Revista Brasileira de Física,Vol. 10, NP 3, 1980
Material Dispersion Measurements in Optical Fibre Waveguides
HARISH R. D. SUNAK
andJOÃO BATISTA DE MELLO AYRES NETO
Instituto de Física, Universidade Estadual de Campinas, 13100 Campinas, SP, Brasil
Recebido ern 17 de Dezembro de 1979
We have c a r r i e d o u t p r e l i m i n a r y measurements o f m a t e r i a l d i s - p e r s i o n on o p t i c a l f i b r e s now b e i n g r o u t i n e l y produced b y
T E L E B R Á S
i n Brasil . This was done by u s i n g two semi conductorl a s e r s emi t t i n g a t the d i f f e r e n t wavelengths o f 800 mmand 904 mm. Our r e s u l t o f -100 ps/nm/km i n germania-doped s i li c a f i b r e s i s -30% h i g h e r than the valeu f o r pure s i l i c a ; t h i s agrees w e l l w i t h r e s u l t s o b t a i n e d i n o t h e r l a b o r a t o r i e s w i t h simil a r f i b r e s . M a t e r i a l d i s p e r s i o n can li m it the bandwidth o f an o p t i-
c a l f i b r e , e s p e c i a l l y when a l i g h t e m i t t i n g diode, o p e r a t i n g i n the 800- 900 mm wavelength region i s used as the l i g h t source i n a f i b r e o p t i c a l communication system havi ng graded- i ndex f i b r e s wi t h an optimum index p r o f i l e .
Concluimos medidas p r e l iminares de dispersão material em f i b r a s Ó t i c a s que vem,sendo produzidas r o t i n e i r a m e n t e pela TELEBRÁS no Brasil
.
Estas medidas foram f e i tas usando-se d o i s l a s e r s semicondutores e m i t i n d o l u z de d i f e r e n t e s comprimentos de onda, 800 nme 904 nm. Nosso r e s u l t a d o de -100 ps/nm/km em f i b r a s de s i l i c a dopadas com germânio
é
- 3 0 % maior que o v a l o r para s i li c a pure, o que e s t á de acordo com resultados o b t i-
dos emo u t r o s l a b o r a t ó r i o s com f i b r a s s i m il a r e s . Dispersão materi a l pode l i m i t a r a l a r g u r a de banda de uma f i b r a ó t i c a , especialmente quando um diodo emissor de luz (LED), operando na r e g i ã oe s p e c t r a l e n t r e 800 nm e 900 nm
é
usado c o m o f o n t e de l u z num sistema de comunicações por f i b r a s Óticas tendo f i b r a s de Í n d i c e graduado com umÓtimo p e r f i l de Í n d i c e .1. INTRODUCTION
I n an e f f o r t t o develop f i b r e o p t i c a w i t h i n B r a z i I, we have a1 ready r e p o r t e d l S 2 r e s u
1 corrununications systems l t s o f t h e measurements o f n o r m a l i s e d r o d e c o n v e r s i o n c o e f f i c i e n t s i n f i b r e s , w h i c h a r e now b e i n g r o u t i n e l y p u l l e d a t t h e "Padre Roberto Landel l de Moural1 Research and Development C e n t r e o f TELEBRhS i n Campinas. I n o u r c o n t i n u i n g programme t o assess these f i b r e s e x p e r i m e n t a l l y , we have used a Nanosecond T e s t
~ a c i l i t y3 t o c a r r y o u t a d e t a i l e d and s y s t e m a t i c s t u dy4 o f p u l s e d i s p e r - s i o n i n these f i b r e s and t h e r e s u l t s w i l l be r e p o r t e d s h o r t l y 5 .
