Method for reducing interference from scattered light/reflected light of interference path by generating carrier through phase

Abstract

A method for reducing interference from scattered light/reflected light of an interference path by generating carrier through phase. Phase modulation is applied on the terminal of a fiber path, and a target signal is separated from an interference signal by selecting a specific working point, to obtain a purer target signal, thereby lengthening the measurement distance. The signal demodulation manner used in this method is different from the traditional manner of modulation performed by generating a carrier through the phase, and does not need to use the modulation frequency as the reference signal during demodulation, so this manner is easily implemented. The method is applicable to long-distance pipeline monitoring and wide-range fiber perimeter security, and especially to an application environment in which the modulation end is far away from the signal demodulation end. The method can also be applied in an application in which measurement is implemented by modulating an optical transmission phase in a feedback device.

Claims

1. A method for reducing interference from scattered light of an interference path by generating a carrier through phase, comprising: (1) concatenating a phase modulator in the tail of a single core feedback sensing fiber, phase modulator introduce the interferometer phase difference .sub.c(t), after reaching feedback means through the phase modulator, the light reflect to an optical cable and occurs interference signal, the signal is expressed as:
P=p.sub.1 cos [.sub.0+.sub.1(t)+.sub.1(t)] wherein, p.sub.1 is a constant coefficient related to system parameters, .sub.0 is an initial phase of an interference structure and a constant, .sub.1(t) is an interference phase difference caused by disturbance; as to the back scattered light caused by a previous path in the phase modulator in an optical fable, changes in the phase are not affected by the signal applied to the phase modulator, part of the optical interference signal is expressed as: $P_B=\underset{i}{.Math.}{p}_{\mathrm{Bi}}\cos \left[{}_{B0i}+{}_{B1i}\left(t\right)\right]$ wherein, p.sub.Bi is an interference coefficient caused by an i-th scattering point of an optical fiber, .sub.B 0 i is an initial phase corresponding to the i-th scattering point, .sub.B1i(t) is an interference phase difference corresponding to the i-th scattering point and caused by disturbance, $\underset{i}{.Math.}.Math.$ represented the sum of all the scattered points along a previous induction fiber of phase modulator; thus, the total output signal change portion indicates as follows:
P.sub.alt=P+P.sub.B J.sub.n order Bessel function expansion of the signal is expression is expressed as: $\begin{array}{c}\begin{array}{c}P=p_1\cos \left[{}_0+{}_1\left(t\right)+{}_m\cos \left(2f_m\right)\right]\\ =p_1\cos \left({}_0+{}_1\left(t\right)\right)\left[J_0\left({}_m\right)+2J_2\left({}_m\right)\cos \left(4f_{m}t\right)+\mathrm{.Math.}\right]+\\ p_1\sin \left({}_0+{}_1\left(t\right)\right)\left[2J_1\left({}_m\right)\cos \left(2f_{m}t\right)+2J_3\left({}_m\right)\cos \left(6f_{m}t\right)+\mathrm{.Math.}\right]\end{array}\mathrm{and}\\ \begin{array}{c}{P}_{\mathrm{alt}}=P+P_B\\ =P_B+p_1\cos \left({}_0+{}_1\left(t\right)\right)\left[J_0\left({}_m\right)+2J_2\left({}_m\right)\cos \left(4f_{m}t\right)+\mathrm{.Math.}\right]+\\ p_1\sin \left({}_0+{}_1\left(t\right)\right)\left[2J_1\left({}_m\right)\cos \left(2f_{m}t\right)+2J_3\left({}_m\right)\cos \left(6f_{m}t\right)+\mathrm{.Math.}\right]\\ =\left[P_B+p_1\cos \left({}_0+{}_1\left(t\right)\right).Math.J_0\left({}_m\right)\right]+\\ p_1\cos \left({}_0+{}_1\left(t\right)\right)[2J_2\left({}_m\right)\cos \left(4f_{m}t\right)+\mathrm{.Math.}]\\ p_1\sin \left({}_0+{}_1\left(t\right)\right)\left[2J_1\left({}_m\right)\cos \left(2f_{m}t\right)+2J_3\left({}_m\right)\cos \left(6f_{m}t\right)+\mathrm{.Math.}\right]\end{array}\end{array}$ (2) Selecting signal frequency f.sub.m loaded in phase modulator, f.sub.m satisfy the following conditions: f.sub.m>f.sub.sBmax+f.sub.s1max, and f.sub.m is located out of the frequency component of .sub.1(t); f.sub.s1max is a maximum frequency of sin(.sub.01+.sub.1(t)) or cos(.sub.01+.sub.1(t)), f.sub.sBmax is a maximum frequency of P.sub.B; (3) applying a sinusoidal signal at the phase modulator, the carrier generated by the modulated signal is expressed as:
.sub.c(t)=.sub.m cos(2f.sub.mt), .sub.m is the amplitude of .sub.c(t); (4) adjusting the amplitude of the sinusoidal signal, so that:
J.sub.0(.sub.m)=0, an frequency component of the effective interference signal formed by feedback means distributes at the sideband of the fundamental frequency and multiple frequency carrier frequency f.sub.m, frequency components are not in the vicinity of zero frequency, effective interference signal P is expressed as: $\begin{array}{c}P=0+p_1\cos \left({}_0+{}_1\left(t\right)\right)\left[2J_2\left({}_m\right)\cos \left(4f_{m}t\right)+\mathrm{.Math.}\right]+\\ p_1\sin \left({}_0+{}_1\left(t\right)\right)\left[2J_1\left({}_m\right)\cos \left(2f_{m}t\right)+2J_3\left({}_m\right)\cos \left(6f_{m}t\right)+\mathrm{.Math.}\right]\\ =p_1\cos \left[{}_0+{}_1\left(t\right)+{}_m\cos \left(2f_{m}t\right)\right]\end{array}$ At this time, only interfering signal P.sub.B formed by backscattered light's interference is in the vicinity of zero frequency; (5) high-pass filtering P.sub.alt to filter out the interference signal P.sub.B and remain effective signal P, separating interference signal with effective signal to get effective signal.

