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3

Lasers

3.1 Introduction

LASER is an acronym for light amplification by stimulated emission of radiation. In 1917, Einstein postulated
that an atom in the excited level is stimulated to emit radiation that has the same frequency and phase as the
radiation [1]. This is known as stimulated emission, which remained a theoretical curiosity until Schewlow
and Townes in the USA [2], and Basov and Prokhorov in the USSR, proposed that stimulated emission can be
used in the construction of lasers. Townes, Gordon, and Zeiger built the first ammonia MASER (microwave
amplification by stimulated emission of radiation) at Columbia University in 1953. This device used stim-
ulated emission in a stream of energized ammonia molecules to produce amplification of microwaves at a
frequency of about 24 GHz. In 1960, Maiman demonstrated the ruby laser, which is considered to be the first
successful optical laser [3]. In 1962, the first semiconductor laser diode was demonstrated by a group led by
Hall [4]. The first diode lasers were homojunction lasers. The efficiency of light generation can be significantly
enhanced using double heterojunction lasers, as demonstrated in 1970 by Alferov and his collaborators [5].
Fiber-optic communications would not have progressed without lasers. Lasers have not only revolutionized
fiber-optic communications, but also found diverse applications in laser printers, barcode readers, optical data
recording, combustion ignition, laser surgery, industrial machining, CD players, and DVD technology.
In this chapter, we first discuss the basic concepts such as absorption, stimulated emission, and spontaneous
emission. Next, we analyze the conditions for laser oscillations. After reviewing the elementary semiconduc-
tor physics, the operating principles of the semiconductor laser are discussed.

3.2 Basic Concepts
Consider two levels of an atomic system as shown in Fig. 3.1. Let the energy of the ground state be E and
1
that of the excited state be E .Let N and N be the atomic densities in the ground state and excited state,
2
1
2
respectively. If radiation at an angular frequency
E − E 1
2
= , (3.1)
ℏ
where ℏ = h∕(2), h = Planck’s constant = 6.626 × 10 −34 J ⋅ s, is incident on the atomic system, it can inter-
act in three distinct ways [1].
(a) An atom in the ground state of energy E absorbs the incident radiation and goes to the excited state
1
of energy E , as shown in Fig. 3.2. In other words, an electron in the atom jumps from an inner orbit to an
2
Fiber Optic Communications: Fundamentals and Applications, First Edition. Shiva Kumar and M. Jamal Deen.
© 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.