Laser doppler vibrometer and vibration measurement method

Abstract

To provide a compact and low-cost laser Doppler vibrometer and a vibration measurement method capable of composing an interferometer and a photodetector in one set even if a long optical fiber is used.SOLUTION: A laser Doppler vibrometer comprises: a first optical coupler for branching laser beams from a laser light source into two and setting one laser beam as irradiation light and the other laser beam as local light; a sensor head for receiving irradiation light generated by the first optical coupler via a first optical path, applying the light to a measurement object via a second optical path, receiving measurement light obtained by reflection of the irradiation light by the measurement object via the second optical path, and propagating the measurement light through the first optical path; a second optical coupler for generating interference light by allowing measurement light propagated through the first optical path to interfere with the local light; and an electric processing system for acquiring information on vibration of the measurement object from the interference light. The sensor head has a Faraday rotor for rotating polarized waves by 45°.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a laser Doppler vibrometer and a vibration measurement method that can be performed with the laser Doppler vibrometer. [Background technology]

[0002] First, an overview of a conventional laser Doppler vibrometer will be given (see, for example, Patent Document 1).

[0003] In conventional laser Doppler vibrometers, a laser beam generated by a laser light source is split into two beams, one as illumination light and the other as local light. When the illumination light is directed at the object to be measured, it is reflected by the object. The measurement light reflected from the illumination light has a frequency shift due to the Doppler effect according to the vibration state of the object to be measured, and this frequency shift appears as a phase change. Therefore, if the phase change caused by the object to be measured can be measured, the vibration state of the object to be measured can be known.

[0004] In general, to extract phase information in a laser Doppler vibrometer, heterodyne detection is used, in which an interference fringe between measurement light and local light, which have different wavelengths, is detected by an optoelectronic converter.

[0005] Conventionally, contact-type vibration sensors, which are widely used for vibration measurement, have had difficulty measuring the vibration of distant objects or objects under high temperatures or magnetic fields. In contrast, the laser Doppler vibrometer is capable of non-contact measurement by irradiating light from a distance, making it possible to measure the vibration of these objects.

[0006] A known laser Doppler vibrometer has a separate sensor head section, which includes an objective lens and other components that input and output light to the object to be measured, and a measurement section that detects interference between local light and signal light, receives the interference light resulting from the interference, and processes the electrical signal obtained by receiving the interference light (see, for example, Patent Document 2).

[0007] By adopting the configuration described in Patent Document 2, it is possible to use the 1.3 μm band or 1.5 μm band used in optical communications as the laser light wavelength. This makes it possible to use a long optical fiber with low loss, and to separate the measurement unit and the sensor head unit by several hundred meters or more. The sensor head unit is basically composed of only small optical components such as an objective lens, and can be made compact.

[0008] A configuration using such a long optical fiber in the optical path of the measurement light is particularly promising in environments where it is difficult to install a large sensor head or measurement unit near the object to be measured due to constraints on the device installation environment. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2001-159560 A [Patent Document 2] Patent Publication No. 2022-67148 [Non-patent literature]

[0010] [Non-Patent Document 1] Hidetoshi Kumagai, Takashi Yamamoto, Masato Koashi, and Nobuyuki Imoto, “Robustness of quantum communication based on a decoherence-free subspace using a counter-propagating weak coherent light pulse,” Physical Review A 87, 052325 - Published 28 May 2013. [Non-Patent Document 2] Amnon Yariv, “Operator algebra for propagation problems involving phase conjugation and nonreciprocal elements,” APPLIED OPTICS, Vol. 26, No. 21, pp. 4538-4540, 1 November,1987. Summary of the Invention [Problem to be solved by the invention]

[0011] One of the problems with laser Doppler vibrometers using long optical fibers is the polarization rotation within the optical fiber.

[0012] Generally, optical fibers have residual birefringence due to manufacturing inaccuracies. The value is quantitatively evaluated by a statistical quantity called polarization mode dispersion (PMD). The PMD of typical optical fibers for optical communication is 0.1 to 1 ps / (km). 1 / 2 It is estimated that an optical phase difference of 2π occurs between orthogonal polarized waves over a propagation distance of about 6 cm to 6 m. This phase difference changes easily due to changes in the environmental temperature, and is very difficult to control.

[0013] When using a long optical fiber of several hundred meters, the polarization state of the measurement light returning from the measurement object is generally unstable and difficult to control. In the worst case, it is assumed that the polarization of the measurement light and the local light input to the optical interferometer will be orthogonal to each other, making it impossible to measure the vibration of the measurement object.

[0014] As one method for solving such problems, Patent Document 2 proposes a laser Doppler vibrometer equipped with a receiver with a polarization diversity configuration. In other words, two local lights with mutually orthogonal polarizations are prepared, and an interference waveform with the local lights is always obtained regardless of the polarization state of the measurement light input to the optical interferometer, making it possible to measure vibration by heterodyne detection or homodyne detection.

[0015] On the other hand, when using a receiver with the above-mentioned polarization diversity configuration, it is necessary to provide an optical interferometer and an optical detector for each of the measurement lights with orthogonal polarizations, so two sets of optical interferometers and optical detectors are required, which results in problems such as the device itself becoming larger and more expensive.

[0016] The present invention has been made in view of the above problems, and an object of the present invention is to provide a small-sized, low-cost laser Doppler vibrometer and a vibration measuring method that can configure an optical interferometer and a photodetector as a single set even when a long optical fiber is used. [Means for solving the problem]

[0017] In order to achieve the above object, the laser Doppler vibrometer of the present invention includes a first optical coupler that splits a laser beam from a laser source into two beams, one of which is used as an illumination beam and the other as a local beam, a sensor head that receives the illumination beam generated by the first optical coupler via a first optical path and irradiates a measurement object via a second optical path, receives measurement beam reflected by the measurement object from the illumination beam via the second optical path, and propagates the measurement beam via the first optical path, a second optical coupler that causes interference between the measurement beam propagated via the first optical path and the local beam and generates interference beam, and an electrical processing system that acquires information on the vibration of the measurement object from the interference beam. The sensor head includes a Faraday rotator that rotates the polarization by 45°.

