Laser Doppler vibrometer and vibration measurement method

Abstract

To provide a simple and compact configuration that is tolerant to deviations in angle θ and working distance D. [Solution] The device is configured to include a light source unit that splits laser light into two to generate illumination light and local light, an interference light detection unit that has a photodetector and irradiates a measurement object with the illumination light, causes the measurement light generated by reflection of the illumination light from the measurement object to interfere with the local light to generate interference light, receives the interference light with the photodetector, and generates an interference signal, which is an electrical signal, and a signal processing unit that acquires information about the vibration of the measurement object from the interference signal. At the light receiving surface of the photodetector, the beam diameter of the local light that constitutes the interference light is larger than the light receiving diameter of the photodetector, and the beam diameter of the measurement light that constitutes the interference light is sufficiently smaller than the light receiving diameter of the photodetector.

Description

[Technical Field]

[0001] This invention relates to a laser Doppler vibrometer and vibration measurement method that utilizes the Doppler effect of light to measure minute vibrations of an object to be measured, and can be used for predictive maintenance and inspection of machinery and equipment installed in factories and plants, as well as various infrastructures and structures. [Background technology]

[0002] Conventional laser Doppler vibrometers use an optical interferometer to detect (heterodyne detection) the phase difference between reflected light (also called measurement light) generated when a portion of a laser beam is irradiated onto a vibrating object being measured, and reference light (also called local light) that is a frequency-shifted portion of the laser beam. This allows the phase fluctuation caused by the Doppler effect in the measurement light to be detected, and as a result, the vibration of the object being measured can be measured (see, for example, Patent Document 1).

[0003] The laser Doppler vibrometer disclosed in Patent Document 1 enables non-contact measurement by irradiating light from a distance, making it possible to measure the vibration of distant objects or objects under high temperatures and strong magnetic fields, which is difficult to do with contact-type vibration sensors that are widely used in vibration measurement. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2001-159560 [Non-patent literature]

[0005] [Non-Patent Document 1] Anthony A. Tovar and Lee W. Casperson, "Generalized beam matrices: gaussian beam propagation in misaligned complex optical systems," J. Opt. Soc. Am. A 12, pp. 1522-1533 (1995). [Non-patent document 2] Kenji Kono, "Fundamentals and Applications of Optical Coupling Systems for Optical Devices," Chapter 3, Gendai Kogakusha, ISBN-13: 978-4874721513 (1991.1) Summary of the Invention [Problem to be solved by the invention]

[0006] Laser Doppler vibrometers use the interference of light to measure vibrations. Therefore, to obtain accurate vibration measurement data, it is essential to have an optically stable optical system that is designed, installed, and adjusted with high precision.

[0007] However, when measuring vibration in an actual field environment, it is not always possible to perform the above-mentioned high-precision adjustment of the optical system. Therefore, there is a need for the development of a laser Doppler vibrometer that does not require high-precision adjustment of the optical system, is easy to set up, and allows for easy establishment of a data acquisition environment, yet is capable of stably measuring vibration with the required accuracy.

[0008] For example, it is desirable that the distance between the Doppler laser vibrometer device body and the object to be measured is not limited to a specific range, but rather that measurements can be made even if there is some deviation from the optimal value. More specifically, the distance between the Doppler laser vibrometer device body and the object to be measured is the distance between the object to be measured and the objective lens, which is the part of the device body that outputs laser light as irradiation light to be irradiated onto the object to be measured and inputs reflected light. Here, this distance is sometimes referred to as the working distance.

[0009] Eliminating the need for high-precision adjustment of the optical system makes it possible to provide a laser Doppler vibrometer that is easier to use and can be easily adapted to the on-site environment. For example, a person or robot can hold the laser Doppler vibrometer and measure the vibrations of the object by shining the laser light emitted from the device onto the object.

[0010] One of the challenges in developing a laser Doppler vibrometer that can be easily adapted to such field environments is to increase the working distance and the tolerance for deviations in the installation angle of the object being measured.

[0011] For example, when irradiated light emitted from a point on a plane strikes a measurement object (assuming the measurement object is a mirror) located a working distance D away at an angle θ to the normal, is reflected, and returns to the plane from which the irradiated light was emitted, the beam center of the reflected light is shifted by Dtan(2θ) from the point from which the irradiated light was emitted.

[0012] Laser Doppler vibrometers measure vibration by causing reflected light to interfere with separately prepared localized light and detecting the interference signal (beat signal). If the positional deviation of the reflected light, which is the returning light, becomes large, the spatial overlap between the reflected light and the localized light becomes small, resulting in a smaller beat signal. Furthermore, if the reflected light returns outside the light-receiving diameter of the photodetector that receives the interference light, it becomes impossible to detect the interference signal in the first place.

[0013] For example, when a person or robot holds a laser Doppler vibrometer for measurement, it is difficult to control the angle θ and working distance D with high precision. This can lead to issues such as unstable vibration measurements or even the inability to perform measurements at all.

[0014] To make the device resistant to deviations in the angle θ and working distance D, there are methods such as deflecting the output direction of the irradiated light (steering) or using a lens system with a variable focal length. However, these methods have the problem of increasing the size and cost of the device.

[0015] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a laser Doppler vibrometer and a vibration measurement method that are tolerant to deviations in the angle θ and the working distance D and that can be configured simply and compactly. [Means for solving the problem]

[0016] In order to achieve the above-mentioned object, the laser Doppler vibrometer of the present invention is configured to include a light source unit that splits a laser beam into two to generate illumination light and local light, an interference light detection unit having a photodetector that irradiates a measurement object with the illumination light and causes interference between measurement light generated by reflection of the illumination light on the measurement object and the local light, receives the interference light with the photodetector, and generates an interference signal that is an electrical signal, and a signal processing unit that acquires information about the vibration of the measurement object from the interference signal. At a light receiving surface of the photodetector, the beam diameter of the local light that constitutes the interference light is larger than the light receiving diameter of the photodetector, and the beam diameter of the measurement light that constitutes the interference light is sufficiently smaller than the light receiving diameter of the photodetector.

[0017] According to a preferred embodiment of the laser Doppler vibrometer of the present invention, the interference light detection unit includes an optical branching / combining circuit, a concave lens, and a condenser lens. The local light is sent to the photodetector via the concave lens, the optical branching / combining circuit, and the condenser lens. The optical branching / combining circuit outputs the irradiation light input from the light source unit to irradiate the measurement object. The measurement light is sent to the photodetector via the optical branching / combining circuit and the condenser lens.

[0018] According to a further preferred embodiment of the laser Doppler vibrometer of the present invention, the optical branching / combining circuit has first to fourth input / output ports, the illumination light is input to the first input / output port and output from the third input / output port, the measurement light is input to the third input / output port and output from the fourth input / output port and sent to the photodetector, and the local light is input to the second input / output port and output from the fourth input / output port and sent to the photodetector.

[0019] Here, the optical branching / combining circuit is preferably a non-polarizing beam splitter.

[0020] In addition, the optical branching / combining circuit is a polarizing beam splitter, and the interference light detection unit includes a polarization rotator in the optical path between the optical branching / combining circuit and the object to be measured, which rotates the polarization plane of the input light and output light by 90° as the light travels back and forth along the optical path, and a polarizer in the optical path between the optical branching / combining circuit and the photodetector, which passes polarized light in a direction tilted 45 degrees from the polarization plane when the light enters and leaves the polarization rotator.

