Linewidth narrowing method and device for fiber-coupled semiconductor laser

Abstract

The technology of the present invention belongs to the field of laser technology, and specifically relates to a method and device for narrowing the line width of a fiber-coupled semiconductor laser; the purpose of the present invention is to achieve a high-power semiconductor laser with precise wavelength, extremely narrow line width, and highly uniform spot output, and the output wavelength of the high-power semiconductor laser can be accurately locked to the spectral line of atoms or molecules; the constructed external cavity semiconductor laser has the characteristics of good central wavelength stability, and the line width can be compressed from the free-running nm level to the 10pm level, and can provide basic support in many fields such as high-energy gas laser pumping, spin exchange pumping, quantum optics, etc.

Description

Technical Field

[0001] The technology of the invention belongs to the field of laser technology, and specifically relates to a method and device for narrowing the line width of a high-power fiber-coupled semiconductor laser based on an atomic filter. Background Art

[0002] High-power semiconductor lasers have developed rapidly in recent years due to their advantages such as high efficiency, compactness, high reliability and strong scalability. Their output power has continued to increase and their prices have continued to fall. They have been widely used in military, material manufacturing, medical and other fields. Among them, fiber-coupled semiconductor lasers based on single-light source beam combining are a common solution for achieving high-power output.

[0003] A high-power fiber-coupled semiconductor laser usually contains multiple laser diodes, each of which is an independent single light source. The output power of a single tube has increased from several watts in the 1990s to tens of watts. The power of semiconductor lasers can be increased by combining the lasers emitted by multiple single light sources. For example, a commercial fiber-coupled semiconductor laser module contains more than a dozen light sources, each of which has a power of about ten watts. The output power of the entire semiconductor laser module can reach hundreds of watts. By combining multiple hundred-watt fiber-coupled semiconductor modules, the output power of the laser can be further increased to kilowatts.

[0004] Single-light source beam combining has the advantages of high brightness, low cost, and good reliability due to the relatively dispersed heat source and relatively low heat flux density. Therefore, it has gradually become the mainstream method of semiconductor laser beam combining. Common beam combining methods include step beam combining and polarization beam combining. First, through step beam combining, the lasers emitted by multiple light sources are spatially expanded to achieve power improvement, and then polarization beam combining is used to superimpose two polarized light paths with mutually perpendicular polarization directions to further increase the power. After beam combining, the semiconductor laser is collimated and shaped by a micro-optical system and coupled into the optical fiber. This method fundamentally changes the output beam of the laser and solves the problem that the beam output from free space is inconsistent in the direction of the fast and slow axes and cannot be flexibly transmitted. When the laser is transmitted in the optical fiber, the light field distribution is transformed from the rectangular spot emitted by the light source to a circular spot suitable for transmission in the optical fiber, and a high-power circular spot is output under the action of the optical fiber; at the same time, this also makes the use of high-power semiconductor lasers extremely convenient, because the optical fiber can be bent to achieve flexible transmission, and the laser energy can be easily directed to any direction. Therefore, the fiber coupling method based on single-light source beam combining has greatly improved the output performance of high-power semiconductor lasers and expanded their application scenarios.

[0005] The output spectrum width of semiconductor lasers under free-running conditions is generally 2-3nm. However, in many applications, not only high power is required, but also the spectrum is required to have a precise central wavelength and a very narrow line width. For example, in the application of spin-polarized light pumping in medical lung polarization imaging, high-power semiconductors are required to excite rubidium atoms. The absorption spectrum of the atom at the D1 line is precisely located at 794.9nm, and the absorption spectrum width is usually ≤0.1nm. The central wavelength and spectral line width of the high-power semiconductor laser are required to be precisely matched to it to achieve efficient pump absorption. In the development of a series of light sources such as alkali metal lasers, metastable inert atom lasers, and highly robust solid-state lasers, it is also necessary to precisely control the spectrum of high-power semiconductor lasers.

