Optical heterodyne detection method based on triangular step frequency modulation pulse signal

Abstract

The invention relates to an optical heterodyne detection method based on a triangular step frequency modulation pulse signal. The optical heterodyne detection method comprises the following steps: step 1, generating an initial pulse signal; step 2, generating a triangular step frequency modulation pulse signal: after the initial pulse signal is circularly processed through an optical fiber frequency shift delay loop, enabling the frequency of the pulse signal to be progressively increased and then progressively decreased according to a fixed step length through an arbitrary waveform generator, and generating the triangular step frequency modulation pulse signal; according to the invention, the frequency of the triangular step frequency modulation pulse signal can be progressively increased and then progressively decreased by using the optical fiber frequency shift time delay loop, and the interference heterodyne signal can carry vibration information by introducing the frequency difference and the fixed time delay. Triangular step frequency modulation pulse signals generated by the optical fiber frequency shift delay ring enter the unbalanced Michelson interferometer along the output end of the coupler, transmission of heterodyne detection signals along the same optical fiber is achieved, noise and interference of the transmission optical fiber are reduced, and high-precision and large-range vibration signal detection can be achieved.

Description

Technical Field

[0001] The present application relates to the technical field of optical fiber sensing systems, and in particular to an optical heterodyne detection method based on a triangular step frequency modulation pulse signal. Background Art

[0002] A time-series multi-frequency pulse signal refers to a pulse signal composed of multiple pulse components with different frequencies within one cycle. Compared with a single-frequency pulse signal, a dual-frequency or multi-frequency pulse system successively emits optical pulse signals of different frequencies and injects them into the sensing optical fiber. Due to the different frequencies of the pulse signals used, the performance of the sensing system is further improved. Compared with single-frequency pulse sensing technology, multi-frequency pulses can avoid the limitation of pulse repetition frequency and thus increase the measurement bandwidth. They can be widely used in radar systems, optical fiber sensing, quantum communications and other fields. At present, a variety of multi-frequency pulse generation technologies have been proposed, including modulation using phase or electro-optical modulators, parallel connection of multiple frequency shifting devices and cyclic frequency shifting pulses, but usually only monotonically increasing or decreasing pulse frequencies can be achieved. If the frequency of a multi-frequency pulse signal can be increased first and then decreased and the technology can be applied to the optical heterodyne detection method, heterodyne detection signals with different frequencies can be transmitted along the same optical fiber to reduce the noise and interference of the transmission optical fiber. At the same time, the characteristic that the frequency of the pulse signal first increases and then decreases is used to make two adjacent pulse signals produce a fixed frequency difference. As the heterodyne frequency, the phase of the heterodyne signal output by the interferometer is demodulated to realize the detection of the vibration signal loaded on the sensor arm of the unbalanced Michelson interferometer with high measurement accuracy and large measurement range. Summary of the invention

[0003] The purpose of the present invention is to propose an optical heterodyne detection method based on a triangular step frequency modulated pulse signal to address the problem that two optical signals with a certain frequency difference cannot be transmitted on the same optical fiber in the optical heterodyne detection method, and are seriously interfered by the environment on the path in the case of long-distance sensing. The optical heterodyne detection method can effectively eliminate the environmental interference on the path in the case of long-distance sensing.

[0004] The present application provides an optical heterodyne detection method based on a triangular step frequency modulated pulse signal using the following technical solution:

[0005] An optical heterodyne detection method based on a triangular step frequency modulated pulse signal comprises the following steps:

[0006] Step 1: Generate an initial pulse signal;

[0007] Step 2, generating a triangular step frequency modulated pulse signal: after the initial pulse signal is cyclically processed by the optical fiber frequency shift delay loop, the pulse signal frequency is increased and then decreased by an arbitrary waveform generator at a fixed step length to generate a triangular step frequency modulated pulse signal;

[0008] Step 3: unidirectionally transmit the triangular step frequency modulated pulse signal to the unbalanced Michelson interferometer through a circulator, and use the reference arm of the interferometer and the delay optical fiber of the sensor to adjust the arm length difference to match the pulse period;

[0009] Step 4: Reflect the light signal through the Faraday rotator mirror and eliminate polarization fading, generate an interference signal carrying vibration information, and convert the interference signal into an electrical signal;

[0010] Step 5: Use the host computer to perform phase demodulation on the electrical signal, extract the vibration information loaded by the sensor arm, and realize optical heterodyne detection.