The t o t a l d i sp e r s i o n6 i n a multimode f i b r e i s . p r i n c i p a l l y t h e sum o f rnodal and m a t e r i a l d i s p e r s i o n . Modal d i s p e r s i o n , sometimes a l s o r e f e r r e d t o as waveguide d i s p e r s i o n , i s due t o t h e d i f f e r e n t group v e l o - c i t i e s o f t h e v a r i o u s modes p r o p a g a t i n g i n t h e f i b r e . T h i s w a s shown7 v e r y c l e a r l y u s i n g a r u b y l a s e r : w i t h t h e e x c i t a t i o n o f a11 t h e modes, a v e r y l a r g e d i s p e r s i o n was o b s e r v e d f o r t h e f i r s t t i m e i n a graded- i n d e x f i b r e . M a t e r i a l d i s p e r s i o n , a l s o r e f e r r e d t o as c h r o m a t i c d i s p e r s i o n , i s due t o t h e wavelength dependence o f t h e r e f r a c t i v e i n d e x o f the f i c o r e m a t e r i a l and hence causes d i f f e r e n c e s i n t h e v e l o c i t i e s among f r e q u e n c y s p e c t r a l components o f t h e 1 ig h t s o u r c e used. 00th moda1 m a t e r i a1 d i s p e r s i on cause broaden i n g o f a narrow, s ub-nanosecond, 1 i p u l s e a f t e r p r o p a g a t i o n i n t h e f i b r e , b u t t h e c o n t r i b u t i o n o f each
b r e t h e and g h t can be markedly d i f f e r e n t depending on t h e l i g h t s o u r c e used and t h e t y p e o f f i b r e e x c i t e d w i t h t h i s source: (a) I n a s t e p - i n d e x multimode f i b r e , w i t h a i 1 modes e x c i t e d by a semiconductor l a s e r , rnodal d i s p e r s i o n i s l a r g e , a b o u t 30 ns/km f o r e v e r y p e r c e n t i ndex d i f f e r e n c e between c o r e and c l a d - d i n g . S i n c e t h e s p e c t r a l w i d t h o f t h e l a s e r i s < 4 nm, m a t e r i a l d i s p e r - s i o n i s n e g l i g i b l e . T h i s i s a l s o v a l i d even w i t h a l i g h t e m i t t i n g d i o d e , w i t h an o r d e r o f magnitude g r e a t e r s p e c t r a l w i d t h , (b) m d a l d i s p e r s i o n can be reduced d r a s t i c a l l y by u s i n g a gradeci- index f i b r e wi t h an o p t i - mum i n d e x p r o f i l e . I f t h e 1 ig h t s o u r c e i s a semiconductor l a s e r a t 900nm, m d a l and m a t e r i a l d i s p e r s i o n a r e comparable, b e i n g l e s s t h a n 1 nsec/km.
However, i f a l i g h t e m i t t i n g d i o d e i s used, m a t e r i a l d i s p e r s i o n can be u p t o t h r e e times g r e a t e r than rnodal d i s p e r s i o n , as shown b y ~ a w s o n ~ .
Modal d i s p e r s i o n can be e l i m i n a t e d a t any wavelength ( A ) by
d e s i g n i n g a single- mode f i b r e f o r t h i s wavelength w i t h V, t h e n o r m a l i s e d frequency parameter, h a v i n g a v a l u e l e s s than 2.405. I t i s g i v e n by
where d i s t h e d i a m e t e r o f f i b r e core, ni i s t h e i n d e x o f f i b r e c o r e and n 2 i s t h e i n d e x o f f i b r e c l a d d i n g . M a t e r i a l d i s p e r s i o n can a l s o be e l i - minatedq when A i s s e l e c t e d i n t h e r e g i o n o f 1.3 pm, t h e e x a c t w a v e l e n g t h f o r z e r o m a t e r i a l d i s p e r s i o n depends on t h e c o r e m a t e r i a l , t h e dopant and i t s c o n c e n t r a t i o n l O .
A t t e n u a t i o n i s a l s o o f g r e a t importance i n t h e d e s i g n o f f i b r e o p t i c a l systems as s i g n a l s have t o be r e g e n e r a t e d . R e c e n t l y i t has been shownll t h a t f i b r e a t t e n u a t i o n s o f O.P'dB/km, v e r y near t h e t h e o r e r i c a l R a y l e i g h s c a t t e r i n g l imi t, can. be achieved a t t h e wavelength o f 1.55 pm.
