Laser interferometer

Abstract

The laser interferometer provided by this invention can improve the accuracy of demodulating the sampling signal from the object being measured based on the received light signal. The laser interferometer includes: a laser source; a first optical splitter that splits the laser into a first split beam and a second split beam; an optical modulator that modulates the first split beam into a reference beam; a second optical splitter that splits the object beam and the reference beam into a third split beam and a fourth split beam, wherein the object beam is light generated by reflection of the second split beam from the object being measured; a first light-receiving element that receives the third split beam; and a second light-receiving element that receives the fourth split beam; the optical axis of the first split beam from the first optical splitter toward the optical modulator is offset from the optical axis of the reference beam from the optical modulator toward the first optical splitter, and the difference between the optical path length from the second optical splitter to the first light-receiving element and the optical path length from the second optical splitter to the second light-receiving element is 20 mm or less.

Description

Technical Field

[0001] This invention relates to laser interferometers. Background Technology

[0002] Patent Document 1 discloses a laser vibrometer as a device for measuring the vibration velocity of an object. In this laser vibrometer, a laser is irradiated onto the object being measured, and the vibration velocity is measured based on the scattered laser light that has undergone a Doppler frequency shift.

[0003] Specifically, the laser vibrometer described in Patent Document 1 includes an acousto-optic modulator (AOM). The acousto-optic modulator shifts the frequency of the laser by changing the frequency of the supplied ultrasonic wave. In the laser vibrometer, the frequency-shifted laser light is used as a reference light. Furthermore, the beat frequency is extracted by aligning the scattered laser light from the object being measured with the reference light from the acousto-optic modulator. The vibration velocity of the object being measured is determined based on this extracted beat frequency.

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

[0005] In laser sources, laser oscillations can sometimes become unstable due to the intrusion of reflected light. In the laser vibrometer described in Patent Document 1, if the laser oscillation becomes unstable, the quality of the laser decreases. Consequently, in the laser vibrometer, the signal-to-noise ratio (S / N ratio) of the received light signal obtained from the scattered laser decreases, or the phase of the laser emitted from the laser source becomes discontinuous. As a result, there is a problem of reduced accuracy in measuring the vibration velocity of an object. Summary of the Invention

[0006] The laser interferometer according to the applicable examples of the present invention is characterized by comprising:

[0007] Laser source, emits laser light;

[0008] A first optical splitter splits the laser beam into a first split beam and a second split beam.

[0009] An optical modulator modulates the first segmented light into reference light with different frequencies;

[0010] The second light splitter splits the object light and the reference light into a third split light and a fourth split light, wherein the object light is the light generated by the second split light being reflected by the object being measured;

[0011] A first light-receiving element receives the third segmented light; and

[0012] The second light-receiving element is positioned at a different location than the first light-receiving element and receives the fourth split light;

[0013] The optical axis of the first split light moving from the first optical splitter toward the optical modulator is offset from the optical axis of the reference light moving from the optical modulator toward the first optical splitter.

[0014] The difference between the optical path length from the second optical splitter to the first light-receiving element and the optical path length from the second optical splitter to the second light-receiving element is less than 20 mm. Attached Figure Description

[0015] Figure 1 This is a functional block diagram illustrating the laser interferometer according to the first embodiment.

[0016] Figure 2 It means Figure 1 The diagram shows a schematic configuration of the sensor head 51.

[0017] Figure 3 It means Figure 2 A perspective view of a first example of a light modulator.

[0018] Figure 4 This is a top view showing a portion of a second configuration example of an optical modulator.

[0019] Figure 5 This is a top view showing a third configuration example of an optical modulator.

[0020] Figure 6 This indicates that in the incident light K i A conceptual diagram illustrating the generation of multiple diffracted beams when light enters from a direction perpendicular to the surface of the vibrating element.

[0021] Figure 7 This explains the composition of the incident light K. i A conceptual diagram of an optical modulator whose direction of travel is 180° from the direction of travel of the reference light L2.

[0022] Figure 8 This explains the composition of the incident light K. i A conceptual diagram of an optical modulator whose direction of travel is 180° from the direction of travel of the reference light L2.

[0023] Figure 9 This explains the composition of the incident light K. i A conceptual diagram of an optical modulator whose direction of travel is 180° from the direction of travel of the reference light L2.

[0024] Figure 10 This is a cross-sectional view showing an optical modulator with a packaged structure.

[0025] Figure 11 It means Figure 1The circuit diagram shown is an example of the configuration of a differential amplifier circuit and a current-to-voltage converter.

[0026] Figure 12 This is a circuit diagram showing the configuration of a single-stage inverter oscillation circuit.

[0027] Figure 13 This is an example of the LCR equivalent circuit of a vibrating element.

[0028] Figure 14 It means from Figure 2 The diagram shows the trace of the emitted light L1 (first segmented light L1a) from the laser source and the trace of the reference light L2 generated by the reflection of the emitted light L1 by the optical modulator as the return light L5 towards the laser source.

[0029] Figure 15 This is a schematic diagram showing the sensor head configuration of the laser interferometer involved in the first modified example.

[0030] Figure 16 It means from Figure 15 The diagram shows the traces of the emitted light L1 and the first segmented light L1a emitted from the laser source, and the trace of the reference light L2 generated by the reflection of the first segmented light L1a by the optical modulator as the return light L5 facing the laser source.

[0031] Figure 17 This is a schematic diagram used to illustrate the impact of optical axis offset on differential amplification processing.

[0032] Figure 18 This is a schematic diagram used to illustrate the impact of optical axis offset on differential amplification processing.

[0033] Figure 19 This is a schematic diagram used to illustrate the impact of optical axis offset on differential amplification processing.

[0034] Figure 20 This is a schematic diagram used to illustrate the impact of optical axis offset on differential amplification processing.

[0035] Figure 21 This is a schematic diagram used to illustrate the impact of optical axis misalignment on differential amplification processing, and it is relative to... Figure 20 The schematic diagram shown illustrates a distance difference ΔL between the first light-receiving element and the second light-receiving element. dif The diagram was modified in a certain way.

[0036] Figure 22 yes Figure 21 A magnified view of a portion of the image.

[0037] Figure 23 It is a schematic representation of arrival. Figure 22The diagram shows the reference light and the optical path of the object light at the position of the second light-receiving element.

[0038] Figure 24 This represents distance WDs relative to distance L. BS The optical axis A of object light L3 at three different modes L3 A graph showing the changes.

[0039] Figure 25 This is a schematic diagram showing the optical system configuration of the laser interferometer involved in the second modification.

[0040] Figure 26 This is a schematic diagram showing the optical system configuration of the laser interferometer involved in the third modification.

[0041] Figure 27 This is a schematic diagram showing the optical system configuration of the laser interferometer involved in the fourth modification.

[0042] Explanation of reference numerals in the attached figures

[0043] 1…Laser interferometer, 2…Laser source, 3…Collimating lens, 4…First light splitter, 5…Second light splitter, 6…1 / 2 wavelength plate, 7…1 / 4 wavelength plate, 8…1 / 4 wavelength plate, 9…Analyzer, 10…First light-receiving element, 11…Second light-receiving element, 12…Optical modulator, 14…Object to be measured, 15…Reflecting element, 16…Setting part, 17…Shielding element, 18…Optical path, 20…Optical path, 22…Optical path, 24…Optical path, 26…Optical path, 28…Optical path, 30…Vibrating element, 30A…Vibrating… Moving element, 30B…vibrating 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, 53…preprocessing unit, 54…oscillation circuit, 55…demodulation processing unit, 56…optical path length changing unit, 70…container, 72…container body, 74…lid, 76…bonding wire, 120…optical modulation oscillator, 172…opening, 301…first electrode, 302… …Second electrode, 303…Diffraction grating mounting portion, 305…Piezoelectric substrate, 306…Comb-shaped electrode, 307…Ground electrode, 311…Surface, 312…Back side, 530…Differential amplifier circuit, 530a…Connection wiring, 530b…Connection wiring, 531…Current-to-voltage converter, 531a…First input terminal, 531b…Second input terminal, 531c…Operational amplifier, 531d…Feedback resistor, 531e…Output terminal, 532…ADC, 533…ADC, 534…First bandpass Filter, 535…Second bandpass filter, 536…First delay adjuster, 537…Second delay adjuster, 538…Multiplier, 539…Third bandpass filter, 540…First AGC, 541…Second AGC, 542…Adder, 551…Multiplier, 552…Multiplier, 553…Phase shifter, 555…First low-pass filter, 556…Second low-pass filter, 557…Divider, 558…Argontangent operator, 559…Output circuit, 721…First recess, 722…Second recess, A L1 …optical axis, A L1a …optical axis, A L1b …optical axis, A L2 …optical axis, A L3 …optical axis, A L5 …optical axis, C0…parallel capacitor, C1…series capacitor, C3…third capacitor, Cd…second capacitor, Cg…first capacitor, GND…GND terminal, I1…photocurrent, I2…photocurrent, Id…differential current, K -2s …diffraction light, K -1s …diffraction light, K 0s …diffraction light, K 1s …diffraction light, K 2s…Diffracted light, K1…Incident light, L…Distance, L1…Series inductance, L1…Outgoing light, L1a…First segmented light, L1b…Second segmented light, L2…Reference light, L3…Object light, L5…Return light, L6a…Third segmented light, L6b…Third segmented light, L… BS …distance, L PD …distance, L pin …distance, N…normal, n1…refractive index, n2…refractive index, O…origin, OL…overlapping portion, P…space, R…optical path, R'…optical path, R1…equivalent series resistance, Rd…limiting resistance, Rf…feedback resistance, R… pin …optical path, R qom …optical path, R sam …optical path, S1…first signal, S2…second signal, Sd…drive signal, Ss…reference signal, Vcc…terminal, WDs…distance, X1…terminal, X2…terminal, Y…terminal, d…deviation amplitude, jp1…bifurcation, jp2…bifurcation, ps1…first signal path, ps2…second signal path, x…signal x, x LD …distance, y…signal y, y ref …deviation range, ΔL PD1 …distance, ΔL PD2 …distance, ΔL dif …Difference in distance, ΔWD ref …Difference in distance, Δy… Deviation range, Δy pin …deviation range, ΔPL n2 …physical length, ΔPL PD1 …physical length, β…incident angle, θ'…deviation angle, θ B …Brightness angle, θ S …tilt angle, θ ref …offset angle, θ sam …offset angle, φ pin …diameter. Detailed Implementation

[0044] The laser interferometer of the present invention will now be described in detail with reference to the embodiments shown in the accompanying drawings.

[0045] 1. First Implementation Method

[0046] First, the laser interferometer involved in the first embodiment will be described.

[0047] Figure 1 This is a functional block diagram illustrating the laser interferometer 1 according to the first embodiment.

[0048] Figure 1The laser interferometer 1 shown has a sensor head 51 and a demodulation circuit 52. The sensor head 51 has an optical system 50, a differential amplifier circuit 530, a current-to-voltage converter 531, and an oscillation circuit 54. The demodulation circuit 52 is input with the light received from the optical system 50.

[0049] 1.1. Sensor Head

[0050] Figure 2 It means Figure 1 A schematic diagram of the sensor head 51 is shown. Furthermore, in Figure 2 In the diagram, we define the X-axis, Y-axis, and Z-axis as three mutually orthogonal axes, and represent them with arrows. We set the tip of the arrow to "positive" and the base of the arrow to "negative". Figure 2 This is a top view (view from above the Z-axis).

[0051] 1.1.1. Optical System

[0052] As previously mentioned, the sensor head 51 has an optical system 50.

[0053] like Figure 2 As shown, the optical system 50 includes a laser light source 2, a collimating lens 3, a first light splitter 4, a second light splitter 5, a half-wave plate 6, a quarter-wave plate 7, a quarter-wave plate 8, an analyzer 9, a first light-receiving element 10, a second light-receiving element 11, a frequency shifter type light modulator 12, and a mounting section 16 for arranging the measurement object 14.

[0054] Laser source 2 emits outgoing light L1 (laser). First light-receiving element 10 and second light-receiving element 11 convert the received light into electrical signals. Optical modulator 12 includes a vibrating element 30, which changes the frequency of the outgoing light L1 to generate reference light L2 containing the modulation signal. Setting unit 16 can be configured as needed to place the object to be measured 14. Outgoing light L1 incident on the object to be measured 14 is reflected as object light L3 containing the Doppler signal, i.e., the sampling signal, from the object to be measured 14.

[0055] The optical path of the emitted light L1 from the laser source 2 is designated as optical path 18. A collimating lens 3 and a half-wave plate 6 are sequentially arranged on optical path 18, starting from the laser source 2 side. Optical path 18 is coupled to optical path 20 via reflection from the first optical splitter 4. On optical path 20, a quarter-wave plate 8 and an optical modulator 12 are sequentially arranged, starting from the first optical splitter 4 side. Furthermore, optical path 18 is coupled to optical path 22 via transmission from the first optical splitter 4. On optical path 22, a quarter-wave plate 7 and a mounting section 16 are sequentially arranged, starting from the first optical splitter 4 side.

[0056] Optical path 20 is coupled to optical path 24 through transmission from the first optical splitter 4. On optical path 24, an analyzer 9 and a second optical splitter 5 are sequentially arranged from the side of the first optical splitter 4. Furthermore, optical path 24 is divided into optical path 26 and optical path 28 by the second optical splitter 5. Optical path 26 extends between the second optical splitter 5 and the first light-receiving element 10. Optical path 28 extends between the second optical splitter 5 and the second light-receiving element 11.

[0057] The emitted light L1 from the laser source 2 is split into two by the first optical splitter 4. One beam enters the optical modulator 12 via optical path 20. The other beam enters the object to be measured 14 via optical path 22. The reference light L2 generated by the optical modulator 12 enters the second optical splitter 5 via optical paths 20 and 24. The object light L3 generated by the reflection from the object to be measured 14 enters the second optical splitter 5 via optical paths 22 and 24.

[0058] The reference light L2 and the object light L3 are split into two by the second light splitter 5. One light is transmitted into the first light receiving element 10 through the light path 26, and the other light is transmitted into the second light receiving element 11 through the light path 28.

[0059] Furthermore, in this specification, "optical path" refers to the path of light travel between optical components. Additionally, "optical axis," as described later, refers to the central axis of the light beam passing through the optical path.

[0060] The following is a further explanation of each part of the optical system 50.

[0061] 1.1.1.1. Laser source

[0062] Laser source 2 is a laser source that emits coherent outgoing light L1. Laser source 2 preferably uses a light source with a linewidth of MHz or less. Specifically, examples include gas lasers such as He-Ne lasers, semiconductor laser elements such as DFB-LD (Distributed feedback-laser diode), FBG-LD (laser diode with fiber bragg grating), VCSEL (Vertical Cavity Surface Emitting Laser), and FP-LD (Fabry-Perot Laser Diode).

