CN113390447A - Optical fiber differential interference sensing system based on frequency response compensation and frequency response compensation method - Google Patents
Optical fiber differential interference sensing system based on frequency response compensation and frequency response compensation method Download PDFInfo
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- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
- G01D5/35306—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement
- G01D5/35335—Aspects of emitters or receivers used by an interferometer in an optical fibre sensor arrangement
Abstract
The invention relates to an optical fiber differential interference sensing system based on frequency response compensation and a frequency response compensation method, wherein the frequency response compensation method comprises the following steps: 1) carrying out phase restoration on the two collected interference signals, setting a disturbance signal as f (t), setting time delay generated by the optical fiber delay coil as tau, and restoring the obtained phase difference signal
2) Converting the solving expression of the disturbance signal f (t) into a discrete form; 3) and after the initial condition is determined, time domain compensation is carried out in a recursion calculation mode. Compared with the prior art, the invention can effectively improve the suppressed low-frequency component in the original optical fiber differential interference sensing system, improve the low-frequency response of the system, improve the signal-to-noise ratio of the low-frequency signal, provide a compensation scheme which is simple in calculation and easy to realize for the optical fiber differential interference sensing system, and is suitable for realizing the fidelity detection of real signals and the effective monitoring of the low-frequency signal.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to an optical fiber differential interference sensing system based on frequency response compensation and a frequency response compensation method.
Background
The optical fiber interferometer is an important component of the optical fiber sensing technology, and is widely applied to detection of various physical quantities such as vibration, temperature, stress and the like due to the advantages of high sensitivity, low cost and electromagnetic interference resistance. The common structures are divided into Michelson, Mach-Zehnder (M-Z), Fabry-Perot and Sagnac forms. The optical fiber interferometer can be divided into direct detection and differential detection, and the sensitivity of direct detection (such as an M-Z interferometer) is high, but the optical fiber interferometer is easily influenced by factors such as environment, temperature, polarization degradation and the like. Differential detection (such as a Sagnac interferometer) can effectively eliminate the influence of temperature and polarization state on the detection result, but the low-frequency performance is poor.
In recent years, an M-Z hybrid Sagnac type optical fiber interference system is widely applied to distributed detection, and has the advantages of simple structure, stable performance and insensitivity to environmental change, however, the information acquired by the differential detection method is the phase difference of two interference lights, a disturbance signal is set as f (t), the time delay generated by an optical fiber delay coil is tau, and the phase difference signal obtained by reduction is
Comprises the following steps:
if the disturbing signal is a single-frequency signal, i.e.
The obtained phase difference signal is restored
Comprises the following steps:
the amplitude of the signal obtained by the differential interference system through phase reduction contains
The term, decreases as the perturbation signal frequency decreases. The differenceThe frequency response of the partial interference system is shown in fig. 3, the system structure causes that the low-frequency response performance is poor, the amplitude of the low-frequency signal is relatively small and is submerged by the high-frequency signal, and the low-frequency part of the detected signal is distorted compared with the real signal after phase restoration.
Along with the extension of the working frequency band of the interference type optical fiber sensing system to the low frequency, the detection of the system to the low frequency signal is seriously influenced by the self structure of the system and the interference of the low frequency background noise, and the true signal fidelity detection and the low frequency signal effective detection cannot be satisfied.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide an optical fiber differential interference sensing system based on frequency response compensation and a frequency response compensation method, and aims to improve the response of the optical fiber differential interference sensing system to low-frequency signals, effectively compensate the original low-frequency signals in the system detection result, effectively improve the low-frequency component of the phase reduction result and reduce real signals.
