A self-correlation active optical fiber cavity ring-down quasi-distributed temperature sensing method and device

Abstract

This invention discloses an autocorrelation active fiber cavity ring-down quasi-distributed temperature sensing method and apparatus. The sensing apparatus includes a light source, a first three-port circulator, a plurality of active fiber cavities, a reference CFBG, and a photodetector. The light source is connected to the first three-port circulator, which is also connected to the reference CFBG. The first three-port circulator is connected to the active fiber cavities. Each active fiber cavity consists of a CFBG, a second three-port circulator, an erbium-doped fiber amplifier, a first fiber coupler, and a second fiber coupler. The plurality of active fiber cavities are connected in series, and the reflected light signals are cyclically transmitted and amplified through the plurality of active fiber cavities to achieve multi-point temperature detection. The output end of each active fiber cavity is connected to the photodetector, and the output end of the photodetector is connected to a data acquisition card. The data acquisition card is connected to an external terminal.

Description

Technical Field

[0001] This invention relates to the field of sensor technology, and in particular to an autocorrelation active fiber cavity quasi-distributed temperature sensing method and apparatus. Background Technology

[0002] A CFBG (chirp Fiber Bragg Grating) is a type of fiber grating where the refractive index variation in the fiber core or the period of the refractive index variation gradually increases (decreases) along the fiber axis. Different wavelengths of incident light can be reflected at different positions along the CFBG axis. Therefore, CFBGs are characterized by a wide reflection spectrum and a gradually changing group delay within the reflection bandwidth. The slope of the group delay curve represents the dispersion value of the fiber grating. Traditional fiber cavity ring-down sensing systems based on pulsed light measure the ring-down time of light pulses within the fiber cavity. They offer advantages such as simple system structure and strong resistance to electromagnetic interference, making them suitable for high-precision temperature and strain detection. However, these systems require strict control over the matching relationship between the pulse width and the fiber cavity length to avoid pulse aliasing. Specifically, the pulse width must be much smaller than the round-trip time of light within the fiber cavity, which is proportional to the cavity length. Therefore, traditional methods typically require cavity lengths of several hundred meters to meet these requirements, directly leading to reduced sensitivity and slower response speed. Furthermore, traditional techniques often employ pulsed light, requiring pulsed lasers or electro-optic modulators to pulse the light source, making it difficult to simplify the sensing system structure and further reduce system costs, significantly limiting their practical application. Additionally, the pulsed light injection scheme results in unstable power within the fiber cavity, causing gain fluctuations in the erbium-doped fiber amplifier and reducing the stability of the sensing system. Summary of the Invention

[0003] In view of the above-mentioned prior art, the present invention provides an autocorrelation active fiber cavity ring-down quasi-distributed temperature sensing method and device, which mainly solves the technical problems existing in the above-mentioned background art.

[0004] To achieve the above objectives, the technical solution of this invention is implemented as follows: The first aspect of this invention discloses an autocorrelation active fiber cavity ring-down quasi-distributed temperature sensing device. The sensing device includes a light source, a first three-port circulator, several series-connected active fiber cavities, a reference CFBG, and a photodetector. The output end of the light source is connected to the first port of the first three-port circulator, the second port of the first three-port circulator is connected to the input end of the reference CFBG, the reflected light of the reference CFBG is transmitted back through the second port of the first three-port circulator and exported from its third port, and the third port of the first three-port circulator is connected to the active fiber cavity. Each active fiber cavity consists of a CFBG, a second three-port circulator, an erbium-doped fiber amplifier, a first fiber coupler, and a second fiber coupler. The reflected light signal is cyclically transmitted and amplified through multiple active fiber cavities to achieve multi-point temperature detection. The output end of each active fiber cavity is connected to the photodetector, the output end of the photodetector is connected to the data acquisition card, and the data acquisition card is connected to an external terminal.

[0005] Optionally, the third port of the first three-port circulator is connected to the input of the first fiber optic coupler via a second isolator; The output port of the first fiber coupler is connected to the input port of the erbium-doped fiber amplifier, and the output port of the erbium-doped fiber amplifier is connected to the first port of the second three-port circulator. The second port of the second three-port circulator is connected to the corresponding CFBG in the CFBG array; The third port of the second three-port circulator is connected to the input port of the second fiber optic coupler, and the output of the second fiber optic coupler is connected to the photodetector.