[0063] The laser source 2 is preferably a semiconductor laser element. This allows for the miniaturization of the laser source 2. Consequently, miniaturization of the laser interferometer 1 is possible. In particular, since the sensor head 51 housing the optical system 50 in the laser interferometer 1 can be miniaturized and lightened, it is also useful in improving the operability of the laser interferometer 1.

[0064] 1.1.1.2. Collimating Lens

[0065] The collimating lens 3 is a convex lens disposed between the laser source 2 and the first light splitter 4. The collimating lens 3 parallelizes the outgoing light L1 emitted from the laser source 2.

[0066] Furthermore, when the outgoing light L1 emitted from the laser source 2 is sufficiently parallelized, the collimating lens 3 can be omitted when the laser source 2 uses a gas laser such as a He-Ne laser.

[0067] On the other hand, when the laser source 2 is a semiconductor laser element, the laser interferometer 1 preferably includes a collimating lens 3 disposed between the laser source 2 and the first optical splitter 4. This allows the emitted light L1 from the semiconductor laser element to be parallelized. As a result, the emitted light L1 becomes collimated, thus suppressing the enlargement of various optical components receiving the emitted light L1 and enabling miniaturization of the laser interferometer 1.

[0068] The collimated outgoing light L1 passes through the 1 / 2 wavelength plate 6 and is converted into linearly polarized light with an intensity ratio of P-polarized light to S-polarized light of, for example, 50:50, and then enters the first optical splitter 4.

[0069] 1.1.1.3. Optical splitter

[0070] The first optical splitter 4 is a polarization beam splitter disposed between the laser source 2 and the optical modulator 12, and between the laser source 2 and the object to be measured 14. The first optical splitter 4 has the function of allowing P-polarized light to pass through and reflecting S-polarized light. Through this function, the first optical splitter 4 splits the outgoing light L1 into a first split beam L1a, which is the reflected light of the first optical splitter 4, and a second split beam L1b, which is the transmitted light of the first optical splitter 4.

[0071] The first segmented light L1a, which is S-polarized light reflected by the first optical splitter 4, is converted into circularly polarized light by the quarter-wave plate 8 and enters the optical modulator 12. The circularly polarized light L1a entering the optical modulator 12 is subjected to f m A frequency shift of [Hz] is used as the reference light L2 for reflection. Therefore, the reference light L2 contains a frequency f. mThe modulation signal is [Hz]. When the reference light L2 passes through the quarter-wave plate 8 again, it is converted into P-polarized light. The P-polarized light of the reference light L2 passes through the first optical splitter 4 and the analyzer 9 and enters the second optical splitter 5.

[0072] The second segmented light L1b, which is P-polarized light that has passed through the first light splitter 4, is converted into circularly polarized light by the quarter-wave plate 7 and then incident on the moving object 14. The circularly polarized light L1b incident on the object 14 is subjected to f d The Doppler frequency shift of [Hz] is reflected as object light L3. Therefore, object light L3 contains frequency f. d The sampling signal is [Hz]. When the object light L3 passes through the 1 / 4 wavelength plate 7 again, it is converted into S-polarized light. The S-polarized light of the object light L3 is reflected by the first optical splitter 4 and passes through the analyzer 9 into the second optical splitter 5.

[0073] As mentioned above, since the emitted light L1 is coherent, the reference light L2 and the object light L3 are used as interference light and are incident on the first light-receiving element 10 and the second light-receiving element 11.

[0074] The second optical splitter 5 is a polarization beam splitter disposed between the analyzer 9 and the first light-receiving element 10 and the second light-receiving element 11. The second optical splitter 5 splits the interference light (reference light L2 and object light L3) into a third split light L6a and a fourth split light L6b. Specifically, the S-polarized light in the interference light is reflected by the second optical splitter 5 and enters the first light-receiving element 10 as the third split light L6a. The P-polarized light in the interference light passes through the second optical splitter 5 and enters the second light-receiving element 11 as the fourth split light L6b.

[0075] Alternatively, a non-polarizing beam splitter can be used instead of a polarizing beam splitter. In this case, the half-wave plate 6, quarter-wave plate 7, and quarter-wave plate 8 are not needed, thus miniaturizing the laser interferometer 1 can be achieved by reducing the number of components. Alternatively, an optical splitter other than a beam splitter can also be used.

[0076] 1.1.1.4. Analyzer

[0077] The S-polarized and P-polarized beams, being orthogonal to each other, are independent and therefore simply coincident without interference causing beat frequencies. Thus, a light wave superimposed with S-polarized and P-polarized light is passed through an analyzer 9 tilted at 45° relative to both S-polarized and P-polarized beams. By using the analyzer 9, light with common components can pass through each other, thus producing interference. As a result, in 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].

[0078] 1.1.1.5: Light-receiving element

[0079] The third segmented light L6a enters the first light-receiving element 10. The fourth segmented light L6b enters the second light-receiving element 11, which is positioned differently from the first light-receiving element 10. The third segmented light L6a and the fourth segmented light L6b are the aforementioned interference lights. The first light-receiving element 10 and the second light-receiving element 11 receive the interference light and output a light-receiving signal. By demodulating the sampled signal based on the light-receiving signal and using the method described later, the motion, i.e., vibration velocity or displacement of the object to be measured 14, can finally be determined. Examples of the first light-receiving element 10 and the second light-receiving element 11 include, for example, photodiodes.

[0080] The first light-receiving element 10 receives the third split light L6a and outputs a photocurrent. The second light-receiving element 11 receives the fourth split light L6b and outputs a photocurrent. In the differential amplifier circuit 530 (described later), the difference between the photocurrent from the first light-receiving element 10 and the photocurrent from the second light-receiving element 11 is obtained and output as a differential current. In the current-to-voltage converter 531 (described later), this differential current is converted into a voltage signal.

[0081] 1.1.1.6. Optical Modulator

[0082] Figure 3 It means Figure 2 A perspective view of a first configuration example of the optical modulator 12 shown.

[0083] 1.1.1.6.1. Summary of the First Configuration Example of an Optical Modulator

[0084] The frequency shifter type optical modulator 12 has an optical modulation oscillator 120. Figure 3 The optical modulation oscillator 120 shown has a plate-shaped vibrating element 30 and a substrate 31 supporting the vibrating element 30.

[0085] The vibrating element 30 is made of a material that repeatedly vibrates in a manner that distorts in the direction along the surface when an electrical potential is applied. In this configuration example, the vibrating element 30 is an AT-cut quartz oscillator that undergoes thickness shear vibration along the vibration direction 36 in the high-frequency region of the MHz band. A diffraction grating 34 is formed on the surface of the vibrating element 30. The diffraction grating 34 has grooves 32 that hold components intersecting the vibration direction 36, that is, a plurality of straight grooves 32 extending in a direction intersecting the vibration direction 36.

[0086] The substrate 31 has a surface 311 and a back surface 312 that are in a face-to-back relationship. A vibrating element 30 is disposed on the surface 311. In addition, pads 33 for applying a potential to the vibrating element 30 are provided on the surface 311. On the other hand, pads 35 for applying a potential to the vibrating element 30 are also provided on the back surface 312.

[0087] The size of the substrate 31 is, for example, approximately 0.5 mm or more and 10.0 mm or less on the long side. Additionally, the thickness of the substrate 31 is, for example, approximately 0.10 mm or more and 2.0 mm or less. As an example, the substrate 31 is a square with one side measuring 1.6 mm, and its thickness is 0.35 mm.

[0088] The size of the vibrating element 30 is, for example, about 0.2 mm or more and about 3.0 mm or less on the long side. In addition, the thickness of the vibrating element 30 is, for example, about 0.003 mm or more and about 0.5 mm or less.

[0089] As an example, the vibrating element 30 is a square with one side of 1.0 mm and a thickness of 0.07 mm. In this case, the vibrating element 30 oscillates at a fundamental oscillation frequency of 24 MHz. Furthermore, by changing the thickness of the vibrating element 30 or taking into account harmonics (overtones), the oscillation frequency can be adjusted in the range of 1 MHz to 1 GHz.

[0090] In addition, Figure 3 In this process, the diffraction grating 34 is formed on the entire surface of the vibrating element 30, but it may also be formed on only a portion of it.

[0091] The magnitude of optical modulation by the optical modulator 12 is given by the inner product of the difference wavenumber vector of the wavenumber vector of the outgoing light L1 incident on the optical modulator 12 and the wavenumber vector of the reference light L2 emitted from the optical modulator 12, and the vector of the vibration direction 36 of the vibrating element 30. In this configuration example, the vibrating element 30 performs thickness shear vibration, but since this vibration is in-plane vibration, optical 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 a diffraction grating 34 on the vibrating element 30, optical modulation can be performed according to the principle described later.

[0092] Figure 3 The diffraction grating 34 shown is a blazed diffraction grating. A blazed diffraction grating is a grating with a stepped cross-sectional shape. The straight grooves 32 of the diffraction grating 34 are arranged such that their extension direction is orthogonal to the vibration direction 36.

[0093] When from Figure 1 and Figure 2 The oscillation circuit 54 shown is directed towards Figure 3When the vibrating element 30 shown is supplied with a drive signal Sd (an AC voltage is applied), the vibrating element 30 oscillates. The power (drive power) required for the oscillation of the vibrating element 30 is not particularly limited, ranging from as low as 0.1 μW to about 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.

[0094] Furthermore, existing optical modulators sometimes require structures to maintain their temperature, making it difficult to reduce their size. Additionally, existing optical modulators consume a lot of power, thus posing a challenge to miniaturization and power saving in laser interferometers. In contrast, in this configuration example, the oscillation element 30 is very small, and the power required for oscillation is also low; therefore, miniaturization and power saving of the laser interferometer 1 are easily achieved.

[0095] 1.1.1.6.2. Methods for forming diffraction gratings

[0096] The method for forming the diffraction grating 34 is not particularly limited. As an example, one method is to fabricate a mold using a mechanical scribing method (scrubbing machine) and then form a groove 32 on the electrode of the vibrating element 30 of the AT-cut quartz oscillator using nanoimprint lithography. The reason for forming the groove on the electrode is that, in the case of the AT-cut quartz oscillator, high-quality thickness shear vibration can theoretically be generated on the electrode. Furthermore, the formation of the groove 32 is not limited to the electrode and can also be on the surface of the material in the non-electrode portion. Alternatively, exposure-based etching methods, electron beam lithography, focused ion beam (FIB) processing, etc., can be used instead of nanoimprint lithography.

[0097] Alternatively, a diffraction grating can be formed on the chip of an AT-cut quartz oscillator using a photoresist material, and a mirror film composed of a metal film or a multilayer dielectric film can be placed on the diffraction grating. By placing a metal film or a mirror film, the reflectivity of the diffraction grating 34 can be improved.

[0098] Furthermore, a resist film can be formed on the chip or wafer of an AT-cut quartz oscillator. After etching, the resist film is removed, and then a metal film or mirror film is formed on the processed surface. In this case, since the resist material is removed, it is not affected by the moisture absorption of the resist material, thus improving the chemical stability of the diffraction grating 34. Additionally, by setting a highly conductive metal film such as Au or Al, it can also be used as an electrode to drive the oscillation element 30.

[0099] In addition, the diffraction grating 34 can also be formed using techniques such as anodic aluminum oxide (porous aluminum oxide).

[0100] 1.1.1.6.3. Other configuration examples of optical modulators

[0101] The vibrating element 30 is not limited to a quartz oscillator; for example, it can also be a Si oscillator, a surface acoustic wave (SAW) device, a ceramic oscillator, etc.

[0102] Figure 4 This is a top view showing a portion of a second configuration example of the optical modulator 12. Figure 5 This is a top view showing a third configuration example of the optical modulator 12.

[0103] Figure 4 The vibrating element 30A shown is a Si oscillator manufactured from a Si substrate using MEMS technology. MEMS (Micro-Electro-Mechanical Systems) is a micro-electromechanical system.

[0104] 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... Figure 4 The left and right directions, that is, along Figure 4 The axis connecting the first electrode 301 and the second electrode 302 vibrates by repeatedly approaching and separating from each other. This allows in-plane vibration to be applied to the diffraction grating 34. The oscillation frequency of the Si oscillator is, for example, around 1 kHz to several hundred MHz.

[0105] Figure 5 The vibrating element 30B shown is a SAW device that utilizes surface waves. SAW (Surface Acoustic Wave) is a type of surface acoustic wave.

[0106] 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, a surface acoustic wave (SAW) is excited through the inverse piezoelectric effect. This allows in-plane vibration to be applied to the diffraction grating 34. The oscillation frequency of the SAW device is, for example, around several hundred MHz to several GHz.

[0107] For devices like the above, optical modulation can also be performed according to the principle described later, in the same way as with AT-cut quartz oscillators, by setting diffraction grating 34.

[0108] On the other hand, when the vibrating element 30 is a quartz oscillator, the extremely high Q value of quartz can be used to generate a high-precision modulation signal. The Q value is an indicator of the sharpness of the resonant peak. In addition, quartz oscillators have the advantage of being less susceptible to external interference. Therefore, by using the modulation signal modulated by the optical modulator 12 equipped with a quartz oscillator, the sampling signal from the object to be measured 14 can be acquired with high precision.

[0109] 1.1.1.6.4. Optical Modulation Based on Vibrating Elements

[0110] Next, the principle of using the vibrating element 30 to modulate light will be explained.

[0111] Figure 6 This indicates that in the 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 30.

[0112] When the incident light K i When the light is incident on a diffraction grating 34 that undergoes thickness shear vibration along the vibration direction 36, such as Figure 6 As shown, multiple diffracted beams K are generated through the diffraction phenomenon. ns n is the diffracted light K ns The number of times, n = 0, ±1, ±2, ... Furthermore, in Figure 6 In the diffraction grating 34 shown, the one illustrated is not... Figure 3 Instead of showing a blazed diffraction grating, this illustration serves as an example of other diffraction gratings, depicting a diffraction grating formed by repeated concave and convex shapes. Furthermore, in Figure 6 In the text, the diffraction light K is omitted. Os The illustration.

[0113] exist Figure 6 In the middle, the incident light K i The light enters from a direction perpendicular to the surface of the vibrating element 30, but this angle of incidence is not particularly limited; the angle of incidence can also be set to be oblique relative to the surface of the vibrating element 30. In the case of oblique incidence, the diffracted light K ns Its direction of travel also changes accordingly.

