A novel mode-locked soliton fiber laser system based on a spatial alignment structure

Abstract

The present invention discloses a novel mode-locked soliton fiber laser system based on a spatial alignment structure, comprising: a pump source outputs pump light and is connected to a wavelength division multiplexer, and the wavelength division multiplexer couples the pump light into a ring cavity; the wavelength division multiplexer is connected to an erbium-doped single-mode fiber, the erbium-doped single-mode fiber is connected to a polarization-independent isolator through a first single-mode fiber, and a first fiber collimator is connected to the output end of the polarization-independent isolator through a second single-mode fiber; the first fiber collimator outputs collimated laser light, and a second fiber collimator couples the laser light output by the first fiber collimator back into the ring cavity to form a spatial alignment structure; the second fiber collimator, a polarization controller and a coupler are connected through a third single-mode fiber, and the coupler and the wavelength division multiplexer are connected through a fourth single-mode fiber to form a closed optical path, and the coupler forms the output end of the system. The structure of the present invention is simple and compact, with high stability, low cost, easy to self-start mode locking, good repeatability, and little interference from the external environment.

Description

Technical Field

[0001] The present invention belongs to the field of ultrafast laser technology, and particularly relates to a novel mode-locked soliton fiber laser system based on a spatial alignment structure. Background Art

[0002] Compared with traditional ultrafast solid laser systems, ultrafast fiber laser systems have obvious advantages such as compact structure, good beam quality, low cost, and easy thermal management, and have been widely used in the fields of fiber communication, biomedicine, optical imaging, frequency metrology, and laser processing. In order to meet the growing application requirements nowadays, it is necessary to further improve the stability and output parameters of ultrafast fiber systems.

[0003] For the optimization of ultrafast fiber systems, it is necessary to consider from two aspects: the type of output pulses of the system and the method of generating pulses. In terms of pulse type, according to the different amounts of net dispersion in the laser system cavity, it can be divided into traditional solitons generated in the negative dispersion region, dispersion management and self-similar pulses generated in the near-zero dispersion region, dissipative solitons and dissipative soliton resonance pulses generated in the positive dispersion region, etc. Among them, in the large net negative dispersion region, traditional soliton pulses generated by the dynamic balance of intracavity saturable absorption, second-order dispersion, and nonlinear effects can maintain a constant pulse width and spectral width during propagation. Compared with other types of pulses, it has natural advantages in terms of system stability. In terms of pulse generation method, compared with active mode locking, passive mode locking can achieve self-starting pulse output without additional modulation devices. This method greatly reduces the cost and complexity of the system and is the mainstream technology in the current research of ultrafast fiber systems. Passive mode locking technology relies on a saturable absorber (SA) to generate pulses, and the performance of the SA directly determines the output parameters of the system. In recent years, extensive research has been carried out on the work of building ultrafast fiber systems based on real saturable absorbers such as semiconductor saturable absorber mirrors (SESAM), low-dimensional materials, etc., or based on saturable absorbers such as nonlinear polarization evolution (NPE), nonlinear loop mirror (NLM), Mamyshev, multimode interference effects, etc.

[0004] However, defects such as low damage threshold and poor reproducibility of two-dimensional materials in SESAM limit the improvement of the system output power and stability. At the same time, the wavelength range generated by the laser system is also limited by the material bandgap. Although the equivalent saturable absorber can overcome the limitations of material damage threshold and bandgap to achieve broadband saturable absorption, problems such as poor environmental stability in NPE, difficulty in self-starting of NLM, complex Mamyshev structure, and large fusion loss between multimode fiber and single-mode fiber still exist. These defects severely limit the application of ultrafast laser systems in some complex environments. Summary of the Invention

[0005] To solve the above problems existing in the prior art, the present invention provides a novel mode-locked soliton fiber laser system based on a spatial alignment structure. The technical problems to be solved by the present invention are realized through the following technical solutions:

[0006] An embodiment of the present invention provides a novel mode-locked soliton fiber laser system based on a spatial alignment structure, including: a pump source, a wavelength division multiplexer, an erbium-doped single-mode fiber, a first single-mode fiber, a polarization-independent isolator, a second single-mode fiber, a first fiber collimator, a second fiber collimator, a polarization controller, a third single-mode fiber, a coupler, and a fourth single-mode fiber; wherein,

