APPLICATIONS OF ELECTRO-OPTIC AND NONLINEAR MATERIALS FOR WAVEGUIDE DEVICES

M J GOODWIN*

1. INTRODUCTION

The a p p l i c a t i o n o f o p t i c a l l y n o n l i n e a r m a t e r i a l s i n waveguide formats i s

p a r t i c u l a r l y a t t r a c t i v e f o r several reasons. F i r s t l y , o p t i c a l n o n l i n e a r i t i e s
a r e g e n e r a l l y weak, r e q u i r i n g h i g h o p t i c a l i n t e n s i t i e s o r e l e c t r i c f i e l d
s t r e n g t h s f o r u s a b l e e f f e c t s . The a d o p t i o n o f a waveguide geometry a l l o w s
t h e s t r o n g f i e l d s t o be achieved r e a d i l y by c o n f i n i n g t h e o p t i c a l f i e l d t o a
very small c r o s s s e c t i o n , t y p i c a l l y o f o r d e r 10,0112. A d d i t i o n a l l y , these h i g h
f i e l d s can be m a i n t a i n e d over l o n g i n t e r a c t i o n l e n g t h s , a l l o w i n g h i g h e r
i n t e r a c t i i o n e f f i c i e n c i e s than a r e a c h i e v a b l e i n b u l k m a t e r i a l s . The second
a t t r a c t i o n o f t h e waveguide format stems f r o m t h e p o s s i b i l i t y o f combining
s e v e r a l d e v i c e f u n c t i o n s on a s i n g l e i n t e g r a t e d o p t i c component, t h e
i n d i v i d u a l d e v i c e elements b e i n g connected by m o n o l i t h i c waveguide s e c t i o n s .
This g r e a t l y increases the functional complexity available from e l e c t r o - o p t i c
and n o n l i n e a r components t o t h e e x t e n t t h a t t h e range o f a p p l i c a t i o n s o f
waveguide d e v i c e s i s now expanding r a p i d l y .

Nonlinear o p t i c a l e f f e c t s can be d i v i d e d c o n v e n i e n t l y i n t o two c a t e g o r i e s ;

e l e c t r o - o p t i c e f f e c t s i n which an a p p l i e d e l e c t r i c f i e l d i n t e r a c t s w i t h t h e
o p t i c a l f i e l d s , and n o n l i n e a r ( o r a l l - o p t i c a l ) e f f e c t s i n which two o r more
o p t i c a l f i e l d s i n t e r a c t d i r e c t l y . The b a s i c f u n c t i o n s o f f e r e d by e l e c t r o -
o p t i c and n o n l i n e a r waveguide devices w i l l be d e s c r i b e d and t h e i r use i n more
complex i n t e g r a t e d o p t i c a l components w i l l be i l l u s t r a t e d w i t h examples f r o m
t h e f i e l d s o f o p t i c a l communications, o p t i c a l sensors and s i g n a l processing.

2. ELECTRO-OPTIC DEVICES

The a p p l i c a t i o n o f an e l e c t r i c f i e l d t o an e l e c t r o - o p t i c m a t e r i a l r e s u l t s i n
a change i n i t s r e f r a c t i v e index, and consequently a l t e r s t h e phase of l i g h t
passing through it. The s i m p l e s t waveguide device i s thus t h e phase
modulator shown s c h e m a t i c a l l y i n F i g . 1. The performance o f a waveguide
phase m o d u l a t o r can be c h a r a c t e r i s e d by t h e d r i v e v o l t a g e p e r u n i t
bandwidth 11j.

