CN111740313A - Mid-infrared all-fiber laser oscillator based on tapered double-clad Raman gain fiber and method for generating mid-infrared laser - Google Patents
Mid-infrared all-fiber laser oscillator based on tapered double-clad Raman gain fiber and method for generating mid-infrared laser Download PDFInfo
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- H—ELECTRICITY
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/30—Structure or shape of the active region; Materials used for the active region
- H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
- H01S5/3401—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers having no PN junction, e.g. unipolar lasers, intersubband lasers, quantum cascade lasers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/02—Structural details or components not essential to laser action
- H01S5/024—Arrangements for thermal management
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
- H01S5/125—Distributed Bragg reflector [DBR] lasers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/40—Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
- H01S5/4012—Beam combining, e.g. by the use of fibres, gratings, polarisers, prisms
Abstract
The invention discloses a mid-infrared all-fiber laser oscillator device based on a conical double-cladding Raman gain fiber and a method for generating mid-infrared laser, wherein laser output of a plurality of high-power mid-infrared quantum cascade lasers enters a fiber combiner through respective multimode tail fibers to be used as a pump, a section of mid-infrared single-mode fiber with a conical double-cladding structure is used as a Raman gain medium, a pair of mid-infrared Bragg fiber gratings are engraved at two ends of the conical double-cladding Raman gain fiber to form a laser resonant cavity, and a section of cladding mode stripper with a fiber structure is welded at the output end of the gain fiber, so that mid-infrared near single transverse mode laser output is realized. The tapered double-cladding Raman gain fiber adopted by the invention can effectively improve the absorption efficiency of the pump light, improve the conversion efficiency from low-brightness pump light to high-brightness laser output, improve the quality of the light beam output by the laser and realize the output of the mid-infrared hundred-watt high-power near single-mode laser above 4 micrometers.
Description
Technical Field
The invention relates to a mid-infrared all-fiber laser oscillator device, in particular to a mid-infrared all-fiber laser oscillator based on a tapered double-clad Raman gain fiber and a method for generating mid-infrared laser.
Background
The mid-infrared 3-14 micron waveband has important practical value, and life-related fingerprint fundamental frequency vibration absorption spectral lines of gas molecules containing carbon, hydrogen, oxygen and nitrogen and volatile organic compounds fall within the waveband, so that the mid-infrared spectrum technology can perform synchronous online, rapid and accurate qualitative and quantitative analysis on multi-component trace gas, and has important requirements in application scenes such as military and national defense, environmental monitoring, industrial production process monitoring, toxic and explosive gas detection, disease diagnosis and the like.
The mid-infrared laser light source with high power, high brightness and near diffraction limit light beam quality has important requirements and application prospects in the technical field of mid-infrared laser represented by mid-infrared spectrum technology.
The optical fiber has the characteristics of bending property, extremely large specific surface area, waveguide mode of near diffraction limit light beam quality and the like, so that the optical fiber technology based on the quartz glass optical fiber plays an important role in the development process of an optical fiber laser with a near infrared 1-2 micron single mode or near single mode and continuous laser output of kilowatt or even ten-kilowatt power.
Because of the limitation of phonon energy and optical fiber loss of the optical fiber matrix material, the rare earth doped quartz glass optical fiber has extremely large loss in the middle infrared band above 3 microns and can not be used as an effective laser gain matrix; the most excellent fluorine zirconium glass optical fiber in the middle infrared non-quartz glass optical fiber is also limited by phonon energy and optical fiber loss of an optical fiber matrix material, and the laser output wavelength and the output power of the rare earth doped fluorine zirconium glass optical fiber are limited, wherein the longest laser wavelength reported in the rare earth doped fluorine zirconium glass optical fiber at present is 3.9 microns, and the maximum output power is only hundreds of milliwatts; meanwhile, due to the characteristic of rare earth energy level electron transition, the output wavelength of the rare earth doped fiber laser is discrete, the gain bandwidth of the rare earth doped fiber laser is limited, and the rare earth doped fiber laser cannot effectively cover the intermediate infrared band.