[0021] Furthermore, according to a preferred embodiment of the laser Doppler vibrometer of the present invention, the light source unit includes a laser light source that generates laser light, and an optical branching circuit that is a non-polarizing beam splitter that branches the laser light into a first light that serves as irradiation light and a second light that serves as local light.

[0022] According to a preferred embodiment of the laser Doppler vibrometer of the present invention, the irradiating light and the local light have polarized lights orthogonal to each other, the optical branching / combining circuit is a polarizing beam splitter, and the optical branching / combining circuit has first to fourth input / output ports, the irradiating light sent from the light source unit is input to the first input / output port and output from the third input / output port, the measuring light is input to the third input / output port and output from the fourth input / output port and sent to the photodetector, the local light sent from the light source unit is input to the first input / output port and output from the second input / output port, and the local light input to the second input / output port is output from the fourth input / output port. The interference light detection unit includes a first polarization rotator in an optical path between the optical branching / combining circuit and the object to be measured, which rotates the plane of polarization of the input light and the output light by 90° as the light travels back and forth along the optical path, a total reflection mirror which reflects the light output from the second input / output port of the optical branching / combining circuit and inputs it to the second input / output port of the optical branching / combining circuit, a second polarization rotator in an optical path between the optical branching / combining circuit and the total reflection mirror, which rotates the plane of polarization of the input light and the output light by 90° as the light travels back and forth along the optical path, and a polarizer in an optical path between the optical branching / combining circuit and the photodetector, which passes polarized light in a direction tilted 45° from the plane of polarization when the light enters and leaves the first polarization rotator.

[0023] Furthermore, according to a preferred embodiment of the laser Doppler vibrometer of the present invention, the light source unit includes a laser light source that generates laser light, an optical branching circuit that is a non-polarizing beam splitter that branches the laser light into a first light that serves as irradiation light and a second light that serves as local light, a polarization rotator that rotates the polarization plane of the second light by 90°, and an optical branching / combining circuit that is a polarizing beam splitter that has first to third input / output ports, outputs the first light input to the first input / output port from the third input / output port as irradiation light, and outputs the second light input to the second input / output port from the third input / output port as local light.

[0024] In order to achieve the above-mentioned object, a vibration measurement method of the present invention includes the steps of: splitting a laser beam into two to generate illumination light and local light; irradiating a measurement object with the illumination light, causing the measurement light generated by reflection of the illumination light on the measurement object to interfere with the local light to generate interference light; receiving the interference light with a photodetector to generate an interference signal, which is an electrical signal; and acquiring information about the vibration of the measurement object from the interference signal. At a light-receiving surface of the photodetector, the beam diameter of the local light constituting the interference light is larger than the light-receiving diameter of the photodetector, and the beam diameter of the measurement light constituting the interference light is sufficiently smaller than the light-receiving diameter of the photodetector.

[0025] According to a preferred embodiment of the vibration measuring method of the present invention, the local light has a beam diameter expanded by a concave lens and then interferes with the measurement light.

[0026] According to a preferred embodiment of the vibration measuring method of the present invention, the irradiating light, the local light, and the measuring light are common polarized light components.

[0027] According to another preferred embodiment of the vibration measuring method of the present invention, the illumination light and the local light are generated with a common polarization component, the local light is polarization-rotated by 90°, and then interferes with the measurement light, and the polarization components of the local light and the measurement light, each tilted by 45°, are received on the light-receiving surface of the photodetector.

[0028] According to another preferred embodiment of the vibration measuring method of the present invention, the illumination light and the local light are generated as orthogonal polarization components, the local light is rotated in polarization by 90°, and the measurement light is rotated in polarization by 90° relative to the illumination light, and then they interfere with each other, and the polarization components of the local light and the measurement light that are tilted by 45° are received at the light receiving surface of the photodetector. [Effects of the Invention]

[0029] According to the present invention, it is possible to provide a simple and compact laser Doppler vibrometer that is tolerant to deviations in the angle θ and the working distance D. [Brief explanation of the drawings]

[0030] [Figure 1] FIG. 2 is a schematic diagram for explaining a first vibrometer. [Figure 2] 3 is a schematic diagram for explaining the relationship between local light and measurement light on the light receiving surface of a photodetector. FIG. [Figure 3] FIG. 10 is a diagram showing the results of a simulation calculation. [Figure 4] FIG. 10 is a diagram showing the results of a verification experiment. [Figure 5] FIG. 4 is a schematic diagram for explaining a second vibrometer. [Figure 6] FIG. 10 is a schematic diagram for explaining a third vibrometer. DETAILED DESCRIPTION OF THE INVENTION

[0031] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and positional relationship of each component are merely shown in a schematic manner to enable understanding of the present invention. Furthermore, while preferred configuration examples of the present invention will be described below, they are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes and modifications can be made that achieve the effects of the present invention without departing from the scope of the configuration of the present invention.

[0032] The laser Doppler vibrometer of this invention detects vibrations of a measurement object from interference light between localized light and measurement light reflected from the measurement object. In this laser Doppler vibrometer, the beam diameter of the localized light constituting the interference light is set larger than the light-receiving diameter on the light-receiving surface of a photodetector that receives the interference light. The measurement light is set sufficiently smaller than the light-receiving diameter on the light-receiving surface of the photodetector.

[0033] Here, an example of heterodyne detection in which measurement light and frequency-shifted local light are interfered with each other will be described, but the basic effect of this invention is not limited to heterodyne detection and can also be applied to homodyne detection.

[0034] (First vibration meter) A laser Doppler vibrometer (hereinafter also referred to as a first vibrometer) according to a first embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is a schematic diagram for explaining the first vibrometer.

[0035] The first vibrometer 101 includes a light source unit 201, an interference light detection unit 301, and a signal processing unit 401. The light source unit 201 generates irradiation light to be irradiated onto the measurement object 1000 and local light. The interference light detection unit 301 receives interference light between the local light and reflected light (measurement light) generated when the irradiation light is reflected on the measurement object 1000, and obtains an interference signal that is a photoelectric conversion signal. The signal processing unit 401 acquires information about the vibration of the measurement object 1000 from the interference signal.

[0036] The light source unit 201 includes a laser light source 210 , an optical branching circuit 220 , and a frequency shifter 230 .

[0037] The laser light source 210 is configured using any suitable conventional light source that generates laser light. The laser light generated by the laser light source 210 is sent to the optical branching circuit 220 via a first optical path L1.

[0038] The optical branching circuit 220 is configured using any suitable conventionally known optical circuit, such as an optical coupler, that branches and outputs input light. The optical branching circuit 220 has first to third input / output ports 221 to 223. Laser light transmitted from the laser light source 210 via the first optical path L1 is input to the first input / output port 221 of the optical branching circuit 220. The light input to the first input / output port 221 is branched into two, a first light and a second light, at a set appropriate branching ratio, and output from the second input / output port 222 and the third input / output port 223 to the second optical path L2 and the third optical path L3, respectively.

[0039] The first light output from the second output port 222 of the optical branching circuit 220 to the second optical path L2 becomes the illumination light that is illuminated onto the measurement object 1000. The illumination light propagating through the second optical path L2 is sent to the interference light detection unit 301.

