Single-longitudinal-mode hollow-core optical fiber gas laser and operation method

Abstract

The invention provides a single-longitudinal-mode hollow-core optical fiber gas laser and an operation method. The single-longitudinal-mode hollow-core optical fiber gas laser comprises a tunable pumping source, a first annular resonant cavity structure arranged on the outer side of the tunable pumping source and a gas filling and vacuumizing system connected with the inner side of the first annular resonant cavity structure. The tunable pumping source controls the output wavelength of the pumping seed source through the direct-current power supply so as to be matched with the absorption wavelength of the gain gas and output pumping light; a first air chamber and a second air chamber in the first annular resonant cavity structure are arranged on the five-dimensional adjusting frame, and the first annular resonant cavity structure is used for generating single longitudinal mode laser; and the gas filling and vacuumizing system is used for performing vacuumizing and gain gas filling on the hollow-core optical fiber in the first annular resonant cavity structure. Through the annular resonant cavity structure and the cavity length design, intermediate infrared single longitudinal mode laser can be effectively generated.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and in particular to a single-longitudinal-mode hollow fiber gas laser and its operating method. Background Technology

[0002] Single-mode lasers, due to their significant advantages in monochromaticity, coherence, and stability, are widely used in precision measurement, gas detection, and precision spectroscopy. Currently, single-mode laser generation methods such as the short-cavity method, the FP etalon method, the ring cavity method, and the compound cavity method have been widely applied in near-infrared lasers. However, for mid-infrared single-mode lasers, traditional lasers struggle to generate single-mode lasers above 4μm due to limitations in the transition energy levels of gain ions in solid-state and fiber lasers. While nonlinear frequency conversion can generate mid-infrared single-mode lasers, it typically requires high pump intensity and involves complex structures and demanding optical components. Quantum cascade lasers, utilizing their inherent short-cavity characteristics, can generate mid-infrared single-mode lasers, offering advantages such as small size and tunable wavelength; however, they require sophisticated design and fabrication and suffer from poor beam quality.

[0003] The wavelength range above 4 μm enters the vibrational-rotational energy level transition region of gas molecules, offering a rich selection of gain gases for the mid-infrared band. With the advent of hollow-core optical fibers, hollow-core fiber gas lasers have attracted attention as a highly efficient method for generating mid-infrared light sources. By filling different gain gases, mid-infrared radiation in different bands can be generated. This type of laser combines the advantages of high gain, high power, and high beam quality of both gas and fiber lasers, providing a reliable solution for wavelength extension of mid-infrared lasers. However, current research on mid-infrared light source generation using gas-filled hollow fibers mainly focuses on single-pass gain spontaneous emission amplification. Due to the lack of cavity structure, the mid-infrared light source generated by this method lacks longitudinal mode characteristics. Summary of the Invention

[0004] In order to overcome the shortcomings of the prior art, the purpose of this invention is to provide a single longitudinal mode hollow fiber gas laser and its operation method, which can transmit pump light and laser light simultaneously and effectively generate mid-infrared single longitudinal mode laser light.

[0005] To achieve the above objectives, the present invention provides the following solution: a single longitudinal mode hollow fiber gas laser, comprising: a tunable pump source, a first annular resonant cavity structure disposed outside the tunable pump source, and a gas filling and vacuuming system connected to the inner side of the first annular resonant cavity structure; The tunable pump source is used to generate pump light, and the output wavelength of the pump seed source is controlled by a DC power supply to match the absorption wavelength of the gain gas. The first annular resonant cavity structure is used to generate single-longitudinal-mode laser; The gas filling and vacuuming system is used to vacuum and fill the hollow fiber in the first annular resonant cavity structure with gain gas.

