Compact wavelength-selectable fiber-coupled monolithic solid state laser and method of operation thereof

Abstract

This invention discloses a compact, wavelength-selectable, fiber-coupled single-mode solid-state laser and its operating method, relating to the intersection of solid-state laser technology and fiber laser technology. The laser includes an LD pump source and a laser gain cavity. The gain cavity is sequentially arranged along the optical axis as a pump input collimator, a gain crystal, a wavelength-selective filter, and an output collimator. The core innovation of this invention lies in: integrating the resonant cavity mirror function onto the coating surface of the fiber collimator, enabling laser oscillation to occur within the collimated optical path, achieving physical unification of optical path transmission and optical feedback; employing a replaceable wavelength-selective filter to achieve flexible wavelength selection within a wide gain spectrum range of 1020–1090 nm; reducing the power density of the coating surface and improving power tolerance through fiber expander end caps; and reusing mature optical isolator packaging technology to achieve miniaturized, highly consistent mass production. This invention has significant advantages such as small size, high output beam quality, wavelength selectability, and direct fiber-coupled output.

Description

Technical Field

[0001] This invention relates to the intersection of solid-state laser technology and fiber laser technology, specifically to a compact selectable wavelength single-mode solid-state laser based on a resonant cavity built into a fiber-coupled collimator and its operating method. Background Technology

[0002] Solid-state lasers are widely used in many fields such as fiber optic sensing, lidar, precision spectroscopy, industrial processing, and medical aesthetics due to their outstanding advantages such as good beam quality, strong coherence, and wide wavelength coverage. Traditional solid-state lasers typically use discrete optical components (such as independent mirrors, lens groups, gain crystals, and mechanical adjustment frames) to build the laser resonant cavity. After the laser oscillates in free space, it is then connected to an optical fiber through an additional collimation coupling system to achieve output.

[0003] However, existing solid-state laser technology faces the following critical technical bottlenecks that urgently need to be addressed: First, they are bulky and complex to assemble and adjust. Traditional solutions rely on separate reflectors and lens assemblies, requiring precise mechanical adjustment frames for optical path calibration. Not only is it difficult to reduce the size, but assembly and debugging are also time-consuming and labor-intensive, making them unsuitable for the urgent needs of modern optoelectronic systems for miniaturization and integration.

[0004] Second, fiber coupling is difficult and has high losses. The output of a traditional solid-state laser is a free-space beam. If fiber output is required, additional collimating lens groups and coupling lenses must be added to couple the free-space light into the fiber. This process not only increases the size, cost, and insertion loss of the system, but also introduces additional alignment errors, reducing the stability and reliability of the system.

[0005] Third, fixed wavelengths or difficult tuning. Traditional designs are usually optimized for a single wavelength, and the reflection spectrum of the resonant cavity mirror is fixed, lacking flexible multi-wavelength switching capabilities. Even with complex tuning mechanisms (such as rotating gratings, piezoelectric ceramic driven cavity mirrors, etc.), they are often structurally complex, costly, and have limited tuning range.

[0006] Fourth, mode control is complex. Traditional solid-state lasers require complex mode control to ensure single-mode (TEM) operation. 00 To achieve the desired output, additional mode selection components (such as pinhole apertures, single-mode fiber filters, etc.) are required. This not only increases cavity loss and reduces efficiency, but also makes the structure more complex.

[0007] Fifth, the coating is easily damaged under high power. In traditional schemes, the pump light is usually focused on a small end face of the gain crystal, resulting in extremely high power density. Under prolonged high-power operation, the coating on this end face is easily burned off due to heat accumulation, severely limiting the reliability and lifespan of the laser.

[0008] In summary, existing technologies have not yet provided a solid-state laser solution that can achieve miniaturized integration, direct fiber coupling, wide-range selectable wavelengths, high power tolerance, and mass production capability while ensuring single-mode output. Therefore, a novel laser architecture that overcomes the aforementioned technological bottlenecks is urgently needed. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of the prior art and provide a compact, wavelength-selectable, fiber-coupled single-mode solid-state laser and its operating method. This laser integrates the resonant cavity function onto the coated surface of the fiber collimator, fundamentally simplifying the cavity structure and achieving physical unification of laser oscillation, mode selection, wavelength selection, and fiber coupling.

