CN113483880A - Vibration sensing system based on few-mode optical fiber - Google Patents
Vibration sensing system based on few-mode optical fiber Download PDFInfo
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- G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
- G01H9/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means
- G01H9/004—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means using fibre optic sensors
Abstract
The invention discloses a vibration sensing system based on few-mode optical fibers. The method comprises the following steps: the signal generating module is used for sending a pulse laser signal; the signal modulation module is used for modulating the pulse laser signal into a detection light signal; the first noise reduction module is used for coupling the detection optical signal to the at least mode optical fiber to excite different optical fiber modes; the optical fiber is also used for outputting a fundamental mode LP01 and a high-order mode LP11 in a time-sharing mode according to the detection optical signal fed back by the few-mode optical fiber; the signal receiving module is used for carrying out noise reduction processing on the detection optical signals of the fundamental mode LP01 and the high-order mode LP 11; the signal processing module is used for respectively demodulating and calculating the detection optical signal of the fundamental mode and the detection optical signal of the high-order mode, and averaging the calculation results to obtain the vibration sensing information. The technical scheme provided by the invention reduces the interference of the detection signal in the few-mode optical fiber vibration sensing system and improves the sensing precision of the few-mode optical fiber based vibration sensing system.
Description
Technical Field
The embodiment of the invention relates to an optical fiber sensing technology, in particular to a vibration sensing system based on few-mode optical fibers.
Background
The optical fiber sensing technology is a novel sensing technology, and optical fibers are transmission media and sensing media and can transmit light wave signals at long distance. Meanwhile, the sensing detection of physical quantities such as temperature, humidity, stress and the like can be realized by detecting optical signals. Optical fiber sensing technology has found increasing application in various fields of detection.
Most of sensing technologies are based on the sensing detection of backscattered light signals in a common single-mode optical fiber, the noise of the sensing signals is large due to the fact that the continuously distributed random backscattered light signals return, the noise signals can interfere the sensing signals, and the sensing precision of the system is reduced.
Disclosure of Invention
The invention provides a vibration sensing system based on a few-mode optical fiber, which reduces the interference of detection signals in the vibration sensing system based on the few-mode optical fiber and improves the sensing precision of the vibration sensing system based on the few-mode optical fiber.
The embodiment of the invention provides a vibration sensing system based on few-mode optical fibers, which comprises:
the signal generating module is used for sending a pulse laser signal;
the signal modulation module is connected with the signal generation module and is used for modulating the pulse laser signal into a detection light signal;
the first noise reduction module is connected with the signal modulation module and the few-mode optical fiber and used for coupling the detection optical signal to the few-mode optical fiber to excite different optical fiber modes; the optical fiber is also used for outputting a fundamental mode and a high-order mode of the optical fiber mode in a time-sharing manner according to the detection optical signal fed back by the few-mode optical fiber;
the signal receiving module is connected with the first noise reduction module and used for carrying out noise reduction processing on the detection optical signals of the fundamental mode and the high-order mode;
and the signal processing module is connected with the signal receiving module and is used for respectively demodulating and calculating the detection optical signal of the fundamental mode and the detection optical signal of the high-order mode, and averaging the calculation result of the detection optical signal of the fundamental mode and the calculation result of the detection optical signal of the high-order mode to obtain the vibration sensing information.
Optionally, the first noise reduction module includes a first circulator, a mode multiplexer, and an optical switch;
a first optical port of the first circulator is connected with an output end of the signal modulation module, a second optical port of the first circulator is connected with a first end of the mode multiplexer, a third optical port of the first circulator is connected with a first end of the optical switch, a second end of the mode multiplexer is connected with the few-mode optical fiber, a third end of the mode multiplexer is connected with a second end of the optical switch, and a third end of the optical switch is connected with the signal receiving module; the first circulator is configured to couple the probe optical signal to the mode multiplexer, and after receiving the fed back high-order mode through the mode multiplexer, the first circulator is output to the optical switch through the third optical port; the mode multiplexer is used for coupling a detection optical signal to the few-mode optical fiber to excite different optical fiber modes and demultiplexing the detection optical signal mode fed back by the few-mode optical fiber into a fundamental mode and a high-order mode; the optical switch is used for outputting a fundamental mode and a high-order mode in a time-sharing mode.
Optionally, the first noise reduction module further includes a coupler;
the input end of the coupler is connected with the output end of the signal modulation module; a first output end of the coupler is connected with a first optical port of the first circulator; the coupler is used for coupling the detection optical signal into two paths of optical signals to be output.
