JP7622582B2 - Vibration meter and vibration measurement method - Google Patents

Description

This invention relates to a vibrometer, such as a laser Doppler vibrometer, and a vibration measurement method.

When scattered light is generated by shining light onto a vibrating object, a vibration component caused by the Doppler effect is superimposed on the scattered light. Therefore, by receiving the scattered light generated when light is irradiated onto a vibrating object and demodulating the received scattered light, the phase fluctuation caused by the Doppler effect can be detected and the vibration of the object can be measured.

A laser Doppler vibrometer is a vibrometer that uses the Doppler effect of light to measure vibrations without contact.

Conventionally, contact-type vibrometers have been widely used for vibration measurement. However, it is difficult to use contact-type vibrometers when the object being measured is far away, or in high temperatures or strong magnetic fields. In addition, contact-type vibrometers have their own inherent frequency. For this reason, they cannot accurately measure vibrations on the order of kHz or higher.

In contrast, the laser Doppler vibrometer is a non-contact type of vibrometer, and can therefore measure vibrations at great distances, at high temperatures, and in strong magnetic fields. Laser Doppler vibrometers can also accurately measure vibrations on the order of kHz or higher.

Laser Doppler vibrometers, which take advantage of their non-contact nature, are expected to play an important role in recent years in predictive maintenance of equipment in factories and plants, and in infrastructure and structure inspection.

The configuration of a typical laser Doppler vibrometer will be described with reference to FIG. 3. FIG. 3 is a schematic diagram for explaining the configuration of a typical laser Doppler vibrometer (see, for example, Patent Document 1).

The laser light source 110 generates continuous light that is linearly polarized and oscillates at a center frequency f ₀ , and emits the continuous light. The continuous light emitted from the laser light source 110 is sent to the coupler 121.

The coupler 121 splits the continuous light into two. One of the split beams is used as a measurement beam to irradiate the object to be measured 200, and the other is used as a reference beam to cause interference with the scattered light.

The reference light is frequency shifted in the frequency shifter 126 and then sent to the optical phase detection circuit 122. The measurement light passes through a polarizing beam splitter (PBS) 124 and is then sent to a λ/4 plate 144. The λ/4 plate 144 converts the measurement light from linearly polarized light to circularly polarized light. The measurement light converted to circularly polarized light propagates through the optical fiber 130 and is emitted into space by the objective lens 150 and irradiated onto the measurement object 200.

A part of the scattered light generated by scattering the measurement light irradiated on the measurement object 200 at the measurement object 200 is captured by the objective lens 150. The scattered light captured by the objective lens 150 propagates through the optical fiber 130 and is sent to the λ/4 plate 144. The λ/4 plate 144 converts the scattered light from circularly polarized light to linearly polarized light. The scattered light converted to linearly polarized light by the λ/4 plate 144 is reflected by the PBS 12.
Sent to 4.

The polarization direction of the scattered light, which is linearly polarized and sent to the PBS 124, is perpendicular to the polarization direction of the measurement light sent to the PBS 124. Therefore, the scattered light is reflected by the PBS 124 and sent to the optical phase detection circuit 122. The scattered light and the reference light sent to the optical phase detection circuit 122 interfere with each other in the optical phase detection circuit 122 to generate interference light. The interference light is sent to the photoreceiver 160. The photoreceiver 160 photoelectrically converts the interference light to generate an interference electric signal. The interference electric signal is sent to an analog-to-digital converter (ADC) 170. The interference electric signal is converted to a digital signal in the ADC 170 and then sent to the signal processing circuit 180. The signal processing circuit 180 calculates the phase fluctuation component of the scattered light resulting from the vibration of the measurement object 200 from the digital signal, and obtains information such as the frequency, displacement, speed, and acceleration of the vibration.

In the general laser Doppler vibrometer described with reference to FIG. 3, the reference light and the measurement light and scattered light travel different paths. Therefore, if disturbance vibration is applied to the optical fiber 130 before the measurement light and scattered light reach the optical phase detection circuit 122 from the coupler 121, the phase fluctuation caused by the vibration of the optical fiber 130 is superimposed on the interference light, resulting in an inaccurate vibration measurement of the object to be measured 200.