Quantum cascade lasers are mid-infrared laser sources that have rapidly developed in recent years. By regulating and controlling the components of the semiconductor material and the periodic structure parameters of the semiconductor material, the laser wavelength can cover any wavelength of 3-20 microns of mid-infrared, the highest output power is close to 10W, and the quality of an output light beam is close to the diffraction limit; meanwhile, the quantum cascade laser is based on electroluminescence, the total electro-optic conversion efficiency of the laser operating at room temperature is close to 30% at present, and can reach more than 40% in theory; therefore, the quantum cascade laser is currently the most promising mid-infrared laser light source with high efficiency, practicability and portability. But the high-power continuous laser output with high beam quality and power level up to 100 watts is a ceiling which cannot be reached by the prior mid-infrared quantum cascade laser technology; with the laser output power exceeding 10 w, the beam quality of the quantum cascade laser can be significantly reduced, and the thermal management problem of the mid-infrared semiconductor laser light source under the high-power condition is also a great technical challenge.
Disclosure of Invention
The invention aims to provide a mid-infrared all-fiber laser oscillator based on a tapered double-clad Raman gain fiber, which can realize high-beam quality and high-power continuous laser output of mid-infrared 4 microns above with the output power reaching 100W.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows: a mid-infrared all-fiber laser oscillator based on a tapered double-clad Raman gain fiber comprises: n high-power intermediate infrared quantum cascade lasers with multimode tail fiber output, a high-power intermediate infrared optical fiber beam combiner for combining N multimode tail fibers into one multimode optical fiber output, a section of conical double-cladding Raman gain optical fiber, an intermediate infrared fiber Bragg grating I and an intermediate infrared fiber Bragg grating II which are inscribed at two ends of the conical double-cladding Raman gain optical fiber, and an optical fiber cladding mode stripper, the intermediate infrared fiber Bragg grating I, the conical double-clad Raman gain fiber and the intermediate infrared fiber Bragg grating II are sequentially connected to form a laser resonant cavity, the output laser of the N quantum cascade lasers is injected into the fiber combiner, then the laser is transmitted out through an optical fiber beam combiner and coupled into the laser resonant cavity, and the laser output by the laser resonant cavity passes through an optical fiber cladding mode stripper outside the cavity and then outputs the laser with the wavelength of over 4 microns of mid-infrared light;
the tapered double-cladding Raman gain fiber consists of a fiber core, a first cladding and a second cladding which are sequentially surrounded outside the fiber core, the refractive index of the first cladding is slightly smaller than that of the fiber core, the refractive index of the second cladding is obviously smaller than that of the first cladding, the diameter of the first cladding is more than 3 times of that of the fiber core, the cross sections of the fiber core and the second cladding are circular, the cross section of the first cladding is close to that of the fiber core of the multimode fiber output by the fiber combiner, the thick end of the tapered double-cladding Raman gain fiber is a pump light input end, the thin end of the tapered double-cladding Raman gain fiber is a Raman laser output end, the fiber size between the thick end and the thin end is continuously reduced along the length direction of the fiber, the diameter ratio of the fiber core and the first cladding is constant along the length direction of the fiber, and the tapering ratio (namely the ratio of the outer,
the intermediate infrared fiber Bragg grating I is positioned at the thick end of the conical double-cladding Raman gain fiber, the intermediate infrared fiber Bragg grating II is positioned at the thin end of the conical double-cladding Raman gain fiber, the working wavelength of the intermediate infrared fiber Bragg grating I is the same as that of the intermediate infrared fiber Bragg grating II, and the intermediate infrared fiber Bragg grating I has higher reflectivity which is R1The medium infrared fiber Bragg grating II has lower reflectivity as R2Satisfy R1>R2So that the laser power finally output from the forward direction (i.e., from the thin end) is much larger than the laser power output from the reverse direction (i.e., from the thick end).
Preferably, the optical fiber core has a large mode field diameter and a low numerical aperture, the mode field diameter is between 5 and 200 microns, the numerical aperture is between 0.01 and 0.2, the first cladding has a large core diameter and a high numerical aperture which are similar to those of the output multimode optical fiber of the optical fiber combiner, the core diameter is between 50 and 500 microns, the numerical aperture is between 0.2 and 1.0, and the geometry of the first cladding is similar to that of the output multimode optical fiber core of the optical fiber combiner.
Preferably, the tapered double-clad raman gain fiber has a length of between 0.5 and 500 meters.