[0006] Optionally, the CFBG array is composed of multiple identical weak reflectivity CFBGs connected in series, adjacent CFBGs are connected by optical fiber delay lines, and the end CFBG is connected in series with a third isolator.

[0007] Optionally, the wavelength range of the light source is 1520nm to 1620nm.

[0008] Optionally, the splitting ratio of the first fiber coupler to the second fiber coupler is not less than 90:10.

[0009] Optionally, the center reflection wavelength of the reference CFBG reference temperature is 1560 nm.

[0010] Optionally, the center reflection wavelength of each CFBG in the CFBG array is 1565nm.

[0011] A second aspect of this invention discloses an autocorrelation-based active fiber cavity ringback quasi-distributed temperature sensing method, characterized in that the sensing method is applied to the sensing device described in any of the preceding claims, and the sensing method includes the following steps: The optical signal is received by the photodetector after being reflected by the CFBG array and transmitted through the optical path. The optical signal is converted into an electrical signal and then transmitted to the data acquisition card, which records and stores the electrical signal data. Autocorrelation operation is performed on the electrical signal data to obtain autocorrelation oscillation curves, which contain autocorrelation peaks corresponding to each CFBG. The autocorrelation peaks in the autocorrelation decay curve are subjected to exponential fitting, and the autocorrelation decay time corresponding to each CFBG is extracted according to the definition of decay time. Based on the linear relationship between the reciprocal difference of autocorrelation ringing time and temperature T, the temperature information of each CFBG location is calculated.

[0012] Optionally, performing autocorrelation calculations on the electrical signal data to obtain an autocorrelation oscillation curve specifically includes: extracting time-domain data within the effective attenuation period of the electrical signal data, multiplying the time-domain data with the time-domain data after a delay time D point by point to obtain a product sequence, integrating the product sequence to obtain the autocorrelation value corresponding to the delay time D, traversing all time delays to be analyzed to obtain a set of autocorrelation coefficients under different delays, and plotting all different delay times D and their corresponding autocorrelation values ​​one by one as a curve that changes with time, which is the autocorrelation oscillation curve.

[0013] Optionally, the established linear relationship between the reciprocal difference of autocorrelation ringing times and temperature T is as follows:

[0014] in, This is the actual autocorrelation oscillation time. denoted as the initial autocorrelation decay time, k as the sensitivity coefficient, and T as the temperature.

[0015] The beneficial effects of the present invention are as follows: (1) Traditional fiber cavity ring-down sensing systems require pulsed lasers or modulators to modulate continuous light into pulsed light, and rely on high-speed detectors and data acquisition equipment; the present invention adopts ASE broadband continuous light input, combined with autocorrelation signal processing technology, without the need for pulsed lasers or modulators, significantly reducing hardware costs and system complexity.

[0016] (2) This invention uses ASE broadband continuous light to replace the traditional pulsed light source, breaking through the strict matching limitation between pulse width and cavity length, and can achieve high-precision sensing without long-distance optical fiber. The pulse width of the autocorrelation pulse obtained by autocorrelation operation is very narrow, thus allowing the use of a shorter cavity length. This short cavity design can significantly improve sensitivity and response speed.

[0017] (3) Continuous light injection into the erbium-doped fiber amplifier (EDFA) can achieve lower gain fluctuations compared to the traditional pulsed light injection scheme. Its core advantage is that continuous light can stabilize the power in the fiber cavity, thereby greatly reducing the gain fluctuations of the EDFA and improving the stability of the sensing system.

[0018] (4) The autocorrelation demodulation technique can effectively remove the influence of ASE noise generated by the erbium-doped fiber amplifier, thereby improving the signal-to-noise ratio of the sensing system, further improving the stability of the sensing system, and thus improving the detection limit of the sensing system.

