laser interferometer

Abstract

The present application provides a laser interferometer which is small in size and high in demodulation accuracy of a sampling signal from an object. The laser interferometer is characterized by comprising: a light source section which emits first laser light; a light modulator which includes a vibration element and modulates the first laser light using the vibration element to generate second laser light including a modulation signal; a light receiving element which receives interference light of third laser light and the second laser light and outputs a light receiving signal, the third laser light including a sampling signal generated by reflection of the first laser light by an object; a demodulation circuit which demodulates the sampling signal based on a reference signal and in accordance with the light receiving signal; and an oscillation circuit which outputs the reference signal to the demodulation circuit, the vibration element being a signal source of the oscillation circuit.

Description

[0001] This application is a divisional application of the invention patent application with application number 202110967137.4, application date August 23, 2021, and invention title "Laser Interferometer". Technical Field

[0002] This invention relates to a laser interferometer. Background Technology

[0003] Patent Document 1 discloses a laser Doppler velocimeter that measures the velocity of a vibrating object by irradiating it with a laser beam and utilizing the frequency of the laser beam, which changes due to the Doppler effect. In this laser Doppler velocimeter, a structure is needed to modulate the light emitted from the laser source in order to detect the directionality of the object's vibration. Therefore, Patent Document 1 discloses a technique using an acousto-optic modulator or an electro-optic modulator.

[0004] In the acousto-optic modulator or electro-optic modulator described in Patent Document 1, an RF (Radio Frequency) signal output from an oscillation circuit is input. This causes a change in the refractive index of the optical elements disposed within these optical modulators, thereby modulating the light.

[0005] When light emitted from a laser source shines on a vibrating object, it is reflected, producing a returning beam. Because the returning beam causes a Doppler frequency shift, a beat frequency signal is generated by interfering this returning beam with a reference beam. By passing this beat frequency signal through a demodulator, the vibration velocity of the object can be determined. Typically, a reference signal generated by an oscillating circuit is used for demodulation in the demodulator.

[0006] An oscillating circuit is a circuit that uses a signal source, such as a crystal oscillator, as a source oscillator to output a signal of a desired frequency. However, the oscillation of a crystal oscillator has temperature characteristics. Therefore, there is a problem that the frequency of the signal output from the oscillating circuit changes when affected by temperature variations.

[0007] On the other hand, the modulation of light performed by the aforementioned optical modulator also has temperature characteristics. Therefore, when affected by temperature changes, the modulation frequency modulated by the optical modulator will change.

[0008] Furthermore, the temperature characteristics of the optical modulator differ from those of the crystal oscillator. This deviation in temperature characteristics affects the measurement results of the laser Doppler velocimeter, resulting in a decrease in the accuracy of the measured velocity.

[0009] Patent Document 1: Japanese Patent Application Publication No. 9-54293 Summary of the Invention

[0010] The laser interferometer described in the application examples of this invention is characterized by having:

[0011] The light source emits the first laser beam;

[0012] An optical modulator having a vibrating element and using the vibrating element to modulate the first laser to generate a second laser containing a modulated signal;

[0013] A light-receiving element receives interference light from a third laser and a second laser, and outputs a light-receiving signal, wherein the third laser includes a sampling signal generated by the reflection of the first laser by an object;

[0014] A demodulation circuit that demodulates the sampled signal based on a reference signal and according to the received light signal;

[0015] An oscillation circuit outputs the reference signal to the demodulation circuit.

[0016] The vibrating element is the signal source of the oscillation circuit. Attached Figure Description

[0017] Figure 1 This is a functional block diagram illustrating the laser interferometer involved in the implementation method.

[0018] Figure 2 To indicate Figure 1 The diagram shows a schematic representation of the sensor head and optical system.

[0019] Figure 3 To indicate Figure 2 A perspective view of a first structural example of an optical modulator.

[0020] Figure 4 This is a top view showing a portion of a second structural example of an optical modulator.

[0021] Figure 5 This is a top view showing a third example of an optical modulator structure.

[0022] Figure 6 To the incident light K i From relative Figure 3 This is a conceptual diagram illustrating the situation where multiple diffracted lights are generated when the surface of the vibrating element is incident in a perpendicular direction.

[0023] Figure 7 To be configured as incident light K i The concept diagram illustrates a light modulator whose angle between the direction of travel of the light beam L1 and the direction of travel of the reference light L2 is 180 degrees.

[0024] Figure 8To be configured as incident light K i The concept diagram illustrates a light modulator whose angle between the direction of travel of the light beam L1 and the direction of travel of the reference light L2 is 180 degrees.

[0025] Figure 9 To be configured as incident light K i The concept diagram illustrates a light modulator whose angle between the direction of travel of the light beam L1 and the direction of travel of the reference light L2 is 180 degrees.

[0026] Figure 10 This is a cross-sectional view showing an optical modulator with a packaged structure.

[0027] Figure 11 A circuit diagram showing the structure of a first-order inverter oscillation circuit.

[0028] Figure 12 An example of the LCR equivalent circuit for a vibrating element.

[0029] Figure 13 A diagram to simplify the representation of the structure of an existing laser interferometer.

[0030] Figure 14 To simplify Figure 1 A diagram showing the structure of a laser interferometer.

[0031] Figure 15 A graph showing an example of the waveform of a received light signal.

[0032] Figure 16 Let C represent the load capacitance of the oscillating circuit. L A graph showing the relationship between the accuracy of the measurement results of the displacement amplitude of the piezoelectric element being measured and the object being measured.

[0033] Figure 17 This is a schematic structural diagram illustrating the mounting structure of the optical system of the laser interferometer involved in the first modified example.

[0034] Figure 18 This is a schematic structural diagram illustrating the mounting structure of the optical system of the laser interferometer involved in the second variation.

[0035] Figure 19 This is a schematic structural diagram illustrating the mounting structure of the optical system of the laser interferometer involved in the third variation.

[0036] Figure 20 This is a schematic structural diagram illustrating the mounting structure of the optical system of the laser interferometer involved in the fourth variation. Detailed Implementation

[0037] The laser interferometer of the present invention will now be described in detail based on the embodiments shown in the accompanying drawings.

[0038] Figure 1 This is a functional block diagram illustrating the laser interferometer involved in the implementation method.

[0039] Figure 1 The laser interferometer 1 shown has a sensor head 51 equipped with an optical system 50, a demodulation circuit 52 that receives light signals from the optical system 50, and an oscillation circuit 54 that outputs a reference signal to the demodulation circuit 52.

[0040] 1. Sensor head

[0041] Figure 2 To indicate Figure 1 The diagram shows a schematic structure of the sensor head 51 and the optical system 50.

[0042] The optical system 50 includes a light source 2, a polarization beam splitter 4, a quarter-wave plate 6, a quarter-wave plate 8, an analyzer 9, a light receiving element 10, a frequency-shifting optical modulator 12, and a mounting section 16 on which the object to be measured 14 is disposed.

[0043] The light source 2 emits outgoing light L1 (first laser) of a predetermined wavelength. The light receiving element 10 converts the received light into an electrical signal. The light modulator 12 includes a vibrating element 30 and modulates the outgoing light L1 to generate a reference light L2 (second laser) containing the modulation signal. Although the mounting unit 16 can be provided as needed, it is configured to be able to mount the object to be measured 14. The outgoing light L1 incident on the object to be measured 14 is reflected as object light L3 (third laser) containing the sampling signal from the object to be measured 14.

[0044] The optical path of the emitted light L1 from the light source 2 is designated as optical path 18. Optical path 18 is combined with optical path 20 by reflection from polarization beam splitter 4. On optical path 20, a quarter-wave plate 8 and an optical modulator 12 are arranged sequentially from the polarization beam splitter 4 side. Furthermore, optical path 18 is combined with optical path 22 by transmission from polarization beam splitter 4. On optical path 22, a quarter-wave plate 6 and a mounting portion 16 are arranged sequentially from the polarization beam splitter 4 side.

