Time division multiplexing system and method based on microstructured fiber EFPI hydrophone

Abstract

The application discloses a time division multiplexing system and method based on a microstructure optical fiber EFPI hydrophone, and the system comprises a tunable laser, a continuous light with continuously switched wavelengths is outputted; an acousto-optic modulator, the continuous light is chopped into time division multiplexing pulse light; a time division multiplexing component, comprising a time division multiplexing sensing array based on the microstructure optical fiber EFPI hydrophone, time division multiplexing interference pulse light with sensing information is outputted; and an optical receiving unit, the time division multiplexing interference pulse light is received and demodulated. The microstructure optical fiber EFPI sensor has the advantages of simple and compact structure, high sensitivity, small volume and large-scale manufacturing, and is a new way to realize miniaturization of the optical fiber hydrophone. The application is applied to the field of the optical fiber hydrophone, when the tunable laser emits a series of continuously switched light at a fixed step, the signal light of each wavelength can generate an interference signal through a time division multiplexing array element, and large-scale time division multiplexing of the optical fiber EFPI hydrophone based on the microstructure is realized.

Description

Technical Field

[0001] This invention relates to the field of fiber optic hydrophone technology, specifically a time-division multiplexing system and method based on a microstructure fiber optic EFPI hydrophone. Background Technology

[0002] Fiber optic underwater acoustic detection uses fiber optic sensing technology to detect sound waves in water, with fiber optic hydrophones forming the foundation. Currently, phase interferometry-based fiber optic hydrophones primarily utilize traditional long-arm interferometer designs, where long-distance sensing fibers are wound around a sensitizing structure. This results in excessively large sensors, increasing the difficulty of transportation and installation. In recent years, underwater unmanned platforms have matured, with underwater unmanned vehicles and gliders widely used for marine environmental observation. Due to the limited endurance and payload capacity of underwater unmanned platforms, the size of the hydrophone arrays they carry is crucial. Furthermore, the complexity of the underwater acoustic environment makes it difficult for a single hydrophone to effectively identify and track underwater targets. Therefore, it is necessary to arrange fiber optic hydrophones into sensor arrays and use hydrophone multiplexing array technology to beamform the sound field signal, enabling the localization and orientation of underwater targets. Microelectromechanical Systems (MEMS) devices offer advantages such as small size, low cost, light weight, and mass production capabilities. Combining MEMS technology with fiber optic hydrophone technology provides a new engineering application direction for the miniaturization and large-scale reuse of fiber optic hydrophones. MEMS devices are typically on the micrometer scale, offering advantages for mass production. This allows fiber optic MEMS sensing technology to fully leverage the advantages of fiber optic sensing while significantly reducing sensor size and manufacturing costs. Furthermore, it facilitates the development of fiber optic hydrophones with the engineering characteristics of large-scale reuse and the ability to be mounted on underwater unmanned platforms.

[0003] Currently, multiplexing techniques for microstructured fiber optic extrinsic Fabry-Perot interferometer (EFPI) sensors (hereinafter referred to as "microstructured fiber optic EFPI hydrophones") fabricated using MEMS mainly include time-division multiplexing, wavelength-division multiplexing, coherent multiplexing, and hybrid multiplexing of several methods. When the number of microstructured fiber optic EFPI hydrophones is large, these methods become more complex and increase costs.

[0004] In particular, time-division multiplexing (TDM) suffers a sharp decline in signal-to-noise ratio (SNR) as the number of microstructured fiber optic EFPI hydrophones increases. Due to the difficulty in serial TDM multiplexing of microstructured fiber optic EFPI hydrophones and the high transmission loss caused by low end-face reflectivity, existing large-scale TDM array technologies are unsuitable for array multiplexing of microstructured fiber optic EFPI hydrophones. For example, T Liu et al. in the UK used two lasers of different wavelengths to achieve multiplexing of wavelength division multiplexing-based EFPI sensors, but this scheme has a small multiplexing scale and complex structure. Yang Panpan et al. from Anhui University of Science and Technology designed a 4S×8T multiplexing array based on time-division and space-division multiplexing, which can achieve 32-channel multiplexing, but has low sensitivity, high noise, and limited multiplexing scale. Zi-jun Wang et al. from the University of Electronic Science and Technology of China proposed a multiplexing sensing system based on a reflective semiconductor optical amplifier (RSOA), which has a good SNR and no significant crosstalk between sensors, but is not suitable for microstructured EFPI sensors. This paper proposes a multiplexing scheme for microstructured fiber EFPI hydrophones based on tunable lasers, which solves the problem of time-division multiplexing of microstructured fiber EFPI hydrophones. Summary of the Invention

[0005] To address the shortcomings of the existing technology, this invention provides a time-division multiplexing system and method based on a microstructured fiber optic EFPI hydrophone. The time-division multiplexing array of the microstructured fiber optic EFPI hydrophone is realized based on a tunable laser. When the tunable laser emits a series of continuously switched lights at a fixed step size, the signal light of each wavelength will pass through the time-division multiplexing array elements to generate an interference signal.