[0019] (5) Each CFBG, together with two high split-ratio fiber couplers, an erbium-doped fiber amplifier, a three-port circulator, and a fiber delay line, constitutes an active fiber cavity to achieve multi-point temperature detection. At the same time, by combining time-division multiplexing technology and precisely setting the fiber delay line length, different sensing channels are distinguished to avoid autocorrelation fading signal aliasing and achieve quasi-distributed temperature detection. Attached Figure Description

[0020] Figure 1 This is a structural block diagram of the sensing device in an embodiment of the present invention; Figure 2 This is a schematic diagram of the ASE source spectrum curve with a wavelength range of 1520nm to 1620nm and the reference CFBG reflection spectrum with a center reflection wavelength of 1560nm in an embodiment of the present invention. Figure 3 This is a schematic diagram illustrating the dynamic change in the overlap area between the reference CFBG reflectance spectrum and the CFBG array reflectance spectrum caused by temperature changes in an embodiment of the present invention. Figure 4 This is a schematic diagram illustrating the time-domain separation of autocorrelation decay signals from different temperature sensing channels using time-division multiplexing technology in an embodiment of the present invention. Figure 5 This is a flowchart of the sensing method in an embodiment of the present invention.

[0021] Explanation of icon numbers: 1-Light source, 2-1-First three-port circulator, 2-2-Second three-port circulator, 3-Reference CFBG, 4-1-First isolator, 4-2-Second isolator, 4-3-Third isolator, 5-1-First fiber coupler, 5-2-Second fiber coupler, 6-Erbium-doped fiber amplifier, 7-1-First fiber delay line, 7-2-Second fiber delay line, 7-3-Third fiber delay line, 8-1-First CFBG, 8-2-Second CFBG, 8-3-Third CFBG, 9-Photodetector, 10-Data acquisition card, 11-External terminal. Detailed Implementation

[0022] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. In the following description, the expression "some embodiments" refers to a subset of all possible embodiments; however, it should be understood that "some embodiments" can be the same subset or different subsets of all possible embodiments and can be combined with each other without conflict.

[0023] In the following description, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention can be practiced without one or more of these details. In other instances, certain technical features well-known in the art have not been described in order to avoid obscuring the invention.

[0024] It should be understood that the present invention can be embodied in various forms and should not be construed as being limited to the embodiments set forth herein. Rather, providing these embodiments will make the disclosure thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Furthermore, the terminology used herein is intended only to describe particular embodiments and is not intended to limit the invention. When used herein, the singular forms “a,” “an,” and “the” are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “compose” and / or “comprising,” when used in this specification, identify the presence of the stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups. When used herein, the term “and / or” includes any and all combinations of the associated listed items.

[0025] It should also be noted that when an element is referred to as being "fixed to" another element, it can be directly attached to the other element or there may be an intervening element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or there may be an intervening element. The terms "vertical," "horizontal," "inner," "outer," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only possible implementation.

[0026] To fully understand this invention, a detailed structure will be presented in the following description to illustrate the technical solution proposed by this invention. Optional embodiments of the invention are described in detail below; however, in addition to these detailed descriptions, the invention may have other embodiments.

[0027] Please refer to the attached document. Figure 1 The first aspect of this invention discloses an autocorrelation active fiber cavity ring-down quasi-distributed temperature sensing device. The sensing device includes a light source 1, a first three-port circulator 2-1, a plurality of active fiber cavities connected in series, a reference CFBG3, and a photodetector 9. The output end of the light source 1 is connected to the first port of the first three-port circulator 2-1, the second port of the first three-port circulator 2-1 is connected to the input end of the reference CFBG3, the reflected light of the reference CFBG3 is transmitted back through the second port of the first three-port circulator 2-1 and exported from its third port, and the third port of the first three-port circulator 2-1 is connected to the active fiber cavity. Each active fiber cavity consists of a CFBG, a second three-port circulator 2-2, an erbium-doped fiber amplifier 6, a first fiber coupler 5-1, and a second fiber coupler 5-2. Multiple active fiber cavities are connected in series to circulate and amplify the reflected light signal, thereby achieving multi-point temperature detection. The output end of each active fiber cavity is connected to the photodetector 9, and the output end of the photodetector is connected to the data acquisition card 10. The data acquisition card 10 is connected to the external terminal 11.

