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

Abstract

The invention provides a multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser, which relates to the technical field of optical fiber sensing and comprises an annular resonance main cavity, a first harmonic oscillator cavity, a second harmonic oscillator cavity, a third harmonic oscillator cavity, a first arbitrary waveform generator, a pulse modulation module and a data acquisition module, the annular 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 chirp grating, a second circulator and a single-mode fiber, and the third end of the second circulator is connected with the first optical isolator. The second end of the second circulator is connected with one end of the chirp grating, the third end of the second circulator is connected with the third end of the first circulator through the single-mode optical fiber, the second end of the first circulator is connected with the other end of the chirp grating, and the first end of the first circulator is connected with the second harmonic oscillator cavity.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber optic sensing, and particularly to a multi-wavelength distributed fiber optic sensing system based on a tunable narrow linewidth laser. Background Art

[0002] Compared with traditional distributed sensing systems, by introducing a large-capacity all-identical weak reflection fiber Bragg grating array, the intensity of the reflected signal is 3-4 orders of magnitude higher than the Rayleigh scattering relied on in traditional distributed sensing system technologies, so that interference light pulses and acoustic signals with higher signal-to-noise ratios can be obtained. This characteristic enables the grating-enhanced distributed fiber optic sensing system to exhibit significant advantages in weak signal detection and long-distance monitoring. However, existing grating-enhanced distributed fiber optic sensing systems generally use single-frequency narrow linewidth lasers. The small wavelength range of the system light source module limits the overall system, and the capacity of the single-wavelength grating sensing array is limited by spectral shadow and multiple reflection crosstalk, which results in serious limitations in aspects such as dynamic range, system bandwidth, and application fields.

[0003] Chinese Patent with Publication No. CN116914543B discloses a broadband continuously tunable laser system, including 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 to the wavelength tunable device and is used to output long-wavelength band laser; the second laser ring cavity structure is connected to the first laser ring cavity structure and the wavelength tunable device and is used to output medium-wavelength band laser; the third laser ring cavity structure is connected to the first laser ring cavity structure, the second laser ring cavity structure, and the wavelength tunable device and is used to output short-wavelength band laser. However, it is difficult to achieve an ideal energy balance among the lasers in each wavelength band in the above multi-laser ring cavity structure, and mode competition is likely to occur. Moreover, the system is relatively sensitive to environmental factors such as temperature and vibration, and is prone to introducing additional phase noise or laser drift, which in turn affects the narrow linewidth performance of the light source. Therefore, it is very necessary to provide a multi-wavelength distributed fiber optic sensing system based on a tunable narrow linewidth laser to help improve the narrow linewidth performance of the light source. Summary of the Invention

[0004] In view of this, the present invention proposes a multi-wavelength distributed fiber optic sensing system based on a tunable narrow linewidth laser. By using a ring resonator main cavity to form a laser oscillation cavity, it is ensured that the laser output has extremely low phase noise and high stability, and the longitudinal mode interval of the laser is effectively adjusted through the first resonator cavity and the second resonator cavity to improve the single longitudinal mode working stability, so as to improve the narrow linewidth performance of the light source.

[0005] The present invention provides a multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser, including a ring resonator main cavity, a first resonator sub-cavity, a second resonator sub-cavity, a third resonator sub-cavity, a first arbitrary waveform generator, a pulse modulation module, and a data acquisition module. Among them,

[0006] The ring resonator main cavity includes 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 to the first resonator sub-cavity. The first tunable filter is also connected to the first arbitrary waveform generator. The first coupler is connected to the pulse modulation module. The ring resonator main cavity is used to generate a narrow linewidth continuous light with high coherence;

[0007] The first resonator sub-cavity is connected to the second resonator sub-cavity. Both the first resonator sub-cavity and the second resonator sub-cavity are used to adjust the longitudinal mode interval of the laser;

[0008] The third resonator sub-cavity includes a first circulator, a chirped grating, a second circulator, and a single-mode optical fiber. The third port of the second circulator is connected to the first optical isolator. The second port of the second circulator is connected to one end of the chirped grating. The third port of the second circulator is connected to the third port of the first circulator through the single-mode optical fiber. The second port of the first circulator is connected to the other end of the chirped grating. The first port of the first circulator is connected to the second resonator sub-cavity. The third resonator sub-cavity is used to construct multiple distribution optical paths to adjust the equivalent cavity length of the third resonator sub-cavity;

