Vibration meter and vibration measurement method
The vibration meter and measurement method address the limitations of polarization-maintaining fibers by using a continuous light source and phase modulator with Fresnel reflection to propagate signal and reference lights along the same path, reducing noise and cost through longer single-mode fibers.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- OKI ELECTRIC INDUSTRY CO LTD
- Filing Date
- 2022-04-11
- Publication Date
- 2026-05-15
AI Technical Summary
Conventional laser Doppler vibrometers using polarization-maintaining fibers face limitations due to increased polarization cross-talk with longer fiber lengths, which are also more expensive than single-mode fibers, restricting their extension and increasing costs.
A vibration meter and measurement method utilizing a continuous light source, phase modulator, circulator, optical fiber, lens, and signal processing unit, where signal and reference lights propagate along the same optical path, using Fresnel reflection for reference light, and employ a bandpass filter and signal processing to extract vibration information without polarization, allowing for longer fiber lengths and reduced component count.
This approach significantly reduces noise from external disturbances, enables longer fiber lengths, reduces component count, and uses less expensive single-mode fibers, thereby lowering the overall cost of the vibration meter.
Smart Images
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Abstract
Description
[Technical Field]
[0001] This invention relates to a vibration meter, such as a laser Doppler vibrometer, and a vibration measurement method. [Background technology]
[0002] First, I will explain the overview of the laser Doppler vibrometer (see, for example, Patent Document 1).
[0003] In a laser Doppler vibrometer, continuous light generated by a continuous light source is split into two wavelengths, with one wavelength used as the signal light and the other as the reference light.
[0004] Signal light E s This can be expressed, for example, by the following equation (1).
[0005]
number
[0006] Here, P s , ω s and φ s These are the intensity, angular frequency, and initial phase of the signal light, respectively. When the signal light is shone on the object to be measured, a frequency shift occurs due to the Doppler effect, depending on the vibration state of the object. This frequency shift is expressed as a phase change φ(t). Therefore, the object to be measured... thing The signal light E is reflected by s This is given by the following equation (2).
[0007]
number
[0008] Here, if the phase change φ(t) in equation (2) above can be calculated, the vibration state of the object being measured can be determined. For phase measurement, generally, reference light E of different wavelengths is used. LO Heterodyne detection is used, in which interference fringes are detected by a photoelectric converter. Reference light E LOis given by the following formula (3).
[0009] [Number]
[0010] Here, P LO , ω LO and φ LO are the intensity, angular frequency, and initial phase of the reference light, respectively. When the signal light scattered by the measurement object represented by the above formula (2) is directly detected, |E s | 2 = P s and the phase information is lost.
[0011] However, in heterodyne detection, the following formula (4) is obtained, so that the phase change φ(t) can be extracted.
[0012] [Number]
[0013] As a laser Doppler vibrometer, there is an optical fiber-based laser Doppler vibrometer that transmits laser light through an optical fiber, which is advantageous from the viewpoint of ease of handling of the laser light (see, for example, Patent Document 2). Referring to FIG. 2, an optical fiber-based laser Doppler vibrometer as a conventional vibrometer will be described. FIG. 2 is a schematic diagram for explaining a conventional vibrometer.
[0014] The continuous light generated by the continuous light source 110 passes through the first beam splitter (BS) 112 and the polarization beam splitter (PBS) 116 and propagates through the polarization-maintaining optical fiber (PM fiber) 118. A part of the continuous light propagating through the PM fiber 118 is transmitted by the half mirror 142, and the rest is reflected. The transmitted light passing through the half mirror 142 is the signal light, and the reflected light reflected by the half mirror 142 is the reference light. The signal light is irradiated onto the measurement object 190 through the λ / 4 plate 144 and the lens 120.
[0015] The signal light scattered by the measurement object 190 is sent to the PBS 116 through the lens 120, the λ / 4 plate 144, the half mirror 142, and the PM fiber 118. The signal light passes through the λ / 4 plate 144 twice during the round trip between the half mirror 142 and the measurement object 190. Therefore, the signal light sent to the PBS 116 through the PM fiber 118 is incident on the PBS 116 with its polarization rotated by 90° with respect to the continuous light sent from the continuous light source 110. As a result, the signal light is reflected by the PBS 116 and incident on the frequency shifter 146.
