Multi-wavelength distributed fiber sensing system based on tunable narrow linewidth laser

Abstract

The application provides a multi-wavelength distributed optical fiber sensing system based on a tunable narrow-line-width laser, relates to the technical field of optical fiber sensing, and comprises a ring resonant main cavity, a first resonant sub-cavity, a second resonant sub-cavity, a third resonant sub-cavity, a first arbitrary waveform generator, a pulse modulation module and a data acquisition module, wherein the ring resonant main cavity comprises a first optical isolator, a first coupler, a first amplifier, a second optical isolator, a first tunable filter and a second amplifier, the third resonant sub-cavity comprises a first circulator, a chirped grating, a second circulator and a single-mode optical fiber, the third end of the second circulator is connected with the first optical isolator, one end of the chirped grating is connected with the second end of the second circulator, the third end of the second circulator is connected with the third end of the first circulator through the single-mode optical fiber, the other end of the chirped grating is connected with the second end of the first circulator, and the first end of the first circulator is connected with the second resonant sub-cavity, so that the narrow-line-width performance of the light source is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical fiber sensing technology, and particularly relates to a multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser. BACKGROUND

[0002] Compared with the traditional distributed sensing system, by introducing a large-capacity all-identical weak reflection fiber Bragg grating array, the reflection signal intensity is 3-4 orders of magnitude higher than the Rayleigh scattering relied on in the traditional distributed sensing system technology, so that higher signal-to-noise ratio interference light pulses and acoustic signals can be obtained. This feature makes the grating enhanced distributed optical fiber sensing system show significant advantages in weak signal detection and long distance monitoring. However, the existing grating enhanced distributed optical fiber sensing system generally uses a single frequency narrow linewidth laser, and the wavelength range of the system light source module is small, which limits the overall system, and the single wavelength grating sensing array capacity is limited by spectral shadowing and multiple reflection crosstalk, which seriously limits the dynamic range, system bandwidth and application fields.

[0003] A wideband continuous tunable laser system is disclosed in Chinese Patent No. CN116914543B, which includes a wavelength tunable device, a first laser ring cavity structure, a second laser ring cavity structure, and a third laser ring cavity structure. The wavelength tunable device is used to adjust the laser wavelength. The first laser ring cavity structure is connected with the wavelength tunable device and is used to output long-waveband laser. The second laser ring cavity structure is connected with the first laser ring cavity structure and the wavelength tunable device and is used to output middle-waveband laser. The third laser ring cavity structure is connected with the first laser ring cavity structure, the second laser ring cavity structure, and the wavelength tunable device and is used to output short-waveband laser. However, it is difficult to achieve ideal energy balance among the waveband lasers in the above-mentioned multi-laser ring cavity structure, and mode competition is likely to occur. Moreover, the system is sensitive to environmental factors such as temperature and vibration, which can easily introduce additional phase noise or laser drift, thereby affecting the narrow linewidth performance of the light source. Therefore, it is necessary to provide a multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser to improve the narrow linewidth performance of the light source. SUMMARY

[0004] Therefore, the present application provides a multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser. By adopting a ring resonant main cavity to constitute a laser oscillation cavity, the laser output with extremely low phase noise and high stability is ensured. Moreover, the longitudinal mode spacing of the laser is effectively adjusted by the first resonant sub-cavity and the second resonant sub-cavity, and the single longitudinal mode working stability is improved to enhance the narrow linewidth performance of the light source.

[0005] The application provides a multi-wavelength distributed optical fiber sensing system based on a tunable narrow line width laser, comprising a ring resonant main cavity, a first resonant sub-cavity, a second resonant sub-cavity, a third resonant sub-cavity, a first arbitrary waveform generator, a pulse modulation module and a data acquisition module, wherein,

[0006] The ring resonant main cavity comprises a first optical isolator, a first coupler, a first amplifier, a second optical isolator, a first tunable filter and a second amplifier connected in sequence, the second amplifier is connected with the first resonant sub-cavity, the first tunable filter is further connected with the first arbitrary waveform generator, the first coupler is connected with the pulse modulation module, and the ring resonant main cavity is used for generating narrow line width continuous light with high coherence.

[0007] The first resonant sub-cavity is connected with the second resonant sub-cavity, and the first resonant sub-cavity and the second resonant sub-cavity are both used for adjusting the longitudinal mode spacing of the laser.

[0008] The third resonant sub-cavity comprises a first circulator, a chirped grating, a second circulator and a single-mode optical fiber, the third end of the second circulator is connected with the first optical isolator, one end of the chirped grating is connected with the second end of the second circulator, the third end of the second circulator is connected with the third end of the first circulator through the single-mode optical fiber, the other end of the chirped grating is connected with the second end of the first circulator, the first end of the first circulator is connected with the second resonant sub-cavity, and the third resonant sub-cavity is used for constructing multiple distributed optical paths to adjust the equivalent cavity length of the third resonant sub-cavity.

[0009] The pulse modulation module is connected with the first arbitrary waveform generator and the data acquisition module respectively.

[0010] On the basis of the above technical scheme, preferably, the pulse modulation module comprises an optical pulse unit, a third amplifier, a second tunable filter, a third circulator, a multi-wavelength weak reflection light grating array and a second arbitrary waveform generator, wherein,

[0011] The optical pulse unit is connected with the third end of the first coupler, the second arbitrary waveform generator and the third amplifier respectively, the third amplifier is connected with the first end of the second tunable filter, the second end of the second tunable filter is connected with the first arbitrary waveform generator, the third end of the second tunable filter is connected with the first end of the third circulator, the second end of the third circulator is connected with the multi-wavelength weak reflection light grating array, the third end of the third circulator is connected with the data acquisition module, and the second arbitrary waveform generator is further connected with the data acquisition module.

