Mid-infrared tunable pure soliton fiber laser based on cascade amplification

Abstract

Disclosed is a mid-infrared tunable pure soliton fiber laser based on cascade amplification, comprising: a femtosecond pulse laser, a first-stage fiber amplifier, and a second-stage fiber amplifier. The femtosecond pulse laser is used for generating a femtosecond pulse signal and transmitting the femtosecond pulse signal to the first-stage fiber amplifier. The first-stage fiber amplifier is used for performing power amplification on the femtosecond pulse signal and generating a Raman soliton. The second-stage fiber amplifier is used for recovering, recirculating, and reusing background signal light to amplify the Raman soliton. By recovering, recirculating, and reusing the background signal light to amplify the Raman soliton, the present invention can achieve higher output power and pulse energy, a wider tunable range, and pulses of pure Raman solitons.

Description

Mid-infrared tunable pure soliton fiber laser based on cascaded amplification Technical Field

[0001] This invention relates to the field of fiber laser technology, and in particular to a mid-infrared tunable pure soliton fiber laser based on cascaded amplification. Background Technology

[0002] High-power mid-infrared femtosecond lasers are of paramount importance due to their wide range of applications in molecular spectroscopy, remote sensing, laser surgery, and materials processing. Compared to solid-state lasers, mid-infrared femtosecond lasers based on fluoride ZBLAN fibers offer significant advantages in system compactness, environmental reliability, and high beam quality. Currently, nonlinear polarization rotation mode-locking technology is an effective means of achieving femtosecond pulse output from mid-infrared fiber lasers; however, the mode-locked pulses are limited to a few wavelengths, including 2.8 μm, 2.9 μm, 3.1 μm, and 3.5 μm, and their wavelength tunability is limited by the gain bandwidth of fluoride rare-earth ions. In many practical applications of molecular spectroscopy and sensing, mid-infrared femtosecond lasers with continuously tunable wavelengths over a wide range are required.

[0003] The soliton self-frequency shift (SSFS) effect in optical fibers can be used to overcome similar wavelength tunability limitations, providing femtosecond pulses with broadband tunability. The SSFS effect allows the original soliton pulse to de-energize from the Kelley sideband of the mode-locked pulse and shift towards longer wavelengths, thus obtaining a very clean and complete Raman soliton. To date, tunable femtosecond lasers based on the SSFS effect have been extensively studied in silica fiber, tellurate fiber, fluoride fiber, and chalcogenide fiber. Due to high loss in the mid-infrared region, Raman solitons in silica fiber are confined to a wavelength of 2.3 μm, while soft glass fiber, benefiting from lower phonon energy, can support further frequency shifts in the mid-infrared region. In soft glass fiber, compared to tellurate and chalcogenide fibers, fluoride fiber has a lower nonlinear refractive index and larger anomalous dispersion, making it possible to generate Raman solitons with high pulse energy and high peak power through fluoride fiber.

[0004] Currently, existing technologies for achieving mid-infrared broadband tunable pulse output based on the SSFS effect in fluoride fiber mainly focus on the following aspects: First, using a near-infrared ultrafast laser amplifier as a pump source to generate high-energy ultrashort pulses, and employing fluoride fiber as the frequency-shifting fiber to achieve mid-infrared tunable ultrafast pulse output. For example, a mid-infrared ultrafast system with a pulse energy of 5 nJ and a wavelength tunable range of 2~4.3 μm was realized by combining an erbium-doped laser amplifier with a highly nonlinear InF3 fiber. Although SSFS can utilize a more convenient near-infrared fiber laser as a pump source, using a mid-infrared fiber laser as a pump source has the advantage of a longer initial pump wavelength, thus facilitating the achievement of larger pulse energies for Raman soliton pulses at longer wavelengths. Second, using an ultrashort pulse oscillator and its amplifier based on fluoride fiber as a mid-infrared pump source, and again employing fluoride fiber as the frequency-shifting fiber, a mid-infrared tunable ultrafast system was developed to achieve high-energy Raman soliton pulse output. For example, by using an Er:ZBLAN fiber oscillator and its amplifier as a pump source, an ultrafast system with continuously tunable watt-level Raman soliton pulses from 2.8 μm to 3.6 μm wavelength can be achieved. However, the above SSFS system is always accompanied by background signal light or second-order Raman solitons, which limits the energy conversion efficiency, Raman frequency shift range, and spectral purity. Further work is needed to select first-order Raman solitons from background signal light or second-order Raman solitons to obtain pure Raman solitons, which will undoubtedly increase the system's structural complexity and operational difficulty. Summary of the Invention

[0005] The main objective of this invention is to provide a mid-infrared tunable pure soliton fiber laser based on cascaded amplification, which can obtain a high-power mid-infrared broadband tunable pure Raman soliton laser and significantly improve the spectral purity of the mid-infrared Raman soliton fiber laser.

