Linear frequency modulation laser pulse generation device with three-stage adjustment

Abstract

The invention relates to the technical field of lasers, in particular to a three-level adjustment linear frequency modulation laser pulse generation device which comprises M narrow linewidth lasers with different wavelengths, a pulse modulation module, a wavelength division chopping module and an acousto-optic frequency shift module. Compared with a traditional OTDR-based distributed optical fiber sound wave sensing technology, the method has the advantages that a higher sound wave sampling frequency can be obtained, and the sound wave sampling frequency can even be improved by more than 100 times under the condition that the sensing distance is particularly large; compared with the technology that a narrow linewidth laser of each wavelength adopts a set of independent modulation and frequency shift equipment, and then laser pulses of all wavelengths are combined, modulation and frequency shift of the laser pulses are completed only through one set of modulation and frequency shift equipment, and the material cost is greatly saved. Therefore, the problem of high hardware cost caused by adopting a multi-wavelength pulse delay modulation technology to improve the sampling frequency is solved.

Description

Technical Field

[0001] The present invention relates to the field of distributed optical fiber sensing, and in particular to a three-level regulated linear frequency modulation laser pulse generating device. Background Art

[0002] In OTDR-based distributed optical fiber acoustic sensing (DAS), the laser pulse repetition frequency (RPF) is the sampling frequency of the acoustic vibration signal. A higher PRF supports a higher frequency of the demodulated acoustic vibration signal, or a greater mechanical vibration amplitude of the demodulated acoustic vibration signal. This, in turn, translates to a higher frequency of the coherent detection output signal supported by the distributed optical fiber sensor. According to the sampling law, the sampling frequency must be greater than twice the signal frequency, so the maximum PRF limits the maximum detection frequency of the acoustic vibration signal. However, in OTDR technology, the PRF must be less than the inverse of the round-trip delay difference of the sensing cable length to ensure that the Rayleigh backscattering of the two laser pulses does not overlap in time and become indistinguishable. For example, if the sensing cable to be measured is 50 km long and has a round-trip delay difference of approximately 0.5 ms, the maximum PRF is 2 kHz, resulting in a maximum frequency of the coherent detection output signal less than 1 kHz, and the supported mechanical vibration frequency is even lower.

[0003] Fiber ring acousto-optic frequency shifting (AOSF) technology can achieve multi-frequency multiplexing and increase the laser pulse repetition rate. However, the increased pulse repetition rate should be limited to maintain a good signal-to-noise ratio (SNR) for the transmitted laser pulses. This is because the frequency shift and fiber delay caused by the laser pulses circulating within the fiber ring, coupled with the need for multiple EDFA optical power amplification, can accumulate excessive ASE noise. Multi-wavelength pulse delay modulation can achieve multi-wavelength multiplexing and increase the laser pulse repetition rate without compromising the transmitted pulse SNR, but the hardware cost will increase exponentially. Summary of the Invention

[0004] The purpose of the present invention is to provide a three-level regulation linear frequency modulation laser pulse generating device, aiming to solve the problem of high hardware cost caused by increasing the sampling frequency by using multi-wavelength pulse delay modulation technology.

[0005] To achieve the above object, the present invention provides a three-level regulated linear frequency modulated laser pulse generating device, comprising M narrow linewidth lasers of different wavelengths, a pulse modulation module, a wavelength division chopping module and an acousto-optic frequency shifting module;

[0006] The M narrow linewidth lasers of different wavelengths are used to emit multi-wavelength continuous laser light and output through the first DWDM module;

[0007] The pulse modulation module is used to perform linear frequency sweeping and pulse modulation on the narrow linewidth laser to output linear frequency sweeping pulse lasers of M wavelengths;

[0008] The wavelength division chopping module is used to de-wavelength the pulsed lasers of each wavelength and select the pulses respectively before combining and outputting them;

[0009] The acousto-optic frequency shift module outputs a phase-continuous pulse sequence after K-1 level acousto-optic frequency shift, forming a wavelength division multiplexed linear frequency modulated laser pulse signal.

[0010] The pulse modulation module is composed of a single-sideband electro-optical modulator or an acousto-optic modulator, which modulates and outputs a series of laser pulses with a period of T1 and a pulse interval of T2, and the number of pulses is N (N≥2).