[0006] At present, the linewidth narrowing of high-power fiber-coupled semiconductor lasers is mainly achieved through the external cavity feedback method, that is, by constructing a semiconductor laser external cavity to control the loss of light of different wavelengths in the external cavity, by adjusting the parameters of the external cavity and selecting a suitable external cavity frequency-selective element, the loss of light of a specific wavelength in the external cavity can be reduced, so that the semiconductor laser is more likely to emit light of the corresponding wavelength, thereby achieving wavelength locking and linewidth narrowing. The application of this method in industry has been very mature, and the commonly used external cavity diffraction element is a volume Bragg grating. At present, the external cavity semiconductor lasers based on volume Bragg grating reported at home and abroad can achieve an output linewidth of the order of 0.1nm under high-power (hundreds of watts to kilowatts) operation (this is mainly limited by the physical limit of Bragg diffraction), and this technical indicator is difficult to meet some application requirements; in addition, due to the background absorption of the material itself, the volume Bragg grating will undergo thermal deformation under actual high-power load conditions, which will lead to the instability of the central wavelength of the external cavity feedback. Based on the above problems, it is necessary to propose a new external cavity narrowing technical solution to design and realize a more stable and reliable high-power fiber-coupled semiconductor laser with extremely narrow spectral linewidth and stable central wavelength.

[0007] The full name of the atomic filter is Faraday anomalous dispersion optical filter (FADOF). It consists of a pair of orthogonally placed polarization beam splitting prisms, an atomic gas chamber, and a magnet that applies a magnetic field. Its specific structure is as follows Figure 1As shown. Among them, 1 and 5 are two orthogonally placed polarization beam splitter prisms, 3 is a vacuum alkali metal atomic gas chamber, and 2 and 4 are magnet pairs for generating magnetic fields. During use, the non-polarized incident light first passes through the polarization beam splitter prism 1, and the polarization state rotates in the atomic gas chamber based on the Faraday anomalous dispersion optical rotation effect, and the rotation angle is closely related to the wavelength. For the light of the target wavelength, by reasonably setting the FADOF working parameters (such as temperature, magnetic field, etc.), the rotation angle of the target wavelength light can be made exactly 90°, and it will be completely transmitted through the polarization beam splitter prism 5 placed orthogonal to the polarization beam splitter prism 1; while the light with a rotation angle greater than or less than 90° wavelength, the transmittance through the polarization beam splitter prism 5 is not as good as the light of the target wavelength. At this time, the overall transmittance of FADOF to the target wavelength reaches a peak, thereby achieving the effect of frequency selective filtering of the target wavelength. As an optical filter device, the atomic filter has the advantages of extremely narrow filtering bandwidth, high out-of-band suppression ratio, and transmission peak corresponding to atomic absorption line. Its filtering effect is as follows Figure 2 As shown, its filtering bandwidth can reach the MHz level.

[0008] At present, there are many types of external cavity semiconductor lasers with atomic filters as external cavity components, but most of them are used in low-power application scenarios, such as atomic clocks, magnetometers, etc., and there are relatively few applications for high power, and they are mainly high-power external cavity semiconductor lasers with spatial output, with output power ranging from tens of watts to hundreds of watts. However, the use of atomic filters for optical fiber coupled output semiconductor lasers to realize external cavity construction is still a blank. Summary of the invention

[0009] The purpose of the invention is to realize a high-power semiconductor laser with precise wavelength, extremely narrow line width and highly uniform spot output, and propose a line width narrowing method and device for a high-power fiber-coupled semiconductor laser based on an atomic filter, which can accurately lock the output wavelength of the high-power semiconductor laser to the spectral line of atoms or molecules. The constructed external cavity semiconductor laser has the characteristics of good central wavelength stability, and the line width can be compressed from the free-running nm level to the 10pm level, which can provide basic support in many fields such as high-energy gas laser pumping, spin exchange pumping (SEOP), quantum optics, etc.

