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Submitted on 1 Jan 1988

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OPTICAL BISTABILITY IN ACTIVE DEVICESHitoshi Kawaguchi

To cite this version:Hitoshi Kawaguchi. OPTICAL BISTABILITY IN ACTIVE DEVICES. Journal de Physique Collo-ques, 1988, 49 (C2), pp.C2-63-C2-68. <10.1051/jphyscol:1988214>. <jpa-00227631>

JOURNAL DE PHYSIQUE C o l l o q u e C2, S u p p l e m e n t au n06, Tome 4 9 , juin 1 9 8 8

OPTICAL BISTABILITY IN ACTIVE DEVICES

H i t o s h i KAWAGUCHI

NTT O p t o - E l e c t r o n i c s L a b o r a t o r i e s , 3-1, M o r i n o s a t o , ~akarniya, A t s u g i - s h i , Kanagawa , 243-01, Japan

A b s t r a c t - R e c e n t p r o g r e s s i n t h e s t u d y o f b o t h a b s o r p t i v e a n d d i s p e r s i v e b i s t a b i l i t y i n s e m i c o n d u c t o r l a s e r s i s r e v i e w e d . I n h o m o g e n e o u s l y e x c i t e d s e m i c o n d u c t o r l a s e r s a s a n a b s o r p t i v e c a s e a n d r e s o n a n t t y p e l a s e r d i o d e a m p l i f i e r s a s a d i s p e r s i v e c a s e , a r e d e s c r i b e d .

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

R e c e n t p r o g r e s s i n t h e s t u d y o f b o t h a b s o r p t i v e a n d d i s p e r s i v e b i s t a b i l i t y i n l a s e r d i o d e s (LDs) i s r e v i e w e d / I / . T h e m o s t d i s t i n c t i v e f e a t u r e o f b i s t a b l e LDs i s t h a t t h e y h a v e o p t i c a l g a i n . O p t i c a l g a i n r e s u l t s i n a d v a n t a g e s such as l o w switching o p t i c a l p o w e r , high ON-OFF r a t i o , a n d l a r g e f a n - o u t .

The f i r s t s t a g e i n t h e s t u d y o f o p t i c a l b i s t a b l e LDs s t a r t e d i n 1 9 6 4 w i t h t h e t a n d e m - t y p e LD p r o p o s e d b y L a s h e r / 2 / . B a s e d u p o n t h e a d v a n c e d s e m i c o n d u c t o r l a s e r t e c h n o l o g y d e v e l o p e d f o r o p t i c a l f i b e r c o m m u n i c a t i o n s , t h e s e c o n d s t a g e o f b i s t a b l e LD s t u d y b e g a n i n 1 9 8 1 b y K a w a g u c h i a n d I w a n e / 3 / a n d H a r d e r e t a l . 1 3 1 . They r e p o r t e d r e m a r k a b l e b i s t a b l e c h a r a c t e r i s t i c s i n I n P / I n G a A s P l a s e r s a n d GaAslAlGaAs l a s e r s , r e s p e c t i v e l y .

S i n c e t h e n , v a r i o u s t y p e s o f b i s t a b l e LDs h a v e b e e n r e p o r t e d a s l i s t e d i n T a b l e 1 . B i s t a b i l i t y c a n b e s e e n i n ON-OFF o f l a s e r o s c i l l a t i o n 1 2 - 4 1 . o p t i c a l g a i n c h a n g e i n r e s o n a n t t y p e LD a m p l i f i e r s 1 5 1 , p o l a r i z a t i o n 1 6 1 , t r a n s v e r s e mode / 7 , 8 / , l a s i n g w a v e l e n g t h / 9 / a s w e l l a s l o c k e d a n d u n l o c k e d s t a t e i n i n j e c t i o n l o c k i n g s y s t e m s / l o / . D e t a i l s a r e d e s c r i b e d i n R e f e r e n c e / 1 1 / . A p p l i c a t i o n s t o p h o t o n i c s w i t c h i n g a n d o p t i c a l f i b e r c o m m u n i c a t i o n s h a v e a l r e a d y b e e n i n v e s t i g a t e d . U s i n g t a n d e m - t y p e l a s e r d i o d e s a s o p t i c a l m e m o r i e s , a n o p t i c a l t i m e - d i v i s i o n s w i t c h i n g s y s t e m w a s d e v e l o p e d by S u z u k i e t al. / 1 2 / . Webb h a s s t u d i e d t h e u s e o f a b i s t a b l e LD a m p l i f i e r a s a c l o c k e d d e c i s i o n g a t e , a n d s t ~ o w n t h a t a l l - o p t i c a l r e g e n e r a t i o n u s i n g s u c h a g a t e i s f e a s i b l e 1 1 3 1 . 2 . A b s o r p t i v e b i s t a b i l i t y