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

[0010] Based on the above technical solutions, preferably, the pulse modulation module includes an optical pulse unit, a third amplifier, a second tunable filter, a third circulator, a multi-wavelength weak reflection optical fiber grating array, and a second arbitrary waveform generator. Among them,

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

[0012] Based on the above technical solutions, preferably, the data acquisition unit includes 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 mirror, and a second Faraday mirror, where,

[0013] One end of the data acquisition unit is connected to the second arbitrary waveform generator, and the other end of the data acquisition unit is respectively connected to the first photodetector, the second photodetector, and the third photodetector. The first photodetector is connected to the third port of the fourth circulator. The first port of the fourth circulator is connected to the third port of the third circulator. The second coupler is respectively connected to the second port of the fourth circulator, the second photodetector, the delay optical fiber, the second Faraday mirror, and the third photodetector. The delay optical fiber is also connected to the first Faraday mirror.

[0014] More preferably, the expression of the free spectral range of the composite resonator satisfied by the ring resonator main cavity, the first resonator cavity, and the second resonator cavity is:

[0015]

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

[0017] More preferably, the third resonator cavity at least includes a first distribution optical path, a second distribution optical path, and a third distribution optical path, where,

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

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

[0020] In the third optical distribution path, the narrow-linewidth continuous light is input from the first port of the first circulator, output from the second port of the first circulator to the chirped grating, and the narrow-linewidth continuous light is reflected on the right side of the chirped grating to re-input the narrow-linewidth continuous light into the second port of the first circulator. The narrow-linewidth continuous light is sequentially input into the single-mode optical fiber through the third port of the first circulator. The narrow-linewidth continuous light is incident on the chirped grating through the first port and the second port of the second circulator, and the narrow-linewidth continuous light passes through the chirped grating to re-input the narrow-linewidth continuous light into the second port of the first circulator, and the narrow-linewidth continuous light is sequentially input into the first port of the second circulator through the third port of the first circulator and the single-mode optical fiber, and the narrow-linewidth continuous light is output from the third port of the second circulator..

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

[0022]

[0023] Among them, path1 represents the proportional weight of the first optical distribution 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 of the chirped grating, L1 represents the cavity length of the first optical distribution path, L2 represents the cavity length of the second optical distribution path, D represents the dispersion coefficient of the single-mode optical fiber, α represents the transmission loss of the single-mode optical fiber, path2 represents the proportional weight of the second optical distribution path, path3 represents the proportional weight of the third optical distribution path, and i represents the serial number of the optical distribution path.

[0024] More preferably, both the first resonator cavity and the second resonator include two optically coupled devices connected end to end.

[0025] More preferably, the first coupler is a 1×2 coupler, and the second coupler is a 3×3 coupler.

[0026] More preferably, the first amplifier is a semiconductor optical amplifier, and the second and third amplifiers are both erbium-doped fiber amplifiers.

[0027] More preferably, the first arbitrary waveform signal generator is used to send trigger signals to the first tunable filter and the second tunable filter to synchronously modulate the bandwidth filtering ranges of the first tunable filter and the second tunable filter, and the second arbitrary waveform signal generator is used to modulate the optical pulse unit and synchronously acquire data.

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

[0029] (1) By adopting a ring resonator main cavity to form a laser oscillation cavity, the generation of highly coherent and narrow linewidth continuous light is realized, ensuring that the laser output has extremely low phase noise and high stability. Moreover, the longitudinal mode interval of the laser is effectively adjusted through the first resonator cavity and the second resonator cavity, improving the single longitudinal mode working stability and suppressing the side mode oscillation, thereby further enhancing the purity and stability of the laser output. The third resonator cavity forms multiple distribution optical paths through the first and second circulators, a chirped grating and a single-mode optical fiber. By utilizing its flexible and adjustable equivalent cavity length, the effective distribution and regulation of multiple paths of light are realized, thereby optimizing the energy distribution in the laser cavity to improve the narrow linewidth performance of the light source and simultaneously meet the requirements of multi-wavelength distributed optical fiber sensing.