[0016] The signal light incident on the frequency shifter 146 is frequency-shifted by the frequency shifter 146 driven by the oscillator 114 and then incident on the second BS 152.
[0017] On the other hand, the reference light whose polarization when incident on the PBS 116 is different from that of the signal light by 90° passes through the PBS 116 and is incident on the first BS 112. The reference light reflected by the first BS 112 is rotated by 90° in polarization by the λ / 2 plate 150. As a result, the signal light and the transmitted light are incident on the second BS 152 with their polarization directions aligned. The interference signal between the signal light and the reference light passes through the second BS 152 and is incident on the photoelectric converter 122. The interference signal is converted into an electrical signal by the photoelectric converter 122, converted into a digital signal by an analog-to-digital converter (ADC) 126, and then sent to the signal processing unit 130. The signal processing unit 130 extracts the phase change φ(t) corresponding to the vibration state of the measurement object 190.
[0018] In the laser Doppler vibrometer described with reference to FIG. 2, since the optical path difference between the transmitted light and the reference light is only the round trip between the half mirror 142 and the measurement object 190, it can be regarded as almost the same. Thus, by propagating the signal light and the reference light along almost the same optical path, it is possible to avoid disturbances such as vibrations applied to the optical fiber from becoming noise and affecting the demodulation of the phase change φ(t).
Prior Art Documents
Patent Documents
[0019] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-159560 [Patent Document 2] Japanese Patent Application Laid-Open No. 4-249719 [Summary of the Invention] [Problems to be Solved by the Invention]
[0020] The laser Doppler vibrometer disclosed in Patent Document 2 described above reduces the influence of disturbances applied to the PM fiber by propagating the signal light and the reference light through the same PM fiber.
[0021] However, since the polarization cross-talk increases as the fiber length of the PM fiber increases, there is a limit to extending the length of the PM fiber. In addition, the PM fiber is more expensive than the commonly used single-mode (SM) fiber.
[0022] This invention has been made in view of the above problems. An object of this invention is to provide a vibrometer and a vibration measuring method capable of reducing the influence of disturbance vibrations applied to an optical fiber without using a PM fiber. [Means for Solving the Problems]
[0023] In order to achieve the above object, the vibrometer of this invention is configured to include a continuous light source, an oscillator, a phase modulator, a circulator, an optical fiber, a lens, a photoelectric converter, a band-pass filter, and a signal processing unit. The continuous light source generates continuous light, and the phase modulator performs a predetermined phase modulation on the continuous light based on the phase modulation drive signal having an angular frequency ω generated by the oscillator. The circulator sends the phase-modulated continuous light to the optical fiber. The continuous light sent from the circulator to the optical fiber passes through the lens as signal light and irradiates the measurement target thing and is irradiated to the measurement target thingThe scattered signal light is sent to the circulator via the lens and optical fiber. A portion of the continuous light sent from the circulator to the optical fiber is reflected as reference light near the end face of the optical fiber, such as at the boundary between the optical fiber and the lens, and this reference light is sent to the circulator via the optical fiber. The signal light and reference light sent to the circulator via the optical fiber are sent to a photoelectric converter, which converts the interference light resulting from the interference of the signal light and reference light into an electrical signal to generate an interference signal, and sends the interference signal to a bandpass filter. The bandpass filter extracts the ω and 2ω components of the interference signal and sends them to the signal processing unit, which uses the ω and 2ω components to process the signal. measurement subject thing Obtain vibration information.
