Method for improving linearity of fabry-perot interferometric displacement measurement system and measurement probe

Abstract

The application provides a method for improving the linearity of a Fabry-Perot interference displacement measurement system and a measurement probe, wherein two optical fibers are inserted into glass ferrules and fixed by bonding; an APC connector is connected with the ground glass ferrules after grinding by using a metal connecting sleeve, and the displacement measurement probe is obtained after bonding and fixing on both sides of the metal connecting sleeve; the cavity length of the Fabry-Perot interference cavity is increased by the displacement measurement probe; after a high-frequency carrier modulation signal is applied, the signal is input into LabVIEW for arctangent algorithm demodulation; after demodulation, the non-linear displacement measurement error is eliminated; the arctangent algorithm demodulation and phase accumulation are performed to obtain the measured displacement value, so that the displacement measurement result with high linearity is obtained, the problem of too small phase modulation depth in the PGC demodulation algorithm is solved, the requirement for the laser frequency tuning ability is reduced, the linearity of the demodulation result is improved, and the requirement for high linearity displacement measurement at a small working distance is met.

Description

Technical Field

[0001] This invention belongs to the fields of fiber optic interferometry and phase generation carrier demodulation technology, and relates to a method and a measurement probe for improving the linearity of a Fabry-Perot interferometric displacement measurement system. Background Technology

[0002] With the increasing demands for precision displacement measurement in fields such as semiconductor manufacturing, surface science research, and aerospace, displacement sensors need to achieve high resolution, large range, and high linearity in displacement measurement. Fiber Fabry-Perot (FP) interferometry is widely used in sensing and measurement due to its simple optical path structure and high sensitivity. The core operation of a fiber FP interferometer-based displacement sensor is to demodulate the optical interference signal to obtain the displacement value. Currently, the signal demodulation methods for fiber FP interferometer displacement sensors mainly include intensity demodulation and phase-generated carrier (PGC) demodulation. Direct intensity demodulation of interferometric displacement sensors results in a smaller measurement range, while PGC-based displacement sensors have the advantages of high resolution and large measurement range, leading to their widespread application. Common PGC demodulation algorithms include Differential Cross Multiplication (DCM) and Arctangent (ATAN). The DCM algorithm involves differentiation and integration, which can cause DC drift in the measurement results when demodulating low-frequency displacement signals. The ATAN demodulation algorithm can suppress laser intensity fluctuations and is more suitable for demodulating low-frequency displacement signals.

[0003] Displacement sensors based on PGC-ATAN demodulation offer advantages such as high resolution and large range. However, high-frequency modulation of the signal is necessary before demodulation. Signal modulation methods can be categorized into internal and external modulation, with internal modulation being widely favored due to its simple structure, high modulation efficiency, and independence from phase modulator influence. However, the limited optical frequency tuning capability of typical lasers restricts the optimal phase modulation depth C, reducing the linearity of the demodulation result and even making signal demodulation impossible.

[0004] Currently reported technologies, such as the phase generation carrier demodulation optimization algorithms disclosed in Chinese patents CN202210115929.3 and CN201710941759.3, eliminate the dependence of the phase demodulation result on the phase modulation depth from the algorithm principle. However, the above two optimization algorithms introduce integration and differentiation operations. If the signal under test contains a DC term or has a low frequency, the demodulation result will have DC drift, which is not suitable for demodulating low-frequency displacement signals, and the algorithm is relatively complex and difficult to implement. Chinese patent CN202310625523.4 proposes an improved PGC algorithm suitable for demodulating low-frequency displacement signals. It is based on third harmonic phase generation carrier mixing, and then uses the arctangent algorithm to demodulate the low-frequency signal. However, the amplitude of the third harmonic signal is small and difficult to detect. Currently, displacement sensors based on the FP interferometry principle mostly use lens-type displacement probes to increase the cavity length of the interferometric cavity, thereby increasing the phase modulation depth, and then use orthogonal detection algorithms such as ATAN to obtain the displacement value to be measured. For example, Attocube's FPS3010 product, used with multiple lens probes, is employed for displacement measurement. In China, Professor Jia Pinggang's research group at North University of China has used self-aligned lens probes to increase the length of the FP interferometer cavity for displacement measurement. However, lens probes are costly, bulky, and require a large working distance. This large working distance results in low space utilization during displacement measurement, hindering small-volume integrated applications. Moreno et al. increased the cavity length through a small-volume tapered fiber tip structure design, but the effect of increasing length was limited. Therefore, improving the phase modulation depth of PGC demodulation to achieve high linearity, large dynamic range, and low cost displacement measurement at small working distances is a problem that needs to be solved. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a method and a measurement probe for improving the linearity of a Fabry-Perot interferometric displacement measurement system. This method effectively increases the phase modulation depth to improve the linearity of the demodulation results, enabling large-range, high-resolution, and low-cost small-distance displacement measurement.