[0010] The technical solution of the present invention is: a method for narrowing the line width of a fiber-coupled semiconductor laser, which is divided into the following steps:

[0011] S1 selects a semiconductor laser whose output spectrum covers the working wavelength of the atomic filter according to the required wavelength;

[0012] S2 uses an optical shaping and modulation system to shape and modulate the output laser of the semiconductor laser. On the one hand, it collimates the output laser beam so that the spot shape remains unchanged during the transmission and return of the beam in the external cavity, thereby improving the external cavity feedback efficiency; on the other hand, it generates linearly polarized light and modulates the polarization direction of the linearly polarized light, thereby adjusting the polarization component incident to the atomic filter, and finally achieving the effect of adjustable feedback light ratio; the specific process is as follows:

[0013] S2.1 beam collimation is achieved through an optical lens. When the laser is output from the energy transmission fiber, its divergence angle is related to the core diameter and numerical aperture of the energy transmission fiber. According to the parameters of the energy transmission fiber, an optical lens with a suitable focal length is selected to make the laser approximately parallel light transmission in the process of going back and forth in the external cavity. The feedback light morphology is basically unchanged compared to the output light morphology, thereby achieving the purpose of improving feedback efficiency;

[0014] S2.2 Modulation of the polarization state of the light beam is achieved through a polarization beam splitter and a half-wave plate: the polarization beam splitter is used to decompose the circularly polarized light emitted from the energy transmission fiber into two linearly polarized lights with polarization directions perpendicular to each other, thereby obtaining the linearly polarized light required for the atomic filter to work, and then the linearly polarized light is modulated by a half-wave plate to achieve output light with any polarization direction;

[0015] After collimation and modulation, the light beam S3 enters the atomic filter for frequency selection. The polarization state of the light deflects in the atomic gas chamber, and the deflection amount is closely related to the wavelength of the light. The transmittance of the light with a polarization state deflected by 90° reaches a peak value. Combined with the absorption of light by the atoms themselves, different wavelengths of light correspond to different transmittances. The feedback light with high transmittance is more likely to gain a competitive advantage in the intracavity mode competition, thereby achieving frequency selection. The working state of the atomic filter is adjusted by adjusting the magnetic field strength and the gas chamber temperature to adjust the atomic concentration, thereby changing the wavelength of the light that has gained an advantage in the mode competition.

[0016] S4 According to the principle of reversibility of optical path, the reflected light will return along the original path and be incident on each light source of the high-power fiber-coupled semiconductor laser. The feedback light changes the competition of the light source mode. The part of the spectral component of the light source that is the same as the feedback spectrum gains an advantage in the competition, and the part that is different from the feedback spectrum is suppressed, ultimately achieving wavelength locking and line width narrowing of each light source and the entire semiconductor laser.

[0017] The present invention also provides a line width narrowing device for a fiber-coupled semiconductor laser of the above method, comprising a semiconductor laser, an energy transmission fiber, a beam shaping and modulation system, an atomic filter, and a reflector;

[0018] The light output by the semiconductor laser is transmitted through the energy transmission fiber and then enters the beam shaping and modulation system to be collimated and modulated. Collimation keeps the spot shape unchanged during the transmission and return of the light beam in the external cavity, so as to improve the feedback efficiency of the external cavity. The modulation is based on the polarization characteristics of the semiconductor laser. On the one hand, linear polarized light is generated, and on the other hand, the polarization direction of the linear polarized light is modulated, thereby adjusting the polarization component incident to the atomic filter, and finally achieving the effect of adjustable feedback light ratio. The collimated and modulated light enters the atomic filter for frequency selection, and the light with the polarization direction rotated by 90 degrees can pass through the atomic filter and be reflected by the reflector. According to the principle of reversibility of the optical path, the reflected light will return along the original path and be incident to each light source of the semiconductor laser. Each light source and the reflector form a laser resonant external cavity. The feedback light changes the light source mode competition: the part of the spectral component of the light source that is the same as the feedback spectrum gains an advantage in the competition, and the part that is different from the feedback spectrum is suppressed, finally achieving the wavelength locking and line width narrowing of each light source and the entire semiconductor laser.

[0019] Furthermore, the semiconductor laser may be a semiconductor laser with stacked array output or linear array output.

[0020] Furthermore, the working medium in the atomic gas chamber of the atomic filter may be alkali metal atoms, such as rubidium, potassium, cesium, etc.;

[0021] Furthermore, the semiconductor laser can be locked to different absorption lines of the working medium in the atomic gas chamber of the atomic filter. For example, rubidium atoms can also be used to lock a 420nm blue light semiconductor laser.

