Laser interferometer

Abstract

To provide a low-cost laser interferometer equipped with an optical modulator that enables frequency modulation of laser light without the use of a diffraction grating. [Solution] A laser interferometer comprising: a laser light source that emits a first laser beam; an optical modulator having a vibration element having a vibration component in a direction intersecting the incident surface of the first laser beam, which modulates the first laser beam using the vibration element and generates a second laser beam including a modulated signal; a photodetector that receives a third laser beam including a sample signal generated when the first laser beam is reflected by an object, and the second laser beam, and outputs a received signal; a demodulation circuit that demodulates the sample signal from the received signal based on a reference signal; and an oscillation circuit that operates with the vibration element as a signal source and outputs the reference signal to the demodulation circuit.

Description

Technical Field

[0001] The present invention relates to a laser interferometer.

Background Art

[0002] Patent Document 1 discloses a laser Doppler measurement device for grasping the movement of a moving object. In the laser Doppler measurement device, a laser beam is irradiated onto the object to be measured, and the movement is measured based on the scattered laser beam that has received a Doppler shift. Specifically, using heterodyne interference, the amount of shift in the frequency of the laser beam is obtained, and from this shift amount, the speed and displacement of the moving object are determined.

[0003] The laser Doppler measurement device described in Patent Document 1 includes a frequency shifter type optical modulator. This optical modulator includes a crystal AT oscillator that vibrates in thickness shear and a diffraction grating including a plurality of grooves provided in the displacement direction of the oscillator. In this optical modulator, since the thickness shear vibration is an in-plane vibration, that is, it vibrates in a direction intersecting the incident direction of the incident laser beam, it is difficult to modulate the frequency of the laser beam. In other words, in order to efficiently modulate the frequency of the laser beam, it is required that the inner product of the difference between the incident wave number vector and the output wave number vector of the laser beam and the vibration vector of the crystal AT oscillator be sufficiently large. However, when only the crystal AT oscillator is used, this inner product becomes almost zero. Therefore, in the optical modulator described in Patent Document 1, a diffraction grating is combined with the crystal AT oscillator. This diffraction grating has grooves in a direction intersecting the vibration direction of the crystal AT oscillator. Thereby, the direction of the vibration vector is converted, and the above-mentioned inner product can be made greater than zero, enabling frequency modulation of the laser beam.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the diffraction grating increases the manufacturing difficulty of the optical modulator and leads to a high cost of the laser interferometer. Therefore, it has been an issue to realize a laser interferometer using an optical modulator that enables frequency modulation of the laser beam without using a diffraction grating.

Means for Solving the Problems

[0006] The laser interferometer according to an application example of the present invention includes a laser light source that emits a first laser beam, an oscillating element having an oscillating component in a direction intersecting the incident surface of the first laser beam, modulating the first laser beam using the oscillating element, and generating a second laser beam including a modulation signal; a light receiving element that receives a third laser beam including a sample signal generated by reflecting the first laser beam from an object and the second laser beam, and outputs a light receiving signal; a demodulation circuit that demodulates the sample signal from the light receiving signal based on a reference signal; an oscillation circuit that operates with the oscillating element as a signal source and outputs the reference signal to the demodulation circuit; and is characterized by including the above.

[0007] The laser interferometer according to an application example of the present invention includes a laser light source that emits a first laser beam, an oscillating element having an oscillating component in a direction intersecting the incident surface of the first laser beam, modulating the first laser beam using the oscillating element, and generating a second laser beam including a modulation signal; a light receiving element that receives a third laser beam including a sample signal generated by reflecting the second laser beam from an object and the modulation signal, and the first laser beam, and outputs a light receiving signal; a demodulation circuit that demodulates the sample signal from the light receiving signal based on a reference signal; An oscillator circuit that operates using the aforementioned vibration element as a signal source and outputs the reference signal to the demodulation circuit, It is characterized by having the following features.

[0008] A laser interferometer according to an application example of the present invention is, A laser light source that emits the first laser beam, An optical modulator comprising a vibrating element having a vibration component in a direction intersecting the incident plane of a third laser beam, which includes a sample signal generated when the first laser beam is reflected by an object, modulates the third laser beam using the vibrating element to generate a second laser beam including the modulated signal, A photodetector that receives the second laser light, which includes the sample signal and the modulation signal, and the first laser light, and outputs a received signal, A demodulation circuit that demodulates the sample signal from the received light signal based on a reference signal, An oscillator circuit that operates using the aforementioned vibration element as a signal source and outputs the reference signal to the demodulation circuit, It is characterized by having the following features. [Brief explanation of the drawing]

[0009] [Figure 1] This is a functional block diagram showing a laser interferometer according to an embodiment. [Figure 2] Figure 1 is a schematic diagram showing the sensor head section. [Figure 3] Figure 2 is a perspective view showing a first example of the configuration of the vibration element included in the optical modulator shown. [Figure 4] Figure 2 is a perspective view showing a first example of the configuration of the vibration element included in the optical modulator shown. [Figure 5] This is a conceptual diagram showing the frequency characteristics of the main vibration mode (in-plane bending vibration mode) and the secondary vibration mode (out-of-plane vibration mode). [Figure 6] Figure 2 is a side view showing a second example of the configuration of the vibration element included in the optical modulator shown in Figure 2. [Figure 7] This is a cross-sectional view showing an optical modulator having a package structure. [Figure 8]This is a cross-sectional view showing a modified example of the package structure in Figure 7. [Figure 9] This is a cross-sectional view showing a modified example of the package structure in Figure 7. [Figure 10] This is a cross-sectional view showing a modified example of the sensor head portion in Figure 1. [Figure 11] This is a cross-sectional view showing a modified example of the sensor head portion in Figure 1. [Figure 12] This is a cross-sectional view showing a modified example of the sensor head portion in Figure 1. [Figure 13] This is a circuit diagram showing the configuration of a single-stage inverter oscillator circuit. [Figure 14] This is an example of an LCR equivalent circuit for a vibrating element. [Figure 15] This graph shows the relationship between the Bessel coefficients J0(B), J1(B), and J2(B) and the phase shift B of the modulated signal. [Figure 16] This is a schematic diagram showing the optical system related to the first modified example. [Figure 17] This is a schematic diagram showing the optical system related to the second modified example. [Figure 18] This is a schematic diagram showing the optical system related to the third modified example. [Figure 19] This is a schematic diagram showing the optical system related to the fourth modified example. [Modes for carrying out the invention]

[0010] The laser interferometer of the present invention will be described in detail below based on the embodiments shown in the accompanying drawings. Figure 1 is a functional block diagram showing a laser interferometer 1 according to an embodiment.

[0011] The laser interferometer 1 shown in Figure 1 comprises a sensor head unit 51 equipped with an optical system 50, a current-voltage converter 531, and an oscillation circuit 54, and a demodulation circuit 52 to which the optical detection signal from the optical system 50 is input. The laser interferometer 1 irradiates a moving object 14 with laser light and detects and analyzes the reflected light. This allows for the measurement of the displacement and velocity of the object 14.

[0012] 1. Sensor head unit Figure 2 is a schematic diagram showing the sensor head unit 51 shown in Figure 1.

[0013] 1.1.Optical system As shown in Figure 2, the optical system 50 includes a laser light source 2, a collimating lens 3, an optical divider 4, a half-wave plate 6, a quarter-wave plate 7, a quarter-wave plate 8, an analyzer 9, a photodetector 10, and a frequency shifter type optical modulator 12.

[0014] The laser light source 2 emits an outgoing light L1 (first laser light). The photodetector 10 converts the received light into an electrical signal. The optical modulator 12 is equipped with a vibrating element 30, which changes the frequency of the outgoing light L1 and generates a reference light L2 (second laser light) that includes the modulation signal. The outgoing light L1 incident on the object 14 is reflected as object light L3 (third laser light) that includes a sample signal, which is a Doppler signal originating from the object 14.

[0015] The optical path connecting the optical divider 4 and the laser light source 2 is designated as optical path 18. The optical path connecting the optical divider 4 and the optical modulator 12 is designated as optical path 20. The optical path connecting the optical divider 4 and the object 14 is designated as optical path 22. The optical path connecting the optical divider 4 and the light-receiving element 10 is designated as optical path 24. In this specification, "optical path" refers to the path through which light travels, set between optical components. On optical path 18, the half-wave plate 6 and the collimating lens 3 are arranged in that order from the optical splitter 4 side. On optical path 20, the quarter-wave plate 8 is arranged. On optical path 22, the quarter-wave plate 7 is arranged. On optical path 24, the analyzer 9 is arranged.

[0016] The emitted light L1 from the laser light source 2 travels through the optical path 18 and is split into two by the optical splitter 4. One of the split emitted light L1, the first split light L1a, travels through the optical path 20 and is incident on the optical modulator 12. The other split emitted light L1, the second split light L1b, travels through the optical path 22 and is incident on the object 14. The reference light L2 generated by the frequency modulation in the optical modulator 12 travels through the optical paths 20 and 24 and is incident on the photodetector 10. The object light L3 generated by reflection from the object 14 travels through the optical paths 22 and 24 and is incident on the photodetector 10.

[0017] The following provides a further explanation of each part of the optical system 50. 1.1.1. Laser light source Laser light source 2 is a laser light source that emits coherent emitted light L1. Preferably, laser 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).

[0018] The laser light source 2 is preferably a semiconductor laser element. This makes it possible to miniaturize the laser light source 2. As a result, the laser interferometer 1 can be miniaturized. In particular, the sensor head section 51, which houses the optical system 50, can be miniaturized and lightened, which is useful in improving the operability of the laser interferometer 1, such as the degree of freedom in installing the sensor head section 51.

[0019] 1.1.2. Collimating Lenses The collimating lens 3 is an optical element placed between the laser light source 2 and the optical divider 4, and an aspherical lens is one example. The collimating lens 3 parallelizes the emitted light L1 from the laser light source 2. However, if the emitted light L1 from the laser light source 2 is sufficiently parallelized, for example, if a gas laser such as a He-Ne laser is used as the laser light source 2, the collimating lens 3 may be omitted.