I n h o m o g e n e o u s l y e x c i t e d LDs s u c h a s t a n d e m t y p e L n s show b i s t a b i l i t y i n t h e c u r r e n t - l i g h t o u t p u t c u r v e a n d o p t i c a l i n p u t - o u t p u t c u r v e . T h i s i s b e c a u s e t h e r e g i o n w h e r e a c a r r i e r i s n o t i n j e c t e d a c t s a s a s a t u r a b l e a b s o r b e r .

S o m e t i m e s s e l f - p u l s i n g a s w e l l a s b i s t a b i l i t y a r e p r e s e n t i n t a n d e m - t y p e LDs. T h e s e d e p e n d o n t h e c a r r i e r l i f e t i m e d i s t r i b u t i o n a l o n g a l a s e r c a v i t y 1 1 4 1 . A

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988214

C2-64 JOURNAL DE PHYSIQUE

e l e c t r o d e was d i v i d e d i n t o two p a r t s a l o n g t h e l a s e r c a v i t y , a n d d r i v e c u r r e n t s w e r e i n j e c t e d s e p a r a t e l y i n t o t h e . d i v i d e d p a r t s a s shown i n F i g . 1. The c a s e where t h e two p a r t s o f t h e a c t i v e r e g i o n w e r e g i v e n d i f f e r e n t c a r r i e r r e c o m b i n a t i o n r a t e s w a s c o n s i d e r e d u s i n g d i f f e r e n t n o n - r a d i a t i v e c a r r i e r l i f e t i m e s f o r e a c h r e g i o n . The t h r e s h o l d pump r a t e a n d o u t p u t s t a b i l i t y a s a f u n c t i o n o f t h e n o n r a d i a t i v e c a r r i e r l i f e t i m e i n r e g i o n 2 , rnr2 , a r e shown i n F i g . 1. The s o l i d l i n e s i n d i c a t e t h e t h r e s h o l d pump r a t e s . When rn i s l o n g e r t h a n 2 n s , t h e l a s e r h a s two t h r e s h o l d pump r a t e s , P th l a n d P t h 2 , 2 e p e n d i n g o n t h e i n i t i a l c o n d i t i o n s . T h e s e b i s t a b l e c h a r a c t e r i s t i c s c a n be a t t r i b u t e d t o t h e f a c t t h a t t h e a c t i v e l a y e r o f r e g i o n 2 a c t s a s a s a t u r a b l e a b s o r b e r . When t h e n o n r a d i a t i v e c a r r i e r l i f e t i m e i s s h o r t e r t h a n 2 n s , t h e l a s e r h a s o n l y one t h r e s h o l d pump r a t e . The s h a d e d r e g i o n r e p r e s e n t s t h e p u l s a t i o n r e g i o n i n t h e f i g u r e . When t h e n o n r a d i a t i v e c a r r i e r l i f e t i m e i s s h o r t e r t h a n 0 . 5 n s , s e l f - p u l s a t i o n o c c u r s o v e r a

T a b l e 1. B i s t a b l e s e m i c o n d u c t o r l a s e r s

nonlinear refractive

15 PI P2

- r , b p h + t - .y g '0 mu Region I Region 11 "b - x - Bistability Pthl W

5-

B J> L

0 0.1 0.2 0.5 1 2 5 10 2 0

NONRADIATIVE LIFETIME I N REGION 11 Tnr, (ns )

Ll*r pth2

h h l Pth2 PI I I I I I I

F i g . 1 - T h r e s h o l d pump r a t e s ( f u l l l i n e s ) a n d p u l s a t i o n r e g i o n ( s h a d e d r e g i o n ) v s . n o n - r a d i a t i v e c a r r i e r l i f e t i m e i n r e g i o n 11. F i g u r e shows a s c h e m a t i c d i a g r a m o f t h e t a n d e m - t y p e l a s e r s t r u c t u r e u s e d i n t h e a n a l y s i s .

fm (GHz)

1.54 i ) f m = IGHz ( i ) fR ' 8 7 5 MHz

I ns/dii (Period 2

1.75 ( i f )

