Fig. 1. Measured attenuation of AR-HCF by a cut-back from 96.8 m to 22 m. Inset: SEM picture of AR-HCF. The core diameter is about 35 µm.

Fig. 2. Schematic of the laser delivery setup. HWP is half wave plate; ISO is isolator; QWP is quarter wave plate; L1, L2, L3, L4 are coated aspherical lenses with f₁ = 50 mm, f₂ = 150 mm, f₃ = 90 mm, f₄ = 50 mm; M1 is a sampler to monitor the power of the laser source and backward Stokes light; M2 is another sampler used to monitor the output power and beam profile or wavelength at the same time; PM1, PM2 are power meters. At BM1 and BM2 positions, pump and Stokes laser beams were characterized by using a power meter, optical spectral analyzer, and pyroelectric array camera, respectively.

Fig. 3. Measured output spectra in the forward direction under 5 bar gas pressure for different pump power. Pumped at 1064 nm, the first rotational Stokes laser (RS1) of H₂ is at 1135 nm, and the second rotational Stokes (RS2) at 1216 nm.

Fig. 4. Far-field patterns at (a) 1064 nm and (b) 1135 nm measured at the output end of AR-HCF at 5 bar gas pressure.

Fig. 5. Measured powers of the residual pump and forward and backward Rot-SRS as a function of coupled pump power with a H₂ pressure at (a) 2.5 bar, (b) 5 bar, (c) 7.5 bar, and (d) 10 bar. In Region (I), Rot-SRS in the forward and backward directions is measured only; (II) bi-directional first Rot-SRS and forward second Rot-SRS, (III) the pump laser failed to work because strong backward Rot-SRS is disturbed. The forward Rot-SRS power (black line) and residual power (red line) refer to the left axis, and the backward Rot-SRS power (blue line) refers to the right axis.

Fig. 6. Power (left axis) and quantum conversion efficiency (right axis) of (a) forward Rot-SRS and (b) backward Rot-SRS as a function of coupled pump power at different gas pressures.

Table 1. Comparison of Pump Thresholds in H₂-Filled FGRLs under Continuous and Pulsed Operation