CN110118532B - Dual-wavelength nonlinear displacement demodulation method and system for optical fiber Fabry-Perot displacement sensor - Google Patents
Dual-wavelength nonlinear displacement demodulation method and system for optical fiber Fabry-Perot displacement sensor Download PDFInfo
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Abstract
本发明公开一种光纤Fabry‑Perot位移传感器的双波长非线性位移解调方法及系统，属于光纤传感技术领域。其中，系统包括：光纤Fabry‑Perot位移传感器、两路波长可调谐激光器、两个光纤耦合器、光纤环形器、波分复用器、两个光电探测器和数据采集卡。方法包括以下步骤：1)获取两路反射谱；2)设置两路激光器工作波长，分别确定初始相位；3)反射谱相位转换、曲线拟合及求解反函数；4)分别计算光相位的改变量，确定腔长变化量；5)根据模型解调非线性位移。本发明能够提高传统光纤Fabry‑Perot位移传感器的测量范围，解调精度高，能克服相位模糊的问题，解调灵活，能对解调结果的可靠性给予评价。
The invention discloses a dual-wavelength nonlinear displacement demodulation method and system of an optical fiber Fabry-Perot displacement sensor, and belongs to the technical field of optical fiber sensing. Among them, the system includes: fiber Fabry-Perot displacement sensor, two wavelength tunable lasers, two fiber couplers, fiber circulator, wavelength division multiplexer, two photodetectors and data acquisition card. The method includes the following steps: 1) acquiring two reflection spectra; 2) setting the working wavelengths of the two lasers to determine the initial phase respectively; 3) converting the reflection spectrum phase, curve fitting and solving the inverse function; 4) respectively calculating the change of the optical phase , determine the cavity length variation; 5) demodulate the nonlinear displacement according to the model. The invention can improve the measurement range of the traditional optical fiber Fabry-Perot displacement sensor, has high demodulation accuracy, can overcome the problem of phase ambiguity, has flexible demodulation, and can evaluate the reliability of demodulation results.
Description
Technical Field
The invention belongs to the technical field of optical fiber sensing, and particularly relates to a displacement demodulation technology of an optical fiber Fabry-Perot displacement sensor.
Background
With the development of science and technology, it is more and more important to monitor and measure the micro-vibration in a complex environment, and the measurement method has the problems that the measurement cannot be adapted to the complex environment, or the high-precision micro-vibration measurement cannot be realized, or the measurement in a large vibration range cannot be satisfied, when the measurement is performed by the commonly used doppler vibrometer, strain gauge, optical fiber displacement sensor and the like. The Rayleigh integral is used as a sound field calculation method, and the sound pressure of any point in a field point can be calculated as long as the vibration velocity distribution of the surface of the transducer is measured. In the measuring environment of high sound pressure of an HIFU sound field, the fiber Fabry-Perot displacement sensor can measure the vibration displacement of the surface of the transducer, and the sound pressure distribution of the HIFU field can be calculated by combining Rayleigh integral. When the fiber Fabry-Perot displacement sensor measures the vibration displacement, three problems are mainly faced: 1) the measuring range is small, and the measuring range is easy to exceed for large displacement measurement. 2) The output voltage of the system is not in direct proportion to the displacement in theory. 3) The working wavelength of the laser is modified to carry out nonlinear displacement demodulation, if the wavelength is set to be close to the trough of the reflection spectrum, positive displacement can be well demodulated, but the demodulation error of negative displacement is large; if the wavelength is set close to the peak of the reflection spectrum, the negative displacement can be well demodulated, but the demodulation error of the positive displacement is large.
Disclosure of Invention
The invention aims to provide a dual-wavelength nonlinear displacement demodulation system and method of an optical fiber Fabry-Perot displacement sensor, aiming at the existing defects, which can effectively improve the problems, improve the measurement range of the traditional optical fiber Fabry-Perot displacement sensor, improve the demodulation precision, increase the demodulation flexibility, overcome the problem of phase ambiguity and evaluate the reliability of the demodulation result.
