Pulse fiber laser with tunable wavelength

Abstract

The invention belongs to the technical field of optical fiber lasers, and particularly relates to a wavelength-tunable pulse optical fiber laser which comprises a laser pumping source, a wavelength division multiplexer, a gain thulium-doped silica optical fiber, an ultrahigh numerical aperture optical fiber, a collimator, a band-pass filter, a polarization dependent isolator, an optical fiber coupler, a polarization controller, a polarization maintaining optical fiber and a common single-mode optical fiber. According to the invention, a spatial light path of the collimator and the band-pass filter is adopted to select the laser wavelength and filter clutters, and meanwhile, the optical property of the band-pass filter is changed by adjusting an incident angle between a collimated light beam and the band-pass filter, so that the tuning of the central wavelength of an output pulse is realized; on the basis that an acousto-optic modulation band-pass filter is not adopted, the 1.7-micron wavelength femtosecond pulse laser system which is compact in structure, low in cost, continuous and tunable in wavelength and stable in wavelength is achieved.

Description

Technical Field

[0001] This invention belongs to the field of fiber laser technology, and specifically relates to a wavelength-tunable pulsed fiber laser. Background Technology

[0002] Ultrafast lasers in the 1.7-micron wavelength range possess unique spectral characteristics. Because the 1.7-micron wavelength range is located at the absorption peak of fat and within the low absorption region of water, it has important targeting effects on fat-rich tissues. Compared to other near-infrared light sources, this wavelength range is often used as an ideal light source for lipid imaging. This wavelength range also covers the absorption peaks of covalent bonds such as CH, and is also used for gas detection, organic polymer processing, and atomic and molecular detection. Therefore, femtosecond-level ultrashort pulse fiber lasers in the 1.7-micron wavelength range have attracted widespread attention from researchers and are one of the research hotspots in the field of ultrafast lasers.

[0003] There are three main methods to achieve 1.7-micron wavelength laser output: 1) Nonlinearly shifting light from other wavelengths to the 1.7-micron band through nonlinear effects such as stimulated Raman scattering and optical parametric oscillations. This method significantly increases the complexity and cost of the laser system, and its efficiency and wavelength range are affected by the power level supported by the fiber. 2) Directly obtaining 1.7-micron wavelength laser output by pumping bismuth-doped fibers or other specially doped fibers. However, the fabrication of bismuth-doped fibers is currently only at the laboratory stage, with low commercial maturity, which greatly limits the application range of this type of fiber and hinders further promotion. 3) Obtaining 1.7-micron wavelength laser by pumping thulium-doped fibers or thulium-holmium co-doped fibers and filtering with fiber Bragg gratings, bandpass filters, etc. Compared to the first two methods, using thulium-doped fibers, which have the highest commercialization rate, is the optimal choice for obtaining 1.7-micron wavelength lasers. The production of gain fibers and corresponding fiber devices required for thulium-doped mode-locked fiber lasers is relatively mature, which can effectively reduce costs.

[0004] CN 113140955 B discloses "A 1.7-micron picosecond ultrafast fiber laser based on SESAM". This technology utilizes SESAM (Saturable Ebsorber Structure with Antireflection Mirror) as a saturable absorber to achieve picosecond-level 1.7-micron ultrafast pulse output. Although stable 1.7-micron ultrafast pulse laser output is achieved, its spectral bandwidth is only 0.9 nm, pulse width is 14 ps, and the center wavelength is controlled by adjusting the length of the gain fiber, which cannot achieve continuous wavelength tuning and increases the complexity of the laser.

[0005] CN 119627599 A discloses "An all-fiber ultrafast laser based on 1.7-micron high energy". This technology achieves high-energy 1.7-micron ultrafast pulse output using a nonlinear polarization rotation mechanism. Although it achieves an increase in single-pulse energy, the spectral bandwidth is narrow, the pulse width is close to 1 ps, and the center wavelength is fixed and untunable. Current 1.7-micron ultrafast fiber lasers all output ultrashort pulses with fixed center wavelengths. These lasers cannot achieve continuously tunable wavelengths, and the direct output pulse width is on the picosecond scale.

