CN106253047B - Tunable mid-infrared light fibre mixed gas cascade Ramar laser - Google Patents

Abstract

The invention discloses a tunable mid-infrared optical fiber mixed gas cascaded Raman laser, which comprises a near-infrared tunable laser pump source, an input coupling component, an input gas cavity, a hollow-core optical fiber, an output gas cavity, and The output guide assembly and the coupling output mirror, the hollow core fiber is filled with two kinds of Stimulated Raman Scattering of the pump light to generate the first-order Raman laser and Stimulated Raman scattering of the first-stage Raman laser. Mixed Raman gain gas for second-stage Raman lasers in the infrared, also including feedback for directing first-stage Raman lasers transmitted out of the output mirror and second-stage Raman lasers in the mid-infrared into the input window system. The tunable mid-infrared fiber mixed gas cascaded Raman laser has the advantages of narrow linewidth, tunability, low pumping threshold, high conversion efficiency and the like.

Description

Tunable mid-infrared fiber-optic mixed-gas cascaded Raman laser

technical field

The invention relates to the technical field of laser generating equipment, in particular to a tunable mid-infrared optical fiber mixed gas cascaded Raman laser.

Background technique

The mid-infrared band is located in the atmospheric window and is widely used in remote sensing detection, lidar, communication, and military fields. At present, the lasers that realize output in the mid-infrared band mainly include quantum cascade lasers, electronic vibration solid-state lasers, optical parametric oscillators, and fiber lasers. Among them, quantum cascade lasers generate more heat during continuous operation, and their excited regions are large, making it difficult to achieve high-power single-mode output; electronic vibration solid-state lasers can achieve 2-5 μm high-efficiency output, but the thermal lens effect limits its Increase in power; optical parametric oscillators can achieve tunable mid-infrared output at a power level of several watts, but they have high requirements for pump source linewidth and polarization state; currently, holmium-doped fluoride fiber lasers can achieve 3-4μm laser output, but the power level and slope efficiency are low, and it is also difficult to expand the wavelength to longer wavelengths.

Gas stimulated Raman scattering has high gain coefficient, large frequency shift range, and flexible medium selection. It has received extensive attention since it was first reported in 1963, and it is an effective means for laser wavelength expansion. However, when gas stimulated Raman scattering is realized in free space, the effective distance is limited by the Rayleigh length, the pumping threshold is high, and many competing Raman lines will be generated.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art and provide a tunable mid-infrared optical fiber mixed gas cascade Raman laser that is compact, narrow linewidth, tunable, low pump threshold power, and high conversion efficiency .

In order to solve the problems of the technologies described above, the present invention adopts the following technical solutions:

A tunable mid-infrared optical fiber mixed gas cascaded Raman laser, comprising a near-infrared tunable laser pump source hollow-core fiber, an input gas cavity, an output gas cavity, and a coupled output mirror, and the two ends of the hollow-core fiber are respectively sealed In the input gas chamber and the output gas chamber;

The input gas chamber is provided with an input window, and an input coupling component for coupling the pump light to the hollow-core optical fiber is provided between the near-infrared tunable laser pump source and the input window;

The hollow-core fiber, the input gas cavity and the output gas cavity are mixed with two or more Raman gain gases, wherein at least one Raman gain gas causes the pump light to undergo stimulated Raman scattering to generate a first-order Raman laser , the remaining Raman gain gas causes the first-stage Raman laser to undergo stimulated Raman scattering to generate a second-stage Raman laser in the mid-infrared band;

The transmission loss of the hollow-core fiber in the pump laser band, the first-level Raman laser band, and the second-level Raman laser band is <0.5dB/m, while the transmission loss in other bands is >5dB/m;

The output gas chamber is provided with an output window, and an output guide assembly for guiding the light emitted from the output window to the outcoupling mirror is arranged between the output window and the outcoupling mirror;

The transmittance of the coupling output mirror to the first-stage Raman laser is greater than 90%, the reflectance to the second-stage Raman laser in the mid-infrared band is 10% to 90%, and the transmittance is 90% to 10%. ;

It also includes a feedback system for guiding the first-stage Raman laser transmitted from the output coupling mirror and the second-stage Raman laser in the mid-infrared band into the input window.