I t i s obvioui; t h a t t h e u l t i m a t e f i b r e o p t i c a l communication system con- s i s t s o f a single- mode f i b r e , designed t o have z e r o m a t e r i a l d i s p e r s i o n a t t h i s wavelength o f minimum a t t e n u a t i o n . Such a f i b r e has been demons- t r a t e d by Cohen e t aZ. 1 2 . S i n c e t h i s t y p e o f system w o u l d t r a n s m i t e x - t r e m e l y h i g h i n f o r m a t i o n capaci t i e s o v e r v e r y l o n g d i s t a n c e s , i t w o u l d be f a i r l y l i r n i t e d i n i t s a p p l i c a t i o n , e.g., l i n k i n g two l a r g e c i t i e s . T h e - r e a r e many o t h e r a p p l i c a t i o n s where t h e systems c o u l d e a s i l y have much l e s s bandwidths, b u t n e v e r t h e l e s s i n t h e tens o f megabi t s p e r second r a n - ge, by u s i n g f a i r l y cheap and h i g h l y r e l i a b l e l i g h t e m i t t i n g d i o d e s o- p e r a t i n g i n t h e 800-900 nm wavelength r e g i o n . W i t h t h i s i n mind, we have made p r e l i m i n a r y m a t e r i a l d i s p e r s i o n measurements on o p t i c a l f i b r e s now b e i n g r o u t i n e l y produced i n B r a s i I, so as t o d e t e r m i n e t h e i r i n f o r m a t i o n c a r r y i n g c a p a c i t i e s f o r d i v e r s e a p p l i c a t i o n s . M a t e r i a l d i s p e r s i o n measu- rements d i r e c t l y on t h e f i b r e i t s e l f a r e i r n p o r t a n t so as t o know t h e t o - t a l e f f e c t o v e r t h e whole l e n g t h o f t h e f i b r e . F u r t h e r , changescan o c c u r due t o s e v e r e thermal p r o c e s s i n g a t about 2000 C d u r i n g t h e O f i b r e d r a - w i n g process, t h e r e b y r e n d e r i n g any i n i t i a l measurements on b u l k samples
i n v a l i d .
2. EXPERIMENTAL SET-UP
Our f i b r e s , being multimode step- index types, have ~ h o w n ~ ' ~ a modal d i s p e r s i o n o f -20 nsec/km u s i n g 0.5 nsec pulses from a GaAs l a s e r . Hence we c o u l d n o t use a l i g h t emí t t i n g diode t o assess the m a t e r i a l d i s - p e r s i o n d i r e c t l y 8 as i t would be d i f f i c u l t t o separate the two e f f e c t s . We have used an i n d i r e c t method, f i r s t r e p o r t e d by Gloge e t az. l 3 and
i l l u s t r a t e d i n f i g u r e 1. The principie o f the m t h o d i s very simple and i s as f o l l o w s : two pulses from lasers o f d i f f e r e n t wavelengths a r e laun- ched simultaneously i n t o the f i b r e . The s e p a r a t i o n o f t h e p u l s e peaks i s measured b e f o r e and a f t e r propagation and the d i f f e r e n c e gives t h e mate- r i a l d i s p e r s i o n . Laser LI was RCA SG 2001 e r n i t t i n g a t 904 nm and Laser L2 was type ~ ~ 6 1 , manufactured by Laser Diode L a b o r a t o r i e s , emi t t i n g a t 800 nm. Both were d r i v e n , from a common p u l s e generator, u s i n g separate i d e n t i c a l c; r c u i ts as described by ~ n d r e w s l ~ . The s e p a r a t i o n i n the wave- lengths AX = 104 nm, was much g r e a t e r than 40 nm, the spread a t the h a l f -power p o i n t s o f a t y p i c a l LED spectrum. This was chosen because our f i - bres were much s h o r t e r (-100 m) than t h a t used Gloge e t aZ.13 ( 1 0 0 0 m ) and the modal d i s p e r s i o n was l a r g e r as w e l l . Hence t o observe a d i s t i n c t
A
u
Silicon
Osciloscope X Y R e c o r d e r
M i c r o s c n n ~
1
,Beam Splitter ObjectivT i m e Dela y
Fig.1 - Experimental set- up used t o measure m a t e r i a l d i s p e r s i o n i n o p t i c a l f i b r e s .