[0012] On the basis of the above technical scheme, preferably, the data acquisition unit comprises a data acquisition unit, a first photodetector, a second photodetector, a third photodetector, a fourth circulator, a second coupler, a delay optical fiber, a first Faraday rotator and a second Faraday rotator, wherein,

[0013] One end of the data acquisition unit is connected with the second arbitrary waveform generator, and the other end of the data acquisition unit is connected with the first photodetector, the second photodetector and the third photodetector respectively, the first photodetector is connected with the third end of the fourth circulator, the first end of the fourth circulator is connected with the third end of the third circulator, the second coupler is connected with the second end of the fourth circulator, the second photodetector, the delay optical fiber, the second Faraday rotator and the third photodetector respectively, and the delay optical fiber is further connected with the first Faraday rotator.

[0014] Further preferably, the ring resonant main cavity, the first resonant sub-cavity and the second resonant sub-cavity satisfy the expression of the complex resonant cavity free spectral range as follows:

[0015]

[0016] wherein FSR represents the complex resonant cavity free spectral range, Q represents the mode coefficient of the ring resonant main cavity, c represents the speed of light, n represents the refractive index of the medium, L represents the cavity length of the ring resonant main cavity, q1 represents the mode coefficient of the first resonant sub-cavity, l1 represents the cavity length of the first resonant sub-cavity, q2 represents the mode coefficient of the second resonant sub-cavity, and l2 represents the cavity length of the second resonant sub-cavity. eff

[0017] Further preferably, the third resonant sub-cavity comprises at least a first distribution light path, a second distribution light path and a third distribution light path, wherein,

[0018] The narrow linewidth continuous light in the first distribution light path is input from the first port of the first circulator, output to the chirped grating through the second port of the first circulator, transmitted through the chirped grating, and output from the third port of the second circulator;

[0019] ​The narrow linewidth continuous light in the second distribution optical path is input by the first port of the first circulator, output to the chirped grating through the second port of the first circulator, reflected at the right side of the chirped grating to re-input the narrow linewidth continuous light to the second port of the first circulator, and sequentially input to the second port of the second circulator and the single-mode fiber through the third port of the first circulator, the narrow linewidth continuous light is input to the chirped grating through the second port of the second circulator, and the narrow linewidth continuous light is re-input to the second port of the second circulator by the reflection of the chirped grating, and output from the third port of the second circulator.

[0020] The narrow linewidth continuous light in the third distribution optical path is input by the first port of the first circulator, output to the chirped grating through the second port of the first circulator, reflected at the right side of the chirped grating to re-input the narrow linewidth continuous light to the second port of the first circulator, and sequentially input to the single-mode fiber through the third port of the first circulator, the narrow linewidth continuous light is sequentially input to the chirped grating through the first port of the second circulator and the second port of the second circulator, the narrow linewidth continuous light is transmitted through the chirped grating to re-input the narrow linewidth continuous light to the second port of the first circulator, and the narrow linewidth continuous light is sequentially input to the first port of the second circulator through the third port of the first circulator and the single-mode fiber, and the narrow linewidth continuous light is output by the third port of the second circulator.

[0021] Further preferably, the proportional weight expressions of the first distribution optical path, the second distribution optical path and the third distribution optical path are respectively:

[0022]

[0023] Wherein, path1 represents the proportional weight of the first distribution optical path, T1η1 represents the transmission efficiency of the first circulator, λ represents the wavelength of the chirped grating, T represents the temperature of the chirped grating, T CFBG (λ, T) represents the transmission spectrum of the chirped grating, T2η2 represents the transmission efficiency of the second circulator, β1 represents the temperature correction coefficient corresponding to the first circulator, β2 represents the temperature correction coefficient corresponding to the second circulator, ΔT represents the temperature change amount of the chirped grating, L1 represents the cavity length of the first distribution optical path, L2 represents the cavity length of the second distribution optical path, D represents the dispersion coefficient of the single-mode fiber, α represents the transmission loss of the single-mode fiber, path2 represents the proportional weight of the second distribution optical path, path3 represents the proportional weight of the third distribution optical path, and i represents the serial number of the distribution optical path.

[0024] Further preferably, the first resonator cavity and the second resonator cavity each comprise two optical couplers connected head to tail.

[0025] Further preferably, the first coupler is a 1x2 coupler, and the second coupler is a 3x3 coupler.

[0026] Further preferably, the first amplifier is a semiconductor optical amplifier, and the second amplifier and the third amplifier are each an erbium-doped fiber amplifier.

[0027] Further preferably, the first arbitrary waveform signal generator is configured to send a trigger signal to the first tunable filter and the second tunable filter to synchronize the bandwidth filtering ranges of the first tunable filter and the second tunable filter, and the second arbitrary waveform signal generator is configured to modulate the optical pulse unit and synchronize the data acquisition.

[0028] The multi-wavelength distributed fiber sensing system based on the tunable narrow-linewidth laser provided by the present application has the following beneficial effects over the prior art:

[0029] (1) By adopting a ring resonant main cavity to form a laser oscillation cavity, high coherence and narrow-linewidth continuous light generation is achieved, ensuring that the laser output has extremely low phase noise and high stability, and the longitudinal mode spacing of the laser is effectively adjusted by the first resonator cavity and the second resonator cavity, improving the single longitudinal mode working stability and suppressing the side mode oscillation, thereby further improving the purity and stability of the laser output, and the third resonator cavity forms multiple distribution light paths with the first and second circulators and the chirped grating and the single-mode fiber, utilizes the flexible and adjustable equivalent cavity length, and realizes effective distribution and regulation of multiple light paths, thereby optimizing the energy distribution in the laser cavity to improve the narrow-linewidth performance of the light source while meeting the needs of multi-wavelength distributed fiber sensing.

