LASER SOURCES TODAY AND TOMORROW

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LASER SOURCES TODAY AND TOMORROW

M. Weber

To cite this version:

M. Weber. LASER SOURCES TODAY AND TOMORROW. Journal de Physique Colloques, 1987,

48 (C7), pp.C7-3-C7-11. �10.1051/jphyscol:1987701�. �jpa-00226906�

JOURNAL DE P H Y S I Q U E

Colloque C7, suppl6ment au n012, Tome 48, dgcembre 1987

LASER SOURCES TODAY AND TOMORROW M. J. WEBER

Lawrence Livermore National Laboratory, University of California, Livermore, C A 94550, U.S.A.

ABSTRACT

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P r o p e r t i e s o f c u r r e n t gas, l i q u i d and s o l i d - s t a t e l a s e r sources a r e surveyed b r i e f l y , i n c l u d i n g t h e p r o s p e c t s f o r t h e i r f u r t h e r

development. O f p a r t i c u l a r i n t e r e s t a r e r e c e n t advances i n semiconductor d i o d e and t u n a b l e s o l i d - s t a t e l a s e r s and t h e expanding e x p e r i m e n t a l a c t i v i t i e s i n x-ray l a s e r s and f r e e e l e c t r o n l a s e r s . Improved e l e c t r o n beams and p e r i o d i c magnetic s t r u c t u r e s ( w i g g l e r s , undul a t o r s ) a r e t e c h n o l o g i e s t h a t f r e e e l e c t r o n l a s e r s share w i t h advanced s y n c h r o t r o n r a d i a t i o n sources. The t u n a b l e , h i g h - b r i g h t n e s s , p a r t i a l l y c o h e r e n t r a d i a t i o n f r o m t h e l a t t e r w i l l a l s o be u s e f u l f o r many a p p l i c a t i o n s .

I n t r o d u c t i o n

The q u a r t e r o f a c e n t u r y t h a t has passed s i n c e t h e advent o f t h e l a s e r has w i t n e s s e d an a s t o n i s h i n g p r o 1 i f e r a t i o n i n t h e number and t y p e s o f l a s e r sources ( 1 ) . S t i m u l a t e d emission i s observed f r o m a l l s t a t e s o f m a t t e r

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gases, l i q u i d s , s o l i d s and plasmas. Today one can s e l e c t l a s e r sources c o v e r i n g t h e wave1 e n g t h range f r o m microwaves t o s o f t x-rays, o p e r a t i n g c o n t i n u o u s l y o r i n u l t r a s h o r t p u l s e s approaching p h y s i c a l l i m i t s , and h a v i n g o u t p u t e n e r g i e s and powers f o r o s c i l l a t o r - a m p l i f i e r systems e x t e n d i n g t o t h e m u l t i p l e k i l o j o u l e and t e r a w a t t ranges. E f f i c i e n c i e s v a r y f r o m t h e abysmal, f o r x-ray l a s e r s , t o 750% f o r semiconductor d i o d e l a s e r s . S p a t i a l l y , l a s e r beams can be d i f f r a c t i o n 1 im i t e d ; s p e c t r a l l y , f r a c t i o n a l s h o r t - t e r m l i n e w i d t h s o f -10-16 have been o b t a i n e d . A t l o w powers, d i o d e l a s e r s have e x t r a p o l a t e d l i f e t i m e s o f 100-1000 y e a r s . The volume o f a c t i v e l a s i n g media can v a r y f r o m about 1000 l ~ m 3 t o many m3. A1 though t h e c o s t s o f l a s e r s range f r o m r e l a t i v e l y i n e x p e n s i v e t o expensive, mass-produced d i o d e l a s e r s can be v e r y cheap whereas l a s e r s f o r f u s i o n r e s e a r c h and defense can be e x t r e m e l y expensive. A number o f l a s e r extrema a r e g i v e n i n T a b l e I. I t i s an i m p r e s s i v e 1 i s t , one r e s u l t i n g f r o m s e v e r a l decades o f i n t e n s e l a s e r r e s e a r c h and development.

TABLE I . Extreme values o f various laser properties.

P r o ~ e r t y Navel ength Pulse duration Peak power Pulse energy Beam qua1 i t y Spectral linewidth Operating lifetime Dimensions Cost

Value

5 nanometers t o microwaves femtoseconds to continuous wave

>lo0 terawatts

>I00 kilojoules diffraction limited 4 0 mHz (for 1 second)

>I00 years

micrometers t o kilometers several dollars t o the gross national product (1 imi t)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987701

C7-4 JOURNAL D E PHYSIQUE

Today some areas o f l a s e r science and technology a r e mature, w h i l e o t h e r s are s t i l l emerging. I n t h e b r i e f space a v a i l a b l e here, one cannot

comprehensively review e i t h e r present o r f u t u r e l a s e r sources. I s h a l l t h e r e f o r e n e c e s s a r i l y be s e l e c t i v e i n the t o p i c s and l a s e r sources discussed.

