A thulium-erbium co-doped fluoride fiber laser based on dual-wavelength pumping

Abstract

The application relates to a thulium-erbium co-doped fluoride fiber laser based on dual-wavelength pumping, which comprises a 793 nm pumping source, a 980 nm pumping source, a double-clad gain fiber co-doped with erbium ions and thulium ions, a pumping coupling system and first and second cavity mirrors respectively located at two ends of the gain fiber; the pumping coupling system couples 793 nm pumping light and 980 nm pumping light to the inner cladding of the gain fiber in a cladding pumping mode; the first and second cavity mirrors have high reflectivity to 2 mu m band laser to form a resonant cavity; the 793 nm pumping light excites thulium ions in the core of the gain fiber to generate 2 mu m band laser; the 2 mu m band laser oscillating in the resonant cavity cooperates with the 980 nm pumping light to excite erbium ions to generate 3.5 mu m band laser; the first or second cavity mirror has partial transmissivity to 3.5 mu m band laser to output 3.5 mu m band laser. The 3.5 mu m band mid-infrared fiber laser of the application can realize efficient laser output in a low-cost and high-stability mode.

Description

Technical Field

[0001] This application belongs to the field of fiber laser technology, specifically relating to a thulium-erbium co-doped fluoride fiber laser based on dual-wavelength pumping. Background Technology

[0002] Mid-infrared lasers in the 3.5 μm band lie within the atmospheric transparency window and the molecular fingerprint spectral region, and also coincide with the strong absorption peak of water molecules, making them irreplaceable in applications such as medical surgery, gas sensing, environmental monitoring, and infrared countermeasures. Currently, the main technical approach to obtaining 3.5 μm lasers is to use a dual-wavelength pump scheme of 980 nm and 2 μm erbium-doped fluoride fiber. However, existing technologies rely on an external 2 μm fiber laser for core pumping, which is costly, difficult to couple, and suffers from severe thermal effects, significantly limiting the improvement of output power. Summary of the Invention

[0003] In view of the above problems, this application provides a thulium-erbium co-doped fluoride fiber laser based on dual-wavelength pumping, which uses thulium-erbium co-doped double-clad fluoride fiber pumped with 793 nm and 980 nm wavelengths as the gain medium, and can achieve efficient laser output in a low-cost and high-stability manner.

[0004] Embodiments of this application provide a thulium-erbium co-doped fluoride fiber laser based on dual-wavelength pumping, comprising: The first pump source is used to generate pump light with a wavelength of 793 nm; The second pump source is used to generate pump light with a wavelength of 980 nm; Gain fiber, wherein the gain fiber is a double-clad fiber, comprising a core, an inner cladding located on the outer periphery of the core, and an outer cladding located on the outer periphery of the inner cladding; the core is co-doped with erbium ions and thulium ions; A pump coupling system for coupling the 793 nm wavelength pump light and the 980 nm wavelength pump light to the inner cladding of the gain fiber in a cladding-pumped manner; and The resonant cavity mirror includes a first cavity mirror and a second cavity mirror located at both ends of the gain fiber, respectively. The first and second cavity mirrors have high reflectivity for 2 μm laser light to form a resonant cavity for 2 μm laser oscillation at the gain fiber. The 793 nm pump light is used to excite thulium ions in the core of the gain fiber to generate 2 μm laser light oscillating in the resonant cavity. The 2 μm laser light oscillating in the resonant cavity works in conjunction with the 980 nm pump light to excite erbium ions to generate 3.5 μm laser light. At least one of the first and second cavity mirrors has partial transmittance for the 3.5 μm laser light to output the 3.5 μm laser light.

[0005] In some possible implementations, the pump coupling system is configured to couple the 793 nm wavelength pump light and the 980 nm wavelength pump light from the same end of the gain fiber to the inner cladding.

[0006] In some possible implementations, the pump coupling system is configured to couple the 793 nm wavelength pump light and the 980 nm wavelength pump light from both ends of the gain fiber to the inner cladding, respectively.

