Physical mode analysis of multimode cascaded nonlinear processes in strongly-coupled waveguides

TL;DR

This paper demonstrates on-chip control and analysis of multimode nonlinear interactions in silicon nitride waveguides, revealing simultaneous supercontinuum and harmonic generation, and introducing a method for better understanding and controlling multimode nonlinear optics.

Contribution

It introduces a novel chip-integrated approach for controlling and analyzing multimode nonlinear processes in waveguides, enabling new possibilities for frequency conversion and nonlinear optics research.

Findings

Simultaneous dual-supermode supercontinuum generation observed.

Cascaded four-wave mixing upconverts third-harmonic radiation.

The method enhances understanding and control of multimode nonlinear interactions.

Abstract

We experimentally investigate on-chip control and analysis of spatially multimode nonlinear interactions in silicon nitride waveguide circuits. Using widely different dispersion of transverse supermodes in a strongly-coupled dual-core waveguide section, and using integrated pairs of input and output single-mode waveguides, we enable controlled excitation of nonlinear processes in multiple supermodes, while a basic physical mode decomposition aids the identification of parallel and cascaded processes. Pumping with ultrashort pulses at 1.5-$\mu$m wavelength (around 195-THz light frequency), we observe simultaneous dual-supermode, near-infrared supercontinuum generation having different spectral widths, in parallel with third-harmonic generation at around 515 nm (582 THz). Cascaded four-wave mixing with supercontinuum components upconverts the third-harmonic radiation toward a set of four shorter blue wavelengths emitted in the range between 485 and 450 nm (617 to 661 THz). The approach taken here, i.e., using chip-integrated spatial multiplexing and demultiplexing for excitation and analysis of broadband transverse nonlinear conversion, can be an advanced tool for better understanding and control in multimode nonlinear optics, such as for extending frequency conversion to wider spectral ranges via extra phase matching paths.