Parasitic loss in microring-waveguide coupling and its impact on wideband nonlinear photonics

TL;DR

This paper investigates how parasitic loss in cutoff waveguides affects the efficiency of wideband nonlinear photonic devices, revealing a significant challenge in coupling widely separated frequencies in microring resonators.

Contribution

It demonstrates experimentally and through simulations that cutoff waveguides can still cause parasitic loss at telecom wavelengths, impacting nonlinear device performance.

Findings

Parasitic loss can occur even in cutoff waveguides designed to prevent it.

Such loss can inhibit the threshold in visible-telecom optical parametric oscillation.

Effective coupling at widely separated frequencies remains a major challenge.

Abstract

Microring resonators enable the enhancement of nonlinear frequency mixing processes, generating output fields at frequencies that widely differ from the inputs, in some cases by more than an octave. The efficiency of such devices depends on effective in- and out-coupling between access waveguides and the microrings at these widely separated frequencies. One successful approach is to separate the coupling task across multiple waveguides, with a cutoff waveguide (a waveguide that does not support guided modes above a certain wavelength) being judiciously used to prevent unwanted excessive overcoupling at low frequencies. Here, we examine how such a cutoff waveguide can still induce parasitic loss in the coupling region of a microring resonator, thereby impacting nonlinear device performance. We verified this parasitic loss channel through both experiment and simulation, showing that a waveguide optimized for 532 nm (visible) and 780 nm (near-infrared), while nominally cut off at 1550 nm, can still introduce significant parasitic loss at telecom wavelengths. This is studied in the context of visible-telecom optical parametric oscillation, where the excess parasitic loss can be strong enough to prevent threshold from being reached. Our finding elucidates a major challenge for wideband integrated nonlinear photonics processes when efficient coupling of widely-separated frequencies is needed.