[0006] This invention is achieved through the following technical solution:

[0007] A displacement measurement probe, comprising,

[0008] The displacement measurement probe is designed as follows: an APC connector is used as the laser emission end. An optical fiber is inserted into a glass ferrule of a predetermined length using an optical fiber connection device and fixed with adhesive to reduce the stress sensitivity of the optical fiber. One end face of the fixed glass ferrule is ground flat, and the other end face is ground at an angle of 0 to 45° to eliminate parasitic interference. A metal connecting sleeve is used to connect the APC connector to the ground glass ferrule. The displacement measurement probe is obtained by fixing the two sides of the metal connecting sleeve with adhesive.

[0009] Preferably, the method for determining the length L0 of the glass insert is as follows:

[0010] The optimal value of the phase modulation depth C is determined based on the Bessel function, and then the optical frequency tuning value Δv of the laser is determined; combined with the displacement detection distance x, the glass ferrule length L0 is calculated.

[0011]

[0012] Where c is the speed of light, n represents the refractive index of the medium inside the FP interferometer cavity, Δv is the optical frequency tuning value of the laser, L0 is the length of the glass ferrule, and x is the displacement detection distance.

[0013] Preferably, the second optical fiber is a single-mode optical fiber or a special optical fiber made of birefringent material, obtained by removing the coating layer on the surface of the optical fiber using physical or chemical methods.

[0014] Preferably, the glass ferrule uses glass or ceramic with a lower coefficient of thermal expansion than the second optical fiber as the ferrule medium. This is to reduce interference phase drift caused by temperature-induced changes in the length and refractive index of the second optical fiber, and to suppress measurement errors L caused by low-frequency noise in the system. t .

[0015] Preferably, the adhesive bonding is done with epoxy resin or photocurable adhesive to reduce interference phase drift caused by stress-induced changes in fiber length and refractive index, thereby reducing measurement errors.

[0016] Preferably, the glass insert can be designed as a spiral or folded shape to reduce the probe volume.

[0017] A method for improving the linearity of a Fabry-Perot interferometric displacement measurement system includes,

[0018] After connecting the displacement measurement probe to the displacement measurement system, a high-frequency carrier signal is generated by the signal generator to modulate the laser, so that the output high-frequency changing light signal is generated. The light signal is reflected back to the circulator (5) after passing through the circulator (5) and the displacement measurement probe (8). The light intensity change caused by the low-frequency displacement of the mirror (9) under test is reflected back to the circulator (5) and coupled with the light signal to obtain the modulated interference signal. The interference signal is converted into a photocurrent signal after detection. The current signal is converted and amplified into a voltage signal, and then input into LabVIEW for demodulation by the arctangent algorithm. After demodulation, the nonlinear displacement measurement error caused by insufficient phase modulation depth is eliminated, thereby obtaining a displacement measurement result with high linearity.

[0019] Preferably, the expression for the modulated interference signal is:

[0020]

[0021] In the formula, A represents the DC bias, and B represents the amplitude of the AC term. The phase to be measured includes temperature effects, C is the phase modulation depth, and cosw c t is the carrier signal;

[0022] Preferably, the process of inputting the data into LabVIEW for arctangent demodulation, and obtaining high-linearity displacement measurement results after demodulation, is as follows:

[0023] Using a frequency-doubled signal Gcosw c t and the second harmonic signal Hcos2w c t is multiplied by the interference signal to obtain a low-harmonic mixing term S1 and a low-harmonic mixing binomial term S2, which are easy to detect:

[0024]

[0025]

[0026] After passing through a cutoff frequency of w c Filtering with a low-pass filter of 2 / 2 yields two orthogonal signals, 1S3 and 2S4, with a phase difference of 90°:

[0027]

[0028] Where: G and H are the constant terms of the first harmonic signal and the second harmonic signal, respectively.

