Flexible waveguide coupling probe for wafer-level optical characterization of planar lightwave circuits

A probe that enables optical coupling to planar lightwave circuits (PLCs) is described. A flexible waveguide is used to form a variable length directional coupler that extracts light from a waveguide in the wafer to the probe. Varying the length enables optimal coupling to be achieved for a wide range of probe-to-waveguide gap, materials, widths and cladding thicknesses present on a chip. In this paper we consider the use of SU-8 polymer as waveguide test probe and polydimethylsiloxane (PDMS) as cladding for the probe. The results indicate that this approach is ideal for characterizing PLC´s as the 3dB bandwidth of the probe covers the whole 1300 - 1700 nm fiber-optic telecommunication range. Coupling length control in the range of 50 - 200 mm leads to maximum coupling in excess of 80% for the range of conditions investigated.