Externo1
-
TriggerC
' O
.d i f f e r e n c e i n the s e p a r a t i o n o f the two p u l s e peaks a f t e r propagation i n the f i bi-e, a l a r g e r AX was r e q u i red. However, knowing t h e normal i s e d ma- t e r i a l d i s p e r s i o n i n ps/nm/km, we can c a l c u l a t e the m a t e r i a l d i s p e r s i o n expected using a 1 i g h t emi t t i n g diode having a peak wavelength o f -850nm and a spread o f 40nm a t the half- power p o i n t s .
The: l a s e r s e m i t t e d l i g h t pulses o f 0.5 nsec i n d u r a t i o n . By u s i n g d i f f e r e n t lengths o f cables from the p u l s e generator, the pulses were separated temporally so as t o d e t e c t a d i f f e r e n c e i n t h e i r separa- t i o n a f t e r propagation. Microscope o b j e c t i v e s A and F (x20, N.A. = 0.54) c01 1 imated the 1 i g h t s p a t i a l l y from these l a s e r s and u s i n g a beam-spl i t - t e r i n the peth o f l a s e r Ln, t h e pulses were l a u n c h e d i n t o t h e f i b r e wi t h the microscope o b j e c t i v e B. We used a x5 o b j e c t i v e so as t o reduce the m d a l d i s p e r s i o n o f the f i b r e by n o t e x c i t i n g a l l the modes o f the f i b r e . Detector Dl detected the pulses b e i n g launched i n t o the f i b r e , a n d hence t h e i r separation. Microscope o b j e c t i v e C c01 1 imated the f i b r e o u t - p u t l i g h t and the o b j e c t i v e D focused i t o n t o d e t e c t o r D2, which detec- t e d the pulses a f t e r propagation i n the f i b r e . These detectorswere s i l i - con avalanche photodetectors having a response o f a few hundred picose- conds. The pulses were d i s p l a y e d on a P h i l l i p s Sampling Oscilloscope,Mo- de1 PM 3400, and subsequently recorded on an X- Y p l o t t e r . Using the e l e c - t r i c a l delay, the o s c i l l o s c o p e was t r i g g e r e d f i r s t t o observe the i n p u t pulses f rom d e t e c t o r D l and immediatel y a f terwards the o u t p u t pulses f rom d e t e c t o r D2.
3.
RESULTSFig. 2 ( Laser L1 blocked.
was 0.6 nsec. W i t the o u t p u t pulse,
( b ) . This was a 1.8 nsec, g i v i n g
a) shows the i n p u t pulse from Laser L2 a t 800 nm, wi t h The f u l l w i d t h a t half-maximum (FWHM) i n t e n s i t y p o i n t s h a1 1 modes e x c i t e d using a x10 microscope o b j e c t i v e B,
from 105 m o f TELEBRAS f i b r e 79, i s shown i n Figure 2 germania-doped s i 1 i ca f i b r e ( ~ e 0 ~
- s i
02).
The FWHM was a modal d i s p e r s i o n o f 1.7 nsec a f t e r deconvolution o f t h e i n p u t FWHM. This f i b r e showed a 1 i n e a r dependence4" o f modal d i s - persion, when we shortened the f i b r e p r o g r e s s i v e l y by 50 m steps from 350 m t o 100 m, i n d i c a t i n g f a i r l y small mode conversion, i . e . , very l i t -F i g . 2
-
( a ) I n p u t p u l s e t o 105 m o f f i b r e 79 from laser L i a t 800 nm.(FWHM = 0,6 n s ) ; (b) Output p u l s e f r o n the f i b r e . (FWHM = 1.8 ns).
t l e change i n t h e a n g l e o f . t h e l i g h t rays launched. Hence we e x t r a p o l a - t e d t h e modal d i s p e r s i o n i n t h i s f i b r e t o be 17 nsec/km.
