Masers and Lasers, 2nd Edition

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7

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.

1

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.