[0011] Wherein, the wavelength division chopping module includes a second DWDM module, M optical switches and a third DWDM module;

[0012] The second DWDM module is used to demultiplex the wavelength modulated signals and connect them to M optical switches;

[0013] The M optical switches are used to select one T2 period from M T2 periods within a T1 period, respectively. The laser pulses selected by adjacent optical switches have a delay difference of T2 and are output;

[0014] The third DWDM module is used to combine M groups of optical pulses with different wavelengths and adjacent delay intervals T3 between each group of wavelengths into one core optical fiber and output them to the delayed optical fiber ring acousto-optic frequency shifting module.

[0015] The acousto-optic frequency shifting module includes a 2×2 fiber coupler, an acousto-optic frequency shifter, a fiber amplifier, a fiber delay line, a bandpass filter and an adjustable attenuator;

[0016] The 2×2 optical fiber coupler is used to input the combined pulses into the acousto-optic frequency shifter;

[0017] The acousto-optic frequency shifter is used to shift the frequency of the pulse;

[0018] The optical fiber amplifier is used to amplify the frequency-shifted pulse;

[0019] The optical fiber delay line is used to add delay to the amplified frequency-shifted pulse and then connect it to the adjustable attenuator 45;

[0020] The bandpass filter and the adjustable attenuator are used to attenuate the pulse and then return it to the 2×2 optical fiber coupler.

[0021] The coherence length of the M narrow-linewidth lasers with different wavelengths transmitted back and forth in the optical fiber is greater than the length of the sensing optical fiber, and the operating wavelength interval between each two narrow-linewidth lasers is not less than 0.8 nm.

[0022] The present invention discloses a three-level regulation linear frequency modulation laser pulse generating device, comprising M narrow-linewidth lasers of different wavelengths, a pulse modulation module, a wavelength division chopping module and an acousto-optic frequency shifting module. The M narrow-linewidth lasers of different wavelengths are used to emit multi-wavelength continuous lasers. The pulse modulation module is used to perform linear frequency sweeping and pulse modulation on the narrow-linewidth lasers, and output linear frequency swept pulse lasers of M wavelengths. The wavelength division chopping module is used to de-wavelength-divide the pulse lasers of each wavelength, select the pulses respectively, and then combine and output them. The acousto-optic frequency shifting module, after K-1 levels of acousto-optic frequency shifting, outputs a phase-continuous pulse sequence, forming a wavelength-division multiplexed linear frequency modulation laser pulse signal. Compared to traditional OTDR-based distributed fiber acoustic wave sensing technology, this invention can achieve higher acoustic wave sampling frequencies. At particularly large sensing distances, the acoustic wave sampling frequency can be increased by more than 100 times. Compared to technologies that use a separate set of modulation and frequency shifting equipment for each wavelength of narrow-linewidth laser before combining the laser pulses of each wavelength, this invention uses only one set of modulation and frequency shifting equipment to complete the modulation and frequency shifting of the laser pulses, significantly saving material costs. This solves the problem of high hardware costs associated with using multi-wavelength pulse delay modulation technology to increase the sampling frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0024] Figure 1 This is a schematic structural diagram of a three-level regulated linear frequency modulated laser pulse generating device provided by the present invention;

[0025] Figure 2 This is the timing diagram of the entire process of Example 1

[0026] Figure 3 1 is a schematic diagram of the pulse modulation timing of the first-stage modulator of Example 1;

[0027] Figure 4 1 is a schematic diagram of the single wavelength pulse output timing of the third-stage delayed fiber ring acousto-optic frequency shifting module of Example 1;

[0028] Figure 51 is a schematic diagram of the timing of five wavelength pulse outputs of the third-stage delayed fiber ring acousto-optic frequency shifting module of Example 1;

[0029] In the figure: 1-M narrow-linewidth lasers of different wavelengths, 11-first DWDM module, 2-pulse modulation module, 3-wavelength division chopping module, 4-acousto-optic frequency shifting module, 31-second DWDM module, 32-M optical switches, 33-third DWDM module, 41-2×2 fiber coupler, 42-acousto-optic frequency shifter, 43-fiber amplifier, 44-fiber delay line, 45-bandpass filter and adjustable attenuator. DETAILED DESCRIPTION

[0030] The following describes embodiments of the present invention in detail, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and are not to be construed as limiting the present invention.