[0011] By adopting the above technical solution, the frequency of the triangular step frequency modulated pulse signal can be increased first and then decreased by using the optical fiber frequency shift delay loop. By introducing the frequency difference and the fixed delay, the heterodyne signal after interference can carry vibration information. The triangular step frequency modulated pulse signals generated by the optical fiber frequency shift delay loop enter the unbalanced Michelson interferometer along the output end of the coupler, realizing the transmission of the heterodyne detection signal along the same optical fiber, thereby reducing the noise and interference of the transmission optical fiber, and realizing high-precision and wide-range vibration signal detection.

[0012] Optionally, the step 1 of generating the initial pulse signal mainly comprises a pulse signal generating optical path, which mainly comprises a narrow linewidth laser, a first acousto-optic modulator and a first erbium-doped fiber amplifier, and the generating steps are:

[0013] Using narrow linewidth laser to emit narrow linewidth continuous light;

[0014] The first acousto-optic modulator is used as an optical switch to modulate the continuous light emitted by the narrow linewidth laser to obtain a frequency of The pulse signal;

[0015] Amplifying the optical power of the pulse signal output by the first acousto-optic modulator using the first erbium-doped fiber amplifier;

[0016] After the initial pulse signal is generated, the pulse signal is input into the optical fiber frequency shift delay loop through the first coupler to achieve frequency shift, and the frequency shifted signal is output from the optical fiber frequency shift delay loop.

[0017] By adopting the above technical solution, a high-stability initial pulse signal is generated through the combination of narrow-linewidth laser, acousto-optic modulator and erbium-doped fiber amplifier to ensure the signal quality of subsequent frequency modulation and detection processes. The narrow-linewidth laser provides pure continuous light, the acousto-optic modulator modulates it into a high-precision pulse signal, and the erbium-doped fiber amplifier compensates for the optical power loss, providing a reliable light source foundation for optical heterodyne detection, which significantly improves the overall performance and stability of the system.

[0018] Optionally, in step 2, the optical fiber frequency shift delay loop includes a second coupler, a second acousto-optic modulator, a third acousto-optic modulator, a third coupler, a second erbium-doped fiber amplifier, a first delay optical fiber and an isolator;

[0019] The second coupler and the third coupler connect the second acousto-optic modulator and the third acousto-optic modulator to the optical fiber frequency shift delay loop;

[0020] The second acousto-optic modulator is an up-shift frequency acousto-optic modulator, which is used to control the increase of the pulse frequency;

[0021] The third acousto-optic modulator is a down-shift acousto-optic modulator, which is used to control the decrease of the pulse frequency;

[0022] The second erbium-doped fiber amplifier is used to compensate for the optical power lost due to the circulation of the pulse signal in the ring;

[0023] The first delay optical fiber is used to delay pulses of different frequencies and control the pulse interval to avoid the nonlinear effect caused by pulse aliasing;

[0024] Isolators are used to shield the When there are frequency shift pulses, The effect of a pulse being reflected at the coupler is reduced, thereby ensuring that the loop only works in the clockwise direction and preventing noise signals from entering the loop and interfering with the system.

[0025] By adopting the above technical solution, the fiber frequency shift delay loop can easily and stably generate a triangular step frequency modulated pulse signal with a frequency that first increases and then decreases through the coordinated work of the up- and down-shift acousto-optic modulators, the erbium-doped fiber amplifier, the delay fiber and the isolator. The delay fiber controls the pulse interval to avoid signal aliasing, and the optical isolator shields the reflected noise to ensure unidirectional transmission of the signal. This design not only simplifies the system structure, but also improves the stability and flexibility of signal generation, providing high-quality frequency modulated pulse signals for heterodyne detection.

