CN112880865A - Ultra-long-distance high-spatial-resolution Raman optical fiber dual-parameter sensing system and method - Google Patents
Ultra-long-distance high-spatial-resolution Raman optical fiber dual-parameter sensing system and method Download PDFInfo
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Abstract
The invention belongs to the technical field of distributed optical fiber sensing, and particularly relates to an ultra-long-distance Raman optical fiber double-parameter sensing system with high spatial resolution and a method thereof, wherein the system comprises a pulse laser source and a chaotic laser source, the chaotic laser source is connected with the input end of a first coupler, the output end of the first coupler is respectively connected with one input end of a second coupler and a second photoelectric detector, the output end of the pulse laser source is connected with the other input end of the second coupler, the output end of the second coupler is connected with a first port of a wavelength division multiplexer, a second port of the wavelength division multiplexer is connected with the input end of a second optical switch, two output ends of the second optical switch are respectively connected with two ends of a sensing optical fiber, and a third port of the wavelength division multiplexer is connected with the first photoelectric detector; according to the invention, the temperature and strain information is calculated through the anti-Stokes light generated by the chaotic pulse laser and the two pulse lasers with different pulse widths in the sensing optical fiber, so that the sensing distance and the spatial resolution are improved.
Description
Technical Field
The invention belongs to the technical field of distributed optical fiber sensing, and particularly relates to an ultra-long-distance Raman optical fiber double-parameter sensing system and method with high spatial resolution.
Background
The distributed optical fiber Raman sensing system can realize continuous distributed temperature monitoring and has the advantages of electromagnetic interference resistance, corrosion resistance, electric insulation, high sensitivity, fire prevention, explosion prevention and the like. In the sensing optical fiber, two Raman scattering signals of Stokes light and anti-Stokes light can be generated when the pulse laser propagates in the optical fiber, wherein the anti-Stokes signal is more easily influenced by temperature and is more sensitive to the temperature. Therefore, the Stokes light and the anti-Stokes light can be used for temperature demodulation, and distributed temperature monitoring along the optical fiber is realized. The distributed optical fiber temperature sensor has a wide application prospect at present, and is mainly applied to the fields of disastrous detection, protection, alarm and the like of coal mines, tunnels, dangerous goods warehouses, high-rise buildings, bridges, expressways and the like.
However, since the detection signal of the distributed fiber raman sensing system is a pulse signal, the pulse width of the light source limits the spatial resolution performance of the system, and reducing the pulse width of the light source can improve the spatial resolution of the system, but also can affect the signal-to-noise ratio of the system and reduce the sensing distance of the system. Therefore, the spatial resolution of the current distributed fiber raman sensor cannot break through the limit of 1 m. Since spontaneous raman scattering has a limit to the maximum value of the power of the incoming fiber, the signal-to-noise ratio of the system decreases as the sensing distance increases, and thus it is difficult to achieve a sensing distance exceeding 100 km. In addition, the sensing distance and the measurement accuracy of the distributed temperature sensing system mainly depend on a temperature demodulation method, at present, the temperature demodulation method comprises single-path demodulation and double-path demodulation, and the single-path demodulation method is to determine the temperature by utilizing Raman anti-Stokes back scattering light through a sensing optical fiber. The two-way demodulation method is to demodulate the temperature using the ratio of the anti-stokes backscattered light to the rayleigh or stokes backscattered light. In the above methods, only one end of the sensing optical fiber is connected to the distributed temperature sensing system, however, the measurement result of the demodulation method is greatly influenced by the outside. In addition, the current distributed fiber raman sensing technology cannot realize simultaneous monitoring of temperature and strain.
Therefore, a brand new distributed optical fiber raman sensing device and method are needed to be invented to solve the problems that the existing distributed optical fiber raman sensing system cannot simultaneously improve the spatial resolution, the sensing distance and the measuring precision and cannot simultaneously measure the temperature and the stress.
Disclosure of Invention
The invention provides a distributed optical fiber Raman sensing device and method based on fusion of differential pulse laser and chaotic laser, aiming at solving the technical problems that the spatial resolution of the existing distributed optical fiber Raman sensing system is difficult to break through 1m, the sensing distance is difficult to break through 100km, the measurement precision is greatly influenced by the outside, and the system cannot realize continuous distributed measurement of two parameters of temperature and strain at the same time, so that the temperature and strain along the optical fiber can be simultaneously and cooperatively monitored, and the millimeter-magnitude spatial resolution along the 100km long-distance optical fiber can be measured.
