(by a factor of π/2) and the power requirement more than doubles (by a factor of 7π2/4), because the effective interac­tion length of the device is shortened due to the softened edges of the interaction region. This is a small price to pay for a tremendous improvement in channel isolation.

R E F E R E N C E S 1. D . A . S m i t h et al., " I n t e g r a t e d o p t i c a c o u s t i c a l l y - t u n a b l e f i l t e r s f o r W D M

n e t w o r k s , " I E E E J . S e l e c t e d A r e a s i n C o m m u n . 8 , 1 9 9 0 , 1 1 5 1 - 1 1 5 9 .

2 . D . A . S m i t h et al., " I n t e g r a t e d a c o u a t i c a l l y - t u n e d o p t i c a l f i l t e r s f o r f i l t e r ­

i n g a n d s w i t c h i n g a p p l i c a t i o n s , " t o b e p u b l i s h e d i n P r o c . I E E E 1 9 9 1

U l t r a s o n i c s S y m p o s i u m , I E E E , 1 9 9 2 , 5 4 7 - 5 4 8 .

3 . D . A . S m i t h a n d J. J . J o h n s o n , " S i d e l o b e s u p p r e s s i o n i n a n a c o u s t o - o p t i c

f i l t e r w i t h a r a i s e d - c o s i n e i n t e r a c t i o n s t r e n g t h , " A p p l . P h y s . L e t t . 6 1 ,

1 9 9 2 , 1 0 2 5 - 1 0 2 7 .

4 . H . H e r r m a n n a n d S t . S c h m i d , " I n t e g r a t e d a c o u s t o - o p t i c a l m o d e - c o n ­

v e r t o r s w i t h w e i g h t e d c o u p l i n g u s i n g s u r f a c e a c o u s t i c w a v e d i r e c t i o n a l

c o u p l e r s , " E l e c t r o n . L e t t . 2 8 , 1 9 2 , 9 7 9 - 9 8 0 .

Monolithically Integrated 21-Wavelength DFB Laser Array with a Star Coupler and Optical Amplifiers By Chung-en Zah and T. P. Lee, Bellcore, Red Bank, N. J.

We have reported previously multi-wavelength distributive feedback DFB laser arrays with as many as 20 wavelengths on a single chip fabricated by the use of strained-layer InGaAs/InGaAsP multi-quantum wells.1 A wavelength span as large as 131 ran in the 1.5 μm wavelength region, a 3 dB modulation bandwidth as high as 16 GHz, and a linewidth as small as 180 kHz have been obtained. More recently, we have demonstrated monolithic integration of the laser array with a star coupler and optical amplifiers on the same chip to simplify fiber pigtailing. These devices have potential application for high density wavelength divisive multiplex­ing W D M systems, as well as for optical networks using wavelength routing.

Traditionally, the increased transmission bandwidth has been accomplished by time-division-multiplexing through

the increase in the transmission speed. This is because the cost in transmission per bit per km of fiber has been decreas­ing continuously up to a speed of 2.5 Gbit/sec. As a result, the transmission capacity has been increasing at a rate about doubling every year from 1980 through 1988. Recently, com­mercial systems at rates of 1.2 and 1.7 Gbit/sec have been

installed, and 2.5 Gbit/sec systems are planned for new installations. Although systems with even higher rates at 10 Gbit/sec have been demonstrated in the laboratories, com­mercial systems probably wil l not be available in the near future due to the unavailability of high speed electronics.

Because of the broad optical bandwidth (30 THz) avail­able in the low-loss transmission window of the optical fiber, it is desirable to exploit the fiber bandwidth by wave­length division multiplexing (WDM) to overcome the elec­tronic limitations, thus to further increase the transmission capacity. For example, early W D M transmission experiments at 10,16, and 100 wavelength channels with the aggregated capacity of 20, 32, and 62 Gbit/sec, 2 - 4 respectively, were reported in 1985,1988, and 1990. In the last two years, there were two major breakthroughs that make the W D M even more attractive. They are (1) the Er-doped fiber amplifiers (EDFAs), and (2) strained layer multiple quantum well laser diodes (SL-MQW-LDs). The EDFA eliminates the need of electronic regenerators, whereas the SL-MQW-LDs have very low threshold currents and large gain spectral width that permit multiple wavelength laser arrays to be fabricated on a single wafer. Furthermore, the EDFA has sufficient band­width to amplify many wavelength channels simultaneously; thus, it is cost effective in conjunction with W D M systems.