[0030] (2) By using the first, second and third distribution optical paths, multi-path splitting of narrow linewidth continuous light is realized. Each distribution optical path enables the optical signal to circulate in multiple paths through different reflection and transmission paths, ensuring that the output signals of each path have the same characteristics of high coherence and narrow linewidth. The third resonator cavity 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 optical fiber sensing system to realize multi-wavelength detection and optimize the sensing response, further enhancing the dynamic adaptation ability and measurement accuracy of the optical fiber sensing system. At the same time, in the second and third distribution optical paths, the light is re-coupled back to the corresponding ports of the first circulator through the reflection of the chirped grating to form a closed feedback loop, which is beneficial to further stabilizing the continuous light output. This feedback mechanism effectively suppresses external interference, enabling the entire optical fiber sensing system to maintain excellent signal stability and long-term operation reliability in a complex environment. Brief Description of the Drawings

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

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

[0033] Description of the reference numerals: 1, main ring resonator; 11, first optical isolator; 12, first coupler; 13, first amplifier; 14, second optical isolator; 15, first tunable filter; 16, second amplifier; 2, first resonator cavity; 3, second resonator cavity; 4, third resonator cavity; 41, first circulator; 42, chirped grating; 43, second circulator; 44, single-mode optical fiber; 5, first arbitrary waveform generator; 6, pulse modulation module; 61, optical pulse unit; 62, third amplifier; 63, second tunable filter; 64, third circulator; 65, multi-wavelength weak reflection fiber 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 optical fiber; 78, first Faraday rotator; 79, second Faraday rotator. Detailed Embodiments

[0034] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0035] Unless otherwise defined, the technical terms or scientific terms used in the present invention shall have the ordinary meanings understood by those of ordinary skill in the art to which the present invention pertains. The terms "first", "second" and similar terms used in the present invention do not denote any order, quantity or importance, but are only used to distinguish different components. Similarly, terms such as "a" or "an" do not denote a quantity limitation, but mean that there is at least one. Terms such as "connected" or "coupled" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. Terms such as "upper", "lower", "left" and "right" are only used to indicate relative position relationships. When the absolute position of the object being described changes, the relative position relationship also changes accordingly.

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

[0037] The ring resonator 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 resonator 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, and the ring resonator main cavity 1 is used to generate a narrow linewidth continuous light with high coherence.

[0038] In this embodiment, the first coupler 12 is a 1×2 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 trigger signals to the first tunable filter 15 and the second tunable filter 63 to synchronously modulate the bandwidth filtering ranges 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. The two optical isolators are respectively placed at the front ends of the two optical amplifiers. One is to ensure that there is no backward light, and the other is to ensure that the best gain is obtained in the cavity. 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 this embodiment, the first resonator cavity 2 is connected to the second resonator cavity 3, and both the first resonator cavity 2 and the second resonator cavity 3 are used to adjust the longitudinal mode interval of the laser. Both the first resonator cavity 2 and the second resonator include two optical couplers connected end to end.

[0040] The first resonator cavity 2 and the second resonator cavity 3 are both composed of two cascaded double-coupler ring resonators. Each single-ring resonator is formed by connecting the tails of two couplers with an optical fiber fusion splicer to form a closed resonator ring. 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 resonator cavity, while the cavity length determines the free spectral range of the filtering spectrum and is inversely proportional to the cavity length difference. During the process of screening with the composite cavity filter in cooperation 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 is between 0.5 and 1 times the bandwidth of the Fabry-Perot filter, and the longitudinal mode interval of the main cavity is between 0.5 and 1 times the filtering bandwidth of the composite cavity. Considering the bandwidth and tuning range of the Fabry-Perot filter, a variety of composite cavity filters with cavity lengths of 21 - 90 cm, cavity length differences and cavity length ratios both less than 1 / 10, and cross-coupling ratios of 50% - 95% are designed. Under the condition of meeting the performance parameters of the Fabry-Perot filter and the matching of the longitudinal mode spacing of the main cavity, the parameters are gradually fine-tuned to increase the side mode suppression ratio and achieve the output of a single longitudinal mode of laser with concentrated energy.

