US Patent 7324718 Optical device and optical module

A PD array is used in an optical module for monitoring signal light. In the optical module, when monitoring is conducted, signal light that passes through an optical fiber array is branched by a branching section disposed in the optical fiber array, wherein branched light from the branching section is detected by a photodetector region. The PD array is mounted on the optical fiber array. On the back of the base of the PD array, a coating film for preventing degradation in characteristics is formed, wherein the coating film includes an anti-reflective multilayer film.