Laser interferometer

Abstract

To provide a laser interferometer that can highly accurately demodulate a Doppler signal derived from a measurement object, not relying on an oscillation condition of an oscillating element.SOLUTION: A laser interferometer comprises: an optical modulation unit that includes a light source emitting laser light, and an oscillation element to be driven by a drive signal, and overlaps a modulation signal on the laser light; a light reception unit that receives the laser light including a sample signal derived from an object and modulation signal, and outputs a light reception signal; a computation unit that demodulates the sample signal from the light reception signal on the basis of a reference signal; and a signal generation unit that outputs the drive signal and reference signal. The optical modulation unit has: the oscillation element that has a first oscillation part and a second oscillation part oscillating in a reverse phase with respect to the first oscillation part; a first optical modulator that is provided in the first oscillation part, and modulates the laser light; a second optical modulator that is provided in the second oscillation part, and modulates the laser light modulated by the first optical modulator; and a bypass optical path that causes the laser light modulated by the first optical modulator to be incident upon the second optical modulator.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Patent Document 1 discloses a laser Doppler velocometer (laser interferometer) that measures the velocity of a vibrating object by irradiating it with a laser beam and utilizing the frequency of the laser beam, which is changed by the Doppler effect. In a laser Doppler velocometer, a structure is required to modulate the light emitted from the laser light source in order to detect the directionality of the object's vibration phenomenon. For this reason, Patent Document 1 discloses the use of an acousto-optic modulator or an electro-optic modulator.

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

[0004] On the other hand, Patent Document 3 discloses a laser Doppler velocometer configured to apply a sinusoidal signal to an optical modulator, receive a reference light beam obtained by frequency shifting the light beam from a laser light source, and a reflected light beam obtained by irradiating the object to be measured with the light beam, receive these with a photodetector, perform predetermined calculation processing on the received signal, and then perform FM demodulation processing. In such a laser Doppler velocometer, by performing predetermined calculation processing before FM demodulation processing, a signal corresponding to the speed of the object to be measured can be obtained from the received signal even when the frequency of the reference light beam is sinusoidally shifted.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] In a laser interferometer using a vibration element, there are cases where the speed of the measurement target cannot be accurately measured. Specifically, depending on the vibration conditions of the vibration element, the intensity of the modulation signal in the reference light becomes extremely small. In such cases, the Doppler signal derived from the measurement target cannot be demodulated with high precision.

Means for Solving the Problems

[0007] The laser interferometer according to an application example of the present invention includes a light source that emits laser light, a vibration element driven by a drive signal, and an optical modulation unit that superimposes a modulation signal on the laser light using the vibration element, a light receiving unit that receives the laser light including a sample signal derived from an object and the modulation signal and outputs a light receiving signal, an arithmetic unit that demodulates the sample signal from the light receiving signal based on a reference signal, a signal generation unit that outputs the drive signal and the reference signal, and includes The optical modulation unit has the vibration element having a first vibration unit and a second vibration unit that vibrates in a reverse phase to the first vibration unit, a first optical modulator provided in the first vibration unit that modulates the laser light, A second optical modulator provided in the second vibrating unit for modulating the laser beam modulated by the first optical modulator; A detour optical path for making the laser beam modulated by the first optical modulator enter the second optical modulator; It has.

Brief Description of Drawings

[0008] [Figure 1] It is a functional block diagram showing a laser interferometer according to the first embodiment. [Figure 2] It is a schematic configuration diagram showing a sensor head unit provided in the laser interferometer of FIG. 1. [Figure 3] It is a diagram showing a schematic configuration of a vibrating element provided in the optical modulation unit of FIG. 2. [Figure 4] It is a graph showing the relationship between the ratio of noise mixed in the modulation signal and the signal-to-noise ratios of two signals PASS1 and PASS2 generated inside the preprocessing unit shown in FIG. 1. [Figure 5] It is a graph showing the relationship between the ratio [%] of the time lag Δt with respect to the vibration period of the vibrating element and the relative value of the B value due to multiple modulations. [Figure 6] It is a schematic configuration diagram showing a sensor head unit provided in a laser interferometer according to a first modification of the first embodiment. [Figure 7] It is a schematic configuration diagram showing a sensor head unit provided in a laser interferometer according to a second modification of the first embodiment. [Figure 8] It is a schematic configuration diagram showing a sensor head unit provided in a laser interferometer according to the second embodiment. [Figure 9] It is a perspective view showing a vibrating element provided in the optical modulation unit of FIG. 8. [Figure 10] It is a schematic configuration diagram showing a sensor head unit provided in a laser interferometer according to a first modification of the second embodiment. [Figure 11] It is a schematic configuration diagram showing a sensor head unit provided in a laser interferometer according to a second modification of the second embodiment. [Figure 12]This is a schematic diagram showing the sensor head portion of the laser interferometer according to the third embodiment. [Figure 13] Figure 12 is a perspective view showing the vibrating element included in the optical modulation section. [Figure 14] This is a conceptual diagram showing the frequency characteristics of the main vibration mode (in-plane vibration mode) and the secondary vibration mode (out-of-plane vibration mode). [Figure 15] This is a schematic diagram showing the sensor head portion of a laser interferometer according to a first modified example of the third embodiment. [Figure 16] This is a schematic diagram showing the sensor head portion of a laser interferometer according to a second modified example of the third embodiment. [Modes for carrying out the invention]

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

[0010] Figure 1 is a functional block diagram showing a laser interferometer 1 according to the first embodiment. Figure 2 is a schematic configuration diagram showing the sensor head 51 included in the laser interferometer 1 of Figure 1.

[0011] The laser interferometer 1 shown in Figure 1 comprises a sensor head 51 and a main body 59. The sensor head 51 is easily miniaturized and lightweight, and is easily portable and easy to install, so it can be placed, for example, near the object 14 shown in Figure 2, which is the object to be measured by the laser interferometer 1. The main body 59 can be placed away from the sensor head 51 and may be housed, for example, in a rack.

[0012] The sensor head unit 51 shown in Figure 1 includes an interference optical system 50 and a signal generation unit 6. The main body unit 59 includes a calculation unit 52.

[0013] 1.1. Sensor head section 1.1.1. Interferometric Optics The interference optical system 50 is a Michelson-type interference optical system. As shown in Figure 2, the interference optical system 50 comprises a light source 2, beam splitters 41 and 42, a quarter-wave plate 43, mirrors 44 and 45, a light modulation unit 12, and a photodetector 10.

[0014] Light source 2 emits laser light L1. The optical modulation unit 12 includes a vibrating element 30 and a bypass optical path 7, which changes the phase of the laser light L1 and superimposes a modulation signal.

[0015] The laser beam L1 emitted from the light source 2 enters the beam splitter 41 and is split into two. One laser beam L1 goes towards the bypass optical path 7 via the mirror 44, and the other laser beam L1 goes towards the beam splitter 42.

[0016] The bypass optical path 7 shown in Figure 2 is an optical path that guides the laser beam L1 so that one of the laser beams L1 is directed towards the vibrating element 30 twice.

[0017] Furthermore, the vibrating element 30 has a first vibrating section 31 and a second vibrating section 32 that vibrate in opposite phases to each other, and the first vibrating section 31 and the second vibrating section 32 are used to superimpose a modulation signal onto the laser light L1. In this specification, the superimposition of a modulation signal onto the laser light due to the interaction between the structure provided on the vibrating element 30 and the laser light is referred to as "superimposing a modulation signal onto the laser light using a vibrating element."

[0018] Upon the first irradiation, when the laser beam L1 is irradiated onto the first vibrating part 31 of the vibrating element 30, a modulation signal is added to modulate the phase, generating the laser beam L21. The laser beam L21 is irradiated onto the second vibrating part 32 of the vibrating element 30 via the bypass optical path 7. In other words, the bypass optical path 7 has the function of diverting the laser beam so that the laser beam is irradiated onto the first vibrating part 31 and the second vibrating part 32, which vibrate in opposite phases to each other. Upon the second irradiation, a modulation signal is again added to the laser beam L21, generating the laser beam L22. In other words, the laser beam L22 is light obtained by performing two phase modulations on the laser beam L1. Here, since the first vibrating part 31 and the second vibrating part 32 vibrate in opposite phases to each other, the phase of the laser beam L1 can be shifted in the same direction for both of them with two irradiations. The generated laser beam L22 is incident on the beam splitter 42 via the mirror 45. The laser light L22 then passes through the beam splitter 42 and is incident on the light-receiving element 10.

[0019] The other laser beam L1 enters the beam splitter 42, then passes through the quarter-wave plate 43 and enters the object 14. A sample signal is added to the laser beam L1 that enters the object 14, depending on the speed and position of the object 14. As a result, the laser beam L1 is reflected as laser beam L3, which contains the sample signal originating from the object 14. The laser beam L3 passes through the quarter-wave plate 43 again and returns to the beam splitter 42, where it is reflected and enters the photodetector 10.

[0020] The light-receiving element 10 receives the incident laser light L22 and L3 and converts its intensity into an electrical signal.

[0021] The following provides a further explanation of each part of the interference optical system 50. 1.1.1.1.Light source Light source 2 is a laser light source that emits coherent laser light L1. Preferably, light source 2 uses a light source with a linewidth of MHz or less. Specifically, examples include gas lasers such as He-Ne lasers, DFB-LDs (Distributed Feedback Laser Diodes), FBG-LDs (Fiber Bragg Grating Laser Diodes), VCSELs (Vertical Cavity Surface Emitting Lasers), and semiconductor laser elements such as FP-LDs (Fabry-Perot Laser Diodes).

