Raman distributed optical fiber temperature sensing device and method based on differential square wave chirp

Abstract

The invention relates to the field of distributed optical fiber sensing, and discloses a Raman distributed optical fiber sensing device and method based on differential square wave chirp. In the device, laser output by a laser is modulated into square wave chirp modulated pulse light and negative square wave chirp modulated pulse light by a pulse light modulator, then the pulse light is amplified by a pulse erbium-doped optical fiber amplifier and then is divided into two beams by a coupler, one beam serves as detection light and enters a sensing optical fiber after passing through a wavelength division multiplexer, and the other beam serves as detection light after passing through a pulse erbium-doped optical fiber amplifier. Positive and negative square wave chirp modulation Raman back scattering signals generated in the sensing optical fiber are output by a wavelength division multiplexer and then are detected by a first photoelectric detector, and the other beam is directly detected by a second photoelectric detector as reference light; a detection signal is acquired by a data acquisition card and then sent to a computer for reconstruction to obtain an equivalent delta pulse Raman scattering response signal, temperature demodulation along the optical fiber is realized, and Raman distributed optical fiber temperature sensing with high spatial resolution and high system signal-to-noise ratio under a long sensing distance can be realized.

Description

Technical Field

[0001] The present invention relates to the field of distributed optical fiber sensing, and in particular to a Raman distributed optical fiber sensing device and method based on differential square wave chirp. Background Art

[0002] The Raman distributed fiber optic sensing system enables continuous, distributed measurement of temperature characteristics along a sensing fiber. In this system, temperature changes in the sensing fiber's environment modulate the intensity of Raman scattered light within the fiber. By collecting and analyzing these Raman scattered signals, which carry temperature information, distributed temperature data along the fiber can be obtained. Due to its strong environmental adaptability, excellent resistance to electromagnetic interference, wide detection range, and high measurement accuracy, this technology is widely used for temperature safety monitoring in coal mines, oil and gas pipelines, bridges, buildings, and other fields.

[0003] In Raman distributed fiber optic sensing systems, spatial resolution and system signal-to-noise ratio (SNR) are two crucial technical specifications. They directly reflect the minimum spatial scale at which the system can detect temperature changes in the optical fiber and its temperature sensitivity. These systems typically use pulsed signals as detection signals, using the pulse time-of-flight method to achieve localized temperature measurement. However, the Raman backscattered signal collected using this method does not originate from a single precise location on the optical fiber, but rather is the sum of signal intensities at all locations within the pulse width. Therefore, the spatial resolution of conventional Raman distributed sensing systems is often limited by the pulse width of the light source. While reducing the pulse width can improve spatial resolution, it also reduces the system SNR, limiting the effective sensing distance. Consequently, existing Raman distributed fiber optic sensing systems generally face the technical bottleneck of achieving a balanced balance between spatial resolution and system SNR over long sensing distances.

[0004] Based on this, it is necessary to invent a new Raman distributed fiber optic sensing method to solve the technical bottleneck that the spatial resolution of the existing Raman distributed fiber optic sensing system is limited by the pulse width, resulting in the inability to take into account both the spatial resolution and the system signal-to-noise ratio at long sensing distances. Summary of the Invention

[0005] In order to solve the technical bottleneck of existing Raman distributed fiber optic sensing systems, in which the spatial resolution is limited by the pulse width, resulting in the inability to balance the spatial resolution and system signal-to-noise ratio at long sensing distances, the present invention proposes a Raman distributed fiber optic temperature sensing device and method based on differential square wave chirp to achieve high spatial resolution and high signal-to-noise ratio temperature measurement along the optical fiber.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows: a Raman distributed optical fiber temperature sensing device based on differential square wave chirp, comprising: a laser, a pulse light modulator, a pulsed erbium-doped fiber amplifier, a coupler, a wavelength division multiplexer, a sensing optical fiber, a first photodetector, a second photodetector, a data acquisition card, and a computer;

[0007] The laser output by the laser is modulated into pulse light modulated by positive square wave chirp modulation and negative square wave chirp modulation by a pulse light modulator, then amplified by a pulsed erbium-doped fiber amplifier and divided into two beams by a coupler, one of which is used as detection light and then incident on the sensing fiber after passing through a wavelength division multiplexer. The positive square wave chirp modulated Raman backscattering signal and the negative square wave chirp modulated Raman backscattering signal generated in the sensing fiber are output by the wavelength division multiplexer and detected by the first photodetector, and the other beam is used as reference light and is directly detected by the second photodetector.

