Method of improving measurement speed of distributed optical fiber sensor by adopting orthogonal signals and system thereof

Abstract

A method of improving measurement speed of distributed optical fiber sensors by adopting orthogonal signals and the system thereof is disclosed, which is related to the optical fiber sensor field and solves the problems that conventional technology will increasing the bandwidth of the received signal, reducing the signal-to-noise ratio of the received signal or distortion the spatial resolution of the system. The method comprises steps of generating N periodic orthogonal optical pulse sequence; injecting the N periodic orthogonal optical pulse sequence into the optical fiber under test(5); collecting the scattered light signal; demodulating the scattered light signal with the local oscillating light and then converting into digital signals; extracting the scatter information of the orthogonal optical pulses from the collected digital signals; and arranging the scattered information in order of precedence of the infusion. The measurement speed of the distributed optical fiber sensors is improved by N1 times.

Claims

1. A method of improving a measurement speed of distributed optical fiber sensors by adopting orthogonal signals, comprising steps of: step 1: generating a periodic orthogonal optical pulse sequence; wherein one cycle contains N mutually orthogonal signals, and these orthogonal signals are coherently demodulated to share same detector bandwidth, wherein N denotes a total number of orthogonal signals step 2: injecting the periodic orthogonal optical pulse sequence into an optical fiber under test in order of precedence and collecting scattered light signals; demodulating the scattered light signals with a local oscillating light, and transforming the demodulated signal into digital signals; and step 3: extracting scatter information of each of orthogonal optical pulses of the periodic orthogonal optical pulse sequence from collected digital signals; arranging the scatter information in order of precedence of the injecting into the optical fiber under test.

2. A method of an electrical signal generating unit generating two channels of orthogonal electrical pulses with a frequency of f.sub.1 named pulses-I and pulses-Q, comprising steps as follow: generating two pulses with an initial phase of 0 and a delay of nL/c as pulses-I, wherein a math expression of an I channel-signal is written as:
V.sub.Ii(t)=V.sub.D cos(2f.sub.1t)rect(t/T)+V.sub.D cos(2f.sub.1t)rect[(tnL/c)/T] meanwhile, generating two pulses with initial phases of 90 and 90 respectively and a delay of nL/c as pulses-Q, wherein a math expression of a Q-channel signal is as below:
V.sub.Qi(t)=V.sub.D cos(2f.sub.1t+/2)rect(t/T)+V.sub.D cos(2f.sub.1t/2)rect[(tnL/c)/T wherein L denotes a length of the optical fiber under test; c denotes a speed of light in vacuum; n denotes a refractive index of an optical fiber under test; V.sub.D denotes a radio-frequency signal amplitude of a modulation; rect denotes a rectangular function; T denotes a pulse width; t denotes a time variable; the pulses-I and pulses-Q are respectively used as the i-channel signal and the q-channel signal input IQ modulator to modulate the continuous-wave light, and a generated signal is expressed as
E.sub.i=E.sub.c cos[2(f.sub.c+f.sub.1)t]rect(t/T)+E.sub.c cos[2(f.sub.cf.sub.1)t]rect[(tnL/c)/T] wherein, f.sub.c denotes a frequency of an continuous-wave light; E.sub.c denotes a signal amplitude of an optical signal after modulation; Ei is one period of a periodic orthogonal pulse sequence, the period of the periodic orthogonal pulse sequence is 2nL/c; injecting the periodic orthogonal optical pulse sequence into an optical fiber under test in order of precedence and collecting scattered light signals; demodulating the scattered light signals with a local oscillating light, a demodulated signal is as below:
E.sub.0=AR(z)exp{j[2f.sub.1(tT.sub.z)2f.sub.cT.sub.z]}.Math.rect[(tT.sub.z)/T]+AR(z)exp{j[2f.sub.1(tT.sub.z)+2f.sub.cT.sub.z]}.Math.rect[(tnL/cT.sub.z)/T] wherein, j denotes an imaginary unit; A denotes a response factor of a probe; T.sub.z denotes a delay of receiving a scattered light signal on point z on the optical fiber under test at a receiver; R(z) denotes a distribution of a scattered light signal amplitude along the optical fiber under test; transforming the demodulated signal into digital signals; wherein the demodulated signal E.sub.0 contains two parts of signal, and it can be divided in to frequency domain; extracting scatter information of each of the orthogonal optical pulses of the periodic orthogonal optical pulse sequence from collected digital signals in the frequency domain; and arranging the scatter information in order of precedence of the injecting into the optical fiber under test.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic structural diagram of the system of the present invention;