[0030] (2) By utilizing the first, second, and third distribution light paths, multi-path light splitting of narrow-linewidth continuous light is achieved, and each distribution light path passes through different reflection and transmission paths, so that the light signal circulates in multiple paths, thereby ensuring that the output signals of each path have the same high coherence and narrow-linewidth characteristics, and the third resonator cavity can effectively adjust the equivalent cavity length of the system by constructing multiple distribution light paths, and different light paths correspond to different cavity length adjustment methods, which provides a flexible means for the distributed fiber sensing system to realize multi-wavelength detection and optimize the sensing response, further enhances the dynamic adaptation capability and measurement accuracy of the fiber sensing system, and meanwhile, in the second and third distribution light paths, the light is recoupled back to the corresponding port of the first circulator through the reflection of the chirped grating, forming a closed feedback loop, which is conducive to further stabilizing the continuous light output, and this feedback mechanism effectively suppresses external interference, so that the entire fiber sensing system can still maintain excellent signal stability and reliability for long-time operation in complex environments. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a schematic diagram of the structure of the multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser provided by the present invention.

[0033] Explanation of reference numerals in the attached figures: 1. Ring resonant main cavity; 11. First optical isolator; 12. First coupler; 13. First amplifier; 14. Second optical isolator; 15. First tunable filter; 16. Second amplifier; 2. First resonant cavity; 3. Second resonant cavity; 4. Third resonant cavity; 41. First circulator; 42. Chirped grating; 43. Second circulator; 44. Single-mode fiber; 5. First arbitrary waveform generator; 6. Pulse modulation module; 61. Optical pulse. 62. Third amplifier; 63. Second tunable filter; 64. Third circulator; 65. Multi-wavelength weak reflection grating array; 66. Second arbitrary waveform generator; 7. Data acquisition module; 71. Data acquisition unit; 72. First photodetector; 73. Second photodetector; 74. Third photodetector; 75. Fourth circulator; 76. Second coupler; 77. Delay fiber; 78. First Faraday rotator; 79. Second Faraday rotator. Detailed Implementation

[0034] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0035] Unless otherwise defined, technical terms used in the present disclosure shall have the ordinary meaning as understood by a person of ordinary skill in the art to which the present disclosure pertains. The terms "first", "second", and similar terms are used herein merely to distinguish one element from another, and are not intended to imply any order or importance. Similarly, the terms "one" and "a" are not limited to one, but rather mean "at least one". The terms "connected" and "coupled" are not limited to direct connections or physical connections, but can include indirect connections or electrical connections. The terms "upper", "lower", "left", "right", and the like are used only to indicate relative positions, and when the absolute positions of the described objects are changed, the relative positions are also changed accordingly.

[0036] Reference Figure 1 The present disclosure provides a multi-wavelength distributed fiber sensing system based on a tunable narrow linewidth laser, which comprises a ring resonant main cavity 1, a first resonant sub-cavity 2, a second resonant sub-cavity 3, a third resonant sub-cavity 4, a first arbitrary waveform generator 5, a pulse modulation module 6, and a data acquisition module 7, wherein,

[0037] The ring resonant main cavity 1 comprises a first optical isolator 11, a first coupler 12, a first amplifier 13, a second optical isolator 14, a first tunable filter 15, and a second amplifier 16 connected in sequence, the second amplifier 16 is connected with the first resonant sub-cavity 2, the first tunable filter 15 is also connected with the first arbitrary waveform generator 5, the first coupler 12 is connected with the pulse modulation module 6, and the ring resonant main cavity 1 is used for generating narrow linewidth continuous light with high coherence.

[0038] In the embodiment, the first coupler 12 is a 1x2 coupler, the first amplifier 13 is a semiconductor optical amplifier, the second amplifier 16 is an erbium-doped fiber amplifier, and the first arbitrary waveform signal generator is used to send a trigger signal to the first tunable filter 15 and the second tunable filter 63 to synchronously modulate the bandwidth filtering range of the first tunable filter 15 and the second tunable filter 63. The semiconductor optical amplifier and the erbium-doped fiber amplifier together provide sufficient gain in the cavity, and two optical isolators are respectively arranged at the front ends of the two optical amplifiers to ensure that there is no reverse light and to ensure that the cavity obtains the best gain. The first coupler 12 divides the light into two parts, one part circulates in the cavity, and the other part is output from the cavity for monitoring.

[0039] In the embodiment, the first resonant sub-cavity 2 is connected with the second resonant sub-cavity 3, and the first resonant sub-cavity 2 and the second resonant sub-cavity 3 are both used for adjusting the longitudinal mode spacing of the laser. The first resonant sub-cavity 2 and the second resonant sub-cavity 3 each comprise two optical couplers connected in head-to-tail manner.