[0022] Furthermore, the working medium in the atomic gas chamber of the atomic filter can be other atoms or molecules with optical rotation effects, such as metastable inert gas atoms, carbon dioxide molecules, etc., which can further extend the working wavelength.

[0023] Furthermore, the atomic filter can be used for external cavity feedback after beam combining of multiple semiconductor lasers to achieve the same wavelength locking and line width narrowing of multiple semiconductor lasers.

[0024] The present invention is based on the following principle: the main difference between the technical solution proposed by the present invention and the existing high-power fiber-coupled semiconductor external cavity architecture lies in the difference in the external cavity frequency selection element. An atomic filter is used as the external cavity frequency selection element. Since the frequency selection element atomic filter has the advantages of extremely narrow filtering bandwidth, high out-of-band suppression ratio, and transmission peak corresponding to atomic absorption line, the constructed external cavity laser also has corresponding advantages.

[0025] In order to make the atomic filter suitable for external cavity feedback of high-power semiconductor lasers, the present invention optimizes the optical path design on the one hand, and uses polarization elements to adjust the polarization component incident on the atomic filter to achieve adjustable feedback light ratio. Under this optical path architecture, only part of the power of the laser passes through the atomic filter, which can significantly reduce the power load of the atomic filter. Under the same conditions, the optical path structure can achieve wavelength locking and line width narrowing of higher-power semiconductor modules within the damage threshold of the atomic filter.

[0026] The technical effects of the present invention are:

[0027] 1. The present invention uses an atomic filter as an external cavity feedback element, and the output wavelength is precisely locked to the atomic absorption line, which can achieve stable wavelength output;

[0028] 2. The present invention uses an atomic filter as an external cavity feedback element, which can achieve an output line width of the order of 10-100pm, and the line width is extremely narrow, which effectively improves the pumping efficiency of the semiconductor laser when pumping the atomic corresponding laser;

[0029] 3. The present invention uses an atomic filter to simultaneously achieve wavelength locking and line width narrowing for multiple light sources and even multiple semiconductor laser modules, and can achieve laser output with high spectral uniformity. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Atomic filter structure diagram;

[0031] Figure 2 Atomic filter filtering effect diagram;

[0032] Figure 3 Structural diagram of the linewidth narrowing device for fiber-coupled semiconductor lasers;

[0033] Figure 4 A structure diagram of a 100-watt-class 795nm fiber-coupled semiconductor laser linewidth narrowing device based on an atomic filter;

[0034] Figure 5 Structural diagram of a kilowatt-class 795nm multi-module beam-combining external cavity fiber-coupled semiconductor laser linewidth narrowing device based on an atomic filter. DETAILED DESCRIPTION

[0035] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0036] Figure 3 This is a structural diagram of a linewidth narrowing device for a fiber-coupled semiconductor laser, including a semiconductor laser, an energy transmission fiber, a beam shaping and modulation system, an atomic filter, and a reflector.

