Near-infrared wavelength scanning pulse fiber laser having low mode-locking threshold

Abstract

Disclosed in the present invention is a near-infrared wavelength scanning pulse fiber laser having a low mode-locking threshold. According to the present invention, a nonlinear amplifying loop mirror and a saturable absorber mirror architecture are used to achieve a low mode-locking threshold and ensure long-term mode-locking stability. After mode-locking self-starting, an automatic control module automatically achieves single-pulse operation by adjusting the intensity of retro-reflected pulses. In the single-pulse operation stage, different frequencies of radio frequency signals and different signal intensities are provided for an acousto-optic tunable filter by means of a radio frequency module to achieve a 1030 nm-1100 nm wavelength scanning and tuning range.

Description

A near-infrared wavelength-swept pulsed fiber laser with a low mode-locking threshold Technical Field

[0001] The invention belongs to the field of laser technology, and in particular relates to a near-infrared wavelength scanning pulse fiber laser with a low mode-locking threshold. Background Art

[0002] Ultrafast laser sources with ultrawideband wavelength scanning and tunability play a vital role in many fields of science and engineering, such as multicolor and multimodal fluorescence microscopy and coherent Raman microscopy. Ultrawideband wavelength scanning and tunable laser output can typically be achieved through: 1) intracavity tunable filtering; and 2) nonlinear frequency conversion, such as optical parametric oscillators, optical parametric amplifiers, and advanced nonlinear spectrum broadening. Intracavity filtering is the most straightforward technique, ensuring stable output power and high signal-to-noise ratio pulse quality.

[0003] The rapid development of mode-locked fiber lasers in recent years has provided new options for achieving ultra-broadband wavelength scanning and tunable lasers. In the life sciences, thanks to the discovery of various fluorescent probes, researchers can combine multiple fluorescent probes to achieve multicolor and multimodal microscopic imaging (Nat Methods 9, 815 (2012)), thereby simultaneously obtaining more comprehensive biological information. This, in turn, places new demands on excitation light sources with ultra-broadband wavelength scanning and tunable ranges. Currently, the most widely used laser sources are based on optical parametric oscillators, with sapphire solid-state lasers being the primary representative. However, these sources suffer from weaknesses such as poor resistance to environmental interference and high cost. However, ultra-broadband wavelength scanning and tunable light sources that utilize intracavity ultra-fast wavelength scanning and mode locking often suffer from high intracavity losses, resulting in extremely high mode-locking thresholds and susceptible to device damage. Therefore, developing high-performance, low mode-locking threshold ultrafast laser sources for ultra-broadband wavelength scanning and tunability is crucial. Summary of the Invention

[0004] In order to solve the above problems, the present invention proposes a near-infrared wavelength swept pulse fiber laser with a low mode-locking threshold.

[0005] The purpose of the present invention is achieved by at least one of the following technical solutions.

[0006] A near-infrared wavelength-swept pulsed fiber laser with a low mode-locking threshold, comprising a pump source, a fiber-type wavelength division multiplexer, a polarization-maintaining gain fiber, a first fiber coupler, a broadband collimator, an electrically controlled adjustable wave plate, a polarization beam splitter, a light intensity flux modulation module, a fiber-type non-reciprocal phase biaser, a fiber-type acousto-optic deflector, a radio frequency driver module, a second fiber coupler, a photodetector, and a microcomputer control module;

[0007] The output end of the pump source is connected to the pump end of the fiber-type wavelength division multiplexer to provide pump light; the common end of the fiber-type wavelength division multiplexer is connected to one end of the polarization-maintaining gain fiber, which is responsible for generating and amplifying signal light; the other end of the polarization-maintaining gain fiber is connected to the first port of the first fiber coupler; the signal light is transmitted in two directions, forming a clockwise transmission light field and a counterclockwise transmission light field; the signal end of the fiber-type wavelength division multiplexer is connected to one end of the fiber-type acousto-optic deflector; the other end of the fiber-type acousto-optic deflector is connected to one end of the fiber-type non-reciprocal phase biaser; in addition, the radio frequency input end of the fiber-type acousto-optic deflector is connected to the output end of the radio frequency control end; the other end of the fiber-type non-reciprocal phase biaser is connected to the second port of the first fiber coupler; the fourth port of the first fiber coupler is connected to the broadband collimator; the above devices are connected to form a nonlinear fiber amplifying ring mirror;