The scheme for solving the technical problems is as follows:
in a first aspect, the invention provides a dual-wavelength nonlinear displacement demodulation method of an optical fiber Fabry-Perot displacement sensor. The method comprises the following steps:
1. acquiring two reflection spectrums: ensuring that the vibration displacement of the measured object is not acted on the optical fiber Fabry-Perot displacement sensor; the two lasers work, and the data acquisition card acquires discrete point data, namely two reflection spectrums, of the output voltage V of the photoelectric detector, which changes along with the wavelength lambda of the corresponding wavelength tunable laser.
2. Setting two paths of working wavelengths of the laser, and respectively determining initial phases: fourier transform is carried out on the reflection spectrum, the cavity length L of the Fabry-Perot cavity is calculated, the working wavelengths of the two paths of lasers are respectively set, and the two working wavelengths cannot be the same; then according to the optical phase
Respectively determining an initial phase phi when the vibration displacement is not applied to the fiber Fabry-Perot displacement sensor10And phi20Wherein n isaIs the refractive index of the air in the Fabry-Perot cavity.
3. Reflection spectrum phase conversion, curve fitting and solving an inverse function: according to
Converting the reflection spectrum into discrete point data of the output voltage V and the optical phase phi, fitting, and establishing a functional relation V (phi) of the output voltage V and the optical phase phi, wherein the fitting form is as follows:
in the formula, y0, A, xc and w are all parameters to be fitted, and an inverse function V of the formula (1) is obtained-1(φ) obtains the optical phase φ as a function of the output voltage V:
wherein k is an integer.
4. Respectively calculating the change quantity of the optical phase, and determining the cavity length change quantity: and (3) simultaneously keeping the working wavelength output set in the step (2) by the two lasers, and then applying the vibration displacement of the object to be measured to the optical fiber Fabry-Perot displacement sensor. Respectively recording output voltages of two paths of photoelectric detectors after the vibration displacement acts on the fiber Fabry-Perot displacement sensor, and respectively calculating a final phase phi after the vibration displacement acts according to a functional relation between the optical phase phi and the output voltages in the step 311And phi21Then combining the initial phase phi01And phi02Respectively obtaining the phase variation delta phi of the two laser beams1And delta phi2And respectively calculating the Fabry-Perot cavity length variation delta L caused by vibration displacement according to the Fabry-Perot cavity length L and the working wavelengths of the two lasers1And Δ L2That is, the nonlinear vibration displacement of the two laser beams of the measured object is DeltaL1And Δ L2。
In a second aspect, the present invention further provides a dual wavelength nonlinear displacement demodulation system of a fiber Fabry-Perot displacement sensor, the system comprising: the device comprises an optical fiber Fabry-Perot displacement sensor, a wavelength tunable laser, an optical fiber component and a detection and feedback component. After light emitted by the wavelength tunable laser reaches the optical fiber Fabry-Perot displacement sensor through the optical fiber assembly, two beams of reflected light of the optical fiber Fabry-Perot displacement sensor, which are modulated by positive and negative displacement, pass through the optical fiber assembly and are received and processed by the detection and feedback assembly, and positive and negative displacement is obtained at the same time.
Further, the optical fiber assembly includes: the optical fiber coupler comprises a first optical fiber coupler, an optical fiber circulator, a wavelength division multiplexer and a second optical fiber coupler. The optical fiber coupler is used for decomposing one laser beam into two laser beams or compounding the two laser beams into one laser beam, and the wavelength division multiplexer only sets the laser with the wavelength through the system.
Further, the detection and feedback assembly comprises: photoelectric detector and data acquisition card. The photoelectric detectors receive the optical signals output by the wavelength division multiplexer, each photoelectric detector outputs two paths of signals, one path of signals outputs displacement signals, the other path of signals is connected to the data acquisition card, and the data acquisition card sets the wavelength of the light source assembly according to the path of signals.