The wavelength ranges f o r l a s e r a c t i o n i n various media a r e shown i n F i g . 1. Gas l a s e r s have by f a r t h e l a r g e s t range

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o v e r l a p p i n g t h e r e g i o n o f microwave o s c i l l a t o r s and masers (e.g., C02-laser-pumped methyl bromide l a s i n g a t 2.7 mm) and now extending i n t o t h e s o f t x-ray r e g i o n . This l a r g e range i n v o l v i n g r o t a t i o n a l - v i b r a t i o n a l t r a n s i t i o n s , outer-she1 1 e l e c t r o n i c t r a n s i t i o n s and i n n e r - s h e l l e l e c t r o n i c t r a n s i t i o n s . Although t h e r e a r e some gaps i n t h e coverage, t h e r e i s n e a r l y quasi-continuum coverage i n most s p e c t r a l regions. Using harmonic generation, frequency m i x i n g and Raman s h i f t i n g techniques, l a s e r r a d i a t i o n i s a v a i l a b l e throughout t h e o p t i c a l r e g i o n . The wavelength range o f condensed matter l a s e r s i s much more r e s t r i c t e d , being l i m i t e d by phonon processes ( i n f r a r e d a b s o r p t i o n edge and n o n r a d i a t i v e d e - e x c i t a t i o n ) a t l o n g wavelengths and by t h e fundamental a b s o r p t i o n edge o f t h e h o s t a t s h o r t wavelengths. Although some s l i g h t extension may be p o s s i b l e f o r s o l i d - s t a t e and l i q u i d l a s e r s , t h e o n l y i m p o r t a n t f r o n t i e r f o r s i g n i f i c a n t l y extending t h e wavelength range o f l a s e r sources i s t h e x-ray r e g i o n and beyond t o gamma rays. A m p l i f i c a t i o n by s t i m u l a t e d emission i s i n p r i n c i p l e p o s s i b l e i n m a t e r i a l media a t picometer wave1 engths.

U l t r a v i o i y'sible

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x-ray~ultraviolet- ^{Soft Vacuum} Infrared &Far infrared*

Gas lasers:

w H

5 nm 2.7 mm

Liquid fasers:

M 0.3 pm 1.3 pm Solid-state lasers:

w H

0.17 ~m 32 pm

0.001 0.01 0.1 1 .o 10 100 1000

Wavelength (pin)

F i g u r e 1. Wavelength ranges i n which l a s e r o s c i l l a t i o n has been obtained f o r d i f f e r e n t media.

Gas Lasers

Laser a c t i o n has been r e p o r t e d f o r about 7000 e l e c t r o n i c , v i b r a t i o n a l and r o t a t i o n a l t r a n s i t i o n s i n n e u t r a l , i o n i z e d and molecular gas species. As can b,e seen from Fig. 2, t h e p e r i o d i c t a b l e has been explored e x t e n s i v e l y f o r d e u t r a l and i o n l a s e r s , b u t n o t exhaustively; a d d i t i o n a l l a s i n g species and t r a n s i t i o n s a r e t h e r e f o r e p o s s i b l e . However, o u t o f t h e l a r g e number o f known l a s i n g gases, t h e number of useful, commercially v i a b l e sources i s v e r y much smaller. The p r i n c i p a l commercial gas l a s e r s a r e He-Ne and He-Cd (mu1 t i p l e l i n e s ) , N2, C02, i o n (Ar, Kr) and excimer (ArF, KrF, XeCQ, XeF); o t h e r s o f l e s s general use i n c l u d i n g metal vapor (Cu, Au), i o d i n e , chemical (HF, DF) and f a r i n f r a r e d l a s e r s . The c u r r e n t state-of-the-art and o p e r a t i n g

c h a r a c t e r i s t i c s o f these l a s e r s are thoroughly surveyed i n Ref. 2. Some o f these l a s e r s a r e t e c h n o l o g i c a l l y mature; f o r o t h e r s one may a n t i c i p a t e f u r t h e r improvements i n o u t p u t power and p u l s e energy, e f f i c i e n c y and r e l i a b i l i t y 13).

F i g u r e 2. P e r i o d i c t a b l e showing those elements (shaded) f o r which l a s e r o s c i l l a t i o n has been obtained from n e u t r a l atoms o r i o n s i n gases.