[0010] The technical solution adopted in this invention is a compact, wavelength-selectable fiber-coupled single-mode solid-state laser, which includes: A single pump source outputs 976nm or 980nm single-mode pump light; A laser gain cavity, along the optical axis, comprises, in sequence: a pump input collimator, a gain crystal, a wavelength selection filter, and an output collimator; The cylindrical spherical lens of the pump input collimator is coated with a 980nm anti-reflection film and a 1064nm high-reflection film, forming the total reflection end mirror of the resonant cavity; The cylindrical spherical lens of the output collimator is coated with a 980nm high-reflection film and a selected wavelength partial reflection film, which constitute the output coupling end mirror of the resonant cavity. The wavelength-selective filter is a replaceable quartz substrate coated bandpass filter. By replacing the filter with a different center wavelength, laser output of different wavelengths in the range of 1020 to 1090 nm can be achieved.

[0011] Furthermore, the pump input collimator includes an optical fiber expander cap, which is composed of a single-mode fiber fused with a section of coreless fiber. The diameter of the coreless fiber is the same as the cladding diameter of the single-mode fiber, which is used to expand the pump light spot area and reduce the power density of the coating surface.

[0012] Furthermore, an outer sealing glass tube is fitted over the outer surface of the fiber optic expander cap. The inner diameter of the outer sealing glass tube is slightly larger than the outer diameter of the coreless fiber, which is used to fix and protect the expander structure. The outer diameter of the outer sealing glass tube matches the outer diameter of the cylindrical spherical lens, and the two are encapsulated together in a metal welded tube.

[0013] Furthermore, the gain crystal is a Yb:YCOB crystal with a 1064nm antireflection coating on its surface, a gain bandwidth covering 1020-1090nm, and a self-frequency doubling effect, which can simultaneously output 532nm green light.

[0014] Furthermore, the gain crystal can also be an Nd:YVO4, Yb:YAG, or Nd:YAG laser crystal to adapt to different wavelength requirements.

[0015] Furthermore, the wavelength-selective filter can be tilted at a certain angle, and the output wavelength can be shifted within a small range by adjusting the tilt angle.

[0016] Furthermore, the pump input collimator and the output collimator are symmetrical structures, each containing a cylindrical spherical lens and an optical fiber expander end cap, which are externally fixed by a metal welded tube.

[0017] Furthermore, the overall package dimensions of the laser are 20mm × 20mm × 10mm.

[0018] A method for operating a compact wavelength-selectable fiber-coupled single-mode solid-state laser as described above, the method comprising the following steps: (1) Pump light injection step: The pump light output by the 976nm or 980nm single-mode LD is transmitted to the input collimator through the optical fiber. The 980nm anti-reflection coating on the surface of the cylindrical spherical lens of the input collimator enables the pump light to be efficiently transmitted into the cavity. (2) Laser gain step: The pump light is absorbed in the Yb:YCOB gain crystal, generating stimulated emission. The gain bandwidth of the crystal covers 1020~1090nm. (3) Frequency selective filtering steps: When the beam reaches the wavelength selective filter, the filter is a quartz substrate coated bandpass filter that only allows a specific center wavelength to pass through. Different wavelength outputs can be achieved by replacing the filter with a different center wavelength. (4) Resonant cavity feedback steps: The 980nm high-reflectivity film on the cylindrical spherical lens surface of the output collimator reflects the unabsorbed pump light back to the cavity to continue pumping. The 1040nm partial reflection film allows part of the laser to be transmitted and output, and part to be reflected back to the cavity to maintain oscillation. (5) Laser output steps: After the laser that meets the threshold condition establishes a stable oscillation in the cavity, it is output from the output collimator end via fiber coupling. Since the entire oscillation is completed in the collimated optical path, other modes that are not the fundamental mode are dissipated during transmission, and only the fundamental mode that matches the transmission mode is preserved.

