OPTICAL BISTABILITY IN SEMICONDUCTOR LASER AMPLIFIERS

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OPTICAL BISTABILITY IN SEMICONDUCTOR LASER AMPLIFIERS

M. Adams

To cite this version:

M. Adams. OPTICAL BISTABILITY IN SEMICONDUCTOR LASER AMPLIFIERS. Journal de

Physique Colloques, 1988, 49 (C2), pp.C2-57-C2-61. �10.1051/jphyscol:1988213�. �jpa-00227630�

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JOURNAL D E PHYSIQUE

Colloque C2, SupplAment au n06, Tome 49, juin 1988

OPTICAL BISTABILITY IN SEMICONDUCTOR LASER AMPLIFIERS

M.

J

.

^ADAMS

British Telecom Research Laboratories. Martlesham Heath, GB-Ipswich IP5 7RE, Great-Britain

Re'sune/ - On d i s c u t e l e me'canisme fondamental de l a b i s t a b i l i t g o?tio,ue dans l e s a m p l i f i c a t e u r s l a s e r semiconducteur e t r e v i s e l e " s t a t e - o f - t h e - a r t a c t u e l des gtudes t h g o r i q u e s e t expgrimentaux.

Abstract

-

The b a s i c p h y s i c a l mechanism o f OB i n semiconductor l a s e r a m p l i f i e r s i s d i s c u s s e d and a review given o f t h e c u r r e n t s t a t e - o f - t h e - a r t i n t h e o r e t i c a l and experimental s t u d i e s .

It has r e c e n t l y become c l e a r t h a t OB i n semiconductor l a s e r a m p l i f i e r s h a s many p o t e n t i a l advantages f o r a p p l i c a t i o n s i n o p t i c a l l o g i c and s i g n a l p r o c e s s i n g . Foremost among t h e s e advantages may be l i s t e d ( i ) ready a v a i l a b i l i t y (compared with ohher b i s t a b l e devices [ 11 ), ( i i ) wavelength c o m p a t i b i l i t y with o p t i c a l canmunications systems, ( i i i ) i n h e r e n t o p t i c a l g a i n ( f o r s e r i a l p m c e s s i h g ) , and ( i v ) a conbination of microwatt s w i t c h i n g powers and nanosecond s w i t c h i n g t h e s l e a d i n g t o femtojoule o p t i c a l s w i t c h i n g e n e r g i e s [ 2,31

.

I n support of item ( i i ) on t h i s l i s t , it i s worth n o t i n g t h a t m p l i f i e r OB has been r e p o r t e d a t 0.8 ym [ 41

,

^1.3um [ 51

,

and

1.5 pm [ 61

,

a l l o f which a r e wavelengths emvloyed f o r o p t i c a l communications systems.

I n t h i s paper t h e b a s i c p h y s i c a l mechanism of OB i n semiconductor l a s e r a m p l i f i e r s i s d i s c u s s e d and it i s shown how a r e s o n a n t c a v i t y with gain can be used t o y i e l d s w i t c h i n g e n e r g i e s o f o r d e r femtojoules. A review i s p r e s e n t e d of t h e o r e t i c a l and experimental s t u d i e s on both Fabry-Perot and d i s t r i b u t e d feedback (DFB) a m p l i f i e r s [ 7 ] . Whilst t h e e a r l i e r work was confined t o t r a n s m i s s i o n through an a m p l i f i e r , more r e c e n t l y an a n a l y s i s has been made of corresponding e f f e c t s o c c u r r i n g i n r e f l e c t i o n from t h e device. I n t h i s c o n f i g u r a t i o n a r i c h v a r i e t y of

n o n l i n e a r and b i s t a b l e e f f e c t s has been p r e d i c t e d

[ a ] ,

i n c l u d i n g a new ' b u t t e r f l y ' h y s t e r e s i s l o o p not widely known h i t h e r t o . Perhaps t h e most u s e f u l a t t r i b u t e o f reflection-mode n o n l i n e a r i t i e s i s t h a t both NANI)/NOR and A N D / ~ R g a t e c h a r a c t e r i s t i c s s h o u l d be a c h i e v a b l e i n p r i n c i p l e i n a s i n g l e device, depending on t h e c o n d i t i o n s of d r i v e c u r r e n t and wavelength detxning.