[0006] To achieve the above objectives, the present invention provides a mid-infrared tunable pure soliton fiber laser based on cascaded amplification, comprising: a femtosecond pulse laser, a first-stage fiber amplifier, and a second-stage fiber amplifier;

[0007] The femtosecond pulse laser is used to generate a femtosecond pulse signal and transmit the femtosecond pulse signal to the first-stage fiber amplifier; the first-stage fiber amplifier is used to amplify the power of the femtosecond pulse signal and generate Raman solitons; the second-stage fiber amplifier is used to recycle and reuse the background signal light to amplify the Raman solitons; the first-stage fiber amplifier includes: a first polarization-correlated isolator, a first half-wave plate, a first quarter-wave plate, a first pump laser, a first pump light collimating lens, a first pump light laser dichroic mirror, a first pump light laser focusing lens, and a first optical fiber; the first polarization-correlated isolator is connected to the first half-wave plate, the first half-wave plate is connected to the first quarter-wave plate, and the first quarter-wave plate is connected to the first pump light laser dichroic mirror. The pump light collimating lens is connected to the first pump laser and the first pump light laser dichroic mirror, respectively, on opposite sides. The first pump light laser dichroic mirror is connected to the first pump light laser focusing lens. The first pump light laser dichroic mirror is used to combine the laser light passing through the quarter-wave plate and the pump light passing through the first pump light collimating lens, and transmit them to the first pump light laser focusing lens. The first pump light laser focusing lens is connected to the first optical fiber. The second-stage fiber amplifier includes: a cladding power stripper, a second optical fiber, a first optical fiber output cap, and a first laser collimating lens. The cladding power stripper is connected to the second optical fiber, the second optical fiber is connected to the first optical fiber output cap, and the first optical fiber output cap is connected to the first laser collimating lens.

[0008] Further, the femtosecond pulsed laser includes: a second pump laser, a second pump light collimating lens, a second pump light laser dichroic mirror, a second pump light laser focusing lens, a third optical fiber, a second optical fiber output cap, a second laser collimating lens, a third pump light laser dichroic mirror, a second half-wave plate, a polarization beam splitter, a second polarization correlation isolator, a gold mirror, and a second quarter-wave plate; the second pump laser is connected to the second pump light collimating lens; the second pump light collimating lens is connected to the second pump light laser dichroic mirror; the second pump light laser dichroic mirror is connected to the second pump light laser focusing lens and the second quarter-wave plate, for combining and transmitting the pump light and laser light from the second pump light collimating lens and the second quarter-wave plate. The second pump light laser focusing lens is connected to the third optical fiber. The output cap of the second optical fiber is connected to the other end of the third optical fiber. The output cap of the second optical fiber is also connected to the second laser collimating lens. The second laser collimating lens is connected to the third pump light laser dichroic mirror. The third pump light laser dichroic mirror is connected to the second half-wave plate. The second half-wave plate is connected to the polarization beam splitter. The polarization beam splitter is connected to the second polarization correlation isolator, which is used to output femtosecond pulse signals to the first-stage fiber amplifier and output laser to the second polarization correlation isolator. The second polarization correlation isolator is connected to the gold mirror. The gold mirror is connected to the second quarter-wave plate.

[0009] Furthermore, the first optical fiber is an erbium-doped fluoride optical fiber, and the first optical fiber is a double-clad optical fiber; the second optical fiber is a dysprosium-doped fluoride optical fiber, and the second optical fiber is a single-clad optical fiber.

[0010] Furthermore, the first optical fiber has an erbium ion doping concentration of 7 mol.%, a length of 3.9 m, a core diameter of 15 μm, a numerical aperture of 0.12, a cladding diameter of 260 μm, and is cut by two planes spaced 240 μm apart, with a cladding numerical aperture of 0.46; the second optical fiber has a dysprosium ion doping concentration of 0.2 mol.%, a length of 11 m, a core diameter of 12.5 μm, a cladding diameter of 125 μm, and a numerical aperture of 0.16.

