Monopulse heterodyne modulation system and method applied to optical fiber hydrophone

Abstract

The invention provides a monopulse heterodyne modulation system applied to an optical fiber hydrophone. A demodulation board outputs a linear frequency modulation signal to a tunable laser; the tunable laser modulates a laser signal through linear frequency modulation signal modulation, and outputs a continuous optical signal to the acousto-optic modulator; the demodulation board also outputs a path of periodic pulse signal to the acousto-optic modulator; wherein the sawtooth width of the linear frequency modulation signal is greater than the pulse width of the periodic pulse signal, and the sawtooth center of the linear frequency modulation signal is aligned with the pulse center of the periodic pulse signal; the acousto-optic modulator is used for modulating the continuous optical signal by adopting the periodic pulse signal and outputting a pulse optical signal; a pulse optical signal enters the optical fiber interferometer through the transmission optical fiber and is reflected by two arms of the optical fiber interferometer, then interference occurs at the output end, and an interference signal is obtained after photoelectric conversion. The scheme is high in vibration interference resistance and link interference resistance, and is favorable for realizing multi-time division multiplexing.

Description

Technical Field

[0001] The present application relates to the field of signal processing, and in particular to a single pulse heterodyne modulation system and method applied to a fiber optic hydrophone. Background Art

[0002] In the field of fiber optic hydrophone sensing, in order to suppress the phase fading problem, commonly used modulation and demodulation methods include phase generation carrier, heterodyne, 3×3 and other schemes. Among them, the heterodyne modulation and demodulation method is used because of its simple algorithm and stable system operation. However, the heterodyne scheme currently used in the industry usually uses a fixed frequency difference dual-pulse heterodyne modulation and demodulation method, that is, a frequency shift delay module is used to generate two pulses, one of which uses an acousto-optic frequency shifter to generate a fixed frequency shift. The frequency shift delay module is essentially a non-equal-arm Mach-Zehnder (MZ) interferometer, which is sensitive to environmental sounds and is therefore not suitable for use on moving platforms. In addition, after the downlink transmission optical fiber is disturbed by the environment, the modulation effect on the dual pulse signal is different, resulting in the final interference and introduction of phase noise, so it is not suitable for use in scenarios with strong link interference or long link transmission distances. In addition, in multiple time division multiplexing, the time length occupied by the dual pulse is longer, which is not conducive to increasing the number of time division multiplexing. Summary of the invention

[0003] The purpose of this application is to propose a single pulse heterodyne modulation method for optical fiber hydrophone, which has strong anti-vibration interference and anti-link interference capabilities and is conducive to realizing multi-time division multiplexing.

[0004] The technical solution adopted in this application is:

[0005] In a first aspect, the present application provides a single pulse heterodyne modulation system applied to a fiber optic hydrophone, comprising: a demodulation board, a tunable laser and an acousto-optic modulator;

[0006] The demodulation board outputs a linear frequency modulation signal to the tunable laser;

[0007] The tunable laser modulates the laser signal by linear frequency modulation signal modulation, so that the output optical frequency changes periodically with the linear frequency modulation signal, and outputs a continuous optical signal to an acousto-optic modulator (AOM);

[0008] A tunable laser is used, and its tunable function enables the laser to receive modulation of a linear frequency modulation signal. The above steps can realize laser linear frequency modulation output, and the output optical signal is still a continuous optical signal in terms of intensity.

[0009] The demodulation board outputs a periodic pulse signal to the acousto-optic modulator;

[0010] Wherein, the sawtooth width of the linear frequency modulation signal is greater than the pulse width of the periodic pulse signal, and the sawtooth center of the linear frequency modulation signal is aligned with the pulse center of the periodic pulse signal;

[0011] Since there is a certain length of optical fiber between the tunable laser and the AOM, in order to make the output pulse light signal frequency linear frequency modulation, the delay of the pulse modulation signal can be set to compensate for the optical fiber delay between the laser and the AOM, so that the centers of the two signals are aligned, that is, the sawtooth center of the linear frequency modulation signal is aligned with the pulse center of the periodic pulse signal, thereby ensuring that the output pulse light is a linear frequency modulation optical signal.

