An optical heterodyne detection method based on triangular stepped frequency modulated pulse signals

Abstract

The present application relates to an optical heterodyne detection method based on a triangular stepped frequency modulation pulse signal, comprising the following steps: Step 1, generating an initial pulse signal; Step 2, generating a triangular stepped frequency modulation pulse signal: circulatingly processing the initial pulse signal through an optical fiber frequency shift delay loop, and making the frequency of the pulse signal increase first and then decrease at a fixed step by means of an arbitrary waveform generator to generate a triangular stepped frequency modulation pulse signal; The present invention can realize the increase first and then decrease of the frequency of the triangular stepped frequency modulation pulse signal by using the optical fiber frequency shift delay loop. By introducing a frequency difference and a fixed delay, the heterodyne signal after interference can carry vibration information, and the triangular stepped frequency modulation 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 enabling the detection of vibration signals with high precision and in a large range.

Description

Technical Field

[0001] This application relates to the technical field of fiber optic sensing systems, and particularly to an optical heterodyne detection method based on a triangular stepped frequency modulation pulse signal. Background Art

[0002] A time-sequential multi-frequency pulse signal refers to a pulse signal composed of multiple pulse components with different frequencies within one period. Compared with a single-frequency pulse signal, a dual-frequency or multi-frequency pulse system successively emits optical pulse signals with different frequencies into a 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 the pulse repetition frequency and thus increase the measurement bandwidth, and can be widely applied in fields such as radar systems, fiber optic sensing, and quantum communication. Currently, a variety of multi-frequency pulse generation technologies have been proposed, including methods such as using phase or electro-optic modulators for modulation, multiple frequency shift devices in parallel, and cyclic frequency shift pulses, but usually only monotonic increase or decrease of the pulse frequency can be achieved. If it is possible to realize the frequency of the multi-frequency pulse signal to increase first and then decrease and apply this technology to the optical heterodyne detection method, it can enable heterodyne detection signals with different frequencies to be transmitted along the same optical fiber to reduce the noise and interference of the transmission optical fiber. At the same time, by utilizing the characteristic that the pulse signal frequency increases first and then decreases, a fixed frequency difference is generated between adjacent two pulse signals As the heterodyne frequency, by demodulating the phase of the heterodyne signal output by the interferometer, high-precision and large-range detection of the vibration signal loaded on the sensing arm of the unbalanced Michelson interferometer can be achieved. Summary of the Invention

[0003] The object of the present invention is to propose an optical heterodyne detection method based on a triangular stepped frequency modulation pulse signal to effectively eliminate the environmental interference on the path in the case of long-distance sensing, aiming at the problem that two optical signals with a certain frequency difference in the optical heterodyne detection method cannot be transmitted through the same optical fiber and are severely affected by environmental interference on the path in the case of long-distance sensing.

[0004] An optical heterodyne detection method based on a triangular stepped frequency modulation pulse signal provided by this application adopts the following technical solution:

[0005] An optical heterodyne detection method based on a triangular stepped frequency modulation pulse signal includes the following steps:

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

[0007] Step 2: Generate a triangular stepped frequency modulation pulse signal: After circularly processing the initial pulse signal through an optical fiber frequency shift delay loop, and making the pulse signal frequency increase first and then decrease at a fixed step through an arbitrary waveform generator, a triangular stepped frequency modulation pulse signal is generated;

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

[0009] Step 4: Reflect the optical signal through a Faraday rotator mirror to 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 on the sensing arm, and realize optical heterodyne detection.

[0011] By adopting the above technical solution, the fiber optic frequency shift delay loop can be used to realize the frequency of the triangular stepped frequency modulation pulse signal to increase first and then decrease. By introducing a frequency difference and a fixed delay, the heterodyne signal after interference can carry vibration information, and the triangular stepped frequency modulation pulse signal generated by the fiber optic frequency shift delay loop enters 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 enabling high-precision and large-range vibration signal detection.

[0012] Optionally, the generation of the initial pulse signal in Step 1 is mainly the pulse signal generation optical path, which mainly includes a narrow linewidth laser, a first acousto-optic modulator, and a first erbium-doped fiber amplifier. The generation steps are as follows:

[0013] Use the narrow linewidth laser to emit narrow linewidth continuous light;

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

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

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

[0017] By adopting the above technical solution, a highly stable initial pulse signal is generated through the combination of a narrow linewidth laser, an acousto-optic modulator, and an erbium-doped fiber amplifier to ensure the signal quality in the 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 basis for optical heterodyne detection, significantly improving the overall performance and stability of the system.