[0008] The third port of the first fiber coupler is connected to the first port of the second fiber coupler; the second port of the second fiber coupler outputs the signal light, and the third port is connected to the optical input end of the photodetector to convert the optical signal into an electrical signal; the electrical signal output end of the photodetector is connected to the input end of the microcomputer control module, and the output end of the microcomputer control module is connected to the control end of the electrically controlled tunable wave plate;

[0009] The electrically controlled adjustable wave plate, polarization beam splitter, and light intensity flux modulation module are all spatial devices, arranged in sequence. The signal light emitted from the broadband collimator passes through the electrically controlled adjustable wave plate, polarization beam splitter, and light intensity flux modulation module in sequence, and is then reflected by the light intensity flux modulation module and returned in the opposite direction of the original path.

[0010] The signal light reflected from the broadband collimator enters the first fiber coupler and is split into two signal light beams. The two beams then return to the first fiber coupler after clockwise and counterclockwise transmission. The counterclockwise transmission field sequentially passes through the first fiber coupler, the polarization-maintaining gain fiber, the fiber-type wavelength division multiplexer, the fiber-type acousto-optic deflector, and the fiber-type non-reciprocal phase shifter before returning to the first fiber coupler. The transmission direction of the clockwise transmission field is opposite to that of the counterclockwise transmission field.

[0011] The photodetector converts the optical signal into an electrical signal, which is then input into a microcomputer control module for processing. The microcomputer control module compares the received electrical signal with a preset pulse repetition frequency setting value, and then adjusts the polarization state of the light on the electrically controlled adjustable wave plate, changing the reflected light intensity to achieve automated control of the single-pulse operation of the laser.

[0012] Furthermore, the polarization-maintaining gain fiber is a rare earth ion ytterbium-doped polarization-maintaining fiber.

[0013] Furthermore, the light intensity flux modulation module includes a variable focus lens, a fixed focus lens and a saturable absorption reflector arranged in sequence;

[0014] During operation, the signal light collimated by the broadband collimator passes through the electrically controlled adjustable wave plate, polarization beam splitter, variable focus lens, fixed focus lens and saturable absorption reflector in sequence, and then is reflected back by the saturable absorption reflector in the opposite direction of the original path, passing through the broadband collimator and the first fiber coupler and returning to the nonlinear fiber amplifying loop mirror;

[0015] The variable focus lens and fixed focus lens are used to change the spot size of the light incident on the saturable absorption mirror;

[0016] The light intensity flux is inversely proportional to the spot size, and changing the spot size can achieve a change in the light intensity flux. Because the reflectivity of the saturable absorption mirror is proportional to the light intensity flux, the locking threshold is reduced.

[0017] Furthermore, the first optical fiber coupler is an optical fiber coupler with a splitting ratio of 40:60;

[0018] During operation, the signal light of the clockwise transmission light field is input into the first fiber coupler through the first port of the first fiber coupler and then split into two signal light beams according to the splitting ratio, of which 40% of the signal light is output to the first port of the second fiber coupler through the third port of the first fiber coupler, and 60% of the signal light is output to the broadband collimator through the fourth port of the first fiber coupler;

[0019] The signal light of the counterclockwise transmitted light field is input into the first fiber coupler through the second port of the first fiber coupler and then split into two beams of signal light according to the splitting ratio, wherein 60% of the signal light is output to the first port of the second fiber coupler through the third port of the first fiber coupler, and 40% of the signal light is output to the broadband collimator through the fourth port of the first fiber coupler; the signal light reflected back by the broadband collimator is input into the first fiber coupler through the fourth port of the first fiber coupler and then split into two beams of signal light according to the splitting ratio, wherein 60% of the signal light is output through the first port of the first fiber coupler, and 40% of the signal light is output through the second port of the first fiber coupler.

