CN118209142A - High-precision demodulation method based on low-definition FP interferometer - Google Patents
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Abstract
The invention provides a high-precision demodulation method based on a low-definition FP interferometer, which comprises a signal modulation module, a rapid peak searching module, an Empirical Mode Decomposition (EMD) module, an arcsin anti-aliasing module, a phase resolving module and a non-uniform fitting and outputting module. The method comprises the steps of realizing suppression of residual light intensity accompanying modulation and residual low-frequency interference by using an empirical mode decomposition module, realizing rapid peak searching of a triangular wave signal by using a rapid peak searching module, realizing anti-aliasing of an arcsin signal by using peak information, realizing unwrapping of a signal phase by using a phase resolving module, realizing accurate demodulation of a phase to be detected, realizing nonlinear correction of a light source by using a non-uniform fitting module, and finally extracting distance information to be detected. The high-precision demodulation of the phase information and the high-precision calculation of the distance information are realized through the processing of the time domain signal of the low-definition FP interferometer. The influence of light intensity disturbance on demodulation signals is eliminated, and the high-precision measurement of the absolute distance is realized, so that the application field of the optical fiber microprobe sensor is greatly expanded.
Description
Technical Field
The invention belongs to the field of optical fiber interferometer phase demodulation algorithms, and particularly relates to a high-precision demodulation method based on a low-definition FP interferometer.
Background
Distance is the distance between two objects in space, and is an unavoidable ring in modern technology as a basic element reflecting the relation of the objects. With the development of technology and the progress of precision equipment, the demands of precision position measurement and topography measurement in the fields of precision equipment manufacturing and assembly, ships, aerospace, building mapping and the like are increasing. In particular for the manufacture of precision parts equipment, a higher positioning accuracy means a better assembly result and at the same time better equipment performance. Among the existing distance measurement methods, the coherent measurement method utilizes the interference phenomenon of light to measure the distance, has higher measurement accuracy and stronger resolution capability for the weak reflection target.
The existing phase resolving method mainly includes a method of obtaining a quadrature interference signal using hardware typified by a polarization quadrature method and a method of demodulating a phase-shifted signal obtained using digital processing typified by a hilbert transform method. The former can obtain a preferable orthogonal signal and extract a phase, but it causes a system to be complicated, while the latter cannot realize a high-speed signal calculation due to the end effect of the hilbert transform. Four-channel signal receiving systems (Wang Li, hou Wenmei) using Wollaston prism, four-channel signal receiving system of single frequency laser interferometer [ J ]. Meter theory, 2006 (04): 313-316.), swept nonlinear correction ranging method based on similar triangular interpolation sampling (CN 112946611A), swept interference ranging signal processing method (CN 113253241A), and swept interference measurement nonlinear synchronization error correction method based on phase compensation (CN 116659395A) proposed by the university of Harbin industrial university Liu Guodong subject group are proposed as proposed by Shanghai university Wang Li et al in 2006.
Disclosure of Invention
The invention aims to provide a high-precision demodulation method based on a low-definition FP interferometer.
The aim of the invention is realized by the following technical scheme:
The invention relates to a high-precision demodulation method of a low-definition FP interferometer, which comprises a signal modulation acquisition module, wherein main interferometer signals and auxiliary interferometer signals acquired by the signal modulation acquisition module sequentially pass through a fast peak searching module, an Empirical Mode Decomposition (EMD) module, an arcsin anti-aliasing module, a phase resolving module and a non-uniform fitting and outputting module, and finally phase demodulation signals and calculated distance values are output; the signal modulation acquisition module realizes synchronous signal modulation and acquisition, the rapid peak searching module is used for identifying signal characteristic peaks, the empirical mode decomposition EMD module is used for suppressing signal distortion, the arcsin module is used for solving signal phases, the arcsin anti-aliasing module is used for extracting arcsin waveform aliasing information, conversion from triangular waves to sawtooth waves after arcsin is realized, the phase resolving module is used for extracting wrapped phases in the sawtooth waves, and the non-uniform fitting and output module is used for outputting final observed quantity results.
Further, the signal modulation acquisition module comprises data acquisition and modulation output, and the data acquisition module is used for acquiring interference signals output by the interferometer, and the interference signals are converted into electric signals from optical signals after photoelectric conversion; the modulation output module outputs triangular waves to the light source modulator for modulating the light source, the modulated light is injected into the interferometer, the modulation frequency is between 0.5kHz and 5kHz, and the tuning range is more than 20G.
