CN102359797B - System and method for reducing noise of phase generated carrier (PGC) system in optical fiber hydrophone - Google Patents

Abstract

The present invention relates to the field of optical fiber hydrophones, in particular to a system and method for noise reduction of a PGC system in an optical fiber hydrophone, the method comprising: a modulator (0), a laser (1), and a 1×2 coupler ( 2), sensing probe (3), reference probe (4), first demodulation module (501), second demodulation module (502), decorrelation module (6); wherein, modulator (0) and laser (1) connected, the laser (1) is connected to a 1×2 coupler (2), and the 1×2 coupler (2) is connected to the sensing probe (3) and the reference probe (4) respectively, The sensing probe (3) is connected to the first demodulation module (501), the reference probe (4) is connected to 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 invention can improve the noise performance of the hydrophone.

Description

System and method for noise reduction of PGC system in fiber optic hydrophone

technical field

The invention relates to the field of optical fiber hydrophones, in particular to a system and method for noise reduction of a PGC system in an optical fiber hydrophone.

Background technique

The optical fiber hydrophone system includes a light source input, a modulation part, a sensing part and a demodulation part. PGC is a modulation method of the modulation part, and the system modulated by PGC is called a PGC system. The performance indicators of the fiber optic hydrophone array include system self-noise, dynamic range, multiplexing scale, and element sensitivity. The dynamic range of the fiber optic hydrophone array using the PGC (Phase Generated Carrier) scheme is mainly determined by the carrier frequency and the noise floor. It is an important research content of the interferometric fiber optic hydrophone array to study the noise performance and design a reasonable noise reduction scheme to make the noise index meet the requirements. For a PGC system using an unbalanced interferometer, the phase noise of the light source is the main source of its output noise. In the PGC space division multiplexing system (referring to the PGC system of space division multiplexing mode), the output results of different sensing units are dominated by homologous noise. One way to optimize the noise performance of the system is to eliminate the noise between different sensing units. Homogeneous noise between them—a process called decorrelation.

The current existing method is to use an insensitive reference probe made of a 2×2 coupler to demodulate it, and then subtract the demodulation results of the sensing probe and the reference probe to remove homologous noise. In this method, the de-correlation result is unsatisfactory because the correlation degree of the two signals is unstable.

Contents of the invention

(1) Technical problems to be solved

The technical problem to be solved by the invention is: how to improve the noise performance of the hydrophone.

(2) Technical solutions

In order to solve the above technical problems, the present invention provides a system for noise reduction of PGC system in fiber optic hydrophone, including: modulator, laser, 1×2 coupler, sensing probe, reference probe, first demodulator module, a second demodulation module, and a de-correlation module; the first demodulation module is used to demodulate the optical signal returned after the sensor signal output by the 1×2 coupler passes through the sensor probe, and the The second demodulation module is used to demodulate the optical signal returned after the reference path signal output by the 1×2 coupler passes through the reference probe; the de-correlation module is used to demodulate the first demodulation module, The signal output by the second demodulation module is subjected to a de-correlation operation to remove homologous noise to obtain an output signal;

Wherein, the modulator is connected to a laser, and the laser is connected to a 1×2 coupler, and the 1×2 coupler is respectively connected to the sensing probe and the reference probe, and the sensing probe is connected to the first The demodulation module is connected, the reference probe is connected with the second demodulation module, and both the first demodulation module and the second demodulation module are connected with the decorrelation module.

Preferably, the sensing probe includes a 2×2 coupler and a first Faraday rotating mirror, and the 2×2 coupler is connected to the first demodulation module; the reference probe includes a 3×2 coupler and a first Faraday mirror. Two Faraday mirrors, the 3×2 coupler is connected to the second demodulation module.

Preferably, the de-correlation module includes a linear transformation module and a third demodulation module connected in sequence, and the linear transformation module is used to linearly combine the two output signals of the 3x2 coupler to obtain a combined signal; The third demodulation module is used to demodulate the combined signal.