In order to solve the technical problems, the invention adopts the technical scheme that: an ultra-long-distance high-spatial-resolution Raman optical fiber double-parameter sensing system comprises a pulse laser source, a chaotic laser source, a first optical switch, a first coupler, a second coupler, a wavelength division multiplexer, a second optical switch, a sensing optical fiber, a first photoelectric detector, a second photoelectric detector, a data acquisition card and a computer;
the output end of the chaotic laser source is connected with the input end of the first coupler, the output end of the first coupler is respectively connected with one input end of the second coupler and the second photoelectric detector, the output end of the pulse laser source is connected with the other input end of the second coupler, the output end of the second coupler is connected with the first port of the wavelength division multiplexer, the second port of the wavelength division multiplexer is connected with the input end of the second optical switch, the two output ends of the second optical switch are respectively connected with the two ends of the sensing optical fiber, and the third port of the wavelength division multiplexer is connected with the first photoelectric detector;
the output ends of the first photoelectric detector and the second photoelectric detector are connected with a data acquisition card, and the output end of the data acquisition card is connected with a computer; one end of the sensing optical fiber is arranged in the thermostatic bath;
the pulse laser source is used for respectively outputting two pulse lasers with pulse widths of M and N, and the chaotic laser source is used for outputting chaotic pulse lasers; the first optical switch is used for controlling and switching the output of the pulse laser source and the chaotic laser source; the second optical switch is used for switching the direction of the pulse laser which is incident into the sensing optical fiber;
the computer is used for calculating and obtaining strain information along the sensing optical fiber according to anti-Stokes light intensity generated by backward Raman scattering of the chaotic pulse laser in the sensing optical fiber and a corresponding reference chaotic pulse signal, and is also used for calculating and obtaining temperature information along the sensing optical fiber according to anti-Stokes light intensity generated by backward Raman scattering of two pulse lasers with different pulse widths in the sensing optical fiber.
The pulse laser source is a pulse laser, and the chaotic laser source comprises a chaotic laser, an isolator, a semiconductor optical amplifier and a pulse erbium-doped optical fiber amplifier;
the input end of the first optical switch is connected with the output ends of the chaotic laser and the pulse laser, the output ends of the first optical switch are respectively connected with the other input end of the second coupler and the input end of the isolator, and the output end of the isolator is sequentially connected with the semiconductor optical amplifier, the pulse erbium-doped optical fiber amplifier and the first coupler.
The ultra-long-distance high-spatial-resolution Raman optical fiber double-parameter sensing system further comprises a third coupler, a fourth coupler, a first semiconductor laser and a second semiconductor laser;
two output ends of the second optical switch are respectively connected with one input ends of the third coupler and the fourth coupler, output ends of the first semiconductor laser and the second semiconductor laser are respectively connected with the other input ends of the third coupler and the fourth coupler, and output ends of the third coupler and the fourth coupler are respectively connected with two ends of the sensing optical fiber.
The ultra-long-distance high-spatial-resolution Raman optical fiber double-parameter sensing system further comprises a signal amplifier, and the output end of the first photoelectric detector is connected with the data acquisition card through the signal amplifier.
The pulse width of the pulse laser is 100ns, the pulse width of the pulse laser is 100.01ns, and the repetition frequency is 1 kHz.
The first optical switch is a 2 x 2 optical switch, the second optical switch is a 1 x 2 optical switch, the first coupler is a 1 x 2 optical fiber coupler, the second coupler is a 2 x 1 optical fiber coupler, and the wavelength division multiplexer is a 1 x 2 wavelength division multiplexer.
The calculation formula of the temperature information along the sensing optical fiber is as follows:
wherein T represents the temperature of the sensing fiber, T0Showing the temperature of the thermostatic bath, Deltav is the Raman frequency shift, h is the Planckian constant, k is the Boltzmann constant,
representing the anti-stokes light intensity generated by forward pulses with pulse widths N and M, respectively;
representing the anti-stokes light intensity generated by backward pulses with pulse widths N and M, respectively;
forward pulses with pulse widths N and M, respectively, are provided in a reference fiber L0The intensity of the anti-stokes light generated at this point,
respectively represent pulse widths ofN and M backward pulses on a reference fiber L0The intensity of the anti-stokes light generated.
The method for calculating the strain information along the sensing optical fiber comprises the following steps:
firstly, calculating a chaos matching coefficient along a sensing optical fiber, wherein the calculation formula is as follows:
wherein,
the chaos matching coefficient at a position l in the sensing optical fiber is represented, T' represents the time length of backward anti-Stokes light generated by the received chaos pulse and a reference chaos pulse signal, Z (l + T) represents the backward anti-Stokes light intensity generated at the optical fiber l, T represents delay time, and X (T) represents the chaos pulse reference signal intensity;
and then, determining strain information along the sensing optical fiber according to the slope of the chaotic matching coefficient.
