Abstract

Upconversion lasers are unconventional sources of short wavelengths generation: visible radiation can be achieved with the use of infrared pump sources. Er³⁺-based upconversion lasers can, for example, emit in the green spectral range, when pumped at ~ 970 nm. Nevertheless, two restrictions have to be observed: non-radiative decay of the emitting manifold must be prevented and a high pump fluence is required for the multiple absorption process. Both conditions can be satisfied in waveguides written in fluoride crystals. The host has a low phonon energy and the waveguide geometry allows for high pump intensities over the whole crystal length. Er³⁺-doped fluoride crystals have been demonstrated as efficient upconversion bulk lasers [1] and fluoride based fibers [2] as a possible application in the waveguide geometry. The common scheme of femtosecond-laser inscription of waveguides in fluorides is a cladding geometry [3, 4]. The first crystalline Er:LiLuF₄-upconversion waveguide laser is reported in [5]. The poor stability and efficiency of the green emission were caused by non-optimized cavity and fabrication parameters. In this paper, the mentioned drawbacks were tried to be overcome and an improvement in stability and output power is shown by the application of a circular cladding geometry. An output power of 10 mW at 552.6 nm could be achieved, with an incident pump power on the crystal of 1.5 W at 974 nm (Fig. 1 (right)).

© 2013 IEEE