[0114] Furthermore, depending on 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 best to set |n|=1. That is, in Figure 2 In the laser interferometer 1, the preferred frequency shifter type optical modulator 12 is configured to use ±1st order diffracted light as reference light L2. This configuration allows for the stabilization of measurements in the laser interferometer 1.

[0115] On the other hand, in the case where 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 ±2nd order or higher as the reference light L2 instead of ±1st order diffracted light. Thus, since higher-order diffracted light can be used, the laser interferometer 1 can be made more frequent and smaller.

[0116] In this embodiment, as an example, the incident light K of the incident light modulator 12 is... i The optical modulator 12 is configured such that the angle between its entry direction and the travel direction of the reference light L2 emitted from the optical modulator 12 is 180°. The following shows... Figures 7 to 9 Three examples will be used to illustrate this.

[0117] Figures 7 to 9 These are descriptions of the composition of incident light K. i A conceptual diagram of an optical modulator 12 whose direction of travel is 180° from the direction of travel of the reference light L2.

[0118] Figure 7 The optical modulator 12 shown includes a resonant element 30 and a reflector 37. The reflector 37 is configured to reflect the diffracted light K. 1s The light is reflected back 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 emitted from mirror 37 and returning to diffraction grating 34... 1s The light is diffracted again at diffraction grating 34, directed toward the incident light K that is incident on the modulator 12. i It travels in the opposite direction to the direction of travel. Therefore, by adding a reflector 37, the aforementioned incident light K can be satisfied. i The condition is that the angle between the direction of entry of the light and the direction of travel of the reference light L2 is 180°.

[0119] Furthermore, the reference light L2 generated by the light modulator 12 is subjected to two frequency modulations via the reflector 37. Therefore, by using the reflector 37, a higher frequency modulation can be achieved compared to using a single vibrating element 30.

[0120] exist Figure 8 In, relative to Figure 6 The configuration causes the vibrating element 30 to tilt. The tilt angle θ at this point... S The incident light K is set to satisfy the aforementioned condition. 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°.

[0121] Figure 9 The diffraction grating 34 shown has a blaze angle θB A blazed diffraction grating. Furthermore, when incident light K travels at an incident angle β relative to the normal N of the surface of the vibrating element 30... i When the light enters the diffraction grating 34, the reference light L2 is relative to the normal N with the blaze angle θ. B The same angle returns. Therefore, by making the incident angle β the same as the blaze angle θ B Equal, which satisfies the aforementioned incident light K i The condition is 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, since no light is used... Figure 7 The reflector 37 shown is not required. Figure 8 As shown, tilting the vibrating element 30 itself satisfies the condition, thus enabling further miniaturization and high-frequency operation of the laser interferometer 1. Especially in the case of a blazed diffraction grating, the configuration that satisfies the condition is called the "Litterrow configuration," which also has the advantage of significantly improving the diffraction efficiency of the diffracted light.

[0122] also, Figure 9 The spacing P represents the spacing of the blazed diffraction grating; for example, the spacing P is 1 μm. Additionally, the blaze angle θ... B For example, 25°. In this case, to satisfy the condition, it is only necessary to adjust the incident light K... i The incident angle β relative to the normal N can also be set to 25°.

[0123] 1.1.1.6.5. Packaging Structure

[0124] Figure 10 This is a cross-sectional view showing an optical modulator 12 with an encapsulation structure.

[0125] Figure 10 The optical modulator 12 shown includes a container 70 as a housing, an optical modulation oscillator 120 housed in the container 70, and circuit elements 45 constituting an oscillation circuit 54. Furthermore, the container 70 is hermetically sealed, for example, in a reduced pressure atmosphere such as a vacuum, or in an inert gas atmosphere such as nitrogen or argon.

[0126] like Figure 10 As shown, the container 70 has a container body 72 and a lid 74. The container body 72 has a first recess 721 disposed inside it 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 terminals and the external terminals.

[0127] Additionally, the opening of the container body 72 is blocked by the cap 74 via a sealing ring (not shown) or a low-melting-point glass or other sealing component. The cap 74 is made of a material that allows laser light to pass through, such as glass.

[0128] 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).

[0129] 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. In addition, circuitry other than the oscillation circuit 54 described later may be disposed on the circuit element 45.

[0130] By employing such a 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. This suppresses external noise from entering the drive signal Sd, or conversely, prevents the drive signal Sd from becoming a noise source. Furthermore, a container 70 can protect both the optical modulation oscillator 120 and the circuit element 45 from external environmental influences. Therefore, the reliability of the laser interferometer 1 can be improved while miniaturizing the sensor head 51.

[0131] Furthermore, the structure of container 70 is not limited to the structure shown in the figure. For example, the optical modulation oscillator 120 and circuit element 45 can also have independent packaging structures. Additionally, although not shown, other circuit elements constituting the oscillation circuit 54 can also be housed in container 70. Moreover, container 70 can be provided as needed and may be omitted.

[0132] Furthermore, the optical modulator 12 is not limited to having the vibrating element 30 described above; for example, it may be an acousto-optic modulator (AOM), an electro-optic modulator (EOM), or the like. In addition, when an AOM or EOM is applied to the optical modulator 12, a light reflection function may be added to the AOM or EOM.

[0133] 1.1.2. Differential Amplifier Circuit and Current-to-Voltage Converter

[0134] Figure 11 It means Figure 1 The circuit diagram shown is an example of the configuration of the differential amplifier circuit 530 and the current-to-voltage converter 531.

[0135] As mentioned above, the first light-receiving element 10 and the second light-receiving element 11 are, for example, photodiodes, each having a cathode and an anode.

[0136] Figure 11 The differential amplifier circuit 530 shown includes a connection wiring 530a connecting the anode of the first light-receiving element 10 and the cathode of the second light-receiving element 11, and a connection wiring 530b connecting the cathode of the first light-receiving element 10 and the anode of the second light-receiving element 11. The differential amplifier circuit 530 performs differential amplification processing to obtain the difference between the photocurrent I1 from the first light-receiving element 10 and the photocurrent I2 from the second light-receiving element 11, and outputs it as a differential current Id.

[0137] Figure 11 The current-to-voltage converter 531 shown includes a first input terminal 531a, a second input terminal 531b, an operational amplifier 531c, a feedback resistor 531d, and an output terminal 531e.

[0138] Connecting wire 530a is connected to the first input terminal 531a, and connecting wire 530b is connected to the second input terminal 531b.

[0139] The first input terminal 531a is connected to the inverting input terminal of the operational amplifier 531c. The second input terminal 531b is connected to the non-inverting input terminal of the operational amplifier 531c. In addition, the second input terminal 531b and the non-inverting input terminal of the operational amplifier 531c are connected to a reference potential such as ground potential.

[0140] Feedback resistor 531d is connected between the output terminal and the inverting input terminal of operational amplifier 531c. Feedback resistor 531d provides negative feedback and converts current into voltage. Through the negative feedback provided by feedback resistor 531d, the potential difference between the inverting and non-inverting input terminals of operational amplifier 531c is made almost zero. Thus, a so-called virtual short is established.

[0141] Output terminal 531e is connected to the output terminal of operational amplifier 531c.

[0142] The current-to-voltage converter 531, also known as a transimpedance amplifier (TIA), converts the differential current Id input to the first input terminal 531a into a voltage signal and outputs it as a light-receiving signal from the output terminal 531e.

[0143] By incorporating the aforementioned differential amplifier circuit 530, the signal-to-noise ratio (S / N ratio) can be improved in the light-receiving signal output from the current-to-voltage converter 531 or in the result of a calculation using the light-receiving signal.

[0144] Specifically, the differential amplifier circuit 530 can perform differential processing to obtain the difference between the photocurrent I1 from the first photoreceiving element 10 and the photocurrent I2 from the second photoreceiving element 11, and output it as a differential current Id. According to the differential processing, the DC components of the photocurrents I1 and I2 are canceled or suppressed. Therefore, the differential current Id contains almost no DC component. This allows for increased conversion gain in the current-to-voltage converter 531, thereby improving the signal-to-noise ratio (S / N ratio) of the received signal. Furthermore, when performing the operation of amplifying the AC signal contained in the received signal using the demodulation circuit 52 (described later), the DC component does not need to be considered. As a result, the accuracy of the operation is increased, and the S / N ratio of the operation result is improved.

[0145] Furthermore, the amplitude of the differential current Id is larger than the amplitudes of the photocurrents I1 and I2. In other words, the photocurrents I1 and I2 are amplified in the differential amplifier circuit 530, resulting in an amplified differential current Id. Therefore, by using this differential current Id, a high signal-to-noise ratio (S / N) light-receiving signal can be obtained.

[0146] Additionally, common-mode noise corresponding to the length of the connecting wires 530a and 530b is introduced. This noise is mostly introduced in phase with respect to both connecting wires 530a and 530b. Therefore, differential processing is implemented to cancel or suppress the noise. This improves the signal-to-noise ratio (S / N) of the received optical signal.

[0147] Furthermore, the configuration of the differential amplifier circuit 530 is not limited to the configuration described above. For example, after converting the photocurrents I1 and I2 into voltage signals independently, the phase of one of the voltage signals can be reversed, and then the differential voltage between the two voltage signals can be calculated, and the calculation result can be used as the received light signal.

[0148] A current-to-voltage converter 531 and a demodulation circuit 52 are configured between them. Figure 1 The ADC532 shown is an analog-to-digital converter that converts analog signals into digital signals at a specified number of sampling bits.

[0149] 1.1.3. Oscillator Circuit

[0150] like Figure 1 As shown, the oscillation circuit 54 outputs a drive signal Sd, which is input to the optical modulator 12 of the optical system 50. Additionally, the oscillation circuit 54 outputs a reference signal Ss, which is input to the demodulation circuit 52.

[0151] The oscillation circuit 54 is any circuit that can make the vibrating element 30 oscillate; there are no particular limitations, and various circuit configurations can be used. Figure 12 This is a circuit diagram illustrating the configuration of a single-stage inverter oscillator circuit as an example of circuit configuration.

[0152] Figure 12 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.

[0153] Circuit element 45 is an inverter IC. Terminals X1 and X2 of circuit element 45 are connected to the internal inverter of circuit element 45, respectively. Terminal GND is connected to ground potential, and terminal Vcc is connected to power supply potential. Terminal Y is used for oscillation output.

[0154] A first capacitor Cg is connected between terminal X1 and the ground potential. Additionally, a limiting resistor Rd and a second capacitor Cd are connected in series between terminal X2 and the ground potential, starting from terminal X2. Furthermore, one end of the 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.

[0155] In addition, 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.

[0156] Figure 13 This is an example of the LCR equivalent circuit of the vibrating element 30.

[0157] like Figure 13 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.

[0158] exist Figure 12 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 It is given by the following equation (a).

[0159]

Mathematical Formula 1

[0160]

[0161] Therefore, the oscillation frequency f output from terminal Y of the oscillation circuit 54 osc It is given by the following equation (b).

[0162]

Mathematical Formula 2

[0163]

[0164] f Q It is the natural frequency of the vibrating element 30.

[0165] According to equation (b) above, by appropriately changing the load capacitance C L It can control the oscillation frequency f of the signal output from terminal Y. osc Make minor adjustments.

[0166] In addition, the natural frequency f of the vibrating element 30 Q The oscillation frequency f of the oscillation circuit 54 osc The difference Δf is given by the following equation (c).

[0167]

Mathematical Expression 3

[0168]

[0169] Here, since C1 << C O C1 << C L Therefore, Δf is approximately given by the following equation (d).

[0170]

Mathematical Expression 4

[0171]

[0172] Therefore, the oscillation frequency f of the oscillation circuit 54 osc Becoming the natural frequency f of the vibrating element 30 Q The corresponding value.

[0173] Here, when the vibrating element 30 is fixed in the container 70, for example, if it is subjected to expansion stress caused by temperature through the fixing part, the natural frequency f Q The change occurs. Furthermore, if the vibrating element 30 is tilted, the natural frequency f... Q It changes due to the influence of gravity and other factors caused by its own weight.

[0174] In the oscillator circuit 54, even if the inherent frequency f is reduced for such reasons... Q The change has occurred; based on the above equation (d), the oscillation frequency f osc It also changes in a manner linked to this change. That is, the oscillation frequency f osc Always from the natural frequency f Q The value deviates from Δf. Therefore, the vibration of the vibrating element 30 is stable, and the displacement amplitude is stable. By stabilizing the displacement amplitude, the modulation characteristics of the optical modulator 12 are stabilized, thus improving the demodulation accuracy of the sampled signal in the demodulation circuit 52.

[0175] As an example, the preferred value is Δf = |f osc -f Q|≤3000[Hz], more preferably 600[Hz].

[0176] As described above, in the laser interferometer 1 of this embodiment, the optical modulator 12 includes a vibration element 30, and the optical modulator 12 uses the vibration element 30 to modulate the first split light L1a.

[0177] Based on this configuration, the optical modulator 12 can be miniaturized and made lighter. Therefore, the laser interferometer 1 can be miniaturized and made lighter.

[0178] In addition, the laser interferometer 1 includes a demodulation circuit 52 and an oscillation circuit 54. Furthermore, the oscillation circuit 54 utilizes the vibrating element 30 as its signal source, and as follows... Figure 1 The demodulation circuit 52 outputs a reference signal Ss. Based on the reference signal Ss, the demodulation circuit 52 demodulates the sampled signal from the object to be measured 14 according to the received light signal.

[0179] Based on this configuration, even the natural frequency f of the vibrating element 30 Q Changes can also alter the oscillation frequency f of the oscillation circuit 54. osc The change is related to the natural frequency f of the vibrating element 30. Q The corresponding values ​​allow for easy stabilization of the vibration of the vibrating element 30. This enables the temperature characteristics of the modulation signal to correspond to the temperature characteristics of the vibrating element 30, thereby stabilizing the modulation characteristics of the optical modulator 12. Consequently, the demodulation accuracy of the sampled signal in the demodulation circuit 52 can be improved.

[0180] 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 vibrating element 30. Therefore, since both the temperature characteristics of the modulation signal and the temperature characteristics of the reference signal correspond to the temperature characteristics of the vibrating element 30, the changes in the modulation signal accompanying temperature changes are consistent with or approximately the changes in the reference signal Ss. Thus, even if the temperature of the vibrating element 30 changes, the impact on demodulation accuracy can be suppressed, and the demodulation accuracy of the sampled signal from the measurement object 14 can be improved.

[0181] Furthermore, since the oscillation circuit 54 has low power consumption, it is easy to achieve power saving in the laser interferometer 1.

[0182] Alternatively, a signal generator, such as a function generator or a signal generator, can be used instead of the oscillation circuit 54.

