A fiber optic sensor array with large dynamic range

Abstract

The present invention proposes a fiber optic sensor array with a large dynamic range, wherein the sensing units are more than two and are connected in sequence through transmission optical fibers, and the sensing units include a multiplexing coupler, a sensing / delay fiber loop, and a fiber optic interferometer. The scheme uses a fiber optic interferometer as the basis of a signal sensing unit, and embeds two high-sensitivity and / or low-sensitivity sensing units with positive and negative phases of a certain time delay in the fiber optic transmission multiplexing link, injects light pulses with a certain time interval and repetition rate, and then returns at the Faraday reflector of the fiber optic interferometer to form an interference pulse sensing sequence. The present invention has a variety of combination methods, and can choose to achieve the sensing characteristics of low noise and high saturation threshold of the array, so that the fiber optic sensor array has a large dynamic range of signal acquisition and restoration capabilities, so that it can adapt to the distortion-free acquisition of signals in various existing environments, and is well applied in the fields of sea and land invasion sound monitoring and seabed mineral resource exploration in complex environments.

Description

Technical Field

[0001] The invention belongs to the field of optical fiber acoustic sensing, in particular to an optical fiber sensor array with a large dynamic range. Background Art

[0002] Fiber optic sensor arrays are effective equipment for underwater target detection, ocean noise environment measurement, perimeter fence intrusion monitoring, etc. They are amphibious and can be deployed flexibly. They can be fixedly laid or carried on platforms. They can be used in single arrays, multiple arrays, or networks. They can monitor and warn intrusion targets in a set area and conduct mineral exploration in specific marine economic zones. As the construction and development of important economic facilities on land and sea in my country continue to deepen, the requirements for their security and defense continue to increase. As passive, easy-to-scale deployment, and wide-range coverage technical equipment, fiber optic sensor arrays are being widely developed and rapidly promoted in their application research and equipment deployment.

[0003] Due to its passive and long-distance transmission characteristics, fiber optic sensor arrays can be used in low-noise environments such as deep sea and desert with extremely low noise. At the same time, due to its wide signal acquisition frequency band and long distance, the application environment usually has high background interference. For example, when acquiring ocean acoustic signals, the busy ship traffic and harsh marine meteorological environment in the vicinity will form a strong interference background. In addition, in application fields such as underwater acoustic countermeasures, strong radiated acoustic signals with high source levels at close range can easily cause sensor whitening. Therefore, fiber optic sensor arrays are usually required to have not only extremely low self-noise levels, but also high whitening thresholds.

[0004] The self-noise level of the fiber optic sensor array system is determined by the demodulation noise and the sensitivity of the sensor unit, that is, N 系统噪声 =N 解调噪声 -M 单元灵敏度 , from which we can see that N 解调噪声 The lower, M 单元灵敏度 The higher the value, the better for self-noise suppression. Because the demodulation system is a complex optoelectronic hybrid hardware and software system, its demodulation noise is subject to many restrictions. Therefore, improving the sensitivity of the fiber optic sensor unit is an effective method for system noise suppression. At the same time, for the fiber optic sensor system, its corresponding signal reception dynamic range is N 系统噪声 ~S 上限阈值 , and the upper threshold S 上限阈值 =S 解调阈值 -M 单元灵敏度 , that is, the upper limit of the dynamic range is limited by the signal demodulation distortion threshold and the sensitivity of the sensor unit. Usually, since the sensitivity M of the optical fiber sensor unit is a fixed value, the dynamic range of the system is actually determined by S 解调阈值 -N 解调噪声At present, the upper limit of the signal receiving dynamic range is determined by the demodulation algorithm. For example, interferometric fiber optic sensors often use the 3×3 method, heterodyne method and phase generated carrier method. The upper limits of the demodulation dynamic range of the three methods decrease in turn, and the upper limit threshold of the algorithm fidelity demodulation is limited by the signal frequency, carrier frequency, etc. Its further improvement will be strictly limited by hardware conditions and system bandwidth. Therefore, it is difficult to increase the dynamic range of the fiber optic sensor array system by improving the signal demodulation end in terms of cost, technology and overall effect.

[0005] The sensitivity of the sensitive units in the optical fiber sensor array can be changed to broaden the dynamic range of the system, especially in signal reception against the interference of complex sound fields or for anti-whitening of large signals. However, the current sensor arrays all have a certain sensitivity. If the sensor units are required to have different sensitivities, the array unit size needs to be increased or the number of effective units needs to be reduced. The corresponding demodulation end capability also needs to be significantly adjusted, which either increases the system complexity and cost or reduces the system performance. These are all non-optimal solutions. Summary of the invention

[0006] Normally, the sensing and delay functions of the fiber loop in the time-division optical path structure are separated and play their respective roles. When the sensitivity of the sensing unit needs to be adjusted, the interferometer parameters of the sensing unit are replaced and the delay fiber loop is adjusted in sequence. Based on the shortcomings of the existing technology, this solution comprehensively considers the design of the delay fiber loop and the sensing unit to achieve a single optical path structure and multiple sensitivity combination units.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0008] A fiber optic sensor array with a large dynamic range, comprising a sensing unit, wherein the sensing units are more than two and are sequentially connected through transmission optical fibers, the sensing unit adopts a reflective time-division optical path structure, comprising a multiplexing coupler, a sensing / delay fiber loop and a fiber optic interferometer, the multiplexing coupler is respectively connected to the sensing / delay fiber loop and the fiber optic interferometer through transmission optical fibers, the sensing / delay fiber loop is connected to the multiplexing coupler of the next sensing unit through the transmission optical fiber, the fiber optic interferometer comprises an interferometer coupler, an interferometer sensing fiber loop and two Faraday reflectors, the interferometer coupler is connected to the Faraday reflector through the sensing fiber , and is connected to another Faraday reflector through a reference fiber. The interferometer sensor fiber coil is set on the sensor fiber. The last sensor unit includes a multiplexing coupler, a sensor / delay fiber coil and two fiber interferometers. The multiplexing coupler is connected to a sensor / delay fiber coil and a fiber interferometer through transmission optical fibers. The sensor / delay fiber coil is connected to another fiber interferometer through transmission optical fibers. The various sensor units are distributed in an array and form a fiber transmission multiplexing link for signal transmission and sensing. After injecting light pulses with a certain time interval and repetition rate, the link returns at the Faraday reflector of each fiber interferometer and forms an interference pulse sensing sequence.

