All-fiber passive mode-locking thulium-doped fiber laser based on SMS structure

Abstract

The invention discloses an all-fiber passive mode-locking thulium-doped fiber laser based on an SMS structure. The laser is an all-fiber structure laser formed by connecting a pumping source 1, an optical fiber wavelength division multiplexer 2, a gain optical fiber 3, an optical fiber coupler 4, an SMS structure saturable absorber 5, a polarization controller 6 and a polarization independent isolator 7 through optical fiber welding, and the SMS structure saturable absorber 5 and the polarization controller 6 are independently and separately arranged. The low-threshold and high-energy mode-locked fiber laser based on the SMS structure is simple in structure, convenient to operate, stable and reliable, and not only can generate stable low-threshold fundamental frequency pulses, but also can realize high-energy ultrashort pulse output.

Description

Technical Field

[0001] The invention relates to the field of optics, and in particular to an all-fiber passively mode-locked thulium-doped fiber laser based on a SMS structure. Background Art

[0002] As one of the most important scientific discoveries in the 20th century, the emergence of laser has greatly promoted the development of science and technology. Passive mode-locked fiber lasers with rare earth-doped optical fibers as gain media have been developed rapidly in recent years due to their wide applications in optical communications, optical data storage, sensor technology, medicine, etc. Ultrashort optical pulses can be generated using passive mode-locking technology. Passive mode-locked lasers have attracted widespread attention for their compact structure, narrow pulse width, and high peak power. They also have the advantages of excellent flexibility, high reliability, and easy system integration.

[0003] The 2μm laser has attracted extensive attention from researchers for its unique advantages. The 2μm laser has good tissue penetration and cutting accuracy. The special wavelength enables the laser energy to act more accurately on the target tissue, achieving more precise cutting and vaporization. Secondly, the 2μm laser is in the eye-safe band, and will not cause eye damage to medical staff and patients when used. In addition, the 2μm laser has a fast cutting speed and less bleeding, which helps to shorten the operation time and reduce the postoperative recovery time. The 2μm laser has great application prospects in medical treatment due to its high safety and high efficiency. On the other hand, compared with traditional 1μm and 1.5μm lasers, 2μm lasers also have significant advantages in processing non-metallic materials. The 2μm laser has a stronger processing ability for non-metallic materials such as plastics, fiber paper, etc., and can achieve high-quality processing without an absorption layer, reduce production costs, and improve production efficiency. With the continuous advancement of technology and the continuous expansion of application fields, 2μm lasers are expected to play a greater role in many fields such as medicine, scientific research, and industry.

[0004] Passively mode-locked fiber lasers are an important part of modern laser technology, and their performance directly affects the application prospects of lasers. In order to achieve stable mode-locked pulses in fiber lasers, saturable absorbers (SA) are one of the most basic and critical mode-locked devices. SA can generally be divided into two categories: real SA and equivalent SA. Real SA refers to materials that have nonlinear absorption properties, and their properties depend on the type and structure of the material. Equivalent SA is an artificial structure designed to manufacture nonlinear absorption behavior, including nonlinear polarization rotation (NPR), nonlinear optical loop mirror (NOLM) and nonlinear amplified loop mirror (NALM). However, fiber lasers based on NPR are very sensitive to environmental vibrations and polarization states, which may lead to poor stability of mode locking. The "8" cavity mode-locked laser based on NOLM or NALM has problems such as low repetition frequency and high mode-locking threshold. In addition, real SA based on two-dimensional nanomaterials has limitations such as low damage threshold and unstable performance, which hinders its further application.

[0005] In addition, Chinese patent publication number CN109616862A provides a mode-locked pulse fiber laser based on an SMS structure. The SMS structure of the fiber laser is wound in a polarization controller (PC), that is, the SMS structure is wound into the PC and used as a mode-locked device. Since the bending loss of the SMS structure fiber in the 2μm band is very large, if the SMS structure is wound in the PC, the loss in the laser cavity will be greatly increased, resulting in the inability to generate stable mode-locked pulses. Therefore, the SMS structure of the fiber laser cannot be used in 2μm fiber lasers.