[0006] Optionally, the first annular resonant cavity structure is configured as a top-view structure. The first annular resonant cavity structure includes a first collimating lens disposed below the tunable pump source, a first dichroic mirror disposed below the first collimating lens, a focusing lens disposed to the left of the first dichroic mirror, a first air chamber disposed to the left of the focusing lens, a hollow optical fiber connected to the left end of the first air chamber, a second air chamber connected to the hollow optical fiber, a second dichroic mirror disposed below the second air chamber, a second collimating lens disposed below the second dichroic mirror, an output coupling mirror disposed below the second collimating lens, and a gold mirror disposed below the output coupling mirror and to the right of the first dichroic mirror. The input end of the hollow optical fiber is connected to the left end of the first air chamber, and the output end of the hollow optical fiber is connected to the top end of the second air chamber. The first air chamber and the second air chamber are mounted on a five-dimensional adjustment frame.

[0007] Optionally, the first collimating lens is used to collimate the pump light; The first dichroic mirror is used to reflect pump light and transmit laser light; The focusing lens is used to focus the pump light and couple it into the hollow fiber. The first air chamber has the same structure as the second air chamber. A window is provided on the right side of the first air chamber and an ultraviolet adhesive is provided on the left side. The window is used to seal the first air chamber and allow pump light and laser to pass through. The window is coated with an anti-reflection film for the laser and pump light bands and is placed at a certain small angle with the laser transmission direction. The ultraviolet adhesive is used to fix the hollow optical fiber. The second dichroic mirror has the same parameters as the first dichroic mirror, and the second dichroic mirror is used to filter out residual pump light; The second collimating lens has the same structure as the focusing lens. The second collimating lens cooperates with the focusing lens to couple the laser into the hollow optical fiber. Both the focusing lens and the second collimating lens are coated with anti-reflection films for the laser and pump light bands. The output coupling mirror is used to output a single longitudinal mode laser. The gold mirror is used to reflect laser light; The focal length ratio of the first collimating lens and the focusing lens is adapted to the mode field matching conditions of the pump light and the hollow fiber.

[0008] Optionally, the hollow fiber is configured as a single-turn anti-resonant hollow fiber, which is used to transmit laser and pump light. The hollow fiber includes a first annular silica layer, a plurality of second annular silica layers disposed inside the first annular silica layer, and an irregular air layer filled inside the first annular silica layer and the second annular silica layers.

[0009] Optionally, the gas filling and vacuum system includes a fifth gas valve communicating with the upper side of the first gas chamber, a sixth gas valve communicating with the left side of the second gas chamber, a valve assembly and a vacuum gauge disposed between the fifth and sixth gas valves, a gas cylinder disposed on the upper side of the valve assembly, and a vacuum molecular pump disposed on the lower side of the valve assembly. The valve assembly includes a first gas valve, a second gas valve, a third gas valve, and a fourth gas valve. The first and second gas valves are used to control the output of the gas cylinder, and the third and fourth gas valves are used to control the output of the vacuum molecular pump.

[0010] The present invention also provides a method for operating a single-longitudinal-mode hollow fiber gas laser, comprising: The first annular resonant cavity structure is fixed in position, the vacuum system is turned on and a vacuum operation is performed for two hours. Then the gas filling system is turned on and the gain gas is filled in. The pressure of the gain gas is greater than the threshold for dual-wavelength operation. The wavelength of the output gas of the tunable pump source is controlled by a DC power supply to match the gain. A helium-neon laser is placed on the output side of the first ring resonant cavity for laser simulation and adjustment. Then the helium-neon laser is removed and laser coupling is performed.

[0011] Optionally, the first annular resonant cavity structure is fixed in position, the vacuum system is turned on, and a two-hour vacuuming operation is performed. Then, the gas filling system is turned on and gain gas is introduced, including: The first and second air chambers are fixed on the five-dimensional adjustment frame. The pump light generated by the tunable pump source is coupled into the hollow fiber using the first collimating lens, the first dichroic mirror and the focusing lens. Then the five-dimensional adjustment frame is adjusted to finely adjust the position and pitch of the hollow fiber. Turn on the vacuum molecular pump and all gas valves, and perform a vacuuming operation for two hours. Then close the first, second, third, fourth, and fifth gas valves, open the gas cylinder and the second gas valve, observe the reading of the vacuum gauge, and fill in a certain pressure of gain gas. Close the second and sixth gas valves and wait for the gain gas to diffuse for four hours.

