CN109974760B - Brillouin optical time domain analysis method based on Brillouin phase shift demodulation - Google Patents
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- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
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Abstract
The invention discloses a Brillouin optical time domain analysis system based on Brillouin phase shift demodulation, which consists of a light path detection part and a circuit demodulation part; a light path adopts a symmetrical double-sideband Brillouin optical time domain analysis system, probe light is generated by cascade modulation of a Mach-Zehnder modulator and a phase modulator, light generated by the first-stage modulation is used as light interacting with a pump, light generated by the second stage is used as auxiliary light to eliminate phase noise, and a receiving end performs coherent detection; the circuit demodulates the signal by envelope detection. The system is simple, easy to realize and easy to adjust; single channel, low sampling rate, small data volume; can obtain high Brillouin phase measurement precision and high stability.
Description
Technical Field
The invention relates to the technical field of distributed optical fiber sensing, in particular to a Brillouin optical time domain analysis method based on Brillouin phase shift demodulation.
Background
In recent years, with the rapid development of oil and gas pipelines, high-speed rails, large buildings and the like, the safety of the distributed optical fiber sensing technology is more and more concerned by various circles, and the distributed optical fiber sensing technology becomes a key technology for sensing external information in a long distance and severe environment due to a large number of advantages of the distributed optical fiber sensing technology. The brillouin optical time domain analysis technique is an important technique in many distributed optical fiber sensing techniques. The method is mainly applied to the fields of oil and gas pipelines, structural health monitoring and the like. In recent years, researches show that the brillouin frequency shift extracted by the brillouin phase has more and better advantages: dynamic measurement and pumping consumption resistance, and higher demodulation precision can be obtained only by a narrower sweep frequency space. Based on this, many researchers have made many studies on the brillouin optical time domain analysis technology based on the brillouin phase spectrum. For intensive research in this small area, demodulation of the brillouin phase spectrum is of paramount importance.
However, the current brillouin phase demodulation techniques mainly have the following categories: 1. radio frequency detection, namely, after an original radio frequency signal obtained by coherent detection is collected by a device with a high sampling rate, phase extraction is carried out on the radio frequency signal in a digital domain (the technology needs a great amount of data); 2. digital IQ demodulation, namely acquiring an original radio frequency signal by high sampling rate equipment, and performing IQ demodulation in a digital domain (also requiring a great data volume); 3. circuit IQ demodulation, wherein a dual-channel acquisition device is required in the technology, two paths of signals must be synchronized, and otherwise, an increased error exists; 4. the baseband demodulation technology utilizes the Sagnac interference effect to realize demodulation, but the technology has a complex structure, needs complex adjustment and dual-channel data acquisition, and has poor demodulation performance.
To summarize, techniques 1 and 2 require acquisition equipment at extremely high sampling rates and acquire a large amount of data, and thus are inefficient. Techniques 3 and 4 achieve a reduction in sampling rate, but both require dual channel acquisition equipment and the two channels need to be well synchronized. Furthermore, the above techniques do not take into account the influence of phase noise, and thus although the above techniques can demodulate the brillouin phase, the demodulation effect is not very good.
Disclosure of Invention
In view of the above problems, an object of the present invention is to provide a simple, high-precision, and high-stability brillouin phase shift demodulation-based brillouin optical time domain analysis method, which solves the deficiencies of the phase demodulation-based brillouin optical time domain analysis technique in practical applications. The technical scheme is as follows:
a Brillouin optical time domain analysis method based on Brillouin phase shift demodulation is characterized in that continuous light output by a tunable laser is divided into continuous light of two branches after passing through an optical coupler;
continuous light of an upper branch circuit passes through a first polarization controller to adjust the light polarization state, then passes through a Mach-Zehnder modulator to complete primary modulation under the drive of a frequency synthesizer, so that carrier suppression double-sideband modulation is realized, then the loss of optical power is compensated through a first continuous erbium-doped fiber amplifier in sequence, the light polarization state is adjusted through a second polarization controller, secondary adjustment is carried out through a phase modulator to generate required detection light, the detection light power is adjusted through an adjustable attenuator, and the detection light is injected into a sensing optical fiber through an optical isolator;
the continuous light of the lower branch circuit is adjusted in light polarization state through a third polarization controller, and then is driven by a pulse generator through an intensity modulator to generate pump light pulses, the pulse light is amplified through a pulse erbium-doped fiber amplifier in sequence, ASE noise is filtered through a light band-pass filter, the polarization state of the pulse light is disturbed through a deflector, and the pulse light is injected into a sensing fiber through a first port of a circulator to generate stimulated Brillouin effect with detection light;
the detection light after the stimulated Brillouin effect reaches a receiving end through a third port of the circulator;
at a receiving end, the detection light is amplified through a second continuous erbium-doped fiber amplifier and then injected into a photoelectric detector to be converted into an electric signal;
the electric signals are amplified through the low-noise amplifier in sequence, filtered through the band-pass filter, demodulated through the envelope detector to obtain the target Brillouin phase shift, collected through the data acquisition card and sent to the upper computer for post-processing.