[0022] The light source 2 is preferably a semiconductor laser element. This makes it possible to miniaturize the light source 2. As a result, the laser interferometer 1 can be miniaturized.

[0023] 1.1.1.2. Beam Splitter The beam splitter 41 is a polarization beam splitter positioned between the light source 2 and the mirror 44. The beam splitter 41 has the function of transmitting P-polarized light and reflecting S-polarized light. Due to this function, the beam splitter 41 splits the laser light L1 into two. Therefore, one of the aforementioned laser light L1 is P-polarized, and the other laser light L1 is S-polarized. Note that the laser light L1 incident on the beam splitter 41 may pass through any optical element so that it becomes linearly polarized, for example, with a P-polarized to S-polarized intensity ratio of 50:50.

[0024] One of the P-polarized laser beams, L1, that passes through the beam splitter 41 is reflected by the mirror 44 and enters the bypass path 7. The laser beam L1 that enters the bypass path 7 has its phase modulated twice by the vibrating element 30, becoming the laser beam L22. The laser beam L22 enters the beam splitter 42.

[0025] The other laser beam L1, which is S-polarized and reflected by the beam splitter 41, is converted to P-polarized by a half-wave plate (not shown) and incident on the beam splitter 42. The beam splitter 42 is a polarization beam splitter positioned between the beam splitter 41 and the quarter-wave plate 43, and between the mirror 45 and the photodetector 10. The beam splitter 42 transmits P-polarized light and reflects S-polarized light. Due to this function, the other laser beam L1 is incident on the object 14 via the quarter-wave plate 43. The laser beam L1 incident on the object 14 is f d It undergoes a Doppler shift of [Hz]. This generates laser light L3 containing the sample signal originating from the object 14. The laser light L3 returns to the beam splitter 42 via the quarter-wave plate 43. Since the laser light L3 is S-polarized, it is reflected by the beam splitter 42 and incident on the photodetector 10.

[0026] The beam splitter 41 may be an unpolarized beam splitter. In that case, optical elements for controlling polarization should be provided between the beam splitter 41 and the bypass optical path 7, and between the beam splitter 41 and the beam splitter 42.

[0027] 1.1.1.3.Detour optical path The bypass optical path 7 is positioned to optically connect mirror 44 and mirror 45 and is configured to irradiate the vibrating element 30 with laser light L1 twice.

[0028] The bypass optical path 7 comprises a beam splitter 72, a quarter-wave plate 73, a mirror 74, a mirror 75, a beam splitter 76, and a quarter-wave plate 77.

[0029] The beam splitter 72 has the function of switching the optical paths of the laser beams L1 and L21 according to their polarization, for example. Specifically, the beam splitter 72 transmits P-polarized light and reflects S-polarized light. The laser beam L1 that has passed through the aforementioned beam splitter 41 and been reflected by the mirror 44 is P-polarized, so it passes through the beam splitter 72 and is incident on the vibrating element 30 via the quarter-wave plate 73.

[0030] The vibrating element 30 has a first vibrating section 31 and a second vibrating section 32 that vibrate in opposite phases to each other. The bypass optical path 7 redirects the laser beam so as to irradiate the first vibrating section 31 and the second vibrating section 32 once each.

[0031] The laser beam L1 is first reflected by the first vibrating part 31 of the vibrating element 30. At this time, the laser beam L1 is +Φ m It undergoes a phase shift of [Hz]. Phase shift amount Φ m is, Φ m =Bsin(2π*f m This is expressed as *t). B is the phase shift (B value) of the modulated signal added by the phase shift, and f m t is the frequency of the modulation signal, and t is time. Due to the phase shift, a modulation signal that modulates the phase is added to the laser light L1, and the laser light L21 is generated. The generated laser light L21 returns to the beam splitter 72 via the quarter-wave plate 73. Since the laser light L21 is S-polarized, it is reflected by the beam splitter 72, and then sequentially reflected by mirrors 74 and 75 before being incident on the beam splitter 76.

[0032] The beam splitter 76 has the function of switching the optical paths of the laser beams L21 and L22 according to their polarization. Specifically, the beam splitter 76 transmits P-polarized light and reflects S-polarized light. As a result, the laser beam L21 is reflected by the beam splitter 76 and incident on the vibrating element 30 via the quarter-wave plate 77.

[0033] The laser beam L21 is reflected by the second vibrating part 32 of the vibrating element 30. At this time, the laser beam L21 is +Φm It undergoes a phase shift of [Hz]. As a result, a modulation signal that modulates the phase is added to the laser beam L21, and the laser beam L22 is generated. Consequently, the laser beam L22 is +2Φ m The system includes a modulation signal corresponding to a phase shift in [Hz]. The generated laser light L22 returns to the beam splitter 76 via the quarter-wave plate 77. Since the laser light L22 is P-polarized, it passes through the beam splitter 76, is reflected by the mirror 45, and enters the beam splitter 42. The laser light L22 then passes through the beam splitter 42 and enters the photodetector 10.

[0034] In the beam splitter 42, the laser beams L22 and L3 interfere, generating interference light. This interference light has a beat with a higher signal-to-noise ratio than conventional beam splitters. Based on the frequency and phase of this beat, the calculation unit 52 can ultimately calculate the velocity and position of the object 14 with high accuracy.

[0035] As described above, in the bypass optical path 7 shown in Figure 2, by using beam splitters 72 and 76 and quarter-wave plates 73 and 77, it is possible to irradiate the vibrating element 30 with laser light twice.

[0036] 1.1.1.4. Photodetector When interfering light is incident on the photodetector 10, the photodetector 10 outputs a photocurrent (received signal) corresponding to the intensity of the interfering light. By demodulating the sample signal from this received signal using a method described later, the movement of the object 14, i.e., its velocity and position, can ultimately be determined. Examples of the photodetector 10 include photodiodes and phototransistors. Note that the light received by the photodetector 10 can be any laser light containing the sample signal and the modulation signal, and is not limited to the interfering light described above, as it varies depending on the configuration of the optical system. Furthermore, "demodulating the sample signal from the received signal" in this specification includes demodulating the sample signal from various signals converted from the photocurrent (received signal).

[0037] 1.1.1.5. Vibration element Figure 3 shows a schematic configuration of the vibration element 30 included in the optical modulation unit 12 of Figure 2. The vibrating element 30 shown in Figure 3 is plate-shaped and, when an electric potential is applied, repeatedly vibrates in a mode that deforms in the direction along the surface (thickness-slip vibration mode). An element that vibrates in such a mode is called a thickness-slip vibration element. Examples of thickness-slip vibration elements include quartz AT resonators and quartz BT resonators. These quartz resonators have a quartz crystal that is a piezoelectric material and electrodes provided on the quartz crystal. In a quartz resonator, the extremely high Q value of the quartz can be used to add a modulation signal with a high signal-to-noise ratio to the laser light L1.

[0038] The vibrating element 30 shown in Figure 3 has a first surface 311 which is the first vibrating part 31, and a second surface 321 which is the second vibrating part 32. The first surface 311 (first vibrating part 31) and the second surface 321 (second vibrating part 32) shown in Figure 3 vibrate in opposite phases to each other. Vibrating in opposite phases to each other means that the first surface 311 and the second surface 321 vibrate along the same vibration axis in opposite phases to each other.

[0039] Furthermore, the first surface 311 is provided with a first diffraction grating 331, which is a first optical modulator 33, and the second surface 321 is provided with a second diffraction grating 341, which is a second optical modulator 34. Examples of the first diffraction grating 331 and the second diffraction grating 341 include the blazed diffraction grating shown in Figure 3, as well as laminar diffraction gratings and the like.

[0040] Such a vibrating element 30 oscillates based on the drive signal Sd output from the signal generation unit 6. The power required for the oscillation of the vibrating element 30 (drive power) is not particularly limited, but is small, ranging from about 0.1 μW to 100 mW. Therefore, by incorporating the vibrating element 30, the laser interferometer 1 can be made smaller, lighter, and more power-efficient. Furthermore, the thickness-sliding vibrating element is also useful because it has superior shock resistance and temperature characteristics compared to elements with other vibration modes.

[0041] FIG. 3 shows solid and dashed arrows indicating the deformation directions of the vibration element 30. The voltages that cause the deformations indicated by the solid arrows and the voltages that cause the deformations indicated by the dashed arrows are in opposite phases to each other. Therefore, the first surface 311 (first vibration portion 31) and the second surface 321 (second vibration portion 32) of the vibration element 30 shown in FIG. 3 vibrate in opposite phases to each other. As a result, after irradiating the first diffraction grating 331 with laser light, if the laser light is also irradiated onto the second diffraction grating 341, the phase of the laser light can be shifted in the same direction for both irradiations.

[0042] For example, when the first surface 311 (first vibration portion 31) shown in FIG. 3 is displaced as indicated by the dashed arrow, the second surface 321 (second vibration portion 32) is displaced as indicated by the dashed arrow in synchronization with the displacement of the first surface 311. Then, after irradiating the first diffraction grating 331 provided on the first surface 311 with the laser light L1 to give a phase shift of +Φ m [Hz], even if the laser light L21 is irradiated onto the second diffraction grating 341 provided on the second surface 321, a phase shift of +Φ m [Hz] can be given. This is because the first surface 311 and the second surface 321 vibrate in opposite phases to each other, and also because the speed of light is very large and the delay amount associated with the two irradiations can be almost ignored. As described above, laser light L22 including a modulation signal corresponding to a phase shift of +2Φ m [Hz] is obtained.