Preferably, the optical fiber core, the first cladding and the second cladding are made of mid-infrared glass, the optical fiber core, the first cladding and the second cladding are highly transparent in a waveband of over 4 microns of mid-infrared, and the longest transparent waveband can reach 20 microns.
Preferably, the N intermediate infrared quantum cascade lasers have similar laser output wavelengths, and the laser output wavelengths are greater than 3.7 micrometers, wherein the similar wavelengths mean that the position difference of the laser output peak wavelengths of the N quantum cascade lasers is within a range of +/-50 nanometers; the mode is continuous laser or quasi-continuous laser output, the output power of the single intermediate infrared quantum cascade laser is 0.1-20W after being coupled into the multimode tail fiber, and the output beam quality M2Between 5 and 100.
Furthermore, the optical fiber combiner is arranged on the heat sink, and the purpose of heat dissipation is achieved through effective heat management, such as water or active cooling by ventilation, so that the temperature of the combining part of the optical fiber combiner is reduced, and the functions of protecting optical fiber devices in the combiner and inhibiting the instability of waveguide characteristics caused by temperature fluctuation in the combiner are achieved.
Preferably, the optical fiber combiner is prepared by a heating tapering method.
Preferably, the intermediate infrared fiber bragg grating i and the intermediate infrared fiber bragg grating ii are respectively inscribed on both ends of the tapered double-clad raman gain fiber by femtosecond lasers.
The invention also aims to provide a method for generating mid-infrared laser by the mid-infrared all-fiber laser oscillator based on the tapered double-clad Raman gain fiber, which comprises the following specific steps:
s1, forming a resonant cavity for generating intermediate infrared laser by adopting a section of conical double-cladding single-mode Raman gain fiber, and an intermediate infrared fiber Bragg grating I and an intermediate infrared fiber Bragg grating II at two ends of the Raman gain fiber;
s2, a plurality of intermediate infrared quantum cascade laser pumping sources with similar wavelengths, close power and multimode pigtail output are adopted, and a Nx1 incoherent multimode optical fiber combiner is used for generating high pumping power of N.eta times (eta is the combining efficiency of the optical fiber combiner, eta <1, but N.eta >1) of the output power of a single intermediate infrared quantum cascade laser;
s3, enabling the mid-infrared high-power multimode pump light to enter a laser resonant cavity through low-loss all-fiber fusion coupling, generating gain in a Raman gain band of the tapered double-cladding single-mode Raman gain fiber, oscillating back and forth in the resonant cavity and outputting high-power laser;
and S4, removing residual pump light in laser output by the high-power laser output from the laser resonant cavity through an optical fiber cladding mode stripper outside the cavity, and realizing the output of the high-power near single-mode laser with the mid-infrared wavelength of more than 4 microns.
Compared with the prior art, the invention has the following beneficial effects:
a) the tapered double-clad Raman gain fiber is essentially different from a common double-clad Raman gain fiber with uniform size, and the laser output beam quality of the output end of the tapered double-clad Raman gain fiber is the same at the input end and the output end, so that the input end and the output end of the single-mode double-clad Raman gain fiber are both single-mode output, and the fiber core of the output end (namely a thin end) is smaller than that of the input end (namely a thick end), so that the fiber core of the input end can support few-mode operation, but only single-transverse-mode operation is supported at the output end, and unidirectional mode selectivity of a laser resonant cavity is formed, so that higher collection capability of low-brightness pump light can be realized at the input end, and better laser beam quality can be;
b) in the conical double-cladding Raman gain fiber, because the reverse stimulated Brillouin scattering is transmitted from the thin end to the thick end, the existence of the taper enables the Brillouin scattering migration frequency to change along with the length of the fiber, the generation threshold of Brillouin laser can be improved, the generation of harmful Brillouin laser can be inhibited, and the output of Raman laser with higher power can be realized;
c) due to the existence of the taper of the tapered double-cladding Raman gain fiber, the pump light in the pump cladding (namely in the first cladding) can more effectively sweep the Raman gain fiber core, so that the absorption efficiency of the pump light is effectively improved;
d) the full optical fiber structure ensures that the whole device structure has high mechanical stability, and is beneficial to the stable use of the device in a complex use environment.
