Dual-wavelength femtosecond fiber laser

Abstract

The present application relates to the technical field of optical fibers, in particular to a dual-wavelength femtosecond fiber laser. The laser comprises a pumping light source, which is used for outputting first-wavelength femtosecond pulse laser and a first excitation light source; a double-clad nonlinear optical fiber, which is connected to an output end of the pumping light source and is used for converting part of the first-wavelength femtosecond pulse laser into second-wavelength femtosecond pulse laser by means of using the soliton self-frequency shift effect, and outputting the remaining first-wavelength femtosecond pulse laser, the second-wavelength femtosecond pulse laser and the first excitation light source; and a holmium-doped gain fiber, which is connected to the output end of the double-clad nonlinear optical fiber and is used for amplifying the first-wavelength femtosecond pulse laser and the second-wavelength femtosecond pulse laser under the excitation of the first excitation light source. Therefore, the present application achieves the output of dual-wavelength high-energy femtosecond pulse laser in full-fiber mid-infrared lasers, achieves a compact and portable structure, good environmental adaptability and wavelength and energy advantages in practical applications such as medicine and material processing, and is applicable to vehicle-mounted scenarios, airborne scenarios, etc.

Description

Dual-wavelength femtosecond fiber laser

[0001] The present application claims priority to the Chinese Patent Application No. 202410404771.0, filed on April 7, 2024, and entitled "Dual-wavelength femtosecond fiber laser", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the field of optical fiber technology, in particular to a dual-wavelength femtosecond fiber laser. BACKGROUND

[0003] Mid-infrared femtosecond laser has a wide application prospect in the fields of biomedicine, environmental monitoring and industrial processing, because there is strong molecular fundamental vibration absorption in this band, and it also contains important atmospheric transmission windows. In particular, dual-wavelength pulsed laser has unique advantages in medical and material processing fields. For example, in laser surgery, dual-wavelength pulsed laser can interact with tissues of different depths, improve ablation efficiency and reduce heat diffusion zone, thereby having the advantages of small surgical wound, fast wound healing and narrow scar width.

[0004] In recent years, with the in-depth research of rare earth ion doped fluoride fiber, mode-locked laser based on rare earth ion doping can directly generate mid-infrared femtosecond laser, but due to the limitation of fiber material and device, dual-wavelength femtosecond laser cannot be directly generated. Raman soliton laser has the ability to realize dual-wavelength femtosecond laser output and allows the construction of all-fiber structure laser system, but the average power and pulse energy of the output femtosecond pulsed laser are generally low, and the practicability is poor.

[0005] SUMMARY

[0006] The present application provides a dual-wavelength femtosecond fiber laser to solve the problem that the conventional fiber laser cannot output high-energy dual-wavelength femtosecond pulsed laser.

[0007] The application provides a dual-wavelength femtosecond fiber laser, comprising: a pump light source, used for outputting first-wavelength femtosecond pulse laser and outputting a first excitation light source; the wavelength of the first-wavelength femtosecond pulse laser is 2.1 μm; a double-clad nonlinear optical fiber, connected with the output end of the pump light source, used for receiving the first-wavelength femtosecond pulse laser and the first excitation light source, converting part of the first-wavelength femtosecond pulse laser into second-wavelength femtosecond pulse laser by using the soliton self-frequency shift effect, the wavelength of the second-wavelength femtosecond pulse laser is 2.9 μm; and outputting the remaining part of the first-wavelength femtosecond pulse laser, the second-wavelength femtosecond pulse laser and the first excitation light source; a holmium-doped gain fiber, connected with the output end of the double-clad nonlinear optical fiber, used for receiving the first excitation light source, the first-wavelength femtosecond pulse laser and the second-wavelength femtosecond pulse laser, and amplifying the energy of the first-wavelength femtosecond pulse laser and the second-wavelength femtosecond pulse laser under the excitation of the first excitation light source; and an end cap, connected with the output end of the holmium-doped gain fiber, used for protecting the output end face of the holmium-doped gain fiber and amplifying the spot energy density of the first-wavelength femtosecond pulse laser and the second-wavelength femtosecond pulse laser.

[0008] In an implementable mode, the pump light source comprises: a mode-locked thulium-doped fiber oscillator, a first amplification stage, a fiber stretcher and a second amplification stage; the mode-locked thulium-doped fiber oscillator is used for generating first femtosecond pulse laser, the wavelength of the first femtosecond pulse laser is 2 μm; the first amplification stage is connected with the output end of the mode-locked thulium-doped fiber oscillator, used for receiving the first femtosecond pulse laser, power-amplifying the first femtosecond pulse laser, and then outputting the first femtosecond pulse laser; the fiber stretcher is connected with the output end of the first amplification stage, used for receiving the first femtosecond pulse laser, time-domain stretching the first femtosecond pulse laser, and then outputting the first femtosecond pulse laser; and the second amplification stage is connected with the output end of the fiber stretcher, used for receiving the first femtosecond pulse laser, power-amplifying the first femtosecond pulse laser again, and then outputting the first femtosecond pulse laser.

[0009] In an implementable mode, the wavelength of the first-wavelength femtosecond pulse laser has a first fluctuation range, the first fluctuation range is greater than or equal to 2.02 μm and less than or equal to 2.12 μm; the second-wavelength femtosecond pulse laser has a second fluctuation range, the second fluctuation range is greater than or equal to 2.89 μm and less than or equal to 3.01 μm; and the first femtosecond pulse laser has a third fluctuation range, the third fluctuation range is greater than or equal to 1.90 μm and less than or equal to 2.05 μm.

[0010] In an implementable manner, the pump light source further comprises a first laser, a second laser, a first fiber coupler and a first thulium-doped gain fiber; the first laser is configured to output a first excitation light source, and the first excitation light source has a wavelength of 1150 nm; the second laser is configured to output a second excitation light source, and the second excitation light source has a wavelength of 793 nm; a first pump input end of the first fiber coupler is connected with an output end of the first laser, a second pump input end of the first fiber coupler is connected with an output end of the second laser, and a signal input end of the first fiber coupler is connected with an output end of the second amplification stage; the first fiber coupler is configured to couple the first femtosecond pulse laser, the first excitation light source and the second excitation light source into the first thulium-doped gain fiber; the first thulium-doped gain fiber is connected with an output end of the first fiber coupler, configured to receive the first femtosecond pulse laser, the first excitation light source and the second excitation light source, and further amplify and compress the pulse width of the first femtosecond pulse laser under the excitation of the second excitation light source and part of the first excitation light source; the first thulium-doped gain fiber is further configured to convert the amplified first femtosecond pulse laser into a first-wavelength femtosecond pulse laser through a soliton self-frequency shift effect generated when the first femtosecond pulse laser with high peak power is transmitted in the first thulium-doped gain fiber; and the first thulium-doped gain fiber is further configured to output the first-wavelength femtosecond pulse laser and the remaining part of the first excitation light source.

[0011] In an implementable manner, the double-clad nonlinear fiber is connected with the output end of the first thulium-doped gain fiber to receive the first-wavelength femtosecond pulse laser and the first excitation light source.

[0012] In an implementable manner, the matrix material of the double-clad nonlinear fiber is fluorotellurite glass; the mode field area of the double-clad nonlinear fiber is smaller than that of the holmium-doped gain fiber; and the zero-dispersion wavelength of the double-clad nonlinear fiber is less than 2 μm.

[0013] In an implementable manner, the matrix material of the holmium-doped gain fiber is zirconium fluoride glass; and the doping concentration of holmium ions in the holmium-doped gain fiber is greater than 2.5 mol. %.

[0014] In an implementable manner, the holmium-doped gain fiber is specifically configured to amplify the energy of the first-wavelength femtosecond pulse laser and the second-wavelength femtosecond pulse laser; during the energy amplification of the first-wavelength femtosecond pulse laser and the second-wavelength femtosecond pulse laser by the holmium-doped gain fiber, an energy level transition process occurs in the holmium-doped gain fiber, and the energy level transition process comprises: the holmium ions in the ground state 5 absorbing energy to transition to a high energy level 5 I6 under the action of the light field of the first excitation light source, so that the population of the holmium ions in the high energy level 5 I6 is greater than the population of the holmium ions in the ground state 5 I8; the population of the holmium ions in the high energy level 5The holmium ion of I6 is excited to a low energy level by the second wavelength femtosecond pulse laser 5 I7 transition and release the same wavelength photon as the second wavelength femtosecond pulse laser; in a low energy level 5 The holmium ion of I7 is excited to the ground state by the first wavelength femtosecond pulse laser 5 I8 transition and release the same wavelength photon as the first wavelength femtosecond pulse laser.