In the above-mentioned tunable mid-infrared fiber-optic mixed gas cascaded Raman laser, preferably, the input coupling component includes a first focusing coupling lens and a dichroic mirror, and the first focusing coupling lens and the dichroic mirror are sequentially along the pumping light emission direction Arrangement; the output guide assembly includes a collimator lens arranged in sequence along the direction of light output from the output window and a first mirror for reflecting the light to the output coupling mirror.

For the tunable mid-infrared optical fiber mixed gas cascaded Raman laser, preferably, the feedback system includes a mirror group, a rotary mirror, a second focusing coupling lens and the dichroic mirror, and the rotary mirror can be moved along The optical path direction is translated with high precision to change the length of the feedback optical path; the first-level Raman laser and the second-level Raman laser in the mid-infrared band are collimated by the mirror group and the rotary mirror, and then focused by the second focusing coupling lens to dichroic mirror.

In the above-mentioned tunable mid-infrared fiber-optic mixed gas cascaded Raman laser, preferably, the reflector group includes a second reflector and a third reflector, and the second reflector, the third reflector and the pair of revolving reflectors The reflectivity of the first-level Raman laser and the second-level Raman laser in the mid-infrared band is greater than 98%.

In the above-mentioned tunable mid-infrared fiber-optic mixed gas cascaded Raman laser, preferably, the transmittance of the dichroic mirror to the pump light is greater than 95%, and the transmittance of the dichroic mirror to the first-stage Raman laser and the first-stage Raman laser in the mid-infrared band is greater than 95%. The reflectivity of the secondary Raman laser is greater than 90%.

In the above-mentioned tunable mid-infrared optical fiber mixed gas cascaded Raman laser, preferably, the pump light generated by the near-infrared tunable laser pump source is a pulsed or continuous near-infrared laser, and passes through the input window sequentially from the dichroic mirror , input gas cavity, hollow-core fiber, output gas cavity, output window, collimator lens, first mirror, coupling output mirror, mirror group, revolving mirror, second focusing coupling lens and back to the loop of dichromatic mirror The optical length of L is L, and the length of L can be adjusted by high-precision translation of the rotating mirror along the direction of the optical path. When the near-infrared tunable laser pump source generates the pump light pulse near-infrared laser, its repetition frequency M and the loop The optical length L satisfies the following relationship:

nL=c/M

In the formula, n is a positive integer, c is the propagation speed of light in vacuum, the unit of L is m, and the unit of M is Hz.

In the above-mentioned tunable mid-infrared optical fiber mixed gas cascaded Raman laser, preferably, the input window and the output window are transparent to the pump light, the first-stage Raman laser and the second-stage Raman laser in the mid-infrared band. The pass rate is greater than 90%.

In the above-mentioned tunable mid-infrared optical fiber mixed gas cascaded Raman laser, preferably, the two or more Raman gain gases in the hollow-core fiber, the input gas cavity and the output gas cavity include hydrogen and more than one alkane gas, The alkane gases include methane, ethane, propane, butane and ethylene.

The above-mentioned tunable mid-infrared optical fiber mixed gas cascaded Raman laser preferably also includes a vacuum pumping and gas filling device for vacuuming the input gas chamber or the output gas chamber and filling two kinds of Raman gain gases.

The principle of the present invention is: the hollow core fiber can provide a nearly ideal environment for gas stimulated Raman scattering, it can effectively confine the pump light in the fiber core of the micron level, greatly improving the pumping intensity and effective The effective gain of each Raman signal can be controlled by reasonably selecting the pump wavelength and Raman gain gas, and designing the transmission loss spectrum of the hollow-core photonic crystal fiber, and combining the two to form a fiber gas laser to achieve mid-infrared laser output . Since the Raman frequency shift range of a single gas is limited, it is not enough to shift the frequency of the common near-infrared pump light in the 1 micron band to the mid-infrared band. Therefore, two or more gases can be filled in a hollow-core fiber at the same time. The pump light acts on one of the gases and undergoes stimulated Raman scattering to shift the wavelength up once to generate the first-order Raman laser. The first-order Raman laser interacts with the other gas to undergo stimulated Raman scattering A wavelength shift is performed to generate a second-order Raman laser in the mid-infrared band. Or through more times of stimulated Raman scattering, the wavelength can be shifted up to generate mid-infrared laser light. By rationally designing the transmission band of the hollow-core fiber, the generation of competing Raman lasers in this process can be effectively suppressed, so that the generation of mid-infrared lasers can achieve a higher conversion efficiency.