[0006] To achieve the above objectives, the present invention provides a time-division multiplexing system based on a microstructure fiber optic EFPI hydrophone, comprising:

[0007] Tunable lasers are used to output continuous light with continuously switchable wavelengths;

[0008] An acousto-optic modulator is used to chop continuous light with continuously switching wavelengths into time-division multiplexed pulsed light.

[0009] The time-division multiplexing component includes a time-division multiplexing sensor array based on a microstructure fiber optic EFPI hydrophone, used to time-division multiplex the time-division multiplexed pulse light and output time-division multiplexed interference pulse light with sensing information.

[0010] An optical receiving unit is used to receive and demodulate the time-division multiplexed interference pulse light.

[0011] In one embodiment, the time-division multiplexing sensor array includes an optical circulator and N-1 couplers C1-C1. N-1 N-1 delay fibers D1-D N-1With N microstructured fiber optic EFPI hydrophones S1-S N The optical circulator and coupler each have a first port, a second port, and a third port.

[0012] The first port of the optical circulator is connected to the output of the acousto-optic modulator via an optical fiber, and the third port of the optical circulator is connected to the input of the optical receiving unit via an optical fiber.

[0013] The first port of coupler C1 is connected to the second port of the optical circulator via an optical fiber, and the third port of coupler C1 is connected to the microstructure fiber optic EFPI hydrophone S1 via an optical fiber.

[0014] Coupler C i The first port is through the delayed fiber D i-1 With coupler C i-1 The second port is connected to the coupler C. i The third port connects to the microstructure fiber optic EFPI hydrophone S via optical fiber. i Connected, of which 1 <i≤N-1;

[0015] Coupler C N-1 The second port is connected to the delayed fiber D. N-1 With microstructure fiber optic EFPI hydrophone S N Connected.

[0016] In one embodiment, the delay fiber D1-D N-1 The lengths are all:

[0017]

[0018] Where L is the delay fiber D1-D N-1 The length of τ is the chopping time of the time division multiplexing array AOM, c is the speed of light in vacuum, n is the refractive index of the fiber, and L0 is the physical spacing between adjacent TDM channels plus fiber redundancy.

[0019] In one embodiment, the coupler C1-C N-1 The coupling ratio is:

[0020]

[0021] Where, χ n For coupler C n The coupling ratio is given by β, where β is the additional loss of the coupler.

[0022] In one embodiment, the optical receiving unit includes:

[0023] A photodetector is used to convert the time-division multiplexed interference pulse light into a time-division multiplexed interference electrical signal;

[0024] A / D analog-to-digital conversion unit is used to convert the time-division multiplexed interference electrical signal from an analog signal to a digital signal;

[0025] The signal processing and control unit is used to receive the digital signal and perform time-division multiplexing and demodulation to obtain sensing information.

[0026] To achieve the above objectives, the present invention also provides a time-division multiplexing method based on a microstructure fiber optic EFPI hydrophone, which uses the aforementioned time-division multiplexing system for time-division multiplexing.

[0027] In one embodiment, the time-division multiplexing method includes the following steps:

[0028] Step 1: Continuous light based on the continuous switching of output wavelength of a tunable laser;

[0029] Step 2: Based on the acousto-optic modulator, the continuous optical wavelength is continuously switched and chopped into time-division multiplexed pulse light;

[0030] Step 3: The time-division multiplexed pulse light is time-division multiplexed using a time-division multiplexed sensor array based on a microstructure fiber optic EFPI hydrophone to obtain time-division multiplexed interference pulse light with sensing information.

[0031] Step 4: Demodulate the time-division multiplexed interference pulse light to obtain the sensing information measured by the time-division multiplexed sensing array.