[0015] In an implementable manner, the gain coefficient of the holmium-doped gain fiber when amplifying the energy of the first wavelength femtosecond pulse laser is determined based on formula one, and the gain coefficient when amplifying the energy of the second wavelength femtosecond pulse laser is determined based on formula two;

[0016] Formula one:

[0017] Wherein, ∫ is the integral symbol, g 2.1μm (z) is the gain coefficient of the first wavelength femtosecond pulse laser at the z position of the holmium-doped gain fiber; τ 2.1 is the upper energy level lifetime of the first wavelength femtosecond pulse laser; τ 2.9 is the upper energy level lifetime of the second wavelength femtosecond pulse laser; β is the branching ratio of the radiation decay between the high energy level 5 I6 and the low energy level 5 I7; σ e2.1 is the stimulated emission cross section corresponding to the first wavelength femtosecond pulse laser, σ a2.1 is the absorption cross section corresponding to the first wavelength femtosecond pulse laser, A eff is the mode field area of the holmium-doped gain fiber, h is the Planck constant, υ p is the frequency of the first excitation light source, P a is the power of the holmium-doped gain fiber absorbing the first excitation light source, n is the doping concentration of holmium ions, and L is the fiber length of the holmium-doped gain fiber;

[0018] Formula two:

[0019] Wherein, ∫ is the integral symbol, g 2.9μm (z) is the gain coefficient of the second wavelength femtosecond pulse laser at the z position of the holmium-doped gain fiber; τ 2.1 is the upper energy level lifetime of the first wavelength femtosecond pulse laser; τ 2.9 is the upper energy level lifetime of the second wavelength femtosecond pulse laser; β is the branching ratio of the radiation decay between the high energy level 5 I6 and the low energy level 5 I7; σ e2.9 is the stimulated emission cross section corresponding to the second wavelength femtosecond pulse laser, σ a2.9 is the absorption cross section corresponding to the second wavelength femtosecond pulse laser, A effA is the mode field area of the holmium-doped gain fiber, h is Planck's constant, and υ is the frequency of the first excitation light source p A is the frequency of the first excitation light source a A is the power absorbed by the holmium-doped gain fiber from the first excitation light source.

[0020] In an implementable manner, the peak powers of the first-wavelength femtosecond pulsed laser and the second-wavelength femtosecond pulsed laser output by the double-clad nonlinear fiber are determined based on the following formula three, and the pulse widths are determined based on the following formula four.

[0021] Formula three:

[0022] wherein, A is the mode field area of the holmium-doped gain fiber, h is Planck's constant, and υ is the frequency of the first excitation light source 2.1 A is the peak power of the first-wavelength femtosecond pulsed laser, and A is the peak power of the second-wavelength femtosecond pulsed laser 2.9μm A is the peak power of the first-wavelength femtosecond pulsed laser, and A is the peak power of the second-wavelength femtosecond pulsed laser 2.1 T represents the pulse width of the first-wavelength femtosecond pulsed laser

[0023] Formula four:

[0024] wherein, T represents the pulse width of the first-wavelength femtosecond pulsed laser, and T represents the pulse width of the second-wavelength femtosecond pulsed laser 2.1 T represents the pulse width of the first-wavelength femtosecond pulsed laser, and T represents the pulse width of the second-wavelength femtosecond pulsed laser 2.9μm A is the peak power of the first-wavelength femtosecond pulsed laser, and A is the peak power of the second-wavelength femtosecond pulsed laser 2.1 A is the peak power of the first-wavelength femtosecond pulsed laser.

[0025] It can be known from the above that the application provides a dual-wavelength femtosecond fiber laser, which comprises a pump light source, a double-clad nonlinear fiber, and a holmium-doped gain fiber.

[0026] The application has the following beneficial effects:

[0027] 1.The present application provides dual-wavelength femtosecond pulse laser by using Raman soliton laser, and the energy is improved by using mid-infrared fiber amplifier, wherein the excitation light source required by the mid-infrared fiber amplifier is input into the laser system through the front-stage quartz-based fiber coupler. Thus, the shortcomings of the existing mid-infrared fiber laser are overcome, the dual-wavelength high-energy femtosecond pulse laser is realized in the mid-infrared laser with all-fiber structure, and the cost is low, the structure is compact, portable, and environmentally friendly, and the wavelength advantage and energy advantage are obtained in medical and material processing applications, which is suitable for vehicle-mounted, airborne and other scenes. Further, by selecting Raman soliton lasers with different wavelengths and rare earth ion doped fibers with different types, dual-wavelength femtosecond pulse lasers with different wavelengths can be output, which has good portability and expandability, and is more conducive to practical application.

[0028] 2.The present application provides a dual-wavelength femtosecond fiber laser, wherein a 1150nm light source required for stimulated absorption of holmium ions is coupled into the laser system in the thulium-doped fiber amplification stage. Part of the 1150nm excitation light source is used to excite the thulium ions to the 3 H6→ 3 H5” energy level transition, which is easy to excite the cross relaxation process compared with the single 793nm excitation light source, significantly increases the slope efficiency of thulium ion laser radiation, and promotes the energy conversion efficiency of 2μm to 2.1μm soliton shift; the remaining part of the 1150nm excitation light source is coupled into the holmium-doped zirconium fluoride glass fiber to pump the holmium ions from the 5 I8 energy level to the 5 I6 energy level to realize population inversion; further, the 2.9μm laser and the 2.1μm laser are simultaneously coupled into the holmium-doped zirconium fluoride glass fiber, the 2.9μm laser induces the stimulated radiation of the 5 I6→ 5 I7” energy level to realize energy amplification of the 2.9μm laser. At the same time, the 2.1μm laser induces the stimulated radiation of the 5 I7→ 5 I8” energy level to realize energy amplification of the 2.1μm laser, which can reduce the population of the 5 I7 energy level, and alleviate the laser transition self-termination phenomenon caused by the shorter upper energy level lifetime than the lower energy level lifetime in the transition from the 5 I6→ 5 I7” energy level, and significantly improve the slope efficiency and output power of the 2.9μm fiber laser amplifier. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the present application, the drawings needed in the embodiments will be briefly introduced as follows. Obviously, other drawings can also be obtained by those skilled in the art without any creative effort based on these drawings.

[0030] Fig. 1 is a structural schematic diagram of a dual-wavelength femtosecond pulse fiber laser provided by the present application;

[0031] Fig. 2 is a simplified energy level diagram of the energy level transition of holmium ions in the amplification process of the first-wavelength femtosecond pulse laser and the second-wavelength femtosecond pulse laser provided by the present application;

[0032] Fig. 3 is a schematic diagram of the evolution in the frequency domain of the first-wavelength femtosecond pulse laser in the transmission in the double-clad fluorotellurite glass fiber and the holmium-doped zirconium fluoride glass fiber provided by the present application;

[0033] Fig. 4 is a spectral diagram of the light output at the tail end of the double-clad fluorotellurite glass fiber and a spectral diagram of the light output at the tail end of the holmium-doped zirconium fluoride glass fiber provided by the present application;

[0034] Fig. 5 is a partial enlarged view of part A in Fig. 4 provided by the present application;

[0035] Fig. 6 is a schematic diagram of the evolution in the time domain of the first-wavelength femtosecond pulse laser in the transmission in the double-clad fluorotellurite glass fiber and the holmium-doped zirconium fluoride glass fiber provided by the present application;

[0036] Fig. 7 is a time-domain pulse curve diagram of the light output at the tail end of the double-clad fluorotellurite glass fiber and a time-domain pulse curve diagram of the light output at the tail end of the holmium-doped zirconium fluoride glass fiber provided by the present application;

[0037] Fig. 8 is a self-correlation trajectory diagram of the 2.9 μm laser output at the tail end of the holmium-doped zirconium fluoride glass fiber provided by the present application;

[0038] Fig. 9 is a structural schematic diagram of a mode-locked thulium-doped fiber oscillator provided by the present application;

[0039] Fig. 10 is a structural schematic diagram of a first amplification stage provided by the present application;

[0040] Fig. 11 is an output spectral diagram of a pump light source 10 provided by the present application;

[0041] Fig. 12 is a self-correlation curve diagram of the pump light source 10 provided by the present application;

[0042] Fig. 13 is a simplified energy level diagram of thulium ions in a thulium-doped gain fiber provided by the present application;

[0043] Fig. 14 is an absorption spectral diagram of thulium ions in the thulium-doped gain fiber provided by the present application;

[0044] Figure 15 is a schematic diagram of a working process of a dual-wavelength femtosecond pulse fiber laser according to an embodiment of the present application.