[0007] The pump light is output from the pump source and connected to the wavelength division multiplexer through a pigtail fiber, and the wavelength division multiplexer couples the pump light into the ring cavity;

[0008] The output end of the wavelength division multiplexer is connected to the erbium-doped single-mode fiber through a passive fiber, the erbium-doped single-mode fiber is connected to the input end of the polarization-independent isolator through the first single-mode fiber, and the first fiber collimator is connected to the output end of the polarization-independent isolator through the second single-mode fiber;

[0009] The first fiber collimator outputs collimated laser light, and the second fiber collimator couples the collimated laser light output by the first fiber collimator back into the ring cavity to form a spatial alignment structure, which generates a saturable absorption effect, and the saturation intensity and unsaturated loss are adjusted by appropriately adjusting the alignment conditions;

[0010] The second fiber collimator, the polarization controller, and the coupler are connected through the third single-mode fiber, and the coupler and the wavelength division multiplexer are connected through the fourth single-mode fiber to form a closed optical path, and the coupler forms the output end of the system.

[0011] In an embodiment of the present invention, the maximum output power of the pump source is 1.5W.

[0012] In one embodiment of the present invention, the erbium-doped single-mode fiber has a peak absorption of 110 dB / m at 1530 nm.

[0013] In one embodiment of the present invention, the output apertures of the first fiber collimator and the second fiber collimator are both 2.0 mm.

[0014] In one embodiment of the present invention, the distance between the first fiber collimator and the second fiber collimator is 5 cm to 20 cm.

[0015] In one embodiment of the present invention, the coupling efficiency between the first fiber collimator and the second fiber collimator is 70%.

[0016] In one embodiment of the present invention, the splitting ratio of the coupler is 20:80; wherein, 20% of the output end of the coupler outputs laser, and 80% of the output end of the coupler is connected to the wavelength division multiplexer.

[0017] In one embodiment of the present invention, the models of the first single-mode fiber, the second single-mode fiber, the third single-mode fiber, and the fourth single-mode fiber are all SMF-28e.

[0018] In one embodiment of the present invention, high-order soliton pulses with a maximum single-pulse energy of 4.73 nJ are output.

[0019] Advantages of the present invention:

[0020] The novel mode-locked soliton fiber laser system based on a spatial alignment structure proposed by the present invention innovatively presents a structure of a mode-locked soliton fiber laser system. Specifically, it includes a pump source, a wavelength division multiplexer, an erbium-doped single-mode fiber, a first single-mode fiber, a polarization-independent isolator, a second single-mode fiber, a first fiber collimator, a second fiber collimator, a polarization controller, a third single-mode fiber, a coupler, and a fourth single-mode fiber. Among them, the pump source outputs pump light and is connected to the wavelength division multiplexer through a pigtail. The wavelength division multiplexer couples the pump light into the ring cavity. The output end of the wavelength division multiplexer is connected to the erbium-doped single-mode fiber through a passive fiber. The erbium-doped single-mode fiber is connected to the input end of the polarization-independent isolator through the first single-mode fiber. The first fiber collimator is connected to the output end of the polarization-independent isolator through the second single-mode fiber. The first fiber collimator outputs collimated laser light, and the second fiber collimator couples the collimated laser light output by the first fiber collimator back into the ring cavity to form a spatial alignment structure. This spatial alignment structure generates a saturable absorption effect, and by appropriately adjusting the alignment conditions, the adjustment of the saturation intensity and unsaturated loss can be realized. The second fiber collimator, the polarization controller, and the coupler are connected through the third single-mode fiber. The coupler and the wavelength division multiplexer are connected through the fourth single-mode fiber to form a closed optical path, and the coupler forms the output end of the system. It can be seen that the structure of the present invention is reasonable. Without introducing any material saturable absorber, an equivalent saturable absorption effect can be generated only by changing the alignment conditions between the first fiber collimator and the second fiber collimator. Compared with the traditional soliton fiber laser system based on a real saturable absorber and the traditional equivalent saturable absorption technology generated by nonlinear effects, the saturable absorption effect generated by the spatial alignment structure innovatively proposed by the present invention is not limited by the material damage threshold and bandgap, nor by the environmental stability, and has a relatively high single-pulse energy. Generally speaking, the mode-locked soliton fiber laser system proposed by the present invention has obvious advantages such as simple and compact structure, high stability, low cost, easy self-starting mode locking, good repeatability, and little interference from the external environment. As a laser light source, it can be used in fields such as optical fiber communication, microfabrication, frequency measurement, biomedicine, and nonlinear optical research.

