Fig. 1. (a) SEM image of the fabricated nested AR-HCF. The core diameter is about 31 µm. (b) Measured attenuation of the nested AR-HCF by cut-back.

Fig. 2. (a) Schematic of H₂-filled FGRL. R1, R2, mirrors; L1, L2, L3, L4, coated aspherical lenses with f1 = 50 mm, f2 = 150 mm, f3 = 75 mm, f4 = 100 mm; ISO, isolator; DM1, DM2, dichroic mirrors; PM, power meter. (b) Measured spectrum of the pump source. (c) Linewidth of the pump source measured by the self-heterodyne method.

Fig. 3. Measured spectra of SRS in the forward direction at 10 bar hydrogen for different coupled pump powers.

Fig. 4. (a) Measured pump threshold powers of forward SRS for different H₂ pressures. (b) Measured forward Stokes output power. (c) Calculated quantum efficiency of forward SRS. (d) Calculated quantum efficiency of backward SRS. (e) Calculated total quantum efficiency of dual directional SRS. (f) Measured residual pump power, as a function of coupled pump power at different gas pressures. Note: The power conversion efficiency is calculated by η_q = P_Stokes/P_pump.

Fig. 5. Measured linewidths of the (a) pump source for 1 W output and (b) forward vibrational Stokes laser for 1 W output at 10 bar hydrogen pressure.

Fig. 6. Measurement of the temporal stability of the output power of forward Stokes. RMS: root-mean-square deviation.