[0183] 1.2. Demodulation Circuit

[0184] The demodulation circuit 52 performs demodulation processing as follows: it demodulates the sampled signal from the object to be measured 14 based on the light-receiving signal output from the current-to-voltage converter 531. The sampled signal includes, for example, phase information and frequency information. Furthermore, the displacement of the object to be measured 14 can be obtained from the phase information, and the velocity of the object to be measured 14 can be obtained from the frequency information. If different physical quantities can be obtained in this way, it can function as a displacement meter or a velocity meter, thus enabling high functionality of the laser interferometer 1.

[0185] The circuit configuration of the demodulation circuit 52 is set according to the modulation processing method. In the laser interferometer 1 according to this embodiment, an optical modulator 12 equipped with a vibrating element 30 is used. The vibrating element 30 is a simple harmonic oscillation element, and therefore its vibration speed changes constantly within the period. Therefore, the modulation frequency also changes with time, and existing demodulation circuits cannot be used directly.

[0186] Existing demodulation circuits refer to circuits that demodulate sampled signals from received optical signals, for example, signals containing modulated signals modulated using an acousto-optic modulator (AOM). In an acousto-optic modulator, the modulation frequency remains constant. Therefore, existing demodulation circuits can demodulate sampled signals from received optical signals containing modulated signals with a constant modulation frequency, but they cannot directly demodulate signals containing modulated signals modulated using an optical modulator 12 with a varying modulation frequency.

[0187] therefore, Figure 1 The demodulation circuit 52 shown includes a preprocessing unit 53 and a demodulation processing unit 55. The light-receiving signal output from the current-to-voltage converter 531 first passes through the preprocessing unit 53 and is then guided to the demodulation processing unit 55. The preprocessing unit 53 performs preprocessing on the light-receiving signal. Through this preprocessing, a signal that can be demodulated using a conventional demodulation circuit is obtained. Therefore, in the demodulation processing unit 55, the sampling signal from the object to be measured 14 is demodulated using a known demodulation method.

[0188] 1.2.1. Composition of the pretreatment unit

[0189] Figure 1 The preprocessing unit 53 shown includes a first bandpass filter 534, a second bandpass filter 535, a first delay adjuster 536, a second delay adjuster 537, a multiplier 538, a third bandpass filter 539, a first AGC 540, a second AGC 541, and an adder 542. Furthermore, AGC stands for Auto Gain Control.

[0190] The light-receiving signal output from the current-to-voltage converter 531 is split into two signals, a first signal S1 and a second signal S2, at the bifurcation point jp1. Figure 1In this process, the path of the first signal S1 is set as the first signal path ps1, and the path of the second signal S2 is set as the second signal path ps2.

[0191] An ADC533 is connected between the oscillator circuit 54 and the second delay adjuster 537. The ADC533 is an analog-to-digital converter that converts analog signals into digital signals at a specified number of sampling bits.

[0192] The first bandpass filter 534, the second bandpass filter 535, and the third bandpass filter 539 are filters that selectively allow signals in specific frequency bands to pass through.

[0193] The first delay adjuster 536 and the second delay adjuster 537 are circuits that adjust the delay of the signals, respectively. The multiplier 538 is a circuit that generates an output signal proportional to the product of the two input signals. The adder 542 is a circuit that generates an output signal proportional to the sum of the two input signals.

[0194] Next, the operation of the preprocessing unit 53 will be explained along the flow of the first signal S1, the second signal S2, and the reference signal Ss.

[0195] After passing through the first bandpass filter 534 configured on the first signal path ps1, the group delay of the first signal S1 is adjusted by the first delay adjuster 536. The group delay adjusted by the first delay adjuster 536 is equivalent to the group delay of the second signal S2 caused by the second bandpass filter 535 (described later). Through this delay adjustment, the delay time generated by the filter circuits passing through the first bandpass filter 534 through which the first signal S1 passes is made consistent between the second bandpass filter 535 and the third bandpass filter 539 through which the second signal S2 passes. After passing through the first delay adjuster 536, the first signal S1 is input to the adder 542 via the first AGC 540.

[0196] The second signal S2 is input to the multiplier 538 after passing through the second bandpass filter 535 configured on the second signal path ps2. In the multiplier 538, the second signal S2 is multiplied by the reference signal Ss output from the second delay adjuster 537. Specifically, the reference signal Ss output from the oscillator circuit 54 is multiplied by cos(ω... m The reference signal Ss, represented by t), is digitally converted in the ADC533, its phase is adjusted in the second delay adjuster 537, and then input to the multiplier 538. ω m t is the angular frequency of the modulation signal of the optical modulator 12, and t is time. Then, after passing through the third bandpass filter 539, the second signal S2 is input to the adder 542 via the second AGC 541.

[0197] Adder 542 outputs an output signal that is proportional to the sum of the first signal S1 and the second signal S2.

[0198] 1.2.2. Basic Principles of Preprocessing

[0199] Next, the basic principle of preprocessing in preprocessing unit 53 will be explained. In the following explanation, as an example, a system in which the frequency of the modulation signal varies sinusoidally and the displacement of the object being measured 14 also varies in simple harmonic motion along the optical axis will be considered. Here, E... m E d φ is set as

[0200]

Mathematical Expression 5

[0201] E m =a m {cos(ω0t+B sinω m t+φ m )+i sin(ω0t+B sinω m t+φ m )} (1)

[0202] E d =a d {cos(ω0t+A sinω d t+φ d )+i sin(ω0t+A sinω d t+φ d (2)

[0203] φ=φ m -φ d (3)

[0204] At that time, the light-receiving signal I output from the current-to-voltage converter 531 PD Theoretically, it can be expressed by the following formula.

[0205]

Mathematical Expression 6

[0206] I PD =<|E m +E d | 2 >

[0207] =<|E m 2 +E d 2 +2E m E d |>

[0208] =a m 2 +a d2 +2a m a d cos(B sinω m tA sinω d t+φ) (4)

[0209] In addition, E m E d φ m φ d , φ, ω m ω d , ω0, a m a d As shown below.

[0210]

Mathematical Expression 7

[0211] E m Electric field component of the modulation signal from the optical modulator

[0212] E d Electric field component from the sampling signal of the object being measured.

[0213] φ m The initial phase of the modulation signal from the optical modulator

[0214] φ d The initial phase of the sampling signal from the object being measured.

[0215] φ: Optical path phase difference of the laser interferometer

[0216] ω m Angular frequency of the modulation signal from the optical modulator

[0217] ω d Angular frequency of the sampled signal from the object being measured.

[0218] ω0: Angular frequency of the emitted light from the light source

[0219] a m :coefficient

[0220] a d :coefficient

[0221] In addition, in equation (4), <> represents time average.

[0222] The first and second terms of equation (4) above represent the DC component, and the third term represents the AC component. If this AC component is set as I... PD·AC , then I PD·AC As shown in the following formula.

[0223]

Mathematical Expression 8

[0224] IPD·AC =2a m a d cos(B sinω m tA sinω d t+φ)

[0225] =2a m a d {cos(B sinω m t)cos(A sinω d t-φ)+sin(B sinω m t)sin(A sin ω d t-φ)} (5)

[0226]

[0227]

[0228] A: Phase shift of the sampled signal

[0229] f dmax Doppler frequency shift of the sampled signal

[0230] f d Frequency of the sampled signal

[0231] B: Phase shift of the modulated signal

[0232] f mmax Doppler frequency shift of the modulated signal

[0233] f m Frequency of the modulating signal

[0234] Here, we know that there are ν-th degree Bessel functions as shown in equations (8) and (9).

[0235]

Mathematical Expression 9

[0236]

[0237]

[0238] If we use the Bessel functions of equations (8) and (9) above to perform a series expansion of equation (5), it can be transformed as shown in equation (10) below.

[0239]

Mathematical Formula 10

[0240] I PD.AC =2a m a d [{J0(B)+2J2(B)cos(2·ω mt)+2J4(B)cos(4·ω m t)+…}cos(A sinω d t-φ)-{2J1(B)sin(1·ω m t)+2J3(B)sin(3·ω m t)+…}sin(A sinω d t-φ)] (10)

[0241] Where J0(B), J1(B), J2(B), ... are Bessel coefficients.

[0242] If the frequency band is modified as described above, then theoretically it can be said that a bandpass filter can be used to extract the frequency band corresponding to a specific number of frequencies.

[0243] Therefore, in the aforementioned preprocessing unit 53, the received light signal is preprocessed according to the following process based on this theory.

[0244] First, the light-receiving signal output from the current-to-voltage converter 531 is split into two signals, a first signal S1 and a second signal S2, at the bifurcation point jp1. The first signal S1 passes through a first bandpass filter 534. The center angular frequency of the first bandpass filter 534 is set to ω. m Therefore, the first signal S1 after passing through the first bandpass filter 534 is represented by the following formula.

[0245]

Mathematical Expression 11

[0246] I pass1 =J1(B){-cos(ω m t+A sinω d t-φ)+cos(ω m tA sinω d t+φ)}

[0247] =-2J1(B)sin(ω) m t)sin(A sinω d t-φ) (11)

[0248] On the other hand, the second signal S2 passes through the second bandpass filter 535. The center angular frequency of the second bandpass filter 535 is set to a value different from the center angular frequency of the first bandpass filter 534. Here, as an example, the center angular frequency of the second bandpass filter 535 is set to 2ω. m Therefore, the second signal S2 after passing through the second bandpass filter 535 is represented by the following formula.

[0249]

Mathematical Expression 12

[0250]

[0251] The second signal S2, after passing through the second bandpass filter 535, is multiplied by the reference signal Ss by the multiplier 538. The second signal S2 after passing through the multiplier 538 is represented by the following formula.

[0252]

Mathematical Expression 13

[0253]

[0254] The second signal S2, after passing through multiplier 538, passes through third bandpass filter 539. The center angular frequency of third bandpass filter 539 is set to the same value as the center angular frequency of first bandpass filter 534. Here, as an example, the center angular frequency of third bandpass filter 539 is set to ω. m Therefore, the second signal S2 after passing through the third bandpass filter 539 is represented by the following formula.

[0255]

Mathematical Expression 14

[0256]

[0257] Then, the phase of the first signal S1 represented by the above formula (11) is adjusted by the first delay adjuster 536, and the amplitude is adjusted by the first AGC 540.

[0258] In addition, the amplitude of the second signal S2 represented by the above formula (14) is also adjusted by the second AGC541, so that the amplitude of the second signal S2 is consistent with the amplitude of the first signal S1.

[0259] Furthermore, the first signal S1 and the second signal S2 are added together by adder 542. The result of the addition is represented by the following equation (15).

[0260]

Mathematical Expression 15

[0261] I 53 =cos(ω m t+A sinω d t-φ) (15)

[0262] As shown in equation (15) above, the result of the addition is that unnecessary terms disappear and necessary terms can be extracted. This result is input into the demodulation processing unit 55.

[0263] 1.2.3. Composition of the demodulation processing unit

[0264] The demodulation processing unit 55 performs demodulation processing as follows: it demodulates the sampled signal from the object to be measured 14 based on the signal output from the preprocessing unit 53. There are no particular limitations on the demodulation processing method; a well-known method of quadrature detection can be cited. Quadrature detection is a method of demodulation processing that involves externally mixing mutually orthogonal signals from the input signal.

[0265] Figure 1 The demodulation processing unit 55 shown is a digital circuit that includes a multiplier 551, a multiplier 552, a phase shifter 553, a first low-pass filter 555, a second low-pass filter 556, a divider 557, an arctangent operator 558, and an output circuit 559.

[0266] 1.2.4. Demodulation processing in the demodulation processing unit

[0267] In the demodulation process, firstly, the signal output from the preprocessing unit 53 is split into two parts using the bifurcation section jp2. In the multiplier 551, one of the split signals is multiplied by the coefficient of performance (cos(ω)) output from the oscillation circuit 54. m The reference signal Ss is represented by t). In multiplier 552, the split signal is multiplied by -sin(ωt), which shifts the phase of the reference signal Ss output from oscillation circuit 54 by -90° through phase shifter 553. m The signal represented by t). The reference signal Ss and the signal whose phase is shifted by the reference signal Ss are signals whose phases are shifted by 90° from each other.

[0268] The signal after passing through multiplier 551 passes through first low-pass filter 555, and then is input as signal x to divider 557. The signal after passing through multiplier 552 passes through second low-pass filter 556, and then is input as signal y to divider 557. In divider 557, the signal y is divided by the signal x, and the output y / x is passed through arctangent operator 558 to obtain the output atan(y / x).

[0269] Then, by passing the output atan(y / x) through the output circuit 559, the phase φ is calculated. d As information from the object being measured 14, the output circuit 559 performs phase unrolling processing to connect the phases when there is a 2π phase jump between adjacent points. Furthermore, the displacement of the object being measured 14 can be calculated from the phase information. Thus, a displacement meter is implemented. Additionally, the velocity can be derived from the displacement. Thus, a speed meter is implemented.

[0270] The circuit configuration of the demodulation processing unit 55 has been described above, but the circuit configuration of the digital circuit described above is an example and is not limited thereto. For example, the configuration of the preprocessing unit 53 is not limited to the above configuration. In addition, the demodulation processing unit 55 is not limited to a digital circuit and may also be an analog circuit. The analog circuit may also include an F / V converter circuit or a ΔΣ counter circuit.

[0271] Alternatively, the circuit configuration of the demodulation processing unit 55 can also be configured to obtain frequency information from the object being measured 14. The speed of the object being measured 14 can be calculated based on the frequency information.

[0272] 1.3. Suppression of Returning Light by Optical Axis Misalignment

[0273] Next, the case of suppressing reflected light by shifting the optical axis will be explained. Furthermore, in the following explanation, "distance" refers to optical distance (optical path length).

[0274] The emitted light L1 from laser source 2 diffuses due to light diffraction. Specifically, from the optical path R... LD The optical path R of the emitted light L1 from the laser source 2 is at a separation distance x LD The position is represented by the diffusion radiation angle θ by the following equation (16).

[0275]

Mathematical Expression 16

[0276] R = R LD +θx LD (16)

[0277] The second term in equation (16) above is the diffraction term. The phenomenon of light diffraction is generally represented by the following equation (16-1).

[0278]

Mathematical Expression 17

[0279] R LD sinθ=mλ (16-1)

[0280] Since the diffraction terms of the second order and higher orders are very small, they are not considered. In this case, m = 1 can be taken. In addition, the diffusion radiation angle θ is generally small, so θ << 1. Therefore, the above equation (16-1) can be transformed into the following equation (16-2).

[0281]

Mathematical Expression 18

[0282]

[0283] Therefore, equation (16) can be transformed into equation (16-3).