The invention also provides a sensing method of the ultra-long-distance high-spatial-resolution Raman optical fiber dual-parameter sensing system, which comprises a strain measurement step and a temperature measurement step, wherein the temperature measurement step comprises the following steps:
s101, controlling pulse laser with the pulse width of M output by a pulse laser source to be incident to a second coupler through a first optical switch, sequentially passing through a wavelength division multiplexer and then being incident to a sensing optical fiber, and receiving anti-Stokes light output from the sensing optical fiber by using a first photoelectric detector; then, changing a second optical switch to enable the next pulse to be injected from the other end of the sensing optical fiber, and receiving the anti-Stokes light output from the sensing optical fiber by using the first photoelectric detector again;
s102, changing the pulse width output by the pulse laser source to be N, and repeating the operation of the step S101;
the method for measuring the strain comprises the following steps: and the first optical switch controls the chaotic laser output by the chaotic laser source to be incident to the first coupler, and the first photoelectric detector and the second photoelectric detector are used for respectively receiving anti-Stokes optical signals and chaotic pulse signals generated in the sensing optical fiber.
In the step S101, the method further includes a step of controlling light emitted by the first semiconductor laser or the second semiconductor laser to enter the sensing fiber in the same direction as the pulse laser;
the method for measuring the strain further comprises the step of controlling light emitted by the first semiconductor laser or the second semiconductor laser and the chaotic pulse laser to enter the sensing optical fiber in the same direction.
Compared with the prior art, the invention has the following beneficial effects:
the invention provides an ultra-long-distance high-spatial-resolution Raman optical fiber dual-parameter sensing system and method. Meanwhile, the differential layered analysis technology is carried out on the Raman scattering signals under different pulse widths by gradually changing the pulse width entering the sensing optical fiber, and the limitation of the pulse width on the system spatial resolution in the traditional method is broken through. Because the bandwidth of the chaotic laser is far larger than that of the common semiconductor laser, and the resolution of the system is influenced by the full width at half maximum of the chaotic signal, the larger the bandwidth is, the smaller the full width at half maximum is, the higher the spatial resolution is, and when the bandwidth of the chaotic laser reaches 50GHz, the theoretical spatial resolution can reach millimeter magnitude.
In addition, the invention utilizes the demodulation principle of the loop double-end structure to extract the temperature of the sensing optical fiber, and the structure can eliminate the local external physical disturbance of the environmental change to the sensing optical fiber and improve the sensing distance. According to the invention, through the semiconductor lasers with the wavelength of 1350nm arranged at the two ends of the sensing optical fiber, the Raman anti-Stokes light amplification with the wavelength of 1450nm is realized by utilizing the stimulated Raman scattering effect of the optical fiber, the signal-to-noise ratio of the system is further improved, and thus the sensing distance of the system can break through 100 km. In addition, the invention also utilizes the loss characteristic of the chaotic Raman anti-Stokes light to realize the cooperative monitoring of temperature and strain.
Drawings
Fig. 1 is a schematic structural diagram of a distributed optical fiber raman sensing apparatus according to an embodiment of the present invention;
in the figure: 1-pulse laser source, 2-chaotic laser source, 3-first optical switch, 4-isolator, 5-semiconductor optical amplifier, 6-pulse erbium-doped optical fiber amplifier, 7-first coupler, 8-second coupler, 9-wavelength division multiplexer, 10-second optical switch, 11-first semiconductor laser, 12-second semiconductor laser, 13-third coupler, 14-fourth coupler, 15-sensing optical fiber, 16-first photoelectric detector, 17-low noise amplifier, 18-attenuator, 19-second photoelectric detector, 20-data acquisition card, 21-computer, 22-thermostatic bath.
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, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments; 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.
Example one
The embodiment of the invention provides an ultra-long-distance high-spatial-resolution Raman optical fiber dual-parameter sensing system, which is based on a measurement principle of combining differential pulse laser and chaotic laser modulation and can realize the measurement effects of long sensing distance, high spatial resolution and high measurement precision.
As shown in fig. 1, a super-long-distance raman optical fiber dual-parameter sensing system with high spatial resolution according to a first embodiment of the present invention includes a pulsed laser source 1, a chaotic laser source 2, a first optical switch 3, an isolator 4, a semiconductor optical amplifier 5, a pulsed erbium-doped optical fiber amplifier 6, a first coupler 7, a second coupler 8, a wavelength division multiplexer 9, a second optical switch 10, a first semiconductor laser 11, a second semiconductor laser 12, a third optical fiber coupler 13, a fourth optical fiber coupler 14, a sensing optical fiber 15, a first photodetector 16, a low-noise amplifier 17, an attenuator 18, a second photodetector 19, a data acquisition card 20, a computer 21, and a constant temperature bath 22.
