle and the reflection angle in mirror 37 is 180°. As a result, the diffracted light K that exits mirror 37 and returns to diffraction grating 34... 1s The light is diffracted again by the diffraction grating 34, and directed towards the incident light K that is incident on the light modulator 12. i It travels in the opposite direction to the direction of travel. Therefore, by adding a reflector 37, the above-mentioned conditions can be met, and the incident light K... i The condition is that the angle between the direction of entry of the light and the direction of travel of the reference light L2 is 180°.

[0205] Furthermore, by using the reflector 37, the reference light L2 generated by the light modulator 12 becomes light that is modulated twice in frequency. Therefore, by using the reflector 37, a higher frequency modulation can be achieved compared to the case of using a single vibrating element 30.

[0206] exist Figure 15 In the middle, the vibrating element 30 is relative to Figure 13 The configuration is tilted. The tilt angle θ is then set to satisfy the above conditions, and the incident light K... i The condition is that the angle between the direction of entry of the light and the direction of travel of the reference light L2 is 180°.

[0207] Figure 16 The diffraction grating 34 shown has a blaze angle θ B A blazed diffraction grating. And, in the incident light K traveling at an incident angle β... i When the normal N of the surface of the vibrating element 30 is incident on the diffraction grating 34, the reference light L2 is relative to the normal N at a blaze angle θ. B The same angle is returned. Therefore, by setting the incident angle β to be equal to the blaze angle θ B It can satisfy the above-mentioned incident light K i The condition that the angle between the direction of entry of the light beam and the direction of travel of the reference light L2 is 180°. In this case, it is possible to avoid using Figure 14 The mirror 37 shown is also inferior to... Figure 15 By tilting the vibrating element 30 itself to satisfy the conditions as shown, further miniaturization and high-frequency operation of the laser interferometer 1 can be achieved. In particular, in the case of a blazed diffraction grating, the configuration that satisfies the conditions is called a "self-collimating configuration (Rito configuration)," which also has the advantage of particularly improving the diffraction efficiency of the diffracted light.

[0208] It should be noted that, Figure 16The spacing P represents the spacing of the blazed diffraction grating; as an example, the spacing P is set to 1 μm. Additionally, the blaze angle θ... B Set to 25°. In this case, to satisfy the aforementioned condition, the incident light K... i The incident angle β relative to the normal N can also be set to 25°.

[0209] 3.5. Packaging Structure

[0210] Figure 17 This is a cross-sectional view showing an optical modulator 12 with an encapsulation structure.

[0211] Figure 17 The optical modulator 12 shown includes: a container 70 serving as a housing, an optical modulation oscillator 120 housed within the container 70, and circuit elements 45 constituting an oscillation circuit 54. It should be noted that the interior of the container 70 is hermetically sealed, for example, under a reduced pressure atmosphere such as a vacuum or an inert gas atmosphere such as nitrogen or argon.

[0212] like Figure 17 As shown, the container 70 has a container body 72 and a lid 74. The container body 72 has a first recess 721 disposed therein and a second recess 722 disposed inside the first recess 721 and deeper than the first recess 721. The container body 72 is made of, for example, a ceramic material or a resin material. Furthermore, although not shown, the container body 72 includes internal terminals disposed on its inner surface, external terminals disposed on its outer surface, and wiring connecting the internal and external terminals.

[0213] Furthermore, the opening of the container body 72 is sealed by the cap 74 via a sealing ring (not shown) or a sealing component such as low-melting-point glass. The cap 74 is made of a material that allows laser light to pass through, such as glass.

[0214] An optical modulation oscillator 120 is disposed on the bottom surface of the first recess 721. The optical modulation oscillator 120 is supported on the bottom surface of the first recess 721 by a bonding member (not shown). In addition, the internal terminals of the container body 72 are electrically connected to the optical modulation oscillator 120, for example, via a bonding wire, a bonding metal, or other conductive material (not shown).

[0215] A circuit element 45 is disposed on the bottom surface of the second recess 722. The circuit element 45 is electrically connected to the internal terminals of the container body 72 via a bonding wire 76. Thus, the optical modulation oscillator 120 and the circuit element 45 are also electrically connected via wiring provided in the container body 72. It should be noted that circuitry other than the oscillation circuit 54 described later may also be provided in the circuit element 45.

[0216] By employing this packaging structure, the optical modulation oscillator 120 and the circuit element 45 can be overlapped, thus bringing their physical distance closer and shortening the wiring length between them. Therefore, it is possible to suppress external noise from entering the drive signal Sd, or conversely, prevent the drive signal Sd from becoming a noise source. Furthermore, a single container 70 can protect both the optical modulation oscillator 120 and the circuit element 45 from external environmental influences. Therefore, not only can the sensor head 51 be miniaturized, but the reliability of the laser interferometer 1 can also be improved.