[0007] In some possible implementations, the cross-sectional shape of the inner cladding of the gain fiber is a double D-shape, D-shape, octagon, or rectangle.

[0008] In some possible implementations, the pump coupling system includes a spatial optical path coupling system.

[0009] In some possible implementations, the pump coupling system includes an optical fiber combiner, through which the output pigtails of the first pump source and the second pump source are fused to the ends of the gain fiber.

[0010] In some possible implementations, the doping concentration ratio of thulium ions to erbium ions in the core of the gain fiber is configured to balance the generation efficiency of the 2 μm band laser with the virtual ground state absorption efficiency of the erbium ions.

[0011] In some possible implementations, the core matrix and inner cladding material of the gain fiber are both fluoride glass, and the outer cladding material is a low-refractive-index polymer.

[0012] In some possible implementations, the fluoride glass is ZBLAN glass.

[0013] In some possible implementations, the core matrix of the gain fiber is fluoride glass or other low phonon energy glass, such as tellurate glass.

[0014] The aforementioned technical features can be combined in various suitable ways or replaced by equivalent technical features, as long as the purpose of this application can be achieved.

[0015] Compared with existing technologies, in order to overcome technical bottlenecks such as high cost, difficult coupling, severe thermal effects, and complex systems, this application proposes a 3.5 μm mid-infrared fiber laser. Based on thulium-erbium co-doped double-clad fluoride fiber, a 2 μm laser is generated inside the fiber using a 793 nm laser diode and coordinated with a 980 nm pump light. Erbium ions are efficiently excited through cladding pumping, ultimately achieving low-cost and high-stability 3.5 μm mid-infrared laser output. Specifically: (1) A 793nm laser diode is used as the source of the 2 μm laser, replacing the expensive dedicated 2 μm fiber laser. The 793nm laser diode is a mature commercial product with low price, which greatly reduces the system cost; (2) The 2 μm laser is directly generated inside the fiber by the 793nm pump Tm³⁺, without the need to couple the external 2 μm laser into the fluoride fiber core, which fundamentally solves the problems of difficult external 2 μm laser coupling and thermal misalignment in the existing technology, and significantly improves the system stability and practicality; (3) Both 793nm and 980nm pump lights are multimode sources and can be clad pumped. They can be coupled into the inner cladding of the double-clad fiber through spatial optical path coupling, avoiding the complex coupling in the existing technology and improving the system reliability and long-term stability; (4) The 793nm pump light is clad pumped and transmitted in the inner cladding. The energy is gradually absorbed by the Tm³⁺ in the fiber core. Compared with the traditional scheme where the 2 μm pump light is strongly absorbed at the fiber input end and forms a local hot spot, the energy of the 2 μm pump light in this scheme is significantly reduced. μm lasers are distributed along the fiber length, which significantly reduces the peak heat load and is beneficial to improving output power and system stability; (5) Er³⁺ and Tm³⁺ are co-doped in the fiber core, and the long lifetime of Er³⁺ is effectively consumed by the energy transfer process between ions. 4 I 13 / 2 Energy levels, reducing particle accumulation in lower energy level regions, are beneficial for establishing The population inversion between the two phases helps to lower the oscillation threshold of the 3.5 μm laser and improve its slope efficiency, enabling more efficient energy extraction. Attached Figure Description

[0016] The present application will be described in more detail below based on embodiments and with reference to the accompanying drawings. Wherein: Figure 1 This is a schematic diagram of the double-clad Er³⁺ / Tm³⁺ co-doped ZBLAN fiber structure of Embodiment 1 of this application.

[0017] Figure 2 This is a schematic diagram of the pump optical coupling scheme of Embodiment 1 of this application.

[0018] Figure 3 This is the energy level transition process of Embodiment 1 of this application. Detailed Implementation

[0019] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art are within the scope of protection of this application.

[0020] The terminology used in the implementation section of this application is for the purpose of explaining specific embodiments of this application only, and is not intended to limit this application.

