Fig. 1. All-fiber scheme of MM GRIN-fiber Raman laser with cavity formed by HR UV-FBG1 and output fs-FBG2 with LD pumping through the fiber pump combiner (or bulk-optic pump coupling scheme^[10] shown in the inset).

Fig. 2. Comparison of HR UV-FBG and output fs-FBG inscribed in GRIN fibers of different core diameters: (a) $62.5~\unicode[STIX]{x03BC}\text{m}$^[10], (b) $85~\unicode[STIX]{x03BC}\text{m}$^[15] and (c) $100~\unicode[STIX]{x03BC}\text{m}$^[16].

Fig. 3. (a) Output Stokes power at 954 nm (filled symbols) as a function of input (coupled to GRIN fiber) LD pump power and (b) corresponding beam-quality parameter $M^{2}$ as a function of Stokes power for different GRIN-fiber core diameters. Arrows mark the threshold of the 2nd-order Stokes wave generation at 996 nm.

Fig. 4. (a) Output spectra for GRIN-fiber Raman laser with 62.5, 85 and $100~\unicode[STIX]{x03BC}\text{m}$ cores at different Stokes powers. (b) Corresponding $-3~\text{dB}$ linewidth as a function of the generated Stokes power at 954 nm fitted by curves ${\sim}x^{a}$ with best-fit parameter $a=0.64$ ($62.5~\unicode[STIX]{x03BC}\text{m}$); 0.45 ($85~\unicode[STIX]{x03BC}\text{m}$); 0.34 ($100~\unicode[STIX]{x03BC}\text{m}$).

Fig. 5. (a) Stable and unstable beam profiles at low power (13 W) for operation at 950 nm (left) and 958 nm (right) in 100-$\unicode[STIX]{x03BC}\text{m}$ GRIN RFL, respectively. (b) Measured Raman gain profile with corresponding marks illustrating 100-$\unicode[STIX]{x03BC}\text{m}$ GRIN RFL operation with 950-, 954- and 958-nm FBGs^[16].

Fig. 6. (a) Output power and (b) beam-quality characteristics of 2nd-order random Raman lasing at 996 nm in 100-$\unicode[STIX]{x03BC}\text{m}$ GRIN RFL together with power of the 1st Stokes output at 954 nm.