The above system may take the following specific form: the wavelength tunable laser passes through an arm of first fiber coupler entering optical fiber circulator one end, another arm of optical fiber circulator one end with the second fiber coupler is connected, an arm of the one end of second fiber coupler with wavelength division multiplexer connects, another arm of the one end of second fiber coupler with the photoelectric detector input is connected, the optical fiber circulator other end with optic fibre Fabry-Perot displacement sensor connects, the wavelength division multiplexer other end with the photoelectric detector input is connected, photoelectric detector output one end is connected data acquisition card, the other end connection output.
The system can adopt another specific form as follows: the wavelength tunable laser device is connected with the data acquisition card through the first optical fiber coupler, and the other end of the wavelength tunable laser device is connected with the output end of the data acquisition card.
In the method, two lasers work cooperatively and are mutually matched, independent and do not interfere with each other. The demodulation method provided by the invention is mutually independent and simultaneously carries out positive and negative displacement measurement, and can truly and accurately demodulate positive and negative displacement by one-time measurement. Compared with the situation that positive and negative displacement measurement can not be carried out at the same moment only by using one laser, positive and negative displacement can be demodulated only by carrying out measurement twice, the method can be used for measuring the positive and negative displacement at the same moment, and the demodulation accuracy is higher.
In addition, the method can evaluate the reliability of the measurement result. Because two paths of laser should be modulated by positive and negative displacements, the information of the negative displacements can be extracted by demodulating one path of signal with the positive displacements, and the information of the positive displacements can be extracted by demodulating one path of signal with the negative displacements. Only if the measurement results are consistent with the above description, the reliability of the measurement result can be determined, otherwise, the fiber Fabry-Perot displacement sensor may be damaged or a system fault may occur when the measurement is not completed.
The invention has the beneficial effects that:
1. the method and the system of the invention utilize wavelength division multiplexing, adopt two paths of laser to demodulate the positive and negative displacement independently and simultaneously, have higher demodulation accuracy, can demodulate the positive and negative displacement by only one-time measurement, and are real simultaneous measurement. When one laser is adopted to measure positive and negative displacement, the sequence is in order, and the positive and negative displacement can be demodulated only by measuring twice, which is not the simultaneous measurement in real sense.
2. According to the method and the system for demodulating the dual-wavelength nonlinear displacement of the fiber Fabry-Perot displacement sensor, due to the fact that the working wavelengths of the two lasers are different, if one path of signal is just close to the direction fuzzy point, the other path of signal cannot be close to the direction fuzzy point, and it is guaranteed that at least one path of signal is not affected by the direction fuzzy.
3. The method and the system provided by the invention adopt two paths of lasers to work cooperatively, and can be matched with each other, so that certain evaluation can be provided for the reliability of the measurement result. The two beams of signals finally output by the invention should contain positive and negative displacement information, if one path of signals is normal and the other path of signals is abnormal, the fiber Fabry-Perot displacement sensor can be determined to be damaged or have a system fault in the measurement, and the result of the measurement can be regarded as invalid. The evaluation capability can greatly improve the reliability and accuracy of measurement and prevent the interference of invalid measurement. Other measurement methods do not have such evaluation capability.
4. The system is simple and compact in structure. The wavelength division multiplexer is adopted in the system for wavelength selection, and the system flexibility is high. Meanwhile, the two lasers work cooperatively and mutually independently without interference.
Therefore, the dual-wavelength nonlinear displacement demodulation system and method of the fiber Fabry-Perot displacement sensor have unique advantages and potential practical values for accurately measuring the vibration displacement.
Drawings
The invention is further described below with reference to the following figures and examples:
fig. 1 is a system block diagram of an embodiment 1 of a two-wavelength nonlinear displacement demodulation system of a fiber-optic Fabry-Perot displacement sensor provided by the invention.
Fig. 2 is a system block diagram of embodiment 2 of the two-wavelength nonlinear displacement demodulation system of the fiber Fabry-Perot displacement sensor provided by the invention.
Fig. 3 is a demodulation flow chart of the demodulation method of the two-wavelength nonlinear displacement of the fiber Fabry-Perot displacement sensor provided by the invention.
Fig. 4 is a first path reflection spectrum curve of embodiment 2 of the two-wavelength nonlinear displacement demodulation system of the fiber Fabry-Perot displacement sensor provided by the invention.