[0021] Mid-infrared lasers in the 3.5 μm band are widely used in medical surgery, gas sensing, environmental monitoring, infrared countermeasures, and spectral analysis. The inventors have discovered that existing 3.5 μm mid-infrared fiber lasers suffer from at least the following unresolved issues: (1) The reliance on a dedicated, single-mode output 2 μm fiber laser as a pump source, which is expensive and bulky, severely limits the commercial application and portable development of 3.5 μm lasers.

[0022] (2) It is necessary to couple an external 2 μm laser into the core of a fluoride fiber through a high-precision spatial optical path. Due to the high thermal expansion coefficient and low melting point of fluoride fiber, thermal deformation of the fiber end face during high-power operation will lead to a decrease in coupling efficiency. The optical path needs to be adjusted in real time, which affects the stability and practicality of the system.

[0023] (3) The 2 μm pump light is directly pumped by the fiber core. The energy is highly concentrated at the micron-scale fiber core input end, resulting in extremely high local thermal load, which can easily cause refractive index distortion, uneven gain distribution, or even end face damage, severely limiting the improvement of output power.

[0024] (4) 980 nm pumping is multimode, cladding pumping, while 2 μm pumping is single-mode, core pumping. The two pump light modes are different, requiring a complex optical beam combining system, which increases the difficulty of system integration.

[0025] In view of this, this application proposes a thulium-erbium co-doped double-clad fluoride fiber laser based on dual-wavelength pumping. Er³⁺ and Tm³⁺ are simultaneously doped into the core of a double-clad fluoride fiber. Both the 793 nm and 980 nm pump lights employ cladding pumping, entering the inner cladding of the double-clad fiber through spatial optical path coupling. Specifically, the 793 nm laser diode serves as the first pump source, directly exciting thulium ions in the fiber core to generate a 2 μm laser beam. This 2 μm laser beam acts as the second pump source, working synergistically with the 980 nm pump light to ultimately produce a 3.5 μm laser output.

[0026] Specifically, embodiments of this application provide a 3.5 μm mid-infrared fiber laser, including a first pump source, a second pump source, a gain fiber, a pump coupling system, and resonant cavity mirrors. The first pump source generates pump light with a wavelength of 793 nm. The second pump source generates pump light with a wavelength of 980 nm. The gain fiber is a double-clad fiber, including a core, an inner cladding located around the core, and an outer cladding located around the inner cladding; the core is co-doped with erbium and thulium ions. The pump coupling system couples the 793 nm and 980 nm pump light to the inner cladding of the gain fiber using cladding pumping. The resonant cavity mirrors include a first cavity mirror and a second cavity mirror located at opposite ends of the gain fiber.

[0027] The first and second cavity mirrors have high reflectivity for 2 μm laser light to form a resonant cavity at the gain fiber for 2 μm laser oscillation. The 793 nm pump light is used to excite thulium ions within the core of the gain fiber to generate 2 μm laser light oscillating within the resonant cavity. The 2 μm laser light oscillating within the resonant cavity works in conjunction with the 980 nm pump light to excite erbium ions to generate 3.5 μm laser light. At least one of the first and second cavity mirrors has partial transmittance for the 3.5 μm laser light to output the 3.5 μm laser light.

[0028] In some embodiments, the pump coupling system is configured to couple the 793 nm wavelength pump light and the 980 nm wavelength pump light from the same end of the gain fiber to the inner cladding.

[0029] In some embodiments, the pump coupling system is configured to couple the 793 nm wavelength pump light and the 980 nm wavelength pump light from both ends of the gain fiber to the inner cladding, respectively.

[0030] In some embodiments, the cross-sectional shape of the inner cladding of the gain fiber is a double D-shape, a D-shape, an octagon, or a rectangle.

[0031] In some embodiments, the pump coupling system includes a spatial optical path coupling system.

