CN105973450A - Optical fiber Fizeau interferometric array distributed vibration sensing system and method - Google Patents

Abstract

The invention discloses an optical fiber Fizeau interferometric array distributed vibration sensing system and method. The system comprises a laser light source, a first optical fiber coupler, a first optical frequency shifter, a second optical frequency shifter, a second optical fiber coupler, a delay device, an optical circulator and an optical fiber Fizeau interferometric array. The system also comprises a photoelectric detector, a data acquisition and control card, a first radio frequency signal source and a second radio frequency signal source; light emitted by the laser light source passes through the first optical fiber coupler so as to be divided into two beams of light, wherein the first beam of light and the second beam of light pass through the first optical frequency shifter and the second optical frequency shifter respectively so as to be subjected to frequency shifting, and therefore, the first beam of light and the second beam of light can be modulated into a first optical pulse and a second optical pulse with different frequencies; after the second optical pulse passes through the delay device, the second optical pulse and the first optical pulse are coupled into a pulse pair at the second optical fiber coupler; the pulse pair passes through the optical circulator and enters the optical fiber Fizeau interferometric array, and as a result, a plurality of reflected pulse pairs can be generated; the reflected pulse pairs go back to the optical circulator; and the reflected pulse pairs are subjected to heterodyne interferometry at the photoelectric detector.

Description

Optical Fiber Fizeau Interference Array Distributed Vibration Sensing System and Method

technical field

The invention relates to the technical field of optical fiber sensors, in particular to an optical fiber Fizeau interference array distributed vibration sensing system and method.

Background technique

Distributed optical fiber vibration sensing uses optical fiber as the sensing element and transmission medium to detect vibration signals around the sensing link. It not only has simple optical fiber sensing technology, but also has high sensitivity, high temperature resistance, corrosion resistance and electromagnetic resistance. interference and other advantages, and can better reflect the advantages of optical fiber distribution and extension. Any point in the sensing link can be modulated by the vibration signal, so as to realize the no-miss detection of the vibration signal. The interferometric distributed optical fiber vibration sensing technology is The vibration sensing technology based on the principle of light wave interference modulation has outstanding advantages such as high sensitivity, large dynamic range and high response frequency. After more than 30 years of development, the interference optical fiber vibration sensing technology has achieved fruitful research results. Instrument, Michelson interferometer and Sagnac interferometer and MZI-MZI, SI-SI and other composite interference structures.

The vibration sensing systems that have been studied based on interference structures such as Mach-Zehnder interferometers and Michelson interferometers are all single-arm sensing, and the reference light and signal light do not travel in the same optical path, so the two arms are easily affected by differences. The interference of environmental noise leads to the instability of the interference fringes. Fizeau interference uses the first reflected light when light is transmitted in the optical system as the reference light, and the second reflected light as the signal light. The reference light and the signal light travel along the same optical path and interfere. In Fizeau interference, the signal light Traveling in the same optical path as the reference light overcomes the instability of interference fringes caused by different external disturbances (such as vibration, temperature fluctuations, etc.).

With the gradual maturity of interferometric distributed optical fiber vibration sensing technology and the continuous development of time division and wavelength division multiplexing technology, in order to realize the networked, large-scale and quasi-distributed measurement of the sensor system, various structures, scales and sensors The optical fiber vibration sensing array with sensing distance has been extensively researched. Looking at the current multiplexed sensing array technologies, it is difficult to have a technology that can achieve the best performance in terms of detection distance, array size, line array diameter and measurement sensitivity. At the same time, it has superior performance.

Contents of the invention

In order to overcome the problems of Mach-Zehnder interferometer and Michelson interferometer, etc., because the signal light and reference light travel in different optical paths and cause interference fringe instability caused by different external disturbances (such as vibration, temperature fluctuations, etc.), a A fiber optic Fizeau interference array distributed vibration sensing system and method.

