Tunable Lasers Handbook

Transition Metal

Solid-State Lasers

Norman P. Barnes

NASA Langley Research Center

Hampton, Virginia

1. INTRODUCTION

Solid-state lasers are becoming the laser of choice for many diverse applica-

tions. Selection of solid-state lasers is based on their performance capabilities,

such as available wavelengths, efficiency, tuning range, reliability, and pulse for-

mat flexibility. These performance capabilities can be directly attributed to spec-

troscopic properties that are often unique to solid-state lasers.

Solid-state lasers can be subdivided into two broad categories, transition

metal lasers and lanthanide series lasers. The spectroscopic and performance

properties of these two broad categories of solid-state lasers are considerably

different. Transition metal lasers have active atoms that come from the fourth

row in the periodic table of the elements, whereas lanthanide series lasers have

active atoms that come from the sixth row. Although many properties of these

two categories are similar, spectral absorption and emission characteristics and,

thus, tunability are significantly different. Transition metal lasers are usually tun-

able over a relatively wide spectral range, whereas the tuning range of lanthanide

series lasers is relatively limited. It is the intent of this chapter to concentrate on

the transition metal lasers.

Tunable Lasers Handbook

Copyright

9

1995 by Academic Press, Inc. All rights of reproduction in any form reserved.

219

220 Norman P. Barnes

Tunability of transition metal solid-state lasers, a prime reason for their

selection, results from the interaction of the transition metal active atom with the

crystal field of the laser material. The electrons that participate in the lasing

process in transition metal lasers are the 3d electrons; the electrons that partici-

pate in the lasing process in lanthanide series lasers are the 4f electrons. Because

of the electronic configuration, the 3d electrons interact strongly with the crystal

field of the laser material, whereas the 4f electrons do not. It is the strong inter-

action of the electrons with the crystal field that produces the tunability. Tuning

ranges can be a very large fraction of the center wavelength. For example, the

ratio of the tuning range to the peak gain wavelength of Ti:A1203 is about 0.5,

providing one of the largest tuning ranges of any laser.

Solid-state lasers operate best in the near-infrared region of the spectrum,

from about 0.7 to 2.1 ktm. Operation at shorter wavelengths tends to be limited by

the lifetime of the upper laser level and laser material considerations. Material

considerations are associated with the pumping process because solid-state lasers

are optically pumped almost exclusively. Because the pump wavelength is almost

always shorter than the laser wavelength, the laser material must be transparent at

wavelengths considerably shorter than the laser wavelength. Because most optical

materials begin absorbing in the near ultraviolet, finding a laser material with the

requisite transparency becomes increasingly difficult as the laser wavelength moves

from the near infrared into the visible. On the other extreme, long-wavelength

operation of solid-state lasers is limited primarily by lifetime and quantum effi-

ciency considerations. As the laser wavelength becomes longer, an increasing

fraction of the excited laser atoms is lost to nonradiative decay processes. Non-

radiative decay processes deplete the upper laser level population density without

the emission of a photon, which, in turn, decreases the lifetime and quantum effi-

ciency. Nonradiative decay processes make it increasingly more difficult to create

a high upper laser level population density and thus reach threshold. High thresh-

olds limit the laser efficiency and if the thresholds are too high, eventually prevent

operation of solid state at the longer wavelengths lasers altogether.

Solid-state lasers offer a large variety of pulse formats, ranging from single

pulses with very large energies to continuous wave (cw) operation. At low pulse

repetition frequencies (prfs), solid-state lasers can be excited or pumped with a

pulsed source. Laser output pulse lengths can range from time intervals com-

mensurate with the pump pulse length to pulse lengths that are a tiny fraction of

the pump pulse length. When the laser operates with pulse lengths controlled by

the pump pulse length, the mode of operation is referred to as

normal mode.

To

obtain the short laser pulse lengths, and the high peak power associated with

them, an optical switch or Q-switch is employed. During most of the pump pulse

the Q-switch prevents lasing. However, because of the long lifetime of the upper

laser level, the pump energy can be stored in the upper laser level efficiently.

Near the end of the pump pulse, the Q-switch is opened and the majority of the