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Integrated optics and thin-film technology
241 I N T E G R A T E D O P T I C S AND T H I N - F I L M T E C H N O L O G Y *
P.K. T I E N Bell Telephone Laboratories, Holmdel, New Jersey 07733, USA Invited paper Research in integrated optics was initiated about six years ago in an effort to form integrated optical circuits which contain miniature lasers, modulators and waveguides. We present in section I a historical review of the subject and discuss, in sections 2 and 3, new wave p h e n o m e n a in optical waveguides, magneto-optic devices and monolithic integrated optics in GaAs-related s e m i c o n d u c t o r compounds. We e m p h a s i z e that various m e t h o d s used in integrated optics can be s u m m a r i z e d in four simple rules.
1. Historical development of integrated optics Integrated optics has two distinct goals: one is to apply thin-film technology to the formation of optical devices and circuits; the other is to integrate a large n u m b e r of optical c o m p o n e n t s on a single small substrate, so forming an optical circuit reminescent of the integrated circuit in microelectronics. When research in integrated optics was initiated six years ago, the idea of forming thin-film optical devices was new. One thought of m a n y advantages of the thin-film optical components. They can easily be miniaturized, are readily integrable and when they are bonded to a substrate, a solid and stable circuit can be produced. One was also intrigued by the success of the microelectronics and the thought that the medium or even the large scale integration in optical systems may be possible was certainly inviting. At that time, those who were engaged with the integrated optics had extensive experience in m i c r o w a v e technology. An idea which naturally followed was that optical waveguides should be used as the basic structures of the optical c o m p o n e n t s so that a light wave can propagate freely in them. It turns out that the simplest waveguide is a thin layer of the dielectric film deposited on a substrate with the refractive index of the film larger than that of the substrate. The waveguide in such a thin-film f o r m is called slab- or film-waveguide. Similarly, a channel waveguide is f o r m e d by depositing a thin strip of the film on a substrate. More complicated optical waveguides include p - n junctions and heterostructures used in the semi * This paper is a shortened version of a review paper in Review of Modern P h y s i c s , which will he published in the April 1977 issue.
Physica 89B (1977) 241-254 © North-Holland
conductor injection laser diodes. One can thus visualize an optical circuit consisting of many c o m p o n e n t s which are, themselves, film or channel waveguides, p - n junctions and heterostructures. By joining those waveguides together, the light wave can be transmitted f r o m one c o m p o n e n t to another. In that sense, the optical circuits would not be much different f r o m the m i c r o w a v e circuits. On the other hand, however, wavelength of the light considered here is about a factor of 10,000 smaller than those of the microwaves, a far more sophisticated technology is required to construct the optical circuits. In the beginning, even the simple experiment of propagating a light w a v e in a film-waveguide was not easy. The light wave can be easily scattered f r o m a slightly rough surface. We recall in 1969 [1] when a sputtered ZnO film was f r s t used as the optical waveguide, the light wave propagated only a distance less than 1 mm in the waveguide before it was completely scattered. The waveguide had then an optical loss of 60 dBlcm. Soon, waveguides made of glass films by Goell and Standley [2] and those of organosilicon films by Tien et al. [I], had losses less than 1 dB/cm. Figure 1 shows photographs of the light paths in (a) a thermally oxidized Ta205 film on glass substrate, (b) a polymerized vinyltrimethyisilane film on pyrex substrate and (c) a single-crystal LiNbO3 film grown epitaxially on a LiTaO3 substrate. In those photographs, a short light path indicates a large optical loss in the waveguide. In the early experiments, how to couple a laser beam onto a very thin waveguide was a serious problem. The edge of the film was usually rough and the thickness of the film was typically less than 1 micron; it was not practical
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and p r e c i s e p o s i t i o n i n g oi lhc light b e a m arc ~ IleCeSSHl}. l ' h e p F i M l l tt'-.CtJ !',)! | h ' , l | plil'po',~_ i ~called prisnl c o u p l e r . Simihi! io lhe !ncl;iJiic w a v e g u i d e u,~ed ~n TlllCft'lwLi\cs. ill} ~)p!ic~:' w a v e g u i d e c a n ..,ll'.;l;lii1 i l l , l i l y { l : i ! i s v c r s c c i c c ! F i , , (TF;) ;ind I r a n s v c i s c m a g n e t i c i I M ) m o d e ..... <' p r o p a g a t i o n . I{ach n/ode h~, ", ~- ,.~v~ ! wa, v v e l o c i t y w h i c h Villies w i t h lhc t h i c k n c s , ~,,t ! h i film and the r e f r a c t i v e indice~ of the film :ind the subslrate. By u>ing lhc >o-called " m-ii!i,: s p e c t r o s c o p y d i s c ~ w e r c d b', I ) c n e~ :d !~}, ~ :. p o s s i b l e to d e l e l n l i i l c the '~c v e i o c i l x , ~t}d thus the m o d e of the [ighl ,a.n~c p!opagaiioi~ ii~ the w a v e g u i d e . It is ab, o p o s s i b l e to demtmsira-:c waveguide mode; visuall\ m ~ pr,~Ljevli~;.q s c r e e n . F i g u r e 2 is ~ p h o t o g l ' a p h of the !!;di~!c, p u b l i s h e d h~ Tien el nl !~,i }~iach hrigh! lmc ~ the p h o t o g r a p h r e p r e s e n t s one w a v e g u i d e m o d e T h e n u m b e r s b e l o w i n d i c a t e ihe o r d e r , of ll'!¢ w a v e g u i d e m o d e s . T h e bright ,,pot in the middle of the p h o t o g r a p h i n d i c a t e s the w a v e g u i d e niodc in w h i c h the light wa~c in the w a ~ e g u M e ~ . ~ propagating. N o w imagine a light w a v e w h i c h i,: p r o p a g a ring in 'a f i l m - w a v e g u i d e Naturally. the t h i c k n e s s of the light he:m! must he c~mtaincd within the thicknes,, of the film: the path of Ihe light b e a m in the phlne of the film f o l l o w s lhc tlsual g e o m e t r i c a l optic,, [:o," e x a m p l e , if ,~l-~c film is not u n i f o r m and lht]> the w a v e ~etoci[~ v a r i e s in different area', of the film. t h e r e w, ill hc r e f r a c t i o n s of light b e t w e e n i h o s e area~. Refle~ lion a n d r e f r a c t i o n of light o b s e r v e d in the two, d i m e n s i o m t l p l a n e of a w a ~ e g t t i d c i~ called t~,.
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c Fig. I P h o t o g r a p h s s h o w the light w a v e pltlpagalit:,n hi thin-fihn w a v e g u i d e s m a d e of (:i) a Taft), fihn till gilts <` ~,uhstrale, fb) :i v i n y l l r i m e t h y l s i l a n e p o l y m e r fihn on p~,rc,,
to f o c u s a light b e a m o n t o the w a v e g u i d c d i r e c t l y t h r o u g h the e d g e of the film. Fort u n a t e l y , it was soon d i s c o v e r e d that by using ai p r i s m , one can c o u p l e a laser b e a m o n t o o r o u l of the waveguJde t h r o u g h the top surface of the
fihn 13.4,51. In this arrangement, tight focusing
A f t e r o b s e r v i n g r e f r a c t i o n of light i!~ wa~,, g u i d e s , the idea o f f o r m i n g thin-film p r i s m s aud l e n s e s c a m e n a t u r a l l y . Yet Ihe possihilil,, ~d f o r m i n g t h o s e f a m i l i a r o p t i c a l c o m p o n e n t s ii~ thin films f a s c i n a t e d !nanv, l ' h i n - f i l m p r i s m s a~,~ l e n s e s w e r e first r e p o r t e d {!~ the [3e'~ic-. R e s e a r c h C o n f e r e n c e held in Rocheste~ :n ,4.6v~ 161. F i g u r e i~ s h o w s p h o l o g r a p h s of ~ ~hin-tih, p r i s m , a lens. a p o l a r i z e r : ! n d ;t c o r n e l {t.'flecI{ lr e p o r t e d in s e v e r a l p a p e r s b,, ] i e H ci i i !7 g! T h e y w e r e m a d e of two hiyer', of the films: ~mc film t y p i c a l l y Z n S (or Nb,().) has :', large Ic f r u c t i v e index n 2..~ and the o t h e r film typically glass (or ~lrganic rnalerials)~ h:l,, ~l t,,iv~ r e f r a c t i v e i n d e x n , I. ~ T h m - f i h n geode,dc tenses r e c e n t l ) reported b) V e r b e r el al. I
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Fig. 2. The bright vertical lines in this photograph are called m-lines. Each line represents one waveguide mode. From the positions of the m-lines, one can determine the refractive and thickness of the film as well as the wave velocities of the waveguide modes. The so-called m-line spectroscopy provides valuable information of the wave propagation in the waveguide. diffraction limit and are able to resolve light b e a m s with an angular separation as small as 3.3 mrad. Another important d e v e l o p m e n t in integrated optics is the use of the gratings for couplers, reflectors, band-rejection filters and more importantly, laser structures. A grating is a corrugated surface which may be f o r m e d either on the top surface or at the film-substrate interface of a waveguide. The waveguides containing gratings are called corrugated waveguides or simply periodic structures. The use of a grating for reflection of light in the waveguide was first proposed by Miller [10]. Subsequently, the use of the gratings for coupling light onto waveguides was reported by Dakss et al. [11] and also by Kogelnik and Sosnowski [12]. In 1970, Kogelnik and Shank [13] proposed the use of a corrugated waveguide as the laser cavity and demonstrated a dye laser based on this principle. They called this type of the lasers distributed f e e d b a c k (DFB) lasers. Soon their idea was extended to form Bragg reflector lasers (BRL) [14]. As of today, the DFB G a A s heterostructure lasers with separate confinement made by Casey et al. 1114] have been operated at
room t e m p e r a t u r e under pulsed conditions [15] with a threshold current of only 2.2 k A / c m 2 and those made by N a k a m u r a et al. [16] have been operated at r o o m t e m p e r a t u r e and continuous wave with a threshold current of 3.5 k A / c m 2. Besides the lasers, one has to switch or modulate the light in a communication system. The use of semiconductor p - n junction diodes for modulation of light can be dated b a c k to the early sixties. For example, Nelson and Reinhart reported light modulation in reverse-biased G a P junctions in 1964 [17]. Bond et al. [18] and Yariv and Leite [19] discussed waveguide modes in p - n junctions in 1963. Since the inception of integrated optics in 1969, Hall et al. experimented with the light modulation in G a A s epitaxial layers [20] and Reinhart and Miller [21] built efficient G a A s / A I G a A s double heterostructure light modulators. Between 1970-1972, Tien et al. [22] reported a magneto-optic switch which used an iron-garnet film as the waveguide. The switch is capable of switching the light at a speed up to 3 0 0 M H z (fig. 4a). Martin and Hall [23] and Taylor et al. [24] studied light modulation in optical waveguides f o r m e d by diffusion in I I - V I c o m p o u n d s such as ZnS/ZnSe and CdS/CdSe.
