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

Abstract

The application discloses a near-infrared wavelength scanning pulse fiber laser with a low mode-locked threshold. The application adopts a nonlinear amplification ring mirror and a saturable absorption mirror framework to realize a low mode-locked threshold and ensure long-term stability of mode locking; after self-starting of mode locking, an automatic control module is used to automatically realize single pulse operation by adjusting the intensity of the back reflection pulse; in the single pulse operation stage, a radio frequency module is used to provide radio frequency signals with different frequencies and different signal intensities to an acousto-optic tunable filter to realize a 1030nm-1100nm wavelength scanning and tuning range.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of laser, and particularly relates to a near-infrared wavelength scanning pulse fiber laser with a low mode-locking threshold. BACKGROUND

[0002] Ultra-wideband wavelength scanning and tuning ultrafast laser source plays an important role in many fields of science and engineering, such as multicolor multimodal fluorescence microscopy, coherent Raman microscopy, etc. Generally, in order to realize the output of ultra-wideband wavelength scanning and tuning laser, the following methods can be used: 1) in-cavity tunable filtering; 2) nonlinear frequency conversion, such as optical parametric oscillator, optical parametric amplifier and advanced nonlinear spectral broadening technology. Among them, the in-cavity filtering method is the most direct technology, which can ensure the stability of the output power and provide high signal-to-noise ratio pulse quality.

[0003] In recent years, with the rapid development of mode-locked fiber lasers, a new choice is provided for realizing the ultra-wideband wavelength scanning and tuning laser. In the field of life science, thanks to the discovery of various fluorescent probes, researchers can realize multicolor multimodal microscopy by combining multiple fluorescent probes (Nat Methods 9, 815 (2012)), so as to obtain more comprehensive biological information at the same time, which also puts forward new requirements for the excitation light source with ultra-wideband wavelength scanning and tuning range. At present, the laser source widely used is based on an optical parametric oscillator, among which a sapphire solid laser is the main representative, but it has the disadvantages of weak environmental interference resistance and high price. However, the ultra-wideband wavelength scanning and tuning light source using in-cavity ultra-high speed wavelength scanning and mode-locking technology usually has a high mode-locking threshold due to the large in-cavity loss, which is easy to damage the device. Therefore, it is very important to obtain a high-performance ultra-wideband wavelength scanning and tuning ultrafast laser source with a low mode-locking threshold. SUMMARY

[0004] In order to solve the above problems, the application provides a near-infrared wavelength scanning pulse fiber laser with a low mode-locking threshold.

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

[0006] The application provides a near-infrared wavelength scanning pulse fiber laser with a low mode-locking threshold, which comprises a pump source, a fiber type wavelength division multiplexer, a polarization maintaining gain fiber, a first fiber coupler, a wideband collimator, an electrically controlled adjustable wave plate, a polarization beam splitter, an optical intensity flux modulation module, a fiber type non-reciprocal phase biasing device, a fiber type acousto-optic deflector, a radio frequency driving module, a second fiber coupler, a photodetector and a microcomputer control module.

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

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

[0009] The electrically controlled adjustable wave plate, the polarization beam splitter and the light intensity flux modulation module are all spatial type devices, which are sequentially arranged; the signal light emitted from the broadband collimator sequentially passes through the electrically controlled adjustable wave plate, the polarization beam splitter and the light intensity flux modulation module, and is then reflected by the light intensity flux modulation module to return along the original path;

[0010] The signal light reflected from the broadband collimator is divided into two beams of signal light after entering the first optical fiber coupler, and returns to the first optical fiber coupler through the clockwise and counterclockwise transmission light fields; the counterclockwise transmission light field sequentially passes through the first optical fiber coupler, the polarization maintaining gain optical fiber, the fiber type wavelength division multiplexer, the fiber type acousto-optic deflector and the fiber type non-reciprocal phase biasing device, and finally returns to the first optical fiber coupler; the transmission direction of the clockwise transmission light field is opposite to that of the counterclockwise transmission light field.

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

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

[0013] Further, the light intensity flux adjusting module comprises a variable focus lens, a fixed focus lens and a saturable absorption mirror arranged in sequence.