Compared with the prior art, the present invention has the advantages of:

1. The tunable mid-infrared optical fiber mixed gas cascaded Raman laser of the present invention utilizes the cascaded stimulated Raman scattering of two or more gases at the same time, and is a new type of mid-infrared laser generated by a near-infrared pump laser device.

2. The present invention utilizes the hollow-core optical fiber to effectively confine the pump light in the micron-scale fiber core, which greatly improves the pump intensity and effective action distance, and enhances the interaction intensity between the pump light and the Raman gain gas; At the same time, taking advantage of the convenient design of the anti-resonance hollow fiber transmission band, the output loss band of the fiber can be specially designed, so that the transmission loss of the pump wavelength and the two-stage Raman laser wavelength is low, and the transmission loss of other bands is high, which can effectively suppress The generation of competing Raman lasers improves the conversion efficiency.

3. The present invention makes the first-level Raman laser continue to interact with the second Raman gain gas by designing the feedback system, further improving the conversion efficiency of the second-level Raman laser in the mid-infrared band, and making the mid-infrared band The second-stage Raman laser propagates multiple times in the loop of the space structure to form resonance, which can reduce the pump threshold power and improve the conversion efficiency.

4. The present invention combines the advantages of high output power of gas laser, high damage threshold and good beam quality of fiber laser, and has great potential advantages in practical application.

Description of drawings

Figure 1 is a schematic diagram of the structure of a tunable mid-infrared optical fiber mixed gas cascaded Raman laser.

Fig. 2 is a scanning electron micrograph of a cross-section of an ice cream-type anti-resonance hollow-core optical fiber.

Fig. 3 is a scanning electron micrograph of the cross-section of the free-boundary anti-resonance hollow-core fiber.

Fig. 4 is a schematic diagram of the wavelength of the pump light after being Raman-shifted by two gases and its relative position in the anti-resonant hollow-core optical fiber transmission belt.

illustration:

1. Near-infrared tunable laser pump source; 2. Hollow-core fiber; 3. First focusing coupling lens; 4. Dichromatic mirror; 5. Input window; 6. Input gas cavity; 7. Output gas cavity; 8. Output Window; 9. Collimating lens; 10. First reflector; 11. Coupling output mirror; 12. Second reflector; 13. Third reflector; 14. Rotary reflector; 15. Second focusing coupling lens; 16 , Vacuum and inflation device.

detailed description

The present invention will be described in further detail below in conjunction with the accompanying drawings and specific embodiments.

As shown in Figure 1, the tunable mid-infrared optical fiber mixed gas cascaded Raman laser of this embodiment includes a near-infrared tunable laser pump source 1, a hollow-core fiber 2, an input gas cavity 6, an output gas cavity 7 and a coupling The output mirror 11 and the two ends of the hollow-core optical fiber 2 are respectively sealed in the input gas chamber 6 and the output gas chamber 7;

The input gas cavity 6 is provided with an input window 5, and an input coupling assembly for coupling the pump light to the hollow-core optical fiber 2 is provided between the near-infrared tunable laser pump source 1 and the input window 5, and the input coupling assembly includes a first focusing The coupling lens 3 and the dichroic mirror 4, the first focusing coupling lens 3 and the dichroic mirror 4 are arranged in sequence along the pumping light emission direction;

The hollow core fiber 2, the input gas cavity 6 and the output gas cavity 7 are filled with a mixed gas mixed with two kinds of Raman gain gases, one of which causes the pump light to undergo stimulated Raman scattering to generate a first-order Raman laser, another Raman gain gas causes the first-stage Raman laser to undergo stimulated Raman scattering to generate a second-stage Raman laser in the mid-infrared band; through the cascaded stimulated Raman of two Raman gain gases Scattering converts the wavelength of the near-infrared pump laser to the mid-infrared band;