[0006] Therefore, it is necessary to explore new structures of SA to make up for the shortcomings of existing SA. Summary of the invention

[0007] The present invention aims to provide an all-fiber passively mode-locked thulium-doped fiber laser based on an SMS structure. The present invention has a low mode-locking threshold and high output energy, and adopts an all-fiber structure, which makes the laser more stable and reliable, and expands the application scenarios of 2μm fiber lasers.

[0008] The SMS-structured saturable absorber-based mode-locked fiber laser is a new solution for achieving mode-locked pulses. Its principle relies on the nonlinear multimode interference effect (NL-MMI) in multimode fibers. Specifically, the SMS structure refers to the formation of an SMF-MMF-SMF structure (abbreviated as SMS structure) by fusing a multimode fiber (MMF) between two single-mode fibers (SMF).

[0009] When the incident light in the SMF enters the MMF, due to the mode coupling effect, the high-order mode components will be excited in the MMF. After propagating through the MMF, each mode will produce different phase shifts due to the nonlinear effect, so that superposition occurs to achieve multimode interference. Finally, the high-intensity light is coupled into the SMF again, and repeatedly circulated and amplified in the laser to finally form a pulse output. This structure can achieve a selective transmission effect similar to saturable absorption, thereby achieving mode locking. Among the many types of MMFs, graded index multimode fiber (GIMF) stands out due to its low inter-mode dispersion, large core diameter, high bandwidth performance, good compatibility and scalability, and has become the only choice for multimode fiber in the SMS structure. In addition, the performance of the SMS structure is stable and is not affected by time changes. In view of these advantages, the mode locking technology based on the SMS structure saturable absorber can lay a solid foundation for the development of high-power, high-stability all-fiber mode-locked lasers.

[0010] In view of this, the present application provides an all-fiber passively mode-locked thulium-doped fiber laser based on SMS structure, wherein the laser is connected by fiber fusion splicing of a pump source 1, a fiber wavelength division multiplexer 2, a gain fiber 3, a fiber coupler 4, a SMS structure saturable absorber 5, a polarization controller 6 and a polarization-independent isolator 7 to form an all-fiber structure laser. The SMS structure saturable absorber (5) and the polarization controller (6) are independently and separately arranged. The SMS structure is separated from the polarization controller, and the two can be adjusted separately, with high flexibility and stability and low loss.

[0011] In a possible implementation, the output end of the pump source 1 is connected to the pump end of the optical fiber wavelength division multiplexer 2, the output end of the optical fiber wavelength division multiplexer 2 is connected to one end of the gain fiber 3, the other end of the gain fiber 3 is connected to the input end of the optical fiber coupler 4, the first output end of the optical fiber coupler 4 serves as the output of the laser, the second output end of the optical fiber coupler 4 is connected to the input end of the SMS structure saturable absorber 5, the output segment of the SMS structure saturable absorber 5 is connected to the input end of the polarization controller 6, the output end of the polarization controller 6 is connected to the input end of the polarization-independent isolator 7, and the output end of the polarization-independent isolator 7 is connected to the signal end of the optical fiber wavelength division multiplexer 2.

[0012] In a possible implementation, the SMS structure saturable absorber 5 is a fully optical SMS structure saturable absorber formed by fusing two sections of single-mode optical fiber SMF with a section of multi-mode optical fiber MMF. For example, a 43 cm multi-mode optical fiber and two sections of single-mode optical fiber, each 50 cm long, can be selected.

[0013] In a possible implementation, the SMS structure saturable absorber 5 is based on the principle of nonlinear multimode interference effect between a multimode optical fiber MMF and a single-mode optical fiber SMF, and is made by fusing two ends of a multimode optical fiber MMF of a set first length to a single-mode optical fiber of a set second length.

[0014] In a possible implementation, the central wavelength of the pump source 1 is 1570 nanometers (nm), the output end is an optical fiber output, and the maximum output power is 1.5 watts (W).

