Fiber optic stimulated Brillouin hydrophone modulation and demodulation system and monitoring method

Abstract

The present invention discloses a fiber-optic stimulated Brillouin hydrophone modulation and demodulation system, comprising: a single-frequency continuous laser, a first coupler, an acousto-optic modulator, an electro-optic modulator, a second coupler, a fiber amplifier, a circulator, a single-mode fiber, a third coupler, a photodetector, and a signal processor. The single-frequency continuous laser generated by the single-frequency continuous laser passes through the first coupler, and one channel of the single-frequency continuous laser is modulated by the acousto-optic modulator, amplified by the fiber amplifier, and then circulated before entering one end of the single-mode fiber and the third coupler. Another channel of the single-frequency continuous laser is modulated by the electro-optic modulator and enters the second coupler, before entering the other end of the single-mode fiber and the third coupler. The output end of the third coupler is connected to the photodetector, and the output end of the photodetector is connected to the signal processor. The present invention has a simple array configuration and can be deployed over tens of kilometers. Signal demodulation is real-time and fast, and the operating frequency band of the linear array is adjustable.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber sensing systems, and in particular to an optical fiber stimulated Brillouin hydrophone modulation and demodulation system and a monitoring method. Background Art

[0002] Fiber-optic hydrophone linear arrays have the advantages of high sensitivity and resistance to electromagnetic interference. As conformal arrays, towed linear arrays and fixed arrays, they play a huge role in ocean acoustic target detection. There are two main methods for preparing fiber optic hydrophone linear arrays. One is composed of single hydrophones connected in series, where the hydrophones can be of interferometer type, fiber Bragg grating type, fiber distributed feedback laser type, etc. The diameter of this type of linear array basically exceeds 10mm, and the single-fiber multiplexing capability is also weak; the other is an integrated hydrophone linear array, in which the hydrophones are naturally inlined in the linear array, the diameter of the linear array is within 10mm, and the single-fiber multiplexing capability is also strong. Its technologies mainly include weak-reflection fiber grating array type, which is prepared by drawing on the edge of a drawing tower and engraving gratings on the edge of a laser system. The process is complex and the cost is high; the Rayleigh scattering fiber type for heterodyne demodulation, in which the local laser and the Rayleigh scattered light in the optical fiber are combined and beat, and then the interference phase signal is calculated by integration. Affected by the Rayleigh scattering effect, the linear array cannot be made too long, and the integral calculation in signal demodulation is time-consuming and complex. Summary of the Invention

[0003] The present invention provides an optical fiber stimulated Brillouin hydrophone modulation and demodulation system and a monitoring method to solve the problems in the prior art of weak single-fiber multiplexing capability, complex process, high cost, and time-consuming and complex demodulation.

[0004] The present invention provides a fiber stimulated Brillouin hydrophone modulation and demodulation system, comprising: a single-frequency continuous laser, a first coupler, an acousto-optic modulator, an electro-optic modulator, a second coupler, a fiber amplifier, a circulator, a single-mode fiber, a third coupler, a photodetector, and a signal processor;

[0005] The single-frequency continuous laser generated by the single-frequency continuous laser forms two single-frequency continuous lasers after passing through the first coupler, and enters the acousto-optic modulator and the electro-optic modulator respectively; one single-frequency continuous laser is modulated by the acousto-optic modulator and amplified by the optical fiber amplifier to form a pulsed single-frequency laser, which enters one end of the single-mode optical fiber and the third coupler respectively through the circulator to form Brillouin scattered light in the single-mode optical fiber; the other single-frequency continuous laser is modulated by the electro-optic modulator to form a modulated laser with the same or similar wavelength as the Brillouin scattered light, and the modulated laser forms two modulated lasers after passing through the second coupler, one modulated laser enters the other end of the single-mode optical fiber, and the other modulated laser enters the third coupler; the output end of the third coupler is connected to the photodetector; and the output end of the photodetector is connected to the signal processor.