The output gas cavity 7 is provided with an output window 8, and an output guide assembly for guiding the light emitted from the output window 8 to the output coupling mirror 11 is provided between the output window 8 and the output coupling mirror 11. A collimator lens 9 arranged in sequence in the direction of light output from the window 8 and a first mirror 10 for reflecting the light to the outcoupling mirror 11;

The transmittance of the output coupling mirror 11 to the first-stage Raman laser is greater than 95%, the reflectance to the second-stage Raman laser in the mid-infrared band is 10%-90%, and the transmittance is 90%-10%;

It also includes a feedback system for reflecting and focusing the first-stage Raman laser transmitted by the output coupling mirror 11 and the second-stage Raman laser in the mid-infrared band to the dichroic mirror 4 , and reflected by the dichroic mirror 4 into the input window 5 .

The feedback system of this embodiment includes a mirror group, a rotary mirror 14, and a second focusing coupling lens 15. After straightening, it is focused to the dichroic mirror 4 by the second focusing coupling lens 15. The reflector group includes a second reflector 12 and a third reflector 13, wherein the second reflector 12, the third reflector 13, and the revolving reflector 14 are used for the first-stage Raman laser and the second-stage Raman laser in the mid-infrared band. The reflectivity of Mann laser is greater than 98%. The transmittance of the dichromatic mirror 4 to the pump light is greater than 95%, and the reflectivity of the dichromatic mirror 4 to the first-stage Raman laser and the second-stage Raman laser in the mid-infrared band is greater than 90%. The rotary mirror 14 is installed on the translation device and can be translated with high precision along the optical path direction to change the length of the feedback optical path to meet the pulse overlap condition under the pulse working condition. The translation device can use an existing high-precision translation platform.

The tunable mid-infrared fiber-gas mixed gas cascade Raman laser is a compact, narrow-linewidth, tunable fiber-optic gas Raman laser. Infrared laser wavelengths are further extended to the mid-infrared direction. When working, the near-infrared laser generated by the near-infrared tunable laser pump source 1 is coupled to the hollow-core fiber 2 filled with two Raman gain mixture gases through the first focusing coupling lens 3, dichroic mirror 4, and input window 5 In the fiber core, the pump light interacts with one of the Raman gain gases in the fiber core to generate stimulated Raman scattering to generate the first-order Raman laser, and the first-order Raman laser and the second Raman gain gas Gas interaction occurs stimulated Raman scattering to generate the second-order Raman laser in the mid-infrared band, and the first-order Raman laser and the second-order Raman laser are incident through the output window 8, the collimator lens 9 and the first mirror 10 To the coupling-out mirror 11, the transmittance of the coupling-out mirror 11 to the first-stage Raman laser is greater than 98%, and the reflectivity to the second-stage Raman laser in the mid-infrared band is about 10%-90%. About 90%-10%, the first-level Raman laser transmitted from the coupling output mirror 11 and the second-level Raman laser in the mid-infrared band pass through the second mirror 12, the third mirror 13, and the rotary mirror 14 in turn. After straightening, it is focused to the dichroic mirror 4 by the second focusing coupling lens 15, then reflected by the dichroic mirror 4 and coupled to the core of the hollow-core fiber 2 through the input window 5, so that the first-order Raman laser continues to be connected with the second This kind of Raman gain gas interaction can further improve the conversion efficiency of the second-stage Raman laser in the mid-infrared band. The second-stage Raman laser in the mid-infrared band propagates multiple times in the loop of this space structure to form resonance, which can Reduce pumping threshold power and improve conversion efficiency.