[0026] Optionally, in step three, the unbalanced Michelson interferometer is a vibration sensing optical path, including a fourth coupler, a second delay optical fiber, a third delay optical fiber, a first Faraday rotator mirror, and a second Faraday rotator mirror;

[0027] The fourth coupler is used to divide the triangular step frequency modulation pulse signal into two paths with equal intensity, which enter the reference arm and the sensor arm of the unbalanced Michelson interferometer respectively, and the two reflected optical signals will interfere at the fourth coupler of the coupler;

[0028] The second delay optical fiber and the third delay optical fiber are used to adjust the arm length difference between the two arms of the unbalanced Michelson interferometer so that it matches the pulse period of the triangular step frequency modulation pulse signal;

[0029] The first Faraday rotator mirror and the second Faraday rotator mirror are used for reflecting the optical signal and can rotate the polarization state of the input light wave by 90° to eliminate the polarization attenuation of the light wave.

[0030] By adopting the above technical solution, the unbalanced Michelson interferometer adjusts the arm length difference between the reference arm and the sensor arm to match the pulse period of the triangular step frequency modulated pulse signal, and combines the Faraday rotator to eliminate polarization fading, thereby significantly improving the signal-to-noise ratio and detection accuracy of the interference signal; the polarization state rotation function of the Faraday rotator effectively suppresses the signal fading caused by optical fiber birefringence, ensures the stability of the interference signal, and provides a reliable technical guarantee for high-precision vibration detection.

[0031] Optionally, in step four, a photodetector is used to convert the optical signal into an electrical signal.

[0032] By adopting the above technical solution, the interference signal is converted into an electrical signal through a photoelectric detector, which is convenient for subsequent signal processing and data acquisition; the high sensitivity and wide frequency response range of the photoelectric detector can accurately capture the difference frequency signal corresponding to the heterodyne frequency, while suppressing the sum frequency and harmonic components, providing high-quality input signals for the extraction of vibration information, and significantly improving the signal-to-noise ratio and dynamic range of the detection system.

[0033] Optionally, the method for generating the triangular step frequency modulation pulse signal in step 2 is: if the first pulse signal is represented as,

[0034]

[0035] is the amplitude of the light pulse, is the pulse width, is the initial frequency;

[0036] The triangular step frequency modulation pulse signal has a period of The light field inside is expressed as,

[0037]

[0038] If there is A triangular step frequency modulation pulse signal whose frequency first increases and then decreases, It is and The frequency of the pulse signal, the frequency step is .

[0039] By adopting the above technical solution, the frequency change law of the triangular step frequency modulation pulse signal is clarified through the formula to ensure that the frequency increases first and then decreases in each cycle, providing a stable frequency difference for heterodyne detection. As the heterodyne frequency; this frequency change law not only simplifies the signal generation process, but also improves the accuracy and stability of heterodyne detection, providing a reliable frequency reference for the extraction of vibration signals.

[0040] Optionally, the second AOM and the third AOM are selected by controlling the waveform duration of the carrier signal output by the arbitrary waveform generator, thereby controlling the increase or decrease of the pulse frequency and achieving synchronization with the first AOM;

[0041] When the pulse frequency is Increase to When the carrier signal output by the arbitrary waveform generator controls the second AOM to work, the third AOM does not work, and the frequency of the signal increases every time it goes around the ring. ;

[0042] When the pulse frequency is Reduce to When the carrier signal output by the arbitrary waveform generator controls the third AOM to work, the second AOM does not work, and the frequency of the signal decreases every time it goes around the ring. .

[0043] By adopting the above technical scheme, the gating timing of the up- and down-shift acousto-optic modulators is precisely controlled by an arbitrary waveform generator to achieve the increase and decrease of the pulse frequency, and at the same time synchronized with the optical switch acousto-optic modulator to ensure the coordinated operation of the system; this synchronous control mechanism not only improves the accuracy and stability of frequency adjustment, but also simplifies the system operation, providing efficient technical support for the generation of triangular step frequency modulation pulse signals.