[0012] Optionally, the output wavelength of the tunable pump source is controlled by a DC power supply to match the absorption wavelength of the gain gas, and a helium-neon laser is placed at the output end of the first ring resonant cavity for laser simulation and adjustment. Then, the helium-neon laser is removed for laser coupling, including: A second dichroic mirror is placed below the second gas chamber, and a power meter for observing spontaneous emission amplified light is placed below the second dichroic mirror. The output wavelength of the tunable pump source is then controlled by a DC power supply to match the absorption wavelength of the gain gas until the highest spontaneous emission amplified light power is obtained. A helium-neon laser is placed at the location of the gold mirror, and coupled into the hollow-core optical fiber to simulate laser light. The position of the second gas chamber is adjusted according to the helium-neon laser. Then, the following operations are performed sequentially: placing the second collimating lens and the output coupling mirror, removing the helium-neon laser, placing the gold mirror, and adjusting the second collimating lens to complete the laser coupling. The hollow-core optical fiber and the length of the spatial optical path must meet the following conditions: ; Where c represents the speed of light. Represents the length of the optical path in space. Represents the length of the hollow fiber. The gain linewidth represents the gain gas. This represents the longitudinal mode spacing of the laser. When the longitudinal mode spacing is greater than the gain linewidth, the laser outputs a single longitudinal mode.

[0013] This invention discloses the following technical effects by providing a single-longitudinal-mode hollow fiber gas laser and its operating method: 1. This invention proposes a single-longitudinal-mode hollow fiber gas laser that can generate single-longitudinal-mode lasers in the mid-infrared band, greatly expanding the wavelength range of single-longitudinal-mode lasers. By changing the gain gas and the matching pump source, mid-infrared single-longitudinal-mode lasers in different bands can be generated, such as acetylene in the 3.1μm band, carbon dioxide in the 4.3μm band, and carbon monoxide in the 4.8μm band.

[0014] 2. The annular cavity structure used in this invention, combined with the narrow-band gain of the gas itself, allows for the generation of single-longitudinal-mode lasers by designing the cavity length, without the need for other special devices, resulting in a simple structure.

[0015] 3. The single longitudinal mode laser generated in this invention has the characteristic of being tunable. The tuning method is to control the wavelength of the pump source to correspond to different absorption lines of the same gain gas. Different absorption lines correspond to different laser wavelengths.

[0016] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A schematic diagram of a single-longitudinal-mode hollow fiber gas laser device provided in an embodiment of the present invention; Figure 2 A schematic diagram of the cross-section of a hollow optical fiber provided in an embodiment of the present invention; Figure 3 This is an operation flowchart of a single-longitudinal-mode hollow fiber gas laser provided in an embodiment of the present invention; Figure 4 The spectral measurement diagram provided in the embodiment of the present invention; Figure 5 Line width measurement diagram provided for embodiments of the present invention; Explanation of reference numerals in the attached figures: 1. Tunable pump source; 2. First collimating lens; 3. First dichroic mirror; 4. Focusing lens; 5. First gas chamber; 6. Hollow-core optical fiber; 61. First annular silicon dioxide layer; 62. Second annular silicon dioxide layer; 63. Irregular air layer; 7. Gas filling and vacuum system; 71. Vacuum molecular pump; 72. Gas cylinder; 73. Vacuum gauge; 74. First gas valve; 75. Second gas valve; 76. Third gas valve; 77. Fourth gas valve; 78. Fifth gas valve; 79. Sixth gas valve; 8. Second gas chamber; 9. Second dichroic mirror; 10. Second collimating lens; 11. Output coupling mirror; 12. Gold mirror. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] like Figure 1 As shown, the present invention provides a single longitudinal mode hollow fiber gas laser, comprising: a tunable pump source 1, a first annular resonant cavity structure disposed outside the tunable pump source 1, and a gas filling and vacuuming system 7 connected to the inner side of the first annular resonant cavity structure.

[0022] 1. Tunable pump source The tunable pump source 1 controls the output wavelength of the seed source through a DC power supply to match the absorption wavelength of the gain gas (carbon dioxide) for outputting pump light.