For many a p p l i c a t i o n s , gas l a s e r s a r e being threatened by t h e emerging challenge o f s o l i d - s t a t e l a s e r s which have undergone p r o g r e s s i v e and

s i g n i f i c a n t advances r e c e n t l y . H i s t o r i c a l l y , vacuum tubes i n e l e c t r o n i c s were replaced by s o l i d - s t a t e devices; gas l a s e r s i n o p t i c a l and o p t o e l e c t r o n i c systems mky s i m i l a r l y be replaced by a l l s o l i d - s t a t e l a s e r s . Gas tubes continue t o be used i n numerous special e l e c t r o n i c s a p p l i c a t i o n s ; gas l a s e r s w i l l undoubtedly p l a y a s i m i l a r r o l e i n o p t i c a l a p p l i c a t i o n s .

L i a u i d La~sers

Organic dye l a s e r s , e i t h e r laser-pumped o r flashlamp-pumped, a r e used e x t e n s i v e l y i n research and t e c h n o l o g i c a l a p p l i c a t i o n s where t h e i r t u n a b i l i t y and short-pulse c h a r a c t e r i s t i c s are advantageous. Various f a m i l i e s o f dyes b r i d g e t h e wavelength range from 0.3 t o 1.3 pm ( 1 ) . As noted above, t h e p o s s i b i l i t y o f extending t h i s s p e c t r a l range i s l i m i t e d . However, i n c r e a s i n g t h e s t a b i l i t y o f the n e a r - i n f r a r e d dyes a g a i n s t photodegradation would be an important development.

Dye l a s e r s a r e a l s o threatened by advances i n tunable s o l i d - s t a t e l a s e r s . Several c r y s t a l l i n e paramagnetic i o n and c o l o r c e n t e r l a s e r s having l a r g e t u n i n g ranges i n t h e v i s i b l e - n e a r I R regions have been i d e n t i f i e d and organic dye molecules have r e c e n t l y been i n c o r p o r a t e d i n t o glasses w i t h obvious l a s i n g p o s s i b i l i t i e s . Development o f these l a s e r s o f f e r s t h e a t t r a c t i o n o f no chemical p r e p a r a t i o n o r dye c i r c u l a t i o n pumps and no r i s k o f exposure t o carcinogenic substances.

I n o r g a n i c l i q u i d l a s e r s were i n v e s t i g a t e d i n t h e mid-1960s through t h e mid-1970s. These i n c l u d e d r a r e - e a r t h chelates and r a r e earths i n a p r o t i c (non-hydrogen c o n t a i n i n g ) solvents (1). The l a s i n g p o s s i b i i i t i e s o f the former are l i m i t e d by r a p i d n o n r a d i a t i v e decay processes and t h e l a t t e r m a t e r i a l s were f r e q u e n t l y c o r r o s i v e o r t o x i c . O v e r a l l , prospects f o r these l a s e r s were n o t promisiqg and work on them has waned.

J O U R N A L DE PHYSIQUE

S o l i d - S t a t e Lasers

The p x t several years have seen a renaissance o f a c t i v i t y i n s o l i d - s t a t e l a s e r s . This was e v i d e n t i n t h i s y e a r ' s CLEO Conference (4) and, t o some degree, i n t h i s conference. The a c t i v i t y has been spurred by developments i n semiconductor and i n s u l a t i n g l a s e r m a t e r i a l s i n c l u d i n g high-power l a s e r diodes and diode arrays, new tunable l a s e r s , and more e f f i c i e n t pumping schemes f o r both small and l a r g e l a s e r systems.

Semiconductor Lasers

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These l a s e r s i l l u s t r a t e both e v o l u t i o n a r y and

r e v o l u t i o n a r y advances r e s u l t i n g from improvements i n m a t e r i a l s , p r e p a r a t i o n methods, device s t r u c t u r e s , and o p e r a t i n g techniques (5). Using molecular beam e p i t a x y (MBE) and metal-organi c chemical vapor d e p o s i t i o n (MOCVD)

,

quantum e f f e c t s can be t a i l o r e d i n a r b i t r a r y forms o f l a y e r e d h e t e r o s t r u c t u r e s (quantum we1 1 devices). This has r e s u l t e d i n AlGaAs diodes w i t h CW o u t p u t power >1 W, phased diode a r r a y s w i t h outputs o f tens o f w a t t s and

i n t e n s i t i e s o f several k ~ l c m 2 , low-power diode e f f i c i e n c i e s approaching 80%

and diode a r r a y e f f i c i e n c i e s o f -40%, e x t r a p o l a t e d l i f e t i m e s o f 10-100 years a t low power (3-5 mW) and modulation frequencies >16 GHz a t 300 K. Although these l a s e r s operate i n t h e near i n f r a r e d , 111-V compounds a r e a l r e a d y being developed f o r t h e 600-700 nm r e g i o n and one can e n v i s i o n compound

semiconductors e v e n t u a l l y o p e r a t i n g from t h e n e a r - u l t r a v i o l e t t o t h e f a r - i n f ra r e d

.