[0018] According to a preferred embodiment of the above-mentioned laser Doppler vibrometer, the laser Doppler vibrometer includes an optical multiplexer / demultiplexer that outputs, from the second input / output port, the illumination light generated by the first optical coupler and input to the first input / output port to propagate along the first optical path, and outputs, from the third input / output port, the measurement light that is input to the second input / output port and has propagated along the first optical path, and a polarization rotator that rotates the polarization of the measurement light output from the third input / output port of the optical multiplexer / demultiplexer by 90° and sends it to the second optical coupler.

[0019] According to another preferred embodiment of the laser Doppler vibrometer, the laser Doppler vibrometer includes a first optical multiplexer / demultiplexer that outputs the illumination light input to the first input / output port and generated by the first optical coupler from the second input / output port to propagate through the first optical path, and outputs the measurement light input to the second input / output port and propagated through the first optical path from the third input / output port to send it to the second optical coupler, a second optical multiplexer / demultiplexer that outputs the local light input to the first input / output port and generated by the first optical coupler from the second input / output port to propagate through the third optical path, and outputs the local light input to the second input / output port and propagated through the third optical path from the third input / output port to send it to the second optical coupler, and a Faraday rotator mirror provided on the opposite side of the second optical multiplexer / demultiplexer of the third optical path. In this case, it is preferable that the optical path length of the third optical path is equal to the sum of the optical path lengths of the first optical path and the second optical path. As a suitable example of the optical multiplexer / demultiplexer, an optical circulator or a polarizing beam splitter can be used.

[0020] The vibration measuring method of the present invention includes the steps of splitting a laser beam into two beams, one of which is used as an illumination beam and the other as a local beam, receiving the illumination beam through a first optical path, rotating the polarization by 45°, and then illuminating the measurement object through a second optical path, receiving the measurement beam reflected by the measurement object through the second optical path, rotating the polarization by 45°, and propagating the measurement beam through the first optical path, causing the measurement beam propagated through the first optical path to interfere with the local beam to generate interference beam, and acquiring information on the vibration of the measurement object from the interference beam. Here, the two 45° rotations of the polarization are non-reciprocal.

[0021] According to a preferred embodiment of the vibration measuring method of the present invention, the method further comprises a step of rotating the polarization of the measurement light propagated through the first optical path by 90°, which is performed before the step of generating the interference light.

[0022] According to another preferred embodiment of the vibration measuring method of the present invention, the method includes the steps of outputting the local light from the second input / output port and propagating the local light through a third optical path, rotating the polarization of the local light propagated through the third optical path by 90°, and propagating the local light whose polarization has been rotated by 90° through the third optical path again, which are performed before the step of generating the interference light. In this case, it is preferable that the optical path length of the third optical path is equal to the sum of the optical path lengths of the first optical path and the second optical path. Effect of the Invention

[0023] According to the laser Doppler vibrometer of the present invention, the sensor head is provided with a Faraday rotator that rotates the polarization by 45°, so that the polarization of the measurement light that has traveled back and forth through the first optical path and the second optical path can be made orthogonal to the polarization of the initial local light, without being affected by the polarization rotation in the first optical path. As a result, even when a long optical fiber is used, the local light and the measurement light can always be interfered with in the same polarization, with a simple configuration. Therefore, it is possible to provide a low-cost, highly stable laser Doppler vibrometer and vibration measurement method that can measure vibrations with the maximum signal-to-noise ratio expected for the same measurement light intensity. [Brief description of the drawings]

[0024] [Figure 1] FIG. 4 is a schematic diagram for explaining a first vibrometer. [Diagram 2] FIG. 13 is a diagram showing the results of calculating the x-polarized intensity |Ex|2 of the output light after a round trip when birefringence and PDL are present. [Diagram 3] FIG. 4 is a schematic diagram for explaining an experimental system for verifying the effect of the first vibrometer. [Figure 4] FIG. 13 is a diagram showing the results of verifying the effect of the first vibrometer. [Diagram 5] FIG. 4 is a schematic diagram for explaining a second vibration meter. [Figure 6] FIG. 2 is a schematic diagram for explaining a Faraday rotator mirror. [Figure 7] 10A and 10B are schematic diagrams for explaining other configuration examples of the sensor head. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Hereinafter, an embodiment of the present invention will be described with reference to the drawings, but the shape, size, and positional relationship of each component are merely shown in a schematic manner to allow the present invention to be understood. In addition, the following describes a preferred configuration example of the present invention, but the material and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiment, and many changes or modifications can be made that can achieve the effects of the present invention without departing from the scope of the configuration of the present invention.

[0026] (1st 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.

[0027] The first vibrometer splits the laser light generated by a laser light source into two, illumination light and local light, and measures the vibration of the object to be measured by using an optical interferometer to detect phase fluctuations due to the Doppler effect contained in the measurement light from the measurement light generated when the illumination light is irradiated onto the object to be measured and the local light.

[0028] Here, an example will be described in which heterodyne detection is performed by causing interference between the measurement light and the frequency-shifted local light, but a configuration in which homodyne detection is performed may also be used.

[0029] The first vibrometer includes, as an optical processing system 100, a laser light source 110, a first optical coupler 121, an optical circulator 130 serving as an optical multiplexer / demultiplexer, a sensor head 140, a frequency shifter 150, a polarization rotator 160, and a second optical coupler 122. The first vibrometer also includes, as an electrical processing system 200, a photodetector 210, an analog-to-digital converter (ADC) 220, and a signal processing circuit 230.

[0030] The laser light source 110 has a center frequency f 0 The laser light generated by the laser light source 110 is sent to a first optical coupler 121.

[0031] The first optical coupler 121 splits the laser light sent from the laser light source 110 into two. One of the split laser light is the irradiated light E in The other of the two split laser beams is sent to the optical circulator 130 as local light E LO The signal is sent to the frequency shifter 150 as

[0032] The power splitting ratio of the first optical coupler 121 is a matter of design. Local light of suitable power for 122 E LO is input and is selected so that the signal-to-noise (SN) ratio of the measurement light from the measurement target does not deteriorate significantly.

[0033] Optical circulator 130 has first to third input / output ports 130-1 to 130-3. In optical circulator 130, light input to first input / output port 130-1 is output from second input / output port 130-2, and light input to second input / output port 130-2 is output from third input / output port 130-3.

[0034] The irradiated light E sent from the first optical coupler 121 to the optical circulator 130 in is input to the first input / output port 130-1 of the optical circulator 130. The irradiated light E in The irradiated light E output from the second input / output port 130-2 is in The light is sent to the sensor head 140 via a first optical path 191. The first optical path 191 is formed of, for example, a long optical fiber.