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Fig. 3, P h o t o g r a p h s of (a) u thin-film prism, (h) a Ion',, (c) ~ p o l a r i z e r and ,~d) a c o r n e r 1eflcc~r m a d e b', the m e l b o d ~i t w o - l a y e r e d c o n s t r u c t i o n Tho,,e thin-fihn c o m p o n e n t , , h a v e lhc si/e,, of r t m g h l v I m m - I mm :rod ~ ];n'ge n u m b e r of them ,:m~ be f o r m e d s i m u l t a n e o u s l y in or~c single p h o t o l i t h o g r a p h i c proces,,
H a m m e r et al. [25] reported electro-optic Bragg modulators in ZnO layers epitaxially grown on sapphire substrates by a close-spaced chemical v a p o r transport method. LiNb()~ has large electro-optic and acousto-optic coefficients and is an ideal material for electro-optic and acousto-optic devices. LiNbO~ optical waveguides were first f o r m e d by K a m i n o w and Carruthers [26] by out-diffusion of Li. Subsequently, they have been formed using in-diffusion of Ti in LiNbO~ by Schmidt and K a m i n o w [27] and in-diffusion of Nb in LiTaO~ by H a m m e r and Philips 1281. LiNb()~ waveguides have also been formed by melt-phase epitaxy and liquid phase epitaxy by
Miyazawa el al. [29] and Ballman et al. [3(}], Between 1973 and 1975, a large number of the papers were published on LiNbO~ waveguidedevices. Some of them are shown in fig, 4. Figure 4b is a channel waveguide phase modulator m':tde by K a m i n o w el al. [31]. Rather remarkably, it has a merit constant of 0.002 m W / M H z ; (c) is an acousto-optic deflector by Schmidt et al. [32L which can achieve I00 percent intensity modulation in the zeroth-order beam using only 250 mW of acoustic power: td~ is an electro-optic lighl beam scanner by Tien el al. [331 which is capable of scanning a light beam in the waveguide over :in angle of t~'
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Fig. 4. (a) A magneto-optic switch made of a Y~Ga,,7~Sco,Fe~7,O,2film on a Gd~Ga~O,2substrate; (b) an electro-optic channel waveguide phase modulator made by diffusing Ti into a LiNbO3 substrate; (c) an electro-optic light beam scanner made of a LiNbO3 film epitaxially grown on a LiTaO3 substrate; (d) an acousto-optic light beam deflector made by diffusing Ti into a LiNbO3 substrate. covering roughly 10 resolvable spots. Many of the thin-film devices described a b o v e have efficiencies considerably larger than their bulk counterparts mainly because, in the waveguides, one can concentrate the light b e a m and the switching electric, magnetic or acoustic field simultaneously in a very small space. Some simple integrated optical circuits have also been made. For example: O s t r o w s k y made a circuit containing a glass waveguide and a silicon photodetector [34]; Martin used diffusion technique forming a circuit involving an optical switch coupled to a branching waveguide [35]; Reinhart and Logan [36] reported a G a A s chip which contains a laser and a modulator. Very recently, Schmidt f o r m e d a matrix of four optical switches [37]. In the past, Topical Meeting on Integrated Optics was held e v e r y two years. The first meeting was held in Las Vegas in 1970 and was attended by more than 250 scientists from 11 different countries. The main topics of discussion were waveguides and waveguide modes. The second meeting was held in N e w Orleans; magneto-optic and electro-optic modulators were then the subjects of interest. The third meeting was held in Salt L a k e City early this year. The lasers, switches, modulators, couplers
reported in this meeting have already reached a certain degree of sophistication. Indeed, a great deal has been accomplished in integrated optics in the past six years.
2. Methods of forming thin-film optical devices and circuits Integrated optics is a diversified field; it involves material science, fabrication techniques, electromagnetic theory, device engineering and quantum electronics. Since 1969, more than 400 papers have been published on the various subjects of integrated optics. In spite of this vast amount of the publication, the basic principles involved in the formation of thin-film optical devices and circuits are rather simple and they can be s u m m a r i z e d in several simple rules, namely, rule of the refractive index, rule of tapered transitions, rule for reflection of light and rule for corrugated waveguides. We will discuss those rules in detail below. In all the systems involving energy-flow or wave-propagation, if the p h e n o m e n o n considered is orderly, the energy flow tends to concentrate in the region where the wave velocity is the slowest. In the optical system, the refractive index is the measure of the inverse of
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the w a v e velocity. We m a y thus state as the rule for the refractive index: In a thin-film s t r u c t u r e parallel to the light w a r e p r o p a g a t i o n , the light w a r e tends t o he c o n t a i n e d a n d guided in the region where the r e f r a c t i r e index is the largest. The light wave propagation can therefore be confined in a thin layer of the film which has a refractive index larger than that of the surroundings. That ix, of course, the basic principle of the dielectric w a v e g u i d e s which were d i s c o v e r e d by H o n d r o s and D e b y e [38] dated back to 1910. In a way, the light w a v e propagation in high-index film is reminescent of the current flow in a c o n d u c t i n g electric circuit, although refractive index and c o n d u c t i v i t y play entirely different roles in the Maxwell equalions. We can explain a large n u m b e r of the w a v e p h e n o m e n a in integrated optics by using the a b o v e rule. Returning again It) the film-waveguide s h o w n in fig. 5a the light w a v e is contained and guided in the film b e c a u s e the film has a larger refractive index than the substrate and the airspace. Similarly a reverse-biased p - n junction used in the s e m i c o n d u c t o r diode is also an optical waveguide. The plasma contribution of the refractive index in a s e m i c o n d u c t o r varies inversely with the square root of the electron c o n c e n t r a t i o n . W h e n the p - n junction is reversely biased, electrons are swept out of the junction which has then a refractive index larger than that in the vicinity, in the G a A s s y s t e m , a h e t e r o s t r u c t u r e is f o r m e d by cladding a G a A s p - n junction b e t w e e n two layers of
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Fig. 5. Figures show(a) a film waveguide,(b) alapcred-film coupler, (c) a junction belween pa,'o film-waveguides and Id) an interconneclion between two thin-film devices. ()no can easily explain the wave phenomena observed m Fho,,c figures hy the rule of the refractive index
A I G a A s . Since the refractive index of the (ktA~, is larger than that of the A 1 G a A s , the entire p-n junction in this case b e c o m e s an optical wave~ guide which, in a laser diode, is often called the aclive region. Figure 5h ,,hows another interesting a~ rangemenl called tapered-fihn coupler which was d i s c o v e r e d by Tien and Martin 1391 in 1971 In that case, the film does not c o v e r the entire substrate and it is tapered to nothing s o m e w h e r e on the substrate. At thin tapered edge, the light beam in the film enters the substrate forming a well-defined collimated beam ilbout 2 ° below the film-substrate interface. In this case, the light beam c h o o s e s to enter the substrate instead of the airspace b e c a u s e the refractive index of the substrate is larger than that of the airspace Recently, tapered-film couplers have been made in the A I G a A s s y s t e m by Mertz el a]. 1401. They have also been used by Reinhart et ai. for coupling a laser to a waveguide [41], for forming Bragg-reflector lasers 1421 and for forming an amplifier-modulator circuit 1431. It is possible to join two waveguides, A and B. together by tapering their edges and overlay ing the tapered edge of one w a v e g u i d e on top ot the other w a v e g u i d e an s h o w n in fig. 5c. The light beam originally in w a v e g u i d e A will then c o n t i n u e to propagate in w a v e g u i d e B through the tapered transition simply b e c a u s e the waveguide', A and B have refractive indices larger than the substrate and the airspace. Similarly. fig. 5d s h o w s a m e t h o d of f o r m i n g light-guiding interc o n n e c t i o n s in an integrated optical circuit. Here. A and B are thin-film devices f o r m e d on a substrate: a film C which has tapered edges at both ends c o v e r s partially the devices A and 13. The light beam in A will enter the device B through the i n t e r c o n n e c t i n g film C only when the refractive indices of A, B and C are larger than those in the s u r r o u n d i n g spaces. The m e t h o d s described a b o v e involving tapered transitions were first reported by Tien et al. 1441 in 1973 and are likely to be one of the principal m e t h o d s of f o r m i n g integrated optical circuits. These m e t h o d s d e m o n s t r a t e the i m p o r t a n c e of the rule of the refractive index. In all the a h o c e cases, ([ a thin -film s t r u c t u r e varies in space, we a s s u m e that it varies s l o w l y c o m p a r e d with the optical wavelength so that the w a v e m a i n t a i n s the s a m e n o r m a l m o d e o f p r o p a g a t i o n during the transition. W h a t we h a v e c o n s i d e r e d here is an
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adiabatic process of the light wave propagation. Another important application of the rule of the index is the two-layered construction shown in fig. 6 [7]. T w o devices A and B are f o r m e d on the substrate; they have relatively high refractive indices n = 2.3. In this case, the interconnecting film C covers the entire surfaces of A and B as well as the rest of the surface of the substrate; the film C has a refractive index n ~- 1.53 which is considerably lower than those of the devices but is still larger than that of the substrate n = 1.47. According to the a b o v e rule, a light beam originally in the film C at the left of the figure will be kicked onto device A in the area occupied by A and then kicked back onto C in the space between A and B. As the light b e a m continues to propagate toward the right, it will be similarly kicked onto device B and then back to C. The film C in this case serves both as the interconnection and the input/output terminals. The wave velocities in devices A and B are quite different f r o m that in film C. The light beam thus refracts according to Snell's law as it enters and leaves the devices. Those refractions can clearly be seen in the photograph of fig. 6. If the device A has a triangular shape on the surface of the
Fig. 6. The top figure illustrates the method of two-layered construction. The film C which covers entirely the devices A and B serves as the interconnection between the devices and their input/output terminals. The lower photograph was taken in an experiment designed to d e m o n s t r a t e the above method of the construction. Two ZnS strips which were deposited diagonally on a glass substrate are the devices A and B s h o w n in the top figure. The ZnS strips were covered by a layer of vinyltrimethylsilane polymer film. The light wave was fed into the polymer film and it propagated from left to right. The refractions of the light observed at the boundaries of the ZnS strips indicate that the light wave truly propagated in the ZnS films in the areas occupied by them and in the polymer film in the rest of the areas.
substrate, it will be the thin-film prism shown in fig. 3a. The other devices shown in fig. 3 were also formed by this method of two-layered construction. The refractions d e m o n s t r a t e d in the photograph of fig. 6 have made deflection of light possible in the thin-film prism and focusing of light in the thin-film lens. It is possible to show rigorously that the light path in the two-dimensional plane of a waveguide obeys geometrical optics, in which the Fermat's principle and Sriell' s law in terms of the wave velocities must hold. We realized in the a b o v e discussion the importance of the tapered transitions. In addition to those described in connection with figs. 5c and 5d, a tapered transition could be a section of the waveguide joining smoothly two waveguides of thicknesses WA and Ws, respectively. It can also be a section of the waveguide joining two waveguides of compositions A and B, respectively, and the composition of the section varies gradually from A to B. In all the tapered transitions described above, the geometry or the composition of the structure varies slowly in space as c o m p a r e d with the optical wavelength, and we should expect the following important adiabatic rule of the tapered transition to apply: The light wave maintains a same normal mode of propagation in a tapered transition and the power carried in that normal mode is conserved. Here, we assume naturally that the wave structure considered has m a n y eigensolutions. Each eigensolution is a normal mode and the light wave is propagating in one of those normal modes. It is always desirable to have the light wave to propagate in a same normal mode throughout the entire optical circuit. According to the a b o v e rule, this can be accomplished by interconnecting all the waveguide c o m p o n e n t s in the circuit by tapered transitions. We have mentioned earlier that light can be easily scattered by the rough spots or discontinuities in the waveguide. Interestingly, the scattering itself seldom f o r m s a coherent reflected wave in the waveguide. Most of the discontinuities in the film cause forward scattering. Even if the scattering were isotropic, the film used for the waveguide is so thin and the solid angle sustained by the thickness of the film is so small that light scattered backward into that solid angle is negligible. On the other hand, however, reflections do occur in optical waveguides from cleaved surfaces, from preferen-
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tiallv etched g r o o v e s and in c o r r u g a t e d u'a~:e,~,uides. This rule o f the reflection i.~ entirely empirical. In integrated optics, we are p a r t i c u l a r l y interested in the reflection of light in a c o r r u g a t e d w a v e g u i d e . As we have m e n t i o n e d earlier, ',~ c o r r u g a t e d w a v e g u i d e ix simply a w a v e g u i d e h a v i n g a grating f o r m e d either on the top surface of the film or at the f l l m - s u b s t r a t e interface. The use of the grating for reflecting light ix well k n o w n and, in fact, the grating s p e c t r o m e t e r ix one of the oldest optical i n s t r u m e n t s . W h e n the s p a c i n g b e t w e e n the c o r r u g a t i o n s is equal to ',~ half of the optical w a v e l e n g t h , the w a v e g u i d e ix called a Bragg reflector. Figure 7a s h o w s a light w a v e A p r o p a g a t i n g t o w a r d a Bragg reflector.