Quantum w e l l technology ( 6 ) w i t h i t s u l t r a l o w t h r e s h o l d c u r r e n t s , possi b i 1 i t y f o r wide s p e c t r a l tunabi 1 i t y , and device geometries a p p r o p r i a t e f o r i n t e g r a t i n g o p t i c a l and e l e c t r o n i c s t r u c t u r e s i s p a r t i c u l a r l y promi s i n g and w i l l make p o s s i b l e i n t e g r a t e d o p t o e l e c t r o n i c c i r c u i t s (IOEC) f o r communications and computers.

The i n c r e a s i n g avai 1 abi 1 i t y o f diode 1 asers having h i g h power, h i g h e f f i c i e n c y , l o n g l i f e t i m e , and low c o s t suggests t h a t these w i l l be t h e l a s e r s o f choice f o r the m a j o r i t y o f a p p l i c a t i o n s . I n a d d i t i o n t o t h e i r use as a primary l a s e r source, diode l a s e r s a r e r e p l a c i n g lamps f o r pumping Nd:YAG and o t h e r s o l i d - s t a t e l a s e r m a t e r i a l s ( 4 ) . Simple, compact, r o b u s t diode-pumped Nd:YAG l a s e r s w i t h excel l e n t mode c o n t r o l are a l r e a d y commercially a v a i l a b l e ; w i t h frequency doubling a Nd:YAG l a s e r can produce b l u e , green and r e d outputs. Medium power (kW) s o l i d - s t a t e l a s e r s u s i n g s l a b geometries f o r e f f i c i e n t heat removal and high-power pulsed l a s e r s f o r i n e r t i a l confinement f u s i o n research can a l s o b e n e f i t from diode pumping. The requirements o f t h e l a t t e r f o r 5-10 MJ o f o p t i c a l energy a t a r e p e t i t i o n r a t e o f 5-10 Hz w i t h an o v e r a l l e f f i c i e n c y o f > l o % now appears achievable ( 7 ) .

Tunable Lasers - Tunable s o l i d - s t a t e l a s e r s based on phonon-terminated l a s e r a c t i o n i s another area o f r e v i v e d i n t e r e s t ( 8 ) . The demonstration o f tunable l a s e r a c t i o n f o r ~ r 3 + i n a l e x a n d r i t e has spurred t h e d i s c o v e r y o f tunable

~ r 3 + l a s i n g i n l o w - f i e l d s i t e s i n c r y s t a l s a t wavelengths r a n g i n g from 0.7 t o 1.1 pm. Approximately the same s p e c t r a l range has been covered by

~ i 3 + - d o ~ e d Al2O3. D i v a l e n t V, N i , Cu and ~ e 3 + p r o v i d e tunable l a s e r a c t i o n i n t h e r e g i o n 1.1-2.5 pm and around 0.3 pm, r e s p e c t i v e l y .

Semiconductor l a s e r s (111-V and 11-VI compounds and l e a d s a l t s ) cover t h e s p e c t r a l r e g i o n 0.33 t o 32 pm (1) and can be tuned by v a r y i n g temperature, pressure o r magnetic f i e l d . Glass l a s e r s , because o f inhomogeneous broadening o f the emission, a l s o o f f e r some small (<lo%) t u n a b i l i t y .

Color c e n t e r l a s e r s g e n e r a l l y have l a r g e t u n i n g ranges (9); those based on F2+ and FA centers i n a1 k a l i ha1 id e s now span t h e range 0.8-4 pm. M a t e r i a l s covering o t h e r s p e c t r a l regions i n c l u d e CaO:F+ (350-420 nm), diamond:Hj (-500-600 nm) and sapphire (540-620 nm and 750-900 nm), however o n l y i s o l a t e d r e p o r t s e x i s t f o r these m a t e r i a l s and f u r t h e r s t u d i e s are needed.

Another p o s s i b i l i t y f o r tunable l a s e r s a r i s e s from the a b i l i t y t o

i n c o r p o r a t e o r g a n i c dye mol ecules i n t o glasses u s i n g e i t h e r sol-gel techniques o r compositions having s u f f i c i e n t l y low m e l t i n g temperatures t h a t dye

decomposition does n o t occur. Work i s c u r r e n t l y underway on b o t h of these approaches.