Optionally, the vibration sensing system of the few-mode optical fiber further includes a second noise reduction unit;
the signal generation module is connected with the signal modulation module through the second noise reduction unit; the second noise reduction unit is used for reducing the transmission noise of the pulse laser signal.
Optionally, the second denoising unit includes: the first amplifier, the first reflective Bragg grating and the second circulator;
the first amplifier is used for amplifying the optical power of the pulse laser signal;
a first optical port of the second circulator is connected with the first amplifier, a second optical port of the second circulator is connected with the first reflective Bragg grating, and a third optical port of the second circulator is connected with an input end of the signal modulation module; the first reflective Bragg grating is used for reflecting the pulse laser signal with a preset wavelength; the second circulator is configured to couple the pulsed laser signal to the signal modulation module.
Optionally, the vibration sensing system of the few-mode optical fiber further includes a third noise reduction unit;
the first noise reduction module is connected with the signal receiving module through the third noise reduction unit; the third noise reduction unit is used for reducing transmission noise of the detection optical signal.
Optionally, the third noise reduction unit includes: a second amplifier, a second reflective Bragg grating and a third circulator;
a first optical port of the third circulator is connected with the second amplifier, a second optical port of the third circulator is connected with the second reflective bragg grating, and a third optical port of the third circulator is connected with the signal receiving module as an output end of the third noise reduction unit; the second reflective Bragg grating is used for reflecting the detection optical signal with a preset wavelength; the third circulator is used for coupling the detection optical signal to the signal receiving module.
Optionally, the signal receiving module includes: an acousto-optic modulator and a non-balanced Mach Zehnder interferometer;
the acousto-optic modulator is connected with a third optical port of the third circulator and is used for modulating the frequency of the detection optical signal;
the unbalanced Mach-Zehnder interferometer is connected with the signal processing module and is used for improving the wavelength division precision of the detection optical signals.
Optionally, the signal processing module includes: the system comprises a calculation subunit, a data acquisition subunit and a photoelectric detector;
the photoelectric detector is connected with the output end of the unbalanced Mach Zehnder interferometer; the photoelectric detector is used for respectively converting the detection optical signals of the fundamental mode and the high-order mode into analog signals;
the data acquisition subunit is connected with the photoelectric detector; the data acquisition subunit is used for converting the analog signal into a digital quantity;
the calculation subunit is connected with the data acquisition subunit, and the calculation subunit is used for inputting the digital quantity into a preset calculation model for noise reduction calculation and obtaining vibration sensing information.
Optionally, the vibration sensing system based on the few-mode optical fiber further includes:
the first pulse generator is connected with the signal generating module and used for providing a first working pulse of the signal generating module;
the second pulse generator is connected with the signal modulation module and the first pulse generator; the first pulse generator is also used for sending a first trigger signal to the second pulse generator; the second pulse generator is used for providing a second working pulse of the signal modulation module according to the first trigger signal; the signal modulation module is used for switching on or switching off according to the second working pulse;
the third pulse generator is connected with the acousto-optic modulator and the second pulse generator;
the second pulse generator is also used for sending a second trigger signal to the third pulse generator; the third pulse generator is used for providing a third working pulse of the signal modulation module according to the second trigger signal; the acousto-optic modulator is used for being switched on or switched off according to the third working pulse.
According to the technical scheme provided by the embodiment of the invention, the few-mode optical fiber is used as a sensing medium, the first noise reduction module is used for outputting the fundamental mode LP01 or the high-order mode LP11 in a time-sharing mode according to the detection optical signal fed back by the few-mode optical fiber, the signal receiving module is used for carrying out noise reduction processing on the detection optical signal of the fundamental mode LP01 or the high-order mode LP11, the signal processing module is used for respectively calculating the detection optical signal subjected to the noise reduction processing on the fundamental mode LP01 and the high-order mode LP11, the interference of the detection signal in the vibration sensing system based on the few-mode optical fiber is reduced through the combined weight of different optical fiber modes of backward Rayleigh scattering light, and the sensing precision of the vibration sensing system based on the few-mode optical fiber is improved.
Drawings
Fig. 1 is a schematic structural diagram of a vibration sensing system based on a few-mode optical fiber according to an embodiment of the present invention.