2. The method according to claim 1, wherein, using effective signal P to further reconstruct signal (t):
(t)=.sub.1(t)+.sub.m cos(2f.sub.mt) filtering (t) to filter out .sub.m cos(2f.sub.mt) to obtain an interference phase difference signal .sub.1(t) caused by the disturbance.

3. The method according to claim 1, wherein the interference structure provided with two interference output ports, the two-way interference signals are expressed as: $P_{3a2}=p_1\cos \left[{}_{11}+{}_1\left(t\right)+{}_c\left(t\right)\right]+\underset{i}{.Math.}{p}_{\mathrm{Bi}}\cos \left[{}_{3a2-B0i}+{}_{B1i}\left(t\right)\right]$ $P_{3a3}=p_1\cos \left[{}_{12}+{}_1\left(t\right)+{}_c\left(t\right)\right]+\underset{i}{.Math.}{p}_{\mathrm{Bi}}\cos \left[{}_{3a3-B0i}+{}_{B1i}\left(t\right)\right]$ and .sub.11.sub.12n, n; is an integer obtaining two signals having a fixed phase difference:
P.sub.3a2=p.sub.1 cos [.sub.11+.sub.1(t)+.sub.m cos(2f.sub.mt)]
P.sub.3a3=p.sub.1 cos [.sub.12+.sub.1(t)+.sub.m cos(2f.sub.mt)] combining the two signals to recover the signal (t)=.sub.1 (t)+.sub.m cos(2f.sub.mt).

4. The method according to claim 1, wherein, the signal frequency on the loading phase modulator f.sub.m>f.sub.smax, low-pass filtering the signal (t)=.sub.1(t)+.sub.m cos(2f.sub.mt) to filter out frequency components of f.sub.m to obtain an interference phase difference signal caused by the disturbance.