Other Laser P o s s i b i l i t i e s

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Although paramagnetic i o n l a s e r s were among t h e e a r l i e s t developed and a continued procession o f new l a s e r s has appeared, a d d i t i o n a l l a s i n g p o s s i b i l i t i e s e x i s t . Elements t h a t have lased as i o n s i n s o l i d s a r e i n d i c a t e d on t h e p e r i o d i c t a b l e i n F i g . 3 and p r o v i d e l a s i n g from 0.17 pm t o -5 pm. Hosts have i n c l u d e d over 300 c r y s t a l s , innumerable glasses, and g l a s s ceramics (10). The number o f commercial l a s e r s i s again v e r y small (3). T r a n s i t i o n and p o s t - t r a n s i t i o n group elements a r e enclosed by b o l d l i n e s i n F i g . 3. Of t h e t r a n s i t i o n metals, several i r o n group (3dn) i o n s have lased; fluorescence from a number of p a l l a d i u m (4dn) and p l a t i n u m (5dn) group i o n s i s known, b u t o n l y w i t h i n t h e p a s t few years has one o f them been lased ~ h ~ + ) . Fluorescence a l s o occurs from f i l l e d s h e l l i o n s

6

such as Cu+ ( 3 d l ) . Although t h e r e have been several attempts t o observe s t i m u l a t e d emission from Cu+ i n c r y s t a l s and glasses, t h e r e has been o n l y one r e p o r t o f l a s i n g t o my knowledge. Many p o s t - t r a n s i t i o n group i o n s having a ns2 e l e c t r o n i c c o n f i g u r a t i o n (e.g., ~ n 2 + , pb2+, ~ i 3 + ) and molecular

i o n complexes (such as tungstates and vanadates) e x h i b i t broad, i n t e n s e fluorescence bands of i n t e r e s t f o r tunable l a s e r s . I n many cases d e l e t e r i o u s e x c i t e d - s t a t e a b s o r p t i o n may prevent o s c i l l a t i o n , b u t t h i s remains t o be q u a n t i f i e d .

I A V J A

F i g u r e 3. P e r i o d i c t a b l e showing those paramagnetic elements (shaded) f o r which l a s e r o s c i 1 la t i o n has been obtained i n s o l i d s .

The r a r e earths, b o t h d i v a l e n t and t r i v a l e n t ions, have been t h e most e x p l o i t e d elements f o r l a s e r s because o f t h e i r many e x c i t e d s t a t e s having l o n g l i f e t i m e s and narrow emission l i n e s (11). This year t h e s e r i e s was completed w i t h the l a s i n g o f Most r a r e earths have lased on more than one t r a n s i t i o n ; w i t h s e l e c t i v e e x c i t a t i o n and cascade l a s i n g schemes many, many more a r e p o s s i b l e and o n l y remain t o be demonstrated. Ions o f t h e a c t i n i d e s e r i e s p r o v i d e a d d i t i o n a l l a s i n g p o s s i h i , l i ti e s , a1 though l e s s numerous and g e n e r a l l y 1 ess a t t r a c t i v e than t h e lanthanides (12). Coupled-ion systems i n v o l v i n g v a r i o u s i o n p a i r i n g s r e s u l t i n new energy l e v e l schemes and o t h e r l a s i n g p o s s i b i l i t i e s .

JOURNAL DE PHYSIQUE

X-Ray Lasers

During t h e p a s t t h r e e years l a s i n g has been extended i n t o t h e extreme u l t r a v i o l e t and s o f t x-ray regions. The s h o r t l i f e t i m e s o f i n n e r - s h e l l e l e c t r o n i c t r a n s i t i o n s r e q u i r e d t h e use o f high-power, short-pulse l a s e r s t o c r e a t e t h e necessary h i g h temperature plasmas, selected i o n d e n s i t i e s , and p o p u l a t i o n i n v e r s i o n s (by c o l l i s i o n a l e x c i t a t i o n and decay o r recombination cascade processes). Lasing schemes have i n c l u d e d 3p-3s t r a n s i t i o n s o f neon-1 i k e i o n s ( ~ e 2 4 + @9+, ~ o 3 2 + ) , 4d-4p t r a n s i t i o n s o f n i c k e l - 1 i k e i o n s ( ~ u 3 5 + , ~ d 3 6 + , ~ b 4 2 + ) Balmer cr t r a n s i t i o n s of hydrogen-1 i ke i o n s ( ~ 5 + , 07+, ~ 8 + ) and 5f-3d t r a n s i t i o n s of l i t h i u m - l i k e i o n s (A!L~O+) (13).

