CN102359797B - System and method for reducing noise of phase generated carrier (PGC) system in optical fiber hydrophone - Google Patents
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Abstract
The invention relates to the field of optical fiber hydrophones, in particular to a system and a method for reducing noise of a phase generated carrier (PGC) system in an optical fiber hydrophone. The system comprises a modulator (0), a laser (1), a 1*2 coupler (2), a sensing probe (3), a reference probe (4), a first demodulation module (501), a second demodulation module (502) and a decorrelation module (6), wherein the modulator (0) is connected with the laser (1); the laser (1) is connected with the 1*2 coupler (2); the 1*2 coupler (2) is connected with the sensing probe (3) and the reference probe (4); the sensing probe (3) is connected with the first demodulation module (501); the reference probe (4) is connected with the second demodulation module (502); and the first demodulation module (501) and the second demodulation module (502) are connected with the decorrelation module (6). The system can improve the noise performance of the hydrophone.
Description
Technical Field
The invention relates to the field of optical fiber hydrophones, in particular to a system and a method for reducing noise of a PGC system in an optical fiber hydrophone.
Background
The optical fiber hydrophone system comprises a light source input, a modulation part, a sensing part and a demodulation part, wherein PGC is a modulation mode of the modulation part, and a system modulated by the PGC is called a PGC system. The performance indexes of the optical fiber hydrophone array comprise system self-noise, dynamic range, multiplexing scale, element sensitivity and the like. The dynamic range of a fiber optic hydrophone array employing a PGC (Phase Generated Carrier) scheme is mainly determined by the Carrier frequency and noise floor. The research on the noise performance and the design of a reasonable noise reduction scheme ensure that the noise index meets the requirements are important research contents of the interference type optical fiber hydrophone array. For PGC systems employing unbalanced interferometers, the phase noise of the light source is the dominant source of its output noise. In a PGC space division multiplexing system (referred to as a PGC system of space division multiplexing), output results of different sensing units are dominated by homologous noise, and one idea for optimizing the noise performance of the system is to eliminate the homologous noise between different sensing units — this process is called decorrelation.
The existing method is to demodulate an insensitive reference probe made of a 2x2 coupler, and then subtract the demodulation results of the sensing probe and the reference probe to remove the homologous noise. The method has the advantage that the decorrelation result is not ideal due to the unstable correlation degree of the two paths of signals.
Disclosure of Invention
Technical problem to be solved
The technical problem to be solved by the invention is as follows: how to improve the noise performance of the hydrophone.
(II) technical scheme
In order to solve the above technical problem, the present invention provides a system for reducing noise of a PGC system in a fiber optic hydrophone, comprising: the device comprises a modulator, a laser, a 1 x2 coupler, a sensing probe, a reference probe, a first demodulation module, a second demodulation module and a decorrelation module; the first demodulation module is used for demodulating an optical signal returned after a sensing path signal output by the 1 × 2 coupler passes through a sensing probe, and the second demodulation module is used for demodulating an optical signal returned after a reference path signal output by the 1 × 2 coupler passes through a reference probe; the decorrelation module is used for performing decorrelation operation on the signals output by the first demodulation module and the second demodulation module to remove homologous noise to obtain an output signal;
the modulator is connected with a laser, the laser is connected with a 1 × 2 coupler, the 1 × 2 coupler is respectively connected with the sensing probe and the reference probe, the sensing probe is connected with the first demodulation module, the reference probe is connected with the second demodulation module, and the first demodulation module and the second demodulation module are both connected with the decorrelation module.
Preferably, the sensing probe comprises a 2 × 2 coupler and a first faraday rotator mirror, and the 2 × 2 coupler is connected with the first demodulation module; the reference probe comprises a 3x2 coupler and a second Faraday rotator mirror, and the 3x2 coupler is connected with the second demodulation module.
Preferably, the decorrelation module includes a linear transformation module and a third demodulation module, which are connected in sequence, and the linear transformation module is configured to perform linear combination on the two output signals of the 3x2 coupler to obtain a combined signal; the third demodulation module is configured to demodulate the combined signal.
The invention also provides a method for carrying out PGC system noise reduction in the optical fiber hydrophone by using the system, which comprises the following steps:
s1, inputting the light emitted by the laser and modulated by the modulator frequency into the 1 x2 coupler for space division multiplexing, wherein one output path is a sensing path signal, and the other output path is a reference path signal;
and S2, respectively demodulating the returned light after the sensing path signal and the reference path signal pass through the sensing probe and the reference probe by the first demodulation module and the second demodulation module, and then performing decorrelation operation by the decorrelation module to remove homologous noise to obtain an output signal.