[0044] Optionally, by adjusting the length difference between the second delay fiber and the third delay fiber in the vibration sensing optical path to be half the length of the first delay fiber in the fiber frequency shift delay loop, the pulse period of the triangular step frequency modulation pulse signal matches the arm length difference of the unbalanced Michelson interferometer, and the pulse signals reflected from the short arm and the long arm coincide in timing, and return to the fourth coupler again to interfere, forming a beat frequency carrying vibration information of heterodyne interference signal.

[0045] By adopting the above technical solution and adjusting the interferometer arm length difference to match the pulse period, the reflected pulses overlap in the time domain and form a stable heterodyne interference signal, which significantly improves the sensitivity and dynamic range of vibration detection. At the same time, signal aliasing and timing jitter are avoided, providing a reliable technical guarantee for high-precision vibration detection, which is particularly suitable for fiber optic sensing applications over long distances and in complex environments.

[0046] Optionally, the duration of the pulse sequence output from the optical fiber frequency shift delay loop and the number of pulses in the sequence can be controlled by controlling the waveform duration of the carrier signal output by the arbitrary waveform generator, that is, selecting the upper and lower limits of the frequency shift.

[0047] By adopting the above technical solution, by controlling the output signal duration of the arbitrary waveform generator, the duration and number of pulses of the pulse sequence can be flexibly adjusted, thereby selecting the upper and lower limits of the frequency shift. This not only meets the needs of different application scenarios, but also improves the flexibility and adaptability of the system, providing more possibilities for the generation and application of triangular step frequency modulation pulse signals.

[0048] In summary, the present application includes at least one of the following beneficial technical effects:

[0049] The present invention utilizes an optical fiber frequency shift delay loop to realize that the frequency of a triangular step frequency modulated pulse signal first increases and then decreases. By introducing a frequency difference and a fixed delay, the heterodyne signal after interference can carry vibration information. The triangular step frequency modulated pulse signals generated by the optical fiber frequency shift delay loop all enter the unbalanced Michelson interferometer along the output end of the coupler, realizing the transmission of the heterodyne detection signal along the same optical fiber, thereby reducing the noise and interference of the transmission optical fiber, and realizing high-precision and wide-range vibration signal detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 This is a schematic diagram of the structure of the optical heterodyne detection method based on the triangular step frequency modulated pulse signal proposed by the present invention;

[0051] Figure 2 The triangular step frequency modulation pulse signal proposed by the present invention forms a beat frequency of Schematic diagram of heterodyne interference signal principle;

[0052] Figure 3 A timing diagram of three AOM control signals generated by the arbitrary waveform generator of the present invention;

[0053] In the figure:

[0054] 1. Pulse signal generating optical path; 101. Narrow line width laser; 102. First acousto-optic modulator; 103. First erbium-doped fiber amplifier;

[0055] 2. a first coupler;

[0056] 3. Fiber frequency shift delay loop; 301. second coupler; 302. second acousto-optic modulator; 303. third acousto-optic modulator; 304. third coupler; 305. second erbium-doped fiber amplifier; 306. first delay fiber; 307. isolator;

[0057] 4. Arbitrary waveform generator;

[0058] 5. Circulator;

[0059] 6. Vibration sensing optical path; 601. Fourth coupler; 602. Second delay optical fiber; 603. Third delay optical fiber; 604. First Faraday rotator mirror; 605. Second Faraday rotator mirror;

[0060] 7. Photoelectric detector;

[0061] 8. Host computer. DETAILED DESCRIPTION

[0062] The present application is further described in detail below in conjunction with the accompanying drawings.

[0063] like Figure 1-3 As shown, the embodiment of the present application discloses an optical heterodyne detection method based on a triangular step frequency modulation pulse signal, comprising the following steps:

[0064] Step 1, generating an initial pulse signal; the initial pulse signal is mainly generated by a pulse signal generating optical path 1, which mainly includes a narrow linewidth laser 101, a first acousto-optic modulator 102 and a first erbium-doped fiber amplifier 103, and the generation steps are:

[0065] Using a narrow linewidth laser 101 to emit narrow linewidth continuous light;

[0066] The first acousto-optic modulator 102 is used as an optical switch to modulate the continuous light emitted by the narrow linewidth laser 101 to obtain a frequency of The pulse signal;

[0067] Amplifying the optical power of the pulse signal output by the first acousto-optic modulator 102 by using the first erbium-doped fiber amplifier 103;

[0068] After the initial pulse signal is generated, the pulse signal is input into the optical fiber frequency shift delay loop 3 through the first coupler 2 to achieve frequency shift, and the frequency-shifted signal is output from the optical fiber frequency shift delay loop 3.