Specifically, the output end of the pulse laser 1 is connected with the a port of the first optical switch 3; the output end of the chaotic laser 2 is connected with a port b of the first optical switch 3; the output end c of the first optical switch 3 is connected with the port a of the second coupler 8, and the output end d of the first optical switch 3 is connected with the input end of the isolator 4; the isolator 4 is connected with the semiconductor optical amplifier 5; the semiconductor optical amplifier 5 is connected with the pulse erbium-doped fiber amplifier 6; the pulse erbium-doped fiber amplifier 6 is connected with the port a of the first coupler 7; the b port of the first coupler 7 is connected with the b port of the second coupler 8; the port c of the second coupler 8 is connected with the port a of the wavelength division multiplexer 9; the b port of the wavelength division multiplexer 9 is connected with the a port of the second optical switch 10; the b port of the second optical switch 10 is connected with the a port of the coupler 13; the port b and the port c of the coupler 13 are respectively connected with the first semiconductor laser 11 and the sensing optical fiber 15; the c port of the second optical switch 10 is connected with the a port of the coupler 14; the port b and the port c of the coupler 14 are respectively connected with the second semiconductor laser 12 and the sensing optical fiber 15; the port c of the wavelength division multiplexer 9 is connected with the first photoelectric detector 16; the first photodetector 16 is connected with a low noise amplifier 17; the low noise amplifier 17 is connected with an acquisition card 20, and the acquisition card 20 is connected with a computer 21; the port c of the first coupler 7 is connected with the input end of the attenuator 18; the attenuator 18 is connected to the second photodetector 19; the second photoelectric detector 19 is connected with the data acquisition card 20; the acquisition card 20 is connected with a computer 21; the 1m length of the sensing fiber 15 is placed in the thermostat 22.
The pulse laser source is used for respectively outputting two pulse lasers with pulse widths of M and N, and the chaotic laser source is used for outputting chaotic pulse lasers; the first optical switch 3 is used for controlling and switching the output of the pulse laser source and the chaotic laser source; the second optical switch is used for switching the direction of the pulse laser incident into the sensing optical fiber 15; the first photodetector 16 is used for receiving the raman scattering anti-stokes light generated and output in the sensing fiber by the pulse laser and the chaotic laser. The second photodetector 19 is used for receiving the chaotic pulse reference laser.
The computer 21 is configured to calculate and obtain strain information along the sensing fiber 15 according to the anti-stokes light intensity generated by backward raman scattering of the chaotic pulse laser in the sensing fiber and a corresponding reference chaotic pulse signal, and is further configured to calculate and obtain temperature information along the sensing fiber 15 according to the anti-stokes light intensity generated by backward raman scattering of two pulse lasers with different pulse widths in the sensing fiber.
After continuous light with the center wavelength of 1350nm emitted by the first semiconductor laser 11 and the second semiconductor laser 12 enters the sensing optical fiber, a stimulated raman scattering effect is generated in the sensing optical fiber, raman scattering light (anti-stokes scattering light and stokes scattering light) generated in the sensing optical fiber by pulse laser generated by the pulse laser 1 and raman stokes scattering light and anti-stokes scattering light in the sensing optical fiber by chaotic pulse laser generated by the chaotic laser 2 after modulation both obtain modulation of the continuous raman scattering generated by the stimulated raman scattering, so that self signals are enhanced, the signal to noise ratio of the system can be improved, and 100km long-distance sensing is realized.
The first optical switch 3 is a 2 × 2 optical switch, the second optical switch 10 is a 1 × 2 optical switch, the first coupler 7 is a 1 × 2 optical fiber coupler, the second coupler 8 is a 2 × 1 optical fiber coupler, and the wavelength division multiplexer 9 is a 1 × 2 wavelength division multiplexer. The pulse laser 1 first emits a pulse laser having a center wavelength of 1550nm for emitting two kinds of pulse lasers different in pulse width. The wavelength of the continuous chaotic laser generated by the chaotic laser 2 is 1550 nm.
(1) Temperature measuring phase
In the temperature measuring stage, the pulse laser 1 emits pulse laser with the center wavelength of 1550nm and the pulse width of M being 100ns, and the repetition frequency is 1kHz, so that the sensing distance can be ensured to reach 100 km. The differential pulse laser is divided into two beams after reaching the wavelength division multiplexer 9 through the first optical switch 3 and the second coupler 8, wherein one beam of pulse laser signal enters the sensing optical fiber 15 through the second optical switch 10, the generated Raman backward scattering signal returns to the wavelength division multiplexer 9 through the second optical switch 10, the anti-Stokes light in the backward scattering light enters the first photoelectric detector 16, the signal is converted into an electric signal and amplified through the signal amplifier 17, the analog electric signal is converted into a digital electric signal through the data acquisition card 20, and finally the digital electric signal enters the computer 21 for storage. Then, the switching state of the second optical switch 10 is changed, and a next pulse having a pulse width M of 100ns is incident from the other end of the sensing fiber 15, and forward scattered light intensity data is obtained after the sensing fiber 15 passes through a path opposite to the above path. Next, the pulse laser width generated by the pulse laser 1 is changed to be N100.01 ns, and the foregoing steps are repeated to acquire intensity data of forward and backward scattered signals again. And demodulating the four acquired forward and backward scattering signals by a computer to acquire temperature information along the optical fiber.