[0009] Preferably, the multiplexing coupler is an unequal splitting ratio coupler, the transmission optical fiber at the large splitting ratio end is connected to the adjacent sensing / delay fiber loop, and the transmission optical fiber at the small splitting ratio end is connected to the optical fiber interferometer, and the interferometer coupler is an equal splitting ratio coupler, and the length difference between the sensing fiber and the reference fiber of each optical fiber interferometer is fixed.

[0010] Preferably, the sensing unit adopts a symmetrical time-division optical path structure, the multiplexing couplers of the front sensing units include 1 input multiplexing coupler and 1 output multiplexing coupler, the number of sensing / delay fiber coils and fiber interferometers are both 1, wherein the input multiplexing coupler is connected to the sensing / delay fiber coil and the fiber interferometer respectively through transmission optical fiber, the sensing / delay fiber coil is connected to the input multiplexing coupler of the next sensing unit through transmission optical fiber, the output multiplexing coupler is connected to the same fiber interferometer through transmission optical fiber, and the output multiplexing coupler is connected to the output multiplexing coupler of the next sensing unit; the last sensing unit includes 1 input multiplexing coupler, 1 fiber interferometer and 1 output multiplexing coupler, and the input multiplexing coupler and the output multiplexing coupler are both connected to the same fiber interferometer through transmission optical fiber.

[0011] Preferably, the optical signal input to the optical fiber transmission multiplexing link is a laser double pulse structure, and the output optical signal is a laser interference pulse sequence structure, the laser pulse interval is equal to the delay of the interferometer sensing fiber coil (the arm difference delay of the optical fiber interferometer), which is 1 / 2 of the sensing / delay fiber coil delay, and the laser pulse repetition period is not less than (multiplexing degree + 1) × 2 × pulse interval.

[0012] Preferably, the splitting ratio corresponding to two adjacent multiplexing couplers satisfies:

[0013]

[0014] Wherein, n is a positive integer not less than 2, Cn is the small splitting percentage of the n-th multiplexing coupler, and Cn-1 is the small splitting percentage of the n-1-th multiplexing coupler.

[0015] Preferably, all interferometer sensing coils have the same phase, all sensing / delay coils have the same phase, and the sensing / delay coils and interferometer sensing coils have opposite phases, forming a fully-sensitized combination of sensing units.

[0016] Preferably, all interferometer sensing fiber coils and sensing / delay fiber coils have the same phase, forming a fully desensitized combination of sensing units.

[0017] Preferably, all interferometer sensing fiber coils have the same phase, one of two adjacent sensing / delay fiber coils has an opposite phase to the interferometer sensing fiber coil and the other has the same phase, and sensing / delay fiber coils with different phases are alternately distributed to form a cross combination of sensing units.

[0018] Preferably, the fiber optic interferometer adopts an external cavity sensing structure, the sensing / delay fiber coil adopts an external cavity sensing structure or an internal cavity sensing structure, and the phases of the same sensing structures are the same, and the phases of different sensing structures are opposite.

[0019] Specifically, the fiber optic interferometer and sensor / delay fiber coil include three skeleton structures, namely I, II and III. The external cavity sensing structure of the fiber optic interferometer is type I. The external cavity fiber optic interferometer includes a core shaft skeleton I. An equal-ratio coupler and two Faraday reflectors are encapsulated in the core shaft skeleton I. One end of the equal-ratio coupler is connected to the two Faraday reflectors through a sensing fiber and a reference fiber respectively, and the other end of the equal-ratio coupler is connected to an input and output optical fiber. A sensitivity enhancement structure is arranged on the core shaft skeleton I, and a sensitivity enhancement air cavity is formed in the sensitivity enhancement structure. An interferometer sensing fiber coil is wound outside the sensitivity enhancement structure. The interferometer sensing fiber coil is connected to the internally encapsulated sensing fiber to achieve contact with the external fluid environment and signal acquisition.

[0020] The external cavity sensing structure of the sensing / delay fiber coil is type II. The external cavity sensing / delay fiber coil includes a core shaft skeleton II. A sensitivity enhancement structure is arranged on the core shaft skeleton II. A sensitivity enhancement air cavity is formed inside the sensitivity enhancement structure. The sensing / delay fiber coil is wound outside the sensitivity enhancement structure. The two ends of the sensing / delay fiber coil are respectively connected to an input optical fiber and an output optical fiber to achieve contact with the external fluid environment and signal acquisition.

[0021] The inner cavity sensing structure of the sensing / delay fiber coil is type III. The inner cavity sensing / delay fiber coil includes a core shaft skeleton III. A sensitivity enhancement structure and an outer sealing shell are arranged on the core shaft skeleton III. An outer sensitivity enhancement air cavity is formed between the outer side of the sensitivity enhancement structure and the outer sealing shell. The sensing / delay fiber coil is wound on the sensitivity enhancement structure. The two ends of the sensing / delay fiber coil are respectively connected to the input optical fiber and the output optical fiber to realize the contact with the internal fluid environment and the signal acquisition.

[0022] The beneficial effects of the present invention are as follows:

[0023] 1. Effectively reduce the noise floor of the array sensor unit: the self-noise of the sensor unit is N=N 解调噪声 -M 单元灵敏度 After the array sensor unit scale, optical path structure and detection scheme are determined, N 解调噪声 It is basically confirmed that improving the unit sensitivity is an effective way to reduce the self-noise background. The sensor array structure of this achievement scheme can be combined to form a unit sensitivity of M after the delayed fiber loop is inverted and sensitized. 组合单元 =M s +M d , based on the multiplexing delay characteristics, M d The sensitivity is M s Therefore, the overall sensitivity is increased by 9.5dB, that is, the sensor noise floor is reduced by 9.5dB.