[0014] In operation, the signal light collimated by the broadband collimator passes through the electrically controlled adjustable wave plate, the polarization beam splitter, the variable focus lens, the fixed focus lens and the saturable absorption mirror in sequence, and is then reflected by the saturable absorption mirror back to the broadband collimator and the first fiber coupler, and then back to the nonlinear fiber amplifier ring.

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

[0016] The light intensity flux is inversely proportional to the size of the light spot, and changing the size of the light spot changes the light intensity flux. Since the reflectivity of the saturable absorption mirror is proportional to the size of the light intensity flux, the effect of reducing the mode-lock threshold is achieved.

[0017] Further, the first fiber coupler is a fiber coupler with a splitting ratio of 40:60.

[0018] In operation, the signal light transmitted clockwise passes through the first port of the first fiber coupler and is then split into two beams according to the splitting ratio, wherein 40% of the signal light is output from the third port of the first fiber coupler to the first port of the second fiber coupler, and 60% of the signal light is output from the fourth port of the first fiber coupler to the broadband collimator.

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

[0020] Further, the second fiber coupler is a fiber coupler with a splitting ratio of 10:90.

[0021] In operation, the signal light output from the third port of the first fiber coupler passes through the first port of the second fiber coupler and is then split into two beams according to the splitting ratio, wherein 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] Further, the radio frequency driving module is a device or equipment that can provide radio frequency signals; the radio frequency driving module provides different radio frequency signals and different radio frequency signal strengths to make the optical fiber type acousto-optic deflector generate a 1030nm-1100nm wavelength tuning range.

[0023] Further, the photoelectric detector is a device or equipment that converts optical signals into electrical signals, including but not limited to a photomultiplier tube and a photodiode.

[0024] Further, the saturable absorption mirror is composed of a material that has a saturable absorption effect on light intensity, including a semiconductor, graphene, and a carbon nanotube.

[0025] Further, the optical fiber type non-reciprocal phase biasing device is used to complete the phase shift difference between the clockwise direction and the counterclockwise direction transmission light field in the nonlinear fiber amplification ring mirror, and the introduced phase shift difference is not more than π / 2.

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

[0027] When working, the microcomputer control module is used to identify the repetition frequency of the electrical signal input by the photoelectric detector, and then compare it with the preset pulse repetition frequency; if they are inconsistent, the electrical controllable adjustable wave plate 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.

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

[0029] The present application utilizes the nonlinear fiber amplification ring mirror and the saturable absorption mirror, realizes the pulse self-starting under the low pump threshold by changing the light intensity flux, and simultaneously realizes the automatic control of the mode-locked state by using the microcomputer controller; meanwhile, the 1030nm-1100nm wavelength scanning and tuning laser output are realized by using the acousto-optic tunable filter. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 It is a structure schematic diagram of a near-infrared wavelength scanning pulse fiber laser with a low mode-locked threshold in an embodiment of the present application.

[0031] Figure 2 It is a 1030nm-1100nm wavelength tuning result graph in an embodiment of the present application.

[0032] Figure 3 It is a schematic diagram of the relationship between the spot radius and the zoom lens in an embodiment of the present application. DETAILED DESCRIPTION

[0033] 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.

[0034] Example:

[0035] A near-infrared wavelength swept pulsed fiber laser with a low mode-locking threshold, such as Figure 1 As shown, it includes 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 non-reciprocal 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;

[0036] 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, and the polarization-maintaining gain fiber 3 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 RF input end of the fiber-type acousto-optic deflector 10 is connected to the output end of the RF driving module 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;

[0037] 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;

[0038] 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.

[0039] The signal light reflected from the broadband collimator 5 enters the first fiber coupler 4 and is divided into two beams of signal light, which are returned to the first fiber coupler 4 through clockwise and counterclockwise transmission light fields, wherein 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, the fiber type non-reciprocal phase biasing device 9, and finally returns 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;

[0040] The photoelectric detector 13 converts the optical signal into an electrical signal, which is then input to the microcomputer control module 14 for processing; the microcomputer control module 14 compares the received electrical signal with the preset pulse repetition frequency setting value, and then adjusts the light polarization state of the electrically controlled adjustable wave plate 6 to change the reflected light intensity, thereby realizing automatic regulation and control of the single-pulse operation of the laser.

[0041] In one embodiment, the polarization maintaining gain fiber 3 is a rare earth ion ytterbium doped polarization maintaining fiber.