[0217] It should be noted that the structure of container 70 is not limited to the structure shown in the figure. For example, the optical modulation oscillator 120 and the circuit element 45 may also have separate packaging structures. In addition, although not shown, another circuit element constituting the oscillation circuit 54 may also be housed in container 70. It should be noted that container 70 can be provided as needed and may also be omitted.

[0218] 3.6. Oscillator Circuit

[0219] like Figure 18 As shown, the oscillation circuit 54 outputs the drive signal Sd input to the light modulator 12 of the optical system 50. Additionally, as... Figure 8 As shown, the oscillation circuit 54 outputs the reference signal Ss that is input to the demodulation circuit 52.

[0220] In the oscillation circuit 54, any circuit that can make the oscillating element 30 oscillate is acceptable; there are no particular limitations, and circuits of various configurations can be used. Figure 18 The circuit diagram shown is an example of the circuit configuration of a single-stage inverter oscillator circuit, which serves as an example of the circuit configuration of the oscillator circuit 54.

[0221] Figure 18 The oscillation circuit 54 shown includes circuit element 45, feedback resistor Rf, limiting resistor Rd, first capacitor Cg, second capacitor Cd, and third capacitor C3.

[0222] Circuit element 45 is an inverter IC. Terminals X1 and X2 of circuit element 45 are respectively connected to the inverter terminals inside circuit element 45. Terminal GND is connected to the ground potential, and terminal Vcc is connected to the power supply potential. Terminal Y is the terminal used for oscillation output.

[0223] A first capacitor Cg is connected between terminal X1 and the ground potential. Additionally, a limiting resistor Rd and a second capacitor Cd, connected in series between terminal X2 and the ground potential, are connected in this order, starting 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.

[0224] Furthermore, one end of the vibrating element 30 is connected between the first capacitor Cg and the feedback resistor Rf, and the other end of the vibrating element 30 is connected between the second capacitor Cd and the limiting resistor Rd. Thus, the vibrating element 30 becomes the signal source of the oscillation circuit 54.

[0225] Figure 19 This is an example of the LCR equivalent circuit of the vibrating element 30.

[0226] like Figure 19 As shown, the LCR equivalent circuit of the vibration element 30 consists of a series capacitor C1, a series inductor L1, an equivalent series resistance R1, and a parallel capacitor C0.

[0227] exist Figure 18 In the oscillation circuit 54 shown, the capacitance of the first capacitor Cg is set to C. g Let the capacitance of the second capacitor Cd be C. d At that time, the load capacitance C L Provided by the following formula (a).

[0228]

[0229] Thus, the oscillation frequency f output from terminal Y of the oscillation circuit 54 osc Provided by the following formula (b).

[0230]

[0231] f Q It is the natural vibration number of the vibrating element 30.

[0232] According to equation (b) above, it can be seen that by appropriately changing the load capacitance C... L This allows the oscillation frequency f of the signal output from terminal Y to be controlled. osc Make minor adjustments.

[0233] In addition, the natural vibration coefficient f of the vibrating element 30 Q The oscillation frequency f of the oscillation circuit 54 osc The difference Δf is provided by the following equation (c).

[0234]

[0235] Where, since C1 << C0 and C1 << C L Therefore, Δf can be approximately provided by the following equation (d).

[0236]

[0237] Therefore, the oscillation frequency f of the oscillation circuit 54osc Becoming the natural vibration number f of the vibrating element 30 Q The corresponding value.

[0238] Here, when the vibrating element 30 is fixed to, for example, the container 70, when subjected to expansion stress caused by temperature via the fixing part, the natural vibration coefficient f Q Changes. Furthermore, when the vibrating element 30 is tilted, it is affected by gravity and other factors due to its own weight, resulting in changes in the natural vibration coefficient f. Q change.

[0239] In the oscillating circuit 54, even the inherent oscillation number f Q Due to this reason, the oscillation frequency f varies based on the above equation (d). osc It changes in a manner in conjunction with this change. That is, the oscillation frequency f... osc Becoming always from the natural vibration number f Q The value deviates from Δf. Therefore, the vibration of the vibrating element 30 is stabilized, which in turn stabilizes the modulation characteristics of the optical modulator 12. As a result, the demodulation accuracy of the sample signal in the demodulation circuit 52 can be improved.

[0240] As an example, the preferred value is Δf = |f osc -f Q |≤3000[Hz], more preferably 600[Hz].

[0241] In the laser interferometer 1 according to this embodiment, the optical modulator 12 includes a vibrating element 30. The optical modulator 12 uses the vibrating element 30 to modulate the transmitted light L1a (the first split light).

[0242] Based on this configuration, the optical modulator 12 can be significantly miniaturized and lightened compared to the first embodiment. This, in turn, enables the laser interferometer 1 to be miniaturized and lightened.

