Origin of the high-finesse limit below lasing threshold at low detuning

Determine the physical origin of the observed high effective-finesse limit that appears below the lasing threshold at low detuning in the coherently driven 357 m SMF-28 fiber ring Kerr resonator with distributed counter-propagating Raman gain at 1455 nm and continuous-wave driving at 1555 nm. The aim is to identify the mechanism responsible for this upper bound on effective finesse in the low-detuning regime while the net round-trip gain remains below the lasing threshold.

Background

The paper demonstrates temporal cavity soliton generation in a 357 m SMF-28 fiber ring resonator whose effective finesse is tuned using distributed counter-propagating Raman amplification. Effective finesse values up to approximately 800 are achieved, enabling low-threshold soliton formation and low-repetition-rate frequency comb generation.

When analyzing soliton existence versus effective finesse and detuning, the authors find that the minimum finesse threshold at a fixed detuning agrees with established cavity soliton theory. However, at low detuning they observe an unexpected upper limit on the effective finesse that occurs before the lasing threshold is reached. The mechanism producing this high-finesse limit is not established in the work and is suggested to possibly arise from experimental constraints, motivating further investigation.