[0024] 2. Improve the upper limit of the saturation threshold of the received signal of the optical fiber sensor array: When the optical path multiplexing scale and demodulation scheme of the optical fiber sensor array are determined, the upper limit of the dynamic range of the corresponding algorithm demodulation is also determined, and the upper limit of the dynamic range of the existing demodulation algorithm is inversely correlated with the signal frequency (D 动态上限 ∝1 / f), such as Figure 3 As shown, by combining the desensitization design of the sensor unit, that is, M 组合单元 =M s -M d , at this time M d The fiber loop length can be between 0~L 延时 (2·L 敏感纤圈 ) length, and the phase is in phase, which can effectively improve the ability of the array unit to resist large signal saturation and whitening as needed, and the absolute value of its upper limit improvement ability reaches |M d |.

[0025] 3. Greatly improve flexibility and scalability: While ensuring the multiplexing degree and sampling and demodulation design of the currently common optical fiber sensor array, the present invention combines the sensing design of the multiplexed delay link fiber loop, so that the same optical fiber sensor fiber loop can be used independently or in combination with the delay link fiber loop to form two or more types of sensing units with high and low sensitivity, thereby improving the saturation threshold of the signal received by the sensor system from the hardware of the sensor end. It is easy to implement in terms of process and flexible in combination, and has a significant effect on improving the anti-interference and large signal receiving capabilities of the optical fiber sensor array. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A diagram of an optical interference pulse sequence formed and outputted by an optical fiber sensor array, an optical fiber interferometer, and optical pulse interference of a reflective structure;

[0027] Figure 2 A diagram of a symmetrically structured optical fiber sensor array, an optical fiber interferometer, and an optical interference pulse sequence formed and output by optical pulse interference;

[0028] Figure 1 and Figure 2 Labeling: 1-unequal splitting ratio multiplexing coupler, 2-sensing / delay fiber coil, 3-transmission fiber, 4-fiber interferometer, 5-interferometer coupler, 6-interferometer sensing fiber coil, 7-reference fiber, 8-Faraday reflector;

[0029] 9- double optical pulses, 10- interference formed by the reflected pulse from the sensor arm in interferometer N-1 and the reflected pulse from the reference arm, 11- interference formed by the reflected pulse from the sensor arm in interferometer N-1 and the delayed fiber loop N-1 and the reflected pulse from the reference arm of interferometer N, 12- interference formed by the reflected pulse from the sensor arm in interferometer N and the reflected pulse from the reference arm, 13- interference optical pulse sequence returned by the optical path of the N sensor unit array.

[0030] Figure 3 Schematic diagram of the dynamic range expansion of the fiber optic sensor array.

[0031] Figure 4 The following are the structure diagrams of three types of optical fiber sensors: Figure 4 (a) is a fiber interferometer (Type I) using an external cavity sensing structure; Figure 4 (b) is a sensing / delay fiber coil (Type II) using an external cavity sensing structure; Figure 4 (c) is a sensing / delay fiber coil (Type III) using an inner cavity sensing structure;

[0032] Figure 4Marking instructions: 2a-first sensing / delay fiber loop, 2b-second sensing / delay fiber loop, 14-input and output optical fibers, 15-core skeleton I, 16-sensitization structure I, 17-sensitization air cavity I, 18-first sensing signal, 16a-sensitization structure II, 17a-sensitization air cavity II, 18a-second sensing signal, 19-input optical fiber, 20-core skeleton II, 21-output optical fiber, 22-core skeleton III, 23-outer sealed shell, 24-sensitization structure III, 25-sensitization air outer cavity, 26-third sensing signal.

[0033] Figure 5 The structure diagram of the window function for arranging the sensitivity of the array sensor unit.

[0034] Figure 6 is the directivity function characteristic diagram of optical fiber sensor arrays of various sizes, where: Figure 6 (a) is a linear array of 8 sensor units; Figure 6 (b) is a linear array of 16 sensor units; Figure 6 (c) is a linear array of 64 sensor units; Figure 6 (d) in the figure is a linear array of 128 sensor units. DETAILED DESCRIPTION

[0035] The specific implementation scheme of the large dynamic range optical fiber sensor array of the present invention is described below in conjunction with the accompanying drawings:

[0036] like Figure 1 , Figure 2 As shown, the present invention provides a large dynamic range optical fiber sensor array, including sensing units, wherein the sensing units are more than two and are sequentially connected through a transmission optical fiber 3, the sensing units include a multiplexing coupler, a sensing / delay fiber coil 2 and an optical fiber interferometer 4, the multiplexing coupler is respectively connected to the sensing / delay fiber coil 2 and the optical fiber interferometer 4 through the transmission optical fiber 3, the sensing / delay fiber coil 2 is connected to the corresponding multiplexing coupler of the next sensing unit through the transmission optical fiber 3, the optical fiber interferometer 4 includes an interferometer coupler 5, an interferometer sensing fiber coil 6 and a Faraday reflector 8, the interferometer coupler 5 is connected to the sensing / delay fiber coil 2 and the optical fiber interferometer 4 through the transmission optical fiber 3, the sensing / delay fiber coil 2 is connected to the corresponding multiplexing coupler of the next sensing unit through the transmission optical fiber 3, the optical fiber interferometer 4 includes an interferometer coupler 5, an interferometer sensing fiber coil 6 and a Faraday reflector 8, the interferometer coupler 5 is connected to the sensing / delay fiber coil 2 and the optical fiber interferometer 4 through the transmission optical fiber 3, the sensing / delay fiber coil 2 is connected to the sensing / delay fiber coil 2 and ... It is connected to a Faraday reflector 8 through a sensing fiber and to another Faraday reflector 8 through a reference fiber 7. An interferometer sensing fiber coil 6 is arranged on the sensing fiber. Each sensing unit is distributed in an array and forms a fiber optic transmission multiplexing link for signal transmission and sensing. The present invention adopts a fiber optic interferometer 4 as the basis of the signal sensing unit, and embeds two types of high-sensitivity and / or low-sensitivity sensing units with positive and negative phases of a certain time delay in the fiber optic transmission multiplexing link. After injecting light pulses with a certain time interval and repetition rate, they return at the Faraday reflector 8 of the fiber optic interferometer 4 and form an interference pulse sensing sequence.