[0018] Optionally, in the second step, the fiber optic 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 fiber, and an isolator;

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

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

[0021] The third acousto-optic modulator is a down-frequency 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 loss due to the pulse signal circulating in the loop;

[0023] The first delay fiber is used to achieve the delay of pulses with different frequencies, control the pulse interval, and thus avoid the non-linear effect caused by pulse aliasing;

[0024] The isolator is used to shield the influence brought by the reflection of the previous pulses at the coupler when generating the nth frequency shift pulse, so as to ensure that the loop only works in the clockwise direction and prevent noise signals from entering the loop to interfere with the system.

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

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

[0027] Among them, the fourth coupler is used to divide the triangular stepped frequency modulation pulse signal into two paths with equal intensity and enter the reference arm and the sensing arm of the unbalanced Michelson interferometer respectively. At the same time, the two reflected optical signals will interfere at the fourth coupler;

[0028] The second delay fiber and the third delay fiber are used to adjust the arm length difference between the two arms of the unbalanced Michelson interferometer to match the pulse period of the triangular stepped frequency modulation pulse signal;

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

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

[0031] Optionally, in the fourth step, 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 by a photodetector, which is convenient for subsequent signal processing and data acquisition; the high sensitivity and wide frequency response range of the photodetector can accurately capture the difference frequency signal corresponding to the heterodyne frequency, while suppressing the sum frequency and harmonic components, providing a high-quality input signal 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 stepped frequency modulation pulse signal in the second step is: if the first pulse signal is expressed as

[0034]

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

[0036] Then the optical field of the triangular stepped frequency modulation pulse signal within one period is expressed as

[0037]

[0038] If there are triangular stepped frequency modulation pulse signals whose frequencies first increase and then decrease within one period, then is the frequency of the th and the th pulse signals, and the frequency step is .

[0039] By adopting the above technical solution, the frequency change law of the triangular stepped frequency modulation pulse signal is clarified by the formula, ensuring that the frequency first increases and then decreases within each period, providing a stable frequency difference for heterodyne detection As the heterodyne frequency; this frequency variation 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, by controlling the waveform duration of the carrier signal output by the arbitrary waveform generator to gate the second acousto-optic modulator and the third acousto-optic modulator, thereby controlling the increase or decrease of the pulse frequency and achieving synchronization with the first acousto-optic modulator simultaneously;

[0041] When the pulse frequency changes from to , the carrier signal output by the arbitrary waveform generator controls the second acousto-optic modulator to work, and the third acousto-optic modulator does not work. At this time, the frequency of the signal increases by each time it loops around.

[0042] When the pulse frequency changes from to , the carrier signal output by the arbitrary waveform generator controls the third acousto-optic modulator to work, and the second acousto-optic modulator does not work. The frequency of the signal decreases by each time it loops around.

[0043] By adopting the above technical solution, the gating timing of the up and down frequency-shifted acousto-optic modulators is precisely controlled by the arbitrary waveform generator to achieve the increase and decrease of the pulse frequency, and at the same time synchronize 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 stepped 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 of the length of the first delay fiber in the fiber frequency-shifted delay loop, at this time the pulse period of the triangular stepped 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 exactly in time sequence and return to the fourth coupler again to interfere, forming a heterodyne interference signal with a beat frequency of .

[0045] By adopting the above technical solution, by adjusting the arm length difference of the interferometer to match the pulse period, the reflected pulses coincide in the time domain and form a stable heterodyne interference signal, significantly improving the sensitivity and dynamic range of vibration detection, and at the same time avoiding signal aliasing and timing jitter, providing a reliable technical guarantee for high-precision vibration detection, and is especially suitable for fiber optic sensing applications in long-distance and complex environments.