[0069] Step 2, generating a triangular step frequency modulated pulse signal: after the initial pulse signal is cyclically processed by the optical fiber frequency shift delay loop 3, the pulse signal frequency is first increased and then decreased according to a fixed step length by the arbitrary waveform generator 4, so as to generate a triangular step frequency modulated pulse signal; in this step, the optical fiber frequency shift delay loop 3 includes a second coupler 301, a second acousto-optic modulator 302, a third acousto-optic modulator 303, a third coupler 304, a second erbium-doped fiber amplifier 305, a first delay optical fiber 306 and an isolator 307;

[0070] The second coupler 301 and the third coupler 304 connect the second acousto-optic modulator 302 and the third acousto-optic modulator 303 to the optical fiber frequency shift delay loop 3;

[0071] The second AOM 302 is an up-shift AOM, which is used to control the increase of the pulse frequency;

[0072] The third AOM 303 is a down-shift AOM, which is used to control the decrease of the pulse frequency;

[0073] The second erbium-doped fiber amplifier 305 is used to compensate for the optical power lost due to the circulation of the pulse signal in the ring;

[0074] The first delay optical fiber 306 is used to delay pulses of different frequencies and control the pulse interval to avoid the nonlinear effect caused by pulse aliasing;

[0075] Isolator 307 is used to shield the When there are frequency shift pulses, The effect of a pulse being reflected at the coupler is reduced, thereby ensuring that the loop only works in the clockwise direction and preventing noise signals from entering the loop and interfering with the system.

[0076] Step 3: The triangular step frequency modulated pulse signal is unidirectionally transmitted to the unbalanced Michelson interferometer through the circulator 5, and the arm length difference is adjusted by using the reference arm of the interferometer and the delay optical fiber of the sensor to match the pulse period;

[0077] The unbalanced Michelson interferometer is a vibration sensing optical path 6, which includes a fourth coupler 601, a second delay optical fiber 602, a third delay optical fiber 603, a first Faraday rotator mirror 604 and a second Faraday rotator mirror 605;

[0078] The fourth coupler 601 is used to divide the triangular step frequency modulation pulse signal into two paths with equal intensity, which enter the reference arm and the sensor arm of the unbalanced Michelson interferometer respectively, and the two reflected optical signals will interfere at the fourth coupler 601 of the coupler;

[0079] The second delay optical fiber 602 and the third delay optical fiber 603 are used to adjust the arm length difference between the two arms of the unbalanced Michelson interferometer so that it matches the pulse period of the triangular step frequency modulation pulse signal;

[0080] The first Faraday rotator mirror 604 and the second Faraday rotator mirror 605 are used for reflecting optical signals and can rotate the polarization state of the input light wave by 90° to eliminate polarization attenuation of the light wave.

[0081] Step 4: Reflect the light signal through the Faraday rotator mirror and eliminate polarization fading, generate an interference signal carrying vibration information, and convert the interference signal into an electrical signal; specifically, use the photodetector 7 to convert the light signal into an electrical signal.

[0082] Step 5: Use the host computer 8 to perform phase demodulation on the electrical signal, extract the vibration information loaded by the sensor arm, and realize optical heterodyne detection.

[0083] The above is an optical heterodyne detection system based on a triangular step frequency modulation pulse signal adopted by the present invention. Next, the principle of the present invention to realize the triangular step frequency modulation pulse signal frequency first increasing and then decreasing is first explained:

[0084] The triangular step frequency modulated pulse signal is composed of multiple pulse signals with different frequencies in one cycle, and all pulses are evenly spaced in the time domain to avoid spectrum aliasing and sampling rate jitter.