P r e l i m i n a r y r e s u l t s have been obtained f o r N i - l i k e Yb a t 5.0 nm, t h e s h o r t e s t wavelength l a s e r t o d a t e (14). This i s v e r y c l o s e t o t h e 2.3-4.4 nm

"water-window" o f i n t e r e s t f o r s t u d i e s o f b i o l o g i c a l m a t e r i a l s .

The above l a s e r s have used exploding f o i l s and f i b e r s as t h e a m p l i f y i n g medium and c o n d i t i o n s o f l a r g e single-pass gain because no m i r r o r s were a v a i l a b l e f o r mu1 t i p a s s operation. Recent developments i n x-ray o p t i c s ( 1 9 , such as x-ray m i r r o r s based on s y n t h e t i c m u l t i l a y e r s t r u c t u r e s , should make t h e use o f resonant c a v i t i e s , as f o r o p t i c a l l a s e r s , p o s s i b l e . (A double-pass a m p l i f i e r has a l r e a d y been demonstrated (14).) This w i l l open up t h e

e x p l o r a t i o n o f o t h e r m a t e r i a l s and c o n f i g u r a t i o n s having lower gain.

The use of h i g h e r Z elements and h i g h e r power l a s e r s w i l l l e a d t o even s h o r t e r wavelength Tasing. Lasers such as the 10-beam Nd:glass NOVA l a s e r (7) should be s u f f i c i e n t t o observe s t i m u l a t e d emission from W and Pb (14).

Beyond x-rays a r e gamma-ray l a s e r s i n v o l v i n g t r a n s i t i o n s between s t a t i o n a r y s t a t e s o f nuclear isomers (16). Present e f f o r t s i n c l u d e n u c l e a r spectroscopy t o a c q u i r e t h e fundamental d a t a on energy l e v e l s and o t h e r r e q u i r e d c h a r a c t e ~ i s t i cs o f i s o m e r i c s t a t e s t h a t might be s u i t a b l e f o r a c h i e v i n g s t i m u l a t e d emission. The r a t e o f progress and eventual success a r e h i g h l y u n c e r t a i n a t present.

Free-Electron Lasers

The p a s t decade has been one o f i n c r e a s i n g a c t i v i t y on f r e e - e l e c t r o n l a s e r s (FEL) and progress toward t h e goals o f h i g h e f f i c i e n c y , h i g h power and tunable r a d i a t i o n a t v a r i a b l e wavelengths (17). Coherent r a d i a t i o n has been demonstrated from microwave t o v i s i b l e (525 nm) wavelengths. E f f i c i e n c i e s as h i g h as 40% have been obtained a t 8.6 mm; tapered w i g g l e r magnets have increased t h e e f f i c i e n c y o b t a i nab1 e a t s h o r t e r wave1 engths. An understanding o f w i g g l e r magnetic f i e l d tolerances r e q u i r e d t o extend t h e technology t o very

long w i g g l e r s has a l s o been obtained.

Free e l e c t r o n l a s e r s i n v o l v e t h e d e f l e c t i o n o f r e l a t i v i s t l c e l e c t r o n s from l i n e a r a c c e l e r a t o r s ( i n d u c t i o n o r r f l i n a c s ) , o t h e r a c c e l e r a t o r s and storage r i n g s i n v a r i o u s p e r i o d i c magnetic f i e l d s . I n p r i n c i p l e t h e y a r e s c a l a b l e t o any wavelength and l a s i n g i n t h e UV and VUV should be achievable soon. O p t i c s capable, o f t o l e r a t i n g t h e powers generated and t h e synchrotroh r a d i a t i o n t h a t may be p r e s e n t a r e needed. Using microwave r e a c c e l e r a t i o n of e l e c t r o n s t o compensate f o r t h e g a i n r e d u c t i o n i n tapered w i g g l e r s , conversion e f f i c i e n c i e s i n excess o f 50% a r e t h e o r e t i c a l l y p o s s i b l e .

Present FELs r e q u i r e l a r g e a c c e l e r a t o r s and r a d i a t i o n s h i e l d i n g , f e a t u r e s which l i m i t t h e i r a t t r a c t i v e n e s s as a w i d e l y a v a i l a b l e tunable r a d i a t i o n source. The use o f short-period e l ectromagnetic w i g g l e r s c o u l d s i g n i f i c a n t l y reduce t h e e l e c t r o n beam energy r e q u i r e d t o reach t h e near-IR and v i s i b l e s p e c t r a l regions. Recently g y r o t r o n s and high-power I R l a s e r s have been proposed as w i g g l e r s f o r a FEL (4).