Distributed feedback laser arrays are attractive as multi-wavelength light sources for wavelength division multi­plexed systems because a single TE cooler can be used to keep the relative wavelength spacing constant that facili­tates a simple wavelength control and stabilization. By tak­ing advantage of the wide gain width of the strained layer multiple quantum wells, we have recently fabricated multi-wavelength DFB laser arrays with as many as 20 wavelength channels in one array.1 The channel spacing ranged from 3-7 nm, which can allow four to eight channels within the band of the Er-doped fiber amplifiers.

To make the DFB laser diode array a practical device, it is desirable to combine the multiwavelength output into a single-mode fiber pigtail. Recently, we have investigated monolithic integration of a multi-wavelength DFB laser di­ode array with a star coupler and an optical amplifier on one chip. After amplification, the combined signals in all wave­lengths in the output waveguides of the star coupler can then be coupled into a single-mode fiber pigtail. The top view of a finished integrated laser array chip is shown in the figure. The 21-wavelength DFB laser array is connected to the star coupler through passive waveguides. Quantum well optical amplifiers were inbeded in two output waveguides near the center of the star coupler, and the remaining output waveguides are passive. The star coupler is formed by radi­ally spacing the input and output waveguides with an angu­lar increment of 0.6° on a 750 μm radius circle centered at the middle of the input and output waveguides. The active layer of the DFB lasers and the optical amplifiers consists of six compressive-strained In 0 . 7Ga 0 . 3As wells. The gratings of dif­ferent periods are generated by e-beam lithography.

Preliminary results show that the cw lasing wavelengths spans from 1512-1578 nm for 18 channels with threshold currents varying from 14-55 mA. The remaining three lasers have much higher thresholds. The channel spacing is 3.7 nm with a standard deviation of 0.38 nm. The side mode sup­pression ratio is typically better than 35 dB.

A micrograph of the (op view of the monolithically integrated 21-wavelength DFB laser array with a star coupler and optical amplifiers.

28 O P T I C S & P H O T O N I C S N E W S / D E C E M B E R 1 9 9 2

Optics in 1992

A C K N O W L E D G M E N T S

W e are g ra te fu l to R. B h a t , C . C a n e a u , a n d M . A . K o z a fo r O M V P E m a t e r i a l s , P . S. D . L i n a n d A . S. G o z d z f o r e - b e a m l i t h o g r a p h y , N . C . A n d r e a d a k i s f o r A R c o a t i n g s , F. F a v i r e a n d B. P a t h a k fo r d e v i c e f a b r i c a t i o n a n d c h a r a c t e r i z a t i o n , a n d L . C u r t i s , D . D . M a h o n e y , R . E . S p i c a r , V . S. S h a h , a n d W . C . Y o u n g fo r f ibe r p i g t a i l i n g .

R E F E R E N C E S

1. C. E. Zah et al., "1.5 μm compressive-strained multiple-quantum-well 20-wavelength distributed-feedback laser arrays," Electron. Lett. 29, 1992, 824-826; C.E. Zah et al., "1.5 μm tensile-strained single quantum well 20-wavelength distributed feedback laser arrays," Electron. Lett. 28,1992,1585-1587.

2. N . A . Olsson et al., "68.3 km transmission with 1.37 Tbit-km/s capacity using wavelength division multiplexing of ten single-frequency lasers at 1.5 μm," Electron. Lett. 21,1985,105-106.