[0040] The first resonator cavity 2 and the second resonator cavity 3 are both composed of two double-coupler ring resonator cavities which are cascaded in an up-down manner, and each single ring resonator cavity is connected into a closed resonant ring by two couplers using a fiber fusion splicer. There are mainly two parameters affecting the filtering characteristics, the cross-coupling ratio of the coupler and the ratio of the cavity length to the cavity length difference. The coupling ratio of the coupler mainly affects the energy storage capacity of the resonant cavity, and the cavity length determines the free spectral range of the filter spectrum, which is inversely proportional to the cavity length difference. In the screening process of the composite cavity filter in combination with the Fabry-Perot filter, the required bandwidth should match the free spectral range of the composite cavity filter, and the filtering bandwidth of the composite cavity filter should match the longitudinal mode interval of the main cavity, that is, the free spectral range of the composite cavity should be between 0.5-1 times the bandwidth of the Fabry-Perot filter, and the longitudinal mode interval of the main cavity should be between 0.5-1 times the filtering bandwidth of the composite cavity. Considering the bandwidth and tuning range of the Fabry-Perot filter, a plurality of composite cavity filters with a cavity length of 21-90 cm and a cavity length difference to cavity length ratio less than 1 / 10 and a cross-coupling ratio of 50%-95% are designed. Under the condition of meeting the performance parameters of the Fabry-Perot filter and matching the longitudinal mode interval of the main cavity, the parameters are gradually fine-tuned to increase the side mode suppression ratio and realize the output of a single longitudinal mode of energy concentration.

[0041] The transmission spectrum of the composite cavity with different coupling ratios and cavity length to cavity length difference ratios is simulated according to the coupled mode theory. Changing the coupling ratio can obtain a comb spectrum with different side mode suppression ratios and different main longitudinal mode bandwidths. With the decrease of the coupling ratio, the transmittance, different side mode suppression ratios and full width at half maximum gradually decrease. With the decrease of the ratio of the cavity length to the cavity length difference, the side mode suppression ratio gradually decreases and the full width at half maximum gradually increases. The standard for selecting the parameters of the composite cavity filter is that after the free spectral range and the full width at half maximum meet the requirements, the smaller the side mode suppression ratio, the higher the transmittance, the better. Considering the influence of each parameter, it is concluded that reasonable design of the selection frequency characteristics of the composite cavity filter can effectively compress the linewidth of the laser. The center wavelength of the main longitudinal mode of the composite cavity filter changes with the cavity length difference when the cavity length difference is kept constant. When the cavity length difference becomes longer, the center wavelength of the longitudinal mode of the comb spectrum gradually becomes longer. Therefore, by changing the cavity length difference, the longitudinal mode selection characteristics of the composite cavity filter can be effectively adjusted, the interval of the center wavelength of the longitudinal mode can be effectively shortened, and the filtering resonant cavity with high repetition accuracy and high wavelength resolution can be realized by combining the Fabry-Perot filter.

[0042] To achieve longitudinal mode energy distribution in the ring resonator, a coherence characteristic optimization method based on a chirped grating 42 is employed. The grating period of the chirped fiber grating is not constant but monotonically varies along the axial direction. Different grating periods correspond to different reflection wavelengths, forming a wide reflection band. Different positions of the chirped grating 42 reflect incident light of different wavelengths, with the reflected incident light wavelength gradually increasing along the axial direction. Since the optical period of the chirped grating 42 gradually changes linearly along the grating axis, it can be considered as a superposition of several gratings with increasing or decreasing reflection wavelengths. Short wavelengths reflected by the chirped grating 42 will be reflected at the grating tail, while longer wavelengths will be reflected at the corresponding front. Different wavelengths of light correspond to the reflection of each small segment of the grating, causing the optical path to monotonically increase or decrease with wavelength. A single-frequency fiber laser with multiple composite cavities based on a chirped grating 42 is constructed. By effectively utilizing both the reflectivity and transmittance of the chirped grating 42, multiple optical path planning is performed, significantly improving the fundamental frequency and coherence length of the entire ring cavity and effectively optimizing the linewidth. A chirped Bragg grating structure built with dual circulators can form a multi-cavity structure within the resonant cavity, allowing single-mode lasers to cyclically and coherently coherently within the resonant cavity. Due to the losses and the influence of transmittance and reflectivity of the circulators and chirped grating 42, more cycles inevitably lead to greater attenuation. The impact of the later optical paths on the fundamental frequency of the entire ring cavity decreases. Therefore, this structure significantly improves the fundamental frequency, which is beneficial for further narrowing the linewidth.

[0043] The expression for the free spectrum of the composite resonant cavity, satisfied by the main ring resonator 1, the first resonator cavity 2, and the second resonator cavity 3, is as follows:

[0044]

[0045] Where FSR represents the free spectrum of the composite resonator, Q represents the mode coefficient of ring resonator main cavity 1, c represents the speed of light, and n eff Let L represent the refractive index of the medium, L represent the cavity length of the main ring resonator 1, q1 represent the mode coefficient of the first resonator cavity 2, l1 represent the cavity length of the first resonator cavity 2, q2 represent the mode coefficient of the second resonator cavity 3, and l2 represent the cavity length of the second resonator cavity 3.

[0046] Furthermore, when the gain bandwidth of the laser is constant, to achieve single-mode operation of the laser, the longitudinal mode spacing of the composite resonator needs to be increased. There are two corresponding methods: The first case is that the length of the main cavity is much larger than that of a single sub-cavity. In this case, the longitudinal mode spacing of the composite resonator is approximately equal to the longitudinal mode spacing of the sub-cavities. As long as the ring length of the sub-cavities is small enough, single-mode operation of the laser can be achieved. The second case is that the difference in cavity length between the two cavities is very small. According to the vernier principle, the corresponding longitudinal mode spacing of the composite resonator will become:

[0047]

[0048] At this time, the longitudinal mode interval of the whole laser is the least common multiple of the longitudinal mode intervals of all the sub-cavities. The longitudinal mode interval of the laser can be controlled by controlling the difference in cavity length, and when the longitudinal mode interval in the compound resonant cavity is greater than the gain bandwidth of the gain medium or greater than the bandwidth of the wavelength initial selection channel, the single longitudinal mode stable operation of the laser can be realized. By reasonably designing the structural parameters of the multiple sub-cavities, the equivalent optical path can also be increased, the photon lifetime is prolonged, and the effect of narrowing the laser linewidth is realized.

