CN104568119A - Optical fiber vibration sensing system of single light source pulse and sensing method thereof - Google Patents

Abstract

The invention discloses an optical fiber vibration sensing system with single light source pulse encoding and a sensing method thereof. Under the control of the FPGA, the optical pulse forms an encoding, the encoded pulse light is formed through a coupler, and injected into the sensing optical fiber through an optical fiber circulator. The backpropagating scattered light generated in the propagation process interferes, enters the coupler through the optical fiber circulator, interferes with the local oscillator light, and generates a beat frequency signal, which is converted into an electrical signal by the photodetector and passed through a band-pass filter circuit. The difference frequency component is filtered out, and then the beat frequency is reduced through the mixer. Finally, the signal is collected and amplified by the acquisition card. In the upper computer, the amplitude modulation and demodulation of each control frequency is realized, and then decoding and positioning processing are performed. The system has the unique characteristics of distributed optical fiber monitoring and detection system, and is less affected by external interference such as electromagnetic, and is easy to install, which can well meet various vibration detection and monitoring applications, especially long-distance pipeline monitoring and peripheral monitoring. Border security, etc.

Description

A fiber optic vibration sensing system with single light source pulse encoding and sensing method thereof

technical field

The invention relates to the field of optical fiber sensing, in particular to a single light source pulse coded optical fiber vibration sensing system and a sensing method thereof.

Background technique

Due to its high sensitivity, immunity to electromagnetic interference, wide detection range, and low cost, distributed optical fiber sensing systems are widely used in long-distance oil and gas pipeline monitoring, perimeter security, and building structure health monitoring. research hotspots.

The Mach-Zehnder/Sagnac interferometer distributed optical fiber sensing system uses the phase difference change caused by external disturbances in the two sensing optical paths to be detected, and is positioned by the correlation time delay estimation method, which can sense vibration well. However, due to the difficulty of determining the time delay in the correlation time delay estimation method itself, the positioning accuracy of this method is not high, and it is difficult to accurately judge the vibration point.

The distributed optical fiber sensing system based on optical time domain reflectometer (OTDR) technology uses the Rayleigh scattering phenomenon that occurs when light waves are transmitted in the optical fiber, and detects the intensity of Rayleigh scattered light in the back to obtain the loss change of the optical fiber and precisely locate the optical fiber. point of failure. Since this technique measures the intensity of Rayleigh scattered light, its measurement sensitivity is relatively low and it can only respond to changes in static loss.

The Φ-OTDR (Phase Sensitive OTDR) technology based on coherent Rayleigh scattering uses a long-term coherent light source to detect the coherent result of the light pulse return light. Its interference method can effectively achieve dynamic response, and can simultaneously achieve high positioning accuracy and high sensitivity detection , especially for the detection of weak disturbance signals. At the same time, because the spatial resolution is related to the pulse length, very narrow pulses are often used for spatial resolution, usually tens to hundreds of nanoseconds, so the return light intensity is weak, and the signal resolution is low after long-distance transmission , limiting the dynamic range.

The Φ-OTDR based on pulse modulation can effectively improve the sensing distance on the basis of ensuring the spatial resolution, but because each modulation pulse needs to be independent of each other, multiple light sources are required to realize the system complexity. , the cost is also higher.

Contents of the invention

The invention provides an optical fiber vibration sensing system and its sensing method with single light source pulse coding. The invention successfully increases the injected light energy and suppresses the shot noise through pulse coding without increasing the complexity of the system. , which increases the sensing distance, see the description below for details:

An optical fiber vibration sensing system with single light source pulse coding, which consists of laser light source, FPGA, acousto-optic modulator group, optical fiber circulator, coupler, photoelectric detector, signal acquisition and conditioning device, band-pass filter circuit, mixer, Composed of host computer and sensing optical fiber,

The laser light source generates continuous light, which is converted into light pulses through acousto-optic modulator groups with different control frequencies. Under the control of FPGA, the light pulses form codes, and the coded pulses are formed through the coupler, which is injected into the sensing fiber through the optical fiber circulator. The backpropagating scattered light generated in the propagation process interferes, enters the coupler through the optical fiber circulator, interferes with the local oscillator light, and generates a beat frequency signal, which is converted into an electrical signal by the photodetector, and the formed electrical signal passes through The band-pass filter circuit filters out the difference frequency components, and then the beat frequency is reduced through the mixer, and finally the acquisition card collects and amplifies the signal, and in the host computer, realizes the respective amplitude modulation and demodulation of each control frequency, and then decodes and locates deal with.