[0020] On the other hand, when the laser light source 2 is a semiconductor laser element, it is preferable that the laser interferometer 1 includes a collimating lens 3 positioned between the laser light source 2 and the optical divider 4. This allows the emitted light L1 from the semiconductor laser element to be parallelized. As a result, the emitted light L1 becomes collimated light, which suppresses the need to enlarge the various optical components that receive the emitted light L1, and thus allows for miniaturization of the laser interferometer 1.

[0021] The collimated light L1 passes through the half-wave plate 6, where it is converted into linearly polarized light with a P-polarized to S-polarized intensity ratio of, for example, 50:50, and then enters the light divider 4.

[0022] 1.1.3.Light splitter The optical splitter 4 is a polarized beam splitter placed between the laser light source 2 and the optical modulator 12, and between the laser light source 2 and the object 14. The optical splitter 4 has the function of transmitting P-polarized light and reflecting S-polarized light. Due to this function, the optical splitter 4 splits the emitted light L1 into a first split beam L1a, which is the light reflected by the optical splitter 4, and a second split beam L1b, which is the light transmitted by the optical splitter 4.

[0023] The first split beam L1a, which is S-polarized and reflected by the optical splitter 4, is converted to circular polarization by the quarter-wave plate 8 and incident on the optical modulator 12. The first split beam L1a incident on the optical modulator 12 is f m It undergoes a frequency shift of [Hz] and is reflected as reference light L2. Therefore, the reference light L2 has a frequency f mThe modulated signal is in [Hz]. The reference light L2 is converted to P-polarized light when it passes through the quarter-wave plate 8 again. The P-polarized light of the reference light L2 passes through the light divider 4 and the analyzer 9 and is incident on the photodetector 10.

[0024] The second split beam L1b, which is P-polarized after passing through the light splitter 4, is converted to circular polarization by the quarter-wave plate 7 and incident on the moving object 14. The second split beam L1b incident on the object 14 is f d It undergoes a Doppler shift of [Hz] and is reflected as object light L3. Therefore, object light L3 has a frequency of f d The sample signal is in [Hz]. The object light L3 is converted to S-polarized light when it passes through the quarter-wave plate 7 again. The S-polarized object light L3 is reflected by the light divider 4, passes through the analyzer 9, and is incident on the photodetector 10.

[0025] As mentioned above, since the emitted light L1 is coherent, the reference light L2 and object light L3 are incident on the photodetector 10 as interfering light.

[0026] Alternatively, a non-polarizing beam splitter may be used instead of a polarizing beam splitter. In this case, the half-wave plate 6, quarter-wave plate 7, and quarter-wave plate 8, etc., become unnecessary, allowing for miniaturization of the laser interferometer 1 by reducing the number of components. Furthermore, other types of optical dividers may be used instead of a beam splitter.

[0027] 1.1.4. Analyzer Since S-polarized and P-polarized light, which are orthogonal to each other, are independent of each other, simply superimposing them will not produce beats due to interference. Therefore, the light wave obtained by superimposing S-polarized and P-polarized light is passed through an analyzer 9 tilted at 45° with respect to both the S-polarized and P-polarized light. By using analyzer 9, light with common components can be transmitted, causing interference. As a result, in analyzer 9, the reference light L2 and the object light L3 interfere, and |f m -f d Interfering light with a frequency of |[Hz] is generated.

[0028] 1.1.5. 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 displacement and velocity, can ultimately be determined. Examples of the photodetector 10 include a photodiode. Note that the light received by the photodetector 10 may include any light containing the reference light L2 and object light L3, and is not limited to these interfering lights alone. 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).

[0029] 1.1.6. Optical Modulator Figures 3 and 4 are perspective views showing a first configuration example of the vibration element 30 included in the optical modulator 12 shown in Figure 2. In Figures 3 and 4, the X, Y, and Z axes are defined as three mutually orthogonal axes and are indicated by arrows. The tip of the arrow is considered "positive," and the base of the arrow is considered "negative." For example, both the positive and negative directions of the X axis are referred to as the "X-axis direction." The same applies to the Y-axis and Z-axis directions.

[0030] 1.1.6.1. Vibration element The frequency shifter type optical modulator 12 is equipped with a vibrating element 30. In Figure 3, a tuning fork type quartz crystal oscillator is used as an example of the vibrating element 30. The vibrating element 30 shown in Figure 3 comprises a base portion 301, two arms 302, 302, electrodes 303, 304, and a light reflecting surface 305.

[0031] The base portion 301 is the part that extends along the X-axis. One arm portion 302 is the part that extends from the X-axis positive end of the base portion 301 toward the Y-axis positive side. The other arm portion 302 is the part that extends from the X-axis negative end of the base portion 301 toward the Y-axis positive side.

[0032] Electrode 303 is a conductive film provided on the surface of the arm portions 302, 302 that is parallel to the XY plane. Although not shown in Figure 3, electrodes 303 are provided on opposing surfaces, and the arms 302 are driven by applying voltages with opposite polarities to each electrode. Also, in Figure 4, the electrodes 303 are not shown.

[0033] Electrode 304 is a conductive film provided on the surface of the arm portions 302, 302 that intersects with the XY plane. Although not shown in Figure 3, electrodes 304 are also provided on opposing surfaces, and the arms 302 are driven by applying voltages with opposite polarities to each electrode. Also, in Figure 4, the electrode 304 is not shown.

[0034] The light-reflecting surface 305 is provided on the tip of the arm portion 302, on a surface parallel to the XY plane, and has the function of reflecting the emitted light L1. The light-reflecting surface 305 shown in Figure 3 is set on the surface of the electrode 303. In other words, the electrode 303 has the function of not only applying voltage to the arm portion 302, but also the function of a light-reflecting surface 305. In addition to the electrode 303, a light-reflecting film may also be provided.

[0035] A tuning fork-type quartz oscillator uses a quartz crystal cut from a quartz substrate. Examples of quartz substrates used in the manufacture of tuning fork-type quartz oscillators include Z-cut quartz plates.

[0036] The vibrating element 30, which is a tuning fork-type quartz crystal oscillator, has both an in-plane bending vibration mode and an out-of-plane vibration mode.

[0037] The in-plane bending vibration mode is a mode in which the two arms 302, 302 repeatedly move closer to or further apart from each other in the XY plane, as shown by the double-headed arrows in Figure 3. Therefore, when the two arms 302, 302 are vibrating in this in-plane bending vibration mode, the light-reflecting surface 305 hardly displaces in the direction intersecting the XY plane. In Figure 3, the outline of the two arms 302, 302 at the moment they are separated from each other while vibrating in the in-plane bending vibration mode is shown by a solid line, and the outline before deformation is shown by a dashed line.

[0038] Out-of-plane vibration modes are vibration modes having a vibration component in the direction that intersects with the light-reflecting surface 305 (the incident surface of the emitted light L1), i.e., in the Z-axis direction of Figure 4, as shown by the double-headed arrows in Figure 4. In this specification, vibrations having such a vibration component are also referred to as "out-of-plane vibrations." Out-of-plane vibrations are not limited to vibrations in the Z-axis direction; any vibration having a vibration component that intersects with the light-reflecting surface 305 is acceptable regardless of its vibration direction.

[0039] When vibrating in an out-of-plane vibration mode, the light-reflecting surface 305 is displaced sufficiently large in the direction intersecting the XY plane. Examples of out-of-plane vibrations include "out-of-plane in-phase bending vibration" in which the two arms 302, 302 are displaced in phase with each other on the positive or negative Z-axis side, "out-of-plane out-of-phase bending vibration" in which they are displaced in opposite phases, "in-phase torsional vibration" in which the two arms 302, 302 are twisted in phase with each other, and "out-of-phase torsional vibration" in which they are twisted in opposite phases. In Figure 4, the outlines of the two arms 302, 302 undergoing out-of-plane out-of-phase bending vibration are shown with solid lines, and the outlines before deformation are shown with dashed lines. Such out-of-plane out-of-plane out-of-phase bending vibration modes are also called walk modes.

[0040] In typical oscillators, when the in-plane bending vibration mode is considered the primary vibration mode, out-of-plane vibration modes such as the walk mode are treated as secondary vibration modes. In this case, the primary vibration mode is used, for example, as the oscillation mode for a clock source, while the secondary vibration modes are either avoided as spurious emissions or suppressed and not used.

[0041] In contrast, in this embodiment, the light reflecting surface 305 is displaced in the Z-axis direction by actively exciting the out-of-plane vibration mode without suppressing it. As a result, as shown in FIG. 4, when the emitted light L1 is incident on the light reflecting surface 305, the light reflecting surface 305 can be vibrated in a direction parallel to the incident direction, so that the interaction between the vibration of the light reflecting surface 305 and the frequency of the emitted light L1 increases. As a result, the optical modulator 12 capable of frequency modulating the emitted light L1 can be realized without using the diffraction grating that is necessary in the conventional optical modulator. Thereby, the manufacturing difficulty of the optical modulator 12 can be reduced by the amount corresponding to the unnecessary diffraction grating, and the cost of the laser interferometer 1 can be reduced.

[0042] FIG. 5 is a conceptual diagram showing the frequency characteristics of the main vibration mode (in-plane bending vibration mode) and the frequency characteristics of the secondary vibration mode (out-of-plane vibration mode). The horizontal axis in FIG. 5 is the vibration frequency, and the vertical axis is the vibration velocity amplitude. In this specification, the characteristics represented by the curve showing the frequency dependence of the vibration velocity amplitude shown in FIG. 5 are referred to as the "frequency characteristics" of each vibration mode.

[0043] The main vibration mode is excited by the drive signal Sd output from the oscillation circuit 54. Let the frequency when the vibration element 30 is excited by the oscillation circuit 54 be the oscillation frequency f osc And let the natural frequency of the main vibration mode of the vibration element 30 alone be f Q On the other hand, let the natural frequencies of the plurality of secondary vibration modes existing in the vibration element 30 alone be a, b, c, d, and e.

[0044] The oscillation frequency f osc of the oscillation circuit 54 is a value corresponding to the natural frequency f Q of the vibration element 30. In the example of FIG. 5, a frequency slightly higher than the natural frequency f Q becomes the oscillation frequency f osc The frequency characteristics of the main vibration mode are represented by a curve that peaks at the natural frequency f Q and decreases toward both sides.