[0049] The third resonant sub-cavity 4 includes a first circulator 41, a chirped grating 42, a second circulator 43, and a single-mode fiber 44. The third end of the second circulator 43 is connected with the first optical isolator 11, one end of the chirped grating 42 is connected with the second end of the second circulator 43, the third end of the second circulator 43 is connected with the third end of the first circulator 41 through the single-mode fiber 44, the other end of the chirped grating 42 is connected with the second end of the first circulator 41, the first end of the first circulator 41 is connected with the second resonant sub-cavity 3, and the third resonant sub-cavity 4 is used to construct multiple distribution light paths to adjust the equivalent cavity length of the third resonant sub-cavity 4.

[0050] The third resonant sub-cavity 4 includes at least a first distribution light path, a second distribution light path, and a third distribution light path, wherein,

[0051] In the first distribution light path, the narrow-linewidth continuous light is input from the first port of the first circulator 41, output to the chirped grating 42 through the second port of the first circulator 41, and transmitted through the chirped grating 42, and the narrow-linewidth continuous light is output from the third end of the second circulator 43;

[0052] In the second distribution light path, the narrow-linewidth continuous light is input from the first port of the first circulator 41, output to the chirped grating 42 through the second port of the first circulator 41, and reflected on the right side of the chirped grating 42 to be re-input into the second port of the first circulator 41, and the narrow-linewidth continuous light is sequentially input into the first port of the second circulator 43 through the third port of the first circulator 41 and the single-mode fiber 44, input into the chirped grating 42 through the second port of the second circulator 43, re-input into the second port of the second circulator 43 by being reflected by the chirped grating 42, and output from the third end of the second circulator 43;

[0053] The narrow linewidth continuous light in the third distribution light path is input from the first port of the first circulator 41, output from the second port of the first circulator 41 to the chirped grating 42, reflected at the right side of the chirped grating 42 to re-input the narrow linewidth continuous light into the second port of the first circulator 41, sequentially input the narrow linewidth continuous light into the single-mode fiber 44 through the third port of the first circulator 41, sequentially input the narrow linewidth continuous light into the chirped grating 42 through the first port of the second circulator 43 and the second port of the second circulator 43, transmit the narrow linewidth continuous light through the chirped grating to re-input the narrow linewidth continuous light into the second port of the first circulator 41, and sequentially input the narrow linewidth continuous light into the first port of the second circulator 43 through the third port of the first circulator 41 and the single-mode fiber 44, and output the narrow linewidth continuous light from the third port of the second circulator 43.

[0054] It can be understood that the first light path is from the 1 port of the first circulator 41 to the 2 port, input into the chirped grating 42, and finally from the 2 port of the second circulator 43 to the 3 port to enter the first coupler 12 to complete a cycle of circulation. Different from the first light path, the light on the second light path is reflected by the chirped grating 42, from the 2 port of the first circulator 41 to the 3 port, enters the second circulator 43 through the single-mode fiber 44, is reflected at the right side of the chirped grating 42 in the figure, and is output from the 3 port of the second circulator 43 to the first coupler 12 to complete circulation. The third light path still enters the first circulator 41 along the two optical amplifiers and the two optical isolators, is reflected by the chirped grating 42, and reaches the right side of the chirped grating 42 through the two circulators. Different from the second light path, the light is transmitted through the chirped grating 42 this time. When the light reaches the left side of the chirped grating 42 in the figure again along the single-mode fiber 44 above and the two circulators, it is reflected, enters the second circulator 43, and reaches the first coupler 12 to complete a cycle of circulation. Each light path does not occur sequentially, and a certain proportion of light passes through the light path.

[0055] In the embodiment, the first, second and third distribution optical paths are used to realize the multi-channel splitting of narrow linewidth continuous light. Each distribution optical path circulates the light signal in multiple channels through different reflection and transmission paths (including left and right side reflection of the chirped grating 42 and transmission of the single-mode optical fiber 44), thereby ensuring that each channel output signal has the same high coherence and narrow linewidth characteristics. The third resonant sub-cavity 4 can effectively adjust the equivalent cavity length of the system by constructing multiple distribution optical paths. Different optical paths correspond to different cavity length adjustment methods, which provides a flexible means for the distributed fiber sensing system to realize multi-wavelength detection and optimize the sensing response, further enhancing the dynamic adaptation capability and measurement accuracy of the sensing system. In the second and third distribution optical paths, the light is recoupled back to the corresponding port of the first circulator 41 through the reflection of the chirped grating 42, forming a closed feedback loop, which is conducive to further stabilizing the continuous light output. This feedback mechanism effectively suppresses external interference, enabling the entire fiber sensing system to maintain excellent signal stability and reliability for long-term operation in complex environments.