The purpose of the invention can be realized by the following technical scheme:
an optical fiber differential interference sensing system based on frequency response compensation comprises a wide-spectrum light source, a 3 x 3 optical fiber coupler, an optical fiber delay coil, a 2 x 2 optical fiber coupler, a sensing optical cable and a Faraday rotator mirror which are sequentially arranged along a light path;
the wide-spectrum light source is connected with one input port of a 3 x 3 optical fiber coupler through an optical fiber and is divided into three beams of light with the same power by the 3 x 3 optical fiber coupler, wherein the first beam of light is coupled out of a fiber core, the second beam of light passes through an optical fiber delay coil and is connected with the first port of the 2 x 2 optical fiber coupler, the third beam of light is connected with the second port of the 2 x 2 optical fiber coupler, the light beam enters a sensing optical cable from the third port of the 2 x 2 optical fiber coupler and is reflected back by a Faraday rotator and enters the 3 x 3 optical fiber coupler again to generate interference, the first photoelectric detector and the second photoelectric detector are respectively connected with the other two input ports of the 3 x 3 optical fiber coupler, and the electrical output ends of the two photoelectric detectors are connected with an amplifier and then are connected with a digital-to-analog converter to carry out phase restoration.
The system demodulates two paths of interference signals detected by the photoelectric detector by a demodulation method to obtain each frequency component amplitude A of the phase difference signaliWith the frequency omegaiOn the other hand, there are:
wherein tau is the time delay generated by the optical fiber delay coil.
A frequency response compensation method of an optical fiber differential interference sensing system comprises the following steps:
1) carrying out phase restoration on the two collected interference signals, setting a disturbance signal as f (t), setting time delay generated by the optical fiber delay coil as tau, and restoring the obtained phase difference signal
Comprises the following steps:
the solution for the perturbation signal f (t) is:
2) converting the solving expression of the disturbance signal f (t) into a discrete form;
3) and after the initial condition is determined, time domain compensation is carried out in a recursion calculation mode.
In the step 2), the solution of the disturbance signal f (t) is converted into a discrete expression:
…
…
where m is the accumulated compensation interval, k represents the discrete time interval number, t1、...、tm+1、tkm+1Respectively, a series of discrete times, at being a time step.
The initial condition is the value of the first discrete time interval f (t)1)，f(tm)]Obtaining the value of [ f (t) in the first discrete time interval1)，f(tm)]And then, obtaining an approximate value of each discrete time point in the kth discrete time interval through recursion calculation.
Initial value of said first time interval [ f (t)1)，f(tm)]The calculation formula of (A) is as follows:
…
wherein, C1,C2,…,CmAre all constant, ideally for a finite length and an initial time t1When t < t1When f (t) is 0, then C1,C2,…,CmThe values are all 0.
The input of the optical fiber differential interference sensing system is f (t), after low-frequency compensation is introduced, the output result is reduced to f (t), and then the frequency response function h (jw) of the frequency response compensation system is as follows:
where e is the natural logarithm, j is the imaginary unit, and ω is the angular frequency.
And (3) for the phenomenon that the disturbance signal f (t) in the whole time domain has a harmonic wave with the frequency f being K/tau after iterative compensation, wherein K is a natural number, and the harmonic wave is eliminated by introducing a low-pass filter for filtering after the time domain compensation.
The accumulated compensation interval adopted by the frequency response compensation method is selected according to the time delay tau generated by the length L of the optical fiber delay coil.
The selection formula of the accumulation compensation interval m is as follows:
m＝L·n/c·fs
wherein, L is the length of the optical fiber delay coil, c is the propagation speed of light in vacuum, n is the refractive index of the optical fiber, and fs is the sampling rate of the system.
Compared with the prior art, the invention has the following advantages:
the method can effectively improve the suppressed low-frequency component in the optical fiber differential interference sensing system, improve the low-frequency response of the system, realize the fidelity detection of real signals, improve the signal-to-noise ratio of the low-frequency signals, and provide a compensation scheme which is simple in calculation and easy to realize for the optical fiber differential interference sensing system.
Drawings
Fig. 1 is a structure of an optical fiber differential interference sensing system employed in the present invention.