[0041] According to the transmission spectra of the composite cavity with different coupling ratios, cavity lengths and cavity length ratios simulated by the coupled mode theory, different comb spectra with different side mode suppression ratios and different main longitudinal mode bandwidths can be obtained by changing the coupling ratio. As the coupling ratio decreases, its transmittance, different side mode suppression ratios, and full width at half maximum gradually decrease. As the ratio of the cavity length to the cavity length difference decreases, its side mode suppression ratio gradually decreases while the full width at half maximum gradually increases. The standard for parameter selection 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 and the higher the transmittance, the better. Considering the influence of various parameters and through research and analysis, it is concluded that reasonable design of the composite cavity filter and its frequency selection characteristics 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 while keeping the cavity length difference unchanged. 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 longitudinal mode center wavelength can be effectively shortened, and a filtering resonator with high repetition accuracy and high wavelength resolution can be realized by cooperating with the Fabry-Perot filter for frequency selection.

[0042] To achieve the longitudinal mode energy distribution of the ring resonator, a method for optimizing the coherence characteristics of the resonator based on the chirped grating 42 is adopted. The grating period of the chirped fiber grating is not a constant, but monotonically changes along the axial direction. Different grating periods correspond to different reflection wavelengths, and a very wide reflection band can be formed. Different positions of the chirped grating 42 reflect incident light of different wavelengths, and the wavelength of the reflected incident light gradually increases along the axial direction. Since the optical period of the chirped grating 42 gradually linearly changes along the grating axis, the chirped grating 42 can be regarded as the superposition of several gratings with increasing or decreasing reflection wavelengths. The short wavelengths reflected by the chirped grating 42 will be reflected at the tail of the grating, and the long wavelengths will be reflected at the front accordingly. The light waves of different wavelengths correspond to the reflections of each small section of the grating, making the optical path monotonically increase or decrease with the wavelength. A single-frequency fiber laser with multiple composite cavities is constructed based on the chirped grating 42. By effectively utilizing the reflectivity and transmissivity of the chirped grating 42 at the same time, the optical path is multiplexed, greatly improving the cavity fundamental frequency and coherence length of the entire ring cavity, and effectively realizing linewidth optimization. By building a chirped Bragg grating structure with a dual circulator, a multi-subcavity structure can be formed in the resonator, enabling the single-mode laser to circulate and cohere in the resonator. Due to the losses of the circulator and the chirped grating 42 and the influence of the transmittance and reflectivity, more circulations will surely bring greater attenuation, and the later the optical path, the smaller the influence on the cavity fundamental frequency of the entire ring cavity. Therefore, this structure greatly improves the cavity fundamental frequency, which is beneficial to further narrowing the linewidth.

[0043] The expression of the free spectral range of the composite resonator satisfied by the ring resonator main cavity 1, the first resonator sub-cavity 2, and the second resonator sub-cavity 3 is as follows:

[0044]

[0045] Among them, FSR represents the free spectral range of the composite resonator, Q represents the mode coefficient of the ring resonator main cavity 1, c represents the speed of light, n eff represents the refractive index of the medium, L represents the cavity length of the ring resonator main cavity 1, q1 represents the mode coefficient of the first resonator sub-cavity 2, l1 represents the cavity length of the first resonator sub-cavity 2, q2 represents the mode coefficient of the second resonator sub-cavity 3, and l2 represents the cavity length of the second resonator sub-cavity 3.

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

[0047]

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

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

[0050] The third resonator cavity 4 includes at least a first distribution optical path, a second distribution optical path, and a third distribution optical path, where

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

[0052] In the second distribution optical path, the narrow-linewidth continuous light 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, 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 into the first port of the second circulator 43 through the third port of the first circulator 41 and the single-mode optical fiber 44. The narrow-linewidth continuous light is input into 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 into the second port of the second circulator 43, and is output from the third port of the second circulator 43;

[0053] In the third optical distribution path, the narrow linewidth continuous light 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, and the narrow linewidth continuous light is reflected on the right side of the chirped grating 42 to re-enter the second port of the first circulator 41. Then the narrow linewidth continuous light is input to the single-mode fiber 44 through the third port of the first circulator 41 in sequence. The narrow linewidth continuous light is incident on the chirped grating 42 through the first port and the second port of the second circulator 43 in sequence. The narrow linewidth continuous light transmits through the chirped grating to re-enter the second port of the first circulator 41, and the narrow linewidth continuous light is 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 in sequence, and the narrow linewidth continuous light is output from the third port of the second circulator 43.