Fig. 2 is a schematic structural diagram of another vibration sensing system based on a few-mode optical fiber according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In recent years, optical fiber sensing technology has been increasingly applied in the fields of communication systems, high-voltage cable state monitoring, pipeline leakage detection, fire detection and the like. The fully distributed sensing technology is mostly based on backscattered light signals in a common single mode fiber, the signal-to-noise ratio of sensing signals is low due to the continuously distributed extremely low random backscattering, great noise influence brings great difficulty to the acquisition and processing of the signals, and the performance index and the detection precision of the system are greatly limited.
In view of this, an embodiment of the present invention provides a vibration sensing system based on a few-mode optical fiber, and fig. 1 is a schematic structural diagram of a vibration sensing system based on a few-mode optical fiber according to an embodiment of the present invention.
Referring to fig. 1, the few-mode fiber vibration sensing system includes:
a signal generating module 110, configured to send a pulsed laser signal;
the signal modulation module 120 is connected to the signal generation module 110, and is configured to modulate the pulse laser signal into a probe optical signal;
the first noise reduction module 130 is connected with the signal modulation module 120 and the few-mode fiber 140 and is used for coupling the probe optical signal to the few-mode fiber 140 to excite different fiber modes; the optical fiber is also used for outputting a fundamental mode and a high-order mode of an optical fiber mode in a time-sharing manner according to the detection optical signal fed back by the few-mode optical fiber 140;
a signal receiving module 150, connected to the first denoising module 140, for denoising the detected optical signals in the fundamental mode and the high-order mode;
the signal processing module 160 is connected to the signal receiving module 150, and configured to perform demodulation calculation on the detected optical signal in the fundamental mode and the detected optical signal in the higher-order mode, and average the calculation result of the detected optical signal in the fundamental mode and the calculation result of the detected optical signal in the higher-order mode to obtain the vibration sensing information.
Specifically, the laser refers to a high-intensity coherent light generated based on the principle of substance stimulated emission, and the laser has the same frequency, direction and strict phase relation. Pulsing the laser with an electrical signal can result in a pulsed laser signal with a pulsed characteristic. Illustratively, the signal generation module 110 may include a laser and a first pulse signal generator. The few-mode fiber 140 is a fiber with a large core area that can transmit parallel data streams using several independent fiber modes. Ideally, the capacity of the few-mode fiber 140 is proportional to the number of fiber modes. The optical fiber mode refers to the distribution of optical electromagnetic waves transmitted by an optical fiber. Generally, different fiber modes have different field structures, and each transmission line has a fundamental mode corresponding thereto. The fundamental mode is the mode with the longest cutoff wavelength. Other modes with short cutoff wavelength than the fundamental mode are called high-order modes. Optical fibers can be classified into single mode fibers, few mode fibers, and multimode fibers according to the number of modes that the fiber can transmit. The few-mode optical fiber has less optical fiber mode output, and is favorable for mode selection and signal processing.
The vibration sensing system based on few-mode optical fiber has the following working principle: the signal generating module 110 generates a laser signal, wherein the laser signal may be a single-wavelength laser or a continuous-wavelength laser. The pulsed laser signal can be obtained by pulsing the laser with an electrical signal. The acousto-optic modulation module 120 receives the pulse laser signal and modulates the pulse laser signal into a probe light signal, for example, the acousto-optic modulation module may adopt an acousto-optic modulator, and the acousto-optic modulator modulates the pulse laser signal to obtain a probe light signal, so that the probe light signal can be coupled to the sensing optical fiber. The probe light signal is coupled to the at least one mode fiber 140 through the first noise reduction module 130, and different fiber modes of the at least one mode fiber 140 can be excited due to different types of transmission fibers in the coupling process, wherein the fiber modes of the at least one mode fiber 140 include a fundamental mode LP01 and a high-order mode LP 11. The detection light signal passes through the few-mode optical fiber 140, and the backward scattering light of the few-mode optical fiber 140 carries vibration information because the vibration signal causes the phase, amplitude and other parameters of the detection light signal to change. The first noise reduction module 130 outputs two paths of backscattered light in different optical fiber modes in a time-sharing manner, where the backscattered light is a detection optical signal. The signal receiving module 150 processes the two paths of backward scattering light within a certain time, so that noise in the backward scattering light is reduced, and sensing precision is improved. The signal processing module 160 calculates the detection optical signal after noise reduction processing for the fundamental mode LP01 and the high-order mode LP11, respectively, and quantizes the frequency, strain, and position of the dynamic disturbance at any position on the few-mode fiber by combining the demodulation results of the backward rayleigh scattered light of different fiber modes, thereby obtaining vibration sensing information. The processing method of the signal processing module 160 may adopt a phase demodulation algorithm to demodulate the frequencies and the strains of the two modes of the sensing fiber at different positions respectively. Illustratively, the processing method of the signal processing module 160 adopts a 3X3 OC phase demodulation algorithm, and obtains demodulated signals of two modes by calculation, and then averages the strain data of corresponding positions of the two modes to obtain data after the two modes are combined, thereby obtaining vibration sensing information.
