Distributed optical fiber vibration sensing method and system based on frequency shift loop

Abstract

The present invention discloses a distributed optical fiber vibration sensing method and system based on a frequency shift loop, which relates to the field of optical fiber sensing technology. Light pulses are respectively input into two frequency shift loops for frequency shifting n times, and a time interval t is generated between the two frequency-shifted light pulses. The optical wave frequency and optical frequency difference of the light pulses are flexibly controlled through the frequency shift loops, while the diversity of the detection pulse frequency is improved, and the influence of the nonlinear effect is reduced. In the distributed optical fiber vibration sensing system based on the frequency shift loop, multiple pairs of detection pulses with different frequency shift times, that is, detection pulses with different optical pulse frequencies, are formed by using two frequency shift loops. Injecting into the sensing optical fiber can suppress interference fading, improve the system sampling rate, enhance the system performance, and reduce the system noise.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber sensing, and particularly to a distributed optical fiber vibration sensing method and system based on a frequency shift loop. Background Art

[0002] Distributed optical fiber sensing technology has great application potential in multiple fields, such as structural health monitoring, temperature monitoring, oil and gas exploration, etc. Distributed optical fiber sensing technology has the advantages of continuous distributed detection, long detection distance, accurate positioning, rich measurement information, intrinsic safety, low cost, etc., and has been widely used in fields such as power, petroleum, bridges, tunnels, slopes, etc.

[0003] Existing distributed optical fiber vibration sensing systems based on dual-pulse detection mainly generate dual pulses with different frequencies in a two-way direct modulation manner. When the two-way pulses of the distributed optical fiber vibration sensing system are directly modulated, the optical wave frequencies and optical frequency differences of the two-way detection pulses are determined, and the optical wave frequencies and optical frequency differences cannot be adjusted in time during the generation of the sensing signal, resulting in poor flexibility of the distributed optical fiber vibration sensing system. Summary of the Invention

[0004] The technical problem to be solved by the present invention is that in a traditional distributed optical fiber vibration sensing system based on dual-pulse detection, since the optical wave frequencies and optical frequency differences of the detection pulses are determined, the optical wave frequencies and optical frequency differences cannot be adjusted in time during the generation of the sensing signal, resulting in poor flexibility of the distributed optical fiber vibration sensing system. The purpose of the present invention is to provide a distributed optical fiber vibration sensing method and system based on a frequency shift loop, so that optical pulses enter two frequency shift loops respectively for n times of frequency shift, and a time interval t is generated between the two frequency-shifted optical pulses. The optical wave frequencies and optical frequency differences of the optical pulses are flexibly controlled through the frequency shift loop, while the diversity of the detection pulse frequencies is improved, and the influence of the nonlinear effect is reduced; the above technical problem is solved.

[0005] The present invention is achieved through the following technical solutions:

[0006] This solution provides a distributed optical fiber vibration sensing method based on a frequency shift loop, which is characterized by including the steps of:

[0007] Modulating the continuous light emitted by the laser into optical pulses;

[0008] Making the optical pulses enter two frequency shift loops respectively for n times of frequency shift, and generating a time interval t between the two frequency-shifted optical pulses;

[0009] Forming an optical pulse pair with two optical pulses having a time interval of t;

[0010] Injecting the optical pulse pair into the sensing optical fiber and generating a sensing signal in the sensing optical fiber.

[0011] Working principle of this solution: In a traditional distributed fiber optic vibration sensing system based on dual-pulse detection, since the optical wave frequency and optical frequency difference of the detection pulse are determined, the optical wave frequency and optical frequency difference cannot be adjusted in a timely manner during the generation of the sensing signal, resulting in poor flexibility of the distributed fiber optic vibration sensing system. The purpose of the present invention is to provide a distributed fiber optic vibration sensing method and system based on a frequency shift loop, enabling optical pulses to enter two frequency shift loops respectively for n frequency shifts, and causing a time interval t to occur between the two frequency-shifted optical pulses. The optical wave frequency and optical frequency difference of the optical pulses are flexibly controlled through the frequency shift loop, while enhancing the diversity of the detection pulse frequency and reducing the influence of the nonlinear effect; the technical problem that the optical wave frequency and optical frequency difference of the two detection pulses cannot be regulated is solved.

[0012] A further optimized solution is that the optical pulses have different single-frequency shift frequencies in the two frequency shift loops.