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

[0047] By adopting the above technical solution, by controlling the output signal duration of the arbitrary waveform generator, the duration of the pulse sequence and the number of pulses are flexibly adjusted, so as to select the upper and lower limits of the frequency shift. This not only meets the requirements of different application scenarios, but also improves the flexibility and adaptability of the system, providing more possibilities for the generation and application of triangular stepped 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 a fiber frequency shift delay loop to be able to achieve the frequency of the triangular stepped frequency modulation pulse signal to increase first and then decrease. By introducing a frequency difference and a fixed delay, the heterodyne signal after interference can carry vibration information, and the triangular stepped frequency modulation pulse signals generated by the 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 being able to achieve high-precision and large-range vibration signal detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 is a schematic structural diagram of an optical heterodyne detection method based on triangular stepped frequency modulation pulse signals proposed by the present invention;

[0051] Figure 2 is a schematic diagram of the principle of the heterodyne interference signal formed by the triangular stepped frequency modulation pulse signal proposed by the present invention with a beat frequency of ;

[0052] Figure 3 is a timing diagram of the control signals of three acousto-optic modulators generated by the arbitrary waveform generator of the present invention;

[0053] In the figure:

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

[0055] 2. 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 optical 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 implementation manners

[0062] The following further describes the present application in detail with reference to the accompanying drawings.

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

[0064] Step 1: Generate an initial pulse signal; generating the initial pulse signal is mainly the pulse signal generation 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. The generation steps are as follows:

[0065] Use the narrow linewidth laser 101 to emit narrow linewidth continuous light;

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

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

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

[0069] Step 2: Generate a triangular stepped frequency modulation pulse signal: After the initial pulse signal is cyclically processed by the fiber frequency shift delay loop 3, and the frequency of the pulse signal is made to increase first and then decrease at a fixed step by an arbitrary waveform generator 4 to generate a triangular stepped frequency modulation pulse signal; in this step, the 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] Among them, 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 fiber frequency shift delay loop 3;

[0071] The second acousto-optic modulator 302 is an up-frequency-shifted acousto-optic modulator, which is used to control the increase of the pulse frequency;

[0072] The third acousto-optic modulator 303 is a down-frequency-shifted acousto-optic modulator, 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 loss due to the pulse signal circulating in the loop;

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

[0075] The isolator 307 is used to shield the influence brought by the reflection of the previous pulses at the coupler when generating the nth frequency-shifted pulse, so as to ensure that the loop operates only in the clockwise direction and prevent noise signals from entering the loop to interfere with the system.

[0076] Step 3: Unidirectionally transmit the triangular stepped frequency-modulated pulse signal to the unbalanced Michelson interferometer through the circulator 5, and use the reference arm of the interferometer and the delay fiber adjustment arm of the sensor to adjust the arm length difference to match the pulse period;

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

[0078] Among them, the fourth coupler 601 is used to divide the triangular stepped frequency-modulated pulse signal into two paths with equal intensity and enter the reference arm and the sensing arm of the unbalanced Michelson interferometer respectively. At the same time, the two reflected optical signals will interfere at the fourth coupler 601 of the coupler;

[0079] The second delay fiber 602 and the third delay fiber 603 are used to adjust the arm length difference between the two arms of the unbalanced Michelson interferometer to match the pulse period of the triangular stepped frequency-modulated pulse signal;

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

[0081] Step 4: Reflect the optical signal through the Faraday rotator and eliminate the polarization fading to generate an interference signal carrying vibration information, and convert the interference signal into an electrical signal; specifically, use the photodetector 7 to convert the optical 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 on the sensing arm, and achieve optical heterodyne detection.

[0083] The above is the optical heterodyne detection system based on triangular stepped frequency modulation pulse signals adopted by the present invention. Next, the principle of the present invention to achieve the frequency of the triangular stepped frequency modulation pulse signal increasing first and then decreasing will be described first:

[0084] The triangular stepped frequency modulation pulse signal is composed of multiple pulse signals with different frequencies within one period, and all pulses are evenly spaced in the time domain to avoid spectral aliasing and sampling rate jitter.

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

[0086]

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

[0088] Then the optical field of the triangular stepped frequency modulation pulse signal within one period is expressed as

[0089]

[0090] If there are triangular stepped frequency modulation pulse signals with frequencies increasing first and then decreasing within one period, then is the frequency of the th and the th pulse signals, and 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 pulse signal with a frequency of . This pulse signal is amplified by the first erbium-doped fiber amplifier 103 and then split into two paths by a coupler. One path directly enters the photodetector 7 and is displayed on the oscilloscope, and the other part enters the circulating fiber frequency shift delay loop 3. Each time the light circulates around the loop, it will cause further frequency shift, and since the path passed through each time is the same, the time interval between adjacent pulse signals with frequency shift is also the same. For the th loop of the loop, an optical pulse with a new frequency equal to will be generated. The number of multi-frequency pulses required is controlled by adjusting the output time of the signal of the arbitrary waveform generator 4.