[0008] The detection signals of the first photodetector and the second photodetector are collected by a data acquisition card and sent to a computer. The computer is used to reconstruct the differential signals of the received positive square wave chirp modulated Raman backscattering signals and the negative square wave chirp modulated Raman backscattering signals according to the reference light signals corresponding to the received pulsed light of positive square wave chirp modulation and negative square wave chirp modulation, and locate and demodulate the temperature mutation points along the optical fiber according to the reconstructed equivalent delta pulse Raman scattering response.

[0009] The computer performs differential signal reconstruction on the received positive square wave chirp modulated Raman backscattering signal and the negative square wave chirp modulated Raman backscattering signal based on the reference optical signals corresponding to the received pulsed light modulated by positive square wave chirp and negative square wave chirp, and obtains the specific formula of equivalent delta pulse Raman scattering response as follows:

[0010]

[0011] Among them, x p ,x n represent the reference light corresponding to the pulse light modulated by positive square wave chirp and the pulse light modulated by negative square wave chirp, respectively. and They represent the square wave chirp modulated Raman backscattering signal I p (t) and negative square wave chirp modulated Raman backscattering signal I n Complex domain representation of (t);

[0012] The computer (10) demodulates the temperature mutation point along the optical fiber according to the reconstructed equivalent delta pulse Raman scattering response signal using the following formula:

[0013]

[0014] Where T represents the demodulation temperature, L represents the position in the sensing fiber, Δν represents the Raman frequency shift, h represents the Planck constant, k represents the Boltzmann constant, and T0 represents the calibration temperature. and are the equivalent delta pulse Raman scattering response signals at the sensing fiber position L obtained during calibration and measurement, respectively. abs is the amplitude operation, and A is the filter matching coefficient.

[0015] Square wave chirp modulated Raman backscattering signal I p (t) and negative square wave chirp modulated Raman backscattering signal I n The complex field of (t) and The formula is:

[0016]

[0017] Where H represents Hilbert transform.

[0018] The Raman distributed optical fiber temperature sensing device based on differential square wave chirp further includes a signal generator, the output end of which is electrically connected to the pulse light modulator, and is used to output a positive square wave chirp modulation signal and a negative square wave chirp modulation signal to drive the pulse light modulator, wherein the positive square wave chirp modulation signal and the negative square wave chirp modulation signal are respectively:

[0019]

[0020] Where X(t) represents the square wave intensity chirped pulse signal, and:

[0021]

[0022] Where i is the imaginary unit, ω0 is the starting frequency of the chirp, β is the sweep rate, t is the time, and sign represents the sign function. Represents a pulse with a pulse width of τ.

[0023] The laser (1) is a semiconductor laser with an operating wavelength of 1550 nm; the Raman backscattering signal detected by the first photodetector (8) is a Raman anti-Stokes scattered light signal.

[0024] In addition, the present invention also provides a Raman distributed optical fiber temperature sensing method based on differential square wave chirp, which is implemented based on a Raman distributed optical fiber temperature sensing device and includes the following steps:

[0025] S1, calibration stage: set the temperature of the sensing fiber to T0, control the pulse light modulator to output square wave chirp modulated pulse light; collect the square wave chirp modulated Raman backscattering signal I generated in the sensing fiber p0(t) and the reference light x corresponding to the square wave chirp modulated pulse light p0 Then, the pulse light modulator is controlled to output a negative square wave chirp modulated pulse light, and the positive square wave chirp modulated Raman backscattering signal I generated in the sensing fiber is collected. n0 (t) and the reference light x corresponding to the square wave chirp modulated pulse light n0 ;

[0026] S2, measurement phase: control the pulse light modulator to output square wave chirp modulated pulse light; collect the square wave chirp modulated Raman backscattering signal I generated in the sensing fiber p (t) and the reference light x corresponding to the square wave chirp modulated pulse light p Then, the pulse light modulator (3) is controlled to output a pulse light modulated by a negative square wave chirp, and the positive square wave chirp modulated Raman backscattering signal I generated in the sensing fiber is collected. n (t) and the reference light x corresponding to the square wave chirp modulated pulse light n ;

[0027] S3. Calculate the equivalent delta pulse Raman scattering response signal during the calibration and measurement phases. The calculation formula is:

[0028]

[0029] in, and They represent the square wave chirp modulated Raman backscattering signal I obtained in the calibration phase. p0 (t) and negative square wave chirp modulated Raman backscattering signal I n0 The complex domain representation of (t) is, and They represent the square wave chirp modulated Raman backscattering signal I measured in the measurement phase. p (t) and negative square wave chirp modulated Raman backscattering signal I n The complex domain representation of (t); ⊕ is the matched filter operation;

[0030] S4. Calculate the temperature information along the optical fiber based on the Raman scattering response signal of the equivalent delta pulse in the calibration and measurement phases. The calculation formula is:

[0031]

[0032] Where T represents the demodulation temperature, A represents the filter matching coefficient, L represents the temperature change position in the sensing fiber, ν represents the Raman frequency shift, h represents the Planck constant, k represents the Boltzmann constant, and T0 represents the calibration temperature. and represent the equivalent delta pulse Raman scattering response signals at the sensing fiber position L obtained during calibration and measurement, respectively; abs is the amplitude operation.

[0033] Square wave chirp modulated Raman backscattering signal I p (t) and negative square wave chirp modulated Raman backscattering signal I n The complex field of (t) and The formula is:

[0034]

[0035] Where H represents Hilbert transform.

[0036] In S1 and S2, the signal generator outputs a positive square wave chirp modulation signal and a negative square wave chirp modulation signal to control the pulse light modulator to output positive square wave chirp modulated pulse light and negative square wave chirp modulated pulse light, and the positive square wave chirp modulation signal and the negative square wave chirp modulation signal are respectively:

[0037]

[0038] Where X(t) represents the square wave intensity chirped pulse signal, and:

[0039]

[0040] Where i is the imaginary unit, ω0 is the starting frequency of the chirp, β is the sweep rate, t is the time, and sign represents the sign function. Represents a pulse with a pulse width of τ.

[0041] In the above S4, the temperature change position in the sensing optical fiber is determined by the pulse flight method.

[0042] The Raman distributed optical fiber temperature sensing method based on differential square wave chirp further includes the step of measuring the chirp matching coefficient A. The measurement method is as follows:

[0043] Set the temperature of the sensing fiber to T1, repeat step S1 to collect the square wave chirp modulated Raman backscattering signal I generated along the sensing fiber. p1 (t) and the reference light x corresponding to the square wave chirp modulated pulse light p1 Then, the pulse light modulator (3) is controlled to output a pulse light modulated by a negative square wave chirp, and the positive square wave chirp modulated Raman backscattering signal I generated along the sensing optical fiber is collected. n1 (t) and the reference light x corresponding to the square wave chirp modulated pulse light n1 ;

[0044] Calculate the equivalent delta pulse Raman scattering response signal corresponding to temperature T1, and the calculation formula is:

[0045]

[0046] Calculate the chirp matching coefficient A using the following formula:

[0047]

[0048] Where ν is the Raman frequency shift, h is the Planck constant, and k is the Boltzmann constant. and They represent the equivalent delta pulse Raman scattering response signals corresponding to the position L in the sensing optical fiber when the temperatures are T1 and T0, respectively.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] The present invention differentially reconstructs the Raman backscattering signal excited by a positive square wave chirped pulse signal and the Raman backscattering signal excited by a negative square wave chirped pulse, restores the amplitude and polarity information of the chirped pulse, and then uses the compressibility of the chirped pulse to obtain the equivalent Raman scattering of an extremely narrow delta pulse, so that its spatial resolution breaks through the pulse width limitation, that is, using a longer chirped pulse to achieve a longer sensing distance and signal-to-noise ratio, and the spatial resolution does not deteriorate with the deterioration of the sensing distance. At the same time, the two groups of pulses have consistent pulse widths and periods, and complementary amplitude and polarity. The joint demodulation improves the signal-to-noise ratio of the Raman distributed optical fiber sensing system, and ultimately realizes high spatial resolution and high signal-to-noise ratio distributed temperature measurement over a long sensing distance. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 A schematic structural diagram of a Raman distributed optical fiber temperature sensing device based on differential square wave chirp provided in Example 1 of the present invention;