[0013] This solution also provides a distributed fiber optic vibration sensing system based on a frequency shift loop for implementing the method described in the above solution, including: a laser, a pulse modulator, a splitter, a frequency shift loop, a first coupler, and a sensing optical fiber;

[0014] The laser is used to emit continuous light;

[0015] The pulse modulator is used to modulate the continuous light into an optical pulse with a pulse width of τ 1 , and a frequency of f 0 ;

[0016] The splitter is used to divide the optical pulse into two paths and input them into the two frequency shift loops respectively;

[0017] The frequency shift loop is used to perform n frequency shifts on the optical pulse to obtain two optical pulses with a time interval of t;

[0018] The first coupler is used to form an optical pulse pair from the two optical pulses with a time interval of t;

[0019] The sensing optical fiber is used to inject the optical pulse pair, and the optical pulse pair generates a sensing signal in the sensing optical fiber.

[0020] A further optimized solution is that the frequency shift loop includes: a frequency shifter, a coupler, and an EDFA; the frequency shifter, the coupler, and the EDFA form a loop;

[0021] The optical pulse enters the loop through the coupler, is frequency-shifted by the frequency shifter, and then amplified by the EDFA and returned to the coupler.

[0022] The EDFA is an optical fiber amplifier used to amplify the optical pulse coming out of the frequency shifter.

[0023] A further optimized solution is that it further includes an optical switch, a delay optical fiber, a circulator, and detection and demodulation;

[0024] An optical switch is installed between each frequency shift loop and the first coupler;

[0025] The delay optical fiber is installed between the first coupler and any one of the frequency shift loops;

[0026] The optical pulse pair is injected into the sensing optical fiber through the circulator, and the sensing signal generated by the optical pulse pair in the sensing optical fiber returns to the detection and demodulation module through the circulator. The detection and demodulation module demodulates the vibration information according to the optical pulse.

[0027] A further optimized solution is that the frequency shifters in the two frequency shift loops have different single - frequency shift frequencies.

[0028] A further optimized solution is that the dimension of the time interval t is in the order of nanoseconds.

[0029] A further optimized solution is that by changing the length of the delay optical fiber, the time interval t between the two optical pulses is changed;

[0030] By changing the opening time of the optical switch, the time interval t between the two optical pulses is changed.

[0031] A further optimized solution is that it further includes a signal generator, and the multi - channel signal generator controls the pulse width of the pulse modulator, the number of frequency shifts of the frequency shifter, and the state of the optical switch in parallel respectively.

[0032] A further optimized solution is that before the frequency shift loop completes n frequency shifts of the optical pulse, the optical switch is in the open state. By controlling the optical switch, single - pulse multiple frequency shifts are realized, a large frequency shift is obtained, and the performance requirements for the frequency shift device are reduced.

[0033] If multiple pulse pairs with different numbers of frequency shifts are to be generated, within the pulse τ 2 duration of controlling the frequency shifter, only one initial optical pulse (with a frequency of f 0 ) enters the frequency shift loop, and the optical switch is in the open state;

[0034] If a pair of detection pulses with a large frequency difference is to be generated, within the pulse τ 2 duration of controlling the frequency shifter, only one initial optical pulse (with a frequency of f 0 ) enters the frequency shift loop, and the optical switch is opened after the frequency shift is completed.

[0035] When the sensing signals generated by injecting multiple pulse pairs with different numbers of frequency shifts into the sensing optical fiber are detected by the demodulation module, frequency - division signal processing is performed in the electrical domain; when the sensing signal generated by injecting a pair of detection pulses with a large frequency difference into the sensing optical fiber, frequency - division signal processing can be performed using an optical filter.

[0036] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0037] The distributed optical fiber vibration sensing method based on a frequency shift loop provided by the present invention enables optical pulses to enter two frequency shift loops respectively for frequency shifting n times, and makes the optical pulses after frequency shifting in the two paths have a time interval t. The optical wave frequency and optical frequency difference of the optical pulses are flexibly controlled through the frequency shift loop, while enhancing the diversity of the detection pulse frequency and reducing the influence of non-linear effects. In the distributed optical fiber vibration sensing system based on a frequency shift loop, multiple pairs of detection pulses with different frequency shift times, that is, detection pulses with different optical pulse frequencies, are formed by using two frequency shift loops. Injecting into the sensing optical fiber can suppress interference fading, improve the system sampling rate, enhance the system performance, and reduce the system noise. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following will briefly introduce the drawings required in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts. In the drawings:

[0039] Figure 1 is a schematic flow chart of a distributed optical fiber vibration sensing method based on a frequency shift loop;

[0040] Figure 2 is a schematic structural diagram of a distributed optical fiber vibration sensing system based on a frequency shift loop;

[0041] Figure 3 is a schematic diagram of multiple pulse pairs generated by the frequency shift loop in Embodiment 3;

[0042] Figure 4 is a schematic diagram of a pulse pair generated by the frequency shift loop in Embodiment 4. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and the drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0044] The existing distributed optical fiber vibration sensing system based on dual-pulse detection mainly generates dual pulses with different frequencies in a two-way direct modulation manner. When the two-way pulses of the distributed optical fiber vibration sensing system are directly modulated, the optical wave frequency and optical frequency difference of the two-way detection pulses are determined, and the optical wave frequency and optical frequency difference cannot be adjusted in time during the generation of the sensing signal, resulting in poor flexibility of the distributed optical fiber vibration sensing system.

[0045] In view of this, the following embodiments are provided to solve the above technical problems:

[0046] Embodiment 1

[0047] This embodiment provides a distributed optical fiber vibration sensing method based on a frequency shift loop, as Figure 1 shown, including the steps of:

[0048] Modulating the continuous light emitted by the laser into optical pulses;

[0049] Making the optical pulses enter two frequency shift loops respectively for n times of frequency shift, and making the optical pulses after frequency shift in the two paths generate a time interval t; the frequencies of the pulse pairs generated by the nth frequency shift are f 0 +nf 1 and f 0 +nf 2 respectively;

[0050] Forming an optical pulse pair with the optical pulses in the two paths having a time interval of t;

[0051] Injecting the optical pulse pair into the sensing optical fiber and generating a sensing signal in the sensing optical fiber.

[0052] The optical pulses have different single - frequency shift frequencies in the two frequency shift loops.

[0053] Through the frequency shift loop, the optical wave frequency and the optical frequency difference of the double pulses can be flexibly controlled. At the same time, the diversity of the detection pulse frequency can be improved, and the influence of the nonlinear effect can be reduced.

[0054] Embodiment 2

[0055] This embodiment provides a distributed optical fiber vibration sensing system based on a frequency shift loop for implementing the method described in the previous embodiment, as Figure 2 shown, including: a laser, a pulse modulator, a splitter, a frequency shift loop, a first coupler, and a sensing optical fiber;

[0056] The laser is used to emit continuous light;

[0057] The pulse modulator is used to modulate the continuous light into optical pulses with a pulse width of τ 1 and a frequency of f 0 ;

[0058] The splitter is used to divide the optical pulses into two paths and input them into the two frequency shift loops respectively;

[0059] The frequency shift loop is used to perform n times of frequency shift on the optical pulses to obtain two optical pulses with a time interval of t; the frequencies of the pulse pairs generated by the nth frequency shift are f 0 +nf 1 and f 0 +nf 2 respectively;

[0060] The first coupler is used to form an optical pulse pair from two optical pulses with a time interval of t.

[0061] The sensing optical fiber is used to inject the optical pulse pair, and the optical pulse pair generates a sensing signal in the sensing optical fiber.

[0062] The frequency shift loop includes: a frequency shifter, a coupler, and an EDFA; the frequency shifter, the coupler, and the EDFA form a loop.

[0063] The optical pulse enters the loop through the coupler, is frequency-shifted by the frequency shifter, amplified by the EDFA, and then returns to the coupler.

[0064] It also includes an optical switch, a delay optical fiber, a circulator, and a detection and demodulation unit.

[0065] An optical switch is installed between each frequency shift loop and the first coupler.

[0066] The delay optical fiber is installed between the first coupler and any one of the frequency shift loops.

[0067] The optical wave frequency and the optical frequency difference of the double pulses can be flexibly controlled through the frequency shift loop. At the same time, the diversity of the detection pulse frequency can be improved, and the influence of the nonlinear effect can be reduced. By controlling the optical switch, single-pulse multiple frequency shifts can be realized, a large frequency shift can be obtained, and the performance requirements for the frequency shift device can be reduced.

[0068] The optical pulse pair is injected into the sensing optical fiber through the circulator, and the sensing signal generated by the optical pulse pair in the sensing optical fiber returns to the detection and demodulation module through the circulator. The detection and demodulation module demodulates the vibration information according to the optical pulse.