Fig. 5 is a second path reflection spectrum curve of embodiment 2 of the two-wavelength nonlinear displacement demodulation system of the fiber Fabry-Perot displacement sensor provided by the invention.
FIG. 6 is a curve of the output signal of the system when the dual-wavelength nonlinear displacement demodulation system of the fiber Fabry-Perot displacement sensor demodulates the forward displacement.
FIG. 7 is a curve of output signals of the system when the dual-wavelength nonlinear displacement demodulation system of the fiber Fabry-Perot displacement sensor demodulates negative displacement.
Wherein the reference numerals are respectively:
101, PC, 102, a first wavelength tunable laser, 103, a second wavelength tunable laser, 104, a first optical fiber, 105, a second optical fiber, 106, a first optical fiber coupler, 107, a third optical fiber, 108, an optical fiber circulator, 109, a fourth optical fiber, 110, an optical fiber Fabry-Perot hydrophone, 111, a measured object, 112, a fifth optical fiber, 113, a second optical fiber coupler, 114, a sixth optical fiber, 115, a seventh optical fiber, 116, a wavelength division multiplexer, 117, an eighth optical fiber, 118, a ninth optical fiber, 119, a first photoelectric detector, 120, a second photoelectric detector, 121, an output end, 122, a data acquisition card, 201, PC, 202, a first wavelength tunable laser, 203, a second wavelength tunable laser, 204, a first optical fiber, 205, a second optical fiber, 206, an optical fiber coupler, 207, a third optical fiber, 208, an optical fiber circulator, 209, a fourth optical fiber, 209, and a third optical fiber, 210. The device comprises a fiber Fabry-Perot hydrophone, a measured object 211, a measured object 212, a fifth fiber 213, a wavelength division multiplexer 214, a sixth fiber 215, a seventh fiber 216, a first photoelectric detector 217, a second photoelectric detector 218, an output end 219 and a data acquisition card.
Detailed Description
The invention is further illustrated by the following figures and examples:
example 1: dual-wavelength nonlinear displacement demodulation system of optical fiber Fabry-Perot displacement sensor
Referring to fig. 1, the system includes: the device comprises a PC (personal computer) 101, a first wavelength tunable laser 102, a second wavelength tunable laser 103, a first optical fiber coupler 106, an optical fiber circulator 108, an optical fiber Fabry-Perot displacement sensor 110, a second optical fiber coupler 113, a wavelength division multiplexer 116, a first photoelectric detector 119, a second photoelectric detector 120, an output end 121, a data acquisition card 122 and the like.
The fiber Fabry-Perot displacement sensor 110 is adjusted to make the light emitting surface parallel to the measured object 111. When the object 111 is not vibrated, two paths of reflection spectrum signals are obtained. The first wavelength tunable laser 102, the second photoelectric detector 120 and the data acquisition card work first to obtain the reflection spectrum of the fiber Fabry-Perot displacement sensor; the second wavelength tunable laser 103, the second photodetector 120 and the data acquisition card work again to obtain the reflection spectrum of the fiber Fabry-Perot displacement sensor.
The PC 101 respectively sets the output wavelength lambda of the first wavelength tunable laser 102 according to the two paths of reflection spectrum signals1The second wavelength tunable laser 103 outputs a wavelength λ2The two paths of wavelengths are different, and the wavelength division multiplexer 116 is set to pass only the wavelength λ1And λ2The laser of (1).
Wavelength of λ1、λ2The two laser beams are respectively transmitted through a first optical fiber 104 and a second optical fiber 105, enter a first optical fiber coupler 106, are coupled into a mixed laser beam, and then enter an optical fiber Fabry-Perot displacement sensor 110 through one end of an optical fiber circulator 108. The mixed laser modulated by the positive and negative vibration displacement is reflected back to the other end of the optical fiber circulator 108, and is divided into two beams of mixed laser in proportion by the second optical fiber coupler 113, wherein one beam of mixed laser is divided into lambda by the wavelength division multiplexer 1161、λ2A laser of a wavelength. Only wavelength lambda1Can enter the first photodetector 119; likewise only wavelength lambda2Can enter the second photodetector 120; so far, the two photodetectors respectively and simultaneously perform photoelectric conversion on the light with two wavelengths, one path of the converted voltage signal is connected to the data acquisition card 122, and the other path is connected to the output end 121. In the present embodiment, the first fiber coupler 106 and the second fiber coupler 113 may be 1 × 2 or 2 × 2 fiber couplers. The optical fiber circulator 108 can be replaced by an optical isolator and an optical fiber coupler, which belong to the same replacement and can also achieve the purpose of the invention.
