Masers and Lasers, 2nd Edition

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Introduction

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

hE

ν=Δ

,

(1.1)

where ΔE is the energy dierence between the two levels, ν the frequency of

emission, and h Planck’s 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

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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 full

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.

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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