[0035] The sensor head 140 detects the irradiated light E inThe sensor head 140 irradiates the measurement light Es toward the measurement object 1000. The sensor head 140 also receives the measurement light Es reflected by the measurement object 1000 and couples it into the first optical path 191.

[0036] The sensor head 140 includes an objective lens 142 and a Faraday rotator 144 .

[0037] The objective lens 142 is provided to efficiently output the irradiation light and input the measurement light. The objective lens 142 is designed according to the size of the measurement object 1000 and the distance between the sensor head 140 and the measurement object 1000.

[0038] It is preferable that the second optical path 192 that optically couples the sensor head 140 and the measurement object 1000 be free space.

[0039] The Faraday rotator 144 rotates the polarization of the input light by 45° non-reciprocally. When the light goes back and forth through the Faraday rotator 144, the polarization is rotated by 45° on the outward and return paths, and by 90° on the round trip. Here, the irradiated light E in The optical path along which the measurement light Es propagates after being reflected by the measurement object 1000 is called the return path.

[0040] Illumination light E sent to the sensor head 140 via the first optical path 191 in The light E is irradiated onto the measurement object 1000 via the objective lens 142 and the Faraday rotator 144, and is reflected by the measurement object 1000. in The measurement light E is reflected by the measurement object 1000. s is sent to the first optical path 191 via the Faraday rotator 144 and the objective lens 142 .

[0041] The arrangement of the objective lens 142 and the Faraday rotator is a matter of design. Here, an example has been described in which the Faraday rotator 144 is closer to the measurement object 1000 than the objective lens 142, but the objective lens 142 may be on the measurement object side.

[0042] Measurement light E propagating through the first optical path 191 s The measurement light E is sent to the optical circulator 130 via the first optical path 191. s The measurement light E is input to the second input / output port 130-2 of the optical circulator 130. The measurement light input to the second input / output port 130-2 is output from the third input / output port 130-3. s is sent to the polarization rotator 160.

[0043] The polarization rotator 160 rotates the measurement light E output from the third input / output port 130-3 of the optical circulator 130. s The polarization of the measurement light E is rotated by 90°. A suitable example of the polarization rotator 160 is a half-wave plate with its optical axis direction tilted by 45°. When the optical circulator 130 and the second optical coupler 122 are configured with polarization-maintaining optical fibers, it is also a suitable example to rotate the optical axis by 90° at the connection point of the optical circulator 130 and the second optical coupler 122. s is sent to the second optical coupler 122.

[0044] The local light E sent from the first optical coupler 121 to the frequency shifter 150 LO is frequency-shifted in the frequency shifter 150. The frequency shifter 150 is, for example, a commercially available acousto-optic modulator (AOM). The frequency shifter 150 shifts the frequency of the input high-frequency driving signal (f shift ) to shift the optical frequency of the input light. The local light E LO is sent to the second optical coupler 122.

[0045] The second optical coupler 122 couples the measurement light E s and the local light E sent from the frequency shifter 150 LOIn the second optical coupler 122, the measurement light and the local light interfere with each other, and the resulting interference light is sent to the photodetector 210.

[0046] The photodetector 210 performs photoelectric conversion on the interference light sent from the second optical coupler 122. The photodetector 210 has a function of converting the received optical signal into an electrical signal. Any suitable conventional photodiode (PD) can be used as the photodetector 210. The analog electrical signal obtained by photoelectric conversion in the photodetector 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 analog electrical signal generated by the photodetector 210 is sent to the ADC 220.

[0047] The ADC 220 digitizes the analog electrical signal to generate a digital electrical signal. The digital electrical signal generated by the ADC 220 is sent to the signal processing circuit 230.

[0048] The signal processing circuit 230 can be configured with any suitable conventional electronic computer such as a personal computer. The signal processing circuit 230 performs arithmetic operations such as filtering on the digital electrical signal to obtain the result of vibration measurement of the vibrating object. Note that, as for a technique for obtaining the 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, a conventional technique such as that disclosed in Patent Document 2 can be used, and therefore a description thereof will be omitted here.

[0049] (operation) By configuring the optical systems that couple the laser light source 110, the first optical coupler 121, the optical circulator 130, the frequency shifter 150, the polarization rotator 160, and the second optical coupler 122 that make up the optical processing system 100 using a spatial optical system or a polarization-maintaining optical fiber, it is relatively easy to maintain the polarization state in the optical processing system.

[0050] On the other hand, the light propagating through the optical fiber, which is the first optical path 191, undergoes polarization rotation.

[0051] Here, the operation of the first vibrometer will be described while comparing the first vibrometer having a Faraday rotator in its sensor head with a conventional technique having no Faraday rotator in its sensor head.

[0052] As described above, in the conventional laser Doppler vibrometer, polarization rotation occurs in the measurement light in the portion corresponding to the first optical path 191 of the present invention. On the other hand, as described above, for the local light, by configuring the optical system that couples the components corresponding to the laser light source 110, the first optical coupler 121, the frequency shifter 150, and the second optical coupler 122 using a spatial optical system or a polarization-maintaining optical fiber, it is easy to keep the polarization state of the local light input to the second optical coupler 122 the same as that of the laser light output from the laser light source 110.

[0053] The polarization state of light input to one end (first end) of the optical fiber transmission line corresponding to the first optical path 191 is expressed as E i =(E xi ,E yi ) T The polarization state of the light propagating through the optical fiber transmission line and output from the other end (second end) of the optical fiber transmission line is expressed as E o =(E xo ,E yo ) T Let us assume that.

[0054] Input polarization E i and output polarization E o is the transfer function M of the optical fiber transmission line from the first end to the second end F When using, the following equation (1) is satisfied.

[0055]

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[0056] Transfer function M F is expressed as a 2 × 2 matrix, and when there is no loss in the optical fiber transmission line, the transfer function M F The determinant of is 1.

[0057] On the other hand, the transfer function M B is expressed by the following formulas (2) and (3) (see, for example, Non-Patent Document 1).

[0058]

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[0059]

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[0060] Here, consider a case where light input to a first end of an optical fiber transmission line is specularly reflected at the second end, propagates in the opposite direction along the same optical fiber transmission line toward the first end, and is output from the first end. The polarization state of the light output from the first end can be expressed as E rt =(E xrt ,E yrt ) T Then, E rt is given by the following equations (4) and (5).