FIG. 2 is a flowchart of a compensation method provided by the present invention.
FIG. 3 is a frequency response of an original differential interference system.
Fig. 4 shows the amplitude-frequency characteristic of the frequency response compensation system.
Fig. 5 is a schematic view of the optical path structure employed in embodiment 1.
FIG. 6 is a comparison of 10Hz signals before and after compensation respectively for the fiber differential interference sensing system of example 1.
Figure 7 is a comparison of the signal from ground walking monitored by the buried pipeline before and after compensation in example 2.
The notation in the figure is:
1. the optical fiber coupler comprises a wide-spectrum light source, 2 and 3 x 3 optical fiber couplers, 2a, 2b and 2c are three homodromous ports of the 3 x 3 optical fiber coupler respectively, 2d and 2e are homodromous ports of the 3 x 3 optical fiber coupler respectively, 3 an optical fiber delay coil, 4 and 2 x 2 optical fiber couplers, 4a and 4b are homodromous ports of the 2 x 2 optical fiber coupler respectively, 5 a sensing optical cable, 6 a Faraday rotator mirror, 7 a first photoelectric detector, 8 a second optical fiber detector, 9 and piezoelectric ceramics.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments.
As shown in fig. 1, the present invention provides an optical fiber differential interference sensing system based on frequency response compensation, which includes: the optical fiber delay line comprises a wide-spectrum light source 1 (a wide-spectrum laser), a 2 x 2 optical fiber coupler 4, a 3 x 3 optical fiber coupler 2, an optical fiber delay coil 3, a sensing optical cable 5, a Faraday rotator mirror 6, a first photoelectric detector 7, a second optical fiber detector 8, an amplifier and a digital-to-analog converter.
In fig. 1, a light source is connected to an input port of a 3 × 3 fiber coupler 2 through an optical fiber, the 3 × 3 fiber coupler divides three beams of light with the same power into three beams, one beam of light is coupled out of a fiber core, the other two beams of light are connected to two input ports of a 2 × 2 fiber coupler 4 through an optical fiber delay coil 3 and the optical fiber, respectively, the light beam enters a sensing optical cable 5 from one port of the 2 × 2 fiber coupler, is reflected by a faraday rotator 6, enters the 3 × 3 fiber coupler 2 again and generates interference, two photodetectors are connected to the other two input ports of the 3 × 3 fiber coupler, and an electrical output end of the photodetector is connected to an amplifier and then to a digital-to-analog converter.
The invention provides an optical fiber differential interference sensing system based on frequency response compensation, which demodulates and compensates frequency response of interference signals, and comprises the following specific steps:
(1) carrying out phase restoration on two paths of interference signals collected by the system, setting a disturbance signal as f (t), setting time delay generated by an optical fiber delay coil as tau, and restoring an obtained phase difference signal
Comprises the following steps:
(2) considering that the system adopted by the invention carries out analog-to-digital conversion on the signal, the discrete point number corresponding to the time delay tau is set as m, and one length is set as nfThe signal of (a), can divide the discrete time interval into [ t ]km+1，tkm+m],k＝0,2…,nf-1, considering the discrete points of the (k-1) th time interval, a series of discrete times tkm+1,tkm+2,…,tkm+mCorresponding step length of 1/fsThat is, τ ═ m · Δ t, then the discrete form of equation (3) can be written as:
…
…
as can be seen from equation (4), the value [ f (t) for the first time interval is determined1)，f(tm)]I.e. from the initial value of the first time interval [ f (t)1)，f(tm)]Find [ f (t)2m)，f(t3m)]Then [ f (t)2m)，f(t3m)]And as an initial value of the next step, by the iterative analogy, solving to obtain an approximate value of f (t) at each discrete time point in the kth time interval.