2 - NONLINEARITIES I N OPTIWL AMPLIFIERS

There a r e two dominant s o u r c e s of o p t i c a l n o n l i n e a r i t y i n l a s e r a m p l i f i e r s , namely g a i n s a t u r a t i o n and n o n l i n e a r r e f r a c t i o n . They a r i s e from t h e r e l a t i v e l y s t r o n g dependences o f m a t e r i a l g a i n E., and r e f r a c t i v e index N on e l e c t r o n c o n c e n t r a t i o n n , f o r wavelengths ( A ) c l o s e t o t h e forbidden band-gap. These dependences can be combined i n t o one parameter, t h e s o - c a l l e d ' l i n e w i d t h enhancement f a c t o r ' b [ 91

,

d e f i n e d a s :

Fig. 1 shows a p l o t of b v e r s u s photon energy, a s measured on a 1 . 5 pm InGaAsP l a s e r s t r u c t u r e , t o g e t h e r w i t h a recent t h e o r e t i c a l r e s u l t f o r t h i s dependence [ 101

.

The l a s i n g photon energy

i s about 35 meV above t h e band gap ( E g )

,

and a t t h i s energy t h e v a l u e of b i s c l o s e t o 5.

The s a t u r a t i o n of t h e n e t o p t i c a l g a i n , g, w i t h i n t e r n a l i n t e n s i t y I i n t h e a c t i v e r e g i o n o f t h e a m p l i f i e r . can be adequately d e s c r i b e d a s f o l l o w s [ 111 :

go

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988213

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F i g . 1

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Linewidth enhancement f a c t o r b v e r s u s photon energy f o r 1 . 5 urn InGaAsP: s o l i d l i n e - numerical c a l c u l a t i o n , broken l i n e - a n a l y t i c a l approximation, c r o s s e s

-

experimental p o i n t s

( a f t e r r e f e r e n c e 1 0 ) .

where go i s t h e u n s a t u r a t e d g a i n and a i s t h e l o s s i n t h e d e v i c e due t o a b s o r p t i o n and

s c a t t e r i n g . S i m i l a r l y , t h e n o n l i n e a r r e f r a c t i o n can be c a s t i n t e r m s of t h e s i n g l e - p a s s phase change 4 i n a resonant c a v i t y :

where L i s t h e c a v i t y l e n g t h and $, i s t h e i n i t i a l detuning. I n e q u a t i o n s ( 2 ) and ( 3 ) , t h e s a t u r a t i o n i n t e n s i t y Is i s d e f i n e d a s hv/[

rr

(dgm/dn)l w e r e

-8 r

i s t h e confinement f a c t o r and ^T i s t h e recombination t i m e ; Is t a k e s , a v a l u e of about 10 w/cm2 f o r 1 .5 pm a m p l i f i e r s .

Equations ( 2 ) and ( 3 ) , when combined w i t h t h e s t a n d a r d e q u a t i o n s f o r a Fabry-Perot c a v i t y w i t h g a i n [ 111 ( o r t h e i r e q u i v a l e n t forms f o r a DFB c a v i t y [ 71 ) , can p r o v i d e t h e b a s i s f o r a u s e f u l model of a m p l i f i e r OB. F i g . 2 g i v e s r e s u l t s f o r t r a n s m i t t e d output v e r s u s o p t i c a l i n p u t f o r an a m p l i f i e r a s c a l c u l a t e d u s i n g t h i s model [ 81. The i n t e n s i t i e s a r e s c a l e d t o t h e s a t u r a t i o n i n t e n s i t y I,, t h e i n i t i a l detuning