In temperature demodulation, the laser pulse width is set as W, and the intensity of Raman anti-Stokes scattering signals (anti-Stokes) at the position of the sensing fiber L is set as follows:
where P is the incident power of the pulsed laser, KasExpressing the coefficients relating to the Raman anti-Stokes backscattering cross-section, S being the backscattering factor of the fibre, vasIs the frequency of the Raman anti-Stokes scattered signal, phieRepresenting the luminous flux of the pulsed light coupled into the fibre, alpha0、αasLoss coefficients of incident light and anti-Stokes light, respectively, per unit length in the sensing fiber, Ras(T) is the temperature modulation function of the anti-Stokes scattered light:
wherein, Deltav is Raman frequency shift, h is Planckian constant, k is Boltzmann constant, and T is sensing fiber temperature.
Actually, the backward raman scattered light generated by the pulsed laser is a continuous signal, and after analog-to-digital conversion by the data acquisition card, the continuous signal is converted into a plurality of discrete data points, and the temperature information contained in each data point corresponds to the length of the sensing fiber, and is:
where L' is the corresponding length in the fiber, c represents the speed at which light is transmitted in vacuum, t represents the pulse width, and n represents the fiber refractive index. Since the temperature information of the corresponding region is all superimposed on one data point, the temperature information in more detail in the range corresponding to this data point cannot be distinguished, which is also a main reason for limiting the spatial resolution. For the same position data obtained by different pulse widths, the length corresponding to 100ns is 10m, the length corresponding to 100.01ns is 10.001m, the two time sequence signals are subjected to differential processing, and then the signals corresponding to the optical fiber length range of 0.001m can be obtained, so that the spatial resolution performance of the system is improved. The pulse widths of pulses M and N are not fixed, and 100ns and 100.01ns are only for convenience of description. Specifically, in this embodiment, the pulse width is increased on the premise that the stimulated raman scattering is not generated, and the sensing distance can be increased by increasing the fiber-entering power. The difference between the pulse widths directly affects the spatial resolution, so that the pulse widths M and N can be as close as possible, that is, the difference can be as small as possible, in this embodiment, | M-N | takes a value of 0.01ns, and the millimeter-scale spatial resolution can be realized under the condition of meeting the software and hardware requirements of the existing system. In addition, the value range of the pulse widths M and N can be 50 ns-150 ns, and the effect of long sensing distance of the invention can be achieved.
When the pulse laser 1 emits a laser pulse with a pulse width M, the laser pulse is injected into the sensing fiber 15 through the second optical switch 10 in a certain state, and forward raman scattering light is generated. Then the high-speed data acquisition card 20 receives the forward raman anti-stokes scattered light at the position of the sensing fiber (the temperature and position along the sensing fiber are denoted by T and L respectively), and the light intensity can be expressed as:
the pulse laser 1 emits a laser pulse having a pulse width M, and injects the laser pulse into the sensor fiber 15 through the second optical switch 10 in a state different from the above state, that is, in a direction opposite to the above direction in which the pulse is injected into the sensor fiber, thereby generating backward raman scattered light. Then, the high-speed data acquisition card receives backward Raman anti-Stokes scattered light at the position of the sensing fiber (the temperature and the position along the sensing fiber are respectively represented by T and L), and the light intensity can be represented as (5):
similarly, the optical switch 10 is in two states, and forward and backward raman anti-stokes scattered lights generated when the pulse laser 1 emits a pulse with a pulse width N are respectively obtained, and the light intensities thereof are respectively expressed as:
when the second optical switch 10 is in the same state, the forward anti-stokes light generated by the pulse laser with the pulse width M and the pulse width N is differentiated to obtain the corresponding forward anti-stokes light intensity difference RFor(T, L), namely:
in the above-mentioned formula,
the light intensities of the forward anti-stokes lights respectively generated by the pulse lasers respectively representing the pulse widths M and N.
Similarly, when the second optical switch 10 is in another state, the pulse with the pulse width M is differentiated from the backward anti-stokes light generated by the pulse laser with the pulse width N to obtain the corresponding backward anti-stokes light intensity difference RBack(T, L), namely:
in the above-mentioned formula,
the light intensities of backward anti-stokes lights respectively generated by the pulse lasers respectively representing the pulse widths M and N.