[0029] Then, by dividing the orthogonal signal 1S3 and the orthogonal signal 2S4, we get:

[0030] Signal

[0031] Wherein, G / H is a constant, and J1(C) / J2(C) is the ratio of the first-order and second-order Bessel functions. When the phase modulation depth C is 2.63 rad, the ratio is 1, thereby eliminating the nonlinear displacement measurement error caused by J1(C) / J2(C) not being 1.

[0032] Finally, the arctangent value of signal S5 is calculated and the phase is accumulated. Based on the relationship between phase and displacement, a displacement measurement result with high linearity is obtained.

[0033] Preferably, the displacement measurement result is expressed as follows:

[0034] The displacement value to be measured, Δx:

[0035]

[0036] in, The phase change value is represented by n, the refractive index of the medium inside the FP interferometer cavity is represented by λ, the center wavelength of the laser is represented by L0, and the length of the glass ferrule is represented by L. t This indicates the change in probe length caused by temperature.

[0037] Preferably, the displacement measurement system includes a signal generator, a laser, a fiber optic circulator, a photoelectric detection module, a signal conversion and acquisition module, and a demodulation module;

[0038] The signal generator is connected to the laser input, the laser output is connected to the fiber optic circulator input port, one output port of the fiber optic circulator is connected to the displacement measurement probe, and the other output port of the fiber optic circulator is connected to the photoelectric detection module. The input and output of the photoelectric detection module are connected to the demodulation module after passing through the signal conversion and acquisition module. One end face of the second fiber in the displacement measurement probe and the mirror under test form an FP interferometer cavity.

[0039] Compared with the prior art, the present invention has the following beneficial technical effects:

[0040] This invention provides a method and probe for improving the linearity of a Fabry-Perot interferometric displacement measurement system. An APC connector is used as the laser emission end. An optical fiber is inserted into a glass ferrule of a predetermined length via an optical fiber connection device and fixed with adhesive. One end face of the fixed glass ferrule is ground flat, and the other end face is ground to an angle of 0–45°. A metal connecting sleeve is used to connect the APC connector to the ground glass ferrule. The two sides of the metal connecting sleeve are then fixed with adhesive to obtain the displacement measurement probe. By designing the displacement measurement probe, the cavity length of the Fabry-Perot interferometer is increased, thereby improving the phase modulation depth C of the PGC demodulation and enhancing the linearity of the displacement demodulation result. After applying a high-frequency carrier modulation signal, the photoelectric detection module detects the change in the interference signal caused by the displacement, and then inputs it into LabVIEW for arctangent algorithm demodulation. After demodulation, nonlinear displacement measurement errors are eliminated. Arctangent algorithm demodulation and phase accumulation are then performed to obtain the displacement value to be measured, thus obtaining a displacement measurement result with high linearity. This invention solves the problem of insufficient phase modulation depth in the PGC demodulation algorithm by designing a displacement measurement probe with low temperature drift characteristics. This reduces the requirements for laser optical frequency tuning capability, improves the linearity of the demodulation results, and meets the requirements for high linearity displacement measurement at small working distances. By designing a displacement measurement probe to increase the phase modulation depth, this invention fully leverages the advantages of the PGC algorithm's large dynamic range, enabling high-resolution, large-range displacement measurement of interferometric displacement sensors at small working distances.

[0041] Furthermore, the present invention has a reasonable solution, a simple displacement measurement probe structure, low cost, and is easy to implement, which can give full play to the advantages of FP interferometric displacement measurement.

[0042] Furthermore, the displacement measurement probe of this invention selects glass, ceramic or other materials with low thermal expansion coefficients as the ferrule medium, which are lower than those of the second optical fiber. The ferrule diameter is as large as possible, and the ferrule is bonded to the second optical fiber, which reduces the sensitivity of the second optical fiber to temperature and stress changes, thereby reducing the displacement measurement error introduced by the probe. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of the FP displacement measurement system in an embodiment of the present invention;

[0044] Figure 2 This is a structural diagram of the displacement measurement probe designed in an embodiment of the present invention;

[0045] Figure 3 These are the interference signals before and after applying the displacement measurement probe in this embodiment of the invention;

[0046] Figure 4 These are the waveform and spectrum diagram of the low-frequency signal after demodulation in this embodiment of the invention; Figure (a) is the waveform diagram and Figure (b) is the spectrum diagram.

