Biophotonics_Concepts_to_Applications

4.4.1 Basic Laser Actions

A lasing medium can be a gas, a liquid, a solid-state crystal, a semiconductor, or an
opticalfiber. Despite their material and physical differences, the basic principle of
operation is the same for each type of laser. Laser action is the result of three key
processes: boosting electrons (or molecules) to a higher energy level by a method
such as photon absorption, a fast decay process of the excited electron to a slightly
lower metastable energy level, and stimulated emission. The simple energy-level
diagram in Fig.4.11represents these three processes. Here E 1 is the ground-state
energy and E 2 and E 3 are two excited-state energies. According to Planck’s law as
described in Sect.2.3, a transition of an electron between the ground state and an
excited state can occur through the absorption of a photon. Vice versa, the transition
from an excited state to a lower state results in the emission of a photon.
For lasing action the lasing material mustfirst absorb energy from some external
source. The external energy raises electrons in the material atoms from a
ground-state energy level E 1 to a higher energy level E 3. Because E 3 is an unstable
state, through a fast decay transition the electrons drop quickly (in the order of fs) to
a lower metastable energy level E 2. This process thus can produce apopulation
inversionbetween levels 1 and 2, which means that the electron population of the
excited states is greater than in the ground state. The process of raising electrons to
higher energy levels is calledpumping. Pumping can be done either electronically
or optically.
From the E 2 state the electron can return to the ground state by emitting a photon
of energy hν 12 =E 2 −E 1. This process can occur without any external stimulation
and is calledspontaneous emission. The electron also can be induced to make a
downward transition from the excited level to the ground-state level by an external
stimulation such as an incoming photon. If a photon of energy hν 12 impinges on the
system while the electron is still in its excited E 2 state, the electron is immediately
stimulated to drop to the ground state and gives off a photon of energy
hν 12 =E 2 −E 1. This emitted photon is in phase with the incident photon, and the
resultant emission is known asstimulated emission. Now the incident and emitted
photons together can induce further stimulated emissions, which can lead to a lasing
action.

Energy level 3

Energy level 2

Energy level 1

Fast

decay

Light emission at 21 > 13

because E 21 = hc/ 21 < E 13 = hc/ 13

Pumping

at 13

Fig. 4.11 Pumping of
electrons to higher energy
levels to produce a population
inversion and the ensuing
light emission

106 4 Fundamentals of Light Sources