Optical Transmitters The role of the optical transmitter is to convert an electrical input signal into the corresponding optical signal and then launch it into the optical fiber serving as a communication channel. The major component of optical transmitters is an optical source. Fiber-optic communication systems often use semiconductor optical sources such as light-emitting diodes (LEDs) and semiconductor lasers because of several inherent advantages offered by them. Some of these advantages are compact size, high efficiency, good reliability, right wavelength range, small emissive area compatible with fibercore dimensions, and possibility of direct modulation at relatively high frequencies. Although the operation of. | Fiber-Optic Communications Systems Third Edition. Govind P. Agrawal Copyright 2002 John Wiley Sons Inc. ISBNs 0-471-21571-6 Hardback 0-471-22114-7 Electronic Chapter 3 Optical Transmitters The role of the optical transmitter is to convert an electrical input signal into the corresponding optical signal and then launch it into the optical fiber serving as a communication channel. The major component of optical transmitters is an optical source. Fiber-optic communication systems often use semiconductor optical sources such as light-emitting diodes LEDs and semiconductor lasers because of several inherent advantages offered by them. Some of these advantages are compact size high efficiency good reliability right wavelength range small emissive area compatible with fibercore dimensions and possibility of direct modulation at relatively high frequencies. Although the operation of semiconductor lasers was demonstrated as early as 1962 their use became practical only after 1970 when semiconductor lasers operating continuously at room temperature became available 1 . Since then semiconductor lasers have been developed extensively because of their importance for optical communications. They are also known as laser diodes or injection lasers and their properties have been discussed in several recent books 2 - 16 . This chapter is devoted to LEDs and semiconductor lasers and their applications in lightwave systems. After introducing the basic concepts in Section LEDs are covered in Section while Section focuses on semiconductor lasers. We describe single-mode semiconductor lasers in Section and discuss their operating characteristics in Section . The design issues related to optical transmitters are covered in Section . Basic Concepts Under normal conditions all materials absorb light rather than emit it. The absorption process can be understood by referring to Fig. where the energy levels E1 and E2 correspond to the ground state and the excited