[0040] The second light output from the third output port 223 of the optical branching circuit 220 to the third optical path L3 is sent to a frequency shifter 230 arranged on the third optical path L3. The frequency shifter 230 is, for example, a commercially available acousto-optic modulator (AOM). The frequency shifter 230 shifts the frequency (f shift ) The light obtained by frequency-shifting the second light in the frequency shifter 230 becomes the local light for heterodyne detection. The local light propagating through the third optical path L3 is sent to the interference light detection unit 301.

[0041] When the wavelength of the laser light source is in a communication wavelength band such as the 1.5 μm band or the 1.3 μm band, the light source unit 201 can be configured by using optical fiber coupled modules as each component and connecting these optical fiber coupled modules. In this case, the first to third optical paths L1 to L3 are configured with optical fibers.

[0042] The interference light detection unit 301 includes an optical branching / combining circuit 310, a concave lens 320, a condensing lens 330, and a photodetector 340. Note that it is desirable that the light input to the optical branching / combining circuit 310 is collimated light. Therefore, the optical branching / combining circuit 310 preferably includes a first collimating lens 351 and a second collimating lens 352. In this case, the first collimating lens 351 and the second collimating lens 352 are provided on the side of the interference light detection unit 301 where the illumination light and the local light are input.

[0043] The illumination light sent to the interference light detection unit 301 via the second optical path L2 is input to the first collimating lens 351 to be collimated, and then sent to the optical branching / combining circuit 310 via the fourth optical path L4. The local light sent to the interference light detection unit 301 via the third optical path L3 is input to the second collimating lens 352 to be collimated, and then sent to the optical branching / combining circuit 310 via the seventh optical path L7.

[0044] The optical branching / combining circuit 310 has first to fourth input / output ports 311 to 314. The illumination light sent to the interference light detection unit 301 propagates through the fourth optical path L4 and is input to the first input / output port 311 of the optical branching / combining circuit 310. A portion of the light input to the first input / output port 311 is output from the third input / output port 313 to the fifth optical path L5. The illumination light output to the fifth optical path L5 propagates through the fifth optical path L5 and is irradiated onto the measurement object 1000 arranged at the end of the fifth optical path L5.

[0045] The reflected light (measurement light) from the measurement object 1000 propagates through the fifth optical path L5 and is input to the third input / output port 313 of the optical branching / combining circuit 310. A portion of the light input to the third input / output port 313 is output to the sixth optical path L6 from the fourth input / output port 314 of the optical branching / combining circuit 310. The measurement light output to the sixth optical path L6 propagates through the sixth optical path L6 and is sent to the photodetector 340 arranged at the end of the sixth optical path L6.

[0046] The local light sent to the interference light detection unit 301 propagates through the seventh optical path L7 and is input to the second input / output port 312 of the optical branching / combining circuit 310. A portion of the light input to the second input / output port 312 is output to the sixth optical path L6 from the fourth input / output port 314. The local light output to the sixth optical path L6 propagates through the sixth optical path L6 and is sent to the photodetector 340 arranged at the end of the sixth optical path L6.

[0047] Here, the optical branching / combining circuit 310 is preferably configured as a free-space type optical branching circuit in which input and output to each port are performed via collimated light propagating in free space.

[0048] Here, the optical branching circuit 220 provided in the light source unit 201 and the optical branching / combining circuit 310 provided in the interference light detection unit 301 are assumed to be non-polarized beam splitters, and the local light, irradiation light, and measurement light are assumed to be linearly polarized light in the same polarization direction (e.g., p-polarized light).

[0049] The condenser lens 330 is provided on the sixth optical path L6 between the optical branching / combining circuit 310 and the photodetector 340. The condenser lens 330 serves to condense the measurement light output from the fourth input / output port 314 of the optical branching / combining circuit 310 onto the light receiving surface of the photodetector 340. Lens parameters such as the focal length of the condenser lens 330 are design matters, and may be appropriately designed depending on the light receiving aperture of the photodetector 340, the focal length of the concave lens 320 described below, and the like.

[0050] The concave lens 320 is provided on the seventh optical path L7 between the second collimator lens 352 and the optical branching / combining circuit 310. The local light passes through a lens train made up of the second collimator lens 352, the concave lens 320, the optical branching / combining circuit 310, and the condenser lens 330. The lens parameters of the concave lens 320 are designed so that the local light is collimated on the light-receiving surface of the photodetector 340 and the beam diameter is larger than the light-receiving diameter of the photodetector 340.

[0051] The measurement light output from the fourth input / output port 314 of the optical branching / combining circuit 310 interferes with the local light, and the interference light passes through the sixth optical path L6 and is received by the photodetector 340, where it is photoelectrically converted. The photodetector 340 has a function of converting the received optical signal into an electrical signal. Any suitable conventional photodiode (PD) can be used as the photodetector 340. The analog electrical signal obtained by photoelectric conversion in the photodetector 340 has a frequency f shift The phase change of this beat signal is the phase change caused by the vibration of the measurement object 1000. The interference signal, which is an analog electrical signal generated by the photodetector 340, is sent to the signal processing unit 401.

[0052] The signal processing unit 401 includes an analog-to-digital converter (ADC) 410 and a signal processing circuit 420 .

[0053] The ADC 410 digitizes the interference signal, which is an analog electrical signal, to generate a digital electrical signal. The digital electrical signal generated by the ADC 410 is sent to the signal processing circuit 420.

[0054] The signal processing circuit 420 can be configured with any suitable conventionally known electronic computer, such as a personal computer. The signal processing circuit 420 performs arithmetic operations, such as filtering, on the digital electrical signal to obtain a result of vibration measurement of the vibrating object 1000. Note that a conventionally known technique can be used to obtain a result of vibration measurement of the vibrating object based on a digital electrical signal obtained from interference light obtained by heterodyne detection of the measurement light and the local light, and therefore a description thereof will be omitted here.

[0055] As described above, if the reflecting surface of the measurement object 1000 is tilted with respect to the optical axis direction of the incident irradiation light, the center position and angle of the reflected light (measurement light) beam will be shifted. As a result, when the measurement light reaches the light-receiving surface of the photodetector 340, the center position and angle of the beam will be shifted.

[0056] The relationship between the local light and the measurement light on the light-receiving surface of the photodetector 340 will be described with reference to Fig. 2. Figs. 2(A) and 2(B) are schematic diagrams for explaining the relationship between the local light and the measurement light on the light-receiving surface of the photodetector 340. Fig. 2(A) shows a case where the beam diameter of the local light and the beam diameter of the measurement light are approximately the same on the light-receiving surface of the photodetector 340. On the other hand, Fig. 2(B) shows a case where the beam diameter of the local light is larger than the beam diameter of the measurement light on the light-receiving surface of the photodetector 340. That is, Fig. 2(A) corresponds to a conventional laser Doppler vibrometer, and Fig. 2(B) corresponds to the first vibrometer.

[0057] As shown in Figure 2(A), when the beam diameter of the local light and the beam diameter of the measurement light are approximately the same on the light-receiving surface of the photodetector 340, even a slight shift in the beam position of the measurement light will result in a large change in the spatial overlap of the measurement light and the local light. The intensity of the interference light detected by the photodetector 340 is proportional to the overlap integral on the light-receiving surface of the measurement light and the local light. In other words, the beat signal that is finally detected is proportional to the beam tilt θ caused by the tilt of the measurement object 1000. tot As a result, the signal-to-noise (SN) ratio of the vibration measurement results changes significantly. In this way, with conventional laser Doppler vibrometers, the measurement results become unstable when the beam position of the measurement light is shifted due to factors such as the tilt of the measurement object 1000, making vibration measurement difficult.