[0092] In order to achieve the frequency increase first and then decrease, the fiber optic frequency shift loop arranges two separate frequency shift paths, and is connected to the frequency shift optical path through two couplers (the second coupler 301 and the third coupler 304). One path includes an up-frequency acousto-optic modulator (the second acousto-optic modulator 302), and one path includes a down-frequency 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 gating of the up-frequency and down-frequency acousto-optic modulators is realized, so as to control the increase or decrease of the pulse frequency, and at the same time realize the synchronization with the optical switch acousto-optic modulator. When the pulse frequency changes from increases to , the carrier signal output by the arbitrary waveform generator 4 controls the up-frequency acousto-optic modulator to work, and the down-frequency acousto-optic modulator does not work. At this time, the frequency of the signal increases by every time it loops around the ring; similarly, when the pulse frequency changes from decreases to , the carrier signal output by the arbitrary waveform generator 4 controls the down-frequency acousto-optic modulator to work, and the up-frequency acousto-optic modulator does not work. The frequency of the signal decreases by every time it loops around the ring. The working time of the two acousto-optic modulators is equal to times the delay time introduced by the signal looping around the ring once. 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 are selected. Finally, using this structure, a triangular stepped frequency modulation pulse signal with a frequency increasing first and then decreasing can be generated at the output end of the coupler.

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

[0094] Let the arm length difference of the unbalanced Michelson interferometer be , and control the length of the fiber optic frequency shift delay loop 3 so that , then the delay time , that is, the pulse period of the triangular stepped 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 respectively coincide in time sequence and interfere at the circulator 5, forming a heterodyne interference signal with a beat frequency of . The expression of the interference optical field is:

[0095]

[0096] where and are the amplitudes of the two light waves respectively, includes the initial phase of the heterodyne interference signal, which is determined by the arm length difference of the two arms of the Michelson interferometer.Decisions, as well as the phase drift introduced by environmental disturbances. For the tiny vibration signal loaded on the sensing arm of the unbalanced Michelson interferometer, the output interference optical field can be expressed as:

[0097]

[0098] The photoelectric signal after mixing contains multiple frequency components, including a DC component, a second harmonic component of the reference light frequency, and a second harmonic component of the signal light frequency. At the same time, it contains the sum frequency and difference frequency components of the reference light and the signal light. It should be noted that if an appropriate frequency response range of the photodetector 7 is selected such that the frequency values corresponding to the second harmonic component and the 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 thus be selectively captured. Therefore, after the interference signal enters the photodetector 7, it is converted into an electrical signal, and its expression is:

[0099]

[0100] where is the amplitude of the DC term, is the amplitude of the AC term, which is related to the input light intensity, the beam splitting ratio of the coupler, and the polarization states of the two beams of light. It can be seen from the above formula that the frequency of the intermediate frequency signal after mixing is the frequency difference between the two signals output by the interferometer. Thus, the relevant information of the signal to be measured can be loaded in the phase difference between 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 signal with a pulse width of and a frequency of . Assuming that the delay introduced by the arm length difference of the interferometer is , then the period of a pulse should be , that is, the time for the signal to travel back and forth along the arm length difference. The frequency of the pulse signal increases from to and then decreases to within a triangular stepped frequency modulation pulse period . Therefore , that is, the laser generates a pulse signal with a pulse width of every time (implemented by an optical switch / modulator). The delay introduced by the loop length of the frequency shift loop should be the same as the delay introduced by the arm length difference, both being , so that the pulse signals reflected back from the two arms of the interferometer coincide in time sequence and interfere. The period of the control signal output by the arbitrary waveform generator 4 is also ​

[0102] The above are all the preferred embodiments of this application, and the protection scope of this application is not limited thereby. Therefore, all equivalent changes made according to the structure, shape, and principle of this application shall be covered within the protection scope of this 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 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. .

7. 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.

8. 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.

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