Silicon Nitride Waveguides and Spot Size Converters with < 1.76 dB Loss Over Broad Wavelength Range from 1010 nm to 1110 nm for OCT Applications

In this work the first spot size converters for edge coupling into the silicon nitride waveguide platform with coupling loss lower than 1.76 dB in the wavelength range from 1010 nm to 1110 nm are presented. The spot size converters consist of a SU-8 waveguide matching the mode field diameter of a single mode fiber. High quality end facets of SU-8 are prepared by carefully dicing through the SU-8 waveguides without edge polishing. The SU-8 waveguide mode is converted to the Si3N4 waveguide mode by an inverse taper. The Si3N4 taper tips are narrower than 200 nm, which is required to avoid reflections. They are fabricated using cost efficient i-line projection lithography with a nominal resolution of 500 nm. The Si3N4 waveguide propagation losses are as low as 0.4 dB/cm. Such low loss broadband photonic platform can find use in e.g. retinal optical coherence tomography. The measured coupling loss is comparable to the best available devices for longer wavelengths.