When performing demodulation in step S2, the demodulation method used is a digital arc tangent demodulation method, and the demodulation results of the sensor path signal and the reference path signal obtained after demodulation are respectively:
wherein,for the phase information of the signal of the sensing path,the noise being the signal of the sensing path being phase noiseLight intensity noise niCircuit noiseIn the systemParameter (d) ofCsThe result of the digital arc tangent demodulation is obtained;for the phase information of the reference path signal,as reference path noise, phase noiseLight intensity noise niCircuit noiseParameters in the systemCrThe result of the digital arc tangent demodulation is obtained;is the output of additive light intensity noise in the demodulation result.
The operation of the decorrelation module in step S2 is as follows:
the two paths of output signals of the 3x2 coupler are linearly combined to obtain a combined signal i, so that the phase of the combined signal contains the phase of the sensing path signalThe information of (2):
wherein ir1Is the first of the two output signals of the 3x2 coupler, ir2Is the second path, K is the splitting ratio of the coupler;
then, demodulating the combined signal i to obtain a phase value:
wherein,
whereinIs a fixed one of the two output signals of the 3x2 couplerPhase difference due to Cr≈CsThus, there are:
the output signal after the decorrelation operation in step S2 is as follows:
(III) advantageous effects
The invention realizes the tracking of the phase of the sensing signal by introducing a 3 multiplied by 2 coupler into a PGC space division multiplexing system and utilizing the characteristic that a fixed phase difference exists between two paths of reference signals, thereby realizing a decorrelation algorithm, achieving the purpose of eliminating homologous noise and greatly improving the noise performance of the hydrophone. The decorrelation system adopting the phase control method can completely eliminate phase noise introduced by a light source, and the cost is that the circuit noise and the light intensity noise are respectively improved by 8.5dB and 14dB at most. Due to the fact that circuit noise and optical path noise are low in the conventional system, the system can still effectively reduce the overall noise of the system.
Drawings
FIG. 1 is a schematic diagram of the system of the present invention;
FIG. 2 is a flow chart of a method of the present invention;
FIG. 3 is a flowchart of a process of FIG. 2 when implemented with an FPGA;
FIG. 4 is a block diagram of an experimental setup for verifying the feasibility of the system of FIG. 1;
FIG. 5 is a diagram of experimental results, i.e., decorrelation results of a 9812 acquisition card.
In the figure, 0: a modulator; 1: a laser; 2: a 1 × 2 coupler; 3: a sensing probe; 301: a 2 × 2 coupler; 302: a first Faraday rotator mirror; 4. a reference probe; 401: a 3 × 2 coupler; 402: a second Faraday rotator mirror; 501: a first demodulation module; 502: a second demodulation module; 6: a decorrelation module; 601: a linear transformation module; 602: and a third demodulation module.
Detailed Description
The following detailed description of embodiments of the present invention is provided in connection with the accompanying drawings and examples. The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.
The invention provides a method for eliminating homologous noise of a multi-channel sensing probe of a PGC system, and designs a noise reduction system for the PGC space division multiplexing system. The greater the degree of correlation, the better the decorrelation effect. In order to obtain a stable decorrelation effect, it is necessary to control the correlation degree of the multiplexing system.
As shown in fig. 1, the system includes a modulator 0, a laser 1, a 1 × 2 coupler 2, a sensing probe 3, a reference probe 4, a first demodulation module 501, a second demodulation module 502, and a decorrelation module 6; the first demodulation module 501 is configured to demodulate light returned after the sensing path signal output by the 1 × 2 coupler 2 passes through the sensing probe 3, and the second demodulation module 502 is configured to demodulate an optical signal returned after the reference path signal output by the 1 × 2 coupler 2 passes through the reference probe 3; the decorrelation module 6 is configured to perform decorrelation operation on the signals output by the first demodulation module 501 and the second demodulation module 502 to remove homologous noise, so as to obtain an output signal
The modulator 0 is connected with the laser 1, the laser 1 is connected with the 1 × 2 coupler 2, the 1 × 2 coupler 2 is respectively connected with the sensing probe 3 and the reference probe 4, the sensing probe 3 is connected with the first demodulation module 501, the reference probe 4 is connected with the second demodulation module 502, and both the first demodulation module 501 and the second demodulation module 502 are connected with the decorrelation module 6.
The sensing probe 3 comprises a 2 × 2 coupler 301 and a first faraday rotator 302, wherein the 2 × 2 coupler 301 is connected with the first demodulation module 501; the reference probe 4 comprises a 3 × 2 coupler 401 and a second faraday rotator 402, the 3 × 2 coupler 401 being connected to the second demodulation module 502.
The 2x2 coupler 301 is the main body of the sensing probe 3, the 3x2 coupler 401 is the main body of the reference probe 4, and the first faraday rotator 302 and the second faraday rotator 402 are respectively connected to the end faces of the arms of the 2x2 coupler 301 and the 3x2 coupler 401 to cancel the influence of the polarization of light.
