  In this work, the radiation responses of silicon photonic passive devices
built in silicon-on-insulator (SOI) technology are investigated through high
energy neutron and 60Co gamma-ray irradiation. The wavelengths of both
micro-ring resonators (MRRs) and Mach-Zehnder interferometers (MZIs) exhibit
blue shifts after high-energy neutron irradiation to a fluence of 1*1012 n/cm2;
the blue shift is smaller in MZI devices than in MRRs due to different
waveguide widths. Devices with SiO2 upper cladding layer show strong tolerance
to irradiation. Neutron irradiation leads to slight changes in the crystal
symmetry in the Si cores of the optical devices and accelerated oxidization for
devices without SiO2 cladding. A 2 um top cladding of SiO2 layer significantly
improves the radiation tolerance of these passive photonic devices.
