Subwatt Threshold cw Raman Fiber-Gas Laser Based on ${\mathrm{H}}_2$-Filled Hollow-Core Photonic Crystal Fiber

We report on what is, to our knowledge, the first cw pumped Raman fiber-gas laser based on a hollow-core photonic crystal fiber filled with hydrogen. The high efficiency of the gas-laser interaction inside the fiber allows operation in a single-pass configuration. The transmitted spectrum exhibits 99.99% of the output light at the Stokes wavelength and a pump power threshold as low as 2.25 W. The study of the Stokes emission evolution with pressure shows that highly efficient Raman amplification is still possible even at atmospheric pressure. The addition of fiber Bragg gratings to the system, creating a cavity at the Stokes wavelength, reduces the Raman threshold power below 600 mW.

Figure 1

(a) Theoretical conversion efficiency from the input pump power to the output power at the first rotational Stokes wavelength as a function of the propagation length and the input power. (b) Calculated output power ratio to the total input power at the pump (dashed line), first Stokes (solid gray line), second Stokes (dotted line) as a function of the coupled power for 30 m of HC-PCF. The anti-Stokes output power is ${10}^{-6}$ weaker than the pump and Stokes lines.

Figure 2

(a) Experimental setup, $\lambda /2$: half-wave plate, $\lambda /4$: quarter-wave plate, PBS: polarization beam splitter, SMF: 980 nm cutoff single mode fiber. (b) Evolution of the dispersed output spectrum as a function of coupled input power: pump (left) and $S_{00}(1)$ (right) at 5 bar ${\mathrm{H}}_2$ pressure. (c) Output optical spectrum for 1 W and (d) 8.7 W coupled input power at 5 bar ${\mathrm{H}}_2$ pressure. Higher conversion efficiency is achieved at (e) 1 bar pressure. The broad spectral feature around 1064 nm is likely to be due to the residual fluorescence of the laser.

Figure 3

(a) Experimental output-input power ratio for the pump (circles and black line) and $S_{00}(1)$ ( gray squares and line) at 5 bar ${\mathrm{H}}_2$ pressure. Theoretical calculations are also given for comparison for the pump (dashed line), the first Stokes (solid gray line), and the second Stokes (short dotted line). The anti-Stokes radiation is too weak to be represented. (b) Experimental (circles) and theoretical (dotted line) evolution of $S_{00}(1)$ threshold power with ${\mathrm{H}}_2$ pressure.

Figure 4

(a) Low threshold, all-fiber cw Raman laser experimental setup. FBG: fiber Bragg grating. (b) Output power versus coupled input power for the pump (circles and black line) and $S_{00}(1)$ (gray squares and line) when using FBGs to form a cavity. The drop in Stokes intensity above 3 W is due to conversion to second Stokes.