[0061]

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[0062]

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[0063] Here, the matrix r is a transformation matrix that performs coordinate transformation between the forward wave on the outward path and the backward wave on the return path. In other words, the matrix r transforms x to -x and leaves y as y.

[0064] Transfer function M of the outward pathF When the components of are given by the following equation (6), the following equation (7) is obtained from the above equation (4).

[0065]

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[0066]

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[0067] Here, the polarization state of the input light is E i =(1,0) T In this case, the polarization state of the output light after traveling back and forth through the optical fiber transmission line is E rt =(-a 2 -c 2 , ab+c) T The parameters a, b, c, and d that define the transfer function of the optical fiber transmission line are indefinite and also vary over time. For this reason, the polarization state of the output light after traveling back and forth through the optical fiber transmission line is generally indefinite and difficult to control.

[0068] Here, in the first vibrometer, a Faraday rotator 144 that rotates the polarization by 45° non-reciprocally is disposed on the second end side of the first optical path 191, which is an optical fiber transmission line. Note that here, a case is considered in which no polarization rotation occurs in the second optical path 192 between the sensor head 140 and the measurement object 1000 when the second optical path 192 travels back and forth. This assumption is valid when the second optical path 192 is spatially coupled and specular reflection is dominant in the reflection at the measurement object 1000.

[0069] When the Faraday rotator 144 is provided in the sensor head 140, the polarization state of the light output from the second end is expressed as E rt =(E xrt ,E yrt ) T Then, E rt is given by the following equation (8).

[0070]

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[0071] Here, F F is the transfer function of the forward Faraday rotator 144, and F B is the transfer function of the return path Faraday rotator 144. When the polarization rotation angle in the Faraday rotator 144 is 45°, F F , F B is given by the following equations (9-a) and (9-b) (see, for example, Non-Patent Document 2).

[0072]

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[0073] As a result, the polarization state of the light output from the second end is given by the following equation (10).

[0074]

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[0075] Here, the polarization state of the input light is E i =(1,0) T The polarization state of the output light after going back and forth through the optical fiber transmission line and the Faraday rotator is expressed as E rt =(0,-(ad-bc)) T This shows that the light is always output as y-polarized light, regardless of the transfer function of the optical fiber transmission line.

[0076] The relationship of the above formula (10) holds even if there is a polarization dependent loss (PDL) in the optical fiber transmission line.

[0077] If the optical system consisting of the laser light source 110, the first optical coupler 121, the frequency shifter 150, the second optical coupler 122, and the optical circulator 130 is a polarization-maintaining optical system, the illumination light output from the first input / output port 130-1 of the optical circulator 130 and the local light input to the second optical coupler 122 can be made to have the same polarization, here, x-polarization.

[0078] On the other hand, as described above, when x-polarized light is output from the second input / output port 130-2 of the optical circulator 130 and input to the optical fiber transmission line constituting the first optical path 191, it passes through the optical fiber transmission line constituting the first optical path 191 and the Faraday rotator 144, is reflected by the object to be measured 1000, propagates through the Faraday rotator 144 and the optical fiber transmission line, returns to the optical circulator 130 again, and the measurement light input to the second input / output port 130-2 of the optical circulator becomes y-polarized light.

[0079] The measurement light, which is y-polarized and input to the second input / output port 130-2 of the optical circulator 130, is output from the third input / output port 130-3 while maintaining the y-polarized state, and sent to the polarization rotator 160. The polarization rotator 160 converts the y-polarized measurement light into x-polarized light, and sends it to the second optical coupler 122.

[0080] Therefore, the local light and the measurement light sent to the second optical coupler 122 are both x-polarized waves.

[0081] As a result, in the first vibrometer, the local light and the measurement light of the same polarization (here, x-polarization) are input to the second optical coupler 122, regardless of the transfer function of the first optical path 191. That is, the local light and the measurement light interfere with each other in the same polarization in the second optical coupler 122. Therefore, the beat light with the maximum modulation depth is always generated from the second optical coupler 122, regardless of the transfer function of the first optical path 191.

[0082] In addition, the interference light output from the second optical coupler 122 has the maximum modulation depth possible because the polarizations of the local light and the measurement light are the same. This enables vibration measurement with the maximum S / N ratio expected for the same measurement light intensity.

[0083] In the above description, an optical circulator is used as the optical multiplexer / demultiplexer. As described above, the polarization of the illumination light input to the first input / output port 130-1 of the optical circulator 130 and output from the second input / output port 130-2 is fixed to x-polarized light, and the polarization of the measurement light input to the second input / output port 130-2 of the optical circulator 130 and output from the third input / output port 130-3 is fixed to y-polarized light. Therefore, polarizing beam splitters (PBS) can also be used as the optical multiplexer / demultiplexer. The PBS is provided so that, for example, when x-polarized light and y-polarized light are input to the second input / output port, x-polarized light is output from the first input / output port, and y-polarized light is output from the third input / output port.

[0084] The x-polarized irradiated light E sent from the first optical coupler 121 to the PBS in is input to the first input / output port of the PBS and output from the second input / output port. Also, the irradiated light E in is sent to the sensor head 140 via the first optical path 191. Also, the y-polarized measurement light E s The measurement light E output from the third input / output port 130-2 is input to the second input / output port of the PBS and output from the third input / output port. s is sent to the polarization rotator 160.

[0085] Here, a case has been described in which no polarization rotation occurs in the second optical path 192, but for a more general discussion, consider a case in which polarization rotation occurs in the second optical path 192 when the light travels back and forth to the location, including reflection from the object to be measured. This assumption corresponds to a case in which the second optical path 192 or the object to be measured 1000 has birefringence or PDL.

[0086] The transfer function of the outward path in the second optical path 192 is L F Then, the return transfer function L B Similarly to equation (2), is expressed by the following equation (11).

[0087]

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[0088] At this time, the total transfer function T total is given by the following equation (12).

[0089]

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[0090] Generally speaking, if the birefringence or PDL generated in the second optical path 192 is large, the effect obtained in the first vibrometer described above, that is, light input as x-polarized light and then travels back and forth, returning as y-polarized light, cannot be obtained.

[0091] On the other hand, if the birefringence and PDL occurring in the second optical path 192 are appropriately small, the effect obtained by the first vibrometer can be ensured to a sufficient degree for practical use.