[0037] In the present invention, the multiplexing coupler is an unequal splitting ratio coupler 1, the transmission optical fiber 3 at the large splitting ratio end is connected to the adjacent sensing / delay fiber loop 2, and the transmission optical fiber 3 at the small splitting ratio end is connected to the optical fiber interferometer 4. Among them, the sensing / delay fiber loop 2 generally adopts the same length of several tens of meters and is wound into a signal sensitive fiber loop structure. The sensing / delay fiber loop 2 is connected between two adjacent splitting ratio couplers through the transmission optical fiber 3, and is responsible for the uniform delay of the optical signals of the adjacent sensor channels and signal sensing. The interferometer coupler 5 is an equal splitting ratio coupler. The number of interferometer couplers 5 and interferometer sensor fiber coils 6 in the fiber optic interferometer 4 is 1 each. The number of Faraday reflectors 8 is 2. The sensor fiber of the fiber optic interferometer 4 is the sensor arm, which is responsible for signal acquisition. The reference fiber 7 is the reference arm. The two maintain a fixed length difference of several tens of meters and are wound into a signal sensitive fiber coil structure. The sensor / delay fiber coil 2 is connected between two adjacent splitting ratio couplers through the transmission optical fiber 3, and is responsible for uniform delay of optical signals of adjacent sensor channels and signal sensing.

[0038] In the present invention, the optical fiber sensor array is a reflective time-division optical path structure or a symmetrical time-division optical path structure. Figure 1 As shown, each of the front sensing units of the reflective time-division optical path structure includes a multiplexing coupler, a sensing / delay fiber loop 2 and a fiber interferometer 4. The multiplexing coupler is connected to the sensing / delay fiber loop 2 and the fiber interferometer 4 respectively through the transmission fiber 3, and the sensing / delay fiber loop 2 is connected to the multiplexing coupler of the next sensing unit through the transmission fiber 3; the last sensing unit includes a multiplexing coupler, a sensing / delay fiber loop 2, and two fiber interferometers 4. The multiplexing coupler is connected to the sensing / delay fiber loop 2 and the fiber interferometer 4 respectively through the transmission fiber 3, wherein the sensing / delay fiber loop 2 is connected to another fiber interferometer 4 through the transmission fiber 3.

[0039] like Figure 2 As shown, each of the front sensing units of the symmetrical time-division optical path structure includes an input multiplexing coupler, a sensing / delay fiber coil 2, a fiber interferometer 4 and an output multiplexing coupler, wherein the input multiplexing coupler is respectively connected to the sensing / delay fiber coil 2 and the fiber interferometer 4 through the transmission fiber 3, the sensing / delay fiber coil 2 is connected to the input multiplexing coupler of the next sensing unit through the transmission fiber 3, the output multiplexing coupler is connected to the same fiber interferometer 4 through the transmission fiber 3, and the output multiplexing coupler is directly connected to the output multiplexing coupler of the next sensing unit; the last sensing unit includes an input multiplexing coupler, a fiber interferometer 4 and an output multiplexing coupler (the sensing / delay fiber coil 2 is not set), the input multiplexing coupler and the output multiplexing coupler are both connected to the same fiber interferometer 4 through the transmission fiber 3, and the output multiplexing coupler is connected to the next output multiplexing coupler of the adjacent sensing unit.

[0040] In the present invention, the optical signal input by the optical fiber transmission multiplexing link is a laser double pulse structure, and the output optical signal is a laser interference pulse sequence structure. The laser pulse interval is equal to the delay of the interferometer sensing fiber coil 6 (the arm difference delay of the optical fiber interferometer 4), which is 1 / 2 of the delay of the sensing / delay fiber coil 2, and the laser pulse repetition period is not less than (multiplexing degree + 1) × 2 × pulse interval.

[0041] In the present invention, the coupler is a 1×2 type with multiple splitting ratio parameters. The splitting ratio and type are determined by the scale of time division multiplexing and the maximum allowable optical loss value, so that the optical loss of adjacent sensor channels is the same. The splitting ratio corresponding to two adjacent multiplexing couplers satisfies:

[0042] Wherein, n is an integer not less than 2, Cn is the small splitting percentage of the nth multiplexing coupler, and Cn-1 is the small splitting percentage of the n-1th multiplexing coupler. By designing a specific splitting ratio of the coupler, the optical power balance between the sensing units is guaranteed, and the optimal visibility of the interference pulse can be achieved.

[0043] In the present invention, the delay relationship corresponding to the difference between the two arms of the long-arm length-difference interferometer of the sensing unit is Ts, and the delay relationship after the interferometer round trip is 2·Ts. The multiplexed optical path realizes the distinction of optical signals between the sensing units through the delay fiber loop, and the corresponding delay is Td. Then the delay of the multiplexed optical path round trip (reflection type) or double trip (symmetrical type) is 2·Td.

[0044] In order to achieve cross interference between the long and short arms of adjacent interferometers, the delay fiber coil and the interferometer arm difference delay are designed to satisfy the following relationship:

[0045]

[0046] That is, Td=2·Ts, and the delay of the sensing / delay fiber loop 2 is twice the delay of the interferometer sensing fiber loop 6. By making the length of the sensing / delay fiber loop 2 twice the difference in the interferometer arm length, it can be ensured that the reflected optical signals can be strictly matched and merged.