[0020] Furthermore, the second optical fiber coupler is an optical fiber coupler with a splitting ratio of 10:90;

[0021] During operation, the signal light output from the third port of the first fiber coupler is input into the second fiber coupler through the first port of the second fiber coupler, and then split into two beams of signal light according to the splitting ratio, of which 90% of the signal light is output from the second port of the second fiber coupler, and 10% of the signal light is output from the third port of the second fiber coupler to the photodetector.

[0022] Furthermore, the RF driver module is a device or apparatus that can provide different RF signals; the RF driver module provides different RF signals and different RF signal intensities so that the fiber-type acousto-optic deflector generates a wavelength tuning range of 1030nm-1100nm.

[0023] Furthermore, the photodetector is a device or apparatus that converts an optical signal into an electrical signal, including but not limited to a photomultiplier tube and a photodiode.

[0024] Furthermore, the saturable absorption mirror is made of a material having a saturable absorption effect on light intensity, including semiconductors, graphene, and carbon nanotubes.

[0025] Furthermore, the fiber-type non-reciprocal phase biaser is used to complete the phase shift difference between the clockwise and counterclockwise transmission light fields in the nonlinear fiber amplifying loop mirror, and the introduced phase shift difference does not exceed π / 2.

[0026] Furthermore, the microcomputer control module is a control system based on FPGA.

[0027] During operation, the microcomputer control module is used to identify the repetition frequency of the electrical signal input by the photodetector, and then compare it with the preset pulse repetition frequency; if there is any inconsistency, the light intensity is adjusted by controlling the electrically controlled adjustable wave plate until the repetition frequency of the electrical signal is consistent with the preset pulse repetition frequency, thereby realizing automatic control of the single-pulse operation of the laser.

[0028] Compared with the prior art, the advantages of the present invention are:

[0029] The present invention utilizes a nonlinear fiber amplifying ring mirror and a saturable absorption reflector to achieve pulse self-starting at a low pump threshold by changing the light intensity flux, while automatically controlling the mode-locking state using a microcomputer controller; at the same time, an acousto-optic tunable filter is used to achieve 1030nm-1100nm wavelength scanning and tuned laser output.

[0030] Description of the drawings:

[0031] FIG1 is a schematic structural diagram of a near-infrared wavelength swept pulse fiber laser with a low mode-locking threshold according to an embodiment of the present invention;

[0032] FIG2 is a diagram showing the wavelength tuning results of 1030 nm to 1100 nm in an embodiment of the present invention;

[0033] FIG3 is a schematic diagram showing the relationship between the spot radius and the zoom lens in an embodiment of the present invention.

[0034] Specific implementation method:

[0035] The features of the present invention and other related features are further described in detail below through embodiments in conjunction with the accompanying drawings to facilitate understanding by those skilled in the art.

[0036] Example:

[0037] A near-infrared wavelength-swept pulsed fiber laser with a low mode-locking threshold, as shown in FIG1 , comprises a pump source 1, a fiber-type wavelength division multiplexer 2, a polarization-maintaining gain fiber 3, a first fiber coupler 4, a broadband collimator 5, an electrically controlled adjustable wave plate 6, a polarization beam splitter 7, a light intensity flux modulation module 8, a fiber-type nonreciprocal phase biaser 9, a fiber-type acousto-optic deflector 10, a radio frequency driver module 11, a second fiber coupler 12, a photodetector 13, and a microcomputer control module 14;

[0038] The output end of the pump source 1 is connected to the pump end of the fiber-type wavelength division multiplexer 2 to provide pump light; the common end of the fiber-type wavelength division multiplexer 2 is connected to one end of the polarization-maintaining gain fiber 3, which is responsible for generating and amplifying signal light; the other end of the polarization-maintaining gain fiber 3 is connected to the first port of the first fiber coupler 4; the signal light is transmitted in two directions, forming a clockwise transmission light field and a counterclockwise transmission light field; the signal end of the fiber-type wavelength division multiplexer 2 is connected to one end of the fiber-type acousto-optic deflector 10; the other end of the fiber-type acousto-optic deflector 10 is connected to one end of the fiber-type non-reciprocal phase biaser 9; in addition, the radio frequency input end of the fiber-type acousto-optic deflector 10 is connected to the output end of the radio frequency control end 11; the other end of the fiber-type non-reciprocal phase biaser 9 is connected to the second port of the first fiber coupler 4; the fourth port of the first fiber coupler 4 is connected to the broadband collimator 5; the above devices are connected to form a nonlinear fiber amplifying ring mirror;