[0031] See also Figures 1 to 5 The present invention provides a three-level regulated linear frequency modulated laser pulse generating device, comprising M narrow linewidth lasers of different wavelengths 1, a pulse modulation module 2, a wavelength division chopping module 3 and an acousto-optic frequency shifting module 4;

[0032] The M narrow linewidth lasers 1 with different wavelengths are used to emit multi-wavelength continuous laser light and output through the first DWDM module 11;

[0033] The pulse modulation module 2 is used to perform linear frequency sweeping and pulse modulation on the narrow linewidth laser to output linear frequency sweeping pulse lasers of M wavelengths;

[0034] The wavelength division chopping module 3 is used to de-wavelength the pulse lasers of each wavelength, select the pulses respectively, and then combine and output them;

[0035] The acousto-optic frequency shift module 4 outputs a phase-continuous pulse sequence after K-1 level acousto-optic frequency shift, forming a wavelength division multiplexed linear frequency modulated laser pulse signal.

[0036] Furthermore, the pulse modulation module 2 is composed of a single-sideband electro-optical modulator or an acousto-optic modulator, which modulates and outputs a series of laser pulses with a period of T1 and a pulse interval of T2, and the number of pulses is N (N≥2).

[0037] Furthermore, the coherence length of the M narrow-linewidth lasers 1 with different wavelengths transmitted back and forth in the optical fiber is greater than the length of the sensing optical fiber, and the operating wavelength interval between each two narrow-linewidth lasers is not less than 0.8 nm.

[0038] In this embodiment, narrow-linewidth lasers with M wavelengths (λ1, λ2, λ3...λM) output continuous laser light to dense wavelength division multiplexer 1 (DWDM-1), which is then combined into an optical fiber and enters pulse modulation module 2 for linear frequency sweep modulation and pulse modulation. The modulation period is T2, corresponding to a modulation frequency of F2. There are N sub-pulses in one T2 period, and the time interval between each sub-pulse is T3, corresponding to a modulation frequency of F3. The laser modulated pulses of M wavelengths are output to dense wavelength division multiplexer 2 (DWDM-2) for wavelength demultiplexing, where each wavelength occupies an optical fiber output, and the pulses of each wavelength enter the corresponding optical switches numbered OS-1, OS-2, OS-3...OS-M. The laser pulses of each wavelength output by DWDM-2 then enter the wavelength division chopper module 3 for selection and output, where M T2 periods are defined as period T1, corresponding to a modulation frequency of F1. A pulse train of length T2 is retained within each wavelength T1 period, and the pulse trains of adjacent wavelengths are delayed by T2. The pulsed laser signals output by the optical switches OS-1, OS-2, OS-3, ..., OS-M are then connected to the M input ports of a dense wavelength division multiplexer 3 (DWDM-3). After being combined, they are output via a single optical fiber to a fiber acousto-optic frequency shifting module. After passing through the acousto-optic frequency shifting fiber ring, K pulse trains with a delay difference of T3 × N are output. This solution overcomes the shortcoming of existing fiber optic acoustic wave sensing technology based on OTDR technology, in which the maximum sampling frequency is less than the inverse of the round-trip delay difference of the sensing optical cable. By increasing the system's sampling frequency by M × N × K times, this solution supports ultra-high acoustic wave sampling frequencies at ultra-long detection distances, and supports high-frequency, large dynamic range acoustic wave sensing at ultra-long distances.

[0039] The laser pulse modulation period T1 is the period of a single-frequency, single-wavelength laser pulse. T1 is greater than the round-trip delay difference of the laser pulse in the total length of the sensing optical cable, and T1=1 / F1.

[0040] The laser pulse modulation period T2 is the time delay difference of a single wavelength in T1 when M narrow linewidth lasers 1 of different wavelengths are modulated simultaneously, and T1=T2×M, T2=1 / F2, where M is a positive integer greater than or equal to 2.