[0024] Furthermore, the vibration measurement method of this invention comprises the process of generating continuous light, the process of applying a predetermined phase modulation to the continuous light based on a phase modulation drive signal with angular frequency ω, the process of propagating the phase-modulated continuous light through an optical fiber, and the process of passing the continuous light propagated through the optical fiber through a lens to the target of measurement as signal light. thing The process of irradiating and the target of measurement thing The process involves: propagating the scattered signal light through the optical fiber via a lens; reflecting a portion of the continuous light propagating through the optical fiber as a reference light near the end face of the optical fiber and propagating the reference light through the optical fiber; converting the interference light resulting from the interference of the signal light and the reference light into an electrical signal to generate an interference signal; extracting the ω and 2ω components of the interference signal; and using the ω and 2ω components, measurement subject thing It includes a process for acquiring information about vibrations. [Effects of the Invention]
[0025] According to the vibration meter and vibration measurement method of this invention, by propagating the signal light and the reference light along almost the same optical path, the effect of external disturbances such as vibrations applied to the optical fiber as noise affecting the demodulation of φ(t) is reduced to a negligible degree. Furthermore, since polarization is not used, the fiber length can be extended compared to conventional technology, the number of required components is greatly reduced, and relatively inexpensive single-mode optical fibers can be used as the optical fibers, thus enabling a reduction in the cost of the vibration meter. [Brief explanation of the drawing]
[0026] [Figure 1] This is a schematic diagram illustrating the vibration meter of this invention. [Figure 2] This is a schematic diagram illustrating a conventional vibration meter. [Modes for carrying out the invention]
[0027] The embodiments of this invention will be described below with reference to the figures, but the shape, size, and arrangement of each component are only shown in a general manner to the extent that the invention can be understood. Furthermore, preferred configuration examples of this invention will be described below, but the material and number of each component will not be explained. The numerical conditions and other factors are merely examples of preferred designs. Therefore, this invention is not limited to the following embodiments, and many modifications or alterations can be made to achieve the effects of this invention without departing from the scope of its configuration. Furthermore, explanations of components common to Patent Documents 1 and 2, etc., may be omitted.
[0028] An embodiment of the vibration meter of this invention will be described with reference to Figure 1. Figure 1 is a schematic diagram illustrating the vibration meter of this invention and shows one example of the configuration of a laser Doppler vibration meter.
[0029] The vibration meter is comprised of a continuous light source 10, a phase modulator 12, an oscillator 14, a circulator 16, an optical fiber 18, a lens 20, a photoelectric converter 22, a bandpass filter 24, an analog-to-digital converter (ADC) 26, and a signal processing unit 30.
[0030] The continuous light source 10 generates continuous light (laser light) and emits it. The continuous light emitted from the continuous light source 10 is sent to the phase modulator 12.
[0031] A modulator drive signal is input to the phase modulator 12 from the oscillator 14 so that a phase modulation signal represented by the following equation (5) is applied to the continuous light.
[0032]
number
[0033] Here, ω, θ1, and β are the angular frequency, initial phase, and modulation depth of the modulator drive signal, respectively.
[0034] Light phase-modulated by the phase modulator 12 is sent to the circulator 16. The circulator 16 is a device that emits light from different positions depending on the incident position. The circulator 16 has, for example, first to third input / output ports 16a to 16c. Light incident on the first input / output port 16a is emitted from the second input / output port 16b, light incident on the second input / output port 16b is emitted from the third input / output port 16c, and light incident on the third input / output port 16c is emitted from the first input / output port 16a. In this example, light phase-modulated by the phase modulator 12 is incident on the first input / output port 16a of the circulator 16 and emitted from the second input / output port 16b.
[0035] Light emitted from the second input / output port 16b of the circulator 16 propagates through the optical fiber 18 used to connect the circulator 16 and the lens 20 and is sent to the lens 20. An SM fiber can be used as the optical fiber 18 used to connect the circulator 16 and the lens 20.
[0036] Light sent to lens 20 passes through lens 20 and is irradiated onto the object to be measured 90 as signal light. The signal light scattered by the object to be measured 90 is again incident on lens 20, propagates through optical fiber 18 and is sent to circulator 16. This signal light is phase-modulated by φ(t) according to the vibration state of the object to be measured 90. As a result, the phase of the signal light is given by equation (6) below.