[0039] The third port of the first fiber coupler 4 is connected to the first port of the second fiber coupler 12; the second port of the second fiber coupler 12 outputs the signal light, and the third port is connected to the optical input end of the photodetector 13 to convert the optical signal into an electrical signal; the electrical signal output end of the photodetector 13 is connected to the input end of the microcomputer control module 14, and the output end of the microcomputer control module 14 is connected to the control end of the electrically controlled tunable wave plate 6;

[0040] The electrically controlled adjustable wave plate 6, the polarization beam splitter 7, and the light intensity flux modulation module 8 are all spatial devices and are arranged in sequence. The signal light emitted from the broadband collimator 5 passes through the electrically controlled adjustable wave plate 6, the polarization beam splitter 7, and the light intensity flux modulation module 8 in sequence, and is then reflected by the light intensity flux modulation module and returned in the opposite direction of the original path.

[0041] The signal light reflected from the broadband collimator 5 enters the first fiber coupler 4 and is split into two signal light beams. The two beams return to the first fiber coupler 4 after clockwise and counterclockwise transmission light fields. The counterclockwise transmission light field sequentially passes through the first fiber coupler 4, the polarization-maintaining gain fiber 3, the fiber-type wavelength division multiplexer 2, the fiber-type acousto-optic deflector 10, and the fiber-type non-reciprocal phase shifter 9 before finally returning to the first fiber coupler 4. The transmission direction of the clockwise transmission light field is opposite to that of the counterclockwise transmission light field.

[0042] The photodetector 13 converts the optical signal into an electrical signal, which is then input into the microcomputer control module 14 for processing; the microcomputer control module 14 compares the received electrical signal with a preset pulse repetition frequency setting value, and then adjusts the polarization state of the electrically controlled adjustable wave plate 6 to change the reflected light intensity, thereby realizing automatic control of the single-pulse operation of the laser.

[0043] In one embodiment, the polarization-maintaining gain fiber 3 is a rare earth ion ytterbium-doped polarization-maintaining fiber.

[0044] In one embodiment, the light intensity flux adjustment module 8 includes a variable focus lens 801, a fixed focus lens 802, and a saturable absorption reflector 803 arranged in sequence;

[0045] During operation, the signal light collimated by the broadband collimator 5 passes through the electrically controlled adjustable wave plate 6, the polarization beam splitter 7, the variable focus lens 801, the fixed focus lens 802, and the saturable absorption mirror 803 in sequence, and is then reflected back by the saturable absorption mirror 803 in the opposite direction of the original path, passing through the broadband collimator 5 and the first fiber coupler 4 and returning to the nonlinear fiber amplifying loop mirror.

[0046] The variable focus lens 801 and the fixed focus lens 802 are used to change the size of the light spot incident on the saturable absorption reflector 803;

[0047] The light intensity flux is inversely proportional to the spot size, and changing the spot size can achieve a change in the light intensity flux. Since the reflectivity of the saturable absorption mirror 803 is proportional to the light intensity flux, the locking threshold is reduced.

[0048] In one embodiment, the first fiber coupler 4 is a fiber coupler with a splitting ratio of 40:60;

[0049] During operation, the signal light of the clockwise transmission light field is input into the first fiber coupler 4 through the first port of the first fiber coupler 4 and then split into two beams of signal light according to the splitting ratio, wherein 40% of the signal light is output to the first port of the second fiber coupler 12 through the third port of the first fiber coupler 4, and 60% of the signal light is output to the broadband collimator 5 through the fourth port of the first fiber coupler 4;

[0050] The signal light of the counterclockwise transmitted light field is input into the first fiber coupler 4 through the second port of the first fiber coupler 4 and then split into two beams of signal light according to the splitting ratio, wherein 60% of the signal light is output to the first port of the second fiber coupler 12 through the third port of the first fiber coupler 4, and 40% of the signal light is output to the broadband collimator 5 through the fourth port of the first fiber coupler 4; the signal light reflected back by the broadband collimator is input into the first fiber coupler 4 through the fourth port of the first fiber coupler 4 and then split into two beams of signal light according to the splitting ratio, wherein 60% of the signal light is output through the first port of the first fiber coupler 4, and 40% of the signal light is output through the second port of the first fiber coupler 4.