[0045] On the other hand, in the example in Figure 5, 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.

[0046] 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 oscillation circuit 54 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.

[0047] As mentioned above, the magnitude of the optical modulation by the optical modulator 12 is given by the dot product of the difference between the incident wave vector of the emitted light L1 incident on the optical reflecting surface 305 and the wave vector of the reference light L2 emitted from the optical modulator 12, and the vibration vector of the optical reflecting surface 305. Therefore, if the main vibration mode is an in-plane bending vibration mode, as in a tuning fork type quartz oscillator, the optical reflecting surface 305 can be vibrated in the Z-axis direction (the direction intersecting the incident plane of the emitted light L1) by exciting the out-of-plane vibration mode, which is a secondary vibration mode. In other words, the out-of-plane vibration mode coupled with the in-plane bending vibration mode can be actively excited.

[0048] When the light-reflecting surface 305 is vibrating in the Z-axis direction, and emitted light L1 is incident on the light-reflecting surface 305, the frequency of the emitted light L1 is modulated by the Doppler effect and emitted as reference light L2. At this time, the dot product of the difference between the incident wavenumber vector of the emitted light L1 and the emitted wavenumber vector of the reference light L2, and the vibration vector of the light-reflecting surface 305, is greater than zero and sufficiently large. This makes it possible to increase the efficiency of optical modulation by the optical modulator 12 without using a diffraction grating. As a result, the difficulty of manufacturing the optical modulator 12 can be reduced, and the cost of the optical modulator 12 and the laser interferometer 1 can be easily reduced.

[0049] Furthermore, the natural frequencies of the secondary vibration modes can be adjusted by the structure of the vibration element 30, such as the length, thickness, and cross-sectional shape of the arms 302, 302, and the arrangement of electrodes 303, 304. 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 natural frequencies b, c, and d that are close to the original frequency. The specific structure can be determined through experiments and simulations. For example, by changing the cross-sectional shape of the arms 302, 302 from a rectangle to a parallelogram, for example, the sub-vibration modes can be more easily excited. Then, by changing the shape of the parallelogram, the natural frequencies of the sub-vibration modes can be adjusted.

[0050] Furthermore, out-of-plane out-of-plane out-of-plane bending vibration is more useful than out-of-plane in-phase bending vibration. The latter has higher oscillation reproducibility compared to the former. In addition, the latter has the advantage of being easier to approach the principal vibration mode, thus simplifying the design of the vibrating element 30.

[0051] 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.

[0052] The shape of the tuning fork-type quartz oscillator is not limited to the two-legged tuning fork type with two arms 302, 302 as shown in Figures 3 and 4, but can also include a three-legged tuning fork type, a four-legged tuning fork type with a cantilevered beam shape, and so on.

[0053] When a drive signal Sd is supplied (an AC voltage is applied) from the oscillation circuit 54 shown in Figures 1 and 2 to the vibrating element 30 shown in Figure 3, the vibrating element 30 oscillates. The power required for the oscillation of the vibrating element 30 (drive power) is not particularly limited, but is small, ranging from about 0.1 μW to 100 mW. Therefore, the drive signal Sd output from the oscillation circuit 54 can be used to make the vibrating element 30 oscillate without amplification.

[0054] Furthermore, conventional optical modulators such as acousto-optic modulators (AOMs) and electro-optic modulators (EOMs) sometimes require a structure to maintain the temperature of the optical modulator, making it difficult to reduce their volume. In addition, these optical modulators consume a lot of power, which poses a challenge in miniaturizing and reducing the power consumption of laser interferometers. In contrast, in this configuration example, the volume of the vibrating element 30 is very small, and the power required for oscillation is also small, making it easy to miniaturize and reduce the power consumption of the laser interferometer 1.

[0055] The vibrating element 30 is not limited to a tuning fork type quartz crystal oscillator, but may be other quartz crystal oscillators such as a quartz AT oscillator or an SC-cut quartz crystal oscillator that have an out-of-plane vibration mode.

[0056] The primary vibration mode of a quartz AT oscillator is a mode that produces thickness-slip vibration; however, by optimizing the structure of the vibration element, such as the shape of the quartz crystal and the arrangement of electrodes, it is possible to excite out-of-plane vibration modes.

[0057] The primary vibration mode of an SC-cut quartz crystal oscillator is also a mode that produces thickness-slip vibration, but the SC-cut quartz crystal oscillator also has out-of-plane vibration modes in a predetermined ratio. Here, if the ratio of thickness-slip vibration modes is m1, the ratio of out-of-plane vibration modes is m3, and the ratio of other modes is m2, then m1:m2:m3 is, for example, about 7:1:2. For SC-cut quartz crystal oscillators, quartz pieces cut from a quartz SC-cut plate are used.

[0058] Figure 6 is a side view showing a second configuration example of the vibration element 30 included in the optical modulator 12 shown in Figure 2. In Figure 6, the X, Y, and Z axes are defined as three mutually orthogonal axes and are indicated by arrows. The tip of the arrow is considered "positive," and the base of the arrow is considered "negative." For example, both the positive and negative directions of the Y axis are referred to as the "Y-axis direction."

[0059] In Figure 6, an SC-cut quartz crystal oscillator is used as the vibrating element 30. The vibrating element 30 shown in Figure 6 has a vibration component in the thickness direction of the quartz piece, i.e., in the Z-axis direction, as indicated by the double-headed arrows in Figure 6. Consequently, the light-reflecting surface 305 vibrates in the Z-axis direction. Figure 6 illustrates the external shape of the SC-cut quartz crystal oscillator vibrating in an out-of-plane vibration mode.

[0060] Furthermore, the vibration element 30 is not limited to a quartz crystal oscillator; any oscillator having an out-of-plane vibration mode may be used, such as a silicon oscillator or a ceramic oscillator. Unlike other oscillators, such as piezoelectric elements, quartz crystal oscillators, silicon oscillators, and ceramic oscillators utilize resonance phenomena, resulting in high Q values ​​and easy stabilization of the natural frequency. In this specification, oscillators that utilize resonance phenomena based on a high Q value are referred to as "self-oscillating oscillators." By using a self-oscillating oscillator as the vibration element 30, the modulation signal can be stabilized, and the oscillation circuit 54 operating with the vibration element 30 as a signal source can output a more accurate reference signal Ss. Moreover, both the modulation signal and the reference signal Ss are processed in real time by the demodulation circuit 52. Therefore, even if both signals are subjected to disturbances, they cancel each other out or are reduced, and the processing result is less likely to be affected. Thus, the sample signal originating from the object 14 can be demodulated with a high S / N ratio (signal-to-noise ratio), and a laser interferometer 1 that can measure the velocity and displacement of the object 14 with higher accuracy can be realized.

[0061] Furthermore, out-of-plane vibrations have lower natural frequencies than in-plane bending vibrations. Therefore, this embodiment, which utilizes out-of-plane vibration modes, allows for lower frequencies of the received signal and reference signal Ss compared to conventional laser interferometers using optical modulators that utilize in-plane bending vibration modes. Specifically, the natural frequency of in-plane bending vibrations is often, for example, 1 MHz or higher, while the natural frequency of out-of-plane vibrations is often, for example, less than 1 MHz. This reduces the processing performance of processors such as analog-to-digital converters (ADCs) and FPGAs (Field-Programmable Gate Arrays) that process these signals. As a result, it becomes easier to reduce the cost of the laser interferometer 1.

[0062] A silicon resonator is a resonator comprising a single-crystal silicon piece manufactured from a single-crystal silicon substrate using MEMS technology, and a piezoelectric film. MEMS (Micro Electro Mechanical Systems) refers to micro-electromechanical systems. Examples of single-crystal silicon piece shapes include cantilever shapes such as two-legged tuning fork type and three-legged tuning fork type. The oscillation frequency of a silicon resonator is, for example, from 1 kHz to several hundred MHz.

[0063] Furthermore, silicon oscillators can be designed to primarily generate out-of-plane vibrations. In this case, for example, the Q-factor can be increased by reducing the thickness of the cantilevered single-crystal silicon piece. Then, out-of-plane bending vibrations can be excited by arranging piezoelectric films, etc.

[0064] 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.

[0065] Furthermore, 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.

[0066] 1.1.6.2. Package Structure The optical modulator 12 may have a package structure. Figure 7 is a cross-sectional view showing an optical modulator 12 having a package structure. The package structure refers to a structure in which the vibrating element 30 is hermetically sealed inside the container 70 (housing) shown in Figure 7. The optical modulator 12 shown in Figure 7 comprises a container 70 having a housing section as an internal space, a vibrating element 30 housed in the container 70, and a circuit element 45 that constitutes part of the oscillation circuit 54.

[0067] As shown in Figure 7, the container 70 comprises a container body 72 and a lid 74. The container body 72 has a first recess 721 provided inside it, and a second recess 722 provided inside the first recess 721 and deeper than the first recess 721. The container body 72 is made of, for example, a ceramic material, a resin material, etc. Although not shown, the container body 72 also includes internal terminals provided on its inner surface, external terminals provided on its outer surface, and wiring connecting the internal terminals and external terminals.

[0068] Furthermore, the opening of the container body 72 is sealed by a lid 74 via a sealing member such as a sealing ring or low-melting-point glass (not shown). The lid 74 is a transmissive window through which the emitted light L1 and reference light L2 are transmitted, and is also a component that forms part of the container 70. The material used for the lid 74 is a material that can transmit laser light, such as glass or crystalline material. It is also preferable that the lid 74 is provided with an anti-reflective coating.

[0069] As mentioned above, the housing section of the container 70 is hermetically sealed. This allows the housing section to be maintained under reduced pressure. By reducing the pressure in the housing section, the air resistance in the out-of-plane vibration of the vibrating element 30 can be reduced. Therefore, the vibration efficiency of the vibrating element 30 housed in the housing section can be increased, and the displacement of the light-reflecting surface 305 can be made larger. Furthermore, the oscillation of the vibrating element 30 can be stabilized. As a result, the signal-to-noise ratio of the modulated signal can be increased, and ultimately, the sampled signal can be demodulated with a higher signal-to-noise ratio.