The technical solution adopted by the present invention to solve its technical problems is:

Provide a fiber optic Fizeau interference array distributed vibration sensing system, including laser light source, first optical fiber coupler, first optical frequency shifter, second optical frequency shifter, second optical fiber coupler, delayer, optical ring The light emitted by the laser source is divided into two beams by the first fiber coupler, and the first beam and the second beam are frequency shifted by the first optical frequency shifter and the second optical frequency shifter respectively. And modulated into the first optical pulse and the second optical pulse of different frequencies, and after the second optical pulse is delayed by the delayer, it is coupled with the first optical pulse at the second optical fiber coupler to form a pulse pair, and the pulse pair passes through the optical ring The circulator enters the optical fiber Fizeau interference array, generates multiple reflected pulse pairs, and then returns to the optical circulator;

The system also includes a photoelectric detector, a data acquisition and control card, a first radio frequency signal source and a second radio frequency signal source, the photodetector is connected to the optical circulator, the data acquisition and control card, the first radio frequency signal source The frequency shifter is connected, and the second radio frequency signal source is connected to the second optical frequency shifter; the latter pulse of the previous reflected pulse pair in two adjacent reflected pulse pairs is used as a reference light, and the former reflected pulse pair of the latter reflected pulse pair The pulse is used as the signal light, and the two have heterodyne interference at the photodetector to generate an interference beat frequency signal; the interference beat frequency signal is output to the data acquisition and control card through the photodetector for demodulation.

In the system of the present invention, the optical fiber Fizeau interference array includes multiple pairs of optical fiber reflection points, and optical fibers are connected between each pair of optical fiber reflection points.

In the system of the present invention, the fiber reflection point is a fiber Bragg grating or a chirped grating, or the fiber reflection point is a sudden change in refractive index caused by laser irradiation on the fiber.

In the system of the present invention, the first optical frequency shifter and the second optical frequency shifter are acousto-optic modulators.

In the system of the present invention, the delay device is a delay optical fiber, and the length of the delay optical fiber is twice the length of the optical fiber between each pair of optical fiber reflection points.

The present invention also provides a kind of optical fiber Fizeau interference array distributed vibration sensing method, comprising the following steps:

generating a first light pulse and a second light pulse having different frequencies;

Delaying the second light pulse, combining the delayed second light pulse with the first light pulse to form a light pulse pair;

The light pulse is incident on the fiber Fizeau interference array to generate multiple reflected pulse pairs;

The latter pulse of the previous reflected pulse pair in two adjacent reflected pulse pairs is used as the reference light, and the former pulse of the latter reflected pulse pair is used as the signal light, and the two undergo heterodyne interference to generate an interference beat frequency signal;

Output the interferometric beat frequency signal and demodulate it.

The beneficial effects produced by the present invention are: in the present invention, the light emitted by the light source is modulated into a double pulse pair, which is incident to the optical fiber Fizeau interference array to generate a plurality of reflected pulse pairs, and the previous reflected pulse pair is used in two adjacent reflected pulse pairs The latter pulse is used as the reference light, and the previous pulse of the latter reflected pulse pair is used as the signal light to carry the phase disturbance caused by the vibration on the optical fiber between adjacent reflection points. By controlling the pulse spacing and the adjacent reflection points The length of the optical fiber between them is the same to realize the interference between the reference light and the signal light, which avoids the system noise caused by the interference of the unbalanced interference arm and reduces the volume of the sensor. By using the Fizeau interference structure, the reference light and the signal light Transmission in the same optical path, thus overcoming the problem of unstable interference fringes caused by different external interference caused by reference light and signal light transmission in different optical paths in the traditional interference structure; and multiplexing large-scale sensors on a single optical fiber, Improvements and enhancements have been made in performance such as detection distance, array scale, and measurement sensitivity.