[0037]
number
[0038] On the other hand, at the connection end face of the optical fiber 18 with the lens 20, a few percent of Fresnel reflected light is generated. Fresnel reflection is a phenomenon in which a portion of light is reflected at the interface between media with different refractive indices. This Fresnel reflected light is also called reference light. The reference light propagates through the optical fiber 18 and is sent to the circulator 16. In this example, we describe an example in which Fresnel reflection at the fiber end face is used as reference light, but we are not limited to this. It is sufficient for a portion of the continuous light to be reflected at the boundary between the optical fiber 18 and the lens 20, and reflected light at the lens end face can also be used as reference light.
[0039] The signal light and reference light propagating through the optical fiber 18 and sent to the circulator 16 are incident on the second input / output port 16b of the circulator 16 and emitted from the third input / output port 16c. The light emitted from the third input / output port 16c of the circulator 16 is sent to the photoelectric converter 22.
[0040] Here, if we let d be the distance from the connection end face of the optical fiber 18 with the lens 20 to the object to be measured 90, and c be the speed of light, then the reference light is reflected 2d / c earlier than the signal light. Therefore, when the phase of the signal light is given by equation (6) above, the phase of the reference light is given by equation (7) below.
[0041]
number
[0042] Here, an example is shown in which the light reflected from the fiber end face is used as the reference light. However, if the lens 20 is a lens group composed of multiple lenses, the light reflected from the lens end face furthest from the fiber end face of the optical fiber 18 may also be used as the reference light. In this case, the distance from the lens end face furthest from the fiber end face to the object to be measured 90 is d.
[0043] When the signal light and reference light emitted from the third input / output port 16c of the circulator 16 are incident on the photoelectric converter 22, they become interference light, as shown in equation (8) below.
[0044]
number
[0045] By approximating and transforming equation (8) above, we obtain the following equation (9).
[0046]
number
[0047] Here, J is a Bessel function of the first kind, and the variable C is expressed by the following equation (10).
[0048]
number
[0049] Here, the variable C in the Bessel function must be time-independent. However, the distance d to the object being measured, which is included in the variable C given by equation (10) above, changes over time due to vibration. For this reason, the transformation to the Bessel function of the first kind is not strictly valid.
[0050] On the other hand, the amplitude of the oscillation is extremely small with respect to distance d, and the ratio can be considered almost zero. Therefore, the transformation to the first kind of Bessel function is performed by approximately treating the variable C as a constant independent of time.
[0051] Interfering light incident on the photoelectric converter 22 is converted into an electrical signal. In other words, the photoelectric converter 22 converts interfering light into an electrical signal to generate an interference signal. The interference signal generated by the photoelectric converter 22 is sent to the bandpass filter 24.
[0052] The bandpass filter 24 extracts the angular frequency components ω and 2ω from the interference signal represented by equation (9) above. As a result, the cosφ(t) and sinφ(t) terms in equation (9) above become equations (11a) and (11b), respectively.
[0053]
number
[0054] The signal containing the components extracted by this bandpass filter 24 is sent to the ADC 26. The ADC26 converts the signal sent from the bandpass filter 24 into a digital signal. The digital signal converted by the ADC26 is sent to the signal processing unit 30.
[0055] The signal processing unit 30 performs digital signal processing to obtain information about the vibration of the object being measured 90. To know the vibration of the object being measured 90, it is sufficient to know the phase change φ(t). The phase change φ(t) can be obtained as the arctanth of sinφ(t) / cosφ(t) if it is possible to calculate it. A person skilled in the art can configure the digital signal processing circuit for the signal processing unit 30 to realize the functions described later.
[0056] First, the modulator drive signal multiplier 32 multiplies the cosφ(t) component of the digital signal given by equation (11a) above by the square of the modulator drive signal given by equation (12) below. Furthermore, it multiplies the sinφ(t) component of the digital signal given by equation (11b) above by the modulator drive signal given by equation (12) below.
[0057]
number
[0058] When the results of the multiplication in the modulator drive signal multiplier 32 are summarized, the following equations (13a) and (13b) are obtained.
