Orthogonally Polarized Kerr Frequency Combs

TL;DR

This paper demonstrates the generation of orthogonally polarized Kerr frequency combs within a single microresonator, revealing interactions between combs and novel polarization dynamics with potential applications in photonics.

Contribution

It reports the first observation of two orthogonally polarized combs coexisting and interacting in a single microresonator, including the mechanism and characteristics of the second comb.

Findings

Second comb generated with as little as 0.1 mW seed power

The second comb exhibits a concave envelope due to frequency mismatch

Repetition rates of the two combs coincide despite different free spectral ranges

Abstract

Kerr optical frequency combs with multi-gigahertz spacing have previously been demonstrated in chip-scale microresonators, with potential applications in coherent communication, spectroscopy, arbitrary waveform generation, and radio frequency photonic oscillators. In general, the harmonics of a frequency comb are identically polarized in a single microresonator. In this work, we report that one comb in one polarization is generated by an orthogonally polarized soliton comb and two low-noise, orthogonally polarized combs interact with each other and exist simultaneously in a single microresonator. The second comb generation is attributed to the strong cross-phase modulation with the orthogonally polarized soliton comb and the high peak power of the intracavity soliton pulse. Experimental results show that a second frequency comb is excited even when a continuous wave light as a "seed"-with power as low as 0.1 mW-is input, while its own power level is below the threshold of comb generation. Moreover, the second comb has a concave envelope, which is different from the sech2 envelope of the soliton comb. This is due to the frequency mismatch between the harmonics and the resonant frequency. We also find that the repetition rates of these two combs coincide, although two orthogonal resonant modes are characterized by different free spectral ranges.