Drawings
FIG. 1 is a schematic diagram of a mid-infrared all-fiber laser oscillator device based on a tapered double-clad Raman gain fiber according to the present invention;
FIG. 2 is a schematic cross-sectional view of an output end of a 37x1 multimode fiber combiner;
FIG. 3 is a schematic structural diagram of a tapered double-clad Raman gain fiber;
FIG. 4 is a schematic cross-sectional view of an input end of a tapered double-clad Raman gain fiber;
FIG. 5 is a schematic diagram of pump light traversing a tapered double-clad Raman gain core from the side in an oblique manner;
FIG. 6 is a normalized Raman spectrum of InF, sulfide, selenide, telluride glasses.
In the figure, 1-1 to 1-37, quantum cascade laser; 2. an optical fiber combiner; 3. the optical fiber comprises a conical double-cladding Raman gain optical fiber 3a, a fiber core 3b, a first cladding 3c, a second cladding 3d, multimode pump light 3e, Raman signals 4a, fiber Bragg gratings I and 4b, fiber Bragg gratings II and 5 and an optical fiber cladding mode stripper.
Detailed Description
The invention is described in further detail below with reference to the figures and specific examples.
The invention relates to a mid-infrared all-fiber laser oscillator based on a tapered double-clad Raman gain fiber, which is described by taking a quantum cascade laser with 37 laser wavelengths of 4.0 microns and single quantum cascade laser tail fiber output power of 5W as an example, synthesizing into 1 path of low-brightness and high-power laser through a 37x1 multimode fiber combiner, pumping a laser resonant cavity formed by a section of mid-infrared single-mode fluorine indium glass tapered double-clad Raman gain fiber and a pair of mid-infrared fiber Bragg gratings directly engraved at two ends of the gain fiber, and realizing the output of near single-mode 5.0 micron high-power laser;
as shown in fig. 1, 37 quantum cascade lasers (1-1 to 1-37) with wavelength of 4.0 micron and tail fiber output power of 5 w are used as pump light sources, the tail fiber is a fluorine indium glass multimode fiber with outer diameter of 150 micron, inner diameter of 120 micron and core numerical aperture of 0.26;
the 37 tail fibers are prepared into a 37x1 high-power incoherent optical fiber combiner 2 by a heating tapering method, and the cross section of the output end of the combiner is shown in fig. 2; the outer diameter of the multimode fiber core array is 400 microns, and the effective diameter of the multimode fiber core array is 350 microns; the beam combining efficiency of the beam combiner is close to 95%;
the output optical fiber of the 37x1 optical fiber combiner 2 and the tapered double-clad Raman gain optical fiber 3 are subjected to low-loss fusion by an optical fiber fusion splicer;
a section of 50-meter-long intermediate infrared single-mode fluorine indium glass conical double-cladding Raman gain fiber 3 and fiber Bragg gratings I4 a and II 4b which are directly written on two ends of the gain fiber through femtosecond laser form a laser resonant cavity; the power of the 4.0-micron multimode pump light coupled into the laser resonant cavity is 175 watts;
the structure of the tapered double-clad raman gain fiber 3 is shown in fig. 3, and the fiber is composed of a raman gain core 3a, and a first cladding 3b (i.e., a pump cladding) and a second cladding 3c which are sequentially surrounded outside the core 3 a; the refractive index of the fiber core 3a is slightly larger than that of the first cladding 3b, the fiber core 3a has a very small numerical aperture NA, and the NA of the fiber core is between 0.01 and 0.2; the first cladding 3b is used as a pumping cladding, has a larger refractive index than the second cladding 3c, thereby having a larger numerical aperture and a larger collection capability for pumping light, and the NA of the first cladding 3b is between 0.2 and 1.0;
the cross section of the input end (i.e. the butt end) of the tapered double-clad raman gain fiber 3 is shown in fig. 4, the outer diameter of the cross section is 400 microns, the cross section of the fiber core 3a is circular, and the diameter of the fiber core 3a is 30 microns; the effective diameter of the first cladding 3b is 350 microns, the size and the cross section shape are the same as or close to those of the 37x1 high-power incoherent optical fiber combiner 2, and the geometric centers of the cross sections of the fiber core 3a, the first cladding 3b and the second cladding 3c are coincident. Preferably, the glass components of the first cladding 3b and the output multimode fiber of the fiber combiner 2 are close to each other, so as to ensure that low-loss fusion can be achieved between the fiber combiner 2 and the tapered double-clad raman gain fiber 3.