[0070] The pressure in the depressurized containment is not particularly limited as long as it is below atmospheric pressure, but it is preferably 100 Pa or less. On the other hand, if you want to maintain the depressurized state well, you may set the lower limit to about 10 Pa.

[0071] Furthermore, sealing the container 70 airtight and reducing the pressure in the containment area are not mandatory and may be omitted.

[0072] A vibrating element 30 is positioned on the bottom surface of the first recess 721. The vibrating element 30 is supported on the bottom surface of the first recess 721 by a bonding member (not shown). Furthermore, the internal terminals of the container body 72 and the vibrating element 30 are electrically connected via a conductive material (not shown), such as a bonding wire or bonding metal.

[0073] A circuit element 45 is positioned on the bottom surface of the second recess 722. The circuit element 45 is electrically connected to the internal terminals of the container body 72 via a conductive material such as a bonding wire 76. This also electrically connects the vibration element 30 and the circuit element 45 via the wiring provided in the container body 72. The circuit element 45 may also be provided with circuits other than the oscillation circuit 54, which will be described later.

[0074] By adopting this package structure, the vibration element 30 and the circuit element 45 can be stacked, reducing their physical distance and shortening the wiring length between them. This suppresses external noise interference to the drive signal Sd, and conversely, prevents the drive signal Sd from becoming a noise source. Furthermore, both the vibration element 30 and the circuit element 45 can be protected from the external environment within a single container 70. This allows for miniaturization of the sensor head 51 while improving the reliability of the laser interferometer 1.

[0075] Furthermore, the package structure can minimize degassing, which can worsen the vacuum level, compared to, for example, hermetically sealing the entire optical system 50. As a result, the optical modulator 12 employing this package structure is more likely to have improved long-term reliability.

[0076] Furthermore, the container 70 constituting the package structure described above can also be manufactured together with the vibrating element 30 in a wafer-level manufacturing process. For this reason, the optical modulator 12 employing the package structure can be easily manufactured at a reduced cost.

[0077] The structure of the container 70 is not limited to the illustrated structure; for example, the vibration element 30 and the circuit element 45 may have separate package structures. Also, although not shown, the container 70 may house other circuit elements constituting the oscillation circuit 54 and other circuit elements. The container 70 may be provided as needed and may be omitted.

[0078] Here, we will describe a modified version of the container 70. Figures 8 and 9 are cross-sectional views showing modified versions of the package structure in Figure 7, respectively.

[0079] The container 70A shown in Figure 8 is the same as the container 70 shown in Figure 7, except that it has a through hole 700 provided in a part of the lid 74 and a transmissive window 71A fitted into the through hole 700. The transmissive window 71A has a curved surface. This allows the transmissive window 71A to not only transmit the outgoing light L1 and the reference light L2, but also to adjust the direction of propagation of these lights. For example, the transmissive window 71A can be given the function of a collimating lens 3. This allows the outgoing light L1 to be focused and the range in which it enters the light reflecting surface 305 to be narrowed. It also allows the reference light L2 to be parallelized and the range in which it enters the light divider 4 to be narrowed. As a result, the optical system 50 can be miniaturized. Furthermore, by using the container 70A shown in Figure 8, the collimating lens 3 can be omitted. This reduces the number of parts in the optical system 50 and makes the laser interferometer 1 less expensive.

[0080] Examples of curved surfaces include convex curved surfaces, and aspherical surfaces are particularly preferred. This reduces various aberrations in the lens and improves the accuracy of parallelization.

[0081] Furthermore, the constituent material of the lid 74 used in the container 70A does not need to be a material that can transmit laser light; it may be a material that does not transmit laser light. It is preferable that the transmission window 71A is provided with an anti-reflective coating.

[0082] The container 70B shown in Figure 9 is the same as the container 70A shown in Figure 8, except that it has a through hole 700 provided in a part of the lid 74 and a transmissive window 71B fitted into the through hole 700. The transmissive window 71B is made of a material that can transmit laser light and is flat in shape. The transmissive window 71B is positioned at an angle with respect to the incident direction of the emitted light L1, that is, at an angle such that the incident angle of the emitted light L1 with respect to the main surface 711 (the surface on which the emitted light L1 is incident) is greater than 0°. This reduces the probability that the emitted light L4, even if it is reflected by the main surface 711 and generates reflected light L4, will be incident on the photodetector 10. If reflected light L4 is incident on the photodetector 10, it will cause a decrease in the S / N ratio of the received signal. For this reason, by using the container 70B having a transmissive window 71B positioned at an angle, the decrease in the S / N ratio of the received signal can be suppressed.

[0083] In Figure 9, the main surface 711 is used as the reference plane when the incident angle of the emitted light L1 is 0°, and the inclination angle θ of the main surface 711 with respect to this reference plane is shown. In this case, the incident angle is 2θ. Then, the diameter of the effective light-receiving surface in the light-receiving element 10 is φ D The distance between the optical modulator 12 and the light-receiving element 10 is set to L. md In this case, the inclination angle θ should be set such that it satisfies the following equation.

[0084]

number

[0085] This makes it possible to suppress the reception of reflected light L4 generated on the main surface 711 by the photodetector 10. As a result, even if reflected light L4 is generated, it is possible to suppress a decrease in the signal-to-noise ratio of the received signal. For example, diameter φ D 0.8mm, distance L mdIf the width is 10 mm, the inclination angle θ should be 1.1° or more. As an example, the inclination angle θ is preferably 0.05° or more and 5.0° or less, and more preferably 0.10° or more and 2.0° or less. It is also preferable that an anti-reflective coating is provided on the transmissive window 71B.

[0086] 1.2.Hermetically sealed structure Figures 7 to 9 illustrate a package structure for hermetically sealing the optical modulator 12, but part or all of the sensor head 51 may also have a hermetically sealed structure.

[0087] Figures 10 to 12 are cross-sectional views showing modified versions of the sensor head 51 in Figure 1, respectively. The hermetic sealing structure refers to a structure in which at least the optical modulator 12 is hermetically sealed inside the cases 502A, 502B, and 502C (housings) shown in Figures 10 to 12.

[0088] The sensor head 51A shown in Figure 10 comprises a case 502A having a housing space as an internal space, an optical system 50 housed in the case 502A, and wiring boards 507, 508, and 509. Note that some of the optical elements of the optical system 50 are omitted from the illustration in Figure 10.

[0089] As shown in Figure 10, case 502A comprises a case body 503 and a transparent window 504A. The case body 503 is made of, for example, a metal material, a resin material, or the like.

[0090] The transmissive window 504A is fitted into a hole provided in the case body 503. The material used for the transmissive window 504A is a material that can transmit laser light, such as glass or crystalline material. The transmissive window 504A may have the same configuration and function as the transmissive window 71B described above. That is, the transmissive window 504A may be installed in an inclined position with respect to the reference plane, or it may be installed in an uninclined position. In the former case, the generation of reflected light L4 can be suppressed.

[0091] The wiring board 507 supports the optical modulator 12 and is electrically connected to the optical modulator 12. The wiring board 508 supports the photodetector 10 and the laser light source 2 and is electrically connected to them. The wiring board 509 is electrically connected to the wiring boards 507 and 508, and is also electrically connected to the outside. Note that "electrically connected" means connected by power lines and communication lines.

[0092] Furthermore, a reflective element 5 is added to the optical system 50 shown in Figure 10. The reflective element 5 is positioned on the optical path 24 and changes the direction of propagation of the reference light L2 and the object light L3.

[0093] The sensor head 51B shown in Figure 11 comprises a case 502B having a housing space as an internal space, an optical system 50 housed in the case 502B, and wiring boards 507, 508, and 509. Note that some of the optical elements of the optical system 50 are omitted from the illustration in Figure 11.

[0094] As shown in Figure 11, case 502B comprises a case body 503 and a transmissive window 504B. The transmissive window 504B is fitted into a hole provided in the case body 503. The transmissive window 504B has the same configuration and function as the transmissive window 71A described above. That is, the transmissive window 504B is provided with the function of a collimating lens 3.

[0095] The sensor head unit 51C shown in Figure 12 comprises a case 502C having a housing space as an internal space, an optical system 50 housed in the case 502C, and wiring boards 507, 508, and 509. Note that some of the optical elements of the optical system 50 are omitted from the illustration in Figure 12.

[0096] Case 502C comprises a first case 505 and a second case 506, as shown in Figure 12. Additionally, collimating lenses 3a, 3b, and 3c and optical fibers 26 and 27 are added to the optical system 50 shown in Figure 12.

[0097] The first case 505 comprises a case body 503 and a transmissive window 504B. The housing section of the first case 505 houses the collimating lens 3, collimating lens 3a, optical divider 4, reflective element 5, and optical modulator 12 of the optical system 50, as well as a wiring board 507. On the other hand, the housing section of the second case 506 houses the laser light source 2, photodetector 10, collimating lens 3b and collimating lens 3c of the optical system 50, as well as wiring boards 508 and 509.

[0098] Furthermore, the optical fibers 26 and 27 are mostly located externally and optically connect the housing section of the first case 505 and the housing section of the second case 506.

[0099] On the optical path 18 connecting the optical splitter 4 and the laser light source 2, the collimating lens 3, optical fiber 26, and collimating lens 3b are arranged in this order from the optical splitter 4 side. On the optical path 24 connecting the optical splitter 4 and the photodetector 10, the reflecting element 5, collimating lens 3a, optical fiber 27, and collimating lens 3c are arranged in this order from the optical splitter 4 side.

[0100] Preferably, the housings in cases 502A, 502B, and 502C are hermetically sealed. This allows the housings to be maintained under reduced pressure. By reducing the pressure in the housings, even if the optical modulator 12 does not have a package structure, the air resistance in out-of-plane vibration of the vibrating element 30 housed in the housing can be reduced. This increases the vibration efficiency of the vibrating element 30 and allows for a larger displacement of the light-reflecting surface 305. As a result, the signal-to-noise ratio (S / N ratio) of the modulated signal can be increased, and ultimately, the sampled signal can be demodulated with a higher S / N ratio.