Because the optical signal generates loss in the transmission process of the optical fiber, the backscattering signal generated at different positions of the sensing optical fiber is related to attenuation, and in order to reduce the measurement error, the invention adopts the distributed temperature sensing system with the double-end structure, and the double-end structure with self-demodulation can demodulate the temperature along the sensing optical fiber. In the double-end distributed temperature sensing system, a section of reference optical fiber is arranged at the front end of the sensing optical fiber, and can generate reference temperature. The temperature coefficient under the double-end structure can be obtained by the geometric mean of the front-back and backward anti-stokes light intensity difference of the normalized single-end system, and the formula is as follows:
wherein R isLoop(T, L) represents a temperature coefficient in the sensing fiber, and therefore, the attenuation coefficient thereof is converted into a constant related to the length of the sensing fiber by the temperature coefficient obtained by the above geometric averaging, and does not change when the state of the sensing fiber does not change. Similarly, in the constant temperature bath 22Temperature of T0The sensing optical fiber of (2) has a corresponding temperature coefficient of:
wherein,
respectively representing the light intensity of backward anti-stokes light generated at the reference fiber by the pulse laser with the pulse width M and the pulse width N.
Respectively representing the light intensity of forward anti-stokes light generated at the reference fiber by the pulse laser with the pulse width of M and the pulse width of N.
By comparing the formula (10) with the formula (11), the attenuation coefficient can be eliminated, and finally the temperature demodulation formula is obtained:
wherein T represents the temperature of the sensing fiber, T0The temperature of the thermostatic bath is represented, Deltav is Raman frequency shift, h is Planckian constant, k is Boltzmann constant, and the temperature value of each point along the sensing optical fiber can be obtained through a formula (12).
(2) Strain measurement phase
In the strain measurement stage, the switching state of the first optical switch 3 is changed, so that the continuous chaotic laser generated by the chaotic laser 2 enters the semiconductor optical amplifier 5 through the isolator 4 to be modulated into chaotic pulse laser, the pulse width of the chaotic pulse laser is 10ns, and the repetition frequency is 1kHz, so that the sensing distance of 100km is realized. Then the chaotic pulse laser is amplified by a pulse erbium-doped fiber amplifier 6 and is divided into 1: and 99, the chaotic pulse laser with strong energy is probe light, the probe light enters the sensing optical fiber 15 after passing through the second coupler 8, the wavelength division multiplexer 9 and the second optical switch 10, and anti-stokes light data generated by backward Raman scattering is obtained. Demodulating the acquired anti-stokes light intensity generated by backward Raman scattering and the acquired reference chaotic pulse signal to obtain strain information along the sensing fiber 15.
Strain demodulation principle: the coefficient can carry out chaotic time sequence matching operation on backward Stokes light generated by the obtained chaotic pulse and a chaotic pulse reference signal, and the position of the sensing optical fiber subjected to strain and the additional loss coefficient caused by the strain are obtained through the correlation of the chaotic time sequence matching operation. The chaos matched filtering formula is as follows:
in the formula,
the chaotic matching coefficient of the sensing optical fiber l is represented, T' represents the time length of backward anti-Stokes light generated by received chaotic pulses and a reference chaotic pulse signal, Z (l + T) represents the backward anti-Stokes light intensity generated at the optical fiber l, T represents delay time, and X (T) represents a chaotic pulse reference signal.
Chaotic time sequence matching operation is carried out on the chaotic pulse reference signal and the anti-Stokes optical signal, and chaotic matching coefficients
The image is a straight line with the slope as the loss coefficient, and the loss coefficient of the anti-Stokes signal from the unstrained area of the sensing optical fiber is alpha0The loss coefficient of the anti-Stokes signal from the strained region is alpha1＝α0+ Δ α, where Δ α is the strain induced parasitic loss. The position of the sensing optical fiber subjected to strain and the additional loss value delta alpha caused by the strain can be obtained through the chaotic matching coefficient. And the additional loss of the sensing fiber is in a positive linear relationship with the stress strain to which the fiber is subjected. Based on the method, the strain information along the optical fiber can be demodulated.
Example two
The second embodiment of the invention provides a sensing method of the ultra-long-distance high-spatial-resolution Raman optical fiber dual-parameter sensing system,
the method comprises the steps of strain measurement and temperature measurement, wherein the step of temperature measurement comprises the following steps:
s101, controlling pulse laser with the pulse width M output by a pulse laser source to be incident to a second coupler 8 through a first optical switch 3, and to be incident to a sensing optical fiber after sequentially passing through a wavelength division multiplexer 9, and receiving anti-Stokes light output from the sensing optical fiber by using a first photoelectric detector 16; then, the second optical switch 10 is changed to make the next pulse enter from the other end of the sensing fiber, and the first photodetector 16 is used again to receive the anti-stokes light output from the sensing fiber;
s102, changing the pulse width output by the pulse laser source 1 to be N, and repeating the operation of the step S101;
the method for measuring the strain comprises the following steps: the chaotic laser output by the chaotic laser source is controlled to be incident to the first coupler 7 through the first optical switch 3, and the anti-stokes optical signal and the chaotic pulse signal generated in the sensing optical fiber are respectively received by the first photoelectric detector 16 and the second photoelectric detector 19.
