Long-distance ultra-weak fiber grating sensing system optical power equalization method

Abstract

The application discloses a long-distance ultra-weak fiber grating sensing system optical power equalization method, calculates the power cost of the ultra-weak fiber grating sensing system optical path, and the size of the total power gain; according to the size of the total power gain, the number of EDFA1, EDFA2 and RFA and the pumping mode are preliminarily determined; the gain equation of the ultra-weak fiber grating at different positions is established; the simulation model of the optical system is constructed by considering the RFA same direction pumping; under the condition that the RFA pumping power is certain, the EDFA1 optimal power gain size, the EDFA2 optimal power gain size and the RFA optimal working position are determined through the model simulation; the RFA pumping power and the RFA optimal working position are determined through the model simulation by fixing the gain size of the EDFA1 and the gain size of the EDFA2. The optimal optical system design scheme when the RFA same direction pumping is determined. The application can realize the balance of the long-distance ultra-weak fiber grating sensing system power, so that the optical signal in the system can maintain stable transmission, and the sensing distance of the existing demodulation system can be extended by several times.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical fiber sensing technology, and in particular to a long-distance ultra-weak fiber Bragg grating (uwFBG) sensing system optical power equalization method. BACKGROUND

[0002] In an optical fiber communication system, the equalization of signal optical power is crucial for the normal operation of the optical fiber communication. For an optical communication system across a span section, the optical power between the optical fiber and the device is matched with each other, and the power fluctuation of each channel is within the allowed range, which is the key to ensure low bit error rate of optical signals in the system and to have a large dynamic range. If the optical power is unbalanced, it will cause problems such as poor signal-to-noise ratio of the channel, degraded performance of the receiver, and affect the performance and stability of the system.

[0003] Unlike long-distance optical fiber communication systems, in long-distance uwFBG sensing systems, signals need to be transmitted back and forth, and due to the effects of fiber transmission loss and nonlinearity, the optical power of the signals will be more severely unbalanced during fiber transmission. This imbalance will cause large power fluctuations in the sensing system, signal distortion, and the power of some position sensors may be lower than the sensitivity of the detector, making the sensing results inaccurate or even unable to work normally. Solving the power equalization problem in the uwFBG sensing system is the key to extending the sensing distance.

[0004] In traditional FBG sensing systems, document [1]: Rao Yunjiang, Feng Sha, Ran Zengling, et al. Ultra-long distance fiber Bragg grating sensing system [J]. Journal of University of Electronic Science and Technology of China, 2011, 40(05): 703-705+736. uses a combination of erbium-doped fiber amplifiers (EDFA) and Raman fiber amplifiers (RFA) to achieve a 300km ultra-long distance sensing system, and the signal-to-noise ratio of the system at 275km is still greater than 15dB. Document [2]: Cao Jianhua, Lin Shengtao, Wang Zinan, et al. Design and implementation of ultra-long distance fiber random laser multi-point sensing system [J]. Acta Optica Sinica, 2021, 41(13): 1306006. uses the characteristic that the power distribution of high-order random laser extends to the tail end, combined with FBG to achieve a 150km multi-point optical fiber sensing system.

[0005] However, the FBG reflectivity in the above system is as high as 90%, the signal strength is more than 30dB higher than that of uwFBG, and the number of FBG sensors is not more than 10, and the signal crosstalk is small. Directly introducing the EDFA+RFA scheme in the uwFBG sensing system can easily cause power imbalance of the signals at the front and back ends of the optical fiber, further degrading the signal-to-noise ratio of the end signals.

[0006] Document [3]: Jin S, Song W, Yang C, et al. 120km long distance distributed fiber vibration sensing system [J]. Optical Communications Research, 2021, 47(3): 20-24. It is proposed to alleviate the power imbalance problem by collecting the front and rear signals through double channels, but this scheme requires doubling the hardware configuration, resulting in a more complex system. In addition, the shadow effect of large-scale multiplexing of the same wavelength uwFBG cannot be ignored. Document [4]: Gao W J, Liu J X, Guo H Y, et al. Multi-Wave length ultra-weak fiber bragg grating arrays for long-distance quasi-distributed sensing [J]. Photonic Sensors, 2021, 12(2): 185-195. 10,000 uwFBG sensing units with a reflectivity of 0.01% are multiplexed on a 10km fiber, and the additional power drop of the tail gratings is more than 8dB, which further aggravates the power imbalance of the uwFBG sensing system.

