Chapter 3
Optical Transmitters
The role of the optical transmitter is to convert an electrical input signal into the cor-
responding optical signal and then launch it into the optical fiber serving as a commu-
nication 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 ad-
vantages offered by them. Some of these advantages are compact size, high efficiency,
good reliability, right wavelength range, small emissive area compatible with fiber-
core 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 con-
tinuously at room temperature became available [1]. Since then, semiconductor lasers
have been developed extensively because of their importance for optical communica-
tions. 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 3.1, LEDs are covered in Section 3.2, while Section 3.3
focuses on semiconductor lasers. We describe single-mode semiconductor lasers in
Section 3.4 and discuss their operating characteristics in Section 3.5. The design issues
related to optical transmitters are covered in Section 3.6.
3.1 Basic Concepts
Under normal conditions, all materials absorb light rather than emit it. The absorption
process can be understood by referring to Fig. 3.1, where the energy levelsE 1 andE 2
correspond to the ground state and the excited state of atoms of the absorbing medium.
If the photon energyhνof the incident light of frequencyνis about the same as the
energy differenceEg=E 2 −E 1 , the photon is absorbed by the atom, which ends up in
the excited state. Incident light is attenuated as a result of many such absorption events
occurring inside the medium.