[0028] The third port of the first three-port circulator 2-1 is connected to the input end of the first fiber optic coupler 5-1 through the second isolator 4-2. The output port of the first fiber coupler 5-1 is connected to the input port of the erbium-doped fiber amplifier 6, and the output port of the erbium-doped fiber amplifier 6 is connected to the first port of the second three-port circulator 2-2. The second port of the second three-port circulator 2-2 is connected to the CFBG array; The third port of the second three-port circulator 2-2 is connected to the input port of the second fiber optic coupler 5-2, and the output port of the second fiber optic coupler 5-2 is connected to the photodetector 9.

[0029] In one or more embodiments of this application, the CFBG array is composed of multiple identical weak reflectivity CFBGs connected in series, adjacent CFBGs are connected by optical fiber delay lines, and the end CFBGs are connected in series with a third isolator 4-3.

[0030] Specifically, light source 1 is an ASE source. The optical signal output from light source 1 first enters the first port of the first three-port circulator 2-1. After being guided by the first three-port circulator 2-1, it is transmitted from the second port to the reference CFBG3. The reference CFBG3 reflects the optical signal of a specific wavelength. The reflected light returns along the original path to the second port of the first three-port circulator 2-1 and is then led out from the third port into the active fiber cavity. In each active fiber cavity, the optical signal is transmitted sequentially through the first fiber coupler 5-1, the erbium-doped fiber amplifier 6, and the second three-port circulator 2-2 to the corresponding CFBG. The CFBG reflects a specific optical signal according to the temperature characteristics of its location. The reflected light is transmitted back through the second three-port circulator 2-2, amplified again by the erbium-doped fiber amplifier 6, and then partially output to the photodetector 9 via the second fiber coupler 5-2. The remaining optical signal continues to circulate and amplify within the cavity. Since multiple active fiber optic cavities are connected in series, the optical signal passes through each cavity in sequence, so that the reflected signal corresponding to each CFBG can be effectively transmitted, amplified and finally captured by the photodetector 9. Then the electrical signal is transmitted to the external terminal 11 for processing and analysis via the data acquisition card 10.

[0031] Multiple cascaded active fiber optic cavities, combined with a CFBG array, enable simultaneous temperature detection at multiple monitoring points, expanding the system's monitoring range and meeting the requirements of quasi-distributed temperature sensing. The erbium-doped fiber amplifier 6 within each active fiber optic cavity cyclically amplifies the optical signal, effectively compensating for signal loss during transmission and improving signal strength and signal-to-noise ratio. This allows for accurate capture of even slight temperature-induced changes in optical signal characteristics. The CFBG settings provide a stable wavelength reference, helping to reduce the impact of environmental interference on measurement results and improving temperature detection accuracy. Simultaneously, the cooperation between the second three-port circulator 2-2 and the first and second fiber couplers 5-1 and 5-2 ensures efficient optical signal circulation within the cavity and effective signal output, guaranteeing system stability and reliability. This makes the entire sensing device promising for applications in industrial and environmental monitoring scenarios.

[0032] Specifically, multiple CFBGs constitute a CFBG array. In this embodiment, the CFBG array includes at least a first CFBG8-1, a second CFBG8-2, a third CFBG8-3, and a third isolator 4-3. The second port of the second three-port circulator 2-2 is connected to the first CFBG8-1 via a first fiber delay line 7-1. The first CFBG8-1 and the second CFBG8-2 are connected via a second fiber delay line 7-2. The second CFBG8-2 and the third CFBG8-3 are connected via a third fiber delay line 7-3. The third CFBG8-3 is connected to the third isolator 4-3.