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

[0055] In this embodiment, by using the first, second, and third optical distribution paths, multi-channel splitting of narrow linewidth continuous light is achieved. Each optical distribution path enables the optical signal to circulate through different reflection and transmission paths (including reflections on the left and right sides of the chirped grating 42 and transmission through the single-mode fiber 44), ensuring that the output signals of all channels have the same high coherence and narrow linewidth characteristics. By constructing multiple optical distribution paths in the third resonator cavity 4, the equivalent cavity length of the system can be effectively adjusted. Different optical paths correspond to different cavity length adjustment methods, providing a flexible means for the distributed fiber optic sensing system to achieve multi-wavelength detection and optimize the sensing response, further enhancing the dynamic adaptation ability and measurement accuracy of the sensing system. In the second and third optical distribution paths, light is recoupled back to the corresponding ports of the first circulator 41 through the reflection of the chirped grating 42, forming a closed feedback loop, which is beneficial for further stabilizing the continuous light output. This feedback mechanism effectively suppresses external interference, enabling the entire fiber optic sensing system to maintain excellent signal stability and long-term operation reliability in a complex environment.

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

[0057]

[0058] where path1 represents the proportional weight of the first optical distribution 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 optical distribution path, L2 represents the cavity length of the second optical distribution path, D represents the dispersion coefficient of the single-mode fiber 434, α represents the transmission loss of the single-mode fiber 44, path2 represents the proportional weight of the second optical distribution path, path3 represents the proportional weight of the third optical distribution path, and i represents the serial number of the optical distribution path.

[0059] Due to the existence of circulator loss and chirped grating loss, the subsequent possible optical paths may have a relatively weak overall light intensity due to loss, and the influence on the main ring cavity is reduced, so they will not be elaborated here one by one. By reasonably controlling the equivalent cavity lengths L i (i = 1, 2, 3) and the coupling weights σ i (i = 1, 2, 3), the fundamental frequency f base of the laser cavity can be effectively increased. The fundamental frequency formed by the three optical paths can be expressed as:

[0060]

[0061] The pulse modulation module 6 is respectively connected to the first arbitrary waveform generator 5 and the data acquisition module 7. 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 weakly reflective fiber grating array 65, and a second arbitrary waveform generator 66. Among them,

[0062] The optical pulse unit 61 is respectively connected to the third end of the first coupler 12, the second arbitrary waveform generator 66, and the third amplifier 62. 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 weakly reflective fiber 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. The second arbitrary waveform signal generator is used to modulate the optical pulse unit 61 and synchronously acquire data.

[0063] In this embodiment, the optical pulse unit 61 is modulated by the second arbitrary waveform generator 66 and works synchronously with the data acquisition module 7 to ensure that the emission of laser pulses and the signal acquisition are precisely matched in time, thereby improving the data acquisition accuracy and the system response speed. The third amplifier 62 and the second tunable filter 63 work together to amplify and adjust the frequency characteristics of the modulated optical pulse signal, which is beneficial to optimizing the signal waveform and enhancing the dynamic range and detection sensitivity of the fiber optic sensing system. The combination of the third circulator 64 and the multi-wavelength weakly reflective fiber grating array 65 realizes the splitting, reflection, and integration of optical signals with different wavelengths, thereby supporting the requirements of multi-wavelength sensing and providing high-quality optical signals for multiple measurement channels. The modules are interconnected through precisely designed connection methods to achieve linkage. The second arbitrary waveform generator 66 not only modulates the optical pulse unit 61 but also is interconnected with the data acquisition module 7, providing a flexible and efficient modular integration solution for the entire pulse modulation and signal acquisition process, which is convenient for future system maintenance and upgrade.