[0023] The tunable pump source 1 consists of a 2μm narrow linewidth tunable semiconductor seed source and two stages of thulium-doped fiber amplifier. The output wavelength of the seed source is controlled by a high-precision DC power supply. In order to match the absorption wavelength of carbon dioxide, the pump wavelength is first adjusted to around 2003.5nm, which corresponds to the R(18) absorption peak of carbon dioxide. The output pigtail of the pump source is a commercial single-mode fiber.

[0024] 2. First ring resonant cavity structure The first annular resonant cavity structure is used to generate a single longitudinal mode laser.

[0025] The first annular resonant cavity structure is configured as a top-view structure. The first annular resonant cavity structure includes a first collimating lens 2 disposed below the tunable pump source 1, a first dichroic mirror disposed below the first collimating lens 2, a focusing lens 4 disposed to the left of the first dichroic mirror, a first air chamber 5 disposed to the left of the focusing lens 4, a hollow optical fiber 6 connected to the left end of the first air chamber 5, a second air chamber 8 connected to the hollow optical fiber, a second dichroic mirror 9 disposed below the second air chamber 8, a second collimating lens 10 disposed below the second dichroic mirror 9, an output coupling mirror 11 disposed below the second collimating lens 10, and a gold mirror 12 disposed below the output coupling mirror 11 and to the right of the first dichroic mirror. The input end of the hollow optical fiber 6 is connected to the left end of the first air chamber 5, and the output end of the hollow optical fiber is connected to the top end of the second air chamber 8.

[0026] The first collimating lens 2 is configured as a zinc selenide aspherical lens coated with a 2μm band anti-reflection film, with a focal length of 12.7mm. The first collimating lens 2 is used to collimate the pump light.

[0027] The first dichroic mirror 3 is a calcium fluoride plate coated with a 2μm band reflective film and a 4.3μm band antireflective film. The first dichroic mirror 3 is used to reflect pump light and transmit laser light.

[0028] The focusing lens 4 is a calcium fluoride plano-convex lens coated with anti-reflective films in the 2μm and 4.3μm bands, with a focal length of 100mm. The focusing lens 4 is used to focus the pump light and couple it into the hollow fiber 6. It should be noted that the focal length ratio of the first collimating lens 2 and the first focusing lens 4 needs to meet the mode field ratio of the single-mode fiber and the hollow fiber.

[0029] The first gas chamber has the same structure as the second gas chamber. A window is located on the right side of the first gas chamber, and UV adhesive is located on the left side. The window is at an 8° angle to the laser transmission direction to prevent optical feedback. The window is a half-inch calcium fluoride plate coated with anti-reflective films in the 2μm and 4.3μm wavelength bands, used to seal the first gas chamber while allowing pump light and laser light to pass through. The UV adhesive is used to fix the hollow optical fiber. The first and second gas chambers are mounted on a five-dimensional adjustment frame.

[0030] The second dichroic mirror 9 has the same parameters as the first dichroic mirror and is used to filter out residual pump light.

[0031] The second collimating lens 10 has the same structure as the focusing lens 4. The second collimating lens 10 works in conjunction with the first collimating lens 2 to couple the laser beam into the hollow fiber; a ring cavity structure is constructed under the condition of satisfying mode field matching. Both the focusing lens and the second collimating lens are coated with anti-reflection films for the laser and pump light bands. The anti-reflection films are provided to prevent parasitic oscillations within the ring cavity from affecting the operating state of the single-longitudinal-mode laser.

[0032] The output coupling mirror 11 is configured as a calcium fluoride plate coated with a 90% reflective film in the 4.3μm band, and the output coupling mirror 11 is used to output a single longitudinal mode laser. The gold mirror 12 is used to reflect laser light to achieve a ring cavity structure.

[0033] The hollow fiber is configured as a single-turn anti-resonant hollow fiber 6. The hollow fiber is used to transmit lasers in the 2μm and 4.3μm bands simultaneously. It is a container for carbon dioxide gain gas. The gain linewidth of carbon dioxide is about 50MHz. The length of the hollow fiber needs to be calculated so that the longitudinal mode spacing is greater than the gain linewidth of carbon dioxide gas.

