Page 322 - Electrical Properties of Materials

304 Lasers

12.6.2 The gaseous discharge laser
When a current is passed through a gas, as happens in a fluorescent lamp or
a neon sign, most of the charged particles making up the current come from
gas atoms that have been ionized by collision. But as well as completely dis-
possessing atoms of their electrons, the collisional process causes some bound
electrons to gain extra energy and go into a higher state, that is, a state de-
scribed by higher quantum numbers. You will remember that we had a formula
for the simplest gas, hydrogen, in Chapter 4:

13.6
E n =– . (12.34)
n 2
This shows that there is an infinite number of excited states above the ground
state at –13.6 eV, getting closer together as the ionization level (0 eV) is
approached.
In the helium–neon laser the active ‘lasing’ gas is neon, but there is about 7–
10 times as much helium as neon present. Consequently, there are quite a lot of
helium atoms excited to states about 20 eV above the ground state (Fig. 12.5).
Now helium atoms in these particular states can get rid of their energy in one
favourable way—by collision with other atoms that also have levels at the same
energies. Since neon happens to have suitably placed energy levels, it can take

over the extra energy making the population of the upper levels (3a ,3b )more
numerous than that of the lower level (2 ), and thus laser action may occur. It

is, of course, necessary to adjust gas pressures, discharge tube dimensions, and
current quite critically to get the inverted population; in particular it is obtained
only in a fairly narrow range of gas pressures around 1 Torr.
The reflectors are external to the tube, as shown in Fig. 12.6. Note that
the windows are optical flats, oriented at the Brewster angle, θ B , in order to

21
3a 3a′
Collisions
20
3b 3b′
Fig. 12.5
The energy levels of interest for a 1.15 μm 632.8 nm
helium–neon laser. Helium atoms get 19
excited to levels 3a and 3b due to the Energy (eV)
impact of accelerated electrons. Neon 2′
atoms, which happen to have the Electron
18 impact

same energy levels (3a ,3b ) collide

with helium atoms and take over the
extra energy. Laser action may now
occur at two distinct wavelengths,
corresponding to radiative transitions

from levels 3a and 3b to a lower 0 1 1′

H u i l e m N o e n
level 2 .