Description of drawings

The present invention will be further described below in conjunction with accompanying drawing and embodiment, in the accompanying drawing:

Fig. 1 is the structural representation of the embodiment of the present invention;

Figure 2 is a comparison chart between the system measurement results and the detection results of the geophone;

detailed description

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

Fig. 1 is a structural schematic diagram of an embodiment of the present invention, the optical fiber Fizeau interference array distributed vibration sensing system includes a laser light source 1, a first optical fiber coupler 2, a first optical frequency shifter 4, a second optical frequency shifter 7, Second optical fiber coupler 11, time delay device, optical circulator 13 and optical fiber Fizeau interference array 14, the light that laser light source 1 sends is divided into two beams of light through first optical fiber coupler 2, and the first beam of light and the second beam of light are respectively The frequency shift is generated by the first optical frequency shifter 4 and the second optical frequency shifter 7 and modulated into a first optical pulse and a second optical pulse with different frequencies. An optical pulse is coupled into a pulse pair at the optical circulator 13, and the pulse pair enters the optical fiber Fizeau interference array 14, and after being reflected by a plurality of optical fiber reflection points of the optical fiber Fizeau interference array 14, multiple reflected pulse pairs are generated, and then passed through the Fizeau interference array 14 Reflected back to the optical circulator 13.

The system also includes photodetector 15, data acquisition and control card 17, first radio frequency signal source 8 and second radio frequency signal source 9, photodetector 15 is connected with optical circulator 13, data acquisition and control card 17, the first The radio frequency signal source 8 is connected to the first optical frequency shifter 4 , and the second radio frequency signal source 9 is connected to the second optical frequency shifter 7 . The reflected pulse pair enters the photodetector 15 through the optical circulator 13, and heterodyne interference occurs at the photodetector 15 to generate an interference beat frequency signal; the interference beat frequency signal is output to the data acquisition and control card 17 through the photodetector 15 to demodulate.

Wherein, the first optical frequency shifter 4 and the second optical frequency shifter 7 are acousto-optic modulators. The time-delay device can be selected as the time-delay fiber 10, and the length of the time-delay fiber 10 is twice the length of the fiber between each pair of fiber reflection points.

The first probe light enters the first optical frequency shifter 4 through the first connecting optical fiber 3 for frequency shift and is modulated into a light pulse whose frequency is f ₁ +υ and then injects into the second optical fiber coupler 11 (3dB) through the second connecting optical fiber 5 ) of the first port 11.1; the second probe light enters the second optical frequency shifter 7 through the third connecting optical fiber 6 to generate a frequency shift and is modulated into an optical pulse with a frequency _f is injected into the second port 11.2 of the second fiber coupler.

The light pulses input from the first port and the second port are combined by the second fiber coupler 11 (3dB) to form a pair of light pulses which are output from the third port 11.3 of the second fiber coupler 11 (3dB) and then pass through the fourth The segment connecting fiber 12 is incident to the first port 13.1 of the optical circulator 13, and the optical pulse pair output from the second port 13.2 of the optical circulator is incident to the optical fiber Fizeau interference array 14.

The optical fiber Fizeau interference array includes multiple pairs of optical fiber reflection points, and optical fibers are connected between each pair of optical fiber reflection points. The fiber reflection point can be a fiber Bragg grating or a chirped grating, or the fiber reflection point is a sudden change in refractive index caused by laser irradiation on the fiber.

In one embodiment of the present invention, the optical fiber Fizeau interference array is made by using the method of online preparation of ultra-weak gratings and using a special platform for fiber drawing and on-line gratings. A total of 1005 Bragg gratings are written in it, and the interval between the Bragg gratings is 10m, the reflectivity of the grating is -35dB—-40dB.

The third port 13.3 of the optical circulator is connected to the photodetector 15, and the electrical output port of the photodetector 15 is connected to the input port 17.3 of the data acquisition and control card 17 through the fifth connection optical fiber 16, and the first port of the data acquisition and control card The control port 17.1 is connected with the control input port 8.1 of the first radio frequency signal source 8 to realize the control of the first radio frequency signal source, and the second control port 17.2 of the signal processing and control card is connected with the control input port of the second radio frequency signal source 9 9.1 The connection realizes the control of the second radio frequency signal source.

The RF output port 8.2 of the first RF signal source 8 is connected to the RF input port 4.3 of the first optical frequency shifter; the RF output port 9.2 of the second RF signal source 9 is connected to the RF input port 7.3 of the second optical frequency shifter.