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the first fringe be B, and that by the s e c o n d fringe be B, and so on. T h e light reflected by' the s e c o n d fringe is excited a h a l f - w a v e l e n g t h later than B~ and o n c e it ix excited, it has Io travel backward an additional half-wavelength in ordel to catch up with B,. T h e phases of B, and B thus differ e x a c t l } b} 2;r. and ,,o B~ and B~ add constructivel_~. Similarly, all o t h e r reflections t-~. B a . . . from the s u c c e e d i n g fringes have the same phase as B~ and 13,, and all of them add c o n s t r u c t i v e l y m a k i n g the Bragg reflector a ver~ efficient s t r u c t u r e for r e f e c t i n g light in waveguides. The Bragg reflection d e s c r i b e d here ix well k n o w n in X-ray c r y s t a l l o g r a p h y : it is also the basic principle of the a c o u s t o - o p t i c interaclion. It is also i m p o r t a n t to note that the Bra~,~ reflector is an inducti~,e element in the sense that the reflected waue has a p h a s e being 90 ° out o j that o f the f o r w a r d wat:e which excites it. We will return to this point latel. Figure g shows several typical structure, of the lasers. A laser is simply an oscillator of the light wave, which consists of a medium capable of amplifying the light and a resonant circuit in the form of an optical cavity. The s e m i c o n d u c tor laser p r e s e n t l y in c o m m e r c i a l p r o d u c t i o n has two cleaved end ,,urfaces f o r m i n g ~ F;ibrv Perot type of the optical cavity as s h o w n in fig. 8a. [ h e light w a v e ix being amplified in the c a v i t y as it is reflected back and forth b e t w e e n lwo c l e a v e d s u r f a c e s , and oscillation begins as the gain of the amplification o v e r c o m e s the loss in the cavity. [ I n f o r t u n a t e l y . this type of the laser s t r u c t u r e c a n n o t be used in integrated oF': \5~1~
~AIIH
[ ~\
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Fig. 7. Figure (at shows a lighl wave in a waveguide v, hich is being reflected by a Bragg reflector. Figure (b) demon strates the principle of the DFB laser. The light wave i,, making a round trip in a corrugated waveguide after being first reflected back from the right and then from the ]efl Unfortunately, exactly at the Bragg condition, the pha~,c relation of the waves is such that the light wave which returns to the center dotted line after having made a round trip cancels the wave which is originated from there at thai time. It is therefore necessary to move away the Bragg condition in the operation of the DFB laser, so that the light wave reinforces itself after each round trip.
~!? \ ( A ,
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As it passes by m a n y fringes of the c o r r u g a t i o n , each of the fringes c a u s e s a d i s t u r b a n c e to the t r a v e l i n g light wave. l~et the light reflected by
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|Cig. 8. (a; a typical diode la',cr ",,',hich ha', two cleaved ends forming a Fabr~-l'erot cavity; Ib) a I)FB laser: (c) a B r a g g reflector laser: (d) a lasei whh Iv,'~, c ~ r n c r r e f l e , c l o r , ,
249 tics. Since once the two ends are cleaved, the laser becomes a discrete unit and is difficult to be incorporated into an optical circuit. An obvious idea is then to replace the two cleaved end surfaces by two Bragg reflectors for reflecting light in the waveguide as shown in fig. 8c. The laser thus formed is called Braggreflector (BR) laser. We could even move one step further by combining two separate Bragg reflectors into a single one forming a so-called distributed feedback (DFB) laser shown in fig. 8b. The corrugated waveguide used in that way is called a distributed feedback circuit. The BR and DFB are the two basic laser structures used in integrated optics, since the corrugated waveguides used in those structures can easily be connected to the other parts of the optical circuit by tapered transitions. Finally, we will discuss the rule for the corrugated waveguides which have been used for reflectors, band-rejection filters, Bragg-reflector (BR) lasers and distributed feedback (DFB) lasers. For reflectors, filters and B R lasers, the waveguide is operated at the Bragg condition that the spacing between the corrugations is equal to a half of the optical wavelength. One can also broaden the response of the filter by introducing a chirp in the corrugation and reduce its side lobes by varying the depth of the corrugation. For D F B lasers, however, the waveguide is not operated at the Bragg condition for two different reasons. Let us divide hypothetically the corrugated waveguide into two equal halves and let a wave be launched toward the right at this dividing line. First, if the Bragg condition were satisfied, the wave would not travel very far in the right half of the waveguide before it is completely reflected and thus could not enjoy fully the gain provided by the amplifying medium. Secondly, we have mentioned earlier that the Bragg reflector is an inductive circuit element. According to fig. 7b, as the light wave is making a round trip after first being reflected by the right portion and then by the left portion of the waveguide, it suffers a net phase shift of 90 ° in the right and another phase shift of 90 ° in the left so that the phase of the wave after a round trip is opposite to that of the original wave started at the dividing line. The wave energy in the D F B laser cannot therefore be built up constructively at the Bragg condition. It is, in fact, necessary to move away from the Bragg condition sufficiently that an
additional phase shift of 7r or of an odd multiple of 7r can be introduced in one round trip of the light wave, and so the wave reinforces itself after each trip. Oscillations of the D F B laser corresponding to different odd multiples of 7r of the added phase shifts as described above are the longtitudinal modes of the laser Thin-film technology and guided-wave optics are the two main subjects in integrated optics. The rules described above are important in that they simplify the analyses of the various wave phenomena observed in the thin-film devices and circuits. More importantly, they are the fundamentals of the guided-wave optics based on them, one can invent new devices and circuits. 3. Magneto-optic Devices in Rare-Earth Garnets and Monolithic Integrated Optics in Semiconductors To develop integrated optics, we need an assortment of thin-film optical devices and the materials suitable for forming those devices. Because of that demand, the research in optical materials has been greatly expanded in the past few years [45]. Amorphous films such as sputtered glass, solution-deposited organic polymers, ZnS, Nb205 and SiO2 are commonly used for forming waveguides, prisms, lenses and other passive optical components. For active optical components such as lasers, modulators and switches, single crystal films must be used; they can be formed by proton bombardment, ion-implantation, diffusion and various methods of epitaxy. Because of the success of the monolithic integrated electronic circuits in silicon, we naturally think of monolithic integrated optics. The best material for that purpose is the GaAsrelated system, which can be used to form lasers, modulators, switches, waveguides and detectors. Next, LiNbO3 is one of the favorable materials for electro-optic modulators and switches as well as acousto-optic devices and delay lines. Excellent LiNbO3 devices have been formed by diffusing Ti into LiNbO3 substrates; the fabrication process is thus particularly simple. We have already discussed several LiNbO3 devices in the first section of this paper. Rare-earth garnets are also interesting in that they can be used to form wave-
25O
g u i d e s , m a g n e t o - o p t i c ',witches, i~onreciproc;iJ e l e m e n t s , m e m o r y d e v i c e s and e v e n opticall~ p u m p e d N d - Y A ( I laser-,. W e will discu,,x ,,omc of l h o s e a p p l i c a t i o n s belox~. Rccentl'>. lit tegrated optical c i r c u i t s have been ft+llncd eli xiiici,n ,,ubstlates+ In lhosc c i r c u i t s - hi}or w h i c h c o v e r s a pal'l of lhc -,tlbMl:itc Fill,. m e t h o d of i s o l a t i n g the s u b s t l a l e ,>o thai device> of l o w r e f r a c t i v e inclice~ can be ft+rined
c o n s i d e r a b l e detail b e l o w . T h e d e v i c e c o n s i s t s of a Y~Ga.,~Sc,+,Fe~+4),. fihn on a i l l 1 I-oriented (id~Ga~()~e s u b s t r a t e . T h e film is e p i t a x i a l l y g r o w n by the d i p p i n g m e t h o d of the liquid p h a s e e p i t a x y . It has a F a r a d a y r o t a t i o n c o n s t a n t 200°/Clil (new Ill:iterlals such as GdPreFe~(),+ has a much hirger r o t a t i o n c o n s t a n t - 1125°/cm but is not a v a i l a b l e in thin-film form). T h e fihn s e r v e s as an o p t i c a l w a v e g u i d e , l+et the film be in the x - y plane and the Iighl waive p r o p a g a t i n g in the w a v e g u i d c a l o n g the x - d i r e c t i o n . A s e r p e n t i n e e l e c t r i c cir cult is f o r m e d d i r e c t l y on lhe s u r f a c e of the fihn
h'+ the Ip, uai phtHt+iithogiaph+c +cchnlqi+c. ~,hc+ ++ the c u r r e n l ill the circuit +, ~ , m +i p r < + d t i c c - : m a g n e t i z i n g field +N lhc + dH'c+.+tior+ t~hich +,capahh." ~+i rt+l~tlill~z !he +:~ugile[i,',tthm ,.c,_[o+ +: lhc film inlo th;+l ,l+!c
d e m a g n e t i z a t i o n field i,, zero ('onseqtlel-ltly if i;. were I10| f o l the a n i s o l r o p v in the t ' l ' ¥ s t a l the ~witch c o u l d be o p c i a l e d v~Hh c \ l r e n l c i , , -,l+hlJ d r i v i n g p o t ~ e r In the t_'xpt'rillJcil{, t~,~,t+ p!l-mlx ;ift' IIscd io c o u p l e the Iighl w.~t~ c |nit+ :.ltld C,tll different d i r e c t i o n +, s e p a r a t e d by un angle of aholll 20 ° b e c a u s e of the b i r e f r i n g e n c e <~f the pr~sln No,<~ c~msidcr, fc,+ e x a m p l e , thai ~1']'F k.+t', c is fed into the filnl t++ the iilptlt prism. T h e wa;,c ellen p r o p a g a t e s iri the filrfi u n d e r the ~,erpentinc ellCeil| ~ x we hil,,e lllel) lit+ned earlier, w h e n the ctlrretil in the Cil-Cllit +s ~+7"+ the niagnetiz.;ilion vcchiF /t4 is :iiOllg {tit.s a m e d i r e c i h m as the light w:i~e p r o p a g a t i o n . T h e effect , accordingly. A 1 . 1 5 # n l H e - N e laser w:p, used to test the o p e r a t i o n o f this l n a g n e l o - o p t i c -,witch. We f o u n d thai b e l o w the ,~witching frequent)< ,~f I M H z , the m o t i o n o f the m a g n e t i z a t i o n wax nlainly that of the d o m a i n walls and the ,,witch c o u l d be o p e r a t e d at ci very .,mall d r i v i n g field (as small as 0. l ()el. A b o v e 1 0 M H y , h o w e ~ e r |he spins in the m a g n e t i c fihn w e r e r o t a t e d ~n unison a c c o r d i n g |o the m o d i f i e d v e r s i o n of the 141ndau-l+ifshit/ e q u a t i o n ((}ilberl's e q u a t i o n ) :
251 where M is the magnetization vector and H is the sum of the rf and dc fields. The addition of the dc field was found necessary for high f r e q u e n c y switching. By neglecting the loss term at the right side of the equation, the term I T I ( M x H ) represents the torque needed for rotating the magnetization M. It is evident that a larger torque must be used for a faster rotation of the magnetization. Therefore, a price must be paid for a large switching field in the high frequency operation. The switch has been operated up to 300 M H z and its p e r f o r m a n c e is summarized in fig. 9. (
SWITCHING FIELD Hs (IN Oa) EXPERIMENTAL THEORETICAL
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Fig. 9. The figure shows the amount of the switching field required to operate a magneto-optic switch. The vertical axis, RTU, indicates the amount of the TE wave which is converted into the TM wave by the effect of the Faraday rotation. The numbers associated with the open circles in this figure are the magnitudes of the switching fields in Oe used in various experiments. The solid lines indicate the theoretical results and the points are the experimental data. The results show that at 300 MHz, a switching field of 3 Oe or more is needed, whereas at 10MHz, a field of 0.1 Oe would be quite sufficient.