[0284]

Mathematical Expression 19

[0285]

[0286] The optical path R of the outgoing light L1 emitted from the laser source 2 and entering the collimating lens 3 can be obtained from the above equation (16-3).

[0287] Figure 14 It means from Figure 2 The diagram shows the trajectory of the emitted light L1 (first segmented light L1a) from the laser source 2, and the reference light L2 generated by the reflection of the emitted light L1 by the light modulator 12 as the trajectory of the return light L5 towards the laser source. Furthermore, in Figure 14 For ease of illustration, the extended optical path refracted by the first optical splitter 4 is shown in the diagram. Additionally, although not shown, the return light L5, besides... Figure 14 In addition to the light from the reference light L2, the light from the object light L3 may also be included. Furthermore, the return light L5 may consist solely of the light from the object light L3.

[0288] exist Figure 14 In this design, the center of the collimating lens 3 is set as the origin O (reference position), and the distance between the origin O and the optical modulator 12 is set as L. Furthermore, if the collimating lens 3 is omitted, the position of the collimated outgoing light L1 is set as the origin O (reference position). For example, in a gas laser such as a He-Ne laser, the collimated outgoing light L1 is emitted from the emission point, thus the emission point becomes the origin. Additionally, the optical path of the returning light L5 when it reaches the collimating lens 3 is set as R'.

[0289] Therefore, the optical path R' of the returning light L5 is represented by the following equation (16-4).

[0290]

Mathematical Expression 20

[0291]

[0292] In this embodiment, by making Figure 14 The optical axis A of the emitted light L1 (first segmented light L1a) shown is... L1a Optical axis A of reference light L2 L2 The return light L5 is offset and suppressed from entering the laser source 2. In this specification, the optical axis A will be... L1a With optical axis A L2 This misalignment is known as "optical axis offset". Figure 2 and Figure 14 In the example, optical axis A is made L2 It is offset from the Z-axis direction relative to the optical axis of positive reflection.

[0293] The returned light L5 is the cause of instability in the laser oscillation in laser source 2. Therefore, if the intensity of the returned light L5 can be reduced by shifting the optical axis, the laser oscillation can be stabilized.

[0294] The instability of laser oscillations caused by the returned light L5 is related to self-mixing coupling in laser source 2. Self-mixing coupling is quantified by an index M, which is the product of the coupling coefficient and the amount of returned light L5. The coupling coefficient is proportional to the reciprocal of the resonator length of laser source 2. Therefore, self-mixing coupling tends to become significant in semiconductor laser elements with short resonator lengths when the amount of returned light L5 increases. It is believed that if self-mixing coupling in semiconductor laser elements is suppressed, self-mixing coupling in most types of light sources will also be suppressed.

[0295] To suppress self-mixing coupling in laser source 2, the index M preferably satisfies M < 10. -6 This is equivalent to satisfying OD6 when the shielding capability of the light intensity during optical axis shift is expressed in terms of optical density (OD value). If the optical axis shift has a shielding capability that satisfies OD6, then the stabilization of laser oscillation can be achieved.

[0296] Based on this, as long as the light intensity P0 of the emitted light L1 and the light intensity P of the returned light L5, which is not blocked by the optical axis shift, are both sufficient... r The following relationship (17) must be satisfied.

[0297]

Mathematical Expression 21

[0298]

[0299] Therefore, the condition under which the returning light L5 does not enter the collimating lens 3 is investigated. When the returning light L5 reaches the collimating lens 3, it becomes a beam with an optical path R'. However, as long as this beam does not overlap with the effective diameter of the collimating lens 3, the light intensity P can be maintained. r In essence, it is zero. That is to say, it satisfies the relationship of the above equation (17).

[0300] The result of the optical axis shift is that, at the position of collimating lens 3, the optical axis A of the outgoing light L1 is shifted. L1 Optical axis A of the return light L5 L5 An offset occurs between them. Figure 14 In the middle, the optical axis A in the collimating lens 3 is... L1 With optical axis A L5 The deviation range is set as Δy [mm]. In addition, the effective diameter of the collimating lens 3 is set as κ [mm]. Thus, the condition required for the deviation range Δy to satisfy the relationship of the above equation (17) is expressed by the following equation (17-1).

[0301]

Mathematical Expression 22

[0302]

[0303] If we substitute the above equation (16-4) into the above equation (17-1), the condition required for the deviation amplitude Δy is expressed by the following equation (A).

[0304]

Mathematical Expression 23

[0305]

[0306] Therefore, the deviation magnitude Δy of the optical axis offset can be selected in a manner that satisfies the above equation (A).

[0307] Here, as Figure 14 As shown, the deviation amplitude Δy is adjusted by the tilting light modulator 12. Specifically, in Figure 14 In the optical system 50 shown, the optical axis A of the reference light L2 generated by the light modulator 12 is... L2 The optical axis A of the first split light L1a incident on the light modulator 12 L1a The optical modulator 12 is tilted by staggering the beams. This tilts the optical axis A of the first split beam L1a incident on the optical modulator 12. L1a The optical axis A of the reference light L2 generated by the optical modulator 12 L2 The resulting angle is set as the deviation angle θ'.

[0308] The deviation magnitude Δy is expressed using the deviation angle θ' by the following equation (17-2).

[0309]

Mathematical Expression 24

[0310] Δy=L tanθ′ (17-2)

[0311] The deviation angle θ' is generally small, so θ' << 1. Therefore, the above equation (17-2) can be transformed into the following equation (17-3).

[0312]

Mathematical Expression 25

[0313]

[0314] Therefore, in order to achieve the deviation magnitude Δy that satisfies the above equation (A), it is only necessary to adjust the distance L and the deviation angle θ' according to the above equation (17-3).

[0315] Furthermore, as long as the deviation amplitude Δy satisfies the aforementioned equation (A), the intensity of the return light L5 incident on the laser source 2 can be sufficiently suppressed. Thus, the laser oscillation in the laser source 2 can be stabilized. As a result, the quality of the emitted light L1 is stable, thereby suppressing the reduction in accuracy of the demodulation sampling signal based on the received light signal in the demodulation circuit 52.

[0316] Furthermore, when the deviation amplitude Δy deviates from the range of Equation (A), the probability of the return light L5 entering the collimating lens 3 increases. Therefore, the intensity of the return light L5 entering the laser source 2 increases, and the laser oscillation may become unstable.

[0317] Furthermore, the deviation amplitude Δy is appropriately adjusted according to other parameters. For example, it is preferable to satisfy 0.10≤Δy≤10.0, more preferably 0.50≤Δy≤10.0, even more preferably 2.10≤Δy≤10.0, and particularly preferably 2.30≤Δy≤10.0. Additionally, considering the size of the sensor head 51, the upper limit of the deviation amplitude Δy is more preferably 6.00 or less, and even more preferably 3.00 or less.

[0318] Furthermore, if the deviation magnitude Δy is lower than the lower limit, the deviation angle θ' becomes too small, potentially reducing the ease of manufacturing the optical system 50. On the other hand, if the deviation magnitude Δy exceeds the upper limit, it is necessary to increase the size of the first optical splitter 4 or the second optical splitter 5, thus making the miniaturization of the sensor head 51 potentially difficult.

[0319] Furthermore, the distance L between the collimating lens 3 and the light modulator 12 is preferably 5.0 mm or more and 200 mm or less, more preferably 10.0 mm or more and 100 mm or less. This helps to suppress the enlargement of the optical system 50.

[0320] 1.3.1. First Calculation Example

[0321] As an example of the case that satisfies equation (A), a first calculation example is shown. The parameters and calculation results of the first calculation example are shown in Table 1.

[0322] Table 1

[0323]

[0324] In the first calculation example, as shown in Table 1, the deviation Δy required to satisfy equation (A) is 0.7 mm. This deviation Δy is the distance x corresponding to the focal length of the collimating lens 3. LD This value can also be achieved in a small optical system 50, where the collimating lens 3 is about 1.0 mm and the distance L between the collimating lens 3 and the light modulator 12 is about 10.0 mm.

[0325] Therefore, in the first calculation example, the realization of the optical system 50 satisfying equation (A) was confirmed. Thus, the incidence of the return light L5 relative to the laser source 2 can be suppressed to a level where laser oscillations do not become unstable.

[0326] 1.3.2. Second Calculation Example

[0327] As an example of the case that satisfies equation (A), a second calculation example is shown. The parameters and calculation results of the second calculation example are shown in Table 2.

[0328] Table 2

[0329]

[0330] In the second calculation example, as shown in Table 2, the deviation Δy required to satisfy equation (A) is 5.7 mm. This deviation Δy is the distance x corresponding to the focal length of the collimating lens 3. LD This value can be fully realized in an optical system 50 with a size of about 30mm and a distance L between the collimating lens 3 and the light modulator 12 of about 100mm.

[0331] Therefore, in the second calculation example, the realization of the optical system 50 satisfying equation (A) was also confirmed. Thus, the incidence of the return light L5 relative to the laser source 2 can be suppressed to a level where laser oscillations do not become unstable.

[0332] 1.4. Optical axis shift and suppression of returning light by shielding elements

[0333] Next, regarding Figure 2 and Figure 14 A first variation of the optical system 50 will be described. In this first variation, the returned light is suppressed not only by optical axis offset, but also by the combined use of optical axis offset and shielding element 17.

[0334] Figure 15 This is a schematic diagram showing the sensor head 51 of the laser interferometer 1 involved in the first modified example. Figure 16 It means from Figure 15 The diagram shows the traces of the emitted light L1 and the first segmented light L1a emitted from the laser source 2, and the trace of the reference light L2 generated by the reflection of the first segmented light L1a by the light modulator 12 as the return light L5 towards the laser source 2.

[0335] Figure 15 The sensor head 51 involved, in addition to Figure 2 In addition to the configuration shown, it also includes a shielding element 17.

[0336] The shielding element 17 is an aperture stop (or aperture barrier) disposed between the collimating lens 3 and the first optical splitter 4. The shielding element 17 has an opening 172 corresponding to the optical path 18. For example... Figure 16 As shown, the shielding element 17 suppresses the return light L5 from the reference light L2 from entering the laser source 2.

[0337] The shielding element 17 only needs to function as a shield for the returned light L5, thus its structure is very simple. This contributes to the simplification of the structure of the laser interferometer 1. Furthermore, the shielding element 17 can also be a slit or a pinhole, etc., and its structure is not particularly limited.

[0338] Since the shielding element 17 is disposed between the collimating lens 3 and the first optical splitter 4, it is only necessary to make the diameter φ of the opening 172 of the shielding element 17... pin If the optical path R' of the returned light L5 is smaller than that of L5, the intensity of the returned light L5 can be reduced. Furthermore, when the opening 172 is not a perfect circle, the diameter φ of the opening 172... pin It is the diameter of the circle inscribed in opening 172.

[0339] In this modified example, only the light intensity P of the emitted light L1 is required. O The light intensity P of the returned light L5 that is not blocked by the optical axis offset and the shielding element 17 r The following relationship (17) must be satisfied.

[0340]

Mathematical Expression 26

[0341]

[0342] Here, the light intensity of the return light L5 that is about to enter the shielding element 17 is set as P'r. The light intensity P'r of the return light L5 is represented by the following equation (18). Furthermore, the following equation (18) is an example of the case where the light intensity distribution of the outgoing light L1 is a Gaussian distribution or a distribution based on it.

[0343]

Mathematical Expression 27

[0344] P′ r =P1×P2×P3 (18)

[0345]

[0346]

[0347]

[0348]

[0349] P3 = 1 - cos 2 (2Δψ) (18-3)

[0350] In equation (18-1) above, P is the peak light intensity of the emitted light L1, r is the radial position of the emitted light L1 from the central axis, and w is the light intensity of the emitted light L1 becoming 1 / e of the peak light intensity. 2The distance between the emitted light L1 and the central axis in the radial direction at the position. Furthermore, in the case of emitted light L1, since θ << 1, it can be set as θ = λ / r. Therefore, w is as shown in equation (18-1-1) above. Thus, equation (18-1) above can be modified as in equation (18-1-2) above. Furthermore, in equation (18-2) above, Refl M It is the light reflectance in the light modulator 12, Refl S It measures the light reflectance in object 14. Furthermore, in the above formula (18-3), Δψ is the error in the setting angle of the half-wavelength plate, which is simulated by the two plates, the half-wavelength plate 6 and the half-wavelength plate 8.

[0351] P1, as expressed in equation (18-1), represents the light intensity of the emitted light L1 passing through the opening 172 of the shielding element 17. P2, as expressed in equation (18-2), represents the reflectivity of the optical modulator 12 and the reflectivity of the object being measured 14. P3, as expressed in equation (18-3), represents the intensity of the light returning to the laser source 2 side after passing through the first optical splitter 4 when polarized light is accidentally generated due to the allowable angular error of the 1 / 2 wavelength plate.

[0352] According to the above formula (18), the light intensity Pr of the return light L5 that is not blocked by the optical axis offset or the shielding element 17 and passes through the opening 172 is represented by the following formula (19).

[0353]

Mathematical Expression 28

[0354]

[0355] diameter φ pin Inside the circle

[0356]

[0357] In equation (19) above, x and y are the positions along two mutually orthogonal axes in the cross-section of the returning light L5. Furthermore, in equation (19-1) above, Lpin is the distance between the collimating lens 3 and the shielding element 17. Moreover, if the intensity distribution of the emitted light L1 is Gaussian, and the standard deviation of the intensity distribution is set as σ, then w = 2σ.

[0358] Therefore, as long as the diameter φ of the opening 172 of the shielding element 17 is selected in such a way that the light intensity Pr calculated using the above equation (19) satisfies the above equation (17), pin That is, a laser interferometer 1 capable of stabilizing laser oscillations can be realized.

[0359] In addition, in this embodiment, the diameter of the opening 172 is set to φ. pinThe optical axis A of the first split light L1a incident on the light modulator 12 is... L1a The optical axis A of the reference light L2 generated by the optical modulator 12 L2 The angle formed is set as the deviation angle θ'. Furthermore, the distance between the shielding element 17 and the optical modulator 12 is set as ΔL [mm]. At this time, the laser interferometer 1 preferably satisfies the following equation (B).

[0360]

Mathematical Expression 29

[0361]

[0362]

Mathematical Expression 30

[0363] Δy=Lθ′ (B-1)

[0364] ΔL=LL pin (B-2)

[0365] Based on this configuration, by using the optical axis offset and shielding element 17 together, the intensity of the reflected light L5 incident on the laser source 2 can be suppressed, and the signal-to-noise ratio in the received signal can be improved. Both the optical axis offset and shielding element 17 can be implemented with simple structures, thus, the miniaturization and weight reduction of the laser interferometer 1 can be easily achieved.