[0058] On the other hand, as shown in Figure 2(B), if the beam diameter of the local light is sufficiently larger than that of the measurement light, even if the beam position of the measurement light is slightly shifted, the overlap integral of the spatial overlap of the measurement light and the local light does not change much. In other words, the beat signal finally detected does not change much. As a result, the vibration measurement does not change much even if the measurement object 1000 is tilted, allowing for stable vibration measurement.

[0059] As described above, the first vibrometer enables stable vibration measurement without significant deterioration of the desired beat signal strength (amplitude) of the interference light, even if the measurement object is slightly tilted.

[0060] It is most desirable that the vibration measurement can be performed stably if the beam position of the measurement light is within the light receiving surface of the photodetector 340. Therefore, the beam diameter ω of the local light L is the light receiving diameter d on the light receiving surface of the photodetector 340. PD Greater than (ω L >d PD ) is desirable. As for the measurement light, the beam diameter ω s is ω L and d PD is sufficiently smaller than (ω s <<ω L , dPD ) is desirable.

[0061] The focal lengths and positions of the concave lens 320, the condenser lens 330, the first collimator lens 351 and the second collimator lens 352 may be designed and arranged so as to satisfy the above conditions.

[0062] Furthermore, if the illumination light propagating through the fifth optical path L5 is collimated when it exits the third input / output port 313 of the optical branching / combining circuit 310, the illumination light and the measurement light, which is the illumination light reflected from the object to be measured, propagate through the fifth optical path L5 in a substantially collimated state. In this case, even if the length (working distance) D of the fifth optical path L5 changes slightly, the beam parameters (beam diameter and curvature) of the measurement light when it finally reaches the photodetector 340 do not change significantly. In other words, tolerance to changes in the working distance D can be achieved. The lens parameters, such as the focal length of the first collimating lens 351, that satisfy this condition are a design matter, and a lens appropriately designed according to the desired working distance tolerance can be used.

[0063] Next, the effect of the first vibrometer was verified by numerical calculations and experimental demonstrations. Here, it is assumed that the beam shapes of the local light and the irradiated light are Gaussian beams. This assumption generally holds when the output light from a general laser light source 210 propagates through an optical system such as a circularly symmetric lens system. The propagation of a Gaussian beam when the Gaussian beam has a positional or angular misalignment is discussed in Non-Patent Document 1, although within the scope of paraxial approximation.

[0064] According to Non-Patent Document 1, the propagation characteristics of a Gaussian beam propagating through an optical system such as a lens (array) or space with an optical axis misalignment (position or angle) can be discussed using a 3x3 system matrix, expressed by the following equation (1), which is a more generalized system matrix (also called an ABCD matrix).

[0065]

number

[0066] Here, parameters A to D are the same as parameters A to D in the conventional system matrix (ABCD matrix), and G and H are parameters that reflect the optical axis deviation of the optical system.

[0067] The beam parameter q2 and the displacement parameter S2, which represents the positional and angular deviations of the Gaussian beam after it has propagated through this optical system, are expressed by the following equations (2) and (3).

[0068]

number

[0069]

number

[0070] Here, q1 and S1 are the beam parameter and displacement parameter, respectively, of the Gaussian beam input to the optical system. The beam parameter q and displacement parameter S are determined by the beam diameter ω, curvature R, and beam center deviation d of the Gaussian beam. a , and slope d a ' is given by the following equations (4) and (5).

[0071]

number

[0072]

number

[0073] where β0 is the wave number.

[0074] By calculating the above equations (2) to (5) for the optical system given by the above equation (1), the beam parameters (beam diameter and curvature R) and displacement parameters (positional deviation and angular deviation) of the Gaussian beam at the time of output from the optical system can be calculated.

[0075] In the first vibrometer, the local light does not irradiate the measurement object 1000 but remains within the device. Therefore, the beam position and angle can be controlled to be constant relative to the light receiving surface of the photodetector 340. Furthermore, the position and angle of the irradiated light can also be controlled to be constant until it irradiates the measurement object 1000.

[0076] Therefore, for the local light and the light irradiated onto the measurement object 1000, the beam parameters can be calculated using a normal ABCD matrix (i.e., G=H=0) without any positional or angular deviation.

[0077] On the other hand, the irradiated light (measurement light) reflected from the measurement object 1000 undergoes an angular shift due to the inclination of the measurement object. m When the reflection is inclined, the system matrix representing this reflection can be expressed by the following equation (6).

[0078]

number

[0079] The system matrix when the measurement light passes through the fifth optical path L5, the optical branching / combining circuit 310, and the sixth optical path L6 is given by the following equation (7).

[0080]

number

[0081] At this time, the positional deviation x0 and the inclination tan(θ tot ) is given by the following equations (8) and (9).

[0082]

number

[0083]

number

[0084] By using the above equations (1) to (9), it is possible to simulate the beam parameters, positional deviation, and angular deviation of the Gaussian beam when the local light and measurement light reach the light receiving surface of the photodetector 340.

[0085] Non-Patent Document 2 discloses the overlap integral of two Gaussian beams when there are beam parameters and positional and angular misalignments. While the main purpose of Non-Patent Document 2 is to model the coupling efficiency when a Gaussian beam is coupled to an optical fiber, the theory can be applied to this invention as is. However, since the amplitude of the beat signal in an optical interferometer is proportional to the square root of the product of the intensities of the local light and the measurement light, the square root of the coupling efficiency discussed in Non-Patent Document 2 can be considered to be proportional to the amplitude of the beat signal in this case.

[0086] Now, when considering a coordinate system based on the optical axis and center of the local light, the optical electric field distribution of the local light in the x-axis direction is given by the following equation (10).

[0087]

number

[0088] Here, the traveling direction of light is defined as the z direction, and the light receiving surface of the photodetector 340 is defined as the xy plane. The optical electric field distribution of the measurement light shifted in position by x0 and inclination by tan θ relative to the coordinate system of the local light is given by the following equation (11).

[0089]

number

[0090] In this case, the coupling efficiency is the square of the integral of the product of the conjugate components in the xy plane in the above equations (10) and (11). The strength of the beat signal is proportional to the coupling efficiency, and the amplitude of the beat signal is proportional to the square root of the coupling efficiency.

[0091] By substituting the positional and angular misalignments obtained by the above equations (8) and (9) for the positional and angular misalignments in the above equation (11) to calculate the in-plane integral, the amplitude and intensity of the beat signal generated by the interference light can be simulated. However, the light-receiving area of ​​the actual photodetector 340 is finite. To take this effect into account, the distribution of the in-plane detection efficiency of the photodetector 340 can be incorporated into the calculation of the in-plane integral.

[0092] An example of the calculation results is shown in Figure 3. In Figure 3, the horizontal axis represents the tilt (unit: °) of the reflector, which is the measurement object, and the vertical axis represents the beat signal strength (unit: dB). In Figure 3, the dotted line represents the case of a configuration similar to the first vibrometer, while the solid line represents the case of a configuration in which the concave lens is removed from the first vibrometer.

[0093] Here, the beat signal intensity on the vertical axis is normalized to the value in the absence of a concave lens in an ideal state where the beam diameters of the local light and the measurement light are the same and there is no center or angular misalignment between them.