The domain wall motion in the magnetic film is interesting but domains with the magnetization parallel to the film are difficult to observe. We have built a special microscope so that the light can be focused on the film which is placed at 45 ° from the optical axis of the microscope. Figure 10 shows photographs of the domains observed in various conditions and fig. 11 illustrates the domains induced by the current in a serpentine circuit. Another interesting feature of this switch involves the serpentine circuit. It is well known that the Faraday rotation effect is not effective
unless the T E and TM waves have identical wave velocities. This requirement is called " p h a s e - m a t c h i n g " . In optical waveguides, the T E and TM w a v e s have different w a v e velocities, but the alternate legs of the serpentine circuit have current-flows in opposite directions such that they excite a grating of the magnetization in the film. The period of the magnetic grating is designed so that the T E w a v e coupled through this grating of the magnetization has exactly the same wave velocity as the TM wave and thus the two waves can be coupled one another effectively. Several attempts have been made in the past few years to f o r m an optically-pumped thin-film Nd ÷3 laser in glasses [46], in yttrium aluminum garnets [47] and in pentaphosphates [48]. Those lasers are not very attractive because of the following reasons: First, N d - p e n t a p h o s p h a t e waveguide lasers cannot be p u m p e d efficiently either longitudinally or transversely. Second, although Nd-glass and Y A G waveguide lasers can be efficiently p u m p e d longitudinally, they are basically low-gain devices and it is difficult to form a thin-film waveguide which has an attenuation small enough on the order of a few percent. Moreover, the grating used in the DFB laser could easily introduce another loss of one decibel or more in the waveguide. Mainly because of those difficulties with the lasers, the garnet-system is not actively pursued at the present time. On the other hand, the G a A s system has m a n y advantages and the technology involved is well trenched since the pioneering work of the r o o m t e m p e r a t u r e continuous wave A I - G a - A s heterostructure laser by M.D. Panish et al. and by Zh. I. Alferov et al. in the late sixties. Moreover, the present electronics industry is geared to the semiconductor technology and the addition of the integrated optics to its domain is certainly welcome. In spite of the fact that G a A s lasers have been studied for more than 15 years, new technology continues to emerge particularly in the last two years. First, as described in the first section of this paper, the work of Casey et al. and that of N a k a m u r a et al. have d e m o n s t r a t e d that efficient DFB lasers can be f o r m e d in the G a A s system. Next, although preferential etching is well known in the silicon technology, it was applied to the G a A s system by C o m e r f o r d and
252
Fig. 10. Photographs of the magnetic domains taken by a specially constructed microscope: la~the domains ~h,,crved wilho,l any external applied field; (b) a uniform film indicating that the domains shown above can easily be wiped out by applying a dc magnetic field parallel to the film on the order of 0.30e or more: {c) the periodic domain,, ob,,crved ,mix ~ :~ field of 0 2 ()c ..r less: (d) domains observed under the stress applied by a stick at the center of lhc disc Z o r y [491 f o r f o r m i n g c o r r u g a t e d w a v e g u i d e s . S o o n , o n e d i s c o v e r e d that w a v e g u i d e s and l a s e r s c o u l d be g r o w n in the p r e f e r e n t i a l l y e t c h e d c h a n n e l s . T h e t e c h n i q u e is called "'emb e d d e d e p i t a x y ' " by S a m i d et al. [50], " e t c h e d b u r i e d s t r u c t u r e " by B u r n h a m and S c i f r e s [51J and " e t c h a n d fill" b y B o e t z et al. and by T s a n g and W a n g [52, 53]. T h i r d , one f o u n d the p o s sibility o f v e r t i c a l a n d lateral c a r r i e r and o p t i c a l c o n f i n e m e n t in b u r i e d h e t e r o s t r u c t u r e s m a d e by T s u k a d a [54]. F o r t h , the m o l e c u l a r b e a m epit a x y has b e c o m e c o m p e t i t i v e b y the p i o n e e r i n g w o r k of J.R. A r t h u r a n d A.Y. Cho. F i n a l l y , by i n t r o d u c i n g a w e d g e in the well c o n t a i n i n g the melt, R.A. L o g a n d e v e l o p e d a n e w t e c h n i q u e in liquid p h a s e e p i t a x y for f o r m i n g t a p e r e d w a v e g u i d e s and w a v e g u i d e s w h i c h h a v e o n e c o m p o s i t i o n at o n e end and a n o t h e r c o m p o s i t i o n at the o t h e r e n d (a t e c h n i q u e c a l l e d spill c o m position). As will be d i s c u s s e d b e l o w , t h e s e new t e c h n i q u e s in o n e w a y o r a n o t h e r h a v e o p e n e d
n e w p o s s i b i l i t i e s for f o r m i n g i n t e g r a t e d optical circuits. As far as we can "~ce l o d a y , ~.tn i n l e g r a t e d o p t i c a l circuit c o n t a i n i n g a large n u m b e r o f the lasers can be f o r m e d by g r o w i n g :i large h e t e r o s t r u c t u r e c o v e r i n g the entire s u r f a c e of the s u b s t r a t e . I n d i v i d u a l l a s e r s are then f o r m e d on the s u b s t r a l e by e t c h i n g a w a y the s p a c e s b e t w e e n the lasers or s i m p l y h} p r o t o n b o m b a r d m e n t so that the c u r r e n ! can o n l y flow in the a r e a s w h e r e the l a s e r s are l o c a t e d . Alter-n a t i v e l y , we can g r o w a p - n j u n c t i o n c o v e r i n g the entire s u b s t r a t e , which will be reversely' b i a s e d in the o p e r a t i o n . W i n d o w s are then o p e n e d in the p - n j u n c t i o n l a y e r s by preferen.tial e t c h i n g and the lasers are s u b s e q u e n t l y g r o w n in t h o s e w i n d o w s . I! is likely that lasers will be g r o w n by liquid p h a s e e p i t a x y wilh the i n t e r c o n n e c t i o n s and o t h e r p a s s i v e c o m p o n e n t s m a d e by m o l e c u l a r b e a m e p i l a x y . H o p e f u l l y , using t h e s e m e t h o d s a m e d i u n l scale i n l e g r a t i o n
253
Fig. 11. Domains excited by the current in a serpentine circuit: The current is zero in (a) increases gradually in (b) and (c) up to I ampere in (d).
of the optical circuit in the G a A s system could be d e m o n s t r a t e d in the next few years. The G a A s system is not without problems. So far, DFB lasers have not been d e m o n s t r a t e d for long life. We do not as yet know how to dissipate the large amount of heat that will be generated in the circuit. Recent m e a s u r e m e n t s show a loss on the order of 5 d B / c m at 0.9 ~ m in G a A s and AIGaAs layers. The loss could be reduced significantly by using AIGaAsSb for the lasers and AIGaAs for the other components. Integrated optics is still a developing field. New ideas and new technology will continue to emerge. For those who work in this field, it has been a stimulating six years. Indeed, integrated optics has opened a new chapter in optical
science. The very principle which makes it possible is the rule of the refractive index which states: "light wave propagation tends to be contained and guided in the region where the refractive index is the largest." References [I] P.K. Tien, Appl. Opt. 10 (1971) 2395. P.K. Tien, G. Smolinsky and R.J. Martin I1 (1972) 637, [2] J.E. Goell and R.D. Standley, Proc. IEEE, 58 (1970) 1504. [31 P.K. Tien, R. Ulrich and R.J. Martin, Appl. Phys. Lett. 14 (1969) 291. [4] J.H. Harris, R. Shubert and J.N. Polky, URSI Conference Abstracts, Washington, D.C., April 1969. [5] L.V. logansen, Soviet Phys. Technical Phys. 7 (1962) 295.
254
I¢,I P.K. l'ien and R. Lilrich. ~, paper on Prism ~,oupler and
Fhin-Film Prisms and l e n s e s , Device Research ('~mference, Rochester 119691. l 7] P.K. r i e n , S. Riva-Sanseverino, N.J. Marlin ~md (; S m o h n s k y , Appl. Phys. l,eH. 24 11974~ ~47 [81 P.K. Tien, R.J. Martin and S. Riva-Sanse~erino, ..\ppl Ph?,',. l.ett. 27 (1975) 2';I [91 M. Verher. D.W. Vahev and V.[{. Wood 28 (19761 514 10] S.E. Miller, Bell System T e c h J. 48 (19691 2059 and 2161. Ill M . I . l)akss, I:. Kuhn. P,F. He(dr(oh ',rod B.,.\. S c o u Appl. Phys. l,etl. 16 (197(!) s23. I21 H. Kogelnik and q'.P. So,,nowski, Bell System I t c h . [ 49 (1970) 1602. 13] H. Kogelnik and ('.V. Shank, Appf. I)h~s. I c t l tx (1971) I~;2; J. App1. F'hy,, 4~; (19721 2~27. 14] S. Wang, [EEE J. Q u a n t u m Electron. QF~-IO 11974~ 41 I~l H . ( ( a ~ e y , .h. S. Somekh and M I]legcnl~,, \ppl t'h*~. [.ett. 27 (197~,t 142 [h t M. N;ikamnra, K. Aiki. ,I ( m e d a and \ . / a l i ~ '\ppt Phys. LeIL 27 11975) 41)3. [ 17] D.F. Nelson and R.K. Reinhart, Appl. Phys. I,etl. q ( 1964i 148. [IXl W,I,. Bond, B.G. ('ohen, R.('.(" l,eile and A Ynri; Appl. Phys. I,etl. 2 (19631 57. [191 A, Yariv and R.('.('. l,eile. Appl. Ph~.s. l,elI. 2 (196:,t '~5. [201 l). Hall, A. Y a r i v a n d E. Garmire. Appl. Phys. I~eU. I v (1970) 127: Opt. C o m m u n i c a t i o n I (197(1) 41)3. 121] F.K. Reinhart and B.I. Miller, Appl. Phys. Let(. 20(197~) 36. [22] P.K. Tien, R.J. Martin, R. Wolfe, R C . Le('raw and S.I Blank, Appl. Phys. LetL 21 (1972)354. [231 W.E. Martin and D.B. Hall, Appl. Phys. I~eu. 21 I b)72i 325. 1241 H.E. Taylor, W.E. Martin, I).B. Hall and V . M Smile?, Appl. Phys. l,ett. 21 (1972) 99. !25[ J.M. H a m m e r , D.J. Channin and M.T. Duffs.. Appl. Ph~.s l.etl. 2t 119731 176. J.H. H a m m e r , l).J. ('hannin. M . I l)uff~,, md I D Wittke. Appl. Phys. l,etl. 21 (1972) t58. [2hi l.P. K a m i n o w a n d J R . ( ' a r r u l h e r s . Appl. PM,,,. l,ctl. 22 (1973~ t26. [27] R.V. Schmidt and I.P. K a m m o w , Appl. Phys. I.eu. 2" i 1974t 458 1281 J.M. Hammer and W. Philips, Appl. t)h}'~ l.cll 24 (1974~ 54g. I29l S. Miyazawa, Appl. Phys. (,ell, 23 (1973i 198. S. Miyazawa, S. Fushimi and S. Kondo, Appl. Phys. I,ell 26 (197g) 8.