In this embodiment, the hollow-core fiber 2 adopts a specially designed anti-resonance hollow-core fiber with a transmission belt, which is a micron-scale hollow-core structure, and confines the pump light in a space with a micron-scale cross section, which can effectively enhance The interaction of the pump light with the Raman gain gas increases the effective distance and is designed to have multiple transmission bands in the near-infrared and mid-infrared, and its transmission loss spectrum is based on the anti-resonant optical waveguide model, that is, the position of the transmission band It can be controlled by the thickness of the capillary wall of the fiber cladding, and has low transmission loss (<0.5dB/m) at the pump band and the two-stage Raman wavelength, and has a higher transmission loss at other Raman signal wavelengths that need to be suppressed Transmission loss (>5dB/m).

In this embodiment, both the input window 5 and the output window 8 are coating windows, and the transmittance of the input window 5 and the output window 8 to the pump light, the first-stage Raman laser and the second-stage Raman laser in the mid-infrared band is equal. Greater than 90%.

In this embodiment, the tunable mid-infrared optical fiber mixed gas cascaded Raman laser also includes a vacuum pumping and gas filling device 16 for vacuuming the input gas chamber 6 or the output gas chamber 7 and filling two kinds of Raman gain gases The vacuum pumping and inflation device 16 can adopt the prior art, and it can also be equipped with a monitoring component for air pressure monitoring.

In this embodiment, the pump light generated by the near-infrared tunable laser pumping source 1 may be a continuous near-infrared laser or a pulsed near-infrared laser. When the pump light generated by the near-infrared tunable laser pump source 1 is a pulsed near-infrared laser, the tunable mid-infrared fiber-optic gas-cascaded Raman laser works in a pulsed state of synchronous pumping. In this state, the two-color Mirror 4 passes through input window 5, input gas cavity 6, hollow core fiber 2, output gas cavity 7, output window 8, collimator lens 9, first reflector 10, coupling output mirror 11, reflector group, gyroreflection Mirror 14, the second focusing coupling lens 15 return to the optical length of the loop of the dichroic mirror 4 is L, this optical length L can be adjusted by the high-precision translation of the rotary mirror 14 in the direction of the optical path, and the near-infrared tunable laser The repetition frequency of pump source 1 is M, and the corresponding relationship between L and M satisfies formula (1):

nL=c/M (1)

In the formula, n is a positive integer, c is the propagation speed of light in vacuum, the unit of L is m, and the unit of M is Hz.

Under this corresponding relationship, any pulse of the first-order Raman laser generated in the anti-resonance hollow-core fiber 2 and the second-order Raman laser in the mid-infrared band propagates in the loop for one or more cycles and then in the two-color Mirror 4 overlaps with the next pump laser pulse, which can reduce the pumping threshold of the two-stage Raman laser and improve the conversion efficiency.

In this embodiment, the two or more Raman gain gases in the hollow core fiber 2, the input gas cavity 6 and the output gas cavity 7 are a combination of common Raman gases, such as hydrogen and one or two alkane gases, Alkane gases include methane, ethane, propane, butane, ethylene, and the like. Or other suitable Raman gain mixed gas, the type of gas is not limited to 2 types, it can be 2 or more types, when there are more than 2 types, the number of stimulated Raman scattering series that occurs is correspondingly more than 2 level until the mid-infrared band Raman laser is generated. The following takes the combination of hydrogen and ethane as an example to illustrate the generation process of the first-stage Raman laser and the second-stage Raman laser in the mid-infrared band and the design of the transmission band in the anti-resonant hollow-core fiber 2 .

In the process of stimulated Raman scattering, the first-order Stokes wavelength can be obtained by formula (2)

In the formula, λ _out is the first-order Stokes wavelength in the stimulated Raman scattering process, and the unit is nm; λ _pump is the pump wavelength, and the unit is nm; Mann frequency shift, the unit is cm ^-1 .