[0037] Example 1: A 100-watt 795nm fiber-coupled semiconductor laser linewidth narrowing device based on an atomic filter Figure 4 It is a 100W-class 795nm fiber-coupled semiconductor laser linewidth narrowing device based on atomic filter. In the figure, component 1 is a semiconductor laser containing multiple light sources, which contains 18 light sources, each of which has a power of about 9W, and the maximum power of the entire semiconductor laser is about 160W; its output center wavelength is 795±2nm, and the spectral linewidth is 2nm; the front end of the semiconductor chip of the light source is plated with a 795±2nm anti-reflection film, and the reflectivity is within 0.5%, which is used to suppress the free spectrum output and achieve a better external cavity spectrum regulation effect; semiconductor laser 1 realizes laser output through an energy transmission fiber with a core radius of 135μm. Component 2 is a collimating lens with a focal length of 50mm, which is used to collimate the laser beam with a certain divergence angle output from the optical fiber, so that the beam radius remains unchanged within a certain distance of transmission, so as to improve the feedback light coupling efficiency. Component 3 is a No. 1 polarization beam splitter prism, which is used to generate linearly polarized light that acts on the rubidium atomic filter. Element 4 is a half-wave plate, and element 5 is a second polarization beam splitter prism. The polarization component transmitted through element 5 can be adjusted through the half-wave plate 4. The combination of the two can adjust the light power entering the rubidium atomic gas chamber 7. Elements 2, 3, and 4 together form a beam shaping modulation system. 6 is a magnetic field excited by a permanent magnet. The average magnetic field strength is 240 Gauss. It is used to split the energy levels of rubidium atoms and rotate the polarization state of light with a wavelength near the atomic absorption line. The rotation angle is closely related to the wavelength of the light, so that the atomic gas chamber has a frequency selection function. Element 7 is a vacuum rubidium atomic gas chamber with a length of 60mm and a radius of 25.4mm. It is filled with 87Rb is used to deflect the polarization state of light, and the deflection amount is closely related to the wavelength of light, and at the same time, the wavelength of the transmitted laser is locked to the absorption line of rubidium atom D2. Component 8 is a third polarization beam splitter prism, which is used to select light with a polarization state of just 90° as feedback light. Components 5, 6, 7, and 8 together form an atomic filter 9. Component 10 is a reflector, which is used to reflect the light of a specific wavelength reflected by the polarization beam splitter prism 8 and return it to the light source for spectrum regulation, thereby achieving wavelength locking and line width narrowing of the semiconductor laser. When the system is working, the lasers output by multiple light sources in the semiconductor laser 1 are coupled to the energy transmission fiber after beam combining, and enter the collimating lens 2 after transmission through the energy transmission fiber to be collimated. Then the light is incident on the first polarization beam splitter prism 3 to generate linearly polarized light, and the linearly polarized light passes through the half-wave plate 4 and the second polarization beam splitter prism 5 and then enters the atomic filter 9 for frequency selection. Due to the frequency selection effect of the atomic filter 9, the polarization direction of the light of a specific wavelength is deflected by 90° and reflected at the third polarization beam splitter prism 8 (the third polarization beam splitter prism 8 is placed in the same direction as the second polarization beam splitter prism 5), and the reflected light is reflected again at the reflector 10. According to the principle of reversibility of the optical path, the reflected light will pass through the third polarization beam splitter prism 8, the vacuum rubidium atomic gas chamber 7, the second polarization beam splitter prism 5, the half-wave plate 4, the first polarization beam splitter prism 3, and the collimating lens 2 along the original path, and then enter the energy transmission fiber, and enter the various light sources of the semiconductor laser 1 along the energy transmission fiber. The feedback light changes the competition of the light source mode. The part of the spectral component of the light source that is the same as the feedback spectrum gains an advantage in the competition, and the part that is different from the feedback spectrum is suppressed, and finally the wavelength locking and line width narrowing of each light source and the entire semiconductor laser are achieved.

[0038] Example 2: A kilowatt-class 795nm multi-module beam-combining external cavity fiber-coupled semiconductor laser linewidth narrowing device based on atomic filter