[0006] In other existing technologies, acousto-optic modulated bandpass filters based on spatial structures have achieved picosecond pulsed laser output with wavelength tuning in the 1.7-micron band. However, acousto-optic modulated bandpass filters are expensive, making the laser costly. Therefore, it is necessary to explore 1.7-micron femtosecond-level mode-locked fiber lasers with continuously tunable wavelengths, which is of great significance for developing compact, low-cost, and stable femtosecond pulsed laser systems. Summary of the Invention

[0007] In view of this, the present invention provides a wavelength-tunable pulsed fiber laser, which uses a spatial optical path of collimator and bandpass filter to select the laser wavelength, filter out spontaneous emission and pump light that is not fully absorbed by the gain fiber, and adjust the incident angle between the collimated beam and the bandpass filter to change the optical properties of the bandpass filter, thereby achieving tuning of the center wavelength of the output pulse, so that the output light wavelength can be tuned within a certain range. The present invention realizes a compact, low-cost, continuously tunable, and wavelength-stable 1.7-micron wavelength femtosecond pulsed laser system without using an acousto-optic modulated bandpass filter.

[0008] To achieve the above-mentioned technical objectives, the specific technical solution adopted by the present invention is as follows: A wavelength-tunable pulsed fiber laser, characterized in that it comprises a laser pump source, a wavelength division multiplexer, a gain thulium-doped silica fiber, an ultra-high numerical aperture fiber, a first collimator, a second collimator, a bandpass filter, a first polarization controller, a polarization-dependent isolator, a polarization-maintaining fiber, a second polarization controller, and a fiber coupler. The laser pump source is fused to the beam-splitting end of the wavelength division multiplexer; the beam-combining end of the wavelength division multiplexer is fused to one end of the gain thulium-doped silica fiber; the other end of the gain thulium-doped silica fiber is connected to a common single-mode fiber and fused to one end of the common single-mode fiber; the other end of the common single-mode fiber is fused to one end of the ultra-high numerical aperture fiber; the other end of the ultra-high numerical aperture fiber is fused to the incident end of the first collimator. The incident end of the second collimator is collimated with the exit end of the first collimator; the bandpass filter is disposed between the first collimator and the second collimator; The output end of the second collimator is fused to one end of the first polarization controller; the other end of the first polarization controller is fused to the input end of the polarization-dependent isolator; the output end of the polarization-dependent isolator is fused to one end of the polarization-maintaining fiber; the other end of the polarization-maintaining fiber is fused to one end of the second polarization controller; the other end of the second polarization controller is fused to the beam-combining end of the fiber coupler; one beam-splitting end of the fiber coupler serves as the output end of the pulsed fiber laser, and the other beam-splitting end is fused to the beam-splitting end of the wavelength division multiplexer. Wherein: the bandpass filter is used to select the laser wavelength, filter out spontaneous emission, and filter out pump light that is not absorbed by the gain thulium-doped silica fiber; The pulsed fiber laser is based on adjusting the orientation of the bandpass filter, causing the transmission peak at the center of the bandpass filter to shift due to the change in the equivalent refractive index. This shifts the center transmission wavelength of the collimated light passing through the bandpass filter towards a shorter wavelength, thereby achieving tuning of the center wavelength of the output pulse.

[0009] Furthermore, the laser pump source injects pump light into the gain thulium-doped silica fiber through the pump end of the wavelength division multiplexer, forming a forward pump light and generating signal gain. The pump light and signal gain pass sequentially through the ultra-high numerical aperture fiber, the first collimator, the bandpass filter, the second collimator, the polarization-dependent isolator, the polarization-maintaining fiber, and the fiber coupler before returning to the resonance of the wavelength division multiplexer. The laser oscillation formed after multiple cycles is output from one of the beam splitters of the fiber coupler.

[0010] Furthermore, the first polarization controller, the second polarization controller, and the polarization-dependent isolator constitute a nonlinear polarization rotation mechanism; the pulsed fiber laser adjusts the polarization state of the light in its resonant cavity to make the high-intensity portion of the pulse peak consistent with the polarization state of the polarization-dependent isolator itself, and removes the energy with inconsistent polarization at the pulse edge; After the pulse circulates multiple times within the resonant cavity, it gradually narrows into a pulse, and then outputs a mode-locked laser through the beam splitter of the fiber coupler.