[0059]
number
[0060] The components given by equations (13a) and (13b) above, which are the result of multiplication by the modulator drive signal multiplier 32, are sent to the low-pass filter (LPF) 34. The low-pass filter 34 blocks the angular frequency components of equations (13a) and (13b) at 4ωt and 2ωt. As a result, the following equations (14a) and (14b) are obtained.
[0061]
number
[0062] The result of blocking angular frequencies 4ωt and 2ωt by the low-pass filter 34 is sent to the inverse-phase multiplier 36.
[0063] The inverse phase multiplier 36 calculates the arguments of equations (14a) and (14b) and multiplies them by the inverse phase, thereby removing the components of the exp function that were generated during the process of removing cos2ω and cosω from equations (14a) and (14b). As a result, the following equation is obtained. (15a) and equation (15b) are obtained.
[0064]
number
[0065] The result obtained by removing the exp component in the inverse phase multiplier 36, as shown by equations (15a) and (15b) above, is sent to the phase calculation unit 38.
[0066] The phase calculation unit 38 calculates the phase by removing the Bessel function. Here, the variable C that satisfies J1(C)=J2(C) is 2.63. Therefore, when modulated with the angular frequency ω given by the following equation (16), J1(C)=J2(C).
[0067]
number
[0068] The optimal value of the angular frequency ω, that is, the angular frequency ω that satisfies equation (16) above, can be determined by sweeping the angular frequency of the modulator drive signal generated by the oscillator 14 at the start of measurement. Let A be the value of the Bessel function when J1(C)=J2(C), and let I and Q be the values of each term, then it can be expressed by the following equations (17a) and (17b).
[0069]
number
[0070] Therefore, the phase change φ(t) due to vibration of the object being measured 90 can be determined from the following equation (18).
[0071]
number
[0072] As explained above, according to the vibration meter and vibration measurement method of this invention, the optical path difference between the signal light and the reference light is only the round trip distance between the end face of the optical fiber 18 on the lens 20 side and the object to be measured 90, and can therefore be considered to be almost the same. In this way, by propagating the signal light and the reference light along almost the same optical path, the effect of reducing the influence of disturbances such as vibrations applied to the optical fiber as noise on the demodulation of the phase change φ(t) to a negligible degree is obtained. Furthermore, since polarization is not used, the fiber length can be extended compared to conventional technology. In addition, the number of required components is greatly reduced, and relatively inexpensive single-fiber optics are used. Since a luminous optical fiber can be used, the cost of the vibration meter can be reduced.
[0073] 10, 110 Continuous light source 12 Phase modulator 14, 114 oscillators 16 Circulator 18 Optical Fibers 20, 120 lenses 22, 122 Photoelectric Converters 24 Bandpass Filter 26,126 ADC 30, 130 Signal Processing Unit 32 Modulator drive signal multiplier 34. Low-pass filter (LPF) 36. Inverse Phase Multiplier 38 Phase extraction means 90, 190 objects to be measured 112, 152 Beam Splitter (BS) 116 Polarizing Beam Splitter (PBS) 118 Polarization-maintaining optical fiber (PM fiber) 142 Half Mirror 144 λ / 4 plate 146 Frequency Shifter 150 λ / 2 plate
Claims
1. It comprises a continuous light source, an oscillator, a phase modulator, a circulator, an optical fiber, a lens, a photoelectric converter, a bandpass filter, and a signal processing unit. The aforementioned continuous light source generates continuous light, The phase modulator applies a predetermined phase modulation to the continuous light based on a phase modulation drive signal with angular frequency ω generated by the oscillator. The circulator sends the phase-modulated continuous light to the optical fiber. The continuous light sent from the circulator to the optical fiber is irradiated onto the object to be measured as signal light, after passing through the lens. The signal light scattered by the object being measured is sent to the circulator via the lens and the optical fiber. A portion of the continuous light sent from the circulator to the optical fiber is reflected as reference light near the end face of the optical fiber. The reference light is sent to the circulator via the optical fiber. The signal light and the reference light, which have been sent to the circulator via the optical fiber, are sent to the photoelectric converter. The photoelectric converter converts the interference light resulting from the interference of the signal light and the reference light into an electrical signal to generate an interference signal, and sends the interference signal to the bandpass filter. The bandpass filter extracts the ω and 2ω components of the interference signal and sends them to the signal processing unit. The signal processing unit uses the ω and 2ω components of the interference signal to acquire information about the vibration of the object being measured. A vibration meter characterized by the following features.