[0006] Furthermore, the first coupler is a 1×2 coupler with a distribution ratio of 50:50; the second coupler is a 1×2 coupler with a distribution ratio of 90:10, wherein the modulated laser with a light intensity of 90% enters the other end of the single-mode optical fiber, and the modulated laser with a light intensity of 10% enters the third coupler; the third coupler is a 3×3 coupler.

[0007] Furthermore, the modulation frequency of the electro-optical modulator is 11 GHz.

[0008] Furthermore, the optical fiber amplifier is an erbium-doped optical fiber amplifier.

[0009] The present invention also provides a monitoring method for a fiber-optic stimulated Brillouin hydrophone modulation and demodulation system, comprising:

[0010] Step 1: Generate single-frequency continuous laser through a single-frequency continuous laser;

[0011] Step 2: modulate the single-frequency continuous laser by an acousto-optic modulator; modulate the single-frequency continuous laser by an electro-optic modulator, so that the wavelengths of the two modulated single-frequency continuous lasers are the same or similar;

[0012] Step 3: The single-frequency continuous laser light after acousto-optic modulation is amplified by a fiber amplifier and then inputted into a coupler and one end of a single-mode fiber respectively; the single-frequency continuous laser light after electro-optic modulation is inputted into a coupler and the other end of a single-mode fiber respectively;

[0013] Stimulated Brillouin scattered light is formed in the single-mode optical fiber; interference light is formed in the coupler;

[0014] Step 4: Select several interference phase signal collection points on the single-mode optical fiber; when the intensity and / or frequency of the interference light obtained by the photodetector changes, calculate the interference phase signal of each interference phase signal collection point from the interference light intensity signal;

[0015] Step 5: Obtain the direction of the external sound wave based on the interference phase signal difference of each interference phase signal collection point.

[0016] Furthermore, in step 3, the single-frequency continuous laser generates a single-frequency continuous laser which passes through a 50:50 1×2 coupler to split the single-frequency continuous laser into two single-frequency continuous lasers with equal intensity, which are respectively supplied to an acousto-optic modulator and an electro-optic modulator.

[0017] Furthermore, in step 3, the electro-optically modulated single-frequency continuous laser passes through a 90:10 1×2 coupler to divide the electro-optically modulated single-frequency continuous laser into a single-frequency continuous laser with a light intensity of 90% and a single-frequency continuous laser with a light intensity of 10%. The single-frequency continuous laser with a light intensity of 90% is input into a single-mode optical fiber, and the single-frequency continuous laser with a light intensity of 10% is input into the coupler.

[0018] Furthermore, in step 4, the light intensity signals of the interference light are detected by three photodetectors; and the interference phase signal is calculated according to the three light intensity signals by an improved homodyne symmetric demodulation method, specifically:

[0019] Get three light intensity signals:

[0020]

[0021] Among them, I i is the light intensity signal, D is the DC signal of the light intensity, S is the AC component coefficient of the light intensity, is the interference phase signal to be determined, i is the sequence number of the three light intensity signals;

[0022] Perform the operation (I1+I2+I3) / 3 to obtain the DC component D;

[0023] After removing the DC operation, the AC signals of the three light intensity signals are obtained:

[0024]

[0025] Among them, I ACi is an AC signal of a light intensity signal;

[0026] Execution I AC1 2 +I AC2 2 +I AC3 2 Operation, we get the coefficient S of the AC component;

[0027] After normalization operation, the normalized AC component is obtained:

[0028]

[0029] After differential operation, the output is:

[0030]

[0031] Execute the operation a1(α′2-α′3)+α2(α′2-α′1)+α3(α′1-α′2), and the signal output is:

[0032]

[0033] Performing the integration operation on N, we can get This is the interference phase signal.

[0034] Furthermore, in step 4, a number of interference phase signal collection points are selected at equal intervals on the single-mode optical fiber.

[0035] Furthermore, in step 4, the spacing between the interference phase signal collection points is selected according to the requirements of the hydrophone line array detection frequency band.