[0039] Figure 5It is a kilowatt-class 795nm multi-module beam-combining external cavity fiber-coupled semiconductor laser linewidth narrowing device based on atomic filter. Part 1 in the figure contains 7 hundred-watt-class 795nm semiconductor laser modules, each of which contains 18 light sources, each of which has a power of about 9W, the maximum power of the entire semiconductor laser module is about 160W, and the total power of the 7 semiconductor laser modules is about 1.1kW; the front end of the semiconductor chip of the light source is plated with a 795±2nm anti-reflection film with a reflectivity within 0.5%, which is used to suppress the free spectrum output and achieve a better external cavity spectrum control effect; component 2 is a fiber combiner, which is used to combine the output light of 7 semiconductor lasers into one beam, and realize laser output through an energy transmission fiber with a core radius of 600μm. Element 3 is a collimating lens made of fused quartz, which is used to reduce temperature rise and temperature-induced expansion under high-power load conditions; its focal length is 50mm, and it is used to collimate the laser beam with a certain divergence angle output from the optical fiber, so that the beam radius remains unchanged within a certain distance of transmission, so as to improve the feedback light coupling efficiency. Element 4 is a No. 1 polarization beam splitter prism, which is used to generate linearly polarized light that acts on the rubidium atomic filter. Element 5 is a half-wave plate, and element 6 is a No. 2 polarization beam splitter prism. The combination of the two can achieve the function of adjusting the optical power entering the rubidium atomic gas chamber 8. Elements 3, 4, and 5 together constitute a beam shaping modulation system. 7 is a magnetic field excited by a permanent magnet, and the average magnetic field strength is 240Gauss, which is used to split the energy levels of rubidium atoms and rotate the polarization state of light with a wavelength near the atomic absorption line, and the rotation angle is closely related to the wavelength of the light, so that the atomic gas chamber has a frequency selection function. Element 8 is a vacuum rubidium atomic gas chamber with a length of 60mm and a radius of 25.4mm, filled with 87Rb is used to deflect the polarization state of specific light to a certain extent, and at the same time lock the wavelength of the transmitted laser to the rubidium atomic absorption line. Component 9 is a third polarization beam splitter prism, which is used to select light with a polarization state of just 90° as feedback light. Components 6, 7, 8, and 9 together form an atomic filter 10. Component 11 is a reflector, which is used to reflect the light of a specific wavelength reflected by the polarization beam splitter prism 9 and return it to the light source for spectral regulation, perform the same wavelength locking and line width narrowing on multiple semiconductor lasers, and achieve highly uniform spectral output of multiple semiconductor lasers. When the system is working, the laser output by the semiconductor laser modules 1-7 is first coupled to the energy transmission fiber, and the 7-way output is combined at the fiber combiner 2 after being transmitted through the energy transmission fiber, and then enters the collimating lens 3 after being transmitted through the energy transmission fiber to be collimated. Then the light is incident on the first polarization beam splitter prism 4 to generate linearly polarized light. After the linearly polarized light passes through the half-wave plate 5 and the second polarization beam splitter prism 6, part of the light with an adjustable proportion is incident on the atomic filter for frequency selection. Due to the frequency selection effect of the atomic filter, the polarization direction of the light of a specific wavelength is deflected by 90° and reflected at the third polarization beam splitter prism 9 (the third polarization beam splitter prism 9 is placed in the same direction as the second polarization beam splitter prism 6), and the reflected light is reflected again at the reflector 11. According to the principle of reversibility of the optical path, the reflected light will pass through the third polarization beam splitter prism 9, the vacuum rubidium atomic gas chamber 8, the second polarization beam splitter prism 6, the half-wave plate 5, the first polarization beam splitter prism 4, and the collimating lens 3 along the original path, and then enter the energy transmission fiber, and enter each light source of each laser module along the energy transmission fiber. The feedback light changes the competition of the light source mode. The part of the spectral component of the light source that is the same as the feedback spectrum gains an advantage in the competition, and the part that is different from the feedback spectrum is suppressed, and finally the wavelength locking and line width narrowing of each light source and the entire semiconductor laser are achieved.

Claims

1. A method for narrowing the line width of a fiber-coupled semiconductor laser, characterized in that: The method is divided into the following steps: S1 selects a semiconductor laser whose output spectrum covers the working wavelength of the atomic filter according to the required wavelength; S2 uses an optical shaping and modulation system to shape and modulate the output laser of the semiconductor laser. On the one hand, it collimates the output laser beam so that the spot shape remains unchanged during the transmission and return of the beam in the external cavity, thereby improving the external cavity feedback efficiency; on the other hand, it generates linearly polarized light and modulates the polarization direction of the linearly polarized light, thereby adjusting the polarization component incident to the atomic filter, and finally achieving the effect of adjustable feedback light ratio; the specific process is as follows: S2.1 beam collimation is achieved through an optical lens. When the laser is output from the energy transmission fiber, its divergence angle is related to the core diameter and numerical aperture of the energy transmission fiber. According to the parameters of the energy transmission fiber, an optical lens with a suitable focal length is selected to make the laser approximately parallel light transmission in the process of going back and forth in the external cavity. The feedback light morphology is basically unchanged compared to the output light morphology, thereby achieving the purpose of improving feedback efficiency; S2.2 Modulation of the polarization state of the light beam is achieved through a polarization beam splitter and a half-wave plate: the polarization beam splitter is used to decompose the circularly polarized light emitted from the energy transmission fiber into two linearly polarized lights with polarization directions perpendicular to each other, thereby obtaining the linearly polarized light required for the atomic filter to work, and then the linearly polarized light is modulated by a half-wave plate to achieve output light with any polarization direction; After collimation and modulation, the light beam S3 enters the atomic filter for frequency selection. The polarization state of the light deflects in the atomic gas chamber, and the deflection amount is closely related to the wavelength of the light. The transmittance of the light with a polarization state deflected by 90° reaches a peak value. Combined with the absorption of light by the atoms themselves, different wavelengths of light correspond to different transmittances. The feedback light with high transmittance is more likely to gain a competitive advantage in the intracavity mode competition, thereby achieving frequency selection. The working state of the atomic filter is adjusted by adjusting the magnetic field strength and the gas chamber temperature to adjust the atomic concentration, thereby changing the wavelength of the light that has gained an advantage in the mode competition. S4 According to the principle of reversibility of the optical path, the reflected light will return along the original path and be incident on each light source of the high-power fiber-coupled semiconductor laser, and the light source mode competition will change through the feedback light; The part of the spectral composition of the light source that is the same as the feedback spectrum gains an advantage in the competition, and the part that is different from the feedback spectrum is suppressed, ultimately achieving wavelength locking and line width narrowing of each light source and the entire semiconductor laser.