[0056] Further, the proportional weight expressions of the first, second and third distribution optical paths are respectively:

[0057]

[0058] wherein path1 represents the proportional weight of the first distribution optical path, T1η1 represents the transmission efficiency of the first circulator, λ represents the wavelength of the chirped grating 42, T represents the temperature of the chirped grating 42, T CFBG (λ,T) represents the transmission spectrum of the chirped grating 42, T2η2 represents the transmission efficiency of the second circulator 43, β1 represents the temperature correction coefficient corresponding to the first circulator 41, β2 represents the temperature correction coefficient corresponding to the second circulator 43, ΔT represents the temperature change of the chirped grating 42, L1 represents the cavity length of the first distribution optical path, L2 represents the cavity length of the second distribution optical path, D represents the dispersion coefficient of the single-mode optical fiber 44, α represents the transmission loss of the single-mode optical fiber 44, path2 represents the proportional weight of the second distribution optical path, path3 represents the proportional weight of the third distribution optical path, and i represents the serial number of the distribution optical path.

[0059] Due to the existence of circulator loss and chirped grating loss, the subsequent possible optical paths may be affected by loss, resulting in weak overall light intensity and reduced influence on the ring main cavity. Therefore, this is not described one by one. By reasonably controlling the equivalent cavity lengths L i (i = 1, 2, 3) and coupling weights σ i (i = 1, 2, 3) of the three optical paths, the laser cavity fundamental frequency f base can be effectively improved. The fundamental frequency formed by the three optical paths can be represented as:

[0060]

[0061] The pulse modulation module 6 is connected with the first arbitrary waveform generator 5 and the data acquisition module 7 respectively. The pulse modulation module 6 comprises an optical pulse unit 61, a third amplifier 62, a second tunable filter 63, a third circulator 64, a multi-wavelength weak reflection light grating array 65 and a second arbitrary waveform generator 66, wherein,

[0062] The optical pulse unit 61 is connected with the third end of the first coupler 12, the second arbitrary waveform generator 66 and the third amplifier 62 respectively. The third amplifier 62 is connected with the first end of the second tunable filter 63. The second end of the second tunable filter 63 is connected with the first arbitrary waveform generator 5. The third end of the second tunable filter 63 is connected with the first end of the third circulator 64. The second end of the third circulator 64 is connected with the multi-wavelength weak reflection light grating array 65. The third end of the third circulator 64 is connected with the data acquisition module 7. The second arbitrary waveform generator 66 is also connected with the data acquisition module 7. The second arbitrary waveform signal generator is used for modulating the optical pulse unit 61 and synchronously collecting data.

[0063] In the embodiment, the optical pulse unit 61 is modulated by the second arbitrary waveform generator 66, and works synchronously with the data acquisition module 7, so as to ensure that the emission of laser pulse and the signal collection are accurately matched in time, thereby improving the data collection accuracy and the system response speed. The third amplifier 62 and the second tunable filter 63 work cooperatively to amplify and adjust the frequency characteristics of the modulated optical pulse signal, which is conducive to optimizing the signal waveform and improving the dynamic range and detection sensitivity of the optical fiber sensing system. The combination of the third circulator 64 and the multi-wavelength weak reflection light grating array 65 realizes the branching, reflection and integration of different wavelength optical signals, thereby supporting the needs of multi-wavelength sensing and providing high-quality optical signals for multiple measurement channels. The connection between the modules is realized through the precise design of the connection mode, the second arbitrary waveform generator 66 not only modulates the optical pulse unit 61, but also interconnects with the data acquisition module 7, thereby providing a flexible and efficient modular integration scheme for the entire pulse modulation and signal collection process, which is convenient for future system maintenance and upgrading.

[0064] The data acquisition unit 71 comprises a data acquisition unit 71, a first photodetector 72, a second photodetector 73, a third photodetector 74, a fourth circulator 75, a second coupler 76, a delay optical fiber 77, a first Faraday rotator 78 and a second Faraday rotator 79, wherein,

[0065] One end of the data acquisition unit 71 is connected with the second arbitrary waveform generator 66, and the other end of the data acquisition unit 71 is connected with the first photoelectric detector 72, the second photoelectric detector 73 and the third photoelectric detector 74 respectively, the first photoelectric detector 72 is connected with the third end of the fourth circulator 75, the first end of the fourth circulator 75 is connected with the third end of the third circulator 64, the second coupler 76 is connected with the second end of the fourth circulator 75, the second photoelectric detector 73, the delay optical fiber 77, the second Faraday rotator 79 and the third photoelectric detector 74 respectively, the delay optical fiber 77 is also connected with the first Faraday rotator 78, the second coupler 76 is a 3*3 coupler, and the third amplifier 62 is an erbium-doped fiber amplifier.

[0066] In the embodiment, the data acquisition unit 71 is connected with the three photoelectric detectors respectively, so that the information from different signal branches can be captured simultaneously, thereby realizing efficient acquisition of multi-wavelength and multi-channel signals, the fourth circulator 75 and the second coupler 76 work together to make the optical signals branch or combine in different paths, so that the signals in each path can be acquired independently, the combination of the delay optical fiber 77 and the Faraday rotator effectively introduces time delay and feedback, the polarization-independent characteristic of the Faraday rotator is used to effectively compensate the signal distortion caused by polarization change, thereby improving the signal stability, the introduction of the third amplifier 62 (erbium-doped fiber amplifier) improves the detection capability of the system for weak signals and enhances the overall signal-to-noise ratio, and the second coupler 76 as a 3*3 optical coupler not only realizes effective branching and combining of multi-channel optical signals, but also provides a flexible interface for system expansion and multi-channel data processing.

[0067] The reasonable coupling between the data acquisition unit 71 and the second arbitrary waveform generator 66 and each detector makes the pulse modulation, signal feedback and data synchronous acquisition form a closed loop, thereby optimizing the time domain modulation and signal acquisition process.

