Liquid combs: broadband light with equidistance but without stability

Broadband light sources with equidistant modes represent a key capability in advancing the fields of spectroscopy, imaging, and communications. Semiconductor lasers designed for broadband coherent multimode operation often exhibit states that appear to have low coherence. These states, identified by a broad RF spectrum with a full width at half maximum typically in the MHz range, have thus received considerably less attention. Here, for the first time, we show that these states, despite lacking long-term stability, possess high mutual coherence and maintain equidistance. We refer to these states as liquid combs. We develop a frequency-resolved characterization technique that proves that the temporal phase differences between the modes of a liquid comb vary identically. Since liquid combs often possess a wider bandwidth than typical frequency combs, they are attractive for spectroscopic applications and may enable more flexible designs of broadband sources.