[0034] like Figure 2 As shown, the hollow optical fiber 6 includes a first annular silicon dioxide layer 61, a plurality of second annular silicon dioxide layers 62 disposed inside the first annular silicon dioxide layer 61, and an irregular air layer 63 filling the interior of the first annular silicon dioxide layer 61 and the second annular silicon dioxide layers 62.

[0035] The hollow optical fiber has a length of 4m, a spatial optical path of 1m, and a corresponding longitudinal mode spacing of 60MHz. The diameter of the first annular silicon dioxide layer is 366μm, and multiple second annular silicon dioxide layers can form an circumscribed circle with a diameter of 116μm.

[0036] 3. Gas filling and vacuuming system The gas filling and vacuuming system 7 is used to vacuum and fill the first annular resonant cavity structure with carbon dioxide gas.

[0037] The gas filling and vacuum system 7 includes a fifth gas valve 78 communicating with the upper side of the first gas chamber 5, a sixth gas valve 79 communicating with the left side of the second gas chamber 8, a valve assembly and a vacuum gauge 73 disposed between the fifth gas valve 78 and the sixth gas valve 79, a gas cylinder 72 disposed on the upper side of the valve assembly, and a vacuum molecular pump 71 disposed on the lower side of the valve assembly. The valve assembly includes a first gas valve 74, a second gas valve 75, a third gas valve 76, and a fourth gas valve 77. The first gas valve 74 and the second gas valve 75 are used to control the output of the gas cylinder 72, and the third gas valve 76 and the fourth gas valve 77 are used to control the output of the vacuum molecular pump 71.

[0038] like Figure 3 As shown, the present invention also provides an operating method for a single-longitudinal-mode hollow-core fiber gas laser, comprising: Step 1: Fix the position of the first ring resonant cavity structure, turn on the gas filling and vacuum system 7, perform a vacuuming operation for two hours, and then fill in the gain gas; wherein, the pressure of the gain gas is greater than the threshold of dual-wavelength operation.

[0039] Step 1 specifically includes: The first gas chamber 5 and the second gas chamber 8 are fixed on the five-dimensional adjustment frame. Using the first collimating lens 2, the first dichroic mirror 3, and the focusing lens 4, the pump light generated by the 2μm tunable pump source 1 is coupled into the hollow fiber 6. The five-dimensional adjustment frame is adjusted to achieve optimal coupling efficiency. The vacuum molecular pump 71 and all gas valves are turned on, and the vacuum is completed in approximately four hours. The first gas valve 74, the second gas valve 75, the third gas valve 76, the fourth gas valve 77, and the fifth gas valve 78 are closed. The gas cylinder 72 is opened, and gas valve 75 is slowly opened. The reading of the vacuum gauge 73 is observed, and approximately 15 mbar of carbon dioxide is introduced. The second gas valve 75 and the sixth gas valve 79 are closed, and the carbon dioxide (gain gas) is allowed to diffuse in for approximately four hours. The pressure of the gain gas is greater than the threshold for dual-wavelength operation; only a single wavelength meets the laser oscillation condition.

[0040] Step 2: Use a DC power supply to control the output wavelength of the tunable pump source 1 to match the absorption wavelength of carbon dioxide, and place a helium-neon laser on the output side of the first ring resonant cavity to perform laser simulation and adjustment. Then remove the helium-neon laser and perform laser coupling.

[0041] Step 2 specifically includes: A second dichroic mirror 9 is placed, and a power meter is placed below it to observe spontaneous emission amplified light of a certain power. The output wavelength of the pump source is tuned using a high-precision DC power supply to match the absorption wavelength of carbon dioxide, with the highest observed spontaneous emission amplified light power being optimal. At this point, the gold mirror 12 has not yet been placed. A helium-neon laser is placed at this position and coupled into a hollow fiber to simulate a laser. The position of the second gas chamber 8 is adjusted according to the helium-neon laser to ensure that its output laser is on the laser path. Then, the second collimating lens 10 and the output coupling mirror 11 are placed. Finally, the helium-neon laser is removed, and the gold mirror 12 is placed. The position of the second collimating lens 10 is finely adjusted to achieve the best coupling effect.

