In optical telecommunication of today, wavelength division multiplexing and demultiplexing (WDM) is extensively used for increasing the accessible bandwidth in a single fiber. In WDM, a series of discrete wavelengths are transmitted through the same fiber (the bus), each one them encoded individually. Adjacent channels are separated from one another by 200 GHz, and typically are modulated to carry signals with 10 or 40 GB/s of information. Over one hundred separate wavelengths (or channels) can be carried on a fiber simultaneously, which means that terabytes of data can be carried on a single fiber. The key issue in WDM systems is finding ways to add and drop individual wavelengths from the fiber while letting the rest pass on to their ultimate destination. This add/drop process requires optical multiplexers and demultiplexers. Fig. 14.1 shows a schematic representation of an optical add/drop multiplexer. A single waveguide carrying a number of discrete wavelengths enters the multiplexor. It is desired that one channel be extracted (“dropped”) while the rest pass through without loss. Conversely, it is necessary that information at one particular wavelength be able to be put onto the waveguide without interfering with the other channels This is called an “add”. Finding an effective and inexpensive way to create ad/drop filters for WDM applications is a major issue today. In this chapter we will look at some of the key technologies involved in this problem, including the fiber Bragg grating, Mach-Zender interferometers, and Hi-Q resonators made from integrated waveguide structures.
KeywordsFiber Bragg Grating Cavity Mode Wavelength Division Multiplex Ring Resonator Couple Mode Theory
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