Ion implanted crystal waveguide lasers

Abstract

This thesis reports the first use of He⁺ implantation to create integrated optic waveguide lasers, by modifying the refractive index of bulk laser host crystals. He+ ion implantation produces a variety of refractive index changes within different target (implanted) crystals which can be used to create planar and channel optical waveguides with propagation losses of the order of 1dB/cm. The spectroscopic properties of the laser impurity dopant in the target crystal are generally not altered drastically by the implantation. The fluorescence lifetime is typically unchanged in the waveguide compared to the bulk crystal, but the fluorescent emission lines are broadened to an extent dependent on the implanted ion dose used to fabricate the waveguide. The wide ranging applicability of the fabrication technique has been demonstrated, with planar waveguide laser action reported in 5 neodymium doped materials, Nd:YAG, Nd:YAP, Nd:MgO:LiNbO₃, Nd:Bi₄Ge₃O₁₂ and Nd:GGG. Diode-pumped laser operation has been demonstrated. with absorbed power thresholds of around 10mW, slope efficiencies up to 30% and output powers up to 40mW.
The fabrication of gold ion-stopping masks several microns thick using photolithographic patterning has allowed the creation of the first index enhancement ion implanted channel waveguides, with channel waveguide laser action demonstrated in Nd:YAG and Nd:GGG. Very low laser thresholds of around 500µW absorbed power have been obtained with diode-pumping of Nd:YAG, comparable with the best results achieved with single-crystal fibre growth. Slope efficiencies of 30% have been achieved in both Nd:YAG and Nd:GGG.
This thesis also reports the first use of ion implantation to create waveguides in tunable laser crystal hosts, Alexandrite and Cr:YSAG. The spectroscopic properties of the chromium laser impurity are found to be little changed by the waveguide fabrication process, offering hope for the future creation of a tunable channel waveguide laser. In addition, the phenomenon of optically written channel waveguides within ion implanted planar waveguides in Bi₄Ge₃O₁₂ is reported here. Launching a moderate power (~50mW) laser beam into a He⁺ implanted planar BGO waveguide creates a permanent channel waveguide structure. Extension of this writing technique to a beam incident on the planar waveguide top surface could allow the production of complex integrated optic circuits, and results of initial experiments of this type are presented.

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