[0021] The following will further elaborate on the present invention in conjunction with the drawings and embodiments. Brief Description of the Drawings

[0022] Figure 1 is a schematic diagram of the structure of the novel mode-locked soliton fiber laser system based on a spatial alignment structure provided by an embodiment of the present invention;

[0023] Figure 2 is a schematic diagram of the output spectrum under different pump powers provided by an embodiment of the present invention;

[0024] Figure 3 is a schematic diagram of the mode-locked pulse sequence under a pump power of 1.5W provided by an embodiment of the present invention;

[0025] Figure 4 It is a schematic diagram of the 6-hour power stability provided by the embodiments of the present invention.

[0026] Explanation of reference numerals:

[0027] 1 - Pump source; 2 - Wavelength division multiplexer; 3 - Erbium-doped single-mode fiber; 4 - First single-mode fiber; 5 - Polarization-independent isolator; 6 - Second single-mode fiber; 7 - First fiber collimator; 8 - Second fiber collimator; 9 - Polarization controller; 10 - Third single-mode fiber; 11 - Coupler; 12 - Fourth single-mode fiber. Detailed implementation manners

[0028] The following further describes the present invention in detail with reference to specific embodiments, but the implementation manners of the present invention are not limited thereto.

[0029] In order to implement a fiber laser system with a simple structure, not limited by the material damage threshold and bandgap, and not limited by the environment, please refer to Figure 1 , the embodiments of the present invention provide a novel mode-locked soliton fiber laser system based on a spatial alignment structure, including: a pump source 1, a wavelength division multiplexer 2, an erbium-doped single-mode fiber 3, a first single-mode fiber 4, a polarization-independent isolator 5, a second single-mode fiber 6, a first fiber collimator 7, a second fiber collimator 8, a polarization controller 9, a third single-mode fiber 10, a coupler 11, and a fourth single-mode fiber 12; wherein,

[0030] The pump source 1 outputs pump light and is connected to the wavelength division multiplexer 2 through a pigtail fiber. The wavelength division multiplexer 2 couples the pump light into the ring cavity; the output end of the wavelength division multiplexer 2 is connected to the erbium-doped single-mode fiber 3 through a passive fiber, and the erbium-doped single-mode fiber 3 is connected to the input end of the polarization-independent isolator 5 through the first single-mode fiber 4. The first fiber collimator 7 is connected to the output end of the polarization-independent isolator 5 through the second single-mode fiber 6; the first fiber collimator 7 outputs collimated laser light, and the second fiber collimator 8 couples the collimated laser light output by the first fiber collimator 7 back into the ring cavity to form a spatial alignment structure. This spatial alignment structure generates a saturable absorption effect, and the saturation intensity and unsaturated loss are adjusted by appropriately adjusting the alignment conditions; the second fiber collimator 8, the polarization controller 9, and the coupler 11 are connected through the third single-mode fiber 10, and the coupler 11 and the wavelength division multiplexer 2 are connected through the fourth single-mode fiber 12 to form a closed optical path, and the coupler 11 forms the output end of the system.

[0031] Preferably, the pump source 1 is a single-mode or multi-mode laser diode (abbreviated as LD); the maximum output power of the pump source 1 is 1.5 W; the pump source 1 outputs 976-nm pump light.

[0032] Preferably, the peak absorption of the erbium-doped single-mode fiber 3 at 1530 nm is 110 dB / m.

[0033] Preferably, the output apertures of both the first fiber collimator 7 and the second fiber collimator 8 are 2.0 mm.

