Random fiber lasers (RFLs) can be realized based on the Rayleigh backscattering in fiber as random distributed feedback, without using the fixed resonator in traditional fiber laser. The cavity-free nature of RFLs not only substantially simplifies the constructions of fiber lasers, but also gives RFLs unique merits, such as high brightness, modeless spectrum and ultra-broad wavelength tunability. The wavelengths of cascaded Raman RFLs (RRFLs) have covered 1-2 µm region with high output power, combining with nonlinear frequency conversions, the RFLs are expected to be ideal driving sources for generating visible and mid-infrared light. Moreover, the visible or mid-infrared light generated by frequency conversion of the RFL can inherit the characteristics such as modeless, low noise, low temporal and/or spatial coherence, which could extend the applications of RFLs which require low-coherence light sources such as temporal ghost imaging and speckle-free imaging to different wavelength regions.
For visible light generation, second-harmonic generation (SHG) of an RRFL and an ytterbium-doped RFL has been reported to generate watt-level visible light. However, until now, broadly tunable visible light covering various colors generated from an RFL still remains underdeveloped. MIR radiation with the tuning wavelengths ranging from 5.5 to 9.5 µm has been experimentally realized. However, in these demonstrations, two different types of RFLs are required in the DFG process and there has been no report of DFG of a dual-wavelength RFL yet. In addition, the ability to generate >10 µm CW DFG with BaGa4Se7 (BGSe) crystal still remains unexplored.
To solve the above problems, Prof. Liang Houkun and his team from Sichuan University collaborated with Prof. Liu Jun from Shenzhen University and Prof. Yao Jiyong from Chinese Academy of Sciences reported a dual-wavelength switchable and tunable RFL, and demonstrated widely tunable CW visible and mid-infrared light via nonlinear frequency conversions of the dual-wavelength RFL for the first time. The relevant research results were published in Photonics Research, Vol. 11, Issue 5,（Han Wu, Weizhe Wang, Bo Hu, Yang Li, Kan Tian, Rui Ma, Chunxiao Li, Jun Liu, Jiyong Yao, Houkun Liang. Widely tunable continuous-wave visible and mid-infrared light generation based on a dual-wavelength switchable and tunable random Raman fiber laser[J]. Photonics Research, 2023, 11(5): 808).
As shown in Fig. 1(a), by using the combination of a tunable pump and two tunable gratings in Littrow configuration that can provide separated point feedback for the two Stokes wavelengths corresponding to silica- and phosphorus-related Raman peaks, the spectrum of an RRFL can be flexibly manipulated for the aim of nonlinear frequency conversions, including single-wavelength tunable emission at the 1.1 µm or 1.2 µm band for second-harmonic generation (SHG), dual-wavelength simultaneously tunable emission at the 1.1 µm and 1.2 µm bands for the sum-frequency generation (SFG), and dual-wavelength separation tunable emission for difference-frequency generation (DFG). As a result, with the combination of SHG and SFG in a periodically poled lithium niobate crystal array, we experimentally demonstrate the broadest tuning range (560–630 nm) of visible light generated from an RRFL as shown in Fig. 1(b). The tunable MIR light in the range of 10.7–12.3 µm is also demonstrated through DFG of an RRFL operating in separation tunable dual-wavelength emission mode in a BGSe crystal as shown in Fig. 1(c).
Fig.1 (a) Schematic of the experimental setup. (b) The experimentally measured spectra of the tunable CW visible light from 560 to 630 nm. (c) The experimentally measured spectra of the CW MIR light from 10.7 to 12.3 µm.
Our experimental results show that the developed dual-wavelength switchable and tunable RFL can provide a compact, robust, and cost-effective platform for performing multifunctional nonlinear frequency conversion processes and generating widely tunable light in both the visible and MIR regions, which can potentially extend the applications of RFLs such as imaging and sensing in various spectral regions.
