A 2.8 μm and 3.5 μm dual-wavelength mid-infrared fiber laser

Abstract

The present invention discloses a 2.8μm and 3.5μm dual-wavelength mid-infrared fiber laser, which adopts a 0.98μm+1.15μm dual-wavelength pumping scheme and uses a fiber combiner to simultaneously couple the two pump beams into a double-clad erbium-doped fluoride fiber. The 0.98μm pump light absorption pumps the ground state erbium ions to 4 I 11 / 2 Energy level, based on 4 I 11 / 2 → 4 I 13 / 2 Energy level transition produces 2.8μm laser radiation; further through 1.15μm pump light absorption 4 I 13 / 2 The erbium ions accumulated on the energy level are pumped to 4 F 9 / 2 Energy level, based on 4 F 9 / 2 → 4 I 9 / 2 The energy level transition produces 3.5μm laser radiation; and the erbium ions that complete the 3.5μm laser emission will return to the 4 I 11 / 2 The invention can realize the output of 2.8μm and 3.5μm dual-wavelength mid-infrared lasers synchronously based on a single-segment double-clad erbium-doped fluoride fiber, with high laser efficiency and compact structure.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lasers, in particular to a 2.8 μm and 3.5 μm dual-wavelength mid-infrared fiber laser. BACKGROUND

[0002] The 2.5-5 μm mid-infrared waveband is located in the atmospheric low-loss transmission window, and contains the characteristic absorption spectral lines of many organic and inorganic molecules such as methane and hydrogen chloride, so that the laser light source in this waveband has extremely important application background in the fields of space communication, military confrontation, high-sensitivity gas detection, high-molecular organic material processing, etc. The fiber laser has become a powerful technical approach for mid-infrared laser light source due to its excellent thermal management performance, good beam quality and high system integration capability. Among them, the mid-infrared fiber laser represented by erbium-doped fluoride fiber has developed rapidly in recent years. The erbium ion energy level structure is rich, and the 0.98 μm single-wavelength pumping and 0.98 μm+1.97 μm dual-wavelength pumping schemes can realize 2.8 μm and 3.5 μm mid-infrared laser emission based on the erbium ion 4 I 11 / 2 → 4 I 13 / 2 and 4 F 9 / 2 → 4 I 9 / 2 The energy level transitions can realize 2.8 μm and 3.5 μm mid-infrared laser emission, respectively. The highest output power of the current 2.8 μm and 3.5 μm mid-infrared erbium-doped fluoride fiber laser has reached 40 W [1] and 15 W [2] , respectively. Since the 2.8 μm and 3.5 μm two-wavelength lasers correspond to non-overlapping energy level transitions, the erbium-doped fluoride fiber laser has the ability to realize dual-wavelength synchronous output.

[0003] However, the existing pumping scheme cannot effectively invert the population of the two wavebands at the same time, resulting in previous researches being aimed at single wavelength. Although the terminal energy levels of the 0.98 μm and 1.97 μm pump absorption correspond to the upper energy levels of the 2.8 μm and 3.5 μm lasers, respectively, it can be considered to use the 0.98 μm+1.97 μm dual-wavelength pumping scheme to simultaneously realize 2.8 μm and 3.5 μm laser output. However, since the virtual ground state of the 1.97 μm pump absorption and the upper energy level of the 2.8 μm laser are the same energy level 4 I 11 / 2 ), this process will reduce the population of the upper energy level of the 2.8 μm laser and reduce the inversion gain. Therefore, under this pumping method, the 2.8 μm laser and the 3.5 μm laser are in a contradictory relationship, and it is difficult to ensure the efficient output of dual-wavelength laser at the same time. In addition, in the conventional 0.98 μm single-wavelength pumped 2.8 μm fiber laser, the erbium ion 4 I 13 / 2The energy level lifetime is relatively long (9.9ms), and it is difficult to achieve effective particle number inversion between the upper and lower energy levels of the 2.8μm laser, and laser self-termination is very likely to occur, which greatly hinders the improvement of the 2.8μm laser efficiency.

[0004] References

[0005] [1]YOAydin, V.Fortin, R.Vallée, and M.Bernier, "Towards power scaling of 2.8μm fiber lasers," Opt.Lett.43(18),4542–4545(2018).