In this embodiment, the wavelength division multiplexer 116 can be replaced by a fiber coupler and a tunable FP filter or a fiber grating, and the object of the invention can also be achieved.
In this embodiment, the second photodetector 120 is a two-input two-output photodetector, and may be replaced by two one-input one-output photodetectors, which can also achieve the purpose of the present invention.
Example 2: dual-wavelength nonlinear displacement demodulation system of optical fiber Fabry-Perot displacement sensor
Referring to fig. 2, the system includes: the device comprises a PC (personal computer) 201, a first wavelength tunable laser 202, a second wavelength tunable laser 203, a fiber coupler 206, a fiber circulator 208, a fiber Fabry-Perot displacement sensor 210, a wavelength division multiplexer 213, a first photoelectric detector 216, a second photoelectric detector 217, an output end 218, a data acquisition card 219 and the like.
The fiber Fabry-Perot displacement sensor 210 is adjusted to have its light emitting surface parallel to the object 211 to be measured. When the object 211 is not vibrated, two reflection spectrum signals are obtained. The first wavelength tunable laser 202, the first photoelectric detector 216, the wavelength division multiplexer 213 and the data acquisition card work first to obtain the reflection spectrum of the fiber Fabry-Perot displacement sensor; the second wavelength tunable laser 203, the second photodetector 220, the wavelength division multiplexer 213 and the data acquisition card work again to obtain the reflection spectrum of the fiber Fabry-Perot displacement sensor.
The PC 201 sets the output wavelength lambda of the first wavelength tunable laser 202 according to the two reflected spectrum signals1The second wavelength tunable laser 203 outputs a wavelength λ2While setting the wavelength division multiplexer 213 to pass only the wavelength λ1And λ2The laser of (1).
Wavelength of λ1、λ2The two laser beams are respectively transmitted through a first optical fiber 204 and a second optical fiber 205, enter an optical fiber coupler 206, are coupled into a mixed laser beam, and then enter an optical fiber Fabry-Perot displacement sensor 210 through one end of an optical fiber circulator 208. The mixed laser modulated by the positive and negative vibration displacement is reflected back to the other end of the fiber circulator 208, and the lambda is transmitted by the wavelength division multiplexer 2131、λ2The laser light of the wavelength is split. Only wavelength lambda1Can enter the first photodetector 216; likewise only wavelength lambda2Can enter the second photodetector 217; so far, the two photoelectric detectors respectively and simultaneously carry out photoelectric conversion on the light with two wavelengths, one path of the converted voltage signal is connected to the data acquisition card and 219, and the other path is connected to the data acquisition card and 219One of which is connected to output 218.
In this embodiment, the optical fiber coupler 206 may be a 1 × 2 or 2 × 2 optical fiber coupler. The optical fiber circulator 208 can be replaced by an optical isolator and an optical fiber coupler, which belong to the same replacement and can also achieve the purpose of the invention.
In this embodiment, the wavelength division multiplexer 213 can be replaced by a fiber coupler and a tunable FP filter or a fiber grating, and the object of the invention can also be achieved.
Example 3: dual-wavelength nonlinear displacement demodulation method of fiber Fabry-Perot displacement sensor
This embodiment is a method applied to the above-mentioned dual-wavelength nonlinear displacement demodulation system of the fiber Fabry-Perot displacement sensor, and the steps of the method will be described in further detail with reference to fig. 3.