[0045] In the figure, 10 is a pump light source; 11 is a mode-locked thulium-doped fiber oscillator; 111 is a 1570 nm semiconductor laser; 112 is an optical wavelength division multiplexer; 113 is a second thulium-doped gain fiber; 114 is a fiber output coupler; 115 is a first fiber isolator; 116 is a saturable absorber assembly; 12 is a first amplification stage; 121 is a 793 nm semiconductor laser; 122 is a second fiber coupler; 123 is a third thulium-doped gain fiber; 124 is a second fiber isolator; 13 is a fiber stretcher; 14 is a second amplification stage; 15 is a first laser; 16 is a second laser; 17 is a first fiber coupler; 171 is a first pump input end; 172 is a second pump input end; 173 is a signal input end; 18 is a first thulium-doped gain fiber; 20 is a double-clad nonlinear fiber; 30 is a holmium-doped gain fiber; and 40 is an end cap. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the present application will be clearly described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0047] Hereinafter, the terms "first", "second", and the like are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first", "second", and the like can explicitly or implicitly include one or more of the features. In the description of the present application, unless otherwise specified, the meaning of "a plurality of" is two or more.

[0048] In addition, in the present application, the orientation terms such as "upper", "lower", "inner", "outer", and the like are defined with respect to the orientation of the components shown in the drawings, and it should be understood that these directional terms are relative concepts, which are used for relative description and clarification, and can be changed accordingly according to the change of the orientation of the components placed in the drawings.

[0049] First, the terms related to the embodiments of the present application are introduced.

[0050] 1. Near-infrared and mid-infrared bands: Near-infrared and mid-infrared bands refer to two different frequency bands in the electromagnetic spectrum. The near-infrared band typically ranges from 700 to 1100 nm, and it is characterized by its ability to penetrate certain thicknesses of biological tissue, glass, plastic, and other materials without being absorbed. Therefore, it has a wide range of applications in medical imaging, remote sensing, infrared thermal imaging, and other fields.

[0051] The mid-infrared band, which generally ranges from 2 to 5 μm in the field of laser technology, is characterized by containing important atmospheric transmission windows and molecular characteristic lines. This band of light can be absorbed by most molecules, so it has a wide range of applications in molecular spectroscopy, material processing, biomedicine, and other fields.

[0052] 2. Femtosecond pulsed laser: A femtosecond pulsed laser is a type of laser with extremely short pulses, with a pulse width in the femtosecond range, where 1 femtosecond = 10 -15 -12 seconds. Femtosecond pulsed lasers have many special physical and chemical effects, and because of their extremely short pulse times, they have very high energy densities, allowing them to perform micro-machining, micro-fabrication, and fine cutting on material surfaces. They can also be applied in the fields of spectroscopy, optical imaging, and medical diagnosis.

[0053] 3. All-fiber: All-fiber refers to a technology that uses fiber devices for all components and elements in an optical system. The goal is to integrate transmission, modulation, amplification, and other functions in an optical system (such as a laser) into a fiber, achieving a completely fiber-based structure. All-fiber technology has a series of advantages. First, as a signal transmission medium, fiber has low loss, high bandwidth, and anti-interference characteristics, which can effectively reduce signal attenuation and distortion. Second, fiber devices are small, lightweight, and highly reliable, less susceptible to external interference and damage, improving system stability and reliability. In addition, all-fiber can simplify system structure, reduce size and weight, and facilitate integration and deployment.

[0054] 4. Gain fiber: Gain fiber is a specially designed fiber used to amplify optical signals. It usually includes a fiber core doped with rare earth elements such as thulium, erbium, holmium, or ytterbium, which can transition when excited by external stimuli, providing amplification to optical signals. The amplification process in gain fiber is achieved by exciting light to excite the doped material to transition. When the excitation light propagates into the doped fiber, it excites the doped material to transition to a high-energy state. This excitation causes the doped material to be in an excited state, and then when the input optical signal passes through the gain fiber, the doped material transitions to a lower energy level, amplifying the optical signal transmitted through the fiber.

[0055] 5、Nonlinear fiber: Nonlinear fiber is a type of optical fiber that exhibits nonlinear optical properties, where the refractive index changes with the intensity of light, leading to nonlinear effects during the propagation of light within it. It is widely used in laser technology, optical communication, and sensors. Some common nonlinear effects include self-focusing, self-phase modulation, stimulated Raman scattering, four-wave mixing, and soliton self-frequency shift.

[0056] 6、Soliton self-frequency shift effect: Specifically, when a pulsed laser with high peak power propagates in a nonlinear fiber, due to the interaction between light and the molecules or lattice in the fiber, stimulated Raman scattering within the pulse occurs, resulting in a shift in the frequency of the light. This frequency shift occurs in the form of a soliton. A soliton is a special wave structure whose energy and shape remain unchanged during propagation. Soliton self-frequency shift is of great significance in optical communication and nonlinear optics research, as it can be used to generate new optical frequencies, achieve frequency conversion of optical signals, and expand the optical spectrum.

[0057] 7、Excitation: In a laser, the excitation process refers to the process of providing the required energy for the excited state through energy input. The excitation source can excite the ground state electrons of ions to the excited state, thereby achieving population inversion, which refers to the state where the number of particles in the excited state is greater than the number of particles in the ground state. This process is called excitation, which is equivalent to injecting energy into the laser medium, enabling it to produce stimulated absorption processes.

[0058] 8、Stimulated radiation: When a light-emitting atom in an excited state transitions to a lower energy state or ground state under the action of an external radiation field, it emits photons. At this time, the energy of the external radiation must be exactly the energy difference between the two energy levels of the atom. The frequency, phase, propagation direction, and polarization state of the emitted photons and external photons are all the same. Stimulated radiation is a necessary condition for the generation of laser light.

[0059] Mid-infrared (MIR) lasers have wide applications in biomedical, environmental monitoring and industrial processing, due to the strong molecular fundamental vibration absorption and the atmospheric transmission window. In recent years, MIR pulse fiber lasers have been rapidly developed, thanks to the mature fabrication technology of rare-earth ion-doped fluoride fibers. Dual-wavelength pulse lasers, which can simultaneously generate 2.1 μm and 2.9 μm laser outputs, have unique advantages in medical and material processing applications. For example, in laser surgery, dual-wavelength pulse lasers can be used for tissue ablation and cutting, such as skin, subcutaneous tissue, muscle, etc. 2.1 μm and 2.9 μm are close to the absorption peaks of hydroxyl and have different absorption coefficients, which can interact with different depth tissues, improve the ablation efficiency and reduce the heat diffusion zone. Femtosecond infrared laser can selectively provide energy for water molecules in the tissue to drive the ablation or cutting process, which is faster than energy heat exchange and shock wave propagation, and does not form plasma or produce ionizing radiation effect, reducing the thermal damage to the surrounding tissue, with the advantages of small surgical wound, fast wound healing, narrow scar width, etc. Therefore, the development of dual-wavelength femtosecond pulse fiber lasers has important scientific significance and application value. At present, the methods to realize 2.1 μm and 2.9 μm dual-wavelength pulse output mainly include two categories: Ho-doped fiber oscillators based on gain modulation or Q-switching technology, and technical solutions combining the advantages of both. The former and the latter are to realize pulse output by periodically modulating the gain and loss in the resonant cavity, respectively, and the pulse width is generally in the order of microseconds or nanoseconds. There are several ways to generate mid-infrared femtosecond laser, at present, the main methods to realize 2.9 μm femtosecond laser include direct generation based on rare-earth ion stimulated radiation and indirect generation based on nonlinear frequency conversion. The former is mainly based on Ho 3+ 's 5 I6→ 5 I7' energy level transition and mode-locked laser oscillator, and the latter mainly includes solid-state lasers and fiber lasers based on optical parametric oscillation, difference frequency, stimulated Raman scattering technology, etc. Among them, fiber lasers are the most promising to realize portable, stable and efficient mid-infrared pulse laser output due to their good beam quality, compact structure and good environmental adaptability, which has attracted widespread attention. However, the mid-infrared mode-locked fiber laser is limited by the properties of mid-infrared glass materials, which makes the mid-infrared fiber laser technology and pulse modulation technology seriously lag behind in the core devices, and the system structure has not been fully fiberized, far from the stability, high efficiency and high power of near-infrared fiber lasers. Raman soliton fiber lasers are easy to realize full-fiber structure, and the whole system has good environmental adaptability. By adjusting the matching conditions of pump light source and nonlinear fiber, 2.1 μm and 2.9 μm dual-wavelength femtosecond pulse outputs can be realized. However, due to the small mode field area, the femtosecond laser pulse energy and power obtained are generally low.