The present invention also provides a method for noise reduction of the PGC system in the optical fiber hydrophone using the system, comprising the steps of:

S1. Input the light emitted by the laser that has been modulated by the modulator to the 1×2 coupler for space division multiplexing, and one of the outputs is a sensor signal, and the other is a reference signal;

S2. After the sensing path signal and the reference path signal respectively pass through the sensing probe and the reference probe, the returned light is respectively demodulated by the first demodulation module and the second demodulation module, and then decorrelated by the decorrelation module operation to remove homologous noise to obtain the output signal.

When performing demodulation in step S2, the demodulation method used is the digital arctangent demodulation method, and the demodulation results of the sensing path signal and the reference path signal obtained after demodulation are respectively:

为传感路信号的相位信息，

为传感路信号的噪声，是相位噪声

光强噪声n_i、电路噪声

在所述系统的参数C^s下经过数字化反正切解调的结果；为参考路信号的相位信息，

为参考路噪声，是相位噪声

光强噪声n_i、电路噪声

在所述系统的参数

C^r下经过数字化反正切解调的结果；

是解调结果中加性光强噪声的输出。in,

is the phase information of the sensor signal,

is the noise of the sensor signal, which is the phase noise

light intensity noise n _i , circuit noise

parameters in the system The result of digitized arctangent demodulation under C ^s ; is the phase information of the reference signal,

is the reference road noise, is the phase noise

light intensity noise n _i , circuit noise

parameters in the system

The result of digitized arctangent demodulation under C ^r ;

is the output of additive light intensity noise in the demodulation result.

The steps for the de-correlation module to perform calculations in step S2 are as follows:

Linearly combine the two output signals of the 3x2 coupler to obtain the combined signal i, so that the phase of the combined signal includes the phase of the sensor signal Information:

Among them, i _r1 is the first path of the two output signals of the 3x2 coupler, i _r2 is the second path, and K is the splitting ratio of the coupler;

The combined signal i is then demodulated to obtain a phase value of:

in,

是3×2耦合器的两路输出信号的一个固定的相位差，由于C^r≈C^s，于是有：in

is a fixed phase difference of the two output signals of the 3×2 coupler, since C ^r ≈ ^{C s} , then:

The signal output after the decorrelation operation in step S2 is as follows:

(3) Beneficial effects

The present invention introduces a 3×2 coupler into the PGC space division multiplexing system, utilizes the characteristics of a fixed phase difference between the two reference signals, realizes the phase tracking of the sensor signal, and realizes the The de-correlation algorithm achieves the purpose of eliminating the same-source noise, which greatly improves the noise performance of the hydrophone. The "phase control method" decorrelation system of the present invention can completely eliminate the phase noise introduced by the light source, at the cost of increasing the circuit noise and light intensity noise by up to 8.5dB and 14dB respectively. Since the circuit noise and optical path noise in the system are usually relatively low, this kind of system can still effectively reduce the overall noise of the system.

Description of drawings

Fig. 1 is a schematic diagram of the system structure of the present invention;

Fig. 2 is method flowchart of the present invention;

Fig. 3 is the program flowchart when the method of Fig. 2 is realized with FPGA;

Fig. 4 is the structural diagram of the experimental device for verifying the feasibility of Fig. 1 system;

Figure 5 is the graph of the experimental results, that is, the graph of the decorrelation results of the 9812 capture card.

In the figure, 0: modulator; 1: laser; 2: 1×2 coupler; 3: sensing probe; 301: 2×2 coupler; 302: first Faraday mirror; 4. Reference probe; 401: 3 ×2 coupler; 402: second Faraday mirror; 501: first demodulation module; 502: second demodulation module; 6: decorrelation module; 601: linear transformation module; 602: third demodulation module.

Detailed ways

The specific implementation manners of the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

The invention proposes a method for eliminating homologous noise of multiple sensing probes in a PGC system, and designs a noise reduction system for the PGC space division multiplexing system. The larger the correlation, the better the de-correlation effect. In order to obtain a stable de-correlation effect, it is necessary to control the correlation degree of the multiplexing system.