The sensing method comprises the following steps:

The host computer controls N acousto-optic modulator groups with different control frequencies through the FPGA, and makes N mutually delayed short pulses form a coded pulse with a code length of N and a pulse width of N*τ at the N*1 coupler Light is injected into the sensing fiber through a fiber optic circulator;

The Rayleigh scattered light of the coded pulse light interferes with the local oscillator light from the laser light source to form beat waves of frequency f _i and Δf _ij , which are converted into corresponding electrical signals by photodetectors;

By selecting the control frequency of the acousto-optic modulator group, the filter result of the band-pass filter circuit is mixed with the preset sine wave to obtain the beat wave frequency after frequency reduction;

The upper computer obtains the Rayleigh scattering signal of a single coded pulse light by performing amplitude modulation and demodulation on the beat signal of the beat wave frequency after frequency reduction;

Vibration along the sensing fiber is obtained by calculating the moving difference of the Rayleigh scattering signal of a single coded pulse light.

The beneficial effects of the technical solution provided by the present invention are: the present invention successfully increases the injected light energy, suppresses shot noise, and improves the sensing distance through pulse coding without increasing the complexity of the system; The detection further improves the detectable minimum light intensity of the system and suppresses background scattering, and on the basis of heterodyne detection, the response of each coded pulse is demodulated, so that coding and decoding can still be realized under the same light source, further improving the sensing distance and system sensitivity. The optical fiber vibration sensing system has a simple structure, high signal resolution, low noise level, high sensitivity, and high positioning accuracy, which overcomes the complex structure and high cost of the existing system, and at the same time, its signal resolution and spatial resolution are mutually restricted and low sensitivity. , The noise level restricts the dynamic range. Moreover, the system has the unique characteristics of distributed optical fiber monitoring and detection system, and is less affected by external interference such as electromagnetic, and is easy to install. It can well meet various vibration detection and monitoring applications, especially long-distance pipeline monitoring and monitoring. Perimeter security, etc.

Description of drawings

Fig. 1 is a structural schematic diagram of a single light source pulse coded optical fiber vibration sensing system;

Fig. 2 is a flow chart of a single light source pulse coded optical fiber vibration sensing method.

In the accompanying drawings, the list of parts represented by each label is as follows:

1. Laser light source; 2. 1:99 coupler;

3. The first multi-channel coupler; 4. Acousto-optic modulator groups with different control frequencies;

5. The second multi-channel coupler; 6. Optical fiber circulator;

7. Sensing optical fiber; 8. 1:1 coupler;

9. Photodetector; 10. Band-pass filter circuit;

11. Mixer; 12. Signal generator;

13. Low-pass filter circuit; 14. Signal acquisition and conditioning module;

15. Host computer; 16. FPGA.

Detailed ways

In order to make the purpose, technical solution and advantages of the present invention clearer, the implementation manners of the present invention will be further described in detail below.

Example 1

A fiber optic vibration sensing system with single light source pulse coding, see Figure 1, the fiber optic vibration sensing system consists of a laser light source 1, a 1:99 coupler 2, a first multi-channel coupler 3, and acousto-optic modulation of different control frequencies Device group 4, second multi-channel coupler 5, optical fiber circulator 6, sensing fiber 7, 1:1 coupler 8, photodetector 9, bandpass filter circuit 10, mixer 11, signal generator 12, A low-pass filter circuit 13, a signal acquisition and conditioning module 14, a host computer 15 and an FPGA 16 are formed.