[0006] [2] M.Lemieux-Tanguay, V.Fortin, T.Boilard, P.Paradis, F.Maes, L.Talbot, R.Vallée, and M.Bernier, "15W monolithic fiber laser at 3.55μm," Opt.Lett.47(2), 289–292(2022). Summary of the Invention

[0007] The present invention provides a 2.8μm and 3.5μm dual-wavelength mid-infrared fiber laser. The present invention adopts a dual-wavelength pumping scheme combining 0.98μm ground state absorption with 1.15μm excited state absorption, and utilizes cascade transitions between different energy levels of erbium ions to simultaneously obtain 2.8μm and 3.5μm dual-wavelength mid-infrared laser emission in a single-segment double-clad erbium-doped fluoride fiber. At the same time, it overcomes the laser self-termination phenomenon caused by the long energy level lifetime of the 2.8μm laser, effectively solving the problems of the existing mid-infrared fiber laser light source with a single output wavelength and low efficiency. See the following description for details:

[0008] A dual-wavelength mid-infrared fiber laser comprises: a first pump source, a second pump source, a fiber combiner, a first Bragg fiber grating, a second Bragg fiber grating, a double-clad erbium-doped fluoride fiber, and a long-pass filter.

[0009] The first pump source is a multimode semiconductor laser with an output wavelength of 0.98 μm. The second pump source is a single transverse mode ytterbium-doped quartz fiber laser with an output wavelength of 1.15 μm. The input end of the fiber combiner includes a single-mode fiber and a multimode fiber. The output fiber is a double-clad fiber, which satisfies the requirements that the 1.15 μm single-mode pump light is transmitted in the fiber core and the 0.98 μm multimode pump light is transmitted in the inner cladding.

[0010] The central wavelength of the first Bragg fiber grating is erbium ion 4 F 9 / 2 →4 I 9 / 2 For any wavelength within the transition emission band, the reflectivity is greater than 99.5%, the full width at half maximum is less than 5nm, and the insertion loss is less than 0.5dB; the central wavelength of the second Bragg fiber grating is erbium ion 4 I 11 / 2 → 4 I 13 / 2 At any wavelength within the transition emission band, the reflectivity is greater than 99.5%, the full width at half maximum is less than 5nm, and the insertion loss is less than 0.5dB; the long-pass filter has a cut-off wavelength of 1.5μm, a reflectivity greater than 95% in the 0.98μm and 1.15μm bands, and a transmittance greater than 95% at the two laser wavelengths.

[0011] The double-clad fluoride optical fiber has a 0° cut output end face, which can provide 4% Fresnel reflection in the entire band, and respectively form a 3.5μm laser resonant cavity and a 2.8μm laser resonant cavity with the first Bragg fiber grating and the second Bragg fiber grating.

[0012] The two pump lights of 0.98 μm and 1.15 μm emitted by the first pump source and the second pump source are combined by the fiber combiner, and then pass through the first Bragg fiber grating and the second Bragg fiber grating to enter the double-clad erbium-doped fluoride fiber. Figure 1 As shown, the 0.98 μm pump light absorption belongs to the ground state absorption process, corresponding to the erbium ion 4 I 15 / 2 → 4 I 11 / 2 Energy level transition; 1.15μm pump light absorption belongs to the excited state absorption process, corresponding to the erbium ion 4 I 13 / 2 → 4 F 9 / 2 Energy level transition. The 0.98μm pump light pumps the ground state erbium ions to 4 I 11 / 2 Energy level, to achieve the first reversal of the number of particles in the upper and lower energy levels of 2.8μm laser, and to form 2.8μm laser oscillation after reaching the threshold. 4 I 13 / 2 The energy level has a longer lifetime, and more erbium ions will accumulate at this energy level. 4 I 13 / 2 Energy level particles are pumped to 4 F 9 / 2 Energy level, on the one hand, it realizes the inversion of the number of particles in the upper and lower energy levels of the 3.5μm laser, provides gain for the 3.5μm laser, and generates 3.5μm laser oscillation; on the other hand, this excited state absorption process can effectively drain 4 I 13 / 2 energy level, and 4 F9 / 2 The erbium ion of the energy level will also return to the ground state through a non-radiative transition after completing the 3.5-micron laser emission 4 I 11 / 2 The energy level is twice arranged in the 2.8-micron laser upper and lower energy levels, effectively solving the laser self-termination problem caused by the particle accumulation in the conventional 2.8-micron mid-infrared fiber laser, thereby realizing efficient 2.8-micron and 3.5-micron dual-wavelength laser output. 4 I 13 / 2 The 0.98-micron and 1.15-micron pump light is not completely absorbed, and the long-pass filter is placed at the output end of the double-clad erbium-doped fluoride fiber to filter out the remaining pump light in the output laser.

[0013] The second pump source can be continuous wave, Q pulse, etc.