Step 1, obtaining two paths of reflection spectrum signals: when the object 211 to be measured does not vibrate, the first wavelength tunable laser 202, the first photodetector 216 and the data acquisition card 219 work to obtain a first path of reflectance spectrum curve of the fiber Fabry-Perot displacement sensor 210, as shown in fig. 4; then, the second wavelength tunable laser 203, the second photodetector 217 and the data acquisition card 219 operate to obtain a second path of reflectance spectrum curve of the fiber Fabry-Perot displacement sensor 210, as shown in fig. 5. As long as the light path of the system is stable, the voltage amplitudes of the two reflection spectrum signals do not need to be completely consistent.
Step 2, setting two paths of working wavelengths of the laser, and respectively determining initial phases: fourier transform is carried out on the reflection spectrum, the Fabry-Perot cavity length L is calculated, and the working wavelengths of the two paths of lasers are respectively set. The wavelengths of the two paths of laser light meet the following conditions: corresponding to two different channels of the wavelength division multiplexer, and the voltages on the corresponding reflection spectrums of the two wavelengths cannot be the same. Because the cavity length variation caused by the same displacement is the same, the variation of the optical phase can be changed by setting different working wavelengths of the laser, and further the system output voltages under different working wavelengths are different.
When the forward displacement is demodulated, the forward displacement is required to cause the same phase variation, and the output voltage signal variation is large enoughAccording to the reflection spectrum shown in fig. 4, the operating wavelength of the laser can be set at a position (a) close to the trough, and at this time, the output voltage corresponding to the phase variation caused by the positive displacement is changed greatly, and the output voltage corresponding to the phase variation caused by the negative displacement is changed little, so that the demodulation of the positive displacement is accurate, and the demodulation error of the negative displacement is large. Similarly, when demodulating the negative displacement, the negative displacement is required to cause the same phase variation, the output voltage signal has a sufficiently large variation, and according to the reflection spectrum shown in fig. 5, the working wavelength of the laser can be set at a position (B) close to the peak, and at this time, the demodulation of the negative displacement is accurate, and the demodulation error of the positive displacement is large. Respectively determining the initial phase phi when the vibration displacement is not applied to the optical fiber Fabry-Perot displacement sensor after the setting of the working wavelength is finished10And phi20The optical phase φ can be expressed as:
in the formula naIs the refractive index of air in the Fabry-Perot cavity.
Step 3, reflection spectrum phase conversion, curve fitting and inverse function solving: the light intensity I reflected back to the photodetector by the fiber Fabry-Perot displacement sensor 210rCan be approximated as:
Ir＝2R(1-cosφ)Ii (2)
in the formula IiThe output light intensity of the wavelength tunable laser is R, and the reflectivity of the air-fiber interface in the Fabry-Perot cavity is R. The formula (2) can obtain that the output reflected light intensity is a double-value function of the wavelength lambda and the cavity length L of the laser and is a single-value function of the phase phi, and the change of the wavelength and the cavity length can cause the change of the reflected light intensity. Since additional phase is introduced during the process of reflecting light from the fiber Fabry-Perot displacement sensor 210 back to the photodetector, and the obtained reflection spectrum is discrete point data, curve fitting needs to be performed on the reflection spectrum signal if the relation between any phase and the output voltage needs to be established. Therefore, the reflection spectrum is converted into discrete point data of the output voltage V and the optical phase phi according to the formula (1), and least square fitting is performed to establish the output voltage V and the optical phase phiPhi, the fitting form is:
in the formula, y0, A, xc and w are all parameters to be fitted, and the inverse function V of the formula (3) is obtained-i(φ) the functional relationship between the optical phase φ and the output voltage V is obtained as:
wherein k is an integer.