Synchrotron R a d i a t i o n

The n e x t generation o f synchrotron r a d i a t i o n f a c i l i t i e s wi 11 have low- emittance e l e c t r o n beams and many s t r a i g h t sections f o r i n s e r t i o n devices ( w i g g l e r s and u n d u l a t o r s ) , thereby p r o v i d i n g an a l t e r n a t i v e source o f intense, tunable r a d i a t i o n from t h e i n f r a r e d through the x-ray r e g i o n f o r many

a p p l i c a t i o n s (18, 19). These f a c i l i t i e s share w i t h FELs t h e requirements o f h i g h c u r r e n t and small e l e c t r o n beam divergence and t h e technology o f p e r i o d i c magnetic s t r u c t u r e s . High-gai n FELs o p e r a t i n g i n t h e r e g i o n o f s e l f-amp1 i f i ed spontaneous emission and w i t h d e n s i t y modulation o f t h e e l e c t r o n beam produce coherent synchrotron r a d i a t i o n ; u n d u l a t o r r a d i a t i o n from synchrotron sources, i n comparison, i s u s u a l l y an incoherent s u p e r p o s i t i o n o f r a d i a t i o n from i n d i v i d u a l e l e c t r o n s .

Undulator r a d i a t i o n possesses many o f the c h a r a c t e r i s t i c s o f l a s e r r a d i a t i o n

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h i g h i n t e n s i t y , monochromaticity, d i r e c t i o n a l i t and coherence.

I t i s h i g h l y d i r e c t i o n a l w i t h an angular divergence -l/yN1Y2, where y i s t h e e l e c t r o n Lorentz f a c t o r and where N i s t h e number o f p e r i o d s o f t h e magnetic u n d u l a t o r . The s p e c t r a l bandwidth i s -1IN. Undulator r a d i a t i o n i s p a r t i a l l y coherent t r a n s v e r s e l y due t o f i n i te,electron-beam emi ttances. The l o n g i t u d i n a l coherence l e n g t h f o r fundamental r a d i a t i o n i s -NX. I t can be shown (19) t h a t t h e coherent power from an u n d u l a t o r i s p r o p o r t i o n a l t o t h e beam c u r r e n t , N, and 1 3 and i n v e r s e l y p r o p o r t i o n a l t o the e l e c t r o n beam emi ttances. M i 11 iw a t t s o f tunable, p o l a r i z e d , coherent power w i t h a s p a t i a l coherence o f -1 p m should be r e a d i l y achievable a t 5 nm (19). However, because o f t h e cubic dependence on wavelength, t h e r e i s a r a p i d decrease i n coherent power a t s h o r t e r wavelengths.

Synchrotron r a d i a t i o n sources a r e f r e q u e n t l y compared i n terms o f s p e c t r a l b r i g h t n e s s (photonsls-mm2-mr2-0.1% bandwidth). On t h i s b a s i s f u t u r e

undulators w i l l p r o v i d e t h e same o r d e r o f s p e c t r a l b r i g h t n e s s as t h e o r i g i n a l seZ4+ XUV l a s e r o p e r a t i n g a t 100 Hz (20). One must keep t h e intended

a p p l i c a t i o n i n mind and be c a r e f u l when making such comparisons. For example, t h e r e p e t i t i o n r a t e o f XUVIx-ray l a s e r s i s p r e s e n t l y a l o n g way from t h e above assumed 100 Hz. I n terms o f peak s p e c t r a l brightness, l a s e r s should g e n e r a l l y be orders o f magnitude b e t t e r than synchrotron sources (13, 20). I f coherence l e n g t h i s important, l a s e r s may again be s u p e r i o r because o f t h e i r narrower l i n e w i d t h . I n a d d i t i o n , synchrotron r a d i a t i o n f a c i l i t i e s a r e n o t l a b o r a t o r y sources, although e f f o r t s a r e underway t o develop compact synchrotron sources f o r x-ray l i t h o g r a p h y and o t h e r a p p l i c a t i o n s . Thus many f a c t o r s e n t e r i n t o t h e usefulness and c o s t e f f e c t i v e n e s s o f l a s e r versus synchrotron r a d i a t i o n photon sources.

Concl udi na Remarks

A n t i c i p a t i n g tomorrow's 1 aser sources based on e x t r a p o l a t i o n i s r e l a t i v e l y easy; i t i s the unexpected

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t h e s u r p r i s e s

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t h a t a r e u n p r e d i c t a b l e . The p a s t year has witnessed an e x p l o s i v e growth o f a c t i v i t y i n s u p e r c o n d u c t i v i t y worldwide t r i g g e r e d by t h e discovery o f h i g h temperature m a t e r i a l s . One may ask whether a s i m i l a r development could occur i n l a s e r s .

By way o f comparison, some h i s t o r i c a l milestones a r e compared i n F i g . 4.