[0041] The laser pulse modulation period T3 is the time delay difference between two laser pulses of adjacent modulation frequencies of a laser source with the same wavelength, and T2=T3×N×K; wherein N is the number of laser pulses modulated by the pulse modulator within a T2 period with adjacent cycle frequencies differing by f and adjacent delays differing by T3, and N is a positive integer greater than or equal to 2; and K is the multiple of the change in the number of laser pulses from the time the laser pulse enters the fiber ring acousto-optic frequency shifter 42 to the time the laser pulse exits the fiber ring acousto-optic frequency shifter 42 within a T2 period, and K is a positive integer greater than or equal to 2.

[0042] The M narrow-linewidth lasers 1 of different wavelengths have a coherence length for round-trip transmission in the optical fiber that is greater than the length of the sensing optical fiber, and the operating wavelength interval between each narrow-linewidth laser is not less than 0.8 nm; the number of laser wavelengths is determined by the period T1 of a single-frequency, single-wavelength laser pulse and the time delay difference T2 occupied by a single-wavelength laser pulse within a T1 period, and M = T1 / T2, where M is a positive integer greater than or equal to 2.

[0043] Compared to traditional OTDR-based distributed fiber acoustic wave sensing technology, this invention can achieve higher acoustic wave sampling frequencies. At particularly large sensing distances, the acoustic wave sampling frequency can be increased by more than 100 times. Compared to technologies that use a separate set of modulation and frequency shifting equipment for each wavelength of narrow-linewidth laser before combining the laser pulses of each wavelength, this invention uses only one set of modulation and frequency shifting equipment to complete the modulation and frequency shifting of the laser pulses, significantly saving material costs. This solves the problem of high hardware costs associated with using multi-wavelength pulse delay modulation technology to increase the sampling frequency.

[0044] Furthermore, the wavelength division chopping module 3 includes a second DWDM module 31, M optical switches 32 and a third DWDM module 33;

[0045] The second DWDM module 31 is used to demultiplex the wavelength modulated signals and connect them to M optical switches 32;

[0046] The M optical switches 32 are used to select one T2 period from the M T2 periods within a T1 period, respectively. The laser pulses selected by adjacent optical switches have a delay difference of T2 and are output;

[0047] The third DWDM module 33 is used to combine M groups of optical pulses with different wavelengths and adjacent delay intervals T3 into one core optical fiber and output them to the delayed optical fiber ring acousto-optic frequency shifting module 4.

[0048] In this embodiment, the dense wavelength division multiplexers DWDM-1, DWDM-2, and DWDM-3 each have a common port and M wavelength division ports, where the M wavelength division ports correspond to laser wavelengths λ1, λ2, λ3, ..., λM. In addition, the delay differences from the combining port of DWDM-2 to the input ports of optical switches OS-1, OS-2, OS-3, ..., OS-M, via the M wavelength division ports, are equal. The delay differences from the output ports of optical switches OS-1, OS-2, OS-3, ..., OS-M, via the M wavelength division ports, to the combining port of DWDM-3, are equal.

[0049] The laser modulator in the pulse modulation module 2 can be an acousto-optic modulator or an optical switch, which modulates to generate a laser pulse in each T2 period; the laser modulator can also be a single-sideband phase modulator, which modulates to generate N laser pulses in each T2 period, with the delay difference between adjacent pulses being T3, and the adjacent pulses generating a frequency shift f.

[0050] The optical switches OS-1, OS-2, OS-3, ..., OS-M respectively select one T2 period from the M T2 periods within one T1 period of the pulse light of each wavelength to pass through. There is a delay difference of T2 between the laser pulses selected by adjacent optical switches.

[0051] Furthermore, the acousto-optic frequency shifting module 4 includes a 2×2 fiber coupler 41, an acousto-optic frequency shifter 42, a fiber amplifier 43, a fiber delay line 44, a bandpass filter and an adjustable attenuator 45;

[0052] The 2×2 fiber coupler 41 is used to input the combined pulses into the acousto-optic frequency shifter 42;

[0053] The acousto-optic frequency shifter 42 is used to shift the frequency of the pulse;

[0054] The optical fiber amplifier 43 is used to amplify the frequency-shifted pulses;

[0055] The optical fiber delay line 44 is used to add delay to the amplified frequency-shifted pulse and then connect it to the adjustable attenuator 45;

[0056] The adjustable attenuator 45 is used to attenuate the pulse and then return it to the 2×2 fiber coupler 41 .