[0032] Compared with the prior art, the present invention has the following beneficial technical effects:

[0033] Due to the unique structure of the microstructure fiber optic EFPI cavity, serial multiplexing is difficult, and parallel multiplexing can only be achieved using couplers. However, its low reflectivity and coupler losses make the signal easily overwhelmed by noise in large-scale time-division multiplexing. Current multiplexing methods often employ intensity demodulation, but the low reflectivity results in low demodulation accuracy and limited practicality. This invention uses a tunable laser to emit laser light of different wavelengths and employs a five-step phase-shift algorithm with multi-wavelength averaging to reduce noise and signal distortion, effectively extracting the true signal, thereby achieving large-scale time-division multiplexing of a microstructure-based fiber optic EFPI hydrophone. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0035] Figure 1 This is a schematic diagram of the time-division multiplexing system based on a microstructure fiber optic EFPI hydrophone in an embodiment of the present invention;

[0036] Figure 2 This is a timing diagram of a time-division multiplexed array based on a microstructure fiber optic EFPI hydrophone in an embodiment of the present invention;

[0037] Figure 3 This is a schematic diagram of the microstructure fiber optic EFPI hydrophone in an embodiment of the present invention.

[0038] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0039] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0040] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0041] like Figure 1 The illustration shows a time-division multiplexing system based on a microstructured fiber optic EFPI hydrophone disclosed in this embodiment. It mainly includes a tunable laser, an acousto-optic modulator (AOM), a time-division multiplexing component, and an optical receiving unit. The tunable laser outputs continuous light with continuously switching wavelengths. The AOM chops the continuously switching wavelength light into time-division multiplexed pulsed light. The time-division multiplexing component includes a time-division multiplexing sensor array based on the microstructured fiber optic EFPI hydrophone. The number of TDM channels in the time-division multiplexing sensor array is the same as the number of wavelength switching channels in the time-division multiplexed pulsed light, used for time-division multiplexing of the pulsed light and outputting time-division multiplexed interference pulsed light carrying sensing information. The optical receiving unit mainly includes a photodetector (PD), an analog-to-digital converter (FPGA), and a signal processing and control unit (PC), used to receive and demodulate the time-division multiplexed interference pulsed light to obtain the sensing information measured by the time-division multiplexing sensor array.

[0042] In this embodiment, the tunable laser is a tunable laser from Uc Instruments Corp., which can output light in the 1528-1568nm band and can achieve a minimum step size of 0.02nm and a wavelength switching speed of 200kHz.

[0043] In practical implementation, the time-division multiplexing sensor array includes an optical circulator and N-1 couplers C1-C1. N-1 N-1 delay fibers D1-D N-1 With N microstructured fiber optic EFPI hydrophones S1-S N The optical circulator and coupler each have a first port, a second port, and a third port. Specifically, the first port of the optical circulator is connected to the output of the acousto-optic modulator via an optical fiber, and the third port of the optical circulator is connected to the input of the photodetector via an optical fiber. The first port of coupler C1 is connected to the second port of the optical circulator via an optical fiber, and the third port of coupler C1 is connected to the microstructured fiber optic EFPI hydrophone S1 via an optical fiber. i The first port is through the delayed fiber D i-1 With coupler C i-1 The second port is connected to the coupler C. i The third port connects to the microstructure fiber optic EFPI hydrophone S via optical fiber. i Connected, coupler C N-1 The second port is connected to the delayed fiber D. N-1 With microstructure fiber optic EFPI hydrophone S N Connected, of which 1 <i≤N-1。

[0044] A tunable laser outputs continuously switching wavelength light I0, in which different wavelengths of light are chopped into time-division multiplexed pulses I after passing through an acousto-optic modulator AOM. in The light then enters the time-division multiplexed sensor array through the second port of the circulator. To ensure that each wavelength of pulsed light can completely enter all the sensing elements (i.e., the microstructured fiber optic EFPI hydrophone) of the time-division multiplexed sensor array and return without pulse overlap, a time delay is achieved using a delay fiber. This requires a reasonable design of the delay fiber length to ensure that the pulsed light generates a time delay of τ / 2 in a single pass through the delay fiber (the pulsed light needs to travel back and forth twice; a single pass through one delay fiber generates a time delay of τ / 2). Let the delay fiber be D1-D... N-1 The lengths of the delay fibers are all equal, both being L (since the sensing elements are the same, it is only necessary to ensure that the delay fiber lengths are the same and reasonable, the pulse intervals will be equal), and the following relationship must be satisfied:

[0045]

[0046] Where τ is the chopping time of the time-division multiplexing array AOM, c is the speed of light in vacuum, n is the refractive index of the optical fiber, and L0 is the physical spacing between adjacent TDM channels plus optical fiber redundancy.

[0047] Taking a 200kHz wavelength switching speed, 8-time-division multiplexed sensor array as an example, the delay time τ for the return pulse of the same wavelength from adjacent sensing elements is 625ns, corresponding to a delay fiber length of 62.5m; the delay time between two adjacent wavelength pulses should be 8τ, i.e. Figure 2 As shown.

