The tap-couplers use a proprietary coating process to selective reflects a small amount of light from the input. As a tap monitor, they possess low PDL and a flat wavelength response across the erbium gain spectrum. They are specifically designed to be used in conjunction with erbium-doped amplifiers and other polarization sensitive applications. As the equipment there is three types, PIC Tap-couplers, PWIC Tap-couplers, PWFC Tap-couplers. For details please refer to them.