[0011] Furthermore, the first pump laser is a 976 nm semiconductor pump laser; the center wavelength of the Raman soliton output by the second-stage fiber amplifier is 3.03 μm-3.63 μm, and the pulse energy is 0.4-31.8 nJ.

[0012] Furthermore, the third optical fiber is Er-doped. 3+ Fluoride ZBLAN optical fiber, Er 3+The ion concentration is 7 mol.%, the length is 2.4 m, the first optical fiber is a double-clad optical fiber with a core diameter of 15 μm, a numerical aperture of 0.12, a cladding diameter of 260 μm, and is cut by two planes spaced 240 μm apart. The cladding numerical aperture is 0.46.

[0013] Furthermore, the second pump laser is a 976 nm semiconductor pump laser; the fiber input end of the first fiber, the first fiber output end cap, the fiber input end of the third fiber, and the second fiber output end cap are all cut at an 8° angle.

[0014] Furthermore, the first fiber output cap and the second fiber output cap are made of zirconium fluoride, and the core diameter is 200μm. The first fiber output cap and the second fiber are connected by fusion splicing, and the second fiber output cap and the third fiber are connected by fusion splicing.

[0015] Furthermore, the Raman soliton generated by the first-stage fiber amplifier is 3 μm, and the background light is 2.8 μm.

[0016] Furthermore, the first optical fiber and the second optical fiber are connected by fusion splicing, and the power stripper is used to strip the power lost due to the fusion splicing of the first optical fiber and the second optical fiber, as well as the remaining 976 nm pump light of the first pump laser; the femtosecond pulse laser and the first-stage fiber amplifier are connected by spatial coupling.

[0017] This invention provides a mid-infrared tunable pure soliton fiber laser based on cascaded amplification. Its advantages are: it can recover and reuse background signal light to amplify Raman solitons, thereby obtaining higher output power and pulse energy, a wider tunable range, and pure Raman soliton pulses. Attached Figure Description

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

[0019] Figure 1 is a schematic block diagram of the structure of a mid-infrared tunable pure soliton fiber laser based on cascaded amplification according to an embodiment of the present invention.

[0020] Figure 2 is a schematic diagram of the first-stage fiber amplifier and the second-stage fiber amplifier of the mid-infrared tunable pure soliton fiber laser based on cascaded amplification in an embodiment of the present invention.

[0021] Figure 3 is a schematic diagram of the femtosecond pulsed laser based on a cascaded amplified mid-infrared tunable pure soliton fiber laser according to an embodiment of the present invention.

[0022] Figure 4 is a spectrum of the output pulse of the first-stage fiber amplifier of the mid-infrared tunable pure soliton fiber laser based on cascaded amplification in an embodiment of the present invention.

[0023] Figure 5 shows the spectrum of the output pulse of the second-stage fiber amplifier of the mid-infrared tunable pure soliton fiber laser based on cascaded amplification in an embodiment of the present invention.

[0024] In the accompanying drawings, the reference numerals indicate:

[0025] 01. Femtosecond pulsed laser; 02. First-stage fiber amplifier; 03. Second-stage fiber amplifier; 1. Second pump laser; 2. Second pump collimating lens; 3. Second pump laser dichroic mirror; 4. Second pump laser focusing lens; 5. Third fiber; 6. Second fiber output cap; 7. Second laser collimating lens; 8. Third pump laser dichroic mirror; 9. Second half-wave plate; 10. Polarizing beam splitter; 11. Second polarization correlation isolator ; 12. Gold mirror; 13. Second quarter-wave plate; 21. First polarization correlation isolator; 22. First half-wave plate; 23. First quarter-wave plate; 24. First pump laser; 25. First pump light collimating lens; 26. First pump light laser dichroic mirror; 27. First pump light laser focusing lens; 28. First optical fiber; 29. ​​Cladding power stripper; 30. Second optical fiber; 31. First optical fiber output cap; 32. First laser collimating lens. Embodiments of the present invention

[0026] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0027] Please refer to Figure 1, which shows a mid-infrared tunable pure soliton fiber laser based on cascaded amplification, including: a femtosecond pulse laser 01, a first-stage fiber amplifier 02, and a second-stage fiber amplifier 03; wherein, the femtosecond pulse laser 01 is used to generate a femtosecond pulse signal and transmit the femtosecond pulse signal to the first-stage fiber amplifier 02; the first-stage fiber amplifier 02 is used to amplify the power of the femtosecond pulse signal and generate Raman solitons; the second-stage fiber amplifier 03 is used to recover and reuse the background signal light to amplify the Raman solitons.