[0012] The acousto-optic modulator modulates the continuous light signal with a periodic pulse signal to convert the continuous light signal into a pulse light signal, and outputs the pulse light signal;

[0013] The pulse light signal enters the fiber interferometer through the transmission optical fiber; wherein the fiber interferometer is a Michelson interferometer with a certain arm difference; after the pulse light signal is reflected by the two arms of the fiber interferometer, interference occurs at the output end, and after photoelectric conversion, an interference signal is obtained, and the interference signal (heterodyne modulation signal) is a difference frequency signal.

[0014] This scheme can use a single pulse to generate a difference frequency signal.

[0015] In a possible implementation manner, the tunable laser is a narrow linewidth tunable laser.

[0016] In a possible implementation manner, the linear frequency modulation signal is a periodic sawtooth wave signal or a periodic triangle wave signal.

[0017] In a possible implementation, the frequency Δf of the difference frequency signal is designed by selecting the fiber interferometer arm difference ΔL and the linear modulation slope k of the tunable laser, and the formula is:

[0018]

[0019] Where n is the refractive index of the optical fiber, which is determined by the type of optical fiber and is usually around 1.45; c is the speed of light in a vacuum, which is a constant and is 2.9979×10 8 .

[0020] The frequency Δf of the difference frequency signal determines the operating frequency band and upper limit of the dynamic range of the system. By designing the fiber interferometer arm difference ΔL and the linear modulation slope k of the tunable laser, the frequency Δf of the difference frequency signal can be made to be in the order of several hundred kHz, thus ensuring the wide bandwidth and large dynamic range performance of the system.

[0021] In a possible implementation, the linear modulation slope k of the tunable laser is determined by setting the voltage modulation efficiency a of the tunable laser and the sawtooth slope m of the linear frequency modulation signal, where k=am.

[0022] Therefore, the frequency Δf of the difference frequency signal of this scheme can be designed through the coordinated design of the laser voltage modulation efficiency a, the sawtooth slope m of the linear frequency modulation signal, and the fiber interferometer arm difference ΔL to ensure sufficient bandwidth and dynamic range of the system. For example, the voltage modulation efficiency of the laser is designed to be a=100MHz / V, and the output signal of the demodulator board is designed to be m=0.001V / ns, then k=100kHz / ns. ΔL is designed to be 1m, and the final output Δf is about 500kHz.

[0023] In a possible implementation, the interference signal is:

[0024]

[0025] Where I is the interference signal intensity, A is the DC amplitude of the interference signal, and B is the AC amplitude of the interference signal. Both parameters are determined by the intensity of the optical signal output by the interferometer. It is the external sound signal detected by the fiber optic interferometer, and its size is determined by the intensity of the external sound signal.

[0026] In a second aspect, the present application provides a single pulse heterodyne modulation method applied to a fiber optic hydrophone, comprising:

[0027] Demodulation board, outputting a linear frequency modulation signal to the tunable laser;

[0028] The tunable laser modulates the laser signal by linear frequency modulation signal modulation, so that the output optical frequency changes periodically with the linear frequency modulation signal, and outputs a continuous optical signal to the acousto-optic modulator;

[0029] The demodulation board outputs a periodic pulse signal to the acousto-optic modulator;

[0030] Wherein, the sawtooth width of the linear frequency modulation signal is greater than the pulse width of the periodic pulse signal, and the sawtooth center of the linear frequency modulation signal is aligned with the pulse center of the periodic pulse signal;

[0031] The acousto-optic modulator uses a periodic pulse signal to modulate a continuous light signal, so that the continuous light signal is converted into a pulse light signal, and outputs a pulse light signal.

[0032] The specific implementation method of the second aspect of the present application can refer to the implementation method of the first aspect, which will not be repeated here.