[0014] The beneficial effects of the technical solutions provided by the present application are:

[0015] 1) The laser self-termination effect caused by the long lifetime of the 2.8-micron laser lower energy level of erbium ions is an important factor limiting the power and efficiency improvement of 2.8-micron mid-infrared laser; the present application innovatively proposes a 0.98-micron+1.15-micron dual-wavelength pumping scheme, which empties the 2.8-micron laser lower energy level particles by 1.15-micron pumping and increases the 2.8-micron laser upper and lower energy level particle inversion by the non-radiative transition of high-energy level particles, thereby significantly improving the output efficiency of 2.8-micron mid-infrared laser;

[0016] 2) The present application introduces 1.15-micron pumping on the basis of conventional 0.98-micron single-wavelength pumping, which improves the output power and efficiency of 2.8-micron laser while providing gain for 3.5-micron laser, thereby realizing simultaneous output of 2.8-micron and 3.5-micron dual-wavelength mid-infrared laser based on a single segment of double-clad erbium-doped fluoride fiber, rich in output wavelengths and high in system integration. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 The energy level structure diagram of erbium ions in fluoride glass and the particle transition process involved in the present application;

[0018] Figure 2 The structure schematic diagram of the dual-wavelength mid-infrared fiber laser provided by the present application is shown.

[0019] The accompanying drawings are provided for the purpose of illustrating the present application. Figure 2 In the drawings, the components represented by the respective reference numerals are listed as follows:

[0020] 1: first pump source; 2: second pump source;

[0021] 3: fiber combiner; 3-1: single-mode input end of the fiber combiner;

[0022] 3-2: Multimode input of fiber combiner; 3-3: Output of fiber combiner;

[0023] 4: First Fiber Bragg Bragg Grating; 5: Second Fiber Bragg Bragg Grating;

[0024] 6: Double-clad erbium-doped fluoride fiber; 7: Long-pass filter. DETAILED DESCRIPTION

[0025] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention are described in further detail below.

[0026] Example 1

[0027] The embodiment of the present invention provides a 2.8 μm and 3.5 μm dual-wavelength mid-infrared fiber laser, which includes: a first pump source 1, a second pump source 2, a fiber combiner 3, a first Bragg fiber grating 4, a second Bragg fiber grating 5, a double-clad erbium-doped fluoride fiber 6, and a long-pass filter 7;

[0028] Among them, the first pump source 1 is a semiconductor laser with multi-mode fiber coupled output, with a maximum output power of 10W and a wavelength of 976nm. The output pigtail is connected to the multi-mode input end 3-2 of the fiber combiner 3; the second pump source 2 is an ytterbium-doped quartz fiber laser with single-mode fiber coupled output, with a maximum output power of 50W and a wavelength of 1150nm. The output pigtail is connected to the single-mode input end 3-1 of the fiber combiner 3.

[0029] The first fiber Bragg grating 4 has a central wavelength of 3540 nm, a reflectivity greater than 99.9%, a full width at half maximum of 2 nm, and an insertion loss of 0.2 dB. The second fiber Bragg grating 5 has a central wavelength of 2820 nm, a reflectivity greater than 99.9%, a full width at half maximum of 2 nm, and an insertion loss of 0.2 dB. The insertion loss of both fiber Bragg gratings is less than 0.3 dB in the 976 nm and 1150 nm bands. The double-clad erbium-doped fluoride fiber 6 has a non-polarization-maintaining structure, with core and inner cladding diameters of 15 μm and 250 μm, respectively, a core numerical aperture of 0.125, an erbium ion doping concentration of 1 mol%, and a length of 10 m. The longpass filter 7 has a cutoff wavelength of 1.5 μm, a reflectivity greater than 95% in the 976 nm and 1150 nm bands, and a transmittance greater than 95% at 2820 nm and 3540 nm.

[0030] In a specific implementation, the 976nm pump light and the 1150nm pump light are combined by the fiber combiner 3 and then incident on the double-clad erbium-doped fluoride fiber 6, wherein the 976nm pump light is transmitted in the inner cladding of the double-clad erbium-doped fluoride fiber 6, and the 1150nm pump light is transmitted in the core of the double-clad erbium-doped fluoride fiber 6. The 976nm pump light first pumps the ground state erbium ions into the core 4 I 11 / 2 Energy level, completed 4 I 11 / 2 When the 976nm pump light power exceeds the 2.8μm laser threshold, 2820nm laser oscillation is first formed in the laser resonant cavity formed by the second Bragg fiber grating 5 and the output end face of the double-clad erbium-doped fluoride fiber 6. 4 I 11 / 2 After the erbium ion completes the laser emission, it transitions to 4 I 13 / 2 energy level, and accumulate at this energy level; further pumped by 1150nm 4 I 13 / 2 The erbium ions at the energy level are pumped to 4 F 9 / 2 Energy level, emptying 4 I 11 / 2 Energy level and realize the inversion of the upper and lower energy level particles of 3.5μm laser. When the 1150nm pump light power reaches the 3.5μm laser threshold, 3540nm laser oscillation is formed in the laser resonant cavity composed of the first Bragg fiber grating 4 and the output end face of the double-clad erbium-doped fluoride fiber 6, and is pumped to 4 F 9 / 2 After completing the laser emission, the erbium ions at the energy level will return to the 4 I 11 / 2 Energy level, to achieve the secondary layout of the number of particles in the upper and lower energy levels of 2.8μm laser. Through multiple layout processes, it can effectively overcome 4 I 13 / 2 The problem of 2.8μm laser self-termination caused by particle accumulation at the energy level is solved, thereby achieving high-efficiency 2.8μm and 3.5μm dual-wavelength mid-infrared laser output based on a single double-clad erbium-doped fluoride fiber.