[0019] Furthermore, in the frequency selective filtering step, by tilting the wavelength selective filter at a certain angle, the angular spectrum characteristics of the interference filter are utilized to achieve a small-range precise drift of the output wavelength.

[0020] The beneficial effects of this invention are: (1) Physical unification of optical path transmission and optical feedback. This invention integrates the function of the resonant cavity mirror onto the coated surface of the fiber collimator, and laser oscillation is completed inside the collimated optical path. This design fundamentally eliminates the need for traditional discrete mirrors, mechanical adjustment frames, and free-space optical paths, enabling the laser structure to leap from discrete component assembly to a functionally integrated device. Traditional solutions require dozens of independent components and precision adjustment mechanisms, while this invention only requires a few integrated components to achieve the same or even better optical performance.

[0021] (2) Natural single-mode output capability. Traditional solid-state lasers require additional pinhole apertures or single-mode fiber filters to ensure single-mode output, which increases cavity loss and reduces efficiency. This invention utilizes the numerical aperture constraint of the collimator itself to confine the entire laser oscillation within the mode space allowed by the collimated optical path. Non-fundamental mode light is naturally dissipated due to diffraction loss during propagation within the cavity, with only the fundamental mode TEM remaining. 00 It can maintain stable operation. This natural mode selection mechanism requires no additional mode selection components, ensuring high beam quality without introducing additional losses.

[0022] (3) Wide range of selectable wavelengths. By replacing wavelength-selective filters (BP filters) with different center wavelengths, this invention can achieve flexible switching of multiple discrete wavelengths (such as 1040nm, 1053nm, 1064nm, etc.) within the 1020-1090nm range covered by the Yb:YCOB gain bandwidth. At the same time, continuous fine-tuning within a range of ± a few nanometers can be achieved by tilting the BP filter angle. This design combines wavelength selection with cavity mirror feedback, which is a technical feature not disclosed in traditional solutions. To achieve similar multi-wavelength selectability, traditional solutions require complex rotating gratings or piezoelectric tuning mechanisms, which are costly and have poor reliability.

[0023] (4) High power tolerance. This invention introduces an optical fiber expander end cap design within the resonant cavity, expanding the pump beam spot from a core diameter of 6 μm to approximately 125 μm, reducing the power density by about two orders of magnitude. This design cleverly resolves the inherent contradiction between high-power pumping and the limited power tolerance of the coating—allowing the use of higher-power LD pump sources to increase laser output power while effectively preventing the coating from burning out due to excessive power density. This design is the key guarantee for the high reliability and long lifespan operation of this invention.

[0024] (5) Direct fiber-coupled output. Traditional solid-state lasers output free-space beams, requiring additional collimating lens groups and coupling lenses to connect to optical fibers, which not only increases size and loss but also introduces alignment errors. In this invention, laser oscillation is directly completed in the collimation path of the fiber collimator. The collimator at the output end is directly connected to the single-mode fiber, and the laser is directly coupled out from the fiber at the output collimator end, without any secondary collimation or coupling operation. This characteristic allows this invention to be directly connected in series to an optical fiber system like ordinary optical fiber devices, greatly simplifying the complexity of system integration.

[0025] (6) Miniaturization and mass production capability. This invention reuses a mature optical isolator packaging process, with an overall package size of approximately 20mm × 20mm × 10mm, which is only one-tenth or even smaller than that of a traditional solid-state laser. Isolator packaging technology is a highly mature technology in the field of optical communication, with a high degree of automation and standardization. This invention fully reuses this production line, without the need to develop new packaging processes, and can achieve high consistency and high yield mass production, significantly reducing manufacturing costs. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the optical path structure of the present invention. Detailed Implementation

[0027] 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. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0028] like Figure 1 As shown, the compact wavelength-selectable fiber-coupled single-mode solid-state laser provided by this invention consists of two main parts: an LD pump source 1 and a laser gain cavity 2. The LD pump source 1 outputs 976nm (or 980nm) single-mode pump light, which is transmitted to the laser gain cavity via optical fiber. The laser gain cavity 2 is provided with the following components along the optical axis: a pump input collimator 3, a Yb:YCOB gain crystal 4, a wavelength-selective filter 5 (BP filter), and an output collimator 6. The entire gain cavity adopts an isolator-style miniaturized package with a package size of approximately 20mm × 20mm × 10mm, which is fully compatible with existing optical isolator packaging technology.

