Optical modulators convert information carried by an electric current in an electromagnet into light. According to the properties of the material that are used to modulate the light beam, modulators are divided into two groups: absorptive modulators and refractive modulators. The beam may be carried over free space, or propagated through an optical waveguide (optical fibre). Optical modules typically have an electrical interface on the side that connects to the inside of the system and an optical interface on the side that connects to the outside. This document describes the basic principles of coherent optical modulation schemes used in Dense Wavelength Division Multiplexed (DWDM) networks. In this case, it is light, in order to encode the binary information.
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