[0042] like Figure 4 The image shows the measured laser spectrum, with a center wavelength of 4366 nm. For this type of gas laser, the generated radiation source typically exhibits dual-wavelength radiation peaks. In this scheme, the gas pressure is increased to suppress one of the radiation peaks, achieving single-longitudinal-mode laser generation with an output power of 302 mW.

[0043] like Figure 5 The image shows the measured laser linewidth, measured using a scanning FP interferometer. This method controls the piezoelectric ceramic with voltage, and uses a scanning voltage to scan the cavity length. Light energy satisfying a specific resonant frequency is received by the detector through the FP interferometer. The free spectral range of this scanning FP interferometer is 1.5 GHz, and it has one and only one isolated sharp peak within this free spectral range, which is typical of a single-longitudinal-mode laser. The calculated laser linewidth is approximately 10.29 MHz.

[0044] The hollow optical fiber and the length of the spatial optical path must meet the following conditions: ; Where c represents the speed of light. Represents the length of the optical path in space. Represents the length of the hollow fiber. The gain linewidth represents the gain gas. This represents the longitudinal mode spacing of the laser. When the longitudinal mode spacing is greater than the gain linewidth, the laser outputs a single longitudinal mode.

[0045] Therefore, the present invention provides a single-longitudinal-mode hollow fiber gas laser and an operating method that can simultaneously transmit pump light and laser light, effectively generating mid-infrared single-longitudinal-mode laser light.

[0046] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0047] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A single-longitudinal-mode hollow-core fiber gas laser, characterized in that, include: A tunable pump source, a first annular resonant cavity structure disposed outside the tunable pump source, and a gas filling and vacuuming system connected to the inner side of the first annular resonant cavity structure; The tunable pump source is used to generate pump light, and the output wavelength of the pump seed source is controlled by a DC power supply to match the absorption wavelength of the gain gas. The first annular resonant cavity structure is used to generate single-longitudinal-mode laser; The gas filling and vacuuming system is used to vacuum and fill the hollow fiber in the first annular resonant cavity structure with gain gas.

2. A single-longitudinal-mode hollow-core fiber gas laser according to claim 1, characterized in that, The first annular resonant cavity structure is configured as a top-view structure. The first annular resonant cavity structure includes a first collimating lens disposed below the tunable pump source, a first dichroic mirror disposed below the first collimating lens, a focusing lens disposed to the left of the first dichroic mirror, a first gas chamber disposed to the left of the focusing lens, a hollow optical fiber connected to the left end of the first gas chamber, a second gas chamber connected to the hollow optical fiber, a second dichroic mirror disposed below the second gas chamber, a second collimating lens disposed below the second dichroic mirror, an output coupling mirror disposed below the second collimating lens, and a gold mirror disposed below the output coupling mirror and to the right of the first dichroic mirror. The input end of the hollow optical fiber is connected to the left end of the first gas chamber, and the output end of the hollow optical fiber is connected to the top end of the second gas chamber. The first gas chamber and the second gas chamber are mounted on a five-dimensional adjustment frame.

3. A single-longitudinal-mode hollow-core fiber gas laser according to claim 2, characterized in that: The first collimating lens is used to collimate the pump light; The first dichroic mirror is used to reflect pump light and transmit laser light; The focusing lens is used to focus the pump light and couple it into the hollow fiber. The first air chamber has the same structure as the second air chamber. A window is provided on the right side of the first air chamber and an ultraviolet adhesive is provided on the left side. The window is used to seal the first air chamber and allow pump light and laser to pass through. The window is coated with an anti-reflection film for the laser and pump light bands and is placed at a certain small angle with the laser transmission direction. The ultraviolet adhesive is used to fix the hollow optical fiber. The second dichroic mirror has the same parameters as the first dichroic mirror, and the second dichroic mirror is used to filter out residual pump light; The second collimating lens has the same structure as the focusing lens. The second collimating lens cooperates with the focusing lens to couple the laser into the hollow optical fiber. Both the focusing lens and the second collimating lens are coated with anti-reflection films for the laser and pump light bands. The output coupling mirror is used to output a single longitudinal mode laser. The gold mirror is used to reflect laser light; The focal length ratio of the first collimating lens and the focusing lens is adapted to the mode field matching conditions of the pump light and the hollow fiber.