After the temperature measurement and the strain measurement are finished, the data are collected and processed through a data collection card and a computer, and the temperature and the strain information along the sensing optical fiber can be obtained through demodulation.
Further, the step S101 further includes a step of controlling light emitted by the first semiconductor laser 14 or the second semiconductor laser 15 to enter the sensing fiber in the same direction as the pulse laser;
the method for measuring the strain further comprises the step of controlling the light emitted by the first semiconductor laser 14 or the second semiconductor laser 15 to enter the sensing optical fiber in the same direction as the chaotic pulse laser.
In summary, the invention provides an ultra-long-distance high spatial resolution raman optical fiber dual-parameter sensing system and method, after continuous chaotic laser generated by a chaotic laser is subjected to pulse modulation through a semiconductor optical amplifier, a chaotic pulse signal and a processed raman scattering signal with chaotic laser characteristics after raman scattering is generated along a sensing optical fiber are subjected to cross-correlation processing with the chaotic pulse laser signal, so as to obtain strain information along the optical fiber. Meanwhile, the differential layered analysis technology is carried out on the Raman scattering signals under different pulse widths by gradually changing the pulse width entering the sensing optical fiber, and the limitation of the pulse width on the system spatial resolution in the traditional method is broken through. Because the bandwidth of the chaotic laser is far larger than that of the common semiconductor laser, and the resolution of the system is influenced by the full width at half maximum of the chaotic signal, the larger the bandwidth is, the smaller the full width at half maximum is, the higher the spatial resolution is, and when the bandwidth of the chaotic laser reaches 50GHz, the theoretical spatial resolution can reach millimeter magnitude.
Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.
Claims (10)
1. A super-long-distance high-spatial-resolution Raman optical fiber double-parameter sensing system is characterized by comprising a pulse laser source, a chaotic laser source, a first optical switch (3), a first coupler (7), a second coupler (8), a wavelength division multiplexer (9), a second optical switch (10), a sensing optical fiber (15), a first photoelectric detector (16), a second photoelectric detector (19), a data acquisition card (20) and a computer (21);
the output end of the chaotic laser source is connected with the input end of the first coupler (7), the output end of the first coupler (7) is respectively connected with one input end of the second coupler (8) and the second photoelectric detector (19), the output end of the pulse laser source is connected with the other input end of the second coupler (8), the output end of the second coupler (8) is connected with the first port of the wavelength division multiplexer (9), the second port of the wavelength division multiplexer (9) is connected with the input end of the second optical switch (10), the two output ends of the second optical switch (10) are respectively connected with the two ends of the sensing optical fiber (15), and the third port of the wavelength division multiplexer (9) is connected with the first photoelectric detector (16);
the output ends of the first photoelectric detector (16) and the second photoelectric detector (19) are connected with a data acquisition card (20), and the output end of the data acquisition card (20) is connected with a computer (21); one end of the sensing optical fiber (15) is arranged in the thermostatic bath (24);
the pulse laser source is used for respectively outputting two pulse lasers with pulse widths of M and N, and the chaotic laser source is used for outputting chaotic pulse lasers; the first optical switch (3) is used for controlling and switching the output of the pulse laser source and the chaotic laser source; the second optical switch is used for switching the direction of the pulse laser which is incident into the sensing optical fiber (15);
the computer (21) is used for calculating and obtaining strain information along the sensing optical fiber (15) according to anti-Stokes light intensity generated by backward Raman scattering of the chaotic pulse laser in the sensing optical fiber and a corresponding reference chaotic pulse signal, and is also used for calculating and obtaining temperature information along the sensing optical fiber (15) according to anti-Stokes light intensity generated by backward Raman scattering of two pulse lasers with different pulse widths in the sensing optical fiber.
2. The ultra-long-distance high-spatial-resolution Raman fiber dual-parameter sensing system according to claim 1, wherein the pulsed laser source is a pulsed laser, and the chaotic laser source comprises a chaotic laser (2), an isolator (4), a semiconductor optical amplifier (5) and a pulsed erbium-doped fiber amplifier (6);
the input end of the first optical switch (3) is connected with the output ends of the chaotic laser (2) and the pulse laser, the output ends of the first optical switch and the pulse laser are respectively connected with the other input end of the second coupler (8) and the input end of the isolator (4), and the output end of the isolator (4) is sequentially connected with the semiconductor optical amplifier (5), the pulse erbium-doped optical fiber amplifier (6) and the first coupler (7).