[0057] In this embodiment, the frequency shift amount generated by a single cycle of the laser pulse in the optical fiber ring is f×N, and the single cycle delay of the optical fiber ring is T3×N; after the laser pulse enters the 2×2 optical fiber coupler 41, it circulates a total of (K-1) times in the optical fiber ring, and then is synthesized with the first pulse that does not pass through the optical fiber ring and directly outputs to the output port of the 2×2 optical fiber coupler 41.

[0058] Example 1

[0059] In this embodiment 1, Figure 1 The three-stage frequency modulation structure of this example is shown. In the first-stage pulse modulation module, multiple narrow-linewidth lasers output continuous laser light to dense wavelength division multiplexer 1 (DWDM-1). The light is then combined into a single optical fiber and then enters the pulse modulation module for linear frequency modulation and pulse modulation.

[0060] In this example, the second-stage wavelength division chopping module outputs M wavelengths of modulated laser pulses to dense wavelength division multiplexer 2 (DWDM-2) for demultiplexing. Each wavelength occupies a separate optical fiber for output. The pulses of each wavelength enter the corresponding optical switches numbered OS-1, OS-2, OS-3, ..., OS-M, which select and output the laser pulses of each wavelength. The pulsed laser signals output by the optical switches OS-1, OS-2, OS-3, ..., OS-M are then connected to the M input ports of dense wavelength division multiplexer 3 (DWDM-3). After being combined, they are output via a single optical fiber to the fiber ring acousto-optic frequency shifting module.

[0061] In this example, the third-stage fiber ring acousto-optic frequency shifting module, the combined pulse enters port 1 of the 2×2 fiber coupler, is output from port 3 and enters the acousto-optic frequency shifter. After frequency shifting, it enters the fiber amplifier for amplification, is connected to the fiber delay line, passes through the bandpass filter and attenuator, returns to port 2 of the 2×2 fiber coupler, and is finally output from port 4.

[0062] In this example, the M narrow-linewidth lasers of different wavelengths have a coherent length for round-trip transmission in the optical fiber that is greater than the length of the sensing fiber, and the operating wavelength interval between each pair of narrow-linewidth lasers is no less than 0.8 nm.

[0063] Figure 2 The full process timing diagram of Example 1 of the three-stage linear frequency modulation laser pulse generator is shown. The multi-wavelength narrow linewidth laser output combines the continuous laser of 5 wavelengths and inputs it into the single sideband modulator in the pulse modulation module. After linear frequency modulation and pulse modulation, the reference pulse group sequence with a period of 10ms and a pulse interval of 2ms is output. Each group has 4 laser pulses, and the delay interval between pulses in the group is 0.1ms. The second-stage adjustment module is a wavelength division chopping module, which first demultiplexes the pulse laser of each wavelength, and then uses 5 optical switches (SOAs) to divide the 5 wavelengths. The pulse laser is pulse selected, and each wavelength of the pulse laser is modulated into a period of 10ms, and the relative delay of each adjacent wavelength is 2ms. Then, the multi-wavelength pulse laser is combined through DWDM-3 and output to the fiber acousto-optic ring frequency shift module; the third level is the fiber acousto-optic ring frequency shift module. The pulse laser output by DWDM-3 undergoes four-level acousto-optic frequency shift through the fiber ring, and then a continuous pulse sequence with an adjacent pulse interval of 0.1ms is output from the 4 ports of the fiber coupler, finally forming a 10kHz linear frequency modulated laser pulse signal with 5 wavelength division multiplexing.

[0064] Figure 3The figure shows a pulse modulation timing diagram of the first-stage modulator of Example 1 of the three-stage linear frequency modulation laser pulse generating device. The first-stage modulator is composed of a single-sideband electro-optical modulator or an acousto-optic modulator, which modulates and generates one laser pulse in each 2ms period. The laser modulator can also be a single-sideband phase modulator, which modulates and generates four laser pulses in each 10ms period. The delay difference between adjacent pulses is 0.1ms, and the frequency shift between adjacent pulses is 100MHz.

[0065] Figure 4 The figure shows a schematic diagram of the single-wavelength pulse output timing of the third-stage delayed optical fiber ring acousto-optic frequency shift module of Example 1 of a three-stage linear frequency modulation laser pulse generating device; the frequency shift amount generated by a single cycle of the laser pulse in the optical fiber ring is 100MHz×4, and the single cycle delay of the optical fiber ring is 0.1ms×4; after the laser pulse enters the 2×2 optical fiber coupler, it circulates a total of 4 times in the optical fiber ring, and forms a pulse signal with a total number of 20 pulses together with the pulse laser directly output through the 2×2 optical fiber coupler.