5. A method for reducing interference from scattered light of an interference path by generating carrier through phase, comprising: concatenating phase modulator in the tail of a single core feedback sensing fiber; selecting sinusoidal modulating signal frequency loaded in phase modulator, so that a change portion of a total output signal includes an interference signal and an effective interference signal, wherein a spectrum of the interference signal and a spectrum of the effective interference signal do not overlap, and a frequency of the sinusoidal modulation signal is located outside a frequency component of interference phase difference caused by a disturbance; applying the sinusoidal modulation signal to the phase modulator to generate a carrier; adjusting an amplitude of the carrier so that a value of a J.sub.0 order Bessel function at the amplitude is equal to zero; and high-pass filtering the change portion of the total output signal, the high-pass filtering filters out the interference signal and remain the effective interference signal.

6. A method according to claim 5, further comprising: reconstructing the remained effective interference signal to obtain a reconstruction signal, and removing the frequency component of the sinusoidal modulation signal from the reduction signal to obtain an interference phase difference signal caused by the disturbance.

7. A method according to claim 6, further comprising a multi-path interference signal output from a plurality of output ports are joined to reconstruct the phase.

8. A method according to claim 6, further comprising Low-pass filtering the reconstruction signal to filter out the frequency component of the sinusoidal modulation signal.

9. A method according to claim 7, wherein, two-path interference signals output from two output ports are joined to reconstruct the phase.

10. A method for reducing interference from scattered light of interference path by generating carrier through phase, comprising: 1) concatenating a phase modulator (9) in the tail of a single core feedback sensing fiber; 2) Selecting signal frequency f.sub.m loaded in phase modulator (9), f.sub.m satisfy the following conditions: f.sub.m>f.sub.sBmax+f.sub.s1max, and f.sub.m is located out of the frequency component of .sub.1(t); 3) applying a sinusoidal signal at the phase modulator, the carrier generated by the modulated signal is expressed as:
.sub.c(t)=.sub.m cos(2f.sub.mt); 4) adjusting the amplitude of the sinusoidal signal, so that:
J.sub.0(.sub.m)=0, an frequency component of the effective interference signal formed by feedback device (2) distributes at the sideband of the fundamental frequency and multiple frequency carrier frequency f.sub.m, frequency components are not in the vicinity of zero frequency, effective interference signal P is expressed as: $\begin{array}{c}P=0+p_1\cos \left({}_0+{}_1\left(t\right)\right)\left[2J_2\left({}_m\right)\cos \left(4f_{m}t\right)+\mathrm{.Math.}\right]+\\ p_1\sin \left({}_0+{}_1\left(t\right)\right)\left[2J_1\left({}_m\right)\cos \left(2f_{m}t\right)+2J_3\left({}_m\right)\cos \left(6f_{m}t\right)+\mathrm{.Math.}\right]\\ =p_1\cos \left[{}_0+{}_1\left(t\right)+{}_m\cos \left(2f_{m}t\right)\right]\end{array}$ At this time, only interfering signal P.sub.B formed by backscattered/backreflected light's interference is in the vicinity of zero frequency; 5) high-pass filtering P.sub.alt to filter out the interference signal P.sub.B and remain effective signal P, separating interference signal with effective signal to get effective signal; and 6) using effective signal P to further reconstruct signal (t):
(t)=.sub.1(t)+.sub.m cos(2f.sub.mt) filtering (t) to filter out .sub.m cos(2f.sub.mt) to obtain an interference phase difference signal .sub.1(t) caused by a disturbance.

Description

BRIEF DESCRIPTION OF FIGURES

(1) FIG. 1 is a positioning schematic diagram of single core feedback sensor;

(2) FIG. 2 is a diagram of single core feedback interference structure;

(3) FIG. 3 is a spectrum of the phase signal demodulated from interference signal, O is frequency notch point.;

(4) FIG. 4 is a schematic diagram of impact by backscattered light;

(5) FIG. 5 is a diagram of light path connection method which use phase generated carrier technology to eliminate the impact of backscattered;

(6) FIG. 6 is a concrete construction which the method of the present invention may be implemented.