3. M. P. Vecchi et al., "High-bit-rate measurements in the L A M B D A N E T multiwavelength optical star network," Optical Fiber Communication Conference Technical Digest, paper W02, Optical Society of America, 1988; C. Lin et al., "Wavelength-tunable 16 optical channel transmission experiment a 2 Gbit /s and 600 Mbi t /s for broadband subscriber distri­bution," Electron. Lett., 24:19,1988,1215.

4. H. Toba et al., "A 100-channel optical F D M transmission/distribution at 622 Mb /s over 50 km," J. Lightwave Technol. 8,1990,1396-1401.

Monolithic InP Grating-Based Wavelength Division Multiplexing Components By J.B.D.Soole and A.Scherer, Bellcore, Red Bank, N . J .

T h e pas t y e a r s a w b r e a k t h r o u g h s i n a n e w c o m p o n e n t t ech ­n o l o g y that p r o m i s e s to p r o v i d e w a v e l e n g t h - s p e c i f i c d e ­v i ces fo r f u t u r e w a v e l e n g t h d i v i s i o n m u l t i p l e x e d ( W D M ) n e t w o r k s that h a v e a h i g h d e g r e e of w a v e l e n g t h c o n t r o l a n d a l o w m a n u f a c t u r i n g cost . T o da te , o n e o f the greates t c h a l ­lenges f a c i n g i m p l e m e n t a t i o n o f p r o p o s e d W D M n e t w o r k s i s the p r a c t i c a l r e a l i z a t i o n of l o w - c o s t w a v e l e n g t h - s p e c i f i c op to -e lec t ron i c c o m p o n e n t s ( lasers, de tec to rs , etc.) that p o s ­sess a n d m a i n t a i n the w a v e l e n g t h a c c u r a c y n e e d e d f o r e f fec­t i v e n e t w o r k o p e r a t i o n . W h i l e m a n y c o m p o n e n t s a re a v a i l a b l e o r u n d e r d e v e l o p m e n t , there is n o t e c h n o l o g y that p r o v i d e s these d e v i c e s w i t h the r e q u i r e d w a v e l e n g t h a c c u ­racy , to le rance , a n d s tab i l i t y at l o w e n o u g h cost to m a k e m a n y o f the n e t w o r k s c o m m e r c i a l l y v i a b l e .

T h e n e w c o m p o n e n t t e c h n o l o g y uses a s i n g l e - c h i p s e m i ­c o n d u c t o r w a v e l e n g t h m u l t i p l e x e r / d e m u l t i p l e x e r i n teg ra ted w i t h d i f f e ren t ac t i ve e l e m e n t s to f o r m a w i d e r a n g e o f c o m ­pa t ib le w a v e l e n g t h - s p e c i f i c c o m p o n e n t s .

T h e hear t o f the m u x / d m u x i s a p l a n a r w a v e g u i d e g r a t i n g spec t rome te r i n t o w h i c h a d i f f r a c t i o n g r a t i n g a n d s t r ipe w a v e g u i d e s h a v e b e e n e t c h e d . 1 , 2 W h e n a m u l t i - w a v e ­l e n g t h s i g n a l is f e d i n t o a n i n p u t s t r i pe g u i d e , i t en te rs the b o d y o f the d e v i c e a n d i s d i s p e r s e d b y a n e t c h e d d i f f r a c t i o n g r a t i n g ; the w a v e l e n g t h - d e m u l t i p l e x e d s i g n a l s t h e n ex i t the d e v i c e v i a o u t p u t s t r i pe g u i d e s . W a v e l e n g t h m u l t i p l e x i n g i s a c h i e v e d b y o p e r a t i n g the d e v i c e i n reve rse . B e c a u s e w a v e ­l e n g t h se lec t i on re l ies o n the d e v i c e g e o m e t r y , h i g h l y a c c u ­rate s p e c i f i c a t i o n o f the w a v e l e n g t h s i s p o s s i b l e .

T h e bas i c I n P g r a t i n g m u x / d m u x w a s r e p o r t e d i n 1991;

1992 s a w i ts i n t e g r a t i o n w i t h ac t i ve e l e m e n t s to f o r m W D M s o u r c e s a n d rece ive rs .