[0043] Note that the vibration element 30 is not limited to a crystal oscillator, and may be a silicon oscillator or a ceramic oscillator. Further, in addition to the first vibration portion 31 and the second vibration portion 32, the vibration element 30 may have a third vibration portion and a fourth vibration portion as additional vibration portions, or may have a larger number of vibration portions. In this case, each additional vibration portion is vibrated in synchronization with the first vibration portion 31 or the second vibration portion 32, and by irradiating each additional vibration portion with laser light, the amount of phase shift can be further increased.

[0044] 1.1.2. Signal generation unit The signal generation unit 6 shown in Figure 1 outputs a drive signal Sd that is input to the vibration element 30, and a reference signal Ss that is input to the calculation unit 52.

[0045] In this embodiment, as shown in Figure 1, the signal generation unit 6 includes an oscillation circuit 61. The oscillation circuit 61 operates using the vibrating element 30 as a signal source and generates a highly accurate periodic signal. As a result, the oscillation circuit 61 outputs a highly accurate drive signal Sd and a reference signal Ss. Consequently, when disturbances occur, the drive signal Sd and the reference signal Ss are affected in the same way. Therefore, the modulation signal added via the vibrating element 30 driven by the drive signal Sd, and the reference signal Ss are also affected in the same way. For this reason, when the modulation signal and the reference signal Ss are used for calculations in the calculation unit 52, the effects of disturbances contained in both can be canceled out or reduced during the calculation process. As a result, the calculation unit 52 can accurately determine the position and velocity of the object 14 even when disturbances occur.

[0046] An example of the oscillator circuit 61 is the oscillator circuit disclosed in Japanese Patent Application Publication No. 2022-38156.

[0047] Furthermore, the signal generation unit 6 may be equipped with a signal generator such as a function generator or a signal generator instead of the oscillation circuit 61.

[0048] 1.2. Main body 1.2.1. Arithmetic section The main unit 59 includes a calculation unit 52. The calculation unit 52 has a pre-processing unit 53, a demodulation processing unit 55, and a demodulated signal output unit 57.

[0049] The arithmetic unit 52 shown in Figure 1 includes a preprocessing unit 53, a demodulation processing unit 55, and a demodulated signal output unit 57. The functions performed by these functional units are realized by hardware, such as a processor, memory, external interface, input unit, and display unit. Specifically, this is realized by the processor reading and executing a program stored in memory. These components can communicate with each other via an internal bus.

[0050] Examples of processors include CPUs (Central Processing Units) and DSPs (Digital Signal Processors). Alternatively, instead of these processors executing software, FPGAs (Field-Programmable Gate Arrays) or ASICs (Application-Specific Integrated Circuits) may be used to implement the aforementioned functions.

[0051] Examples of memory include HDD (Hard Disk Drive), SSD (Solid State Drive), EEPROM (Electrically Erasable Programmable Read-Only Memory), ROM (Read-Only Memory), and RAM (Random Access Memory).

[0052] External interfaces include, for example, digital input / output ports such as USB (Universal Serial Bus) and Ethernet® ports.

[0053] Examples of input devices include keyboards, mice, touch panels, and touchpads. Examples of display devices include liquid crystal display panels and organic EL (Electro-Luminescence) display panels.

[0054] External interfaces, input units, and display units may be provided as needed, but may be omitted.

[0055] For example, the pre-processing unit 53 and demodulation unit disclosed in Japanese Patent Application Publication No. 2022-38156 can be applied to the pre-processing unit 53 and demodulation unit 55.

[0056] The preprocessing unit 53 performs preprocessing on the received light signal based on the reference signal Ss. The preprocessing involves splitting the received light signal into two signals, PASS1 and PASS2, multiplying one of them by the reference signal, and then summing the two signals PASS1 and PASS2 to output the preprocessed signal.

[0057] The demodulation processing unit 55 demodulates a sample signal corresponding to the speed and position of the object 14 based on the reference signal Ss, from the pre-processed signal output from the pre-processing unit 53.

[0058] The demodulated signal output unit 57 performs phase connection on the demodulated signal output from the demodulated processing unit 55 by applying, for example, phase unwrapping. This allows the position of the object 14 to be calculated. As a result, the laser interferometer 1 becomes a displacement meter. Furthermore, the velocity can be determined from the position of the object 14. As a result, the laser interferometer 1 becomes a speedometer.

[0059] 1.2.2. Relationship between the phase shift (B value) of the modulated signal and the signal-to-noise ratio (S / N ratio) The modulation signal added by the optical modulation unit 12 has a parameter called phase shift (B value). This B value affects the signal-to-noise ratio (S / N ratio) of the pre-processed signal output from the pre-processing unit 53. Specifically, the larger the B value, the higher the S / N ratio of the pre-processed signal. By increasing the S / N ratio of the pre-processed signal, the sample signal (Doppler signal) originating from the object 14, which is ultimately calculated by the calculation unit 52, can be demodulated with higher accuracy, regardless of vibration conditions, such as the small amplitude of vibration of the vibrating element 30. As a result, a laser interferometer 1 with high accuracy in calculating the position and velocity of the object 14 can be realized.

[0060] Figure 4 is a graph showing the relationship between the proportion of noise mixed into the modulated signal and the signal-to-noise ratio of the two signals PASS1 and PASS2 generated inside the preprocessing unit 53 shown in Figure 1. In Figure 4, the above relationship is shown as a curve for each B value of the modulated signal added to the laser light by the optical modulation unit 12 shown in Figure 1.

[0061] In Figure 4, it can be seen that as the B value increases from 0.5 to 1.5, the S / N ratio of the two signals PASS1 and PASS2 tends to increase, regardless of the noise ratio. Thus, the larger the B value, the higher the S / N ratio of the two signals PASS1 and PASS2 can be. Therefore, as described above, this embodiment is configured to provide a bypass optical path 7 within the interference optical system 50 and to irradiate the first diffraction grating 331 and the second diffraction grating 341 provided on the vibrating element 30 with laser light, respectively. In other words, the interference optical system 50 is configured to irradiate the vibrating element 30 with laser light twice.

[0062] As mentioned above, the phase shift amount Φ m is, Φ m =Bsin(2π*f m This is expressed as *t). Therefore, the phase shift (B value) of the modulated signal is proportional to the amount of phase shift of the laser light. Consequently, if the amount of phase shift can be doubled, the B value can be doubled.

[0063] In this embodiment, phase modulation is performed twice on the laser light, but this number may be three or more. When n is an integer of 2 or more, if the number of phase modulations on the laser light can be set to n, the B value can theoretically be multiplied by n.

[0064] 1.2.3. Influence of time lag in multiple phase modulations on the B value In this embodiment, as described above, multiple (n) phase modulations are performed. As previously mentioned, since the speed of light is very large, the time lag Δt between the first phase modulation and the nth phase modulation can be almost ignored. However, depending on the configuration of the bypass optical path 7, the optical distance traveled by the laser light between the first irradiation and the nth irradiation may become long, and the time lag Δt may not be negligible. Therefore, we will consider the configuration required for the optical device 1 based on the relationship between the time lag Δt and its effect on the B value.

[0065] The frequency at which the vibration element 30 is driven by the drive signal Sd is f M Let the frequency be [Hz], and let Lt [m] be the optical distance the laser beam travels from the time it is modulated once until it is modulated n times. In this case, it is preferable that the bypass optical path 7 satisfies the following equation (1). Lt ≤ 9 × 10 6 / f M (1)

[0066] By ensuring that the bypass optical path 7 satisfies equation (1) above, the influence of the time lag Δt on the B value can be kept sufficiently small.

[0067] Figure 5 is a graph showing the relationship between the ratio of the time lag Δt to the vibration period of the vibrating element 30 [%] and the relative values ​​of the B values ​​obtained from multiple modulations. In the following explanation, the ratio of the time lag Δt to the vibration period of the vibrating element 30 is also referred to as the "normalized time lag." The relative value of the B value is the relative value of each B value when the B value when the time lag Δt is zero is set to 100%.

[0068] As shown in Figure 5, the relative value of the B value tends to decrease as the normalization time lag increases, but within a sufficiently small range of normalization time lag, the decrease in the relative value of the B value is relatively small. For example, if the normalization time lag can be kept below 3%, a relative value of 99% or more of the B value when the normalization time lag is 0% can be secured.

[0069] The optical distance Lt [m] mentioned above is expressed as Lt = c * Δt, where c is the speed of light. In this equation, c = 3 × 10⁻¹⁰ 8 Substituting [m / s] and Δt=0.03*T, we get Lt=9×10 6 T is the vibration period of the vibrating element 30, and is 1 / f M Therefore, the condition for ensuring a relative value of B of 99% or more is Lt ≤ 9 × 10 6 / f M This leads to the conclusion.

[0070] On the other hand, if the number of phase modulation operations increases, the optical distance Lt increases accordingly, and the time lag Δt increases. For the nth irradiation, the delay between the vibration of the diffraction grating irradiated by the laser light and the vibration of the diffraction grating located on the opposite side should be kept to 1 / 4 or less of the vibration period. By satisfying this condition, the probability of shifting the phase in a direction that increases the relative value of B can be increased for all irradiations from the first to the nth irradiation. Conversely, if, for example, the optical distance Lt increases and the delay exceeds 1 / 4 of the vibration period, some irradiations may shift the phase in a direction that decreases the relative value of B.