[0034] Preferably, the distance between the first fiber collimator 7 and the second fiber collimator 8 is 5 cm to 20 cm.

[0035] Preferably, the coupling efficiency between the first fiber collimator 7 and the second fiber collimator 8 is 70%.

[0036] Preferably, the splitting ratio of the coupler 11 is 20:80; among them, 20% of the output end of the coupler 11 outputs laser, and 80% of the output end of the coupler 11 is connected to the wavelength division multiplexer 2.

[0037] Preferably, the models of the first single-mode fiber 4, the second single-mode fiber 6, the third single-mode fiber 10, and the fourth single-mode fiber 12 are all SMF-28e.

[0038] Preferably, high-order soliton pulses with a maximum single-pulse energy of 4.73 nJ are output.

[0039] Through research by the inventor, the fiber laser system proposed in the present invention can achieve a saturable absorption effect through a spatial alignment structure. The principle is due to the dependence of the peak power of the coupling efficiency of the second fiber collimator 8 receiving laser. Since the laser spot output by the first fiber collimator 7 diverges slightly as the propagation distance increases. At this time, the core of the second fiber collimator 8 is equivalent to a hard aperture, which will introduce additional coupling losses. As the input laser peak power rises, the peak power density at the core of the second fiber collimator 8 also continuously rises. When the input peak power reaches a certain threshold, a self-focusing effect caused by the change of the nonlinear refractive index will occur, thereby reducing the coupling loss caused by the spot divergence and improving the coupling efficiency. As the input power is further increased, the rising speed of the coupling efficiency gradually levels off and finally reaches saturation. Thus, an equivalent saturable absorption effect with high coupling efficiency for high-power pulses and low coupling efficiency for low-power pulses is achieved. And this effect is only related to the optical power received by the second fiber collimator 8. Therefore, by changing the alignment conditions between the two collimators and reducing the power density reaching the second collimator 8, the modulation depth and saturation power of this equivalent saturable absorption effect can also be changed.

[0040] In order to verify the effectiveness of the novel mode-locked soliton fiber laser system based on the spatial alignment structure provided by the embodiments of the present invention, the following experiments are carried out for verification.

[0041] The pump light output by the LD pump source 1 with a central wavelength of 976 nm and a maximum average power of 1.5 W is connected to the input end of the wavelength division multiplexer 2 through a single-mode HI-1060 optical fiber as a pigtail. The wavelength division multiplexer 2 couples the 976 nm pump light into the ring cavity. The wavelength division multiplexer 2 is connected to the erbium-doped single-mode fiber 3 through a section of SMF-28e passive optical fiber. The core layer and cladding diameter of the erbium-doped single-mode fiber 3 are 4 / 125 μm, and the peak absorption coefficient of the core at 1530 nm is 110 dB / m. If the length of the erbium-doped single-mode fiber 3 is too short, the energy of the pump light cannot be fully absorbed, resulting in a low output power and the presence of residual pump light in the output light. If the length of the erbium-doped single-mode fiber 3 is too long, the generated laser will be reabsorbed by itself, and the output power will also decrease. The erbium-doped single-mode fiber 3 is connected to the polarization-independent isolator 5 through a section of SMF-28e first single-mode fiber 4. The isolation degree of the polarization-independent isolator 5 is 60 dB, and the maximum average power it can withstand is 3 W, ensuring that the optical signal runs unidirectionally in the ring cavity.