[0048] The time-division multiplexing sensor array includes a time-division link and a microstructured fiber optic EFPI hydrophone probe. The time-division link comprises an optical circulator (CIR), couplers (C1-C7), delay fibers (D1-D7), in-array transmission cables, and several fiber optic splices. The input pulse signal is I... in The light enters through port 1 of the CIR and connects to the time-division link via port 2. In the time-division link, the signal light with wavelength λ1 enters the first sensing element, passes through port C1-1 to the first coupler, and then a portion of the signal light enters probe S1 through coupler port C1-3. The remaining portion of the signal light passes through the other port C1-2 of the coupler and enters subsequent sensing elements, repeating this process until the signal light reaches the last sensing element S8. Only when the λ1 signal light has passed through all sensing elements and returned to the circulator will the λ2 signal light enter the time-division link and repeat the optical path of the λ1 signal light. Subsequent tunable lasers with different switching wavelengths then sequentially undergo the same operation in this time-division multiplexed link.

[0049] To ensure optical power uniformity across all channels in a time-division multiplexing (TDM) array system and to maintain approximately the same self-noise level across all channels, the coupling ratio of the couplers (C1-C7) in the link needs to be designed. Taking an 8-layer TDM array as an example, there is an additional coupler loss β. 耦合器 , weld point loss β 熔接点 and delay loop loss β 延时环 Additional losses. Let β n This represents the total additional loss for each level of TDM channel.

[0050] Let the optical power entering the TDM array be P. in The reflectivity of the microstructured fiber optic EFPI hydrophone is 4%, and the optical power reaching the circulator after passing through each stage of TDM channels is P. n (n≤7), then we have:

[0051]

[0052] As can be seen from the above, to ensure optical power balance in each TDM channel, the optical power of each TDM channel must be the same. From equation (2), we can obtain:

[0053]

[0054] Among them, P n =P n+1 For n = 1, 2, 3...7, the following relationship can be obtained between the coupling ratio and the total additional loss:

[0055]

[0056] Where, χ n β is the coupling ratio of the nth coupler. n This represents the additional loss at stage n.

[0057] Since the values ​​of various additional losses are relatively small, in actual calculations, it is assumed that the additional losses β at each level are small. n If they are equal and of size β, then a unified expression for the coupling ratio can be obtained:

[0058]

[0059] Where N is the number of multiplexing in the time-division multiplexing array, and in this example N = 8. The coupling ratio of each coupler in the 8-time-division multiplexing array link can be calculated according to equation (5).

[0060] Take the additional loss β of the coupler 耦合器 =0.15×2=0.3dB, weld point loss β 熔接点 =0.2dB and delay loop loss β 延时环 =0.1×2=0.2dB, therefore the total additional loss is β=0.7dB. Therefore, the coupling ratios of each stage of the coupler can be obtained as follows:

[0061] Coupler number 1 2 3 4 5 6 7 Coupling ratio (%) 9.27 11.07 13.5 16.91 22.06 30.69 47.99

[0062] Therefore, from equation (2), we can obtain:

[0063] P n ≈2.9×10 -4 P in (6)

[0064] The output power of the tunable laser in this system is 13mW and the front-end loss is 3dB. According to equation (6), the reflected power of each element is approximately 1.9μW.

[0065] Couplers (C1-C7), delay fibers (D1-D7), and in-array transmission optical cables are connected in series, which can proportionally distribute the input pulse light to eight microstructure fiber EFPI hydrophone elements (S1-S8).

[0066] The optical structure of the fiber optic hydrophones S1-S8 is of the EFPI type. Each hydrophone consists of an optical fiber, a fiber optic ceramic ferrule, and a sensing diaphragm. (Reference) Figure 3 The input light enters the ceramic ferrule from the optical fiber. Part of the light is reflected back through the end face of the ceramic ferrule, forming reflected light I1. The other part is transmitted through the ceramic ferrule, reflected by the sensing diaphragm outside the ferrule, coupled back into the ferrule, and finally returns to the optical fiber, forming reflected light I2. Reflected light I1 and reflected light I2 interfere within the optical fiber. When an external acoustic signal causes the sensing diaphragm to vibrate, the distance between the sensing diaphragm and the end face of the ceramic ferrule changes, thus altering the interference signal. The interference signals from each hydrophone element pass through a coupler and a delay fiber, generating a time-division multiplexing array channel delay of τ / 2, entering port 2 of the circulator, and outputting from port 3, forming output light I. out .

