The role of photomask resolution on the performance of arrayed-waveguide grating devices

The crosstalk performance of an arrayed-waveguide grating (AWG) multi-/demultiplexer is primarily caused by random optical phase errors introduced in the arrayed-waveguides. Since the layout of waveguides on a wafer is patterned by photomask through photolithography process, the resolution of a photomask has a direct influence on the phase errors of an AWG. This paper presents a theoretical analysis on the phase error caused by photomask resolution along with other basic design parameters. Both calculation and measurement results show that a high-resolution photomask (better than 25 nm) is a critical requirement to produce low-crosstalk (less than -30 dB) AWG demultiplexers. We also investigated the non-ideal power distribution in the array waveguides since it contributes considerable phase errors when material impurity is not well controlled during wafer fabrication. Basic criteria of power profile truncation, number of grating waveguides, and material index variation are summarized in this paper as well.