Further, the fast peak searching module performs normalization scaling on the received interference electric signal, sends the normalized signal to a threshold detector, extracts all extreme point sequences crossing the threshold respectively, sends the extracted sequences to a peak analyzer, analyzes the peak width, peak position and peak interval information according to the obtained peak index information, and outputs the required peak position information according to the occurrence rule of the peak characteristics.
Further, the EMD module processes the received peak position information, reads peak amplitude values according to peak indexes, respectively interpolates maximum values and minimum values to obtain an upper envelope and a lower envelope of the signal, and scales the signal by using the two envelopes to eliminate direct current and envelope fluctuation of the signal and ensure the phase stability of the interference signal.
Further, the arcsin module performs arcsin operation on the signal, and directly extracts the phase of the signal, wherein a value exceeding the arcsin definition domain + -1 is zoomed to the corresponding definition domain boundary.
Further, the arcsin anti-aliasing module peak screening analyzes the peak value obtained by the rapid peak searching module, judges a specific peak to be compensated according to signal characteristics, skips the peak to be compensated, judges the condition to be the coefficient threshold of average peak spacing, and then transmits the peak to be compensated obtained by screening to the alternate sequencing; the alternating sequencing judges the signal output by the arcsin module alternately according to the index of the peak, and the interval of the even number is inverted, namely multiplied by-1, so that the transition from the triangular wave to the sawtooth wave is realized.
Further, the phase resolving module uses a differentiator to process the sawtooth waves obtained by alternating sequencing to obtain derivatives of the sawtooth waves, the derivatives are transmitted to threshold detection, threshold detection outputs parameters of threshold value, aliasing compensation is determined according to the sampling rate, and phase quantity to be compensated is determined according to the threshold value, so that phase unwrapping is realized.
Further, the phase resolving module is used for respectively obtaining phase information after aliasing compensation of the main interferometer and the auxiliary interferometer by non-uniform fitting, processing the phase information, and directly carrying out phase domain non-uniform fitting on the phase of the main interferometer relative to the phase of the auxiliary interferometer by adopting a least square method in a fitting mode to realize the correction of the nonlinearity of the frequency sweep of the light source; and transmitting the fitted data to coefficient scaling, performing optical path scaling according to the optical path of the auxiliary interferometer and the refractive index of the measurement space, and outputting the measured absolute distance information with high precision.
The invention has the beneficial effects that:
1. The invention can realize high-precision demodulation under the condition of using a smaller sweep frequency range, and the demodulation precision is directly limited by the phase resolution;
2. Compared with the traditional method, the method has the advantages that the resolving speed is faster, and the testing speed is also greatly improved compared with the traditional method due to the small tuning range, namely, the testing speed is fast;
3. the invention can eliminate the influence of light intensity accompanying modulation on signals, so that the demodulation result has high signal-to-noise ratio, the accuracy of signal amplitude detection and the stability of a demodulation system are improved, and the invention can be widely applied to high-precision distance sensing systems;
4. According to the invention, the EMD is used for inhibiting different frequency signals of the non-signal light, so that the real-time inhibition of the strong random noise and the extra low-frequency interference can be realized;
5. According to the invention, the phase is directly extracted by using asin, so that the problem of end effect caused by Hilbert transformation is avoided, and the phase extraction with higher precision can be realized;
6. According to the method, the nonlinear error of the sweep frequency is eliminated through nonuniform fitting of the phase domain, uncertainty of a demodulation result caused by factors such as unstable light source is avoided, the demodulation result has high precision and high resolution, and the accuracy of the demodulation result of the system and the stability of the demodulation system are improved;
7. The invention has low calculation complexity and good compatibility with the system, and can be widely applied to high-precision optical fiber measurement and sensing systems.
Drawings
FIG. 1 is a flow chart of a high-precision demodulation method for a low-finesse FP interferometer;
FIG. 2 is a diagram of an interferometric modem probe optical path arrangement;
FIG. 3 is a schematic diagram of obtaining a peak base position by peak detection;
FIG. 4 is a schematic diagram of the signal after EMD to eliminate other interference and concomitant modulation of light intensity;
FIG. 5 is a schematic of a triangular wave of phase aliasing obtained by directly extracting the phase by arcsin;
FIG. 6 is a schematic diagram of arcsin antialiasing to convert a triangle wave into an unwrapped sawtooth wave;
FIG. 7 is a phase diagram of phase resolution extraction wrapped in a sawtooth;
Fig. 8 is a schematic diagram of a phase information fitting straight line obtained by phase non-uniform fitting.