[0015] In a possible implementation, the wavelength division multiplexer 2 is a 1570 / 1950 nm optical fiber wavelength division multiplexer, and the pump light emitted by the pump source 1 is injected into the gain optical fiber 3 through the coupling of the wavelength division multiplexer 2 .

[0016] In a possible implementation, the gain optical fiber 3 is a single-mode thulium-doped optical fiber.

[0017] In a possible implementation, the optical fiber coupling ratio of the optical fiber coupler 4 is one of 10:90, 20:80, 30:70, and 40:60.

[0018] In a possible implementation, the polarization controller 6 is a three-ring optical fiber polarization controller or a polarization controller with a squeeze structure.

[0019] In a possible implementation, the polarization-independent isolator is a fiber isolator, which is used to control unidirectional transmission of light in a laser cavity.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention provides an all-fiber passively mode-locked thulium-doped fiber laser based on an SMS structure. The SMS structure saturable absorber manufactured in the embodiment of the present application has a simple structure, good stability and low cost, and can be applied to a mode-locked fiber laser to obtain a low-threshold mode-locked pulse and a high-energy ultrashort pulse.

[0022] The low-threshold and high-energy mode-locked fiber laser based on SMS structure has the advantages of simple structure, convenient operation, stability and reliability. It can not only generate stable low-threshold fundamental frequency pulses, but also realize high-energy ultrashort pulse output. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0024] Figure 1 The present invention provides an all-fiber passively mode-locked thulium-doped fiber laser based on SMS structure;

[0025] Figure 2 A schematic structural diagram of a saturable absorber based on an SMS structure provided by the present invention;

[0026] Figure 3 A schematic diagram of the experimental principle of a dual-arm detection technology provided by the present invention;

[0027] Figure 4 A nonlinear optical absorption curve of a saturable absorber based on SMS structure provided for the invention;

[0028] Figure 5 A schematic diagram of the low-threshold mode-locked pulse output characteristics of the SMS-structured all-fiber passively mode-locked thulium-doped fiber laser provided by the present invention at 140 mW;

[0029] Figure 6 A schematic diagram of the high-energy mode-locked pulse output characteristics of the SMS-structured all-fiber passively mode-locked thulium-doped fiber laser provided by the present invention at 520 mW;

[0030] Figure 7 This is a relationship diagram between the output energy and average output power of the high-energy mode-locked pulse at different pump powers provided by the present invention. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the present application will be described below in conjunction with the accompanying drawings in the embodiments of the present application. In the present application, "at least one" refers to one or more, and "multiple" refers to two or more. "And / or" describes the association relationship of the associated objects, indicating that there may be three relationships, for example, A and / or B, which can represent: A exists alone, A and B exist at the same time, and B exists alone, where A and B can be singular or plural. The character " / " generally indicates that the associated objects before and after are in an "or" relationship. "At least one of the following" or its similar expressions refers to any combination of these items, including any combination of single or plural items. For example, at least one of a, b or c can represent: a, b, c, a and b, a and c, b and c or a and b and c, where a, b and c can be single or multiple. It is worth noting that "at least one" can also be interpreted as "one or more items".

[0032] It should be noted that, in this application, words such as "exemplary" or "for example" are used to indicate examples, illustrations or descriptions. Any embodiment or design described as "exemplary" or "for example" in this application should not be interpreted as being more preferred or more advantageous than other embodiments or designs. Specifically, the use of words such as "exemplary" or "for example" is intended to present related concepts in a specific way.

[0033] The first, second, etc. descriptions appearing in the embodiments of the present application are only used for illustration and distinction of the description objects. There is no order, nor do they indicate any special limitation on the number of devices in the embodiments of the present application, and cannot constitute any limitation on the embodiments of the present application.

