Page 261 - Tunable Lasers Handbook

6 Transition Metal Solid-state Lasers 1

energy stored in the laser material can be extracted in a single pulse. Pulsed
pumping and Q-switching can generate pulses at prfs up to frequencies on the
order of a kilohertz. To produce even higher prfs, a continuous pump source can
be used. Transition from a pulsed pump to a continuous pump source usually
occurs at a prf on the order of the inverse of the upper laser level lifetime. By
using the appropriate optical switching technique, a train of pulses can be
obtained even though a continuous pump source is used.
At frequencies above the kilohertz range, continuous pumping and repetitive
optical switching can produce a train of laser pulses. At pulse repetition frequen-
cies on the order of kilohertz, continuous pumping and repetitive Q-switching
can be employed to produce the desired prf. If even higher prfs are desired-up
to about a inegaHertz frequency-a technique known as cavity dumping can be
used. Energy storage methods are the primary difference between these two
techniques. With repetitive Q-switching, the energy is stored in the laser mate-
rial. whereas with cavity dumping the energy is stored in the optical field. If
even higher prfs are desired, mode locking can be employed. Mode locking pro-
duces pulses by coupling the various frequencies or modes comprising the laser
output. By coupling the modes, pulses with short pulse lengths are produced at a
frequency associated with the round-trip time interval of the laser resonator.
These coupled modes can produce prfs on the order of 100 MHz. Finally? cw
operation of many solid-state lasers is possible using cw pumping.
Long upper laser level lifetimes, characteristic of most solid-state lasers. are
the key to the large variety of possible pulse formats. The upper laser level life-
times for solid-state lasers can be as long as many milliseconds. Virtually all
other types of lasers have short upper laser level lifetimes, on the order of
nanoseconds. A long upper laser level lifetime allows the optical pump pulse for
the solid-state laser to be long, yet still maintain efficient storage of the pump
energy in the upper laser level. In other types of lasers, the stored energy escapes
from the upper laser level virtually as fast as the pump puts it in, Thus, when
other types of lasers operate pulsed, they act much like a quasi CVJ laser that is
only operating for a short time interval. On the other hand. having a long upper
laser level lifetime allows solid-state lasers to store the pump energy and extract
it in a pulse that is short compared with the pumping time interval.
Having a long upper laser level can also lower the threshold for cw opera-
tion. Analysis shows that the threshold for cui operation is proportional to the
inverse of the lifetime. Thus, if this were the only v'ariable. threshold would be
lower for longer lifetime lasers. Offsetting this is the relationship between the
lifetime and the stimulated emission cross section. In many instances, the prod-
uct of these two factors is approximately constant for a particular laser atom. In
these cases. an increase in the lifetime indicates a decrease in the stimulated
emission cross section. Consequently, in these cases. the threshold tends to be
independent of the lifetime to first order.