Detailed Description
The invention is further described below with reference to the accompanying drawings.
In order to clearly illustrate the high-precision demodulation method of the low-definition FP interferometer of the present invention, the present invention will be further described with reference to the drawings and the embodiments, but the scope of the present invention should not be limited thereto.
The high-precision demodulation method of the low-definition FP interferometer distance sensor based on the space light path comprises the following specific processes:
(1) The system operates the signal modulation module 113, and the computer 241 controls the arbitrary waveform generator AWG212 to output a voltage signal via the first data transmission line 242 to perform frequency modulation on the light source 211, so that the light source emits a modulation signal with a triangular wave variation in frequency. Light exiting from the a2 port of the first coupler 213 and entering the main interferometer enters the b1 port of the optical fiber circulator 2211, and exits from the b2 port, and enters the optical fiber microprobe 2213 with the reference surface after passing through the probe long tail fiber 2212 with any length, and is split by the optical fiber microprobe, and light caused by reflection of the reference surface enters the b2 port of the optical fiber circulator 2211 after passing through the probe long tail fiber 2212 with any length again, wherein the optical fiber is the 0 optical path difference reflected light of the low-definition FP interferometer. The light transmitted by the reference surface is collimated by the optical fiber microprobe to form collimated laser with small light spots, the collimated laser passes through the distance to be detected 2214 and irradiates on the target object 2215, the collimated laser is reflected by the target object 2215 and returns to the optical fiber microprobe 2213 with the reference surface again to be split, so that the reflecting surface of the optical fiber microprobe and the target object form an FP cavity with the fringe fineness changing along with the reflectivity of the target, and the light reflected by the FP cavity forms FP cavity interference fringes, wherein the optical path difference of each adjacent beam of light is twice the distance to be detected. This interference fringe is brought into a state of a low finesse FP cavity by limiting the reflectivity of the target. The interference signal enters the b2 port of the optical fiber circulator 2211 after passing through the probe long tail fiber 2212 with any length, is transmitted out from the b3 port, and enters the photoelectric detector PD2311 after passing through the single-mode optical fiber. Light exiting from port a3 of coupler number one 213 and entering the interferometer enters port c1 of coupler number two 2221 and is split into two arms that assist the interferometer. Light transmitted from the c2 port of the second coupler passes through the delay optical fiber 2222 of a known length and enters the d1 port of the third coupler 2224; the light coming out from the c3 port of the second coupler passes through the second delay optical fiber 2223 with known length and then enters the d2 port of the third coupler 224, and the MZI interferometer formed by the second coupler 2221, the first delay optical fiber 2222 with known length, the second delay optical fiber 2223 with known length and the third coupler 2224 is the auxiliary interferometer of the system. Before use, the arm length difference of the MZI interferometer needs to be calibrated, high-precision length calibration can be realized through an OFDR technology, and the calibration directly influences the precision of the final test distance. The interference signal coming out of the d3 port of the third coupler 2224 enters the photodetector PD2321 through a single mode fiber.
(2) The data acquisition module 112 acquires the interference signals of the two photodetectors and inputs them into a subsequent demodulation algorithm. The received interference electric signal is normalized and scaled 121, the normalized signal is sent to a threshold detector 122, all extreme point sequences crossing the threshold are extracted respectively, the extracted sequences are sent to a peak analyzer 123, the information of peak width, peak position, peak interval and the like is analyzed according to the obtained peak index information, and the required peak position information is output according to the occurrence rule of the peak characteristics.
(3) And processing the received peak position information, reading peak amplitude according to the peak index, respectively interpolating the maximum value and the minimum value to obtain an upper envelope and a lower envelope of the signal, amplifying and shrinking the signal by using the two envelopes, eliminating direct current and envelope fluctuation of the signal, and ensuring the phase stability of the interference signal.
(4) And carrying out arcsin operation on the signal, and directly extracting the phase of the signal, wherein the value exceeding the arcsin definition domain (+ -1) is proximally scaled to the corresponding definition domain boundary.
(5) The peak value screening 151 analyzes the peak value obtained by the fast peak searching module 12, judges the specific peak to be compensated according to the signal characteristics, skips the peak to be compensated, judges the condition to be the coefficient threshold of the average peak interval, and then transmits the screened peak to be compensated to the alternating sorting 152. The alternating sequence 152 performs alternating judgment on the signal output by the arcsin module 14 according to the index of the peak, and performs inversion (multiplication-1) on the interval of the even number to realize the conversion from the triangular wave to the sawtooth wave.