[0045] Optical path 20 is combined with optical path 24 through the transmission of polarization beam splitter 4. On optical path 24, an analyzer 9 and a light-receiving element 10 are arranged sequentially from the side of polarization beam splitter 4.

[0046] The emitted light L1 from the light source 2 passes through optical path 18 and optical path 20 and enters the light modulator 12. Furthermore, the emitted light L1 passes through optical path 18 and optical path 22 and enters the object to be measured 14. The reference light L2 generated by the light modulator 12 passes through optical path 20 and optical path 24 and enters the light receiving element 10. The object light L3 generated by reflection from the object to be measured 14 passes through optical path 22 and optical path 24 and enters the light receiving element 10.

[0047] The following sections will describe each part of the laser interferometer 1 in turn.

[0048] 1.1. Light Source Section

[0049] Light source 2 is a laser source that emits a fine-width outgoing beam L1 with interferometric properties. When the linewidth is expressed in terms of frequency difference, a laser source with a linewidth of MHz or less is preferably used. Specifically, examples include gas lasers such as HeNe lasers, DFB-LDs (Distributed feedback-laser diodes), FBG-LDs (laser diodes with fiber bragg gratings), and semiconductor laser elements such as VCSELs (vertical cavity surface-emitting lasers).

[0050] The light source unit 2 is particularly preferably equipped with a semiconductor laser element. This allows for the miniaturization of the light source unit 2. Consequently, the laser interferometer 1 can be miniaturized. In particular, the miniaturization and weight reduction of the sensor head 51 housing the optical system 50 in the laser interferometer 1 are also beneficial in improving the operability of the laser interferometer 1.

[0051] 1.2. Polarization beam splitter

[0052] The polarization beam splitter 4 is an optical element that splits incident light into transmitted light and reflected light. Furthermore, the polarization beam splitter 4 has the function of transmitting P-polarized light and reflecting S-polarized light, and can separate the polarization state of the incident light into orthogonal components. Hereinafter, we consider the case where an outgoing light L1, which is linearly polarized and has a P-polarized light to S-polarized light ratio of, for example, 50:50, is incident on the polarization beam splitter 4.

[0053] In polarization beam splitter 4, as described above, the S-polarized light of the outgoing light L1 is reflected, while the P-polarized light is transmitted.

[0054] The S-polarized light L1 reflected from the polarization beam splitter 4 is converted into circularly polarized light by the quarter-wave plate 8 and then incident on the optical modulator 12. The circularly polarized light L1 incident on the optical modulator 12 is subjected to f m A frequency shift of [Hz] is applied, and the light is reflected as a reference light L2. Therefore, the reference light L2 contains a modulation frequency f. m The modulation signal is [Hz]. The reference light L2 is converted into P-polarized light when it passes through the quarter-wave plate 8 again. The P-polarized light of the reference light L2 passes through the polarization beam splitter 4 and the analyzer 9 and is incident on the light receiving element 10.

[0055] The P-polarized light L1 transmitted through the polarization beam splitter 4 is converted into circularly polarized light by the quarter-wave plate 6 and incident on the moving test object 14. The circularly polarized light L1 incident on the test 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 frequency signal is [Hz]. The object light L3 is converted into S-polarized light when it passes through the 1 / 4 wavelength plate 6 again. The S-polarized light of the object light L3 is reflected by the polarization beam splitter 4 and passes through the analyzer 9 before entering the light receiving element 10.

[0056] As mentioned above, since the outgoing light L1 is interferometric, the reference light L2 and the object light L3 are used as interference lights and incident on the light receiving element 10.

[0057] Alternatively, a non-polarization beam splitter can be used instead of a polarization beam splitter. In this case, since the quarter-wave plate 6 and the quarter-wave plate 8 are not required, the laser interferometer 1 can be miniaturized due to the reduction in the number of components.

[0058] 1.3. Polarizer

[0059] Since the S-polarized and P-polarized light, which are orthogonal to each other, are independent, simply overlapping them will not cause interference. Therefore, the light wave causing the S-polarized and P-polarized light to overlap passes through an analyzer 9 tilted at 45° relative to both the S-polarized and P-polarized light. By using the analyzer 9, the light of mutually common components can be transmitted through each other, and interference can occur. As a result, in the analyzer 9, the modulation signal and the sampling signal interfere, thereby generating a signal with fi. m -f d Interference light with a frequency of [Hz].

[0060] 1.4. Light-receiving element

[0061] Reference light L2 and object light L3 are incident on the light-receiving element 10 via polarization beam splitter 4 and analyzer 9. Consequently, reference light L2 and object light L3 undergo heterodyne interference, thus exhibiting fi... m -f d Interference light with a frequency of [Hz] is incident on the light-receiving element 10. By demodulating the sampled signal from this interference light using the method described later, the motion, i.e., velocity or vibration of the object being measured 14 can ultimately be determined. Examples of light-receiving elements 10 include photodiodes.

[0062] 1.5. Optical Modulator

[0063] Figure 3 To indicate Figure 2 A perspective view of a first structural example of the optical modulator 12 shown.

[0064] 1.5.1. Overview of the optical modulator involved in the first structural example

[0065] The frequency-shifting optical modulator 12 has an optical modulation oscillator 120. The optical modulation oscillator 120 has a plate-shaped vibrating element 30 and a substrate 31 that supports the vibrating element 30.

[0066] The vibrating element 30 is made of a material that repeatedly vibrates by applying a potential to deform in a direction along its surface. In this embodiment, the vibrating element 30 is a crystal AT 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 a structure in which a plurality of linear grooves 32 are periodically arranged.

[0067] The substrate 31 has a surface 311 and a back surface 312 that are opposite to each other. A vibrating element 30 is disposed on the surface 311. In addition, a pad 33 for applying a potential to the vibrating element 30 is provided on the surface 311. On the other hand, a pad 35 for applying a potential to the vibrating element 30 is also provided on the back surface 312.

[0068] The size of the substrate 31 is, for example, set to a length of 0.5 mm or more and 10.0 mm or less. Furthermore, the thickness of the substrate 31 is, for example, set to a length of 0.1 mm or more and 2.0 mm or less. As an example, the shape of the substrate 31 is set to a square with one side of 1.6 mm, and its thickness is set to 0.35 mm.

[0069] The size of the vibrating element 30 is, for example, set to a length of 0.2 mm or more and 3.0 mm or less. Furthermore, the thickness of the vibrating element 30 is, for example, set to a thickness of 0.003 mm or more and 0.5 mm or less.

[0070] As an example, the shape of the vibrating element 30 is set to a square with one side of 1.0 mm and its thickness is set to 0.07 mm. In this case, the vibrating element 30 oscillates at a fundamental oscillation frequency of 24 MHz. In addition, by changing the thickness of the vibrating element 30 or taking harmonics into account, the oscillation frequency can be adjusted in the range of 1 MHz to 1 GHz.

[0071] In addition, although Figure 3 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.

[0072] The intensity of light modulation implemented by the light modulator 12 is given by the dot product of the difference between the wave vector of the outgoing light L1 incident on the light modulator 12 and the wave vector of the outgoing light L2 emitted from the light modulator 12, and the vector of the vibration direction 36 of the vibrating element 30. Although the vibrating element 30 performs thickness shear vibration in this embodiment, since this vibration is in-plane vibration, light modulation cannot be performed even if light is incident perpendicularly to the surface of the vibrating element 30. Therefore, in this embodiment, by providing a diffraction grating 34 in the vibrating element 30, light modulation can be performed according to the principle described later.

[0073] Figure 3 The diffraction grating 34 shown is a blazed grating. A blazed grating is a diffraction grating whose cross-sectional shape is stepped. The straight grooves 32 of the diffraction grating 34 are configured such that their extension direction is orthogonal to the vibration direction 36.

[0074] When from Figure 1 The oscillation circuit 54 shown is directed towards Figure 3 When the vibrating element 30 shown is supplied with a drive signal S1 (an AC voltage is applied), the vibrating element 30 oscillates. Although the power (drive power) required for the oscillation of the vibrating element 30 is not particularly limited, it is as low as 0.1 μW to 100 mW. Therefore, the vibrating element 30 can be made to oscillate without amplifying the drive signal S1 output from the oscillation circuit 54.