When the central wavelength of the near-infrared tunable laser pump source 1 is 1064nm, and the two Raman mixed gases in the anti-resonant hollow-core fiber 2 are hydrogen and ethane, the pump light first interacts with hydrogen in the hollow-core fiber 2 Stimulated Raman scattering occurs to generate the first-order Raman laser. The Raman frequency shift of hydrogen is 4155cm ^-1 . Substituting the pump wavelength and the Raman frequency shift of hydrogen into formula (2), the wavelength of the first-order Raman laser is 1907nm. The first-stage Raman laser at 1907nm is used as the second stimulated Raman scattering pump source to interact with ethane to generate the second-stage Raman laser, and the Raman frequency shift of 1907nm and ethane by 2954cm ^-1 is substituted into the formula (2 ), the second-order Raman laser wavelength can be obtained, that is, the central wavelength of the output laser of the tunable mid-infrared fiber-optic Raman laser proposed by the present invention is 4367nm. When the central wavelength of the near-infrared tunable laser pump source 1 is tuned, the corresponding first-order Raman laser wavelength and the corresponding central wavelength of the mid-infrared laser final output laser proposed by the present invention can be obtained from formula (2).

In the two-stage stimulated Raman scattering process, competing Raman lines such as rotational Raman and higher-order Raman may be generated, and the pump light of the near-infrared tunable laser pump source 1 may be combined with the second Stimulated Raman scattering occurs through the alkane interaction to generate the first-order Raman laser at 1553nm. In order to prevent the occurrence of these situations, the transmission band of the anti-resonance hollow-core fiber 2 can be reasonably designed. Type anti-resonance hollow-core fiber 2, the light-colored area of the fiber cross-section in the figure is the quartz structure, and the dark-colored area is the air hole. The wavelength position of the high-loss region of the anti-resonant hollow-core fiber 2 can be obtained by formula (3).

In the formula, λ _m is the resonance wavelength in nm, that is, the wavelength in the high transmission loss region, d is the wall thickness of the cladding capillary in nm, m is a positive integer, n ₂ and n ₁ are cladding quartz and fiber The refractive index of the core region. By controlling the thickness of the cladding capillary wall and then the transmission band of the anti-resonance hollow-core fiber 2 can be designed, so that the pump center wavelength λ ₀ (1064nm), the first-order Raman laser wavelength λ ₁ (1907nm) and the laser proposed by the present invention The relative position of the final output central wavelength λ ₂ (4367nm) of the λ and the transmission spectrum of the optical fiber is shown in FIG. 4 . It can be seen from Fig. 4 that the three wavelengths are respectively located in the three transmission bands of the optical fiber, and the pump light λ ₀ is shifted to the mid-infrared band λ ₂ by two successive Raman frequency shifts of the two gases. Through this design, the generation of competing Raman lines can be suppressed, and the conversion efficiency of mid-infrared laser can be maximized.

The above-mentioned pump light with a center wavelength of 1064nm can also first react with ethane to generate stimulated Raman scattering to generate a first-order Raman laser at 1553nm. At this time, it is only necessary to design the transmission band at λ1 in Figure ₄ to 1553nm That's it. Similarly, when the Raman gain mixture gas in the anti-resonant hollow-core fiber 2 is other gases, the two-stage Raman laser wavelength can be calculated by formula (2) and the transmission band of the hollow-core fiber 2 can be reasonably adjusted.

The above descriptions are only preferred implementations of the present invention, and the scope of protection of the present invention is not limited to the above examples. For those skilled in the art, improvements and transformations obtained without departing from the technical concept of the present invention should also be regarded as the protection scope of the present invention.

Claims (8)