v/Af = xR [z] (PA) [q] (1)

where nf i s t h e bandwidth, R t h e t e r m i n a t i n g r e s i s t a n c e , E f f t h e d i e l e c t r i c
constant, n t h e r e f r a c t i v e index, r t h e e l e c t r o - o p t i c c o e f f i c i e n t , p a f a c t o r
determined by t h e modulator type, h t h e wavelength, d t h e e l e c t r o d e
separation, 5 a f u n c t i o n o f t h e e l e c t r o d e dimensions and r a f a c t o r r e l a t i n g
t o t h e degree o f o v e r l a p between t h e o p t i c a l and a p p l i e d e l e c t r i c f i e l d s w i t h
a v a l u e between 0 and 1. Thus, m i n i m i s i n g V / A f f a v o u r s m a t e r i a l s w i t h l o w
d i e l e c t r i c c o n s t a n t , h i g h e l e c t r o - o p t i c c o e f f i c i e n t s and h i g h r e f r a c t i v e
index. Also, t h e e l e c t r o d e c o n f i g u r a t i o n should be designed t o maximise r,
which i n turn f a v o u r s s t r u c t u r e s a l l o w i n g t h e p l a c i n g o f e l e c t r o d e s above and

*M J Goodwin, Plessey Research Caswell L i m i t e d , Caswell, Towcester,

Northants, England, "12 8EQ

611
below t h e waveguide, though t h i s i s n o t p o s s i b l e w i t h a i l e l e c t r o - o p t i c
m a t e r i a l systems.

The device bandwidth i s l i m i t e d by t h e RC t i m e c o n s t a n t o f t h e e l e c t r o d e

s t r u c t u r e , and t h i s p r e s e n t s a t r a d e - o f f between reduced capacitance (C-L)
f o r h i g h speed, and t h e consequent i n c r e a s e i n d r i v e v o l t a g e r e q u i r e d f o r a
g i v e n degree o f m o d u l a t i o n (V-l/L). The l i m i t a t i o n s imposed by lumped
e l e c t r o d e s can be overcome by employing a t r a v e l l i n g - w a v e e l e c t r o d e s t r u c t u r e
i n which t h e o p t i c a l and e l e c t r i c a l s i g n a l s propagate t o g e t h e r along t h e
device, y i e l d i n g a c u m u l a t i v e phase s h i f t . Very h i g h bandwidths up t o
100 GHz have been r e p o r t e d [ 2 , though a c h i e v i n g low e l e c t r i c a l losses and
matched e l e c t r i c a l and o p t i c a v e l o c i t i e s i n t h e s e s t r u c t u r e s r e q u i r e s
careful attention.

Of w i d e r i n t e r e s t and a p p l i c a t i o n a r e e l e c t r o - o p t i c devices y i e l d i n g
amplitude m o d u l a t i o n o r s w i t c h i n g . The two s t r u c t u r e s shown i n F i g u r e 2, t h e
Mach-Zehnder i n t e r f e r o m e t e r and t h e d i r e c t i o n a l c o u p l e r , a r e t h e most
commonly employed c o n f i g u r a t i o n s t l ] [ 3 j . The design o f these devices i s
g e n e r a l l y concerned w i t h a c h i e v i n g h i g h m o d u l a t i o n r a t e f o r a minimum expend-
i t u r e o f e l e c t r i c a l power, and t h e design t r a d e o f f s f o r these two r e q u i r e -
ments a r e s i m i l a r t o t h o s e f o r t h e phase modulator and w e l l documented 141.
Broadband performance t o 40 GHz has been demonstrated i n LiNbO 5 I, and
several schemes f o r f r a c t i o n a l bandwidth m o d u l a t i o n can be empfoied t o
r e s t r i c t t h e m o d u l a t i o n bandwidth w i t h t h e aim o f reduced d r i v e power
requirement a6-1[7][8]. Equation 1 y i e l d s a f i g u r e of m e r i t , F, f o r comparing
i n t e n s i t y mo u f a t o r s , F = hfh/V, The d e v i c e shown i n F i g u r e 3 d i s p l a y e d t h e
h i q h e s t r e D o r t e d v a l u e o f F=6.2 G~z.m.v-1. T h i s was a GaAs/ GaAlAs
t r a v e l l i n g ' wave Mach Zehnder s t r u c t u k e employing an e l e c t r i c a l slow-wave
e l e c t r o d e system t o achieve near v e l o c i t y matching 1 9 3 .