[0051] In one embodiment, the second fiber coupler 12 is a fiber coupler with a splitting ratio of 10:90;

[0052] During operation, the signal light outputted from the third port of the first fiber coupler 4 is inputted into the second fiber coupler 12 through the first port of the second fiber coupler 12 and then split into two beams of signal light according to the splitting ratio, wherein 90% of the signal light is outputted from the second port of the second fiber coupler 12 and 10% of the signal light is outputted from the third port of the second fiber coupler 12 to the photodetector 13.

[0053] In one embodiment, as shown in FIG. 2 , the RF driver module 11 is a device or apparatus that can provide different RF signals. The RF driver module 11 provides different RF signals and different RF signal intensities so that the fiber-type acousto-optic deflector 10 generates a wavelength tuning range of 1030 nm to 1100 nm.

[0054] Furthermore, the photodetector 13 is a device or apparatus that converts an optical signal into an electrical signal, including but not limited to a photomultiplier tube and a photodiode.

[0055] Furthermore, the saturable absorption mirror 803 is made of a material having a saturable absorption effect on light intensity, including semiconductors, graphene, and carbon nanotubes.

[0056] Furthermore, the fiber-type non-reciprocal phase biaser 9 is used to complete the phase shift difference between the clockwise and counterclockwise transmission light fields in the nonlinear fiber amplifying loop mirror, and the introduced phase shift difference does not exceed π / 2.

[0057] Furthermore, the microcomputer control module 14 is a control system based on FPGA.

[0058] During operation, the microcomputer control module 14 is used to identify the repetition frequency of the electrical signal input by the photodetector 13, and then compare it with the preset pulse repetition frequency; if they are inconsistent, the light intensity is adjusted by controlling the electrically controlled adjustable wave plate 6 until the repetition frequency of the electrical signal is consistent with the preset pulse repetition frequency, thereby realizing automatic control of the single-pulse operation of the laser.

[0059] In one embodiment, the fiber coupler 12 splits the laser light for output, and its 10% output end is connected to the photodetector 13. The photodetector 13 converts the optical signal into an electrical signal, which is then transmitted to the microcomputer control module 14 via a radio frequency line. After being processed by the microcomputer control module 14, a control signal is generated and input to the electrically controlled adjustable wave plate 6 to achieve twisting of the wave plate. The change in the wave plate angle affects the light intensity of the reflected light, which plays a role in adjusting the loss in the cavity, thereby enhancing or reducing the energy of the resonant cavity. The purpose is to automatically identify the single pulse operation state in the cavity and maintain this working state.

[0060] In one embodiment, as shown in FIG3 , the light intensity flux modulation works as follows:

[0061] The size of the light intensity flux is inversely proportional to the size of the light spot area. The light intensity flux modulator 8 is composed of a zoom lens 801, a fixed focus lens 802 and a saturable absorption reflector. The modulation of the light intensity flux is mainly completed by the zoom lens 801 and the fixed focus lens 802. That is, by changing the focal length of the zoom lens 801, the magnification of the light spot is changed, thereby changing the size of the light spot area, affecting the light intensity flux value incident on the semiconductor saturable reflector.

