T   tracing out the history of the development of the
fundamental ideas on which masers and lasers are based.
Although this development has been deeply impressed in the minds of
all of the early researchers in this eld, it may well escape the knowledge of
young researchers approaching this area for the rst time. Graduate students
in physics and electronics can prot from such knowledge, and the level of
exposition has been chosen accordingly. Whenever possible, original authors
have been allowed to speak for themselves through their papers, and short
biographies of the leaders in the eld have been included. In this second edi-
tion, the development of lasers has been retrieved up to the present, including
nonlinear optics, ultrashort pulses, and some special issues.
1.1 Principle of Operation
Masers and lasers have in common their principle of operation, which is based
on the use of stimulated emission of electromagnetic radiation in a medium of
molecules or atoms, with more particles in the upper (excited) state than in the
lower state (i.e., with an inverted population).
An electron bound to a molecule or an atom may change its energy state
by jumping from one energy level to another with the emission or absorption
of a photon, in which process it will have, respectively, lost or gained energy.
If the particle is initially in an upper state with respect to the fundamental
ground state, it decays spontaneously to the ground state by the emission of a
photon of energy
where ΔE is the energy dierence between the two levels, ν the frequency of
emission, and h Plancks constant.
is is the normal emission process (spontaneous emission) which takes
place every time a material de-excites itself having been suitably excited. Due
Masers and Lasers
to the random nature of spontaneous decay, photons are emitted by various
particles in an independent way and the resultant emission is incoherent.
e probability of spontaneous emission increases with the cube of fre-
quency. It is, therefore, negligible in microwave transitions wherein thermal
relaxation processes are predominant.
If electromagnetic radiation is present with a frequency so as to full
Equation 1.1, two processes can be distinguished:
1. e photon interacts with a particle which is in its lower energy
level: in this case, radiation is absorbed and the particle is forced to
go to the upper level.
2. e photon interacts with a particle which is already in an upper
state. In this case, the particle is forced to go to the lower energy
state by emitting another photon of the same frequency as the inci-
dent photon. is process is called stimulated emission.
In spontaneous emission, both the direction and polarization of photons
are randomly distributed, whereas in the case of stimulated emission they
coincide with those of the incident photon.
In general, the particles of an ensemble in equilibrium are more in the lower
energy level, according to the Maxwell–Boltzmann distribution law. By making
the system interact with radiation of frequency equal to the dierence between
two levels (the ground and the excited levels), processes of type (1) and (2) take
place simultaneously, with a prevalence of the former because more particles are
in the lower state. However, if, in some way, the distribution between levels is
altered, so that more particles are present in the excited state than in the ground
state (one usually says that an inversion of population has been obtained), then
in the interaction process between the radiation and the particles, a net excess
of emitted photons will take place over the absorbed photons, that is, an ampli-
cation process occurs.
e radiation emitted this way will be monochromatic—because it is emit-
ted in correspondence with a well-dened transition—and coherent—because
it is a forced emission which produce at the end an amplication of the wave.
is is the fundamental working principle common to masers and lasers.
1.2 The Devices
Masers are devices emitting in the microwave region and, therefore, molecu-
lar roto-vibrational states or atomic Zeeman levels are usually used. Lasers are
devices emitting in the infrared to ultraviolet region and they can, therefore,
use either molecular or atomic levels.
Once suitable energy levels have been chosen, the next problem is how to
obtain an appreciable amount of radiation and how then to couple that radia-
tion eciently to the particles. is is achieved by the use of a suitable reso-
nant cavity, that is, a microwave cavity for masers and an optical cavity for
lasers. e eect of such a cavity is not only to increase the residence time