[0042] In one embodiment, the light intensity flux modulation module 8 includes a variable focus lens 801, a fixed focus lens 802, and a saturable absorption mirror 803 arranged in sequence.

[0043] In operation, the signal light collimated and output by the broadband collimator 5 sequentially 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, and is then reflected back by the saturable absorption mirror 803 against the original path, and returns to the nonlinear fiber amplification ring mirror through the broadband collimator 5 and the first fiber coupler 4;

[0044] The variable focus lens 801 and the fixed focus lens 802 are used to change the spot size of the light incident on the saturable absorption mirror 803.

[0045] The light intensity flux size is inversely proportional to the spot size, and changing the spot size changes the light intensity flux; because the reflectivity of the saturable absorption mirror 803 is proportional to the size of the light intensity flux, the effect of reducing the mode-locking threshold is achieved.

[0046] In one embodiment, the first fiber coupler 4 is a fiber coupler with a splitting ratio of 40:60.

[0047] In 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 is divided 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.

[0048] 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.

[0049] In one embodiment, the second fiber coupler 12 is a fiber coupler with a splitting ratio of 10:90;

[0050] 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.

[0051] In one embodiment, Figure 2 As shown, the RF driving module 11 is a device or equipment that can provide different RF signals; the RF driving 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 1030nm-1100nm.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] Furthermore, the microcomputer control module 14 is a control system based on FPGA.

[0056] In operation, the microcomputer control module 14 is used to identify the frequency of the electrical signal input by the photodetector 13, and then compare it with the preset pulse repetition frequency. If they are not consistent, the electrically controllable adjustable 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, thereby achieving automatic control of the single-pulse operation of the laser.

[0057] In one embodiment, the fiber coupler 12 splits the laser light and outputs it from the 10% output end, which 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. The microcomputer control module 14 processes the signal and generates a control signal that is input to the electrically controllable adjustable wave plate 6 to adjust the twist of the wave plate. The change in the angle of the wave plate affects the intensity of the back-reflected light, which serves to adjust 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.

[0058] In one embodiment, as shown in FIG. 8, the light intensity flux modulation works as follows: Figure 3

[0059] The size of the light intensity flux is inversely proportional to the size of the spot area. The light intensity flux modulation module 8 is composed of a zoom lens 801, a fixed focus lens 802, and a saturable absorption mirror. 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 spot is changed, thereby changing the size of the spot area and affecting the light intensity flux value incident on the semiconductor saturable mirror.​

Claims

1. A near-infrared wavelength-swept pulsed fiber laser with a low mode-locked threshold, characterized in that, The application relates to a nonlinear fiber amplifier ring mirror, which 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 non-reciprocal phase biasing device (9), a fiber type acousto-optic deflector (10), a radio frequency driving module (11), a second fiber coupler (12), a photoelectric detector (13) and a microcomputer control module (14). The output end of the pump source (1) is connected with 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 with one end of the polarization maintaining gain fiber (3), and the polarization maintaining gain fiber (3) is responsible for generating and amplifying signal light; the other end of the polarization maintaining gain fiber (3) is connected with the first port of the first fiber coupler (4); the signal light is divided into two directions to form clockwise transmission light field and counterclockwise transmission light field; the signal end of the fiber type wavelength division multiplexer (2) is connected with one end of the fiber type acousto-optic deflector (10); the other end of the fiber type acousto-optic deflector (10) is connected with one end of the fiber type non-reciprocal phase biasing device (9); in addition, the radio frequency input end of the fiber type acousto-optic deflector (10) is connected with the output end of the radio frequency driving module (11); the other end of the fiber type non-reciprocal phase biasing device (9) is connected with the second port of the first fiber coupler (4); the fourth port of the first fiber coupler (4) is connected with the broadband collimator (5); the above devices are connected to form the nonlinear fiber amplifier ring mirror. The third port of the first fiber coupler (4) is connected with the first port of the second fiber coupler (12); the second port of the second fiber coupler (12) outputs signal light, and the third port is connected with the light input end of the photoelectric detector (13) to convert the optical signal into an electrical signal; the electrical signal output end of the photoelectric detector (13) is connected with the input end of the microcomputer control module (14), and the output end of the microcomputer control module (14) is connected with the control end of the electrically controlled adjustable wave plate (6). The electrically controlled adjustable wave plate (6), the polarization beam splitter (7) and the light intensity flux modulation module (8) are all spatial type devices and are sequentially arranged; the signal light emitted from the broadband collimator (5) sequentially passes through the electrically controlled adjustable wave plate (6), the polarization beam splitter (7) and the light intensity flux modulation module (8), and is then reflected by the light intensity flux modulation module to return along the original path; The signal light reflected from the broadband collimator (5) is divided into two beams after entering the first fiber coupler (4) and returns to the first fiber coupler (4) through the clockwise and counterclockwise transmission light field; 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 biasing device (9) and finally returns 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. The photoelectric detector (13) converts the optical signal into an electrical signal, which is then input to the microcomputer control module (14) for processing; the microcomputer control module (14) compares the received electrical signal with the preset pulse repetition frequency setting value, and then adjusts the light 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.

