Novel rare-earth-doped planar waveguide lasers

Abstract

The planar guided-wave geometry is compatible with a wide range of materials, waveguide architectures and fabrication techniques. This thesis reports a series of studies of rare-earth-doped planar waveguide lasers, exploiting and extending the diverse range of possibilities that can be applied to planar technology. The areas investigated include the characterisation of guided-wave lasers produced by novel fabrication techniques, the design and implementation of new channel and planar waveguide structures and the demonstration of compact diode-bar pumped planar waveguide lasers.

Laser operation of buried planar waveguides fabricated by a novel solid-state ion-exchange technique based on direct bonding is reported. The fabrication process allows buried waveguide structures to be fabricated by a single-step process, and the resulting guides are shown to have optical losses of < 0.4 dB/cm. Direct UV writing is utilised to fabricate multiple channel sources on this buried platform, and absorbed pump power thresholds as low as 3 mW are demonstrated. These structures offer a new and versatile route to the large-scale production of low-loss components for glass integrated-optics.

Lanthanum fluoride thin-films fabricated by molecular beam epitaxy are investigated. The first laser action from a dielectric waveguide fabricated by molecular beam epitaxy is demonstrated using Nd-doped LaF₃ thin films grown on CaF₂ substrates. Channel waveguide lasers are designed and fabricated on these thin films by two techniques; ion-beam etching and a novel technique that employs a photo-definable polymer. These structures have a maximum phonon-energy of 380 cm^-1, which constitutes the lowest phonon energy dielectric to show laser waveguide emission to date, offering the potential for developing compact mid-infrared sources based on this technology.

The use of planar waveguides for very compact high-power sources is considered. Proximity coupling - the direct coupling of the diode-bar pump radiation is demonstrated for the first time and is used to realise a very compact and rugged laser system. Double-clad planar waveguide structures are utilised for spatial mode-control in the guided axis of the waveguide. To be compatible with planar fabrication techniques, and also due to the desire to retain a compact system, a double-clad design with large multimode core region is used. The theory of fundamental mode selection through confined doping for such structures is developed. This theory is applied to design the first double-clad planar waveguide and diffraction-limited performance in the guided axis is demonstrated from an Yb³⁺ doped device.

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