[0075] 1.5.2. Methods for forming diffraction gratings

[0076] While the method for forming the diffraction grating 34 is not particularly limited, one example is a method that uses a mechanical scribing method (scrubbing machine) to create a model and forms the groove 32 on an electrode that has been deposited on the surface of the vibrating element 30 of the crystal AT oscillator using nanoimprinting. Here, the reason for forming the groove on the electrode is that, in principle, high-quality thickness shear vibration can be generated on the electrode when using a crystal AT oscillator. Furthermore, the formation of the groove 32 is not limited to the electrode and can also be on the surface of a material other than the electrode. Additionally, processing methods such as exposure and etching, electronic line drawing lithography, and focused ion beam processing (FIB) can be used instead of nanoimprinting.

[0077] Alternatively, a diffraction grating can be formed on the chip of the crystal AT oscillator using a photoresist material, and a metal film or a mirror film formed of a dielectric multilayer film can be disposed thereon. By disposing of a metal film or a mirror film, the reflectivity of the diffraction grating 34 can be improved.

[0078] Furthermore, a method can be adopted whereby a resist film is formed on the chip or wafer of the crystal AT oscillator, and after processing by 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, there is no influence caused by the moisture absorption of the resist material, thereby improving the stability of the diffraction grating 34. In addition, by setting a metal film with high conductivity such as Au or Al, it can also be used as an electrode to drive the oscillation element 30.

[0079] Alternatively, the diffraction grating 34 can also be formed using techniques such as anodic aluminum oxide (porous aluminum oxide).

[0080] 1.5.3. Optical modulators involved in other structural examples

[0081] The vibrating element 30 is not limited to a crystal oscillator; for example, it can also be a Si oscillator, a surface wave (SAW) device, etc.

[0082] Figure 4 This is a top view showing a portion of a second structural example of the optical modulator 12. Figure 5 This is a top view showing a third structural example of the optical modulator 12.

[0083] Figure 4 The vibrating element 30A shown is a Si oscillator manufactured using MEMS technology. MEMS (Micro-Electro-Mechanical Systems) refers to micro-electromechanical systems.

[0084] 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 utilize electrostatic attraction as a driving force, for example. Figure 4 The oscillator vibrates in a repetitive pattern of approaching and moving away from each other in the left-right direction. This allows in-plane vibration to be applied to the diffraction grating 34. The oscillation frequency of the Si oscillator is, for example, in the range of 1 kHz to several hundred MHz.

[0085] Figure 5 The vibrating element 30B shown is a SAW device that utilizes surface waves. SAW (Surface Acoustic Wave) refers to elastic surface waves.

[0086] 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, a surface wave is excited by 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, in the range of several hundred MHz to several GHz.

[0087] For devices like the one described above, similar to the case of a crystal AT oscillator, optical modulation can be achieved by setting a diffraction grating 34, according to the principles described later.

[0088] On the other hand, when the vibrating element 30 has a crystal oscillator, the extremely high Q value of the crystal can be used to generate a high-precision modulation signal. The Q value is an indicator of the sharpness of the resonance peak. In addition, the crystal oscillator also has the advantage of being difficult to be affected by external interference. Therefore, by using the modulation signal modulated by the optical modulator 12 equipped with the crystal oscillator, it is possible to obtain the sampling signal from the object being measured 14 with high precision.

[0089] 1.5.4. Optical Modulation Achieved by Vibrating Elements

[0090] Next, use Figure 3 The light modulator 12 shown illustrates the principle of light modulation.

[0091] Figure 6 To the incident light K i From relative to Figure 3 This is a conceptual diagram illustrating the situation where multiple diffracted beams are generated when the surface of the vibrating element 30 is incident in a direction perpendicular to it.

[0092] When the incident light K iWhen incident on a diffraction grating 34 that undergoes thickness shear vibration along the vibration direction 36, as Figure 6 As shown, multiple diffracted beams K are generated through diffraction phenomena. ns n is the diffraction light K ns The number of times, and n = 0, ±1, ±2, ... Additionally, in Figure 6 In the diffraction grating 34 shown, it is not Figure 3 The blazed grating shown is illustrated, while a diffraction grating formed by repeating concave and convex patterns is illustrated as another example of a diffraction grating.

[0093] Although Figure 6 In the middle incident light K i The light is incident perpendicularly to the surface of the vibrating element 30, but this incident angle is not particularly limited. Alternatively, the incident angle can be set by incident at an angle relative to the surface of the vibrating element 30. In the case of oblique incident light, the diffracted light K... ns The direction of travel also changes accordingly.

[0094] Furthermore, due to the design of the diffraction grating 34, higher-order light with |n|≥2 may not sometimes appear. Therefore, to obtain a stable modulation signal, it is desirable to set |n|=1. That is, in Figure 2 In the laser interferometer 1, the frequency-shifting optical modulator 12 is preferably configured to use ±1 order diffracted light as reference light L2. This configuration enables the stabilization of measurements performed by the laser interferometer 1.

[0095] On the other hand, if higher-order light with |n|≥2 appears from the diffraction grating 34, the optical modulator 12 can be configured such that it uses any diffraction light of ±2 or higher order instead of ±1 order diffraction light as the reference light L2. Therefore, since higher-order diffraction light can be utilized, the laser interferometer 1 can be made more frequent and smaller.

[0096] In this embodiment, as an example, the incident light K incident on the light modulator 12 is... i The optical modulator 12 is configured such that the angle between the direction of entry of the light and the direction of travel of the reference light L2 emitted from the optical modulator 12 is 180 degrees. Three structural examples will be described below.

[0097] Figures 7 to 9 These are respectively for the incident light K i A conceptual diagram illustrating an optical modulator 12 constructed such that the angle between the direction of travel of the light L2 and the direction of travel of the reference light L2 is 180 degrees.

[0098] exist Figure 7In addition to the vibrating element 30, the optical modulator 12 also includes a mirror 37. The mirror 37 is configured to focus on the diffracted light K. 1s The light is reflected and returned to the diffraction grating 34. At this point, the diffracted light K relative to mirror 37... 1s The angle between the incident angle and the reflection angle on mirror 37 is 180 degrees. As a result, the diffracted light K emitted from mirror 37 and returning to diffraction grating 34... 1s The light is diffracted again using diffraction grating 34, becoming the incident light K incident on the light modulator 12. i It travels in the opposite direction to the direction of travel. Therefore, by adding mirror 37, it is possible to satisfy the aforementioned requirement that the incident light K... i The condition is that the angle between the direction of entry of the light and the direction of travel of the reference light L2 is 180 degrees.

[0099] Furthermore, by using mirror 37 in this manner, the reference light L2 generated by the optical modulator 12 becomes light that has undergone two frequency modulations. Therefore, compared to using a single vibrating element 30, by using mirror 37, a higher frequency modulation can be achieved.

[0100] exist Figure 8 In, relative to Figure 6 The vibrating element 30 is tilted due to the configuration of the configuration. The tilt angle θ is then set to satisfy the aforementioned incident light K. i The condition is that the angle between the direction of entry of the light and the direction of travel of the reference light L2 is 180 degrees.

[0101] Figure 9 The diffraction grating 34 shown has a blaze angle θ B The blazed grating. And, the incident light K traveling at an angle β relative to the normal N of the surface of the vibrating element 30. i When incident on diffraction grating 34, reference light L2 is relative to normal N at a blaze angle θ. B The same angle returns. Therefore, by making the incident angle β the same as the blaze angle θ B They are equal, thus satisfying the aforementioned incident light K. i The condition that the angle between the direction of entry of the light beam and the direction of travel of the reference light L2 is 180 degrees. In this case, since it is possible to achieve this without using... Figure 7 Mirror 37 as shown and as Figure 8 As shown, by ensuring that the vibrating element 30 itself is not tilted, the condition is satisfied, thus enabling further miniaturization and high-frequency operation of the laser interferometer 1. In particular, in the case of a blazed grating, the configuration that satisfies the condition is called the "Litterrow configuration," which also has the beneficial effect of particularly improving the diffraction efficiency of the diffracted light.