[0047] In the present invention, different schemes of sensitivity enhancement combination, sensitivity reduction combination or cross combination are selected according to the difference of reducing the sensor noise floor or increasing the upper limit of the saturation threshold. Including: (1) Full sensitivity enhancement combination: all interferometer sensor fiber coils 6 have the same phase, all sensor / delay fiber coils 2 have the same phase, and the sensor / delay fiber coils 2 and the interferometer sensor fiber coils 6 have opposite phases; (2) Full sensitivity reduction combination: all interferometer sensor fiber coils 6 and sensor / delay fiber coils 2 have the same phase; (3) Cross combination: all interferometer sensor fiber coils 6 have the same phase, one of the two adjacent sensor / delay fiber coils 2 has the opposite phase to the interferometer sensor fiber coil 6 and the other has the same phase, and sensor / delay fiber coils 2 with different phases are distributed alternately.

[0048] It is understandable that, in addition to the above three schemes, other sensitization combinations or desensitization combinations may also be used, and the present invention is not limited to this.

[0049] The present invention provides a large dynamic range optical fiber sensor array. According to the multiplexing unit scale and light sampling density, the demodulation end transmits a double pulse laser with a certain pulse width, delay interval and repetition rate, obtains the target signal through the delayed optical path of the time-division optical path and the sensor unit, and returns the target signal. Figure 1 , Figure 2 The alignment interference pulse sequence shown in the figure finally realizes the target signal extraction.

[0050] Specifically, a dual optical pulse 9 is input into the optical fiber sensor array to form a sensor end signal acquisition. The dual optical pulse 9 is reflected by the two arms of the interferometer, and the front and rear pulses are staggered and delayed to interfere with each other, forming an interference pulse signal with information acquired by the sensor unit, and the information acquired by the front main delay fiber loop is the same for both arms and cancels each other out; and the time interval between the sensor fiber in the front sensor unit and the reference fiber 7 in the rear sensor unit is 2·Td-2·Ts=2·Ts, so an interference pulse can also be formed, and its corresponding sensor signal comes from the sensor fiber loop of the previous sensor unit and the delay fiber loop between the two sensor units. Figure 1 What is reflected is the interference light pulse sequence 13 returned by the optical path of the N sensor unit array, and the corresponding interference pulses include the interference 10 formed by the reflected pulse of the sensor arm in interferometer N-1 and the reflected pulse of the reference arm, the interference 11 formed by the reflected pulse of the sensor arm in interferometer N-1 and the delay fiber loop N-1 and the reflected pulse of the reference arm of interferometer N, and the interference 12 formed by the reflected pulse of the sensor arm in interferometer N and the reflected pulse of the reference arm. The corresponding relationship between them and the sensor units is from head to tail as follows: ..., sensor unit n-1, sensor unit n-1+delay fiber loop n-1, sensor unit n, ...

[0051] The long-arm fiber of the sensor unit interferometer is used as the sensitive part, and the short-arm fiber is used as the reference part, and its signal acquisition sensitivity is Ms; the delay fiber loop in the multiplexed optical path is designed to be sensitive while bearing the delay of the optical signal, and its corresponding sensitivity is Md. According to the interference pulse formation mechanism in the multiplexed optical path design, the channel sensitivity characteristics corresponding to the interference pulse sequence are..., Ms, Ms-Md,... In order to increase or decrease the sensitivity of the combined sensor unit (corresponding to reducing the self-noise of the sensor array and increasing the sensor saturation threshold, respectively), it is necessary to design a sensitive fiber loop sensitivity enhancement scheme to achieve phase difference, that is, or .

[0052] Because the length of the delay fiber loop is designed to be twice that of the sensing fiber loop, When the sensitivity is increased by 3 times, M 组合单元 =Ms +9.5dB, at this time, the self-noise floor of the sensor array system can be reduced by 9.5dB, that is, the lower limit of the dynamic range is extended by 9.5dB, which is N 噪声下限 -9.5dB. By adjusting the length of the delay fiber loop and combining the same phase Design, then M 组合单元 =M s -M d , the strong signal whitening threshold is increased by |M d The value of | is N 饱和阈值 +|M d |, and its design value range is determined by the value of the strong signal exceeding the upper limit of the saturation threshold of the single sensor unit in the application scenario. The above corresponds to Figure 3 Schematic diagram of dynamic range expansion of fiber optic sensor array shown.

[0053] Here, the scheme design, working principle and combination optimization of the present invention are further explained:

[0054] 1. Construction of fiber optic sensor array

[0055] A reflective or symmetrical time-division optical path structure is used to form a single-fiber loaded multi-fiber sensor unit. The optical splitting coupler is designed with a specific splitting ratio according to the time-division scale and its hierarchical position in the multiplexed optical path to ensure that the optical transmission loss of the multiplexed channel is the same, thereby achieving a uniform balance in the intensity of the optical pulse signal returned by the optical fiber sensor unit.

[0056] The small split ratio end of the multiplexing coupler is connected to the optical path structure of the fiber interferometer 4. In the reflective multiplexing structure, the interferometer uses a single fiber input and output. In the symmetrical multiplexing structure, the fiber interferometer 4 uses dual fibers for input and output respectively, and a pair of multiplexing couplers is used to realize the input and return of optical signals. The large split ratio end of the multiplexing coupler is connected to the sensor / delay fiber coil 2, and the other end is connected to the input fiber 19 of the next multiplexing coupler, while the small split ratio of the multiplexing coupler is still connected to the fiber interferometer 4.

[0057] The fiber interferometer 4 is an equal-split ratio coupler connecting the sensing fiber loop (interferometer sensing arm) and the reference fiber 7 (interferometer reference arm), and the tails of the two fibers are butted against a Faraday rotator mirror.

[0058] In terms of fiber coil delay relationship, the length of the sensing / delay fiber coil 2 is twice that of the interferometer sensing fiber coil 6.