[0032] In some embodiments, the pump coupling system includes an optical fiber combiner, through which the output pigtails of the first pump source and the second pump source are fused to the ends of the gain optical fiber.

[0033] In some embodiments, the doping concentration ratio of thulium ions to erbium ions in the core of the gain fiber is configured to balance the generation efficiency of the 2 μm band laser with the excitation efficiency of the erbium ions.

[0034] In some embodiments, the core matrix and inner cladding material of the gain fiber are both fluoride glass, and the outer cladding material is a low refractive index polymer.

[0035] Optionally, the fluoride glass is ZBLAN glass.

[0036] In some embodiments, the core matrix of the gain fiber is tellurate glass. Example

[0037] like Figure 1 As shown, the gain medium used is double-clad fluoride fiber, preferably ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) fiber. ZBLAN glass has the advantage of low phonon energy, which can effectively suppress non-radiative relaxation and improve mid-infrared luminescence efficiency. Its core is also doped with Er. 3+ Ions and Tm 3+ Ions, utilizing the synergistic effect of both, enable the generation of a 2 μm laser excited by the 793 nm cladding pump within the fiber core, achieving the highest spatial overlap with Er³⁺ and providing an efficient excitation path for the virtual ground state absorption (VGSA) process. The inner cladding is made of pure fluoride glass, designed in a double D shape to enhance the interaction between the pump light and the fiber core, improving pump absorption efficiency. The outer cladding is made of a low-refractive-index polymer, used to confine the pump light within the inner cladding for transmission.

[0038] Pump optical coupling scheme such as Figure 2 As shown.

[0039] The pump light output from the 980 nm pump laser diode is collimated by lens L1 and then incident perpendicularly onto dichroic mirror M1 (M1 has high transmittance for 980 nm pump light and high reflectance for 3.5 μm laser light). The transmitted 980 nm pump light is then focused by lens L2 and incident onto dichroic mirror DM1. DM1 has high transmittance for 980 nm pump light, and the pump light passes through DM1 and couples into the Er... 3+ / Tm 3+ Inner cladding of co-fluoride-doped optical fiber.

[0040] The pump light output from the 793 nm pump laser diode is collimated by lens L4, focused by lens L3, and then incident on the dichroic mirror DM2. DM2 has high transmittance for the 793 nm pump light; the pump light passes through DM2 and couples into the Er... 3+ / Tm 3+ Inner cladding of co-fluoride-doped optical fiber.

[0041] DM1 and DM2 are highly reflective to a wavelength of 2 μm, forming a 2 μm laser resonant cavity. The 2 μm laser generated by the 793 nm pump oscillates within this resonant cavity and does not output, serving as a pump source for virtual ground state absorption.

[0042] DM2 has high reflectivity for 3.5 μm wavelength, while DM1 partially reflects 3.5 μm laser light. The generated 3.5 μm laser light passes through DM1 and is output by dichroic mirror M1.

[0043] The energy level transition process in the embodiments of this application is as follows: Figure 3 As shown in the figure. The figure illustrates Tm. 3+ and Er 3+ The complete energy level structure, and all the key energy transfer processes involved in this invention.

[0044] (1) 793 nm pump-excited Tm 3+ ion The 793 nm pump light propagates in the inner cladding and is absorbed by Tm³⁺ in the fiber core each time it passes through the core: Tm 3+ : 3 H6→ 3 H4 (ground state absorption, GSA), through the cross-relaxation (CR) process: Tm 3+ : 3 H4+ Tm 3+ : 3 H6→Tm 3+ : 3 F4 + Tm 3+ : 3 F4, transferring the particle number to 3 The F4 energy level, from which particles... 3 The F4 energy level transitions back to the ground state.3 H6 emits a ~2 μm laser, which is generated within the fiber core and oscillates within a resonant cavity composed of DM1 and DM2, without outputting anything.