1. A tunable mid-infrared optical fiber mixed gas cascaded Raman laser, characterized in that it includes a near-infrared tunable laser pump source (1), a hollow-core optical fiber (2), an input gas cavity (6), and an output gas A cavity (7) and a coupling output mirror (11), the two ends of the hollow-core optical fiber (2) are respectively sealed in the input gas cavity (6) and the output gas cavity (7); The input gas cavity (6) is provided with an input window (5), and between the near-infrared tunable laser pumping source (1) and the input window (5) is a device for coupling the pump light to a hollow-core optical fiber (2 ) input coupling components; The hollow-core fiber (2), the input gas chamber (6) and the output gas chamber (7) are filled with two or more Raman gain gases, at least one of which makes the pump light stimulated to pull Man scattering produces the first-order Raman laser, and the rest of the Raman gain gas causes the first-stage Raman laser to undergo stimulated Raman scattering to generate the second-stage Raman laser in the mid-infrared band; The transmission loss of the hollow-core fiber (2) in the pump laser band, the first-level Raman laser band, and the second-level Raman laser band is <0.5dB/m, while the transmission loss in other bands is >5dB/m; The output gas chamber (7) is provided with an output window (8), and a device for guiding the light emitted from the output window (8) to the outcoupling mirror (11) is provided between the output window (8) and the outcoupling mirror (11). (11) the output bootstrap component; The transmittance of the coupling output mirror (11) to the first-stage Raman laser is greater than 90%, the reflectance to the second-stage Raman laser in the mid-infrared band is 10% to 90%, and the transmittance is 90%. ~10%; It also includes a feedback system for guiding the first-stage Raman laser transmitted from the output coupling mirror (11) and the second-stage Raman laser in the mid-infrared band into the input window (5); The two or more Raman gain gases in the hollow-core fiber (2), the input gas cavity (6) and the output gas cavity (7) include hydrogen and more than one alkane gas, and the alkane gas includes methane, ethyl Alkanes, propanes, butanes and ethylene. 2. The tunable mid-infrared fiber mixed gas cascaded Raman laser according to claim 1, characterized in that: the input coupling component includes a first focusing coupling lens (3) and a dichroic mirror (4), and the first A focusing coupling lens (3) and a dichroic mirror (4) are sequentially arranged along the direction of pump light emission; the output guide assembly includes a collimator lens (9) arranged sequentially along the direction of output light from the output window (8) and for A first reflector (10) reflects light to an outcoupling mirror (11). 3. The tunable mid-infrared optical fiber mixed gas cascaded Raman laser according to claim 2, characterized in that: the feedback system includes a mirror group, a rotary mirror (14), a second focusing coupling lens (15) and the dichroic mirror (4), the rotary mirror (14) can be translated with high precision along the optical path direction to change the length of the feedback optical path; the first-stage Raman laser and the second-stage Raman laser in the mid-infrared band After being collimated by the mirror group and the rotary mirror (14), it is focused to the dichroic mirror (4) by the second focusing coupling lens (15). 4. The tunable mid-infrared fiber mixed gas cascaded Raman laser according to claim 3, characterized in that: the mirror group includes a second mirror (12) and a third mirror (13), the The reflectivity of the second reflector (12), the third reflector (13) and the revolving reflector (14) to the first-order Raman laser and the second-order Raman laser in the mid-infrared band is greater than 98%. 5. The tunable mid-infrared optical fiber mixed gas cascaded Raman laser according to claim 3, characterized in that: the transmittance of the dichroic mirror (4) to pump light is greater than 95%, and the dichroic mirror (4) ) The reflectivity of the first-level Raman laser and the second-level Raman laser in the mid-infrared band is greater than 90%. 6. The tunable mid-infrared optical fiber mixed gas cascaded Raman laser according to claim 3, characterized in that: the pump light generated by the near-infrared tunable laser pump source (1) is pulsed or continuous near-infrared Laser, and from the dichroic mirror (4) through the input window (5), input gas cavity (6), hollow core fiber (2), output gas cavity (7), output window (8), collimating lens (9 ), the first mirror (10), the outcoupling mirror (11), the mirror group, the revolving mirror (14), the second focusing coupling lens (15) and the optical length of the loop back to the dichroic mirror (4) is L, and the length of L can be adjusted by high-precision translation along the direction of the optical path of the rotating mirror (14). Satisfy the following relationship with the loop optical length L: nL = c / M In the formula, n is a positive integer, c is the propagation speed of light in vacuum, the unit of L is m, and the unit of M is Hz. 7. The tunable mid-infrared optical fiber mixed gas cascaded Raman laser according to claim 1, characterized in that: the input window (5) and the output window (8) are opposite to the pump light, the first stage pull Both the Raman laser and the second-stage Raman laser transmittance in the mid-infrared band are greater than 90%. 8. The tunable mid-infrared fiber-optic mixed gas cascade Raman laser according to claim 1, characterized in that: it also includes a vacuum pumping and charging chamber for the input gas chamber (6) or the output gas chamber (7). Vacuumizing and filling device (16) for two kinds of Raman gain gases.