[0066] Figure 5 This is a schematic diagram of the five wavelength pulse output timing of the third-stage delayed fiber ring acousto-optic frequency shifting module of the three-stage linear frequency modulation laser pulse generating device embodiment 1 of the present invention; a single wavelength laser pulse is selected to pass through a 2ms period from five 2ms periods within a 10ms period, and the laser pulse is selected to pass through the optical switch corresponding to the different wavelength pulse lasers, so that there is a 2ms delay difference between the pulse lasers of adjacent wavelengths. The above embodiments are only specific examples to further explain the purpose, technical solutions and beneficial effects of the present invention in detail, and the present invention is not limited thereto. Any modifications, equivalent substitutions, improvements, etc. made within the scope of the disclosure of the present invention are included in the protection scope of the present invention.

[0067] The above disclosure is merely a preferred embodiment of a three-level regulated linear frequency modulated laser pulse generating device of the present invention. It is certainly not intended to limit the scope of the present invention. A person skilled in the art will understand that implementing all or part of the processes of the above embodiment and making equivalent changes in accordance with the claims of the present invention still fall within the scope of the invention.

Claims

1. A three-level linear frequency modulation laser pulse generator, characterized in that: It includes M narrow linewidth lasers of different wavelengths, a pulse modulation module, a wavelength division chopping module and an acousto-optic frequency shifting module; The M narrow linewidth lasers of different wavelengths are used to emit multi-wavelength continuous laser light and output through the first DWDM module; The pulse modulation module is used to perform linear frequency sweeping and pulse modulation on the narrow linewidth laser to output linear frequency sweeping pulse lasers of M wavelengths; The wavelength division chopping module is used to de-wavelength the pulse lasers of each wavelength, select the pulses, and then combine and output them; The acousto-optic frequency shift module outputs a phase-continuous pulse sequence after K-1 level acousto-optic frequency shift, forming a wavelength division multiplexed linear frequency modulated laser pulse signal.

2. The three-level linear frequency modulation laser pulse generator according to claim 1, characterized in that: The pulse modulation module is composed of a single-sideband electro-optical modulator or an acousto-optic modulator, which modulates and outputs a series of laser pulses with a period of T1 and a pulse interval of T2. The number of pulses in each wavelength group is N (N≥2).

3. The three-level linear frequency modulation laser pulse generator according to claim 1, characterized in that: The wavelength division chopping module includes a second DWDM module, M optical switches and a third DWDM module; The second DWDM module is used to demultiplex the wavelength modulated signals and connect them to M optical switches respectively; The M optical switches are used to select one T2 period from M T2 periods within a T1 period, respectively. The laser pulses selected by adjacent optical switches have a delay difference of T2 and are output; The third DWDM module is used to combine M groups of optical pulses with different wavelengths and adjacent delay intervals T3 into one core optical fiber and output them to the delayed optical fiber ring acousto-optic frequency shifting module.

4. The three-level linear frequency modulation laser pulse generator according to claim 1, characterized in that: The acousto-optic frequency shifting module includes a 2×2 fiber coupler, an acousto-optic frequency shifter, a fiber amplifier, a fiber delay line, a bandpass filter and an adjustable attenuator; The 2×2 optical fiber coupler is used to input the combined pulses into the acousto-optic frequency shifter; The acousto-optic frequency shifter is used to shift the frequency of the pulse; The optical fiber amplifier is used to amplify the frequency-shifted pulse; The optical fiber delay line is used to add delay to the amplified frequency-shifted pulse; The bandpass filter and the adjustable attenuator are used to filter and attenuate the pulses and then return them to the 2×2 optical fiber coupler.

5. The three-level linear frequency modulation laser pulse generator according to claim 1, characterized in that: The coherence length of the M narrow-linewidth lasers of different wavelengths transmitted back and forth in the optical fiber is greater than the length of the sensing optical fiber, and the operating wavelength interval between each two narrow-linewidth lasers is not less than 0.8 nm.

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