[0071] Based on the above, it is preferable that the bypass optical path 7 satisfies the following equation (2). n≦c / (4Lf M ) (2)

[0072] By ensuring that the number of times n irradiates the vibrating element 30 with laser light, which is set in the bypass optical path 7, satisfies equation (2) above, the drawbacks of having too large a number n, that is, the problem of not being able to obtain the effect of increasing the B value in proportion to the number n, can be avoided. In other words, by satisfying equation (2) above, each irradiation can contribute to increasing the B value, thus making it possible to increase the B value more reliably. As a result, the S / N ratio of the pre-processed signal output by the pre-processing unit 53 can be more reliably improved.

[0073] In equation (2) above, L is the optical distance [m] traveled by the laser light from the time the laser light is modulated n-1 times until it is modulated n times. For example, in the case of the bypass optical path 7 shown in Figure 2, n=2, so the optical distance L can be defined as the product of the physical distance the laser light travels from the time the laser light L1 is modulated by the first vibrating part 31 of the vibrating element 30 until the laser light L21 is modulated by the second vibrating part 32 of the vibrating element 30, and the refractive index of the medium through which the laser light L21 passes.

[0074] As an example, when the optical distance L is 0.1m and the vibrating element 30 is driven by the drive signal Sd, the frequency f M When the frequency is 100 MHz, it is preferable that the number of times n irradiates the vibrating element 30 with laser light satisfies n ≤ 75.

[0075] 2. First modified example of the first embodiment Next, a laser interferometer according to a first modified example of the first embodiment will be described.

[0076] Figure 6 is a schematic diagram showing the sensor head portion 51 of the laser interferometer 1 according to a first modified example of the first embodiment.

[0077] The following describes a first modified example of the first embodiment. In this description, the focus will be on the differences from the first embodiment, and similar aspects will be omitted. In Figure 6, components similar to those in the first embodiment are denoted by the same reference numerals.

[0078] The first modified example is the same as the first embodiment, except that the vibrating element 30 is configured to be irradiated with laser light three times.

[0079] The interference optical system 50 shown in Figure 6 comprises a light source 2, a beam splitter 41, a light modulation unit 12, a polarizer 46, and a light receiving element 10.

[0080] In the bypass optical path 7 provided by the optical modulation unit 12 shown in Figure 6, one of the two beams of light split by the beam splitter 41, laser beam L1, is incident. The bypass optical path 7 shown in Figure 6 is configured to irradiate the vibrating element 30 three times with laser beam L1. The other laser beam L1 is incident on the target object 14 and then incident on the photodetector 10 as laser beam L3.

[0081] The beam splitter 41 is an unpolarized beam splitter, and it splits the laser light at a predetermined splitting ratio regardless of polarization. In the configuration shown in Figure 6, it is desirable that the laser light L1 incident on the beam splitter 41 be P-polarized.

[0082] The bypass optical path 7 shown in Figure 6 comprises a beam splitter 72, a quarter-wave plate 73, a mirror 74, a mirror 75, and a mirror 78.

[0083] The laser light L1 incident on the bypass optical path 7 passes through the beam splitter 72 and, via the quarter-wave plate 73, is incident on the first vibrating section 31 of the vibrating element 30. As a result, a modulation signal is added to the laser light L1, and laser light L21 is generated. The generated laser light L21 returns to the beam splitter 72 again via the quarter-wave plate 73. The laser light L21 is incident on the second vibrating section 32 of the vibrating element 30 via the mirrors 74, 75, and 78. As a result, a modulation signal is added to the laser light L21, and laser light L22 is generated. The generated laser light L22 returns to the beam splitter 72 again via the mirrors 78, 75, and 74. This laser light L22 is incident on the first vibrating section 31 again via the quarter-wave plate 73. As a result, a modulation signal is added to the laser light L23, and laser light L23 is generated. The laser light L23 returns to the beam splitter 72 via the quarter-wave plate 73, and then enters the photodetector 10 via the beam splitter 41 and polarizer 46.

[0084] As described above, the bypass optical path 7 shown in Figure 6 allows the laser beam to be irradiated onto the vibrating element 30 three times. This makes it possible to shift the phase of the laser beam in the same direction each of the three irradiations. As a result, the amount of phase shift of the laser beam can be tripled, and the B value can be tripled. In the first modified example described above, the same effects as in the first embodiment can be obtained.

[0085] 3. Second modified example of the first embodiment Next, a laser interferometer relating to a second modified example of the first embodiment will be described.

[0086] Figure 7 is a schematic diagram showing the sensor head portion 51 of the laser interferometer 1 according to a second modified example of the first embodiment.

[0087] The following describes a second modified example of the first embodiment. In this description, the focus will be on the differences between this second modified example and the first embodiment, and similar aspects will be omitted. In Figure 7, components similar to those in the first embodiment and the first modified example are denoted by the same reference numerals.

[0088] The second modified example is the same as the first modified example, except that the vibrating element 30 is configured to be irradiated with laser light four times.

[0089] The interference optical system 50 shown in Figure 7 comprises a light source 2, a beam splitter 41, a light modulation unit 12, a polarizer 46, and a photodetector 10. In the configuration shown in Figure 7, it is desirable that the laser light L1 incident on the beam splitter 41 be P-polarized.

[0090] In the bypass optical path 7 provided by the optical modulation unit 12 shown in Figure 7, one of the two beams of light split by the beam splitter 41, laser beam L1, is incident. The bypass optical path 7 shown in Figure 7 is configured to irradiate the vibrating element 30 four times with laser beam L1. The other laser beam L1 is incident on the target object 14 and then incident on the photodetector 10 as laser beam L3.

[0091] The bypass optical path 7 shown in Figure 7 comprises a beam splitter 72, a quarter-wave plate 73, a mirror 74, a mirror 75, a beam splitter 76, a quarter-wave plate 77, and a mirror 79.

[0092] The laser light L1 incident on the bypass optical path 7 passes through the beam splitter 72 and, via the quarter-wave plate 73, is incident on the first vibrating section 31 of the vibrating element 30. As a result, a modulation signal is added to the laser light L1, and laser light L21 is generated. The generated laser light L21 returns to the beam splitter 72 again via the quarter-wave plate 73. The laser light L21 is incident on the second vibrating section 32 of the vibrating element 30 via the mirrors 74, 75 and the beam splitter 76. As a result, a modulation signal is added to the laser light L21, and laser light L22 is generated. The generated laser light L22 is reflected by the mirror 79 via the quarter-wave plate 77 and the beam splitter 76, and then incident on the second vibrating section 32 again via the beam splitter 76 and the quarter-wave plate 77. As a result, a modulation signal is added to the laser light L22, and laser light L23 is generated. The generated laser light L23 is incident on the first vibrating section 31 again via the quarter-wave plate 77, beam splitter 76, mirrors 75 and 74, beam splitter 72, and quarter-wave plate 73. As a result, a modulation signal is added to the laser light L23, and laser light L24 is generated. The laser light L24 returns to the beam splitter 72 via the quarter-wave plate 73, and is then incident on the photodetector 10 via the beam splitter 41 and polarizer 46.

[0093] As described above, the bypass optical path 7 shown in Figure 7 makes it possible to irradiate the vibrating element 30 with laser light four times. This allows the phase of the laser light to be shifted in the same direction each of the four irradiations. As a result, the amount of phase shift of the laser light can be quadrupled, and the B value can be quadrupled. For example, if a B value of 1.3 is obtained by irradiating the vibrating element 30 with laser light once, theoretically, a B value of 2.6 can be obtained with two irradiations, a B value of 3.9 can be obtained with three irradiations, and a B value of 5.2 can be obtained with four irradiations. In the second modified example described above, the same effects as in the first embodiment can be obtained.

[0094] 4. Second Embodiment Next, a laser interferometer according to the second embodiment will be described.

[0095] Figure 8 is a schematic diagram showing the sensor head 51 of the laser interferometer 1 according to the second embodiment. Figure 9 is a perspective view showing the vibration element 30 of the optical modulation unit 12 in Figure 8. In Figure 9, the X, Y, and Z axes are set as three mutually orthogonal axes. Each axis is indicated by an arrow, with the tip of the arrow being called the "positive side" of each axis and the base of the arrow being called the "negative side" of each axis.

[0096] The second embodiment will be described below, focusing on the differences from the first embodiment, and similar matters will be omitted from the description. In Figures 8 and 9, components similar to those in the above embodiment and its modified form are denoted by the same reference numerals. The second embodiment is the same as the first embodiment, except that the shape of the vibration element 30 is different.

[0097] The vibrating element 30 shown in Figure 8 is a tuning fork-type element having in-plane vibration modes that vibrate in the X-Y plane, as shown in Figure 9. The vibrating element 30 shown in Figure 9 has a first arm 312 which is a first vibrating part 31, and a second arm 322 which is a second vibrating part 32. The first arm 312 (first vibrating part 31) and the second arm 322 (second vibrating part 32) shown in Figure 9 vibrate in opposite phases to each other in the XY plane.

[0098] Furthermore, a first reflective surface 332, which is a first optical modulator 33, is provided on the surface of the first arm portion 312, and a second reflective surface 342, which is a second optical modulator 34, is provided on the surface of the second arm portion 322. The first reflective surface 332 and the second reflective surface 342 are not particularly limited as long as they are surfaces that reflect laser light, but examples include a thin metal film provided on a surface intersecting the X axis. The vibrating element 30, although not shown, includes, for example, a piezoelectric material and an electrode provided on the piezoelectric material. Of these, the electrode is made of a metal material. Therefore, the surface of the electrode can be used as the first reflective surface 332 and the second reflective surface 342.