[0036] Beneficial effects of the present invention:

[0037] By using the frequency variation of stimulated Brillouin scattered light in an optical fiber as a proxy for changes in external sound pressure, the linear array of fiber-optic hydrophones can be made longer, meaning more hydrophones can be multiplexed on a single fiber, and the array aperture can be increased. Using an improved 3×3 homodyne symmetric demodulation algorithm, an interference phase signal can be calculated from the heterodyne interference light intensity. This interference phase signal is dependent solely on the frequency variation of the stimulated Brillouin scattered light and is unaffected by changes in the intensity of the stimulated Brillouin scattered light. By selecting the interference phase signal at an appropriate position on the optical fiber and performing phase comparison, the direction of the sound wave is determined. This technology allows for arbitrary changes in the operating frequency band of the linear array of fiber-optic hydrophones. This system is simple to form and can be extended to tens of kilometers. Signal demodulation is fast and real-time, and the operating frequency band of the linear array is adjustable. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The features and advantages of the present invention will be more clearly understood by referring to the accompanying drawings, which are schematic and should not be construed as limiting the present invention in any way. In the accompanying drawings:

[0039] Figure 1 A system diagram of a specific embodiment of the present invention. DETAILED DESCRIPTION

[0040] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making any creative efforts shall fall within the scope of protection of the present invention.

[0041] The embodiment of the present invention provides a fiber optic stimulated Brillouin hydrophone modulation and demodulation system, such as Figure 1 As shown, the present invention utilizes a fiber-stimulated Brillouin hydrophone linear array technology based on heterodyne interference. A single-frequency continuous laser generates a single-frequency continuous laser, which is divided into two beams of equal intensity by a 50:50 1×2 coupler. One beam is labeled L1 in the diagram, and the other is labeled L2. A digital signal generator generates a pulse signal to drive an acousto-optic modulator, converting L1 into a pulsed single-frequency laser. This is then amplified by an erbium-doped fiber amplifier, labeled P1 in the diagram. P1 is then injected into one end of a single-mode fiber through a circulator, generating back-scattered Brillouin light within the fiber, labeled SB1 in the diagram. An electro-optic modulator modulates L2 at an 11 GHz frequency, generating a modulated laser identical to or similar to the Brillouin scattered light wave, labeled L3 in the diagram. L3 is split into two modulated laser beams of unequal intensity by a 90:10 1×2 coupler, designated L4 and L5 in the figure. L4's intensity is 90% of L3's, while L5's is 10% of L3's. L4 is injected into a single-mode fiber from the other end, where it combines with the Brillouin scattered light SB1 to generate stimulated Brillouin scattered light (SBS1). L5 and SBS1 are combined, and the two laser beams, with similar frequencies, produce interference light (I) after combining. When the single-mode fiber is subjected to external acoustic pressure perturbations, both the intensity and frequency of stimulated Brillouin scattered light SSB1 change, causing variations in the intensity of interference light I. The interference light intensities detected by three photodetectors are recorded as I1, I2, and I3, respectively. The intensity signals received by the photodetectors are collected by an acquisition card and fed into a computer's signal processor, where an improved 3×3 homodyne symmetric demodulation algorithm is used to calculate the interference phase signal from the interference intensity signal. According to the detection frequency band of the optical fiber hydrophone linear array, the interference phase signals at corresponding positions on the single-mode optical fiber are selected in sequence at equal intervals, and the phase signals of these points are compared to obtain the direction of the external sound wave.

[0042] The derivation process of the improved 3×3 homodyne symmetric demodulation algorithm to calculate the interference phase signal is as follows:

[0043] The intensity and frequency of stimulated Brillouin scattered light are related to the external sound pressure. When the sound pressure changes with time t, the intensity and frequency of the scattered light are also functions of t, which can be expressed as:

[0044] P B =P B (t)

[0045] w B =w B (t)

[0046] Then the field intensity of stimulated Brillouin scattered light SBS1 is:

[0047]

[0048] in, is the initial phase of the stimulated Brillouin scattered light, which is a constant.

[0049] After the local continuous single-frequency laser is electro-optically modulated at 11 GHz, it generates a laser with a frequency close to that of the stimulated Brillouin scattered light. The field intensity of the laser L5 is:

[0050]

[0051] Among them, E 50 is the field intensity coefficient of laser L5, w 50 、 is the angular frequency, which is a constant.