W D M d e t e c t o r a r r a y s w e r e f o r m e d b y i n t e g r a t i n g w a v e g u i d e p - i - n pho tode tec to r s w i t h the o u t p u t w a v e g u i d e s . D e v i c e s w i t h b o t h 42 a n d 92 c h a n n e l s w e r e r e p o r t e d . 3 , 4 C h a n ­n e l s e p a r a t i o n w a s 4 n m a n d 1 nm, r e s p e c t i v e l y , w i t h 2 n m a n d 0.6 n m c h a n n e l p a s s b a n d s . T h e de tec to r a r r a y s o p e r a t e d i n the 1.5 μ m f iber b a n d a n d e m p l o y e d h i g h l y e f f i c ien t (90%) w a v e g u i d e p h o t o d e t e c t o r s .

A 1 6 - w a v e l e n g t h W D M lase r , c a l l e d the " M A G I C " ( M u l t i - s t r i p e A r r a y G r a t i n g In teg ra ted C a v i t y ) laser , w a s a l s o r e p o r t e d . 5 T h i s h a s ac t i ve g a i n e l e m e n t s i n t e g r a t e d w i t h the g r a t i n g - b a s e d c a v i t y . T h e p l a n a r g u i d e b o d y a n d the p u m p e d s t r i pes f o r m the lase r c a v i t y , w h i l e the g r a t i n g p r o v i d e s the w a v e l e n g t h se lec t i ve i n t r a - c a v i t y e l emen t . B y i n j ec t i on p u m p i n g o n e o u t p u t s t r i pe a n d d i f f e ren t " s e c o n d " s t r i pes , l a s i n g w a s o b t a i n e d f r o m the s i n g l e o u t p u t at d i f f e r ­en t w a v e l e n g t h s . L i k e the d m u x a n d de tec to r a r r a y , the laser o p e r a t e d at w a v e l e n g t h s p r e c i s e l y d e t e r m i n e d b y the d e v i c e g e o m e t r y .

T h e bas i c t r io o f W D M d e v i c e s — s o u r c e , f i l ter , r e c e i v e r — h a s n o w b e e n d e m o n s t r a t e d u s i n g the i n t e g r a t e d g r a t i n g t e c h n o l o g y . T h e i r d e v e l o p m e n t a n d c o m b i n a t i o n is expec ted . T h e grea t p o t e n t i a l o f the s e m i c o n d u c t o r g r a t i n g - b a s e d tech ­n o l o g y , w h i c h a r i ses f r o m i ts a b i l i t y to set the w a v e l e n g t h s a c c u r a t e l y a n d the fact that f a b r i c a t i o n i s r e a d i l y sca lab le to l a r g e - v o l u m e m a n u f a c t u r e , assu res that i t w i l l b e a c t i v e l y p u r s u e d i n the y e a r s to c o m e .

R E F E R E N C E S 1. J.B.D.Soole et al., "Monolithic InP/InGaAsP/InP grating spectrometer

for the 1.48-1.56 μm wavelength band," Appl . Phys. Lett. 58,1991,1949-1951.

2. C.Cremer et al., "Grating spectrograph in InGaAsP/InP for dense wave­length division multiplexing," Appl . Phys. Lett. 59,1991, 627-629.

3. C. Cremer et al., "Grating spectrograph integrated with photodiode array in InGAsP/InGaAs/InP," IEEE Phot. Tech. Lett. 4,1992,108-110.

4. J.B.D.Soole et al., "Monolithic wavelength demultiplexer and high den­sity p-i-n array for 1.5 μm W D M applications," Integrated Photonics Research Topical Meeting, paper PD-1, New Orleans, La., Apr i l , 1992.

5. J.B.D.Soole et al., "A MAGIC laser for W D M applications," Device Re­search Conference, paper BIII-7, Boston, Mass., 1992.

W D M components based on the InP grating mux/dmux. The combination of detector and laser yields an "electronic island" or node capable of intelligent wavelength conversion or translation.

OPTICS & PHOTONICS N E W S / D E C E M B E R 1992 29