[0034] The present invention provides an all-fiber passively mode-locked thulium-doped fiber laser based on SMS structure, see Figure 1 The SMS-structured all-fiber passively mode-locked thulium-doped fiber laser is composed of a pump source 1, a fiber wavelength division multiplexer 2, a gain fiber 3, a fiber coupler 4, an SMS-structured saturable absorber 5, a polarization controller 6 and a polarization-independent isolator 7 connected by fiber fusion to form an all-fiber structure laser with a ring cavity.

[0035] Furthermore, an all-fiber passively mode-locked thulium-doped fiber laser based on an SMS structure includes the SMS structure saturable absorber, which is connected to the laser, and also includes a pump source 1, an optical fiber wavelength division multiplexer 2, a gain fiber 3, an optical fiber coupler 4, a polarization controller 6 and a polarization-independent isolator 7 connected in sequence by optical fiber fusion splicing, the saturable absorber 5 connects the optical fiber coupler 4 and the polarization controller 6, and the pump source 1, the optical fiber wavelength division multiplexer 2, the gain fiber 3, the optical fiber coupler 4, the SMS structure saturable absorber 5, the polarization controller 6 and the polarization-independent isolator 7 constitute a closed-loop cavity laser.

[0036] The beneficial effect of the present invention is that the mode locking can be started at a relatively low input pump power and stably operate in the mode locking state, that is, the all-fiber passively mode-locked thulium-doped fiber laser based on the SMS structure provided by the present application has a relatively low mode locking threshold. The lower threshold mode locking is related to the parameters of the saturable absorber, such as the modulation depth. The saturable absorber of the SMS structure produced by the present application has a high modulation depth. The measured modulation depth is 47.1%, and the modulation depth of the fitting curve is 75.8%, which is the highest among the SMS structure devices in the 2μm band. It can be fully said that it is very suitable for the 2μm band mode locking, especially for achieving low threshold and high energy pulse output.

[0037] This characteristic greatly improves the pump utilization efficiency. Moreover, the all-fiber mode-locked thulium-doped fiber laser based on the SMS structure in the present invention has the advantages of good stability, simple structure, and high damage threshold. Figure 2 As shown, the SMS structure saturable absorber is used as a mode-locked device, which is independent of the polarization controller, that is, it does not need to be wrapped in the polarization controller. The SMS structure is an all-fiber structure. Compared with the saturable absorber mode-locked device based on two-dimensional materials, the all-fiber mode-locked device is simpler, has better stability, and has a higher damage threshold, so that the laser has better environmental adaptability. In addition, the present invention can achieve high-energy mode-locked pulse output, which is suitable for high-energy pulse light source application scenarios. A stable output pulse can be obtained by using an all-fiber mode-locked thulium-doped fiber laser based on the SMS structure, and its maximum output average power is 116mW, and the corresponding pulse energy is about 18.67nJ, which is much larger than the output energy of traditional solitons (~0.1nJ). This passively mode-locked fiber laser has a sophisticated structure and is easy to operate. It not only has far-reaching significance in the field of academic research, but also shows great potential and value in the field of industrial applications.

[0038] The nonlinear optical properties of the SMS structure saturable absorber were further studied. Here, the double-arm detection technology was used to measure the nonlinear optical absorption characteristics of the SMS structure saturable absorber in the 2μm band.

[0039] The experimental principle of dual-arm detection technology is as follows Figure 3 As shown, the light source is a laboratory-made ultrashort pulse light source (central wavelength ~1880nm, repetition rate ~5.3MHz, pulse width ~1ps). An adjustable optical attenuator is used to adjust the optical power input to the SMS structure saturable absorber. The light passing through the attenuator is divided into two paths by a fiber coupler, one path is directly connected to the power meter as a reference arm, and the other path is connected to the power meter as a signal light after passing through the sample. By adjusting the optical attenuator, the transmittance of the SMS structure saturable absorber under different input powers is measured, and finally the nonlinear optical absorption curve of the SMS structure saturable absorber is obtained, as shown in Figure 4 shown.