(6) The saw-tooth wave obtained by the alternative ordering 152 is processed by a differentiator 161 to obtain the derivative of the saw-tooth wave, the derivative is transmitted to a threshold detection 162, the threshold detection 162 outputs a parameter of the threshold value, the parameter is determined according to the sampling rate, and an aliasing compensation 163 determines the phase quantity to be compensated according to the threshold value, so as to realize phase unwrapping.
(7) The non-uniform fitting 171 obtains and processes the phase information after the aliasing compensation 163 of the main interferometer and the auxiliary interferometer respectively, and uses a least square method to directly perform the non-uniform fitting of the phase domain of the main interferometer relative to the auxiliary interferometer phase by using the fitting mode, thereby realizing the correction of the non-linearity of the frequency sweep of the light source. The fitted data is transferred to the coefficient scaling 172, and the optical path scaling is performed according to the optical path of the auxiliary interferometer and the refractive index of the measurement space, so that the absolute distance information with high accuracy is output.
In the above process, the signal modulation module 11 includes a data acquisition 112 module and a modulation output module 113, where the modulation output module 113 outputs a triangle to the light source modulator for modulating the light source to generate frequency tuning, so that the light frequency generates linear changeWhere α is the instantaneous sweep speed of the light source, Δω is the tuning range of the light source, and T m is the period of the modulated signal. By controlling the reflectivity of the target object to make the FP cavity in a low-definition working state, the signals received by the photodetectors PD2311 and PD2321 at this time can be regarded as two-beam interference fringes. When the dual-beam beat interference occurs, the interference signal can be written as:
Wherein I (tau, t) is the light intensity of the interference signal, E 1、E2 is the complex amplitude of the two beams of light involved in interference, E 01、E02 is the amplitude of the two beams of light involved in interference, alpha is the instantaneous sweep speed omega 0 of the light source, the frequency I 1I2 corresponding to the central wavelength of the interference laser is the light intensity tau of the two beams of light involved in interference, and the light intensity tau of the two beams of light is the two-beam time delay. At this time, the normalized ac term is:
Then for the primary interferometer and the secondary interferometer, the detected interference light signal is expressed as:
φm0＝ω0τmωm＝ατmφr0＝ω0τrωr＝ατr
wherein I m、Ir is the measured light intensity, omega m、ωr is the frequency of the measured beat signal, phi r0、φm0 is the initial phase of the measured beat signal, alpha is the instantaneous sweep speed of the light source, tau m is the two-beam time delay caused by the arm length difference of the known auxiliary interferometer, and tau r is the interference light time delay caused by the spatial distance to be measured.
The time-varying terms can be obtained after the measurement interferometer and the auxiliary interferometer are respectively subjected to phase demodulation:
φr＝ατrt、φm＝ατmt
the corresponding distance information can be obtained by using a phase comparison method and converting:
Wherein n is the refractive index of the optical fiber, L r is the arm length difference of the known auxiliary interferometer, phi m、φr is the time-varying term for measuring the phase in the signals of the interferometer and the auxiliary interferometer, and L is the distance to be measured.
From the above deductions, it can be seen that the alternating term of the obtained interference signal is sinusoidal, and thus can be processed according to the above-mentioned method, and the settlement result
The invention relates to an algorithm improvement of a phase demodulation algorithm, and the principle of the improved algorithm is shown in figure 1; the peak position thereof can be obtained by peak detection as shown in fig. 3; random light intensity disturbance can be corrected by empirical mode decomposition EMD, as in FIG. 4; triangular waves of the phase term can be extracted through arcsin, as shown in fig. 5; triangular waves can be converted to saw-tooth waves by arcsin antialiasing, as in fig. 6; the wrapped phases in the sawtooth wave can be extracted through phase calculation, as shown in fig. 7; the phase information of the measurement signal can be obtained by non-uniform fitting, where the auxiliary interferometer phase is the x-axis and the main interferometer phase is the y-axis, using a phase domain for fitting, in which case the fitting would be non-uniform, but the error due to the source tuning non-linearity can be perfectly corrected, as in fig. 8.
