Fig. 1. (a) Schematic diagram of the narrow-linewidth fiber oscillator. (b) Measured spectrum of the OC-FBG. (c) Spectrum of the output. Inset: fine spectrum around the central wavelength.

Fig. 2. (a) Experimental setup of the H₂-filled FGRL. L: lens; R: reflector; PM: power meter; OSA: optical spectrum analyzer. Inset: thermal image of the input end of HC-PCF. (b) HCF end-cap. (c) Measured loss of the HC-PCF. Inset: cross-section of the HC-PCF.

Fig. 3. (a) Measured spectrum of the output laser at different pump powers when the fiber length is 5 m with the H₂ pressure of 30 bar. (b) Measured spectrum of the output laser at different H₂ pressures when the fiber length is 5 m with the maximum pump power. Inset: fine spectra of the pump, first-order Stokes laser and second-order Stokes laser. 3 dB linewidth of the output laser at (c) different pump powers and (d) different gas pressures.

Fig. 4. (a) First-order Stokes power, (b) first-order Stokes efficiency and power spectral density of the pump, (c) second-order Stokes power and (d) residual pump power vary with coupled pump power at different gas pressures with the fiber length of 5 m.

Fig. 5. First-order Stokes power, second-order Stokes power and residual pump power vary with coupled pump power at 36 bar H₂ pressure with the fiber length of 5 m. Inset: beam profile of the Stokes laser.

Fig. 6. (a) Experimental and simulated results of the Stokes and residual pump powers varying with the coupled pump power. The dotted lines represent the simulated results and the scatters represent the measured results. (b) Simulated first-order Stokes power varying with the coupled pump power at different fiber lengths.