Development and optical characterization of vertical tapers in SiON waveguides using gray-scale-lithography

We describe the development of optical waveguides and adiabatic tapers based on SiON-glasses for a lab-on-chip device with fluorescence optical detection. Sensing is based on evanescent near-field excitation of biomolecules captured on the surface of a thin waveguide. First, the composition of SiON waveguides was optimized to reduce losses for visible light. Waveguides with refractive index of ~1.63 showed propagation losses of ~0.8-0.9 dB/cm at 633 nm. A low loss adiabatic taper was developed to convert efficiently the light from a multi mode waveguide into a thin mono-modal one. Design of the taper was done by calculating numerically the transmission efficiency using a finite-difference time-domain method (FDTD). Simulation results show that losses lower than ~1 dB are obtained for taper lengths of 100 to 300 micron, which indicates an efficient mode conversion. Based on this, tapers of different lengths were realized by grayscale lithography and by reactive ion etching. Their optical testing shows best insertion losses of ~1 dB at 633 nm for multimode to monomode waveguide transitions.