4. A single-longitudinal-mode hollow fiber gas laser according to claim 3, characterized in that, The hollow fiber is configured as a single-turn anti-resonant hollow fiber, which is used to transmit laser and pump light. The hollow fiber includes a first annular silica layer, a plurality of second annular silica layers disposed inside the first annular silica layer, and an irregular air layer filled inside the first annular silica layer and the second annular silica layers.

5. A single-longitudinal-mode hollow fiber gas laser according to claim 4, characterized in that, The gas filling and vacuum system includes a fifth gas valve communicating with the upper side of the first gas chamber, a sixth gas valve communicating with the left side of the second gas chamber, a valve assembly and a vacuum gauge disposed between the fifth and sixth gas valves, a gas cylinder disposed on the upper side of the valve assembly, and a vacuum molecular pump disposed on the lower side of the valve assembly. The valve assembly includes a first gas valve, a second gas valve, a third gas valve, and a fourth gas valve. The first and second gas valves are used to control the output of the gas cylinder, and the third and fourth gas valves are used to control the output of the vacuum molecular pump.

6. A method for operating a single-longitudinal-mode hollow-core fiber gas laser, characterized in that, include: The first annular resonant cavity structure is fixed in position, the vacuum system is turned on and a vacuum operation is performed for two hours. Then the gas filling system is turned on and the gain gas is filled in. The pressure of the gain gas is greater than the threshold for dual-wavelength operation. The wavelength of the output gas of the tunable pump source is controlled by a DC power supply to match the gain. A helium-neon laser is placed on the output side of the first ring resonant cavity for laser simulation and adjustment. Then the helium-neon laser is removed and laser coupling is performed.

7. The operating method of a single-longitudinal-mode hollow fiber gas laser according to claim 6, characterized in that, The first annular resonant cavity structure was fixed in position, the vacuum system was turned on, and a two-hour vacuuming operation was performed. Then, the gas filling system was turned on and the gain gas was filled in, including: The first and second air chambers are fixed on the five-dimensional adjustment frame. The pump light generated by the tunable pump source is coupled into the hollow fiber using the first collimating lens, the first dichroic mirror and the focusing lens. Then the five-dimensional adjustment frame is adjusted to finely adjust the position and pitch of the hollow fiber. Turn on the vacuum molecular pump and all gas valves, and perform a vacuuming operation for two hours. Then close the first, second, third, fourth, and fifth gas valves, open the gas cylinder and the second gas valve, observe the reading of the vacuum gauge, and fill in a certain pressure of gain gas. Close the second and sixth gas valves and wait for the gain gas to diffuse for four hours.

8. The operating method of a single-longitudinal-mode hollow fiber gas laser according to claim 7, characterized in that, The output wavelength of a tunable pump source is controlled by a DC power supply to match the absorption wavelength of the gain gas. A helium-neon laser is placed at the output end of the first ring resonant cavity for laser simulation and adjustment. Then, the helium-neon laser is removed, and laser coupling is performed. A second dichroic mirror is placed below the second gas chamber, and a power meter for observing spontaneous emission amplified light is placed below the second dichroic mirror. The output wavelength of the tunable pump source is then controlled by a DC power supply to match the absorption wavelength of the gain gas until the highest spontaneous emission amplified light power is obtained. A helium-neon laser is placed at the location of the gold mirror, and coupled into the hollow-core optical fiber to simulate laser light. The position of the second gas chamber is adjusted according to the helium-neon laser. Then, the following operations are performed sequentially: placing the second collimating lens and the output coupling mirror, removing the helium-neon laser, placing the gold mirror, and adjusting the second collimating lens to complete the laser coupling. The hollow-core optical fiber and the length of the spatial optical path must meet the following conditions: ； Where c represents the speed of light. Represents the length of the optical path in space. Represents the length of the hollow fiber. The gain linewidth represents the gain gas. This represents the longitudinal mode spacing of the laser. When the longitudinal mode spacing is greater than the gain linewidth, the laser outputs a single longitudinal mode.

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