[0243] Furthermore, the laser interferometer 1 according to this embodiment includes a demodulation circuit 52 and an oscillation circuit 54. For example... Figure 8 As shown, the oscillation circuit 54 outputs a reference signal Ss to the demodulation circuit 52. The demodulation circuit 52 demodulates the sample signal originating from the measured object 14 from the light-received signal based on the reference signal Ss. Furthermore, the vibration element 30 serves as the signal source for the oscillation circuit 54.

[0244] Based on this configuration, even the natural vibration coefficient f of the vibrating element 30 Q Variation can also change the oscillation frequency f of the oscillation circuit 54. osc The natural vibration number f of the vibrating element 30 is transformed into QThe corresponding values ​​thus easily stabilize the vibration of the vibrating element 30. Consequently, the temperature characteristics of the modulation signal correspond to the temperature characteristics of the vibrating element 30, stabilizing the modulation characteristics of the optical modulator 12. As a result, the demodulation accuracy of the sample signal in the demodulation circuit 52 is improved.

[0245] 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 correspond to the temperature characteristics of the vibration element 30. Thus, since both the temperature characteristics of the modulation signal and the temperature characteristics of the reference signal correspond to the temperature characteristics of the vibration element 30, the change in the modulation signal with temperature variations is consistent with or approximately similar to the change in the reference signal Ss. Therefore, even if the temperature of the vibration element 30 changes, the impact on demodulation accuracy can be suppressed, and the demodulation accuracy of the sample signal originating from the measurement object 14 can be improved.

[0246] Furthermore, in the first embodiment described above, the crystal temperature needs to be maintained in the AOM60, which tends to result in relatively high power consumption of the laser interferometer 1. In contrast, in this embodiment, since the oscillation circuit 54 consumes low power, power saving of the laser interferometer 1 can be easily achieved.

[0247] The third embodiment has been described above, but the same effect as the first embodiment can be obtained in this third embodiment.

[0248] 4. Fourth Implementation Method

[0249] Next, the laser interferometer according to the fourth embodiment will be described.

[0250] Figure 20 This is a schematic diagram of the sensor head 51 of the laser interferometer according to the fourth embodiment. Figure 21 yes Figure 20 A magnified view of a portion of the image.

[0251] The fourth embodiment will now be described, but the description will focus on the differences from the third embodiment, and the same items will be omitted. It should be noted that in the figures, the same reference numerals are used to label the same components as in the third embodiment.

[0252] In the fourth embodiment, the configuration of the optical path length changing section is different, and it is the same as the third embodiment.

[0253] 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... Figure 20 As shown, 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 disposed in the optical path 20 in which the transmitted light L1a (first split light) travels, and its refractive index changes according to the input signal. The input unit 172 inputs the signal to the refractive index variable element 171.

[0254] In this optical path length changing section 15A, the optical path length of the optical path 20 can be changed by altering the refractive index. Furthermore, the optical path length changing section 15A has no movable parts, thus exhibiting higher resistance to aging and greater reliability.

[0255] The refractive index variable 171 is a light-transmitting medium arranged in the optical path 20, such as a medium whose refractive index changes when controlled by parameters (signals) such as electric field, magnetic field, heat, and light. For example, polymer-dispersed liquid crystals are known as media controlled by an electric field. The refractive index of polymer-dispersed liquid crystals changes according to the magnitude of the applied electric field.

[0256] By using a polymer-dispersed liquid crystal as the refractive index variable 171, the refractive index can be changed using an easily controllable electric field as the control parameter. Therefore, the configuration of the optical path length changing section 15A can be simplified.

[0257] It should be noted that, as examples other than polymer-dispersed liquid crystals, media with temperature-dependent refractive index can be cited. Examples of such media include inorganic materials such as quartz glass and organic materials such as acrylic resin. Furthermore, in this case, the input unit 172 is a temperature adjustment unit that inputs heat as a control parameter; for example, a heat exchange element such as a Peltier element can be cited.

[0258] The optical path length L of the optical path 20 in the fourth embodiment 20 It can be obtained by the following formula (2).

[0259] L 20 =n0(l q1 +l q3 )+n r l q twenty two)

[0260] n0: Refractive index of the atmosphere

[0261] n r The refractive index of refractive index variant 171

[0262] l q1 The distance from the center of polarization beam splitter 4 to the end of refractive index variable 171

[0263] k q2Length of refractive index variable 171

[0264] l q3 The distance from the end of the refractive index variable 171 to the center of the optical modulator 12

[0265] The l in equation (2) above q1 l q2 and l q3 Equivalent to in Figure 21 The physical distance is schematically shown in the diagram.

[0266] With the refractive index of the refractive index variable 171 adjusted, the optical path length L of optical path 20 can be calculated based on the above equation (2). 20 And based on this, the optical path length L can be... 20 Set it as the target value.