[0366] Furthermore, when the diameter φ of opening 172 pin When the light intensity is below the lower limit, the intensity of the emitted light L1 through the opening 172 decreases. Consequently, the signal-to-noise ratio (S / N) in the received signal decreases, potentially reducing the accuracy of the displacement or velocity measurement of the object 14. On the other hand, when the diameter φ of the opening 172... pin When the upper limit is exceeded, the light intensity that can be reduced by the shielding element 17 becomes smaller. Therefore, it may not be possible to sufficiently reduce the light intensity of the returned light L5 incident on the laser source 2.

[0367] Here, the derivation process of the above equation (B) will be explained.

[0368] To prevent the returning light L5 from entering the opening 172, the optical axis A of the outgoing light L1 in the shielding element 17 is... L1 Optical axis A of the return light L5 L5 deviation Δy pin The conditions required by [mm] are expressed by the following equation (19-2) according to the above equation (17-1).

[0369]

Mathematical Expression 31

[0370]

[0371] In the above equation (19-2), R pin[mm] is the optical path of the return light L5 in the shielding element 17.

[0372] optical path R pin It is represented by the following formula (19-3).

[0373]

Mathematical Expression 32

[0374]

[0375] Deviation Δy pin It is represented by the following formula (19-4).

[0376]

Mathematical Expression 33

[0377] Δy pin =(LL pin )tanθ′ (19-4)

[0378] The deviation angle θ' is generally small, therefore θ' << 1. Therefore, the above equation (19-4) can be transformed into the following equation (19-5).

[0379]

Mathematical Expression 34

[0380]

[0381] Therefore, in order to achieve the deviation magnitude Δy that satisfies equation (A) pin Simply adjust the distance ΔL and the deviation angle θ' according to the above formula (19-5).

[0382] Here, if we substitute equations (19-3) and (19-5) into equation (19-2), we derive the following equation (B-3).

[0383]

Mathematical Expression 35

[0384]

[0385] Furthermore, from the above equation (B-3), we can derive the quadratic inequality of the following equation (B-4).

[0386]

Mathematical Expression 36

[0387] φ pin 2 -ΔLθ′φ pin +ΔLλ≤0 (B-4)

[0388] Furthermore, ΔL in the above equation (B-4) is defined by the following equation (B-5).

[0389]

Mathematical Expression 37

[0390] ΔL=(LL pin(B-5)

[0391] If we solve the quadratic inequality in equation (B-4) above, we can derive the following equation (B-6).

[0392]

Mathematical Expression 38

[0393]

[0394] Here, the terms in the above equation (B-6) represented by the following equation (B-7) can be approximated as shown in the following equation (B-8).

[0395]

Mathematical Expression 39

[0396]

[0397]

[0398] If the above formula (B-8) is used, then formula (B-6) can be transformed into formula (B) as the derived object.

[0399]

Mathematical Expression 40

[0400]

[0401] As described above, derive formula (B).

[0402] In addition, as a specific numerical range, the diameter φ of the opening 172 pin [mm] Preferably, the following relationship (20) is satisfied.

[0403]

Mathematical Expression 41

[0404] 0.10≤φ pin ≤10.0 (20)

[0405] When the diameter φ of opening 172 pin Within this range, the probability of the returned light L5 being blocked by the blocking element 17 increases. Therefore, the intensity of the returned light L5 reaching the laser source 2 can be suppressed, thereby preventing laser oscillation from becoming unstable.

[0406] In addition, the distance between the light modulator 12 and the first light-receiving element 10 and the second light-receiving element 11 is set as L. PD Here, it is assumed that the distance between the optical modulator 12 and the first light-receiving element 10 is equal to the distance between the optical modulator 12 and the second light-receiving element 11. In this case, the laser interferometer 1 preferably satisfies the following equation (21).

[0407]

Mathematical Expression 42

[0408] ΔL=LL pin ≤LPD (twenty one)

[0409] In equation (21) above, L is the distance between the collimating lens 3 and the optical modulator 12. Additionally, L... pin It is the distance between the collimating lens 3 and the shielding element 17.

[0410] Based on this configuration, the distance L can be ensured. PD The distance is relatively long, therefore, the distance between the laser interferometer 1 and the measurable object 14, i.e., the working distance, can be further increased.

[0411] Furthermore, the distance L between the collimating lens 3 and the shielding element 17 pin Preferably, the diameter is 0.5 mm or more and 15.0 mm or less, more preferably 1.0 mm or more and 10.0 mm or less. This avoids the need for a large optical system 50 and allows the shielding element 17 to function effectively.

[0412] 1.4.1. Third Calculation Example

[0413] The diameter φ of the opening 172, as defined by equation (B), is used for calculation. pin The example given is the third calculation example. The parameters and calculation results of the third calculation example are shown in Table 3.

[0414] Table 3

[0415]

[0416] In the third calculation example, as shown in Table 3, the deviation amplitude Δy of the returned light L5 in the collimating lens 3 is set to 0.80 mm, and the distance x between the collimating lens 3 and the laser source 2 is set to... LD When set to 4.0mm, the diameter φ of the opening 172 of the shielding element 17 is increased. pin Setting the diameter to 0.096–0.633 mm is sufficient to adequately block the return light L5. Therefore, in the third calculation example, the return light L5 can be suppressed relative to the laser source 2 to a level where laser oscillations do not become unstable.

[0417] 1.4.2. Fourth Calculation Example

[0418] The diameter φ of the opening 172, as defined by equation (B), is used for calculation. pin Examples of the range are shown in Table 4, illustrating the fourth calculation example. The parameters and calculation results for the fourth calculation example are shown in Table 4.

[0419] Table 4

[0420]

[0421] In the fourth calculation example, as shown in Table 4, the deviation amplitude Δy of the returned light L5 in the collimating lens 3 is set to 0.80 mm, and the distance x between the collimating lens 3 and the laser source 2 is set to... LD When set to 3.0mm, the diameter φ of the opening 172 of the shielding element 17 is increased. pin Setting the diameter to 0.032–0.581 mm is sufficient to adequately block the return light L5. Therefore, in the fourth calculation example, the return light L5 can be suppressed relative to the laser source 2 to a level where laser oscillations do not become unstable.

[0422] 1.4.3. Fifth Calculation Example

[0423] Next, as an example of the light intensity Pr calculated using the above equation (19) satisfying the above equation (17), a fifth calculation example is shown. The parameters and calculation results of the fifth calculation example are shown in Table 5.

[0424] Table 5

[0425]

[0426] In the fifth calculation example, as shown in Table 5, the diameter φ of the opening 172 of the shielding element 17 is... pin The deviation amplitude Δy of the returned light L5 in the collimating lens 3 is set to 2.10mm (≈3.0σ), and the distance x between the collimating lens 3 and the laser source 2 is set to 1.80mm. LD Setting it to 4.0mm, the error Δψ of the half-wavelength plate's setting angle is suppressed to 0.2deg, thus P r The result of / P0 calculation is less than 1×10 -6 In other words, it has been confirmed that by making the deviation amplitude Δy satisfy the relationship in equation (A), the intensity of the return light L5 incident on the laser source 2 can be suppressed. Therefore, in the fifth calculation example, the return light L5 relative to the incident laser source 2 can be suppressed to a degree that the laser oscillation will not become unstable.

[0427] 1.4.4. Calculation Examples Six through Nine

[0428] As an example of the light intensity Pr calculated using equation (19) satisfying equation (17), the sixth to ninth calculation examples are shown. The parameters and calculation results of the sixth to ninth calculation examples are shown in Table 6.

[0429] Table 6

[0430]

[0431] In the sixth calculation example, as shown in Table 6, the diameter φ of the opening 172 of the shielding element 17 is... pinThe deviation amplitude Δy of the returned light L5 in the collimating lens 3 is set to 2.10mm, and the distance x between the collimating lens 3 and the laser source 2 is set to 2.30mm. LD Setting it to 4.0mm suppresses the error Δψ of the half-wavelength plate's setting angle to 1.0deg, thus P r The result of / P0 calculation is less than 1×10 -6 .

[0432] In the seventh calculation example, as shown in Table 6, the diameter φ of the opening 172 of the shielding element 17 is... pin The deviation amplitude Δy of the returned light L5 in the collimating lens 3 is set to 1.60mm, and the distance x between the collimating lens 3 and the laser source 2 is set to 1.40mm. LD Setting it to 3.0mm suppresses the error Δψ of the half-wavelength plate's setting angle to 0.2deg, thus P r The result of / P0 calculation is less than 1×10 -6 .

[0433] In the eighth calculation example, as shown in Table 6, the diameter φ of the opening 172 of the shielding element 17 is... pin The deviation amplitude Δy of the returned light L5 in the collimating lens 3 is set to 1.90mm, and the distance x between the collimating lens 3 and the laser source 2 is set to 2.10mm. LD Setting it to 3.0mm suppresses the error Δψ of the half-wavelength plate's setting angle to 1.0deg, thus P r The result of / P0 calculation is less than 1×10 -6 .

[0434] In the ninth calculation example, as shown in Table 6, the diameter φ of the opening 172 of the shielding element 17 is... pin The deviation amplitude Δy of the returned light L5 in the collimating lens 3 is set to 6.00mm, and the distance x between the collimating lens 3 and the laser source 2 is set to 2.00mm. LD Setting it to 40mm, the error Δψ of the half-wavelength plate's setting angle is suppressed to 0.2deg, thus P r The result of / P0 calculation is less than 1×10 -6 .

[0435] Therefore, in the fifth to ninth calculation examples, it was confirmed that by making the deviation amplitude Δy satisfy the relationship of the above equation (A), the incident light L5 relative to the laser source 2 was suppressed to the point that the laser oscillation would not become unstable.

[0436] 1.5. The impact of optical axis misalignment on differential amplification processing

[0437] Next, the impact of optical axis offset on differential amplification processing will be explained.

[0438] Figure 17 , Figure 18 , Figure 19 as well as Figure 20 This is a schematic diagram used to illustrate the impact of optical axis offset on differential amplification processing. Figure 17 and Figure 18 This is a diagram showing the composition of the comparative examples. Figure 19 and Figure 20 This is a diagram illustrating the configuration of this embodiment. Furthermore, in Figures 17 to 20 For ease of illustration, the light path refracted by the first light splitter 4 and the second light splitter 5 is extended for illustration.

[0439] exist Figures 17 to 20 The diagram schematically illustrates the optical path 18 between the laser source 2 and the first optical splitter 4, the optical path 20 between the first optical splitter 4 and the optical modulator 12, the optical path 22 between the first optical splitter 4 and the object to be measured 14, the optical path 24 between the first optical splitter 4 and the second optical splitter 5, the optical path 26 between the second optical splitter 5 and the first light-receiving element 10, and the optical path 28 between the second optical splitter 5 and the second light-receiving element 11.

[0440] In addition, in each figure, the distance between the first optical splitter 4 and the optical modulator 12 is set as L. BS The distance between the first light splitter 4 and the object 14 to be measured is set as WDs. Furthermore, in each figure, the distance between the second light splitter 5 and the first light-receiving element 10 is set as ΔL. PD1 The distance between the second light splitter 5 and the second light receiving element 11 is set as ΔL. PD2 .

[0441] First, regarding the comparative example Figure 17 and Figure 18 Explanation will be provided. In Figure 17 and Figure 18 In the middle, the distance ΔL PD1 With distance ΔL PD2 Different. In this case, based on the distance L BS Due to the relationship with distance WDs, sometimes the interference light (reference light L2 and object light L3) does not enter either the first light-receiving element 10 or the second light-receiving element 11.

[0442] Specifically, Figure 17 The diagram shows the distance L. BS In the special case where the distance WDs is equal, the optical axis A of the first split beam L1a from the first optical splitter 4 toward the optical modulator 12 will be... L1a The optical axis A of the reference light L2 from the optical modulator 12 toward the first optical splitter 4L2 The angle formed is set as "deviation angle θ" ref On the other hand, the optical axis A of the second split light L1b, which will move from the first light splitter 4 toward the object to be measured 14, will be... L1b The optical axis A of the object light L3 from the measuring object 14 toward the first optical splitter 4 L3 The angle formed is set as "deviation angle θ" sam ".

[0443] If the distance is L BS In the special case where the distance WDs is equal, the deviation angle θ ref and deviation angle θ sam like Figure 17 The reference light L2 and the object light L3 can be directed into both the first light-receiving element 10 and the second light-receiving element 11.

[0444] However, in the aforementioned special case, the distance WDs is fixed, which is impractical. When measuring displacement or velocity of the object 14, it is practical to adjust the distance WDs according to the measurement scenario. Therefore, next, the distance L... BS Studies were conducted on cases different from those involving distance WDs.

[0445] Figure 18 The diagram shows the distance L. BS The situation differs from that of distance WDs. At distance L... BS In cases different from distance WDs, the deviation angle θ ref and deviation angle θ sam They are also different from each other. Therefore, it is impossible for both the reference light L2 and the object light L3 to enter the first light-receiving element 10 and the second light-receiving element 11. The reason is explained below, as follows... Figure 18 As shown, if the distance ΔL PD1 With distance ΔL PD2 If the reference light L2's emission point, the first light-receiving element 10, and the second light-receiving element 11 are not aligned, even if they are on the same straight line, it is still impossible to align the object light L3's emission point, the first light-receiving element 10, and the second light-receiving element 11 on the same straight line. Therefore, light cannot pass through these three points.

[0446] Next, regarding the embodiment described in this article... Figure 19 and Figure 20 Explanation will be provided. In Figure 19 and Figure 20 In the middle, the distance ΔL PD1 With distance ΔL PD2 They are not equal. In this case, regardless of the distance L... BSRegardless of the relationship with the distance WDs, the interference light (reference light L2 and object light L3) can be incident on both the first light-receiving element 10 and the second light-receiving element 11.

[0447] Specifically, Figure 19 The diagram shows the distance L. BS The special case where the distance to WDs is equal. In this case, such as Figure 19 As shown, the deviation angle θ ref and deviation angle θ sam They are equal to each other. Therefore, both the reference light L2 and the object light L3 can be directed into both the first light-receiving element 10 and the second light-receiving element 11.

[0448] Figure 20 The diagram shows the distance L. BS In cases different from distance WDs, when distance LBS differs from distance WDs, the deviation angles θref and θsam also differ. However, since distance ΔLPD1 is equal to distance ΔLPD2, both the reference light L2 and the object light L3 can be incident on both the first light-receiving element 10 and the second light-receiving element 11. The reason for this can be explained as follows. Figure 20 As shown, if the distance ΔLPD1 is equal to the distance ΔLPD2, then when the emission point of the reference light L2, the first light-receiving element 10, and the second light-receiving element 11 are aligned on the same straight line, the emission point of the object light L3, the first light-receiving element 10, and the second light-receiving element 11 can be arranged on another straight line. Therefore, light can pass through these three points.