3. An ultra-long-distance high spatial resolution raman optical fiber dual parameter sensing system according to claim 1 further comprising a third coupler (13), a fourth coupler (14), a first semiconductor laser (11) and a second semiconductor laser (12);
two output ends of the second optical switch (10) are respectively connected with one input ends of a third coupler (11) and a fourth coupler (12), output ends of the first semiconductor laser (11) and the second semiconductor laser (12) are respectively connected with the other input ends of the third coupler (13) and the fourth coupler (14), and output ends of the third coupler (13) and the fourth coupler (14) are respectively connected with two ends of a sensing optical fiber (15).
4. An ultra-long-distance high spatial resolution raman optical fiber dual parameter sensing system according to claim 1, further comprising signal amplifiers (17), wherein the output terminals of said first photodetectors (16) are respectively connected to the data acquisition card (20) through the signal amplifiers (17).
5. An ultra-long reach high spatial resolution raman optical fiber dual parameter sensing system according to claim 1 wherein said pulsed laser has a pulse width of M =100ns, N =100.01ns, and a repetition rate of 1 kHz.
6. An ultra-long-reach high spatial resolution raman optical fiber dual parameter sensing system according to claim 1 wherein said first optical switch (3) is a 2 x 2 optical switch, said second optical switch (10) is a 1 x 2 optical switch, said first coupler (7) is a 1 x 2 optical fiber coupler, said second coupler (8) is a 2 x 1 optical fiber coupler, and said wavelength division multiplexer (9) is a 1 x 2 wavelength division multiplexer.
7. The system of claim 1, wherein the temperature information along the sensing fiber (15) is calculated by the following formula:
wherein T represents the temperature of the sensing fiber, T0Which indicates the temperature of the thermostatic bath,Δνin order to be the raman shift frequency,his the constant of the planck, and is,kis the boltzmann constant and is,
representing the anti-stokes light intensity generated by forward pulses with pulse widths N and M, respectively;
representing the anti-stokes light intensity generated by backward pulses with pulse widths N and M, respectively;
forward pulses with pulse widths N and M, respectively, are provided in a reference fiber L0The intensity of the anti-stokes light generated at this point,
backward pulses with pulse widths N and M, respectively, are provided in a reference fiber L0The intensity of the anti-stokes light generated.
8. The system of claim 1, wherein the strain information along the sensing fiber (15) is calculated by a method comprising the following steps:
firstly, calculating a chaos matching coefficient along a sensing optical fiber, wherein the calculation formula is as follows:
wherein,
indicating position in sensing fiberlT' represents the time length of backward anti-Stokes light generated by the received chaotic pulse and a reference chaotic pulse signal,Z（l+t) Is shown in the optical fiberlWhere the resulting backward anti-stokes light intensity, t represents the delay time,X(t) represents the chaotic pulse reference signal strength;
and then, determining strain information along the sensing optical fiber according to the slope of the chaotic matching coefficient.
9. The sensing method of the ultra-long-distance high-spatial-resolution Raman optical fiber dual-parameter sensing system according to claim 1, comprising a strain measurement step and a temperature measurement step, wherein the temperature measurement step comprises:
s101, controlling pulse laser with the pulse width M output by a pulse laser source to be incident to a second coupler (8) through a first optical switch (3), and incident to a sensing optical fiber after sequentially passing through a wavelength division multiplexer (9), and receiving anti-Stokes light output from the sensing optical fiber by using a first photoelectric detector (16); then, changing a second optical switch (10), enabling the next pulse to enter from the other end of the sensing optical fiber, and receiving the anti-Stokes light output from the sensing optical fiber by using a first photoelectric detector (16) again;
s102, changing the pulse width output by the pulse laser source (1) to be N, and repeating the operation of the step S101;
the method for measuring the strain comprises the following steps: the chaotic laser output by the chaotic laser source is controlled to be incident to the first coupler (7) through the first optical switch (3), and the anti-Stokes optical signal and the chaotic pulse signal generated in the sensing optical fiber are respectively received by the first photoelectric detector (16) and the second photoelectric detector (19).
10. The sensing method of an ultra-long-distance high spatial resolution raman optical fiber dual-parameter sensing system according to claim 9, wherein the step S101 further comprises the step of controlling the light emitted by the first semiconductor laser (14) or the second semiconductor laser (15) to enter the sensing optical fiber in the same direction as the pulse laser;
the method for measuring the strain further comprises the step of controlling light emitted by the first semiconductor laser (14) or the second semiconductor laser (15) to enter the sensing optical fiber in the same direction as the chaotic pulse laser.
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