A p e r i o d o f about 45 years passed between the d i s c o v e r y o f s u p e r c o n d u c t i v i t y and a u n i v e r s a l l y accepted theory; a s i m i l a r p e r i o d passed between t h e f o r m u l a t i o n o f t h e concept o f s t i m u l a t e d emission and Maiman's demonstration o f t h e r u b y l a s e r . Whereas t h e BCS t h e o r y was f o l l o w e d by continued research and p r a c t i c a l a p p l i c a t i o n s o f s u p e r c o n d u c t i v i t y , b u t no s i g n i f i c a n t increase i n c r i t i c a l temperature u n t i l the p a s t year, t h e r u b y l a s e r prompted a f l u r r y o f a c t i v i t y and a s e r i e s o f developments and advances t h a t i s s t i l l

expanding. I n t h i s decade we may have already seen t h e beginning o f a r e v o l u t i o n ( a l b e i t i n a q u i e t e r way and w i t h o u t t h e quantum jump t h a t has

C7-10 JOURNAL DE PHYSIQUE

Discovery BCS High T,

(K. Onnes) theory materials

A "

1911 1957 1987

Lasers

Stimulated emission Ruby

(A. Einstein) laser

A " ???

-

1916 1960

Year

F i g u r e 4. Important milestones i n s u p e r c o n d u c t i v i t y and l a s e r s .

occurred i n superconductivty) i n areas such as t h e p r o d u c t i o n o f quantum w e l l s t r u c t u r e s and t h e r e a l i z a t i o n o f x-ray l a s i n g . But a r e t h e r e o t h e r

unexpected l a s e r d i s c o v e r i e s , say i n energy storage o r e x t r a c t i o n mechanisms, t h a t c o u l d have dramatic consequences? And what are they?

Acknowledaments

Work performed under t h e auspices,of t h e U.S. Department o f Energy by t h e Lawrence Livermore N a t i o n a l Laboratory under c o n t r a c t number W-7405-ENG-48.

References

1. Handbook o f Laser Science and Technolouy: Volume I

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Lasers and Masers;

Volume I1

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Gas Lasers, Weber, M. J., ed.

(CRC

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2. Hecht, J., The Laser Guidebook (McGraw-Hill, New York, 1986).

3. Jacobs, R. R., "Lasers: The Next Q u a r t e r Century," Lasers and A p p l i c a t i o n s (December 1985) 47.

4. D i g e s t o f Technical Papers, Conference o f Lasers and El ectro-Optics (CLEO), Baltimore, Apri 1 1987.

5. See, f o r example, Holonyak, N. and Rediker, R., Ref. 4, p. 94.

6. "Physics and A p p l i c a t i o n s o f Semiconductor Quantum-Well S t r u c t u r e s , "

Chemla, P. S. and Pinczuk, A., eds., J. Quantum E l e c t r o n . QL.22 (1986) 1609; another s p e c i a l i s s u e on t h i s s u b j e c t i s scheduled f o r p u b l i c a t i o n i n August 1988.

7. H o l z r i c h t e r , J. F., "High-Power S o l i d S t a t e Lasers," Nature, 316 (1985) 309.

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9. Mol lenauer, L. F. "Color Center Lasers" i n

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R e i s f e l d , R. and Jorgensen, C. K., Lasers and E x c i t e d States o f

Rare

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Weber, M. J., "Lanthanide and A c t i n i d e Lasers," i n Lanthanide

and

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Key M. H., "Laboratory Production o f X-Ray Lasers," mature, 316 (1985) 314 and references c i t e d t h e r e i n .

Matthews, 0. and McGowan, B. ( p r i v a t e communication).

Underwood, J.

H.

and Attwood, D. T.. "The Renaissance o f X-Ray Optics,"

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Baldwin. G. C.. Solem, J. C. and G o l ' d a n s k i i , V.

I.,

"Approaches t o t h e Development o f Gamma-Ray Lasers," Rev. Mod. Phys. 53 (1981) 687.

For reviews o f f r e e - e l e c t r o n l a s e r s , see IEEE J. Quantum E l e c t r o n .

OE-21

(1985) 804 and references t h e r e i n ; another s p e c i a l i s s u e i s scheduled f o r p u b l i c a t i o n i n August 1987.

Eisenberger, P., "A 6-GeV Storage Ring: An Advanced Photon Research F a c i l i t y , " Science

231

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Attwood, D., Halbach, K . . and Kim, K-J., "Tunable Coherent X-Rays,"

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Rhodes, C. "Lasers i n t h e Vacuum U l t r a v i o l e t and X-Ray Region," i n Report on VUV and X-Ray Sources f o r Atomic and Molecular Science Workshop (National Academy Press, Washington, D.C., 1986) 44.