[0040] The results show that the measured modulation is about 47.1%, the modulation depth of the fitting curve is ~75.8%, and the non-saturable loss is about ~20%. This result means that the SMS structure saturable absorber can be used as a mode locker in thulium-doped fiber lasers to achieve ultrashort pulse output. This shows that the SMS structure saturable absorber exhibits excellent saturable absorption characteristics and can be used to make mode-locked devices for fiber lasers.

[0041] It should be noted that when performing optical fiber fusion splicing, the loss needs to be controlled within 0.1dB.

[0042] It should be noted that the pump source 1 is a fiber laser with a central wavelength of 1570 nm, and its output end is a fiber output with a maximum output power of 1.5 W.

[0043] It should be noted that the wavelength division multiplexer 2 is a 1570 / 1950 nm optical fiber wavelength division multiplexer, and the pump light is coupled and injected into the gain optical fiber 3 through the multiplexer.

[0044] It should be noted that the gain fiber 3 is a single-mode thulium-doped fiber, and different lengths can be selected according to different absorption coefficients and dispersion values.

[0045] It should be noted that the coupler is a fiber coupler 4, which has multiple options and can be any one of the fiber coupling ratios of 10:90, 20:80, 30:70, and 40:60.

[0046] It should be noted that the multimode optical fiber is a gradient refractive index multimode optical fiber, and its core diameter is 50μm. The multimode optical fiber and the single-mode optical fiber form an SMS structure, and the saturable absorption characteristics are realized by using the nonlinear multimode interference effect. Preferably, the single-mode optical fiber is a single-mode optical fiber dedicated to the 2μm band (for example, SM1950 or SM2000).

[0047] It should be noted that the polarization controller 6 is a three-ring optical fiber polarization controller for changing the polarization state of the laser, and can also be a polarization controller of a squeezed structure. If the polarization controller 6 is a three-ring optical fiber polarization controller, the optical fiber wound in the polarization controller 6 is a single-mode optical fiber dedicated to the 2 μm band (for example, SM1950 or SM2000).

[0048] It should be noted that the polarization-independent isolator 7 is a fiber isolator, which is used to control the unidirectional transmission of light in the laser cavity.

[0049] The beneficial effect of the present invention is that the all-fiber passively mode-locked thulium-doped fiber laser based on the SMS structure has a lower mode-locking threshold, which greatly improves the energy utilization efficiency, and the present invention can achieve high energy output, which is suitable for high-energy application scenarios. In addition, the present invention adopts an all-fiber structure to improve the stability and reliability of the laser, making it more environmentally adaptable. This passively mode-locked thulium-doped fiber laser has a sophisticated structure and is easy to operate, and has important academic research significance and industrial application potential.

[0050] It should be noted that this embodiment is based on the technical solution and provides a detailed implementation method and a specific operation process, but the protection scope of the present invention is not limited to this embodiment.

[0051] In a specific embodiment, see Figure 1 The output end of the pump source 1 is connected to the pump end a of the optical fiber wavelength division multiplexer 2, and the output end c of the optical fiber wavelength division multiplexer 2 is connected to one end of the gain fiber 3; the other end of the gain fiber 3 is connected to the input end of the optical fiber coupler 4, and the first output end d of the optical fiber coupler 4 is used as the output of the laser, and the second output end e of the optical fiber coupler 4 is connected to the input end of the SMS structure saturable absorber 5, and the output end of the SMS structure saturable absorber 5 is connected to the input end of the polarization controller 6, and the output end of the polarization controller 6 is connected to the input end of the polarization-independent isolator 7, and the output end of the polarization-independent isolator 7 is connected to the signal end b of the optical fiber wavelength division multiplexer 2.

[0052] Preferably, the gain fiber 3 is a single-mode thulium-doped fiber, which is SCF-Tm-8 / 125 fiber from Coractive of Canada, with an optional length of 2 m, a numerical aperture of 0.15, and a group velocity dispersion of about 69 ps at a wavelength of about 1900 nm. 2 / km;

[0053] Preferably, the optical fiber coupler 4 has a coupling ratio of 20:80, of which 20% is the output end;

[0054] Preferably, the cavity length of the entire laser is about 32.2 m.