[0060] In summary, the Ho-doped fiber oscillator cannot directly output 2.1 μm and 2.9 μm dual-wavelength femtosecond pulse lasers, and lacks a mid-infrared fiber device, so that the all-fiber structure of the femtosecond fiber oscillator cannot be realized; the Raman soliton fiber laser can realize all-fiber output of dual-wavelength femtosecond pulses, but the average power and pulse energy of the femtosecond pulse laser are generally low.

[0061] The embodiment of the present application provides a dual-wavelength femtosecond pulse fiber laser, which can simultaneously output 2.1 μm and 2.9 μm dual-wavelength femtosecond pulse lasers, can realize all-fiber of the laser system, and has practical significance.

[0062] Fig. 1 is a structural schematic diagram of the dual-wavelength femtosecond pulse fiber laser provided by the embodiment of the present application.

[0063] As shown in Fig. 1, the dual-wavelength femtosecond pulse fiber laser provided by the embodiment of the present application comprises a pump light source 10, a double-clad nonlinear optical fiber 20, a Ho-doped gain optical fiber 30 and an end cap 40, which can be sequentially connected in order by means of fusion splicing.

[0064] The pump light source 10 is used to output a first-wavelength femtosecond pulse laser and a first excitation light source. The wavelength of the first-wavelength femtosecond pulse laser can be 2.1 μm, and the wavelength of the first excitation light source can be 1150 nm. That is, the pump light source 10 can output 2.1 μm femtosecond pulse laser and 1150 nm continuous laser.

[0065] In actual application, the first excitation light source can have a certain wavelength fluctuation range, which can be ±10 nm, that is, the wavelength range of the first excitation light source is 1140 nm-1160 nm, which is not limited in the embodiment of the present application.

[0066] In the embodiment of the present application, the wavelength of the first-wavelength femtosecond pulse laser can have a first fluctuation range, which is greater than or equal to 2.02 μm and less than or equal to 2.12 μm.

[0067] The double-clad nonlinear optical fiber 20 is specifically connected with the output end of the pump light source 10, and is used to receive the first-wavelength femtosecond pulse laser and the first excitation light source. By using the soliton self-frequency shift effect, part of the first-wavelength femtosecond pulse laser is subjected to frequency conversion, and the wavelength is accordingly shifted to form a second-wavelength femtosecond pulse laser. The wavelength of the second-wavelength femtosecond pulse laser is 2.9 μm.

[0068] That is, the 2.1 μm femtosecond pulse laser (first wavelength femtosecond pulse laser) can excite the dispersion and nonlinear effects in the double-clad nonlinear optical fiber 20 to generate a new component frequency, that is, the 2.1 μm femtosecond pulse laser is shifted to a 2.9 μm wavelength.

[0069] In the embodiment of the present application, the wavelength of the second wavelength femtosecond pulse laser can have a second wavelength fluctuation range, and the second wavelength fluctuation range is greater than or equal to 2.89 μm and less than or equal to 3.01 μm.

[0070] Further, the double-clad nonlinear optical fiber 20 can output the residual first wavelength femtosecond pulse laser and the newly generated second wavelength femtosecond pulse laser. At the same time, the double-clad nonlinear optical fiber 20 can output the first excitation light source, that is, the double-clad nonlinear optical fiber 20 can play a role of transmitting the first excitation light source.

[0071] Further, the double-clad nonlinear optical fiber 20 has a double-clad structure. The laser propagates in the core, and the excitation light propagates in the inner cladding around the core.

[0072] The holmium-doped gain optical fiber 30 is specifically connected with the output end of the double-clad nonlinear optical fiber 20, used to receive the first excitation light source, the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser, and amplify the energy of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser under the excitation of the first excitation light source.

[0073] In this way, the finally output laser can have a larger energy, which has practical significance.

[0074] FIG. 2 is a simplified energy level diagram of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser in the process of energy level transition of holmium ions in the amplification process according to the embodiment of the present application.

[0075] As shown in FIG. 2, the first excitation light source can be used to excite the holmium ions to realize the population inversion through stimulated absorption.

[0076] In the embodiment of the present application, in order to distinguish from the "pump light source" required by the soliton self-frequency shift effect, the light source used for population inversion is defined as "excitation light source", and the excitation light source "pumps" the ions to transition from the lower energy level to the upper energy level, which is called stimulated absorption and realizes the population inversion.

[0077] The holmium-doped gain optical fiber 30 is specifically used for amplifying the energy of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser;

[0078] In the process of amplifying the energy of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser by the holmium-doped gain optical fiber 30, an energy level transition process occurs in the holmium-doped gain optical fiber 30, and the energy level transition process includes: the holmium ions in the ground state5 I8 of holmium ions absorbs energy under the action of the light field of the first excitation light source to jump to a high energy level 5 I6, that is, the 1150nm laser pumping process occurs, so that the holmium ions on the high energy level 5 The population of holmium ions on I6 is greater than that of the ground state 5 The population of holmium ions on I8 is inverted; the holmium ions are in a high energy level 5 The holmium ions on I6 jump to a low energy level under the excitation of the second wavelength femtosecond pulse laser 5 I7 jumps and releases a photon with the same wavelength as the second wavelength femtosecond pulse laser; the holmium ions are in a low energy level 5 The holmium ions on I7 jump to the ground state under the excitation of the first wavelength femtosecond pulse laser 5 I8 jumps and releases a photon with the same wavelength as the first wavelength femtosecond pulse laser, achieving energy amplification.

[0079] It should be noted that N0 in FIG. 2 represents the population of holmium ions in the ground state 5 I8, N1 represents the population of holmium ions in the low energy level 5 I7, and N2 represents the population of holmium ions in the high energy level 5 I6.

[0080] Continuing to refer to FIG. 2, 5 The energy level lifetime of the I6 energy level is τ = 3.5ms, 5 The energy level lifetime of the I7 energy level is τ = 12.0ms, that is, the upper energy level lifetime is shorter than the lower energy level lifetime, which easily causes laser transition self-quenching. Laser transition self-quenching refers to a phenomenon in which, under certain conditions, an increase in the number of excited state particles in a laser medium leads to a decrease or stop in the efficiency of laser transition. Specifically, when the number of particles in the excited state in the laser medium increases, the interaction (such as collision, energy transfer, etc.) between them also increases. In this way, the average lifetime of the excited state particles is shortened, thereby reducing the laser transition between the excited state particles, and thus the efficiency of the laser transition is reduced or stopped.

[0081] The holmium ions in 5 The holmium ions in the I7 energy level are excited by the first wavelength femtosecond pulse laser to fall to 5 I8, which can reduce 5 The population of holmium ions on the I7 energy level, alleviating the problem of 5 I6→ 5 I7” energy level transition, which causes laser transition self-quenching due to the upper energy level lifetime being shorter than the lower energy level lifetime, and improves the slope efficiency of the 2.9μm fiber amplifier, that is, improves the ratio between the output optical power of the holmium-doped gain fiber and the excitation optical power.

[0082] Continuing to refer to FIG. 1, the end cap 40 is specifically connected to the output end of the holmium-doped gain fiber 30. That is, the 2.1 μm and 2.9 μm femtosecond pulse lasers (the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser) are coupled into the holmium-doped gain fiber 30 to be amplified by stimulated emission, to generate high-energy 2.1 μm and 2.9 μm femtosecond pulse lasers, which are output through the fiber end cap 40.

[0083] The end cap 40 can be used to protect the output end face of the holmium-doped gain fiber 30, effectively protecting the output end face of the holmium-doped gain fiber 30 from the external environment (water vapor, etc.), thereby ensuring the stability and durability of the laser. At the same time, the end cap 40 can shape the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser, and amplify the spot energy density of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser. Since the spot of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser directly output by the holmium-doped gain fiber 30 is relatively small and the density is relatively large, the laser can be adjusted to adapt to different application scenarios.

[0084] It should be noted that the pulse width of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser is in the order of femtoseconds (fs), and can be 50 fs, 200 fs or 500 fs, which is not limited in the embodiments of the present application.

[0085] FIGS. 3-5 are simulation experimental results of the second wavelength femtosecond pulse laser generation process and the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser amplification process in the frequency domain according to the embodiments of the present application. FIGS. 6-8 are simulation experimental results of the second wavelength femtosecond pulse laser generation process and the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser amplification process in the time domain according to the embodiments of the present application.

[0086] In order to facilitate understanding of the evolution process of the laser in the frequency domain and the time domain when the laser is transmitted along the optical fiber in the double-clad fluorotellurite glass fiber and the holmium-doped zirconium fluoride glass fiber, simulation experiments are performed, wherein the double-clad fluorotellurite glass fiber is one of the double-clad nonlinear optical fibers 20, and the holmium-doped zirconium fluoride glass fiber is one of the holmium-doped gain fibers 30.