[0033] Beneficial effects of this application:

[0034] This application realizes single-pulse linear frequency modulation by applying a linear frequency modulation signal to a tunable laser and cooperating with an acousto-optic modulator. In combination with the arm difference of the Michelson interferometer in the fiber optic hydrophone, the interference signals of the two arms of the fiber optic interferometer have a certain frequency difference, thereby realizing a single-pulse heterodyne modulation scheme. This scheme overcomes the problems of the dual-pulse scheme in terms of poor resistance to vibration interference, poor resistance to link interference, and difficulty in realizing multi-time division multiplexing, and has a good effect on application scenarios where the system is located in an environment with vibration or long-distance transmission. It has the following advantages:

[0035] 1. Single-pulse heterodyne modulation is realized through linear frequency modulation signals, which can effectively solve the anti-vibration interference and link interference problems introduced by double pulses.

[0036] 2. The optical path is simpler, and modulation is achieved using only tunable lasers and acousto-optic modulators, reducing the number of components in the modulation system.

[0037] 3. The use of single-pulse heterodyne modulation is beneficial to increasing the amount of time-division multiplexing and the size of the array. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 Schematic diagram of the traditional double-pulse heterodyne modulation system.

[0039] Figure 2 Schematic diagram of a single pulse heterodyne modulation system in one embodiment of the present application. DETAILED DESCRIPTION

[0040] The implementation of the present application is further described below in conjunction with the accompanying drawings and specific examples.

[0041] Embodiment 1:

[0042] The embodiment of the present application provides a single pulse heterodyne modulation system applied to a fiber optic hydrophone, comprising: a demodulation board, a tunable laser and an acousto-optic modulator;

[0043] The demodulation board outputs a linear frequency modulation signal to the tunable laser;

[0044] The tunable laser modulates the laser signal by linear frequency modulation signal modulation, so that the output optical frequency changes periodically with the linear frequency modulation signal, and outputs a continuous optical signal to the acousto-optic modulator;

[0045] A tunable laser is used, and its tunable function enables the laser to receive modulation of a linear frequency modulation signal. The above steps can realize laser linear frequency modulation output, and the output optical signal is still a continuous optical signal in terms of intensity.

[0046] The demodulation board outputs a periodic pulse signal to the acousto-optic modulator;

[0047] Wherein, the sawtooth width of the linear frequency modulation signal is greater than the pulse width of the periodic pulse signal, and the sawtooth center of the linear frequency modulation signal is aligned with the pulse center of the periodic pulse signal;

[0048] Since there is a certain length of optical fiber between the tunable laser and the AOM, in order to make the output pulse light signal frequency linear frequency modulation, the delay of the pulse modulation signal can be set to compensate for the optical fiber delay between the laser and the AOM, so that the centers of the two signals are aligned, that is, the sawtooth center of the linear frequency modulation signal is aligned with the pulse center of the periodic pulse signal, thereby ensuring that the output pulse light is a linear frequency modulation optical signal.

[0049] The acousto-optic modulator modulates the continuous light signal with a periodic pulse signal to convert the continuous light signal into a pulse light signal, and outputs the pulse light signal;

[0050] The pulse light signal enters the fiber interferometer through the transmission optical fiber; wherein the fiber interferometer is a Michelson interferometer with a certain arm difference; after the pulse light signal is reflected by the two arms of the fiber interferometer, interference occurs at the output end, and after photoelectric conversion, an interference signal is obtained, and the interference signal (heterodyne modulation signal) is a difference frequency signal.

[0051] This scheme can use a single pulse to generate a difference frequency signal.

[0052] In some embodiments, the tunable laser is a narrow linewidth tunable laser.

[0053] In some embodiments, the linear frequency modulation signal is a periodic sawtooth wave signal or a periodic triangle wave signal.

[0054] Figure 1 It is a schematic diagram of the traditional double-pulse heterodyne modulation system. Figure 2 The schematic diagram of a single pulse heterodyne modulation system using an embodiment of the present application is shown in FIG. 1 , wherein the linear frequency modulation signal adopts a periodic sawtooth wave signal. 1 ~S 16 , S 1 ~S n The time division multiplexing effect of the conventional double-pulse heterodyne modulation system and the single-pulse heterodyne modulation system of the embodiment of the present application is explained by comparing the two.