[0029] The core design concept of this invention lies in directly integrating the mirror function of the resonant cavity onto the coated surface of the fiber collimator, allowing laser oscillation to occur within the collimated optical path acted upon by the collimator. This design fundamentally eliminates the need for traditional discrete mirrors and free-space optical paths, achieving a physical unity between optical path transmission and optical feedback.

[0030] Traditional resonant cavities consist of discrete mirrors, requiring independent mechanical adjustment frames for fixation and adjustment. This invention cleverly utilizes the coated surfaces of two collimator cylindrical spherical lenses to directly construct a laser resonant cavity, as detailed below: Input end face (total reflection mirror): The cylindrical spherical lens surface of the pump input collimator is coated with a dual-color film system—a 980nm antireflection coating (HT) and a 1064nm high reflectivity coating (HR). The 980nm antireflection coating allows the pump light from the LD to be efficiently transmitted into the cavity (transmittance > 99%), while the 1064nm high reflectivity coating reflects the laser generated inside the cavity back into the cavity (reflectivity > 99.9%), thus forming the total reflection mirror of the resonant cavity.

[0031] Output end face (output coupling mirror): The cylindrical spherical lens surface of the output collimator is also coated with a dual-color film system—a 980nm high-reflectivity film (HR) and a selected wavelength (e.g., 1040nm) partial-reflectivity film (PR). The 980nm high-reflectivity film reflects unabsorbed residual pump light back into the cavity to continue participating in the pumping process, improving pumping efficiency; the selected wavelength partial-reflectivity film (e.g., 95% reflection, 5% transmission) allows a small amount of laser light to be transmitted and output, while most of it is reflected back into the cavity to maintain laser oscillation, forming the output coupling mirror of the resonant cavity.

[0032] The optical length between the two coated surfaces is the cavity length of the resonant cavity. The laser oscillates back and forth in the collimated optical path defined by these two coated surfaces. Due to the excellent collimation performance of the collimator itself, the light remains essentially parallel as it propagates within the cavity. Other modes, not the fundamental mode, are naturally dissipated during transmission due to diffraction loss, and only the fundamental mode (TEM) of the collimator's transmission mode is matched. 00 It can remain stable within the cavity. Therefore, this invention can naturally achieve single-mode output without the need for additional mode selection elements.

[0033] The pump input collimator employs an internal fiber optic beam expander design, specifically comprising the following structural layers: The first layer—single-mode fiber: core diameter 6μm, cladding diameter 125μm, responsible for transmitting the pump light output from the LD to the input collimator.

[0034] The second layer—the coreless fiber expansion section: A section of coreless fiber (125 μm in diameter and several millimeters in length) is fused to the front end of the single-mode fiber. The function of the coreless fiber is to expand the pump light spot area, extending it from a core diameter of 6 μm to a cladding diameter of nearly 125 μm. After the spot area is expanded, the power density reaching the coating surface is reduced to approximately (6 / 125) of the original. 2 ≈1 / 435, effectively preventing the coating from burning out due to heat accumulation under high power. Let the core diameter of the single-mode fiber be d1 and the diameter of the coreless fiber be d2. The approximate ratio of the spot area before and after beam expansion is: A1 / A2≈(d1 / d2)² Substituting d1=6μm and d2=125μm, the power density is reduced by approximately two orders of magnitude. This design cleverly resolves the contradiction between high-power pumping and the power tolerance of the coating, enabling the invention to operate safely and stably at higher pumping power.

[0035] The third layer—the outer sealing glass tube: with an inner diameter of approximately 126 μm, it is tightly fitted around the coreless optical fiber to protect the expansion structure and provide mechanical strength. The inner diameter of the outer sealing glass tube is slightly larger than the outer diameter of the coreless optical fiber (125 μm), forming a gap of approximately 1 μm. After being fixed with adhesive, it forms a stable composite structure.

