Octave-Spanning Supercontinuum Generated in As₂S₃-Silica Waveguides Pumped by Tm-doped All-fibre MOPA

Fiber-based supercontinuum sources in the mid-IR wavelength range are highly desirable for many applications [1] - [3]. Here we report the generation of octave-spanning supercontinua in As 2 S 3 -silica hybrid waveguides pumped by a thulium-doped all-fiber master-oscillator-power-amplifier (MOPA). The narrow As 2 S 3 core of the step-index waveguide has an extremely high nonlinearity, and by balancing its anomalous geometrical dispersion against the normal dispersion of the glass, the zero dispersion wavelength of the guided mode can be shifted towards the pump wavelength by suitable design. The nanospikes at both ends of the hybrid waveguide (see Fig. 1a ) are used to boost the launch efficiency into the fundamental core mode [3] , [4]. A thulium-doped all-fiber MOPA centred at 1.9m [5] , [6] was used as pump source. It emits pulses with a duration of 78 fs and a maximum peak power of 200 kW at a repetition rate of 23.8 MHz. Dual nanospike waveguides with core diameters d = 1.7m and 1.2m were tested in the experiment. The length of both waveguides was 3 mm. Fig. 1b plots the calculated dispersion parameter of the two waveguides. For d = 1.7m the pump lies within ~ 50 nm of a zero dispersion wavelength, in which case for a launched energy of ~32 pJ the measured spectrum at the output extends from 1.1m to 2.5m at the -10 dB level. The total output average power was measured as ~ 1.2 mW. When d = 1.2m, the pump wavelength lies in between two zero dispersion wavelengths, and for 23 pJ launched energy the spectrum extends from 1.7m to 2.3m, while a weak dispersive wave appears at 963 nm. In both cases the third harmonic components are visible at 630 nm (inset of Fig. 1c ), which fall within the bandgap of the As 2 S 3 material, limiting further increase of the pump energy. Numerically simulated spectra (black-dotted curves) calculated for the experimental pulse energy agree well with the measurements.