According to the technical scheme provided by the embodiment of the invention, the few-mode optical fiber is used as a sensing medium, the first noise reduction module is used for outputting the fundamental mode LP01 or the high-order mode LP11 in a time-sharing mode according to the detection optical signal fed back by the few-mode optical fiber, the signal receiving module is used for carrying out noise reduction processing on the detection optical signal of the fundamental mode LP01 or the high-order mode LP11, the signal processing module is used for respectively calculating the detection optical signal subjected to the noise reduction processing on the fundamental mode LP01 and the high-order mode LP11, the interference of the detection signal in the vibration sensing system based on the few-mode optical fiber is reduced through the combined weight of different optical fiber modes of backward Rayleigh scattering light, and the sensing precision of the vibration sensing system based on the few-mode optical fiber is improved.
Fig. 2 is a schematic structural diagram of another vibration sensing system based on a few-mode optical fiber according to an embodiment of the present invention. Referring to fig. 2, the first noise reduction module includes a first circulator 210, a mode multiplexer 220, and an optical switch 230;
a first optical port 211 of the first circulator 210 is connected to an output end of the signal modulation module, a second optical port 212 of the first circulator 210 is connected to a first end of a mode multiplexer 220, a third optical port 213 of the first circulator 210 is connected to a first end of an optical switch 230, a second end of the mode multiplexer 220 is connected to the few-mode optical fiber 140, a third end of the mode multiplexer 220 is connected to a second end of the optical switch 230, and a third end of the optical switch 230 is connected to the signal receiving module; the first circulator 210 is configured to couple the probe optical signal to the mode multiplexer 220, and output the probe optical signal to the optical switch 230 through the third optical port 213 after receiving the fed back high-order mode through the mode multiplexer 220; the mode multiplexer 220 is configured to couple the probe optical signal to the at least mode fiber 140 to excite different fiber modes, and demultiplex the detection optical signal mode fed back by the at least mode fiber 140 into a fundamental mode and a high-order mode; the optical switch 230 is used to time-divisionally output a fundamental mode and a high-order mode.
In particular, a circulator is a non-reciprocal device having a plurality of ends. The signal inputted from the first optical port can be outputted only from the second optical port, and similarly, the signal inputted from the second optical port can be outputted only from the third optical port. The detection optical signal enters the first optical port 211 of the first circulator 210, enters the mode multiplexer 220 through the second optical port 212, and is coupled and transmitted to the few-mode fiber 140 to excite different fiber modes after mode multiplexing. The detection light signal returns to the mode multiplexer 220 through the backward scattering light of the few-mode optical fiber 140, and is divided into two paths for transmission after mode demultiplexing, wherein a port corresponding to the LP01 mode on the mode multiplexer 220 outputs a first path of signal, and a port corresponding to the LP11 mode on the mode multiplexer 220 outputs a second path of signal. The first signal goes directly to the optical switch, and the optical path outputs the fundamental mode LP 01. The second signal returns to the second optical port 212 of the first circulator 210 and enters the optical switch 230 through the third optical port 213, and the optical path outputs the high-order mode LP 11. The first path of light path and the second path of light path are switched on by the light switch 230 in a time-sharing mode, two paths of different back scattering light are output in a time-sharing mode, the two paths of back scattering light collected within a certain time are processed, noise in the back scattering light can be reduced, sensing precision is improved, and the problems in the prior art are solved. For example, the optical switch may be connected to the computing subunit 610 or connected to the processor of the computing subunit 610 through the I/O card, and the computing subunit 610 controls the optical switch to recognize the input fundamental mode LP01 or the high-order mode LP11 at this time to control the optical switch to selectively switch on the first optical path or the second optical path in a time-sharing manner, so as to avoid confusion of data input, which may cause an error in average computation of the computation result of the detection optical signal of the fundamental mode and the computation result of the detection optical signal of the high-order mode by the computing subunit 610.
Optionally, the first noise reduction module further comprises a coupler 240;
the input end of the coupler 240 is connected with the output end of the signal modulation module; a first output terminal of the coupler 240 is connected to the first optical port 211 of the first circulator 210; the coupler 240 is used for coupling the detection optical signal into two optical signals for outputting.