[0036] The fourth layer—a cylindrical spherical lens: The outer sealing glass tube is inserted into the central hole of the cylindrical spherical lens, with the spherical surface of the lens facing outwards, and the coated surface being the outermost surface of the spherical surface. The outer diameter of the lens matches the outer diameter of the outer sealing glass tube.

[0037] The fifth layer – the metal welded tube: The lens, glass tube and fiber optic assembly are encapsulated in the metal welded tube, providing final mechanical fixation, heat dissipation channel and electromagnetic shielding.

[0038] Wavelength selective filters (BP filters) are multilayer interference filters with a dielectric film coated on a quartz substrate. Their working principle is based on the Fabry-Perot interference effect: when light is incident perpendicularly, only specific wavelengths that meet the conditions for constructive interference can be transmitted efficiently, while the remaining wavelengths are reflected.

[0039] The design of the BP chip incorporates two levels of wavelength selection functionality: Coarse adjustment (replacing the BP wafer): Select a BP wafer with a corresponding center wavelength bandpass film based on the target output wavelength. For example, a BP wafer with a 1040nm bandpass film allows laser light near 1040nm to pass through and oscillate, outputting 1040nm laser light; replacing it with a BP wafer with a 1053nm bandpass film outputs 1053nm laser light. The gain bandwidth of Yb:YCOB crystals covers 1020–1090nm, therefore, by replacing BP wafers with different center wavelengths, flexible switching of multiple discrete wavelengths within this range can be achieved.

[0040] Fine-tuning (tilting the BP sheet): When the BP sheet is tilted at a certain angle θ, its effective optical thickness changes, and the center wavelength of the transmission peak shifts. Let the center wavelength of the BP sheet when incident along the normal direction be λ0. When the tilt angle is θ, the center wavelength λ(θ) of the transmission peak approximately satisfies: λ(θ)≈λ0·cos(θ / n), Where n is the refractive index of the filter substrate. By precisely adjusting the tilt angle of the BP filter, continuous fine-tuning of the output wavelength can be achieved within ± a few nanometers. This function is particularly important for applications requiring precise matching of specific absorption lines or communication windows.

[0041] This invention preferably uses Yb:YCOB (ytterbium tetraborate oxycalcium yttrium, Yb:Ca4O(BO3)3) as the gain medium, which has the following outstanding advantages: (1) Wide gain bandwidth: The gain bandwidth of Yb:YCOB covers 1020~1090nm (about 70nm), which is much wider than that of Yb:YAG (about 30nm, 1020~1050nm), providing sufficient gain support for wide range of wavelength selection.

[0042] (2) Narrow linewidth characteristic: Under the same pumping conditions, the laser linewidth output by Yb:YCOB is narrower than that of Yb:YAG, which means higher coherence and longer coherence length, making it suitable for precision interferometry and spectroscopy applications.

[0043] (3) Self-frequency doubling effect: Yb:YCOB crystals have a non-centrosymmetric structure and possess self-frequency doubling capability. While generating 1040nm fundamental frequency laser, 532nm green light can be directly generated through second-order nonlinear effects without the need for an external frequency doubling crystal. This characteristic has unique advantages in applications requiring dual-wavelength output.

[0044] As an alternative, commonly used laser crystals such as Nd:YVO4 (output 1064nm), Yb:YAG (output 1030-1050nm), or Nd:YAG (output 1064nm) can also be selected for the gain crystal. Different crystals have different gain spectra and physical properties, which can be flexibly selected according to specific application requirements, demonstrating the good versatility of the architecture of this invention.

[0045] Combination Figure 1 The working process of this invention is carried out according to the following steps: Step 1: Pump Light Injection. The pump light output from the 976nm (or 980nm) single-mode LD is transmitted to the input collimator via a single-mode fiber. The pump light first passes through the fiber expander cap, expanding the spot size from a 6μm core diameter to approximately 125μm, thus reducing the power density. Subsequently, the pump light reaches the coated surface of the cylindrical spherical lens, where the 980nm antireflection coating allows the pump light to penetrate into the cavity with extremely low loss (<1%).

