Power limits and a figure of merit for stimulated Brillouin scattering in the presence of third and fifth order loss

TL;DR

This paper develops design guidelines and a figure of merit for optimizing stimulated Brillouin scattering in on-chip waveguides, considering various loss mechanisms to maximize amplification and power limits.

Contribution

It introduces a dimensionless parameter to determine optimal conditions and provides practical expressions for waveguide design, accounting for nonlinear and linear losses.

Findings

2PA-induced FCA limits SBS in silicon and germanium at specific wavelengths

Optimal operating conditions depend on a new figure of merit involving gain and loss

Three-photon absorption is negligible for SBS in studied materials

Abstract

We derive a set of design guidelines and a figure of merit to aid the engineering process of on-chip waveguides for strong Stimulated Brillouin Scattering (SBS). To this end, we examine the impact of several types of loss on the total amplification of the Stokes wave that can be achieved via SBS. We account for linear loss and nonlinear loss of third order (two-photon absorption, 2PA) and fifth order, most notably 2PA-induced free carrier absorption (FCA). From this, we derive an upper bound for the output power of continuous-wave Brillouin-lasers and show that the optimal operating conditions and maximal realisable Stokes amplification of any given waveguide structure are determined by a dimensionless parameter $\mathcal{F}$ involving the SBS-gain and all loss parameters. We provide simple expressions for optimal pump power, waveguide length and realisable amplification and demonstrate their utility in two example systems. Notably, we find that 2PA-induced FCA is a serious limitation to SBS in silicon and germanium for wavelengths shorter than 2200nm and 3600nm, respectively. In contrast, three-photon absorption is of no practical significance.