26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

Active loss compensation in a synchronously driven fibre Kerr resonator

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral ANZOS | Photonics and Optics (ANZCOP)

Speaker

Wenhao Liu (Institute of Photonics and Optical Science (IPOS), School of Physics, University of Sydney, NSW 2006, Australia. ARC Centre of Excellence for Optical Microcombs for Breakthrough Science (COMBS), School of Physics, University of Sydney, NSW, Australia)

Description

Driven optical Kerr resonators have attracted considerable interest as platforms for nonlinear dynamics, frequency comb generation and temporal cavity solitons [1,2]. While microresonators have achieved high quality factors, implementing intra-cavity dispersion control and spectral filtering remains challenging and can introduce loss, increasing the power required. These challenges have motivated active cavity designs, where intracavity gain compensates roundtrip losses while operating below the lasing threshold [3]. However, the use of an intracavity gain medium purely as a loss compensation element in driven Kerr resonators remains less explored.

Here we investigate a synchronously driven fibre ring resonator incorporating erbium-doped fibre (EDF) as the gain element. The resonator is driven by a train of 100-ps pulses with a repetition rate matched to the cavity roundtrip time, while the EDF length and pump power are chosen to keep the cavity below the lasing threshold. Increasing the EDF pump power, we observe narrowing of the cavity resonance linewidth, corresponding to an increase in the effective finesse from 7 in the uncompensated cavity to approximately 25. Further improvement is expected through optimisation of the EDF length.

Under suitable drive power and drive-cavity detuning, we observe modulation instability patterns with pulsed driving. The measured MI sideband frequency near onset and its evolution with drive power are qualitatively consistent with linear stability analysis. These patterns are observed with gain compression levels from 0.5 to 2.5 dB relative to the small-signal gain, suggesting that MI dynamics are robust against gain saturation. These results demonstrate that intracavity gain can improve the effective finesse of an otherwise lossy resonator and offer a flexible testbed for studying frequency comb dynamics.

References
[1] T. J. Kippenberg et al., Science 361, eaan8083 (2018).
[2] F. Leo et al., Nat. Photon. 4, 471–476 (2010).
[3] N. Englebert et al., Nat. Photon. 15, 536–541 (2021).