[0092] For an optical component having birefringence or PDL, the transfer function T can be expressed by the following equation (13) (see, for example, Non-Patent Document 1).

[0093]

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[0094] Here, γ x and γ y are the amplitude losses for x-polarized and y-polarized waves, respectively, and correspond to PDL. θ represents the rotation direction of the optical axis. Also, the optical phase changes for x-polarized and y-polarized waves are -φ and +φ, respectively. 2φ represents the phase difference due to birefringence.

[0095] In general terms, the transfer function of the second optical path 192 can be expressed as a function of various (γ x ,γ y , θ, φ) by multiplying the transfer functions of optical components having the combinations.

[0096] However, in the first vibrometer, the second optical path 192 includes only the objective lens 142, which is an optical lens, the measurement object 1000, and the spatial coupling system. The spatial coupling system and the optical lens may be considered to have no birefringence or PDL. Therefore, the birefringence and PDL of the second optical path 192 may be considered to be caused almost entirely by the measurement object 1000.

[0097] From the above considerations, in the following discussion, the transfer function for a single optical component given by the above equation (13) will be referred to as the transfer function L F Let us assume that.

[0098] In this case, the polarization state after the light has traveled back and forth through the first optical path 191 and the second optical path 192 is given by the following equation (14).

[0099]

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[0100] The desired effect in the first vibrometer is to change the polarization state of the input light to x-polarized (1,0) T Then, the polarization state after the round trip is y-polarized (0,1) T In this case, even if some x-polarized component remains, the effect of the invention can be obtained as long as it does not significantly reduce the optical intensity of the y-polarized component.

[0101] Total transfer function T total The matrix elements of T ij If (i,j=1,2), the polarization state after the round trip is (T 11 ,T 21 ) T Therefore, the desired effect in the first vibrometer is given by |T 11 | / |T 21 This is obtained when | is sufficiently small.

[0102] From the above formulas (12) and (13), T 11 , T 21 is given by the following equations (15-a) and (15-b).

[0103]

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[0104] Here, the rotation of the optical axis in the second optical path, the phase change due to birefringence, and the PDL are all sufficiently small, and Δθ, Δφ, and γ y =(1+Δγ)γ x If we assume that (Δθ, Δφ, Δr<<1), the above equations (15-a) and (15-b) can be expressed as the following equations (16-a) and (16-b).

[0105]

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[0106] In the above formulas (16-a) and (16-b), T 11 , T 21 In both cases, the loss γ x 2 applies to the entire formula, so |T 11 | / |T 21 When discussing the ratio of |, the loss γ x 2 can be ignored.

[0107] When Δφ and Δr are sufficiently small, T given by the above equation (16-b)21 The value of is dominated by the second term on the right-hand side, and approaches -(ad-bc), which coincides with the value given by the above formula (10) when the second optical path 192 has no birefringence or PDL.

[0108] Therefore, |T 11 | / |T 21 To discuss the ratio of |T 11 We need only pay attention to | and discuss the absolute value of the value of the following equation (17).

[0109]

number

[0110] Due to the characteristics of the transfer function of the optical fiber transmission line, the absolute values ​​of a, b, c, and d are at most about 1. In other words, the absolute value of the value in [ ] on the first right-hand side of the above formula (17) is at most about 1. Therefore, the value of the above formula (17) can be determined by considering how much the value in [ ] on the second right-hand side increases due to birefringence and PDL.

[0111] In FIG. 2, when there is birefringence and PDL, the intensity of the x-polarized wave of the output light after a round trip |E x | 2 The results of the calculation are shown below. x = 1, and the value in the first brackets [ ] on the right-hand side of the above formula (15-a) is also set to 1.

[0112] FIG. 2(A) shows the calculation results of the PDL dependence when the birefringence is set to zero (φ=0), and FIG. 2(B) shows the calculation results of the birefringence dependence when the PDL is set to zero (Δr=0).

[0113] For example, the intensity of x-polarized light |E x | 2If the tolerance is set to 0.01 (1% of the total intensity) or less, or 0.1 (also 10%), the allowable PDL is estimated to be 0.8 dB (about 20%) or less, and 2.1 dB (about 62%) or less, respectively. Also, the birefringence is estimated to be 0.015π or less as the phase difference between the x-polarized and y-polarized waves, and 0.05π or less, respectively.

[0114] As is well known, the amplitude of the detection signal obtained by homodyne detection or heterodyne detection is proportional to the square root of the measured light intensity. If the x-polarized light intensity fluctuates between 0 and 0.1 due to the influence of polarization rotation, the y-polarized light intensity fluctuates between 0.9 and 1. On the other hand, the amplitude of the detection signal fluctuates between 0.95 and 1, which is not as large as the fluctuation of the measured light intensity.

[0115] That is, even if the x-polarized wave intensity fluctuates by about 0.1, and therefore the intensity of the y-polarized wave to be measured fluctuates by about 0.1, the effect on the measurement performance of the vibration measurement is considered to be very small. Therefore, the effect of the first embodiment of the present invention can be obtained even for a measurement target having birefringence and PDL that do not affect the measurement performance of the vibration measurement as described above. The allowable fluctuation range of the y-polarized wave intensity is a design issue, and depends on, for example, the sensitivity and noise of the photodetector 210.

[0116] An experiment was conducted to demonstrate the effectiveness of the first vibration meter.

[0117] 3 is a schematic diagram showing an experimental system for verifying the effect of the first vibrometer. A laser beam of x-polarized waves with a wavelength of 1550 nm passes through a polarization-maintaining optical circulator 135, a polarizing beam splitter (PBS) 125, a λ / 2 wave plate 182, a λ / 4 wave plate 184, a single-mode optical fiber 100 m 190, a collimator lens (manufactured by Thorlabs, model number: C20APC-C) 143, and a Faraday rotator (manufactured by Thorlabs, model number I1550R5) 145, and is then irradiated onto a mirror 1010. The optical path consisting of the λ / 2 wave plate 182, the λ / 4 wave plate 184, and the single-mode optical fiber 100 m corresponds to a first optical path 191 in the first vibrometer. The part consisting of the collimator lens 143 and the Faraday rotator 145 corresponds to a sensor head 140 in the first vibrometer.