Furthermore, the Mach-Zehnder modulator works at a carrier suppression point, and the driving frequency of the Mach-Zehnder modulator is the frequency sweeping frequency fsAnd the frequency sweep range covers the Brillouin gain interval of the sensing optical fiber.
Furthermore, the driving frequency of the phase modulator is a fixed frequency f1，f1Is greater than the width of the Brillouin gain spectrum, and
wherein, beta(2)Representing group velocity dispersion; l is the length of the optical fiber,
is a phase offset caused by chromatic dispersion, and
furthermore, the bandwidth of the photodetector is larger than the driving frequency f of the phase modulator1。
Further, the envelope detector is a linear envelope detector.
The invention has the beneficial effects that:
1) the method is simple: although two-stage modulation is adopted, the method is simple, easy to implement and easy to adjust;
2) the invention is a single channel, low sampling rate (small data volume): a system which is also based on a coherent detection phase demodulation scheme but designed according to the principle can obtain the brillouin phase (baseband signal) only through an envelope detector, so that the brillouin phase spectrum can be recovered only by a single channel and a lower sampling rate (a smaller amount of data) (which cannot be realized in the prior art);
3) the invention has high precision and strong stability: not only a phase demodulation scheme, but also phase noise caused by optical fiber transmission can be eliminated, so that high Brillouin phase measurement accuracy and high stability can be obtained.
Drawings
Fig. 1 is a schematic diagram of a brillouin optical time domain analysis method based on brillouin phase shift demodulation according to the present invention.
FIG. 2 shows the results of a Brillouin phase spectrum test; a) brillouin phase spectrum measured at 5km of the optical fiber; b) brillouin phase spectrum measured at 20km of the optical fiber; c) brillouin phase spectrum measured at the position of 30km of the optical fiber; d) brillouin frequency shift distribution of the whole optical fiber; e) brillouin frequency shift of the heating position.
FIG. 3 is a graph of the results of a heating test at 39.1km of the fiber tail end; a) the results are Brillouin frequency shift test results at different temperatures; b) the relationship between the brillouin frequency shift and the applied temperature was measured.
FIG. 4 is a graph of the verification results of the phase noise cancellation of this scheme; a)20 sets of test results of IQ demodulation; b)20 test results of the present invention; c) comparing the measured results of the two methods; d) the two methods compare the standard deviation of the test results.
In the figure: 1-a tunable laser; 2-an optical coupler; 3-a first polarization controller; 4-a frequency synthesizer; a 5-Mach Zehnder modulator; 6-first continuous erbium-doped fiber amplifier; 7-a second polarization controller; an 8-phase modulator; 9-an adjustable attenuator; 10-an optical isolator; 11-a sensing fiber; 12-a third polarization controller; 13-an intensity modulator; 14-a pulse generator; 15-pulsed erbium-doped fiber amplifier; 16-optical bandpass filter; 17-a polarization scrambler; 18-a circulator; 19-a second continuous erbium-doped fiber amplifier; 20-a photodetector; 21-a low noise amplifier; 22-band pass filter; 23-an envelope detector; 24-a data acquisition card; 25-an upper computer.
Detailed Description
The invention is described in further detail below with reference to the figures and specific embodiments.
The invention relates to a Brillouin optical time domain analysis method based on Brillouin phase shift demodulation.A symmetrical double-sideband Brillouin optical time domain analysis system is adopted in a light path, probe light is generated by cascade modulation of a Mach-Zehnder modulator and a phase modulator, wherein light generated by the first-stage modulation is used as light interacting with a pump, light generated by the second-stage modulation is used as auxiliary light to eliminate phase noise, and a receiving end performs coherent detection; the circuit demodulates the signal by envelope detection.
As shown in fig. 1, the continuous light output by the tunable laser 1 is divided into two branches of continuous light after passing through an optical coupler 2; continuous light of an upper branch path is adjusted in light polarization state through the first polarization controller 3, primary modulation is completed under the driving of the frequency synthesizer 4 through the Mach-Zehnder modulator 5, carrier suppression double-sideband modulation is achieved, loss of optical power is compensated through the first continuous erbium-doped fiber amplifier 6 in sequence, the light polarization state is adjusted through the second polarization controller 7, secondary adjustment is conducted through the phase modulator 8 to generate required detection light, detection light power is adjusted through the adjustable attenuator 9, and the detection light is injected into the sensing optical fiber 11 through the optical isolator 10.