[0045] (2) 980 nm pump-excited Er 3+ ion The 980 nm pump light propagates in the inner cladding and is absorbed by Er³⁺ ions in the core: Er 3+ : 4 I 15 / 2 → 4 I 11 / 2 (Ground state absorption, GSA). 4 I 11 / 2 It has a long energy level lifetime and can be used as a virtual ground state (VGS).

[0046] (3) 2 μm laser-pumped Er 3+ ion A 2 μm laser beam generated by 793 nm pump-excited Tm³⁺ ions was placed in a state where Er³⁺ ion absorption at energy levels: (VGSA), this process will Er 3+ Ions excited to the upper laser level 4 F 9 / 2 .

[0047] (4) Generation of 3.5 μm laser In 4 F 9 / 2 Er of energy level 3+ Ion transition to 4 I 9 / 2 Energy level: Er 3+ : 4 F 9 / 2 → 4 I 9 / 2 It emits a ~3.5 μm laser, which passes through DM1 and is output by the dichroic mirror M1.

[0048] (5) Tm 3+ and Er 3+ Energy transfer process The ET1 process will Tm 3+3 Energy from the H4 level is transferred to Er. 3+ to stimulate 4 I 9 / 2 Energy level, which relaxes rapidly to 4 I 11 / 2 Increase the particle source of VGSA; the ET2 process will Er 3+4 I 11 / 2 Energy is transferred from the energy level to Tm³⁺, causing it to be excited to3 H5 energy level, which relaxes rapidly to 3 F4, enhanced 2μm laser, ET3 process consumes Er 3+4 I 13 / 2 Energy levels promote population inversion and enhance 2μm laser performance; the CR1 process will increase Tm 3+3 Energy from the H4 level is transferred to Er. 3+ Meanwhile, Tm 3+ Self-relaxation 3 F4, enhances 2μm laser and increases Er 3+4 I 13 / 2 Number of particles.

[0049] Compared with the prior art, Embodiment 1 of this application has the following technical effects: (1) Unlike existing technologies (980 nm cladding pump and 2 μm core pump), in this invention, both the 793 nm and 980 nm pump lights are cladding pumped and are multimode sources. The two are coupled into the inner cladding of the double-clad fiber through a simple spatial optical path, avoiding the complexity of mixed coupling of multimode pump (980 nm) and single-mode pump (2 μm) in existing technologies, and significantly improving the reliability and stability of the system.

[0050] (2) Er 3+ and Tm 3+ Simultaneously doped in the core of the double-clad fluoride fiber. This structure enables the generation of a 2 μm laser excited by 793 nm cladding pump within the core, and is compatible with Er... 3+ The spatial overlap is the highest, which is conducive to the efficient occurrence of the VGSA process. Meanwhile, Er... 3+ Being located in the same region as Tm³⁺ facilitates efficient energy transfer processes and effectively consumes Er. 3+ Long lifespan 4 I 13 / 2 Energy levels, reducing particle accumulation in lower energy level regions, are beneficial for establishing 4 F 9 / 2 and 4 I 9 / 2 The population inversion between the two phases reduces the oscillation threshold of the 3.5 μm laser and improves its slope efficiency.

[0051] (3) A 793 nm laser diode was used as the first pump source to excite Tm in the fiber core by cladding pumping. 3+ Ions are used to directly generate a 2 μm laser beam inside the optical fiber, which oscillates within a high-reflectivity resonant cavity without output, serving as the pump source for the VGSA. Unlike existing technologies, this invention eliminates the need for an external 2 μm fiber laser and the coupling of the 2 μm laser beam into the fiber core.

[0052] (4) Since the 793 nm pump light is cladding pumped, the 2 μm laser is generated in the fiber core and distributed along the fiber length direction, which avoids the severe absorption of the external pump light at the fiber core end, reduces the peak thermal load, and is conducive to the improvement of output power and long-term system stability.

