Page 321 - Electrical Properties of Materials

Some practical laser systems 303

realizations could easily take up all our time. I shall be able to do no more than
describe a few of the better-known lasers.

12.6.1 Solid state lasers

The first laser constructed in 1960 was a ruby laser. The energy-level diagram
for the transitions in ruby (Cr ions in an Al 2 O 3 lattice) is given in Fig. 12.3. I re-
marked above that ruby owed its characteristic red colour, to absorption bands
of the complementary colour, green. This absorption is used in the pumping
process. A typical arrangement is sketched in Fig. 12.4, which shows how
the light from a xenon discharge flash tube ‘pumps’ the ruby to an excited
state. Now the emission process is somewhat different here from that which
I sketched previously for three-level systems. The atoms go from level 3 into
level 2 by giving up their energy to the lattice in the form of heat. They spend a
∗
long enough time in level 2 to permit the population there to become greater ∗ Energy levels in which atoms can
than that of level 1. So laser action may now take place between levels 2 and 1, pause for a fairly long time (a few mil-
liseconds in the present case) are called
giving out red light. ‘metastable’.
The ruby itself is an artificially grown single crystal that is usually a cyl-
inder, with its ends polished optically flat. The ends have dielectric (or metal)
mirrors evaporated on to them. Thus, as envisaged in the previous section, the
Pump
resonator comprises two reflectors. Some power is certainly lost by diffraction, levels 3
Non-radiative
but these losses are small provided the dimensions of the mirror are much lar- transitions
ger than the wavelength. It needs to be noted that one of the mirrors must be 2
imperfect in order to get the power out.
3+
Another notable representative of solid-state crystalline lasers is Nd :
Laser
YAG, that is neodymium ions in an yttrium–aluminium–garnet. It is a four- transition
level laser radiating at a wavelength of 1.06 μm, pumped by a tungsten or
Ground state 1
mercury lamp.
Laser operation at the same frequency may be achieved by putting the neo- Fig. 12.3
dymium ions into a glass host material. Glasses have several advantages in Energy levels of the Cr 3+ ioninruby.
comparison with crystals: they are isotropic, they can be doped at high con- The pump levels are broad bands in
centrations with excellent uniformity, they can be fabricated by a variety of the green and blue, which efficiently
processes (drilling, drawing, fusion, cladding), they can have indices of refrac- absorb the flash tube light. Level 2 is
tion in a fairly wide range, and last but not least, they are considerably cheaper really a doublet (two lines very close
than crystalline materials. Their disadvantage is low thermal conductivity, to each other) so that the laser light
consists of the two red lines of
which makes glass lasers unsuitable for high average-power applications.
wavelengths 694.3 and 692.9 nm.

Mirrored ends Ruby rod

Parallel laser
beam
Fig. 12.4
General arrangement of a ruby laser.
The ruby and the flash tube are
Flash tube
mounted along the foci of the elliptic
cylinder reflector for maximum
Elliptic cylinder reflecting container transference of pump light.