[0094] Here, the lens parameters and other simulation conditions used in the calculations were: a laser light wavelength of 1550 nm; second optical path L2 and third optical path L3 were communication optical fibers with a mode field diameter of 10.4 μm; first and second collimating lenses 351 and 352 were Thorlabs fiber collimators with a focal length of 18.75 mm and equivalent to F280-1550; concave lens 320 was a Thorlabs plano-concave lens with a focal length of −50 mm and equivalent to LC1439-C; condenser lens 330 was a Thorlabs plano-convex lens with a focal length of +100 mm and equivalent to LA1509-C; and a working distance of 500 mm. The photodetector 340 had a light receiving diameter of 2000 μm (full width at half maximum), and the in-plane detection efficiency distribution was assumed to be a second-order super-Gaussian distribution. The distance between the concave lens 320 and the condenser lens 330 was set to about 50 mm, which is the length for collimating the local light.

[0095] When the local light was focused without the concave lens 320, as in the conventional method, the beam diameter of the local light and the measurement light was approximately 57 μm. In this case, as shown by the solid line in Figure 3, the beat signal intensity rapidly deteriorates as the tilt of the measurement object increases.

[0096] 3, the maximum beat signal intensity using the first vibrometer is about 26 dB (about 1 / 20 of the amplitude) lower than that shown by the solid line (when no concave lens is provided), but the deterioration is not significant even when the angle deviation is about 0.1 degrees. In the optical system of the first vibrometer, the beam diameter of the measurement light when it reaches the light-receiving area of ​​the photodetector 340 is about 57 μm. Meanwhile, the beam diameter of the localized light at this time is about 2400 μm, which is expanded to about the light-receiving diameter of the photodetector 340 (2000 μm diameter).

[0097] Note that the maximum value of the beat signal intensity of the first vibrometer (shown by the dotted line) is lower than the result when the localized light is focused (shown by the solid line), because only a part of the localized light overlaps with the measurement light.

[0098] On the other hand, the beat signal intensity is proportional to the intensity of the local light, so the reduction in beat signal intensity can be improved by increasing the intensity of the local light, for example, by appropriately adjusting the branching ratio of the optical branching circuit 120 or the optical branching / combining circuit 310.

[0099] The results of a verification experiment conducted based on the calculation results of Fig. 3 are shown in Fig. 4. Fig. 4 is a diagram showing the results of the verification experiment.

[0100] In Figure 4, the horizontal axis represents the tilt (unit: °) of the reflector, which is the object to be measured, and the vertical axis represents the beat signal strength (unit: dB). In Figure 4, the open circles (◯) represent the results of a verification experiment using a configuration in which the concave lens is removed from the first vibrometer, and the solid line represents the results of numerical calculations corresponding to this verification experiment. Also, in Figure 4, the closed circles (●) represent the results of a verification experiment using the first vibrometer configuration with the concave lens, and the dotted line represents the results of numerical calculations corresponding to this verification experiment.

[0101] The lens parameters used here are the same as those used in the numerical calculation of Fig. 4. However, due to experimental constraints, the photodetector 340 used had a light receiving diameter of 200 µm.

[0102] As shown by the black circles and dotted lines in Figure 4, the experimental results for the first vibrometer with a concave lens and the corresponding numerical calculation results are in excellent agreement.

[0103] The interference light on the light receiving surface of the photodetector 340 is detected by the photodetector 340 and converted into an electrical signal. This electrical signal after photoelectric conversion has a frequency f shift The signal contains a beat signal whose phase change corresponds to the phase change caused by the vibration of the measurement object, etc. Then, the analog-to-digital converter 410 and the signal processing circuit 420 perform arithmetic operations such as filtering similar to conventional heterodyne detection to obtain the desired vibration measurement results.

[0104] The first vibrometer can provide a laser Doppler vibrometer with excellent tolerance to angular and positional deviations. Its components (optical branching circuits and lenses) are similar to those of conventional laser Doppler vibrometers and can be constructed from commercially available optical components. In other words, it can provide a low-cost, highly stable laser Doppler vibrometer with excellent tolerance to angular and positional deviations.

[0105] (Second vibration meter) A laser Doppler vibrometer according to a second embodiment of the present invention (hereinafter also referred to as a second vibrometer) will be described with reference to Fig. 5. Fig. 5 is a schematic diagram for explaining the second vibrometer.

[0106] The first and second vibrometers are different in the configuration of the interference light detection unit, but are otherwise similar in configuration, and therefore, illustrations and detailed descriptions of parts configured similarly to the first vibrometer may be omitted.

[0107] The second vibrometer 102 differs from the first vibrometer in that the optical branching / combining circuit 360 of the interference light detecting section 302 is a polarizing beam splitter.

[0108] The optical branching / combining circuit 360 has first to fourth input / output ports 361 to 364. When light is input to the first input / output port 361 of the optical branching / combining circuit 360, the p-polarized component is output from the third input / output port 363, and the s-polarized component is output from the second input / output port. When light is input to the second input / output port 362 of the optical branching / combining circuit 360, the p-polarized component is output from the fourth input / output port 364, and the s-polarized component is output from the first input / output port 361. When light is input to the third input / output port 363 of the polarizing beam splitter 360, the p-polarized component is output from the first input / output port 361, and the s-polarized component is output from the fourth input / output port 364.

[0109] The second vibrometer 102 also includes a polarization rotator 370 on a fifth optical path L5 along which light output from the third input / output port 363 of the optical branching / combining circuit 360 or light input to the third input / output port propagates. The polarization rotator 370 rotates the polarization direction of linearly polarized light by 90° as the light travels back and forth through the polarization rotator 370. Therefore, when the light output from the third input / output port 363 of the optical branching / combining circuit 360 to the fifth optical path L5 is p-polarized, the light that travels back and forth along the fifth optical path L5 and is input to the third input / output port 363 of the optical branching / combining circuit 360 is converted to s-polarized light. The polarization rotator 370 may be a λ / 4 wave plate or a Faraday rotator that non-reciprocally rotates the polarization direction by 90° per round trip.

[0110] Furthermore, a polarizer 380 is provided on the sixth optical path L6 along which the light output from the fourth input / output port 364 of the optical branching / combining circuit 360 propagates. The polarizer 380 selectively transmits only polarized light that is tilted 45° from p-polarized light.

[0111] As with the first vibrometer, the second vibrometer preferably uses a free-space type optical branching circuit as the optical branching / combining circuit 360, which is a polarizing beam splitter, in which input and output to each port are performed via collimated light propagating in free space.

[0112] Here, it is assumed that the illumination light and local light output from the light source unit 201 after passing through the second optical path L2 and the third optical path L3 are both p-polarized linearly polarized light. The illumination light passes through the fourth optical path L4 while maintaining its p-polarized state, and is input to the first input / output port 361 of the optical branching / combining circuit 360. The local light passes through the seventh optical path L7 while maintaining its p-polarized state, and is input to the second input / output port 362 of the optical branching / combining circuit 360.

[0113] The local light is output as p-polarized light from the fourth input / output port 364 of the optical branching / combining circuit 360, and propagates along the sixth optical path L6.