F i g . 3(a) shows t h e i n p u t p u l s e s t o t h e f i b r e f r o m t h e two l a - s e r s , as d e t e c t e d by d e t e c t o r Di. The p u l s e X = 904 nm i s t o t h e r i g h t and t h e p u l s e A = 800 nm i s t o t h e l e f t . The p u l s e s e p a r a t i o n was nsec. F i g . 3(b) shows t h e p u l s e s a f t e r p r o p a g a t i o n i n t h e f i b r e . l a u n c h i n g o b j e c t i v e B was changed t o xS t o reduce modal d i s p e r s i o n . p u l s e s e p a r a t i o n reduced t o 9.4 nsec due t o m a t e r i a l d i s p e r s i o n . The d u c t i o n t a k e s p l a c e because t h e i n d e s o f r e f r a c t i o n a t 904 nm i s t h a n t h a t a t 800 nm and t h e former p u l s e t r a v e l s a t a f a s t e r v e l o c
0.6 The The r e - ess t Y . Hence w i t h A A = 104 nm, we observed a m a t e r i a l d i s p e r s i o n o f 1.2 nsec i n 105 m o f f i b r e 79. N o r m a l i s i n g , as i s u s u a l l y done w i t h m a t e r i a l d i s p e r - s i o n , we o b t a i n e d 110 psec/nm/km. The n u m e r i c a l a p e r a t u r e (N.A.) o f t h i s
f i b r e was m?asured t o be 0.21. Another f i b r e , n ? 74, a l s o o f Ge02
-
S i 0 2v a r i e t y , (N.A. = 0.10), was a l s o t e s t e d , g i v i n g a v a l u e o f 96psec/nm/km.
F i g . 3
-
The d i F f e r e n t wavelength pulses launched i n t o t h e f i b r e , ( a ) , and d e t e c t e d a f t e r propagation, (b), i n 105 m of f i b r e 79. l h e A c904 nm pulse i s on the r i g h t and the h = 800 nm pulse i s on t h e 1eFt.Pul- se separation i n ( a ) i 5 10.6 ns and i n (b) i s 9.4 ns.Ths m a t e r i a l d i s p e r s i o n ( A T ) , as deterrnined f r o m t h e o r y 1 3 i s g i v e n by
where L 1s l e r i g t h o f f i b r e ,
X
i s t h e peak wavelength, AA i s t h e ç p e c t r a l spread o f t h e l i g h t s o u r c e used and n i s t h e r e f r a c t i v e i n d e x o f c o r e .d2n The second d e r i v a t i v e o f t h e i n d e x w i t h r e s p e c t t o t h e wavelength
1-1
d
x
d e t e r m i n e s t h e m a t e r i a l d i s p e r s i o n . We can compare o u r r e s u l t s w i t h c a l - c u l a t i o n s made f o r p u r e s i l i c a . We do s o by t a k i n g , f o r f i b r e 79, L=lOSm, C = 3 x 108 m/s, iIh = 104 nm, h = 8 5 0 nm, and (h2-) = 0 . 0 2 2 f r o m f i - g u r e
3
o f G l o g e t s paper15. Hence we o b t a i n A+ = 0,94 nsec compared t o a dh2 v a l u e o f 1.2 nsec observed e x p e r i m e n t a l l y , beii-ig a p p r o x i m a t e l y 30% h i - gher. Comparing t h i s w i t h t h e r e s u l t s o b t a i n e d i n o t h e r l a b o r a t o r i e s ( r e - ferences 10, 13, 16, 17) we n o t e t h a t a s i m i l a r e f f e c t o f i n c r e a s e d ma- t e r i a l d i s p e r s i o n was a l s o measured i n Ge02-
S i 0 2 f i b r e s a t 850 nm. We have summarized these r e s u l t s i n Table 1 . We have n o t c o n s i d e r e d measu- rements on b u l k s a ~ r i ~ l e s ~ ~ ~ ~ ~ , f o r reasons a l r e a d y o u t l i n e d , and a l s o r e - s u l t s 2 0 o u t s i d e t h e wavelength o f 850 nm, w h i c h i s o f main i n t e r e s t i n t h i s paper. M a t e r i a l d i s p e r s i o n o f p u r e s i l i c a a t 850 nm i s85
psec/nm/km. S i n c e t h e germania c o n c e n t r a t i o n s a r e n o t known i n a l l t h e f i b r e s i n T a b l e 1, we have i n d i r e c t l y assessed t h i s by e v a l u a t i n g t h e n u m e r i c a l a- p e r a t u r e o f each f i b r e f r o m t h e d a t a p r o v i d e d ; an i n c r e a s e d Ge02 concen-
Presby and
~ anow16 ~ i Reference
Payne and
~ a r t o ~ l O
I
900I
Peak A (nm)
M i y a s h i t a e t aZ. l7
Table I
-
M a t e r i a l d i s p e r s i o n measurements i n germania- doped s i l i c a f i - b r e s.