Claims

1. A near-infrared wavelength-scanning pulsed fiber laser with a low mode-locking threshold, characterized in that, It includes a pump source (1), a fiber optic wavelength division multiplexer (2), a polarization-maintaining gain fiber (3), a first fiber coupler (4), a broadband collimator (5), an electrically controlled tunable wave plate (6), a polarization beam splitter (7), an optical intensity flux modulation module (8), a fiber optic non-reciprocal phase biasing device (9), a fiber optic acousto-optic deflector (10), a radio frequency driving module (11), a second fiber coupler (12), a photodetector (13), and a microcomputer control module (14); The output end of the pump source (1) is connected to the pump end of the fiber optic wavelength division multiplexer (2) to provide pump light; the common end of the fiber optic wavelength division multiplexer (2) is connected to one end of the polarization-maintaining gain fiber (3), and the polarization-maintaining gain fiber (3) is responsible for generating and amplifying the signal light; the other end of the polarization-maintaining gain fiber (3) is connected to the first port of the first fiber coupler (4); the signal light is split into two directions of transmission to form a clockwise transmission optical field and a counterclockwise transmission optical field; the signal end of the fiber optic wavelength division multiplexer (2) is connected to one end of the fiber optic acousto-optic deflector (10); the other end of the fiber optic acousto-optic deflector (10) is connected to one end of the fiber optic non-reciprocal phase biasing device (9); in addition, the radio frequency input end of the fiber optic acousto-optic deflector (10) is connected to the output end of the radio frequency control end (11); the other end of the fiber optic non-reciprocal phase biasing device (9) is connected to the second port of the first fiber coupler (4); the fourth port of the first fiber coupler (4) is connected to the broadband collimator (5); the above devices are connected to form a nonlinear fiber amplifying loop mirror; The third port of the first fiber coupler (4) is connected to the first port of the second fiber coupler (12); the second port of the second fiber coupler (12) outputs the signal light, and the third port is connected to the optical input end of the photodetector (13) to convert the optical signal into an electrical signal; the electrical signal output end of the photodetector (13) is connected to the input end of the microcomputer control module (14), and the output end of the microcomputer control module (14) is connected to the control end of the electrically controlled tunable wave plate (6); The electrically controlled tunable wave plate (6), the polarization beam splitter (7), and the optical intensity flux modulation module (8) are all spatial devices and are arranged in sequence; the signal light emitted from the broadband collimator (5) passes through the electrically controlled tunable wave plate (6), the polarization beam splitter (7), and the optical intensity flux modulation module (8) in sequence, and then is reflected by the optical intensity flux modulation module and returns along the original path; The signal light reflected from the broadband collimator (5) enters the first fiber coupler (4) and is divided into two beams of signal light, and returns to the first fiber coupler (4) through the clockwise and counterclockwise transmission optical fields. Among them, the counterclockwise transmission optical field passes through the first fiber coupler (4), the polarization-maintaining gain fiber (3), the fiber optic wavelength division multiplexer (2), the fiber optic acousto-optic deflector (10), and the fiber optic non-reciprocal phase biasing device (9) in sequence, and finally returns to the first fiber coupler (4); the transmission direction of the clockwise transmission optical field is opposite to that of the counterclockwise transmission optical field; The photodetector (13) converts the optical signal into an electrical signal, which is then input into the microcomputer control module (14) for processing; the microcomputer control module (14) compares the received electrical signal with a preset pulse repetition frequency set value, and then adjusts the optical polarization state of the electro-optically tunable waveplate (6) to change the reflected light intensity, thereby realizing the automatic control of the single-pulse operation of the laser.

2. The near-infrared wavelength-scanning pulsed fiber laser with a low mode-locking threshold according to claim 1, wherein The polarization-maintaining gain fiber (3) is a polarization-maintaining fiber doped with rare-earth ion ytterbium.

3. The near-infrared wavelength scanning pulsed fiber laser with a low mode-locking threshold according to claim 1, characterized in that, The optical intensity flux adjustment module (8) includes a variable-focus lens (801), a fixed-focus lens (802), and a saturable absorption mirror (803) arranged in sequence; During operation, the signal light collimated and output by the broadband collimator (5) sequentially passes through the electro-optically tunable waveplate (6), the polarization beam splitter (7), the variable-focus lens (801), the fixed-focus lens (802), and the saturable absorption mirror (803), and then is reflected back along the original path by the saturable absorption mirror (803), and returns to the nonlinear fiber amplifier loop mirror through the broadband collimator (5) and the first fiber coupler (4); The variable-focus lens (801) and the fixed-focus lens (802) are used to change the spot size incident on the saturable absorption mirror (803); The optical intensity flux is inversely proportional to the spot size. By changing the spot size, the change of the optical intensity flux is realized; and because the reflectivity of the saturable absorption mirror (803) is proportional to the size of the optical intensity flux, the effect of reducing the mode-locking threshold is realized.