[0069] The frequency shifters in the two frequency shift loops have different single-frequency shift frequencies. In the figure, the single-frequency shift frequency of frequency shifter 1 is f 1 , and the single-frequency shift frequency of frequency shifter 2 is f 2 .

[0070] The dimension of the time interval t is on the order of nanoseconds.

[0071] By changing the length of the delay optical fiber, the time interval t between the two optical pulses is changed.

[0072] By changing the opening time of the optical switch, the time interval t between the two optical pulses is changed. The time interval between the two pulses is controlled by controlling the length of the delay optical fiber after the frequency shift loop or by controlling the opening time of the optical switch through the synchronous control circuit.

[0073] It also includes a signal generator. The multi-channel signal generator controls the pulse width of the pulse modulator, the number of frequency shifts of the frequency shifter, and the state of the optical switch in parallel.

[0074] The state in which the optical switch is in the open state before the optical frequency shift loop performs n optical frequency shifts on the optical pulse.

[0075] Embodiment 3

[0076] Based on the previous embodiment, in the distributed optical fiber vibration sensing system based on the optical frequency shift loop of this embodiment, multiple pulse pairs with different optical frequency shift times need to be generated. During the pulse τ 2 duration, only one initial optical pulse (with a frequency of f 0 ) enters the optical frequency shift loop, and the optical switch is in the open state;

[0077] Specifically, the following steps are executed:

[0078] S1. The continuous light emitted by the laser is modulated into an optical pulse (pulse width is τ 1 , frequency is f 0 );

[0079] S2. The optical pulse enters two optical frequency shift optical paths through a splitter for optical frequency shift. The single optical frequency shift frequency of one optical frequency shifter is f 1 , and the single optical frequency shift frequency of the other optical frequency shifter is f 2 , and the optical switch is in the open state. The multiple optical pulse pairs generated by the optical frequency shift are as shown in Figure 3 . The frequencies of the pulse pairs generated by the first optical frequency shift are f 0 + f 1 , f 0 + f 2 ; The frequencies of the pulse pairs generated by the second optical frequency shift are f 0 + 2f 1 , f 0 + 2f 2 ... The frequencies of the pulse pairs generated by the nth optical frequency shift are f 0 + nf 1 , f 0 + nf 2 .

[0080] S3. The two optical frequency shifted optical pulses are injected into the sensing optical fiber as optical pulse pairs at a certain time interval through a coupler and a circulator. The frequencies of the pulse pairs generated by the nth optical frequency shift are f 0 + nf 1 , f 0 + nf 2 , and the optical pulse pairs generate sensing signals in the sensing optical fiber;

[0081] S4. The sensing signals return to the detection and demodulation module, and the external vibration information is demodulated by the detection and demodulation module.

[0082] Embodiment 4

[0083] Based on Embodiment 2, the distributed optical fiber vibration sensing system in this embodiment is based on a frequency shift loop. To generate a pair of detection pulses with a large frequency difference, during the pulse τ 2 duration of the frequency shifter control, only one initial optical pulse (with a frequency of f 0 ) enters the frequency shift loop, and the optical switch is opened after the frequency shift is completed.

[0084] Specifically, the following steps are executed:

[0085] S1. The continuous light emitted by the laser is modulated into an optical pulse (with a pulse width of τ 1 , and a frequency of f 0 );

[0086] S2. The optical pulse enters two frequency shift optical paths through a splitter for frequency shift. The single - frequency shift of one frequency shifter is f 1 , and the single - frequency shift of the other frequency shifter is f 2 ;

[0087] S3. The optical switch is initially in the closed state. After n frequency shifts, the optical switch is opened, generating a pair of pulses with frequencies of f 0 +nf 1 and f 0 +nf 2 respectively, which are injected into the sensing optical fiber as shown in Figure 4 , and a sensing signal is generated in the sensing optical fiber;

[0088] S4. The sensing signal returns to the detection and demodulation module, and the external vibration information is demodulated by the detection and demodulation module.