[0031] Under the above device, when the 976nm pump light power is 10W and the 1150nm pump light power is 50W, 20W of 2820nm laser and 10W of 3540nm mid-infrared laser output can be obtained synchronously.

[0032] Example 2

[0033] In the above embodiment, the second pump source may be an ytterbium-doped quartz fiber laser or a Raman fiber laser, as long as it can provide sufficient output power in the 1150 nm band. This embodiment of the present invention does not impose any limitation on this.

[0034] Unless otherwise specified, the embodiments of the present invention do not limit the models of the components. Any component that can perform the above functions may be used.

[0035] Those skilled in the art will understand that the accompanying drawings are only a schematic diagram of a preferred embodiment, and the serial numbers of the embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0036] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A 2.8μm and 3.5μm dual-wavelength mid-infrared fiber laser, characterized in that: The laser comprises: a first pump source, a second pump source, a fiber combiner, a first Bragg fiber grating, a second Bragg fiber grating, a double-clad erbium-doped fluoride fiber, and a long-pass filter; The output end face of the double-clad erbium-doped fluoride optical fiber is cut at 0°, and forms a 3.5μm laser resonant cavity with the first Bragg fiber grating, and forms a 2.8μm laser resonant cavity with the second Bragg fiber grating; The first pump source and the second pump source correspond to erbium ions respectively 4 I 15 / 2 → 4 I 11 / 2 Ground state absorption and 4 I 13 / 2 → 4 F 9 / 2 The excited state absorption wavelength enters the double-clad erbium-doped fluoride optical fiber through the optical fiber combiner to provide gain for 2.8μm and 3.5μm lasers, thereby overcoming the laser self-termination caused by the long energy level lifetime of the 2.8μm laser. Based on a single section of the double-clad erbium-doped fluoride optical fiber, the synchronous output of 2.8μm and 3.5μm dual-wavelength mid-infrared lasers is achieved.

2. The 2.8μm and 3.5μm dual-wavelength mid-infrared fiber laser according to claim 1, characterized in that: The first pump source is a multi-mode semiconductor laser with an output wavelength of 0.98 μm.

3. The 2.8μm and 3.5μm dual-wavelength mid-infrared fiber laser according to claim 1, characterized in that: The second pump source is a single transverse mode ytterbium-doped quartz fiber laser with an output wavelength of 1.15 μm.

4. The 2.8μm and 3.5μm dual-wavelength mid-infrared fiber laser according to claim 1, characterized in that: The input end of the fiber combiner includes a single-mode fiber and a multi-mode fiber, and the output fiber is a double-clad fiber, which meets the requirements of 1.15μm single-mode pump light being transmitted in the fiber core and 0.98μm multi-mode pump light being transmitted in the cladding.

5. The 2.8μm and 3.5μm dual-wavelength mid-infrared fiber laser according to claim 1, characterized in that: The central wavelength of the first Bragg fiber grating is erbium ion 4 F 9 / 2 → 4 I 9 / 2 For any wavelength within the transition emission band, the reflectivity is greater than 99.5%, the full width at half maximum is less than 5nm, and the insertion loss is less than 0.5dB.

6. The 2.8μm and 3.5μm dual-wavelength mid-infrared fiber laser according to claim 1, characterized in that: The central wavelength of the second Bragg fiber grating is erbium ion 4 I 11 / 2 → 4 I 13 / 2 For any wavelength within the transition emission band, the reflectivity is greater than 99.5%, the full width at half maximum is less than 5nm, and the insertion loss is less than 0.5dB.

7. The 2.8 μm and 3.5 μm dual-wavelength mid-infrared fiber laser according to claim 1, characterized in that: The long-pass filter has a cut-off wavelength of 1.5 μm, a reflectivity greater than 95% at 0.98 μm and 1.15 μm, and a transmittance greater than 95% at 2.8 μm and 3.5 μm.

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