[0101] Furthermore, in cases 502A, 502B, and 502C, the laser light source 2 can also be kept under reduced pressure. This makes it possible to suppress the degradation of the laser light source 2 due to changes in humidity and atmospheric pressure, specifically, fluctuations in the oscillation wavelength.

[0102] Furthermore, some of the optical elements constituting the optical system 50 may be located outside the cases 502A, 502B, and 502C.

[0103] 1.3. Current-Voltage Converter The current-voltage converter 531, also known as a transimpedance amplifier (TIA), converts the photocurrent (received signal) output from the photodetector 10 into a voltage signal and outputs it as a photodetection signal.

[0104] An ADC 532, as shown in Figure 1, is positioned between the current-voltage converter 531 and the demodulation circuit 52. The ADC 532 is an analog-to-digital converter that converts an analog signal into a digital signal with a predetermined number of sampling bits. The ADC 532 is located in the sensor head unit 51.

[0105] The optical system 50 may include multiple photodetectors 10. In this case, by providing a differential amplifier circuit between the multiple photodetectors 10 and the current-voltage converter 531, differential amplification processing can be applied to the photocurrent, thereby increasing the signal-to-noise ratio of the photodetection signal. The differential amplification processing may also be applied to the voltage signal.

[0106] 1.4. Oscillator Circuit The oscillation circuit 54 outputs a drive signal Sd to the vibrating element 30. The oscillation circuit 54 also outputs a reference signal Ss to the demodulation circuit 52.

[0107] The oscillation circuit 54 is not particularly limited as long as it is a circuit capable of oscillating the vibration element 30, and various circuit configurations can be used. As an example of a circuit configuration, Figure 13 shows a circuit diagram illustrating the configuration of a single-stage inverter oscillation circuit.

[0108] The oscillator circuit 54 shown in Figure 13 comprises a circuit element 45, a feedback resistor Rf, a limiting resistor Rd, a first capacitor Cg, a second capacitor Cd, and a third capacitor C3.

[0109] Circuit element 45 is an inverter IC. Terminals X1 and X2 of circuit element 45 are connected to the inverter inside circuit element 45, respectively. Terminal GND is connected to ground potential, and terminal Vcc is connected to power supply potential. Terminal Y is the terminal for oscillation output.

[0110] A first capacitor Cg is connected between terminal X1 and ground potential. A limiting resistor Rd and a second capacitor Cd are connected in series between terminal X2 and ground potential, in that order from terminal X2. Furthermore, one end of a feedback resistor Rf is connected between terminal X1 and the first capacitor Cg, and the other end of the feedback resistor Rf is connected between terminal X2 and the limiting resistor Rd.

[0111] Furthermore, one end of the vibrating element 30 is connected between the first capacitor Cg and the feedback resistor Rf, and the other end of the vibrating element 30 is connected between the second capacitor Cd and the limiting resistor Rd. As a result, the vibrating element 30 becomes the signal source for the oscillation circuit 54.

[0112] Figure 14 shows an example of the LCR equivalent circuit of the vibration element 30. As shown in Figure 14, the LCR equivalent circuit of the vibration element 30 consists of a series capacitance C1, a series inductance L1, an equivalent series resistance R1, and a parallel capacitance C0.

[0113] In the oscillator circuit 54 shown in Figure 13, the capacitance of the first capacitor Cg is set to C g And the capacitance of the second capacitor Cd is set to C d In this case, the load capacity C L This is given by equation (a) below.

[0114]

number

[0115] Then, the oscillation frequency f is the frequency of the signal output from terminal Y of the oscillation circuit 54. osc This is given by equation (b) below.

[0116]

number

[0117] f Q This is the natural frequency of the vibrating element 30. According to equation (b) above, the load capacity C L By appropriately changing the value, the oscillation frequency f of the signal output from terminal Y can be changed. osc It can be seen that it is possible to fine-tune it.

[0118] Furthermore, the natural frequency f of the vibrating element 30 Q The oscillation frequency f of the oscillation circuit 54 osc The difference Δf between and is given by the following equation (c).

[0119]

number

[0120] Here, C1< <C0、C1<<C L Therefore, Δf is approximately given by the following equation (d).

[0121]

number

[0122] Therefore, the oscillation frequency f of the oscillator circuit 54 osc The natural frequency f of the vibrating element 30 is Q The value will be determined accordingly.

[0123] Here, when the vibrating element 30 is fixed to, for example, the container 70, it is subjected to thermal expansion stress through the fixing part, and the natural frequency f Q The frequency f f changes. Also, when the vibrating element 30 is tilted, the natural frequency f changes due to the influence of gravity and other factors. Q It fluctuates.

[0124] For this reason, the oscillator circuit 54 has a natural frequency f Q Even if f f changes, the oscillation frequency f will be adjusted in conjunction with that change based on equation (d) above. osc This means that the oscillation frequency f will change. osc It is always Δf, with natural frequency f Q The value deviates from the original value. As a result, the vibration of the vibrating element 30 stabilizes, and the displacement amplitude stabilizes. Because the displacement amplitude stabilizes, the modulation characteristics of the optical modulator 12 stabilize, and the signal-to-noise ratio of the modulated signal can be further improved. As a result, the demodulation accuracy of the sampled signal in the demodulation circuit 52 can be improved.

[0125] For example, Δf = |f osc -f Q It is preferable that |≦3000[Hz], and more preferably that Δf≦600[Hz].

[0126] The laser interferometer 1 also includes a demodulation circuit 52 and an oscillation circuit 54. The demodulation circuit 52 demodulates the sample signal originating from the object 14 from the photodetection signal based on the photocurrent (received light signal) based on a reference signal Ss. The oscillation circuit 54 operates with the vibrating element 30 as a signal source and outputs the reference signal Ss to the demodulation circuit 52, as shown in Figure 1.

[0127] With this configuration, the natural frequency f of the vibrating element 30 Q Even if it fluctuates, the oscillation frequency f of the oscillation circuit 54 osc The natural frequency f of the vibrating element 30 Q Since the value can be changed accordingly, the vibration of the vibrating element 30 can be easily stabilized. This makes it possible to match the temperature characteristics of the modulated signal to the temperature characteristics of the vibrating element 30, and stabilize the modulation characteristics of the optical modulator 12. As a result, the demodulation accuracy of the sampled signal in the demodulation circuit 52 can be improved.

[0128] Furthermore, in the above configuration, the temperature characteristics of the reference signal Ss output from the oscillation circuit 54 to the demodulation circuit 52 can also be made to correspond to the temperature characteristics of the vibrating element 30. In this case, both the temperature characteristics of the modulated signal and the temperature characteristics of the reference signal correspond to the temperature characteristics of the vibrating element 30, so the behavior of the fluctuations of the modulated signal and the behavior of the fluctuations of the reference signal Ss due to temperature changes will match or approximate. For this reason, even if the temperature of the vibrating element 30 changes, the impact on demodulation accuracy can be suppressed, and the demodulation accuracy of the sample signal originating from the object 14 can be improved.

[0129] Furthermore, because the oscillation circuit 54 consumes little power, it is easy to reduce the power consumption of the laser interferometer 1.

[0130] 2. Demodulation Circuit The demodulation circuit 52 performs demodulation processing to demodulate the sample signal originating from the object 14 from the photodetection signal output from the current-voltage converter 531. The sample signal includes, for example, phase information and frequency information. From the phase information, the displacement of the object 14 can be obtained, and from the frequency information, the velocity of the object 14 can be obtained. By obtaining different physical quantities in this way, the laser interferometer 1 can be given functions as a displacement meter and a velocity meter, thereby enhancing its functionality.

[0131] In the demodulation circuit 52, the circuit configuration is set according to the modulation processing method. In the laser interferometer 1 according to this embodiment, an optical modulator 12 equipped with a vibrating element 30 is used. Since the vibrating element 30 is an element that undergoes simple harmonic motion, the vibration velocity changes moment by moment within the period. For this reason, the modulation frequency also changes with time, and conventional demodulation circuits cannot be used as is.

[0132] Conventional demodulation circuits refer to circuits that demodulate a sampled signal from a received light signal that includes a modulated signal modulated using, for example, an acousto-optic modulator (AOM). In an acousto-optic modulator, the modulation frequency does not change. Therefore, conventional demodulation circuits can demodulate a sampled signal from a received light signal that includes a modulated signal that does not change in modulation frequency, but they cannot demodulate a signal that includes a modulated signal modulated by an optical modulator 12 whose modulation frequency changes.

[0133] Therefore, the demodulation circuit 52 shown in Figure 1 comprises a pre-processing unit 53 and a demodulation processing unit 55. The photodetection signal output from the current-voltage converter 531 is first pre-processed in the pre-processing unit 53 and then led to the demodulation processing unit 55. This pre-processing yields a signal that can be demodulated by a conventional demodulation circuit. Accordingly, the demodulation processing unit 55 demodulates the sample signal originating from the object 14 using a known demodulation method.

[0134] The functions of the demodulation circuit 52 described above are realized by hardware, such as a processor, memory, external interface, input unit, and display unit. Specifically, this is achieved by the processor reading and executing a program stored in memory. These components are able to communicate with each other via an internal bus.

[0135] 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.

[0136] 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).

[0137] External interfaces include, for example, digital input / output ports such as USB (Universal Serial Bus) and Ethernet® ports.

[0138] 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.

[0139] 2.1. Configuration of the pre-processing unit The pre-processing unit 53 shown in Figure 1 includes a first bandpass filter 534, a second bandpass filter 535, a first delay adjuster 536, a second delay adjuster 537, a multiplier 538, a third bandpass filter 539, a first AGC unit 540, a second AGC unit 541, and an adder 542. AGC stands for Auto Gain Control.

[0140] The photodetection signal output from the current-voltage converter 531 is split into two signals, a first signal S1 and a second signal S2, at the branching section jp1. In Figure 1, the path of the first signal S1 is designated as the first signal path ps1, and the path of the second signal S2 is designated as the second signal path ps2.