[0052] In the figure: 1-laser, 2-pulse signal generator, 3-pulse optical modulator, 4-pulse erbium-doped fiber amplifier, 5-coupler, 6-wavelength division multiplexer, 7-sensing fiber, 8-photodetector, 9-data acquisition card, 10-computer. DETAILED DESCRIPTION

[0053] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, not all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0054] Example 1

[0055] like Figure 1 As shown, the first embodiment of the present invention provides a Raman distributed optical fiber temperature sensing device based on differential square wave chirping, comprising: a laser 1, a pulse light modulator 3, a pulsed erbium-doped fiber amplifier 4, a coupler 5, a wavelength division multiplexer 6, a sensing optical fiber 7, a first photodetector 8, a second photodetector 11, a data acquisition card 9, and a computer 10;

[0056] The continuous laser light output by the laser 1 is modulated into pulsed light with positive square wave chirp modulation and negative square wave chirp modulation by a pulse light modulator 3, and then amplified by a pulsed erbium-doped fiber amplifier 4 and split into two beams by a coupler 5. One beam is used as a detection light and then incident on a sensing fiber 7 after passing through a wavelength division multiplexer 6. The positive square wave chirp modulated Raman backscattering signal and the negative square wave chirp modulated Raman backscattering signal generated in the sensing fiber 7 are output by the wavelength division multiplexer 6 and detected by a first photodetector 8. The other beam is used as a reference light and is directly detected by a second photodetector 11.

[0057] The detection signals of the first photodetector 8 and the second photodetector 11 are collected by the data acquisition card 9 and sent to the computer 10. The computer 10 is used to reconstruct the differential signals of the received positive square wave chirp modulated Raman backscattering signals and negative square wave chirp modulated Raman backscattering signals based on the reference light signals corresponding to the received pulsed light with positive square wave chirp modulation and negative square wave chirp modulation, and locate and demodulate the temperature mutation points along the optical fiber based on the reconstructed equivalent delta pulse Raman scattering response.

[0058] Specifically, in this embodiment, the laser 1 is a semiconductor laser with an operating wavelength of 1550 nm.

[0059] Specifically, in this embodiment, the Raman backscattered signal detected by the first photodetector 8 is a Raman anti-Stokes scattered light signal.

[0060] Specifically, in this embodiment, end a of the wavelength division multiplexer 6 is connected to one output end of the coupler 5 , end b of the wavelength division multiplexer 6 is connected to the sensing optical fiber 7 , and end c of the wavelength division multiplexer 6 is connected to the first photodetector 8 .

[0061] Specifically, this embodiment further includes a signal generator 2 , the output end of which is electrically connected to the pulse light modulator 3 , for outputting a positive square wave chirp modulation signal and a negative square wave chirp modulation signal to drive the pulse light modulator 3 .

[0062] The sensing principle of the present invention is introduced below.

[0063] The analytical expression of the square wave intensity chirped pulse signal in the embodiment of the present invention is:

[0064]

[0065] i is the imaginary unit, ω0 is the starting frequency of the chirp, β is the sweep rate, t is the time, sign represents the sign function, Represents a pulse with a pulse width of τ. The sign function generates a square wave chirp, and the rect function is a rectangular window used to limit the pulse width to τ. Compared with the traditional sinusoidal chirp, the square wave chirp has a higher extinction ratio and lower peak sidelobe after matched filtering. Therefore, the signal-to-noise ratio of the system modulated by square wave chirp is better than that of the sinusoidal chirp. The chirped pulse signal can be matched filtered with itself to obtain a δ pulse with an extremely narrow width. Then we can obtain the equivalent Raman scattering response curve of the δ pulse through matched filtering. At this time, the spatial resolution of the system R S It is equal to the full width at half maximum (FWHM) of the δ pulse, that is, it is inversely proportional to the sweep rate and pulse width. This overcomes the limitation of pulse width on the spatial resolution of Raman distributed fiber sensing system in principle.

[0066]

[0067] Where c represents the speed of light, n represents the refractive index of the optical fiber, B represents the chirp rate of the chirp signal, and τ represents the pulse width of the delta pulse.