[0007] Therefore, optical power balancing is the key to extending the sensing distance of the uwFBG system and is a necessary condition to ensure the accuracy and stability of the uwFBG sensing system. However, the current power balancing methods have limitations, and a new optical power balancing method is urgently needed. SUMMARY

[0008] To solve the above technical problems, the present application provides a long-distance ultra-weak fiber grating (uwFBG) sensing system optical power balancing method, which is based on the different characteristics of EDFA, RFA and amplification circuit, and uses a joint amplification method to realize the optical power balancing of the system at the optical and circuit levels, providing an excellent optical power balancing method for long-distance ultra-weak fiber grating sensing.

[0009] The technical scheme adopted by the present application is:

[0010] A long-distance ultra-weak fiber grating sensing system optical power balancing method, comprising the following steps:

[0011] Step 1: Calculate the power cost of the uwFBG sensing system optical path and the size of the total power gain required;

[0012] Step 2: According to the size of the total power gain calculated in step 1, preliminarily determine the number and pumping method of the erbium-doped fiber power amplifier EDFA1, the erbium-doped fiber power amplifier EDFA2 and the Raman optical amplifier RFA;

[0013] Step three: considering the fiber line loss, the dynamic gain of EDFA1, the dynamic gain of RFA, the dynamic gain of EDFA2, the gain equation of the ultra-weak fiber grating at different positions is established;

[0014] Step four: considering the co-directional pumping of RFA, the simulation model of the optical system is constructed, and the optimal power gain size of EDFA1, the optimal power gain size of EDFA2 and the optimal working position of RFA are determined through model simulation under the condition that the pumping power of RFA is constant; total

[0015] Step five: taking the imbalance range of G total as the constraint condition, fixing the gain size of EDFA1 and the gain size of EDFA2, the pumping power of RFA and the optimal working position of RFA are determined through model simulation.

[0016] Step six: comparing the simulation results of steps 4 and 5, the optimal optical system design scheme when the RFA is co-directionally pumped is determined.

[0017] Step seven: considering the reverse pumping of RFA, the simulation model of the optical system is used, the working distance of the system is set, and steps four to six are repeated to determine the optimal optical system design scheme.

[0018] Step eight: when the imbalance range of G total is greater than the threshold value, the segmented acquisition method is used, the gain g e of the programmable electric amplifier is set, the signal intensity of different acquisition segments is optimized, and the imbalance range of G total is ensured to be less than the threshold value.

[0019] Step nine: the parameter configuration of the programmable electric amplifier is determined, the segmented length is determined, the actual ultra-weak fiber grating sensing system is built, and the gain g e of the programmable electric amplifier is optimized according to the actual optical power balancing effect.

[0020] In step one, the working distance of the ultra-weak fiber grating sensing system, the output power of the light source, the reflectivity of the ultra-weak fiber grating, the dynamic range and the sensitivity of the photoelectric detector are considered, the power cost of the entire optical path and the size of the required total power gain are calculated, and the specific calculation is as follows:

[0021] G total = P0-2αL-D+r-P R

[0022] In the formula, G total ​The total gain size of the optical path is determined by the parameters on the right side of the equation; P0 is the in-fiber pulse optical power of the light source; a is the transmission loss coefficient of the signal light; L is the fiber length; D is the transmission loss coefficient of the signal light; r is the reflectivity of the same-wavelength grating in the array; P R is the sensitivity of the detector.

[0023] In step two, the method for preliminarily determining the number of erbium-doped fiber power amplifiers EDFA1, erbium-doped fiber power amplifiers EDFA2 and Raman optical amplifiers RFA and the pumping mode comprises:

[0024] According to the length of the transmission distance, the transmission loss coefficient and the reflectivity of the grating, in the case of long-distance transmission, a single amplifier and a single pumping mode cannot meet the gain requirement of the total power, at least two erbium-doped fiber amplifiers EDFA are required to preliminarily meet the gain requirement in the form of bidirectional pumping in front and back, and in addition, considering the case of wavelength division multiplexing, at least one Raman optical amplifier RFA is required to realize optical signal amplification in a wider wavelength range.