[0011] Furthermore, the laser pump source is a single-wavelength laser; the output laser of the laser pump source has a wavelength of 1500-1600 nm, a spectral width of 0.07-0.13 nm, and an output power of ≤1W.

[0012] Furthermore, the bandpass filter is designed with a center wavelength of 1700 nm and a high-pass band range of not less than 90 nm.

[0013] Furthermore, the bandpass filter is an interference filter; when the incident angle of the collimated beam deviates from the normal incident angle, the center transmission wavelength of the bandpass filter shifts towards the shorter wavelength direction.

[0014] Furthermore, the ultra-high numerical aperture fiber is a dispersion-compensating fiber used to adjust intracavity dispersion, with a core numerical aperture of 0.3-0.4 and a cutoff wavelength of 1050±50nm at short wavelengths.

[0015] Furthermore, the isolation of the polarization-dependent isolator is greater than 40 dB.

[0016] Furthermore, the beam splitting ratio of the fiber coupler is 7 / 93-13 / 87; and the output beam splitting ratio of the output end of the pulsed fiber laser is 7%-13%.

[0017] By adopting the above technical solution, the present invention can bring the following beneficial effects: 1. This invention uses a spatial optical path with a collimator and a bandpass filter to select the laser wavelength and filter out spontaneous emission and pump light that is not completely absorbed by the gain fiber. Furthermore, it makes it easier to tune the incident angle between the collimated beam and the bandpass filter, allowing the output light wavelength to be tuned within a certain range. 2. The laser optical path structure in this invention is compact, stable in operation, convenient to use, low in development cost, and has a high damage threshold. With reasonable control of the net dispersion in the cavity, it can directly output femtosecond-level wavelength tunable ultrashort pulses. It has broad application prospects in the fields of advanced polymer laser manufacturing and biological tissue imaging in the future. 3. This invention cleverly uses a section of polarization-maintaining fiber to form a Lyot filter (a type of optical filter that achieves high-performance wavelength selection or tuning based on birefringence and polarization interference principles) within the cavity. Combined with a bandpass filter, it can finely tune the wavelength of the output light. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of this disclosure, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this disclosure. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the structure of a wavelength-tunable pulsed fiber laser in a specific embodiment of the present invention; Figure 2 This is a mode-locked laser spectrum diagram in a specific embodiment of the present invention; Figure 3 This is a diagram of the mode-locked laser pulse sequence in a specific embodiment of the present invention; Figure 4 This is a mode-locked laser spectrum diagram in a specific embodiment of the present invention; Figure 5 This is a mode-locked laser autocorrelation trajectory diagram in a specific embodiment of the present invention; Figure 6 This is a mode-locked laser wavelength tuning spectrum diagram in a specific embodiment of the present invention; The components are: 1. Laser pump source; 2. Wavelength division multiplexer; 3. Gain thulium-doped silica fiber; 4. Ultra-high numerical aperture fiber; 5. First collimator; 6. Bandpass filter; 7. Second collimator; 8. First polarization controller; 9. Polarization correlation isolator; 10. Polarization-maintaining fiber; 11. Second polarization controller; 12. Fiber coupler. Detailed Implementation

[0020] The embodiments of this disclosure will now be described in detail with reference to the accompanying drawings.

[0021] The following specific examples illustrate the implementation of this disclosure. Those skilled in the art can easily understand other advantages and effects of this disclosure from the content disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of this disclosure, and not all of them. This disclosure can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of this disclosure. It should be noted that, in the absence of conflict, the following embodiments and features in the embodiments can be combined with each other. Based on the embodiments in this disclosure, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this disclosure.

[0022] It should be noted that various aspects of embodiments within the scope of the appended claims are described below. It will be apparent that the aspects described herein can be embodied in a wide variety of forms, and any particular structure and / or function described herein is merely illustrative. Based on this disclosure, those skilled in the art will understand that one aspect described herein can be implemented independently of any other aspect, and two or more of these aspects can be combined in various ways. For example, any number of aspects set forth herein can be used to implement the device and / or practice the method. Additionally, this device and / or method can be implemented using other structures and / or functionalities besides one or more of the aspects set forth herein.

