As we know, optical fiber communication is one of the enabling technologies for Internet and changed the world. The advantage of optical fiber communication is to transmit dozens of wavelengths in a single optical fiber, which is called wavelength-division multiplexing (WDM). The basic devices for WDM transmission are optical filters, which can be realized by fused biconical taper (FBT), thin film filter (TFF), arrayed waveguide grating (AWG) and optical interleaver. TFF and AWG are most commonly used in the WDM system. This paper discusses TFF-based WDM devices.
Thin Film Filter
Fabry-Perot interferometer (FPI) is a commonly used interferometer for spectrum filtering. The structure of a FPI is shown in Fig.1, which consists of two glass plates spaced by a spacer with precise thickness. The inner surfaces of the plates are coated for partial reflection and the outer surfaces are usually anti-reflection (AR) coated.
Optical Passive Components 