S. Kondo. S. Miyazawa, S. Fushimi and K. Sugii, '\ppf. Phys. Letl. 26 (1975) 489 jl0[ A.A. Ballman. H. Brown, P K. I'ien and S. Riga-Sam sever(no, J. Crystal Growlh t11 { 19751 17; 29 [I97fi1 289 I~,ll ].P. Kaminow, I_W. Stulz and E.H. I'urner, A p p [ Phys. l,elt. 2"7 (197"~ s ~ t+2I R V Schmidt and I.P. K a n i m o ~ . IF]EE J Qnantunl Electron. (,)E-II ( l q 7 ~ '3" l~31 P.K. Tien, S. Riva-Sanse~erino and A.A. Hallman, Appl. Phy',. LeH. 25 (1974) S6a. 1~4] D.B. Ostrovsky, R. Poirier. I_.M. Reiber and ('. l ) e verdun, Appl. Phys, l,ett. 22 (1973) 463. {3st W.E. Martin. Appl. Phys. l.ett. 26 (1975) 562. [~61 F.K. Reinhart and R.A I,ogan, Appl. P h y s I,ett 2"v 11975) s32. i171 R.V. Schmidt. Device Research ~ onference, Salt l,ake ('ily, 11976). 13Xl H. Hondro', and P I)ehye, ,~,i13der Physik 32 (lglO) 46"; 1391 ILK. Tien and R . J Marlin. \ p p l P h i , , (.ell IX (1'471; ~98. 1401 J . l , . M e r , ' , R . l , . l , o g a m W . W i e g m a n n a n d A . ( ' . f i o s s a r d . Appl. Phys. I,ett. 26 (197';I 337 I4ll F . K Reinhart and R.A. l,ogan, Appl. Phys. I~ell. 26 i1975) 516. I42] F.K. Reinhart, R.A. Logan and ( . V . Shank, Appl. P h y s l,etl. 27 (1975) 45. [43] F.K. Reinhart and R.A. [,ogan, Appl. Phys. l.eU. 27 ( 19751 532. 1441 P.K. Tien, R.J. Marlin and (i. Smolinsky, Appl. Opt. i2 (1973) 1909. [45] P.K. Tien and A.A. Ballman, J. V a c u u m Science and Technology 12 (1975) 892. [46] Masatoski Saruwatari and 3 a l s u o Izawa, AppL Phys. l.ett. 24 (1974) 603. [471 J . D Van der Ziel, W.A. Bonner. 1. Kopb, S. Singh and I,G. Van Uilert, Appl. Phys. l,ett. 22 (1973) 656. 1481 K e n j u O t s u k a a n d T r o m o a k i Y a m e d a . Appl. Phvs. L e u 26 (1975) Ill. 149] l . ( ' o m e r f o r d and P Zonv. -\ppl Phvs. I~etl. 25 tl9741 2O8. 1501 I Samid, ('.P l . e e . . \ (lover :rod X Y a m , ~.ppl Phxs I,etl. 27 (197~1 405 [51] R,D. B u r n h a m and I).R Scifre',, Appl. Phvs. l e t l . 2" ! 19751 51(L 1~21 D. Beetz, W.T. T s a n g a n d S Wang, Appl. P h v s l e t ( 28 1976) 234. 1531 W.T. Tsang. ( ' . ( Tseng and S Wang, Appl. ~ p I i4 11975! 1200 1541 T. Tsukada, J. Appl. Phys. 4 ~' 11~74) 4899
P.K. T I E N Bell Telephone Laboratories, Holmdel, New Jersey 07733, USA Invited paper Research in integrated optics was initiated about six years ago in an effort to form integrated optical circuits which contain miniature lasers, modulators and waveguides. We present in section I a historical review of the subject and discuss, in sections 2 and 3, new wave p h e n o m e n a in optical waveguides, magneto-optic devices and monolithic integrated optics in GaAs-related s e m i c o n d u c t o r compounds. We e m p h a s i z e that various m e t h o d s used in integrated optics can be s u m m a r i z e d in four simple rules.
1. Historical development of integrated optics Integrated optics has two distinct goals: one is to apply thin-film technology to the formation of optical devices and circuits; the other is to integrate a large n u m b e r of optical c o m p o n e n t s on a single small substrate, so forming an optical circuit reminescent of the integrated circuit in microelectronics. When research in integrated optics was initiated six years ago, the idea of forming thin-film optical devices was new. One thought of m a n y advantages of the thin-film optical components. They can easily be miniaturized, are readily integrable and when they are bonded to a substrate, a solid and stable circuit can be produced. One was also intrigued by the success of the microelectronics and the thought that the medium or even the large scale integration in optical systems may be possible was certainly inviting. At that time, those who were engaged with the integrated optics had extensive experience in m i c r o w a v e technology. An idea which naturally followed was that optical waveguides should be used as the basic structures of the optical c o m p o n e n t s so that a light wave can propagate freely in them. It turns out that the simplest waveguide is a thin layer of the dielectric film deposited on a substrate with the refractive index of the film larger than that of the substrate. The waveguide in such a thin-film f o r m is called slab- or film-waveguide. Similarly, a channel waveguide is f o r m e d by depositing a thin strip of the film on a substrate. More complicated optical waveguides include p - n junctions and heterostructures used in the semi * This paper is a shortened version of a review paper in Review of Modern P h y s i c s , which will he published in the April 1977 issue.
Physica 89B (1977) 241-254 © North-Holland
conductor injection laser diodes. One can thus visualize an optical circuit consisting of many c o m p o n e n t s which are, themselves, film or channel waveguides, p - n junctions and heterostructures. By joining those waveguides together, the light wave can be transmitted f r o m one c o m p o n e n t to another. In that sense, the optical circuits would not be much different f r o m the m i c r o w a v e circuits. On the other hand, however, wavelength of the light considered here is about a factor of 10,000 smaller than those of the microwaves, a far more sophisticated technology is required to construct the optical circuits. In the beginning, even the simple experiment of propagating a light w a v e in a film-waveguide was not easy. The light wave can be easily scattered f r o m a slightly rough surface. We recall in 1969 [1] when a sputtered ZnO film was f r s t used as the optical waveguide, the light wave propagated only a distance less than 1 mm in the waveguide before it was completely scattered. The waveguide had then an optical loss of 60 dBlcm. Soon, waveguides made of glass films by Goell and Standley [2] and those of organosilicon films by Tien et al. [I], had losses less than 1 dB/cm. Figure 1 shows photographs of the light paths in (a) a thermally oxidized Ta205 film on glass substrate, (b) a polymerized vinyltrimethyisilane film on pyrex substrate and (c) a single-crystal LiNbO3 film grown epitaxially on a LiTaO3 substrate. In those photographs, a short light path indicates a large optical loss in the waveguide. In the early experiments, how to couple a laser beam onto a very thin waveguide was a serious problem. The edge of the film was usually rough and the thickness of the film was typically less than 1 micron; it was not practical
242
and p r e c i s e p o s i t i o n i n g oi lhc light b e a m arc ~ IleCeSSHl}. l ' h e p F i M l l tt'-.CtJ !',)! | h ' , l | plil'po',~_ i ~called prisnl c o u p l e r . Simihi! io lhe !ncl;iJiic w a v e g u i d e u,~ed ~n TlllCft'lwLi\cs. ill} ~)p!ic~:' w a v e g u i d e c a n ..,ll'.;l;lii1 i l l , l i l y { l : i ! i s v c r s c c i c c ! F i , , (TF;) ;ind I r a n s v c i s c m a g n e t i c i I M ) m o d e ..... <' p r o p a g a t i o n . I{ach n/ode h~, ", ~- ,.~v~ ! wa, v v e l o c i t y w h i c h Villies w i t h lhc t h i c k n c s , ~,,t ! h i film and the r e f r a c t i v e indice~ of the film :ind the subslrate. By u>ing lhc >o-called " m-ii!i,: s p e c t r o s c o p y d i s c ~ w e r c d b', I ) c n e~ :d !~}, ~ :. p o s s i b l e to d e l e l n l i i l c the '~c v e i o c i l x , ~t}d thus the m o d e of the [ighl ,a.n~c p!opagaiioi~ ii~ the w a v e g u i d e . It is ab, o p o s s i b l e to demtmsira-:c waveguide mode; visuall\ m ~ pr,~Ljevli~;.q s c r e e n . F i g u r e 2 is ~ p h o t o g l ' a p h of the !!;di~!c, p u b l i s h e d h~ Tien el nl !~,i }~iach hrigh! lmc ~ the p h o t o g r a p h r e p r e s e n t s one w a v e g u i d e m o d e T h e n u m b e r s b e l o w i n d i c a t e ihe o r d e r , of ll'!¢ w a v e g u i d e m o d e s . T h e bright ,,pot in the middle of the p h o t o g r a p h i n d i c a t e s the w a v e g u i d e niodc in w h i c h the light wa~c in the w a ~ e g u M e ~ . ~ propagating. N o w imagine a light w a v e w h i c h i,: p r o p a g a ring in 'a f i l m - w a v e g u i d e Naturally. the t h i c k n e s s of the light he:m! must he c~mtaincd within the thicknes,, of the film: the path of Ihe light b e a m in the phlne of the film f o l l o w s lhc tlsual g e o m e t r i c a l optic,, [:o," e x a m p l e , if ,~l-~c film is not u n i f o r m and lht]> the w a v e ~etoci[~ v a r i e s in different area', of the film. t h e r e w, ill hc r e f r a c t i o n s of light b e t w e e n i h o s e area~. Refle~ lion a n d r e f r a c t i o n of light o b s e r v e d in the two, d i m e n s i o m t l p l a n e of a w a ~ e g t t i d c i~ called t~,.