M a t e r i a l d i s p e r - s i o n measured.
( ps/nm/km)
850
Sunak and Ayres Neto
( t h i s work) Sunak and AY r e s Neto ( t h i s work)
F i b r e n u m e r i c a l a p e r t u r e
9 1
850
850
0.13
96
110
0.10
0.21
t r a t i o n would g i v e the f i b r e core a h i g h e r r e f r a c t i v e index nl and hence increase i t s numerical aperature, given by (n:
-
n 2 ) l l 2 , where n 2 i s the r e f r a c t i v e index o f the pure s i 1 i c a c l a d d i n g .We have p l o t t e d m a t e r i a l d i s p e r s i o n as a f u n c t i o n o f numerical aperature i n f igu r e 4 . A f a i r l y good l i n e a r r e l a t i o n s h i p i s observed.
From the formula given above, we n o t e t h a t o n l y the second d e r i v a t i v e o f d2n
the core index w i t h respect t o the wavelength
( 7 )
changes t h e m a t e r i a l d Ad i s p e r s i o n , w i t h L, A X and A h e l d constant. The r e s u l t s o f f i g u r e 4 c o u l d be j u s t i f i e d o n l y i f the slope o f the r e f r a c t i v e index curve, w i t h wave-
length, v a r i e d p r o g r e s s i v e l y more r a p i d l y w i t h i n c r e a s i n g c o n c e n t r a t i o n o f Ge02, and hence i n c r e a s i n g numerical aperature. This was indeed shown very c l e a r l y , by the measurements made by Kobayashi e t a2.18, on b u l k samples o f pure s i l i c a doped w i t h d i f f e r e n t amounts o f germanium. This would a l s o seem t o imply t h a t measurements c a r r i e d o u t on t h e f i b r e i t -
s e l f are i n q u a l i t a t i v e agreement w i t h measurements made on small b u l k samples. Wnrk i s t o be c a r r i e d o u t t o i n v e s t i g a t e t h i s f u r t h e r and t o o b t a i n quanti t a t i ve r e s u l t s of t h e dependence o f materi a1 d i s p e r s i o n on Ge02 c o n c e n t r a t i o n measured i n the f i b r e i t s e l f . From the r e s u l t s o f Presby and ~ a m i n o w l ~ , i t can be seen t h a t , o f the s i x dopants used, ger- manium has the most pronounced e f f e c t i n i n c r e a s i n g t h e m a t e r i a l d i s p e r - s i o n . In B2O3
- Si02
f i b r e s , the m a t e r i a l d i s p e r s i o n d e c r e a s e s very0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.32 N U W R I C A L APERTURE
F i g . 4 - M a t e r i a l d i s p e r s i o n , a t 850 nm, i n Ge02 - S i 0 2 f i b r e s as a f u n c t i o n o f n u m e r i c a l a p e r a t u r e ( O
-
a f t e r Presby and ~ a m i n o w ' ~ ; + -a f t e r H i y a s h i t a e t a 2 . l ' ; x
-
t h i s work,-
m a t e r i a l d i s p e r s i o n o f pure s i l i c a ) .s ! i g h t l y whereas i n P205
-
S i 0 2 f i b r e s , no change as compared t o p u r e s i l i c a , i s observed. As s t a t e d e a r l i e r , o n l y i n single-mode o r optim'um graded- index multimode f i b r e s , t h e e f f e c t o f each dopant on t h e f i b r e bandwidth would have t o be c o n s i d e r e d c a r e f u l l y f o r systems d e s i g n a t a p a r t i c u l a r wavelength. However, i n s t e p - i n d e x multimode f i b r e s , t h e e f - fec: o f i n c r e a s e d m a t e r i a l d i s p e r s i o n , even w i t h h i g h Ge02 c o n c e n t r a - t i o n s , i s n e g l i g i b l e as d i s c u s s e d below.Our f i b r e 79 h a d a modal d i s p e r s i o n ( r ) o f l i nsec/km. Assu- ming t h a t we used a l i g h t emi t t i n g d i o d e o f 40 nm s p e c t r a l w i d t h a t 850 g nm, we would have an addi t i o n a l m a t e r i a l d i s p e r s i o n ( T ~ ) o f 4.4 tisec/km.