[0047] Figure 5 Figure 1 shows the structure of the glass insert of the displacement probe in an embodiment of the present invention; Figure 2(a) shows a spiral shape and Figure 3(b) shows a folded shape.

[0048] In the diagram, 1. Signal generator; 2. Signal generator output; 3. Laser; 4. Circulator input port one; 5. Fiber optic circulator; 6. Circulator output port three; 7. Circulator output port two; 8. Displacement measurement probe; 9. Mirror under test; 10. Photoelectric detection module; 11. Signal conversion and acquisition module; 12. Demodulation module; 13. APC connector; 14. Fiber optic cable one; 15. Metal connecting sleeve; 16. Glass ferrule; 17. Fiber optic cable two. Detailed Implementation

[0049] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0050] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0051] A method for improving the linearity of a Fabry-Perot interferometric displacement measurement system includes the following steps:

[0052] S1. First, design the displacement measurement probe to increase the cavity length of the FP interferometer: first determine the phase modulation depth C required for displacement demodulation PGC-ATAN algorithm, the laser optical frequency tuning value Δν, and combine the displacement detection distance x to determine the glass ferrule length L0.

[0053] S2, the specific method for designing the displacement measurement probe is as follows: After bonding the glass ferrule 16 to the optical fiber with the coating removed, one end face is ground flat, and the other end is ground at an 8° angle. A metal connecting sleeve 15 is used to connect the APC connector 13 to the ground glass ferrule 16 to obtain the displacement measurement probe 8. The left end face of the second optical fiber 17 and the mirror surface 9 to be measured form an FP interference cavity. The displacement measurement probe is then connected to the displacement measurement system. The first optical fiber 14 is inserted into the APC connector 13, as follows... Figure 5 As shown, the structure of the glass insert can be designed as a spiral or folded shape to reduce the probe volume.

[0054] S3, the signal generator 1 outputs a carrier signal, which is modulated by a laser to produce a high-frequency optical signal. The optical signal is reflected back into the circulator 5 after passing through the circulator 5 and the displacement measurement probe 8. The low-frequency displacement signal generated by the mirror 9 under test is reflected into the circulator 5 and coupled with the optical signal to obtain a modulated interference signal. The interference signal is then detected by the photoelectric detection module. After signal conversion and amplification, the signal is acquired and sent to the demodulation module. After demodulation, the nonlinear displacement measurement error is eliminated, thus obtaining a high-linearity displacement measurement result. The design of the displacement measurement probe realizes the phase modulation depth value of 2.63 rad in the ATAN demodulation algorithm, and the high-linearity displacement measurement result is obtained after demodulation.

[0055] Preferably, the specific method for determining the glass ferrule length L0 in S1 is as follows: Taking the PGC-ATAN demodulation algorithm as an example, firstly, the optimal value of the phase modulation depth C is determined according to the Bessel function; then, the laser optical frequency tuning value Δv is determined; then, the displacement detection distance x is determined; finally, the glass ferrule length L0 is calculated.

[0056]

[0057] Where c is the speed of light and n represents the refractive index of the medium inside the FP interferometer cavity.

[0058] Preferably, the specific method for designing the displacement measurement probe in S2 is as follows: Figure 2An APC connector is used as the laser emission end. The 8° tilt angle of the APC connector reduces the reflection of light from one end of the optical fiber. After removing the surface coating of a section of optical fiber using physical or chemical methods, a second optical fiber is obtained, avoiding axial deformation of the optical fiber caused by environmental changes in the coating material. The second optical fiber is inserted into the glass ferrule using an optical fiber connection device and fixed with epoxy resin. One end face of the bonded ferrule is ground flat, and the other end face is ground to an 8° tilt angle. A metal connecting sleeve is used to connect the ground ferrule to the APC connector, and finally, the two sides of the metal connecting sleeve are glued together for fixation.