dimensional optic.v ~
c Fig. I P h o t o g r a p h s s h o w the light w a v e pltlpagalit:,n hi thin-fihn w a v e g u i d e s m a d e of (:i) a Taft), fihn till gilts <` ~,uhstrale, fb) :i v i n y l l r i m e t h y l s i l a n e p o l y m e r fihn on p~,rc,,
to f o c u s a light b e a m o n t o the w a v e g u i d c d i r e c t l y t h r o u g h the e d g e of the film. Fort u n a t e l y , it was soon d i s c o v e r e d that by using ai p r i s m , one can c o u p l e a laser b e a m o n t o o r o u l of the waveguJde t h r o u g h the top surface of the
fihn 13.4,51. In this arrangement, tight focusing
A f t e r o b s e r v i n g r e f r a c t i o n of light i!~ wa~,, g u i d e s , the idea o f f o r m i n g thin-film p r i s m s aud l e n s e s c a m e n a t u r a l l y . Yet Ihe possihilil,, ~d f o r m i n g t h o s e f a m i l i a r o p t i c a l c o m p o n e n t s ii~ thin films f a s c i n a t e d !nanv, l ' h i n - f i l m p r i s m s a~,~ l e n s e s w e r e first r e p o r t e d {!~ the [3e'~ic-. R e s e a r c h C o n f e r e n c e held in Rocheste~ :n ,4.6v~ 161. F i g u r e i~ s h o w s p h o l o g r a p h s of ~ ~hin-tih, p r i s m , a lens. a p o l a r i z e r : ! n d ;t c o r n e l {t.'flecI{ lr e p o r t e d in s e v e r a l p a p e r s b,, ] i e H ci i i !7 g! T h e y w e r e m a d e of two hiyer', of the films: ~mc film t y p i c a l l y Z n S (or Nb,().) has :', large Ic f r u c t i v e index n 2..~ and the o t h e r film typically glass (or ~lrganic rnalerials)~ h:l,, ~l t,,iv~ r e f r a c t i v e i n d e x n , I. ~ T h m - f i h n geode,dc tenses r e c e n t l ) reported b) V e r b e r el al. I
243
Fig. 2. The bright vertical lines in this photograph are called m-lines. Each line represents one waveguide mode. From the positions of the m-lines, one can determine the refractive and thickness of the film as well as the wave velocities of the waveguide modes. The so-called m-line spectroscopy provides valuable information of the wave propagation in the waveguide. diffraction limit and are able to resolve light b e a m s with an angular separation as small as 3.3 mrad. Another important d e v e l o p m e n t in integrated optics is the use of the gratings for couplers, reflectors, band-rejection filters and more importantly, laser structures. A grating is a corrugated surface which may be f o r m e d either on the top surface or at the film-substrate interface of a waveguide. The waveguides containing gratings are called corrugated waveguides or simply periodic structures. The use of a grating for reflection of light in the waveguide was first proposed by Miller [10]. Subsequently, the use of the gratings for coupling light onto waveguides was reported by Dakss et al. [11] and also by Kogelnik and Sosnowski [12]. In 1970, Kogelnik and Shank [13] proposed the use of a corrugated waveguide as the laser cavity and demonstrated a dye laser based on this principle. They called this type of the lasers distributed f e e d b a c k (DFB) lasers. Soon their idea was extended to form Bragg reflector lasers (BRL) [14]. As of today, the DFB G a A s heterostructure lasers with separate confinement made by Casey et al. 1114] have been operated at
room t e m p e r a t u r e under pulsed conditions [15] with a threshold current of only 2.2 k A / c m 2 and those made by N a k a m u r a et al. [16] have been operated at r o o m t e m p e r a t u r e and continuous wave with a threshold current of 3.5 k A / c m 2. Besides the lasers, one has to switch or modulate the light in a communication system. The use of semiconductor p - n junction diodes for modulation of light can be dated b a c k to the early sixties. For example, Nelson and Reinhart reported light modulation in reverse-biased G a P junctions in 1964 [17]. Bond et al. [18] and Yariv and Leite [19] discussed waveguide modes in p - n junctions in 1963. Since the inception of integrated optics in 1969, Hall et al. experimented with the light modulation in G a A s epitaxial layers [20] and Reinhart and Miller [21] built efficient G a A s / A I G a A s double heterostructure light modulators. Between 1970-1972, Tien et al. [22] reported a magneto-optic switch which used an iron-garnet film as the waveguide. The switch is capable of switching the light at a speed up to 3 0 0 M H z (fig. 4a). Martin and Hall [23] and Taylor et al. [24] studied light modulation in optical waveguides f o r m e d by diffusion in I I - V I c o m p o u n d s such as ZnS/ZnSe and CdS/CdSe.
244
Fig. 3, P h o t o g r a p h s of (a) u thin-film prism, (h) a Ion',, (c) ~ p o l a r i z e r and ,~d) a c o r n e r 1eflcc~r m a d e b', the m e l b o d ~i t w o - l a y e r e d c o n s t r u c t i o n Tho,,e thin-fihn c o m p o n e n t , , h a v e lhc si/e,, of r t m g h l v I m m - I mm :rod ~ ];n'ge n u m b e r of them ,:m~ be f o r m e d s i m u l t a n e o u s l y in or~c single p h o t o l i t h o g r a p h i c proces,,
H a m m e r et al. [25] reported electro-optic Bragg modulators in ZnO layers epitaxially grown on sapphire substrates by a close-spaced chemical v a p o r transport method. LiNb()~ has large electro-optic and acousto-optic coefficients and is an ideal material for electro-optic and acousto-optic devices. LiNbO~ optical waveguides were first f o r m e d by K a m i n o w and Carruthers [26] by out-diffusion of Li. Subsequently, they have been formed using in-diffusion of Ti in LiNbO~ by Schmidt and K a m i n o w [27] and in-diffusion of Nb in LiTaO~ by H a m m e r and Philips 1281. LiNb()~ waveguides have also been formed by melt-phase epitaxy and liquid phase epitaxy by
Miyazawa el al. [29] and Ballman et al. [3(}], Between 1973 and 1975, a large number of the papers were published on LiNbO~ waveguidedevices. Some of them are shown in fig, 4. Figure 4b is a channel waveguide phase modulator m':tde by K a m i n o w el al. [31]. Rather remarkably, it has a merit constant of 0.002 m W / M H z ; (c) is an acousto-optic deflector by Schmidt et al. [32L which can achieve I00 percent intensity modulation in the zeroth-order beam using only 250 mW of acoustic power: td~ is an electro-optic lighl beam scanner by Tien el al. [331 which is capable of scanning a light beam in the waveguide over :in angle of t~'
245
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Fig. 4. (a) A magneto-optic switch made of a Y~Ga,,7~Sco,Fe~7,O,2film on a Gd~Ga~O,2substrate; (b) an electro-optic channel waveguide phase modulator made by diffusing Ti into a LiNbO3 substrate; (c) an electro-optic light beam scanner made of a LiNbO3 film epitaxially grown on a LiTaO3 substrate; (d) an acousto-optic light beam deflector made by diffusing Ti into a LiNbO3 substrate. covering roughly 10 resolvable spots. Many of the thin-film devices described a b o v e have efficiencies considerably larger than their bulk counterparts mainly because, in the waveguides, one can concentrate the light b e a m and the switching electric, magnetic or acoustic field simultaneously in a very small space. Some simple integrated optical circuits have also been made. For example: O s t r o w s k y made a circuit containing a glass waveguide and a silicon photodetector [34]; Martin used diffusion technique forming a circuit involving an optical switch coupled to a branching waveguide [35]; Reinhart and Logan [36] reported a G a A s chip which contains a laser and a modulator. Very recently, Schmidt f o r m e d a matrix of four optical switches [37]. In the past, Topical Meeting on Integrated Optics was held e v e r y two years. The first meeting was held in Las Vegas in 1970 and was attended by more than 250 scientists from 11 different countries. The main topics of discussion were waveguides and waveguide modes. The second meeting was held in N e w Orleans; magneto-optic and electro-optic modulators were then the subjects of interest. The third meeting was held in Salt L a k e City early this year. The lasers, switches, modulators, couplers
reported in this meeting have already reached a certain degree of sophistication. Indeed, a great deal has been accomplished in integrated optics in the past six years.