4. The near-infrared ultrafast wavelength-scanning passively mode-locked pulsed fiber laser with a low mode-locking threshold according to claim 1, characterized in that: The first fiber coupler (4) is a fiber coupler with a splitting ratio of 40:60; During operation, the signal light with a clockwise-propagating optical field is input into the first fiber coupler (4) through the first port of the first fiber coupler (4) and is divided into two beams of signal light according to the splitting ratio. Among them, 40% of the signal light is output through the third port of the first fiber coupler (4) to the first port of the second fiber coupler (12), and 60% of the signal light is output through the fourth port of the first fiber coupler (4) to the broadband collimator (5); The signal light with a counterclockwise-propagating optical field is input into the first fiber coupler (4) through the second port of the first fiber coupler (4) and is divided into two beams of signal light according to the splitting ratio. Among them, 60% of the signal light is output through the third port of the first fiber coupler (4) to the first port of the second fiber coupler (12), and 40% of the signal light is output through the fourth port of the first fiber coupler (4) to the broadband collimator (5); the signal light reflected back by the broadband collimator is input into the first fiber coupler (4) through the fourth port of the first fiber coupler (4) and is divided into two beams of signal light according to the splitting ratio. Among them, 60% of the signal light is output through the first port of the first fiber coupler (4), and 40% of the signal light is output through the second port of the first fiber coupler (4).

5. The near-infrared ultrafast wavelength-scanning passively mode-locked pulsed fiber laser with a low mode-locking threshold according to claim 1, wherein: The second fiber coupler (12) is a fiber coupler with a splitting ratio of 10:90; During operation, the signal light output from the third port of the first optical fiber coupler (4) enters the second optical fiber coupler (12) through the first port of the second optical fiber coupler (12), and then is divided into two beams of signal light according to the splitting ratio. Among them, 90% of the signal light is output from the second port of the second optical fiber coupler (12), and 10% of the signal light is output from the third port of the second optical fiber coupler (12) to the photodetector (13).

6. The near-infrared ultrafast wavelength-scanning passively mode-locked pulsed fiber laser with a low mode-locking threshold according to claim 1, characterized in that: The radio frequency driving module (11) is a device or equipment that can provide different radio frequency signals; the radio frequency driving module (11) provides different radio frequency signals and different radio frequency signal intensities so that the fiber optic acousto-optic deflector (10) generates a wavelength tuning range of 1030 nm - 1100 nm.

7. The near-infrared ultrafast wavelength-scanning passively mode-locked pulsed fiber laser with a low mode-locking threshold according to claim 1, characterized in that: The photodetector (13) is a device or equipment that converts an optical signal into an electrical signal, including but not limited to a photomultiplier tube and a photodiode.

8. The near-infrared ultrafast wavelength-scanning passively mode-locked pulsed fiber laser with a low mode-locking threshold according to claim 1, wherein: The material of the saturable absorption mirror (803) is a material with a saturable absorption effect on the light intensity, including semiconductors, graphene, and carbon nanotubes.

9. The near-infrared ultrafast wavelength-scanning passively mode-locked pulsed fiber laser with a low mode-locking threshold according to claim 1, wherein: The fiber optic non-reciprocal phase biasing device (9) is used to complete the phase shift difference between the clockwise and counterclockwise propagating optical fields in the nonlinear fiber amplifying loop mirror, and the introduced phase shift difference does not exceed π / 2.

10. The near-infrared ultrafast wavelength-scanning passively mode-locked pulsed fiber laser with a low mode-locking threshold according to claim 1, characterized in that: The microcomputer control module (14) is a control system based on FPGA; During operation, the microcomputer control module (14) is used to identify the repetition frequency of the electrical signal input by the photodetector (13), and then compare it with the preset pulse repetition frequency; if they are inconsistent, the electro-optic tunable wave plate (6) is controlled to adjust the light intensity until the repetition frequency of the electrical signal is consistent with the preset pulse repetition frequency, so as to realize the automatic control of the single-pulse operation of the laser.

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