Specifically, the coupler 240 includes two outputs for coupling the probe light signal into two outputs. The first output light 241 is used to provide the main power of the detection light signal, and the second output light 242 is used to monitor the peak power and waveform of the detection signal. Illustratively, the power of the first output light 241 is 98% of the total power of the probe light signal, and is used as the probe light signal. The power of the second output light 242 is 2% of the total power of the detection light signal, and can be used to monitor the peak power and waveform of the detection signal. Wherein the splitting ratio of the coupler is 98: 2.
Optionally, the vibration sensing system of the few-mode optical fiber further includes a second noise reduction unit;
the signal generation module is connected with the signal modulation module 120 through the second noise reduction unit; the second noise reduction unit is used for reducing the transmission noise of the pulse laser signal.
Illustratively, the second noise reduction unit outputs the pulse laser signal satisfying a preset wavelength by filtering the pulse laser signal.
Optionally, the second denoising unit includes: a first amplifier 310, a first reflective bragg grating 320, and a second circulator 330;
the first amplifier 310 is used for amplifying the optical power of the pulsed laser signal;
the first optical port 331 of the second circulator 330 is connected to the first amplifier 310, the second optical port 332 of the second circulator 330 is connected to the first reflective bragg grating 320, and the third optical port 333 of the second circulator 330 is connected to the input end of the signal modulation module 120; the first reflective bragg grating 320 is used for reflecting a pulse laser signal with a preset wavelength; the second circulator 330 is used to couple the pulsed laser signal to the signal modulation module 120.
Specifically, the input end of the first amplifier 310 is connected to the output end of the laser 340, and illustratively, the first amplifier 310 may adopt an erbium-doped amplifier, and the power of the pulsed laser signal emitted by the laser 340 is amplified by the erbium-doped amplifier, so as to improve the sensing distance. The second circulator 330 and the first reflective bragg grating 320 are used for filtering the pulse laser signal, reducing noise and increasing sensing accuracy. Similarly, a filter may be used instead of the second circulator 330 and the first reflective bragg grating 320 to achieve the same function, which is only illustrated and not specifically limited by the embodiment of the present invention. Illustratively, the optical path transmission process of the second circulator 330 and the first reflective bragg grating 320 is as follows: when the pulse laser signal is input to the first optical port 331 of the second circulator 330, the pulse laser signal enters the first reflective bragg grating 320 through the second optical port 332, is filtered by the first reflective bragg grating 320, is reflected back to the second optical port 332, and then enters the signal modulation module 120 through the third optical port 333 of the second circulator 330. The first bragg grating 320 may reflect light with a certain wavelength range, so as to filter the pulsed laser signal.
Based on the above embodiment, optionally, the vibration sensing system of the few-mode optical fiber further includes a third noise reduction unit;
the first noise reduction module is connected with the signal receiving module through a third noise reduction unit; the third noise reduction unit is used for reducing transmission noise of the detection optical signal.
Specifically, the third noise reduction unit performs filtering processing on the detection optical signal, so that the detection optical signal meets the preset wavelength output.
Based on the foregoing embodiment, with continuing reference to fig. 2, optionally, the third denoising unit includes: a second amplifier 410, a second reflective bragg grating 420, and a third circulator 430;
a first optical port of the third circulator 430 is connected with the second amplifier 410, a second optical port 432 of the third circulator 430 is connected with the second reflective bragg grating 420, and a third optical port 433 of the third circulator 430 is connected with the signal receiving module as an output end of the third noise reduction unit; the second reflective bragg grating 420 is configured to reflect the detection optical signal with a preset wavelength; the third circulator 430 is used to couple the detection optical signal to the signal receiving module.
Illustratively, the second amplifier 410 may employ an erbium doped amplifier, which amplifies the power of the detected optical signal to increase the sensing distance. The third circulator 430 and the second reflective bragg grating 410 are used for filtering the pulsed laser signal, reducing noise and increasing sensing accuracy. Similarly, a filter may be used instead of the third circulator 430 and the second reflective bragg grating 410 to achieve the same function, which is only illustrated and not specifically limited by the embodiment of the present invention. Illustratively, the optical path transmission process of the third circulator 430 and the second reflective bragg grating 410 is as follows: when the detection optical signal is input to the first optical port 430 of the third circulator 430, the detection optical signal enters the second reflective bragg grating 420 through the second optical port 432, is filtered by the second reflective bragg grating 420, is reflected back to the second optical port 432, and then enters the signal receiving module through the third optical port 433 of the third circulator 430. The second bragg grating 420 may reflect light with a certain wavelength range, so as to filter the detected optical signal.