[0042] The polarization-independent isolator 5 is connected to the first fiber collimator 7 through a section of SMF-28e second single-mode fiber 6. The output aperture of the first fiber collimator 7 is 2.0 mm, the working distance is 20 cm, and the maximum power it can withstand is 2 W. The collimator lens is coated with an antireflection film of 1.5 μm, which not only ensures good output beam quality but also prevents the F-P cavity effect caused by Fresnel reflection at the output end face. The first fiber collimator 7 is horizontally placed to output collimated laser with a diameter of 2 mm, and its beam waist is located 10 cm behind the collimator. The output aperture of the second fiber collimator 8 is 2.0 mm, the working distance is 20 cm, and the maximum power it can withstand is 2 W. The second fiber collimator 8 is placed 20 cm behind the first fiber collimator 7 and has the same height as the first fiber collimator 7, which is 45 cm. The second fiber collimator 8 is used to couple and receive the spatial light output by the first fiber collimator 7, and the coupling efficiency of the spatial structure is adjusted by changing the angle between the second fiber collimator 8 and the first fiber collimator 7. By changing the alignment situation, an equivalent saturable absorption effect with a high transmittance for the high-power part and a low transmittance for the low-power part when the pulse passes through can be achieved. This makes the transmittance of the central part of the strongest pulse in the cavity the highest, and the transmittance of the pulse wings and other stray pulses lower, realizing the phase locking between oscillation frequencies and finally achieving mode locking.

[0043] The second fiber collimator 8, polarization controller 9, and coupler 11 are connected by a section of SMF-28e third single-mode fiber 10. The polarization controller 9 can adjust the polarization state in the cavity and can also play a role in fine-tuning the cavity length. The splitting ratio of the coupler 11 is 20:80. The 20% port of the coupler 11 serves as the output end of the system, and the 80% output end of the coupler 11 is connected to one end of the wavelength division multiplexer 2 through a section of SMF-28e fourth single-mode fiber to form a closed loop. The lengths of the first single-mode fiber 4, the second single-mode fiber 6, and the third single-mode fiber 10 are selected because when using a fiber fusion splicer to splice fiber connection devices, if the fiber length is too short, it is not easy to realize the fusion splicing process. At least a length of more than 1 m is left to reserve enough margin when different devices need to be refusion spliced during the subsequent system optimization process. The fourth single-mode fiber 12 is not only for reserving enough margin when different devices need to be refusion spliced during the subsequent system optimization process. Moreover, because the group velocity dispersion of the single-mode fiber SMF-28e at 1.5 μm is negative, traditional solitons are usually generated in a large intracavity net negative dispersion region. Selecting a sufficient length of single-mode fiber to provide a dispersion delay line provides a large net negative dispersion environment, which is conducive to the generation of traditional solitons.

[0044] In addition, the fundamental solitons generated at a shorter cavity length are restricted by the soliton area theorem, and the output single-pulse energy is usually lower than 0.1 nJ. The fourth single-mode fiber 12 enables the total cavity length to be much larger than the soliton period, which can support the generation of higher-order solitons to further improve the output parameters, and the system finally obtains a soliton output of up to 4.73 nJ. This oscillator is a fully negative-dispersion mode-locked fiber laser system. In a fully negative-dispersion fiber laser system, the key role is played by the soliton formation mechanism. Through the dynamic balance among the negative dispersion effect, the saturable absorption effect, and the nonlinear effect, mode-locked pulses with almost zero chirp are generated.

[0045] In the embodiment of the present invention, the polarization controller 9 is adjusted to an appropriate position, and the state of the laser in the ring cavity is controlled through the polarization controller 9. By changing the pitching angle of the second fiber collimator 8, the coupling efficiency of the spatial structure is adjusted to an appropriate position. Stable mode locking can be generated when the pump power is at least 800 mW, and the mode-locking central wavelength is 1560.9 nm.

[0046] Figure 2 Schematically shows the output spectrum under different pump powers. The different color lines in the figure only schematically distinguish the output spectra at different wavelengths and do not contain information on the technical solution. Figure 2 In it, the abscissa represents the wavelength (Wavelength / nm), and the ordinate represents the spectral intensity (Intensity / dB). From Figure 2It can be seen that the mode locking remains stable when gradually increasing the pump power to 1000, 1100, 1200, 1300, 1400, and 1500 mW. After the pump power exceeds 1300 mW, obvious Kelly sidebands generated by the dispersion wave resonance appear in the spectrum, proving the generation of traditional soliton pulses. By linearly adjusting the alignment condition between the collimators, a slight tuning of the center wavelength of the soliton spectrum towards the short-wavelength direction can be achieved, with a range from 1560.9 to 1559.0 nm.