To solve this problem, a technique has been proposed in which the reference light and the measurement light are propagated through a first optical fiber, the measurement light is then irradiated onto the object to be measured, and the reference light and the scattered light are propagated through a second optical fiber (see, for example, Patent Document 2). In the technique disclosed in Patent Document 2, the laser light propagated through the first optical fiber is incident on a PBS with its polarization axis tilted at 45 degrees, and separated into reference light and measurement light that are orthogonal to each other. The reference light is incident on the PBS and transmitted through the PBS after its polarization axis is rotated 90 degrees. On the other hand, the scattered light generated by the scattering of the measurement light at the object to be measured is incident on the PBS and reflected with its polarization axis rotated 90 degrees relative to the measurement light. The reference light that has passed through the PBS and the scattered light reflected by the PBS are propagated through the second optical fiber and sent to an optical phase detection circuit.

In the technology disclosed in Patent Document 2, even if disturbance vibration is applied to the first optical fiber or the second optical fiber, the same phase fluctuation occurs in the reference light and the measurement light, and the same phase fluctuation occurs in the reference light and the scattered light. Therefore, the original vibration of the measurement object can be measured.

However, the technology disclosed in the above-mentioned Patent Document 1 requires two optical fibers, a first optical fiber and a second optical fiber, making the system complicated and expensive.

On the other hand, a technique has been proposed in which the reference light and measurement light are propagated through an optical fiber, the measurement light is then irradiated onto the object to be measured, and the reference light and scattered light are propagated through the same optical fiber (see, for example, Patent Document 3). In the technique disclosed in Patent Document 3, laser light is propagated through an optical fiber and then separated by a half mirror. The component reflected by the half mirror becomes the reference light, and the component transmitted through the half mirror becomes the measurement light. The measurement light is scattered by the object to be measured. The scattered light generated by the object to be measured passes through the half mirror with its polarization axis rotated 90 degrees relative to the measurement light and reference light. This reference light and scattered light are propagated through the same optical fiber as the laser light and sent to the optical phase detection circuit.

Even in the technology disclosed in Patent Document 3, if disturbance vibration is applied to the optical fiber, the same phase fluctuation occurs in the reference light and the measurement light, and the same phase fluctuation occurs in the reference light and the scattered light. Therefore, the original vibration of the measurement object can be measured.

JP 2001-159560 A Japanese Patent Application Publication No. 4-218730 Japanese Patent Application Publication No. 4-249719

Kenta Yokota, "Comparison of Ellipse Fitting Accuracy: From Least Squares Method to Ultra-Precise Renormalization Method," Information Processing Society of Japan Research Report, Vol. 2012-CVIM-180 No. 24, January 19, 2012

However, in the technology disclosed in the above-mentioned Patent Document 3, polarization crosstalk may occur between orthogonal polarization components of the laser light propagating through the optical fiber, or between the reference light and the scattered light. It is known that the longer the optical fiber, which is a polarization-maintaining fiber, the greater the polarization crosstalk.

When polarization crosstalk becomes large, the required signal becomes relatively small, making accurate measurement difficult. Furthermore, if the polarization crosstalk fluctuates due to fluctuations in the refractive index of the optical fiber, measurement becomes even more difficult.

This invention was made in consideration of the above problems. The purpose of this invention is to provide a vibrometer and a vibration measurement method that can measure the inherent vibration of the object being measured without being affected by external vibrations applied to the optical fiber, and can accurately measure the vibration of the object being measured even if polarization crosstalk occurs in the optical fiber.

To achieve the above-mentioned objective, the vibrometer of the present invention is configured with a laser light source, a polarization-maintaining optical fiber, a light distribution section, an interference optical system, a photodetector, and a signal processing circuit.

The laser light source generates continuous light, which is propagated through a polarization-maintaining optical fiber and sent to the light distribution section. The light distribution section sends a portion of the continuous light to the polarization-maintaining optical fiber as reference light, and irradiates a portion of the continuous light as measurement light onto the object to be measured. The measurement light is scattered by the object to be measured, and the scattered light is sent to the polarization-maintaining optical fiber via the light distribution section. The reference light and scattered light propagate through the polarization-maintaining optical fiber and are sent to the interference optical system. The interference optical system causes the reference light and scattered light to interfere with each other, generating interference light.

The photodetector converts the interference light into an electrical interference signal. The signal processing circuit calculates the parameters of an ellipse obtained by plotting the interference signal on a complex plane, and uses the parameters to correct the ellipse into a circle to obtain a phase angle, and calculates the vibration of the object to be measured from the phase angle.