2. The reference light is either the light reflected by Fresnel reflection at the end face of the optical fiber, or the light reflected at the end face of the lens. The vibration meter according to feature 1.
3. The initial phase and modulation degree of the phase modulation drive signal are θ, respectively. １ When β is such that the distance between the boundary between the optical fiber and the lens and the object to be measured is d, the speed of light is c, and the phase change that the signal light undergoes due to the vibration of the object to be measured is φ(t), then the signal incident on the photoelectric converter is given by the following equation (1) for the Bessel function of the first kind J: [Number 19] The variable C is expressed by the following equation (2): [Number 20] The ω and 2ω components extracted by the bandpass filter are given by the following equations (3a) and (3b), respectively: [Math 21] The signal processing unit comprises a modulator drive signal multiplier, a low-pass filter, an inverse phase multiplier, and a phase calculation means. The modulator drive signal multiplier multiplies the 2ω and ω components extracted by the bandpass filter with an initial phase of θ. ２ Multiply the modulator drive signal represented by the following equation (4) to calculate the following equations (5a) and (5b), [Number 22] [Number 23] The low-pass filter blocks the 4ωt and 2ωt components from the components given by equations (5a) and (5b), respectively, and obtains the components given by the following equations (6a) and (6b): [Number 24] The inverse phase multiplier obtains the components given by equations (6a) and (6b) by calculating their respective phase angles and multiplying them by the inverse phase to obtain the components given by the following equations (7a) and (7b): [Number 25] The phase calculation means removes the Bessel function from the components given by equations (7a) and (7b) to calculate the phase change φ(t) due to vibration of the object being measured. The vibration meter according to feature 1 or 2.
4. The process of generating continuous light, A process of applying a predetermined phase modulation to the continuous light based on a phase modulation drive signal with angular frequency ω, The process of propagating the phase-modulated continuous light through an optical fiber, The process involves irradiating the object to be measured with signal light after passing through the optical fiber and through a lens, The process of propagating the signal light scattered by the object to be measured through the lens and then through the optical fiber, A process in which a portion of the continuous light propagating through the optical fiber is reflected as a reference light near the end face of the optical fiber, and the reference light is propagated through the optical fiber, The process of generating an interference signal by converting the interference light obtained when the signal light and the reference light interfere into an electrical signal, The process of extracting the ω and 2ω components of the interference signal, The process of obtaining vibration information of the object being measured using the components ω and 2ω, and A vibration measurement method characterized by comprising the following features.
5. When the initial phase and modulation degree of the phase-modulated drive signal are θ1 and β, respectively, the distance between the boundary between the optical fiber and the lens and the object to be measured is d, the speed of light is c, and the phase change that the signal light undergoes due to the vibration of the object to be measured is φ(t), then the interference light is given by the following equation (1) for the first kind Bessel function J: 【Number 26】 The variable C is expressed by the following equation (2): [Number 27] The ω and 2ω components of the extracted interference signal are given by equations (3a) and (3b) below, respectively. [Number 28] The process of calculating the following equations (5a) and (5b) by multiplying the 2ω and ω components given by equations (3a) and (3b) by the modulator drive signal represented by the following equation (4) with an initial phase of θ², [Number 29] [Number 30] From equations (5a) and (5b), the process of blocking the 4ωt and 2ωt components, respectively, to obtain the following equations (6a) and (6b), [Number 31] The process involves determining the respective phase angles from equations (6a) and (6b) and multiplying them by the inverse phase to obtain the following equations (7a) and (7b), [Number 32] The process of removing the Bessel function from equations (7a) and (7b) to calculate the phase change φ(t) due to vibration of the object being measured, The vibration measurement method according to claim 4, characterized by comprising the above.