[0068] The multi-wavelength grating enhanced distributed optical fiber sensing system uses the constructed tunable narrow linewidth laser, the continuous light generated by the tunable narrow linewidth laser is modulated into pulsed light by the optical pulse module, the pulsed light is amplified to improve the signal-to-noise ratio because part of the light is attenuated in the modulation process, and a tunable filter is used to filter the amplified spontaneous emission noise outside the wavelength range of the laser. The narrow optical pulse is effectively amplified by the erbium-doped fiber amplifier and passes through the circulator, and is directly incident into the multi-wavelength weak reflection fiber Bragg grating array, the pulse sequence reflected back by the multi-wavelength weak reflection fiber Bragg grating array occurs coherent interference in the unbalanced Michelson interferometer through path matching interference technology. The unbalanced Michelson interferometer is composed of a 3*3 optical fiber coupler, a first Faraday rotator 78, a second Faraday rotator 79 and a 5m delay optical fiber 77. The three-way output interference signals of the 3*3 optical fiber coupler pass through photoelectric detectors in sequence to complete photoelectric conversion and channel high-speed acquisition card to complete data acquisition, phase demodulation and sensing signal recovery.

[0069] In the embodiment, the laser oscillation cavity is formed by adopting the annular resonant main cavity 1, the generation of high coherence and narrow linewidth continuous light is realized, the laser output is ensured to have extremely low phase noise and high stability, the longitudinal mode interval of the laser is effectively adjusted by the first resonant sub-cavity 2 and the second resonant sub-cavity 3, the single longitudinal mode working stability is improved, the side mode oscillation is suppressed, and the purity and stability of the laser output are further improved, the third resonant sub-cavity 4 forms multiple distribution light paths by the first and second circulators 43, the chirped grating 42 and the single mode fiber 44, the flexible and adjustable equivalent cavity length is utilized to realize the effective distribution and regulation of multiple light, and the energy distribution in the laser cavity is optimized to improve the narrow linewidth performance of the light source and meet the needs of multi-wavelength distributed fiber sensing.

[0070] The above only describes the preferred embodiments of the present application and is not intended to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A multi-wavelength distributed fiber optic sensing system based on a tunable narrow-linewidth laser, characterized in that, It includes a ring resonant main cavity (1), a first resonant sub-cavity (2), a second resonant sub-cavity (3), a third resonant sub-cavity (4), a first arbitrary waveform generator (5), a pulse modulation module (6), and a data acquisition module (7), wherein, The ring resonant main cavity (1) includes a first optical isolator (11), a first coupler (12), a first amplifier (13), a second optical isolator (14), a first tunable filter (15), and a second amplifier (16) connected in sequence. The second amplifier (16) is connected to the first resonant cavity (2). The first tunable filter (15) is also connected to the first arbitrary waveform generator (5). The first coupler (12) is connected to the pulse modulation module (6). The ring resonant main cavity (1) is used to generate highly coherent narrow-linewidth continuous light. The first resonant cavity (2) is connected to the second resonant cavity (3), and both the first resonant cavity (2) and the second resonant cavity (3) are used to adjust the longitudinal mode spacing of the laser. The third resonant cavity (4) includes a first circulator (41), a chirped grating (42), a second circulator (43), and a single-mode fiber (44). The third end of the second circulator (43) is connected to the first optical isolator (11), the second end of the second circulator (43) is connected to one end of the chirped grating (42), the third end of the second circulator (43) is connected to the third end of the first circulator (41) through the single-mode fiber (44), the second end of the first circulator (41) is connected to the other end of the chirped grating (42), and the first end of the first circulator (41) is connected to the second resonant cavity (3). The third resonant cavity (4) is used to construct multiple distribution optical paths to adjust the equivalent cavity length of the third resonant cavity (4). The pulse modulation module (6) is connected to the first arbitrary waveform generator (5) and the data acquisition module (7) respectively; The pulse modulation module (6) includes an optical pulse unit (61), a third amplifier (62), a second tunable filter (63), a third circulator (64), a multi-wavelength weak reflection grating array (65), and a second arbitrary waveform generator (66), wherein, The optical pulse unit (61) is connected to the third end of the first coupler (12), the second arbitrary waveform generator (66), and the third amplifier (62), respectively. The third amplifier (62) is connected to the first end of the second tunable filter (63). The second end of the second tunable filter (63) is connected to the first arbitrary waveform generator (5). The third end of the second tunable filter (63) is connected to the first end of the third circulator (64). The second end of the third circulator (64) is connected to the multi-wavelength weak reflection grating array (65). The third end of the third circulator (64) is connected to the data acquisition module (7). The second arbitrary waveform generator (66) is also connected to the data acquisition module (7).

2. The multi-wavelength distributed fiber optic sensing system based on a tunable narrow-linewidth laser as described in claim 1, characterized in that, The data acquisition module (7) includes a data acquisition unit (71), a first photodetector (72), a second photodetector (73), a third photodetector (74), a fourth circulator (75), a second coupler (76), a delay fiber (77), a first Faraday rotator (78), and a second Faraday rotator (79), wherein, One end of the data acquisition unit (71) is connected to the second arbitrary waveform generator (66), and the other end of the data acquisition unit (71) is connected to the first photodetector (72), the second photodetector (73), and the third photodetector (74), respectively. The first photodetector (72) is connected to the third end of the fourth circulator (75), and the first end of the fourth circulator (75) is connected to the third end of the third circulator (64), respectively. The second coupler (76) is connected to the second end of the fourth circulator (75), the second photodetector (73), the time-delay fiber (77), the second Faraday rotator (79), and the third photodetector (74), respectively. The time-delay fiber (77) is also connected to the first Faraday rotator (78).