[0059] 2. Interference light pulse sequence formation

[0060] The photoelectric demodulation device outputs a dual optical pulse signal with a certain pulse width, delay and period, which is input into the multiplexed sensing optical path and is divided into two paths with large and small split ratios by the multiplexed coupler. The small split ratio path is connected to the optical fiber interferometer 4, and the large split ratio path is connected to the sensor / delay fiber coil 2. The former dual pulse light is divided into two by the equal split ratio coupler 5. One path passes through the sensing fiber coil (interferometer sensing arm), is reflected by the Faraday rotation mirror, and then passes through the sensing fiber coil to return to the equal split ratio coupler 5; the other path passes through the reference fiber 7 (interferometer reference arm), is reflected by the Faraday mirror, and returns to the coupler. The light pulses with aligned time delays form an odd-numbered optical interference pulse signal and non-interference pulses before and after here. At the same time, the three pulses return to the multiplexed coupler through one arm with a small split ratio; the latter optical signal enters the next multiplexed coupler through the sensor / delay fiber coil 2 and the transmission optical fiber 3, and is combined with the previous one. The splitting of the multiplexing couplers is the same, and the dual optical pulse signal is also split into two paths here, and the small splitting ratio path is connected to the interferometer, and returns to the first-level multiplexing coupler through the sensing fiber loop, the equal splitting ratio coupler 5, the multiplexing coupler, and the sensing / delay fiber loop 2. According to the delay optical fiber relationship, the first non-interference pulse of the three optical pulses (non-interference-interference-non-interference) returned by the latter path forms an aligned interference with the last non-interference pulse of the three optical pulses returned by the previous path, and the subsequent multiplexing units are analogous to this, and finally an interference optical pulse sequence returned by the array optical path is formed, which is output to the optoelectronic demodulation device end for signal restoration.

[0061] 3. Fiber optic sensor unit packaging and acoustic sensitivity design

[0062] The fiber interferometer 4 and the sensor / delay fiber loop 2 of the present invention include three skeleton structures: I, II, and III. The external cavity sensing structure of the fiber interferometer 4 is type I ( Figure 4 In (a)), the external cavity fiber interferometer 4 includes a core shaft skeleton 15, in which an equal-split ratio coupler and two Faraday reflectors 8 are packaged, one end of the equal-split ratio coupler 5 is connected to the two Faraday reflectors 8 through the sensing fiber and the reference fiber 7 respectively, and the other end of the equal-split ratio coupler 5 is connected to the input and output optical fiber 14, and a sensitivity enhancement structure I16 is arranged on the core shaft skeleton 15, and a sensitivity enhancement air cavity I17 is formed in the sensitivity enhancement structure I16, and an interferometer sensing fiber ring 6 is wound outside the sensitivity enhancement structure I16, and the interferometer sensing fiber ring 6 is connected to the internally packaged sensing fiber to achieve contact with the external fluid environment and obtain the first sensing signal 18. The I-type structure adopts an air back cavity structure, and the sensitivity enhancement air cavity I17 ensures the high sensitivity response of the sensitivity enhancement structure I16, and its signal phase response is based on 0°. The sensing unit adopts dual-fiber input and output, and is connected to the small split ratio end of the multiplexing coupler.

[0063] The external cavity sensing structure of the sensing / delay fiber ring 2 is type II ( Figure 4In (b)), the external cavity sensing / delay fiber loop 2 includes a core shaft skeleton II20, a sensitivity enhancement structure II16a is arranged on the core shaft skeleton II20, a sensitivity enhancement air cavity II17a is formed in the sensitivity enhancement structure II16a, a first sensing / delay fiber loop 2a is wound outside the sensitivity enhancement structure II16a, and the two ends of the first sensing / delay fiber loop 2a are respectively connected to an input optical fiber 19 and an output optical fiber 21 to achieve contact with the external fluid environment and obtain the second sensing signal 18a. The phase response of the type II sensing signal is the same as that of the type I, with a difference of 0°. The sensing unit is connected to the front and rear two multiplexing couplers with an input pigtail and an output pigtail to form a multiplexing optical path trunk.

[0064] The inner cavity sensing structure of the sensing / delay fiber ring 2 is type III ( Figure 4 In (c)), the inner cavity sensing / delay fiber loop 2 includes a core shaft skeleton III22, a sensitivity enhancement structure III24 and an outer sealed shell 23 are arranged on the core shaft skeleton III22, and a sensitivity enhancement air outer cavity 25 is formed between the outer side of the sensitivity enhancement structure III24 and the outer sealed shell 23. A second sensing / delay fiber loop 2b is also wound outside the sensitivity enhancement structure III24, and the two ends of the second sensing / delay fiber loop 2b are respectively connected to the input optical fiber 19 and the output optical fiber 21 to achieve contact with the internal fluid environment and obtain the third sensing signal 26. The phase response of the type III sensing signal is opposite to that of the type I, with a difference of 180°. The sensitive characteristic of the sensing unit is the increase in the upper limit of the dynamic range. The sensing unit can be replaced with the type II to form a multiplexed optical path trunk.

[0065] According to the optical path Figure 1 and Figure 2 The multiplexed optical path structure and the delayed interference relationship of the optical pulses shown in the figure can form three independent and combined relationships of type I, type I+type II, and type I+type III according to the packaging structure and combination design of the above three types of sensor units. The even-numbered optical interference pulses in the returned interference pulse sequence correspond to the return signals of the type I sensor unit, and the odd-numbered optical interference pulses correspond to the return signals of the type I+type II or type I+type III combined sensor units. The three signals can be returned simultaneously and restored in the photoelectric demodulation device. Through the flexible selection of information processing, the dynamic range of the optical fiber sensor array can be greatly expanded, see Figure 3 By adjusting the sensitivity values ​​and phases of the II and III sensors according to the design, the sensitivity spatial distribution of the sensing units in the array can be designed to achieve the effect of controlling the beam main lobe width and side lobe height.