[0033] As the core detection unit of temperature sensing, the CFBG array consists of three identical weak reflectivity devices: the first CFBG8-1, the second CFBG8-2, and the third CFBG8-3. These devices are arranged in series. Adjacent CFBGs are connected by optical paths through the first fiber delay line 7-1, the second fiber delay line 7-2, and the third fiber delay line 7-3, respectively. The third CFBG8-3 at the end is connected to the third isolator 4-3, forming a complete array structure. When the optical signal is output from the second port of the second three-port circulator 2-2, it is injected into the first CFBG 8-1 through the first fiber delay line 7-1. 10% of the light of the CFBG's center reflection wavelength is reflected back to the second port of the second three-port circulator 2-2 through the first fiber delay line 7-1. The remaining light passes through the first CFBG 8-1 and is injected into the second CFBG 8-2 through the second fiber delay line 7-2. 10% of the light of the CFBG's center reflection wavelength is reflected back to the first CFBG 8-1 through the second fiber delay line 7-2. The remaining light passes through the second CFBG 8-2 and is injected into the third CFBG 8-3 through the third fiber delay line 7-3. 10% of the light of the CFBG's center reflection wavelength is reflected back to the first CFBG 8-1 through the third fiber delay line 7-3. The second CFBG8-2, and the other light passes through the third CFBG8-3 and is injected into the third isolator 4-3 to prevent reflected light interference. 90% of the reflected light from the second CFBG8-2 and the third CFBG8-3 will pass through the first CFBG8-1 and the second CFBG8-2, and be injected into the second port of the second three-port circulator 2-2 through the fiber delay line. These reflected light signals are transmitted to the second fiber coupler 5-2 through the third port of the second three-port circulator 2-2. Part of the optical signal is sent to the photodetector 9 through its output port, converted into an electrical signal, and then transmitted to the external terminal 11 for processing through the data acquisition card 10, ultimately realizing temperature monitoring. The remaining optical signal is absorbed or exported by the third isolator 4-3 to prevent its back propagation from interfering with the entire optical path system.

[0034] The erbium-doped fiber amplifier 6 in the active fiber cavity provides continuous gain for the optical signal, effectively compensating for transmission loss, enhancing the optical signal strength, and improving the system's ability to detect subtle temperature changes. The cooperation of the first fiber coupler 5-1 and the second fiber coupler 5-2 achieves reasonable distribution of the optical signal, ensuring both the optical power entering the CFBG array and providing a stable detection signal for the photodetector 9. The 10% low reflectivity design of each grating in the CFBG array significantly reduces signal interference caused by multiple reflections. Combined with the timing differentiation function of the fiber delay line, the signals at each monitoring point are clearly separated in the time domain, facilitating accurate identification. The third isolator 4-3 further blocks interference from end-reflected light, ensuring optical path stability.

[0035] See Figure 4The collaborative design of multiple cascaded active fiber optic cavities, combined with time-division multiplexing technology implemented using fiber delay lines, enables accurate acquisition and identification of temperature information from multiple monitoring points, effectively realizing quasi-distributed temperature sensing functionality. Time-division multiplexing technology refers to distinguishing the autocorrelation and fading signals of multiple temperature measurement channels in the time domain by setting fiber delay lines of different lengths, thereby avoiding aliasing. The length L of the fiber delay line for the nth channel... n Set it according to the following formula:

[0036] In the formula, T max is the initial temperature autocorrelation ring-down time of the previous channel, c is the speed of light, and n is the refractive index of the fiber.

[0037] In one or more embodiments of this application, the splitting ratio of the first fiber coupler 5-1 to the second fiber coupler 5-2 is not less than 90:10, while in this embodiment, the splitting ratio of the first fiber coupler 5-1 and the second fiber coupler 5-2 is 90:10.

[0038] In one or more embodiments of this application, the wavelength range of the light source 1 is 1520 nm to 1620 nm, the center reflection wavelength of the reference CFBG3 is 1560 nm, and the center reflection wavelength of each CFBG in the CFBG array is 1565 nm.