Step 4, respectively calculating the variation of the optical phase, and determining the variation of the cavity length: and (3) simultaneously keeping the working wavelength set in the step (2) by two lasers, and then applying the vibration displacement of the object to be measured to the optical fiber Fabry-Perot displacement sensor. The output voltages of the two paths of photoelectric detectors after the vibration displacement is applied to the fiber Fabry-Perot displacement sensor are respectively recorded, the output signal of the demodulation positive displacement is shown in figure 6, and the output signal of the demodulation negative displacement is shown in figure 7. Respectively calculating the final phase phi after the vibration displacement according to the functional relation between the optical phase and the output voltage in the step 3)11And phi21Then combining the initial phase phi01And phi02Respectively obtaining the phase variation delta phi of the two laser beams1And delta phi2And respectively calculating the Fabry-Perot cavity length variation delta L caused by ultrasonic waves according to the Fabry-Perot cavity length L and the working wavelengths of the two lasers1And Δ L2That is, the nonlinear vibration displacement of the two laser beams of the measured object is DeltaL1And Δ L2。
In this embodiment, the operating wavelengths of the two lasers may also be set near the peak position (C) and the valley position (D), which also achieves the purpose of the invention, but the system output voltage signals are opposite, that is, a negative voltage is output when the system is shifted in the forward direction.
In this embodiment, the working wavelengths of the two lasers may also be set at the positions of the wave crest and the wave trough, which also can achieve the purpose of the invention, but the system output signal will immediately generate the frequency doubling phenomenon, and is not beneficial to the demodulation of the signal.
The demodulation method provided by the invention is used for measuring the displacement independently and simultaneously. Because the working wavelengths of the two lasers are different, if one path of signal is just near the direction fuzzy point, the other path of signal is certainly not near the direction fuzzy point, and at least one path of signal is guaranteed not to be affected by the direction fuzzy. In addition, because two paths of laser are modulated by sound pressure at the same time, and the cavity length change is the same, the measurement result can be determined to be reliable only if the demodulated displacement is the same, otherwise, the fiber Fabry-Perot displacement sensor is damaged or has system fault if the measurement is not finished, the gross error can be avoided, the interference of invalid measurement is prevented, and the reliability and the accuracy of displacement demodulation can be greatly improved.
Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention and not for limiting, and although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions can be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, which should be covered by the claims of the present invention.
Claims (6)
1. A demodulation method of the nonlinear displacement of the dual wavelength of the fiber Fabry-Perot displacement sensor is characterized in that: the method is realized by a system consisting of an optical fiber Fabry-Perot displacement sensor, an optical fiber coupler, an optical fiber circulator, a wavelength division multiplexer, a data acquisition card, two wavelength tunable lasers and two photoelectric detectors; two paths of laser are respectively emitted by two wavelength tunable lasers, enter an optical fiber coupler to be coupled into a path of mixed laser, enter an optical fiber Fabry-Perot displacement sensor through one end of an optical fiber circulator, and the mixed laser modulated by positive and negative vibration displacement is reflected back to the other end of the optical fiber circulator and is separated into lambda through a wavelength division multiplexer1、λ2The laser with the wavelength respectively enters the two photoelectric detectors, and the two photoelectric detectors realize the light with the two wavelengthsRespectively and simultaneously carrying out photoelectric conversion, wherein one path of the converted voltage signal is connected to a data acquisition card, and the other path of the converted voltage signal is connected to an output end;
the method comprises the following steps:
(1) acquiring two reflection spectrums: ensuring that the vibration displacement of the measured object is not acted on the optical fiber Fabry-Perot displacement sensor; the two lasers work, and the data acquisition card acquires discrete point data of the output voltage V of the photoelectric detector changing along with the wavelength lambda of the wavelength tunable laser corresponding to the two lasers, namely two reflection spectrums;
(2) setting two paths of working wavelengths of the laser, and respectively determining initial phases: fourier transform is carried out on the reflection spectrum, the cavity length L of the Fabry-Perot cavity is calculated, the working wavelengths of the two paths of lasers are respectively set, and the two working wavelengths cannot be the same; then according to the optical phase
Respectively determining an initial phase phi when the vibration displacement is not applied to the fiber Fabry-Perot displacement sensor10And phi20Wherein n isaIs the refractive index of air in a Fabry-Perot cavity;
(3) reflection spectrum phase conversion, curve fitting and solving an inverse function: according to
Converting the reflection spectrum into discrete point data of the output voltage V and the optical phase phi, fitting, establishing a functional relation V (phi) of the output voltage V and the optical phase phi, and solving an inverse function to obtain a functional relation V of the optical phase phi and the output voltage V-1(phi); the curve fitting is that the output voltage V and the optical phase phi adopt least square fitting, and the fitting form is as follows:
wherein y0, A, xc, w are all parameters to be fitted;
(4) respectively calculating the change quantity of the optical phase, and determining the cavity length change quantity: two-way laserThe optical device still keeps the working wavelength output set in the step (2) at the same time, then the vibration displacement of the object to be measured is acted on the optical fiber Fabry-Perot displacement sensor, the output voltage of the two paths of photoelectric detectors after the vibration displacement is acted on the optical fiber Fabry-Perot displacement sensor is respectively recorded, and the final phase phi after the vibration displacement action is respectively calculated according to the functional relation between the optical phase phi and the output voltage in the step (3)11And phi21Then combining the initial phase phi01And phi02Respectively obtaining the phase variation delta phi of the two laser beams1And delta phi2And respectively calculating the Fabry-Perot cavity length variation delta L caused by vibration displacement according to the Fabry-Perot cavity length L and the working wavelengths of the two lasers1And Δ L2Namely the nonlinear vibration displacement of the measured object under the two laser beams.