The decorrelation module 6 includes a linear transformation module 601 and a third demodulation module 602, which are connected in sequence, where the linear transformation module 601 is configured to perform linear combination on the two output signals of the 3x2 coupler 401 to obtain a combined signal; the third demodulation module 602 is configured to demodulate the combined signal.
According to theoretical analysis, the factors influencing the correlation degree of the two paths of signals are the modulation depth C and the direct current phase of the two paths of sensorsDue to the fact thatIn a multiplexing system, the modulation depth C can be controlled by adjusting the arm length difference of a hydrophone interferometer, and the direct current phase is difficult to directly control due to the action of environmental factors such as temperature and pressure. In order to improve the correlation, the dc phase of the two sensors must be controlled, and therefore, the following "phase control method" is proposed, the principle of which is shown in fig. 1.
Light emitted by the light source is subjected to frequency modulation and input into a 1 multiplied by 2 coupler (space division multiplexing is realized), wherein one path is a sensing path, and the other path is a reference path. And demodulating the signals returned by the two probes, performing decorrelation operation, and removing homologous noise to obtain a final output signal. The key of the 'phase control method' is the use of a 3 × 2 coupler, and two output signals of the 3 × 2 coupler have a fixed phase differenceThe tracking of the phase of the sensor path can be realized by utilizing the principle. As shown in fig. 2, the method of the present invention comprises the following steps:
s1, inputting the light which is emitted by the laser 1 and frequency-modulated by the modulator 0 into the 1 x2 coupler 2 for space division multiplexing, wherein one output path is a sensing path signal, and the other output path is a reference path signal;
s2, the optical signals returned after the sensing path signal and the reference path signal pass through the sensing probe 3 and the reference probe 4 respectively are demodulated by the first demodulation module 501 and the second demodulation module 502 respectively, and then decorrelation operation is performed by the decorrelation module 6 to remove homologous noise, so as to obtain a final output signal.
When the signals of the sensing path and the reference path are demodulated, the demodulation method is a digital arc tangent demodulation method, and only one of the signals of the two arms of the reference path (the other one can be selected from the signals of the two arms of the reference path) needs to be demodulatedCalculated), the demodulation results of the sensing path and the reference path are obtained as follows:
wherein,for the phase information of the signal of the sensing path,the noise being the sensing path being phase noiseLight intensity noise niCircuit noiseIn the system parameterCsThe result of DAT (Digital Arc-finger) demodulation;for the phase information of the reference path signal,as reference path noise, phase noiseLight intensity noise niCircuit noiseIn the system parameterCrThe result of next DAT demodulation;is the output of additive light intensity noise in the demodulation result.
The operation of the decorrelation module in step S2 is as follows:
the linear transformation module 601 linearly combines the two output signals of the 3x2 coupler to obtain a combined signal i, so that the phase of the combined signal includes the phase of the sensing path signalThe information of (2):
wherein ir1Is the first of the two output signals of the 3x2 coupler, ir2Is the second path, K is the splitting ratio of the coupler;
then, the third demodulation module 602 is used to demodulate the combined signal, and the phase value obtained by demodulation is
Wherein,
in a real system, Cr≈CsIs then provided with
Then an output signal is obtained:
ncthe power spectrum density of the reference probe is formed by combining circuit noises of two output signals of the reference probe, and is as follows:
the circuit noise spectrums of the sensing path and the reference path in the actual system are consistent.
As can be seen from the expression of y,
by theoretical analysis, the decorrelation system of the "phase control method" can completely eliminate the phase noise introduced by the light source, which is the advantage of the patent. The cost is that the circuit noise and the light intensity noise are respectively improved by 8.5dB and 14dB at most. Due to the fact that circuit noise and optical path noise in the system are low in general, the method can still effectively reduce the overall noise of the system.
The flow chart of the FPGA program of the above algorithm is shown in fig. 3:
compared with the existing system, the new program structure adds a DAT module, a linear transformation module, a time delay module and a subtraction module in each sensing path. The maximum resource consumption is a low-pass filter after the carrier is multiplied in the DAT module, when the FPGA is realized, 8 paths of signals pass through one low-pass filter module in series, so that in an eight-path system of space division, only one common low-pass filter module (about 100 th order) needs to be added integrally, and the resource consumption can be ignored relative to anti-aliasing filtering (200-300 th order) and high-pass filtering (higher than 500 th order) before data output. The delay module is used for aligning data, and the total delay time is the sum of the delay of the linear transformation module and the delay of the DAT module.
