Patent Application
20240275120
This application claims priority from the Chinese patent application 2023100986607 filed Feb. 10, 2023, the content of which are incorporated herein in the entirety by reference.
The present disclosure belongs to the technical field of fiber laser, especially relates to a 2.8 μm and 3.5 μm dual-wavelength mid-infrared fiber laser.
Mid-infrared wavelength region of 2.5-5 μm have received great attention due to the fact that it overlaps with transparent window of the atmosphere and includes abundant molecular absorption lines corresponding to bond resonances such as C—H and O—H. These spectra features allow the mid-infrared laser sources widely used in space communication, infrared countermeasures, high-sensitivity gas detection, and polymer processing. Compared with other laser sources, compact rare-earth-doped fiber laser exhibits the advantages on power scalability, wavelength tunability, thermal management and beam quality, which is taken as a promising approach to obtain high-performance mid-infrared lasing. Hitherto the mid-infrared fiber laser demonstrated by Erbium (Er)-doped fluoride fiber has been rapidly developed in recent years to achieve output power scaling. Er ions can provide two mid-infrared emission peaks in fluoride host, i.e., 2.8 μm and 3.5 μm, corresponding to the 4I1/2→4I13/2 and 4F9/2→4I9/2 transitions, respectively. The lasing in these two wavelength regions can be achieved with 0.98 μm single-wavelength pumping scheme and 0.98 μm+1.97 μm dual-wavelength pumping scheme, respectively. Over the past decade, benefited from the development of fluoride fibers and fluoride fiber-based devices, the performance of mid-infrared Er-doped fluoride fiber lasers have been promoted significantly. Hitherto the maximum output powers of 2.8 μm and 3.5 μm single-wavelength Er-doped fluoride fiber lasers reach up to 40 W and 15 W, respectively. In view of the fact that the two transitions mentioned above are independent and have no overlap on start and terminate energy levels, cascaded lasing with dual-wavelength mid-infrared output can be expected by constructing two separate resonators with single piece of Er-doped fluoride fiber.
However, the existing pumping scheme cannot achieve efficient inversion of the particles in 3.5 μm transition and 2.8 μm transitions simultaneously, that's why previous studies are focusing on a single wavelength. Although the final levels of 0.98 μm and 1.97 μm pump absorption respectively correspond to the upper levels of 2.8 μm and 3.5 μm pump absorption, 0.98 μm+1.97 μm dual-wavelength pumping scheme seems feasible since it covers both of the 2.8 μm and 3.5 μm transitions. However, due to that the virtual ground state of 1.97 μm absorption and the upper level of 2.8 μm laser share the same level (4I11/2), the absorption of 1.97 μm pump (4I11/2→4F9/2) depletes the population in 4I11/2 level and thus reduces the gain of 2.8 μm transition. This behavior would induce the 2.8 μm laser efficiency even lower than that achieved with the 1.97 μm single-wavelength pumping scheme. Hence, the “0.98 μm+1.97 μm” dual-wavelength pumping scheme is actually paradoxical for 3.5 μm and 2.8 μm dual-wavelength generation and not applicable to realize efficient cascaded mid-infrared lasing in Er-doped fluoride fiber laser. Furthermore, in the conventional 0.98 μm pumped 2.8 μm single-wavelength mid-infrared fiber lasers, a large amount of Er ions would accumulate in the long-lived 4I13/2 level (˜9.9 ms). Such behavior would reduce the inversion population and induce the self-termination of lasing, which limits the power scaling of 2.8 μm laser.
The present disclosure discloses a 2.8 μm and 3.5 μm dual-wavelength mid-infrared fiber laser, which adopts dual-wavelength pumping scheme of performing ground state absorption at 0.98 μm and excited state absorption at 1.15 μm to simultaneously achieve 2.8 μm and 3.5 μm lasing in single piece of double cladding Er-doped fluoride fiber based on two cascaded transitions, the present disclosure can well address the self-termination of 2.8 μm lasing resulted from the long lifetime, effectively scaling the operating wavelength and improving the efficiency of mid-infrared fiber laser sources. The details are as follows.
A dual-wavelength mid-infrared fiber laser, comprising a first pump source, a second pump source, a fiber combiner, a first fiber Bragg grating, a second fiber Bragg grating, a double cladding Er-doped fluoride fiber and a long pass filter.
The first pump source is multimode laser diode with output wavelength of 0.98 μm, the second pump source is single-transverse-mode Ytterbium (Yb)-doped fiber laser with output wavelength of 1.15 μm, the input port of the fiber combiner includes a single-mode fiber and a multimode fiber, and the output fiber is a double cladding fiber which is capable of propagating 1.15 μm single-mode pump laser inside the core and propagating 0.98 μm multimode pump laser inside the inner cladding.
The central wavelength of the first fiber Bragg grating is a certain wavelength within the emission band of Er ion 4F9/2→4I9/2 transition, the reflectivity of the first fiber Bragg grating is greater than 99.5%, the FWHM is narrower than 5 nm and the insertion loss at pump wavelengths is lower than 0.5 dB; the central wavelength of the second fiber Bragg grating is a certain wavelength within the emission band of Er ion 4I1/2→4I13/2 transition, the reflectivity of the second fiber Bragg grating is greater than 99.5%, the FWHM is narrower than 5 nm and the insertion loss at pump wavelengths is lower than 0.5 dB; the long pass filter has a cutoff wavelength of 1.5 μm, the reflectivity at 0.98 μm and 1.15 μm are greater than 95%, as well as transmission at both of the 2.8 μm and 3.5 μm are greater than 95%.