[0118] A part of the reflected light from mirror 1010 was taken in by Faraday rotator 145 and collimator lens 143 and propagated backward through first optical path 191 consisting of 100 m of single mode optical fiber, and the x-polarized wave intensity and y-polarized wave intensity of the light after backward propagation were measured by power meters 186 and 188. By rotating the optical axes of λ / 2 wave plate 182 and λ / 4 wave plate 184, the polarization rotation occurring in first optical path 191 was simulated, and the changes in the x-polarized wave intensity and y-polarized wave intensity at that time were measured. The total propagation loss of the optical system used in the experiment (including the reflectance of mirror 1010) was about -3 dB.

[0119] The experimental results are shown in Figure 4. Figure 4(A) shows the results when the optical axis of the λ / 2 wave plate is rotated, and Figure 4(B) shows the results when the optical axis of the λ / 4 wave plate is rotated. In the figure, the solid line represents the x-polarized wave intensity, and the dotted line represents the y-polarized wave intensity. In the figure, the thick line represents the result when a Faraday rotator is inserted, and the thin line represents the result for the conventional configuration without a Faraday rotator. Figure 4(A) shows the rotation angle (°) of the λ / 2 wave plate on the horizontal axis, and Figure 4(B) shows the rotation angle (°) of the λ / 4 wave plate on the horizontal axis. In Figures 4(A) and (B), the vertical axis represents the intensity (dB) normalized to the total propagation loss.

[0120] In a conventional configuration in which no Faraday rotator is inserted, the x-polarized wave intensity and the y-polarized wave intensity fluctuate significantly with the rotation of the optical axes of the λ / 2 wave plate 182 and the λ / 4 wave plate 184. However, their sum is almost constant. This means that in the conventional configuration, the polarization state of the measurement light fluctuates significantly due to the effect of the polarization rotation occurring in the first optical path 191.

[0121] On the other hand, in the first vibrometer with the Faraday rotator inserted, the fluctuation of the y-polarized wave intensity was almost unchanged at 0.1 dB or less even when the optical axes of the λ / 2 wave plate 182 and the λ / 4 wave plate 184 were rotated. From the above, the effectiveness of the first vibrometer was verified.

[0122] The first vibrometer is expected to have the following effects. That is, even when a long optical fiber is used, the local light and the measurement light can always interfere with each other in the same polarization with a simple configuration, without being affected by polarization rotation in the optical fiber. As a result, vibration measurement is possible with the maximum signal-to-noise ratio expected for the same measurement light intensity, and a low-cost, highly stable laser Doppler vibrometer can be provided.

[0123] (2nd vibration meter) A laser Doppler vibrometer (hereinafter also referred to as a second vibrometer) according to a second embodiment of the present invention will be described with reference to Fig. 5. Fig. 5 is a schematic diagram for explaining the second vibrometer.

[0124] The second vibrometer, like the first vibrometer, measures the vibration of the measurement target by detecting phase fluctuations caused by the Doppler effect contained in the measurement light using an optical interferometer from the measurement light and the local light. Here, an example of heterodyne detection in which the measurement light and the frequency-shifted local light are made to interfere with each other is described, but a configuration of homodyne detection may also be used.

[0125] The second vibrometer includes, as an optical processing system, a laser light source 110, a first optical coupler 121, a first optical circulator 131 as a first optical multiplexer / demultiplexer, a sensor head 140, a frequency shifter 150, a second optical circulator 132 as a second optical multiplexer / demultiplexer, a Faraday rotator mirror 162, and a second optical coupler 122. The second vibrometer also includes, as an electrical processing system, a photodetector 210, an analog-to-digital converter (ADC) 220, and a signal processing circuit 230.

[0126] The configurations and operations of the laser light source 110, the first optical coupler 121, the sensor head 140, and the frequency shifter 150 are similar to those of the first vibrometer, and therefore a duplicated description will be omitted.

[0127] The first optical circulator 131 is configured similarly to the optical circulator of the first vibrometer, and therefore a description that overlaps with the optical circulator of the first vibrometer may be omitted. The irradiated light sent from the first optical coupler 121 to the first optical circulator 131 is input to a first input / output port 131-1 of the first optical circulator 131. The irradiated light input to the first input / output port 131-1 is output from a second input / output port 131-1. The irradiated light output from the second input / output port 132 is sent to the sensor head 140 via a first optical path 191.

[0128] The sensor head 140 is similar to the first vibrometer. The sensor head 140 detects the irradiated light E in The sensor head 140 irradiates the measurement light Es toward the measurement object 1000. The sensor head 140 also receives the measurement light Es reflected by the measurement object 1000 and couples it into the first optical path 191.

[0129] The light propagating through the first optical path 191 is sent to the first optical circulator 131. The measurement light sent to the first optical circulator 131 via the first optical path 191 is input to the second input / output port 131-2 of the first optical circulator 131. The measurement light input to the second input / output port 131-2 is output from the third input / output port 131-3. The measurement light output from the third input / output port 131-3 is sent to the second optical coupler 122.

[0130] The local light E sent from the first optical coupler 121 to the frequency shifter 150 LO is frequency shifted in the frequency shifter 150. The frequency shifter 150 is similar to the first vibrometer.

[0131] The second optical circulator 132 is configured similarly to the first optical circulator 131, and therefore a description overlapping with that of the first optical circulator 131 may be omitted. The local light sent from the frequency shifter 150 to the second optical circulator 132 is input to a first input / output port 132-1 of the second optical circulator 132. The local light input to the first input / output port 132-1 is output from a second input / output port 132-2. The local light output from the second input / output port 132 is sent to the Faraday rotator mirror 162 via a third optical path 193.

[0132] The third optical path 193 is composed of a long optical fiber, similar to the first optical path 191. The length of the third optical path 193 is adjusted so that the propagation delay time occurring when traveling back and forth through the third optical path 193 is approximately the same as the propagation delay time occurring when traveling back and forth through the first optical path 191 and the second optical path 192. In other words, it is preferable that the optical path length of the third optical path 193 is equal to the sum of the optical path lengths of the first optical path 191 and the second optical path 192, but the difference in delay time between them should be equal to or less than the coherence time of the laser light source 110 at most.

[0133] The Faraday rotator mirror 162 utilizes the Faraday effect to rotate the light input from the input / output end by 90 degrees non-reciprocally with respect to the polarization state, and outputs the rotated light from the input / output end.

[0134] An example of the configuration of a Faraday rotator mirror will be described with reference to Fig. 6. Fig. 6 is a schematic diagram for explaining a Faraday rotator mirror.