[0028] The mid-infrared tunable pure soliton fiber laser provided in this embodiment achieves higher output power and pulse energy, a wider tunable range, and pure Raman soliton pulses by recycling and reusing background signal light to amplify Raman solitons. This solution addresses the problem that SSFS cannot directly provide sufficiently high spectral purity mid-infrared broadband tunable femtosecond pulses.

[0029] Please refer to Figure 2. The first-stage fiber amplifier 02 includes: a first polarization-correlated isolator 21, a first half-wave plate 22, a first quarter-wave plate 23, a first pump laser 24, a first pump light collimating lens 25, a first pump light laser dichroic mirror 26, a first pump light laser focusing lens 27, and a first optical fiber 28. The first polarization-correlated isolator 21 is connected to the first half-wave plate 22, the first half-wave plate 22 is connected to the first quarter-wave plate 23, and the first quarter-wave plate 23 is connected to the first pump light... The laser dichroic mirror 26 is connected to the first pump laser 24 and the first pump laser dichroic mirror 26 respectively on opposite sides of the first pump light collimating lens 25. The first pump laser dichroic mirror 26 is connected to the first pump laser focusing lens 27. The first pump laser dichroic mirror 26 is used to combine the laser light that has passed through the first quarter-wave plate 23 and the pump light that has passed through the first pump light collimating lens 25, and transmit them to the first pump laser focusing lens 27. The first pump laser focusing lens 27 is connected to the first optical fiber 28.

[0030] Please refer to Figure 2. The second-stage fiber amplifier 03 includes: a cladding power stripper 29, a second fiber 30, a first fiber output cap 31, and a first laser collimating lens 32. The cladding power stripper 29 is connected to the second fiber 30, the second fiber 30 is connected to the first fiber output cap 31, and the first fiber output cap 31 is connected to the first laser collimating lens 32.

[0031] In this embodiment, the first optical fiber 28 is an erbium-doped fluoride optical fiber and is a double-clad optical fiber; the second optical fiber 30 is a dysprosium-doped fluoride optical fiber and is a single-clad optical fiber.

[0032] The first optical fiber 28 has an erbium doping concentration of 7 mol.%, a length of 3.9 m, a core diameter of 15 μm, a numerical aperture of 0.12, and a cladding diameter of 260 μm. It is intersected by two planes spaced 240 μm apart, and the cladding numerical aperture is 0.46. The second optical fiber 30 has a dysprosium doping concentration of 0.2 mol.%, a length of 11 m, a core diameter of 12.5 μm, a cladding diameter of 125 μm, and a numerical aperture of 0.16. The erbium ions are trivalent erbium ions (Er). 3+ The dysprosium ion is a trivalent dysprosium ion (Dy). 3+ .

[0033] In this embodiment, the pulse output of the femtosecond pulse laser 01 passes through the first polarization correlation isolator 21 to prevent reflected light from the cascaded amplifier from affecting the femtosecond pulse laser 01. The first half-wave plate 22 and the first quarter-wave plate 23 are used to adjust the polarization state of the seed pulse input to the cascaded amplifier. The first pump light collimating lens 25 is used to collimate the pump light output from the first pump laser 24, wherein the first pump laser 24 is a 976 nm semiconductor pump laser. After the pump light and the seed pulse laser are combined by the first pump light laser dichroic mirror 26, they are coupled into the first optical fiber 28 by the first pump light laser focusing lens 27 to form a forward pumping structure.

[0034] The Raman soliton generated by the first-stage fiber amplifier 02 is 3μm, while the background light is 2.8μm.

[0035] In this embodiment, the first-stage fiber amplifier 02 is used as an amplifier to increase the power of the 2.8 μm femtosecond pulse signal light and as a frequency shifter to generate 3 μm Raman solitons. Then, the 3 μm Raman solitons and the 2.8 μm background signal light are injected into the second-stage fiber amplifier 03, while the remaining 976 nm pump light is removed using the cladding power stripper 29.

