Mysterious frequency combs in erbium-doped fiber lasers at low temperatures

TL;DR

This paper reports the discovery of unusual, ordered frequency combs in erbium-doped fiber lasers at low temperatures, with nonuniform spacings that diminish as temperature approaches 14 K, suggesting a novel origin related to multi-lasing lines and gain broadening.

Contribution

It introduces a new observation of nonuniform frequency combs in fiber lasers at low temperatures and proposes a novel explanation involving inhomogeneous gain broadening and multi-lasing lines.

Findings

Frequency combs contain ~240 lines with decreasing spacing from ~25 GHz to ~100 MHz.

Combs are observed between 3 K and 14 K, disappearing at 14 K.

Proposed origin involves multi-lasing lines and gain broadening effects.

Abstract

We report on the observation of puzzling nonuniform, but ordered, frequency combs in erbium-doped fiber lasers at low temperatures, between 3 K and 14 K. At ~14 K the combs disappear. The combs contain ~240 lines and have nonuniform frequency spacings that starts from ~25 GHz at ~1540.5 nm and decreases, almost monotonically, to ~100 MHz within (1.8-2) nm. We discuss possibilities that they result from enlarged regular mode-comb spacings, from prime numbers-based solitary waves free of four-wave mixing or result from gain gratings. However, we think that the combs originate from multi-lasing lines allowed in inhomogeneous gain broadening, dominant at low temperatures, together with a small power dependent homogeneous broadening part, responsible for the spacing between the lines.