[0059] The displacement measuring probe can be connected in any way, not limited to epoxy resin adhesive or UV-cured adhesive.

[0060] During the grinding of the displacement measurement probe, after the glass ferrule and the optical fiber are bonded together, one end face can be ground flat, and the grinding angle of the other end face is not limited to 8°, but can be in the range of 0 to 45°.

[0061] The glass insert is designed to be spiral or folded depending on its length, and is not limited to a straight insert structure, in order to reduce the size of the probe.

[0062] The glass ferrule is made of glass, ceramic, or other materials with a low coefficient of thermal expansion lower than that of SMF-28 optical fiber. The ferrule diameter is as large as possible, and the ferrule is bonded to the second optical fiber. This reduces the sensitivity of the second optical fiber to temperature and stress changes, thereby reducing the displacement measurement error introduced by the probe.

[0063] Furthermore, the specific process of displacement demodulation in S3 is as follows: As shown in the figure, the displacement measurement probe and the output port of the circulator are connected via SMF-28. Further, the specific process of displacement demodulation in S3 is as follows: Figure 1 As shown, a signal generator produces a high-frequency carrier-modulated laser, which outputs an optical signal and couples it to the first input port of the circulator. A displacement measurement probe is connected to the second output port of the circulator via an SMF-28 optical fiber. The left end face of the second optical fiber inside the sensing probe and the mirror under test form a Fabry-Perot interferometer cavity. The displacement of the mirror under test is applied to the modulated interference signal and coupled to the third output port of the circulator. The modulated interference signal at the third output port of the circulator is detected by a photoelectric detection module. This modulated interference signal can be expressed as:

[0064]

[0065] Where A is the DC bias and B is the AC term amplitude. Let C be the phase to be measured, and cosw be the phase modulation depth. c t is the carrier signal; to obtain orthogonal signals S3 and S4 and thus satisfy the ATAN algorithm demodulation, the first harmonic signal Gcosw is first used.c t and the second harmonic signal Hcos2w c t is multiplied by the interference signal to obtain the low-harmonic mixing terms S1 and S2, which are easy to detect:

[0066]

[0067]

[0068] Then, after passing through a cutoff frequency of w c Filtering with a low-pass filter of 2 / 2 yields two orthogonal signals S3 and S4 with a phase difference of 90°:

[0069]

[0070] Where G and H are the constant amplitude values ​​of the first harmonic signal and the second harmonic signal, respectively, and the result is obtained by dividing the two orthogonal signals S3 and S4. G / H is a constant, and J1(C) / J2(C) is the ratio of the first-order and second-order Bessel functions, which is 1 when the phase modulation depth C is 2.63 rad. The displacement measurement probe achieves a phase modulation depth C of 2.63 rad, eliminating the nonlinear displacement measurement error caused by J1(C) / J2(C) not being 1. Finally, the arctangent value of signal S5 is calculated and the phase is accumulated. Based on the relationship between phase and displacement, the high linearity of the measured displacement value Δx is obtained.

[0071]

[0072] in, The phase change value is represented by n, the refractive index of the medium inside the FP interferometer cavity is represented by λ, the center wavelength of the laser is represented by L0, and the length of the glass ferrule is represented by L. t This indicates the change in probe length caused by temperature.

[0073] The interference signal is converted and amplified by the photoelectric detection module 10 and the signal conversion and acquisition module 11, and then acquired by the data acquisition card and input into LabVIEW for demodulation using the arctangent algorithm.

[0074] The displacement measurement system includes a signal generator, a laser, a fiber optic circulator, a displacement measurement probe, a photoelectric detection module, a signal conversion and acquisition module, and a LabVIEW-based virtual instrument module 12;

[0075] The laser 3 is connected to the input of the signal generator 1. The output of the laser 3 is connected to the input port of the fiber optic circulator 5. The laser 3 is connected to the fiber optic circulator input port 4 of the fiber optic circulator 5. The fiber optic circulator output port 7 is connected to the displacement measurement probe 8. The left end face of the second fiber 17 and the mirror 9 under test form a Fabry-Perot (FP) interferometer cavity. The fiber optic circulator output port 6 is connected to the photoelectric detection module 10. The input and output of the photoelectric detection module 10 are connected to the demodulation module 12 after passing through the signal conversion and acquisition module 11. One end face of the second fiber 17 in the displacement measurement probe 8 and the mirror 9 under test form a Fabry-Perot (FP) interferometer cavity.