The outer diameter of the tapered double-clad Raman gain fiber 3 is reduced in proportion from a thick end to a thin end, the tapering ratio is 1.1, and the diameter proportion of the fiber core and the first cladding is constant along the length direction of the fiber.
The reflectivities of the fiber Bragg grating I4 a and the fiber Bragg grating II 4b are respectively R1And R2Satisfy R1>R2Conditions of (1) ensuring from R2The central wavelengths of the two gratings are positioned at 5.02 microns;
after the multimode pump light 3d is coupled into the first cladding 3b of the tapered double-cladding raman gain fiber 3, multiple reflections occur between the interfaces of the first cladding 3b and the second cladding 3c due to the total internal reflection principle, the multimode pump light 3d passes through the single-mode raman gain fiber core 3a multiple times, and a stimulated raman signal 3e is generated in the single-mode fiber core 3a (as shown in fig. 5);
the wavelength of the stimulated raman signal is within the raman gain bandwidth of the indium fluoride glass. Under the pumping light excitation effect, the generated stimulated Raman signal repeatedly reciprocates and is amplified into high-power output in a laser resonant cavity formed by the conical double-cladding Raman gain fiber 3 and the fiber Bragg gratings 4a and 4b, and the laser output is preferentially output to the thin end of the conical double-cladding Raman gain fiber 3 because the reflectivity of the fiber Bragg grating 4b is lower;
and a cladding mode stripper 5 at the output end removes residual pump light, so that high-beam-quality, near-single-mode, 5.02-micrometer and hectowatt-level high-power laser output is obtained.
The glass system of the tapered double-clad raman gain fiber 3 is not limited to the fluorine indium glass of the present embodiment, and may be other mid-infrared glass, such as sulfide glass, selenide glass or telluride glass. FIG. 6 shows InF glass (InF)3Base), sulfide glass (As component)2S3) Selenide glass (As component)2Se3) Or a telluride glass (composition is GeTe)4) Normalized raman lines of (a). The glass is highly transparent in the wave band above 4 microns in the middle infrared, and the longest transparent wave band can reach 20 microns.
In addition, the length of the tapered double-clad raman gain fiber 3 is not limited to 50 meters in the present embodiment, and a raman gain fiber between 0.5 and 500 meters may be used.
In order to reduce the temperature of the beam combining part of the optical fiber beam combiner, protect optical fiber devices in the beam combiner and inhibit the instability of waveguide characteristics caused by temperature fluctuation in the beam combiner, the optical fiber beam combiner can be arranged on a heat sink, and the purpose of heat dissipation is achieved through effective heat management, such as a water-feeding or air-feeding active cooling method.
The foregoing is a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.
Claims (9)
1. A mid-infrared all-fiber laser oscillator based on a tapered double-clad Raman gain fiber is characterized by comprising: n high-power intermediate infrared quantum cascade lasers with multimode tail fiber output, a high-power intermediate infrared optical fiber beam combiner for combining N multimode tail fibers into one multimode optical fiber output, a section of conical double-cladding Raman gain optical fiber, an intermediate infrared fiber Bragg grating I and an intermediate infrared fiber Bragg grating II which are inscribed at two ends of the conical double-cladding Raman gain optical fiber, and a section of optical fiber cladding mode stripper, the intermediate infrared fiber Bragg grating I, the conical double-clad Raman gain fiber and the intermediate infrared fiber Bragg grating II are sequentially connected to form a laser resonant cavity, the output laser of the N quantum cascade lasers is injected into the fiber combiner, then the laser is transmitted out through an optical fiber beam combiner and coupled into the laser resonant cavity, and the laser output by the laser resonant cavity passes through an optical fiber cladding mode stripper outside the cavity and then outputs the laser with the wavelength of over 4 microns of mid-infrared light;
the tapered double-cladding Raman gain fiber consists of a fiber core, a first cladding and a second cladding, wherein the first cladding and the second cladding are sequentially surrounded outside the fiber core, the refractive index of the first cladding is slightly smaller than that of the fiber core, the refractive index of the second cladding is remarkably smaller than that of the first cladding, the diameter of the first cladding is more than 3 times of that of the fiber core, the cross section of the fiber core is circular, the cross section of the first cladding is close to that of the fiber core of the multimode fiber output by the fiber combiner, the thick end of the tapered double-cladding Raman gain fiber is a pump light input end, the thin end of the tapered double-cladding Raman gain fiber is a Raman laser output end, the size of the fiber between the thick end and the thin end is continuously reduced along the length direction of the fiber, the diameter proportion of the fiber core and the;
the intermediate infrared fiber Bragg grating I is positioned at the thick end of the conical double-cladding Raman gain fiber, the intermediate infrared fiber Bragg grating II is positioned at the thin end of the conical double-cladding Raman gain fiber, the operating wavelengths of the intermediate infrared fiber Bragg grating I and the intermediate infrared fiber Bragg grating II are the same, and the reflectivity of the intermediate infrared fiber Bragg grating I is larger than that of the intermediate infrared fiber Bragg grating II.