[0087] FIG. 3 is a schematic diagram of the evolution process of the first wavelength femtosecond pulse laser in the frequency domain when the first wavelength femtosecond pulse laser is transmitted in the double-clad fluorotellurite glass fiber and the holmium-doped zirconium fluoride glass fiber according to the embodiments of the present application, wherein the horizontal axis is the wavelength (μm) and the vertical axis is the fiber length (m). The double-clad fluorotellurite glass fiber corresponds to the fiber length of 0-0.61 meters in the vertical axis, and the holmium-doped zirconium fluoride glass fiber corresponds to the fiber length of 0.61-3.11 meters in the vertical axis.

[0088] Figure 4 is a spectrum diagram of the tail end output of the double-clad fluorotellurite glass fiber and the tail end output of the holmium-doped zirconium fluoride glass fiber according to the embodiment of the present application.

[0089] Figure 5 is a partial enlarged view of part A in Figure 4 according to the embodiment of the present application.

[0090] Specifically, the spectrum diagram of the tail end output of the double-clad fluorotellurite glass fiber is the solid line part shown in part A of Figure 4, and please refer to the partial enlarged view of part A in Figure 4, i.e. Figure 5. The spectrum diagram of the tail end output of the holmium-doped zirconium fluoride glass fiber is the dotted line part in Figure 4. The abscissa in Figure 4 and Figure 5 is wavelength (μm), and the ordinate is intensity (a.u.).

[0091] Figure 6 is a schematic diagram of the evolution process of the first wavelength femtosecond pulse laser in the time domain when it is transmitted in the double-clad fluorotellurite glass fiber and the holmium-doped zirconium fluoride glass fiber according to the embodiment of the present application. The abscissa is time (ps), and the ordinate is the fiber length (m). The double-clad fluorotellurite glass fiber corresponds to the fiber length of 0-0.61 meters in the ordinate, and the holmium-doped zirconium fluoride glass fiber corresponds to the fiber length of 0.61-3.11 meters in the ordinate.

[0092] As shown in Figure 3 and Figure 6, the 2.1 μm and 2.9 μm lasers (i.e. the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser) are generated in the double-clad fluorotellurite glass fiber and are amplified in energy in the holmium-doped zirconium fluoride glass fiber.

[0093] As shown in Figure 4 and Figure 5, the center wavelengths of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser are 2.07 μm and 2.94 μm respectively. Due to the promotion of laser energy after amplification, there is a relatively obvious intensity contrast in the spectrum, and the total average output power is 5.62 W, in which the average power of the 2.9 μm laser is 3.73 W.

[0094] As shown in Figure 6, after the second wavelength femtosecond pulse laser is generated by the soliton splitting of the first wavelength femtosecond pulse laser, the shortest soliton (i.e. the second wavelength femtosecond pulse laser, whose pulse width is smaller than that of the first wavelength femtosecond pulse laser) is separated from the main part of the pulse. Since the speed of the soliton along the fiber becomes slower and slower, the trajectory of the soliton continuously bends to the right.

[0095] Figure 7 is a time-domain pulse curve diagram of the tail end output of the double-clad fluorotellurite glass fiber and the tail end output of the holmium-doped zirconium fluoride glass fiber according to the embodiment of the present application. The abscissa is time (ps), and the ordinate is intensity (a.u.). The time-domain pulse curve diagram of the tail end output of the double-clad fluorotellurite glass fiber is the solid line in Figure 7, and the time-domain pulse curve diagram of the tail end output of the holmium-doped zirconium fluoride glass fiber is the dotted line in Figure 7.

[0096] Fig. 8 is a self-correlation trajectory diagram of the 2.9 μm laser output from the tail end of the holmium-doped zirconium fluoride glass optical fiber provided by the embodiment of the present application, the abscissa is time (ps), and the ordinate is intensity (a.u.). The 2.9 μm laser is the second wavelength femtosecond pulse laser.

[0097] The soliton pulse is amplified under the action of the excitation light source when it is transmitted in the holmium-doped zirconium fluoride glass optical fiber, and further walk-off phenomenon occurs in the time domain. The time domain pulse curve output from the tail end of the double-clad fluorotellurite glass optical fiber (solid line) and the tail end of the holmium-doped zirconium fluoride glass optical fiber (dashed line) is shown in Fig. 7. The pulse width at 2.9 μm wavelength is 987 fs, and the self-correlation curve is shown in Fig. 8.

[0098] From the above technical solutions, it can be seen that the embodiment of the present application provides a dual-wavelength femtosecond fiber laser. The laser includes a pump light source 10 for outputting a first wavelength femtosecond pulse laser and outputting a first excitation light source; the wavelength of the first wavelength femtosecond pulse laser is 2.1 μm; a double-clad nonlinear optical fiber 20 connected with the output end of the pump light source 10, for receiving the first wavelength femtosecond pulse laser and the first excitation light source, converting part of the first wavelength femtosecond pulse laser into a second wavelength femtosecond pulse laser by using the soliton self-frequency shift effect, and outputting the remaining part of the first wavelength femtosecond pulse laser, the second wavelength femtosecond pulse laser and the first excitation light source; a holmium-doped gain optical fiber 30 connected with the output end of the double-clad nonlinear optical fiber 20, for receiving the first excitation light source, the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser, and amplifying the energy of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser under the excitation of the first excitation light source. In this way, the dual-wavelength high-energy femtosecond pulse laser is realized in the all-fiber structure of the mid-infrared laser, and has the advantages of low cost, compact structure, portability, good environmental adaptability, wavelength advantage and energy advantage in medical and material processing applications, and is suitable for vehicle-mounted, airborne and other scenes.

[0099] Further, the gain coefficient of the holmium-doped gain optical fiber 30 when amplifying the first wavelength femtosecond pulse laser is determined based on the following formula one, and the gain coefficient when amplifying the second wavelength femtosecond pulse laser is determined based on the following formula two.

[0100] Formula one:

[0101] Formula two:

[0102] Wherein, ∫ is the integral symbol, g 2.1μm (z) is the gain coefficient of the first wavelength femtosecond pulse laser at the z position of the holmium-doped gain optical fiber 30, g 2.9μm(z) is the gain coefficient of the second wavelength femtosecond pulse laser at the z position of the holmium-doped gain fiber 30; τ 2.1 is the upper energy level lifetime of the first wavelength femtosecond pulse laser, τ 2.9 is the upper energy level lifetime of the second wavelength femtosecond pulse laser; β is the high energy level 5 I6 and low energy levels 5 Radiation attenuation branching ratio between I7; σ e2.1 is the stimulated emission cross section corresponding to the first wavelength femtosecond pulse laser, σ a2.1 is the absorption cross section corresponding to the first wavelength femtosecond pulse laser; σ e2.9 is the stimulated emission cross section corresponding to the second wavelength femtosecond pulse laser, σ a2.9 is the absorption cross section corresponding to the second wavelength femtosecond pulse laser; A eff is the mode field area of ​​the holmium-doped gain fiber 30, h is the Planck constant, υ p is the frequency of the first excitation light source, P a is the power of the first excitation light source absorbed by the holmium-doped gain fiber 30 , n is the doping concentration of holmium ions, and L is the fiber length of the holmium-doped gain fiber 30 .

[0103] In practical applications, the efficiency and output parameters of the holmium-doped gain fiber 30 can be controlled by adjusting one or more of the parameters such as the holmium ion doping concentration, the fiber length of the holmium-doped gain fiber, the power of the first excitation light source, the energy ratio of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser.

[0104] In some implementations, the doping concentration of holmium ions in the holmium-doped gain fiber 30 is greater than 2.5 mol%. This embodiment of the present application does not specifically limit this.

[0105] In practical applications, the transmission loss of the holmium-doped gain fiber 30 in the 0.8 μm to 3.5 μm band is less than 1 dB / m.

[0106] Furthermore, the peak power of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser output by the double-clad nonlinear optical fiber 20 is determined based on the following formula 3, and the pulse width is determined based on the following formula 4:

[0107] Formula 3:

[0108] Formula 4:

[0109] Among them, P 2.1 is the peak power of the first wavelength femtosecond pulse laser, P 2.9μm is the peak power of the second wavelength femtosecond pulse laser; T 2.1 represents the pulse width of the first wavelength femtosecond pulse laser, T2.9μm is the pulse width of the second wavelength femtosecond pulse laser; N is the soliton order, β2 is the second order dispersion parameter of the double-clad nonlinear optical fiber 20, and γ is the nonlinear coefficient of the double-clad nonlinear optical fiber 20.