According to a preferred embodiment of the vibrometer of this invention, the light distribution unit includes a half mirror and a λ/4 plate. The half mirror reflects a portion of the continuous light and sends it to the polarization-maintaining optical fiber as reference light, and transmits a portion of the continuous light and sends it to the λ/4 plate as measurement light. The measurement light is converted from linearly polarized light to circularly polarized light by the λ/4 plate, and then irradiated onto the measurement object. The scattered light is converted from circularly polarized light to linearly polarized light by the λ/4 plate, and then sent to the half mirror.

The interference optical system includes a polarizing beam splitter, a first beam splitter, a second beam splitter, a frequency shifter, and a λ/2 plate. The polarizing beam splitter transmits the reference light and sends it to the first beam splitter, and reflects the scattered light and sends it to the second beam splitter via the first optical path. The reference light sent to the first beam splitter is sent to the second beam splitter via the second optical path. The frequency shifter is provided in either the first optical path or the second optical path, and applies a predetermined frequency shift. The λ/2 plate is provided in either the first optical path or the second optical path, and rotates the direction of polarization by 90 degrees.

The vibration measurement method of the present invention includes the steps of generating continuous light, propagating the continuous light through a polarization-maintaining optical fiber, propagating a portion of the continuous light as a reference light through the polarization-maintaining optical fiber, irradiating a portion of the continuous light as a measurement light onto a measurement object, and propagating the scattered light generated when the measurement light is scattered by the measurement object through the polarization-maintaining optical fiber, causing the reference light and the scattered light to interfere with each other to generate interference light, converting the interference light into an interference signal that is an electrical signal, calculating the parameters of an ellipse obtained by drawing the interference signal on a complex plane, correcting the ellipse to a circle using the parameters to obtain a phase angle, and calculating the vibration of the measurement object from the phase angle.

The vibrometer and vibration measurement method of this invention allow accurate measurement of the vibration of an object to be measured even when polarization crosstalk occurs.

FIG. 2 is a schematic diagram for explaining a vibrometer according to the present invention. 3A and 3B are schematic diagrams for explaining input and output light of a frequency shifter; FIG. 1 is a schematic diagram for explaining a conventional vibrometer.

Below, 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 roughly to the extent that the invention can be understood. In addition, below, a preferred configuration example of the present invention will be described, 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. In addition, explanations of components common to Patent Documents 1 to 3 may be omitted.

An embodiment of the vibrometer of the present invention will be described with reference to FIG. 1. FIG. 1 is a schematic diagram for explaining the vibrometer of the present invention, showing one configuration example of a laser Doppler vibrometer.

The vibrometer is composed of a laser light source 10, a polarization-maintaining optical fiber 30, a light distribution unit 40, an interference optical system 20, a photodetector 60, and a signal processing circuit 80. The light distribution unit 40 includes a half mirror 42 and a λ/4 plate 44. The light distribution unit 40 generates reference light and measurement light from the continuous light. The interference optical system 20 includes a polarizing beam splitter (PBS) 24, a first beam splitter (BS) 21, a second beam splitter (BS) 22, a frequency shifter 26, and a λ/2 plate 28. The interference optical system 20 generates interference light by causing interference between the reference light and the scattered light.

The laser light source 10 generates linearly polarized continuous light (laser light) oscillating at a center frequency f ₀ and emits the continuous light.

The continuous light emitted from the laser light source 10 passes through the first BS 21 and the PBS 24 of the interference optical system 20 and the polarization-maintaining optical fiber 30 in this order, and is sent to the half mirror 42 of the light distribution unit 40. The half mirror 42 transmits part of the continuous light and reflects the other part.
The transmitted light that has passed through the half mirror 2 is used as measurement light that is irradiated onto the measurement object 100. On the other hand, the reflected light that has been reflected by the half mirror 42 is used as reference light that is to interfere with scattered light generated by the measurement object 100.

The measurement light is sent from the half mirror 42 to the λ/4 plate 44. The λ/4 plate 44 converts the measurement light from linearly polarized light to circularly polarized light. The measurement light converted to circularly polarized light is emitted into space by the objective lens 50 and irradiated onto the measurement object 100.