[0141] An ADC 533 is connected between the oscillation circuit 54 and the second delay adjuster 537. The ADC 533 is an analog-to-digital converter that converts an analog signal to a digital signal with a predetermined number of sampling bits. The ADC 533 is located in the sensor head unit 51.

[0142] The first bandpass filter 534, the second bandpass filter 535, and the third bandpass filter 539 are filters that selectively transmit signals in a specific frequency band.

[0143] The first delay adjuster 536 and the second delay adjuster 537 are circuits that adjust the delay of the signal, respectively. The multiplier 538 is a circuit that generates an output signal proportional to the product of two input signals. The adder 542 is a circuit that generates an output signal proportional to the sum of two input signals.

[0144] Next, the operation of the pre-processing unit 53 will be explained in accordance with the flow of the first signal S1, the second signal S2, and the reference signal Ss.

[0145] The first signal S1 is passed through a first bandpass filter 534 located on the first signal path ps1, and then its group delay is adjusted by a first delay adjuster 536. The group delay adjusted by the first delay adjuster 536 corresponds to the group delay of the second signal S2 by the second bandpass filter 535, which will be described later. This delay adjustment makes it possible to equalize the delay time between the first bandpass filter 534 through which the first signal S1 passes, and the second bandpass filter 535 and third bandpass filter 539 through which the second signal S2 passes. The first signal S1, having passed through the first delay adjuster 536, is input to the adder 542 via the first AGC unit 540.

[0146] The second signal S2 is passed through a second bandpass filter 535 located on the second signal path ps2, and then input to a multiplier 538. In the multiplier 538, the second signal S2 is multiplied by the reference signal Ss output from the second delay adjuster 537. Specifically, the cos(ω) output from the oscillator circuit 54 is multiplied. m The reference signal Ss, represented by t), is digitally converted by the ADC 533, its phase is adjusted by the second delay adjuster 537, and then input to the multiplier 538. mis the angular frequency of the modulated signal from the optical modulator 12, and t is time. Subsequently, the second signal S2 is passed through the third bandpass filter 539, then through the second AGC unit 541, and then input to the adder 542. The adder 542 outputs an output signal that is proportional to the sum of the first signal S1 and the second signal S2.

[0147] 2.2 Basic Principles of Pretreatment Next, the basic principle of preprocessing in the preprocessing unit 53 will be explained. In the following explanation, as an example, we consider a system in which the frequency of the modulated signal changes sinusoidally, and the displacement of the object 14 also changes in simple harmonic motion in the optical axis direction. Here, E m , E d φ,

[0148]

number

[0149] In this case, the photodetection signal I output from the current-voltage converter 531 PD Theoretically, it can be expressed by the following equation.

[0150]

number

[0151] Note E m , E d , φ m , φ d ,φ,ω m , ω d ,ω0,a m a d These are as follows:

[0152]

number

[0153] Furthermore, the <> in equation (4) represents the time average. The first and second terms of equation (4) above represent the DC component, and the third term represents the AC component. This AC component is I PD·AC Therefore, I PD·AC The equation is as follows:

[0154]

number

[0155] Here, the ν-th order Bessel functions shown in equations (8) and (9) below are known.

[0156]

number

[0157] Expanding equation (5) above into a series using the Bessel functions in equations (8) and (9) above, it can be transformed into equation (10) below.

[0158]

number

[0159] However, J0(B), J1(B), J2(B), ... are Bessel coefficients, respectively.

[0160] As shown above, theoretically, it is possible to extract a frequency band corresponding to a specific order using a bandpass filter. Therefore, the aforementioned preprocessing unit 53 performs preprocessing on the photodetection signal according to the following flow based on this theory.

[0161] First, the photodetection signal output from the current-voltage converter 531 is split into two signals, a first signal S1 and a second signal S2, at the branching section jp1. The first signal S1 is passed through the first bandpass filter 534. The first bandpass filter 534 has a center angular frequency of ω mIt is set to this. As a result, the first signal S1 after passing through the first bandpass filter 534 is expressed by the following equation.

[0162]

number

[0163] Meanwhile, the second signal S2 is passed through the second bandpass filter 535. The center angle frequency of the second bandpass filter 535 is set to a different value from the center angle frequency of the first bandpass filter 534. Here, as an example, the center angle frequency of the second bandpass filter 535 is set to 2ω m This setting is used. As a result, the second signal S2 after passing through the second bandpass filter 535 is expressed by the following equation.

[0164]

number

[0165] The second signal S2, after passing through the second bandpass filter 535, is multiplied by the reference signal Ss using the multiplier 538. The second signal S2 after passing through the multiplier 538 is expressed by the following equation.

[0166]

number

[0167] The second signal S2, after passing through the multiplier 538, is passed through the third bandpass filter 539. The center angle frequency of the third bandpass filter 539 is set to the same value as the center angle frequency of the first bandpass filter 534. Here, as an example, the center angle frequency of the third bandpass filter 539 is set to ω m This setting is used. As a result, the second signal S2 after passing through the third bandpass filter 539 is expressed by the following equation.

[0168]

number

[0169] Subsequently, the first signal S1 represented by the above equation (11) has its phase adjusted by the first delay adjuster 536 and its amplitude adjusted by the first AGC unit 540.

[0170] Furthermore, the amplitude of the second signal S2, represented by the above equation (14), is also adjusted by the second AGC unit 541 so that the amplitude of the second signal S2 matches the amplitude of the first signal S1.

[0171] The first signal S1 and the second signal S2 are then added together by the adder 542. The result of the addition is expressed by the following equation (15).

[0172]

number

[0173] As shown in equation (15) above, the addition results in the elimination of unnecessary terms and the extraction of necessary terms. In other words, the addition result I expressed in equation (15) 53 This is a signal from which the frequency modulation component has been extracted. This summation result I 53 This is input to the demodulation processing unit 55.

[0174] As mentioned above, J0(B), J1(B), and J2(B) are Bessel coefficients, and they change according to the phase shift B of the modulated signal.

[0175] Figure 15 is a graph showing the relationship between the Bessel coefficients J0(B), J1(B), and J2(B) and the phase shift B of the modulated signal. From this graph, when J1(B) = J2(B), the S / N ratio of the first signal S1, expressed by equation (11) above, and the S / N ratio of the second signal S2, expressed by equation (14) above, can be made equivalent. As a result, the summation result I 53 This can further improve the signal-to-noise ratio.

[0176] In Figure 15, when J1(B)=J2(B) holds, the phase shift B of the modulated signal is approximately 2.6. The phase shift B of the modulated signal depends on the displacement amplitude of the out-of-plane vibration of the vibrating element 30, i.e., the displacement in the Z-axis direction. From the viewpoint of ensuring the minimum required measurement accuracy in the laser interferometer 1, it is preferable that the phase shift B of the modulated signal satisfies 0.05≦B, more preferably 0.50≦B, and even more preferably π / 3≦B. In this embodiment, it is possible to increase the phase shift B of the modulated signal by actively exciting the out-of-plane vibration of the vibrating element 30.

[0177] For example, there is experimental data showing that when a tuning fork-type quartz oscillator with a resonant frequency of 32 kHz is driven at a voltage of 3 V, the displacement Lq in the Z-axis direction at the tip of the arm is approximately 175 nm, even under atmospheric pressure. Based on this displacement Lq, the phase shift B of the modulated signal when using laser light with a wavelength of λ = 850 nm can be calculated using the formula B = 4πLq / λ, resulting in approximately B = 2.6.

[0178] Furthermore, for example, there is measured data showing that when an SC-cut quartz oscillator with a resonant frequency of 10.26 MHz is driven at a voltage of 3 V, the displacement Lq in the Z-axis direction is approximately 13 nm, even under atmospheric pressure. Based on this displacement Lq, the phase shift B of the modulated signal when using laser light with a wavelength of λ = 850 nm can be calculated to be approximately B = 0.19.

[0179] Therefore, according to the vibrating element 30 that is excited for out-of-plane vibration, the summation result I has a high S / N ratio. 53 It is possible to obtain this.

[0180] 2.3. Configuration of the demodulation processing unit The demodulation processing unit 55 performs demodulation processing to demodulate a sample signal originating from the object 14 from the signal output from the preprocessing unit 53. The demodulation processing is not particularly limited, but a known method is quadrature detection. The quadrature detection method is a method of demodulation processing that involves mixing mutually orthogonal signals from an external source with the input signal.

[0181] The demodulation processing unit 55 shown in Figure 1 is a digital circuit comprising a multiplier 551, a multiplier 552, a phase shifter 553, a first low-pass filter 555, a second low-pass filter 556, a divider 557, an inverse tangent unit 558, and an output circuit 559.

[0182] Multipliers 551 and 552 are circuits that generate an output signal proportional to the product of two input signals. Phase shifter 553 is a circuit that generates an output signal with the phase of the input signal inverted without changing the amplitude. The first low-pass filter 555 and the second low-pass filter 556 are filters that cut out signals in the high-frequency band, respectively.

[0183] The divider 557 is a circuit that generates an output signal proportional to the quotient of two input signals. The inverse tangent arithmetic unit 558 is a circuit that outputs the inverse tangent of the input signals. The output circuit 559 takes the phase φ obtained by the inverse tangent arithmetic unit 558 and outputs the phase φ as information originating from the object 14. d The output circuit 559 calculates the phase. Furthermore, it performs phase unwrapping to connect phases when there is a 2π phase gap between two adjacent points. Then, it calculates the displacement of the object 14 from the obtained phase information. This realizes a displacement sensor. Additionally, the velocity of the object 14 can be determined from the displacement. This realizes a speedometer.

[0184] The circuit configuration of the demodulation processing unit 55 described above is merely an example and is not limited thereto. For example, the demodulation processing unit 55 is not limited to a digital circuit but may also be an analog circuit. The analog circuit may include an F / V converter circuit or a ΔΣ counter circuit.

[0185] 2.4. Demodulation processing by the demodulation processing unit In the demodulation process, first, the signal output from the preprocessing unit 53 is split into two by the branching unit jp2. For one of the resulting signals, the multiplier 551 multiplies the output from the oscillator circuit 54, cos(ω mThe reference signal Ss represented by t) is multiplied. For the other signal after division, the multiplier 552 shifts the phase of the reference signal Ss output from the oscillator circuit 54 by -90° using the phase shifter 553, resulting in -sin(ω m The signal represented by t) is multiplied. The reference signal Ss and the signal obtained by shifting the phase of the reference signal Ss are signals that are 90° out of phase with respect to each other.