[0102] in addition, Figure 9 The tooth pitch P represents the tooth pitch of the blazed grating; as an example, the tooth pitch P is set to 1 μm. Furthermore, the blaze angle θ... B It is set to 25 degrees. In this case, to satisfy the condition, it is only necessary to adjust the angle relative to the incident light K. i The incident angle β of the normal N can also be set to 25 degrees.

[0103] 1.5.5. Packaging Structure

[0104] Figure 10 This is a cross-sectional view showing an optical modulator 12 with an encapsulation structure.

[0105] Figure 10 The optical modulator 12 shown includes a container 70 as a frame, an optical modulation oscillator 120 housed in the container 70, and circuit elements 45 constituting an oscillation circuit 54. In addition, the container 70 is hermetically sealed in a reduced pressure atmosphere such as a vacuum, or an inert gas atmosphere such as nitrogen or argon.

[0106] 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 therein and a second recess 722 disposed inside the first recess 721 and deeper than the first recess 721. The container body 72 is made of, for example, a ceramic material or a resin material. Furthermore, although not shown, the container body 72 includes internal terminals disposed on its inner surface, external terminals disposed on its outer surface, and wiring connecting the internal and external terminals.

[0107] Furthermore, the opening of the container body 72 is blocked by the cover 74 through a sealing ring (not shown) or a low-melting-point glass or other sealing component. The cover 74 is constructed of a material that allows laser light to pass through, such as glass.

[0108] 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 connecting member (not shown). Furthermore, the internal terminals of the container body 72 are electrically connected to the optical modulation oscillator 120 via a conductive material (not shown), such as a connecting lead or a connecting metal.

[0109] 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 lead 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 provided in the circuit element 45.

[0110] As described above, the optical modulator 12 according to this embodiment includes a container 70 that serves as a frame for housing the vibrating element 30. Furthermore, the oscillation circuit 54 is also housed within the container 70.

[0111] By employing this packaging structure, the physical distance between the optical modulation oscillator 120 and the circuit element 45 can be reduced by overlapping them, thereby shortening the wiring length between them. Therefore, it is possible to suppress noise from the outside entering the drive signal S1, or conversely, for the drive signal S1 to become a noise source. Furthermore, a container 70 can protect both the optical modulation oscillator 120 and the circuit element 45 from the influence of the external environment. Thus, the reliability of the laser interferometer 1 can be improved while miniaturizing the sensor head 51.

[0112] Furthermore, the structure of container 70 is not limited to the structure shown in the figure. For example, the optical modulation oscillator 120 and the circuit element 45 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 omitted as needed.

[0113] 2. Oscillator circuit

[0114] like Figure 1 as well as Figure 2 As shown, the output of the oscillation circuit 54 is a drive signal S1 input to the optical modulator 12 of the optical system 50. Furthermore, the output of the oscillation circuit 54 is a reference signal S2 input to the demodulation circuit 52.

[0115] In the oscillating circuit 54, there is no particular limitation on the circuit that can make the oscillating element 30 oscillate; various circuit structures can be used. As an example, Figure 11 A circuit diagram showing the structure of a first-order inverter oscillation circuit.

[0116] Figure 11 The oscillation circuit 54 shown includes circuit element 45, feedback resistor Rf, first limiting resistor R1, second limiting resistor R2, first capacitor Cg, second capacitor Cd and third capacitor C3.

[0117] Circuit element 45 is an inverter IC. Terminals X1 and X2 of circuit element 45 are connected to the inverter respectively. Terminal GND is connected to ground potential, and terminal Vcc is connected to power supply potential. Terminal Y is used for oscillation output.

[0118] A first capacitor Cg is connected between terminal X1 and ground potential. Furthermore, a first limiting resistor R1 and a second capacitor Cd are connected in series between terminal X2 and ground potential, starting from terminal X2. Also, one end of a feedback resistor Rf is connected between terminal X1 and the first capacitor Cg, and the other end of the feedback resistor Rf is connected between terminal X2 and the first limiting resistor R1.

[0119] Furthermore, one end of the second limiting resistor R2 is connected between the first limiting resistor R1 and the second capacitor Cd. Moreover, the aforementioned vibrating element 30 is connected between the first capacitor Cg, the feedback resistor Rf, and the other end of the second limiting resistor R2. In other words, the vibrating element 30 serves as the signal source for the oscillation circuit 54.

[0120] also, Figure 12 An example of the LCR equivalent circuit for the vibrating element 30.

[0121] like Figure 12 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.

[0122] In this oscillation circuit 54, the capacitance of the first capacitor Cg is set to C. g The capacitance of the second capacitor Cd is set to C. d At that time, the load capacitance C L It is given by the following formula (a).

[0123] Formula 1

[0124]

[0125] Thus, the oscillation frequency f output from terminal Y of the oscillation circuit 54 osc It is given by the following formula (b).

[0126] Formula 2

[0127]

[0128] f Q The natural vibration number of the vibrating element 30.

[0129] According to equation (b) above, by appropriately changing the load capacitance C L This allows the oscillation frequency f of the signal output from terminal Y to be controlled. osc Make minor adjustments.

[0130] In addition, the natural vibration coefficient f of the vibrating element 30 Q The oscillation frequency f of the oscillation circuit 54 oscThe difference Δf is given by the following equation (c).

[0131] Formula 3

[0132]

[0133] Here, since C1 << C0 and C1 << C L Therefore, Δf is approximately given by the following equation (d).

[0134] Formula 4

[0135]

[0136] Therefore, the oscillation frequency f of the oscillation circuit 54 osc Becoming the natural vibration number f of the vibrating element 30 Q The corresponding value.

[0137] When the vibrating element 30 is fixed in the container 70, for example, and subjected to expansion stress caused by temperature through the fixing part, the natural vibration coefficient f Q Changes occur. Furthermore, when the vibrating element 30 is tilted, it is affected by the gravitational effect of its own weight, thus causing the natural vibration coefficient f to change. Q Changes have occurred.

[0138] In the oscillating circuit 54, even if the inherent oscillation number f is reduced for such reasons... Q A change has occurred, and based on the above equation (d), the oscillation frequency f will also change in a manner linked to this change. osc The oscillation frequency f changes. osc Always becomes from the natural vibration number f Q The value of Δf has deviated. Therefore, the vibrating element 30 can stably obtain the displacement amplitude L0. Although this displacement amplitude L0 is described below as the value of B, it is an important parameter in the optical modulation performed by the optical modulator 12. Therefore, as long as the displacement amplitude L0 can be stabilized, the modulation characteristics of the optical modulator 12 can be stabilized, thereby improving the demodulation accuracy of the sampled signal in the demodulation circuit 52.

[0139] As an example, Δf = f is preferred. osc -f Q The value is ≤600 Hz, more preferably 240 Hz ≤ Δf ≤ 450. In this case, since the displacement amplitude L0 and the B value become particularly high, the modulation characteristics of the optical modulator 12 can be made particularly stable.

[0140] Furthermore, if a signal generator, such as a function generator or a signal generator, is used instead of the oscillation circuit 54, the oscillation frequency f cannot be adjusted as described above.osc With natural vibration number f Q The changes in the amplitude of displacement L0 may become unstable due to the interaction between the two.

[0141] As described above, the laser interferometer 1 according to this embodiment includes a light source unit 2, a light modulator 12, a light receiving element 10, a demodulation circuit 52, and an oscillation circuit 54. The light source unit 2 emits an outgoing light L1 (first laser). The light modulator 12 includes a vibration element 30 and uses the vibration element 30 to modulate the outgoing light L1, thereby generating a reference light L2 (second laser) containing a modulation signal. The light receiving element 10 receives the object light L3 (third laser) generated by the reflection of the outgoing light L1 on the object to be measured 14 (object), which contains a signal originating from the object to be measured 14, and the interference light with the reference light L2, and outputs a received light signal. The demodulation circuit 52 demodulates the signal originating from the object to be measured 14 based on the reference signal S2 and according to the received light signal. The oscillation circuit 54 outputs the reference signal S2 that is input to the demodulation circuit 52. Furthermore, the vibration element 30 serves as the signal source for the oscillation circuit 54.

