
Types of Lasers 71.1 Gas Lasers 1037
the near UV through the visible part of the spectrum.
In the electrical-discharge excitation process, electrons
collide with neutral atoms in their ground states, trans-
ferring enough energy to ionize them and leave the
ions in several possible excited states. For example, low
discharge currents produce Ar
+
giving rise to visible
emission lines, while high discharge currents produce
Ar
2+
giving rise to UV emission lines. Radiative emis-
sion then occurs to lower excited levels of the ions,
followed by subsequent spontaneous emission to the
ground state of the ion, and then radiationless relaxation
back to the neutral atom ground state. This transition
scheme limits the wall plug efficiency to about 0.1%
for visible and 0.01% for UV operation. Heat manage-
ment is accomplished through either water cooling or
air cooling.
In the visible region of the spectrum, argon lasers
have blue and green emission lines with the strongest
ones at 488 and 515 nm, respectively. Krypton lasers
have several strong emission lines in the green and red,
with the most prominent ones at 521, 568, and 647 nm.
Mixed gas lasers can produce all of these lines. In the
near UV, argon has a strong laser line at 351 nm as well
as several other emission lines down to 275 nm. The
same methods for selecting specific laser lines on neu-
tral atom lasers, discussed in Sect. 71.1.1, are used for
ion lasers. These lasers can operate at powers of over
20 W of cw emission in the visible and at powers of sev-
eral watts cw in the UV. The Doppler-limited linewidth
of noble gas ion lasers is generally ≈ 5–10 GHz. By
using special techniques for stabilization of the cavity,
linewidths of ≈ 500 MHz with drifts of 100 MHz/hr can
be obtained. By mode-locking argon or krypton lasers
with 10 GHz linewidths, it is possible to produce trains
of pulses with pulse lengths of 100 ps and peak powers
of 1 kW, at a pulse repetition frequency of 150 MHz.
Cavity-dumping produces narrow pulses at pulse repe-
tition frequencies of ≈ 1 MHz with peak powers over
100 times the cw power. Lower power cw lasers of this
type typically have beam divergences of 1.5 mrad and
a stability of 5%, while high power lasers have beam
divergences of ≈ 0.4 mrad with a stability of 0.5%.
71.1.3 Metal Vapor Lasers
These lasers can operate with either neutral atoms or
ions. Their excitation process begins with vaporizing
a solid or liquid to produce the gas for lasing, followed
by normal electrical discharge pumping. Either cw or
pulsed operation can be obtained, with laser emission
lines in the near UV and visible spectral regions.
One important ion laser of this class is the Helium–
Cadmium laser. For the excitation processes, metallic
Cd is evaporated and mixed with He. Then a d.c. elec-
tric discharge excites the He ions and ionizes the Cd.
The excited He atoms transfer their energy to the Cd
atoms and the laser transitions take place between elec-
tronic levels of the Cd atom. The main emission line of
a He–Cd laser is the blue line at 441.6 nm. This typ-
ically has a cw output from 130 mW for single-mode
operation up to 150 mW for multimode operation. The
laser linewidth can be as narrow as 0.003 nm. This sys-
tem also has an important laser emission at 325.029 nm,
which typically has cw powers between 5 and 10 mW
single-mode and 100 mW for multimode emission. The
wall plug efficiency is between 0.002% and 0.02%.
The most important neutral atom metal vapor laser
of this type is the copper vapor laser. This has impor-
tant emission lines in the green at 510.55 nm and in the
yellow at 578.21 nm. These lasers operate in the pulsed
mode with temporal pulse widths between 10 and 20 ns
at pulse repetition rates of up to 20 000 pps. Typical pulse
energies are ≈ 1 mJ, yielding average powers of 20 W. It
is possible to increase the repetition rate significantly to
achieve average powers of 120 W or even higher. How-
ever, this laser is self-terminating since the lower levels
of the laser transitions are metastable. This restricts the
pulse sequencing of the laser and requires fast discharge
risetimes. Copper vapor lasers have high gain (10% to
30%/cm), and very high wall plug efficiency (≈ 1.0%).
Gold vapor lasers have similar properties but operate in
the red at 624 nm at several watts of power.
71.1.4 Molecular Lasers
There are several types of molecular lasers that can be
classified with respect to their spectral emission range,
their mode of excitation, or the energy levels involved
in the lasing transition. In the far IR, molecular lasers
operate on transitions between rotational energy levels.
These include water vapor lasers that emit between 17
and 200 µm, cyanide lasers at 337 µm, methyl fluoride
lasers emitting between 450 and 550 µm, and ammonia
lasers that operate at 81 µm. These are generally excited
through optical pumping by a CO
2
laser. They are built
with a metal or dielectric wave guide cavity. The former
design results in lower thresholds but gives multimode,
mixed polarization output, while the latter design re-
sults in propagation losses, giving higher thresholds but
linearly polarized outputs.
CO
2
lasers are some of the most widely used, with
a variety of medical and industrial applications. They
Part F 71.1