High energy passive Q-switching of an erbium fiber laser using a nonlinear liquifying gallium mirror

Abstract

It has been recently reported that the reflectivity of a gallium:glass interface becomes strongly nonlinear at temperatures close to, but below gallium's melting point of ~30°C. The nonlinearity is attributed to a surface-assisted, optically-induced structural phase transition from the stable phase of solid gallium (alpha-gallium) to some other, as yet unidentified, metastable phase of higher reflectivity. It has been observed both in the visible and the near infrared, and characterised fully at 1550 nm. Optical intensities of -1kW/cm are sufficient to change the optical reflectivity of a gallium:glass interface from 50% to 65% at 1550nm, with a turn-on time shorter than 10ns, and temperature dependent turn-off time in the range 10-1000 ns. Initial experiments had shown that this nonlinearity could be used for passive Q-switching of an erbium fiber laser ring cavity. Using a 15m cavity and a fiber butt-coupled gallium mirror we obtained 1 - 2 ps pulses with peak powers of 0.5 W. Whilst this was an important result, the first practical demonstration of the concept, the laser performance itself was not particularly impressive. In this paper we present more recent results with a more optimized cavity which show that liquefying gallium mirrors can be used to generate much shorter pulses of much higher intensity and indeed give comparable results to those achieved with state-of-the-art semiconductor saturable absorbers.

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Identifiers

ORCID for P. Petropoulos: orcid.org/0000-0002-1576-8034

ORCID for D.J. Richardson: orcid.org/0000-0002-7751-1058

ORCID for N.I. Zheludev: orcid.org/0000-0002-1013-6636

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