WDM systems I

Module 19 : WDM Components

Lecture : WDM Components - I

	WDM Components:
	The essential components of a WDM system are primarily those of any network, viz., transmitters, link and receivers. In addition, the system would require other components such as switches, modulators, amplifiers etc. In case of WDM technology, the transmitters are laser sources with stable tunable wavelengths. Before sending the signal through the link, multiplexers mix the wavelengths. Link is low loss optical fiber while at the receiver end there are photo detectors and wavelength demultiplexers. As we have earlier discussed sources and detectors in detail, in this module we will concentrate on components which are specific to the WDM core technology.
	Optical Couplers:
	Optical couplers are devices which split light to divert them into multiple paths or combine light from multiple paths to channel them into a single path. Light signal propagates differently from electric signal. An electric signal passes through a receiver to the ground. However, a light signal is absorbed by a receiver so that if one puts a series of optical receivers at the output end almost no signal will get past the first receiver. Thus it is necessary to split the beam and put the receivers in a parallel fashion.

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	In Directional couplers , light energy generally flows in one direction though they are capable of allowing flow in the other direction as well. For instance, in the Y-shaped $1\times 2$ coupler shown here, a signal arriving at port 1 would be distributed to port 2 and 3 and would travel from left to right. However, if a signal arrives at port 2 (or 3), it would only go to port 1 because of geometry. Directional couplers can be designed such that a predetermined percentage of optical power is output into a particular port. Star couplers are passive devices which connect multiple inputs with i multiple outputs. Star couplers can be both directional and non-directional. Couplers may be designed to be wavelength selective which channel different wavelengths in different directions. These are used in making wavelength division multiplexers and demultiplexers.