Optionally, the signal receiving module includes: an acousto-optic modulator 510 and an unbalanced mach-zehnder interferometer 520;
the acousto-optic modulator 510 is connected to the third optical port 433 of the third circulator 430, and the acousto-optic modulator 510 is configured to modulate the frequency of the detection optical signal;
the unbalanced mach-zehnder interferometer 520 is connected to the signal processing module, and the unbalanced mach-zehnder interferometer 520 is configured to improve the wavelength division accuracy of the detection optical signal.
Specifically, the backscattered light of the detection optical signal including the fundamental mode LP01 and the high-order mode LP11 passes through the optical switch and enters the acousto-optic modulator in a time-sharing manner. When the backscattered light reaches the front end of the sensing fiber, the acousto-optic modulator 510 is turned on, and the amplified backscattered light passes through the unbalanced mach zehnder interferometer 520. The interference effect of the unbalanced mach zehnder interferometer 520 is utilized to improve the phase detection sensitivity and further improve the wavelength division accuracy of the detection optical signal.
Optionally, the signal processing module includes: a calculation subunit 610, a data acquisition subunit, and a photodetector 630;
the photodetector 630 is connected to the output of the unbalanced mach zehnder interferometer 520; the photodetector 630 is configured to convert the detection optical signals of the fundamental mode and the high-order mode into analog signals, respectively;
the data acquisition subunit is connected with the photodetector 630; the data acquisition subunit is used for converting the analog signal into a digital quantity;
the calculation subunit 610 is connected to the data acquisition subunit, and the calculation subunit 610 is configured to input the digital quantity into a preset calculation model to perform noise reduction calculation, and obtain vibration sensing information.
Specifically, the number of photodetectors 630 matches the number of outputs of the unbalanced mach zehnder interferometer 520, and the photodetectors 630 convert the optical signals into corresponding electrical signals. Illustratively, the data acquisition subunit may employ a data acquisition card 620, and the data acquisition card 620 samples the photodetector 630 to convert the analog quantity of the electrical signal into a digital quantity for calculation.
Illustratively, the calculating subunit 610 may include a computer with a calculating function, and the digital signal is subjected to calculating processing by the computer to obtain the final sensing information. And the noise of the detection signal can be further reduced through analysis and calculation.
Optionally, the vibration sensing system based on the few-mode optical fiber further includes:
the first pulse generator 710 is connected with the signal generation module, and the first pulse generator 710 is used for providing a first working pulse of the signal generation module;
a second pulse generator 720 connected to the signal modulation module and the first pulse generator 710; the first pulse generator 710 is further configured to send a first trigger signal to the second pulse generator 720; the second pulse generator 720 is used for providing a second working pulse of the signal modulation module according to the first trigger signal; the signal modulation module 120 is configured to turn on or off according to the second working pulse;
a third pulse generator 730 connected to the acousto-optic modulator 510 and the second pulse generator 720;
the second pulse generator 720 is further configured to send a second trigger signal to the third pulse generator 730; the third pulse generator 730 is configured to provide a third working pulse of the signal modulation module according to the second trigger signal; the acousto-optic modulator 510 is used to turn on or off according to the third operating pulse.
Specifically, the first pulse generator 710 generates a first working pulse to modulate the laser, and the first pulse generator 710 further generates a first trigger signal, which is output to the second pulse generator 720. The amplified pulsed optical signal may generate spontaneously amplified radiation. To avoid this phenomenon, the second pulse generator 720 is turned on by the timing control signal modulation module during the pulse forming stage to allow the probe laser signal to be transmitted to the subsequent optical path; while in the phase of detecting the optical signal, the signal modulation module 120 is turned off to prevent spontaneous amplified radiation from the first erbium doped amplifier. The second pulse generator 720 also sends out a second trigger signal, the second trigger signal is output to the third pulse generator 730, and the backscattered light of the fundamental mode LP01 and the high-order mode LP11 passes through the optical switch and enters the acousto-optic detector in a time-sharing manner. During pulse formation, the third pulse generator 730 controls the acousto-optic detector to remain off through timing to prevent Fresnel reflection from the acousto-optic detector by the mode multiplexer.