[0023] It should also be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of this disclosure. The illustrations only show the components related to this disclosure and are not drawn according to the number, shape and size of the components in actual implementation. In actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0024] Furthermore, specific details are provided in the following description to facilitate a thorough understanding of the examples. However, those skilled in the art will understand that the described aspects can be practiced without these specific details.

[0025] In one embodiment of the invention, a wavelength-tunable pulsed fiber laser is proposed, which modulates the frequency of the output laser based on the central optical properties of the bandpass filter 6; such as Figure 1 As shown, the pulsed fiber laser in this embodiment includes a laser pump source 1, a wavelength division multiplexer 2, a gain thulium-doped silica fiber 3, an ultra-high numerical aperture fiber 4, two collimators, a bandpass filter 6, a polarization correlation isolator 9, an optical fiber coupler 12, two polarization controllers, a polarization-maintaining fiber 10, and a common single-mode fiber.

[0026] Among them, such as Figure 1 As shown, the laser pump source 1 is fused to the beam-splitting end of the wavelength division multiplexer 2; the beam-combining end of the wavelength division multiplexer 2 is fused to one end of the gain thulium-doped silica fiber 3; the other end of the gain thulium-doped silica fiber 3 is fused to one end of a section of ordinary single-mode fiber; the other end of the ordinary single-mode fiber is fused to one end of an ultra-high numerical aperture fiber 4; the other end of the ultra-high numerical aperture fiber 4 is fused to the incident end of the first collimator 5; the incident end of the second collimator 7 is collimated to the exit end of the first collimator 5; a bandpass filter 6 is disposed between the first collimator 5 and the second collimator 7. The output end of the second collimator 7 is fused to one end of the first polarization controller 8; the other end of the first polarization controller 8 is fused to the input end of the polarization-dependent isolator 9; the output end of the polarization-dependent isolator 9 is fused to one end of the polarization-maintaining fiber 10; the other end of the polarization-maintaining fiber 10 is fused to one end of the second polarization controller 11; the other end of the second polarization controller 11 is fused to the beam-combining end of the fiber coupler 12; one beam-splitting end of the fiber coupler 12 serves as the output end of the pulsed fiber laser, and the other beam-splitting end is fused to the beam-splitting end of the wavelength division multiplexer 2.

[0027] The working principle of this embodiment is as follows: The laser pump source 1 injects pump light into the gain thulium-doped silica fiber 3 through the pump end of the wavelength division multiplexer 2, forming a forward pump light and generating signal gain. The light passes sequentially through the ultra-high numerical aperture fiber 4, the first collimator 5, the bandpass filter 6, the second collimator 7, the polarization-dependent isolator 9, the polarization-maintaining fiber 10, and the fiber coupler 12, and finally returns to the resonance. This cycle repeats multiple times, eventually forming laser oscillation. At the same time, the laser contains a nonlinear polarization rotation mechanism core composed of a polarization controller and the polarization-dependent isolator 9. By adjusting the polarization state of the light in the resonant cavity, the high-intensity part of the pulse peak is made consistent with the polarization state of the isolator itself. The pulse edge loses more energy due to the inconsistency with the polarization state of the isolator itself. After the pulse cycles multiple times in the resonant cavity, a pulse narrowing effect is gradually formed. Finally, the mode-locked laser is output through the beam splitter end of the fiber coupler 12. The bandpass filter 6 is used to select the laser wavelength and filter out spontaneous emission and pump light that is not completely absorbed by the gain thulium-doped silica fiber 3. At the same time, by adjusting the incident angle between the collimated beam and the bandpass filter 6, it deviates from the normal incident angle. As a result, the transmission peak of the bandpass filter 6 is shifted due to the change in the equivalent refractive index. After the interference properties at the center of the bandpass filter 6 are finely adjusted, the central transmission wavelength will shift to a shorter wavelength, thus achieving tuning of the center wavelength of the output pulse.