[0089] In the above embodiments, the light emitted by the laser is modulated into an optical pulse and then divided into two paths to enter the frequency shift loop for frequency shift. The two frequency - shifted optical pulses form a pulse pair one after the other and are injected into the sensing optical fiber through a circulator, generating a sensing signal that returns to the distributed optical fiber vibration sensing system and the vibration signal is demodulated. When the optical switch in the frequency shift loop is in the normally open state, multiple pulse pairs with different frequency shift times formed by the frequency shift loop are injected into the sensing optical fiber. When the optical switch in the frequency shift loop is controlled by a signal generator, two frequency shift loops can form a pair of detection pulses with a large frequency difference (during the frequency shift process, the optical switch is in the closed state, and the optical switch is opened after the frequency shift is completed for output) and injected into the sensing optical fiber. Injecting multiple pulse pairs with different frequency shift times into the sensing optical fiber can suppress interference fading, improve the system sampling rate, and enhance the system performance.

[0090] Throughout the specification, references to "one embodiment", "an embodiment", "an example" or "an example" mean that a particular feature, structure, or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present utility model. Thus, the phrases "one embodiment", "an embodiment", "an example" or "an example" that appear throughout the specification do not necessarily all refer to the same embodiment or example. In addition, the specific features, structures, or characteristics may be combined in any suitable combination and / or sub-combination in one or more embodiments or examples. Further, those of ordinary skill in the art should understand that the diagrams provided herein are for illustrative purposes only and are not necessarily drawn to scale. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0091] The specific embodiments described above further elaborate on the object, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only the specific embodiments of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A distributed optical fiber vibration sensing method based on a frequency shift loop, characterized in that, it includes the steps of: modulating the continuous light emitted by a laser into optical pulses; making the optical pulses enter two frequency shift loops respectively for n times of frequency shift, and making the two frequency-shifted optical pulses generate a time interval t; forming an optical pulse pair with the two optical pulses with a time interval of t; injecting the optical pulse pair into a sensing optical fiber and generating a sensing signal in the sensing optical fiber; the optical pulses have different single frequency shift frequencies in the two frequency shift loops; A system for implementing the distributed optical fiber vibration sensing method based on a frequency shift loop includes: a laser, a pulse modulator, a splitter, a frequency shift loop, a first coupler and a sensing optical fiber; the laser is used to emit continuous light; The pulse modulator is used to modulate continuous light into optical pulses with a pulse width of , and a frequency of ; the splitter is used to divide the optical pulses into two paths and input them into the two frequency shift loops respectively; the frequency shift loop is used to perform n times of frequency shift on the optical pulses to obtain two optical pulses with a time interval of t; the first coupler is used to form an optical pulse pair with the two optical pulses with a time interval of t; the sensing optical fiber is used to inject the optical pulse pair, and the optical pulse pair generates a sensing signal in the sensing optical fiber; It also includes an optical switch, a delay optical fiber, a circulator and a detection and demodulation; an optical switch is installed between each frequency shift loop and the first coupler; the delay optical fiber is installed between the first coupler and any one of the frequency shift loops; the optical pulse pair is injected into the sensing optical fiber through the circulator, and the sensing signal generated by the optical pulse pair in the sensing optical fiber returns to the detection and demodulation module through the circulator, and the detection and demodulation module demodulates the vibration information according to the optical pulse; During the pulse duration for controlling the frequency shifter, an initial optical pulse with a frequency of is made to enter the frequency shift loop, and the optical switch is in the open state to generate multiple pairs of optical pulses with different numbers of frequency shifts. During the pulse duration of controlling the frequency shifter, an initial optical pulse with a frequency of is made to enter the frequency shift loop, and the optical switch is turned on after the frequency shift is completed to generate an optical pulse pair with a relatively large frequency difference; by changing the length of the delay optical fiber, the time interval t of the two optical pulses is changed; by changing the opening time of the optical switch, the time interval t of the two optical pulses is changed.

2. The distributed optical fiber vibration sensing method based on a frequency shift loop according to claim 1, characterized in that, the frequency shift loop includes: a frequency shifter, a coupler and an EDFA; the frequency shifter, the coupler and the EDFA form a loop; the optical pulse enters the loop through the coupler, is frequency-shifted by the frequency shifter and then amplified by the EDFA and returns to the coupler.

3. The distributed optical fiber vibration sensing method based on a frequency shift loop according to claim 1, characterized in that, the dimension of the time interval t is in the order of nanoseconds.

4. The distributed optical fiber vibration sensing method based on a frequency shift loop according to claim 1, characterized in that, it also includes a signal generator, and the multi-channel signal generator controls the pulse width of the pulse modulator, the frequency shift times of the frequency shifter and the state of the optical switch in parallel respectively.

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