[0085] If the first pulse signal is expressed as:

[0086]

[0087] is the amplitude of the light pulse, is the pulse width, is the initial frequency;

[0088] The triangular step frequency modulation pulse signal has a period of The light field inside is expressed as

[0089]

[0090] If there is A triangular step frequency modulation pulse signal whose frequency first increases and then decreases, It is and The frequency of the pulse signal, the frequency step is .

[0091] The continuous light emitted by the narrow linewidth laser 101 is modulated by the first acousto-optic modulator 102 used as an optical switch to obtain a frequency of The pulse signal is amplified by the first erbium-doped fiber amplifier 103 and then split into two paths by the coupler. One path directly enters the photodetector 7 and is displayed on the oscilloscope, and the other part enters the cyclic fiber frequency shift delay loop 3. Each time the light circulates around the ring, it will cause further frequency shift, and because the path of each cycle is the same, the time interval between two adjacent pulse signals that undergo frequency shift is also the same. loop, will generate a new frequency equal to The number of multi-frequency pulses required can be controlled by adjusting the output time of the control signal of the arbitrary waveform generator 4.

[0092] In order to achieve the frequency increase first and then decrease, the optical fiber frequency shift loop arranges two separate frequency shift paths and connects to the frequency shift optical path through two couplers (the second coupler 301 and the third coupler 304). One path includes an up-shift acousto-optic modulator (the second acousto-optic modulator 302), and the other path includes a down-shift acousto-optic modulator (the third acousto-optic modulator 303). By controlling the waveform duration of the carrier signal output by the arbitrary waveform generator 4, the up-shift and down-shift acousto-optic modulators are selected, thereby controlling the increase or decrease of the pulse frequency and achieving synchronization with the optical switch acousto-optic modulator. When the pulse frequency is changed by Increase to When the carrier signal output by the arbitrary waveform generator 4 controls the up-shift acousto-optic modulator to work, the down-shift acousto-optic modulator does not work, and the frequency of the signal increases every time it goes around the ring. Similarly, when the pulse frequency is Reduce to When the carrier signal output by the arbitrary waveform generator 4 controls the down-shift acousto-optic modulator to work, the up-shift acousto-optic modulator does not work, and the frequency of the signal decreases every time it goes around the ring. The working time of the two AOMs is equal to The delay time introduced by the signal going around the loop is times the delay time. By controlling the duration of the carrier signal, the duration of the pulse sequence output from the loop and the number of pulses in the sequence can be controlled, that is, the upper and lower limits of the frequency shift can be selected. Finally, this structure can generate a triangular step frequency modulation pulse signal at the output end of the coupler, with the frequency increasing first and then decreasing.

[0093] Next, the principle of using the triangular step frequency modulation pulse signal for heterodyne detection technology in this embodiment is described:

[0094] Assume that the arm length difference of the unbalanced Michelson interferometer is , control the length of the optical fiber frequency shift delay loop 3 so that , then the delay time , that is, the pulse period of the triangular step frequency modulation pulse signal matches the arm length difference of the unbalanced Michelson interferometer, then the pulse signals reflected from the short arm and the long arm coincide in timing, and interfere at the circulator 5, forming a beat frequency of The heterodyne interference signal, the expression of the interference light field is:

[0095]

[0096] in and are the amplitudes of the two light waves, Including the initial phase of the heterodyne interference signal, which is determined by the arm length difference between the two arms of the Michelson interferometer. Determined by the phase drift introduced by the environmental disturbance. For the tiny vibration signal loaded on the sensor arm of the unbalanced Michelson interferometer, the output interference light field can be expressed as:

[0097]

[0098] The photoelectric signal after mixing processing contains multiple frequency components, including DC component, second harmonic component of reference light frequency and second harmonic component of signal light frequency, and also contains sum frequency and difference frequency components of reference light and signal light. It is worth noting that if a suitable frequency response range of the photodetector 7 is selected so that the frequency values ​​corresponding to the second harmonic component and sum frequency component in the signal exceed its response range, only the difference frequency component is within the effective working frequency band of the detector and can be selectively captured. Therefore, the interference signal is converted into an electrical signal after entering the photodetector 7, and its expression is:

[0099]

[0100] in is the magnitude of the DC term, is the amplitude of the AC term, which is related to the input light intensity, the coupler splitting ratio, and the polarization state of the two beams. From the above formula, it can be seen that the frequency of the intermediate frequency signal after mixing is the frequency difference of the two signals output by the interferometer, so that the relevant information of the signal to be measured can be loaded into the phase difference of the two signals. After filtering out the DC term at the demodulation end, the information of the vibration signal to be measured can be obtained through the corresponding phase demodulation algorithm.