[0039] Specifically, the wide wavelength range of the light source, from 11520nm to 1620nm, fully covers the 1560nm center reflection wavelength of the reference CFBG3 and the 1565nm center reflection wavelength of the CFBG array, ensuring that both receive sufficient optical signals for reflection and sensing. The 5nm difference in the center reflection wavelength between the reference CFBG3 and the CFBG array initially ensures a certain spectral overlap for signal correlation while also allowing for spectral shifts due to temperature changes. When the temperature changes, the reflection spectrum of the CFBGs in the CFBG array shifts towards longer or shorter wavelengths, significantly changing the overlap area with the reference CFBG3's reflection spectrum. This alters the ring-down characteristics of the active fiber cavity, making the change easily detectable and quantifiable, thus improving the sensitivity and resolution of temperature sensing. See Figure 5 The second aspect of this invention discloses an autocorrelation active fiber cavity ringback quasi-distributed temperature sensing method, characterized in that the sensing method is applied to the sensing device described in any of the preceding claims, and the sensing method includes the following steps: S1. The optical signal reflected by the CFBG array and transmitted through the optical path is received by the photodetector 9. The optical signal is converted into an electrical signal and then transmitted to the data acquisition card 10. The data acquisition card 10 records and stores the electrical signal data. S2. Perform autocorrelation operation on the electrical signal data to obtain autocorrelation oscillation curve, wherein the autocorrelation oscillation curve contains autocorrelation peaks corresponding to each CFBG; S3. Perform exponential fitting on the autocorrelation peaks in the autocorrelation decay curve, and extract the autocorrelation decay time corresponding to each CFBG according to the definition of decay time. S4. Based on the linear relationship between the reciprocal difference of autocorrelation ringing time and temperature T, the temperature information of each CFBG location is calculated.

[0040] Specifically, CFBGs exhibit high sensitivity to external parameters such as temperature, pressure, strain, and refractive index. Based on this, the temperature demodulation method achieves this by monitoring the change in ring-down time caused by the transformation of the overlap area between the reference CFBG3 reflection spectrum and the CFBG array reflection spectrum. See also Figure 2 The diagram shows a schematic of the ASE source spectrum with a wavelength range of 1520 nm to 1620 nm and the reflection spectrum of the reference CFBG3 with a center reflection wavelength of 1560 nm. After being filtered by the reference CFBG3, the broadband continuous light output from ASE source 1 has bandwidth and center wavelength that are almost identical to the reflection spectrum characteristics of the CFBG. (See also...) Figure 3 , Figure 3 The diagram illustrates the dynamic change in the overlap area between the reflection spectrum of the reference CFBG3 and the reflection spectrum of the CFBG array caused by temperature variations. At the initial temperature (T0), the overlap area between the reflection spectrum of the CFBG array and the reflection spectrum of the reference CFBG3 reaches its maximum value, corresponding to the highest reflected light power. This means that at the initial temperature, the CFBG loss B caused by the CFBGs in the CFBG array is the lowest, resulting in the longest ringing-out time. When the temperature at a certain location of a CFBG in the CFBG array rises to T0+ΔT, that CFBG experiences a wavelength redshift (Δλ) due to thermo-optical effects and thermal expansion. B =η*λ*ΔT), causing its reflection spectrum to shift towards longer wavelengths, significantly reducing the overlap area with the reference CFBG3 reflection spectrum. This results in a relative decrease in the optical power reflected back to the fiber cavity per round trip, which can be regarded as an increase in CFBG loss B. The CFBG loss B can be calculated by the following formula:

[0041] In the formula, P(λ) is the ASE spectral function and R(λ) is the FBG reflectance spectral function.

[0042] Erbium-doped fiber amplifier 6 is used to provide gain for the continuous optical signal. The amplified optical signal exhibits random characteristics in the time domain, and its autocorrelation function shows a very narrow delta-like characteristic of the FWHM (Fiber-Wide-Hill-Mix) wave. The CFBG (Continuous-Built-Glass Array) array achieves filtering by selectively reflecting light near its center wavelength (transmitting other wavelength components). It is noteworthy that although the CFBG alters the spectral composition, the reflected narrowband light retains its original time-domain random characteristics. This key characteristic allows the system output optical signal to form an autocorrelation-decayed signal after autocorrelation calculation. The autocorrelation calculation involves introducing a time-delayed replica signal and performing correlation analysis with the original signal: when the delay time equals the round-trip time t of the fiber cavity... r When the delay time is an integer multiple of t, the system will generate an autocorrelated ringing signal that decays exponentially. This correlation mechanism originates from the optical feedback design of the fiber optic cavity; the optical signal retained in each round trip provides the subsequent loop signal with a correlation to the original signal. Specifically, by dividing the ringing time into n time windows, when the delay time is set to i*t... τ At that time, the i-th time window of the original signal is precisely aligned with the first window of the replica signal. This mechanism ensures a strong correlation between the optical signal and the original signal at a specific time point, thereby generating a gradually decaying autocorrelation oscillation signal.

