Study of Highly Coherent Mid-Infrared Supercontinuum Generation in CMOS Compatible Si-Rich SiN Tapered Waveguide

A promising 2-mm-long highly coherent air-clad suspended core planar Si-rich SiN tapered waveguide is proposed for supercontinuum generation in the midinfrared. Supercontinuum evolution is investigated by tuning the waveguide for both of the transverse-electric (TE) and transverse-magnetic (TM) polarizations employing pump at 1.55 m with a pulse duration of 50-fs and peak power of 50 W. Optimum waveguide dimensional parameters including width and thickness are obtained between 1.5-3 $\mu$ m and 250 nm for TE excitation and 270–330 and 1.5 $\mu$ for TM excitation, respectively, keeping slab thickness constant at 100 nm for both the designs. Initially, SC coverages are investigated for two uniform sections situated at the terminal points of the waveguide. Spectral coverages are also studied for two types of tapered approaches: increasing order tapering (IOT) and decreasing order tapering (DOT). In the case of uniform waveguide, simulation results of either excitation show that SC coverage can be predicted beyond 5 $\mu$ in the midinfrared. On the other hand, width varying IOT approach for TE and DOT approach for TM excitation make better impacts both in spectral extension and spectral flatness owing to simultaneous variations of dispersion and nonlinearity along the pulse propagation direction. Thus, the midinfrared supercontinuum extension up to 6 $\mu$ by TE and 6.3 $\mu$ by TM with better spectral flatness can be predicted by the proposed taper compared to the traditional waveguide design. Finally, the coherence of the generated SC is also studied for the various tapering coefficients at the proposed waveguide output.