(3) The time domain compensation method is a recursion method, and the core of the time domain compensation method is to determine an initial condition, namely to determine the value [ f (t) of a first time interval1)，f(tm)]The value of the first interval is then as follows:
…
in the compensation method of the invention, an initial value [ f (t) of a first time interval is determined1)，f(tm)]It is necessary to determine C in the formula (5)1,C2,…,CmIdeally, the initial time point for a signal is t1When t < t1And f (t) is 0, then C is considered1,C2,…,CmAre all 0.
The frequency response function of the frequency response compensation system of the optical fiber differential interference sensing system is described as follows:
the input of the differential phase detection system is f (t), and the output is
After the low-frequency compensation system is introduced, the output result is reduced to f (t), and the frequency response function of the frequency response compensation system is as follows:
fig. 4 shows the amplitude-frequency characteristics of the fiber differential interference sensing system after frequency response compensation is introduced. As can be seen from the figure, for the disturbance signal f9t in the entire time domain), after iterative compensation is performed on the signal in the actual system, a harmonic with the frequency f being K/τ (K being a natural number) is generated, and this problem can be solved by introducing a low-pass filter for filtering after the time domain compensation.
In the optical fiber differential interference sensing system based on frequency response compensation, a parameter tau required by a compensation method is determined by the length L of an optical fiber delay coil adopted in the system, wherein tau is L.n/c, c is the propagation speed of light in vacuum, and n is the refractive index of an optical fiber.
Example 1
The optical path structure of the optical fiber differential interference sensing system adopted in the present embodiment is shown in fig. 5. The wide-spectrum light source 1 adopts a superradiance light emitting diode, piezoelectric ceramics 9(PZT) driven by 10Hz voltage is connected to a sensing optical cable, and single-frequency signal disturbance of 10Hz is simulated. According to the optical path structure, there are mainly four beams, respectively:
light beam A: 1 → 2a → 2d → 3 → 4a → 9 → 6 → 9 → 4b → 2e
Beam B: 1 → 2a → 2d → 3 → 4a → 9 → 6 → 9 → 4a → 3 → 2d
And (3) light beam C: 1 → 2a → 2e → 4b → 9 → 6 → 9 → 4a → 3 → 2d
The length of the optical fiber delay coil 3 adopted in the embodiment is 2km, which is far longer than the coherence length of a laser, so that the optical paths of the light beam a and the light beam C are the same, interference is generated at a 3 × 3 coupler, photoelectric conversion is performed by a first photoelectric detection 7 and a second photoelectric detection 8, two paths of interference signals are collected, and after phase reduction, the obtained signals are:
since the frequency of the low-frequency disturbing signal applied in the embodiment is 10Hz, the amplitude of the signal after phase reduction is small, and the signal-to-noise ratio is poor. Will the signal
The compensation is performed according to the flow of the frequency response compensation method shown in fig. 2, and fig. 6 shows that the signal-to-noise ratio of the signal is effectively improved by comparing the detected 10Hz low-frequency signal before and after the frequency response compensation.
Example 2: high-frequency components are absorbed by soil when events such as walking, excavation and the like from the ground are transmitted to the embedded pipeline monitoring position, and in order to accurately detect the invasion behaviors, a low-frequency signal of vibration needs to be effectively detected. In this embodiment, the optical fiber differential interference system shown in fig. 1 is used to bury the sensing optical cable 5 in the soil at a depth of 0.15m, and detect the walking event on the soil surface.
FIG. 7 is a comparison graph of walking signals from the soil surface detected by the sensing optical cables buried in the soil at a depth of 0.15m after uncompensated and frequency response compensated respectively, wherein the frequency response compensation method can obviously improve the signal-to-noise ratio of the walking signals.