[0047] Figure 3 The mode-locked pulse train at a pump power of 1.5 W is shown. Figure 3 In which, the abscissa represents time (Time / μs), and the ordinate represents the spectral intensity (Intensity / dB). It can be seen from Figure 3 that when the pump power is fixed at 1.5 W, the interval between stable mode-locked pulse trains is 260 ns, which coincides with the cavity length of the laser system.

[0048] Figure 4 The output power situation of the laser system after 6 hours is shown. Figure 4 In which, the abscissa represents time (Time / h), and the ordinate represents the output power (Output Power / mW). It can be seen from Figure 4 that when the average output power of the system is 17 mW, the root mean square (rms) value of the power within 6 hours is 0.23%.

[0049] In summary, the novel mode-locked soliton fiber laser system based on the spatial alignment structure proposed in the embodiments of the present invention innovatively proposes a structure of a mode-locked soliton fiber laser system. Specifically: a pump source 1, a wavelength division multiplexer 2, an erbium-doped single-mode fiber 3, a first single-mode fiber 4, a polarization-independent isolator 5, a second single-mode fiber 6, a first fiber collimator 7, a second fiber collimator 8, a polarization controller 9, a third single-mode fiber 10, a coupler 11, and a fourth single-mode fiber 12; wherein, the pump light is output by the pump source 1 and connected to the wavelength division multiplexer 2 through a pigtail, and the wavelength division multiplexer 2 couples the pump light into the ring cavity; the output end of the wavelength division multiplexer 2 is connected to the erbium-doped single-mode fiber 3 through a passive fiber, and the erbium-doped single-mode fiber 3 is connected to the input end of the polarization-independent isolator 5 through the first single-mode fiber 4, and the first fiber collimator 7 is connected to the output end of the polarization-independent isolator 5 through the second single-mode fiber 6; the first fiber collimator 7 outputs the collimated laser, and the second fiber collimator 8 couples the collimated laser output by the first fiber collimator 7 back into the ring cavity to form a spatial alignment structure, which produces a saturable absorption effect, and the alignment conditions are appropriately adjusted to realize the adjustment of the saturation intensity and the unsaturated loss; the second fiber collimator 8, the polarization controller 9, and the coupler 11 are connected through the third single-mode fiber 10, and the coupler 11 and the wavelength division multiplexer 2 are connected through the fourth single-mode fiber 12 to form a closed optical path, and the coupler 11 forms the output end of the system. It can be seen that the structure of the present invention is reasonable, and no material saturable absorber needs to be introduced. Only by changing the alignment conditions of the first fiber collimator 7 and the second fiber collimator 8 can an equivalent saturable absorption effect be generated. Compared with the traditional soliton fiber laser system based on a real saturable absorber and the traditional equivalent saturable absorption technology generated by nonlinear effects, the saturable absorption effect generated based on the spatial alignment structure proposed innovatively in the embodiments of the present invention is not limited by the material damage threshold and the bandgap, nor by the environmental stability, and has a relatively high single-pulse energy. Generally speaking, the mode-locked soliton fiber laser system proposed in the embodiments of the present invention has obvious advantages such as simple and compact structure, high stability, low cost, easy self-starting mode locking, good repeatability, and small interference from the external environment. As a laser light source, it can be used in fields such as optical fiber communication, microfabrication, frequency measurement, biomedicine, and nonlinear optical research.

[0050] It has been experimentally proved that by adjusting the polarization controller 9 to an appropriate position, when the pump power is 1.5 W, the tunable range of the central wavelength is from 1559.0 nm to 1560.9 nm at a wavelength of 1.5 μm, and high-stability high-order soliton pulses with a maximum single-pulse energy of 4.73 nJ and a pulse width of 3.5 ps can be obtained, that is, an ultrafast laser is realized. The single-pulse energy of 4.73 nJ overcomes the limitation of the fundamental soliton area theorem, proving the generation of high-order solitons. After 6 hours of power stability test, the calculated rms value is 0.23%, proving the stability of the system.

[0051] In the description of the present invention, it should be understood that the terms "first" and "second" are used for descriptive purposes only and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, "a plurality of" means two or more, unless otherwise specifically defined.