[0043] The attenuation of the autocorrelation ringing signal is due to the cumulative loss of the continuous optical signal in the fiber cavity. This fiber cavity loss includes the loss from the splitting ratio of the fiber coupler, fiber transmission loss, and CFBG loss B. As the round-trip time increases, the continuous optical signal in the fiber cavity gradually attenuates; therefore, the corresponding delay time is i*t. τ The autocorrelation signal exhibits an exponential decay pattern.

[0044] Furthermore, autocorrelation operation is performed on the electrical signal data to obtain the autocorrelation oscillation curve. Specifically, this includes: extracting time-domain data within the effective attenuation period of the electrical signal data, multiplying the time-domain data with the time-domain data after the delay time D point by point to obtain a product sequence, integrating the product sequence to obtain the autocorrelation value corresponding to the delay time D, traversing all the time delays to be analyzed to obtain a set of autocorrelation coefficients under different delays, and plotting all different delay times D and their corresponding autocorrelation values ​​one by one as a curve that changes with time, which is the autocorrelation oscillation curve.

[0045] Furthermore, the autocorrelation ringback time can be defined as the time required for the amplitude of the autocorrelation peak to decrease to 1 / e of the initial amplitude. At the initial temperature, the autocorrelation ringback time is... for:

[0046] In the formula, This is the initial cavity loss of the fiber optic cavity. It is the inherent cavity loss. is the gain of the EDFA, L is the cavity length, n is the refractive index of the fiber core, and c is the speed of light.

[0047] When the sensor head is heated or cooled, the CFBG reflection spectrum undergoes a redshift or blueshift, and the spectral overlap area decreases or increases. This can be considered as an increase or decrease in CFBG loss B, thereby shortening or lengthening the additional loss α caused by temperature. At this time, the autocorrelation ringback time will become:

[0048] In the formula, the additional losses of the sensor head due to temperature are... η is the loss coefficient caused by temperature.

[0049] The established linear relationship between the reciprocal difference of autocorrelation ringing times and temperature T is as follows:

[0050] in, This is the actual autocorrelation oscillation time. Let T be the initial autocorrelation decay time, k be the sensitivity coefficient, and T be the temperature. By monitoring the autocorrelation decay time, the temperature information can be obtained.

[0051] In summary, the scheme disclosed in this application employs a continuous light injection method, eliminating the need for an AOM or EOM to modulate continuous light into pulsed light, thus reducing the complexity of the system structure. The continuous light injection method ensures power stability within the fiber cavity, significantly reducing gain fluctuations in the erbium-doped fiber amplifier and greatly improving the stability of the sensing system. Simultaneously, the collaborative design of multiple cascaded active fiber cavities, combined with the time-domain differentiation mechanism achieved through fiber delay lines, enables accurate acquisition and identification of temperature information from multiple monitoring points, effectively realizing quasi-distributed temperature sensing and expanding the system's monitoring range and application scenarios. Furthermore, the use of autocorrelation demodulation technology results in an extremely narrow full width at half maximum (FWHM) of the autocorrelation pulse, greatly shortening the cavity length and improving the system's sensitivity and response speed. More importantly, the autocorrelation demodulation technology effectively reduces the impact of ASE noise generated by the erbium-doped fiber amplifier, thereby improving the signal-to-noise ratio, further enhancing the stability of the sensing system, and ultimately increasing the detection limit of the sensing system.

