Tapered waveguide lasers

Hettrick, Simon James (2003) Tapered waveguide lasers. University of Southampton, Faculty of Engineering, Science and Mathematics, Optoelectronics Research Centre, Doctoral Thesis, 223pp.

Record type: Thesis (Doctoral)

Abstract

The work reported in this thesis concerns the design and fabrication of tapered waveguide lasers to allow efficient, high-power diode-pumping and produce diffraction-limited, near-symmetrical output, whilst maintaining a monolithic design compatible with integrated optical circuits.

The design rules required for efficient operation of a tapered waveguide are derived, and applied to the fabrication of tapered waveguides by thermal ion exchange (TIE) in Nd-doped borosilicate glass. Ti:sapphire pumped laser operation of these waveguides indicates efficient (40%) and low-loss (0.2dBcm^-1) operation, comparable to that of a channel waveguide fabricated within the same substrate. Parabolic and linear taper shapes are compared, and the linear shape is found to provide superior operation at wide taper expansions (up to 250µm, over a length of ~ 22mm), and hence is more suited to end-pumping with multi-watt broad-stripe-diodes. Broad-stripe-diode end-pumping is also investigated, and the laser performance is characterised, indicating a need to control the refractive index profile to maximise the overlap of the multimode-pump and single-mode laser. Field-assisted ion exchange is therefore investigated to improve the overlap of the diode-pump and laser modes in a tapered waveguide. Waveguides produced by this technique are shown to exhibit the desired step-like index profile, but require further optimisation to improve the laser performance.

Low-loss (0.1dBcm^-1) operation of a tapered waveguide laser is reported in a hybrid Nd-doped/undoped borosilicate glass. The hybrid glass is proposed as a useful substrate for reducing the quasi-3-level reabsorption loss of Yb³⁺-doped lasers. As an initial step towards this goal, channel waveguides are fabricated in standard Yb³⁺-doped phosphate glass, which exhibit efficient (50-75%) and moderate-loss (0.6dBcm^-1) laser operation.

The design requirements for a power scalable diode-side-pumped tapered waveguide laser are also investigated, and YVO₄ is proposed as a suitable substrate material due to its high absorption coefficient. The indiffusion of Nd, Gd and Ti is characterised for the first time, and the fabrication of the first Nd-indiffused waveguide in YVO₄ is reported.