[0135] The Faraday rotator mirror 162 includes a collimator lens 163, a Faraday rotator 164, and a total reflection mirror 165. The Faraday rotator 164 is configured similarly to the Faraday rotator 144 included in the sensor head 140, and non-reciprocally rotates the input light by 45°.

[0136] The input light Ea propagating through the third optical path 193 and input to the Faraday rotator mirror 162 is collimated by the collimator lens 163. The light collimated by the collimator lens 163 passes through the Faraday rotator 164 and is reflected by the total reflection mirror 165. After that, the light propagates in the reverse direction through the Faraday rotator 164 and is collected by the collimator lens 163, and output light Eb is output. The output light Eb is input to the third optical path 193 and propagates in the reverse direction through the third optical path 193 toward the second optical circulator 132.

[0137] At this time, due to the effect of the Faraday rotator 164, the polarization of the output light Eb output from the Faraday rotator mirror 162 is rotated by 90 degrees with respect to the polarization of the input light Ea. As a result, when the local light output from the second input / output port 132 passes through the third optical path 193 and the Faraday rotator mirror 162 and propagates in the reverse direction again through the third optical path 193, the polarization of the local light after reverse propagation becomes y-polarized when the initial local light output from the laser light source is x-polarized. Such Faraday rotator mirrors are commercially available, and a suitable one from among them can be used.

[0138] The output light Eb of the Faraday rotator mirror 162 propagates in the reverse direction through the third optical path 193, and is then input to the second input / output port 132-2 of the second optical circulator 132 and output from the third input / output port 132-3. The local light output from the third input / output port 132-3 is sent to the second optical coupler 122.

[0139] Note that, although an example has been described here in which the frequency shifter 150 is provided between the first coupler 121 and the second optical circulator 132, the frequency shifter 150 may also be disposed in the optical path connecting the third input / output port 162-3 of the second optical circulator 162 and the second optical coupler 122.

[0140] The second optical coupler 122 causes interference between the measurement light sent from the first optical circulator 131 and the local light sent from the second optical circulator 132. In the second optical coupler 122, interference light obtained by interference between the measurement light and the local light is sent to the photodetector 210.

[0141] Although the first and second optical circulators are used as the first and second optical multiplexers and demultiplexers, respectively, the present invention is not limited to this. As in the first vibrometer, a PBS may be used as the optical multiplexer and demultiplexer instead of the optical circulator.

[0142] The photodetector 210, the ADC 220, and the signal processing circuit 230 included in the second vibrometer as an electrical signal system have the same configuration and operation as those of the first vibrometer, and therefore a duplicated description will be omitted.

[0143] In general, a light source has a finite spectral width (linewidth), and its coherence time is inversely proportional to the linewidth. If the output of the same light source is split using an optical coupler or the like, and one of the split light waves is given a delay time that exceeds the coherence time, and then recombined to cause interference, the two interfering light waves will not interfere well even if they have the same polarization, making it difficult to detect the phase difference between the two light waves.

[0144] From the above, in the laser Doppler vibrometer, local light E LO and measurement light E s In order to measure vibration by interfering with each other, the local light E LO and measurement light E sThe delay time difference between them needs to be equal to or less than the coherence time of the laser light source 110. If the optical fiber that is the first optical path 191 is lengthened, there is a concern that the delay time difference described above will become larger than the coherence time of the laser light source 110.

[0145] To solve this problem, local light E LO A delay time compensation optical path having a length for compensating for the delay time difference may be disposed on the optical path from the first optical coupler 121 to the second optical coupler 122. This delay time compensation optical path is preferably an optical fiber transmission line having a length approximately equal to the sum of the optical path lengths of the first optical path 191 and the second optical path 192.

[0146] On the other hand, in this delay time compensation optical path, polarization rotation of the local light occurs during propagation, similar to the first optical path 191. Therefore, the polarizations of the measurement light and the local light input to the second optical coupler 122, which is an optical interferometer, may not match each other (in the worst case, they may be orthogonal), which may make it impossible to measure the vibration of the measurement target.

[0147] The second vibrometer solves the above problem by configuring a delay time compensation optical path with a second optical circulator 132, a third optical path 193 made of an optical fiber having a length approximately equal to the sum of the optical path lengths of the first optical path 191 and the second optical path 192, and a Faraday rotator mirror 162.

[0148] As explained in the operation of the first vibrometer, the local light input as x-polarized light to the first input / output port 132-1 of the second optical circulator 132 is always output as y-polarized light from the third input / output port 132-3 due to the effect of the second Faraday rotator 164 contained in the Faraday rotator mirror 162. However, it is assumed here that the second optical circulator 132 is configured as a polarization-maintaining optical system such as a polarization-maintaining fiber.

[0149] On the other hand, the polarization state of the measurement light output from the third input / output port 131-3 of the first optical circulator 131 is also always y-polarized.

[0150] Therefore, in the second vibrometer, even without the polarization converter 160, the measurement light and the local light are always combined as y-polarized waves in the second optical coupler 122, so that the same effect as in the first vibrometer can be obtained. Thus, the polarization converter 160 is not necessary in the second vibrometer.

[0151] According to the second vibrometer of the present invention, in addition to the effects obtained by the first vibrometer, the following effects can be obtained: That is, even if the first optical path 191 is composed of a long optical fiber having a delay time exceeding the coherence time of the laser light source 110, the second vibrometer has a third optical path 193 composed of an optical fiber having a length similar to that of the first optical path 191, so that the measurement light and the local light can be interfered with each other with a delay time difference within the coherence time of the laser light source 110, thereby enabling vibration measurement.

[0152] Here, the operation of the first vibrometer and the second vibrometer has been described with reference to heterodyne detection as an example, but the effects of the present invention are not limited to heterodyne detection and can also be applied to homodyne detection.

[0153] The electrical signal processing systems of the first and second vibrometers are similar to those used in vibration measurement using a conventional homodyne or heterodyne method, and therefore any suitable conventional technology may be used. For example, signal processing may be performed using the method and circuit configuration disclosed in Patent Document 2.

[0154] The configuration of the sensor head 140 is not limited to the configuration shown in FIG. 1 in which the objective lens 142 and the Faraday rotator 144 are integrated into a module.