[0036] In one embodiment, the first optical fiber 28 and the second optical fiber 30 are connected by fusion splicing, and the power stripper is used to strip the power lost due to the fusion splicing of the first optical fiber 28 and the second optical fiber 30; the femtosecond pulse laser 01 and the first-stage fiber amplifier 02 are connected by spatial coupling.

[0037] In this embodiment, the first optical fiber 28 and the second optical fiber 30 are connected by using fusion splicing technology. Due to the slight mode field mismatch between the two optical fibers, the actual signal light transmission transmittance at the splice point is measured to be 82%. The power loss caused by the fusion splicing is also removed by the cladding power stripper 29.

[0038] Referring to Figure 3, in one embodiment, the femtosecond pulsed laser 01 includes: a second pump laser 1, a second pump light collimating lens 2, a second pump light laser dichroic mirror 3, a second pump light laser focusing lens 4, a third optical fiber 5, a second optical fiber output cap 6, a second laser collimating lens 7, a third pump light laser dichroic mirror 8, a second half-wave plate 9, a polarization beam splitter prism 10, a second polarization correlation isolator 11, a gold mirror 12, and a second quarter-wave plate 13.

[0039] The second pump laser 1 is connected to the second pump light collimating lens 2; the second pump light collimating lens 2 is connected to the second pump light laser dichroic mirror 3; the second pump light laser dichroic mirror 3 is connected to the second pump light laser focusing lens 4 and the second quarter-wave plate 13, used to combine the pump light and laser light from the second pump light collimating lens 2 and the second quarter-wave plate 13, and transmit them to the second pump light laser focusing lens 4; the second pump light laser focusing lens 4 is connected to the third optical fiber 5, and the second optical fiber output cap 6 is connected to the other end of the third optical fiber 5. The fiber optic output cap 6 is also connected to the second laser collimating lens 7, which is connected to the third pump laser dichroic mirror. The third pump laser dichroic mirror is connected to the second half-wave plate 9, which is connected to the polarization beam splitter prism 10. The polarization beam splitter prism 10 is connected to the second polarization correlation isolator 11, which is used to output femtosecond pulse signals to the first-stage fiber amplifier 02 and output laser to the second polarization correlation isolator 11. The second polarization correlation isolator 11 is connected to the gold mirror 12, which is connected to the second quarter-wave plate 13.

[0040] Among them, the third optical fiber 5 is Er-doped. 3+ Fluoride ZBLAN optical fiber, Er 3+ The ion concentration is 7 mol.%, the length is 2.4 m, the first fiber 28 is a double-clad fiber with a core diameter of 15 μm, a numerical aperture of 0.12, a cladding diameter of 260 μm, and is cut by two planes spaced 240 μm apart. The cladding numerical aperture is 0.46.

[0041] The second pump laser 1 is a 976 nm semiconductor pump laser, emitting continuous pump light with a wavelength of 976 nm. This light is focused into the inner cladding of the third fiber 5 by the second pump light collimating lens 2 and the second pump light focusing lens 4. The second and third pump light dichroic mirrors are used to combine and separate the 2.8 μm laser and the 976 nm pump light, respectively. The second polarization-correlated isolator 11 ensures unidirectional circulation of the laser within the ring cavity and, together with the second half-wave plate 9 and the second quarter-wave plate 13, forms a passive mode-locking device to initiate and maintain mode-locked operation. The polarization beam splitter prism 10 serves as the pulse output port, and the gold mirror 12 ensures that the optical path forms a ring laser resonator. The femtosecond pulse laser 01 generates a mode-locked pulse operating in the soliton region with a pulse width of 257 fs.

[0042] In this embodiment, both the first pump laser 24 and the second pump laser 1 are 976 nm semiconductor pump lasers, which are fiber-coupled multimode semiconductor lasers.

[0043] In one embodiment, the first fiber output cap 31 and the second fiber output cap 6 are made of zirconium fluoride and have a core diameter of 200 μm. The first fiber output cap 31 and the first fiber 28 are connected by fusion splicing, and the second fiber output cap 6 and the second fiber 30 are connected by fusion splicing.

[0044] The fiber input end of the first fiber 28, the first fiber output end cap 31, the fiber input end of the third fiber 5, and the second fiber output end cap 6 are all cut at an 8° angle.

[0045] The second fiber output cap 6, cut at an 8° angle, is fused to the output end of the third fiber 5 to prevent damage to the fiber end face caused by prolonged laser operation. Its length is approximately 350 μm.