[0046] Step Two: Pump Absorption and Stimulated Emission. The transmitted pump light is effectively absorbed in the Yb:YCOB gain crystal. 3+ Ions from ground state 2 F7 / 2 transitions to the excited state. 2 F5 / 2, resulting in population inversion. When stimulated emission conditions are met, spontaneous emission light with wavelengths in the range of 1020–1090 nm is produced.

[0047] Step 3: Frequency Selective Filtering. The generated spontaneous emission light reaches the wavelength-selective filter (BP filter). The BP filter only allows narrowband light matching its center wavelength to pass through (e.g., 1040nm ± 0.5nm), while light of other wavelengths is reflected. By replacing BP filters with different center wavelengths, different laser output wavelengths can be selected.

[0048] Step 4: Resonant Cavity Feedback and Laser Establishment. The light passing through the BP plate continues to propagate to the output collimator. The 980nm high-reflectivity film on the output collimator reflects the residual pump light back into the cavity (pump recirculation), while the 1040nm partial reflectivity film allows approximately 5% of the 1040nm laser to be transmitted and output, with approximately 95% reflected back into the cavity. The reflected light returns along its original path to the input collimator, where the 1064nm high-reflectivity film completely reflects it back into the cavity. This process repeats, and the laser forms a stable oscillation within the cavity.

[0049] Step 5: Fundamental Mode Selection Output. Since the entire oscillation occurs within the collimated optical path acted upon by the collimator, the optical field distribution is strictly constrained by the collimator's numerical aperture. Non-fundamental modes (such as higher-order transverse modes) experience significant diffraction loss during propagation within the cavity due to the mismatch with the collimator aperture, and rapidly attenuate and dissipate after multiple round trips. Only the fundamental mode is selected for TEM. 00 The beam is highly matched with the transmission mode of the collimator, and can oscillate continuously within the cavity with extremely low loss, and is finally output from the output collimator in the form of single-mode fiber coupling.

[0050] Step Six: Wavelength Fine-tuning. For fine adjustment of the output wavelength, the BP filter can be tilted slightly. According to the aforementioned wavelength-angle relationship λ(θ)≈λ0·cos(θ / n), for every 1° change in tilt angle, the wavelength drift is approximately 0.1 to 0.5 nm (depending on the filter design), achieving high-precision wavelength tuning without mechanical moving parts.

[0051] The laser gain cavity of this invention uses the same packaging process and external dimensions as the optical isolator. The isolator-style packaging has the following characteristics: (1) Standardized interface: Both the input and output ends adopt standard fiber optic interfaces (such as FC / PC, SC, etc.), which can be directly connected to the existing fiber optic system without additional coupling adjustment.

[0052] (2) Miniaturization: The overall package size is about 20mm×20mm×10mm (or a tubular package with a length of about 34mm), which is only one-tenth or even smaller than the volume of a traditional solid-state laser.

[0053] (3) High consistency: The mature automated packaging production line for optical isolators is reused. The internal alignment accuracy of each device is guaranteed by precision capillary and welding process, resulting in extremely high batch consistency, which is suitable for large-scale mass production.

[0054] (4) Good thermal management: The metal welded tube not only provides mechanical fixation, but also serves as a heat dissipation channel to conduct the heat generated by the gain crystal and LD to the external heat dissipation structure, ensuring that the laser works stably at high power.

[0055] Finally, it should be emphasized that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A compact, wavelength-selectable fiber-coupled single-mode solid-state laser, characterized in that, The laser includes: One pump source (1) outputs 976nm or 980nm single-mode pump light; A laser gain cavity (2) includes, along the optical axis, the following components in sequence: a pump input collimator (3), a gain crystal (4), a wavelength selection filter (5), and an output collimator (6); Among them, the cylindrical spherical lens surface of the pump input collimator (3) is coated with a 980nm anti-reflection film and a 1064nm high-reflection film, which constitute the total reflection end mirror of the resonant cavity; The cylindrical spherical lens of the output collimator (6) is coated with a 980nm high-reflection film and a selected wavelength partial reflection film, which constitute the output coupling end mirror of the resonant cavity; The wavelength-selective filter (5) is a replaceable quartz substrate coated bandpass filter. By replacing the filter with a different center wavelength, laser output of different wavelengths in the range of 1020 to 1090 nm can be achieved.

