Page 262 - Tunable Lasers Handbook
P. 262
222 Norman P. Barnes
Solid-state lasers can operate with reasonable efficiency, even if conven-
tional optical pumping techniques are employed. Solid-state lasers were initially
optically pumped by flashlamps. The very first laser [l], a Cr:A1,0, or ruby
laser, was pumped with a flashlamp similar to the flashlamps used for photo-
graphic purposes. The first lasers were very inefficient, but substantial progress
has been macle even with these optical pump sources. Commercial Nd:YAG
lasers, using flashlamp or arc lamp optical pumping, operate with an electrical to
optical efficiency in the approximate range of 0.01 to 0.05. A primary reason for
this limitation to the efficiency is the poor spectral match of the flashlamp emis-
sion spectrum with the absorption spectrum of the laser material. Because of the
poor spectral match, much of the flashlamp radiation is not absorbed by the laser
material and therefore does not contribute to the laser output. Transition metal
lasers can be more efficient than lanthanide series lasers in this respect because
they can have broad absorption as well as broad emission bands. Broad absorp-
tion bands are more efficient absorbers of the wide spectral bandwidth emission
from the lamps used for optical pumping.
Efficient absorption of flashlamp radiation can be enhanced by using more
than one species of atom in a laser material. Absorption of the optical pump
radiation can be performed by one type of atom and the absorbed energy can be
efficiently transferred to another type of atom that participates in the lasing
process. The former is referred to as a sensirizer and the latter is referred to as
the active atom. Through the use of sensitizers, often transition metal atoms, the
efficiencies of solid-state lasers can be increased by a factor of 2 or more. One
example of such a laser is the Nd:Cr:GSGG laser [2]. Cr, with its broad absorp-
tion bands, is the sensitizer and Nd is the active atom.
With the advent of light-emitting and laser diodes, the prospect of even
more efficient solid-state lasers was realized [3-51. While light-emitting diodes
were used initially: laser diodes, with their narrower spectral bandwidth and
emission angles, have become the norm. Laser diodes have an advantage over
flashlamps by concentrating the optical pump radiation in a relatively narrow
spectral band. By matching the laser diode emission with the absorption bands
of the solid-state laser material, virtually all of the optical pump radiation from
the laser diode can be absorbed by the laser material. Using laser diodes for opti-
cal pumping can increase the efficiency of solid-state lasers, particularly lan-
thanide series lasers, by a factor that may approach an order of magnitude.
Because lanthanide series lasers are often used as optical pumps for transition
metal lasers. increases in the efficiency of the former can have a beneficial effect
on the latter.
The efficiency of solid-state lasers, both in cw and pulsed modes of opera-
tion, is enhanced by the favorable stimulated emission cross section. Efficient
lasers should have stimulated emission cross sections in the midrange, about
10-23 m2. Many solid-state lasers can meet these requirements. If the stimulated
emission cross section of a laser is too large. energy stored in the laser material