2. The demodulation method of the nonlinear displacement of two wavelengths of the fiber Fabry-Perot displacement sensor of claim 1, wherein: and (3) simultaneously outputting lasers with different wavelengths by the two lasers in the step (2).
3. The demodulation method of the nonlinear displacement of two wavelengths of the fiber Fabry-Perot displacement sensor of claim 1, wherein: the final phase of the optical phase in the step (4) is calculated according to the formula (2), that is, the inverse function V of the formula (1) is solved-1(φ) the functional relationship between the optical phase φ and the output voltage V is obtained as:
wherein k is an integer.
4. A dual wavelength nonlinear displacement demodulation system of a fiber Fabry-Perot displacement sensor for implementing the method of any of claims 1-3, characterized in that: the system comprises an optical fiber Fabry-Perot displacement sensor, an optical fiber coupler, an optical fiber circulator, a wavelength division multiplexer, a data acquisition card and two wavelength tunable lasersAnd two photodetectors; two paths of laser are respectively emitted by two wavelength tunable lasers, enter an optical fiber coupler to be coupled into a path of mixed laser, enter an optical fiber Fabry-Perot displacement sensor through one end of an optical fiber circulator, and the mixed laser modulated by positive and negative vibration displacement is reflected back to the other end of the optical fiber circulator and is separated into lambda through a wavelength division multiplexer1、λ2The laser with wavelength respectively enters two photoelectric detectors, the two photoelectric detectors respectively and simultaneously carry out photoelectric conversion on the light with the two wavelengths, one path of the converted voltage signal is connected to a data acquisition card, the data acquisition card carries out wavelength setting on the wavelength tunable laser according to the path of the signal, and the other path of the converted voltage signal is connected to an output end to output a displacement signal.
5. The dual wavelength nonlinear displacement demodulation system of the fiber-optic Fabry-Perot displacement sensor of claim 4, wherein: the optical fiber coupler is arranged between the return optical fiber circulator and the wavelength division multiplexer, reflects the mixed laser at the other end of the return optical fiber circulator, and is proportionally separated into two beams of mixed laser through the other optical fiber coupler, wherein one beam of mixed laser is separated into lambda through the wavelength division multiplexer1、λ2A laser of a wavelength.
6. The dual wavelength nonlinear displacement demodulation system of the fiber-optic Fabry-Perot displacement sensor of claim 4, wherein: the optical fiber Fabry-Perot displacement sensor comprises an optical fiber collimator and an interference cavity, wherein a reflection increasing film is plated on a light-emitting surface of the collimator, and an air cavity between the light-emitting surface of the optical fiber collimator and a measured object is used as the interference cavity; the Fabry-Perot cavity of the Fabry-Perot displacement sensor is naturally formed between the two reflecting surfaces of the air cavity.
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| CN113358037B (en) * | 2021-08-10 | 2021-11-09 | 中国计量科学研究院 | Laser displacement measuring device and method |
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