(2) FIG. 2 is a schematic diagram of generating an up-converted optical pulse signal and a down-converted optical pulse signal according to the present invention;

(3) FIG. 3 is a schematic diagram of a time interval between the up-conversion optical pulse signal and the down-conversion optical pulse signal of the present invention;

(4) FIG. 4 is a schematic diagram of processing the collected signals by adopting a positive and negative beat signal separation algorithm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(5) In order to better illustrate the target, technical solution and advantages of the present invention, referring to the figures and embodiment, the present invention is further explained. The embodiment described is just for better illustrating the present invention and is not a limit.

(6) As shown in FIG. 1, a system of improving a measurement speed of distributed optical fiber sensors by adopting orthogonal signals, comprises a signal generating unit for a periodic orthogonal optical pulse sequence; a signal optical path detection unit for injecting the N periodic orthogonal optical pulse signal sequence into an optical fiber under test in order of precedence, collecting scattered light signals, modulating and converting a local oscillating light and the scattered light signals into digital signals; a signal processing unit for extracting scatter information of each of orthogonal optical pulses respectively from collected digital signals and sorting sensing information contained in demodulated two signals according to the order of the orthogonal signals injected into the optical fiber under test; the signal generating unit for a periodic orthogonal optical pulse sequence comprises a narrow linewidth laser 1; a modulation unit 3 for modulating laser emitted from the narrow linewidth laser 1 to an up-conversion optical pulse signal or a down-conversion optical pulse signal; and a signal generating unit 6 for shifting frequency electric signals to drive the modulation unit 3. The modulation unit 3 is a dual parallel Mach-Zehnder modulator (I/Q modulator). An up-conversion optical pulse signal and a down-conversion optical pulse signal are generated by changing a phase of two channels of signal outputted by the signal generating unit 6 to 90 or 90; the signal generating unit is a two channels output signal generator; the signal optical path detection unit comprises a circulator 4 connected to the I/Q modulator; a 90/10 coupler 2 connected to the narrow linewidth laser 1; a 90-degree optical hybrid 7 connected to the 90/10 coupler 2 and a third port of the circulator 4 respectively; the optical fiber under test 5 connected to a second port of the circulator 4; a balance detector 8 connected to the 90-degree optical hybrid 7; a data acquisition card 9 or an oscilloscope connected to the balance detector 8; the signal processing unit separate scatted (reflected) light signals generated by the up-conversion optical pulse signal and the down-conversion optical pulse signal by a positive and negative beat signal separation algorithm and demodulating sensing information contained in the separated two signals (positive frequency part and negative frequency part signal after IFFT) which is arranged in order of precedence of the infusion into the optical fiber under test.

(7) The detailed structure of the embodiment is as follow: an output end of the narrow linewidth laser 1 is connected to an input port of the 90/10 coupler 2; the 10 percent port of the 90/10 coupler 2 is connected to the local oscillating signal (LO) input end of the 90-degree optical hybrid 7; the 90 percent port of the 90/10 coupler is connected to an input end of the UQ modulator; the radio frequency port of the UQ modulator is connected to the two channels of orthogonal signals of the signal generating unit; I/Q modulator is connected to the signal generating unit; the output port of the UQ modulator is connected to a first port of the circulator 4; the second port of the circulator 4 is connected to the optical fiber under test 5; the third port of the circulator 4 is connected to signal input (SI) port of the 90-degree optical hybrid 7; two channels of UQ output of the 90-degree optical hybrid 7 are connected to two input port of the balanced detector 8; the UQ demodulated light signals are converted to electric signals by the balanced detector 8; the electric signals are converted to the digital signals by the data acquisition card 9 or the oscilloscope.