Assuming Gaussian p u l s e shapes, t h e t o t a l b r o a d e n i n g ( r ) i n 1 km o f f i - b r e w o u l d be r 2 = 1 7 ~
+
4.b2, g i v i n g T = 17.6 n s e c . Hence i t i s c l e a r t h a t t h e bandwidth o f o u r f i b r e i s I imi t e d by moda1 d i s p e r s i o n . T h i s was t o be expected as f ; b r e 79 i s o f t h e mul timode s t e p - i n d e x t y p e . We can c a l c u l a t e , c o n s e r v a t i v e l y , t h e maximum p u l s e b í t r a t e (B) p o s s i b l e i n o u r f i b r e s u s i n g B = ( 2 ~ ) - I f o r u n i t y mark/space r a t i o . Hence B = 28.4 M b i t s /sec o v e r a I km l e n g t h . I f T i s roduced t o 4 nsecikni by d e s i g n i n g a graded- index mul timode f i b r e , u s i n g t h e same l i g h t source, we would ob- 9 t a i n r = 6 nsec and B = 8 4 Mbi t s / s e c . Hence f a i r l y l a r g e bandwidths a r e s t i l l p o s s i b l e u s i n g h i g h l y r e l i a b l e l i g h t e m i t t i n g diodes o p e r a t i n g a t 850 m, and systerns u s i n g t h e s e sources w i 11 have many d i v e r s e a p p l i c a - t i o n s .5. CONCLUSION
We have used two GaAs l a s e r s o f d i f f e r e n t wavelengths and mea- s u r e d a m a t e r i a l d i s p e r s i o n o f 110 psec/nm/km i n Ge02
-
S i 0 2 f i b r e s a t a wavelength o f 850 nm. T h i s r e s u l t i s i n good a g r e e m n t w i t h p r e v i o u sr e s u l t s , showing an i n c r e a s e o f -30% f r o m t h e v a l u e o f 85 psec/nm/km f o r F u r e s i 1 ic a . We a l s o c o n c l ude t h a t modal d i s p e r s i o n dorninates m a t e r i a l d i s p e r s i o n i n rnultimode s t e p - i n d e x f i b r e s b u t t r a n s m i s s i o n r a t e s o f many t e n s o f megabi t s p e r second o v e r one k i lo m e t e r can s t i
We wouid 1 i k e t o thank Telecomunica@es Bras
1 be a c h i e v e d .
l e i r a s S/A
ele-
b r á s ) f o r f i n a n c i a l l y s u p p o r t i n g t h i s w o r k and s u p p l y i n g t h e f i b r e s . One o f us, (J.B.M. Ayres ~ e t o ) a l s o thanks FAPESP f o r a p o s t - g r a d u a t e scho- l a r s h i p , and i s now w i t h C e n t r o de Pesquisas e ~esenvolvimento,Telebrás, Campinas, São P a u l o .
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