Output end of the double cladding Er-doped fluoride fiber is perpendicularly cleaved, which can provide a 4% Fresnel reflectivity of the full band for both of the 2.8 μm and 3.5 μm laser, thus, the cleaved output end and the first fiber Bragg grating form a 3.5 μm resonator, while the cleaved output end and the second fiber Bragg grating form a 2.8 μm resonator.
0.98 μm and 1.15 μm pump light launched by the first pump source and the second pump source are combined by the fiber combiner and injected into the double cladding Er-doped fluoride fiber through the first fiber Bragg grating and the second fiber Bragg grating. As shown in
Further, the second pump source can be a continuous-wave laser or a pulsed laser.
The beneficial effects of the technical scheme according to the present disclosure are:
In which:
To make the objectives, technical solutions, beneficial effects of the present disclosure clearer, the technical solutions in the embodiments of the present disclosure will be clearly and completely described below with reference to the drawings provided in the embodiments of the present disclosure.
A 2.8 μm and 3.5 μm dual-wavelength mid-infrared fiber laser, as shown in
Wherein, the first pump source 1 is a 976 nm wavelength multimode laser diode providing a maximum output power of 10 W, an output fiber thereof is connected with the multimode fiber input port 3-2 of the fiber combiner 3; the second pump source 2 is a 1150 nm wavelength single-transverse-mode Yb-doped fiber laser providing a maximum output power of 50 W, an output fiber thereof is connected with the single-mode fiber input port 3-1 of the fiber combiner 3.
Wherein, the central wavelength of the first fiber Bragg grating 4 is 3540 nm, at which the first fiber Bragg grating 4 provides a reflectivity >99.9% and FWHM of 2 nm, and the insertion loss is 0.2 dB; the central wavelength of the second fiber Bragg grating 5 is 2820 nm, at which the second fiber Bragg grating 5 provides a reflectivity >99.9% and FWHM of 2 nm, and the insertion loss is 0.2 dB. The insertion loss of both the first fiber Bragg grating 4 and the second fiber Bragg grating 5 at 976 nm and 1150 nm are lower than 0.3 dB. Wherein, the double cladding Er-doped fluoride fiber 6 is non polarization maintaining fiber. It has a core and inner cladding diameters of 15 μm and 250 μm, respectively, the core numerical aperture, doping concentration, and length of the fiber are 0.125, 1 mol. %, and 10 m, respectively. Wherein, the long pass filter 7 has a cutoff wavelength of 1.5 μm, it provides a greater than 95% reflectivity at 976 nm and 1150 nm as well as a greater than 95% transmission both at 2820 nm and 3540 nm.
In practice, 976 nm and 1150 nm pump lights are first combined by the fiber combiner 3 and then injected into the double cladding Er-doped fluoride fiber 6, where the 976 nm and 1150 nm pump lights propagate in the inner cladding and core, respectively. The Er ions in the ground state are first promoted to 4I11/2 level by the 976 nm pump light, realizing the first population inversion of 4I11/2 level. After a 976 nm pump power exceeds the 2.8 μm laser threshold, the 2820 nm lasing can be generated in the laser resonator formed between the second fiber Bragg grating 5 and the output end facet of the double cladding Er-doped fluoride fiber 6. Followed by the 2820 nm laser transition, the Er ions would transit to 4I13/2 level and accumulate gradually in this level. These Er ions would be further promoted to 4F9/2 level by the 1150 nm pump light, de-populating the 4I13/2 level and realizing the population inversion of 3.5 μm transition. After a 1150 nm pump power exceeds the 3.5 μm laser threshold, the 3540 nm lasing can be generated in the laser resonator formed between the first fiber Bragg grating 4 and the output end facet of the double cladding Er-doped fluoride fiber 6. Followed by the 3540 nm laser transition, the Er ions would rapidly decay to 4I11/2 level via non radiative transition, realizing the re-population of 4I11/2 level and enhancing the 2.8 μm laser gain. Such multi population behavior can effectively suppress the self-termination of 2.8 μm laser, enabling efficient 2.8 μm and 3.5 μm dual-wavelength operation based on a single piece of double cladding Er-doped fluoride fiber.
In the fiber laser of the present disclosure, at 976 nm pump power of 10 W and 1150 nm pump power of 50 W, the 2820 nm and 3540 nm output power can reach up to 20 W and 10 W, respectively.
In the embodiment of the present disclosure, the second pump source can be an Yb-doped silica fiber laser or a Raman fiber laser, as long as it can provide sufficient output power at 1150 nm, the type of second pump source is not limited.
In the embodiment of the present disclosure, the model of each device is not limited except for special instructions.
Those skilled in the art must understand that the accompanying drawings are only schematic diagrams of a preferred embodiment, and the serial numbers of the above-mentioned embodiments of the present disclosure are only for description, and do not represent the superiority or inferiority of the embodiments.
The foregoing embodiments and specific examples are merely for describing the technical solutions of the present disclosure and not intended to limit the present disclosure. Although the present disclosure has been described in details by the foregoing embodiments, it should be understood by a person of ordinary skill in the art that modifications may be made to the technical solutions recorded in the foregoing embodiments or equivalent replacements may be made to some or all of the technical features, and these modifications or replacements shall not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present disclosure. Non-essential improvements and adjustments or replacements made according to the content of this specification by those skilled in the art shall fall into the protection scope of the present disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2023100986607 | Feb 2023 | CN | national |