[0076] The signal generator 1 generates a high-frequency carrier-modulated laser 3, which outputs a high-frequency modulated optical signal. The optical signal is transmitted through the fiber optic circulator 5 to the output port 7 of the fiber optic circulator. After being reflected by the mirror on the surface of the displacement measuring probe 8, a beam of light enters the circulator 5. The light emitted from the mirror 9 under test is coupled to the circulator, and the two beams are coupled to obtain an interference signal. The interference signal is detected by the photoelectric detection module 10 at the output port 6 of the fiber optic circulator and then converted into a current signal. The current signal is converted into a voltage signal after passing through the signal conversion, amplification, and acquisition module 11. The signal is acquired by the data acquisition board and input to the virtual instrument LabVIEW for demodulation. After demodulation, the nonlinear displacement measurement error is eliminated, thereby obtaining a displacement measurement result with high linearity.

[0077] Laser 3 is connected to the fiber optic circulator input port 1 via FC / APC.

[0078] The signal generator is used to generate a high-frequency modulated carrier signal and modulate the laser.

[0079] The laser is used to output optical signals with high-frequency variations.

[0080] The photoelectric detection module is used to detect optical interference signals;

[0081] The signal conversion and acquisition module is used to convert, amplify, and acquire the flow pressure signal;

[0082] The demodulation module based on LabVIEW virtual instrument is used to complete the demodulation and phase accumulation of the arctangent algorithm and obtain the corresponding displacement value.

[0083] Preferably, the photoelectric detection module is a light intensity detection device such as a charge-coupled device or a photodiode.

[0084] Preferably, the demodulation module based on LabVIEW virtual instrument can also be implemented using any technology such as a microcontroller or FPGA.

[0085] The laser module consists of a distributed feedback semiconductor DFB laser with a wavelength of 1550nm and corresponding circuit driving it.

[0086] The signal conversion and acquisition module consists of a transimpedance amplifier circuit and an NI acquisition board USB-6351, which realizes the acquisition of voltage signals after current-to-voltage conversion and amplification.

[0087] The demodulation module based on LabVIEW virtual instrument performs PGC-ATAN phase demodulation algorithm and phase accumulation operation to demodulate the signal in real time and display the displacement value to be measured.

[0088] The working process of Example 1 is as follows:

[0089] By designing a displacement measurement probe, increasing the cavity length of the Fabry-Perot interferometer cavity in the displacement measurement system, and improving the phase modulation depth of the PGC-ATAN demodulation algorithm, the interference signal is acquired and then demodulated and accumulated to obtain the displacement value to be measured.

[0090] One embodiment of the present invention describes a method and measuring probe for improving the linearity of a Fabry-Perot interferometric displacement measurement system, which mainly includes the following steps:

[0091] Step 1: Based on the Bessel function, the optimal phase modulation depth C for the displacement demodulation ATAN algorithm is determined to be 2.63. The fast tuning value Δν of the DFB laser is determined to be 3GHz. The displacement detection distance x is designed to be 1mm, and the glass ferrule length L0 is obtained.

[0092]

[0093] Where c is the speed of light and n is the refractive index of the medium in the FP interferometer cavity. The glass insert length L0 is designed to be 2 cm.

[0094] Step 2: Fix the glass ferrule using a V-groove, remove the surface coating of a section of optical fiber, insert the coated optical fiber into the glass ferrule using a CCD camera and a six-axis displacement stage, and then bond it with epoxy resin. Subsequently, grind one side flat and the other side at an 8° angle. Use a metal connecting sleeve to connect and bond the ground ferrule to the APC connector to obtain the displacement measurement probe.

[0095] Step 3: Connect the displacement measurement probe to port 2 of the circulator, and acquire the interference signal using the photoelectric detection module. Compare the interference signal curves before and after installing the displacement measurement probe to evaluate the effect of the displacement measurement probe in increasing the phase modulation depth. Figure 3 It can be seen that after the measurement system is operational, the modulation depth of the interference signal is significantly increased after the displacement measurement probe is installed. The interference signal can be represented as:

[0096]

[0097] A represents the DC bias term, and B represents the AC term amplitude. Let C be the phase to be measured, and cosw be the phase modulation depth. c t is the carrier signal.

