In this publication we present the results of a detailed study into directly written multimode waveguides for astronomical applications. We show that waveguides up to 100 m across can be inscribed with the cumulative heating form of this technique. The waveguides have 2 concentric guiding regions which are elliptical; a core that has an average ellipticity of 1.1±0.1 and an outer cladding with an ellipticity of 0.15±0.03. It was demonstrated that the ellipticity of the waveguides could be reduced by creating "structured" waveguides which consist of several waveguides stacked together. The 7 mm long waveguides demonstrated insertion losses at 800 nm as low as 39% when light was launched and collected by a standard multimode fibre (50 m core diameter and numerical aperture of 0.12), which is representative of the fibres currently used on astronomical installations. More importantly, we show for the first time that structured waveguides designed to have outer cladding regions which match the dimensions of the core of the launch and collection fibers, have lower insertion losses than structured waveguides designed to have matching core dimensions. It is believed that by moving to longer wavelengths of operation and exploring other structuring and beam shaping techniques it may be possible to reduce the losses even further and make these waveguides of practical use for astronomy.
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