[0028] The bandpass filter 6 in this embodiment adopts a conventional structure, and its filtering range meets the center transmission requirements within a certain range around 1.7 micrometers. The basic structure includes a substrate and a surface interference film. The surface interference film is used to interfere with the center transmitted light and achieve filtering, allowing only the required wavelength to pass through its center. In this embodiment, by adjusting the angle between the bandpass filter 6 and the collimated light, an angle is formed between the collimated light, the film, and the substrate, thereby changing the central optical properties of the bandpass filter. After the refractive index changes, the wavelength of the central transmission peak shifts, specifically to a shorter wavelength. Thus, the wavelength of the pulsed fiber laser in this embodiment is tunable based on a simple structure. The main body of the device for adjusting the angle of the bandpass filter 6 in this embodiment adopts a housing with openings at both ends. The first collimator 5 and the second collimator 7 are respectively installed at the two openings, and the bandpass filter 6 is rotatably installed in the housing. A manual knob is set in the middle of the housing, which is connected to the bandpass filter 6 and realizes the rotation of the bandpass filter 6.

[0029] In some embodiments, the angle change of the bandpass filter 6 can also be achieved based on stepper motor control.

[0030] Compared with existing solutions based on SESAM, nonlinear polarization rotation, or acousto-optic modulation filters, the technical solution of this embodiment has the following significant advantages: Continuously Tunable Wavelength: This invention innovatively employs a tunable bandpass filter 6 as the core wavelength selection element. By simply rotating the filter and changing the beam incident angle, continuous and smooth tuning of the output laser center wavelength can be achieved. This overcomes the fundamental limitations of existing technologies (such as adjusting the gain fiber length in CN 113140955 B) which cannot be continuously tuned, or (such as CN 119627599 A) which have a fixed wavelength, providing users with flexible wavelength output capabilities and greatly expanding the application scenarios of lasers.

[0031] Femtosecond-level ultrashort pulses are expected: This embodiment employs a mode-locking mechanism based on nonlinear polarization rotation (NPR). This mechanism can generate and maintain femtosecond-level ultrashort pulses. By precisely adjusting the two polarization controllers within the cavity, the nonlinear phase shift can be optimized, thereby achieving more significant pulse compression. This effectively solves the technical bottleneck of existing technologies where the output pulse width generally remains in the picosecond range (e.g., 14 ps or close to 1 ps), providing higher temporal resolution.

[0032] Combining high system stability with low cost, the core gain medium of this invention is a highly commercialized thulium-doped silica fiber 3. Its related devices are mature and cost-controllable, avoiding the use of bismuth-doped fibers or complex nonlinear frequency conversion systems from the laboratory stage. Simultaneously, the entire resonant cavity is an all-fiber structure (except for the filtering section), significantly improving the laser's resistance to environmental interference and facilitating long-term stable operation. Although a spatial filtering optical path is introduced, its structure is far simpler than that of an expensive acousto-optic tunable filter (AOTF), effectively controlling the overall system cost and complexity while achieving wavelength tuning.

[0033] This laser achieves a multi-functional integrated design: a single system simultaneously integrates three major advantages: wavelength tunability, femtosecond pulse operation (potential), and high stability through all-fiber technology. This design cleverly balances performance, flexibility, and cost, providing a compact, high-performance, and cost-effective solution for applications requiring specific wavelength femtosecond light sources, such as biomedical imaging and spectroscopy.

[0034] In summary, the wavelength-tunable pulsed fiber laser proposed in this embodiment successfully solves the key problems existing in the current 1.7-micron ultrafast fiber laser field, such as fixed wavelength, wide pulse width, complex system or high cost, and provides an important technical approach to promote the practical application of laser technology in scientific research and industry.

[0035] In some embodiments of the present invention, the bandpass filter 6 is designed with a center wavelength of 1700nm and a high-pass band range of 90nm. Furthermore, the filter is an interference filter, and when the incident angle of the collimated beam deviates from the normal incident angle, its center transmission wavelength will shift towards the shorter wavelength direction.