[0098] Step 4: Based on the demodulation module of LabVIEW virtual instrument, demodulate the electrical signal detected by the photodetector and perform phase accumulation, demodulate the signal in real time and display the displacement value to be measured.

[0099] The phase signal demodulated by the PGC-ATAN algorithm is represented as follows:

[0100]

[0101] After phase accumulation, the displacement value between any two time points is obtained as follows:

[0102]

[0103] The device selection and parameters for the PGC-ATAN algorithm demodulation low-frequency vibration displacement signal testing device are as follows:

[0104] 1. Signal generator 1: Model KEYSIGHT-33500B, minimum output voltage resolution is 1mV;

[0105] 2. Laser 3: A DFB laser with a center wavelength of 1550nm, model BNLD-402PM-FA, with an output power of 20mW, a linewidth of 200kHz, and a maximum fast-tuning optical frequency of approximately 11GHz;

[0106] 3. Fiber optic circulator 5: operating wavelength 1550nm, model 6015-3-APC, insertion loss ≤0.35dB, isolation ≥40dB;

[0107] 4. Photodetector module 10: Based on a photodetector and related current-to-voltage hardware circuit, the type is InGaAs / PIN photodetector, model LSIPD-H2; the connection mode is FC / APC, and the working wavelength is 800-1700nm;

[0108] 5. Signal conversion and acquisition module: based on current-to-voltage hardware circuit and data acquisition card USB-6351. Data acquisition card: model USB-6351, provides 16 channels of 16-bit 1.25MS / s analog input channels and 2 channels of 16-bit 2.86MS / s analog output channels.

[0109] use Figure 1An experiment was conducted using a Fabry-Perot interferometric displacement measurement system. A 320 Hz intra-carrier modulated laser was applied, and the metal mirror under test was fixed on a nanoscale displacement stage. The nanoscale displacement stage generated a low-frequency vibration displacement of 5 Hz with an amplitude of 500 nm. The displacement was simultaneously tracked and measured using an FPS3010. The demodulation results are as follows: Figure 4 As shown in the figure, the vibration displacement signal is effectively extracted.

[0110] It should be noted that the terms "comprising" and "having" and any variations thereof in the specification, claims and accompanying drawings of this invention are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are explicitly listed, but may include other steps or units that are not explicitly listed or that are inherent to such processes, methods, products or devices.

[0111] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0112] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Those skilled in the art can readily implement the present invention based on the accompanying drawings and the above description. However, any modifications, alterations, or variations made by those skilled in the art without departing from the scope of the present invention, utilizing the disclosed technical content, are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, or variations made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the present invention.

Claims

1. A displacement measuring probe, characterized in that, include, The displacement measuring probe is designed as follows: an APC connector (13) is used as the laser emission end. An optical fiber (17) is inserted into a glass ferrule (16) of a certain length through an optical fiber connection device and fixed with adhesive. One end face of the fixed glass ferrule (16) is ground flat, and the other end face is ground to an inclination angle of 0~45°. A metal connecting sleeve (15) is used to connect the APC connector (13) to the ground glass ferrule (16). The displacement measuring probe is obtained after the two sides of the metal connecting sleeve (15) are fixed with adhesive. Among them, the length of the glass insert (16) L The method for determining 0 is as follows: The phase modulation depth in the phase generation carrier demodulation algorithm is determined based on the Bessel function. C The optimal value is then determined, and then the optical frequency tuning value Δ of the laser is determined. v Combined with displacement detection distance x The length of the glass insert was calculated. L 0: in, c At the speed of light, n Δ represents the refractive index of the medium inside the FP interferometer cavity. v This is the optical frequency tuning value of the laser. L 0 represents the length of the glass insert. x Displacement detection distance; The glass ferrule (16) uses glass or ceramic with a thermal expansion coefficient lower than that of optical fiber 2 (17) as the ferrule medium, and the glass ferrule (16) is processed into a spiral or folded structure.