[0110] In the embodiments of the present application, by adjusting one or more of the peak power or the pulse width of the first wavelength femtosecond pulse laser, the dispersion parameter or the nonlinear coefficient of the double-clad nonlinear optical fiber 20, etc., the peak power and the pulse width of the second wavelength femtosecond pulse laser can be controlled.

[0111] In the embodiments of the present application, the matrix material of the double-clad nonlinear optical fiber 20 can be fluorotellurite glass. Further, the mode field area of the double-clad nonlinear optical fiber 20 is smaller than the mode field area of the holmium-doped gain optical fiber 30; the zero dispersion wavelength of the double-clad nonlinear optical fiber 20 is less than 2 μm. Further, the transmission loss of the double-clad nonlinear optical fiber 20 in the 0.8 μm to 3.5 μm waveband is less than 2 dB / m, and the fiber length is less than 1 meter.

[0112] Continuing to refer to FIG. 1, the pump light source 10 can include a mode-locked thulium-doped fiber oscillator 11, a first amplification stage 12, a fiber stretcher 13, and a second amplification stage 14, which are sequentially fused together.

[0113] The mode-locked thulium-doped fiber oscillator 11 is configured to generate a first femtosecond pulse laser, and the wavelength of the first femtosecond pulse laser can be 1.95 μm.

[0114] FIG. 9 is a structural schematic diagram of a mode-locked thulium-doped fiber oscillator according to an embodiment of the present application.

[0115] As shown in FIG. 9, in the embodiments of the present application, the mode-locked thulium-doped fiber oscillator 11 can be a ring laser oscillator, which is composed of a 1570 nm semiconductor laser 111, an optical wavelength division multiplexer 112, a second thulium-doped gain optical fiber 113, an optical fiber output coupler 114, a first optical fiber isolator 115, and a saturable absorber assembly 116. The optical wavelength division multiplexer 112, the second thulium-doped gain optical fiber 113, the optical fiber output coupler 114, the first optical fiber isolator 115, and the saturable absorber assembly 116 can be sequentially connected in the above order by means of fusion, and the output end of the saturable absorber assembly 116 and the input end of the optical wavelength division multiplexer 112 are fused together, and the output end of the 1570 nm semiconductor laser 111 and the pump input end of the optical wavelength division multiplexer 112 are fused.

[0116] Further, the first amplification stage 12 can be connected with the output end of the mode-locked thulium-doped fiber oscillator 11, used to receive the first femtosecond pulse laser, power amplify the first femtosecond pulse laser, and output the amplified first femtosecond pulse laser. In actual application, the first amplification stage 12 can amplify the average power of the first femtosecond pulse laser to 20 mW while maintaining the spectral profile.

[0117] FIG. 10 is a structural schematic diagram of the first amplification stage provided in the embodiment of the present application.

[0118] As shown in FIG. 10, the first amplification stage 12 can include a 793 nm semiconductor laser 121, a second fiber coupler 122, a third thulium-doped gain fiber 123, and a second fiber isolator 124. The second fiber coupler 122, the third thulium-doped gain fiber 123, and the second fiber isolator 124 are connected in the above order by fusion, and the 793 nm semiconductor laser 121 is fused with the pump output end of the second fiber coupler 122.

[0119] The fiber stretcher 13 is specifically connected with the output end of the first amplification stage 12, used to receive the first femtosecond pulse laser, stretch the time domain of the first femtosecond pulse laser, and output the stretched first femtosecond pulse laser. In actual application, an ultrahigh numerical aperture single-mode fiber can be used as the fiber stretcher 13.

[0120] The second amplification stage 14 is specifically connected with the output end of the fiber stretcher 13, used to receive the first femtosecond pulse laser, power amplify the first femtosecond pulse laser again, and output the amplified first femtosecond pulse laser.

[0121] In the embodiment of the present application, the second amplification stage 14 has the same structure as the first amplification stage 12, and specific reference can be made to FIG. 10, which will not be described herein.

[0122] Further, the pump light source 10 further includes a first laser 15, a second laser 16, a first fiber coupler 17, and a first thulium-doped gain fiber 18.

[0123] It is worth noting that the first fiber coupler 17 is a quartz-based fiber combiner, prepared by using quartz fiber, and provides a coupling excitation light source for a mid-infrared fiber amplifier (i.e., a thulium-doped fiber amplifier and a holmium-doped fiber amplifier).

[0124] The first laser 15 is used to output a first excitation light source, and the wavelength of the first excitation light source is 1150 nm. That is, in the embodiment of the present application, the first laser 15 is a 1150 nm fiber laser.

[0125] The second laser 16 is configured to output a second excitation light source, and the wavelength of the second excitation light source is 793 nm, that is, in the embodiment of the present application, the second laser 16 is a 793 nm semiconductor laser.

[0126] The first pump input end 171 of the first fiber coupler 17 is connected with the output end of the first laser 15, the second pump input end 172 of the first fiber coupler 17 is connected with the output end of the second laser 16, and the signal input end 173 of the first fiber coupler 17 is connected with the output end of the second amplification stage 14; the first fiber coupler 17 is configured to couple the first femtosecond pulse laser, the first excitation light source and the second excitation light source into the first thulium-doped gain fiber 18.

[0127] It is worth noting that the embodiment of the present application does not involve a non-quartz-based mid-infrared fiber coupler. At present, the manufacturing process of the non-quartz-based fiber coupler is still in the research and development stage, the coupling efficiency of the beam combiner is not stable, the heat dissipation problem under high power still needs to be solved, and the cost is high. The cost of labor is high. Compared with the scheme using the mid-infrared fiber coupler, the system provided by the scheme of the present application is more stable, compact and economical.

[0128] The first thulium-doped gain fiber 18 is connected with the output end of the first fiber coupler 17. It can be understood that the mode-locked thulium-doped fiber oscillator 11, the first amplification stage 12, the fiber stretcher 13, the second amplification stage 14, the first fiber coupler 17 and the first thulium-doped gain fiber 18 are connected in the above order by fusion splicing. The first thulium-doped gain fiber 18 is configured to receive the first femtosecond pulse laser, the first excitation light source and the second excitation light source, and to amplify the power and compress the pulse width of the first femtosecond pulse laser again under the excitation of the second excitation light source and part of the first excitation light source. In addition, the first femtosecond pulse laser with high peak power generates a soliton self-frequency shift effect when it is transmitted in the first thulium-doped gain fiber, converts the amplified first femtosecond pulse laser into a first wavelength femtosecond pulse laser, and outputs the first wavelength femtosecond pulse laser and the remaining part of the first excitation light source.

[0129] It can be understood that the double-clad nonlinear fiber 20 is connected with the output end of the first thulium-doped gain fiber 18 to receive the first wavelength femtosecond pulse laser and the first excitation light source.

[0130] FIG. 11 is an output spectrum diagram of the pump light source 10 provided by the embodiment of the present application;

[0131] Figure 11 is a graph of wavelength (pm) versus normalized intensity (a.u.) showing that the output of the pump light source 10 includes two peaks at 1.96 pm and 2.1 pm, with the 2.1 pm laser (first wavelength femtosecond pulsed laser) having a power fraction of about 93.1%, i.e., the efficiency of the first femtosecond pulsed laser converting to the first wavelength femtosecond pulsed laser is more than 93%.

[0132] Figure 12 is a graph of autocorrelation of the pump light source 10 according to an embodiment of the present application.

[0133] Figure 12 is a graph of autocorrelation of the pump light source 10 according to an embodiment of the present application.

[0134] In some implementations, the first thulium-doped gain fiber 18 has a core and cladding size of 10 pm and 130 pm, an absorption of more than 5 dB / m for the pump wavelength, and a fiber length of less than 5 m.

[0135] In some implementations, the energy conversion efficiency of the 2 pm femtosecond laser (first femtosecond pulsed laser) in the pump light source 10 to the 2.1 pm wavelength (first wavelength femtosecond pulsed laser) can be greater than 70%, which can be achieved by adjusting one or more of the lengths of the various fibers in the system, the pump power, the nonlinear coefficient, and the amount of dispersion.

[0136] Figure 13 is a simplified energy level diagram of thulium ions in a thulium-doped gain fiber according to an embodiment of the present application.

[0137] Figure 14 is an absorption spectrum of thulium ions in a thulium-doped gain fiber according to an embodiment of the present application.