[0036] In some embodiments, the ultra-high numerical aperture fiber 4 (UHNA4) is used as a dispersion compensation fiber to adjust the intracavity dispersion. The fiber core numerical aperture is 0.350 and the cutoff wavelength at short wavelengths is 1050±50nm.

[0037] In some embodiments, the laser pump source 1 is a single-wavelength laser with a laser output wavelength of 1568nm, a spectral width of 0.1nm, and an output power of ≤1W.

[0038] In some embodiments, the laser pump source 1 can be a single-wavelength laser from Pulse Optoelectronics, with an output wavelength of 1568nm, a spectral width of 0.1nm, and an output power ≤1W; the wavelength division multiplexer 2 operates at a wavelength of 1570 / 1750±5nm, with an insertion loss of less than 0.3dB; the gain thulium-doped silica fiber 3 is an OFS-manufactured fiber, model TmDF-200, with a length of 780mm; the bandpass filter 6 is designed with a center wavelength of 1700nm and a high-pass band range of 90nm, and this filter is an interference filter, so when the incident angle of the collimated beam deviates from the normal incident angle, its center transmission wavelength will shift towards a shorter wavelength; the polarization-dependent isolator 9 has an isolation greater than 40dB; the fiber coupler 12 has a beam splitting ratio of 10 / 90; and the ordinary single-mode fiber can be Corning SMF28 fiber. The parameters of each of the above fiber optic devices can be adjusted within a small range, but the parameters of each device are matched.

[0039] Using the above-mentioned devices, a single-wavelength laser is used as the laser pump source 1, thulium-doped silica fiber is used as the gain fiber medium, and an NPR structure composed of a polarization controller and a polarization-dependent isolator 9 is used as a saturable absorber for mode locking. A bandpass filter 6 is used as the laser wavelength selection device. The laser pump source 1 injects pump light into the gain thulium-doped silica fiber 3 through the pump end of the wavelength division multiplexer 2, forming a forward pump light and generating signal gain. The light then passes sequentially through an ultra-high numerical aperture fiber 4, a first collimator 5, a bandpass filter 6, a second collimator 7, a polarization-dependent isolator 9, and a polarization-maintaining fiber. 10. Fiber coupler 12 returns to the resonance, repeating this cycle multiple times until laser oscillation is formed. Simultaneously, the laser contains a core structure of a nonlinear polarization rotation mechanism composed of a polarization controller and a polarization-dependent isolator 9. By adjusting the polarization state of the light within the resonant cavity, the high-intensity portion of the pulse center is made consistent with the polarization state of the isolator itself, while the pulse edges lose more energy due to inconsistency with the isolator's polarization state. After multiple cycles within the resonant cavity, the pulse gradually narrows, ultimately outputting a mode-locked laser through the 10% beam splitter of fiber coupler 12.

[0040] Figures 2 to 5 The figure shows the mode-locked pulse characteristic curves obtained under the conditions of a pump power of 230mW and a gain fiber (gain thulium-doped silica fiber 3) length of 780mm. As can be seen from the figure, the laser center wavelength is 1730.98nm, the full width at half maximum (FWHM) is 26.50nm, the pulse interval is 51.94ns, the corresponding repetition rate is 19.26MHz, the signal-to-noise ratio is 64.90dB, and the pulse width is 552fs. Therefore, the fiber laser in this embodiment can achieve stable 1.7-micron femtosecond pulse output with good self-starting performance. Figure 6 To adjust the pump light power to 700mW, the center wavelength of the output pulse can be continuously tuned between 1719.19-1731.14nm by changing the incident angle between the collimated beam and the bandpass filter 6, demonstrating the good tunability of the laser. It provides a convenient and stable light source for laser manufacturing of polymers with different absorption peaks.

[0041] The above description is merely a specific embodiment of this disclosure, but the scope of protection of this disclosure is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this disclosure should be included within the scope of protection of this disclosure. Therefore, the scope of protection of this disclosure should be determined by the scope of the claims.