(8) L denotes a length of the optical fiber under test; c denotes a speed of light in vacuum; n denotes a refractive index of the optical fiber under test; a periodic repetition of the up-conversion optical pulse signal and the down-conversion optical pulse optical pulse signal is 2nL/c; nL/c denotes a time interval between the up-conversion optical pulse signal and the down-conversion optical pulse signal; As shown in the FIG. 3, the rectangle with forward slash denotes the down-conversion optical pulse signal; the rectangle with back slash denotes the up-conversion optical pulse signal. A denotes a response factor of a probe; R(z) denotes a distribution of a scattered light signal amplitude along the optical fiber under test. T.sub.z denotes a delay of receiving a scattered (reflected) light signal on point z on the optical fiber under test at a receiving end; f.sub.c denotes a frequency of incident light; f.sub.1 denotes a frequency shift of the modulation unit; E.sub.c denotes a signal amplitude of an optical signal after modulation; V.sub.D denotes a radio-frequency signal amplitude of a modulation. The method of improving measurement speed of a distributed optical fiber sensor by adopting orthogonal signals and a system thereof comprises steps of:

(9) (a) generating two channels of orthogonal signals with a frequency of f.sub.1

(10) generating two pulses with an initial phase of 0 and a delay of nL/c as in-phase signals which are I-signals, where a math expression of the I-signals is as below:
V.sub.Ii(t)=V.sub.D cos(2f.sub.1t)rect(t/T)+V.sub.D cos(2f.sub.1t)rect[(tnL/c)/T]

(11) meanwhile, generating two pulses with initial phases of 90 and 90 respectively and a delay of nL/c as the orthogonal signals which are Q-signals, where a math expression of the Q-signals is as below:
V.sub.Qi(t)=V.sub.D cos(2f.sub.1t+/2)rect(t/T)+V.sub.D cos(2f.sub.1t/2)rect[(tnL/c)/T

(12) where rect denotes a rectangular function; T denotes a pulse width; t denotes a time variable.

(13) (b) modulating the two channels UQ signal into the up-conversion optical pulse signal and the down-conversion optical pulse signal; where in the math expression of the output signal is:
E.sub.i=E.sub.c cos [2(f.sub.c+f.sub.1)t]rect(t/T)+E.sub.c cos [2(f.sub.cf.sub.1)t]rect[(tnL/c)/T]

(14) (c) UQ demodulating the backscatting sensing signal and the local oscillating light; where the demodulated signal is:
E.sub.0=AR(z)exp{j[2f.sub.1(tT.sub.z)2f.sub.cT.sub.z]}.Math.rect[(tT.sub.z)/T]+AR(z)exp{j[2f.sub.1(tT.sub.z)+2f.sub.cT.sub.z]}.Math.rect[(tnL/cT.sub.z)/T]

(15) (d) processing the collected signal with the positive and negative beat signal separation algorithm; separating the scatted light generated by the up-conversion optical pulse signal and the down-conversion optical pulse signal; demodulating the sensing information of the separated two signals; arranging the two signals in order of precedence of infusion into the optical fiber; where a signal measurement with a measurement time of nL/c is carried out on the optical fiber of L. The processing is illustrated in FIG. 4. The scattered signal comprises positive and negative beat signals; where the scattered signal is covered into two parts of positive beat signal and negative beat signal by the positive and negative beat signal separation algorithm; demodulating the two parts and arranging the demodulated signal in order of precedence of infusion into the optical fiber; the sensing information along the optical fiber is captured. Compared to the conventional measurement system, the present invention improves the measurement speed by N1 times; where in the embodiment N=2.

(16) The embodiment is just an illustration of the present invention and not a limitation. Any alteration and modification in the spirit and principle of the present invention is within the protection range of the present invention.