[0066] 4. Optimization of sensor structure and sensor combination scheme

[0067] The unit dynamic range of the fiber optic sensor array in the scheme is mainly determined by the sensitivity of the unit while keeping the algorithm and demodulation hardware unchanged. The sensitivity of the single sensor structure is the benchmark for the dynamic range. It is a long-arm difference interferometer optical path structure, which is formed by the air back cavity sensitivity enhancement design. Figure 4 The external cavity sensitive acoustic sensor structure (type I) in (a) includes the winding of the long arm optical fiber coil on the sensitivity enhancement structure 16 and the installation of the interferometer optical device. There are two configurations of the delay fiber coil combined with the back cavity sensitivity enhancement design, which are divided into Figure 4 (b) External cavity sensing structure (Type II) and Figure 4 In the intracavity sensing structure (type III) of (c), when subjected to acoustic signals, the optical fiber states of the two are respectively extended / shortened or shortened / extended, and the absolute length of the change in the optical path of the fiber loop to the signal is obtained as |△L|. Both structures are combined with interferometer-type sensing units. Due to the functional differences of increasing sensitivity to expand the lower limit of self-noise and reducing sensitivity to increase the upper limit of saturation threshold, they maintain the same phase (0°) and anti-phase (180°) with the interference sensor respectively. At the same time, the former fully utilizes the length of the delayed fiber loop to maximize sensitivity, while the latter needs to design the desensitization value according to the interference source level of the target environment.

[0068] When the optical fiber sensor array is used, the interferometer sensor unit is used as the basic framework of the array application. Through the algorithm, functions such as target enhancement and azimuth discrimination can be realized. The delay sensor fiber coil is combined with the interferometer long-arm sensor fiber coil. The dynamic range expansion supported by the sensor hardware can be formed by the full-sensitivity delay fiber coil, the full-desensitivity delay fiber coil, and the staggered combination of the sensitivity fiber coil and the desensitization fiber coil. The three cases correspond to the following:

[0069] (1) Sensitization combination: ... - interferometer fiber coil N-1 - (interferometer fiber coil N-1 + delay fiber coil N-1) - interferometer fiber coil N - ...;

[0070] (2) Desensitization combination: ... - interferometer fiber coil N-1 - (interferometer fiber coil N-1 - delay fiber coil N-1) - interferometer fiber coil N - ...;

[0071] (3) Cross combination: ... - interferometer fiber loop N-1 - (interferometer fiber loop N-1 + delay fiber loop N-1) - interferometer fiber loop N - (interferometer fiber loop N-1 - delay fiber loop N-1) - ...;

[0072] The sensor units of combinations (1), (2) sensitivity enhancement and sensitivity reduction have the same aperture as the interferometer sensor units and the same directional characteristics as the non-combined sensor array. The dynamic range is expanded in one direction from noise reduction or anti-interference. By using combination (3) cross combination, the sensor units not only have a lower noise floor, but also have an enhanced ability to resist strong signals. However, for the entire array, the upper limit of the corresponding operating frequency of the sensor array with an expanded dynamic range is reduced to half of the original.

[0073] In the design of the sensor unit structure, the sensitivity enhancement structure is designed to be of the same size and specifications in consideration of interchangeability. The length of the sensor fiber loop is designed and calculated according to the characteristics of the sensor array usage scenario to achieve specific requirements for the sensitivity combination to ensure the large dynamic range of the sensor unit. At the same time, due to the flexibility of the sensitivity design of the combined sensor unit, the sensitivity of each combined unit can be spatially filtered to construct sidelobe suppression or a specific configuration of the sensor array directivity characteristics.

[0074] 5. Construct a spatial filter window to optimize the array directivity characteristics

[0075] By constructing a spatial filter window, the angular leakage and fence effect of the spatial spectrum are realized. When the linear array composed of N sensor units with the same sensitivity, phase and equal spacing d corresponds to the working wavelength λ, the acoustic beam diagram function with the heading angle θ of the sensor array is as follows: .

[0076] For the combined sensing unit, by adjusting the sensitive length and phase of the delayed fiber loop, it can be achieved in the insensitive ~M s +9.6dB, that is, for the sensor array, the sensitivity distribution of the sensor units can form a variety of spatial arrangement configurations of functions, and the corresponding window function can be expressed as the following formula: ,

[0077] The window function corresponding to ω corresponds to the amplitude of n, where n can be the time dimension or the space dimension, n=0,1,...,N-1, and k is the order of the window function, λ k is the corresponding weight.

[0078] For the Hanning window function of N sensor units, the corresponding function is expressed as follows:

[0079] .

[0080] Four spatial filter window configurations using 128 sensor units as an example, typically Figure 5 The sensitivity arrangement window function structure diagram shown in the figure, the corresponding spatial acoustic beam diagram function expression is as follows:

[0081] ,

[0082] Among them, M h (θ) is the modulation parameter of the beam function, including θ-related terms and constant terms, which adjusts the spatial directivity. Taking the linear array of sensor units of various scales (8 sensor units, 16 sensor units, 64 sensor units and 128 sensor units) as an example, its beam function characteristics will include the modulation characteristics of the spatial window function, see Figure 6 (a) Figure 6 (b) Figure 6 (c) Figure 6 In (d), by means of sensitivity windowing modulation, the directivity function of the array at the working wavelength can be constructed to have a significant suppression effect on the side lobes, while at the same time having a certain degree of control over the main lobe.

[0083] In summary, the present invention has three combinations of high sensitivity, conventional sensitivity and low sensitivity. Without increasing the configuration of terminal electronic equipment and software algorithms, the sensing characteristics of low noise and high saturation threshold of the array can be realized through the selective sampling of the working channel (interference pulse), so that the optical fiber sensor array has the ability to obtain and restore signals in a large dynamic range, and then obtain and identify the characteristics of the sound source with high fidelity under complex backgrounds and strong sound interference. On this basis, the present invention, combined with the preferred application of the large dynamic demodulation range algorithm, will further improve the signal reception threshold, adapt to the distortion-free acquisition of signals in various existing environments, and has broad application prospects in the fields of sea and land intrusion sound monitoring and seabed mineral resource exploration in complex environments.