[0099] Figure 9 shows solid and dashed arrows indicating the deformation direction of the vibrating element 30. The voltages that cause deformation indicated by the solid arrows and the voltages that cause deformation indicated by the dashed arrows are in opposite phases. Therefore, the first arm 312 (first vibrating part 31) and the second arm 322 (second vibrating part 32) of the vibrating element 30 shown in Figure 9 vibrate in opposite phases. As a result, if laser light is irradiated onto the first reflective surface 332 and then onto the second reflective surface 342, the phase of the laser light can be shifted in the same direction on both surfaces with just two irradiations.

[0100] Furthermore, the vibrating element 30 shown in Figure 9 is configured such that, when the XY plane is the arm-forming plane, the first arm 312 and the second arm 322 are aligned within the plane of the arm-forming plane. The aforementioned in-plane vibration mode is a vibration mode in which, within the plane of the arm-forming plane, the first arm 312 and the second arm 322 repeatedly undergo deformations that cause them to move away from each other, as shown by the solid arrows in Figure 9, and deformations that cause them to move closer together, as shown by the dashed arrows in Figure 9. In other words, the in-plane vibration mode is a vibration mode in which the first arm 312 is displaced to the negative side of the X axis and the second arm 322 is displaced to the positive side of the X axis, and deformations that cause the first arm 312 to move to the positive side of the X axis and the second arm 322 to move to the negative side of the X axis.

[0101] Furthermore, since the in-plane vibration mode involves bending vibration of the first arm 312 and the second arm 322, it is easier to ensure a larger amplitude for the first reflective surface 332 and the second reflective surface 342 compared to the thickness shear vibration mode. For this reason, even just irradiating the vibrating element 30 with laser light once can yield a large B value of 17.7, for example. In this case, theoretically, two irradiations would yield a B value of 35.4, three irradiations a B value of 53.1, and four irradiations a B value of 70.8.

[0102] Furthermore, the B value can be reduced if you wish to control it to a specific value. For example, the B value can be reduced by utilizing higher-order vibration modes for the vibration element 30.

[0103] Such a vibrating element 30 oscillates based on the drive signal Sd output from the signal generation unit 6. The power required for the oscillation of the vibrating element 30 (drive power) is not particularly limited, but is small, ranging from about 0.1 μW to 100 mW. Therefore, by incorporating the vibrating element 30, the laser interferometer 1 can be made smaller, lighter, and more power-efficient. Furthermore, the tuning fork type element does not require the provision of diffraction gratings, and the electrode surface can be used as the first optical modulator 33 and the second optical modulator 34, making it useful in simplifying the structure.

[0104] The length of the vibration element 30 in the Y-axis direction is, for example, approximately 0.2 mm to 5.0 mm. The thickness of the vibration element 30 in the Z-axis direction is, for example, approximately 0.003 mm to 0.5 mm.

[0105] The shape of the tuning fork element is not limited to the two-legged tuning fork type shown in Figure 9, but can also include a three-legged tuning fork type, a four-legged tuning fork type with a cantilevered beam shape, and so on.

[0106] Furthermore, examples of the vibrating element 30 shown in Figure 8 include a quartz crystal resonator, a silicon resonator, a ceramic resonator, and the like.

[0107] A quartz oscillator comprises a piezoelectric quartz crystal and electrodes provided on the quartz crystal. Such a quartz oscillator utilizes the extremely high Q factor of the quartz to add a highly signal-to-noise ratio modulated signal to the laser beam L1. The quartz crystal is, for example, cut from a quartz substrate such as a quartz Z-cut plate. The oscillation frequency of the quartz oscillator is preferably, for example, between 1 kHz and 100 kHz, and more preferably between 10 kHz and 100 kHz.

[0108] Furthermore, the thickness of the quartz piece is preferably between 100 μm and 300 μm. With this thickness, even when the electrode surfaces formed on the side surface of the quartz piece, that is, the cut surface when it is cut from the quartz substrate, become the first reflective surface 332 and the second reflective surface 342, a sufficient area can be secured.

[0109] A silicon resonator is a resonator comprising a single-crystal silicon piece manufactured from a single-crystal silicon substrate using MEMS technology, a piezoelectric film, and electrodes. MEMS (Micro Electro Mechanical Systems) refers to micro-electromechanical systems. Examples of the shape of the single-crystal silicon piece include cantilever shapes such as two-legged tuning fork type and three-legged tuning fork type, as well as double-supported beam shapes. The oscillation frequency of a silicon resonator is, for example, from 1 kHz to several hundred MHz.

[0110] A ceramic resonator is a resonator comprising a piezoelectric ceramic piece manufactured by firing piezoelectric ceramics, and electrodes. Examples of piezoelectric ceramics include lead zirconate titanate (PZT) and barium titanate (BTO). The oscillation frequency of a ceramic resonator is, for example, several hundred kHz to several tens of MHz. In the second embodiment described above, the same effects as in the first embodiment can be obtained.

[0111] 5. First modified example of the second embodiment Next, a laser interferometer according to the first modified example of the second embodiment will be described. Figure 10 is a schematic diagram showing the sensor head portion 51 of the laser interferometer 1 according to the first modified example of the second embodiment.

[0112] The following describes a first modified example of the second embodiment. In this description, the focus will be on the differences between the first embodiment, its modified example, and the second embodiment, and similar matters will be omitted from the explanation. In Figure 10, the same reference numerals are used for components similar to those in the first embodiment, its modified example, and the second embodiment.

[0113] The first modified example is the same as the second embodiment, except that the vibrating element 30 is configured to be irradiated with laser light three times.

[0114] The interference optical system 50 shown in Figure 10 comprises a light source 2, a beam splitter 41, a light modulation unit 12, a polarizer 46, and a photodetector 10. In the configuration shown in Figure 10, it is desirable that the laser light L1 incident on the beam splitter 41 be P-polarized.

[0115] In the bypass optical path 7 provided by the optical modulation unit 12 shown in Figure 10, one of the two beams of light split by the beam splitter 41, laser beam L1, is incident. The bypass optical path 7 shown in Figure 10 is configured to irradiate the vibrating element 30 with laser beam L1 three times. The other laser beam L1 is incident on the target object 14 and then incident on the photodetector 10 as laser beam L3.

[0116] The beam splitter 41 is an unpolarized beam splitter, and it splits the laser light at a predetermined splitting ratio regardless of polarization.

[0117] The bypass optical path 7 shown in Figure 10 comprises a beam splitter 72, a quarter-wave plate 73, a mirror 74, a mirror 75, and a mirror 78.

[0118] The laser light L1 incident on the bypass optical path 7 passes through the beam splitter 72 and, via the quarter-wave plate 73, is incident on the first vibrating section 31 of the vibrating element 30. As a result, a modulation signal is added to the laser light L1, and laser light L21 is generated. The generated laser light L21 returns to the beam splitter 72 again via the quarter-wave plate 73. The laser light L21 is incident on the second vibrating section 32 of the vibrating element 30 via the mirrors 74, 75, and 78. As a result, a modulation signal is added to the laser light L21, and laser light L22 is generated. The generated laser light L22 returns to the beam splitter 72 again via the mirrors 78, 75, and 74. This laser light L22 is incident on the first vibrating section 31 again via the quarter-wave plate 73. As a result, a modulation signal is added to the laser light L23, and laser light L23 is generated. The laser light L23 returns to the beam splitter 72 via the quarter-wave plate 73, and then enters the photodetector 10 via the beam splitter 41 and polarizer 46.

[0119] As described above, the bypass optical path 7 shown in Figure 10 allows the laser beam to be irradiated onto the vibrating element 30 three times. This makes it possible to shift the phase of the laser beam in the same direction each of the three irradiations. As a result, the amount of phase shift of the laser beam can be tripled, and the B value can be tripled. In the first modified example described above, the same effects as in the second embodiment can be obtained.

[0120] 6. Second Modification of the Second Embodiment Next, a laser interferometer relating to a second modified example of the second embodiment will be described.

[0121] Figure 11 is a schematic diagram showing the sensor head portion 51 of the laser interferometer 1 according to a second modified example of the second embodiment.

[0122] The following describes a second modified example of the second embodiment. In this description, the focus will be on the differences between this embodiment and the first embodiment, its modified example, and the second embodiment, and similar matters will be omitted from the explanation. In Figure 11, the same reference numerals are used for components similar to those in the first embodiment, its modified example, and the second embodiment.

[0123] The second modified example is the same as the first modified example, except that the vibrating element 30 is configured to be irradiated with laser light four times.

[0124] The interference optical system 50 shown in Figure 11 comprises a light source 2, a beam splitter 41, a light modulation unit 12, a polarizer 46, and a photodetector 10. In the configuration shown in Figure 11, it is desirable that the laser light L1 incident on the beam splitter 41 be P-polarized.

[0125] In the bypass optical path 7 provided by the optical modulation unit 12 shown in Figure 11, one of the two beams of light split by the beam splitter 41, laser beam L1, is incident. The bypass optical path 7 shown in Figure 11 is configured to irradiate the vibrating element 30 four times with laser beam L1. The other laser beam L1 is incident on the target object 14 and then incident on the photodetector 10 as laser beam L3.

[0126] The bypass optical path 7 shown in Figure 11 comprises a beam splitter 72, a quarter-wave plate 73, a mirror 74, a mirror 75, a beam splitter 76, a quarter-wave plate 77, and a mirror 79.

