Theoretical and experimental study of stimulated and cascaded Raman scattering in ultra-high-Q optical microcavities

TL;DR

This paper combines theoretical modeling and experimental validation of stimulated Raman scattering in ultra-high-Q silica microcavities, demonstrating low-threshold nonlinear optical phenomena and comparing spherical and toroid microcavity performance.

Contribution

It provides a comprehensive analysis of SRS in microcavities, including threshold, efficiency, and cascading, validated by experiments in the telecommunication band.

Findings

Thresholds below 100 μW achieved in high-Q microcavities

Good agreement between theory and experiment on SRS properties

Microtoroids exhibit single-mode Raman oscillation over wide pump ranges

Abstract

Stimulated Raman scattering (SRS) in ultra-high-Q surface-tension-induced spherical and chip-based toroid microcavities is considered both theoretically and experimentally. These microcavities are fabricated from silica, exhibit small mode volume (typically 1000 $\mu m^{3}$) and possess whispering-gallery type modes with long photon storage times (in the range of 100 ns), significantly reducing the threshold for stimulated nonlinear optical phenomena. Oscillation threshold levels of less than 100 $\mu $% -Watts of launched fiber pump power, in microcavities with quality factors of 100 million are observed. Using a steady state analysis of the coupled-mode equations for the pump and Raman whispering-gallery modes, the threshold, efficiencies and cascading properties of SRS in UHQ devices are derived. The results are experimentally confirmed in the telecommunication band (1550nm) using tapered optical fibers as highly efficient waveguide coupling elements for both pumping and signal extraction. The device performance dependence on coupling, quality factor and modal volume are measured and found to be in good agreement with theory. This includes analysis of the threshold and efficiency for cascaded Raman scattering. The side-by-side study of nonlinear oscillation in both spherical microcavities and toroid microcavities on-a-chip also allows for comparison of their properties. In addition to the benefits of a wafer-scale geometry, including integration with optical, electrical or mechanical functionality, microtoroids on-a-chip exhibit single mode Raman oscillation over a wide range of pump powers.