[0142] Here, the structure of an existing laser interferometer is compared with the structure of the laser interferometer 1 involved in this embodiment.

[0143] Figure 13 A diagram illustrating the structure of a conventional laser interferometer 90 in a simplified manner. Figure 14 To simplify Figure 1 A diagram showing the structure of the laser interferometer 1.

[0144] Figure 13 The existing laser interferometer 90 shown includes an optical modulator 91 such as an acousto-optical modulator (AOM) and an electro-optical modulator (EOM), a light-receiving element 92, a demodulation circuit 93, an oscillation circuit 94, a crystal oscillator 95, and an amplifier 96. The oscillation circuit 94 generates a drive signal by oscillating the crystal oscillator 95. The drive signal, amplified by the amplifier 96, is input to the optical modulator 91, causing the optical modulator 91 to operate. Furthermore, the oscillation circuit 94 outputs a reference signal required for demodulating the sampled signal in the demodulation circuit 93.

[0145] In the existing laser interferometer 90, there is a possibility of temperature characteristic deviation between the optical modulator 91, which performs the function of optical modulation, and the crystal oscillator 95. Therefore, this temperature characteristic deviation causes a decrease in the measurement accuracy of the laser interferometer 90.

[0146] In contrast, Figure 14The laser interferometer 1 shown in this embodiment includes an optical modulator 12 with a vibrating element 30, a light-receiving element 10, a demodulation circuit 52, and an oscillation circuit 54. The oscillation circuit 54 operates the optical modulator 12 by oscillating the vibrating element 30. Furthermore, the oscillation circuit 54 uses the vibrating element 30 as a signal source and outputs a reference signal S2 required for demodulating the sampled signal in the demodulation circuit 52.

[0147] Therefore, according to the structure of this embodiment, since the vibrating element 30 performs the function of optical modulation in the optical modulator 12, the temperature characteristics of the modulation signal correspond to the temperature characteristics of the vibrating element 30. On the other hand, since the vibrating element 30 functions as the signal source of the oscillation circuit 54, the temperature characteristics of the reference signal S2 output from the oscillation circuit 54 to the demodulation circuit 52 also correspond to the temperature characteristics of the vibrating element 30. In this way, 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 change in the modulation signal caused by temperature changes is consistent with or approximately the change in the reference signal. Therefore, even if the temperature of the vibrating element 30 changes, the influence on demodulation accuracy can be suppressed, thereby improving the demodulation accuracy of the sampling signal originating from the measured object 14.

[0148] Furthermore, the existing optical modulator 91 also requires maintaining its temperature, making it difficult to reduce its size. In addition, due to the high power consumption of the optical modulator 91, there are challenges in miniaturizing and reducing the power consumption of the laser interferometer 90.

[0149] In contrast, in this embodiment, since the vibration element 30 is very small and the power required for oscillation is also small, it is easier to miniaturize and save power for the laser interferometer 1.

[0150] Furthermore, as mentioned above, the vibrating element 30 modulates the emitted light L1 (first laser) into a reference light L2 (second laser) by vibration. Additionally, the oscillation circuit 54 uses the vibration of the vibrating element 30 as a source oscillation to generate a reference signal S2.

[0151] According to this structure, the vibration of the vibrating element 30 contributes to both the modulation of the emitted light L1 and the generation of the reference signal S2. Therefore, the vibration frequency of the vibrating element 30 directly affects both the modulation frequency of the emitted light L1 and the frequency of the reference signal S2. Thus, even if the modulation frequency deviates due to temperature changes in the vibrating element 30, since the reference signal S2 also deviates, temperature changes are unlikely to have an impact on the operations in the multiplier 538 of the preprocessing unit 53 (described later) or the operations in the first multiplier 551 and the second multiplier 552 of the demodulation unit 55. Therefore, in principle, since environmental changes such as temperature changes are unlikely to affect the operation process, it is particularly effective in suppressing the decrease in demodulation accuracy.

[0152] 3. Demodulation circuit

[0153] The demodulation circuit 52 performs demodulation processing on the sampled signal originating from the object under test 14 based on the light-receiving signal output from the light-receiving element 10. The sampled signal is, for example, a phase signal or a frequency signal. Displacement information of the object under test 14 can be obtained from the phase signal. Furthermore, velocity information of the object under test 14 can be obtained from the frequency signal. If different information can be obtained in this way, the laser interferometer 1 can be highly functionalized because it functions as a displacement meter or a velocity meter.

[0154] The demodulation circuit 52 sets its circuit structure 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. Since the vibrating element 30 is a single vibrating element, its vibration speed changes constantly. Therefore, the modulation frequency also changes, making it impossible to use a conventional demodulation circuit.

[0155] Existing demodulation circuits refer to circuits that demodulate sampled signals based on received optical signals containing modulated signals modulated using an acousto-optical modulator (AOM). In an acousto-optical modulator, the modulation frequency does not change. Therefore, although existing demodulation circuits can demodulate sampled signals based on received optical signals containing modulated signals modulated by an optical modulator with a fixed modulation frequency, they cannot demodulate signals containing modulated signals modulated by an optical modulator 12 with a changing modulation frequency.

[0156] therefore, Figure 1The demodulation circuit 52 shown includes a preprocessing unit 53 and a demodulation unit 55. The light-receiving signal output from the light-receiving element 10 is first guided to the demodulation unit 55 after passing through the preprocessing unit 53. 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 unit 55, the sampling signal originating from the object under test 14 is demodulated using a known demodulation method.

[0157] 3.1. Structure of the pretreatment unit

[0158] 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.

[0159] Furthermore, between the preprocessing unit 53 and the light-receiving element 10, a current-to-voltage converter 531 and an ADC 532 are sequentially connected from the light-receiving element 10 side.

[0160] Furthermore, an ADC533 is connected between the oscillation circuit 54 and the second delay adjuster 537.

[0161] The current-to-voltage converter 531 is a current-to-voltage converter that converts the current output from the light-receiving element 10 into a voltage signal. ADCs 532 and 533 are analog-to-digital converters that convert analog signals into digital signals with a predetermined number of sampling bits.

[0162] The first bandpass filter 534, the second bandpass filter 535, and the third bandpass filter 539 are filters that selectively allow signals of specific frequency bands to pass through.

[0163] The first delay adjuster 536 and the second delay adjuster 537 are circuits that adjust the delay of the signal, respectively. The multiplier 538 is a circuit that generates an output signal proportional to the product of the two input signals. The first AGC 540 and the second AGC 541 are circuits that align the amplitudes of the signals. The adder 542 is a circuit that generates an output signal proportional to the sum of the two input signals.

[0164] The current output from the light-receiving element 10 is converted into a voltage signal using a current-to-voltage converter 531. The voltage signal is then converted into a digital signal using an ADC 532 and split into two signals: a first signal and a second signal.

[0165] After the first signal passes through the first bandpass filter 534, its group delay is adjusted using 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 caused by the second bandpass filter 535 (described later). Through this delay adjustment, the delay time formed by the passing of the filter circuits is made consistent between the first bandpass filter 534 through which the first signal passes and the second bandpass filter 535 and third bandpass filter 539 through which the second signal passes. The first signal passing through the first delay adjuster 536 is then input to the adder 542 via the first AGC 540.

[0166] The second signal is input to the multiplier 538 after passing through the second bandpass filter 535. In the multiplier 538, the second signal is multiplied by the reference signal cos(ω) output from the second delay adjuster 537. m Specifically, the reference signal S2 output from the oscillation circuit 54 is digitally converted using the ADC 533 and its phase is adjusted using the second delay adjuster 537 before being output to the multiplier 538. Then, after passing through the third bandpass filter 539, the second signal is input to the adder 542 via the second AGC 541. In the adder 542, an output signal proportional to the sum of the first and second signals is output to the demodulation unit 55.