[0067] Output light I out After the photoelectric signal is converted into a time-division multiplexed interference signal by the photodetector PD in the optical receiving unit, the time-division multiplexed interference signal is converted from an analog signal to a digital signal by the A / D analog-to-digital conversion unit FPGA. Then, it enters the signal processing and control unit PC. The phase demodulation method based on five-step phase shift is used to demodulate the reflection spectrum. The data is processed by methods such as envelope elimination, ellipse fitting, and phase shift signal extraction. Two orthogonal signals are generated by utilizing the phase relationship between the interference signals. Finally, the dynamic cavity length change of the EFPI sensor is obtained by using the arctangent algorithm and the relationship between cavity length and phase, thus completing the demodulation and subsequent processing of the array interference phase information.

[0068] The above are merely preferred embodiments of the present invention and do not limit the scope of the patent. Any equivalent structural transformations made using the contents of the specification and drawings of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the scope of patent protection of the present invention.

Claims

1. A time-division multiplexing system based on a microstructure fiber optic EFPI hydrophone, characterized in that, include: Tunable lasers are used to output continuous light with continuously switchable wavelengths; An acousto-optic modulator is used to chop continuous light with continuously switching wavelengths into time-division multiplexed pulsed light. The time-division multiplexing component includes a time-division multiplexing sensor array based on a microstructure fiber optic EFPI hydrophone, used to time-division multiplex the time-division multiplexed pulse light and output time-division multiplexed interference pulse light with sensing information. An optical receiving unit is used to receive and demodulate the time-division multiplexed interference pulse light; The time-division multiplexing sensor array includes an optical circulator and N-1 couplers C1-C1. N-1 N-1 delay fibers D1-D N-1 With N microstructured fiber optic EFPI hydrophones S1-S N The optical circulator and coupler each have a first port, a second port, and a third port. The first port of the optical circulator is connected to the output of the acousto-optic modulator via an optical fiber, and the third port of the optical circulator is connected to the input of the optical receiving unit via an optical fiber. The first port of coupler C1 is connected to the second port of the optical circulator via an optical fiber, and the third port of coupler C1 is connected to the microstructure fiber optic EFPI hydrophone S1 via an optical fiber. Coupler C i The first port is through the delayed fiber D i-1 With coupler C i-1 The second port is connected to coupler C. i The third port connects to the microstructure fiber optic EFPI hydrophone S via optical fiber. i Connected, of which 1 <i≤N-1; Coupler C N-1 The second port is connected to the delayed fiber D. N-1 With microstructure fiber optic EFPI hydrophone S N Connected; The coupler C1-C N-1 The coupling ratio is: in, For coupler C n Coupling ratio, This refers to the additional losses of the coupler.

2. The time-division multiplexing system based on a microstructure fiber optic EFPI hydrophone according to claim 1, characterized in that, The delay fiber D1-D N-1 The lengths are all: Where L is the delay fiber D1-D N-1 Length, denoted as c, where c is the chopping time of the time-division multiplexed array AOM, c is the speed of light in vacuum, and n is the refractive index of the optical fiber. Add fiber redundancy to the physical spacing between adjacent TDM channels in time-division multiplexing.

3. The time-division multiplexing system based on a microstructure fiber optic EFPI hydrophone according to claim 1 or 2, characterized in that, The optical receiving unit includes: A photodetector is used to convert the time-division multiplexed interference pulse light into a time-division multiplexed interference electrical signal; A / D analog-to-digital conversion unit is used to convert the time-division multiplexed interference electrical signal from an analog signal to a digital signal; The signal processing and control unit is used to receive the digital signal and perform time-division multiplexing and demodulation to obtain sensing information.

4. A time-division multiplexing method based on a microstructure fiber optic EFPI hydrophone, characterized in that, Time division multiplexing is performed using the time division multiplexing system described in any one of claims 1 to 3.

5. The time-division multiplexing method based on a microstructure fiber optic EFPI hydrophone according to claim 4, characterized in that, The time-division multiplexing method includes the following steps: Step 1: Continuous light based on the continuous switching of output wavelength of a tunable laser; Step 2: Based on the acousto-optic modulator, the continuous optical wavelength is continuously switched and chopped into time-division multiplexed pulse light; Step 3: The time-division multiplexed pulse light is time-division multiplexed using a time-division multiplexed sensor array based on a microstructure fiber optic EFPI hydrophone to obtain time-division multiplexed interference pulse light with sensing information. Step 4: Demodulate the time-division multiplexed interference pulse light to obtain the sensing information measured by the time-division multiplexed sensing array.

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