[0138] Specifically, Figure 14 is a ground state absorption spectrum of the first thulium-doped gain fiber 18, with wavelength (nm) on the horizontal axis and absorption cross-section (10 -25 m 2 ) on the vertical axis. As shown in Figures 13 and 14, after the first pump light source is coupled into the first thulium-doped gain fiber 18, a portion of the energy can be used to cause the thulium ions to undergo a “H6→ H5” energy level transition, and correspondingly, after the second pump light source is coupled into the first thulium-doped gain fiber 18, a portion of the energy can also be used to cause the thulium ions to undergo a “H6→ H5” energy level transition. 3 H6→ 3 H5” energy level transition. As shown in Figures 13 and 14, after the first pump light source is coupled into the first thulium-doped gain fiber 18, a portion of the energy can be used to cause the thulium ions to undergo a “H6→ H5” energy level transition, and correspondingly, after the second pump light source is coupled into the first thulium-doped gain fiber 18, a portion of the energy can also be used to cause the thulium ions to undergo a “H6→ H5” energy level transition. 3 H6→ 3F4" level transition, compared with the single 793 nm laser (second excitation light source) excitation of the first thulium-doped gain fiber 18 amplification stage, is prone to excite cross-relaxation (CR) process, improve the quantum efficiency of the first thulium-doped gain fiber 18, and promote the energy conversion efficiency of the soliton frequency shift from 2 μm to 2.1 μm.

[0139] It can be understood that the 1150 nm continuous laser (first excitation light source) output by the first laser 15 is coupled into the laser by the first fiber coupler 17, transmitted through the first thulium-doped gain fiber 18 and the double-clad nonlinear optical fiber 20, and then coupled into the holmium-doped gain fiber 30, and the first thulium-doped gain fiber 18 and the holmium-doped gain fiber 30 have absorption to the 1150 nm excitation light source.

[0140] It should be noted that the wavelength of the first femtosecond pulse laser can have a third fluctuation range, which is greater than or equal to 1.90 μm and less than or equal to 2.05 μm, that is, the wavelength of the first femtosecond pulse laser has a fluctuation of ±0.5 μm. For example, the wavelength of the first femtosecond pulse laser can be 1.964 μm.

[0141] It is worth noting that the embodiment of the present application can simultaneously realize the fiber output of the 2.1 μm femtosecond pulse laser and the 2.9 μm femtosecond pulse laser, realize the all-fiber laser system structure, and does not involve lenses and other devices.

[0142] FIG. 15 is a schematic diagram of the working process of the dual-wavelength femtosecond pulse fiber laser provided by the embodiment of the present application.

[0143] As shown in FIG. 15, the 2 μm mode-locked pulse laser (first femtosecond pulse laser) is input into the thulium-doped fiber amplification stage (first amplification stage 12 to first thulium-doped gain fiber 18), and after stage 1: 2 μm laser energy amplification and pulse compression, and stage 2: 2 μm→2.1 μm femtosecond laser wavelength shift, a high-power 2.1 μm femtosecond pulse laser (first wavelength femtosecond pulse laser) is generated, and then the first wavelength femtosecond pulse laser is input into the Raman soliton frequency shift fiber laser (double-clad nonlinear optical fiber 20), and based on the soliton self-frequency shift effect in the mid-infrared nonlinear optical fiber, the frequency conversion of the femtosecond pulse laser is realized, and a second wavelength femtosecond pulse laser of 2.9 μm is obtained, and then the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser are input into the holmium-doped zirconium fluoride fiber amplifier (holmium-doped gain fiber 30), and based on the "self-frequency shift" of holmium ions in the holmium-doped gain fiber 30, the wavelength of the first wavelength femtosecond pulse laser is further shifted to 2.9 μm, and a 2.9 μm femtosecond pulse laser (second wavelength femtosecond pulse laser) is generated. 5 I6→ 5 I7" and " 5 I7→ 5The I8" energy level transition realizes energy amplification of 2.1 μm femtosecond pulse laser (first wavelength femtosecond pulse laser) and 2.9 μm femtosecond pulse laser (second wavelength femtosecond pulse laser), and high-energy 2.1 μm and 2.9 μm dual-wavelength femtosecond pulse laser (first wavelength femtosecond pulse laser and second wavelength femtosecond pulse laser) is obtained.

[0144] It can be seen from the above that the embodiment of the application provides a 2.1 μm and 2.9 μm dual-wavelength femtosecond fiber laser which can output high-energy 2.1 μm and 2.9 μm femtosecond laser simultaneously in a compact, all-fiber mid-infrared laser amplification system. The mode-locked thulium-doped fiber oscillator 11 outputs 2 μm band ultrashort pulse laser (first femtosecond pulse laser), which is amplified by the first amplification stage 12. The amplified laser passes through the fiber stretcher 13 to stretch the laser pulse. The stretched laser is secondarily amplified by the second amplification stage 14. The secondarily amplified laser is coupled into the first thulium-doped gain fiber 18 through the first fiber coupler 17 to realize pulse time domain compression and soliton self-frequency shift process and generate 2.1 μm wavelength femtosecond pulse laser (first wavelength femtosecond pulse laser). Further, the first fiber coupler 17 simultaneously couples the 793 nm semiconductor laser (second laser 16) output 793 nm excitation light source (second excitation light source) and the 1150 nm laser (first laser 15) output 1150 nm excitation light source (first excitation light source) into the first thulium-doped gain fiber 18. The first thulium-doped gain fiber 18 outputs 2.1 μm femtosecond pulse laser (first wavelength femtosecond pulse laser) and 1150 nm excitation light source (first excitation light source) which are coupled into the double-clad nonlinear fiber 20. The 2.1 μm femtosecond pulse laser excites the dispersion and nonlinear effect in the double-clad nonlinear fiber 20, generates a new component frequency, and is frequency-shifted to 2.9 μm wavelength under the pumping of the 2.1 μm femtosecond pulse laser, and outputs 2.1 μm and 2.9 μm femtosecond pulse laser at the output end of the double-clad nonlinear fiber 20, that is, outputs first wavelength femtosecond pulse laser and second wavelength femtosecond pulse laser. The 1150 nm excitation light source is coupled into the holmium-doped gain fiber 30 after being transmitted through the double-clad nonlinear fiber 20. The 1150 nm excitation light source (first excitation light source) is absorbed after being coupled into the holmium-doped gain fiber 30, and the absorbed energy is converted into 2.9 μm wavelength femtosecond pulse laser (second wavelength femtosecond pulse laser) which is output from the holmium-doped gain fiber 30. 5 The holmium ions on the I8 energy level are pumped to 5The I6 energy level, the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser are coupled into the holmium-doped gain optical fiber 30 to generate stimulated emission amplification, and high-energy 2.1 μm femtosecond pulse laser (first wavelength femtosecond pulse laser) and 2.9 μm femtosecond pulse laser (second wavelength femtosecond pulse laser) are generated, and output through the optical fiber end cap 40. The technical scheme provided by the embodiment of the application can realize the all-fiber structure of the 2.1 μm and 2.9 μm dual-wavelength femtosecond pulse laser, and realize the power and energy improvement, and can improve the practical application effect in the field of laser medical treatment and material processing.

[0145] It is to be understood that other embodiments of the application can be utilized and functional and / or structural modifications can be made without departing from the scope of the present application. Therefore, the foregoing description is only exemplary of the general nature of the application, and is not to be taken in a limiting sense. The true scope of the application is indicated only by the appended claims. It is also to be understood that the specific devices, materials, or the like that are shown are indicative of the generic class of which such devices and materials are a representation. It is not to be understood that restrictions are implied or that the claims are not equally applicable to the equipment and materials, such as shown.

Claims

1. A dual-wavelength femtosecond fiber laser, characterized in that: include: A pump light source (10) is used to output a first wavelength femtosecond pulse laser and a first excitation light source; The wavelength of the first wavelength femtosecond pulse laser is 2.1 μm; A double-clad nonlinear optical fiber (20) is connected to the output end of the pump light source (10), and is used to receive the first wavelength femtosecond pulse laser and the first excitation light source, convert a portion of the first wavelength femtosecond pulse laser into a second wavelength femtosecond pulse laser by using the soliton self-frequency shift effect, wherein the wavelength of the second wavelength femtosecond pulse laser is 2.9 μm; and output the remaining portion of the first wavelength femtosecond pulse laser, the second wavelength femtosecond pulse laser, and the first excitation light source; A holmium-doped gain fiber (30) is connected to the output end of the double-clad nonlinear fiber (20), and is used to receive the first excitation light source, the first wavelength femtosecond pulse laser, and the second wavelength femtosecond pulse laser, and amplify the energy of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser under the excitation of the first excitation light source; An end cap (40) is connected to the output end of the holmium-doped gain optical fiber (30) and is used to protect the output end face of the holmium-doped gain optical fiber (30) and amplify the spot energy density of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser.