[0025] In step three, the gain equations of the ultra-weak fiber gratings at different positions are as follows:

[0026] G total ≤G E1 +10lg[(1-r) 2(n-1) r]-2a(L0+nd)+G Rn +G E2n

[0027] In the formula, G total represents the total gain size of the optical path after correction considering the number of gratings and the positions of the amplifiers; n is the grating number in the grating array; G E1 is the gain of the power optical amplifier EDFA1; G Rn is the gain of the RFA at the nth grating position; L0 is the fiber extension line; d is the spacing between the gratings; G E2n is the gain of the EDFA2 at the nth grating position.

[0028] In step four, the method for determining the optimal power gain size of EDFA1, the optimal power gain size of EDFA2 and the optimal working position of RFA comprises:

[0029] An optical system simulation model is used to calculate the curve of the signal power distribution on the fiber with the distance, and then the power fluctuation range size of the curve corresponding to different gain sizes and working positions is calculated, and when the unbalanced range threshold value is lower than G total , it is the optimal parameter value.

[0030] The step four, after determining the optimal working position of RFA, the method for determining the optimal configuration of the input fiber optical power comprises:

[0031] Using the simulation model of the optical system, the distribution of the signal power on the fiber with different input fiber optical powers is calculated, and when the maximum optical power of the remote signal and the power fluctuation range are lower than the imbalance range threshold of G total , the input fiber optical power is the optimal configuration.

[0032] The step five, the method for determining the pump power of RFA and the optimal working position of RFA comprises:

[0033] Using the simulation model of the optical system, the gain size of EDFA1 and the gain size of EDFA2 are fixed, the distribution of the signal power on the fiber with different pump powers and working positions of RFA is calculated, and then the power fluctuation range of the corresponding curve is calculated, and when it is lower than the imbalance range threshold of G total , it is the optimal parameter value.

[0034] The step six, when the RFA is co-pumped, the method for determining the optimal optical system design scheme when the RFA is co-pumped comprises:

[0035] Step four and step five respectively obtain two groups of optical system design schemes, which include the optimal power gain size of EDFA1, the optimal power gain size of EDFA2, the pump power of RFA and the optimal working position of RFA, and other main optical configurations. First, compare the final optical power cost and optical power gain brought by different schemes, and the scheme with smaller cost and larger gain is better; then compare the imbalance range of G total of different schemes, and the smaller scheme is better; and then compare the noise performance, power consumption cost, system compatibility and other parameters of the overall scheme, and finally determine the optimal optical system design scheme.

[0036] The step seven, when the RFA is counter-pumped, the method for determining the optimal optical system design scheme when the RFA is counter-pumped comprises:

[0037] In the case of fixing the counter-pumping mode of RFA, other processes are determined according to steps four to six to determine the optimal optical system design scheme when the RFA is counter-pumped.

[0038] The step eight, when the imbalance range of G total is greater than 3dB, the segmented acquisition method is used to set the gain g e of the programmable electric amplifier for the optical signal power of different segments, and the signal intensity of different acquisition segments is optimized to ensure that the imbalance range of G total is less than 3dB. The specific method comprises:

[0039] In the case of determining the EDFA1 optimal power gain size, the EDFA2 optimal power gain size, the pump power of the RFA and the optimal working position of the RFA and other main optical configurations, the signal power distribution on the optical fiber with the distance is calculated by using the simulation model of the optical system, the signal power is segmented according to the transmission distance, the range of the total gain size between different segments is calculated, and when the range value is greater than the unbalanced range threshold value of G total , the segment with the minimum total gain is set with a larger electrical amplifier gain, so as to achieve the effect of balanced amplification.

[0040] In step nine, the gain g e of the programmable electrical amplifier is optimized according to the actual optical power balancing effect.

[0041] In the case of the gain of the programmable electrical amplifier determined in step eight, the optical power distribution of the actual sensing system is tested, the signal power is segmented according to the transmission distance, the range of the total gain size between different segments is calculated, and if the expected value is not reached, the gain of the programmable electrical amplifier is dynamically adjusted through the feedback circuit.