The measurement light irradiated to the measurement object 100 is scattered by the measurement object 100, and a part of the scattered light generated by the scattering is captured by the objective lens 50. The scattered light captured by the objective lens 50 is sent to the λ/4 plate 44. The λ/4 plate 44 converts the scattered light from circularly polarized light to linearly polarized light. The polarization direction of the scattered light converted to linearly polarized light by this λ/4 plate 44 is perpendicular to the polarization direction of the continuous light generated by the laser light source 10. The scattered light converted to linearly polarized light by the λ/4 plate 44 is sent to the polarization-maintaining optical fiber 30 via the half mirror 42.

Meanwhile, a reference light obtained by reflecting a portion of the continuous light by the half mirror 42 is also sent to the polarization-maintaining optical fiber 30. The polarization direction of this reference light is parallel to the polarization direction of the continuous light generated by the laser light source 10.

The reference light and scattered light sent to the polarization-maintaining optical fiber 30 propagate through the polarization-maintaining optical fiber 30 and are incident on the PBS 24 of the interference optical system 20. The polarization direction of the reference light incident on the PBS 24 is parallel to the continuous light, and the polarization direction of the scattered light incident on the PBS 24 is perpendicular to the continuous light. Therefore, the reference light incident on the PBS 24 passes through the PBS 24 and is sent to the first BS 21. On the other hand, the scattered light incident on the PBS 24 is reflected by the PBS 24 and sent to the λ/2 plate 28.

The λ/2 plate 28 rotates the polarization direction of the scattered light reflected by the PBS 24 by 90 degrees. As a result, the polarization direction of the scattered light output from the λ/2 plate 28 becomes parallel to the polarization direction of the continuous light and the reference light. The scattered light output from the λ/2 plate 28 is reflected or transmitted by the second BS 22 and sent to the photodetector 60.

The reference light sent to the first BS 21 is reflected by the first BS 21 and then sent to the frequency shifter 26. Fig. 2 is a schematic diagram showing the spectrum of the input/output light of the frequency shifter 26. Fig. 2(A) shows the spectrum of the reference light input to the frequency shifter 26, and Fig. 2(B) shows the spectrum of the reference light output from the frequency shifter 26. The frequency shifter 26 imparts a frequency shift of frequency f shift to the input reference light of frequency f _0. The reference light that has been frequency shifted by frequency f _{shift is} sent to the second BS 22.

The reference light sent to the second BS 22 is transmitted through or reflected by the second BS 22 and sent to the photodetector 60. The scattered light and reference light sent to the second BS 22 have polarization directions parallel to the continuous light, i.e., parallel to each other. Therefore, the scattered light and the reference light interfere with each other to generate interference light.

Here, of the optical paths from the PBS 24 to the second BS 22, the optical path of the scattered light is referred to as the first optical path, and the optical path of the reference light is referred to as the second optical path. In this example, an example in which the λ/2 plate 28 is provided in the first optical path has been described, but the λ/2 plate 28 may be provided in either the first optical path or the second optical path. That is, the polarization direction of either the reference light or the scattered light may be rotated by 90 degrees. Also, in this example, an example in which the frequency shifter 26 is provided in the second optical path has been described, but the frequency shifter 26 may be provided in either the first optical path or the second optical path. That is, a frequency shift may be applied to either the reference light or the scattered light.

The photodetector 60 obtains a current signal from the interference light obtained by the interference between the reference light and the scattered light. The current signal obtained by the photodetector 60 is sent to an analog-to-digital converter (ADC) 70.

The ADC 70 samples the current signal, which is an analog signal, at a predetermined sampling frequency to obtain a digital signal. The digital signal generated by the ADC 70 is sent to the signal processing circuit 80.

The signal processing circuit 80 is, for example, a personal computer (PC). The signal processing circuit 80 is a part that processes digital signals. In the signal processing circuit 80, for example, a central processing unit (CPU: Central Processing Unit) executes a program to realize a desired function.

The signal processing circuit 80 performs signal processing on the digital signal obtained from the interference light obtained by interference between the reference light and the scattered light. When the measurement object 100 is in a vibrating state, the vibration component of the measurement object is added to the scattered light due to the Doppler effect. The signal processing circuit 80 obtains the phase change of the interference light by demodulating the digital signal obtained from the interference light, and can calculate the vibration state of the measurement object 100 based on this phase change.