[0114] The irradiated light is output as p-polarized light from the third input / output port 363 of the optical branching / combining circuit 360, propagates through the fifth optical path L5, and is irradiated onto the measurement object 1000 arranged at the end of the fifth optical path L5. The reflected light (measurement light) travels along the fifth optical path L5 and is input to the third input / output port 363 of the optical branching / combining circuit 360. Because the fifth optical path L5 is provided with a polarization rotator 370, the measurement light returning to the third input / output port 363 of the optical branching / combining circuit 360 becomes s-polarized light, and is then output as s-polarized light from the fourth input / output port 364 of the optical branching / combining circuit 360 and propagates through the sixth optical path L6.

[0115] The local light and measurement light propagating through the sixth optical path L6 do not interfere with each other as they are, since their polarization directions are orthogonal. On the other hand, the sixth optical path L6 is provided with a polarizer 380 that selectively transmits only polarized light tilted by 45° from p-polarized light (and s-polarized light). Therefore, the output light from the polarizer 380 is an interference light output between the local light and measurement light components whose polarizations are tilted by 45°.

[0116] As with the first vibrometer, this interference light contains a phase change due to the vibration of the object to be measured 1000. Therefore, by detecting this interference light with the photodetector 340 arranged at the end of the sixth optical path L6, vibration detection becomes possible in the same way as with the first vibrometer.

[0117] In the second vibrometer, the loss of measurement light that occurs in principle (hereinafter also referred to as the principle loss) is lower than in the first vibrometer. That is, in the first vibrometer, the measurement light passes through the optical splitting / combining circuit 310, which is a non-polarizing beam splitter, twice (first as illumination light and second as measurement light), and therefore a principle loss of 6 dB occurs when the splitting ratio of the non-polarizing beam splitter is 50:50.

[0118] On the other hand, in the second vibrometer of the present invention, the measurement light passes through the first optical splitting / combining circuit 360, which is a polarizing beam splitter, twice. However, the first time, the measurement light passes as p-polarized illumination light, and the second time, the measurement light passes as s-polarized measurement light, so no fundamental loss occurs here. The fundamental loss that occurs in the measurement light is only 3 dB when it passes through the polarizer 380. In other words, in the second vibrometer, the fundamental loss of the measurement light can be improved by 3 dB compared to the first vibrometer.

[0119] For localized light, the principle loss is 3 dB in both the first and second vibrometers, and there is no difference.

[0120] As described above, in addition to the effects obtained by the first vibrometer, the second vibrometer can improve the loss of the measurement light by 3 dB, thereby improving the signal-to-noise ratio.

[0121] (Third Vibration Meter) A laser Doppler vibrometer according to a third embodiment of the present invention (hereinafter also referred to as a third vibrometer) will be described with reference to Fig. 6. Fig. 6 is a schematic diagram for explaining the third vibrometer.

[0122] The third vibrometer 103 differs from the first and second vibrometers in that the local light and the irradiated light are input to the interference light detection unit 303 via the same optical path. Detailed descriptions of components that perform the same functions as those of the first and second vibrometers may be omitted.

[0123] Like the first and second vibrometers, the third vibrometer 103 is composed of a light source unit 203 that generates irradiation light and local light to be irradiated onto the measurement object, combines them again, and outputs the combined light; an interference light detection unit 303 that receives interference light between the local light and reflected light (measurement light) from the measurement object 1000, and obtains a photoelectric conversion signal; and a signal processing unit 401 that performs the desired vibration measurement.

[0124] The light source unit 203 includes a laser light source 210 , an optical branching circuit 220 , a frequency shifter 230 , an optical branching / combining circuit 240 , and a polarization rotator 250 .

[0125] The laser light source 210 is configured using any suitable conventional light source that generates laser light. The laser light generated by the laser light source 210 passes through a first optical path L1 and is sent to the optical branching circuit 220. Here, the description will be given assuming that the laser light is linearly p-polarized light.

[0126] The optical branching circuit 220 is configured using any suitable conventional optical circuit, such as an optical coupler, that branches and outputs input light. The optical branching circuit 220 has first to third input / output ports 221 to 223. Laser light transmitted from the laser light source 210 passes through a first optical path L1 and is input to the first input / output port 221 of the optical branching circuit 220. The light input to the first input / output port 221 of the optical branching circuit 220 is branched into a first light and a second light, which are output from the second input / output port 222 and the third input / output port 223 to the second optical path L2 and the third optical path L3, respectively, at a set appropriate branching ratio. At this time, the p-polarized laser light from the laser light source 210 is branched and output to the second optical path L2 and the third optical path L3 while maintaining its polarization state (remaining p-polarized).

[0127] The first light output from the second output port 222 of the optical branching circuit 220 to the second optical path L2 becomes the irradiated light that is irradiated onto the measurement object 1000. The irradiated light output to the second optical path L2 is sent to the optical branching / combining circuit 240 as it is, remaining as p-polarized light.

[0128] The second light output from the third output port 223 of the optical branching circuit 220 to the third optical path L3 is sent to the frequency shifter 230 arranged on the third optical path L3. The frequency shifter 230 shifts the frequency of the input high-frequency driving signal (f shift) The light obtained by frequency-shifting the second light in the frequency shifter 230 becomes the local light for heterodyne detection. The local light propagating through the third optical path L3 is sent to the optical branching / combining circuit 240.

[0129] A polarization rotator 250 is provided in the third optical path L3 between the frequency shifter 230 and the optical branching / combining circuit 240.

[0130] The polarization rotator 250 rotates the polarization direction of linearly polarized light by 90° when the light passes through the polarization rotator 250. In other words, it converts p-polarized input light into s-polarized light. Such a polarization rotator 250 can be a λ / 2 wave plate, or, if the optical path in which the polarization rotator 250 is provided is constructed with polarization-maintaining optical fiber, a fusion splice in which the slow axis and fast axis of the polarization-maintaining optical fiber are rotated by 90° can be used.

[0131] The polarized beam splitter serving as the optical branching / combining circuit 240 has first to third input / output ports 241 to 243. Light that has passed through the second optical path L2 and is input to the first input / output port 241 has its p-polarized component output from the third input / output port 243 to the eighth optical path L8. Light that has passed through the third optical path L3 and is input to the second input / output port 242 has its s-polarized component output from the third input / output port 243 to the eighth optical path L8.

[0132] The arrangement of the frequency shifter 230 and the polarization rotator 250 is not limited to the above example. The frequency shifter 230 and the polarization rotator 250 can be arranged in any order, and the light output from the optical branching circuit 220 to the third optical path L3 may be sent to the optical branching / combining circuit 240 via the polarization rotator 250 and the frequency shifter 230 in this order. In either arrangement, the light output from the optical branching circuit 220 to the optical path L3 has an optical frequency f shift The light is shifted by 90°, and its polarization state is rotated by 90°, and the light is sent to the optical branching / combining circuit 240 as s-polarized light.

[0133] As described above, the illumination light is sent to the optical branching / combining circuit 240 as p-polarized light, input to the first input / output port 241 of the optical branching / combining circuit 240, and most of it is output as p-polarized light via the third input / output port 243 to the eighth optical path L8.

[0134] In addition, the local light is sent to the optical branching / combining circuit 240 as s-polarized light, input to the second input / output port 242 of the optical branching / combining circuit 240, and most of it is output as s-polarized light via the third input / output port 243 to the eighth optical path L8.

[0135] In this way, in the third vibrometer 103, the illumination light and the local light are output from the light source unit 203 via the same eighth optical path L8, combined with their polarizations perpendicular to each other, and sent to the interference light detection unit 303.