[0042] An optical power balancing method of a long-distance ultra-weak fiber grating sensing system, which adopts EDFA, RFA and amplification circuit for joint amplification, and determines the peak power of the in-fiber pulse light amplified by the EDFA1 according to the reflected signal intensity and nonlinear noise.

[0043] Based on the particularity of the position of the EDFA1, the amplification effect will affect all subsequent signal powers. When the peak power of the in-fiber pulse light amplified by the EDFA1 is low, the reflected signal intensity is low, the signal-to-noise ratio is reduced, and higher requirements are put forward for the subsequent amplification configuration; when the peak power of the in-fiber pulse light amplified by the EDFA1 is high, nonlinear effects occur in the optical fiber, the signal is distorted, and the sensing performance is affected. Therefore, determining the peak power of the in-fiber pulse light amplified by the EDFA1 is a very important step in the whole power amplification and power balancing method. The joint amplification method based on the optical simulation system proposed in the present application can quantitatively and intuitively determine the peak power of the in-fiber pulse light amplified by the EDFA1.

[0044] The optical power balancing method of the long-distance ultra-weak fiber grating sensing system has the following technical effects:

[0045] 1) The present application can balance the power of the long-distance ultra-weak fiber grating sensing system, so that the optical signal in the system can maintain stable transmission, the sensing distance of the existing demodulation system can be extended by several times, the sensitivity and reliability of the sensing system are improved, and the application field of the ultra-weak fiber grating is widened.

[0046] 2) Considering the influence of nonlinear effect, the application limits the gain range of EDFA1, and at the same time, considering the economy of RFA, limits the RFA pump power to 600mA, simplifies the Optisystem simulation model, and quickly simulates the optimal working position of RFA. After determining the optimal working position of RFA, the simplified Optisystem simulation model is used to analyze the power distribution on the fiber under different input fiber optical powers. When the optical power of the far-end signal is maximum and the power fluctuation is small, the input fiber optical power at this time is the optimal configuration.

[0047] 3) The application also proposes to jointly optimize the optical amplifier and the electronic circuit amplifier in the time domain to realize dynamic equalization gain. This method controls the modulation time of the semiconductor laser amplifier and the sampling time of the ADC circuit through FPGA logic circuit programming, collects signals on different spatial lengths, and adjusts the dynamic gain of the signals of the specified segmented grating array through programming control of the preamplification circuit.

[0048] 4) Multi-stage configuration of the amplifier, good power equalization effect:

[0049] Combining the preamplifier with the pre-EDFA, post-EDFA and Raman fiber amplifier for adjustment, the method dynamically adjusts the gain of signals of different sensing sections, and solves the problems of optical signal attenuation and equalization.

[0050] 5) High sensitivity of long-distance sensing temperature and strain, good detection effect:

[0051] After the optical power equalization, the demodulation accuracy at the front end of the optical fiber is ±0.2pm, and at 50km, it can still be stabilized within 6pm, the system response frequency is 0.3Hz, and the overall performance is better than that of the traditional BOTDR. The linear fitting degrees of the temperature and strain calibration curves reach 0.999.

[0052] 6) The segmented acquisition method reduces the requirements of the system on data cache and computing power, and has low development difficulty:

[0053] By segmenting the long-distance grating array, the single segment length is within the bearing range of the existing acquisition hardware, and the existing embedded hardware can be directly used for demodulation, which greatly reduces the requirements of the system on hardware, shortens the development cycle, and saves costs. BRIEF DESCRIPTION OF DRAWINGS

[0054] The application will be further described below in combination with the drawings and examples:

[0055] Figure 1 The application is a kind of optical power equalization method flow chart of ultra-weak fiber grating system.

[0056] Figure 2This is a diagram of an optical power equalization device for an ultra-weak fiber Bragg grating system according to the present invention.

[0057] Figure 2 In the middle: 1-Erbium-doped fiber power amplifier EDFA1, 2-Erbium-doped fiber power amplifier EDFA2, 3-Raman amplifier RFA, 4-Amplifier circuit, 5-50km ultra-weak fiber grating array, 6-Circulator, 7-Demodulation module, 8-PC terminal.