The complex signal of the scattered light incident on the photodetector 60 is given by the following equation (1):

Here, j is an imaginary unit, A _SI is the complex amplitude of the scattered light, ω ₀ (=2πf ₀ ) is the central angular frequency of the continuous light generated by the laser light source 10, t is time, and φ(t) is the phase fluctuation component of the light resulting from the vibration of the measurement object 100.

On the other hand, the complex signal of the reference light incident on the photodetector 60 is given by the following equation (2):

Here, A _LO is the complex amplitude of the reference light, and ω _shift (=2πf _shift ) is the amount of angular frequency shift of the frequency shifter 26 .

In the photodetector 60, the current signal obtained from the interference light is given by the following equation (3):

Here, R is the conversion efficiency of the photodetector 60, P _SI and P _LO are the powers of the scattered light and the reference light, respectively, which satisfy the relationships of the following equations (4) and (5).

Here, taking into consideration the polarization crosstalk in the polarization-maintaining optical fiber 30, the complex signals of the scattered light and reference light incident on the photodetector 60 are expressed by the following equations (6) and (7).

Here, A _SI1 is the complex amplitude of the scattered light, A _SI2 is the complex amplitude of the leaked light caused by the polarization crosstalk of the reference light, A _LO1 is the complex amplitude of the reference light, and A _LO2 is the complex amplitude of the leaked light caused by the polarization crosstalk of the scattered light. The second terms of the above formulas (6) and (7) are signal components caused by the polarization crosstalk. When the polarization crosstalk increases, A _SI2 and A _LO2 in the above formulas (6) and (7) increase.

The current signal obtained from the interference light obtained by the interference of the scattered light and reference light given by (6) and (7) above is given by the following equation (8).

Next, when the signal component of ω _shift is extracted from the above equation (8) by a band-pass filter, the following equation (9) is obtained.

Here, to simplify the equation, the conversion efficiency R is set to 1.

The bandpass filter may be configured as an analog circuit or a digital circuit.

When the bandpass filter is configured as an analog circuit, the bandpass filter is provided after the photodetector 60. The output of the bandpass filter is input to the signal processing circuit 80 via the ADC 70.

When the bandpass filter is configured as a digital circuit, the output of the photodetector 60 is sampled by the ADC 70 at a sufficiently large sampling frequency that is at least twice the amount of frequency shift, i.e., at least 2f _shift , and then a digital bandpass filter is applied by the signal processing circuit 80.

After extracting the signal component of ω _shift using a band-pass filter, the above equation (9) is multiplied by exp(−jωt), the signal of frequency 2f _shift is removed using a low-pass filter, and the equation is transformed to obtain the following equation (10).

Here, under conditions in which no polarization crosstalk occurs, P _SI2 ∼ 0 and P _LO2 ∼ 0 can be set, and therefore the following equation (11) can be obtained approximately from the above equation (10).

Therefore, in a situation where no polarization crosstalk occurs, vibration information can be extracted by calculating the phase angle of the above equation (11).

However, in a situation where polarization crosstalk occurs, it is difficult to derive the above formula (11) from the above formula (10). Therefore, signal processing is required to derive the above formula (11) from the above formula (10). Therefore, the signal processing for deriving the above formula (11) from the above formula (10) will be explained.

When the above formula (10) is plotted on a complex plane, an ellipse is drawn. When this ellipse is fitted using any suitable conventionally known least squares method, the parameters of the ellipse can be obtained. Ellipse fitting is disclosed, for example, in Non-Patent Document 1.

Ellipse fitting calculates the center coordinates, major axis length, minor axis length, and inclination as parameters of the ellipse. Using these calculated parameters, the ellipse is corrected to a circle with a radius of r and centered at the origin. With this correction, (P _SI1 P _LO1 ) ^1/2 + (P _SI2 P _LO2 ) ^1/2 and (P _SI1 P _LO1 ) ^1/2 - (P _SI2 P _LO2 ) ^1/2 in the above formula (10) all become r, and (P _SI1 P _LO2 ) ^1/2 and (P _SI2 P _LO1 ) ^1/2 both become 0. Therefore, the following formula (12) is obtained.

The phase angle is obtained from the above equation (12), and the vibration of the object to be measured 100 can be calculated.

