ABSTRACT
Under coherent transmission it is essential that the linewidth of the laser source remain as stable and narrow as possible so that the mixing with a local oscillator laser would give the baseband signals. This chapter deals with the modulation formats employed to manipulate the phase and amplitude of the electromagnetic fields of lightwave sources by electro-optic modulators. It presents the techniques for generation of lightwaves and modulation techniques of lightwaves, directly or externally. The chapter describes the modulation techniques for optical communication systems. Advanced modulation formats have recently attracted much attention for enhancement of the transmission efficiency since the mid-1980s for coherent optical communications. The electro-absorption modulator employs the Franz and Keldysh effect, which is observed as lengthening the wavelength of the absorption edge of a semi-conductor medium under the influence of an electric field. Integrated electro-optic modulator is an optical modulation device in which the lightwaves are confined to optical waveguides.