4,

i s -0.3 IT, and t h e u n s a t u r a t e d g a i n i s s c a l e d with r e s p e c t t o t h e l a s i n g t h r e s h o l d v a l u e o v e r a range from 0.65 t o 0.95. For a t y p i c a l a c t i v e a r e a o f 0.5

urn2,

t h e minimum i n p u t power l e v e l f o r b i s t a b i l i t y can be deduced from F i g . 2 t o be about 100 uW. However, t h i s i s by no means o p t i m a l , and by working with reduced d e t u n i n g g o , power l e v e l s a s low a s 1 VW can g i v e r i s e t o OB, a r e s u l t which i s e x p e r i m e n t a l l y confirmed f o r InGaAsP a m p l i f i e r s [ 2,5--71

.

3 - RECENT RESULTS FOR NONLINEAR AND BISTABLE AMPLIFIERS

Recently, an a n a l y s i s of e f f e c t s o c c u r r i n g i n r e f l e c t i o n from a m p l i f i e r s has been given [ 8 1 . The model used i s t h e same a s t h a t o u t l i n e d above, except t h a t now t h e s t a n d a r d Fabry-Pe_rot r e s u l t f o r t h e r e f l e c t e d wave js used. R e s u l t s a r e shown i n F i g . 3 f o r t h e same paramete!i-s a s f o r F i g . 2.

It i s c l e a r t h a t t h e r e f l e c t e d s i g n a l shows a f a r wider range of behaviour t h a n does t h e t r a n s - m i t t e d o u t p u t . It has been shown [81 t h a t t h e d i f f e r e n c e i n behaviour of t h e r e f l e c t e d and t r a n s m i t t e d i n t e n s i t i e s i s due t o g a i n s a t u r a t i o n e f f e c t s . I n f a c t , t h e r e a r e t h r e e d i s t i n c t t y p e s of h y s t e r e s i s l o o p i n r e f l e c t i o n , . a n d t h e s e a r e i n d i c a t e d s c h e m a t i c a l l y f o r convenience i n Fig. 4. F i r s t , a t v a l u e s of g a i n c l o s e t o t h r e s h o l d , t h e l o o p ( 1 ) i s s i m i l a r t o t h a t o c c u r r i n g i n t r a n s m i s s i o n . Second, a t r e l a t i v e l y low v a l u e s o f g a i n , t h e l o o p ( 2 ) i s similar t o t h a t f o r a p a s s i v e c a v i t y i n r e f l e c t i o n , a s d i s c u s s e d i n d e t a i l by Wherrett [ 121. T h i r d , a t i n t e r m e d i a t e v a l u e s of g a i n , t h e l o o p ( 3 ) i s o f t h e s o - c a l l e d ' b u t t e r f l y t y p e f i r s t r e r o r t ed ( f o r an e n t i r e l y d i f f e r e n t system) by Goldstone and Garmire 131

.

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Fig. 2 - Calculated v a r i a t i o n o f normalised t r a n s r r i t t ed output w i t h o p t i c a l i n p u t f o r b = 5 an&

c j ~ ~ = - 0 . 3 a ( a f t e r r e f e r e n c e 8 ) .

S t r o n g l y n o n l i n e a r o r b i s t a b l e c h a r a c t e r i s t i c s , such a s t h o s e o f Figs. 2-4, can be e x p l o i t e d i n t h e c o n s t r u c t i o n of l o g i c g a t e s . For example, l o o p s o f t h e kind l a b e l l e d ( 1 ) i n Fig.

4

can be used i n AND and OR g a t e s , and a high-contrast AND g a t e with gain u s i n g a m p l i f i e r t r a n s m i s s i o n has been demonstrated by S h a r f i n and Dcgenais [ 141. S i m i l a r l y , l o o p s of t y p e ( 2 ) can be used i n NANE a n d NOR g a t e s . For conventional p a s s i v e n o n l i n e a r e t a l o n s

,

PNC/OR l o g i c i s o b t a i n e d i n

t r a n s m i s s i o n where a s NANDINOR l o g i c i s o b t a i n e d i n r e f l e c t i o n ( s e e , f o r exanrple,

1

151

1.