[0068] The square wave intensity chirp pulse signal X(t) is a bipolar signal, while the optical pulse signals are all positive unipolar signals. Therefore, it is split into two parts by differential means to retain the positive and negative information of the chirp signal amplitude, that is, the positive square wave chirp modulation signal X p (t) and negative square wave chirp modulation signal X n (t), whose expressions are:

[0069]

[0070] Among them, the square wave chirp modulation signal X p (t) and negative square wave chirp modulation signal X n (t) The following conditions are met:

[0071] X(t)=X p (t)-X n (t); (4)

[0072] In this embodiment, the square wave chirp modulation signal X p (t) and negative square wave chirp modulation signal X n (t) The pulse optical modulator 3 is driven to output square wave chirped modulated optical pulses x p and negative square wave chirp modulated optical pulse x n .x p ,xn The Raman backscattered signal stimulated by the pulse in the optical fiber can be expressed as:

[0073]

[0074] Where K a represents the coefficient related to the Raman anti-Stokes backscattering cross section, λ is the wavelength of the Raman anti-Stokes scattering signal, α0, α as are the loss coefficients of incident light and anti-Stokes light per unit length in the sensing fiber, c is the speed of light, n is the refractive index, represents convolution, R as [T] is the temperature modulation function of the anti-Stokes scattered light, which is:

[0075]

[0076] Where ν is the Raman frequency shift, h is the Planck constant, k is the Boltzmann constant, and T is the temperature of the sensing fiber.

[0077] Raman scattering is insensitive to phase, and phase information is not necessary. Complex domain amplitude demodulation can eliminate phase noise, extract the effective amplitude envelope in the signal, and improve the system signal-to-noise ratio. However, the Raman backscattered signals collected in the device are all positive real signals, and the Hilbert transform H[·] is needed to convert them from the real domain to the complex domain, that is,

[0078]

[0079] Among them, H represents Hilbert transform, which can be written in integral form, namely:

[0080]

[0081] Where τ represents the integral variable, and I(τ) represents the Raman scattering signal intensity at time τ.

[0082] In this embodiment, the two sets of Raman backscattered signals generated by positive square wave chirp modulation and negative square wave chirp modulation and the reference signal are differentiated and matched filtered in the complex domain to obtain an equivalent delta pulse Raman scattering response signal. The specific calculation process can be expressed as follows:

[0083]

[0084] Where ⊕ is the matched filter operation. As mentioned above, the positive and negative amplitude statistical characteristics of the original square wave chirp are retained through the differential operation, which will not affect the subsequent amplitude demodulation. From the above process, it can be seen that the equivalent δ pulse Raman scattering response signal and the temperature modulation function R as[T] is directly related. Assuming that the chirp matched filter coefficient is A, the integral effect of the non-negative signal in the matched filter increases the amplitude at temperature T by a factor of A. Therefore, we have:

[0085]

[0086] Where abs is the amplitude operation, then combined with formula (7), it can be seen that the temperature at the optical fiber position L can be expressed as:

[0087]

[0088] When using formula (13) to demodulate the temperature, the chirp matching filter coefficient needs to be calibrated. The calibration process is as follows: collect the Raman scattered light intensity of the entire optical fiber at room temperature, and obtain Then set a certain temperature T1 to get The chirp matching coefficient A can be calculated using formulas (7) and (12).

[0089] Therefore, in this embodiment, the computer 10 performs differential signal reconstruction on the received positive square wave chirp modulated Raman backscattering signal and the negative square wave chirp modulated Raman backscattering signal based on the reference light signals corresponding to the received pulsed light modulated by positive square wave chirp and negative square wave chirp, and obtains the equivalent δ pulse Raman scattering response signal by the specific formula as shown in the above formula (11). The formula for demodulating the temperature mutation point along the optical fiber based on the reconstructed equivalent δ pulse Raman scattering response signal is shown in the above formula (13).

[0090] Example 2

[0091] A second embodiment of the present invention provides a Raman distributed optical fiber temperature sensing method based on differential square wave chirp, which is implemented based on the Raman distributed optical fiber temperature sensing device described in the first embodiment and includes the following steps:

[0092] S1, calibration stage: set the temperature of the sensing fiber to T0, control the pulse light modulator 3 to output square wave chirp modulated pulse light; collect the square wave chirp modulated Raman backscattering signal I generated along the sensing fiber 7 p0 (t) and the reference light x corresponding to the square wave chirp modulated pulse light p0 Then, the control pulse light modulator 3 outputs a negative square wave chirp modulated pulse light, collecting the positive square wave chirp modulated Raman backscattering signal I generated along the sensing fiber 7 n0 (t) and the reference light x corresponding to the square wave chirp modulated pulse light n0 .