[0084] It should be noted that the above implementation scheme only represents a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Equivalent modifications made on the basis of the above implementation scheme all fall within the protection scope of the present invention.

Claims

1. A fiber optic sensor array with a large dynamic range, comprising a sensing unit, characterized in that: The sensing units are more than two and are connected in sequence through transmission optical fibers. The sensing units adopt a reflective time-division optical path structure, including a multiplexing coupler, a sensing / delay fiber coil and a fiber interferometer. The multiplexing coupler is connected to the sensing / delay fiber coil and the fiber interferometer respectively through transmission optical fibers. The sensing / delay fiber coil is connected to the multiplexing coupler of the next sensing unit through transmission optical fibers. The fiber interferometer includes an interferometer coupler, an interferometer sensing fiber coil and two Faraday reflectors. The interferometer coupler is connected to the sensing / delay fiber coil and the fiber interferometer respectively through transmission optical fibers. The sensor fiber is connected to a Faraday reflector through a sensing fiber, and is connected to another Faraday reflector through a reference fiber. The interferometer sensing fiber coil is set on the sensing fiber. The last sensing unit includes a multiplexing coupler, a sensing / delay fiber coil and two fiber interferometers. The multiplexing coupler is connected to a sensing / delay fiber coil and a fiber interferometer through transmission optical fiber. The sensing / delay fiber coil is connected to another fiber interferometer through transmission optical fiber. Each sensing unit is distributed in an array and forms a fiber transmission multiplexing link for signal transmission and sensing. The multiplexing coupler is an unequal splitting ratio coupler, the transmission optical fiber at the large splitting ratio end is connected to the adjacent sensing / delay fiber loop, and the transmission optical fiber at the small splitting ratio end is connected to the optical fiber interferometer. The interferometer coupler is an equal splitting ratio coupler, the length difference between the sensing fiber and the reference fiber of each optical fiber interferometer is fixed, and the splitting ratio corresponding to two adjacent multiplexing couplers satisfies: , Wherein, n is a positive integer not less than 2, Cn is the small splitting percentage of the nth multiplexing coupler, and Cn-1 is the small splitting percentage of the n-1th multiplexing coupler; The optical signal input by the optical fiber transmission multiplexing link is a laser double pulse structure, and the output optical signal is a laser interference pulse sequence structure. The laser pulse interval is equal to the delay of the interferometer sensing fiber coil and is 1 / 2 of the delay of the sensing / delay fiber coil. The laser pulse repetition period is not less than (multiplexing degree + 1) × 2 × pulse interval; All interferometer sensing coils have the same phase, all sensing / delay coils have the same phase, the sensing / delay coils and the interferometer sensing coils have opposite phases, forming a full sensitivity increase combination of the sensing unit, or all interferometer sensing coils and sensing / delay coils have the same phase, forming a full sensitivity reduction combination of the sensing unit, or all interferometer sensing coils have the same phase, one of the two adjacent sensing / delay coils has the opposite phase to the interferometer sensing coil and the other has the same phase, and sensing / delay coils with different phases are alternately distributed to form a cross combination of sensing units; The fiber optic interferometer adopts an external cavity sensing structure, which includes a core shaft skeleton I, in which an equal-split ratio coupler and two Faraday reflectors are packaged, one end of the equal-split ratio coupler is connected to the two Faraday reflectors through a sensing fiber and a reference fiber respectively, and the other end of the equal-split ratio coupler is connected to an input and output optical fiber, and a sensitivity enhancement structure is arranged on the core shaft skeleton I, a sensitivity enhancement air cavity is formed in the sensitivity enhancement structure, an interferometer sensing fiber coil is wound outside the sensitivity enhancement structure, and the interferometer sensing fiber coil is connected to the internally packaged sensing fiber to achieve external fluid environment contact and signal acquisition; The sensing / delay fiber coil is an external cavity sensing structure or an internal cavity sensing structure. The external cavity sensing / delay fiber coil includes a core shaft skeleton II, a sensitivity enhancement structure is arranged on the core shaft skeleton II, a sensitivity enhancement air cavity is formed in the sensitivity enhancement structure, a sensing / delay fiber coil is wound outside the sensitivity enhancement structure, and the two ends of the sensing / delay fiber coil are respectively connected to an input optical fiber and an output optical fiber to achieve external fluid environment contact and signal acquisition; The inner cavity sensing / delay fiber coil includes a core shaft skeleton III, on which a sensitivity enhancement structure and an outer sealing shell are arranged, and an outer sensitivity enhancement air cavity is formed between the outer side of the sensitivity enhancement structure and the outer sealing shell, and a sensing / delay fiber coil is wound outside the sensitivity enhancement structure, and the two ends of the sensing / delay fiber coil are respectively connected to an input optical fiber and an output optical fiber to achieve contact with the internal fluid environment and signal acquisition.

2. The optical fiber sensor array with a large dynamic range according to claim 1, characterized in that: The sensing unit adopts a symmetrical time-division optical path structure. The multiplexing couplers of the front sensing units include 1 input multiplexing coupler and 1 output multiplexing coupler. The number of sensing / delay fiber coils and fiber interferometers are both 1. The input multiplexing coupler is connected to the sensing / delay fiber coil and fiber interferometer respectively through transmission optical fiber. The sensing / delay fiber coil is connected to the input multiplexing coupler of the next sensing unit through transmission optical fiber. The output multiplexing coupler is connected to the same fiber interferometer through transmission optical fiber, and the output multiplexing coupler is connected to the output multiplexing coupler of the next sensing unit. The last sensing unit includes 1 input multiplexing coupler, 1 fiber interferometer and 1 output multiplexing coupler. The input multiplexing coupler and the output multiplexing coupler are both connected to the same fiber interferometer through transmission optical fiber.

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