The polarization state of the continuous light of the lower branch is adjusted through a third polarization controller 12, the continuous light is driven by a pulse generator 14 through an intensity modulator 13 to generate pump light pulses, the pulse light is amplified through a pulse type erbium-doped fiber amplifier 15 in sequence, ASE noise is filtered through a light band-pass filter 16, the polarization state of the pulse light is disturbed through a deflector 17, and the pulse light is injected into a sensing fiber 11 through a first port of a circulator 18 to generate stimulated Brillouin effect with detection light.
The detection light after the stimulated brillouin effect reaches a receiving end through a third port of the circulator 18; at the receiving end, the detection light is amplified by a second continuous erbium-doped fiber amplifier 19 and then injected into a photoelectric detector 20 to be converted into an electric signal; the electric signals are amplified through a low noise amplifier 21 in sequence, filtered through a band-pass filter 22, demodulated through an envelope detector 23 to obtain target Brillouin phase shift, collected through a data acquisition card 24 and then sent to an upper computer 25 for post-processing.
The principle analysis is as follows:
after the probe light generated by the two-stage modulation and the pumping pulse generate the stimulated Brillouin action, the optical field is as follows:
in the formula, E1Intensity of light generated for first order modulation, E2Generating intensity of light, g, for second order modulationBIs the Brillouin gain, f0Frequency of light output from the laser, fsFor frequency sweep (first order modulation drive frequency, f)1For second order modulation of the drive frequencyTo generate the auxiliary light),
is the brillouin phase shift and,
is the phase of the light at its corresponding frequency.
The radio frequency signal extracted by coherent detection and a band-pass filter is:
wherein the content of the first and second substances,
consider that
Is generally less than 0.05 and is,
is adjustable, and an approximation of the above formula is reasonable.
Likewise, the portion without stimulated brillouin effects may yield the following results:
after envelope detection is adopted, the output signals are as follows:
from the above analysis, it is clear that the present invention is possible in principle. Besides the Brillouin phase demodulation, the scheme can also completely eliminate phase noise, and the demodulation precision and stability of the Brillouin phase are obviously improved.
Two-stage modulation for generating probe light in the upper branch is realized by a Mach-Zehnder modulator 5 and a phase modulator 8 respectively, and the Mach-Zehnder modulator 5 works at a carrier suppression point; and the drive frequency of the Mach-Zehnder modulator 5 of the upper branch is the sweep frequency fsThe sweep range of which needs to cover the brillouin gain region of the sensing fiber 11. The drive frequency of the phase modulator 8 of the upper branch is a fixed frequency f1The value of which is required to be represented by the formula
Wherein beta is(2)Representing group velocity dispersion; l is the length of the optical fiber,
is a phase offset caused by chromatic dispersion, and
furthermore f1Must be larger than the width of the brillouin gain spectrum to avoid interference noise.
The continuous light of the lower branch generates pumping light pulses by an electro-optical intensity modulator 13, so that a symmetrical double-sideband Brillouin optical time domain analysis system is formed together with the probe light of the upper branch.
The bandwidth of the photodetector at the receiving end must be larger thanDrive frequency f of second order modulation1(ii) a The frequency of the radio frequency signal after the receiving end is filtered by the band-pass filter (22) is f1And the phase of the signal after transformation by the sum-difference product is transferred to the signal strength, and wherein the phase noise caused by the transmission is removed to the clean, leaving only the required brillouin phase shift and the fixed phase offset caused by the dispersion. Since the brillouin phase shift (demodulation target) of the target signal at the receiving end has been shifted to the signal strength, the target phase can be demodulated by envelope detection.
A target brillouin phase shift (demodulation target) is obtained by envelope detection, and in order to maintain the vector characteristic (sign) of the phase, the phase offset due to dispersion
Must be greater than the maximum of the brillouin phase (in brillouin optical time domain analysis systems the maximum brillouin phase is typically less than 0.03, i.e.
The condition must be satisfied).
The envelope detector used for demodulating the radio frequency signal must be a linear envelope detector (not a non-linear detector such as a logarithmic detector). Although this solution is still based on coherent detection, because the output signal of the envelope detection technique is a baseband signal, the required sampling rate only needs to meet the requirement of spatial resolution (i.e. the required sampling rate is very low as in the gain spectrum measurement method).
In practice, the electrical domain bandwidth of the photodetector 20 is greater than the frequency value f of the secondary modulation driving signal1. The electric band-pass filter 22 has a center frequency f1The bandwidth needs to be larger than the frequency value corresponding to the pumping pulse; the envelope detector must be a linear envelope detector.