2. The displacement measuring probe according to claim 1, characterized in that, The second optical fiber (17) is a single-mode optical fiber or a special optical fiber made of birefringent material, obtained by removing the coating layer on the surface of the optical fiber using physical or chemical methods.

3. A displacement measuring probe according to claim 1, characterized in that, The adhesive bonding and fixing uses epoxy resin glue or light-cured glue.

4. A method for improving the linearity of a Fabry-Perot interferometric displacement measurement system, based on the displacement measurement probe according to any one of claims 1-3, characterized in that, include, After connecting the displacement measurement probe to the displacement measurement system, the high-frequency carrier signal generated by the signal generator (1) modulates the laser (3), so that the output high-frequency changing light signal is generated. The light signal is reflected back to the circulator (5) after passing through the circulator (5) and the displacement measurement probe (8). The light intensity change caused by the low-frequency displacement of the mirror (9) under test is reflected back to the circulator (5) and coupled with the light signal to obtain the modulated interference signal. After the interference signal is detected, it becomes a photocurrent signal. The current signal is converted and amplified into a voltage signal, which is then input into LabVIEW for demodulation using the arctangent algorithm. After demodulation, the nonlinear displacement measurement error is eliminated, thereby obtaining a displacement measurement result with high linearity. The expression for the modulated interference signal is: In the formula, A DC bias, B For the amplitude of the exchange term, φ s (t) represents the phase to be measured, which includes the effect of temperature. C For phase modulation depth, cos w c t It is a carrier signal; The specific process of inputting the data into LabVIEW for arctangent demodulation, and obtaining high-linearity displacement measurement results after demodulation, is as follows: Using a frequency-doubled signal G cos w c t and second harmonic signal H cos2 w c t Multiplying each term by the interference signal yields a low-harmonic mixing term S1 and a low-harmonic mixing term S2, which are easily detectable by the signal: S 1= G cos w c t ( ) S 2= H cos2 w c t ( ) After passing through the cutoff frequency w c Filtering with a low-pass filter of 2 / 2 yields two orthogonal signals, 1S3 and 2S4, with a phase difference of 90°: S 3= GBJ 1( C )sinφ s (t); S 4= HB J 2( C )cos φ s (t); in: G and H These are the constant term amplitudes for the first harmonic signal and the second harmonic signal, respectively. Then, the quadrature signals 1S3 and 2S4 are divided to obtain signal S5: S5= GJ 1( C ) / HJ 2( C ) tan φ s (t); in, G / H It is a constant. J 1( C ) / J 2( C The ratio of the first-order to the second-order Bessel function is 1 when the phase modulation depth C is 2.63 rad, thus eliminating the... J 1(C) / J 2(C) represents the nonlinear displacement measurement error caused when C is not 1; Finally, the arctangent value of signal S5 is calculated and the phase is accumulated. Based on the relationship between phase and displacement, a displacement measurement result with high linearity is obtained.

5. A method for improving the linearity of a Fabry-Perot interferometric displacement measurement system according to claim 4, characterized in that, The displacement measurement results are expressed as follows: The displacement value to be measured △ x : Among them, △ φ s Indicates the phase change value. n This represents the refractive index of the medium inside the FP interferometer cavity. λ Indicates the center wavelength of the laser. L 0 indicates the length of the glass insert. L t This indicates the change in probe length caused by temperature.

6. A method for improving the linearity of a Fabry-Perot interferometric displacement measurement system according to claim 4, characterized in that, The displacement measurement system includes a signal generator (1), a laser (3), an optical fiber circulator (5), a photoelectric detection module (10), a signal conversion and acquisition module (11), and a demodulation module (12). The signal generator (1) is connected to the input end of the laser (3), the output end of the laser (3) is connected to the input port of the fiber optic circulator (5), one output port of the fiber optic circulator (5) is connected to the displacement measurement probe (8), the other output port of the fiber optic circulator (5) is connected to the photoelectric detection module (10), the input end of the photoelectric detection module (10), and the output end of the photoelectric detection module (10) are connected to the demodulation module (12) after passing through the signal conversion and acquisition module (11); one end face of the second fiber (17) in the displacement measurement probe (8) and the mirror surface (9) under test form an FP interference cavity.

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