[0136] As with the first vibrometer, when the wavelength of the laser light source 210 is in a communication wavelength band such as the 1.5 μm band or the 1.3 μm band, the light source unit 203 can be configured by connecting optical fiber coupled modules. In this case, the first to third optical paths L1 to L3 and the eighth optical path L8 are configured with optical fibers.

[0137] The interference light detection unit 303 includes an optical branching / combining circuit 360 , a concave lens 320 , a condenser lens 330 , a photodetector 340 , a first polarization rotator 371 , a second polarization rotator 372 , a polarizer 380 , and a total reflection mirror 390 .

[0138] Here, it is desirable that the light input to the first input / output port 361 of the optical branching / combining circuit 360 via the ninth optical path L9 be collimated light. Therefore, in order to collimate the light output from the eighth optical path L8, it is preferable to place a collimating lens 350 at the input end of the ninth optical path L9, as in the first vibrometer. In this case, the collimating lens 350 is provided on the side of the interference light detection unit 303 where the illumination light and the local light are input. This configuration is particularly preferable when the light source unit 203 is configured using an optical fiber-coupled module as described above.

[0139] Here, the optical branching / combining circuit 360 is a polarized beam splitter. The optical branching / combining circuit 360 has first to fourth input / output ports 361 to 364. Note that the optical branching / combining circuit 360 is preferably a free-space type optical branching circuit in which input and output to each port are performed via collimated light propagating in free space.

[0140] When light is input to the first input / output port 361 of the optical branching / combining circuit 360, the p-polarized component is output from the third input / output port 363, and the s-polarized component is output from the second input / output port 362. When light is input to the second input / output port 362 of the optical branching / combining circuit 360, the p-polarized component is output from the fourth input / output port 364, and the s-polarized component is output from the first input / output port 361. When light is input to the third input / output port 363 of the polarizing beam splitter 360, the p-polarized component is output from the first input / output port 361, and the s-polarized component is output from the fourth input / output port 364.

[0141] The illumination light and local light sent to the interference light detection unit 303 via the eighth optical path L8 are input to the collimating lens 350 to be converted into collimated light, and then sent to the optical branching and combining circuit 360 via the ninth optical path L9.

[0142] The illumination light input to the first input / output port 361 of the optical branching / combining circuit 360 via the ninth optical path L9 is p-polarized light, and is therefore output from the third input / output port 363 to the fifth optical path L5.

[0143] The p-polarized illumination light output from the third input / output port 363 of the optical branching / combining circuit 360 propagates through the fifth optical path L5 and is irradiated onto the object to be measured 1000 placed at the end of the fifth optical path L5, and the reflected light (measurement light) propagates through the fifth optical path L5 and is input to the third input / output port 363 of the optical branching / combining circuit 360.

[0144] The fifth optical path L5 is provided with a first polarization rotator 371. Therefore, the polarization direction of the measurement light returning to the third input / output port 363 of the optical branching / combining circuit 360 is rotated by 90° and the measurement light is input as s-polarized light to the third input / output port 363. The measurement light is then output as s-polarized light from the fourth input / output port 364 of the optical branching / combining circuit 360, propagates through the sixth optical path L6, and is sent to the photodetector 340.

[0145] The local light input to the first input / output port 361 of the optical branching / combining circuit 360 via the ninth optical path L9 is s-polarized light, and is therefore output from the second input / output port 362 to the tenth optical path L10.

[0146] The s-polarized local light output from the second input / output port 362 of the optical branching / combining circuit 360 propagates along the tenth optical path L10 and is irradiated onto a total reflection mirror 390 located at the end of the tenth optical path L10, and the reflected light propagates along the tenth optical path L10 and is input to the second input / output port 362 of the optical branching / combining circuit 360.

[0147] The tenth optical path L10 is provided with a second polarization rotator 372. Therefore, the polarization direction of the localized light returning to the second input / output port 362 of the optical branching / combining circuit 360 is rotated by 90° to become p-polarized light, and the localized light is input to the second input / output port 362. The localized light is then output as p-polarized light from the fourth input / output port 364 of the optical branching / combining circuit 360, propagates through the sixth optical path L6, and is sent to the photodetector 340.

[0148] As in the second vibrometer, the local light and measurement light propagating through the sixth optical path L6 do not interfere with each other because their polarization directions are orthogonal. Meanwhile, as in the second vibrometer, the sixth optical path L6 is equipped with a polarizer 380 that selectively transmits only polarized light tilted 45° from p-polarized light (and s-polarized light). Therefore, the output light from the polarizer 380 is an interference light output between the polarization components of the local light and measurement light that are tilted 45°. As in the first vibrometer, this interference light contains a phase shift due to the vibration of the measurement object 1000. Therefore, by detecting this interference light with a photodetector 340 located at the end of the sixth optical path L6, vibration detection is possible, just like the first vibrometer.

[0149] Similar to the polarization rotator 370 included in the second vibrometer, the first polarization rotator 371 and the second polarization rotator 372 rotate the polarization direction of linearly polarized light by 90° when light travels back and forth through the first polarization rotator 371 and the second polarization rotator 372. As such a polarization rotator, a λ / 4 wave plate or a Faraday rotator that non-reciprocally rotates the polarization direction by 90° per round trip can be used, as described above.

[0150] Similar to the first vibrometer, the condenser lens 330 is provided on the sixth optical path L6. The condenser lens 330 serves to condense the measurement light output from the fourth input / output port 364 of the optical branching / combining circuit 360 onto the light receiving surface of the photodetector 340. Lens parameters such as the focal length of the condenser lens 330 are design matters, and may be appropriately designed depending on the light receiving aperture of the photodetector 340, the focal length of the concave lens 320 (described later), and the like.

[0151] Similarly to the first vibrometer, the concave lens 320 is provided on the tenth optical path L10. Considering that the local light passes through a lens train made up of the concave lens 320, the optical branching / combining circuit 360, the second polarization rotator 372, and the condenser lens 330 and travels back and forth along the tenth optical path L10, the lens parameters of the concave lens 320 are designed so that the local light is collimated on the light-receiving surface of the photodetector 340 and the beam diameter is larger than the light-receiving diameter of the photodetector.

[0152] The measurement light and the local light output from the fourth input / output port 364 of the optical branching / combining circuit 360 and passing through the polarizer 380 interfere with each other, and the interference light passes through the sixth optical path L6 and is received by the photodetector 340 arranged at the end of the sixth optical path L6, where it is photoelectrically converted.

[0153] The configuration and operation of the signal processing unit 401 are similar to those of the signal processing unit of the first vibrometer, and therefore a detailed description thereof will be omitted here.

[0154] In the light source unit 203, the illumination light is input as p-polarized light to a first input / output port 241 of the optical branching / combining circuit 240, and the local light is input as s-polarized light to a second input / output port 1242 of the optical branching / combining circuit 240. Because the optical branching / combining circuit 240 is a polarizing beam splitter, the illumination light and the local light are combined without loss in principle, and sent from a third input / output port 243 of the optical branching / combining circuit 240 to the interference unit 203.

[0155] The polarizations of the illumination light and the local light sent to the interference unit 203 are orthogonal to each other. Therefore, in the interference unit 203, the light is again polarized and separated without any loss in principle by the optical branching / combining circuit 360, which is a polarizing beam splitter.