[0449] Based on the above, in this embodiment, the distance ΔL PD1 With distance ΔL PD2 Set to be equal. Therefore, even at a distance L... BS In cases where the distance WDs differs, both the reference light L2 and the object light L3 can be incident on both the first light-receiving element 10 and the second light-receiving element 11. As a result, an optical system 50 that employs optical axis offset and is capable of differential amplification can be realized.

[0450] Furthermore, theoretically, it is required that the distance ΔL PD1 With distance ΔL PD2 Equal, but depending on factors such as the distance WDs or the size of the optical system 50, a distance ΔL may be allowed to some extent. PD1 With distance ΔL PD2 There is a deviation. Specifically, when the distance ΔL is... PD1 With distance ΔL PD2 Let the difference be ΔL dif Time difference ΔL dif It satisfies the following formula (C).

[0451]

Mathematical Expression 43

[0452] ΔL dif ≤20mm (C)

[0453] As long as the difference is ΔL dif Within this range, an optical system 50 that employs optical axis offset and is capable of differential amplification can be realized.

[0454] The following explains the process of deriving equation (C) above.

[0455] Figure 21 This is a schematic diagram used to illustrate the impact of optical axis misalignment on differential amplification processing, and it is relative to... Figure 20 The schematic diagram shown illustrates a distance difference ΔL between the first light-receiving element 10 and the second light-receiving element 11. dif The diagram was modified in a certain way. Figure 22 yes Figure 21 A magnified view of a portion of the image.

[0456] exist Figure 21 In this configuration, the first light-receiving element 10 and the second light-receiving element 11 are arranged in a manner consistent with the optical axis AL2 of the reference light L2. Therefore, a distance difference ΔL exists between the first light-receiving element 10 and the second light-receiving element 11. dif Therefore, for example, on the optical axis A of the object's light L3. L3 Under the condition that it is consistent with the first light-receiving element 10, it is theoretically impossible to make the optical axis A L3 It is consistent with the second light-receiving element 11.

[0457] like Figure 22 As shown, optical axis A L2 With optical axis A L3 The angle formed is determined by |θ ref -θ sam | indicates. Therefore, the optical axis A at the position of the second light-receiving element 11 is... L2 With optical axis A L3 The deviation d is represented by the following equation (22).

[0458]

Mathematical Expression 44

[0459] d=ΔL dif tan(|θ ref -θ sam |) (22)

[0460] Here, the optical path of the reference light L2 at the position of the second light-receiving element 11 is set as R. qom Set the optical path of object light L3 to R. sam .

[0461] exist Figure 21 In this process, the optical path of the emitted light L1 is reduced by the shielding element 17. Therefore, the R of the reference light L2... qom Calculated according to the above formula (16-3), and expressed as in the following formula (23).

[0462]

Mathematical Expression 45

[0463]

[0464] On the other hand, the optical path R of object light L3 sam It is also calculated according to the above formula (16-3) and expressed as in the following formula (24).

[0465]

Mathematical Expression 46

[0466]

[0467] Here, as an example, assume L = L PD .then, Figure 21 The optical axis A of the reference light L2 at the position of the first light-receiving element 10 shown is... L2 deviation y ref and Figure 16 The optical axis A at the position of the collimating lens 3 shown L5 The deviations Δy are equal. Therefore, through tanθ ref =Δy / L PD ≈θ ref Find θ in equation (23) above. ref Additionally, through tanθ sam =Δy / (L PD +ΔWD ref )≈θ sam Find θ in equation (24) above. sam .

[0468] The parameters used in the above equations (23) and (24) can take values ​​such as the following.

[0469] Wavelength λ = 400–1600 nm

[0470] • The distance L between the collimating lens 3 and the optical modulator 12 is 90mm.

[0471] • The distance L between the collimating lens 3 and the shielding element 17 pin =8mm

[0472] Optical axis A at the position of collimating lens 3 L5 The deviation Δy = 1.6 mm

[0473] • The diameter φ of the opening 172 of the shielding element 17pin =1.50mm

[0474] The distance L between the optical modulator 12 and the first light-receiving element 10 PD =90mm

[0475] 2 Distance L BS The difference between ΔWD and distance WDs ref =30~60mm

[0476] Using equations (23) and (24) above, the optical path R of the reference light L2 at the position of the second light-receiving element 11 can be calculated. qom The optical path R of object light L3 at the position of the second light-receiving element 11 sam .

[0477] Figure 23 It is a schematic representation of arrival. Figure 22 The diagram shows the optical paths of the reference light L2 and the object light L3 at the position of the second light-receiving element 11.

[0478] If the optical axis A at the position of the second light-receiving element 11 L2 With optical axis A L3 If the deviation d is below the specified value, then as follows: Figure 23 As shown, the object light L3 overlaps with the reference light L2. The interference light received by the first light-receiving element 10 and the second light-receiving element 11 is equivalent to... Figure 23 The overlap portion OL between the object light L3 and the reference light L2 is shown. Therefore, in order to output a high S / N ratio light signal from the first light-receiving element 10 and the second light-receiving element 11, the overlap pattern between the object light L3 and the reference light L2 needs to be considered.

[0479] The light intensity distributions of reference light L2 and object light L3 can be considered Gaussian. When the deviation amplitude d increases, even the overlapping portion OL experiences light intensity attenuation. Considering the allowable S / N ratio, the light intensity attenuation in the overlapping portion OL needs to be suppressed to, for example, less than 1%. Therefore, the allowable deviation amplitude d calculated using the above parameters is, for example, less than 0.12 mm. Thus, the distance ΔL that satisfies the allowable deviation amplitude d is calculated. PD1 With distance ΔL PD2 The difference ΔL dif Allowed values.

[0480] First, the above equation (22) is transformed as shown in equation (25).

[0481]

Mathematical Expression 47

[0482]

[0483] Here, as mentioned earlier, assuming L = LPD , and y ref When =Δy, the above equation (25) can be transformed as shown in the following equation (26).

[0484]

Mathematical Expression 48

[0485]

[0486] By inputting the aforementioned parameters into equation (26), equation (C) is derived.

[0487] 1.5.1. Tenth Calculation Example

[0488] As an example of a case satisfying equation (C), a tenth calculation example is shown. The parameters and calculation results of the tenth calculation example are shown in Table 7. In this calculation example, the aforementioned optical axis offset achieves the following effect: the incidence of the return light L5 relative to the laser source 2 is suppressed to a level where laser oscillation will not become unstable, and the attenuation of light quantity in the overlapping portion OL is suppressed. Therefore, the distance ΔL is calculated using the following parameters. PD1 With distance ΔL PD2 The difference ΔL dif Examples of allowed values.

[0489] Table 7

[0490]

[0491] In the tenth calculation example, as shown in Table 7, the diameter φ of the opening 172 of the shielding element 17 is... pin Set to 1.50mm. In this case, if the difference ΔWD ref For diameters below 1000mm, the optical system volume V... 50 Within a realistic range where the output light intensity is 10cc or higher and the wavelength λ of the emitted light L1 is 400–1600 mm, the difference ΔL can be minimized. dif The allowable value is below 20mm. That is to say, in this calculation example, it is confirmed that if the difference ΔL... dif By setting the allowable value to be below 20mm, it is possible to suppress the return light L5 by optical axis offset and suppress the aforementioned deviation amplitude d to below the specified value, thereby enabling differential amplification processing.

[0492] Furthermore, when the diameter φ pin When the setting is 1.50mm, even if the shielding element 17 is omitted, the return light L5 incident on the laser source 2 is shielded to the same extent as when the shielding element 17 is provided. Therefore, the tenth calculation example can also be regarded as a calculation example with the shielding element 17 omitted.

[0493] In addition, the diameter φ of the optical system 50 For the volume V of the optical system50 The overall size of the laser interferometer 1 also has an impact. If we consider both the operability and ease of manufacture of the laser interferometer 1, then the diameter φ of the optical system... 50 Preferably, the diameter is 15mm or more and 200mm or less, more preferably 20mm or more and 150mm or less. Furthermore, the optical system diameter φ... 50 It refers to the diameter of the circle inscribed in optical system 50 when viewed from the Z-axis.

[0494] 1.5.2. Calculation Examples Eleven through Sixteen

[0495] As examples of cases satisfying Equation (C), calculation examples eleven through sixteen are shown. The parameters and calculation results for calculation examples eleven through sixteen are shown in Table 8. In these calculation examples, the aforementioned optical axis offset achieves the following effect: the incidence of the return light L5 relative to the laser source 2 is suppressed to a level where laser oscillation will not become unstable, and the attenuation of light quantity in the overlapping portion OL is suppressed. Therefore, the distance ΔL is calculated using the following parameters. PD1 With distance ΔL PD2 The difference ΔL dif Examples of allowed values.

[0496] Table 8

[0497]

[0498] As shown in Table 8, these calculation examples also confirm that if the difference ΔL dif If the setting is below 20mm, it is possible to suppress the return light L5 by optical axis offset and perform differential amplification.

[0499] As described above, the laser interferometer 1 according to this embodiment includes a laser source 2, a first light splitter 4, an optical modulator 12, a second light splitter 5, a first light-receiving element 10, and a second light-receiving element 11. The laser source 2 emits outgoing light L1 (laser). The first light splitter 4 splits the outgoing light L1 into a first split light L1a and a second split light L1b. The optical modulator 12 modulates the first split light L1a into a reference light L2 with a different frequency. The second light splitter 5 splits the interference light between the object light L3 and the reference light L2 into a third split light L6a and a fourth split light L6b. The object light L3 is generated by the reflection of the second split light L1b from the object 14 being measured. The first light-receiving element 10 receives the third split light L6a, and the second light-receiving element 11 is disposed at a different position from the first light-receiving element 10 and receives the fourth split light L6b.

[0500] Furthermore, in the laser interferometer 1 according to this embodiment, the optical axis A of the first split beam L1a from the first optical splitter 4 toward the optical modulator 12 is... L1aThe optical axis A of the reference light L2 from the optical modulator 12 toward the first optical splitter 4 L2 The light is staggered. Additionally, the distance ΔL from the second light splitter 5 to the first light-receiving element 10... PD1 (Optical path length) and the distance ΔL from the second optical splitter 5 to the second light-receiving element 11 PD2 The difference ΔL (optical path length) dif It is less than 20mm.

[0501] With this configuration, differential amplification can be performed using the first light-receiving element 10 and the second light-receiving element 11. Furthermore, the incidence of the return light L5 relative to the laser source 2 can be suppressed by optical axis offset. Moreover, by adjusting the difference ΔL... dif By suppressing the optical axis offset to a specified range, the adverse effects of optical axis misalignment on differential amplification processing can be suppressed. As a result, the signal-to-noise ratio (S / N) of the received light signal can be effectively improved, and the accuracy of demodulating the sampled signal from the object to be measured 14 based on the received light signal can be improved.

[0502] In addition, when the difference ΔL dif When the upper limit is exceeded, the deviation between the reference light L2 and the object light L3 becomes larger, making differential amplification processing difficult.

[0503] Additionally, the distance ΔL from the second light splitter 5 to the first light receiving element 10 PD1 (Optical path length) and the distance ΔL from the second optical splitter 5 to the second light-receiving element 11 PD2 The difference ΔL (optical path length) dif Preferably, the difference is less than 10 mm. That is, the difference ΔL dif Preferably, the following formula (C-1) is satisfied.

[0504]

Mathematical Expression 49

[0505] ΔL dif ≤10mm (C-1)

[0506] Therefore, the difference ΔL dif Compared to the case satisfying equation (C) above, the distance L can be extended. BS The difference between ΔWD and distance WDs ref The difference ΔWD ref These are parameters representing the working distances of the left and right laser interferometers 1. Therefore, by making the difference ΔL... dif By satisfying the above equation (C), the operability of the laser interferometer 1 is improved.

[0507] Additionally, as mentioned earlier, the laser interferometer 1 may also include a shielding element 17. The shielding element 17 is disposed between the laser source 2 and the first optical splitter 4, and has an opening 172 for the emitted light L1 (laser) to pass through.

[0508] With this configuration, the shielding element 17 does not obstruct the passage of the outgoing light L1 but shields the returning light L5. Therefore, by employing optical axis offset and using the shielding element 17, the incidence of the returning light L5 relative to the laser source 2 can be suppressed in particular.

[0509] Furthermore, the diameter φ of the opening 172 of the shielding element 17 pin [mm] Preferably, the relationship in equation (20) above is satisfied, but more preferably, 0.10 ≤ φ is satisfied. pin ≤6.00, further preferred to satisfy 0.30≤φ pin ≤3.00.

[0510] The diameter φ of the opening 172 of the shielding element 17 pin [mm] Preferably, the φ value is 0.50≤φ pin ≤1.50.

[0511] Therefore, the shielding element 17 does not obstruct the passage of the emitted light L1 and has sufficient shielding capability for the returned light L5. As a result, the signal-to-noise ratio (S / N ratio) of the light received signals output from the first light-receiving element 10 and the second light-receiving element 11 can be improved in particular.

[0512] In addition, when the diameter φ pin When the light path is below the lower limit, depending on the optical path of the emitted light L1, the shielding element 17 may obstruct the passage of the emitted light L1, thereby reducing the intensity of the interference light incident on the first light-receiving element 10 and the second light-receiving element 11. On the other hand, when the diameter φ pin When the upper limit is exceeded, the return light L5 can easily pass through the opening 172, so the return light L5 injected into the laser source 2 may increase.

[0513] Additionally, the distance ΔL from the second light splitter 5 to the first light receiving element 10 PD1 (Optical path length) and the distance ΔL from the second optical splitter 5 to the second light-receiving element 11 PD2 The difference ΔL (optical path length) dif Preferably, the difference is less than 8mm. That is, the difference ΔL dif Preferably, the following formula (C-2) is satisfied.

[0514]

Mathematical Expression 50

[0515] ΔL dif ≤8mm (C-2)

[0516] Therefore, even when using the diameter φ of opening 172 pinEven with a small shielding element 17 of 0.50 mm, differential amplification can still be performed. As a result, it is particularly possible to suppress the return light L5 by optical axis offset, and a high signal-to-noise ratio of the received signal can be achieved through differential amplification.

[0517] Furthermore, the distance ΔL from the second light splitter 5 to the first light receiving element 10 PD1 (Optical path length) and the distance ΔL from the second optical splitter 5 to the second light-receiving element 11 PD2 The difference ΔL (optical path length) dif Preferably, the difference is less than 1 mm. That is, the difference ΔL dif Preferably, the following formula (C-3) is satisfied.