[0052] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. The scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A quasi-distributed temperature sensing device with autocorrelation active fiber cavity ringback quasi-distribution, characterized in that, The sensing device includes a light source, a first three-port circulator, several series-connected active fiber optic cavities, a reference CFBG, and a photodetector. The output end of the light source is connected to the first port of the first three-port circulator, the second port of the first three-port circulator is connected to the input end of the reference CFBG, the reflected light of the reference CFBG is transmitted back through the second port of the first three-port circulator and exported from its third port, and the third port of the first three-port circulator is connected to the active fiber cavity. Each active fiber cavity consists of a CFBG, a second three-port circulator, an erbium-doped fiber amplifier, a first fiber coupler, and a second fiber coupler. The reflected light signal is cyclically transmitted and amplified through multiple active fiber cavities to achieve multi-point temperature detection. The output end of each active fiber cavity is connected to the photodetector, the output end of the photodetector is connected to the data acquisition card, and the data acquisition card is connected to an external terminal. The third port of the first three-port circulator is connected to the input of the first fiber optic coupler via a second isolator. The output port of the first fiber coupler is connected to the input port of the erbium-doped fiber amplifier, and the output port of the erbium-doped fiber amplifier is connected to the first port of the second three-port circulator. The second port of the second three-port circulator is connected to the corresponding CFBG in the CFBG array; The CFBG array is composed of multiple CFBGs with identical weak reflectivity connected in series. Adjacent CFBGs are connected by optical fiber delay lines, and the end CFBG is connected in series with a third isolator. The third port of the second three-port circulator is connected to the input port of the second fiber optic coupler, and the output of the second fiber optic coupler is connected to the photodetector.

2. The autocorrelation active fiber cavity ring-down quasi-distributed temperature sensing device according to claim 1, characterized in that, The wavelength range of the light source is 1520nm to 1620nm.

3. The autocorrelation active fiber cavity ring-down quasi-distributed temperature sensing device according to claim 2, characterized in that, The splitting ratio of the first fiber coupler to the second fiber coupler is not less than 90:

10.

4. The autocorrelation active fiber cavity ring-down quasi-distributed temperature sensing device according to claim 1, characterized in that, The center reflection wavelength of the reference CFBG reference temperature is 1560 nm.

5. The autocorrelation active fiber cavity ring-down quasi-distributed temperature sensing device according to claim 1, characterized in that, The center reflection wavelength of each CFBG in the CFBG array is 1565nm.

6. A method for autocorrelation-based active fiber cavity ring-down quasi-distributed temperature sensing, characterized in that, The sensing method is applied to the sensing device as described in any one of claims 1-5, the sensing method comprising the following steps: The optical signal is received by the photodetector after being reflected by the CFBG array and transmitted through the optical path. The optical signal is converted into an electrical signal and then transmitted to the data acquisition card, which records and stores the electrical signal data. Autocorrelation operation is performed on the electrical signal data to obtain autocorrelation oscillation curves, which contain autocorrelation peaks corresponding to each CFBG. The autocorrelation peaks in the autocorrelation decay curve are subjected to exponential fitting, and the autocorrelation decay time corresponding to each CFBG is extracted according to the definition of decay time. Based on the linear relationship between the reciprocal difference of autocorrelation ringing time and temperature T, the temperature information of each CFBG location is calculated.

7. The autocorrelation active fiber cavity ringback quasi-distributed temperature sensing method according to claim 6, characterized in that, To obtain the autocorrelation cascading curve by performing autocorrelation operation on the electrical signal data, the following steps are taken: extracting time-domain data within the effective attenuation period of the electrical signal data, multiplying the time-domain data with the time-domain data after a delay time D point by point to obtain a product sequence, integrating the product sequence to obtain the autocorrelation value corresponding to the delay time D, traversing all the time delays to be analyzed to obtain a set of autocorrelation coefficients under different delays, and plotting all different delay times D and their corresponding autocorrelation values ​​one by one as a curve that changes with time, which is the autocorrelation cascading curve.

8. The autocorrelation active fiber cavity ringback quasi-distributed temperature sensing method according to claim 7, characterized in that, The established linear relationship between the reciprocal difference of autocorrelation ringing times and temperature T is as follows: in, This is the actual autocorrelation oscillation time. denoted as the initial autocorrelation decay time, k as the sensitivity coefficient, and T as the temperature.

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