[0046] An 8° angle-cut first fiber optic output cap 31 is spliced ​​to the output end of the second fiber optic cable 30 to prevent damage to the fiber optic output end under high-power pulse output. (Dy-doped) 3+ The output beam of the fluoride fiber amplifier is collimated by the first laser collimating lens 32 for data measurement.

[0047] Please refer to Figure 4, which shows the spectral evolution of the output pulse of the first-stage fiber amplifier 02. As the pump power increases, the power of the 2.8 μm femtosecond pulse is amplified, and a Raman soliton is formed at 3 μm, gradually shifting to longer wavelength regions. The center wavelength of the Raman soliton output by the first-stage fiber amplifier 02 is always within the radiation cross-section of the second fiber 30. This is beneficial for the second-stage fiber amplifier 03 to amplify the Raman soliton by recycling and reusing the 2.8 μm background signal light.

[0048] Please refer to Figure 5, which shows the spectral evolution of the output pulse of the second-stage fiber amplifier 03. With increasing pump power, the center wavelength of the Raman soliton can be continuously tunable from 3.03 μm to 3.63 μm. No 2.8 μm background signal light or second-order Raman soliton was observed in the output spectrum, maintaining a pure Raman soliton state with excellent spectral purity throughout. Finally, at a pump power of 20 W, the pure Raman soliton with a center wavelength of 3.63 μm achieved a pulse energy of 31.8 nJ and a pulse duration of 210 fs, corresponding to a peak power of 151 kW.

[0049] The mid-infrared tunable pure soliton fiber laser based on cascaded amplification provided in this embodiment demonstrates that the present invention innovatively amplifies Raman solitons by recycling and reusing background signal light based on cascaded amplification, providing an effective approach for developing high-power, high-pulse-energy, broadband wavelength-tunable, and pure Raman soliton pulsed fiber lasers.

[0050] It should be noted that the above embodiments are examples for illustrating the technical solution of the present invention in detail, and not for limiting it. The present invention can also be applied to other pure Raman soliton fiber laser systems based on SSFS, such as Er-doped... 3+ and doping with Tm 3+ Fiber lasers with cascaded fiber amplification and Tm doping 3+ and Ho 3+ Fiber lasers with cascaded fiber amplification, etc. Any modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the appended claims.

[0051] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0052] The above is a description of a mid-infrared tunable pure soliton fiber laser based on cascaded amplification provided by the present invention. For those skilled in the art, based on the ideas of the embodiments of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A mid-infrared tunable pure soliton fiber laser based on cascaded amplification, characterized in that, include: Femtosecond pulsed laser, first-stage fiber amplifier, and second-stage fiber amplifier; The femtosecond pulse laser is used to generate femtosecond pulse signals and transmit the femtosecond pulse signals to the first-stage fiber amplifier; the first-stage fiber amplifier is used to amplify the power of the femtosecond pulse signals and generate Raman solitons; The second-stage fiber amplifier is used to recover and reuse the background signal light to amplify the Raman soliton; The first-stage fiber amplifier includes: a first polarization-dependent isolator, a first half-wave plate, a first quarter-wave plate, a first pump laser, a first pump collimating lens, a first pump laser dichroic mirror, a first pump laser focusing lens, and a first optical fiber; the first polarization-dependent isolator is connected to the first half-wave plate, the first half-wave plate is connected to the first quarter-wave plate, the first quarter-wave plate is connected to the first pump laser dichroic mirror, the opposite sides of the pump laser collimating lens are respectively connected to the first pump laser and the first pump laser dichroic mirror, the first pump laser dichroic mirror is connected to the first pump laser focusing lens, the first pump laser dichroic mirror is used to combine the laser light passing through the quarter-wave plate and the pump light passing through the first pump laser collimating lens, and transmit them to the first pump laser focusing lens, the first pump laser focusing lens is connected to the first optical fiber; The second-stage fiber amplifier includes: a cladding power stripper, a second fiber, a first fiber output cap, and a first laser collimating lens; the cladding power stripper is connected to the second fiber, the second fiber is connected to the first fiber output cap, and the first fiber output cap is connected to the first laser collimating lens.