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

3. The near-infrared wavelength-swept pulsed fiber laser with low mode-lock threshold according to claim 1, characterized in that, The light intensity flux modulation module (8) comprises a variable focus lens (801), a fixed focus lens (802) and a saturable absorption mirror (803) arranged in sequence. During operation, the signal light output 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 by the saturable absorption mirror (803) back to the broadband collimator (5) and the first fiber coupler (4) to return to the nonlinear fiber amplification ring mirror. 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 mirror (803). The light intensity flux is inversely proportional to the size of the light spot, and changing the size of the light spot changes the light intensity flux; because the reflectivity of the saturable absorption mirror (803) is proportional to the size of the light intensity flux, the effect of reducing the mode-locking threshold is achieved.

4. The near-infrared wavelength-swept pulsed fiber laser with low mode-lock 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 transmitted clockwise through the first port of the first fiber coupler (4) is input into the first fiber coupler (4) and then split into two beams according to the splitting ratio, of which 40% is output through the third port of the first fiber coupler (4) to the first port of the second fiber coupler (12), and 60% is output through the fourth port of the first fiber coupler (4) to the broadband collimator (5). The signal light transmitted counterclockwise through the second port of the first fiber coupler (4) is input into the first fiber coupler (4) and then split into two beams according to the splitting ratio, of which 60% is output through the third port of the first fiber coupler (4) to the first port of the second fiber coupler (12), and 40% is output through the fourth port of the first fiber coupler (4) to the broadband collimator (5); the signal light reflected 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 according to the splitting ratio, of which 60% is output through the first port of the first fiber coupler (4), and 40% is output through the second port of the first fiber coupler (4).

5. The near-infrared wavelength swept pulse fiber laser with low mode-lock threshold according to claim 1, characterized in that: The second fiber coupler (12) is a fiber coupler with a splitting ratio of 10:

90. In 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 is divided 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).

6. The near-infrared wavelength swept pulsed fiber laser with low mode-lock threshold according to claim 1, characterized in that: The radio frequency driving module (11) is a device or equipment capable of providing different radio frequency signals; the radio frequency driving module (11) provides different radio frequency signals and different radio frequency signal strengths so that the fiber type acousto-optic deflector (10) generates a wavelength tuning range of 1030nm-1100nm.

7. The near-infrared wavelength-swept pulsed fiber laser with low mode-lock threshold according to claim 1, characterized in that: The photodetector (13) is a device or equipment for converting optical signals into electrical signals, including a photomultiplier tube and a photodiode.

8. The near-infrared wavelength-swept pulsed fiber laser with low mode-lock threshold according to claim 3, characterized in that: The saturable absorption mirror (803) is made of a material having a saturable absorption effect on light intensity, including a semiconductor, graphene and a carbon nanotube.

9. The near-infrared wavelength-swept pulsed fiber laser with low mode-lock threshold according to claim 1, characterized in that: The fiber type non-reciprocal phase biasing device (9) is used to complete the phase shift difference between the clockwise and counterclockwise transmission light fields in the nonlinear fiber amplification ring mirror, and the introduced phase shift difference is not more than π / 2.

10. The near-infrared wavelength swept pulsed fiber laser with low mode-lock threshold according to claim 1, characterized in that: The microcomputer control module (14) is a control system based on FPGA. In operation, the microcomputer control module (14) is used to identify the repetition frequency of the electrical signal inputted by the photodetector (13), and then compare it with the preset pulse repetition frequency; if they are inconsistent, the electrical controllable adjustable 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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