[0058] Figure 3(a) shows the signal power distribution on the optical fiber under different Raman pump power and input fiber power of 35dBm (under a fixed pumping condition of 600mW) in simulation analysis.

[0059] Figure 3(b) shows the signal power distribution on the optical fiber under simulation analysis when the Raman pump power is 600mW and the input optical power is different (under the fixed pumping condition of 600mW).

[0060] Figure 4(a) shows the simulation analysis of G when the RFA working position moves backward from the fiber front end under different input fiber optical power. R The change chart;

[0061] Figure 4(b) shows the power distribution on the optical fiber under different input optical power at the optimal working position of the fixed RFA, as analyzed by simulation.

[0062] Figure 5 To dynamically adjust the gain coefficient of AD8366, the reflected light intensity of uwFBG in different sections of 50km was segmented and then adjusted for gain.

[0063] Figure 5 In the diagram: the horizontal axis represents the propagation distance of the optical signal, and the vertical axis represents the magnitude of the optical signal power. Detailed Implementation

[0064] A method for optical power equalization in a long-distance ultra-weak fiber Bragg grating sensing system is proposed, which uses EDFA, RFA and amplification circuit for joint amplification, and determines the peak power of the input pulse light after EDFA1 amplification based on the intensity of the reflected signal and nonlinear noise.

[0065] by Figure 2The shown optical fiber sensing system is an example, in Optisystem simulation software, in turn, set the tunable laser, semiconductor laser amplifier, EDFA1, uwFBG array, RFA, pre-EDFA2, photodetector, etc. Model and connection. Input is the laser power, the parameters can be set variable includes EDFA1 power gain size, EDFA2 power gain size, RFA pump power and RFA working position, the output is the signal power of the system. By fixing other parameters, adjusting the EDFA1 power gain size, analyzing the signal power distribution, determining the peak power of the in-fiber pulse light after EDFA1 amplification.

[0066] The signal power distribution on the optical fiber 5 under different Raman pump light power conditions of RFA3 is analyzed to determine the optimal value of the Raman pump light power; under the condition of fixed pumping, the signal power distribution on the optical fiber under different in-fiber power conditions is analyzed; the gain value change is analyzed when RFA is in different working positions under different in-fiber light power, and when the gain value is at the maximum, the working position of RFA is the optimal configuration point; the optimal working position of RFA is fixed, and the power distribution on the optical fiber under different in-fiber light power is analyzed, and when the far-end signal light power is maximum and the power fluctuation is small, the in-fiber light power at this time is the optimal configuration. In the signal acquisition stage, the reflected signal is collected in sections, and the working timing of the preamplifier circuit 4 is accurately controlled by the FPGA to amplify the reflected signal in different sections to different degrees.

[0067] The method comprises the following steps:

[0068] Step 1: According to the working distance, the output power of the light source, the reflectivity of the ultra-weak fiber grating, the dynamic range and the sensitivity of the photoelectric detector in the ultra-weak fiber grating sensing system, the power cost of the optical path of the ultra-weak fiber grating sensing system is calculated, and the size of the total power gain required is calculated.

[0069] G total = P0-2αL-D+r-P R

[0070] In the formula, P0 is the in-fiber pulse light power of the light source, α is the transmission loss coefficient of the signal light, L is the length of the optical fiber, D is the transmission loss coefficient of the signal light, r is the reflectivity of the same wavelength grating in the array, and P is the sensitivity of the detector. R

[0071] Step 2: According to the size of the total power gain calculated in step 1, the number of erbium-doped fiber power amplifiers EDFA1, erbium-doped fiber power amplifiers EDFA2 and Raman optical amplifiers RFA and the pumping mode are preliminarily determined.

[0072] ​Step 3: considering the fiber line loss, the dynamic gain of erbium-doped fiber power amplifier EDFA1, the dynamic gain of Raman optical amplifier RFA, and the dynamic gain of erbium-doped fiber power amplifier EDFA2, the gain equation of the ultra-weak fiber grating at different positions is established:

[0073] G total ≤G E1 +10lg[(1-r) 2(n-1) r]-2α(L0+nd)+G Rn +G E2n

[0074] In the formula, n is the grating number in the grating array, G E1 is the gain of power optical amplifier EDFA1, G Rn is the RFA gain at the nth grating position, L0 is the fiber extension line, d is the spacing between gratings, and G E2n is the EDFA2 gain at the nth grating position.

