Characterization of Low Loss Waveguides Using Bragg Gratings

Abstract

A novel approach is developed for measuring small losses in highly transparent Si3N4/SiO 2 waveguides on a silicon chip. The approach is particularly applicable to waveguides written by high-resolution patterning techniques, such as e-beam lithography, whose lengths cannot be easily increased beyond several centimeters. This method is based on measuring the transmission of an optical cavity formed by two highly reflective ( R at least 0.999) simple Bragg gratings and a uniform waveguide between the two gratings whose length can be varied to increase the loss fitting accuracy. A theoretical model based on an ABCD matrix method is developed and used for the final loss value fitting. Experimentally, a cavity with extinction ratio over –70 dB and quality factor Q = 1.02×106 is realized. The fitting results show a waveguide loss of 0.24 ± 0.01 dB/cm and a grating loss of 0.31 ± 0.01 dB/cm. These results are obtained with relatively high index contrast ( Δn > 0.001) gratings with 0.1-pm wavelength scanning resolution. It is expected that with better design and wavelength scanning technique, this approach is applicable more generally to measure waveguide loss coefficients as low as 0.001 dB/cm.