As shown in Figure 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 used to demodulate the light returned after the sensor signal output by the 1×2 coupler 2 passes through the sensor probe 3, and the second demodulation module 502 is used to demodulate The reference path signal output by the 1×2 coupler 2 is demodulated by the returned optical signal after passing through the reference probe 3; the de-correlation module 6 is used for the first demodulation module 501, the second demodulation module The signal output by 502 is de-correlated to remove homologous noise, and the output signal is obtained

Wherein, the modulator 0 is connected to the laser 1, and the laser 1 is connected to the 1×2 coupler 2, and the 1×2 coupler 2 is respectively connected to the sensing probe 3 and the reference probe 4, and the sensing probe 3 is connected to the first demodulation module 501, the reference probe 4 is connected to the second demodulation module 502, and both the first demodulation module 501 and the second demodulation module 502 are connected to the decorrelation module 6 .

The sensing probe 3 includes a 2×2 coupler 301 and a first Faraday mirror 302, the 2×2 coupler 301 is connected to the first demodulation module 501; the reference probe 4 includes a 3×2 coupler 401 and the second Faraday mirror 402 , the 3×2 coupler 401 is connected to the second demodulation module 502 .

The 2x2 coupler 301 is the main part of the sensing probe 3, the 3x2 coupler 401 is the main part of the reference probe 4, and the first Faraday rotation mirror 302 and the second Faraday rotation mirror 402 are respectively connected to the 2×2 coupler 301 and the 3× 2 The end face of the arm of the coupler 401 is used to eliminate the influence of light polarization.

The de-correlation module 6 includes a linear transformation module 601 and a third demodulation module 602 connected in sequence, the linear transformation module 601 is used to linearly combine 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.

由于在复用系统中，可以通过调整水听器干涉仪的臂长差来控制调制深度C，而直流相位由于受到温度压力等环境因素的作用，很难直接控制。为了提高相关度，必须控制两路传感器的直流相位，为此提出如下“相位控制法”，原理如图1所示。According to theoretical analysis, the factors that affect the correlation between the two signals are the modulation depth C and the DC phase of the two sensors

In the multiplexing system, the modulation depth C can be controlled by adjusting the arm length difference of the hydrophone interferometer, while the DC phase is difficult to directly control due to environmental factors such as temperature and pressure. In order to improve the correlation, it is necessary to control the DC phase of the two sensors. For this reason, the following "phase control method" is proposed, and the principle is shown in Figure 1.

The light emitted by the light source is frequency-modulated and then input to a 1×2 coupler (realizing space division multiplexing), one of which is the sensing path and the other is the reference path. The signals returned by the two probes are demodulated, and de-correlation operation is performed to remove homologous noise to obtain the final output signal. The key to the "phase control method" is the use of the 3×2 coupler, and the two output signals of the 3×2 coupler will have a fixed phase difference This principle can be used to track the phase of the sensor path. As shown in Figure 2, the concrete steps of the method of the present invention are as follows:

S1. Input the light emitted by the laser 1 through the frequency modulation of the modulator 0 into a 1×2 coupler 2 for space division multiplexing, and one output is a sensor signal, and the other is a reference signal;

S2. The optical signals returned after the sensing path signal and the reference path signal respectively pass through the sensing probe 3 and the reference probe 4 are respectively demodulated by the first demodulation module 501 and the second demodulation module 502, and then demodulated by the The correlation module 6 performs a de-correlation operation to remove homologous noise to obtain a final output signal.

算出来)，得到传感路和参考路的解调结果为：When demodulating the signals of the sensor path and the reference path, the demodulation method used is a digital arctangent demodulation method, and only one of the signals of the two arms of the reference path needs to be demodulated (the other can be determined by

Calculated), the demodulation results of the sensing path and the reference path are:

为传感路信号的相位信息，

为传感路的噪声，是相位噪声

光强噪声n_i、电路噪声

在系统参数

C^s下经过DAT(Digital Arc-Tangent，数字化反正切)解调的结果；

为参考路信号的相位信息，为参考路噪声，是相位噪声

光强噪声n_i、电路噪声

在系统参数

C^r下经过DAT解调的结果；

是解调结果中加性光强噪声的输出。in,

is the phase information of the sensor signal,

is the noise of the sensing path, and is the phase noise

light intensity noise n _i , circuit noise

in system parameter

The result of DAT (Digital Arc-Tangent, digital arc tangent) demodulation under C ^s ;

is the phase information of the reference signal, is the reference road noise, is the phase noise

light intensity noise n _i , circuit noise

in system parameter

The result of DAT demodulation under C ^r ;

is the output of additive light intensity noise in the demodulation result.