[0186] The signal that has passed through the multiplier 551 is then passed through the first low-pass filter 555 and input to the divider 557 as signal x. The signal that has passed through the multiplier 552 is then passed through the second low-pass filter 556 and input to the divider 557 as signal y. In the divider 557, a division is performed by dividing signal y by signal x, and the output y / x is passed through the arctangent arithmetic unit 558 to obtain the output atan(y / x).

[0187] Subsequently, by passing the output atan(y / x) through the output circuit 559, the phase φ is obtained as information originating from the object 14. d This is required. In the output circuit 559, phase unwrapping is performed to connect the phases when there is a 2π phase gap between adjacent points. Then, the displacement of the object 14 can be calculated from the phase information. This realizes a displacement sensor. Furthermore, the velocity can be determined from the displacement. This realizes a speedometer.

[0188] On the other hand, the output circuit 559 may be configured to obtain frequency information. Based on the frequency information, the velocity of the object 14 can be calculated.

[0189] 3. Effects of the Embodiment As described above, the laser interferometer 1 according to the embodiment comprises a laser light source 2, an optical modulator 12, a light receiving element 10, a demodulation circuit 52, and an oscillation circuit 54.

[0190] The laser light source 2 emits an emitted light L1 (first laser light). The optical modulator 12 is equipped with a vibrating element 30, which modulates the emitted light L1 and generates a reference light L2 (second laser light) containing the modulated signal. The vibrating element 30 has a vibration component in a direction intersecting the incident plane of the emitted light L1. The photodetector 10 receives the reference light L2 and object light L3 (third laser light) containing the sample signal generated when the emitted light L1 is reflected by the object 14, and outputs a received signal. The demodulation circuit 52 demodulates the sample signal from the received signal based on the reference signal Ss. The oscillation circuit 54 operates with the vibrating element 30 as a signal source and outputs the reference signal Ss to the demodulation circuit 52.

[0191] With this configuration, the vibration component (out-of-plane vibration component) of the emitted light L1 in the vibrating element 30 that intersects the incident plane can be used for frequency modulation of the emitted light L1, thereby increasing the interaction between the vibration of the vibrating element 30 and the frequency of the emitted light L1. As a result, an optical modulator 12 capable of frequency modulation of the emitted light L1 can be realized without using a diffraction grating, which was required in conventional optical modulators. Consequently, the manufacturing difficulty of the optical modulator 12 can be reduced, and the cost of the laser interferometer 1 can be reduced.

[0192] Furthermore, by utilizing the out-of-plane vibration component of the vibrating element 30, the phase shift B of the modulated signal can be easily increased. This improves the signal-to-noise ratio of the received signal, ultimately enabling the realization of a laser interferometer 1 with high measurement accuracy for displacement, velocity, and other parameters.

[0193] Furthermore, since the vibrating element 30 is the signal source for the oscillation circuit 54, the temperature characteristics of the modulated signal and the reference signal Ss can be made to correspond to the temperature characteristics of the vibrating element 30, respectively. Since both the modulated signal and the reference signal Ss are processed in real time by the demodulation circuit 52, the behavior of the fluctuations in the modulated signal and the behavior of the fluctuations in the reference signal Ss due to temperature changes match or approximate each other. Therefore, even if the temperature of the vibrating element 30 changes, the impact on demodulation accuracy can be suppressed, and the demodulation accuracy of the sample signal originating from the object 14 can be improved. This makes it possible to realize a laser interferometer 1 with excellent resistance to disturbances.

[0194] Furthermore, the optical modulator 12 includes a container 70, 70A, 70B or case 502A, 502B, 502C, which is a housing having a section for housing the vibrating element 30. The housing is preferably under reduced pressure.

[0195] This configuration allows for increased vibration efficiency of the vibration element 30 housed in the housing, thereby increasing the vibration displacement. Furthermore, it is possible to stabilize the oscillation of the vibration element 30. As a result, the signal-to-noise ratio of the modulated signal can be further improved.

[0196] Furthermore, the containers 70, 70A, 70B and cases 502A, 502B, 502C (housings) have lids 74 or transmissive windows 71A, 71B, 504A, 504B that separate the housing from the outside and allow the emitted light L1 (first laser light) and reference light L2 (second laser light) to pass through. Depending on the configuration of the optical system 50, object light L3 (third laser light) may also be transmitted, as will be described later. For this reason, the transmissive windows only need to be able to transmit the emitted light L1, reference light L2, and object light L3.

[0197] With this configuration, even if the housing is hermetically sealed, the emitted light L1 can be irradiated onto the vibrating element 30 through the transmissive window, and the reference light L2 can be emitted to the outside. Furthermore, the containers 70, 70A, and 70B can be manufactured together with the vibrating element 30 in a wafer-level manufacturing process. For this reason, the optical modulator 12 equipped with the containers 70, 70A, and 70B can be easily manufactured at a reduced cost.

[0198] Furthermore, the surfaces of the transparent windows 71A and 504B have a curved shape. An example of a curved shape is an aspherical shape.

[0199] With this configuration, the transmission windows 71A and 504B can be given not only the function of transmitting the outgoing light L1 and the reference light L2, but also the function of adjusting the direction of propagation of these lights. This makes it possible to narrow the range in which light is incident, thereby miniaturizing the optical system 50, and also reduces the number of parts in the optical system 50 by replacing the function of the collimating lens 3 with the transmission windows 71A and 504B.

[0200] Furthermore, the transmissive window 71B is positioned at an angle to the incident direction of the emitted light L1 (incident light).

[0201] With this configuration, even if the emitted light L1 is reflected by the main surface 711 of the transmissive window 71B (the surface into which the emitted light L1 is incident) and reflected light L4 is generated, the probability of it incident on the photodetector 10 can be reduced. If reflected light L4 is incident on the photodetector 10, it will cause a decrease in the signal-to-noise ratio of the received light signal. Therefore, by using a transmissive window 71B that is installed in an inclined position, the decrease in the signal-to-noise ratio of the received light signal can be suppressed.

[0202] Furthermore, the vibrating element 30 is a quartz crystal resonator, a silicon resonator, or a ceramic resonator. Because these oscillators utilize the resonance phenomenon, they have a high Q factor, making it easy to stabilize their natural frequencies. This allows for an increased signal-to-noise ratio (S / N ratio) of the modulated signal and improved accuracy of the reference signal Ss. As a result, the sample signal originating from the object 14 can be demodulated with a high S / N ratio, enabling the realization of a laser interferometer 1 capable of measuring the velocity and displacement of the object 14 with greater precision.

[0203] Furthermore, the quartz crystal oscillator is preferably a tuning fork type quartz crystal oscillator or an SC-cut quartz crystal oscillator.

[0204] In these oscillators, the out-of-plane vibrations, which are secondary vibrations, easily couple with the in-plane vibrations, which are the primary vibrations, making it easier to secure large displacements for the out-of-plane vibrations. This makes it easy to increase the phase shift B of the modulated signal, and thus improve the signal-to-noise ratio of the received signal.

[0205] Furthermore, the vibrating element 30 may be an oscillator whose main vibration mode is an out-of-plane vibration mode, or it may be an oscillator having a main vibration mode and a secondary vibration mode in which the displacement in the direction intersecting the incident plane of the emitted light L1 (displacement of out-of-plane vibration) is greater than that of the main vibration mode.

[0206] The latter oscillator can be easily manufactured by adjusting the design of an existing oscillator whose main vibration mode is in-plane vibration. Therefore, the latter oscillator can be manufactured at low cost, contributing to the cost reduction of the laser interferometer 1. Furthermore, even if the out-of-plane vibration is excited as a secondary vibration mode, the displacement amplitude of the out-of-plane vibration is sufficiently ensured to satisfy, for example, 0.05 ≤ B. Thus, by using such a vibration element 30, it is possible to realize a laser interferometer 1 with excellent measurement accuracy while reducing costs.

[0207] 4. Modified Optical Systems Next, the first to fourth modified examples of the optical system 50 will be described.

[0208] Figure 16 is a schematic diagram showing the optical system 50A according to the first modified example. Figure 17 is a schematic diagram showing the optical system 50B according to the second modified example. Figure 18 is a schematic diagram showing the optical system 50C according to the third modified example. Figure 19 is a schematic diagram showing the optical system 50D according to the fourth modified example.

[0209] The following describes the first to fourth modified examples of the optical system 50. The following explanation will focus on the differences from the optical system 50 described above, omitting explanations of similar items. In Figures 16 to 19, the same reference numerals are used for items similar to those in Figure 2. Furthermore, some optical elements are omitted from the illustration in Figures 16 to 19.

[0210] The optical system 50A shown in Figure 16 is the same as the optical system 50 shown in Figure 2, except that the light incident on the photodetector 10, the optical modulator 12, and the object 14 is different. Specifically, in the optical system 50A shown in Figure 16, the emitted light L1 (first laser light) is incident on the photodetector 10 and the optical modulator 12. The optical modulator 12 shown in Figure 16 modulates the emitted light L1 to generate a reference light L2 (second laser light) containing the modulation signal. This reference light L2 is then incident on the object 14. The object light L3 (third laser light) containing the sample signal, generated by the reflection of the reference light L2 from the object 14, is then incident on the photodetector 10. Therefore, the photodetector 10 shown in Figure 16 receives the object light L3 containing the sample signal and the modulation signal, as well as the emitted light L1.

[0211] The optical system 50B shown in Figure 17 is the same as the optical system 50A shown in Figure 16, except that the arrangement of the light-receiving element 10, the optical modulator 12, and the object 14 is different.

