Fig. 1. (a) Taper formation by heating and stretching an SMF. The light gray section represents the fiber cladding, and the dark gray section represents the fiber core, in which both are not to scale. (b) Schematic diagram of the tapered SMF inserted into the HCF.

Fig. 2. Schematic of the experimental setup based on coupling between the tapered solid-core fiber and HCF. AOM, acousto-optical modulator; EDFA, erbium-doped fiber amplifier; PM, power meter; XYZ, manual 3D translation stage; W, uncoated sapphire window; L, plano-convex lens; M, mirror on flipper mount; IBF, infrared bandpass filter. Inserts: the measured transmission loss spectrum (red curve and black curve represent fiber loss from 3000 to 3300 nm and from 1500 to 1600 nm, respectively) and the scanning cross-section of the HCF.

Fig. 3. (a) Measured spectrum of the mid-infrared laser from acetylene-filled HCF lasers taken at 3.5 mbar with 2 m length of HCF. P(3)–P(25) displayed in upper right corner represent corresponding odd pump lines. (b) The simplified molecular energy-level diagram of acetylene for 3 μm emissions when pumped at 1.5μm absorption band, and the right table displays the odd P branch absorption lines of acetylene molecules used in experiments and the corresponding measured laser wavelengths using the OSA.

Fig. 4. Measured laser pulse energy with respect to absorbed pump pulse energy for a 2 m long HCF filled with various pressures of acetylene gas when pumped with (a) P(11) and (b) P(13) absorption lines, respectively.