[0267] The fourth embodiment has been described above, but the same effect as the third embodiment can be obtained in this fourth embodiment.

[0268] The laser interferometer and its control method have been described above based on the illustrated embodiments. However, the laser interferometer of the present invention is not limited to the described embodiments, and the configuration of each part can be replaced with any configuration having the same function. Furthermore, other arbitrary components may be added to the laser interferometer described in the embodiments.

[0269] In addition, the laser interferometer of the present invention may also be a laser interferometer composed of any two or more of the embodiments and configuration examples described above.

[0270] Furthermore, the control method of the laser interferometer of the present invention can also be a control method with any target process added in the above embodiments.

Claims

1. A laser interferometer, characterized in that, have: A laser light source emits outgoing light; A light splitter splits the emitted light into a first split light and a second split light incident on the object to be measured. An optical modulator is disposed in the optical path in which the first split light travels. The optical modulator has a vibrating element, which causes the first split light to be incident on the vibrating element, which vibrates by being supplied with a driving signal, thereby changing the frequency of the first split light and modulating it into a reference light including the modulated signal. An optical path length changing unit is disposed between the optical splitter and the optical modulator to change the first optical path length, which serves as the optical path length between the optical splitter and the optical modulator. The light-receiving element receives the interference light between the object light generated by the reflection of the emitted light from the object being measured and the reference light, and outputs a light-receiving signal; The control unit controls the operation of the optical path length changing unit based on 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. The oscillation circuit uses the vibrating element as a signal source and outputs the driving signal and the reference signal. as well as A demodulator, input to the reference signal output from the oscillation circuit, demodulates the sample signal originating from the measured object from the received light signal based on the reference signal. The oscillation circuit has a first capacitor. One end of the vibrating element of the optical modulator is connected to the first capacitor. When the optical path difference between the first optical path length and the second optical path length is set as d[m], and the linewidth of the emitted light is set as Δf[Hz], the control unit controls the operation of the optical path length changing unit to satisfy 0≤dΔf≤1×10 8 .

2. The laser interferometer according to claim 1, characterized in that, 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. The laser interferometer according to claim 2, characterized in that, The optical path difference between the length of the first optical path and the length of the second optical path is less than 100m.

4. The laser interferometer according to claim 1, characterized in that, The optical path length changing unit has: Multiple optical elements switch the optical path of the first split light; and The drive unit changes the distance between the optical elements by moving at least one of the plurality of optical elements.

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

6. The laser interferometer according to claim 5, characterized in that, The optical element is a right-angle prism reflector or a roof prism reflector.

7. The laser interferometer according to claim 5 or 6, characterized in that, The light-reflecting surface reflects the first segmented light multiple times.

8. The laser interferometer according to claim 1, characterized in that, The oscillation circuit includes a feedback resistor, a second capacitor, and a limiting resistor. One end of the vibrating element of the optical modulator is connected between the first capacitor and the feedback resistor. The other end of the vibrating element of the optical modulator is connected between the second capacitor and the limiting resistor.

9. The laser interferometer according to claim 1, characterized in that, Let the natural vibration number of the vibrating element be f. Q The oscillation frequency of the oscillation circuit is set to f. osc Then |f osc -f Q | ≤3000[Hz].

10. The laser interferometer according to claim 1, characterized in that, The optical path length changing unit has: A refractive index variable element is configured in the optical path of the first split light, the refractive index varying according to the input signal; and The input unit inputs the signal to the refractive index variable.

11. The laser interferometer according to claim 10, characterized in that, The refractive index variable is composed of a polymer-dispersed liquid crystal.

12. The laser interferometer according to claim 1, characterized in that, The laser source is a vertical cavity surface-emitting laser diode or a Fabry-Perot type semiconductor laser diode.

13. The laser interferometer according to claim 1, characterized in that, The laser source is a He-Ne laser or a laser diode with a fiber Bragg grating.

14. A control method for a laser interferometer, characterized in that, Control of the laser interferometer according to any one of claims 1 to 13, The control method for the laser interferometer includes the following steps: Prepare a sample with known displacement or velocity as the object of the measurement; When the optical path length between the optical splitter and the optical modulator is a first length, the received light signal is acquired, and a first measurement value as the displacement or velocity of the sample is calculated based on the acquired light signal. Control the operation of the optical path length changing unit so that the optical path length between the optical splitter and the optical modulator reaches the second length; When the optical path length between the optical splitter and the optical modulator is the second length, the received light signal is acquired, and a second measurement value as the displacement or velocity of the sample is calculated based on the acquired light signal. as well as When the first measured value is close to the known displacement or velocity of the sample, the optical path length between the optical splitter and the optical modulator is set to the first length; when the second measured value is close to the known displacement or velocity of the sample, the optical path length between the optical splitter and the optical modulator is set to the second length.

Images