The present invention uses two beams of pulses with different frequencies to generate heterodyne interference, and obtains external vibration signals by demodulating beat frequency signals, which can be applied to the design of interference-type optical fiber hydrophones, and the demodulation by using heterodyne detection methods can effectively Improve the system detection sensitivity and dynamic range of the fiber optic hydrophone, and at the same time overcome environmental disturbances and the system can carry a large signal bandwidth.

The sensing method realized by the optical fiber Fizeau interference array distributed vibration sensing system of the above embodiment mainly includes the following steps:

The light output by the laser light source 1 passes through the first fiber coupler 2 and is divided into two beams of probe light, namely the first beam of probe light and the second beam of probe light;

The first probe light enters the first optical frequency shifter 4 through the first connecting optical fiber 3 to produce a frequency shift and is modulated into an optical pulse with a frequency of f ₁ +υ, and then injected into the second optical fiber coupler 11 through the second connecting optical fiber 5 The first port 11.1; the second probe light enters the second optical frequency shifter 7 through the third connecting optical fiber 6 to produce a frequency shift and is modulated into an optical pulse with a frequency of f ₂ +υ, and then injected into the second optical pulse through the delay optical fiber 10 Second port 11.2 of the second fiber optic coupler.

After the second optical fiber coupler combines the light pulses input from the first port and the second port, the fourth connecting fiber 12 is incident on the first port 13.1 of the optical circulator 13, and the second port 13.2 of the optical circulator outputs The light pulse is incident on the optical fiber Fizeau interference array 14;

The light pulse pair reflected by the first Bragg grating returns to the optical circulator, exits from the third port of the optical circulator and enters the photodetector through the fifth connecting fiber, and its electric field intensity is expressed as:

E _1-2 is the electric field intensity of the last pulse in the optical time domain reflection pulse pair reflected by the first Bragg grating, c is the speed of light in vacuum, L is the length of the delay line, The total phase introduced for the connecting fiber as well as the delay fiber.

The light pulse pair reflected by the second Bragg grating returns to the optical circulator, and the length of the optical fiber between the two reflection points is l; it exits from the third port of the circulator and enters the photodetector through the fifth section of connecting optical fiber, and its electric field strength Expressed as:

E _2-1 is the electric field intensity of the previous pulse in the optical time domain reflection pulse pair reflected by the second Bragg grating, c is the speed of light in vacuum, l is the first fiber reflection point and the second fiber reflection point The length of the fiber between, n _eff is the effective refractive index of the fiber between the first fiber reflection point and the second fiber reflection point, The total phase introduced for the connecting fiber.

The length of the delay fiber and the length of the fiber between adjacent Bragg gratings satisfy the following relationship:

L=2l

The latter optical pulse of the first pulse pair reflected by the first Bragg grating and the previous pulse of the pulse pair reflected by the second Bragg grating form synchronous heterodyne interference at the photodetector, and the two paths after interference The response I _1,2 of the probe light in a balanced detector can be expressed as:

The output I of the detector is:

$\mathrm{<span\; class="notranslate">\; I</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; \&lsqb;</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; (</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&upsi;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&upsi;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&rsqb;</span>$

ignored here fixed phase difference.

When the optical fiber between the first Bragg grating and the second Bragg grating is vibrated, n _eff will change, and it can be seen from the above formula that the light intensity of the interference beat signal c1 changes. The photoelectric detector, data acquisition and control card receive the interference beat frequency signal c1 and after demodulation, restore the external vibration suffered by the optical fiber between the first optical fiber reflection point and the second optical fiber reflection point. In the experiment, a geophone is placed at the vibration point to detect the vibration signal. Figure 2 is a comparison chart between the system measurement results and the geophone detection results. It can be seen from the figure that the external vibration signal received by the system is demodulated , consistent with the vibration signal detected by the detector.

The two optical pulses enter the Fizeau interference array and repeat the above steps after passing through the third Bragg grating and the fourth Bragg grating to generate the interference beat signal c2, because the receiving time of the interference beat signal c1 and the interference beat signal c2 is different , so the interference beat frequency signals of the optical fibers between different adjacent Bragg gratings can be measured respectively, so as to sequentially demodulate the external vibrations on the optical fibers between different adjacent Bragg gratings.