The continuous light with narrow line width is generated by the laser light source 1, and the probe light and the local oscillator light are generated through the 1:99 coupler 2. The optical modulator group 4 forms multiple channels of pulsed light, wherein the acousto-optic modulator group 4 is written by the host computer 15 to control the on-off sequence of the program in FGPA16, and the corresponding time delays are sequentially generated, and the generated pulsed light conforms to the set code. Matrix S, multi-channel pulsed light forms coded pulsed light at the second multi-way coupler 5, and the coded pulsed light is injected into the sensing fiber 7 through the optical fiber circulator 6. During the propagation process, the generated backscattered light and the input coded pulse The direction of light propagation is opposite, and the light pulse Rayleigh scattered light inside the code produces interference effect, and the Rayleigh pulse light between the codes also produces unfavorable interference, and finally enters the 1:1 coupler 8 through the optical fiber circulator 6 in reverse, at 1 : 1 The scattered light in the coupler 8 interferes with the local oscillator light to form the beat wave of the AOM control frequency and the difference frequency of the AOM control frequency, and forms an electrical signal through the photodetector 9, and filters the difference frequency signal through the band-pass filter circuit 10, The beat frequency is reduced by mixing the preset sine wave generated by the mixer 11 and the signal generator 12, the signal quality is further improved by the low-pass filter circuit 13, and the digital signal is formed by amplifying and analog-to-digital conversion by the signal acquisition and conditioning module 14, Send it to the host computer 15 to complete AM demodulation, decoding and positioning, and obtain the vibration conditions along the distributed sensors.

In the embodiments of the present invention, unless otherwise specified, the models of the devices are not limited, as long as they can complete the above functions.

Example 2

A distributed optical fiber vibration sensing method based on the principles of pulse code modulation, heterodyne detection, and phase-sensitive OTDR, see Figure 1, the method includes the following processes:

101: The host computer 15 controls N acousto-optic modulator groups 4 (AOM) with different control frequencies through the FPGA16, and at the N*1 road coupler 5, N mutually delayed short pulses are formed with a code length of N, and the pulse The coded pulsed light with a width of N*τ is injected into the sensing fiber 7 through the optical fiber circulator 6;

A sensing optical fiber 7 is laid in the range where vibration needs to be detected, and the sensing optical fiber 7 is used as a sensor and a signal transmission medium to realize vibration detection and monitoring. Select coding matrix S, its size is N*N, selects each row (length is N) of coding matrix S successively as coding, then upper computer 15 controls N different control frequencies by FPGA16 (control frequency is respectively f _i (i= 1, 2,..., N)) of the acousto-optic modulator group 4, respectively modulate the continuous light wave from the narrow-linewidth light source into N short pulse lights with respective pulse widths of τ with mutual time delay, in an N*1 At the road coupler 5, make N short pulse lights with mutual time delays form a coded pulse light P _i (i=1,2,...,N) with a code length of N and a pulse width of N*τ, and pass through the optical fiber ring The sensor 6 is injected into the sensing fiber 7.

102: The Rayleigh scattered light of the coded pulsed light interferes with the local oscillator light from the laser light source 1 to form beat waves of frequency f _i and Δf _ij , which are converted into corresponding electrical signals by the photodetector 9;

When the coded pulsed light propagates in the sensing fiber 7, it will form Rayleigh backscattering, the Rayleigh scattered light of each short pulse interferes with each other within the respective pulse width τ, and the interference between each code is unfavorable for detection. Via the fiber optic circulator 6, all the scattered light interferes with the local oscillator light from the laser light source 1 at the 1:1 coupler 8 in front of the photodetector 9 to form frequencies f _i (i=1, 2, ..., N) and The beat waves of Δf _ij =f _i −f _j (i, j=1, 2, . . . , N) are converted into corresponding electrical signals via the photodetector 9 .

103: By selecting the control frequency of the acousto-optic modulator group 4, mixing the filtering result of the band-pass filter circuit 10 with the preset sine wave to obtain the down-frequency beat wave frequency;

That is, by selecting a similar AOM control frequency so that its difference frequency is smaller than its control frequency, that is, f _i >Δf _ij (i,j=1,2,...,N), through the band-pass filter circuit 10, its control frequency The beat wave below is selected, and the band-pass filtering result is mixed with the preset sine wave (its frequency is f ₀ ) by the mixer 11, and the beat wave frequency is reduced to tens of megahertz, namely f _i -f ₀ (i=1,2,…,N), collected at a rate of 100MHz via the acquisition card.