[0518]

Mathematical Expression 51

[0519] ΔL dif ≤1mm (C-3)

[0520] Therefore, regardless of the presence or absence of the shielding element 17, the volume of the optical system V50 can be reduced to approximately 10 cc. As a result, a laser interferometer 1, which is particularly capable of miniaturization, can be realized.

[0521] 1.6. Configuration of optical modulator and light-receiving element

[0522] Next, the configuration of the optical modulator 12, the first light-receiving element 10, and the second light-receiving element 11 will be described.

[0523] In the laser interferometer 1, the working distance may vary depending on the measurement scenario. Therefore, regardless of the value of the distance WDs, it is necessary to use the shielding element 17 to suppress the return light L5 from entering the laser source 2. The configuration of the first light-receiving element 10 and the second light-receiving element 11 is set according to the configuration of the optical modulator 12. Therefore, the relationship between the configuration of the optical modulator 12 and the configuration of the first light-receiving element 10 and the second light-receiving element 11 will be explained in the following description.

[0524] Figure 24 This represents distance WDs relative to distance L. BS The optical axis A of object light L3 at three different modes L3 The graph shows the change in distance L. BS This is the distance between the first optical splitter 4 and the optical modulator 12, and the distance WDs is the distance between the first optical splitter 4 and the object being measured 14. Figure 24 The diagram shows the measured object 14 located at position P. 14-1 P 14-2 P 14-3 The situation is as follows: When the object being measured, 14, is located at position P. 14-1 At that time, L BS=WDs are true. When the measured object 14 is located at P 14-2 At that time, L BS >WDs are established. When the measured object 14 is located at P 14-3 At that time, L BS <WDs established.

[0525] In addition, Figure 24 The diagram shows the distance LL. pin Relative to distance L PD The positional relationship between the shielding element 17 and the first light-receiving element 10 and the second light-receiving element 11 at different times in three modes. Distance L PD This is the distance between the light modulator 12 and the first light-receiving element 10 and the second light-receiving element 11. Figure 24 The diagram shows the shielding element 17 located at position P. 17-1 P 17-2 P 17-3 The situation.

[0526] Additionally, distance L is the distance between collimating lens 3 and optical modulator 12. pin It is the distance between the collimating lens 3 and the shielding element 17.

[0527] In the following explanation, distance L will be... PD With distance LL pin The size relationship is explained in three modes. In addition, in all cases, the tilt of the light modulator 12 is adjusted so that the reference light L2 enters the first light-receiving element 10 and the second light-receiving element 11. 1.6.1.

[0529] LL pin =L PD hour

[0530] The shielding element 17 is provided to shield the return light L5 from the reference light L2. Therefore, the distance LL between the optical modulator 12 and the shielding element 17 is... pin The distance is set to block the return light L5 from the reference light L2 and the return light L5 from the object light L3. Additionally, the diameter φ of the opening 172 of the blocking element 17... pin It is also set to be the diameter that can block the return light L5.

[0531] When L BS =WDs, the return light L5 from the reference light L2 and the return light L5 from the object light L3 are both located at position P. 17-1 The shielding element 17 provides shielding.

[0532] The tilt of the object 14 is also adjusted so that the object light L3 enters the first light-receiving element 10 and the second light-receiving element 11. Therefore, the return light L5 from the object light L3 reaches a position outside the opening 172 of the shielding element 17 and is shielded.

[0533] Additionally, due to LL pin =L PD Therefore, the arrival position of the return light L5 from the object light L3 in the shielding element 17 is fixed, and the distance WDs is related to the distance L. BS The relationship is unrelated. That is to say, whether it is L... BS >WDs or L BS When <WDs, the return light L5 from object light L3 is blocked by the blocking element 17. Therefore, when LL pin =L PD When established, theoretically the return light L5 will not pass through the shielding element 17. 1.6.2.

[0535] LL pin <L PD hour

[0536] When LL pin <L PD At that time, with LL pin =L PD Compared to the time, it is located at position P 17-2 The shielding element 17 is close to the first light splitter 4. Therefore, the arrival position of the return light L5 from the object light L3 in the shielding element 17 is determined by the distance WDs and the distance L. BS The relationship changes accordingly.

[0537] When L BS =WDs, the return light L5 from object light L3 is located at position P 17-2 The shielding element 17 provides shielding.

[0538] When L BS When <WDs, with L BS Compared to when =WDs, the arrival position of the return light L5 from the object light L3 is farther away from the opening 172 of the shielding element 17. Therefore, the return light L5 from the object light L3 is shielded by the shielding element 17.

[0539] When L BS >WDs, with L BSCompared to when the light source is WDs, the return light L5 from the object light L3 arrives closer to the opening 172 of the shielding element 17. Therefore, the return light L5 from the object light L3 may pass through the opening 172. However, even if the return light L5 from the object light L3 passes through the opening 172, the return light L5 after passing through the opening 172 travels away from the laser source 2. Therefore, the probability of the return light L5 entering the laser source 2 is very low. 1.6.3.

[0541] LL pin >L PD hour

[0542] When LL pin >L PD At that time, with LL pin =L PD Compared to the time, it is located at position P 17-3 The shielding element 17 is moved away from the first light splitter 4. Therefore, the arrival position of the return light L5 from the object light L3 in the shielding element 17 is determined by the distance WDs and the distance L. BS The relationship changes accordingly.

[0543] When L BS =WDs, the return light L5 from object light L3 is located at position P 17-3 The shielding element 17 provides shielding.

[0544] When L BS >WDs, with L BS Compared to when =WDs, the arrival position of the return light L5 from the object light L3 is farther away from the opening 172 of the shielding element 17. Therefore, the return light L5 from the object light L3 is shielded by the shielding element 17.

[0545] When L BS When <WDs, with L BS Compared to when L = WDs, the arrival position of the return light L5 from the object light L3 is closer to the opening 172 of the shielding element 17. Therefore, the return light L5 from the object light L3 may pass through the opening 172. Moreover, when L BS When <WDs, the deviation angle θ sam The relative size decreases, therefore the probability that the return light L5 from the object light L3 will enter the laser source 2 after passing through the opening 172 may also increase. Therefore, it is preferable that the positional relationship between the shielding element 17 and the first light-receiving element 10 and the second light-receiving element 11 does not satisfy LL. pin >L PD In other words, the optical system 50 of the laser interferometer 1 preferably satisfies LL. pin ≤L PD .

[0546] As described above, the distance LL from the optical modulator 12 to the shielding element 17 pin The preferred optical path length is the distance L from the optical modulator 12 to the first light-receiving element 10. PD (Optical path length) and below.

[0547] With this configuration, regardless of the distance WDs equivalent to the working distance, the probability of the returned light L5 entering the laser source 2 can be significantly reduced. This suppresses laser oscillation from becoming unstable, thereby preventing a decrease in the signal-to-noise ratio (S / N) of the received light signal.

[0548] 3. Second to fourth variations of the laser interferometer

[0549] Next, the laser interferometers of the second to fourth variations will be described.

[0550] Figure 25 This is a schematic diagram showing the optical system configuration of the laser interferometer 1 involved in the second modification. Figure 26 This is a schematic diagram showing the optical system configuration of the laser interferometer 1 involved in the third modification. Figure 27 This is a schematic diagram showing the optical system configuration of the laser interferometer 1 involved in the fourth modification.

[0551] The second to fourth variations will be described below, but the description will focus on the differences from the described embodiments, omitting descriptions of identical items. Furthermore, in Figures 25 to 27 In this document, the same reference numerals are assigned to configurations that are identical to those described in the embodiments.

[0552] 3.1. Second variation

[0553] In the embodiment described above, the optical axis A is positioned in the Z-axis direction. L2 A L3 Offset. In contrast, in the second variation, such as Figure 25 As shown, in the XY plane, the optical axis A is... L2 A L3 Offset. Even if the optical axis A is offset. L2 A L3 The effects of optical axis offset are the same regardless of the direction of the offset. Therefore, the same effect as the described embodiment can be obtained in the second variation.

[0554] 3.2. Third variation

[0555] In the described embodiment, the second optical splitter 5 has a polarization beam splitter. In contrast, in a third variation, such as... Figure 26As shown, the second light splitter 5 has a triangular prism. Regardless of the polarization state of the interfering light, the triangular prism can split the interfering light into two. Therefore, in the third variation, the same effect as the described embodiment can also be obtained.

[0556] 3.3. Fourth Variation Example

[0557] The fourth variation also includes a reflective element 15. The extension direction of the optical path 28 is altered by the reflective element 15. Furthermore, the physical length between the second light splitter 5 and the second light receiving element 11 is longer than the physical length between the second light splitter 5 and the first light receiving element 10.

[0558] like Figure 27 As shown, the laser interferometer 1 in the fourth modification also includes an optical path length changing unit 56. The optical path length changing unit 56 is disposed on the optical path 26, that is, between the second optical splitter 5 and the first light receiving element 10, and changes the optical path length between the second optical splitter 5 and the first light receiving element 10.

[0559] With this configuration, by lengthening the optical path 26 passing through the optical path length changing section 56, the optical path lengths between the second optical splitter 5 and the first light-receiving element 10, and between the second optical splitter 5 and the second light-receiving element 11, can be made closer to each other. Therefore, even if the physical lengths cannot be made closer due to various obstacles, the optical path lengths can be made closer. As a result, an optical system 50 satisfying the above formula (C) can be realized even in the presence of various obstacles.

[0560] In addition, the optical path length changing unit 56 is disposed at the position where the third split light L6a is incident, and has a refractive index variable type with a refractive index change.

[0561] With this configuration, the optical path length of the optical path 26 passing through the optical path length changing unit 56 can be easily changed simply by altering the refractive index.

[0562] Examples of refractive index variations include optical multilayer filters. By using high refractive index materials in the multiple layers of an optical multilayer filter, the optical path length can be increased even with a thin film. Since it is a multilayer film, the material or thickness of the film can be easily changed, thus easily achieving the desired optical path length.

[0563] The physical length of the optical path length changing unit 56 with refractive index variable is set to ΔPL. n2 The refractive index of the variable-index body is set to n2. Furthermore, the physical length between the second light splitter 5 and the first light-receiving element 10 is set to ΔPL. PD1The refractive index of the air surrounding the optical path length changing section 56 is set to n1. Therefore, the optical path length, i.e., the distance ΔL, between the second optical splitter 5 and the first light-receiving element 10 is... PD1 It is represented by the following formula (27).

[0564]

Mathematical Expression 52

[0565] ΔL PD1 =n1(ΔPL) PD1 -ΔPL n2 )+n2ΔPL n2 (27)

[0566] In the fourth variation described above, the same effect as the described embodiment can also be obtained.

[0567] The laser interferometer of the present invention has been described above with reference to 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 involved in the described embodiments. Additionally, the laser interferometer of the present invention may include any two or more of the described embodiments and their variations.

[0568] In addition to the aforementioned displacement gauges and velocities, the laser interferometer of the present invention can also be applied to vibratory meters, inclinometers, rangefinders (length measuring devices), etc. Furthermore, as applications of the laser interferometer of the present invention, examples include optical comb interferometry technology capable of measuring distance, 3D imaging, and beam splitting, as well as fiber optic gyroscopes such as angular velocity sensors and angular acceleration sensors.

[0569] Furthermore, two or more of the laser source, light modulator, first light-receiving element, and second light-receiving element can be mounted on the same substrate. This facilitates miniaturization and weight reduction of the optical system and improves assembly ease.

[0570] Furthermore, while the various embodiments and modifications described herein have a so-called Michelson-type interferometric optical system, the laser interferometer of the present invention can also be applied to laser interferometers having other types of interferometric optical systems, such as Mach-Zehnder-type interferometric optical systems.

Claims

1. A laser interferometer, characterized in that, have: Laser source, emits laser light; A first optical splitter splits the laser beam into a first split beam and a second split beam. An optical modulator modulates the first segmented light into reference light with different frequencies; The second light splitter splits the object light and the reference light into a third split light and a fourth split light, wherein the object light is the light generated by the second split light being reflected by the object being measured; The first light-receiving element receives the third segmented light; The second light-receiving element is positioned at a different location than the first light-receiving element and receives the fourth split light; as well as A collimating lens is disposed between the laser source and the first optical splitter. The optical axis of the first split beam from the first optical splitter toward the optical modulator is offset from the optical axis of the reference beam from the optical modulator toward the first optical splitter, so as to suppress the return light from entering the laser source, the return light being the light from the reference beam or the object beam traveling toward the laser source. The difference between the optical path length from the second optical splitter to the first light-receiving element and the optical path length from the second optical splitter to the second light-receiving element is less than 20 mm. The position where the collimated light is generated in the collimating lens is the reference position, the optical axis of the collimated light is the first optical axis, and the optical axis of the returned light is the second optical axis. Let R be the optical path of the collimated light, L be the distance between the collimating lens and the optical modulator, λ be the wavelength of the laser, κ be the effective diameter of the collimating lens, and Δy be the deviation between the first optical axis and the second optical axis in the collimating lens. Δy satisfies the following equation (A). 。 2. The laser interferometer according to claim 1, characterized in that, The difference between the optical path length from the second optical splitter to the first light-receiving element and the optical path length from the second optical splitter to the second light-receiving element is less than 10 mm.

3. The laser interferometer according to claim 2, characterized in that, The difference between the optical path length from the second optical splitter to the first light-receiving element and the optical path length from the second optical splitter to the second light-receiving element is less than 1 mm.

4. The laser interferometer according to claim 1 or 2, characterized in that, The laser interferometer includes a shielding element disposed between the laser source and the first optical splitter, and has an opening for the laser to pass through.

5. The laser interferometer according to claim 4, characterized in that, The diameter of the opening is greater than 0.50 mm and less than 1.50 mm.

6. The laser interferometer according to claim 4, characterized in that, The difference between the optical path length from the second optical splitter to the first light-receiving element and the optical path length from the second optical splitter to the second light-receiving element is less than 8 mm.

7. The laser interferometer according to claim 4, characterized in that, The optical path length from the light modulator to the shielding element is less than the optical path length from the light modulator to the first light-receiving element.

8. The laser interferometer according to claim 1, characterized in that, The laser source is a semiconductor laser element.

9. The laser interferometer according to claim 1, characterized in that, The laser interferometer includes an optical path length changing unit, which is disposed between the second optical splitter and the first light-receiving element to change the optical path length between the second optical splitter and the first light-receiving element.

10. The laser interferometer according to claim 9, characterized in that, The optical path length changing unit is disposed at the position where the third segmented light is incident, and the optical path length changing unit has a refractive index variable type with a refractive index change.

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