[0093] S2, measurement phase: controlling the pulse light modulator (3) to output square wave chirp modulated pulse light; collecting the square wave chirp modulated Raman backscattering signal I generated in the sensing optical fiber (7) p (t) and the reference light x corresponding to the square wave chirp modulated pulse light p Then, the pulse light modulator (3) is controlled to output a pulse light modulated by a negative square wave chirp, and the positive square wave chirp modulated Raman backscattering signal I generated in the sensing optical fiber (7) is collected. n (t) and the reference light x corresponding to the square wave chirp modulated pulse light n .

[0094] In step S1 and step S2, the signal generator 2 outputs a positive square wave chirp modulation signal and a negative square wave chirp modulation signal to control the pulse light modulator 3 to output positive square wave chirp modulated pulse light and negative square wave chirp modulated pulse light.

[0095] S3. Calculate the equivalent delta pulse Raman scattering response signal during the calibration and measurement phases. The calculation formula is:

[0096]

[0097] in, and They represent the square wave chirp modulated Raman backscattering signal I obtained in the calibration phase. p0 (t) and negative square wave chirp modulated Raman backscattering signal I n0 The complex domain representation of (t) is, and They represent the square wave chirp modulated Raman backscattering signal I measured in the measurement phase. p (t) and negative square wave chirp modulated Raman backscattering signal I n The complex domain representation of (t); ⊕ is the matched filter operation.

[0098] Among them, the square wave chirp modulated Raman backscattering signal I p (t) and negative square wave chirp modulated Raman backscattering signal I n The complex domain representation of (t) is achieved through Hilbert transform.

[0099] S4. Calculate the temperature information along the optical fiber based on the Raman scattering response signal of the equivalent delta pulse in the calibration stage and the measurement stage. The calculation formula is the above formula (13).

[0100] Specifically, in step S4, the temperature change position in the sensing optical fiber is determined by a pulse flight method.

[0101] In addition, the Raman distributed optical fiber temperature sensing method based on differential square wave chirp of this embodiment further includes the step of measuring the chirp matching coefficient A. The measurement method is as follows:

[0102] (1) Set the temperature of the sensing fiber to T1, and repeat step S1 to collect the square wave chirp modulated Raman backscattering signal I generated along the sensing fiber 7) p1 (t) and the reference light x corresponding to the square wave chirp modulated pulse light p1 Then, the pulse light modulator (3) is controlled to output a negative square wave chirp modulated pulse light, and the positive square wave chirp modulated Raman backscattering signal I generated along the sensing optical fiber 7 is collected. n1 (t) and the reference light x corresponding to the square wave chirp modulated pulse light n1 ;

[0103] (2) Calculate the equivalent delta pulse Raman scattering response signal corresponding to temperature T1. The calculation formula is:

[0104]

[0105] (3) Calculate the chirp matching coefficient A.

[0106] Combining formulas (7) and (12), the calculation formula for the chirp matching coefficient A can be derived, specifically:

[0107]

[0108] Where ν is the Raman frequency shift, h is the Planck constant, and k is the Boltzmann constant. and They represent the equivalent delta-pulse Raman scattering response signals corresponding to position L in the sensing fiber at temperatures T1 and T0, respectively. Specifically, the chirp matching coefficient A can be calibrated by only obtaining the equivalent delta-pulse Raman scattering response signals at two known temperatures at one position in the fiber.