3. INTEGRATED ELECTRO-OPTIC COMPONENTS

I n i t i a l i n t e g r a t i o n o f d i s c r e t e e l e c t r o - o p t i c d e v i c e elements has been a med

a t a c h i e v i n g l a r g e a r r a y s of waveguide switches f o r space-, time- and
w a v e l e n g t h - d i v i s i o n s w i t c h i n g f o r telecommunications, and more r e c e n t l y f o r
s w i t c h i n g r.f. s i g n a l s c a r r i e d on f i b r e o p t i c t r a n s m i s s i o n l i n e s . I n
a d d i t i o n t o t h e i r h i g h bandwidth c a p a b i l i t y , these s t r u c t u r e s a r e a l s o
a t t r a c t i v e because o f t h e i r transparency t o t h e frequency c o n t e n t o r
modulation f o r m a t o f t h e data they convey, i n marked c o n t r a s t t o e q u i v a l e n t
e l e c t r i c a l s w i t c h i n g a r r a y s . The most commonly used s w i t c h i n g element
employed i s t h e d i r e c t i o n a l c o u p l e r , and a r r a y s up t o 8 x 8 i n Ti:LiNbO,
waveguides i l O j [ l l j and 4 x 4 i n InP semiconductor waveguides L 1 2 ] have been
reported.

I n terms o f f u n c t i o n a l complexity, t h e most demanding i n t e g r a t e d component

c u r r e n t l y b e i n g i n v e s t i g a t e d i s t h a t used i n t h e f i b r e o p t i c gyroscope (FOG),
shown s c h e m a t i c a l l y i n F i g u r e 4. These components a r e c u r r e n t l y f a b r i c a t e d
i n Ti:LiNbO, and i n c l u d e d i r e c t i o n a l c o u p l e r , phase modulator and phase and
p o l a r i s a t i o n f i l t e r elements i n t e g r a t e d i n a s i n g l e i n t e g r a t e d o p t i c
component. The FOG i s c o n f i g u r e d as a Sagnac i n t e r f e r o m e t e r i n which
r o t a t i o n o f t h e f i b r e c o i l induces a phase s h i f t between t h e two c o u n t e r
p r o p a g a t i n g l i g h t waves [13j. A p p l i c a t i o n o f an equal and o p p o s i t e e l e c t r o -
o p t i c phase s h i f t c a n c e l s t h i s , and t h e v o l t a g e r e q u i r e d i s p r o p o r t i o n a l t o
the r o t a t i o n rate.
4. NONLINEAR WAVEGUIDE DEVICES

Second-order n o n l i n e a r i n t e r a c t i o n s i n c l u d e second harmonic generation, and

p a r a m e t r i c a m p l i f i c a t i o n and o s c i l l a t i o n . These have been i n v e s t i g a t e d i n
waveguide s t r u c t u r e s , p r i n c i p a l l y i n LiNbO, based m a t e r i a l systems ~ 1 4 1.
More r e c e n t l y , o r g a n i c m a t e r i a l s have r e c e i v e d c o n s i d e r a b l e a t t e n t i o n 115 J,
o f f e r i n g 1a r g e r n o n l inear c o e f f ic i e n t s and t h e consequent p o s s i b i 1it y o f
e f f i c i e n t o p e r a t i o n a t o p t i c a l power l e v e l s a v a i l a b l e f r o m s o l i d s t a t e l a s e r
diodes, and s i m p l e l o w - c o s t waveguide f a b r i c a t i o n processes.

T h i r d - o r d e r n o n l inear waveguide devices a r e based on t h e in t e n s i ty-dependent