2. The mid-infrared all-fiber laser oscillator based on the tapered double-clad raman gain fiber of claim 1, wherein the mode field diameter of the fiber core is between 5 and 200 microns, the numerical aperture is between 0.01 and 0.2, the first cladding has a large core diameter and a high numerical aperture close to the output multimode fiber of the fiber combiner, the core diameter is between 50 and 500 microns, and the numerical aperture is between 0.2 and 1.0.
3. The mid-infrared all-fiber laser oscillator based on a tapered double-clad raman gain fiber according to claim 1, wherein the tapered double-clad raman gain fiber has a length between 0.5 and 500 meters.
4. The mid-infrared all-fiber laser oscillator based on the tapered double-clad Raman gain fiber according to claim 1, wherein the fiber core, the first cladding and the second cladding are made of mid-infrared glass, and are highly transparent in a mid-infrared band of 4 microns or more, and the longest transparent band reaches 20 microns.
5. The mid-infrared all-fiber laser oscillator based on the tapered double-clad Raman gain fiber of claim 1, wherein N mid-infrared quantum cascade lasers have similar laser output wavelengths, and the laser output wavelengths are greater than 3.7 micronsThe mode is continuous laser or quasi-continuous laser output, the output power of the single intermediate infrared quantum cascade laser is 0.1-20W after being coupled into the multimode tail fiber, and the output beam quality M2Between 5 and 100.
6. The mid-infrared all-fiber laser oscillator based on the tapered double-clad raman gain fiber according to claim 1, wherein the fiber combiner is disposed on a heat sink.
7. The mid-infrared all-fiber laser oscillator based on the tapered double-clad Raman gain fiber of claim 1, wherein the fiber combiner is prepared by a hot tapering method.
8. The mid-infrared all-fiber laser oscillator based on the tapered double-clad Raman gain fiber according to claim 1, wherein the mid-infrared fiber Bragg grating I and the mid-infrared fiber Bragg grating II are respectively written on two ends of the tapered double-clad Raman gain fiber by femtosecond laser.
9. The mid-infrared all-fiber laser oscillator based on the tapered double-clad raman gain fiber according to any one of claims 1 to 8, characterized by the specific steps of:
s1, forming a resonant cavity for generating intermediate infrared laser by adopting a section of conical double-cladding single-mode Raman gain fiber, and an intermediate infrared fiber Bragg grating I and an intermediate infrared fiber Bragg grating II at two ends of the Raman gain fiber;
s2, a plurality of intermediate infrared quantum cascade laser pumping sources with similar wavelengths, close power and multimode tail fiber output are adopted, and a Nx1 incoherent multimode fiber combiner is used for generating high pumping power of N.eta times of the output power of a single intermediate infrared quantum cascade laser;
s3, enabling the mid-infrared high-power multimode pump light to enter a laser resonant cavity through low-loss all-fiber fusion coupling, generating gain in a Raman gain band of the tapered double-cladding single-mode Raman gain fiber, oscillating back and forth in the resonant cavity and outputting high-power laser;
and S4, removing residual pump light in laser output by the high-power laser output from the laser resonant cavity through an optical fiber cladding mode stripper outside the cavity, and realizing the output of the high-power near single-mode laser with the mid-infrared wavelength of more than 4 microns.
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