[0155] Fig. 7 is a schematic diagram showing another example of the configuration of the sensor head. Fig. 7 shows an example in which a sensor head 400 is configured using a Faraday rotator module 410 and a fiber collimator module 420, in which the Faraday rotator unit and the objective lens are separated as separate optical modules.

[0156] The Faraday rotator module 410 is an optical module that inputs and outputs light to and from a Faraday rotator 414 via pigtail optical fibers 411 and 418. Since the light needs to be converted into a parallel beam when it is input to and output from the Faraday rotator 414, collimator lenses 412 and 416 are provided inside.

[0157] The fiber collimator module 420 performs processes such as collimating the fiber-propagated light from the pigtail optical fiber 428, and inputs and outputs the light to and from the external free space. For inputting and outputting the light to and from the external free space, an objective lens 422 similar to that used in the first vibrometer is provided.

[0158] The sensor head 400 is configured by optically coupling a Faraday rotator module 410 and a fiber collimator module 420 via pigtail optical fibers 418 and 428 .

[0159] Such Faraday rotator module 410 and fiber collimator module 420 are both commercially available, and an appropriate one can be used depending on the wavelength, etc.

[0160] 7, pigtail optical fibers 418 and 428 are present in the optical path between the Faraday rotator 414 and the measurement object 1000. As discussed in the description of the operation of the first vibrometer regarding the effects of birefringence and PDL, the influence of the polarization rotation occurring in these two pigtail optical fibers remains even if the Faraday rotator 414 is used.

[0161] In order to prevent such polarization rotation, it is desirable that the pigtail optical fibers 418 and 428 are constructed of polarization-maintaining optical fibers, that their lengths be equal, and that their optical axes be rotated 90° at the connection point 430. [Explanation of symbols]

[0162] 100, 101 Optical processing system 110 Laser light source 121, 122 Optical coupler 125 Polarizing Beam Splitter (PBS) 130, 131, 132 Optical circulator 135 Polarization-Maintaining Optical Circulator 140, 400 sensor head 142, 422 Objective Lens 144 Faraday Rotator 150 Frequency Shifter 160 Polarization Rotator 162 Faraday Rotator Mirror 143, 163, 412, 416 Collimator Lenses 164, 414 Faraday rotator 165 Total Reflection Mirror 182 λ / 2 wave plate 184 λ / 4 wave plate 186, 188 Power meter 191, 192, 193 light path 200 Electrical Processing System 210 Photodetector 220 ADC 230 Signal Processing Circuit 410 Faraday Rotator Module 411, 418, 428 Optical Pigtail 420 Fiber Collimator Module 1000 Measurement objects 1010 Mirror

Claims

1. a first optical coupler that splits a laser beam from a laser light source into two beams, one of which is used as an irradiation beam and the other of which is used as a local beam; a sensor head that receives the illumination light generated by the first optical coupler via a first optical path, illuminates the object to be measured via a second optical path, receives measurement light reflected by the object to be measured from the illumination light via the second optical path, and propagates the measurement light along the first optical path; a second optical coupler that generates interference light by causing the measurement light propagating through the first optical path to interfere with the local light; an electrical processing system for acquiring information on vibration of the measurement object from the interference light; Equipped with The sensor head is equipped with a Faraday rotator that rotates the polarization by 45°. Laser Doppler vibrometer.

2. an optical multiplexer / demultiplexer that outputs the illumination light generated by the first optical coupler and input to a first input / output port from a second input / output port to propagate the illumination light along the first optical path, and outputs the measurement light that is input to the second input / output port and has propagated along the first optical path from a third input / output port; a polarization rotator that rotates the polarization of the measurement light output from the third input / output port of the optical multiplexer / demultiplexer by 90° and sends the measurement light to the second optical coupler; The laser Doppler vibrometer of claim 1 .

3. a first optical multiplexer / demultiplexer that outputs irradiation light generated by the first optical coupler and input to a first input / output port from a second input / output port to propagate the first optical path, and outputs measurement light that is input to the second input / output port and has propagated through the first optical path from a third input / output port to send the measurement light to the second optical coupler; a second optical multiplexer / demultiplexer that outputs local light that is input to a first input / output port and generated by the first coupler from a second input / output port to propagate the local light through a third optical path, and that outputs local light that is input to the second input / output port and has propagated through the third optical path from a third input / output port to send the local light to the second optical coupler; a Faraday rotator mirror provided on the opposite side of the third optical path from the second optical multiplexer / demultiplexer; The laser Doppler vibrometer of claim 1 .

4. The optical path length of the third optical path is equal to the sum of the optical path lengths of the first optical path and the second optical path.

4. The laser Doppler vibrometer according to claim 3.

5. The optical multiplexer / demultiplexer is an optical circulator. The laser Doppler vibrometer according to any one of claims 2 to 4.

6. The optical multiplexer / demultiplexer is a polarizing beam splitter. The laser Doppler vibrometer according to any one of claims 2 to 4.

7. A process of splitting a laser beam into two beams, one of which is used as an illumination beam and the other is used as a local beam; receiving the irradiated light via a first optical path, rotating the polarization by 45°, and then irradiating the measured object via a second optical path; receiving measurement light reflected by a measurement object through the second optical path, rotating the polarization by 45°, and propagating the measurement light through the first optical path; generating interference light by causing the measurement light propagated through the first optical path to interfere with the local light; obtaining information on the vibration of the measurement object from the interference light; Equipped with The two 45° rotations of polarization are non-reciprocal Vibration measurement methods.

8. The step of generating the interference light is performed before the step of generating the interference light. rotating the polarization of the measurement light propagating through the first optical path by 90 degrees; The vibration measuring method according to claim 7 , further comprising:

9. The step of generating the interference light is performed before the step of generating the interference light. propagating the localized light through a third optical path; rotating the polarization of the local light propagated through the third optical path by 90 degrees; a step of propagating the local light whose polarization has been rotated by 90° through the third optical path again; Equipped The vibration measuring method according to claim 7.

10. The process of rotating the polarization of the local light by 90° includes: rotating the polarization of the local light propagating through the third optical path by 45° and then totally reflecting the local light; and rotating the polarization of the totally reflected localized light by 45° and then propagating the localized light through the third optical path again. The two 45° polarization rotations performed on the local light are non-reciprocal The vibration measuring method according to claim 9.

11. The optical path length of the third optical path is equal to the sum of the optical path lengths of the first optical path and the second optical path. The vibration measuring method according to claim 9 or 10.

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