REFERENCE NUMERAL

(7) 1: end of the sensing optical fiber 6, 2: feedback device, 3: for the N*M (N, M are integers) coupler, 4: P*Q (P, Q are integers) coupler, 5: optical fiber delayer, delay , 6: sensor optical fiber (optical cable) and feedback device 2 constituted, 3a1, 3a2, . . . , 3aN, 3b1, 3b2: port of coupler 3, 3a1, 3a2, . . . , 3aN: co-rotating ports with a total of N, 3b1, 3b2: two ports in another group co-rotating ports (with a total of M) of coupler 3. 4a1, 4a2, 4b1: ports of coupler 4, 4a1, 4a2: two ports in a group co-rotating ports (with a total of P) of coupler 4, 4b1: two ports in another group co-rotating ports (with a total of Q) of coupler 4. 7, 8: scattering point in optical fiber, 9: phase modulator.

EMBODIMENT

(8) The measurement system of the embodiment use interference structure shown in FIG. 3. Length of sensing optical cable 6 is 30 km. Light source is S03-B type super super radiation diode (SLD) produced by 44 research institute of the Institute of Industrial Electronics Group Corporation, with the operating wavelength of 1310 nm. Coupler 3 uses average of 3*3 Optical Fiber tapered single mode coupler. Coupler 4 uses average of 2*2 Optical Fiber tapered single mode Coupler. Both of them are produced by Wuhan Research Institute of Posts and Telecommunications. Fiber used by fiber delayer is G652 single-mode fiber. Photoelectric converter used in photoelectric conversion and information processing is GT322C500 of InGaAs photodetector produced by 44 research institute. Feedback device 2 is produced by optical fiber end steamed aluminized production, reflectance greater than 95%. Phase modulator 9 concatenating at the tail end is produced by winding optical fiber on a piezoelectric ceramic made. Interference signal baseband bandwidth <10 kHz, frequency of sinusoidal signal loaded at phase modulator is 60 kHz.

(9) In the single core sensing path, an active joint connection point is 10 km from end of sensing optical cable 6 (feedback device 2), at which point reflection >2 dB, disturbance applied near the port 4b1 to sensor cable 6. If do not use this The method of the invention, the system can not properly positioned. After use the modulation and demodulation method, the system can locate accurately.

(10) FIG. 6 is a concrete construction which the method of the present invention may be implemented. In this configuration, the coupler 3 is coupled using average 3*3 device, light input to port 3a1, two interference signals output from two ports 3a3 and 3a2, these two interference signals can be represented as:

(11) $\begin{array}{cc}{P}_{3a2}=p_1\cos \left[\frac{2}{3}+{}_1\left(t\right)+{}_c\left(t\right)\right]+\underset{i}{.Math.}{p}_{\mathrm{Bi}}\cos \left[{}_{3a2-B0i}+{}_{B1i}\left(t\right)\right]& \left(16\right)\\ P_{3a3}=p_1\cos \left[-\frac{2}{3}+{}_1\left(t\right)+{}_c\left(t\right)\right]+\underset{i}{.Math.}{p}_{\mathrm{Bi}}\cos \left[{}_{3a2-B0i}+{}_{B1i}\left(t\right)\right]& \left(17\right)\end{array}$

(12) Suppose the max frequency of .sub.1(t)) is f.sub.smax, then:
f.sub.m>f.sub.sBmax+f.sub.s1maxf.sub.m>f.sub.smax(18)

(13) according to the method described above, the phase modulation amplitude set so as to satisfy equation (7), high-pass filter interference signal to filter out stray light caused by interference, will receive the following signals:

(14) $\begin{array}{cc}{P}_{3a2}=p_1\cos \left[\frac{2}{3}+{}_1\left(t\right)+{}_m\cos \left(2f_{m}t\right)\right]& \left(19\right)\\ P_{3a3}=p_1\cos \left[-\frac{2}{3}+{}_1\left(t\right)+{}_m\cos \left(2f_{m}t\right)\right]& \left(20\right)\end{array}$

(15) then, use formula (19) and formula (20), signal (t) can be restored (Reference: Wu Hongyan, etc; fiber interference positioning system based signal demodulation technique [J]; sensors and micro systems, 2007, 26 (5): p 45-51):
(t)=.sub.1(t)+.sub.m cos(2f.sub.mt)(16)

(16) Low-pass filter (t), we can obtain .sub.1(t).