However, t h e p r e d i c t i o n t h a t b o t h l o g i c f a m i l i e s c m be achieved i n r e f l e c t i o n from t h e same a m p l i f i e r , merely by changing t h e e l e c t r i c a l b i a s , i s a unique f e z t u r e o f a m p l i f i e r behaviour.

Fig. 3 - C a l c u l a t e d v a r i a t i o n o f normalised r e f l e c t e d o u t p u t with opt: c a l i n p u t f o r t h e same p a r a m t e n zs Fig. 2 ( a f t e r r e f e r e n c e 8 ) .

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Fig. 4 - Schematic i l l u s t r a t i o n of t h e t h r e e main t y p e s of h y s t e r e s i s l o o p o c c u r r i n g i n F i g . 3 ( a f t e r r e f e r e n c e 8

1.

I n o r d e r t o e x p l o i t t h e f u l l p o t e n t i a l of o p t i c a l l o g i c , it i s widely recognised t h a t p a r a l l e l p r o c e s s i n g should bb employed ( s e e , f o r example, [ 151 ) . There i s t h e r e f o r e a requirement f o r two-dimensional a r r a y s of g a t e s , and it i s not a t a l l obvious how t h i s can be achieved with o p t i c a l a m p l i f i e r s , s i n c e t h e y c o n v e n t i o n a l l y u s e a waveguide s t r u c t u r e with t h e guide i n t h e p l a n e of t h e e p i t a x i a l l a y e r s . However, from t h e l i t e r a t u r e it appears t h a t t h e r e a r e t h r e e p o s s i b l e o p t i o n s f o r 2-D a m p l i f i e r a r r a y s , a s f o l l o w s :

( i ) Grating-surface-snitting distributed-Bragg-reflector l a s e r s [ 161

.

Linear a r r a y s of t h e s e d e v i c e s have been s u c c e s s f u l l y f a b r i c a t e d i n t h e GaAs/AlGaAs m a t e r i a l s system, and o p e r a t e d i n a phase-locked mode a s a means of a c h i e v i n g very narrow f a r - f i e l d s [ 171.

( i i ) Edge-emitting Fabry-Perot l a s e r s with d e f l e c t i n g m i r r o r s [ 181. Two-dimensional a r r a y s of t h e s e d e v i c e s have been o p e r a t e d with a s many a s 112 elements a t 1 . 3 [ 181 and 22 elements a t 0.8 um 1191.

( i i i ) S h o r t - c a v i t y s u r f a c e - e m i t t i n g l a s e r s [ 201. Although t h e o n l y a r r a y s t o have been

c o n s t r u c t e d with t h e s e d e v i c e s have been r e l a t i v e l y small-scale ( 2 x 3 ) and o p e r a t e d a t 77 K [ 201

,

t h e r e c e n t achievement of cw r o o m t e m p e r a t u r e o p e r a t i o n of a s i n g l e s u r f a c e - e m i t t e r with a t h r e s h o l d of 2 mA [ 2 1 ] i n d i c a t e s t h a t l a r g e r a r r a y s with low e l e c t r i c a l power consumption should be p o s s i b l e .

Of t h e t h r e e o p t i o n s , t h e t h i r d appears t h e most a t t r a c t i v e f o r a p p l i c a t i o n s i n p a r a l l e l p r o c e s s i n g , s i n c e it should permit t h e h i g h e s t packing d e n s i t y and lowest power consumption per element. A f u r t h e r advantage i s t h a t t h e r e may be some r e l a x a t i o n of t h e t o l e r a n c e requirement f o r wavelength c o n t r o l , s i n c e s h o r t c a v i t i e s (- 10 urn) imply mode spacings some 20 - 30 t i m e s l a r g e r t h a n f o r conventional a m p l i f i e r c a v i t i e s . A d i s c u s s i o n of means f o r t e m p e r a t u r e t u n i n g a m p l i f i e r s and t h e i m p l i c a t i o n s f o r l a r g e a r r a y s h a s been g i v e n by S h a r f i n and Dagenais 1221.