3. The multi-wavelength distributed fiber optic sensing system based on a tunable narrow-linewidth laser as described in claim 1, characterized in that, The expression for the free spectrum of the composite resonant cavity satisfied by the ring resonant main cavity (1), the first resonant sub-cavity (2), and the second resonant sub-cavity (3) is as follows: ； in, FSR Represents the free spectrum of the composite resonant cavity. Q This represents the mode coefficients of the ring resonant main cavity (1). c Represents the speed of light. n eff Indicates the refractive index of the medium. L This indicates the cavity length of the ring resonant main cavity (1). q 1 represents the mode coefficient of the first resonator cavity (2). l 1 represents the cavity length of the first resonator cavity (2). q 2 represents the mode coefficient of the second resonator cavity (3). l 2 represents the cavity length of the second resonator cavity (3).

4. The multi-wavelength distributed fiber optic sensing system based on a tunable narrow-linewidth laser as described in claim 1, characterized in that, The third resonator cavity (4) includes at least a first distribution optical path, a second distribution optical path, and a third distribution optical path, wherein, In the first distribution optical path, the narrow linewidth continuous light is input from the first port of the first circulator (41), output to the chirped grating (42) through the second port of the first circulator (41), the narrow linewidth continuous light is transmitted through the chirped grating (42), and the narrow linewidth continuous light is output from the third end of the second circulator (43); In the second distribution optical path, the narrow linewidth continuous light is input from the first port of the first circulator (41), output to the chirped grating (42) through the second port of the first circulator (41), the narrow linewidth continuous light is reflected on the right side of the chirped grating (42) to re-input the narrow linewidth continuous light into the second port of the first circulator (41), and the narrow linewidth continuous light is sequentially input to the first port of the second circulator (43) through the third port of the first circulator (41) and the single-mode fiber (44), the narrow linewidth continuous light is input to the chirped grating (42) through the second port of the second circulator (43), the narrow linewidth continuous light is reflected by the chirped grating (42) and re-input to the second port of the second circulator (43), and output from the third end of the second circulator (43); In the third optical distribution path, the narrow-linewidth continuous light is input from the first port of the first circulator (41), output to the chirped grating (42) through the second port of the first circulator (41), and the narrow-linewidth continuous light is reflected on the right side of the chirped grating (42) to re-input the narrow-linewidth continuous light into the second port of the first circulator (41). The narrow-linewidth continuous light is then sequentially input into the single-mode fiber (44) through the third port of the first circulator (41). The first port of the second circulator (43) and the second port of the second circulator (43) are incident on the chirped grating (42). The narrow linewidth continuous light is transmitted through the chirped grating to re-input the narrow linewidth continuous light into the second port of the first circulator (41). The narrow linewidth continuous light is then sequentially input into the first port of the second circulator (43) through the third port of the first circulator (41) and the single-mode fiber (44). Finally, the narrow linewidth continuous light is output from the third end of the second circulator (43).

5. The multi-wavelength distributed fiber optic sensing system based on a tunable narrow-linewidth laser as described in claim 4, characterized in that, The proportional weight expressions for the first, second, and third optical distribution paths are as follows: ； ； ； in, path 1 indicates the proportional weight of the first allocated optical path. T 1 η 1 represents the transmission efficiency of the first circulator (41). λ This indicates the wavelength of the chirped grating (42). T This indicates the temperature of the chirped grating (42). T CFBG ( λ , T ) represents the transmission spectrum of the chirped grating (42). T 2 η 2 represents the transmission efficiency of the second circulator (43). β 1 represents the temperature correction coefficient corresponding to the first circulator (41). β 2 represents the temperature correction factor corresponding to the second circulator (43), Δ T This indicates the temperature change of the chirped grating (42). L 1 represents the cavity length of the first optical distribution path. L 2 represents the cavity length of the second optical distribution path. D This represents the dispersion coefficient of the single-mode fiber (44). α This represents the transmission loss of the single-mode fiber (44). path 2 represents the proportional weight of the second optical path allocation. path 3 represents the proportional weight of the third optical path allocation. i This indicates the sequence number of the assigned optical path.

6. The multi-wavelength distributed fiber optic sensing system based on a tunable narrow-linewidth laser as described in claim 1, characterized in that, Both the first resonator cavity (2) and the second resonator include two optical couplers connected end to end.

7. The multi-wavelength distributed fiber optic sensing system based on a tunable narrow-linewidth laser as described in claim 2, characterized in that, The first coupler (12) is a 1×2 coupler, and the second coupler (76) is a 3×3 coupler.

8. The multi-wavelength distributed fiber optic sensing system based on a tunable narrow-linewidth laser as described in claim 1, characterized in that, The first amplifier (13) is a semiconductor optical amplifier, and the second amplifier (16) and the third amplifier (62) are both erbium-doped fiber amplifiers.

9. The multi-wavelength distributed fiber optic sensing system based on a tunable narrow-linewidth laser as described in claim 1, characterized in that, The first arbitrary waveform generator (5) is used to send trigger signals to the first tunable filter (15) and the second tunable filter (63) to synchronously modulate the bandwidth filtering range of the first tunable filter (15) and the second tunable filter (63), and the second arbitrary waveform generator (66) is used to modulate the optical pulse unit (61) and synchronously acquire data.

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