[0052] Although the present invention has been described in conjunction with various embodiments, however, in the process of implementing the claimed invention, those skilled in the art can understand and implement other variations of the disclosed embodiments by referring to the specification and its accompanying drawings. In the specification, the word "comprising" does not exclude other components or steps, and "a" or "one" does not exclude a plurality of cases. Certain measures are recited in different embodiments, but this does not mean that these measures cannot be combined to produce good results.

[0053] The above content is a further detailed description of the present invention in conjunction with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited only to these descriptions. For those of ordinary skill in the technical field to which the present invention pertains, without departing from the concept of the present invention, several simple deductions or substitutions can be made, and all should be regarded as belonging to the protection scope of the present invention.

Claims

1. A novel mode-locked soliton fiber laser system based on a spatial alignment structure, characterized in that, Including: A pump source (1), a wavelength division multiplexer (2), an erbium-doped single-mode fiber (3), a first single-mode fiber (4), a polarization-independent isolator (5), a second single-mode fiber (6), a first fiber collimator (7), a second fiber collimator (8), a polarization controller (9), a third single-mode fiber (10), a coupler (11), and a fourth single-mode fiber (12); wherein, The pump source (1) outputs pump light and is connected to the wavelength division multiplexer (2) through a pigtail fiber, and the wavelength division multiplexer (2) couples the pump light into the ring cavity; the output end of the wavelength division multiplexer (2) is connected to the erbium-doped single-mode fiber (3) through a passive fiber, the erbium-doped single-mode fiber (3) is connected to the input end of the polarization-independent isolator (5) through the first single-mode fiber (4), and the first fiber collimator (7) is connected to the output end of the polarization-independent isolator (5) through the second single-mode fiber (6); the first fiber collimator (7) outputs collimated laser light, and the second fiber collimator (8) couples the collimated laser light output by the first fiber collimator (7) back into the ring cavity to form a spatial alignment structure, which produces a saturable absorption effect, and the saturation intensity and unsaturated loss are adjusted by appropriately adjusting the alignment conditions; the second fiber collimator (8), the polarization controller (9), and the coupler (11) are connected through the third single-mode fiber (10), and the coupler (11) and the wavelength division multiplexer (2) are connected through the fourth single-mode fiber (12) to form a closed optical path, and the coupler (11) forms the output end of the system.

2. The novel mode-locked soliton fiber laser system based on a spatial alignment structure according to claim 1, characterized in that, The maximum output power of the pump source (1) is 1.5 W.

3. The novel mode-locked soliton fiber laser system based on a spatial alignment structure according to claim 1, characterized in that, The peak absorption of the erbium-doped single-mode fiber (3) at 1530 nm is 110 dB / m.

4. The novel mode-locked soliton fiber laser system based on a spatial alignment structure according to claim 1, characterized in that, The output apertures of the first fiber collimator (7) and the second fiber collimator (8) are both 2.0 mm.

5. The novel mode-locked soliton fiber laser system based on a spatial alignment structure according to claim 1, characterized in that, The distance between the first fiber collimator (7) and the second fiber collimator (8) is 5 cm to 20 cm.

6. The novel mode-locked soliton fiber laser system based on a spatial alignment structure according to claim 1, characterized in that, The coupling efficiency between the first fiber collimator (7) and the second fiber collimator (8) is 70%.

7. The novel mode-locked soliton fiber laser system based on a spatial alignment structure according to claim 1, characterized in that, The splitting ratio of the coupler (11) is 20:80; wherein, 20% of the output end of the coupler (11) outputs laser light, and 80% of the output end of the coupler (11) is connected to the wavelength division multiplexer (2).

8. The novel mode-locked soliton fiber laser system based on a spatial alignment structure according to claim 1, characterized in that, The models of the first single-mode fiber (4), the second single-mode fiber (6), the third single-mode fiber (10), and the fourth single-mode fiber (12) are all SMF-28e.

9. The novel mode-locked soliton fiber laser system based on a spatial alignment structure according to claim 1, characterized in that, Output high-order soliton pulses with a maximum single-pulse energy of 4.73 nJ.

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