Focal shifts in thin film filters for MUX/DEMUXs

Optical properties of thin film filters (TFFs) are investigated for the design of multiplexer/demultiplexers (MUX/DEMUXs) in a zigzag configuration. Focal shifts are observed in reflection and transmission of the TFFs, and the focal shifts in reflection can be explained by reflection and refraction effects of the curved surfaces of the TFFs, while the focal shifts in transmission are significantly larger than those by the refraction effects of the curved surfaces. The focal lengths are calculated using a transmission model of TFFs, and it is confirmed that they are equal to the focal lengths obtained from the transmission loss trends.