[0064] The data acquisition unit 71 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. Among them,

[0065] 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 respectively connected to the first photodetector 72, the second photodetector 73, and the third photodetector 74. The first photodetector 72 is connected to the third port of the fourth circulator 75. The first port of the fourth circulator 75 is connected to the third port of the third circulator 64. The second coupler 76 is respectively connected to the second port of the fourth circulator 75, the second photodetector 73, the delay optical fiber 77, the second Faraday rotator 79, and the third photodetector 74. The delay optical fiber 77 is also connected to 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 this embodiment, by connecting the data acquisition unit 71 to three photodetectors respectively, information from different signal branches can be captured simultaneously, so as to realize the efficient acquisition of multi-wavelength and multi-channel signals. The coordinated action of the fourth circulator 75 and the second coupler 76 enables the optical signal to be split or combined between different paths, facilitating the independent acquisition of each path signal. The combination of the delay optical fiber 77 and the Faraday rotator effectively introduces time-domain delay and feedback. By using the polarization-independent characteristic of the Faraday rotator, the signal distortion caused by polarization change is effectively compensated, thereby improving the signal stability. The introduction of the third amplifier 62 (erbium-doped fiber amplifier) enhances the detection ability of the system for weak signals and improves the overall signal-to-noise ratio. As a 3×3 optical coupler, the second coupler 76 not only realizes the effective splitting and combining of multiple 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, the second arbitrary waveform generator 66, and each detector forms a closed loop for pulse modulation, signal feedback, and data synchronous acquisition, thereby optimizing the time-domain modulation and signal acquisition process.

[0068] The multi-wavelength grating enhanced distributed fiber optic sensing system adopts the constructed tunable narrow linewidth laser. The continuous light generated by it is modulated into pulsed light by the optical pulse module. Since there is attenuation of some light during the modulation process, it is necessary to amplify to improve the signal-to-noise ratio, and a tunable filter is used to filter the amplified spontaneous emission noise outside the laser wavelength range. The narrow optical pulse is effectively amplified by the erbium-doped fiber amplifier and passes through the circulator, and is directly incident on the multi-wavelength weak reflection fiber Bragg grating array. The pulse sequence reflected back by the multi-wavelength weak reflection fiber Bragg grating array undergoes coherent interference in the unbalanced Michelson interferometer through path matching interference technology. The unbalanced Michelson interferometer consists of a 3×3 fiber coupler, the first Faraday rotator 78, the second Faraday rotator 79, and a 5m delay optical fiber 77. The three output interference signals of the 3×3 fiber coupler sequentially pass through the photodetector to complete photoelectric conversion and the channel high-speed acquisition card to complete data acquisition, and then perform phase demodulation and sensing signal recovery.

[0069] In this embodiment, by adopting the ring resonator main cavity 1 to form a laser oscillation cavity, the generation of highly coherent and narrow-linewidth continuous light is achieved, ensuring that the laser output has extremely low phase noise and high stability. Moreover, by the first resonator cavity 2 and the second resonator cavity 3, the longitudinal mode interval of the laser is effectively adjusted, the working stability of the single longitudinal mode is improved, and the side mode oscillation is suppressed, thereby further enhancing the purity and stability of the laser output. The third resonator cavity 4 forms multiple distribution optical paths with the single-mode optical fiber 44 through the first and second circulators 43 and the chirped grating 42. By using its flexible and adjustable equivalent cavity length, the effective distribution and regulation of multiple optical paths are realized, thereby optimizing the energy distribution in the laser cavity to improve the narrow-linewidth performance of the light source and meet the requirements of multi-wavelength distributed optical fiber sensing at the same time.

[0070] The above are only the preferred embodiments of the present invention and are not intended to limit 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 multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser, characterized in that, It includes a ring resonator main cavity (1), a first resonator cavity (2), a second resonator cavity (3), a third resonator cavity (4), a first arbitrary waveform generator (5), a pulse modulation module (6), and a data acquisition module (7). Among them, the ring resonator 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 resonator 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 resonator main cavity (1) is used to generate high-coherence narrow-linewidth continuous light; the first resonator cavity (2) is connected to the second resonator cavity (3). Both the first resonator cavity (2) and the second resonator cavity (3) are used to adjust the longitudinal mode interval of the laser; the third resonator cavity (4) includes a first circulator (41), a chirped grating (42), a second circulator (43), and a single-mode optical fiber (44). The third port of the second circulator (43) is connected to the first optical isolator (11). The second port of the second circulator (43) is connected to one end of the chirped grating (42). The third port of the second circulator (43) is connected to the third port of the first circulator (41) through the single-mode optical fiber (44). The second port of the first circulator (41) is connected to the other end of the chirped grating (42). The first port of the first circulator (41) is connected to the second resonator cavity (3). The third resonator cavity (4) is used to construct multiple distribution optical paths to adjust the equivalent cavity length of the third resonator cavity (4); the pulse modulation module (6) is respectively connected to the first arbitrary waveform generator (5) and the data acquisition module (7).