Claims

1. A wavelength tunable pulsed fiber laser, characterized by, The laser pumping source, a wavelength division multiplexer, a gain-doped thulium-doped quartz optical fiber, a super high numerical aperture fiber, a first collimator, a second collimator, a band-pass filter, a first polarization controller, a polarization dependent isolator, a polarization maintaining fiber, a second polarization controller and a fiber coupler are included. The laser pumping source is fused to a beam splitting end of the wavelength division multiplexer; a beam combining end of the wavelength division multiplexer is fused to one end of the gain-doped thulium-doped quartz optical fiber; the other end of the gain-doped thulium-doped quartz optical fiber is connected with a common single-mode fiber and fused to one end of the common single-mode fiber; the other end of the common single-mode fiber is fused to one end of the super high numerical aperture fiber; the other end of the super high numerical aperture fiber is fused to an incident end of the first collimator; The incident end of the second collimator is collimated with the exit end of the first collimator; the band-pass filter is arranged between the first collimator and the second collimator; The exit end of the second collimator is fused to one end of the first polarization controller; the other end of the first polarization controller is fused to an incident end of the polarization dependent isolator; an exit end of the polarization dependent isolator is fused to one end of the polarization maintaining fiber; the other end of the polarization maintaining fiber is fused to one end of the second polarization controller; the other end of the second polarization controller is fused to a beam combining end of the fiber coupler; one beam splitting end of the fiber coupler is used as an output end of the pulse fiber laser, and the other beam splitting end is fused to a beam splitting end of the wavelength division multiplexer; The band-pass filter is used for selecting laser wavelength, filtering out spontaneous emission and filtering out pump light not absorbed by the gain-doped thulium-doped quartz optical fiber. The pulse fiber laser adjusts the position of the band-pass filter, causes the transmission peak of the collimated light passing through the band-pass filter to shift in the short wavelength direction due to the change of the equivalent refractive index, and realizes the tuning of the output pulse center wavelength.

2. The pulsed fiber laser of claim 1, wherein, The laser pumping source injects pump light into the gain-doped thulium-doped quartz optical fiber through the pump end of the wavelength division multiplexer, forms a forward pump light and generates signal gain; The pump light and the signal gain pass through the super high numerical aperture fiber, the first collimator, the band-pass filter, the second collimator, the polarization dependent isolator, the polarization maintaining fiber and the fiber coupler in turn, return to the resonance of the wavelength division multiplexer, and form laser oscillation after multiple cycles, which is output from one beam splitting end of the fiber coupler.

3. The pulsed fiber laser of claim 2, wherein, The first polarization controller, the second polarization controller and the polarization dependent isolator constitute a nonlinear polarization rotation mechanism; The pulse fiber laser adjusts the polarization state of the light in the resonant cavity, so that the pulse peak high intensity part is consistent with the polarization state of the polarization dependent isolator itself and the energy with inconsistent polarization is removed at the pulse edge; After multiple cycles in the resonant cavity, the pulse narrows gradually, and the mode-locked laser is output through the beam splitting end of the fiber coupler.

4. The pulsed fiber laser of claim 1, wherein, The laser pump source is a single-wavelength laser; the output laser of the laser pump source has a wavelength of 1500-1600 nm, a spectral width of 0.07-0.13 nm, and an output power of ≤1 W.

5. The pulsed fiber laser of claim 1, wherein, The design center wavelength of the band-pass filter is 1700 nm, and the high-pass waveband range is not less than 90 nm.

6. The pulsed fiber laser of claim 1, wherein, The band-pass filter is an interference filter; when the incident angle of the collimated light beam deviates from the normal incident angle, the center transmission wavelength of the band-pass filter deviates towards the short wavelength direction.

7. The pulsed fiber laser of claim 1, wherein, The super-high numerical aperture optical fiber is a dispersion-compensated optical fiber, used for adjusting the intra-cavity dispersion, the core numerical aperture is 0.3-0.4, and the short-wavelength cutoff wavelength is 1050±50 nm.

8. The pulsed fiber laser of claim 1, wherein, The polarization-dependent isolator has an isolation degree greater than 40 dB.

9. The pulsed fiber laser of claim 1, wherein, The splitting ratio of the optical fiber coupler is 7 / 93-13 / 87; wherein the output splitting ratio of the output end of the pulsed fiber laser is 7%-13%.

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