Laser Properties and
Progresses in Novel Lasers
7.1 Introduction
In the rst experiments on lasers, some of their principal properties were
observed. In the case of solid-state lasers, one of the most important char-
acteristics was spiking. e emission consisted in thousand short pulses of
a microsecond duration separated by 1-μs or so one from the other, during
about 1-ms emission. e result was that the power emitted in each spike was
in the order of kW even if a total 1 J of energy was emitted, and the emission
occurred randomly in time.
e rst experiments on He–Ne gas laser revealed its mode structure. e
resonators that are used in lasers are inherently multimode devices. e reso-
nant modes that can exist in such devices may be classied as longitudinal and
transverse modes. e longitudinal mode order is determined by the number
of eld variations along the axis of the cavity, while the transverse mode order
is determined by the number of eld variations in the plane of the mirrors. For
each longitudinal mode order, there exist a set of transverse modes. e num-
ber of modes that can partake in the oscillations of a laser is dependent on the
geometry and the losses of the resonator, the width of the atomic resonance of
the active material, and the degree of population inversion. Practically, a laser
will oscillate in several modes simultaneously unless special steps are taken
to suppress the unwanted ones. e result is an emission on a number of dif-
ferent very closely spaced frequencies. Unfortunately, the number of modes
operating at each time was varying randomly which resulted in a uctuating
power emission.
e research focused immediately on the tentative to dominate these eects
and correct them so to have more stable and reliable emissions. We start this
chapter considering rst these eorts.

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