[0053] (5) By optimizing Tm 3+ With Er 3+ By adjusting the doping concentration ratio, the generation efficiency of 2 μm laser and the virtual ground state absorption efficiency are balanced to achieve efficient output of 3.5 μm laser. Example

[0054] A co-directional pumping scheme is adopted, in which the 793 nm and 980 nm pump lights are injected from the same end of the optical fiber after passing through a spatial optical path, and the rest is the same as in Example 1. Co-directional pumping can simplify the optical path structure and is suitable for application scenarios with high requirements for system simplicity. Example

[0055] The optical fiber uses other fluoride optical fibers besides ZBLAN fluoride or low phonon energy tellurate glass optical fibers, and the other parts are the same as in Example 1. Example

[0056] The inner cladding layer is D-shaped, octagonal, or rectangular, and all other parts are the same as in Example 1. Example

[0057] The coupling method uses an optical fiber combiner for pump coupling, and the 793 nm and 980 nm pump pigtails are fused to the inner cladding of the double-clad optical fiber to achieve an all-fiber structure. All other parts are the same as in Example 1.

[0058] While this application has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of this application. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of this application as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A thulium-erbium co-doped fluoride fiber laser based on dual-wavelength pumping, characterized in that, include: The first pump source is used to generate pump light with a wavelength of 793 nm; The second pump source is used to generate pump light with a wavelength of 980 nm; Gain fiber, wherein the gain fiber is a double-clad fiber, comprising a core, an inner cladding located on the outer periphery of the core, and an outer cladding located on the outer periphery of the inner cladding; the core is co-doped with erbium ions and thulium ions; A pump coupling system for coupling the 793 nm wavelength pump light and the 980 nm wavelength pump light to the inner cladding of the gain fiber in a cladding-pumped manner; and The resonant cavity mirror includes a first cavity mirror and a second cavity mirror located at both ends of the gain fiber, respectively. The first and second cavity mirrors have high reflectivity for 2 μm laser light to form a resonant cavity for 2 μm laser oscillation at the gain fiber. The 2 μm laser light is confined within the resonant cavity and acts as a pump source for virtual ground state absorption, without being output externally. The 793 nm pump light is used to excite thulium ions within the core of the gain fiber to generate 2 μm laser light oscillating within the resonant cavity. The 2 μm laser light oscillating within the resonant cavity works in conjunction with the 980 nm pump light to excite erbium ions to generate 3.5 μm laser light. At least one of the first and second cavity mirrors has partial transmittance for the 3.5 μm laser light to output the 3.5 μm laser light.

2. The laser according to claim 1, characterized in that, The pump coupling system is configured to couple the 793 nm wavelength pump light and the 980 nm wavelength pump light from the same end of the gain fiber to the inner cladding.

3. The laser according to claim 1, characterized in that, The pump coupling system is configured to couple the 793 nm wavelength pump light and the 980 nm wavelength pump light from both ends of the gain fiber to the inner cladding, respectively.

4. The laser according to any one of claims 1-3, characterized in that, The cross-sectional shape of the inner cladding of the gain fiber is double D-shaped, D-shaped, octagonal, or rectangular.

5. The laser according to any one of claims 1-3, characterized in that, The pump coupling system includes a spatial optical path coupling system.

6. The laser according to any one of claims 1-3, characterized in that, The pump coupling system includes an optical fiber combiner, and the output pigtails of the first pump source and the second pump source are fused to the ends of the gain optical fiber through the optical fiber combiner.

7. The laser according to any one of claims 1-3, characterized in that, In the core of the gain fiber, the doping concentration ratio of thulium ions to erbium ions is configured to balance the generation efficiency of the 2 μm band laser with the virtual ground state absorption efficiency of the erbium ions.

8. The laser according to any one of claims 1-3, characterized in that, The core matrix and inner cladding material of the gain fiber are both fluoride glass, and the outer cladding material is a low refractive index polymer.

9. The laser according to claim 8, characterized in that, The fluoride glass is ZBLAN glass.

10. The laser according to any one of claims 1-3, characterized in that, The core matrix of the gain fiber is fluoride ZBLAN glass or other low phonon energy glass.

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