[0109] In summary, the present invention uses pulsed light modulated by positive square wave chirp as the detection signal, which has lower peak sidelobes and system signal-to-noise ratio. The square wave chirp is divided into positive square wave chirp and negative square wave chirp, and the two groups of Raman backscattered signals excited by it are differentially reconstructed, retaining the amplitude characteristics of the chirped Raman scattering signal; the delta pulse equivalent Raman scattering response is obtained by utilizing the delta pulse compression property and matched filtering operation of the chirped pulse signal. The new temperature-varying zone signal eliminates the limitation of the pulse width on the spatial resolution, making the system spatial resolution depend on the chirped pulse sweep rate and the inverse of the pulse width; and the complex domain amplitude demodulation eliminates the phase noise of Raman scattering and improves the system signal-to-noise ratio, ultimately realizing Raman distributed optical fiber temperature sensing with high spatial resolution and high system signal-to-noise ratio over a long sensing distance.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A Raman distributed optical fiber temperature sensing device based on differential square wave chirp, characterized in that: include: Laser (1), pulse light modulator (3), pulsed erbium-doped fiber amplifier (4), coupler (5), wavelength division multiplexer (6), sensing fiber (7), first photodetector (8), second photodetector (11), data acquisition card (9), computer (10); The laser light output by the laser (1) is modulated into pulse light modulated by positive square wave chirp and negative square wave chirp by a pulse light modulator (3), and then amplified by a pulsed erbium-doped fiber amplifier (4) and divided into two beams by a coupler (5), one of which is incident on a sensing optical fiber (7) after passing through a wavelength division multiplexer (6) as a detection light, and the positive square wave chirp modulated Raman backscattering signal and the negative square wave chirp modulated Raman backscattering signal generated in the sensing optical fiber (7) are output by the wavelength division multiplexer (6) and detected by a first photodetector (8), and the other beam is directly detected by a second photodetector (11) as a reference light; The detection signals of the first photodetector (8) and the second photodetector (11) are collected by a data acquisition card (9) and then sent to a computer (10). The computer (10) is used to reconstruct the differential signals of the received positive square wave chirp modulated Raman backscattering signals and negative square wave chirp modulated Raman backscattering signals according to the reference light signals corresponding to the received pulsed light of positive square wave chirp modulation and negative square wave chirp modulation, and locate and demodulate the temperature mutation points along the optical fiber according to the reconstructed equivalent delta pulse Raman scattering response.

2. The Raman distributed optical fiber temperature sensing device based on differential square wave chirp according to claim 1, characterized in that: The computer (10) performs differential signal reconstruction on the received positive square wave chirp modulated Raman backscattering signal and the negative square wave chirp modulated Raman backscattering signal based on the reference light signals corresponding to the received pulsed light modulated by positive square wave chirp and negative square wave chirp, and obtains the specific formula of equivalent delta pulse Raman scattering response as follows: Among them, x p ,x n represent the reference light corresponding to the pulse light modulated by positive square wave chirp and the pulse light modulated by negative square wave chirp, respectively. and They represent the square wave chirp modulated Raman backscattering signal I p (t) and negative square wave chirp modulated Raman backscattering signal I n Complex domain representation of (t); The computer (10) demodulates the temperature mutation point along the optical fiber according to the reconstructed equivalent delta pulse Raman scattering response signal using the following formula: Where T represents the demodulation temperature, L represents the position in the sensing fiber, Δν is the Raman frequency shift, h is the Planck constant, k is the Boltzmann constant, and T0 represents the calibration temperature. and are the equivalent delta pulse Raman scattering response signals at the sensing fiber position L obtained during calibration and measurement, respectively. abs is the amplitude operation, and A is the filter matching coefficient.

3. The Raman distributed optical fiber temperature sensing device based on differential square wave chirp according to claim 1, characterized in that: Square wave chirp modulated Raman backscattering signal I p (t) and negative square wave chirp modulated Raman backscattering signal I n The complex field of (t) and The formula is: Where H represents Hilbert transform.

4. The Raman distributed optical fiber temperature sensing device based on differential square wave chirp according to claim 1, characterized in that: The device further comprises a signal generator (2), the output end of which is electrically connected to a pulse light modulator (3) and is used to output a positive square wave chirp modulation signal and a negative square wave chirp modulation signal to drive the pulse light modulator (3), wherein the positive square wave chirp modulation signal and the negative square wave chirp modulation signal are respectively: Where X(t) represents the square wave intensity chirped pulse signal, and: Where i is the imaginary unit, ω0 is the starting frequency of the chirp, β is the sweep rate, t is the time, and sign represents the sign function. Represents a pulse with a pulse width of τ.

5. The Raman distributed optical fiber temperature sensing device based on differential square wave chirp according to claim 1, characterized in that: The laser (1) is a semiconductor laser with an operating wavelength of 1550 nm; the Raman backscattering signal detected by the first photodetector (8) is a Raman anti-Stokes scattered light signal.