The steps for the de-correlation module to perform calculations in step S2 are as follows:

的信息：The linear transformation module 601 linearly combines the two output signals of the 3x2 coupler to obtain the combined signal i, so that the phase of the combined signal includes the phase of the sensor signal

Information:

Among them, i _r1 is the first path of the two output signals of the 3x2 coupler, i _r2 is the second path, and K is the splitting ratio of the coupler;

Then, the third demodulation module 602 is used to demodulate the combined signal, and the demodulated phase value is

in,

In a practical system, C ^r ≈ C ^s , so we have

Then get the output signal:

_nc is formed by combining the circuit noise of the two output signals of the reference probe, and its power spectral density is:

In the actual system, the circuit noise spectra of the sensing circuit and the reference circuit are consistent.

From the expression of γ, we can see that,

Through theoretical analysis, the "phase control method" decorrelation system can completely eliminate the phase noise introduced by the light source, which is the advantage of this patent. The price is that circuit noise and light intensity noise are increased by up to 8.5dB and 14dB respectively. Since the circuit noise and optical path noise in the system are usually relatively low, this method can still effectively reduce the overall noise of the system.

The FPGA program flow chart of the above algorithm is shown in Figure 3:

Compared with the existing system, the new program structure adds a DAT module, a linear transformation module, a delay module and a subtraction module to each sensing path. Among them, the largest resource consumption is the low-pass filter after multiplying the carrier in the DAT module. When the FPGA is implemented, the 8-channel signal passes through a low-pass filter module in series. Therefore, in the space division eight-channel system, only one common low-pass filter needs to be added as a whole. Pass filter module (about 100 orders) - this resource consumption is negligible compared to the anti-aliasing filter (200-300 orders) and high-pass filter (higher than 500 orders) before data output. The function of the delay module is to align data, and the total delay is the sum of the delays of the linear transformation module and the DAT module.

In order to verify the feasibility of the "phase control method" decorrelation system, a practical and feasible scheme is shown in Figure 3. The experimental instruments used are: RIO narrow linewidth laser, 10.6m arm length difference hydrophone (2×2 coupler, that is, the sensor probe in Figure 3), 10.6m arm length difference hydrophone (3×2 coupling device, that is, the reference probe in Figure 3), 9812 acquisition card (sampling rate 200kHz), PXI446124-bit DA (carrier frequency 20kHz). The experimental results are shown in Figure 4, and the following conclusions are drawn:

1. The result of direct decorrelation can suppress the low-frequency 1/f noise, but it cannot reduce the noise spectrum above 1kHz;

2. The de-correlation result of "phase control method" completely eliminates the low-frequency 1/f noise and reduces the high-frequency noise by 15dB;

3. The system circuit noise source is AD noise and photoelectric conversion circuit noise, in which the photoelectric conversion circuit noise is higher than AD noise, about -102dB/Hz, and the decorrelation result is -95dB/Hz, which is 7dB higher than the circuit noise, which is consistent with theory and The simulation results are in good agreement.

The above is only a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the technical principle of the present invention, some improvements and modifications can also be made, these improvements and modifications It 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 i_r2Carrying 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 i_r1Is the first of the two output signals of the 3x2 coupler, i_r2Is 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 noise

Light intensity noise n_iCircuit noise

Parameters in the system

The result of the digital arc tangent demodulation is obtained;

for the phase information of the reference path signal,

as reference path noise, phase noise

Light intensity noise n_iCircuit noise

Parameters in the system

C^rThe 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 signal

The information of (2):

wherein i_r1Is the first of the two output signals of the 3x2 coupler, i_r2Is the second path, K is the splitting ratio of the coupler;

then, demodulating the combined signal i to obtain a phase value:

wherein

Wherein

Is a fixed phase difference of the two output signals of the 3x2 coupler due to C^r≈C^sThus, there are:

3. the method of claim 2, wherein the output signal after the decorrelation operation in step S2 is as follows:

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