[0075] Step 4: considering the RFA co-directional pumping, an optical system simulation model is constructed using Optisystem simulation software, the optical parameters of each device are configured according to the actual system, the working distance of the system is assumed to be 50 km, and the optimal power gain size of EDFA1, the optimal power gain size of EDFA2, and the optimal working position of RFA are determined through model simulation under the constraint condition that the imbalance range of G total is less than 3 dB, 6 dB, and 9 dB, and the RFA pump power is 600 mA.

[0076] Step 5: under the constraint condition that the imbalance range of G total is less than 3 dB, 6 dB, and 9 dB, the gain size of EDFA1 and the gain size of EDFA2 are fixed according to the engineering design, and the pump power of RFA and the optimal working position of RFA are determined through model simulation.

[0077] Step 6: the simulation results of steps 4 and 5 are compared to determine the optimal optical system design scheme when the RFA is co-directionally pumped.

[0078] Step 7: considering the RFA reverse pumping, an optical system simulation model is constructed using Optisystem simulation software, the optical parameters of each device are configured according to the actual system, the working distance of the system is assumed to be 50 km, and the optimal optical system design scheme is determined by repeating steps 4 to 6.

[0079] Step 8: when the imbalance range of G total is greater than 3 dB, a segmented acquisition method is adopted, the gain g e of the programmable electric amplifier is set, the signal intensity of different acquisition segments is optimized, and the G totalThe imbalance range is less than 3dB.

[0080] Step 9: Determine the parameter configuration and segment length of the programmable amplifier, build the actual ultra-weak fiber optic grating sensing system, and optimize the gain g of the programmable amplifier based on the actual optical power equalization effect. e .

[0081] The above are the specific steps of the optical power equalization method for the ultra-weak fiber Bragg grating sensing system. Through these steps, the optical power of each part of the system can be balanced, ensuring that the signal is not lost or attenuated too much during transmission, thereby guaranteeing the stability and accuracy of the system.

[0082] Example:

[0083] A simulation model of the uwFBG system was built using Optisystem:

[0084] Assuming that the number of uwFBGs of the same wavelength in the grating array is n and the fiber length is L, the optical power P when the reflected light from the uwFBG at the end of the fiber reaches the PD is represented by equation (1).

[0085] To ensure the strength of the reflected signal from the ultra-weak grating while avoiding nonlinear noise, the peak power P0 of the input pulse after the modulated optical pulse is amplified by the power EDFA1 is around 35dBm to 45dBm.

[0086] The power loss in the transmission link is mainly amplified and equalized by the pre-amplifier EDFA2 and RFA. Among them, RFA is based on the stimulated Raman scattering process in optical fiber, using the transmission fiber as the gain medium to achieve distributed amplification of the signal light.

[0087] The signal optical power under the action of RFA can be expressed as:

[0088]

[0089] In equation (1): P s (z) represents the optical power of the signal light at point z in the optical fiber, P R For Raman pump light power, α s and α p The transmission loss coefficients of the signal light and pump light are respectively, g eff The effective Raman gain coefficient. From equation (2), it can be seen that the power of the signal light transmitted to different positions in the optical fiber is related to the Raman pump light power.

[0090] The length of the optical fiber is set to 50 km, the reflectivity of the uwFBG in the grating array is 0.01%, the interval is 2 m, there are 25000 in total, the transmission loss coefficient of the optical fiber is 0.2 dB / km, and under the condition of 600 mW fixed pumping, the signal power distribution on the optical fiber under the condition of different Raman pump powers and different input powers is simulated and analyzed.

