Flat-focal-field integrated spectrometer using a field-flattening lens

We present a new flat-focal-field arrayed-waveguide grating (AWG) design that utilizes an integrated field-flattening lens placed in the second star coupler. The effective index difference between slab and lens region is obtained by introducing a thin silicon nitride (SiN) layer to a silicon oxynitride environment. Depending upon the SiN layer position, two different lens designs are implemented. As a proof of concept two 81-channel AWGs, one with and one without the lens, are designed, fabricated, and characterized for each lens design. The measurements show that the adjacent crosstalk at the peripheral channels is improved by 2 dB, an improvement which is predicted to become more pronounced for AWGs with higher number of output waveguides (e.g., ~16 dB for 200 output waveguides). Only 0.4 dB of extra excess loss is introduced by the lens.