Fig. 1. Experimental setup. (a) Schematic of experimental setup; (b) SEM image of cross section of ice-cream type HCF;(c) SEM image of cross section of node-less type HCF

Fig. 2. Output spectra, measured loss spectrum, and energy level transition diagram of output spectrum of HCF1. (a) Output spectra and measured loss spectrum of HCF1 at pressures of 400 kPa and 800 kPa under pump powers of 25, 50, and 90 mW; (b) energy level transition diagram of output spectrum of HCF1 at pressure of 800 kPa under pump power of 90 mW (corresponding Raman frequency shifts are shown within brackets)

Fig. 3. Raman light power as a function of coupled pump power and output spectra of HCF under maximum coupled pump power at different pressures. (a)(b) 400 kPa; (c)(d) 600 kPa; (e)(f) 800 kPa; (g)(h) 1000 kPa

Fig. 4. Measured patterns. (a) Pump light from pump source; (b) pump light at output end of HCF; (c) 1561-nm Raman light under low power; (d) 1561-nm Raman light under high power

Fig. 5. Output spectra, measured loss spectrum, and energy level transition diagram of output spectrum of HCF2. (a) Output spectrum and measured loss spectrum of HCF2 at pressure of 1400 kPa, where S1 and S2 represent first- and second-order Stokes respectively and inset shows fine spectrum near 2924.9 nm with resolution of 0.02 nm; (b) energy level transition diagram of output spectrum of HCF2 at pressure of 1400 kPa