[0212] The laser interferometers equipped with the optical systems 50A and 50B according to the first and second modified examples described above include a laser light source 2, an optical modulator 12, a photodetector 10, and a demodulation circuit and an oscillation circuit (not shown in Figures 16 and 17). The laser light source 2 emits an outgoing light L1 (first laser light). The optical modulator 12 includes a vibrating element having a vibration component in a direction intersecting the incident plane of the outgoing light L1, and modulates the outgoing light L1 using the vibrating element to generate a reference light L2 (second laser light) including the modulated signal. The photodetector 10 receives the object light L3 (third laser light) including the sample signal and modulated signal generated when the reference light L2 is reflected by the object 14, as well as the outgoing light L1, and outputs a received signal. The demodulation circuit demodulates the sample signal from the received signal based on the reference signal. The oscillation circuit operates using the vibrating element as a signal source and outputs a reference signal to the demodulation circuit.

[0213] With this configuration, the same effects as in the above embodiment can be obtained. That is, an optical modulator 12 capable of frequency modulation can be realized without using a diffraction grating. As a result, the difficulty of manufacturing the optical modulator 12 can be reduced, and the cost of the laser interferometer can be reduced. In addition, a laser interferometer with high measurement accuracy and excellent resistance to disturbances can be obtained.

[0214] The optical system 50C shown in Figure 18 is the same as the optical system 50A shown in Figure 16, except that the arrangement of the optical modulator 12 and the object 14 is different, and the light incident on the photodetector 10, optical modulator 12, and object 14 is different. Specifically, in the optical system 50C shown in Figure 18, the emitted light L1 (first laser light) is incident on the photodetector 10 and the object 14. The emitted light L1 is reflected by the object 14 to generate object light L3 (third laser light). This object light L3 is then incident on the optical modulator 12. The optical modulator 12 shown in Figure 18 modulates the object light L3 to generate reference light L2 (second laser light) which includes the sample signal and the modulation signal. This reference light L2 is incident on the photodetector 10. Therefore, the photodetector 10 shown in Figure 18 receives the reference light L2 which includes the sample signal and the modulation signal, as well as the emitted light L1.

[0215] The optical system 50D shown in Figure 19 is the same as the optical system 50C shown in Figure 18, except that the arrangement of the photodetector 10, the optical modulator 12, and the object 14 is different.

[0216] The laser interferometers equipped with the optical systems 50C and 50D according to the third and fourth modified examples described above include a laser light source 2, an optical modulator 12, a photodetector 10, and a demodulation circuit and an oscillation circuit (not shown in Figures 18 and 19). The laser light source 2 emits an outgoing light L1 (first laser light). The optical modulator 12 includes a vibrating element having a vibration component in a direction intersecting the incident plane of the object light L3 (third laser light), which includes a sample signal generated when the outgoing light L1 is reflected by the object 14. The vibrating element modulates the object light L3 and generates a reference light L2 (second laser light) which includes the modulated signal. The photodetector 10 receives the object light L2, which includes the sample signal and the modulated signal, as well as the outgoing light L1, and outputs a received signal. The demodulation circuit demodulates the sample signal from the received signal based on a reference signal. The oscillation circuit operates using the vibrating element as a signal source and outputs a reference signal to the demodulation circuit.

[0217] With this configuration, the same effects as in the above embodiment can be obtained. That is, an optical modulator 12 capable of frequency modulation can be realized without using a diffraction grating. As a result, the difficulty of manufacturing the optical modulator 12 can be reduced, and the cost of the laser interferometer can be reduced. In addition, a laser interferometer with high measurement accuracy and excellent resistance to disturbances can be obtained.

[0218] Although the laser interferometer of the present invention has been described above based on the illustrated embodiment and its modified form, the laser interferometer of the present invention is not limited to the above embodiment and its modified form, and the configuration of each part can be replaced with any configuration having a similar function. Furthermore, the laser interferometer according to the above embodiment and its modified form may have other arbitrary components added to it.

[0219] In addition to the displacement meter and speed meter described above, the laser interferometer of the present invention can also be applied to, for example, vibration meters, tilt meters, distance meters (length measuring instruments), etc. Further, as applications of the laser interferometer of the present invention, there are optical coherence interferometry technologies that enable distance measurement, 3D imaging, spectroscopy, etc., and optical fiber gyroscopes that realize angular velocity sensors, angular acceleration sensors, etc.

[0220] Also, two or more of the laser light source, optical modulator, and light receiving element may be placed on the same substrate. Thereby, miniaturization and weight reduction of the optical system can be easily achieved, and the ease of assembly can be enhanced.

[0221] Furthermore, the above-described embodiment and its modified examples have a so-called Michelson-type interference optical system, but the laser interferometer of the present invention can also be applied to those having other types of interference optical systems, for example, Mach-Zehnder-type interference optical systems.

Explanation of Reference Signs

[0222] 1…Laser interferometer, 1A…Laser interferometer, 2…Laser light source, 3…Collimating lens, 3a…Collimating lens, 3b…Collimating lens, 3c…Collimating lens, 4…Optical divider, 5…Reflecting element, 6…Half wave plate, 7…Quarter wave plate, 8…Quarter wave plate, 9…Analyzer, 10…Photodetector, 12…Optical modulator, 14…Object, 18…Optical path, 20…Optical path, 22…Optical path, 24…Optical path, 26…Optical fiber, 27…Optical fiber, 30…Vibrating element, 45…Circuit element, 50…Optical system, 50A…Optical system, 50B…Optical system, 50C…Optical system, 50D…Optical system, 51… Sensor head section, 51A... Sensor head section, 51B... Sensor head section, 51C... Sensor head section, 52... Demodulation circuit, 53... Pre-processing section, 54... Oscillation circuit, 55... Demodulation processing section, 70... Container, 70A... Container, 70B... Container, 71A... Transmitting window, 71B... Transmitting window, 72... Container body, 74... Lid, 76... Bonding wire, 301... Base section, 302... Arm section, 303... Electrode, 304... Electrode, 305... Light reflecting surface, 502A... Case, 502B... Case, 502C... Case, 503... Case body, 504A... Transmitting window, 504B... Transmitting window, 505... First case, 5 06…Second case, 507…Wiring board, 508…Wiring board, 509…Wiring board, 531…Current-voltage converter, 532…ADC, 533…ADC, 534…First bandpass filter, 535…Second bandpass filter, 536…First delay regulator, 537…Second delay regulator, 538…Multiplier, 539…Third bandpass filter, 540…First AGC section, 541…Second AGC section, 542…Adder, 551…Multiplier, 552…Multiplier, 553…Phase shifter, 555…First lowpass filter, 556…Second lowpass filter, 557…Divider, 558…Inverse tangent converter Calculator, 559…Output circuit, 700…Through hole, 711…Main surface, 721…First recess, 722…Second recess, C0…Parallel capacitance, C1…Series capacitance, C3…Third capacitor, Cd…Second capacitor, Cg…First capacitor, GND…GND terminal, L1…Emitted light, L1…Series inductance, L1a…First split light, L1b…Second split light, L2…Reference light, L3…Object light, L4…Reflected light, R1…Equivalent series resistance, Rd…Limiting resistor, Rf…Feedback resistor, S1…First signal, S2…Second signal, Sd…Drive signal, Ss…Reference signal, Vcc…Terminal, X1…Terminal, X2…Terminal, Y…Terminala...Natural frequency, b...Natural frequency, c...Natural frequency, d...Natural frequency, e...Natural frequency, f, Q ...natural frequency, f osc ...oscillation frequency, jp1...branching point, jp2...branching point, ps1...first signal path, ps2...second signal path, x...signal x, y...signal y, θ...tilt angle

Claims

1. A laser light source that emits the first laser beam, An optical modulator comprising a vibrating element having a vibration component in a direction intersecting the incident plane of the first laser beam, modulating the first laser beam using the vibrating element to generate a second laser beam including a modulated signal, A photodetector receives a third laser beam containing a sample signal generated when the first laser beam is reflected by an object, and the second laser beam, and outputs a received signal. A demodulation circuit that demodulates the sample signal from the received light signal based on a reference signal, An oscillator circuit that operates using the aforementioned vibration element as a signal source and outputs the reference signal to the demodulation circuit, A laser interferometer characterized by being equipped with the following features.

2. A laser light source that emits the first laser beam, An optical modulator comprising a vibrating element having a vibration component in a direction intersecting the incident plane of the first laser beam, modulating the first laser beam using the vibrating element to generate a second laser beam including a modulated signal, A photodetector receives a sample signal generated by the reflection of the second laser beam from an object, a third laser beam including the modulated signal, and the first laser beam, and outputs a received signal. A demodulation circuit that demodulates the sample signal from the received light signal based on a reference signal, An oscillator circuit that operates using the aforementioned vibration element as a signal source and outputs the reference signal to the demodulation circuit, A laser interferometer characterized by being equipped with the following features.

3. A laser light source that emits the first laser beam, An optical modulator comprising a vibrating element having a vibration component in a direction intersecting the incident plane of a third laser beam, which includes a sample signal generated when the first laser beam is reflected by an object, modulates the third laser beam using the vibrating element to generate a second laser beam including the modulated signal, A photodetector receives the second laser beam, which includes the sample signal and the modulation signal, and the first laser beam, and outputs a received signal. A demodulation circuit that demodulates the sample signal from the received light signal based on a reference signal, An oscillator circuit that operates using the aforementioned vibration element as a signal source and outputs the reference signal to the demodulation circuit, A laser interferometer characterized by being equipped with the following features.

4. The optical modulator comprises a housing having a housing portion for housing the vibrating element, The laser interferometer according to any one of claims 1 to 3, wherein the housing is depressurized.

5. The laser interferometer according to claim 4, wherein the housing separates the housing from the outside and has a transmission window through which the first laser beam, the second laser beam, and the third laser beam can pass.

6. The laser interferometer according to claim 5, wherein the surface of the transmission window has a curved shape.

7. The laser interferometer according to claim 5 or 6, wherein the transmission window is provided in a position inclined with respect to the direction of incidence of the incident light.

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

9. The laser interferometer according to claim 8, wherein the quartz crystal oscillator is a tuning fork type quartz crystal oscillator or an SC cut quartz crystal oscillator.

10. The laser interferometer according to any one of claims 1 to 9, wherein the vibrating element has a primary vibration mode and a secondary vibration mode in which the displacement in the direction intersecting the incident surface is greater than that of the primary vibration mode.

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