>

5 ii) fm' I GHz Z w 1.81 fR '820 MHz I- Z ( i i i )

Zns/dk (Chaotic Y

( b ) Temporal plot Spectrum F i g . 2 - ( a ) C a l c u l a t e d o u t p u t r e s p o n s e o t s e l t -

2-00 p u l s a t i n g l a s e r f o r s i n u s o i d a l c u r r e n t m o d u l a t i o n . ( i v ) ( i ) P e r i o d 1 , ( i i ) p e r i o d 2 , ( i i i ) p e r i o d 4 , a n d

I I ( i v ) c h a o t i c . 0 5 10 15 20 ( b ) O b s e r v e d o u t p u t r e s p o n s e . ( i ) p e r i o d 2 and ( i i )

( a ) TIME ( ns ) c h a o t i c .

w i d e r a n g e f r o m j u s t a b o v e t h e l a s e r t h r e s h o l d t o a v e r y h i g h e x c i t a t i o n l e v e l . Due t o t h e i r n o n l i n e a r i t y , s e m i c o n d u c t o . r l a s e r s e x h i b i t i n t e r e s t i n g c h a r a c t e r i s t i c s when t h e c u r r e n t a n d c a r r i e r l i f e t i m e d i s t r i b u t i o n a r e c h a n g e d a l o n g t h e l a s e r c a v i t y .

One e x a m p l e of o p t i c a l c h a o s i n LDs i s b r i e f l y d i s c u s s e d . We c o n s i d e r e d t h e l a s e r w h i c h shows s e l f - p u l s a t i o n b e c a u s e t h e c a r r i e r l i f e t i m e o f r e g i o n 2 i s s h o r t . F u r t h e r m o r e , t h e pump r a t e i n r e g i o n I , i s m o d u l a t e d by s i n u s o i d a l RF c u r r e n t s u p e r i m p o s e d on t h e D C b i a s c u r r e n t . T h i s n o n l i n e a r l a s e r s y s t e m shows c h a o t i c b e h a v i o r t h r o u g h p e r i o d - d o u b l i n g b i f u r c a t i o n s w i t h i n c r e a s i n g m o d u a t i o n f r e q u e n c y a s s e e n i n F i g . 2 ( a ) 1 1 4 1 . A t 1 . 5 4 GHz, t h e l a s e r showed s u s t a i n e d p u l s e o s c i l l a t i o n w i t h t l i e m o d u l a t i o n f r e q u e n c y . The f i r s t s u b h a r m o n i c a p p e a r e d a t 1 . 7 5 GHz. A t 1 .81 GHz, p e r i o d 4 o s c i l l a t i o n was o b t a i n e d . F u r t h e r m o r e , c h a o t i c o u t p u t s w e r e s e e n a t a b o v e 2 GHz.

T h e s e c h a r a c t e r i s t i c s w e r e a l s o o b s e r v e d i n t h e e x p e r i m e n t 1 1 5 1 . E x p e r i m e n t a l waveforms and s p e c t r a f r o m a GaAlAs LD a r e shown i n F i g . 2 ( b ) . By i n c r e a s i n g t h e d i f f e r e n c e b e t w e e n t h e r e s o n a n c e f r e q u e n c y f a n d m o d u l a t i o n f r e q u e n c y , f m , p e r i o d 2 a n d c h a o t i c o s c i l l a t i o n were o b s e r v e d .

' R ?

When c a r r i e r l i f e t i m e i s a b o u t t h e same o v e r t h e whole l a s e r c a v i t y , t andem t y p e LDs show b i s t a b l e o u t p u t c h a r a c t e r i s t i c s . The InGaAsP d i s t r i b u t e i l - f e e d b a c k l a s e r s t r u c t u r e o p e r a t i n g a t 1 . 5 5 pm i s shown i n F i g . 3 . The p - t y p e e l e c t r o d e was d i v i d e d i n t o t h r e e p a r t s , a n d t h e r e s i s t a n c e b e t w e e n t h e p - t y p e e l e c t r o d e s was w i t h i n a f e w h u n d r e d . T h u s , t h e d i v i d e d r e g i o n s c a n be e x c i t e d i n d e p e n d e n t l y t h r o u g h t h e e l e c t r o d e s . I f I i s s e t a t z e r o o r a low v a l u e , r e g i o n 1 a c t s a s a s a t u r a b l e a b s o r b e r . I t i s t i e n p o s s i b l e t o o b t a i n b i s t a b l e c h a r a c t e r i s t i c s i n b o t h t h e c u r r e n t - l i g h t o u t p u t a n d o p t i c a l i n p u t - o u t p u t c u r v e s .