6. A Raman distributed optical fiber temperature sensing method based on differential square wave chirp, implemented based on a Raman distributed optical fiber temperature sensing device, characterized in that: The following steps are involved: S1, calibration stage: set the temperature of the sensing optical fiber to T0, control the pulse light modulator (3) to output square wave chirp modulated pulse light; collect the square wave chirp modulated Raman backscattering signal I generated in the sensing optical fiber (7) p0 (t) and the reference light x corresponding to the square wave chirp modulated pulse light p0 Then, the pulse light modulator (3) is controlled to output a pulse light modulated by a negative square wave chirp, and the positive square wave chirp modulated Raman backscattering signal I generated in the sensing optical fiber (7) is collected. n0 (t) and the reference light x corresponding to the square wave chirp modulated pulse light n0 ; S2, measurement phase: controlling the pulse light modulator (3) to output square wave chirp modulated pulse light; collecting the square wave chirp modulated Raman backscattering signal I generated in the sensing optical fiber (7) p (t) and the reference light x corresponding to the square wave chirp modulated pulse light p Then, the pulse light modulator (3) is controlled to output a pulse light modulated by a negative square wave chirp, and the positive square wave chirp modulated Raman backscattering signal I generated in the sensing optical fiber (7) is collected. n (t) and the reference light x corresponding to the square wave chirp modulated pulse light n ; S3. Calculate the equivalent delta pulse Raman scattering response signal during the calibration and measurement phases. The calculation formula is: in, and They represent the square wave chirp modulated Raman backscattering signal I obtained in the calibration phase. p0 (t) and negative square wave chirp modulated Raman backscattering signal I n0 The complex domain representation of (t) is, and They represent the square wave chirp modulated Raman backscattering signal I measured in the measurement phase. p (t) and negative square wave chirp modulated Raman backscattering signal I n Complex domain representation of (t); is the matched filter operation; S4. Calculate the temperature information along the optical fiber based on the Raman scattering response signal of the equivalent delta pulse in the calibration and measurement phases. The calculation formula is: Where T represents the demodulation temperature, A represents the filter matching coefficient, L represents the temperature change position in the sensing fiber, ν represents the Raman frequency shift, h represents the Planck constant, k represents the Boltzmann constant, and T0 represents the calibration temperature. and represent the equivalent delta pulse Raman scattering response signals at the sensing fiber position L obtained during calibration and measurement, respectively; abs is the amplitude operation.

7. The Raman distributed optical fiber temperature sensing method based on differential square wave chirp according to claim 6, characterized in that: Square wave chirp modulated Raman backscattering signal I p (t) and negative square wave chirp modulated Raman backscattering signal I n The complex field of (t) and The formula is: Where H represents Hilbert transform.

8. The Raman distributed optical fiber temperature sensing method based on differential square wave chirp according to claim 6, characterized in that: In S1 and S2, the signal generator (2) outputs a positive square wave chirp modulation signal and a negative square wave chirp modulation signal to control the pulse light modulator (3) to output a positive square wave chirp modulated pulse light and a negative square wave chirp modulated pulse light, wherein the positive square wave chirp modulation signal and the negative square wave chirp modulation signal are respectively: Where X(t) represents the square wave intensity chirped pulse signal, and: Where i is the imaginary unit, ω0 is the starting frequency of the chirp, β is the sweep rate, t is the time, and sign represents the sign function. Represents a pulse with a pulse width of τ.

9. The Raman distributed optical fiber temperature sensing method based on differential square wave chirp according to claim 6, characterized in that: In the above S4, the temperature change position in the sensing optical fiber is determined by a pulse flight method.

10. The Raman distributed optical fiber temperature sensing method based on differential square wave chirp according to claim 6, characterized in that: The method also includes the step of measuring the chirp matching coefficient A, wherein the measurement method is as follows: Set the temperature of the sensing fiber to T1, repeat step S1 to collect the square wave chirp modulated Raman backscattering signal I generated along the sensing fiber (7) p1 (t) and the reference light x corresponding to the square wave chirp modulated pulse light p1 Then, the pulse light modulator (3) is controlled to output a pulse light modulated by a negative square wave chirp, and the positive square wave chirp modulated Raman backscattering signal I generated along the sensing optical fiber (7) is collected. n1 (t) and the reference light x corresponding to the square wave chirp modulated pulse light n1 ; Calculate the equivalent delta pulse Raman scattering response signal corresponding to temperature T1, and the calculation formula is: Calculate the chirp matching coefficient A using the following formula: Where ν is the Raman frequency shift, h is the Planck constant, and k is the Boltzmann constant. and They represent the equivalent delta pulse Raman scattering response signals corresponding to the position L in the sensing optical fiber when the temperatures are T1 and T0, respectively.

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