[0055] When using Figure 1In the dual-pulse heterodyne modulation scheme, the pulse width can be set to 150ns, the pulse period is 5000ns, the downlink output dual pulse width is 300ns, 5000 / 300=16.66, that is, 16-time division multiplexing is achieved. Since the frequency-shift delay module itself is an MZ fiber interferometer, the vibration in the environment will be introduced into the optical phase signal through it, that is, there is a problem of anti-vibration interference. In addition, during long-distance transmission, the interference on the link also produces different phase modulations on the dual pulses, introducing the link acoustic signal into the optical phase signal, that is, the problem of poor anti-link interference.

[0056] When using Figure 2 In the single pulse heterodyne modulation scheme, when the pulse width is also set to 150ns and the pulse period is 5000ns, the downlink output pulse width is 150ns, 5000 / 150=33.33, that is, the system can achieve 32-hour time division multiplexing, so it has a higher multiplexing efficiency. In addition, since the modulation part cancels the frequency shift delay module, the vibration in the environment will not be introduced into it, and there is no vibration interference problem. In addition, due to the use of single pulse output, the interference on the link will not affect the single pulse, that is, there is no link interference problem. In the end, the noise characteristics of this scheme are better, which solves the shortcomings of the original scheme under mobile platform and long-distance transmission conditions.

[0057] In some embodiments, the frequency Δf of the difference frequency signal is designed by selecting the fiber interferometer arm difference ΔL and the linear modulation slope k of the tunable laser, and the formula is:

[0058]

[0059] Where n is the refractive index of the optical fiber, which is determined by the type of optical fiber and is usually around 1.45; c is the speed of light in a vacuum, which is a constant and is 2.9979×10 8 .

[0060] The frequency Δf of the difference frequency signal determines the operating frequency band and upper limit of the dynamic range of the system. By designing the fiber interferometer arm difference ΔL and the linear modulation slope k of the tunable laser, the frequency Δf of the difference frequency signal can be made to be in the order of several hundred kHz, thus ensuring the wide bandwidth and large dynamic range performance of the system.

[0061] In some embodiments, the linear modulation slope k of the tunable laser is determined by setting the voltage modulation efficiency a of the tunable laser and the sawtooth slope m of the linear frequency modulation signal, where k=am.

[0062] Therefore, the frequency Δf of the difference frequency signal of this scheme can be designed through the coordinated design of the laser voltage modulation efficiency a, the sawtooth slope m of the linear frequency modulation signal, and the fiber interferometer arm difference ΔL to ensure sufficient bandwidth and dynamic range of the system. For example, the voltage modulation efficiency of the laser is designed to be a=100MHz / V, and the output signal of the demodulator board is designed to be m=0.001V / ns, then k=100kHz / ns. ΔL is designed to be 1m, and the final output Δf is about 500kHz.

[0063] In some embodiments, the interference signal is:

[0064]

[0065] Where I is the interference signal intensity, A is the DC amplitude of the interference signal, and B is the AC amplitude of the interference signal. Both parameters are determined by the intensity of the optical signal output by the interferometer. It is the external sound signal detected by the fiber optic interferometer, and its size is determined by the intensity of the external sound signal.

[0066] Embodiment 2:

[0067] The present application provides a single pulse heterodyne modulation method applied to a fiber optic hydrophone, comprising:

[0068] Demodulation board, outputting a linear frequency modulation signal to the tunable laser;

[0069] The tunable laser modulates the laser signal by linear frequency modulation signal modulation, so that the output optical frequency changes periodically with the linear frequency modulation signal, and outputs a continuous optical signal to the acousto-optic modulator;

[0070] The demodulation board outputs a periodic pulse signal to the acousto-optic modulator;

[0071] Wherein, the sawtooth width of the linear frequency modulation signal is greater than the pulse width of the periodic pulse signal, and the sawtooth center of the linear frequency modulation signal is aligned with the pulse center of the periodic pulse signal;

[0072] The acousto-optic modulator uses a periodic pulse signal to modulate a continuous light signal, so that the continuous light signal is converted into a pulse light signal, and outputs a pulse light signal.