[0127] The laser light L1 incident on the bypass optical path 7 passes through the beam splitter 72 and, via the quarter-wave plate 73, is incident on the first vibrating section 31 of the vibrating element 30. As a result, a modulation signal is added to the laser light L1, and laser light L21 is generated. The generated laser light L21 returns to the beam splitter 72 again via the quarter-wave plate 73. The laser light L21 is incident on the second vibrating section 32 of the vibrating element 30 via the mirrors 74, 75 and the beam splitter 76. As a result, a modulation signal is added to the laser light L21, and laser light L22 is generated. The generated laser light L22 is reflected by the mirror 79 via the quarter-wave plate 77 and the beam splitter 76, and then incident on the second vibrating section 32 again via the beam splitter 76 and the quarter-wave plate 77. As a result, a modulation signal is added to the laser light L22, and laser light L23 is generated. The generated laser light L23 is incident on the first vibrating section 31 again via the quarter-wave plate 77, beam splitter 76, mirrors 75 and 74, beam splitter 72, and quarter-wave plate 73. As a result, a modulation signal is added to the laser light L23, and laser light L24 is generated. The laser light L24 returns to the beam splitter 72 via the quarter-wave plate 73, and is then incident on the photodetector 10 via the beam splitter 41 and polarizer 46.

[0128] As described above, the bypass optical path 7 shown in Figure 11 allows the laser beam to be irradiated onto the vibrating element 30 four times. This makes it possible to shift the phase of the laser beam in the same direction each of the four irradiations. As a result, the amount of phase shift of the laser beam can be quadrupled, and the B value can be quadrupled. In the second modified example described above, the same effects as in the second embodiment can be obtained.

[0129] 7. Third Embodiment Next, a laser interferometer according to the third embodiment will be described.

[0130] Figure 12 is a schematic diagram showing the sensor head 51 of the laser interferometer 1 according to the third embodiment. Figure 13 is a perspective view showing the vibration element 30 of the optical modulation unit 12 in Figure 12. In Figure 13, the X, Y, and Z axes are set as three mutually orthogonal axes. Each axis is indicated by an arrow, with the tip of the arrow being called the "positive side" of each axis and the base of the arrow being called the "negative side" of each axis.

[0131] The third embodiment will be described below, focusing on the differences from the second embodiment, and similar matters will be omitted from the description. In Figures 12 and 13, components similar to those in the above embodiment and its modified examples are denoted by the same reference numerals.

[0132] The third embodiment is the same as the second embodiment except that the vibration mode of the vibration element 30 is different.

[0133] The vibrating element 30 shown in Figure 12 is a tuning fork-type element that, in addition to the in-plane vibration mode described above, has an out-of-plane vibration mode that vibrates in the Z-axis direction, as shown in Figure 13. The vibrating element 30 shown in Figure 13 has a first arm 312 which is the first vibrating part 31, and a second arm 322 which is the second vibrating part 32. Of these, in the out-of-plane vibration mode, the first arm 312 (first vibrating part 31) and the second arm 322 (second vibrating part 32) shown in Figure 13 vibrate along the Z-axis in opposite phases to each other. Specifically, as shown by the solid arrows in Figure 13, this is a vibration mode in which deformation occurs in which the first arm 312 is displaced to the positive side of the Z-axis and the second arm 322 is displaced to the negative side of the Z-axis, and deformation occurs in which the first arm 312 is displaced to the negative side of the Z-axis and the second arm 322 is displaced to the positive side of the Z-axis, as shown by the dashed arrows in Figure 13.

[0134] Furthermore, a first reflective surface 332, which is a first optical modulator 33, is provided on the surface of the first arm portion 312, and a second reflective surface 342, which is a second optical modulator 34, is provided on the surface of the second arm portion 322. The first reflective surface 332 and the second reflective surface 342 are not particularly limited as long as they are surfaces that reflect laser light, but examples include a thin metal film provided on a surface intersecting the Z axis. The vibrating element 30, although not shown, includes, for example, a piezoelectric body and an electrode provided on the piezoelectric body. Of these, the electrode is made of a metallic material. Therefore, the surface of the electrode can be used as the first reflective surface 332 and the second reflective surface 342. Also, the surface intersecting the Z axis provides more space for providing a thin metal film, etc., compared to the surface intersecting the X axis. For this reason, the first reflective surface 332 and the second reflective surface 342 shown in Figure 13 have high tolerance for deviations in the irradiation position of the laser light. As a result, the laser interferometer 1 according to the third embodiment also has the advantage of being easy to manufacture the interference optical system 50.

[0135] In the vibration element 30, when the in-plane vibration mode is treated as the main vibration mode, the out-of-plane vibration mode is treated as a secondary vibration mode. In typical applications, the main vibration mode is used, for example, as the oscillation mode for a clock source, while the secondary vibration modes are avoided as spurious emissions or suppressed and not used.

[0136] In contrast, in this embodiment, this out-of-plane vibration mode is actively excited without being suppressed. This makes it easier to increase the B value by ensuring sufficient displacement of the first reflective surface 332 and the second reflective surface 342 in the direction of laser beam propagation.

[0137] Figure 14 is a conceptual diagram showing the frequency characteristics of the principal vibration mode (in-plane vibration mode) and the secondary vibration mode (out-of-plane vibration mode). In Figure 14, the horizontal axis represents the vibration frequency, and the vertical axis represents the vibration velocity amplitude. In this specification, the characteristic represented by the curve showing the frequency dependence of the vibration velocity amplitude shown in Figure 14 is referred to as the "frequency characteristic" of each vibration mode.

[0138] The main vibration mode is excited by the drive signal Sd output from the signal generation unit 6. The frequency at which the vibration element 30 is excited by the signal generation unit 6 is called the oscillation frequency f. osc Let f be the natural frequency of the principal vibration mode of the vibration element 30 alone. Q Let's assume that the natural frequencies of the multiple sub-vibration modes present in the single vibrating element 30 are a, b, c, d, and e.

[0139] Oscillation frequency f of signal generation unit 6 osc The natural frequency f of the vibrating element 30 is Q The value will be corresponding to the natural frequency f. In the example in Figure 14, the natural frequency f Q A frequency slightly higher than that is the oscillation frequency f osc The frequency characteristics of the principal vibration mode are given by the natural frequency f. Q It can be represented by a curve that peaks at a certain point and decreases on both sides.

[0140] On the other hand, in the example in Figure 14, there are five types of sub-vibration modes, and their natural frequencies a, b, c, d, and e are equal to natural frequency f Q The distribution is symmetrical. The frequency characteristics of the sub-vibration modes are represented by curves with peaks at natural frequencies a, b, c, d, and e, respectively, and decreasing on both sides.

[0141] Among the frequency characteristics of the sub-vibration modes, the frequency characteristics with peaks at natural frequencies b, c, and d show that a portion of the curve corresponds to the oscillation frequency f. osc This overlaps with the main vibration mode. In this case, the sub-vibration modes with natural frequencies b, c, and d are energetically coupled with the main vibration mode and excited. Therefore, the oscillation frequency f of the signal generation unit 6 is set so that the main vibration mode is excited. osc By setting this, these sub-vibration modes can also be excited. In contrast, the frequency characteristics with peaks at natural frequencies a and e are such that the curves show the oscillation frequency f osc These do not overlap. Therefore, these secondary vibration modes do not couple with the main vibration mode and are not excited.

[0142] Furthermore, sub-vibration modes are more susceptible to frequency instability due to temperature characteristics compared to the primary vibration mode. For this reason, in typical applications, it is necessary to keep the natural frequencies of the sub-vibration modes separate from those of the primary vibration mode.

[0143] However, when the vibrating element 30 is a signal source for outputting a reference signal Ss, demodulation processing is performed in real time by the calculation unit 52 based on the reference signal Ss, which includes frequency fluctuations. Therefore, even if the reference signal Ss contains frequency fluctuations, these fluctuations can be canceled out or reduced during the demodulation process. This has the advantage of reducing constraints on the natural frequencies of the sub-vibration modes, making the design and manufacturing of the vibrating element 30 easier.

[0144] Furthermore, the natural frequencies of the secondary vibration modes can be adjusted by the structure of the vibration element 30, such as the shape of the vibration element 30, including the length of the first arm 312 and the second arm 322 in the Y-axis direction, the thickness in the Z-axis direction, and the cross-sectional shape of the XZ plane, as well as the arrangement of electrodes (not shown). In other words, the natural frequency f of the main vibration mode Q The structure of the vibrating element 30 can be adjusted to achieve a natural frequency close to the primary frequency. The specific structure can be determined through experiments and simulations. For example, by changing the cross-sectional shape of the first arm 312 and the second arm 322 from a rectangle to a parallelogram, the sub-vibration modes are more easily excited. Then, by changing the shape of the parallelogram, the natural frequencies of the sub-vibration modes can be adjusted.

[0145] Furthermore, in the out-of-plane vibration mode, it is easier to ensure larger amplitudes for the first reflective surface 332 and the second reflective surface 342 compared to the thickness-sliding vibration mode. For this reason, even just irradiating the vibrating element 30 with laser light once can yield a large B value of 2.6, for example. In this case, theoretically, two irradiations would yield a B value of 5.2, three irradiations a B value of 7.8, and four irradiations a B value of 10.4.

[0146] The shape of the tuning fork element is not limited to the two-legged tuning fork type shown in Figure 13, but can also include a three-legged tuning fork type, a four-legged tuning fork type with a cantilevered beam shape, and so on.

[0147] Furthermore, examples of the vibrating element 30 shown in Figure 8 include a quartz crystal resonator, a silicon resonator, a ceramic resonator, and the like.

