Tunable Solid-State Lasers
`
`
`PETER F. MOULTON, SENIOR MEMBER,
`
`ll:El:'
`
`Invited Paper
`
`/rare
`Tunable [users based on i()r1—(/(rpm! so/id-stale mcditt
`emerged from the laboratory into crrrrirrtertiitl proz/limo/1 in the
`/(IS! decade, and have found significant application in p/t0tonic—
`systerrzs resmrt:/'1. In the future,
`they may be (1 ('riIi('(1/ crmtporzcrtl
`of space—bnsed remote sensing svstenis. communicutrons S)‘.\l(’)71.S
`and laser medicine. In this article we review the basic p/ivsics
`of tunable wlidas/nit» [avers and tliscitss /he C/1((1‘tI('((’I‘i.S'l‘iiL'S oftlzc
`most sigrzifictmt esmblislzed and emerging .ry.s‘te1n.s'.
`
`I.
`
`lN'rRoou<:TioN
`
`In the last decade tunable solid-state lasers have become
`
`an increasingly significant component of quantum ClCC-
`tronics. While they were first demonstrated in the l96()‘s,
`they have only recently emerged from being a laboratory
`curiosity to playing key roles in variety of electro—optics
`systems. Compared to what, until recently, was the most
`widely used tunable system, the liquid—rnedium dye laser,
`solid-state laser media offer unlimited operating and “shell”
`lifetimes along with the capability to store energy and thus
`generate high peak powers via Q—switching.
`In addition,
`for some systems one can obtain either a broader tuning
`range than a given dye or an extension to longer infrared
`wavelengths. Current applications of tunable solicl—state
`lasers include basic scientific investigations in atomic,
`molecular and solid—state spectroscopy, laboratory studies
`of new semiconductor and fiber—optic devices, generation
`of ultrashort pulses and amplification of such pulses to high
`peak powers. Future applications now under development
`include aircraft-and space-based remote-sensing lidars, sub-
`marine communication systems and laser medicine. This
`article will
`review the field by tirst providing a brief
`discussion of the physics of tunable solid-state lasers and
`then examining a variety of laser systems, as categorized
`by the laser—active ion.
`(To be precise. we will cover
`paramagnetioion lasers only; other solid-state laser media
`employing so—called color centers are considered as an
`entirely different class of laser systems.)
`
`ll.
`
`PHYSICAI. BACKGROUND
`
`Solid-statc lasers operate on stimulated transitions be-
`tween electronic levels of ions (activators) contained in
`fvlzmuscript received August 14, l99|; revised October 29. Wu].
`The author is with Schwartz l:'lcctro~Optics. lnc.. Concord. MA (H742.
`ll-IEE Log Number 9t0os33.
`
`solid crystalline or glassy media (hosts). To date, activators
`of practical significance have been positively charged ions
`from the rare Earth or 3d transition—metal groups of the
`periodic table. While all solid—state lasers can operate over
`some range of wavelengths, and thus are tunable,
`it
`is
`common to consider “tunable" lasers as those capable of
`covering a wavelength range greater than several percent of
`the laser central wavelength. In the following we discuss
`some basic laser concepts and consider the mechanisms
`important for making a laser “tunable."
`
`A. Littewidt/I. Cross Section, and Llfi’![I)lV
`
`The physical characteristic determining tuning range is
`the linewidth of the laser transition. Two other quantities
`of general interest to laser design are the gain cross section
`and the lifetime of the upper laser level.
`If we establish, by optical pumping, a population density.
`N (per cm3), of ions in upper laser level. and limit our
`discussion to four—level lasers where the population of ions
`in the lower level can be neglected, then the optical gain.
`G,
`in a laser medium of length i (in cm) is given by the
`expression
`
`G : <~xptcr(/\)iW)
`
`where at/\) is the gain cross section in cmi, as a function
`of wavelength A. The linewidth of the laser transition is a
`measure of the range of wavelengths over which the cross
`section is large, and is usually given as the full-width at
`half—maximum, i.e..
`the span between the half—gain points
`on either side of the peak wavelength. (Unfortunately.
`in
`the ficld of quantum electronics, wavelength and frequency
`are interchanged all to frequently. We will generally stay
`with wavelength as the convention for this paper when
`referring to laser radiation. except for cases where the use
`of frequency is common.)
`the population of the upper laser level. in the absence of
`laser action, decays at a rate determined by the combined
`effects of radiative (or, spontaneous) emission and other
`nonrarliative processes considered below. In simple systems
`the rate is constant, which leads to an exponential decay
`of the population after the pumping is turned off. and a
`characteristic lifetime given by the inverse ofthc decay rate.
`l.it'ctimes for lcvels used in solid-state lasers are in the range
`
`348
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`l’R(')('ljLl)|N(lS OF THE IEEE. VOL. NI), NO. 3. MARCH W‘):
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`()(ll8—‘)3l‘)/‘J2$l)3_tlt) © 1002 lFF.l7,
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`ASML 1118
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`ASML 1118