[0156] In the third vibrometer, similar to the second vibrometer, the fundamental loss of the measurement light is 3 dB lower than that of the first vibrometer. Furthermore, the illumination light and the local light are output from the same optical path (L8) of the light source unit 203. As a result, only one optical pull-in section is required for the illumination light and the local light to the interference light detection unit 303. In contrast, the first and second vibrometers require two optical pull-in sections corresponding to the fourth optical path L4 and the seventh optical path L7. That is, in addition to the effects of the first and second vibrometers, the third vibrometer enables cost reduction by miniaturizing the device and reducing the number of parts.

[0157] Here, heterodyne detection has been described as an example of the operation of the laser Doppler vibrometer, but the effect of increasing angular deviation tolerance, among the effects of the present invention, is not limited to heterodyne detection and can also be applied to homodyne detection.

[0158] Furthermore, in the present invention, the operations, device configuration, signal processing, etc. after the photodetector 340 are the same as those in vibration measurement using the conventional homodyne or heterodyne method, so that the operations, device configuration, signal processing, etc. after the photodetector can be performed by selecting an appropriate method from the conventional methods. [Explanation of symbols]

[0159] 101, 102, 103 Vibration meter 201, 203 Light source section 210 Laser Light Source 220 Optical branch circuit 230 Frequency Shifter 240, 310, 360 Optical branching and combining circuit 250, 370, 371, 372 Polarization rotators 301, 302, 303 Interference light detection unit 320 concave lens 330 Condenser Lens 340 Photodetector 350, 351, 352 Collimating Lenses 380 Polarizer 390 Total Reflection Mirror 401 Signal Processing Unit 410 Analog-to-Digital Converter (ADC) 420 Signal Processing Circuit 1000 measurement objects

Claims

1. a light source unit that splits a laser beam into two beams to generate irradiation light and local light; an interference light detection unit having a photodetector, which irradiates a measurement object with the irradiation light, causes measurement light generated by reflection of the irradiation light on the measurement object to interfere with the local light, generates interference light, receives the interference light with the photodetector, and generates an interference signal that is an electrical signal; a signal processing unit that acquires information about the vibration of the object to be measured from the interference signal; Equipped with On the light receiving surface of the photodetector, the beam diameter of the local light constituting the interference light is larger than the light receiving diameter of the photodetector, and the beam diameter of the measurement light constituting the interference light is sufficiently smaller than the light receiving diameter of the photodetector. Laser Doppler vibrometer.

2. the interference light detection unit includes an optical branching / combining circuit, a concave lens, and a condenser lens; the localized light is sent to the photodetector via the concave lens, the optical branching / combining circuit, and the condensing lens; the optical branching / combining circuit outputs the irradiation light input from the light source unit and irradiates the object to be measured; The measurement light is sent to the photodetector via the optical branching / combining circuit and the condenser lens.

2. The laser Doppler vibrometer according to claim 1.

3. the optical branching / combining circuit has first to fourth input / output ports, the irradiated light is input to the first input / output port and output from the third input / output port; the measurement light is input to the third input / output port, output from the fourth input / output port, and sent to the photodetector; The local light is input to the second input / output port, output from the fourth input / output port, and sent to the photodetector.

3. The laser Doppler vibrometer according to claim 2.

4. The optical branching / combining circuit is a non-polarizing beam splitter.

4. The laser Doppler vibrometer according to claim 3.

5. the optical branching / combining circuit is a polarizing beam splitter, The interference light detection unit a polarization rotator disposed in an optical path between the optical branching / combining circuit and the object to be measured, which rotates the polarization planes of input light and output light by 90° when the light travels back and forth along the optical path; a polarizer that passes polarized light in a direction tilted by 45 degrees from the plane of polarization when light is input and output from the polarization rotator, in an optical path between the optical branching / combining circuit and the photodetector; The laser Doppler vibrometer according to claim 3 , comprising:

6. The light source unit is a laser light source that generates laser light; an optical branching circuit that is a non-polarizing beam splitter that branches the laser light into two beams, a first beam that serves as irradiation light and a second beam that serves as local light; 6. The laser Doppler vibrometer according to claim 4, further comprising:

7. the illumination light and the local light have polarized lights that are orthogonal to each other, the optical branching / combining circuit is a polarizing beam splitter, the optical branching / combining circuit has first to fourth input / output ports, the irradiation light sent from the light source unit is input to the first input / output port and output from the third input / output port; the measurement light is input to the third input / output port, output from the fourth input / output port, and sent to the photodetector; the local light transmitted from the light source unit is input to the first input / output port and output from the second input / output port; the local light input to the second input / output port is output from the fourth input / output port and sent to the photodetector; The interference light detection unit a first polarization rotator disposed in an optical path between the optical branching / combining circuit and the object to be measured, the first polarization rotator rotating the plane of polarization of input light and output light by 90° when the light travels back and forth along the optical path; a total reflection mirror that reflects the light output from the second input / output port of the optical branching / combining circuit and inputs the light to the second input / output port of the optical branching / combining circuit; a second polarization rotator disposed in an optical path between the optical branching / combining circuit and the total reflection mirror, which rotates the polarization planes of the input light and the output light by 90° when the light travels back and forth along the optical path; a polarizer that passes polarized light in a direction tilted by 45 degrees from the plane of polarization when light is input and output from the first polarization rotator, in an optical path between the optical branching / combining circuit and the photodetector; The laser Doppler vibrometer according to claim 2 , comprising:

8. The light source unit is a laser light source that generates laser light; an optical branching circuit that is a non-polarizing beam splitter that branches the laser light into two beams, a first beam that serves as irradiation light and a second beam that serves as local light; a polarization rotator that rotates the polarization plane of the second light by 90°; an optical branching / combining circuit that is a polarized beam splitter, which has first to third input / output ports, outputs a first light input to the first input / output port from the third input / output port as irradiation light, and outputs a second light input to the second input / output port from the third input / output port as local light; The laser Doppler vibrometer according to claim 7 , comprising:

9. a step of splitting the laser light into two beams to generate irradiation light and local light; a step of irradiating the irradiation light onto a measurement object, causing measurement light generated by reflection of the irradiation light from the measurement object to interfere with the local light, and receiving the interference light with the photodetector to generate an interference signal that is an electrical signal; obtaining information about the vibration of the object to be measured from the interference signal; Equipped with On the light receiving surface of the photodetector, the beam diameter of the local light constituting the interference light is larger than the light receiving diameter of the photodetector, and the beam diameter of the measurement light constituting the interference light is sufficiently smaller than the light receiving diameter of the photodetector. Vibration measurement methods.

10. The local light has a beam diameter expanded by a concave lens and then interferes with the measurement light. The vibration measurement method according to claim 9.

11. The illumination light, the local light, and the measurement light are common polarization components. The vibration measuring method according to claim 9 or 10.

12. the illumination light and the local light are generated with a common polarization component; The local light is polarized and rotated by 90°, and then interferes with the measurement light. The local light and the measurement light are received at a light receiving surface of the photodetector, and the polarization components inclined at 45° are received. The vibration measuring method according to claim 9 or 10.

13. the illumination light and the local light are generated with orthogonal polarization components; the local light is polarized and rotated by 90°, and the measurement light is polarized and rotated by 90° relative to the illumination light, and then the local light interferes with the measurement light; The local light and the measurement light are received at a light receiving surface of the photodetector, and the polarization components inclined at 45° are received. The vibration measuring method according to claim 9 or 10.

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