[0055] The feasibility of the present invention is verified by experimental data as follows:

[0056] When the pump power was increased to 120mW, the polarization state in the cavity was changed by continuously adjusting the deflection angle of the polarization controller, and the resonant cavity continued to output continuous light. Subsequently, the pump power was further increased to 140mW, and unstable oscillations were observed on the oscilloscope. In this case, the pump power was kept unchanged and the polarization controller was fine-tuned to obtain a mode-locked pulse with stable timing. Figure 5 The output characteristics of the stable mode-locked pulse at a pump power of 140mW are shown, including the spectrum, pulse timing, RF diagram in a large range, and RF diagram in a small range. Figure 5 As shown in sub-figure (a), the central wavelength of the spectrum is located at 1910nm, its 3dB bandwidth is 3.17nm, and the spectrum has a good shape. There are obvious Kelly sidebands on both sides, which indicates that the mode-locked operation is in the soliton mode-locked state. Figure 5 The (b) sub-figure shows the corresponding pulse time domain variation diagram. The interval between adjacent pulses is 161ns, which corresponds to the time required for the optical signal to transmit one circle in the laser loop. In addition, only one pulse appears in a time period, and the intensity distribution of the timing pulse is uniform. The mode-locked pulse is in a relatively stable operating state. Figure 5 Sub-figure (c) records the changes in the RF spectrum within the range of 500MHz, with a scanning resolution of 1kHz. Figure 5 The sub-figure (d) in the figure is a small-scale RF spectrum with a scanning resolution of 100Hz. These spectrum measurement results show that the repetition frequency of the mode-locked pulse is 6.21MHz and the signal-to-noise ratio is 75dB, which means that the mode-locked state has high temporal stability. Moreover, in the absence of external interference, the laser can maintain mode-locked operation for a long time. The output power in this mode-locked state is 1.16mW, and the calculated single pulse energy is about 0.19nJ.

[0057] As the pump power increases, high-energy ultrashort pulses can also be obtained. Figure 6 The output characteristics of high energy pulses were recorded when the pump power was 520mW. Figure 6 The spectral characteristics of the output pulse can be observed in the spectral curve of sub-figure (a). The measurement results show that its bandwidth is 0.81nm and the central wavelength is at 1911nm. Figure 6 The (b) sub-figure in Figure 1 shows the timing pulse diagram within the time range of 2.5μs. It can be seen that the time interval between adjacent pulses is 161ns, and the pulse amplitude is relatively stable within the global time range. Figure 6In the RF diagram of sub-graph (c), it can be observed that the peak appears at 6.21MHz, and there are also small peaks on both sides of the central peak, and the noise floor is about 25dB. The measured signal-to-noise ratio is 64dB, indicating that the mode-locked operation state is relatively stable. In addition, Figure 6 The (d) sub-figure in the figure shows the experimentally measured autocorrelation curve, and the full width at half maximum of the pulse is 2.21ps. Figure 7 The average output power and output energy of the high-energy mode-locked pulses are shown as a function of the pump power. When the pump power is 1020mW, the maximum average output power reaches 116mW, and the corresponding pulse energy reaches 18.68nJ. Obviously, compared with the pulse energy of ordinary traditional solitons, the energy of this mode-locked pulse is much higher. These research results will lay a solid foundation for the development and application of high-energy pulse mode-locked fiber lasers.

[0058] The SMS-structured all-fiber passively mode-locked thulium-doped fiber laser provided in this application can start mode locking at a relatively low energy input and can achieve high-energy ultrashort pulse output. Moreover, the all-fiber structure of the laser improves the stability and reliability of the laser. These advantages make it very important in both basic research and industrial applications.

[0059] The technical means disclosed in the present application are not limited to the technical means disclosed in the above-mentioned implementation methods, but also include technical solutions composed of any combination of the above technical features. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of this application, and these improvements and modifications are also regarded as the protection scope of this application.