2. The multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser according to claim 1, wherein, 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 fiber grating array (65), and a second arbitrary waveform generator (66). Among them, the optical pulse unit (61) is respectively connected to the third port of the first coupler (12), the second arbitrary waveform generator (66), and the third amplifier (62). 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 fiber 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).

3. The multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser according to claim 2, wherein The data acquisition unit (71) 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 optical fiber (77), a first Faraday mirror (78), and a second Faraday mirror (79), where, 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 respectively connected to the first photodetector (72), the second photodetector (73), and the third photodetector (74). The first photodetector (72) is connected to the third port of the fourth circulator (75). The first port of the fourth circulator (75) is connected to the third port of the third circulator (64). The second coupler (76) is respectively connected to the second port of the fourth circulator (75), the second photodetector (73), the delay optical fiber (77), the second Faraday mirror (79), and the third photodetector (74). The delay optical fiber (77) is further connected to the first Faraday mirror (78).

4. The multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser according to claim 1, wherein, The expression of the free spectral range of the composite resonator satisfied by the ring resonator main cavity (1), the first resonator cavity (2), and the second resonator cavity (3) is: Among them, FSR represents the free spectral range of the composite resonator, Q represents the mode coefficient of the ring resonator main cavity (1), c represents the speed of light, n eff represents the refractive index of the medium, L represents the cavity length of the ring resonator main cavity (1), q1 represents the mode coefficient of the first resonator cavity (2), l1 represents the cavity length of the first resonator cavity (2), q2 represents the mode coefficient of the second resonator cavity (3), and l2 represents the cavity length of the second resonator cavity (3).

5. The multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser according to claim 1, characterized in that, The third resonator cavity (4) at least includes a first distribution optical path, a second distribution optical path, and a third distribution optical path, where, In the first distribution optical path, the narrow linewidth continuous light 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). The narrow linewidth continuous light transmits through the chirped grating (42), and the narrow linewidth continuous light is output from the third port 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 from the second port of the first circulator (41) to the chirped grating (42). 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 sequentially input into the first port of the second circulator (43) through the third port of the first circulator (41) and the single-mode optical fiber (44). The narrow linewidth continuous light is input into 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 into the second port of the second circulator (43), and is output from the third port 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 from the second port of the first circulator (41) to the chirped grating (42), 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 input into the single-mode fiber (44) through the third port of the first circulator (41) in sequence, the narrow linewidth continuous light is incident on the chirped grating (42) through the first port and the second port of the second circulator (43) in sequence, the narrow linewidth continuous light transmits through the chirped grating to re-input the narrow linewidth continuous light into the second port of the first circulator (41), and the narrow linewidth continuous light is 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) in sequence, and the narrow linewidth continuous light is output from the third port of the second circulator (43).

6. The multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser according to claim 5, wherein The proportional weight expressions of the first optical distribution path, the second optical distribution path, and the third optical distribution path are respectively as follows: Among them, path1 represents the proportional weight of the first distribution optical path, T1η1 represents the transmission efficiency of the first circulator (41), 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 amount 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.

7. The multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser according to claim 1, wherein Both the first resonator cavity (2) and the second resonator include two optically coupled devices connected end to end.

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

9. The multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser according to claim 2, wherein The first amplifier (13) is a semiconductor optical amplifier, and both the second amplifier (16) and the third amplifier (62) are erbium-doped fiber amplifiers.

10. The multi-wavelength distributed optical fiber sensing system based on a tunable narrow linewidth laser according to claim 2, characterized in that, The first arbitrary waveform signal generator is used to send trigger signals to the first tunable filter (15) and the second tunable filter (63) to synchronously modulate the bandwidth filtering ranges of the first tunable filter (15) and the second tunable filter (63), and the second arbitrary waveform signal generator is used to modulate the optical pulse unit (61) and synchronously acquire data.

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