In this way, according to the vibrometer and vibration measurement method of this invention, the reference light, measurement light, and scattered light propagate through the same polarization-maintaining optical fiber, so even if an external vibration occurs in the optical fiber, the same phase fluctuation occurs in the reference light, measurement light, and scattered light, making it possible to measure the original vibration of the object being measured.

Furthermore, even if polarization crosstalk occurs in a polarization-maintaining optical fiber, the effects of polarization crosstalk can be mitigated and the vibration of the target object can be accurately measured by plotting the electrical signal obtained by the photodetector on a complex plane and correcting the drawn ellipse to a circle.

However, slight variations in the refractive index of the optical fiber may cause the shape of the ellipse to change over time. In this case, it is advisable to perform ellipse fitting for each time length during which the shape of the ellipse can be considered constant, and then correct it to a circle.

Note that if an outlier suddenly occurs in the parameters of the ellipse, it can cause a deterioration in accuracy. Therefore, it is preferable to estimate the parameters using signal processing such as a conventionally known Kalman filter.

10, 110 Laser light source 20 Interference optical system 21, 22 Beam splitter (BS)
24, 124 Polarizing beam splitter (PBS)
26, 126 Frequency shifter 28 λ/2 plate 30 Polarization-maintaining optical fiber 40 Light distribution section 42 Half mirror 44, 144 λ/4 plate 50, 150 Objective lens 60 Photodetector 70, 170 Analog-to-digital converter (ADC)
80, 180 Signal processing circuit 100, 200 Measurement object 121 Coupler 122 Optical phase detection circuit 130 Optical fiber 160 Photoreceiver

Claims (3)

The optical fiber includes a laser light source, a polarization-maintaining optical fiber, a light distribution unit, an interference optical system, a photodetector, and a signal processing circuit.
the laser light source generates continuous light, and transmits the continuous light to the optical distribution unit by propagating the continuous light through the polarization-maintaining optical fiber;
the light distribution unit sends a portion of the continuous light to the polarization-maintaining optical fiber as a reference light, and irradiates a portion of the continuous light to a measurement object as a measurement light;
The measurement light is scattered by the object to be measured, and the scattered light is sent to the polarization-maintaining optical fiber via the light distribution unit.
the reference light and the scattered light are transmitted through the polarization-maintaining optical fiber to the interference optical system;
the interference optical system causes the reference light and the scattered light to interfere with each other to generate interference light;
The photodetector converts the interference light into an interference signal that is an electrical signal,
the signal processing circuit calculates parameters of an ellipse obtained by plotting the interference signal on a complex plane, corrects the ellipse to a circle using the parameters to obtain a phase angle, and calculates the vibration of the object to be measured from the phase angle. the light distribution unit includes a half mirror and a λ/4 plate,
the half mirror reflects a portion of the continuous light and sends it to the polarization-maintaining optical fiber as the reference light, and transmits a portion of the continuous light to the λ/4 plate as the measurement light;
The measurement light is converted from linearly polarized light to circularly polarized light by the λ/4 plate, and then irradiated onto the measurement object,
The scattered light is converted from circularly polarized light to linearly polarized light by the λ/4 plate, and then sent to the half mirror.
the interference optical system includes a polarizing beam splitter, a first beam splitter, a second beam splitter, a frequency shifter, and a λ/2 plate;
the polarizing beam splitter transmits the reference light and sends it to the first beam splitter, and reflects the scattered light and sends it to the second beam splitter via a first optical path;
the reference light transmitted to the first beam splitter is transmitted to the second beam splitter via a second optical path;
the frequency shifter is provided in either the first optical path or the second optical path and provides a predetermined frequency shift;
2. The vibrometer according to claim 1, wherein the half-wave plate is provided in either the first optical path or the second optical path and rotates a direction of polarization by 90 degrees. generating a continuous light;
propagating the continuous light through a polarization-maintaining optical fiber;
a step of propagating a portion of the continuous light as a reference light through the polarization-maintaining optical fiber, irradiating a measurement object with the portion of the continuous light as a measurement light, and propagating scattered light generated by scattering the measurement light at the measurement object through the polarization-maintaining optical fiber;
causing the reference light and the scattered light to interfere with each other to generate an interference light;
converting the interference light into an interference signal that is an electrical signal;
A step of calculating parameters of an ellipse obtained by plotting the interference signal on a complex plane;
using the parameters to correct the ellipse into a circle to obtain a phase angle;
and calculating the vibration of the object to be measured from the phase angle.

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