To verify the feasibility of a "phase control method" decorrelation system, a practical solution is shown in fig. 3. The experimental apparatus used had: RIO narrow linewidth laser, 10.6m arm length difference hydrophone (2 x2 coupler, i.e. sensing probe in figure 3), 10.6m arm length difference hydrophone (3 x2 coupler, i.e. reference probe in figure 3), 9812 acquisition card (sampling rate 200kHz), PXI446124 bit DA (carrier frequency 20 kHz). The results of the experiment are shown in fig. 4, leading to the following conclusions:
1. the direct decorrelation result can inhibit low-frequency 1/f noise, but cannot reduce the noise spectrum above 1 kHz;
the decorrelation result of the phase control method completely eliminates low-frequency 1/f noise and reduces high-frequency noise by 15 dB;
3. the noise sources of the system circuit are AD noise and photoelectric conversion circuit noise, wherein the noise of the photoelectric conversion circuit is higher than the AD noise and is about-102 dB/Hz, the decorrelation result is-95 dB/Hz and is 7dB higher than the circuit noise, and the theory and simulation results are matched.
The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.
Claims (3)
1. A system for PGC system noise reduction in a fiber optic hydrophone, comprising: the device comprises a modulator (0), a laser (1), a 1 x2 coupler (2), a sensing probe (3), a reference probe (4), a first demodulation module (501), a second demodulation module (502) and a decorrelation module (6); the first demodulation module (501) is used for demodulating an optical signal returned after a sensing path signal output by the 1 × 2 coupler (2) passes through the sensing probe (3), and the second demodulation module (502) is used for demodulating an optical signal returned after a reference path signal output by the 1 × 2 coupler (2) passes through the reference probe (3); the decorrelation module (6) is configured to perform decorrelation operation on the signals output by the first demodulation module (501) and the second demodulation module (502) to remove homologous noise, so as to obtain an output signal;
the modulator (0) is connected with a laser (1), the laser (1) is connected with a 1 × 2 coupler (2), the 1 × 2 coupler (2) is respectively connected with the sensing probe (3) and a reference probe (4), the sensing probe (3) is connected with the first demodulation module (501), the reference probe (4) is connected with the second demodulation module (502), and the first demodulation module (501) and the second demodulation module (502) are both connected with the decorrelation module (6);
the sensing probe (3) comprises a 2x2 coupler (301) and a first Faraday rotator mirror (302), wherein the 2x2 coupler (301) is connected with the first demodulation module (501); the reference probe (4) comprises a 3x2 coupler (401) and a second faraday rotator mirror (402), the 3x2 coupler (401) is connected with the second demodulation module (502);
the decorrelation module (6) comprises a linear transformation module (601) and a third demodulation module (602) which are connected in sequence, wherein the linear transformation module (601) is used for outputting two paths of output signals i of the 3x2 coupler (401)r1And ir2Carrying out linear combination:a combined signal is obtained in which, among other things,as a result of the superposition of noise on the sensor path signal,outputting the result of the noise superposition of the first path signal for the reference path,outputting the result of the second path signal superimposed noise for the reference path ir1Is the first of the two output signals of the 3x2 coupler, ir2Is the second path, K is the splitting ratio of the coupler; the third demodulation module (602) is configured to perform digital arc-tangent demodulation on the combined signal.
2. A method for PGC system noise reduction in a fiber optic hydrophone using the system of claim 1, comprising the steps of:
s1, inputting the light which is emitted by the laser (1) and is frequency-modulated by the modulator (0) into the 1 x2 coupler (2) for space division multiplexing, wherein one output path is a sensing path signal, and the other output path is a reference path signal;
s2, respectively demodulating returned light after the sensing path signal and the reference path signal pass through the sensing probe (3) and the reference probe (4) through a first demodulation module (501) and a second demodulation module (502), and then performing decorrelation operation through a decorrelation module (6) to remove homologous noise to obtain an output signal;
when performing demodulation in step S2, the demodulation method used is a digital arc tangent demodulation method, and the demodulation results of the sensor path signal and the reference path signal obtained after demodulation are respectively:
wherein,for the phase information of the signal of the sensing path,the noise being the signal of the sensing path being phase noiseLight intensity noise niCircuit noiseParameters in the systemThe result of the digital arc tangent demodulation is obtained;for the phase information of the reference path signal,as reference path noise, phase noiseLight intensity noise niCircuit noiseParameters in the systemCrThe result of the digital arc tangent demodulation is obtained;is the output of additive light intensity noise in the demodulation result;
the operation of the decorrelation module in step S2 is as follows:
the two paths of output signals of the 3x2 coupler are linearly combined to obtain a combined signal i, so that the phase of the combined signal contains the phase of the sensing path signalThe information of (2):
wherein ir1Is the first of the two output signals of the 3x2 coupler, ir2Is the second path, K is the splitting ratio of the coupler;
then, demodulating the combined signal i to obtain a phase value:
wherein
WhereinIs a fixed phase difference of the two output signals of the 3x2 coupler due to Cr≈CsThus, there are:
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