104: The upper computer 15 obtains a Rayleigh scattering signal of a single coded pulse light by performing amplitude modulation and demodulation on the beat signal of the beat frequency after frequency reduction;

In the host computer 15, by performing AM demodulation on beat signals with frequencies f _i -f ₀ (i=1,2,...,N), the demodulated scattered light echo sequence L _i (i=1,2 ,...,N). Take L _i as the row vector to form a signal matrix $L=\left[\begin{array}{c}{L}_1\\ L_2\\ .\\ .\\ .\\ L_N\end{array}\right],$ Applying the decoding matrix to obtain the Rayleigh scattering signal of a single pulse ${\mathrm{\ω}}^{\′}=\left[\begin{array}{c}{{\mathrm{\ϖ}}_1}^{\′}\\ {{\mathrm{\ω}}_2}^{\′}\\ .\\ .\\ .\\ {{\mathrm{\ω}}_N}^{\′}\end{array}\right]=S^{-1}\·\left[\begin{array}{c}{L}_1\\ L_2\\ .\\ .\\ .\\ L_N\end{array}\right],$ Here S ^-1 is the inverse matrix of the encoding matrix S, and ω _i ' is the Rayleigh scattering signal obtained when the i-th encoding pulse of the N-bit encoding acts on the system alone theoretically.

105: Obtain the vibration condition along the sensing optical fiber 7 by performing mobile differential calculation on the Rayleigh scattering signal of a single coded pulsed light.

When the sensing optical fiber 7 of the optical fiber sensing system is affected by vibration, it will cause the change of the scattering signal at the response position. By calculating the moving difference of the scattering signal, the vibration along the sensing optical fiber 7 can be detected, and the distributed vibration sensor.

Those skilled in the art can understand that the accompanying drawing is only a schematic diagram of a preferred embodiment, and the serial numbers of the above-mentioned embodiments of the present invention are for description only, and do not represent the advantages and disadvantages of the embodiments.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection of the present invention. within range.

Claims (2)

1. the optical fiber vibration sensing system of a single light source pulse code, be made up of LASER Light Source, FPGA, acousto-optic modulator group, optical fiber circulator, coupling mechanism, photodetector, signal collection modulation device, bandwidth-limited circuit, frequency mixer, host computer and sensor fibre, it is characterized in that

LASER Light Source produces continuous light, the acousto-optic modulator group different via controlled frequency is converted to light pulse, under the control of FPGA, light pulse forms coding, coded pulse light is formed by coupling mechanism, sensor fibre is injected by optical fiber circulator, the scattered light of propagation dorsad produced in its communication process produces interferes, coupling mechanism is entered via optical fiber circulator, interference with local oscillator light, produce beat signal, photodetector is converted into electric signal, the electric signal formed is through bandwidth-limited circuit filtering difference frequency component, the reduction of beat frequency is realized again via frequency mixer, last by capture card collection and amplifying signal, in host computer, realize the respective demodulation of each controlled frequency, carry out again decoding and localization process.

2., for a method for sensing for the optical fiber vibration sensing system of a kind of single light source pulse code according to claim 1, it is characterized in that, described method for sensing comprises the following steps:

Host computer controls the different acousto-optic modulator group of N number of controlled frequency by FPGA, and making the short light pulse of N number of mutual time delay form code length at coupling mechanism place, N*1 road is N, and pulse width is the coded pulse light of N* τ, and injects sensor fibre via optical fiber circulator;

The Rayleigh scattering light of coded pulse light is interferenceed with the local oscillator light from LASER Light Source, forming frequency f _iand Δ f _ijbat ripple, be converted into corresponding electric signal via photodetector;

By choosing the controlled frequency of acousto-optic modulator group, the filter result of bandwidth-limited circuit and default sine wave being carried out mixing, obtains the bat ripple frequency after frequency reducing;

Host computer, by carrying out demodulation to the bat signal of the bat ripple frequency of frequency after frequency reducing, obtains the Rayleigh scattering signal of single encoded pulsed light;

By carrying out mobile Difference Calculation to the Rayleigh scattering signal of single encoded pulsed light, obtain the Vibration Condition that sensor fibre is along the line.