Program gain and loss for broadband soliton microcombs

TL;DR

This paper introduces a meta-coupler design for silicon nitride microresonators that significantly broadens soliton microcomb spectra and enhances power output by exploiting programmable gain and loss mechanisms.

Contribution

It presents a novel meta-coupler that enables programmable control of gain and loss, improving soliton microcomb bandwidth and power without extra pump power.

Findings

Broader soliton spectra nearly doubled 3 dB bandwidths

Up to 12 dB higher central comb-line power

Up to fivefold increase in emitted comb power

Abstract

Soliton microcombs provide compact, broadband, coherent light sources for precision metrology, spectroscopy, communications, and microwave photonics. Extending their spectral span while retaining useful output power remains challenging and often requires impractically high pump power. Existing approaches mainly tailor the dispersion and pumping conditions, but they do not exploit the coupling spectrum as a programmable aspect of soliton operation. Here we introduce a meta-coupler whose lithographically programmed coupling spectrum concentrates strong pump access near the pumped resonance while leaving most comb lines close to the intrinsic loss rate. Si$_3$N$_4$ microresonators incorporating a meta-coupler exhibit broader circulating soliton spectra, nearly twofold larger 3 dB soliton bandwidths, up to about 12 dB higher central comb-line power, and up to about fivefold greater emitted comb power, without an additional pump-power penalty. Our work unlocks gain and loss as simultaneous programmable knobs for realizing high-performance soliton microcombs.