[0091] Fig. 3(a) is a diagram of the power distribution of the signal light at different positions of 50 km when the input optical power is 35 dBm and the Raman pump light power is different; Fig. 3(b) is a diagram of the power distribution of the signal light at different positions of 50 km when the Raman pump light power is 600 mW and the input optical power is different. As shown in Fig. 3(a) and Fig. 3(b), the greater the pump power, the stronger the gain, the greater the input signal power, the faster the signal power decays, and the power in the whole optical fiber is obviously uneven. This is mainly caused by the stimulated Brillouin scattering (SBS) effect in the optical fiber after the watt-level input optical power is amplified by the RFA gain, which not only limits the highest power of the system, but also accelerates the power decay of the signal light.

[0092] In addition, during the transmission of the pump light, the far-end pump signal is obviously weakened due to the absorption of the Raman effect and the increase of the optical fiber loss, and the Raman gain of the signal at the far end is obviously reduced.

[0093] It is assumed that the pre-EDFA2 provides a gain of 20 dB to 30 dB, in order to obtain the optimal gain configuration of the RFA, the change of G R under the condition of different input optical powers and different working positions of the RFA is simulated and analyzed.

[0094] Fig. 4(a) shows that when the working position of the RFA moves from the front end of the optical fiber to the rear, G R first increases and then decreases, and when G R is at the maximum value, the working position of the RFA is the optimal configuration point. That is, when the input optical power P0 is in the range of 35 dBm to 45 dBm, the optimal working position of the RFA is in the range of 9.2 km to 11.2 km of the optical fiber. When P0 is 35 dBm, the maximum value of G R is 17.2 dBm, and the probe power P of the signal at the far end of 50 km is -8.8 dBm, which meets the design requirements of the system.

[0095] At the same time, in order to obtain the best power balance effect and fix the optimal working position of the RFA, the power distribution on the optical fiber under different input optical powers is analyzed as shown in Fig. 4(b). The results show that when the input optical power is set to 40 dBm, the optical power of the far-end signal is the maximum, and the power fluctuation is less than 6.3 dB.

[0096] Figure 5The 50km in the UWFBG array reflected light intensity after segmented gain by the amplification circuit. total The range of the amplitude of the light power between different segments is 2.2dB, which is far less than the minimum threshold of G

[0097] In conclusion, the application discloses a long-distance ultra-weak fiber grating system light power equalization method, solves the problems of signal attenuation and uneven gain, makes the monitorable distance of the ultra-weak fiber grating sensing system increase to 50km, realizes online monitoring of long-distance oil logging, forest fire and the like, and widens the application field of the ultra-weak fiber grating.

Claims

1. A method for optical power equalization in a long distance ultra-weak fiber grating sensing system, characterized in that The method comprises the following steps: Step one: calculating the power cost of the optical path of the ultra-weak fiber grating sensing system and the size of the total power gain required; Step two: according to the size of the total power gain calculated in step one, preliminarily determining the number and pumping mode of the erbium-doped fiber power amplifier EDFA1, the erbium-doped fiber power amplifier EDFA2 and the Raman optical amplifier RFA; Step three: considering the fiber line loss, the dynamic gain of the erbium-doped fiber power amplifier EDFA1, the dynamic gain of the Raman optical amplifier RFA and the dynamic gain of the erbium-doped fiber power amplifier EDFA2, establishing the gain equation of the ultra-weak fiber grating at different positions; Step four: considering the RFA co-directional pumping, a simulation model of the optical system is constructed to Under the condition of a certain RFA pumping power, the optimal power gain size of EDFA1, the optimal power gain size of EDFA2 and the optimal working position of RFA are determined through model simulation with the unbalanced range as the constraint condition. Step 5: Based on the total gain of the optical path The imbalance range is used as a constraint. The gain of EDFA1 and the gain of EDFA2 are fixed. The pump power of RFA and the optimal operating position of RFA are determined by model simulation. Step six: comparing the simulation results of step four and step five to determine the optimal optical system design scheme when the RFA is forward pumped; Step seven: considering the RFA backward pumping, using the simulation model of the optical system, setting the working distance of the system, repeating steps four to six to determine the optimal optical system design scheme; Step eight: in The uneven range is greater than the threshold value, the segmented acquisition method is used, the gain of the programmable electric amplifier is set The signal strength of different acquisition segments is optimized to ensure The uneven range is less than the threshold value; Step nine: determine the programmable electric amplifier parameter configuration, segment length, build the actual ultra-weak fiber grating sensing system, and optimize the gain of the programmable electric amplifier according to the actual optical power balance effect .

