Tapered InP/InGaAsP waveguide structure for efficient fibre-chip coupling

A novel passive InP/InGaAsP waveguide structure for low-loss coupling of monomode fibres to semiconductor devices having waveguides with small elliptical modes has been fabricated. The device consists of a fibre-matched waveguide, a tapered waveguide structure for the necessary mode transformation, and a small spot waveguide. The transformation of the fibre mode into a mode with a spot of 2.0 mu m lateral and 1.5 mu m vertical extension (FWHM) is demonstrated. Nearly polarisation independent transmission losses, as low as 4.9 dB, are measured for uncoated devices with a 900 mu m long tapered section.