The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (8)
1. A high-precision demodulation method of a low-definition FP interferometer is characterized in that: the system comprises a signal modulation acquisition module (11), wherein main interferometer signals and auxiliary interferometer signals acquired by the signal modulation acquisition module (11) sequentially pass through a rapid peak searching module (12), an Empirical Mode Decomposition (EMD) module (13), an arcsin module (14), an arcsin anti-aliasing module (15), a phase resolving module (16) and a non-uniform fitting and outputting module (17), and finally phase demodulation signals and calculated distance values are output; the signal modulation acquisition module (11) realizes synchronous signal modulation and acquisition, the rapid peak searching module (12) is used for identifying signal characteristic peaks, the empirical mode decomposition EMD module (13) is used for suppressing signal distortion, the arcsin module (14) is used for solving signal phases, the arcsin anti-aliasing module (15) is used for extracting arcsin waveform aliasing information, conversion from triangular waves to sawtooth waves after arcsin is realized, the phase resolving module (16) is used for extracting wrapped phases in the sawtooth waves, and the non-uniform fitting and output module (17) is used for outputting final observed quantity results.
2. The high-precision demodulation method of a low-definition FP interferometer according to claim 1, wherein: the signal modulation acquisition module (11) comprises a data acquisition module (112) and a modulation output module (113), wherein the data acquisition module (112) is used for acquiring interference signals output by an interferometer, and the interference signals are converted into electric signals from optical signals after photoelectric conversion; the modulation output (113) module outputs triangular waves to the light source modulator for modulating the light source, the modulated light is injected into the interferometer, the modulation frequency is between 0.5kHz and 5kHz, and the tuning range is more than 20G.
3. The high-precision demodulation method of a low-definition FP interferometer according to claim 1, wherein: the rapid peak searching module (12) normalizes and expands (121) the received interference electric signals, sends the normalized signals to the threshold detector (122), extracts all extreme point sequences passing the threshold respectively, sends the extracted sequences to the peak analyzer (123), analyzes the information of peak width, peak position and peak interval according to the obtained peak index information, and outputs the required peak position information according to the occurrence rule of the peak characteristics.
4. The high-precision demodulation method of a low-definition FP interferometer according to claim 1, wherein: the EMD module (13) processes the received peak position information, reads peak amplitude values according to peak indexes, respectively interpolates maximum values and minimum values to obtain an upper envelope and a lower envelope of the signal, and utilizes the two envelopes to scale the signal, so that direct current and envelope fluctuation of the signal are eliminated, and phase stability of the interference signal is ensured.
5. The high-precision demodulation method of a low-definition FP interferometer according to claim 1, wherein: the arcsin module (14) performs arcsin operation on the signal, and directly extracts the signal phase, wherein a value exceeding the arcsin definition domain + -1 is zoomed to the corresponding definition domain boundary.
6. The high-precision demodulation method of a low-definition FP interferometer according to claim 1, wherein: the arcsin anti-aliasing module (15) peak screening (151) analyzes the peak value obtained by the rapid peak searching module (12), judges a specific peak to be compensated according to signal characteristics, skips the peak to be compensated, judges that the condition is a coefficient threshold of average peak spacing, and then transmits the peak to be compensated obtained by screening to the alternate sequencing (152); the alternating sequencing (152) judges the signal output by the arcsin module (14) alternately according to the index of the peak, and inverts the interval of the even number, namely, multiplies-1, so as to realize the conversion from the triangular wave to the sawtooth wave.
7. The high-precision demodulation method of a low-definition FP interferometer according to claim 1, wherein: the phase resolving module (16) uses a differentiator (161) to process the sawtooth waves obtained by the alternate sequencing (152) to obtain derivatives of the sawtooth waves, the derivatives are transmitted to a threshold detection (162), the threshold detection (162) outputs a parameter exceeding a threshold value, the aliasing compensation (163) determines the phase quantity to be compensated according to the threshold value and the phase unwrapping is realized.
8. The high-precision demodulation method of a low-definition FP interferometer according to claim 1, wherein: the phase resolving module (17) is used for enabling the non-uniform fitting (171) to respectively obtain phase information after the aliasing compensation (163) of the main interferometer and the auxiliary interferometer, processing the phase information, and directly carrying out phase domain non-uniform fitting on the phase of the main interferometer relative to the phase of the auxiliary interferometer by adopting a least square method in a fitting mode to realize the correction of the nonlinearity of the frequency sweep of the light source; and transmitting the fitted data to coefficient scaling (172), performing optical path scaling according to the optical path of the auxiliary interferometer and the refractive index of the measurement space, and outputting the measured absolute distance information with high precision.
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