Physics and Engineering of Radiation Detection

1.6. General Properties and Sources of Particles and Waves 37

ground state, the system is said to have reachedpopulation inversion. Laser light is
emitted for as long as this population inversion is maintained.
We saw earlier in the section on radioactive decay that the rate of spontaneous
emission is proportional to the number of nuclei in the sample. In the case of
stimulated emission this rate is proportional to the product of the number of atoms
or molecules of the lasing medium and the radiation densityρ(ν) of the incident
photons.

∂N

∂t

=−B 21 ρ(ν)N (1.6.2)

HereB 21 is a constant known as Einstein’sBcoefficient and depends on the type
of atoms. Fig.1.6.6 shows the principle of operation of a typical gas laser. To make
stimulated emission possible, energy mustbe provided from some external source.
This so calledpumpcan be a simple light source. A semi-transparent mirror at the
exiting end and an opaque mirror at the other end of the laser cavity reflects enough
light to maintain the population inversion. A focusing lens at the other end is used
for further collimation of the laser light.

Lasing Medium

Pump

Laser Beam

Focusing Lens

Semi−Transparent Mirror

Mirror

Figure 1.6.6: Principle of lasing action in a gas.

Lasers can be produced either as continuous waves or in the form of short dura-
tion pulses by a variety of materials in different states. Following are some of the
common types of lasers.

Gas Lasers

The active medium of such a laser is in gaseous atomic, molecular, ionic, or
metallic vapor form.

Atoms: A very common example of such a laser is He-Ne laser. It emits

different wavelengths of laser, such as 632.8nm, 1152nm, and 543.5nm.


Molecules: These lasers are produced by molecules of gases such as carbon

dioxide and nitrogen. Simple organic molecules, such asCF 4 , can also be

employed for lasing action. However since these molecules have very narrow

energy widths, they must be pumped with another laser, such asCO 2 .The

wavelengths of these lasers fall in the far infrared region of the spectrum. Lasers

can also be produced by molecules in short-lived excited states. These so

called Excimer (excited dimer) lasers are generally produced by bonding a noble