A t y p i c a l m e a s u r e d c u r r e n t - l i g h t o u t p u t c u r v e i s shown i n F i g . 4(a). A n o t i c e a b l e h y s t e r e s i s l o o p i s s e e n i n t h e c u r v e . The c u r r e n t r a n g e o f h i s t a b i l i t y d e c r e a s e s w i t h i n c r e a s i n g i n t h e c u r r e n t i n j e c t e d i n t o t h e s a t u r a b l e a b s o r b e r . The c u r r e n t c a n o p t i m i z e t h e b i s t a b l e c u r r e n t r a n g e .

When t h e b i a s c u r r e n t i s s e t a t j u s t b e l o w t h e t u r n - o f f t h r e s h o l d , t h e o p t i c a l i n p u t - o u t p u t c u r v e e x h i b i t s b i s t a b i l i t y r e s u l t i n g f rom a n i n j e c t i o n o f o p t i c a l power i n t o t h e s a t u r a b l e a b o s r b e r a s shown i n F i g . 4 ( b ) . The b i a s c u r r e n t v a l u e s a r e i n d i c a t e d b y t h e c l o s e d c i r c l e s (1) t o ( 3 ) i n F i g . 4 ( a ) . An i n c r e a s e i n t h e b i a s c u r r e n t m a r k e d l y i n c r e a s e s t h e i n p u t o p t i c a l power r a n g e o f b i s t a b i l i t y . When t h e b i a s c u r r e n t i s i n t h e c u r r e n t r a n g e o f b i s t a b i l i t y , t h e b i s t a b l e l a s e r

JOURNAL DE PHYSIQUE

ELECTRODE \ I,

Fig. 4 - (a) Obs curve. (b) Observed op The bias current

a 3) 28.6 the closed circl

0.1 0.2 0.3

erved

tical value

es (1)

bistable DFB LD. Il Region 1 acts as a

current-light output

input-ou tput curve. s are indicated by to ( 3 ) in (a).

(a) CURRENT (rnA) (b ) POWER IN (mW)

emits coherent light when input optical power is injected. The laser stays in an "ON state" even when the input optical power is reset at zero. This means that the bistable laser acts as an optical memory device.

Recently, a tunable optical-wavelength conversion device for photonic switching has been demonstrated using this same structure /16/ . 3. Dispersive bistability

A resonant type LD amplifier can act as a nonlinear Fabry-Perot interferometer, and shows bistabillty. This is because the active layer refractive index changes due to gain saturation by light injection.

Among the many types of bistable semiconductor lasers studied previously, only Fabry-Perot type LD amplifiers appear to have multiple bistability or multistability potential. In the LD amplifier, there are many constructive interference transmission peaks. If cavity Q stays constant, the device shows multistability for strong light input intensities. However, cavity Q decreases as the injected opticinl intensity is increased through gain saturation. Utilizing a long Fabry-Perot cavity LD amplifier, optical multiple bistability is demonstrated /17/ . Let us consider the relationship between inputs and outputs for a resonant-type LD amplifier with two optical inputs 1181 . Using two optical inputs, a NAND gate and a NOR gate can be obtained. The initial detuning of a signal beam is equal to X: radian, that is, the signal beam wavelength coincides with the resonance wavelength ?f the LD amplifier when P =

0. When Pinl is increased, carrier density decreases, cawsing the refraftlve index of the active layer to become larger. This results in the lowering of the

cavity resonance frequency. Therefore, bistable characteristics are observed in t h e P -

in1 'out1 curve. In the P - P curve, reverse (clockwise) hysteresis is obtained as shown in Fig. *nfa)(iqu.t2 On the other hand, when wavelength detuning of the signal beam from the resonance wavelength is greater than that of the control beam, normal (counterclockwise) hysteresises are observed in both the

'in1 - pout^* and the P - in1 'out2

curves as shown in Fig. 5 (a) (ii) . The experimental results are shown in Fig. 5(b). In Fig. 5(i), the signal beam detuning is estimated to be R radian from a comparison between the calculated and experimental results of the P - P curve. By increasing the LD2

out2 current, the wavelength of LD2 is s&??!ted toward the longer wavelength side. In Fig. 5(ii), a 2.2 radian signal beam detuning state is observed. Therefore, normal (counterclockwise) hysteresises are observed in both the P - Poutl and the P - Pou 2 curves as shown in Fig. 5(ii). The experimenta!ln.n?esults agree well 2Pth the t6eoretica.1 results shown previously.

