New setup for end-fire coupling excitation of planar waveguides at low temperatures

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Abstract
This work describes a new experimental setup for end-fire coupling excitation of planar waveguides in the 300–10 K temperature range, which allows us to analyze their optical and spectroscopicproperties through different techniques such as transmission, luminescence, or waveguideRaman spectroscopy. Two main novelties are relevant in this setup; (1) the use of long working distance microscope objectives, placed out of the cryostat, for both coupling and collecting guided light, and (2) the original set of mirrors, combined with different translation and rotation stages, which allows fine x,y,z,θ displacements of the beam focusing point preserving the laser beam alignment along the optical axis of the coupling objective. The setup has been tested in a series of Nd 3+ doped ZBLA ( ZrF 4 - BaF 2 - LaF 3 - AlF 3 ) fluoride glass channel waveguides (from 2 to 40 μm width and 2 μm depth) obtained by ion-exchange and photolithography methods. The Nd 3+ guided luminescence has been analyzed at low temperatures and compared to the spectra obtained for the bulk substrate.