2. Methods of forming thin-film optical devices and circuits Integrated optics is a diversified field; it involves material science, fabrication techniques, electromagnetic theory, device engineering and quantum electronics. Since 1969, more than 400 papers have been published on the various subjects of integrated optics. In spite of this vast amount of the publication, the basic principles involved in the formation of thin-film optical devices and circuits are rather simple and they can be s u m m a r i z e d in several simple rules, namely, rule of the refractive index, rule of tapered transitions, rule for reflection of light and rule for corrugated waveguides. We will discuss those rules in detail below. In all the systems involving energy-flow or wave-propagation, if the p h e n o m e n o n considered is orderly, the energy flow tends to concentrate in the region where the wave velocity is the slowest. In the optical system, the refractive index is the measure of the inverse of
246
the w a v e velocity. We m a y thus state as the rule for the refractive index: In a thin-film s t r u c t u r e parallel to the light w a r e p r o p a g a t i o n , the light w a r e tends t o he c o n t a i n e d a n d guided in the region where the r e f r a c t i r e index is the largest. The light wave propagation can therefore be confined in a thin layer of the film which has a refractive index larger than that of the surroundings. That ix, of course, the basic principle of the dielectric w a v e g u i d e s which were d i s c o v e r e d by H o n d r o s and D e b y e [38] dated back to 1910. In a way, the light w a v e propagation in high-index film is reminescent of the current flow in a c o n d u c t i n g electric circuit, although refractive index and c o n d u c t i v i t y play entirely different roles in the Maxwell equalions. We can explain a large n u m b e r of the w a v e p h e n o m e n a in integrated optics by using the a b o v e rule. Returning again It) the film-waveguide s h o w n in fig. 5a the light w a v e is contained and guided in the film b e c a u s e the film has a larger refractive index than the substrate and the airspace. Similarly a reverse-biased p - n junction used in the s e m i c o n d u c t o r diode is also an optical waveguide. The plasma contribution of the refractive index in a s e m i c o n d u c t o r varies inversely with the square root of the electron c o n c e n t r a t i o n . W h e n the p - n junction is reversely biased, electrons are swept out of the junction which has then a refractive index larger than that in the vicinity, in the G a A s s y s t e m , a h e t e r o s t r u c t u r e is f o r m e d by cladding a G a A s p - n junction b e t w e e n two layers of
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Fig. 5. Figures show(a) a film waveguide,(b) alapcred-film coupler, (c) a junction belween pa,'o film-waveguides and Id) an interconneclion between two thin-film devices. ()no can easily explain the wave phenomena observed m Fho,,c figures hy the rule of the refractive index
A I G a A s . Since the refractive index of the (ktA~, is larger than that of the A 1 G a A s , the entire p-n junction in this case b e c o m e s an optical wave~ guide which, in a laser diode, is often called the aclive region. Figure 5h ,,hows another interesting a~ rangemenl called tapered-fihn coupler which was d i s c o v e r e d by Tien and Martin 1391 in 1971 In that case, the film does not c o v e r the entire substrate and it is tapered to nothing s o m e w h e r e on the substrate. At thin tapered edge, the light beam in the film enters the substrate forming a well-defined collimated beam ilbout 2 ° below the film-substrate interface. In this case, the light beam c h o o s e s to enter the substrate instead of the airspace b e c a u s e the refractive index of the substrate is larger than that of the airspace Recently, tapered-film couplers have been made in the A I G a A s s y s t e m by Mertz el a]. 1401. They have also been used by Reinhart et ai. for coupling a laser to a waveguide [41], for forming Bragg-reflector lasers 1421 and for forming an amplifier-modulator circuit 1431. It is possible to join two waveguides, A and B. together by tapering their edges and overlay ing the tapered edge of one w a v e g u i d e on top ot the other w a v e g u i d e an s h o w n in fig. 5c. The light beam originally in w a v e g u i d e A will then c o n t i n u e to propagate in w a v e g u i d e B through the tapered transition simply b e c a u s e the waveguide', A and B have refractive indices larger than the substrate and the airspace. Similarly. fig. 5d s h o w s a m e t h o d of f o r m i n g light-guiding interc o n n e c t i o n s in an integrated optical circuit. Here. A and B are thin-film devices f o r m e d on a substrate: a film C which has tapered edges at both ends c o v e r s partially the devices A and 13. The light beam in A will enter the device B through the i n t e r c o n n e c t i n g film C only when the refractive indices of A, B and C are larger than those in the s u r r o u n d i n g spaces. The m e t h o d s described a b o v e involving tapered transitions were first reported by Tien et al. 1441 in 1973 and are likely to be one of the principal m e t h o d s of f o r m i n g integrated optical circuits. These m e t h o d s d e m o n s t r a t e the i m p o r t a n c e of the rule of the refractive index. In all the a h o c e cases, ([ a thin -film s t r u c t u r e varies in space, we a s s u m e that it varies s l o w l y c o m p a r e d with the optical wavelength so that the w a v e m a i n t a i n s the s a m e n o r m a l m o d e o f p r o p a g a t i o n during the transition. W h a t we h a v e c o n s i d e r e d here is an
247
adiabatic process of the light wave propagation. Another important application of the rule of the index is the two-layered construction shown in fig. 6 [7]. T w o devices A and B are f o r m e d on the substrate; they have relatively high refractive indices n = 2.3. In this case, the interconnecting film C covers the entire surfaces of A and B as well as the rest of the surface of the substrate; the film C has a refractive index n ~- 1.53 which is considerably lower than those of the devices but is still larger than that of the substrate n = 1.47. According to the a b o v e rule, a light beam originally in the film C at the left of the figure will be kicked onto device A in the area occupied by A and then kicked back onto C in the space between A and B. As the light b e a m continues to propagate toward the right, it will be similarly kicked onto device B and then back to C. The film C in this case serves both as the interconnection and the input/output terminals. The wave velocities in devices A and B are quite different f r o m that in film C. The light beam thus refracts according to Snell's law as it enters and leaves the devices. Those refractions can clearly be seen in the photograph of fig. 6. If the device A has a triangular shape on the surface of the
Fig. 6. The top figure illustrates the method of two-layered construction. The film C which covers entirely the devices A and B serves as the interconnection between the devices and their input/output terminals. The lower photograph was taken in an experiment designed to d e m o n s t r a t e the above method of the construction. Two ZnS strips which were deposited diagonally on a glass substrate are the devices A and B s h o w n in the top figure. The ZnS strips were covered by a layer of vinyltrimethylsilane polymer film. The light wave was fed into the polymer film and it propagated from left to right. The refractions of the light observed at the boundaries of the ZnS strips indicate that the light wave truly propagated in the ZnS films in the areas occupied by them and in the polymer film in the rest of the areas.
substrate, it will be the thin-film prism shown in fig. 3a. The other devices shown in fig. 3 were also formed by this method of two-layered construction. The refractions d e m o n s t r a t e d in the photograph of fig. 6 have made deflection of light possible in the thin-film prism and focusing of light in the thin-film lens. It is possible to show rigorously that the light path in the two-dimensional plane of a waveguide obeys geometrical optics, in which the Fermat's principle and Sriell' s law in terms of the wave velocities must hold. We realized in the a b o v e discussion the importance of the tapered transitions. In addition to those described in connection with figs. 5c and 5d, a tapered transition could be a section of the waveguide joining smoothly two waveguides of thicknesses WA and Ws, respectively. It can also be a section of the waveguide joining two waveguides of compositions A and B, respectively, and the composition of the section varies gradually from A to B. In all the tapered transitions described above, the geometry or the composition of the structure varies slowly in space as c o m p a r e d with the optical wavelength, and we should expect the following important adiabatic rule of the tapered transition to apply: The light wave maintains a same normal mode of propagation in a tapered transition and the power carried in that normal mode is conserved. Here, we assume naturally that the wave structure considered has m a n y eigensolutions. Each eigensolution is a normal mode and the light wave is propagating in one of those normal modes. It is always desirable to have the light wave to propagate in a same normal mode throughout the entire optical circuit. According to the a b o v e rule, this can be accomplished by interconnecting all the waveguide c o m p o n e n t s in the circuit by tapered transitions. We have mentioned earlier that light can be easily scattered by the rough spots or discontinuities in the waveguide. Interestingly, the scattering itself seldom f o r m s a coherent reflected wave in the waveguide. Most of the discontinuities in the film cause forward scattering. Even if the scattering were isotropic, the film used for the waveguide is so thin and the solid angle sustained by the thickness of the film is so small that light scattered backward into that solid angle is negligible. On the other hand, however, reflections do occur in optical waveguides from cleaved surfaces, from preferen-
248
tiallv etched g r o o v e s and in c o r r u g a t e d u'a~:e,~,uides. This rule o f the reflection i.~ entirely empirical. In integrated optics, we are p a r t i c u l a r l y interested in the reflection of light in a c o r r u g a t e d w a v e g u i d e . As we have m e n t i o n e d earlier, ',~ c o r r u g a t e d w a v e g u i d e ix simply a w a v e g u i d e h a v i n g a grating f o r m e d either on the top surface of the film or at the f l l m - s u b s t r a t e interface. The use of the grating for reflecting light ix well k n o w n and, in fact, the grating s p e c t r o m e t e r ix one of the oldest optical i n s t r u m e n t s . W h e n the s p a c i n g b e t w e e n the c o r r u g a t i o n s is equal to ',~ half of the optical w a v e l e n g t h , the w a v e g u i d e ix called a Bragg reflector. Figure 7a s h o w s a light w a v e A p r o p a g a t i n g t o w a r d a Bragg reflector.
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the first fringe be B, and that by the s e c o n d fringe be B, and so on. T h e light reflected by' the s e c o n d fringe is excited a h a l f - w a v e l e n g t h later than B~ and o n c e it ix excited, it has Io travel backward an additional half-wavelength in ordel to catch up with B,. T h e phases of B, and B thus differ e x a c t l } b} 2;r. and ,,o B~ and B~ add constructivel_~. Similarly, all o t h e r reflections t-~. B a . . . from the s u c c e e d i n g fringes have the same phase as B~ and 13,, and all of them add c o n s t r u c t i v e l y m a k i n g the Bragg reflector a ver~ efficient s t r u c t u r e for r e f e c t i n g light in waveguides. The Bragg reflection d e s c r i b e d here ix well k n o w n in X-ray c r y s t a l l o g r a p h y : it is also the basic principle of the a c o u s t o - o p t i c interaclion. It is also i m p o r t a n t to note that the Bra~,~ reflector is an inducti~,e element in the sense that the reflected waue has a p h a s e being 90 ° out o j that o f the f o r w a r d wat:e which excites it. We will return to this point latel. Figure g shows several typical structure, of the lasers. A laser is simply an oscillator of the light wave, which consists of a medium capable of amplifying the light and a resonant circuit in the form of an optical cavity. The s e m i c o n d u c tor laser p r e s e n t l y in c o m m e r c i a l p r o d u c t i o n has two cleaved end ,,urfaces f o r m i n g ~ F;ibrv Perot type of the optical cavity as s h o w n in fig. 8a. [ h e light w a v e ix being amplified in the c a v i t y as it is reflected back and forth b e t w e e n lwo c l e a v e d s u r f a c e s , and oscillation begins as the gain of the amplification o v e r c o m e s the loss in the cavity. [ I n f o r t u n a t e l y . this type of the laser s t r u c t u r e c a n n o t be used in integrated oF': \5~1~
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Fig. 7. Figure (at shows a lighl wave in a waveguide v, hich is being reflected by a Bragg reflector. Figure (b) demon strates the principle of the DFB laser. The light wave i,, making a round trip in a corrugated waveguide after being first reflected back from the right and then from the ]efl Unfortunately, exactly at the Bragg condition, the pha~,c relation of the waves is such that the light wave which returns to the center dotted line after having made a round trip cancels the wave which is originated from there at thai time. It is therefore necessary to move away the Bragg condition in the operation of the DFB laser, so that the light wave reinforces itself after each round trip.