[0167] 3.2. Principles of preprocessing performed by the preprocessing unit

[0168] Next, the principle of preprocessing in the preprocessing unit 53 will be explained. First, when the following relationship is established, namely:

[0169] E m =a m cos{2π(f0t+∫f m (t)dt)+φ m0} (1)

[0170] E d =a d cos{2π(f0t+∫f d (t)dt)+φ d0} (2)

[0171] φ0=φ m0 φ d0 (3)

[0172] The intensity I of the light-receiving signal output from the light-receiving element 10 PD Logically, it can be represented by the following formula.

[0173] Formula 6

[0174]

[0175] In addition, E m E d φ m0 φ d0 ,φ0,f m (t), f d (t), f0, a m a d Separately represent the following content.

[0176] Formula 7

[0177] E m Electric field component of the modulated signal

[0178] E d Electric field component originating from the sampling signal of the measured object

[0179] φ m0 Initial phase value of optical path 20

[0180] φ d0 Initial phase value of optical path 22

[0181] φ0: Optical path phase difference of the laser interferometer

[0182] f m (t): Frequency of the modulation signal originating from the optical modulator

[0183] f d (t): Frequency of the sampling signal originating from the measured object

[0184] f0: Frequency of the emitted light from the light source

[0185] a m :coefficient

[0186] a d :coefficient

[0187] In addition, in equation (4), <> represents time average.

[0188] Additionally, f0, as an example, is around 300THz, f m (t) is taken as an example, around 100kHz to 100MHz, f d (t) is taken as an example as approximately 1kHz to 10MHz.

[0189] The first term in equation (4) above represents the DC component, and the second 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.

[0190] Formula 8

[0191]

[0192]

[0193] X(t)=-∫f d (t)dt+φ0 (7)

[0194] f mdmax Maximum modulation frequency of the optical modulator

[0195] f m The modulation frequency of the optical modulator

[0196] Moreover, I PD.AC It can be deformed in the following ways.

[0197] Formula 9

[0198] I PD.AC =Acos(Bsin2πf) m t+X(t))

[0199] =A{cosBsin2πf m t·cosX(t)-sinBsin2πf m t·sinX(t)} (8)

[0200] A = a m a d (9)

[0201]

[0202] Here, we know a Bessel function of degree ν as shown in the following equation.

[0203] Formula 10

[0204] cos{ζsin(2πf v t)}=J0(δ)+2J2(ζ)cos(2.2πf v t)+2J4(ζ)cos(4.2πf v t)+… (11)

[0205] sin{ζsin(2πf ν t)}=2J1(ζ)sin(1.2πf v t)+2J3(ζ)sin(3.2πf v t)+... (12)

[0206] If the series expansion of equation (8) is performed using the Bessel functions of equations (11) and (12), it can be transformed in the following way.

[0207] Formula 11

[0208] I PD . AC =A[{J0(B)+2J2(B)cos(2·2πf m t)+2J4(B)cos(4·2πf m t)+...}cos X(t)

[0209] -{2J1(B)sin(1·2πf m t)+2J3(S)sin(3.2πf m t)+…}sinX(t)] (13)

[0210] However, J0(B), J1(B), J2(B), ... are Bessel coefficients.

[0211] Furthermore, the relationship between the order and coefficient of the vibration frequency in the expanded terms is shown in Table 1 below.

[0212] Table 1

[0213]

[0214] When unfolded in the manner described above, it can be said that logically, a band corresponding to a specific number of times can be extracted using a bandpass filter.

[0215] Therefore, in the aforementioned preprocessing unit 53, the light-receiving signal is preprocessed according to the following process based on this logic.

[0216] First, the digital signal output from the aforementioned ADC 532 is split into two signals: a first signal and a second signal. The first signal passes through a first bandpass filter 534. The first bandpass filter 534 sets the center angular frequency to ω. m Therefore, the result of further adjusting the phase and amplitude of the first signal after passing through the first bandpass filter 534 using the first delay adjuster 536 and the first AGC 540 is expressed by the following formula.

[0217] Formula 12

[0218] I pas1 =Csin(2πf) m t)•sinX(t) (14)

[0219] On the other hand, the second signal 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 after passing through the second bandpass filter 535 is represented by the following formula.

[0220] Formula 13

[0221] I BPF2 =2J2(a)cos(2·2πf) m t)·cosX(t)

[0222] =J2(B){cos(2·2πf m t+X(t))+cos(2·2πf m tX(t))} (15)

[0223] The second signal after passing through the second bandpass filter 535 is compared with the reference signal cos(ω) using the multiplier 538. m t) multiply. The second signal after multiplication is represented by the following formula.

[0224] Formula 14

[0225] I cos (ω m t)=J2(B){cos(2·2πf m t+X(t))+cos(2·2πf m tX(t))}·cos(2πf m t)

[0226] =J2(B){cos(3·2πf m t+X(t))+cos(1·2πf m t+X(i))

[0227] +cos(3·2πf m tX(t))+cos(1·2πf m tX(t))} (16)

[0228] The second signal, 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 after passing through the third bandpass filter 539 is represented by the following formula.

[0229] Formula 15

[0230] I BPF3 =J2(B){cos(2πf m t+X(t))+cos(2πf m txt))}

[0231] =2J2(B)cos(2πf) m t)·cosX(t) (17)

[0232] The result of the second signal, after passing through the third bandpass filter 539, having its amplitude matched that of the first signal by utilizing the second AGC 541, is expressed by the following formula.

[0233] Formula 16

[0234] I pass2 =Ccos(2πf) m t)·cosX(t) (18)

[0235] The first signal represented by equation (14) above and the second signal represented by equation (18) above are added together by adder 542. The result of the addition is represented by the following equation.

[0236] Formula 17

[0237] I 53 =cos(2πf) m tX(t)) (19)

[0238] As shown in equation (19) above, unnecessary terms can be eliminated and necessary terms can be extracted as a result of addition. This result is output to the demodulation unit 55. In addition, although the digital processing using the ADC has been described using the preprocessing unit 53, an analog circuit structure without an ADC can also be used.

[0239] 3.3. Structure of the demodulation unit

[0240] The demodulation unit 55 performs demodulation processing on the sampled signal originating from the object under test 14 based on the signal output from the preprocessing unit 53. Although not particularly limited to demodulation processing, the well-known quadrature detection method can be cited as an example. The quadrature detection method is a method of demodulation processing by performing an operation of mixing mutually orthogonal signals from the outside on the input signal.

[0241] Figure 1 The demodulation unit 55 shown is a digital circuit that includes a first multiplier 551, a second 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 a signal output circuit 559.

[0242] 3.4. Principles of demodulation processing performed by the demodulation unit

[0243] In the demodulation process, firstly, the signal output from the preprocessing unit 53 is split into two signals. In the first multiplier 551, one of the split signals is multiplied by the frequency signal cos(ω) of the reference signal S2 output from the oscillation circuit 54. m The signal is multiplied by t). In the second multiplier 552, the divided signal is multiplied by the frequency signal -sin(ω) of the reference signal S2 output from the oscillator circuit 54, which is shifted by -90° using the phase shifter 553. m Multiply by t. Frequency signal cos(ω) m t) and frequency signal -sin(ω) m t) represents a signal whose phases are offset from each other by 90°.

[0244] The signal passing through the first multiplier 551 passes through the first low-pass filter 555 and is then input as signal x to the divider 557. The signal passing through the second multiplier 552 also passes through the second low-pass filter 556 and is then input as signal y to the divider 557. The divider 557 performs a division operation of signal y by signal x, and then passes signal y / x through the arctangent operator 558 to obtain signal atan(y / x).

[0245] Then, by passing the signal atan(y / x) through the signal output circuit 559, the phase φ is determined. d This is used as a sampling signal originating from the measured object 14. Furthermore, it is possible to base the signal on the phase φ. d The displacement information of the object 14 to be measured is calculated. Thus, a displacement meter for measuring the displacement of the object 14 is realized. Furthermore, velocity information can be derived from the displacement information. Thus, a velocity meter for measuring the velocity of the object 14 is realized.