High speed switching has been demonstrated employing modulation of an 800 Mbit/s pulse Pattern 1191 . In this experiment, the b2 input was kept constant, while the h1 input was directly modulated.

4. Future Prospects

Major remaining problems in bistable semiconductor lasers are the realization of two dimensional arrays and reduction in threhsold current.

Three types of surface-emitting bistable lasers have been proposed. Vertical

Cii) Ci)

CU C 3

Pin 2 a"

(Signal 1 0 TC

f

- 4- 3 0 a

t

1.0 5 a I- 3 0

- - (ii) d2= 2.2

(b) pin^(') - Pin , (ii) Pout I I P S

Fig. 5 - (a) Calculated normalized output

------___ power Poutl/Ps and

PoUt2/Ps VS. input power / P for Fabry-Perot semiconduc?o~~'I~se?c amplifier with two holding beams. (i) J2 =x and (ii) J2 = 2.2.

2 (b) Observed P VS' 'in1

and P VS.

P curves. ?yfl a2 =r( and if^^^^^^ = (a 23:

C2-68 JOURNAL DE PHYSIQUE

c a v i t y - l a s e r t y p e o p t i c a l l o g i c a n d g a t e d e v i c e s w e r e p r o p o s e d 1 2 0 1 . T h e s e d e v i c e s i n c l u d e a s a t u r a b l e a b s o r b e r i n t h e v e r t i c a l l a s e r c a v i t y . A s u r f a c e e m i t t i n g b i s t a b l e LD u s i n g o u t p u t c o u p l i n g b y s e c o n d o r d e r g r a t i n g o f a d i s t r i b u t e d B r a g g r e f l e c t o r 1 2 1 1 a s w e l l a s t h e l a s e r w i t h a n i n t e g r a t e d beam d e f l e c t o r 1 2 2 1 h a v e a l s o b e e n d e m o n s t r a t e d . H o w e v e r , t h e t h r e s h o l d c u r r e n t s o f t h e s e d e v i c e s a r e m o r e t h a n 5 0 m A a t p r e s e n t . To c o n s t r u c t two d i m e n s i o n a l a r r a y s , t h e t h r e h s o l d c u r r e n t s h a v e t o b e r e d u c e d .

S e m i c o n d u c t o r l a s e r s w i t h t h r e s h o l d c u r r e n t s o f l e s s t h a n 1 mA h a v e b e e n a l r e a d y r e p o r t e d . F o r e x a m p l e , a g a l l i u m - a r s e n i d e s i n g l e - q u a n t u m - w e l l , g r a d e d - i n d e x s e p a r a t e - c o n f i n e m e n t , b u r i e d - h e t e r o s t r u c t u r e l a s e r h a s r e c e n t l y a c h i e v e d a 0 . 5 5 m A t h r e s h o l d 1 2 3 1 . I t i s b e l i e v e d u s i n g a d v a n c e d t e c h n o l o g i e s s u c h a s t h e q u a n t u m c o n f i n e d e f f e c t s , LDs w i t h a much l o w e r t h r e s h o l d c u r r e n t , 1 p A t h r e s h o l d LDs, w i l l b e a c h i e v e d 1 2 4 1 . I n t h a t c a s e , c o n s t r u c t i o n o f lOOOxlOO0 two d i m e n s i o n a l a r r a y s may become p o s s i b l e i n t h e f u t u r e .

5 . Summary

R e c e n t p r o g r e s s i n t h e s t u d y o f a b s o r p t i v e a n d d i s p e r s i v e b i s t a b i l i t y i n s e m i c o n d u c t o r l a s e r s w a s r e p o r t e d . The b i s t a b l e s e m i c o n d u c t o r l a s e r s w i l l b e a d v a n t a g e o u s i n t h e f i e l d o f t r a n s m i s s i o n a n d e x c h a n g e , u t i l i z i n g t h e m e r i t s o f o p t i c a l g a i n . When t h r e s h o l d c u r r e n t s a r e r e d u c e d f u r t h e r , t w o - d j m e n s i o n a l a r r a y s o f b i s t a b l e LDs w i l l become i m p o r t a n t i n o p t i c a l i n f o r m a t i o n p r o c e s s i n g .

REFERENCES

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