Evolution of Optical Transmission Technology in Data Center With the popularization and application of mobile Internet, data center has developed rapidly and become an important infrastructure in the information society. The data center consists of a large number of servers. High speed and large capacity data transmission and exchange are needed between servers. The traditional cable transmission cannot meet the speed requirements. Optical fiber transmission technology has entered the data center since 2010, and has become the mainstream transmission technology.
In the first chapter (WDM Devices — AWG with Flat Response), the reasons for the Flat Response required, cause for Gaussian Passband, and three main passband optimization proposals are introduced in brief. This chapter is about two other passband optimization proposals.
4) Shaping of Phase Transfer Function Let’s review the proposals of adding MMI at the input and taper at the output. The core feature is to flatten the focused optical field or the eigen mode of the output waveguide. Thus the correlation function between the two optical fields is flattened. Anyway, the correlation between two mismatched optical fields will introduce excess power loss. The more is the mismatch, the more is the power loss. The AWG designers need to balance the passband width and the loss penalty.
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.