Caltech researchers, led by Alireza Marandi, demonstrate a chip-scale optical frequency comb source using topological solitons in a degenerate optical parametric oscillator based on lithium niobate.

In this system, the DOPO produces a frequency comb with components at half the input frequency, and the output field can exist in two phase states (+1 and -1) that form a topological boundary known as a dark pulse or topological soliton.

The DOPO leverages strong quadratic nonlinearity to achieve nonlinear effects with less stringent resonator quality and easier fabrication compared with cubic-nonlinearity approaches.

Published in Nature, the work points to simplified on-chip frequency comb generation and broader potential applications beyond the lab, with ongoing work to further characterize soliton behavior and explore new uses.

The topological soliton appears as a stable, ultrafast pulse of about 60 femtoseconds in time, corresponding to a broad half-frequency comb that can extend into mid-infrared regions when starting from near-infrared lasers.

As a proof-of-concept, researchers directly coupled an electrically driven laser diode to the DOPO chip, achieving a two-soliton state and a soliton crystal state with 16 evenly spaced dark pulses.