2. The mid-infrared tunable pure soliton fiber laser based on cascaded amplification according to claim 1, characterized in that, The femtosecond pulsed laser includes: Second pump laser, second pump collimating lens, second pump laser dichroic mirror, second pump laser focusing lens, third optical fiber, second optical fiber output cap, second laser collimating lens, third pump laser dichroic mirror, second half-wave plate, polarization beam splitter, second polarization correlation isolator, gold mirror, second quarter-wave plate; The second pump laser is connected to the second pump light collimating lens; the second pump light collimating lens is connected to the second pump light laser dichroic mirror; the second pump light laser dichroic mirror is connected to the second pump light laser focusing lens and the second quarter-wave plate, for combining the pump light and laser light from the second pump light collimating lens and the second quarter-wave plate, and transmitting them to the second pump light laser focusing lens; the second pump light laser focusing lens is connected to the third optical fiber, the second optical fiber output cap is connected to the other end of the third optical fiber, the second optical fiber output cap is also connected to the second laser collimating lens, the second laser collimating lens is connected to the third pump light laser dichroic mirror, the third pump light laser dichroic mirror is connected to the second half-wave plate, the second half-wave plate is connected to the polarization beam splitter, the polarization beam splitter is connected to the second polarization correlation isolator, for outputting femtosecond pulse signals to the first-stage fiber amplifier and outputting laser light to the second polarization correlation isolator, the second polarization correlation isolator is connected to the gold mirror, and the gold mirror is connected to the second quarter-wave plate.

3. The mid-infrared tunable pure soliton fiber laser based on cascaded amplification according to claim 1, characterized in that, The first optical fiber is an erbium-doped fluoride optical fiber, and the first optical fiber is a double-clad optical fiber; The second optical fiber is a dysprosium-doped fluoride optical fiber, and the second optical fiber is a single-layer clad optical fiber.

4. The mid-infrared tunable pure soliton fiber laser based on cascaded amplification according to claim 3, characterized in that, The first optical fiber has an erbium ion doping concentration of 7 mol.%, a length of 3.9 m, a core diameter of 15 μm, a numerical aperture of 0.12, a cladding diameter of 260 μm, and is cut by two planes spaced 240 μm apart. The cladding numerical aperture is 0.

46. The second optical fiber has a dysprosium ion doping concentration of 0.2 mol.%, a length of 11 m, a core diameter of 12.5 μm, a cladding diameter of 125 μm, and a numerical aperture of 0.

16.

5. The mid-infrared tunable pure soliton fiber laser based on cascaded amplification according to claim 1, characterized in that, The first pump laser is a 976 nm semiconductor pump laser; The center wavelength of the Raman soliton output by the second-stage fiber amplifier is 3.03 μm-3.63 μm, and the pulse energy is 0.4-31.8 nJ.

6. The mid-infrared tunable pure soliton fiber laser based on cascaded amplification according to claim 2, characterized in that, The third optical fiber is Er-doped. 3+ Fluoride ZBLAN optical fiber, Er 3+ The ion concentration is 7 mol.%, the length is 2.4 m, the first optical fiber is a double-clad optical fiber with a core diameter of 15 μm, a numerical aperture of 0.12, a cladding diameter of 260 μm, and is cut by two planes spaced 240 μm apart. The cladding numerical aperture is 0.

46.

7. The mid-infrared tunable pure soliton fiber laser based on cascaded amplification according to claim 2, characterized in that, The second pump laser is a 976 nm semiconductor pump laser; The fiber input end of the first optical fiber, the cap of the first optical fiber output end, the fiber input end of the third optical fiber, and the cap of the second optical fiber output end are all cut at an 8° angle.

8. The mid-infrared tunable pure soliton fiber laser based on cascaded amplification according to claim 2, characterized in that, The first fiber output cap and the second fiber output cap are made of zirconium fluoride and have a core diameter of 200 μm. The first fiber output cap and the first fiber are connected by fusion splicing, and the second fiber output cap and the second fiber are connected by fusion splicing.

9. The mid-infrared tunable pure soliton fiber laser based on cascaded amplification according to claim 1, characterized in that, The Raman soliton generated by the first-stage fiber amplifier is 3 μm, while the background light is 2.8 μm.

10. The mid-infrared tunable pure soliton fiber laser based on cascaded amplification according to claim 1, characterized in that, The first optical fiber and the second optical fiber are connected by fusion splicing. The power stripper is used to strip the power lost due to the fusion splicing of the first optical fiber and the second optical fiber, as well as the remaining 976nm pump light of the first pump laser. The femtosecond pulsed laser and the first-stage fiber amplifier are connected by spatial coupling.

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