[0148] Furthermore, out-of-plane vibration modes often have lower natural frequencies than in-plane vibration modes. Therefore, by utilizing out-of-plane vibration modes, the frequencies of the modulation signal and reference signal Ss can be reduced. Specifically, since the natural frequencies of out-of-plane vibration modes are often less than 1 MHz, for example, the frequencies of the modulation signal and reference signal Ss can also be reduced to this level. As a result, the processing performance of processors such as analog-to-digital converters (ADCs) and FPGAs (Field-Programmable Gate Arrays) that process these signals can be reduced. Consequently, it becomes easier to reduce the cost of the laser interferometer 1.

[0149] In silicon oscillators, it is possible to design them so that out-of-plane vibrations are the primary vibrations. In this case, for example, the Q-factor can be increased by reducing the thickness of the single-crystal silicon piece forming a cantilever beam. Then, out-of-plane vibrations can be excited by arranging piezoelectric films, etc.

[0150] Ceramic oscillators can utilize not only bending vibrations but also out-of-plane vibrations coupled to in-plane vibrations such as length vibrations and spreading vibrations.

[0151] In the third embodiment described above, the same effects as those of the first and second embodiments can be obtained.

[0152] 8. First Modification of the Third Embodiment Next, a laser interferometer according to a first modified example of the third embodiment will be described.

[0153] Figure 15 is a schematic diagram showing the sensor head portion 51 of the laser interferometer 1 according to the first modified example of the third embodiment.

[0154] The following describes a first modified example of the third embodiment. In this description, the differences from the first and second embodiments and their modified examples will be the focus, and similar matters will be omitted. In Figure 15, the same reference numerals are used for components similar to those in the first and second embodiments and their modified examples.

[0155] The first modified example is the same as the third embodiment, except that the vibrating element 30 is configured to be irradiated with laser light three times.

[0156] The interference optical system 50 shown in Figure 15 comprises a light source 2, a beam splitter 41, a light modulation unit 12, a polarizer 46, and a photodetector 10. In the configuration shown in Figure 15, it is desirable that the laser light L1 incident on the beam splitter 41 be P-polarized.

[0157] In the bypass optical path 7 provided by the optical modulation unit 12 shown in Figure 15, one of the two beams of light split by the beam splitter 41, laser beam L1, is incident. The bypass optical path 7 shown in Figure 15 is configured to irradiate the vibrating element 30 with laser beam L1 three times. The other laser beam L1 is incident on the target object 14 and then incident on the photodetector 10 as laser beam L3.

[0158] The beam splitter 41 is an unpolarized beam splitter, and it splits the laser light at a predetermined splitting ratio regardless of polarization.

[0159] The bypass optical path 7 shown in Figure 15 comprises a beam splitter 72, a quarter-wave plate 73, a mirror 74, a mirror 75, and a mirror 78.

[0160] The laser light L1 incident on the bypass optical path 7 passes through the beam splitter 72 and, via the quarter-wave plate 73, is incident on the first vibrating section 31 of the vibrating element 30. As a result, a modulation signal is added to the laser light L1, and laser light L21 is generated. The generated laser light L21 returns to the beam splitter 72 again via the quarter-wave plate 73. The laser light L21 is incident on the second vibrating section 32 of the vibrating element 30 via the mirrors 74, 75, and 78. As a result, a modulation signal is added to the laser light L21, and laser light L22 is generated. The generated laser light L22 returns to the beam splitter 72 again via the mirrors 78, 75, and 74. This laser light L22 is incident on the first vibrating section 31 again via the quarter-wave plate 73. As a result, a modulation signal is added to the laser light L23, and laser light L23 is generated. The laser light L23 returns to the beam splitter 72 via the quarter-wave plate 73, and then enters the photodetector 10 via the beam splitter 41 and polarizer 46.

[0161] As described above, the bypass optical path 7 shown in Figure 15 allows the laser beam to be irradiated onto the vibrating element 30 three times. This makes it possible to shift the phase of the laser beam in the same direction each of the three irradiations. As a result, the amount of phase shift of the laser beam can be tripled, and the B value can be tripled. In the first modified example described above, the same effects as those of the third embodiment can be obtained.

[0162] 9. Second Modified Example of the Third Embodiment Next, a laser interferometer according to a second modified example of the third embodiment will be described.

[0163] Figure 16 is a schematic diagram showing the sensor head portion 51 of the laser interferometer 1 according to a second modified example of the third embodiment.

[0164] The following describes a second modified example of the third embodiment. In this description, the differences from the first and second embodiments and their modified examples will be the focus, and similar matters will be omitted. In Figure 16, the same reference numerals are used for components similar to those in the first and second embodiments and their modified examples.

[0165] The second modified example is the same as the first modified example, except that the vibrating element 30 is configured to be irradiated with laser light four times.

[0166] The interference optical system 50 shown in Figure 16 comprises a light source 2, a beam splitter 41, a light modulation unit 12, a polarizer 46, and a photodetector 10 (photodetector). In the configuration shown in Figure 16, it is desirable that the laser light L1 incident on the beam splitter 41 be P-polarized.

[0167] In the bypass optical path 7 shown in Figure 16, one of the two beams of light split by the beam splitter 41, laser beam L1, is incident. The bypass optical path 7 shown in Figure 16 is configured to irradiate the vibrating element 30 four times with laser beam L1. The other laser beam L1 is incident on the object 14 and then incident on the photodetector 10 as laser beam L3.

[0168] The bypass optical path 7 shown in Figure 16 comprises a beam splitter 72, a quarter-wave plate 73, a mirror 74, a mirror 75, a beam splitter 76, a quarter-wave plate 77, and a mirror 79.

[0169] The laser light L1 incident on the bypass optical path 7 passes through the beam splitter 72 and, via the quarter-wave plate 73, is incident on the first vibrating section 31 of the vibrating element 30. As a result, a modulation signal is added to the laser light L1, and laser light L21 is generated. The generated laser light L21 returns to the beam splitter 72 again via the quarter-wave plate 73. The laser light L21 is incident on the second vibrating section 32 of the vibrating element 30 via the mirrors 74, 75 and the beam splitter 76. As a result, a modulation signal is added to the laser light L21, and laser light L22 is generated. The generated laser light L22 is reflected by the mirror 79 via the quarter-wave plate 77 and the beam splitter 76, and then incident on the second vibrating section 32 again via the beam splitter 76 and the quarter-wave plate 77. As a result, a modulation signal is added to the laser light L22, and laser light L23 is generated. The generated laser light L23 is incident on the first vibrating section 31 again via the quarter-wave plate 77, beam splitter 76, mirrors 75 and 74, beam splitter 72, and quarter-wave plate 73. As a result, a modulation signal is added to the laser light L23, and laser light L24 is generated. The laser light L24 returns to the beam splitter 72 via the quarter-wave plate 73, and is then incident on the photodetector 10 via the beam splitter 41 and polarizer 46.

[0170] As described above, the bypass optical path 7 shown in Figure 16 makes it possible to irradiate the vibrating element 30 with laser light four times. This allows the phase of the laser light to be shifted in the same direction each of the four irradiations. As a result, the amount of phase shift of the laser light can be quadrupled, and the B value can be quadrupled. In the second modified example described above, the same effects as those of the third embodiment can be obtained.

[0171] 10. Effects of each embodiment As described above, the laser interferometer 1 according to the embodiment comprises a light source 2, an optical modulation unit 12, a photodetector 10 (photodetector), a calculation unit 52, and a signal generation unit 6. The light source 22 emits laser light. The optical modulation unit 12 includes a vibrating element 30 driven by a drive signal Sd, and uses the vibrating element 30 to superimpose a modulation signal onto the laser light. The photodetector 10 receives laser light including a sample signal and a modulation signal originating from the object 14, and outputs a received signal. The calculation unit 52 demodulates the sample signal from the received signal based on a reference signal Ss. The signal generation unit 6 outputs a drive signal Sd and a reference signal Ss.

[0172] The optical modulation unit 12 also includes a vibrating element 30, a first optical modulator 33, a second optical modulator 34, and a bypass optical path 7. The vibrating element 30 has a first vibrating section 31 and a second vibrating section 32 that vibrates in opposite phase to the first vibrating section 31. The first optical modulator 33 is provided on the first vibrating section 31 and modulates the laser light. The second optical modulator 34 is provided on the second vibrating section 32 and modulates the laser light modulated by the first optical modulator 33. The bypass optical path 7 causes the laser light modulated by the first optical modulator 33 to be incident on the second optical modulator 34.

[0173] With this configuration, the phase shift (B value) of the modulated signal can be increased, thereby improving the signal-to-noise ratio of the signal processed by the calculation unit 52. As a result, the sample signal (Doppler signal) originating from the object 14 can be demodulated with higher accuracy, regardless of the vibration conditions of the vibrating element 30. Consequently, a laser interferometer 1 with high accuracy in calculating the position and velocity of the object can be realized.

[0174] Furthermore, the vibrating element 30 may be a thickness-slip vibrating element having a thickness-slip vibration mode, with a first surface 311 which is a first vibrating part 31 and a second surface 321 which is a second vibrating part 32, having a front-back relationship with each other. In this case, the first optical modulator 33 is a first diffraction grating 331 provided on the first surface 311 of the thickness-slip vibrating element, and the second optical modulator 34 is a second diffraction grating 341 provided on the second surface 321 of the thickness-slip vibrating element.

[0175] This configuration allows for miniaturization, weight reduction, and reduced power consumption of the laser interferometer 1. Furthermore, the thickness-sliding vibration element is useful because it offers superior shock resistance and temperature characteristics compared to elements with other vibration modes.