Based on the above embodiments, with continued reference to fig. 2, an exemplary few-mode fiber vibration sensing analysis process is as follows: in the system, a laser 340 generates a pulse optical signal with the pulse duration of 18ns and the pulse repetition rate of 25 mu s after being modulated. The first reflective bragg grating 320 and the second reflective bragg grating 420 have a center wavelength λ 0 of 1550.2nm, a bandwidth Δ λ of 0.4nm, and a reflectivity of 99.9%. The first amplifier 310 and the second amplifier 410 both use erbium-doped amplifiers. The unbalanced mach zehnder interferometer 520 uses a 3x3 specification so that the coupler at the output of the unbalanced mach zehnder interferometer 520 matches 3 photodetectors 630 with a 40v/mA transimpedance, 125mhz bandwidth. The photodetector converts the reflected optical signal into a corresponding detected electrical signal. The data acquisition card uses a 500MHz bandwidth oscilloscope to sample the photoelectric detector at a sampling rate of 1.25GSa/s, and converts the analog quantity of the detection electric signal into digital quantity which can be used for calculation. The sensor uses a few-mode fiber 140, wound 1.8m of the few-mode fiber 140 on a ring piezoelectric transducer (PZT) of 115mm diameter. 1073m and 25m long stress-free unheated optical fibers are respectively used in front of and behind the annular piezoelectric transducer to separate the disturbance area from the far end of the sensing optical fiber, so that the external environment interference is reduced as much as possible.
It was found by testing that the channels LP01-LP01, i.e. the fundamental mode LP01, observed periodic oscillations at a distance 1073m from the fiber front end, with a minimum detectable strain of 20n epsilon. The peak strain level of 1500Hz monotonic vibration at 1073m was 102.9n ε. Similar results were obtained for vibration analysis of channels LP01-LP11, the higher order mode LP 11. The location of the sinusoidal perturbations is matched to the vibration location of the PZT to which the sensing fiber is attached. Furthermore, the frequency and amplitude of the oscillation is consistent with the frequency and amplitude of the PZT input voltage, indicating that the sensing system has the ability to locate the perturbation position without any cross-talk between the perturbation zone and the non-perturbation zone. The trajectories of the strain spatial distributions LP01-LP01 along the sensing fiber at fixed frequencies are shifted spatially to the left relative to the trajectories LP01-LP 11.
The combined analysis of the two channels gave an average value which was 2.52dB less Δ NF1 than the channels LP01-LP01 and 2.63dB less Δ NF2 than the channels LP01-LP11, the combined results indicating that the present solution provides a sensor system with an improved noise floor of at least 2.5dB over the prior art.
The result shows that the sensing system in the scheme can acquire the back scattered light of two paths of different optical fiber modes of the fundamental mode LP01 and the high-order mode LP11, the back scattered light of the different optical fiber modes is separately separated and analyzed by a computer, and the sensing system can realize the quantification of the frequency, the strain and the position of the dynamic disturbance at any position on the few-mode optical fiber. Meanwhile, experimental results prove that the sensing system provided by the scheme can improve the sensitivity and the measurement range of the system and reduce the bottom noise by combining the phases of a plurality of modes and combining the backscattered light from different modes.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (10)
1. A vibration sensing system based on few-mode optical fibers, comprising:
the signal generating module is used for sending a pulse laser signal;
the signal modulation module is connected with the signal generation module and is used for modulating the pulse laser signal into a detection light signal;
the first noise reduction module is connected with the signal modulation module and the few-mode optical fiber and used for coupling the detection optical signal to the few-mode optical fiber to excite different optical fiber modes; the optical fiber is also used for outputting a fundamental mode and a high-order mode of the optical fiber mode in a time-sharing manner according to the detection optical signal fed back by the few-mode optical fiber;
the signal receiving module is connected with the first noise reduction module and used for carrying out noise reduction processing on the detection optical signals of the fundamental mode and the high-order mode;
and the signal processing module is connected with the signal receiving module and is used for respectively demodulating and calculating the detection optical signal of the fundamental mode and the detection optical signal of the high-order mode, and averaging the calculation result of the detection optical signal of the fundamental mode and the calculation result of the detection optical signal of the high-order mode to obtain the vibration sensing information.