[0052] When the two beams above combine and beat, interference light I is generated, and the light intensity is:

[0053]

[0054] Let w in formula (1) 50 -w B =Δw, from formula (1), we can see that the intensity of the interference light I is not only affected by P B Affected by w B The DC and AC coefficients are no longer constants, so it is necessary to separate w from the light intensity I. B , with w B to sense the change in sound pressure.

[0055] On the basis of the basic homodyne symmetric demodulation algorithm, the order of operation of the algorithm can be changed to eliminate P B The influence of w B The changes are as follows:

[0056] 3×3 coupler's 3-way output light intensity signal I i The DC components are equal and the phase difference is 2π / 3, I i It is expressed by the following formula:

[0057]

[0058] Execute the "I1+I2+I3" operation to get I i The DC component D:

[0059]

[0060] After removing DC operation, the three AC signals are:

[0061]

[0062] The next operation step of the normal homodyne symmetrical demodulation algorithm is differentiation operation. Since the coefficients of the AC signal are also variables, the AC signal coefficients need to be normalized first.

[0063] Execute "I AC1 2 +I AC2 2 +I AC3 2 " operation, the signal output is:

[0064]

[0065] Then the normalized AC component is:

[0066]

[0067] After differential operation, the signal output is:

[0068]

[0069] Execute the operation “α1(α′2-α′3)+a2(α′3-α′1)+a3(α′1-α′2)”, and the signal output is:

[0070]

[0071] Performing the integration operation on N, we can get Signal.

[0072] according to The solution of the signal is to obtain the frequency change of stimulated Brillouin scattering at each position on the single-mode fiber. By referring to the beam forming method of the serial fiber distributed feedback laser hydrophone linear array, the single-mode fiber stimulated Brillouin scattering can be regarded as a continuous number of fiber distributed feedback laser hydrophones, and the frequency change caused by the frequency change at each position of the single-mode fiber can be picked at equal intervals. According to this principle, the interval length of the single-mode optical fiber position is regarded as the interval of a single hydrophone. The length of the interval can be designed and selected according to the requirements of the linear array detection frequency band, so as to achieve the purpose of adjusting the linear array working frequency band.

[0073] Although the embodiments of the present invention have been described with reference to the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the present invention. Such modifications and variations are all within the scope defined by the appended claims.

Claims

1. A fiber stimulated Brillouin hydrophone modulation and demodulation system, characterized in that: include: Single-frequency continuous laser, first coupler, acousto-optic modulator, electro-optic modulator, second coupler, optical fiber amplifier, circulator, single-mode optical fiber, third coupler, photodetector, signal processor; The single-frequency continuous laser generated by the single-frequency continuous laser forms two single-frequency continuous lasers after passing through the first coupler, and enters the acousto-optic modulator and the electro-optic modulator respectively; one single-frequency continuous laser is modulated by the acousto-optic modulator and amplified by the optical fiber amplifier to form a pulsed single-frequency laser, which enters one end of the single-mode optical fiber and the third coupler respectively through the circulator to form Brillouin scattered light in the single-mode optical fiber; the other single-frequency continuous laser is modulated by the electro-optic modulator to form a modulated laser with the same or similar wavelength as the Brillouin scattered light, and the modulated laser forms two modulated lasers after passing through the second coupler, one modulated laser enters the other end of the single-mode optical fiber, and the other modulated laser enters the third coupler; the output end of the third coupler is connected to the photodetector; the output end of the photodetector is connected to the signal processor, Among them, the first coupler is a 1×2 coupler with a distribution ratio of 50:50; the second coupler is a 1×2 coupler with a distribution ratio of 90:10, wherein the modulated laser with a light intensity of 90% enters the other end of the single-mode optical fiber, and the modulated laser with a light intensity of 10% enters the third coupler; the third coupler is a 3×3 coupler.

2. The fiber stimulated Brillouin hydrophone modulation and demodulation system according to claim 1, wherein: The modulation frequency of the electro-optical modulator is 11 GHz.