FIGS. 2 and 3 are graphs showing the results of tests using the apparatus of the present invention, as shown in the figures, 39.1km of optical fiber is used for the tests, FIG. 2-a is a Brillouin phase spectrum at 5km, FIG. 2-b is a Brillouin phase spectrum at 20km, FIG. 2-c is a Brillouin phase spectrum at 30km, FIG. 2-d is a Brillouin frequency shift profile of the whole optical fiber, and FIG. 2-e is a detailed graph of Brillouin frequency shift at a hot spot; 3-a and 3-b are raw Brillouin frequency shift plots and Brillouin frequency shift versus applied temperature plots for temperature tests.
Fig. 4 is a diagram of the verification result of the phase noise cancellation of the scheme of the present invention, wherein: a)20 sets of test results of IQ demodulation; b)20 test results of the present invention; c) comparing the measured results of the two methods; d) the two methods compare the standard deviation of the test results.
Claims (5)
1. A Brillouin optical time domain analysis method based on Brillouin phase shift demodulation is characterized in that continuous light output by a tunable laser (1) is divided into continuous light of two branches after passing through an optical coupler (2);
continuous light of an upper branch circuit is adjusted in light polarization state through a first polarization controller (3), primary modulation is completed through a Mach-Zehnder modulator (5) under the drive of a frequency synthesizer (4), carrier suppression double-sideband modulation is achieved, loss of optical power is compensated through a first continuous erbium-doped fiber amplifier (6) in sequence, the light polarization state is adjusted through a second polarization controller (7), secondary adjustment is conducted through a phase modulator (8) to generate required detection light, the power of the detection light is adjusted through an adjustable attenuator (9), and the detection light is injected into a sensing optical fiber (11) through an optical isolator (10);
the polarization state of the continuous light of the lower branch is adjusted through a third polarization controller (12), the continuous light is driven by a pulse generator (14) through an intensity modulator (13) to generate pump light pulses, the pulse light is amplified through a pulse erbium-doped fiber amplifier (15) in sequence, ASE noise is filtered through a light bandpass filter (16), the polarization state of the pulse light is disturbed through a deflector (17), and the pulse light is injected into a sensing fiber (11) through a first port of a circulator (18) to generate stimulated Brillouin effect with detection light;
the detection light after the stimulated Brillouin effect reaches a receiving end through a third port of the circulator (18);
at a receiving end, the detection light is amplified by a second continuous erbium-doped fiber amplifier (19) and then injected into a photoelectric detector (20) to be converted into an electric signal;
the electric signals are amplified through a low-noise amplifier (21) in sequence, filtered through a band-pass filter (22), demodulated through an envelope detector (23) to obtain target Brillouin phase shift, collected through a data acquisition card (24) and then sent to an upper computer (25) for post-processing.
2. Brillouin optical time domain analysis method based on Brillouin phase shift demodulation according to claim 1, characterized in that the Mach-Zehnder modulator (5) operates at a carrier rejection point with a frequency sweep frequency fsThe sweep frequency range covers the Brillouin gain section of the sensing optical fiber (11).
3. Brillouin optical time domain analysis method based on Brillouin phase shift demodulation according to claim 1, characterized in that the driving frequency of the phase modulator (8) is a fixed frequency f1，f1Is greater than the width of the Brillouin gain spectrum, and
wherein, beta(2)Representing group velocity dispersion; l is the length of the optical fiber,
is a phase offset caused by chromatic dispersion, and
4. brillouin optical time domain analysis method based on Brillouin phase shift demodulation according to claim 3, characterized in that the bandwidth of the photodetector (20) is larger than the driving frequency f of the phase modulator (8)1。
5. Brillouin optical time domain analysis method based on Brillouin phase shift demodulation according to claim 3, characterized in that the envelope detector (23) is a linear envelope detector.
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| CN111220189B (en) * | 2020-01-17 | 2021-04-13 | 西南交通大学 | Brillouin optical time domain analysis sensing device and non-local effect compensation method |
| CN111141318B (en) * | 2020-01-17 | 2022-02-01 | 安捷光通科技成都有限公司 | Brillouin optical time domain clash type distributed optical fiber sensor |
| CN111721338B (en) * | 2020-06-08 | 2022-05-10 | 太原理工大学 | Brillouin optical time domain analysis system for alternately modulating high frequency and low frequency of pump light |
| CN111609918A (en) * | 2020-06-09 | 2020-09-01 | 重庆大学 | Optical fiber distributed vibration sensing system based on envelope detection circuit |
| CN113114370B (en) * | 2021-03-23 | 2022-07-01 | 暨南大学 | DP-QPSK modulator and PM series-connected phase encoding signal generation device and method |
| CN113091783B (en) * | 2021-04-29 | 2022-05-10 | 太原理工大学 | High-sensitivity sensing device and method based on two-stage Brillouin scattering |
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