Another important q u e s t i o n f o r would-be a p p l i c a t i o n s o f a m p l i f i e r s a s l o g i c g a t e s concerns t h e speed o f response. The t r a n s i e n t behaviour of a m p l i f i e r OB was t h e o r e t i c a l l y p r e d i c t e d [23,24,25]

t o be dominated by t h e e l e c t r o n recombination t i m e , a r e s u l t which h a s been v e r i f i e d experimentally f o r both DFB 171 and Fabry-Perot [261 l a s e r s . I n p r a c t i c e , t h i s l i m i t s t h e speed of o p e r a t i o n , and although a r e c e n t demonstration u s i n g d i f f e r e n t wavelengths f o r t h e c o n t r o l and s i g n a l beams has achieved 800 Mb/s 1271, it seems u n l i k e l y t h a t h i g h e r speeds w i l l be a t t a i n e d u n l e s s c a r r i e r l i f e t i m e s can be reduced.

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4 - CONCLUSION

The b a s i c p h y s i c a l mechanisms u n d e r l y i n g a m p l i f i e r OB, namely g a i n s a t u r a t i o n and n o n l i n e a r r e f r a c t i o n , have been d i s c u s s e d with p a r t i c u l a r r e f e r e n c e t o InGaAsP d e v i c e s . A review has been p r e s e n t e d o f r e c e n t r e s u l t s o b t a i n e d on such d e v i c e s , with p a r t i c u l a r emphasis on f e a t u r e s which may be of i n t e r e s t f o r o p t i c a l l o g i c . It i s worth n o t i n g t h a t i n a d d i t i o n t o t h e c a s e of o p t i c a l

switching with a s i n g l e beam a s d i s c u s s e d h e r e , t h e r e i s i n t e r e s t a l s o i n t h e u s e of s w i t c h i n g o f one o p t i c a l s i g n a l by another a t a d i f f e r e n t wavelength [ 2 7 , 2 8 ] , and i n c u r r e n t - c o n t r o l l e d

s w i t c h i n g [ 291. M u l t i p l e b i s t a b i l i t y and m u l t i s t a b i l i t y (compatible w i t h multiple-value l o g i c ) have a l s o been demonstrated 1301. I n a d d i t i o n t h e r e a r e s e v e r a l o t h e r a p p l i c a t i o n s f o r a m p l i f i e r OB which have a s y e t r e c e i v e d l i t t l e a t t e n t i o n . These i n c l u d e u s e s f o r o p t i c a l l i m i t i n g , o p t i c a l memory, wavelength t r a n s l a t i o n i n wavelength-multiplexed networks, and a l l - o p t i c a l s i g n a l

r e g e n e r a t i o n . The l a t t e r t o p i c has a l r e a d y a t t r a c t e d i n t e r e s t with t h e demonstration of d i r e c t r e t i m i n g of an o p t i c a l s i g n a l [ 3 1 ] , a l t h o u g h t h e key f e a t u r e of o p t i c a l t i m i n g r e c o v e r y ( s o f a r o n l y achieved w i t h SEED d e v i c e s [32] ) has y e t t o be demonstrated w i t h o p t i c a l a m p l i f i e r s . ACKNOWLEDGEMENTS

The a u t h o r wishes t o thank h i s c o l l a b o r a t o r s E. J . Westlake, M. J . O'Mahony, R . Wyatt, and L. D. Westlake f o r t h e i r many c o n t r i b u t i o n s t o t h e work d e s c r i b e d . The D i r e c t o r of Research, B r i t i s h Telecom, i s thanked f o r permission t o p u b l i s h .

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