[0176] Furthermore, the vibrating element 30 may be a tuning fork type element having a first arm portion 312 which is a first vibrating part 31, and a second arm portion 322 which is a second vibrating part 32. In this case, the first optical modulator 33 is a first reflective surface 332 provided on the surface of the first arm portion 312, and the second optical modulator 34 is a second reflective surface 342 provided on the surface of the second arm portion 322.

[0177] This configuration allows for miniaturization, weight reduction, and lower power consumption of the laser interferometer 1. Furthermore, the tuning fork type element is useful because it eliminates the need for diffraction gratings and other components, and the electrode surface can be used as the first optical modulator 33 and the second optical modulator 34, thus simplifying the structure.

[0178] Furthermore, the tuning fork-type element may have an in-plane vibration mode. This in-plane vibration mode is a mode that vibrates within the plane of the arm-forming surface where the first arm portion 312 and the second arm portion 322 are aligned.

[0179] This allows the tuning fork-type element to ensure a large amplitude at the first reflective surface 332 and the second reflective surface 342. This allows for a larger B value.

[0180] Furthermore, the tuning fork-type element may have an out-of-plane vibration mode. This out-of-plane vibration mode is a mode that vibrates in a direction intersecting the arm-forming surface where the first arm portion 312 and the second arm portion 322 are aligned.

[0181] As a result, the tuning fork-type element can ensure a large amplitude for the first reflective surface 332 and the second reflective surface 342. This allows for a larger B value. Furthermore, by utilizing out-of-plane vibration modes in the modulation of the laser light, the first reflective surface 332 and the second reflective surface 342, which have large areas, can be used as optical reflective surfaces. Because these have high tolerance for deviations in the irradiation position of the laser light, it is possible to realize a laser interferometer 1 that is easy to manufacture.

[0182] Furthermore, the signal generation unit 6 includes an oscillation circuit 61 that uses the vibration element 30 as a signal source, outputs a drive signal Sd to the vibration element 30, and outputs a reference signal Ss by causing the vibration element 30 to oscillate.

[0183] As a result, the drive signal Sd and the reference signal Ss are affected equally by disturbances. Consequently, the modulation signal and the reference signal Ss added via the vibrating element 30 driven by the drive signal Sd are also affected equally by each other. Therefore, when the modulation signal and the reference signal Ss are used for calculations in the calculation unit 52, the effects of disturbances contained in both can be canceled out or reduced during the calculation process. As a result, the calculation unit 52 can accurately determine the position and velocity of the object 14 even when disturbances are present.

[0184] Furthermore, the vibrating element 30 is preferably a quartz crystal oscillator. Because quartz crystal oscillators have a high Q factor, they can add a modulation signal with a high S / N ratio to the laser light L1.

[0185] Furthermore, when n is an integer of 2 or more, the optical modulation unit 12 has the function of modulating the laser light L1 n times or more. The frequency when the vibration element 30 is driven by the drive signal Sd is f M Let the frequency be [Hz], and let Lt [m] be the optical distance the laser beam travels from the time it is modulated once until it is modulated n times. Then the detour path 7 is Lt ≤ 9 × 10 6 / f M It is preferable that the following conditions be met.

[0186] This makes it possible to sufficiently minimize the influence of the time lag between the first and nth phase modulations on the B value. As a result, it is possible to increase the probability of phase modulation in a direction that increases the B value for all irradiations from the first to the nth irradiation.

[0187] Let L [m] be the optical distance the laser beam travels from the point where it has been modulated n-1 times until it has been modulated n times, and let c [m / s] be the speed of light. Then the detour path 7 is n ≤ c / (4Lf M It is preferable that the following conditions be met.

[0188] This allows for an increase in the B value with each irradiation, thus making it possible to more reliably increase the B value. As a result, the S / N ratio of the pre-processed signal output by the pre-processing unit 53 can be more reliably improved.

[0189] Although 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 embodiments or their modifications, and the configuration of each part can be replaced with any configuration having a similar function. In addition, other arbitrary components may be added to the laser interferometer according to the embodiments or their modifications. Furthermore, the laser interferometer of the present invention may include two or more of the embodiments or their modifications. In addition, each functional part of the laser interferometer of the present invention may be divided into multiple elements, or multiple functional parts may be integrated into one.

[0190] The laser interferometer of the present invention can be applied not only to the displacement meters and speedometers mentioned above, but also to, for example, vibration meters, inclinometers, distance meters (length measuring instruments), etc. Furthermore, applications of the laser interferometer of the present invention include optical comb interferometry technology that enables distance measurement, 3D imaging, spectroscopy, etc., optical fiber gyroscopes that realize angular velocity sensors, angular acceleration sensors, etc., and Fourier spectrometers equipped with moving mirror devices.

[0191] Furthermore, two or more of the light source, light modulation unit, and photodetector may be mounted on the same substrate. This facilitates miniaturization and weight reduction of the interference optical system, as well as improving ease of assembly.

[0192] Furthermore, although the above embodiments and their modifications have a so-called Michelson-type interference optical system, the laser interferometer of the present invention is also applicable to other types of interference optical systems, such as those having a Mach-Zehnder-type interference optical system. [Explanation of symbols]

[0193] 1…Laser interferometer, 2…Light source, 6…Signal generation unit, 7…Detour optical path, 10…Photodetector, 12…Optical modulation unit, 14…Object, 22…Light source, 30…Vibration element, 31…First vibration unit, 32…Second vibration unit, 33…First optical modulator, 34…Second optical modulator, 41…Beam splitter, 42…Beam splitter, 43…Quarter wave plate, 44…Mirror, 45…Mirror, 46…Polarizer, 50…Interference optical system, 51…Sensor head unit, 52…Calculation unit, 53…Preprocessing unit, 55…Demodulation processing unit, 57…Demodulated signal output unit, 59…Main unit, 61…Oscillation circuit, 72…Beam splitter, 73… 1 / 4 wave plate, 74...Mirror, 75...Mirror, 76...Beam splitter, 77...1 / 4 wave plate, 78...Mirror, 79...Mirror, 311...First surface, 312...First arm, 321...Second surface, 322...Second arm, 331...First diffraction grating, 332...First reflecting surface, 341...Second diffraction grating, 342...Second reflecting surface, L1...Laser light, L21...Laser light, L22...Laser light, L23...Laser light, L24...Laser light, L3...Laser light, Sd...Drive signal, Ss...Reference signal, a...Natural frequency, b...Natural frequency, c...Natural frequency, d...Natural frequency, e...Natural frequency, f Q ...natural frequency, f osc ...oscillation frequency

Claims

1. A light source that emits laser light, An optical modulation unit comprising a vibrating element driven by a drive signal, which uses the vibrating element to superimpose a modulation signal onto the laser light, A light receiving unit that receives the laser light including the sample signal originating from the object and the modulated signal, and outputs the received signal, A calculation unit that demodulates the sample signal from the received light signal based on a reference signal, A signal generation unit that outputs the drive signal and the reference signal, Equipped with, The optical modulation unit is The vibrating element has a first vibrating part and a second vibrating part that vibrates in opposite phase to the first vibrating part, A first optical modulator is provided in the first vibrating section for modulating the laser light, A second optical modulator is provided in the second vibrating section and modulates the laser light modulated by the first optical modulator, A bypass optical path that causes the laser light modulated by the first optical modulator to be incident on the second optical modulator, A laser interferometer characterized by having the following features.

2. The vibrating element has a first surface which is the first vibrating part and a second surface which is the second vibrating part, having a front-back relationship with each other, and is a thickness-slip vibrating element having a thickness-slip vibration mode. The first optical modulator is a first diffraction grating provided on the first surface of the thickness-sliding vibration element, The laser interferometer according to claim 1, wherein the second optical modulator is a second diffraction grating provided on the second surface of the thickness-sliding vibration element.

3. The vibrating element is a tuning fork type element having a first arm which is the first vibrating part, and a second arm which is the second vibrating part. The first optical modulator is a first reflective surface provided on the surface of the first arm, The laser interferometer according to claim 1, wherein the second optical modulator is a second reflective surface provided on the surface of the second arm.

4. The laser interferometer according to claim 3, wherein the tuning fork-type element has an in-plane vibration mode that vibrates in the plane of the arm-forming surface where the first arm and the second arm are aligned.

5. The laser interferometer according to claim 3, wherein the tuning fork-type element has an out-of-plane vibration mode that vibrates in a direction intersecting the arm-forming surface where the first arm and the second arm are aligned.

6. The laser interferometer according to claim 1, wherein the signal generation unit comprises an oscillation circuit that uses the vibrating element as a signal source, outputs the drive signal to the vibrating element, and causes the vibrating element to oscillate, thereby outputting the reference signal.

7. The laser interferometer according to claim 1, wherein the vibrating element is a quartz crystal oscillator.

8. When n is an integer of 2 or more, the optical modulation unit has the function of modulating the laser light n or more times. The frequency at which the vibrating element is driven by the drive signal is f. Ｍ Let the frequency be [Hz], and let Lt [m] be the optical distance the laser light travels from the time it is modulated once until it is modulated n times. Then the detour optical path is Lt ≤ 9 × 10 ６ / f Ｍ A laser interferometer according to claim 1 that satisfies the requirements.

9. When L [m] is the optical distance the laser light travels from the time it is modulated n-1 times until it is modulated n times, and the speed of light is c [m / s], the detour optical path is n ≤ c / (4Lf Ｍ A laser interferometer according to claim 8 that satisfies the following conditions.

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