As the refractive index of silica glass depends on its temperature, the operation of basic PLC devices also depends heavily on temperature. With AWGs  – one of the most common types of PLC devices – this temperature dependence can seriously impair a device’s performance outside of a narrow temperature range. Specifically, a temperature change affects the waveguides’ refractive index and therefore their effective lengths – which means the propagation angles out of the waveguide are not matched to couple efficiently into the target waveguide(s). So an AWG device needs additional processing to reduce its sensitivity to temperature when used in a purely passive system.