[0060] The above is a detailed introduction to an all-fiber passively mode-locked thulium-doped fiber laser based on an SMS structure provided in an embodiment of the present application. This article uses specific examples to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method and its core idea of ​​the present application; at the same time, for ordinary technicians in this field, according to the idea of ​​the present application, there will be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be understood as a limitation on the present application. Although the present application is described in detail with reference to the aforementioned embodiments, ordinary technicians in this field should understand that they can still modify the technical solutions recorded in the aforementioned embodiments, or replace some of the technical features therein by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. An all-fiber passively mode-locked thulium-doped fiber laser based on SMS structure, characterized in that: The laser is a fully optical fiber structured laser formed by connecting a pump source (1), an optical fiber wavelength division multiplexer (2), a gain optical fiber (3), an optical fiber coupler (4), an SMS structure saturable absorber (5), a polarization controller (6) and a polarization-independent isolator (7) through optical fiber fusion splicing; wherein the SMS structure saturable absorber (5) and the polarization controller (6) are independently and separately arranged.

2. The all-fiber passively mode-locked thulium-doped fiber laser based on SMS structure according to claim 1, characterized in that: The output end of the pump source (1) is connected to the pump end of the optical fiber wavelength division multiplexer (2), the output end of the optical fiber wavelength division multiplexer (2) is connected to one end of the gain fiber (3), the other end of the gain fiber (3) is connected to the input end of the optical fiber coupler (4), the first output end of the optical fiber coupler (4) serves as the output of the laser, the second output end of the optical fiber coupler (4) is connected to the input end of the SMS structure saturable absorber (5), the output section of the SMS structure saturable absorber (5) is connected to the input end of the polarization controller (6), the output end of the polarization controller (6) is connected to the input end of the polarization-independent isolator (7), and the output end of the polarization-independent isolator (7) is connected to the signal end of the optical fiber wavelength division multiplexer (2).

3. The all-fiber passively mode-locked thulium-doped fiber laser based on SMS structure according to claim 1, characterized in that: The SMS structure saturable absorber (5) is a fully optical SMS structure saturable absorber formed by fusing two sections of single-mode optical fibers SMF with one section of multi-mode optical fiber MMF.

4. The all-fiber passively mode-locked thulium-doped fiber laser based on SMS structure according to any one of claims 1 to 3, characterized in that: The SMS structure saturable absorber (5) is based on the principle of nonlinear multimode interference effect between a multimode optical fiber MMF and a single-mode optical fiber SMF, and is manufactured by respectively fusing two ends of a multimode optical fiber MMF of a set first length to a single-mode optical fiber of a set second length.

5. The all-fiber passively mode-locked thulium-doped fiber laser based on SMS structure according to claim 1, characterized in that: The central wavelength of the pump source (1) is 1570 nanometers, the output end is an optical fiber output, and the maximum output power is 1.5 watts.

6. The all-fiber passively mode-locked thulium-doped fiber laser based on SMS structure according to claim 1, characterized in that: The wavelength division multiplexer (2) is a 1570 / 1950nm optical fiber wavelength division multiplexer, and the pump light emitted by the pump source (1) is injected into the gain optical fiber (3) through the coupling of the wavelength division multiplexer (2).

7. The all-fiber passively mode-locked thulium-doped fiber laser based on SMS structure according to claim 1, characterized in that: The gain optical fiber (3) is a single-mode thulium-doped optical fiber.

8. The all-fiber passively mode-locked thulium-doped fiber laser based on SMS structure according to claim 1, characterized in that: The optical fiber coupling ratio of the optical fiber coupler (4) is one of 10:90, 20:80, 30:70 and 40:

60.

9. The all-fiber passively mode-locked thulium-doped fiber laser based on SMS structure according to claim 1, characterized in that: The polarization controller (6) is a three-ring optical fiber polarization controller or a polarization controller with an extrusion structure.

10. The all-fiber passively mode-locked thulium-doped fiber laser based on SMS structure according to claim 1, characterized in that: Polarization-independent isolators are fiber optic isolators used to control the unidirectional transmission of light in a laser cavity.

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