[0101] The continuous light emitted by the laser is modulated by the modulator to obtain a pulse width of , the frequency is Assume that the delay introduced by the interferometer arm length difference is , then the period of a pulse should be , that is, the time it takes for the signal to travel back and forth along the arm length difference. A triangular step frequency modulation pulse cycle The internal pulse signal frequency is Increase to Reduce to , a total of times the delay time, so , that is, the laser is Generates a pulse width of The pulse signal (realized by the optical switch / modulator) of the frequency shift loop should have the same delay as the delay introduced by the arm length difference, both of which are , so that the pulse signals reflected back from the two arms of the interferometer overlap in timing and thus interfere. The period of the control signal output by the arbitrary waveform generator 4 is also .

[0102] The above are all preferred embodiments of the present application, and the protection scope of the present application is not limited thereto. Therefore, any equivalent changes made according to the structure, shape, and principle of the present application should be included in the protection scope of the present application.

Claims

1. An optical heterodyne detection method based on a triangular step frequency modulated pulse signal, characterized in that: The following steps are involved: Step 1: Generate an initial pulse signal; Step 2: Generate a triangular step frequency modulated pulse signal: After the initial pulse signal is cyclically processed by the optical fiber frequency shift delay loop (3), the pulse signal frequency is increased and then decreased according to a fixed step length by an arbitrary waveform generator (4), thereby generating a triangular step frequency modulated pulse signal; Step 3, transmitting the triangular step frequency modulated pulse signal unidirectionally to the unbalanced Michelson interferometer through the circulator (5), and adjusting the arm length difference of the reference arm of the interferometer and the delay optical fiber of the sensor to match the pulse period; Step 4: Reflect the light signal through the Faraday rotator mirror and eliminate polarization fading, generate an interference signal carrying vibration information, and convert the interference signal into an electrical signal; Step 5: Use the host computer (8) to perform phase demodulation on the electrical signal, extract the vibration information loaded by the sensor arm, and realize optical heterodyne detection.

2. The optical heterodyne detection method based on a triangular step frequency modulated pulse signal according to claim 1, characterized in that: The step 1 of generating the initial pulse signal mainly comprises a pulse signal generating optical path (1), which mainly comprises a narrow linewidth laser (101), a first acousto-optic modulator (102) and a first erbium-doped fiber amplifier (103), and the generating steps are: Using a narrow linewidth laser (101) to emit narrow linewidth continuous light; The first acousto-optic modulator (102) is used as an optical switch to modulate the continuous light emitted by the narrow linewidth laser (101) to obtain a frequency of The pulse signal; Amplifying the optical power of the pulse signal output by the first acousto-optic modulator (102) using a first erbium-doped fiber amplifier (103); After the initial pulse signal is generated, the pulse signal is input into the optical fiber frequency shift delay loop (3) through the first coupler (2) to achieve frequency shift, and the frequency shifted signal is output from the optical fiber frequency shift delay loop (3).

3. The optical heterodyne detection method based on a triangular step frequency modulated pulse signal according to claim 1, characterized in that: In the step 2, the optical fiber frequency shift delay loop (3) comprises a second coupler (301), a second acousto-optic modulator (302), a third acousto-optic modulator (303), a third coupler (304), a second erbium-doped fiber amplifier (305), a first delay optical fiber (306) and an isolator (307); The second coupler (301) and the third coupler (304) connect the second acousto-optic modulator (302) and the third acousto-optic modulator (303) to the optical fiber frequency shift delay loop (3); The second acousto-optic modulator (302) is an up-shift frequency acousto-optic modulator, used to control the increase of the pulse frequency; The third acousto-optic modulator (303) is a down-shift acousto-optic modulator, used to control the decrease of the pulse frequency; A second erbium-doped fiber amplifier (305) is used to compensate for the optical power lost due to the circulation of the pulse signal in the ring; A first delay optical fiber (306) is used to delay pulses of different frequencies and control pulse intervals to avoid nonlinear effects caused by pulse aliasing; Isolators are used to shield the When there are frequency shift pulses, The effect of a pulse being reflected at the coupler is reduced, thereby ensuring that the loop only works in the clockwise direction and preventing noise signals from entering the loop and interfering with the system.