Claims (10)
1. An optical fiber differential interference sensing system based on frequency response compensation is characterized by comprising a wide-spectrum light source (1), a 3 x 3 optical fiber coupler (2), an optical fiber delay coil (3), a 2 x 2 optical fiber coupler (4), a sensing optical cable (5) and a Faraday rotating mirror (6) which are sequentially arranged along an optical path;
the wide-spectrum light source (1) is connected with an input port of the 3 x 3 optical fiber coupler (2) through an optical fiber, and is divided into three beams of light with the same power by the 3 x 3 optical fiber coupler (2), the first light is coupled out of the fiber core, the second light passes through the optical fiber delay coil (3) and then is connected with a first port of the 2 x 2 optical fiber coupler (4), the third light is connected with a second port of the 2 x 2 optical fiber coupler (4), the light enters the sensing optical cable from a third port of the 2 x 2 optical fiber coupler (4), is reflected back by the Faraday rotary mirror (6) and then enters the 3 x 3 optical fiber coupler (2) again to generate interference, the first photoelectric detector (7) and the second photoelectric detector (8) are respectively connected with the other two input ports of the 3 x 3 optical fiber coupler (2), and the electrical output ends of the two photoelectric detectors are connected with the amplifier and then are connected with the digital-to-analog converter to carry out phase restoration.
2. The optical fiber differential interference sensing system based on frequency response compensation of claim 1, wherein the system demodulates two interference signals detected by the photodetector by a demodulation method to obtain the amplitude A of each frequency component of the phase difference signaliWith the frequency omegaiOn the other hand, there are:
wherein tau is the time delay generated by the optical fiber delay coil.
3. A method for compensating frequency response using the fiber optic differential interference sensing system of claim 1, comprising the steps of:
1) carrying out phase restoration on the two collected interference signals, setting a disturbance signal as f (t), setting time delay generated by the optical fiber delay coil as tau, and restoring the obtained phase difference signal
Comprises the following steps:
the solution for the perturbation signal f (t) is:
2) converting the solving expression of the disturbance signal f (t) into a discrete form;
3) and after the initial condition is determined, time domain compensation is carried out in a recursion calculation mode.
4. A method as claimed in claim 3, wherein in step 3), in step 2), the solution of the disturbance signal f (t) is converted into a discrete form as shown in the following expression:
where m is the accumulated compensation interval, k represents the discrete time interval number, t1、...、tm+1、tkm+1Respectively, a series of discrete times, at being a time step.
5. A frequency response compensation method according to claim 4, wherein in step 3), said initial condition is the value [ f (t) of the first discrete time interval1)，f(tm)]Obtaining the value of [ f (t) in the first discrete time interval1)，f(tm)]And then, obtaining an approximate value of each discrete time point in the kth discrete time interval through recursion calculation.
6. A method as claimed in claim 5, wherein in step 3), the initial value [ f (t) of the first time interval1)，f(tm)]The calculation formula of (A) is as follows:
wherein, C1,C2,…,CmAre all constant, ideally for a finite length and an initial time t1When t < t1When f (t) is 0, then C1,C2,…,CmThe values are all 0.
7. A frequency response compensation method according to claim 3, wherein the input of the optical fiber differential interference sensing system is f (t), and after introducing the low frequency compensation, the output result is reduced to f (t), and then the frequency response function h (jw) of the frequency response compensation system is:
where e is the natural logarithm, j is the imaginary unit, and ω is the angular frequency.
8. A frequency response compensation method according to claim 7, characterized in that, for the disturbance signal f (t) in the whole time domain, after iterative compensation, the phenomenon of harmonic with frequency f ═ K/τ is carried out, where K is a natural number, and then the harmonic is eliminated by introducing a low-pass filter for filtering after time domain compensation.
9. A method as claimed in claim 3, wherein the cumulative compensation interval used in the method is selected according to the delay τ generated by the length L of the fiber delay coil.
10. A method as claimed in claim 9, wherein the accumulated compensation interval m is selected from the following equations:
m＝L·n/c·fs
wherein, L is the length of the optical fiber delay coil, c is the propagation speed of light in vacuum, n is the refractive index of the optical fiber, and fs is the sampling rate of the system.
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