2. The dual-wavelength femtosecond fiber laser according to claim 1, characterized in that: The pump light source (10) comprises: a mode-locked thulium-doped fiber oscillator (11), a first amplifier stage (12), a fiber stretcher (13), and a second amplifier stage (14); The mode-locked thulium-doped fiber oscillator (11) is used to generate a first femtosecond pulse laser, wherein the wavelength of the first femtosecond pulse laser is 2 μm; The first amplifier stage (12) is connected to the output end of the mode-locked thulium-doped fiber oscillator (11), and is used for receiving the first femtosecond pulse laser, amplifying the power of the first femtosecond pulse laser, and then outputting the first femtosecond pulse laser; The fiber stretcher (13) is connected to the output end of the first amplifier stage (12), and is used to receive the first femtosecond pulse laser, stretch the time domain of the first femtosecond pulse laser, and then output the first femtosecond pulse laser; The second amplifier stage (14) is connected to the output end of the fiber stretcher (13) and is used for receiving the first femtosecond pulse laser, amplifying the power of the first femtosecond pulse laser again, and then outputting the first femtosecond pulse laser.

3. The dual-wavelength femtosecond fiber laser according to claim 2, characterized in that: The wavelength of the first-wavelength femtosecond pulse laser has a first fluctuation range, and the first fluctuation range is greater than or equal to 2.02 μm and less than or equal to 2.12 μm; The second wavelength femtosecond pulse laser has a second fluctuation range, and the second fluctuation range is greater than or equal to 2.89 μm and less than or equal to 3.01 μm; The first femtosecond pulse laser has a third fluctuation range, and the third fluctuation range is greater than or equal to 1.90 μm and less than or equal to 2.05 μm.

4. The dual-wavelength femtosecond fiber laser according to claim 2, characterized in that: The pump light source (10) further includes a first laser (15), a second laser (16), a first fiber coupler (17), and a first thulium-doped gain fiber (18); The first laser (15) is used to output the first excitation light source, and the wavelength of the first excitation light source is 1150 nm; The second laser (16) is used to output a second excitation light source, and the wavelength of the second excitation light source is 793 nm; The first pump input end (171) of the first fiber coupler (17) is connected to the output end of the first laser (15), the second pump input end (172) of the first fiber coupler (17) is connected to the output end of the second laser (16), and the signal input end (173) of the first fiber coupler (17) is connected to the output end of the second amplifier stage (14); the first fiber coupler (17) is used to couple the first femtosecond pulse laser, the first excitation light source, and the second excitation light source into the first thulium-doped gain fiber (18); The first thulium-doped gain fiber (18) is connected to the output end of the first fiber coupler (17), and is used to receive the first femtosecond pulse laser, the first excitation light source, and the second excitation light source, and to perform power amplification and pulse width compression on the first femtosecond pulse laser again under the excitation of the second excitation light source and part of the first excitation light source; The first femtosecond pulse laser with high peak power generates a soliton self-frequency shift effect when transmitted in the first thulium-doped gain optical fiber (18), converting the amplified first femtosecond pulse laser into the first wavelength femtosecond pulse laser; The first thulium-doped gain fiber (18) is also used to output the first wavelength femtosecond pulse laser and the remaining part of the first excitation light source.

5. The dual-wavelength femtosecond fiber laser according to claim 4, characterized in that: The double-clad nonlinear optical fiber (20) is connected to the output end of the first thulium-doped gain optical fiber (18) to receive the first wavelength femtosecond pulse laser and the first excitation light source.

6. The dual-wavelength femtosecond fiber laser according to claim 1, characterized in that: The matrix material of the double-clad nonlinear optical fiber (20) is fluorotellurate glass; The mode field area of ​​the double-clad nonlinear optical fiber (20) is smaller than the mode field area of ​​the holmium-doped gain optical fiber (30); The zero dispersion wavelength of the double-clad nonlinear optical fiber (20) is less than 2 μm.

7. The dual-wavelength femtosecond fiber laser according to claim 1, characterized in that: The matrix material of the holmium-doped gain optical fiber (30) is zirconium fluoride glass; The doping concentration of holmium ions in the holmium-doped gain optical fiber (30) is greater than 2.5 mol%.

8. The dual-wavelength femtosecond fiber laser according to claim 1, characterized in that: The holmium-doped gain optical fiber (30) is specifically used to amplify the energy of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser; During the process of the holmium-doped gain fiber (30) amplifying the energy of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser, an energy level transition process occurs in the holmium-doped gain fiber (30), and the energy level transition process includes: being in a ground state 5 The holmium ions of I8 absorb energy and transition to a high energy level under the action of the light field of the first excitation light source. 5 I6, so that the high energy level 5 The population of holmium ions on I6 is greater than the ground state 5 The number of holmium ions on I8; in the high energy level 5 The holmium ion of I6 is excited to the low energy level by the second wavelength femtosecond pulse laser. 5 I7 transitions and releases photons with the same wavelength as the second wavelength femtosecond pulse laser; in the low energy level 5 The holmium ion of I7 is excited to the ground state by the first wavelength femtosecond pulse laser. 5 I8 transitions and releases photons with the same wavelength as the first-wavelength femtosecond pulse laser.

9. The dual-wavelength femtosecond fiber laser according to claim 8, characterized in that: The gain coefficient of the holmium-doped gain optical fiber (30) when amplifying the first wavelength femtosecond pulse laser energy is determined based on the following formula 1, and the gain coefficient of the holmium-doped gain optical fiber (30) when amplifying the second wavelength femtosecond pulse laser energy is determined based on the following formula 2; Formula 1: Where ∫ is the integral symbol, g 2.1μm (z) is the gain coefficient of the first wavelength femtosecond pulse laser at the z position of the holmium-doped gain optical fiber (30); τ 2.1 is the upper energy level lifetime of the first wavelength femtosecond pulse laser; τ 2.9 For the said The upper energy level lifetime of the two-wavelength femtosecond pulse laser; β is the upper energy level 5 I6 and the low energy level 5 Radiation attenuation branching ratio between I7; σ e2.1 is the stimulated emission cross section corresponding to the first wavelength femtosecond pulse laser, σ a2.1 is the absorption cross section corresponding to the first wavelength femtosecond pulse laser, A eff is the mode field area of ​​the holmium-doped gain fiber (30), h is the Planck constant, υ p is the frequency of the first excitation light source, P a is the power of the first excitation light source absorbed by the holmium-doped gain fiber (30), n is the doping concentration of holmium ions, and L is the fiber length of the holmium-doped gain fiber (30); Formula 2: Where ∫ is the integral symbol, g 2.9μm (z) is the gain coefficient of the second wavelength femtosecond pulse laser at the z position of the holmium-doped gain optical fiber (30); τ 2.1 is the upper energy level lifetime of the first wavelength femtosecond pulse laser; τ 2.9 is the upper energy level lifetime of the second wavelength femtosecond pulse laser; β is the upper energy level 5 I6 and the low energy level 5 Radiation attenuation branching ratio between I7; σ e2.9 is the stimulated emission cross section corresponding to the second wavelength femtosecond pulse laser, σ a2.9 is the absorption cross section corresponding to the second wavelength femtosecond pulse laser, A eff is the mode field area of ​​the holmium-doped gain fiber (30), h is the Planck constant, υ p is the frequency of the first excitation light source, P a The holmium-doped gain optical fiber (30) absorbs the power of the first excitation light source.

10. The dual-wavelength femtosecond fiber laser according to claim 1, characterized in that: The peak powers of the first wavelength femtosecond pulse laser and the second wavelength femtosecond pulse laser output by the double-clad nonlinear optical fiber (20) are determined based on the following formula 3, and the pulse widths are determined based on the following formula 4: Formula 3: Among them, P 2.1 is the peak power of the first wavelength femtosecond pulse laser, P 2.9μm is the peak power of the second wavelength femtosecond pulse laser, N is the soliton order, β2 is the second-order dispersion parameter of the double-clad nonlinear optical fiber (20), γ is the nonlinear coefficient of the double-clad nonlinear optical fiber (20), T 2.1 represents the pulse width of the first wavelength femtosecond pulse laser; Formula 4: Among them, T 2.1 represents the pulse width of the first wavelength femtosecond pulse laser, T 2.9μm is the pulse width of the second wavelength femtosecond pulse laser, N is the soliton order, β2 is the second-order dispersion parameter of the double-clad nonlinear optical fiber (20), and γ is the nonlinear coefficient of the double-clad nonlinear optical fiber (20); P 2.1 is the peak power of the first wavelength femtosecond pulse laser.

Images