[0246] While the circuit structure of the demodulation unit 55 has been described above, the above-described digital circuit structure is merely an example and is not limiting. Furthermore, the demodulation unit 55 is not limited to a digital circuit; it can also be an analog circuit. In an analog circuit, it may also include an F / V inverter circuit or a ΔΣ counter circuit.

[0247] Alternatively, the demodulation unit 55 described above can be used to obtain a frequency signal, which can then be used as a sampling signal originating from the object being measured 14. This allows for the calculation of the velocity information of the object being measured 14 based on the frequency signal.

[0248] 4. Amplitude of the phase

[0249] As can be seen from the principle of preprocessing performed by the aforementioned preprocessing unit 53, in order to stably perform preprocessing, the AC component of the received light signal needs to have the 1·ω shown in Table 1 above. m The signal components and 2·ω m The two sides of the signal components.

[0250] Figure 15 This is a graph illustrating an example of the waveform of the received light signal. As mentioned earlier, the received light signal is divided into a DC component and an AC component. Figure 15 In order to reproduce the various possible states of the optical path phase difference φ0 in laser interferometer 1, a smooth periodic variation of φ0 is given. Therefore, in Figure 15 In the equation, the DC component cos(φ0) corresponds to the period of the long wave, and the AC component cos(ψ) m -ψ d +φ0) corresponds to the period of shortwave. ψ m Let be the phase of the modulation signal generated by the optical modulator 12, and let ψ be the phase of the modulation signal generated by the optical modulator 12. m =Bsin(ω) m Let t) represent this. ψ d This refers to the phase of the sampling signal originating from the measured object 14. Additionally, in Figure 15 Let ψ be the middle part. d =0. Furthermore, as an example, B is set to 0.27.

[0251] The periods of long and short waves will vary depending on the measurement conditions. Therefore, regardless of the motion of the object being measured 14, in order to perform stable measurements, it is also desirable to record the line representing the received light signal as... Figure 15 The "optimal region" refers to the two regions within the "optimal region." The optimal region is defined as the region where the aforementioned 1·ω appears in the AC component of the received light signal. m The signal components and 2·ω m The regions of both sides of the signal components. That is, such as Figure 15 As shown, when deviating from the optimal region, 1·ω m The signal component will disappear, or 2·ω m The signal components will disappear.

[0252] Therefore, in order to enter the aforementioned "optimal region", it is only necessary to make the ψ in the AC component... m The amplitude of +φ0 should be greater than π / 3. Furthermore, it is preferable to have an amplitude greater than π / 2.

[0253] Based on this, the phase ψ of the modulated signal m Let the amplitude of the change be Δψ m and the phase ψ of the sampled signal d Let the amplitude of the change be Δψd At that time, as long as at least Δψ m +Δψ d The condition >π / 3 is sufficient. Furthermore, Δψ m Ideally, the value should be as large as possible, approaching π / 3. This enables stable measurement.

[0254] Furthermore, according to the aforementioned ψ m =Bsin(ω) m The formula for t), the phase ψ of the modulated signal. m The change in amplitude Δψ m The value of B is chosen. Therefore, when selecting the vibrating element 30, the value of B is preferably as large as possible, tending towards π / 3. As an example, the value of B is preferably greater than 0.5, and more preferably greater than π / 3. Thus, even if the displacement of the object being measured 14 is small, the measurement can be carried out stably.

[0255] Furthermore, the value of B can be converted into the displacement amplitude L0 of the vibrating element 30. For example, when the wavelength of the emitted light L1 is 632 nm, in order to satisfy B > π / 3, it is only necessary to make L0 > 69.5 nm. Furthermore, when the wavelength of the emitted light L1 is 850 nm, in order to satisfy B > π / 3, it is only necessary to make L0 > 93.4 nm. Therefore, when selecting the vibrating element 30, the displacement amplitude L0 can also be selected as a standard.

[0256] In addition, these values ​​are obtained in the following way.

[0257] According to equation (10), B = f mdmax / f m In addition, f mdmax =(4π·f m ·L0·sinθ) / λ. Where λ is the wavelength of the emitted light L1, and θ is... Figure 8 The tilt angle shown.

[0258] Therefore, (4πL0sinθ) / λ>π / 3 holds true. The result is that L0>λ / (12sinθ). Thus, for example, when θ=49.3°, in the case of λ=632nm mentioned above, L0>69.5nm[=632 / {12×sin(49.3)}]. Furthermore, in the case of λ=850nm, L0>93.4nm[=850 / {12×sin(49.3)}].

[0259] Figure 16 To represent the load capacitance C of the oscillator circuit 54 L A graph showing the relationship between the accuracy of the measurement results of the displacement amplitude of the piezoelectric element, which is the measured object 14, and the accuracy of the measurement results. Additionally, Figure 16The vertical axis represents the ratio of the standard deviation σ of the displacement amplitude of the piezoelectric element measured using laser interferometer 1 to the true value of 80.3 nm. Hereinafter, this value will be referred to as the "demodulation accuracy".

[0260] exist Figure 16 In the middle, it is believed that by adjusting the load capacitor C of the oscillation circuit 54... L Setting it to a predetermined value or higher achieves demodulation accuracy below 1%. Figure 16 In the example, by using the load capacitor C L Setting it to 50pF or higher enables demodulation accuracy below 1%. The natural vibration coefficient f of the vibrating element 30... Q When the Hz is 4,971,886 Hz, the load capacitance C is adjusted based on equation (d). L Setting a threshold of 50 pF or higher, when converted to Δf, results in Δf ≤ 600 [Hz]. Therefore, by satisfying this condition, good demodulation accuracy can be achieved in the laser interferometer 1.

[0261] 5. Variations

[0262] Next, the laser interferometer involved in the modified example will be explained.

[0263] Figure 17 This is a schematic structural diagram illustrating the mounting structure of the optical system of the laser interferometer involved in the first modified example. Figure 18 This is a schematic structural diagram illustrating the mounting structure of the optical system of the laser interferometer involved in the second variation. Figure 19 This is a schematic structural diagram illustrating the mounting structure of the optical system of the laser interferometer involved in the third variation. Figure 20 This is a schematic structural diagram illustrating the mounting structure of the optical system of the laser interferometer involved in the fourth variation.

[0264] Although variations will be described below, the description will focus on the differences from the described embodiment, and similar points will be omitted. Furthermore, in Figures 17 to 20 In the text, the same symbols are used to mark the same structures as those in the described embodiment.

[0265] Figure 17 The optical system 50A of the laser interferometer 1 shown includes a substrate 39. The light source 2, the light modulator 12, and the light-receiving element 10 are respectively mounted on this substrate 39. Furthermore, along with the optical system 50A of the laser interferometer 1, the optical system 50A includes a substrate 39. Figure 17 The light path 22 shown has a light receiving element 10, a light source 2, and a light modulator 12 arranged in sequence in orthogonal directions.

[0266] also, Figure 17The optical system 50A shown includes prisms 40 and 42. Prism 40 is disposed on the optical path 24 between the light-receiving element 10 and the analyzer 9. Prism 42 is disposed on the optical path 20 between the light modulator 12 and the quarter-wave plate 8.

[0267] and, Figure 17 The optical system 50A shown includes a convex lens 44. The convex lens 44 is disposed on the optical path 18 between the light source 2 and the polarization beam splitter 4. By providing the convex lens 44, the emitted light L1 from the light source 2 is focused, thereby enabling its effective utilization.

[0268] Figure 18 The optical system 50B of the laser interferometer 1 shown is identical to that of the laser interferometer 1, except for the different configuration of components, etc. Figure 17 The optical system shown is the same as that of the 50A.

[0269] exist Figure 18 On the substrate 39 shown, along with Figure 18 The light source 2, the light receiving element 10, and the light modulator 12 are arranged sequentially in an orthogonal direction in the optical path 22 shown. The prism 40 is disposed on the optical path 18, and the prism 42 is disposed on the optical path 20.