2. The method of claim 1, wherein the method further comprises: determining the optical power of the light source; and adjusting the optical power of the light source to maintain the optical power of the light source at a predetermined level. In step one, the working distance of the ultra-weak fiber grating sensing system, the output power of the light source, the reflectivity of the ultra-weak fiber grating, the dynamic range and the sensitivity of the photoelectric detector are considered to calculate the power cost of the entire optical path and the size of the total power gain required, which is as follows: ； wherein Gtotal represents the total gain size of the optical path; P 0 is the in-fiber pulse optical power of the light source; is the transmission loss coefficient of the signal light; is the fiber length; is the transmission loss coefficient of the signal light; r is the reflectivity of the same-wavelength grating in the array; P R is the sensitivity of the detector.

3. The method of claim 1, wherein the method further comprises: determining the optical power of the light source; and adjusting the optical power of the light source to maintain the optical power of the light source at a predetermined level. In step three, the gain equation of the ultra-weak fiber grating at different positions is as follows: ； In the formula, represents the total gain of the optical path after considering the number of gratings and the position correction of the amplifier; n is the serial number of the grating in the grating array; is the gain of the erbium-doped fiber power amplifier EDFA1; is the RFA gain of the nth grating position; is the fiber extension line; is the spacing between the gratings; is the EDFA2 gain of the nth grating position; is the transmission loss coefficient of the signal light; r is the reflectivity of the same wavelength grating in the array.

4. The method of claim 1, wherein the method further comprises: In step four, the method for determining the optimal power gain size of EDFA1, the optimal power gain size of EDFA2 and the optimal working position of RFA is as follows: ​ The curve of signal power distribution along the distance on the optical fiber is calculated, and then the power fluctuation range of the curves corresponding to different gain sizes and working positions is calculated. When the range is lower than the threshold of the gain imbalance, the parameter value is the optimal one. total of the gain imbalance.

5. The method of claim 1, wherein the method further comprises: The step four is to determine the best configuration of the input fiber light power after determining the best working position of the RFA. Specifically, the distribution of the signal power on the fiber with the distance under different input fiber light powers is calculated. When the light power of the remote signal is the largest and the power fluctuation range is lower than the imbalance range threshold of G total , the input fiber light power is the best configuration. ​ 6. The method of claim 1, wherein the method further comprises: In step five, the method for determining the pumping power of RFA and the optimal working position of RFA is as follows: ​ Fixing the gain size of EDFA1 and the gain size of EDFA2, the signal power distribution on the fiber with different pump power and working position of RFA is calculated, and then the power fluctuation range size of the corresponding curve is calculated, and when it is lower than the threshold of the imbalance range of G total , it is the optimal parameter value.

7. The method of claim 1, wherein the method further comprises: In step six, when the RFA is forward pumped, the method for determining the optimal optical system design scheme when the RFA is forward pumped is as follows: ​ Step four and step five get two groups of optical system design scheme, which include the best power gain size of EDFA1, the best power gain size of EDFA2, the pump power of RFA and the best working position optical configuration of RFA. First, compare the final optical power cost and optical power gain brought by different schemes, the scheme with smaller cost and larger gain is better. Then compare the imbalance range size of G total in different schemes, the smaller scheme is better. Second, compare the noise performance, power consumption cost and system compatibility parameters of the overall scheme to determine the best optical system design scheme.

8. The method of claim 1, wherein the method further comprises: In step eight, when When the imbalance range is greater than 3dB, a segmented acquisition method is adopted, and the gain of the programmable amplifier is set for the optical signal power of different segments. Optimize signal strength in different acquisition segments to ensure The imbalance range is less than 3dB. ​ 9. The method of claim 1, wherein the method further comprises: In step nine, the gain of the programmable electrical amplifier is optimized according to the actual optical power balance effect The specific method is as follows: ​ Under the gain of the programmable electric amplifier determined in step eight, the optical power distribution of the actual sensing system is tested, the signal power is segmented according to the transmission distance, the range of the total gain size between different segments is calculated, and if the expected value is not reached, the gain of the programmable electric amplifier is dynamically adjusted through the feedback circuit.

Images