Why Is Flat Response Required?
In the all optical network (AON), the optical signals passed tens of nodes before reaching the destination node, as shown in Fig.1. The ROADM nodes are usually composed of wavelength selective switches (WSS), multiplexers/demultiplexers and optical switches. The wavelength multiplexers/demultiplexers are optical filters, including TFF-based WDM devices, arrayed waveguide gratings (AWG) and optical interleavers.
Why is AWG demanded?
As we know, DWDM technology enables transmission of dozens of wavelengths in a single fiber, which expands the capacity of optical fiber communication enormously. The first mux/demux modules for DWDM system are based on thin-file filters (TFFs), as shown in Fig.1 and Fig.2. Both are designed in serial structure. Different wavelengths travel different number of devices in the module and result in different power loss. The loss uniformity degrades with increment of port number. Meanwhile, the maximum loss at the last port is another limitation on the port number. Thus the TFF-based WDM modules are usually limited to be ≤16 channels.
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.