4. The optical heterodyne detection method based on a triangular step frequency modulated pulse signal according to claim 1, characterized in that: In the step 3, the unbalanced Michelson interferometer is a vibration sensing optical path (6), comprising a fourth coupler (601), a second delay optical fiber (602), a third delay optical fiber (603), a first Faraday rotation mirror (604) and a second Faraday rotation mirror (605); The fourth coupler (601) is used to divide the triangular step frequency modulation pulse signal into two paths with equal intensity, which enter the reference arm and the sensor arm of the unbalanced Michelson interferometer respectively, and the two reflected optical signals will interfere at the fourth coupler (601) of the coupler; The second delay optical fiber (602) and the third delay optical fiber (603) are used to adjust the arm length difference between the two arms of the unbalanced Michelson interferometer so that it matches the pulse period of the triangular step frequency modulation pulse signal; The first Faraday rotator mirror (604) and the second Faraday rotator mirror (605) are used for reflecting optical signals and can rotate the polarization state of the input light wave by 90 degrees to eliminate the polarization attenuation of the light wave.

5. The optical heterodyne detection method based on a triangular step frequency modulated pulse signal according to claim 1, characterized in that: In step 4, a photodetector (7) is used to convert the optical signal into an electrical signal.

6. The optical heterodyne detection method based on a triangular step frequency modulated pulse signal according to claim 1, characterized in that: The method for generating the triangular step frequency modulation pulse signal in step 2 is as follows: if the first pulse signal is represented as, ， is the amplitude of the light pulse, is the pulse width, is the initial frequency; The triangular step frequency modulation pulse signal has a period of The light field inside is expressed as, ， If there is A triangular step frequency modulation pulse signal whose frequency first increases and then decreases, It is and The frequency of the pulse signal, the frequency step is .

7. The optical heterodyne detection method based on a triangular step frequency modulated pulse signal according to claim 6, characterized in that: The second acousto-optic modulator (302) and the third acousto-optic modulator (303) are selected by controlling the waveform duration of the carrier signal output by the arbitrary waveform generator (4), thereby controlling the increase or decrease of the pulse frequency and achieving synchronization with the first acousto-optic modulator (102); When the pulse frequency is Increase to When the carrier signal output by the arbitrary waveform generator controls the second AOM to work, the third AOM does not work, and the frequency of the signal increases every time it goes around the ring. ; When the pulse frequency is Reduce to When the carrier signal output by the arbitrary waveform generator controls the third AOM to work, the second AOM does not work, and the frequency of the signal decreases every time it goes around the ring. .

8. The optical heterodyne detection method based on a triangular step frequency modulated pulse signal according to claim 4, characterized in that: By adjusting the length difference between the second delay optical fiber (602) and the third delay optical fiber (603) in the vibration sensing optical path (6) to be half the length of the first delay optical fiber (306) in the optical fiber frequency shift delay loop (3), the pulse period of the triangular step frequency modulation pulse signal matches the arm length difference of the unbalanced Michelson interferometer, and the pulse signals reflected from the short arm and the long arm respectively coincide in timing and return to the fourth coupler (601) again to interfere, forming a beat frequency carrying vibration information of heterodyne interference signal.

9. The optical heterodyne detection method based on a triangular step frequency modulated pulse signal according to claim 1, characterized in that: By controlling the waveform duration of the carrier signal output by the arbitrary waveform generator (4), the duration of the pulse sequence output from the optical fiber frequency shift delay loop (3) and the number of pulses in the sequence can be controlled, that is, the upper and lower limits of the frequency shift can be selected.

Images