r e f r a c t i v e i n d e x ( n 2 ) , whereby

n (I) = no + n 2 I (2)
where no i s t h e l i n e a r r e f r a c t i v e i n d e x and I i s t h e o p t i c a l i n t e n s i t y . I n
a guided-wave device, t h i s leads t o an i n t e n s i t y - d e p e n d e n t p r o p a g a t i o n
constant. C l e a r l y , t h e e l e c t r o - o p t i c device concepts discussed above have an
a l l - o p t i c a l e q u i v a l e n t , i n which t h e r o l e o f t h e a p p l i e d e l e c t r i c f i e l d i s
played by t h e o p t i c a l i n t e n s i t y i t s e l f . F o r example, i n t h e n o n l i n e a r
d i r e c t i o n a l c o u p l e r , an i n c r e a s e i n o p t i c a l i n t e n s i t y o f t h e i n p u t s i g n a l
induces a change i n p r o p a g a t i o n c o n s t a n t and switches t h e o u t p u t s i g n a l from
one o u t p u t p o r t t o t h e o t h e r . T h i s t y p e o f a l l - o p t i c a l s w i t c h i n g a l l o w s
o p t i c a l l o g i c gates t o be d e v i s e d which have p o t e n t i a l a p p l i c a t i o n s f o r
s i g n a l processing, p a r t i c u l a r l y s i n c e t h e s w i t c h i n g t i m e s a s s o c i a t e d w i t h
some t h i r d - o r d e r m a t e r i a l s l i e i n t h e sub-picosecond regime, f a r beyond t h e
speeds a c c e s s i b l e i n e l e c t r o n i c l o g i c gates.

To date t h e r e have been few demonstrations o f a l l - o p t i c a l s w i t c h i n g

( r e f e r e n c e 16 p r o v i d e s a r e c e n t r e v i e w o f t h e area) and t h e optimum m a t e r i a l
has y e t t o be i d e n t i f i e d , p r i m a r i l y because o f t h e somewhat severe r e q u i r e -
ments imposed by t h e h i g h o p t i c a l i n t e n s i t i e s encountered i n these devices.
Various f i g u r e s of m e r i t have been proposed and, by most, s i l i c a g l a s s i s
ranked h i g h l y : i n i t i a l l y s u r p r i s i n g s i n c e s i l i c a processes a very small n 2
c o e f f i c i e n t , though i n p r a c t i c e t h i s i s o f f s e t by i t s very low a t t e n u a t i o n
and l o w a b s o r p t i o n a c h i e v e d i n o p t i c a l f i b r e geometries. It i s i n t h e s i l i c a
system t h a t t h e f a s t e s t a l l - o p t i c a l s w i t c h i n g has been r e p o r t e d w i t h a
response t i m e l e s s t h a n 100 f s e c 1171. C l e a r l y , f u r t h e r m a t e r i a l s
development i s r e q u i r e d t o a l l o w h i s c l a s s o f devices t o f u l f i l i t s
potential.

5. CONCLUSIONS

E l e c t r o - o p t i c and n o n l i n e a r waveguide devices a r e a t t r a c t i v e f o r a range o f

e x i s t i n g and p o t e n t i a l a p p l i c a t i o n s i n o p t i c a l comnunications, o p t i c a l
sensors and s i g n a l p r o c e s s i n g . C u r r e n t l y , LiNbO, and t h e 111-V compound
semiconductor GaAs and InP based m a t e r i a l s systems a r e t h e most advanced f o r
e l e c t r o - o p t i c and second-order n o n l i n e a r devices, and s i l i c a f i b r e s f o r t h i r d
o r d e r wavegui de d e v i ces . New m a t e r i a1 s o f f e r i n g 1a r g e r e l e c t r o - o p t i c and
n o n l i n e a r c o e f f i c i e n t s would r e s u l t i n improved d e v i c e performance, though
a d d i t i o n a l r e q u i r e m e n t s f o r p r o c e s s i b i l i t y , low-loss, s t a b i l i t y and o p t i c a l
damage t h r e s h o l d s must a l s o be addressed.
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 Figure 1: Electro-optic Waveguide Phase Modulator

0 V.L 0 V.L

( a ) Mach-Zehnder Interferometer (b) Directional Coupler

Figure 2: Electro-optic Waveguide Amp1 itude Modulators and t h e i r

Transmission Characteristics
 +ve d.c. waveguide

4
cw
light
in

F i g u r e 3: T r a v e l 1 ing-Wave Mach-Zehnder Modulator i n AlGaAs/GaAs

-
light output
to detaxor
plariser/mode filter

integrated

F i g u r e 4: I n t e g r a t e d O p t i c F i b r e O p t i c Gyroscope