METHOD FOR REDUCING INTERFERENCE FROM SCATTERED LIGHT/REFLECTED LIGHT OF INTERFERENCE PATH BY GENERATING CARRIER THROUGH PHASE

摘要

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.

主权项

1. A method for reducing interference from scattered light of interference path by generating carrier through phase, characterized by comprising the following steps:
(1) concatenating phase modulator in the tail of a single core feedback sensing fiber, phase modulator introduce the interferometer phase difference ΔΦc (t), after reaching feedback means through the phase modulator, the light reflect to the optical cable and occurs interference signal, the signal is expressed as:
P=p1cos[Φ0+ΔΦ1(t)+ΔΦc(t)]  (1) wherein, p1 is a constant coefficient related to the system parameters, Φ0 is an initial phase of the interference structures and a constant, ΔΦ1 (t) is an interference phase difference caused by disturbance ; as to the back scattered light caused by previous path in the phase modulator in 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:PB=∑ipBicos[φB0i+ΔφB1i(t)](2) wherein, pBi is an interference coefficient caused by the i-th scattering point of the optical fiber, Φ B 0 i is an initial phase corresponding to the i-th scattering point, ΔΦB1i (t) is an interference phase difference corresponding to the i-th scattering point and caused by disturbance,∑i.represented the sum of all the scattered points along the previous induction fiber of phase modulator ;
thus, the total output signal change portion indicates as follows:
Palt=P+PB   (3) Jn order Bessel function expansion of Formula (1) is expressed as:P=p1cos[φ0+Δφ1(t)+φmcos(2πfm)]=p1cos(φ0+Δφ1(t))[J0(φm)+2J2(φm)cos(4πfmt)+…]+p1sin(φ0+Δφ1(t))[2J1(φm)cos(2πfmt)+2J3(φm)cos(6πfmt)+…]and(5)Palt=P+PB=PB+p1cos(φ0+Δφ1(t))[J0(φm)+2J2(φm)cos(4πfmt)+…]+p1sin(φ0+Δφ1(t))[2J1(φm)cos(2πfmt)+2J3(φm)cos(6πfmt)+…]=[PB+p1cos(φ0+Δφ1(t))·J0(φm)]+p1cos(φ0+Δφ1(t))[2J2(φm)cos(4πfmt)+…]p1sin(φ0+Δφ1(t))[2J1(φm)cos(2πfmt)+2J3(φm)cos(6πfmt)+…](6) (2) Selecting signal frequency fm loaded in phase modulator, fm, satisfy the following conditions: fm>fsBmax+fs1max, and fm is located out of the frequency component of ΔΦ1 (t); fs1max is the maximum frequency of sin (Φ01+ΔΦ1 (t)) or cos (Φ01+ΔΦ1 (t)), fsBmax is the maximum frequency of PB; (3) applying a sinusoidal signal at the phase modulator, the carrier generated by the modulated signal is expressed as:
ΔΦc(t)=Φmcos(2πfmt), Φm is the amplitude of Δ Φ c (t); (4) adjusting the amplitude of the sinusoidal signal, so that:
J0(Φm)=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 fm, frequency components are not in the vicinity of zero frequency, effective interference signal P is expressed as:P=0+p1cos(φ0+Δφ1(t))[2J2(φm)cos(4πfmt)+…]+p1sin(φ0+Δφ1(t))[2J1(φm)cos(2πfmt)+2J3(φm)cos(6πfmt)+…]=p1cos[φ0+Δφ1(t)+φmcos(2πfmt)] At this time, only interfering signal PB formed by backscattered light's interference is in the vicinity of zero frequency; (5) high-pass filtering Palt to filter out the interference signal PB and remain effective signal P, separating interference signal with effective signal to get effective signal.