2. The few-mode fiber based vibration sensing system of claim 1, wherein said first noise reduction module comprises a first circulator, a mode multiplexer, and an optical switch;
a first optical port of the first circulator is connected with an output end of the signal modulation module, a second optical port of the first circulator is connected with a first end of the mode multiplexer, a third optical port of the first circulator is connected with a first end of the optical switch, a second end of the mode multiplexer is connected with the few-mode optical fiber, a third end of the mode multiplexer is connected with a second end of the optical switch, and a third end of the optical switch is connected with the signal receiving module; the first circulator is configured to couple the probe optical signal to the mode multiplexer, and after receiving the fed back high-order mode through the mode multiplexer, the first circulator is output to the optical switch through the third optical port; the mode multiplexer is used for coupling a detection optical signal to the few-mode optical fiber to excite different optical fiber modes and demultiplexing the detection optical signal mode fed back by the few-mode optical fiber into a fundamental mode and a high-order mode; the optical switch is used for outputting a fundamental mode and a high-order mode in a time-sharing mode.
3. The few-mode fiber based vibration sensing system of claim 2, wherein the first noise reduction module further comprises a coupler;
the input end of the coupler is connected with the output end of the signal modulation module; a first output end of the coupler is connected with a first optical port of the first circulator; the coupler is used for coupling the detection optical signal into two paths of optical signals to be output.
4. The few-mode optical fiber based vibration sensing system according to claim 2, further comprising a second noise reduction unit;
the signal generation module is connected with the signal modulation module through the second noise reduction unit; the second noise reduction unit is used for reducing the transmission noise of the pulse laser signal.
5. The few-mode optical fiber based vibration sensing system according to claim 4, wherein said second noise reduction unit comprises: the first amplifier, the first reflective Bragg grating and the second circulator;
the first amplifier is used for amplifying the optical power of the pulse laser signal;
a first optical port of the second circulator is connected with the first amplifier, a second optical port of the second circulator is connected with the first reflective Bragg grating, and a third optical port of the second circulator is connected with an input end of the signal modulation module; the first reflective Bragg grating is used for reflecting the pulse laser signal with a preset wavelength; the second circulator is configured to couple the pulsed laser signal to the signal modulation module.
6. The few-mode optical fiber based vibration sensing system according to claim 2, further comprising a third noise reduction unit;
the first noise reduction module is connected with the signal receiving module through the third noise reduction unit; the third noise reduction unit is used for reducing transmission noise of the detection optical signal.
7. The few-mode optical fiber based vibration sensing system according to claim 6, wherein said third noise reduction unit comprises: a second amplifier, a second reflective Bragg grating and a third circulator;
a first optical port of the third circulator is connected with the second amplifier, a second optical port of the third circulator is connected with the second reflective bragg grating, and a third optical port of the third circulator is connected with the signal receiving module as an output end of the third noise reduction unit; the second reflective Bragg grating is used for reflecting the detection optical signal with a preset wavelength; the third circulator is used for coupling the detection optical signal to the signal receiving module.
8. The few-mode fiber based vibration sensing system of claim 7, wherein said signal receiving module comprises: an acousto-optic modulator and a non-balanced Mach Zehnder interferometer;
the acousto-optic modulator is connected with a third optical port of the third circulator and is used for modulating the frequency of the detection optical signal;
the unbalanced Mach-Zehnder interferometer is connected with the signal processing module and is used for improving the wavelength division precision of the detection optical signals.
9. The few-mode fiber based vibration sensing system of claim 8, wherein said signal processing module comprises: the system comprises a calculation subunit, a data acquisition subunit and a photoelectric detector;
the photoelectric detector is connected with the output end of the unbalanced Mach Zehnder interferometer; the photoelectric detector is used for respectively converting the detection optical signals of the fundamental mode and the high-order mode into analog signals;
the data acquisition subunit is connected with the photoelectric detector; the data acquisition subunit is used for converting the analog signal into a digital quantity;
the calculation subunit is connected with the data acquisition subunit, and the calculation subunit is used for inputting the digital quantity into a preset calculation model for noise reduction calculation and obtaining vibration sensing information.
10. The few-mode fiber based vibration sensing system of claim 9, further comprising:
the first pulse generator is connected with the signal generating module and used for providing a first working pulse of the signal generating module;
the second pulse generator is connected with the signal modulation module and the first pulse generator; the first pulse generator is also used for sending a first trigger signal to the second pulse generator; the second pulse generator is used for providing a second working pulse of the signal modulation module according to the first trigger signal; the signal modulation module is used for switching on or switching off according to the second working pulse;
the third pulse generator is connected with the acousto-optic modulator and the second pulse generator;
the second pulse generator is also used for sending a second trigger signal to the third pulse generator; the third pulse generator is used for providing a third working pulse of the signal modulation module according to the second trigger signal; the acousto-optic modulator is used for being switched on or switched off according to the third working pulse.
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