3. The fiber stimulated Brillouin hydrophone modulation and demodulation system according to claim 1, wherein: The optical fiber amplifier is an erbium-doped optical fiber amplifier.

4. A monitoring method for a fiber optic stimulated Brillouin hydrophone modulation and demodulation system, according to the fiber optic stimulated Brillouin hydrophone modulation and demodulation system according to any one of claims 1 to 3, characterized in that: The monitoring method of the optical fiber stimulated Brillouin hydrophone modulation and demodulation system comprises the following steps: Step 1: Generate single-frequency continuous laser through a single-frequency continuous laser; Step 2: modulate the single-frequency continuous laser by an acousto-optic modulator; modulate the single-frequency continuous laser by an electro-optic modulator, so that the wavelengths of the two modulated single-frequency continuous lasers are the same or similar; Step 3: The single-frequency continuous laser light after acousto-optical modulation is amplified by an optical fiber amplifier and then inputted into one end of the single-mode optical fiber and the third coupler respectively; the single-frequency continuous laser light after electro-optical modulation is inputted into the other end of the single-mode optical fiber and the third coupler respectively after passing through the second coupler; Stimulated Brillouin scattered light is generated in the single-mode optical fiber; interference light is generated in the third coupler; Step 4: Select several interference phase signal collection points on the single-mode optical fiber; when the intensity and / or frequency of the interference light obtained by the photodetector changes, calculate the interference phase signal of each interference phase signal collection point from the interference light intensity signal; Step 5: Obtain the direction of the external sound wave based on the interference phase signal difference of each interference phase signal collection point.

5. The monitoring method of the fiber stimulated Brillouin hydrophone modulation and demodulation system according to claim 4, characterized in that: In step 1, the single-frequency continuous laser generates a single-frequency continuous laser which passes through a 50:50 1×2 coupler, i.e., a first coupler, and is split into two single-frequency continuous lasers with equal light intensity, which are respectively supplied to an acousto-optic modulator and an electro-optic modulator.

6. The monitoring method of the fiber stimulated Brillouin hydrophone modulation and demodulation system according to claim 4 or 5, characterized in that: In step 3, the electro-optically modulated single-frequency continuous laser passes through a 90:10 1×2 coupler, i.e., a second coupler, and the electro-optically modulated single-frequency continuous laser is divided into a single-frequency continuous laser with a light intensity of 90% and a single-frequency continuous laser with a light intensity of 10%. The single-frequency continuous laser with a light intensity of 90% is input into a single-mode optical fiber, and the single-frequency continuous laser with a light intensity of 10% is input into a third coupler.

7. The monitoring method of the fiber stimulated Brillouin hydrophone modulation and demodulation system according to claim 4, characterized in that: In step 4, the light intensity signals of the interference light are detected by three photodetectors; and the interference phase signal is calculated according to the three light intensity signals by an improved homodyne symmetric demodulation method, specifically: Get light intensity signal: in, I i is the light intensity signal, D is the DC signal of light intensity, S is the AC component coefficient of light intensity, is the interference phase signal to be determined, i is the sequence number of the three light intensity signals; implement( I 1+ I 2+ I 3) / 3 operation, get the DC component D ; After removing the DC operation, the AC signal of the light intensity signal is obtained: in, I ACi is an AC signal of a light intensity signal; implement Operation, get the coefficient of AC component S ; After normalization operation, the normalized AC component is obtained: After differential operation, the output is: implement Operation, the signal output is: right N Performing the integral operation, we can get , which is the interference phase signal.

8. The monitoring method of the fiber stimulated Brillouin hydrophone modulation and demodulation system according to claim 4, characterized in that: In step 4, a number of interference phase signal collection points are selected at equal intervals on the single-mode optical fiber.

9. The monitoring method of the optical fiber stimulated Brillouin hydrophone modulation and demodulation system according to claim 4 or 8, characterized in that: In step 4, the spacing between the interference phase signal collection points is selected according to the requirements of the hydrophone line array detection frequency band.

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