2. A linewidth narrowing device for a fiber-coupled semiconductor laser based on the method of claim 1, characterized in that: Including semiconductor lasers, energy transmission optical fibers, beam shaping and modulation systems, atomic filters, and reflectors; The light output by the semiconductor laser is transmitted through the energy transmission fiber and then enters the beam shaping and modulation system to be collimated and modulated. The collimation keeps the spot shape unchanged during the transmission and return process of the beam in the external cavity, so as to improve the feedback efficiency of the external cavity. Modulation targets the polarization characteristics of semiconductor lasers. On the one hand, it generates linearly polarized light. On the other hand, it modulates the polarization direction of the linearly polarized light, thereby adjusting the polarization component incident to the atomic filter, and finally achieving the effect of adjustable feedback light ratio. The light after collimation and modulation enters the atomic filter for frequency selection. The light with polarization direction rotated by ninety degrees can pass through the atomic filter and be reflected by the reflector. According to the principle of reversibility of the optical path, the reflected light will return along the original path and be incident to each light source of the semiconductor laser. Each light source and the reflector form a laser resonant external cavity. The feedback light changes the competition of the light source mode: the part of the spectral component of the light source that is the same as the feedback spectrum gains an advantage in the competition, and the part that is different from the feedback spectrum is suppressed, finally achieving wavelength locking and line width narrowing of each light source and the entire semiconductor laser.

3. A linewidth narrowing device for a fiber-coupled semiconductor laser according to claim 2, characterized in that: The semiconductor laser may be a stacked array output or a linear array output semiconductor laser.

4. A linewidth narrowing device for a fiber-coupled semiconductor laser according to claim 2, characterized in that: The working medium in the atomic gas chamber of the atomic filter is alkali metal atoms.

5. A linewidth narrowing device for a fiber-coupled semiconductor laser according to claim 4, characterized in that: The alkali metal atoms in the atomic gas chamber of the atomic filter are rubidium, potassium and cesium.

6. A linewidth narrowing device for a fiber-coupled semiconductor laser according to claim 2, characterized in that: The semiconductor laser can be locked to different absorption lines of the working medium in the atomic gas chamber of the atomic filter. For example, rubidium atoms can also be used to lock a 420nm blue light semiconductor laser.

7. A linewidth narrowing device for a fiber-coupled semiconductor laser according to claim 2, characterized in that: The working medium in the atomic gas chamber of the atomic filter is other atoms or molecules with optical rotation effect.

8. A linewidth narrowing device for a fiber-coupled semiconductor laser according to claim 7, characterized in that: The working medium in the atomic gas chamber of the atomic filter is metastable inert gas atoms and carbon dioxide molecules, which can further extend the working wavelength.

9. A linewidth narrowing device for a fiber-coupled semiconductor laser according to any one of claims 1 to 8, characterized in that: The atomic filter can be used for external cavity feedback after beam combining of multiple semiconductor lasers to achieve the same wavelength locking and line width narrowing of multiple semiconductor lasers.

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