[0073] The specific implementation method of the embodiment of the present application can refer to the specific implementation method of the above-mentioned embodiment 1, which will not be repeated here.

[0074] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A single pulse heterodyne modulation system for optical fiber hydrophone, characterized in that: include: Demodulation boards, tunable lasers, and acousto-optic modulators; The demodulation board outputs a linear frequency modulation signal to the tunable laser; The tunable laser modulates the laser signal by linear frequency modulation signal modulation, so that the output optical frequency changes periodically with the linear frequency modulation signal, and outputs a continuous optical signal to the acousto-optic modulator; The demodulation board outputs a periodic pulse signal to the acousto-optic modulator; Wherein, the sawtooth width of the linear frequency modulation signal is greater than the pulse width of the periodic pulse signal, and the sawtooth center of the linear frequency modulation signal is aligned with the pulse center of the periodic pulse signal; The acousto-optic modulator modulates the continuous light signal with a periodic pulse signal to convert the continuous light signal into a pulse light signal, and outputs the pulse light signal; The pulse light signal enters the fiber interferometer through the transmission optical fiber; wherein the fiber interferometer is a Michelson interferometer with a certain arm difference; after the pulse light signal is reflected by the two arms of the fiber interferometer, interference occurs at the output end, and after photoelectric conversion, an interference signal is obtained, and the interference signal is a difference frequency signal.

2. The system according to claim 1, characterized in that The tunable laser is a narrow linewidth tunable laser.

3. The system according to claim 3, characterized in that The linear frequency modulation signal is a periodic sawtooth wave signal or a periodic triangle wave signal.

4. The system according to claim 2, characterized in that By selecting the fiber interferometer arm difference ΔL and the linear modulation slope k of the tunable laser, the frequency Δf of the difference frequency signal is designed, and the formula is: Where n is the refractive index of the optical fiber and c is the speed of light in a vacuum.

5. The system according to claim 4, characterized in that The interference signal is: Where, I is the interference signal intensity, A is the DC amplitude of the interference signal, and B is the AC amplitude of the interference signal; It is the external sound signal detected by the fiber optic interferometer.

6. A single pulse heterodyne modulation method applied to optical fiber hydrophone, characterized in that: include: Demodulation board, outputting a linear frequency modulation signal to the tunable laser; The tunable laser modulates the laser signal by linear frequency modulation signal modulation, so that the output optical frequency changes periodically with the linear frequency modulation signal, and outputs a continuous optical signal to the acousto-optic modulator; The demodulation board outputs a periodic pulse signal to the acousto-optic modulator; Wherein, the sawtooth width of the linear frequency modulation signal is greater than the pulse width of the periodic pulse signal, and the sawtooth center of the linear frequency modulation signal is aligned with the pulse center of the periodic pulse signal; The acousto-optic modulator modulates the continuous light signal with a periodic pulse signal to convert the continuous light signal into a pulse light signal, and outputs the pulse light signal; The pulse light signal enters the fiber interferometer through the transmission optical fiber; wherein the fiber interferometer is a Michelson interferometer with a certain arm difference; after the pulse light signal is reflected by the two arms of the fiber interferometer, interference occurs at the output end, and after photoelectric conversion, an interference signal is obtained, and the interference signal is a difference frequency signal.

7. The system according to claim 6, characterized in that The tunable laser is a narrow linewidth tunable laser.

8. The system according to claim 7, characterized in that The linear frequency modulation signal is a periodic sawtooth wave signal or a periodic triangle wave signal.

9. The system according to claim 8, characterized in that By selecting the fiber interferometer arm difference ΔL and the linear modulation slope k of the tunable laser, the frequency Δf of the difference frequency signal is designed, and the formula is: Where n is the refractive index of the optical fiber and c is the speed of light in a vacuum.

10. The system according to claim 9, characterized in that The interference signal is: Where, I is the interference signal intensity, A is the DC amplitude of the interference signal, and B is the AC amplitude of the interference signal; It is the external sound signal detected by the fiber optic interferometer.

Images