2. The compact wavelength-selectable fiber-coupled single-mode solid-state laser according to claim 1, characterized in that: The pump input collimator (3) contains an optical fiber expansion cap, which is composed of a single-mode optical fiber fused with a section of coreless optical fiber. The diameter of the coreless optical fiber is the same as the cladding diameter of the single-mode optical fiber, which is used to expand the pump light spot area and reduce the power density of the coating surface.

3. The compact wavelength-selectable fiber-coupled single-mode solid-state laser according to claim 2, characterized in that: An outer sealing glass tube is fitted over the fiber optic expander cap. The inner diameter of the outer sealing glass tube is slightly larger than the outer diameter of the coreless fiber, which is used to fix and protect the expander structure. The outer diameter of the outer sealing glass tube matches the outer diameter of the cylindrical spherical lens, and the two are encapsulated together in a metal welded tube.

4. The compact wavelength-selectable fiber-coupled single-mode solid-state laser according to claim 1, characterized in that: The gain crystal (4) is a Yb:YCOB crystal with a 1064nm antireflection coating on its surface. Its gain bandwidth covers 1020-1090nm and has a self-frequency doubling effect, which can simultaneously output 532nm green light.

5. The compact wavelength-selectable fiber-coupled single-mode solid-state laser according to claim 4, characterized in that: The gain crystal (4) can also be an Nd:YVO4, Yb:YAG or Nd:YAG laser crystal to adapt to different wavelength requirements.

6. The compact wavelength-selectable fiber-coupled single-mode solid-state laser according to claim 1, characterized in that: The wavelength-selective filter (5) can be tilted at a certain angle, and the output wavelength can be shifted within a small range by adjusting the tilt angle.

7. The compact wavelength-selectable fiber-coupled single-mode solid-state laser according to claim 1, characterized in that: The pump input collimator (3) and the output collimator (6) are symmetrical structures, each containing a cylindrical spherical lens and an optical fiber expander end cap, which are fixed externally by a metal welded tube.

8. The compact wavelength-selectable fiber-coupled single-mode solid-state laser according to claim 1, characterized in that: The overall package dimensions of the laser are 20mm × 20mm × 10mm.

9. A method for operating a compact wavelength-selectable fiber-coupled single-mode solid-state laser as described in any one of claims 1 to 8, characterized in that, Includes the following steps: (1) Pump light injection step: The pump light output by the 976nm or 980nm single-mode LD is transmitted to the input collimator through the optical fiber. The 980nm anti-reflection coating on the surface of the cylindrical spherical lens of the input collimator enables the pump light to be efficiently transmitted into the cavity. (2) Laser gain step: The pump light is absorbed in the Yb:YCOB gain crystal, generating stimulated emission. The gain bandwidth of the crystal covers 1020~1090nm. (3) Frequency selection filtering steps: When the light beam reaches the wavelength selection filter, the filter is a quartz substrate coated bandpass filter that only allows a specific center wavelength to pass through. Different wavelength outputs can be achieved by replacing the filter with a different center wavelength. (4) Resonant cavity feedback steps: The 980nm high-reflectivity film on the cylindrical spherical lens surface of the output collimator reflects the unabsorbed pump light back to the cavity to continue pumping, and the 1040nm partial reflection film allows part of the laser to be transmitted and output, and part to be reflected back to the cavity to maintain oscillation. (5) Laser output steps: After the laser that meets the threshold condition establishes a stable oscillation in the cavity, it is output from the output collimator end via fiber coupling. Since the entire oscillation is completed in the collimated optical path, other modes that are not fundamental modes are dissipated during transmission, and only the fundamental mode that matches the transmission mode is preserved.

10. The working method according to claim 9, characterized in that: In the frequency selective filtering step, by tilting the wavelength selective filter at a certain angle, the angular spectrum characteristics of the interference filter are utilized to achieve a small-range precise drift of the output wavelength.

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