To sum up, in the present invention, the light emitted by the light source is modulated into a double pulse pair, and the second pulse of the previous reflected pulse pair in two adjacent reflected pulse pairs is used as the reference light, and the former pulse of the latter reflected pulse pair is used as reference light. As the signal light carries the phase disturbance caused by the vibration on the optical fiber between adjacent reflection points, the interference between the reference light and the signal light is realized by controlling the pulse spacing to be the same as the length of the optical fiber between adjacent reflection points, avoiding the use of non- Balance the system noise caused by the interference of the interference arm and reduce the size of the sensor. By using the Fizeau interference structure, the reference light and the signal light are transmitted in the same optical path, thus overcoming the transmission of the reference light and the signal light in the traditional interference structure. Transmission in different optical paths leads to unstable interference fringes caused by different external disturbances, and multiplexes large-scale sensors on a single optical fiber, which has improved and improved performance in terms of detection distance, array scale, and measurement sensitivity.

It should be understood that those skilled in the art can make improvements or changes based on the above description, and all these improvements and changes should belong to the protection scope of the appended claims of the present invention.

Claims (6)

1. An optical fiber Fizeau interference array distributed vibration sensing system, is characterized in that, comprises laser light source, the first optical fiber coupler, the first optical frequency shifter, the second optical frequency shifter, the second optical fiber coupler, delay Timer, optical circulator and fiber Fizeau interference array. The light emitted by the laser source is divided into two beams by the first fiber coupler. The first beam and the second beam pass through the first optical frequency shifter and the second optical shifter respectively. The frequency shifter produces a frequency shift and modulates into a first optical pulse and a second optical pulse with different frequencies. After the second optical pulse is delayed by the delayer, it is coupled with the first optical pulse at the second optical fiber coupler to form a pulse pair. , the pulse pair enters the optical fiber Fizeau interference array through the optical circulator, generates multiple reflected pulse pairs, and then returns to the optical circulator; The system also includes a photoelectric detector, a data acquisition and control card, a first radio frequency signal source and a second radio frequency signal source, the photodetector is connected to the optical circulator, the data acquisition and control card, the first radio frequency signal source The frequency shifter is connected, and the second radio frequency signal source is connected to the second optical frequency shifter; the latter pulse of the previous reflected pulse pair in two adjacent reflected pulse pairs is used as a reference light, and the former reflected pulse pair of the latter reflected pulse pair The pulse is used as the signal light, and the two have heterodyne interference at the photodetector to generate an interference beat frequency signal; the interference beat frequency signal is output to the data acquisition and control card through the photodetector for demodulation. 2. The system according to claim 1, wherein the optical fiber Fizeau interference array includes multiple pairs of optical fiber reflection points, and an optical fiber is connected between each pair of optical fiber reflection points. 3. The system according to claim 2, wherein the fiber reflection point is a fiber Bragg grating or a chirped grating, or the fiber reflection point is a sudden change in refractive index caused by laser irradiation on the fiber. 4. The system of claim 1, wherein the first optical frequency shifter and the second optical frequency shifter are acousto-optic modulators. 5. The system according to claim 2, wherein the delay device is a delay fiber, and the length of the delay fiber is twice the length of the fiber between each pair of fiber reflection points. 6. An optical fiber Fizeau interference array distributed vibration sensing method, is characterized in that, comprises the following steps: generating a first light pulse and a second light pulse having different frequencies; Delaying the second light pulse, combining the delayed second light pulse with the first light pulse to form a light pulse pair; The light pulse is incident on the optical fiber Fizeau interference array, and after reflection, multiple reflected pulse pairs are generated; The latter pulse of the previous reflected pulse pair in two adjacent reflected pulse pairs is used as the reference light, and the former pulse of the latter reflected pulse pair is used as the signal light, and the two undergo heterodyne interference to generate an interference beat frequency signal; Output the interferometric beat frequency signal and demodulate it.