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249 tics. Since once the two ends are cleaved, the laser becomes a discrete unit and is difficult to be incorporated into an optical circuit. An obvious idea is then to replace the two cleaved end surfaces by two Bragg reflectors for reflecting light in the waveguide as shown in fig. 8c. The laser thus formed is called Braggreflector (BR) laser. We could even move one step further by combining two separate Bragg reflectors into a single one forming a so-called distributed feedback (DFB) laser shown in fig. 8b. The corrugated waveguide used in that way is called a distributed feedback circuit. The BR and DFB are the two basic laser structures used in integrated optics, since the corrugated waveguides used in those structures can easily be connected to the other parts of the optical circuit by tapered transitions. Finally, we will discuss the rule for the corrugated waveguides which have been used for reflectors, band-rejection filters, Bragg-reflector (BR) lasers and distributed feedback (DFB) lasers. For reflectors, filters and B R lasers, the waveguide is operated at the Bragg condition that the spacing between the corrugations is equal to a half of the optical wavelength. One can also broaden the response of the filter by introducing a chirp in the corrugation and reduce its side lobes by varying the depth of the corrugation. For D F B lasers, however, the waveguide is not operated at the Bragg condition for two different reasons. Let us divide hypothetically the corrugated waveguide into two equal halves and let a wave be launched toward the right at this dividing line. First, if the Bragg condition were satisfied, the wave would not travel very far in the right half of the waveguide before it is completely reflected and thus could not enjoy fully the gain provided by the amplifying medium. Secondly, we have mentioned earlier that the Bragg reflector is an inductive circuit element. According to fig. 7b, as the light wave is making a round trip after first being reflected by the right portion and then by the left portion of the waveguide, it suffers a net phase shift of 90 ° in the right and another phase shift of 90 ° in the left so that the phase of the wave after a round trip is opposite to that of the original wave started at the dividing line. The wave energy in the D F B laser cannot therefore be built up constructively at the Bragg condition. It is, in fact, necessary to move away from the Bragg condition sufficiently that an
additional phase shift of 7r or of an odd multiple of 7r can be introduced in one round trip of the light wave, and so the wave reinforces itself after each trip. Oscillations of the D F B laser corresponding to different odd multiples of 7r of the added phase shifts as described above are the longtitudinal modes of the laser Thin-film technology and guided-wave optics are the two main subjects in integrated optics. The rules described above are important in that they simplify the analyses of the various wave phenomena observed in the thin-film devices and circuits. More importantly, they are the fundamentals of the guided-wave optics based on them, one can invent new devices and circuits. 3. Magneto-optic Devices in Rare-Earth Garnets and Monolithic Integrated Optics in Semiconductors To develop integrated optics, we need an assortment of thin-film optical devices and the materials suitable for forming those devices. Because of that demand, the research in optical materials has been greatly expanded in the past few years [45]. Amorphous films such as sputtered glass, solution-deposited organic polymers, ZnS, Nb205 and SiO2 are commonly used for forming waveguides, prisms, lenses and other passive optical components. For active optical components such as lasers, modulators and switches, single crystal films must be used; they can be formed by proton bombardment, ion-implantation, diffusion and various methods of epitaxy. Because of the success of the monolithic integrated electronic circuits in silicon, we naturally think of monolithic integrated optics. The best material for that purpose is the GaAsrelated system, which can be used to form lasers, modulators, switches, waveguides and detectors. Next, LiNbO3 is one of the favorable materials for electro-optic modulators and switches as well as acousto-optic devices and delay lines. Excellent LiNbO3 devices have been formed by diffusing Ti into LiNbO3 substrates; the fabrication process is thus particularly simple. We have already discussed several LiNbO3 devices in the first section of this paper. Rare-earth garnets are also interesting in that they can be used to form wave-
25O
g u i d e s , m a g n e t o - o p t i c ',witches, i~onreciproc;iJ e l e m e n t s , m e m o r y d e v i c e s and e v e n opticall~ p u m p e d N d - Y A ( I laser-,. W e will discu,,x ,,omc of l h o s e a p p l i c a t i o n s belox~. Rccentl'>. lit tegrated optical c i r c u i t s have been ft+llncd eli xiiici,n ,,ubstlates+ In lhosc c i r c u i t s -
c o n s i d e r a b l e detail b e l o w . T h e d e v i c e c o n s i s t s of a Y~Ga.,~Sc,+,Fe~+4),. fihn on a i l l 1 I-oriented (id~Ga~()~e s u b s t r a t e . T h e film is e p i t a x i a l l y g r o w n by the d i p p i n g m e t h o d of the liquid p h a s e e p i t a x y . It has a F a r a d a y r o t a t i o n c o n s t a n t 200°/Clil (new Ill:iterlals such as GdPreFe~(),+ has a much hirger r o t a t i o n c o n s t a n t - 1125°/cm but is not a v a i l a b l e in thin-film form). T h e fihn s e r v e s as an o p t i c a l w a v e g u i d e , l+et the film be in the x - y plane and the Iighl waive p r o p a g a t i n g in the w a v e g u i d c a l o n g the x - d i r e c t i o n . A s e r p e n t i n e e l e c t r i c cir cult is f o r m e d d i r e c t l y on lhe s u r f a c e of the fihn
h'+ the Ip, uai phtHt+iithogiaph+c +cchnlqi+c. ~,hc+ ++ the c u r r e n l ill the circuit +, ~ , m +i p r < + d t i c c - : m a g n e t i z i n g field +N lhc + dH'c+.+tior+ t~hich +,capahh." ~+i rt+l~tlill~z !he +:~ugile[i,',tthm ,.c,_[o+ +: lhc film inlo th;+l ,l+!c
d e m a g n e t i z a t i o n field i,, zero ('onseqtlel-ltly if i;. were I10| f o l the a n i s o l r o p v in the t ' l ' ¥ s t a l the ~witch c o u l d be o p c i a l e d v~Hh c \ l r e n l c i , , -,l+hlJ d r i v i n g p o t ~ e r In the t_'xpt'rillJcil{, t~,~,t+ p!l-mlx ;ift' IIscd io c o u p l e the Iighl w.~t~ c |nit+ :.ltld C,tll
251 where M is the magnetization vector and H is the sum of the rf and dc fields. The addition of the dc field was found necessary for high f r e q u e n c y switching. By neglecting the loss term at the right side of the equation, the term I T I ( M x H ) represents the torque needed for rotating the magnetization M. It is evident that a larger torque must be used for a faster rotation of the magnetization. Therefore, a price must be paid for a large switching field in the high frequency operation. The switch has been operated up to 300 M H z and its p e r f o r m a n c e is summarized in fig. 9. (
SWITCHING FIELD Hs (IN Oa) EXPERIMENTAL THEORETICAL
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Fig. 9. The figure shows the amount of the switching field required to operate a magneto-optic switch. The vertical axis, RTU, indicates the amount of the TE wave which is converted into the TM wave by the effect of the Faraday rotation. The numbers associated with the open circles in this figure are the magnitudes of the switching fields in Oe used in various experiments. The solid lines indicate the theoretical results and the points are the experimental data. The results show that at 300 MHz, a switching field of 3 Oe or more is needed, whereas at 10MHz, a field of 0.1 Oe would be quite sufficient.
The domain wall motion in the magnetic film is interesting but domains with the magnetization parallel to the film are difficult to observe. We have built a special microscope so that the light can be focused on the film which is placed at 45 ° from the optical axis of the microscope. Figure 10 shows photographs of the domains observed in various conditions and fig. 11 illustrates the domains induced by the current in a serpentine circuit. Another interesting feature of this switch involves the serpentine circuit. It is well known that the Faraday rotation effect is not effective
unless the T E and TM waves have identical wave velocities. This requirement is called " p h a s e - m a t c h i n g " . In optical waveguides, the T E and TM w a v e s have different w a v e velocities, but the alternate legs of the serpentine circuit have current-flows in opposite directions such that they excite a grating of the magnetization in the film. The period of the magnetic grating is designed so that the T E w a v e coupled through this grating of the magnetization has exactly the same wave velocity as the TM wave and thus the two waves can be coupled one another effectively. Several attempts have been made in the past few years to f o r m an optically-pumped thin-film Nd ÷3 laser in glasses [46], in yttrium aluminum garnets [47] and in pentaphosphates [48]. Those lasers are not very attractive because of the following reasons: First, N d - p e n t a p h o s p h a t e waveguide lasers cannot be p u m p e d efficiently either longitudinally or transversely. Second, although Nd-glass and Y A G waveguide lasers can be efficiently p u m p e d longitudinally, they are basically low-gain devices and it is difficult to form a thin-film waveguide which has an attenuation small enough on the order of a few percent. Moreover, the grating used in the DFB laser could easily introduce another loss of one decibel or more in the waveguide. Mainly because of those difficulties with the lasers, the garnet-system is not actively pursued at the present time. On the other hand, the G a A s system has m a n y advantages and the technology involved is well trenched since the pioneering work of the r o o m t e m p e r a t u r e continuous wave A I - G a - A s heterostructure laser by M.D. Panish et al. and by Zh. I. Alferov et al. in the late sixties. Moreover, the present electronics industry is geared to the semiconductor technology and the addition of the integrated optics to its domain is certainly welcome. In spite of the fact that G a A s lasers have been studied for more than 15 years, new technology continues to emerge particularly in the last two years. First, as described in the first section of this paper, the work of Casey et al. and that of N a k a m u r a et al. have d e m o n s t r a t e d that efficient DFB lasers can be f o r m e d in the G a A s system. Next, although preferential etching is well known in the silicon technology, it was applied to the G a A s system by C o m e r f o r d and
252
Fig. 10. Photographs of the magnetic domains taken by a specially constructed microscope: la~the domains ~h,,crved wilho,l any external applied field; (b) a uniform film indicating that the domains shown above can easily be wiped out by applying a dc magnetic field parallel to the film on the order of 0.30e or more: {c) the periodic domain,, ob,,crved ,mix ~ :~ field of 0 2 ()c ..r less: (d) domains observed under the stress applied by a stick at the center of lhc disc Z o r y [491 f o r f o r m i n g c o r r u g a t e d w a v e g u i d e s . S o o n , o n e d i s c o v e r e d that w a v e g u i d e s and l a s e r s c o u l d be g r o w n in the p r e f e r e n t i a l l y e t c h e d c h a n n e l s . T h e t e c h n i q u e is called "'emb e d d e d e p i t a x y ' " by S a m i d et al. [50], " e t c h e d b u r i e d s t r u c t u r e " by B u r n h a m and S c i f r e s [51J and " e t c h a n d fill" b y B o e t z et al. and by T s a n g and W a n g [52, 53]. T h i r d , one f o u n d the p o s sibility o f v e r t i c a l a n d lateral c a r r i e r and o p t i c a l c o n f i n e m e n t in b u r i e d h e t e r o s t r u c t u r e s m a d e by T s u k a d a [54]. F o r t h , the m o l e c u l a r b e a m epit a x y has b e c o m e c o m p e t i t i v e b y the p i o n e e r i n g w o r k of J.R. A r t h u r a n d A.Y. Cho. F i n a l l y , by i n t r o d u c i n g a w e d g e in the well c o n t a i n i n g the melt, R.A. L o g a n d e v e l o p e d a n e w t e c h n i q u e in liquid p h a s e e p i t a x y for f o r m i n g t a p e r e d w a v e g u i d e s and w a v e g u i d e s w h i c h h a v e o n e c o m p o s i t i o n at o n e end and a n o t h e r c o m p o s i t i o n at the o t h e r e n d (a t e c h n i q u e c a l l e d spill c o m position). As will be d i s c u s s e d b e l o w , t h e s e new t e c h n i q u e s in o n e w a y o r a n o t h e r h a v e o p e n e d
n e w p o s s i b i l i t i e s for f o r m i n g i n t e g r a t e d optical circuits. As far as we can "~ce l o d a y , ~.tn i n l e g r a t e d o p t i c a l circuit c o n t a i n i n g a large n u m b e r o f the lasers can be f o r m e d by g r o w i n g :i large h e t e r o s t r u c t u r e c o v e r i n g the entire s u r f a c e of the s u b s t r a t e . I n d i v i d u a l l a s e r s are then f o r m e d on the s u b s t r a l e by e t c h i n g a w a y the s p a c e s b e t w e e n the lasers or s i m p l y h} p r o t o n b o m b a r d m e n t so that the c u r r e n ! can o n l y flow in the a r e a s w h e r e the l a s e r s are l o c a t e d . Alter-n a t i v e l y , we can g r o w a p - n j u n c t i o n c o v e r i n g the entire s u b s t r a t e , which will be reversely' b i a s e d in the o p e r a t i o n . W i n d o w s are then o p e n e d in the p - n j u n c t i o n l a y e r s by preferen.tial e t c h i n g and the lasers are s u b s e q u e n t l y g r o w n in t h o s e w i n d o w s . I! is likely that lasers will be g r o w n by liquid p h a s e e p i t a x y wilh the i n t e r c o n n e c t i o n s and o t h e r p a s s i v e c o m p o n e n t s m a d e by m o l e c u l a r b e a m e p i l a x y . H o p e f u l l y , using t h e s e m e t h o d s a m e d i u n l scale i n l e g r a t i o n
253
Fig. 11. Domains excited by the current in a serpentine circuit: The current is zero in (a) increases gradually in (b) and (c) up to I ampere in (d).
of the optical circuit in the G a A s system could be d e m o n s t r a t e d in the next few years. The G a A s system is not without problems. So far, DFB lasers have not been d e m o n s t r a t e d for long life. We do not as yet know how to dissipate the large amount of heat that will be generated in the circuit. Recent m e a s u r e m e n t s show a loss on the order of 5 d B / c m at 0.9 ~ m in G a A s and AIGaAs layers. The loss could be reduced significantly by using AIGaAsSb for the lasers and AIGaAs for the other components. Integrated optics is still a developing field. New ideas and new technology will continue to emerge. For those who work in this field, it has been a stimulating six years. Indeed, integrated optics has opened a new chapter in optical
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