[0270] Figure 19 The optical system 50C of the laser interferometer 1 shown has a configuration in which an optical modulator 12 is installed in the optical path connecting the object to be measured 14 and the light receiving element 10.

[0271] exist Figure 19 On the substrate 39 shown, along with Figure 19 The light source 2, the light modulator 12, and the light receiving element 10 are arranged sequentially in an orthogonal direction in the optical path 22 shown. The prism 40 is disposed on the optical path 18, and the prism 42 is disposed on the optical path 24.

[0272] According to the above... Figures 17 to 19 The mounting structure shown allows for easy miniaturization of the laser interferometer 1. Furthermore, the component configuration is not limited to the arrangement shown in the figure.

[0273] exist Figures 17 to 19 In the mounting structure shown, the size of the light-receiving element 10 is, for example, 0.1 mm square, the size of the light source 2 is, for example, 0.1 mm square, and the size of the light modulator 12 is, for example, 0.5 to 10 mm square. Furthermore, the size of the substrate 39 on which they are mounted is, for example, 1 to 10 mm square. Thus, the laser interferometer 1 can be miniaturized to the size of this substrate 39.

[0274] Figure 20The optical system 50D of the laser interferometer 1 shown is the same as the optical system 50 of the embodiment except for the following differences.

[0275] First, the optical system 50D replaces the light-receiving element 10 of the aforementioned embodiment with light-receiving elements 10a and 10b. Furthermore, in addition to the polarization beam splitter 4, the optical system 50D also includes a polarization beam splitter 4a. The polarization beam splitter 4a allows P-polarized light to pass through and reflects S-polarized light. Moreover, the optical system 50D includes half-wavelength plates 17a and 17b, a convex lens 44, and a mirror 48. It also includes a substrate 39 for mounting the light source unit 2 and the light-receiving elements 10a and 10b.

[0276] A half-wave plate 17a, a polarization beam splitter 4a, and a mirror 48 are arranged sequentially from the polarization beam splitter 4 side on the optical path 24 between the polarization beam splitter 4 and the light-receiving element 10b.

[0277] A half-wavelength plate 17b and a convex lens 44 are arranged sequentially on the optical path 18, starting from the polarization beam splitter 4 side.

[0278] also, Figure 20 The laser interferometer 1 shown has a differential amplifier circuit 56, which performs differential processing and amplification processing on the light-receiving signal output from the light-receiving element 10a and the light-receiving signal output from the light-receiving element 10b.

[0279] S-polarized light, reflected by the polarization beam splitter 4a, is incident on the light-receiving element 10a. On the other hand, P-polarized light, which has passed through the polarization beam splitter 4a and is reflected by the mirror 48, is incident on the light-receiving element 10b.

[0280] The received light signal intensity I of the S-polarized light incident on the light-receiving element 10a PD1 It can be represented by the following formula.

[0281] Formula 18

[0282]

[0283] The received light signal intensity I of the P-polarized light incident on the light-receiving element 10b PD2 It can be represented by the following formula.

[0284] Formula 19

[0285]

[0286] In the above equations (20) and (21), the first term represents the DC component and the second term represents the AC component.

[0287] Therefore, the differential amplifier circuit 56 implements the control of the received light signal intensity I. PD1With the intensity of the received light signal I PD2 Difference I PD1 -I PD2 Differential processing is performed during the calculation. As a result of this processing, the DC component is eliminated, and the differential I... PD1 -I PD2 It is represented by only the amplified AC component, as shown in the following formula.

[0288] Formula 20

[0289] I PD1-PD2 =2a m a d cos(2πf m tX(t)) (22)

[0290] Therefore, according to this modified example, by using two light-receiving elements 10a and 10b to perform differential processing, the DC component that becomes noise in the demodulation of the sampled signal can be eliminated, and the AC component can be obtained with greater intensity. Thus, the demodulation accuracy of the sampled signal can be further improved.

[0291] In the variations described above, the same effects as the embodiments described above can be obtained.

[0292] While the laser interferometer of the present invention has been described above based on the illustrated embodiments, the laser interferometer of the present invention is not limited to the described embodiments, and the structure of each part can be replaced with any structure having the same function. Furthermore, other arbitrary structures may be added to the laser interferometer involved in the described embodiments.

[0293] Explanation of symbols

[0294] 1…Laser interferometer; 2…Light source; 4…Polarization beam splitter; 4a…Polarization beam splitter; 6…1 / 4 wavelength plate; 8…1 / 4 wavelength plate; 9…Analyzer; 10…Light receiving element; 10a…Light receiving element; 10b…Light receiving element; 12…Optical modulator; 14…Measured object; 16…Setting part; 17a…1 / 2 wavelength plate; 17b…1 / 2 wavelength plate; 18…Optical path; 20…Optical path; 22…Optical path; 24…Optical path; 30…Vibrating element; 30A…Vibrating element; 30B…Vibrating element; 31…Substrate; 32…Groove; 33…Pad; 34…Diffraction grating; 35…Pad; 36…Vibration direction; 37…Mirror; 39…Substrate; 40…Prism ; 42…prism; 44…convex lens; 45…circuit element; 48…mirror; 50…optical system; 50A…optical system; 50B…optical system; 50C…optical system; 50D…optical system; 51…sensor head; 52…demodulation circuit; 53…preprocessing unit; 54…oscillation circuit; 55…demodulation unit; 56…differential amplifier circuit; 70…container; 72…container body; 74…lid; 76…connecting lead; 90…laser interferometer; 91…optical modulator; 92…light receiving element; 93…demodulation circuit; 94…oscillation circuit; 95…crystal oscillator; 96…amplifier; 120…optical modulation oscillator; 301…first electrode; 302…second electrode; 3 03… Diffraction grating mounting section; 305… Piezoelectric substrate; 306… Comb-shaped electrode; 307… Ground electrode; 311… Surface; 312… Back side; 531… Current-to-voltage converter; 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; 542… Adder; 551… First multiplier; 552… Second multiplier; 553… Phase shifter; 555… First low-pass filter; 556… Second low-pass filter; 557… Divider; 558… Arctangent operator; 559… Signal Output circuit; 721… First recess; 722… Second recess; 540… First AGC; 541… Second AGC; C0… Parallel capacitor; C1… Series capacitor; C3… Third capacitor; Cd… Second capacitor; Cg… First capacitor; GND… Terminal; L1… Outgoing light; L1… Series inductor; L2… Reference light; L3… Object light; N… Normal; P… Tooth pitch; R1… Equivalent series resistance; R1… First limiting resistor; R2… Second limiting resistor; Rf… Feedback resistor; S1… Drive signal; S2… Reference signal; Vcc… Terminal; X1… Terminal; X2… Terminal; Y… Terminal; x… Signal; y… Signal; β… Angle of incidence; θ… Tilt angle; θ B …Shining corner.

Claims

1. A laser interferometer, characterized in that, have: The light source emits the first laser beam; An optical modulator having a vibrating element and using the vibrating element to modulate the first laser to generate a second laser containing a modulated signal; A light-receiving element receives interference light from a third laser and a second laser, and outputs a light-receiving signal, wherein the third laser includes a sampling signal generated by the reflection of the first laser by an object; A demodulation circuit that demodulates the sampled signal based on a reference signal and according to the received light signal; An oscillation circuit, having a first capacitor, oscillates through the vibrating element to output the reference signal to the demodulation circuit. The vibrating element is connected to the first capacitor. The vibrating element becomes the signal source of the oscillation circuit.

2. The laser interferometer as described in claim 1, characterized in that, The oscillation circuit has a feedback resistor.

3. The laser interferometer as described in claim 2, characterized in that, The vibrating element is connected between the first capacitor and the feedback resistor.

4. The laser interferometer as described in claim 1, characterized in that, The oscillation circuit includes a second capacitor.

5. The laser interferometer as described in claim 4, characterized in that, The vibrating element is connected to the second capacitor.

6. The laser interferometer as described in claim 4, characterized in that, The oscillation circuit has a first limiting resistor.

7. The laser interferometer as described in claim 6, characterized in that, The first limiting resistor is connected between the vibrating element and the second capacitor.

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