Supercontinuum enhancement using Bragg solitons on a CMOS-compatible chip

Abstract

The appeal of on-chip broadband supercontinuum generation (SCG) comes from its potential to pave the way to full integration of various ultrafast optics applications in frequency metrology, wavelength division multiplexing, and sensing. However, the generation of octave-spanning supercontinuum requires either the use of exceedingly short femtosecond pulses or large footprints. One promising method to achieve broadband supercontinuum is to exploit the high-order soliton fission. Bragg solitons leverage the large anomalous dispersion at the photonic band edge of nonlinear Bragg gratings, therefore they can facilitate high-order soliton fission in much shorter waveguide lengths and significantly lower powers. Soliton dynamics, especially fission, on CMOS-compatible platforms have been limited due to the nonlinear losses such as two-photon absorption and free carrier effects in silicon or low optical nonlinearities in traditional silicon nitride. We use compositionally engineered ultra-silicon-rich nitride (USRN) that possesses a large Kerr nonlinearity in the absence of two-photon absorption. Utilizing ideal nonlinear properties of USRN platform in conjunction with our monolithically integrated cascaded grating-waveguide design, we experimentally demonstrate × 4 spectral broadening enhancement, from 79 nm in the 7 mm long reference waveguide to 311 nm at the cascaded Bragg grating and waveguide device of the same footprint, using input pulses of 1.68 ps FWHM. This result is promising for generating wide supercontinuum, without the need to use sub-picosecond pulses or increasing the device footprint, by exploiting the high-order soliton dynamics availed through the simple photonic chip design consisting of a nonlinear Bragg grating and nonlinear waveguide.