Resonance coupled optical coupler with semiconductor waveguide layer comprising a multi-quantum-well structure

An electro-optic coupler made of consecutively deposited layers of semiconductor material has an one waveguide layer a multiple-quantum-well structure which exhibits a strong index of refraction dispersion in response to an electric field. Another waveguide layer separated from the multiple-quantum-well structure by a coupling layer is made of a bulk semiconductor material having an index of refraction which is comparatively unaffected by the electric field and which is substantially equal to one of the values of the index of refraction that the quantum well structure can assume. Resonant coupling of the waveguide layers is affected by a uniform electric field generated by a voltage applied between metalization on a confinement layer covering the top waveguide and a substrate on which the waveguide layers and coupling layer are grown over a lower confinement layer. When the indices of refraction of the two waveguides are equal, light injected into one waveguide is switched to the other. On the other hand, when the indices of refraction of the two waveguides are not equal, a parallel propagation condition exists. The coupler can be used either as a switch or an attenuator.

BACKGROUND OF INVENTION 
1. Field of the Invention 
This invention relates to semiconductor devices selectively coupling light 
from one waveguide to another for such applications as switching light 
between waveguides and attenuating or modulating light in a waveguide. 
More particularly, the present invention is directed to an optical switch 
in which light is resonant coupled between stacked, semiconductor 
waveguide layers grown on a substrate, separated by a coupling layer and 
bounded by containment layers with one waveguide layer being composed of a 
superlattice of multiple-quantum-well (MQW) material having an index of 
refraction which varies substantially with an applied electric field and 
the other waveguide layer composed of a material having an index of 
refraction which varies relatively little with the electric field. The 
electric field varies the index of refraction of the MQW waveguide layer 
between a value compatible with the index of refraction of the other 
waveguide to resonantly couple light between the waveguides, and a 
substantially different index of refraction in which no substantial amount 
of light is coupled between the waveguides. 
2. Background Information 
Semiconductor devices for switching light between waveguides are known. In 
one type of such devices the waveguides are formed side by side in a 
common plane with a suitable coupling material between. Typically, the 
index of refraction in the two waveguides is identical and that of the 
coupling material is lower so that there is resonant coupling between the 
waveguides in a cross propagation or "switch" condition. A "no switch" or 
parallel propagation condition is commonly created by an induced phase 
change in one of the guides, normally caused by an electric field induced 
change of the index of refraction (dn/dE) in one waveguide. The value of 
dn/dE is a factor which determines the magnitude of the electric field 
required to switch the light. The larger dn/dE, the smaller the voltage 
required for a given geometric configuration. 
Other factors affecting the operation of such devices are the index of 
refraction and width of the coupling material. Within limits, the smaller 
the difference between the indices of refraction of the waveguides and the 
coupling material, and the narrower the width of the coupling material, 
the shorter is the length of the parallel waveguides required for cross 
coupling. In turn, a shorter device length results in decreased device 
capacitance, and thus an increase in the maximum switching speed and a 
decrease of the energy required per switching cycle. 
Thus, material and fabrication constraints have set the performance and 
dimensional limits of such devices. Conventionally, the material used has 
been LiNbO.sub.3 or GaAs. Fabrication techniques, namely lithography and 
the need for well defined etching, usually set a lower limit of 3 
micrometers for the width of the coupling material. This results in a 
required length of about 5 mm. Because of the relatively small dn/dE value 
for GaAs (7.10.sup.-10 cm/V), the required voltage for switching from 
cross-to-parallel propagation is on the order of 20 volts and the required 
energy is estimated to be about 240 pj. All of these values are too large 
for high level integration of the devices for complex applications. 
It has been suggested in U.S. Pat. No. 4,048,591 that discrete waveguide 
elements can be stacked one on top of the other with a film of dielectric 
material in between, however, this patent concludes that it is preferable 
to place the two discrete waveguides side by side with a third coupling 
waveguide and the film of dielectric material overlapping both of them. 
It has also been suggested in published United Kingdom patent application 
No. GB 2174212A that optical switching devices can be constructed from 
layers of semiconductor materials grown on the substrate. The waveguide 
layers are made of the same material and thus have identical indices of 
refraction. They are separated by a coupling layer having an index of 
refraction which varies substantially to effect switching between parallel 
and cross propagation. In these devices, the index of refraction of the 
coupling layer is controlled by current rather than voltage. The coupling 
layer is doped to provide the free carriers required to support the 
current. Such current operated devices require significantly more power 
than voltage operated devices. 
Most of the optical switches utilize homogeneous materials such as 
LiNbO.sub.3, GaAs or ternary GaAlAs. U.S. Pat. No. 4,737,003 suggests the 
use of thin layers of multiple-quantum-well material as a selectively 
reflective layer at the intersection of two waveguides in a common plane. 
The index of refraction of the multiple-quantum-well reflective layer is 
varied through the injection of carriers to either reflect light into the 
intersecting waveguide or to let it pass straight through. This is a very 
complex device to construct, and again, using carrier injection to control 
the refraction index, it has a high power consumption. 
Multiple-quantum-well structures comprise alternating very thin layers of 
materials having different conduction band energy levels to create 
quantum-wells between barrier layers. Such materials are known to exhibit 
an unusually strong dispersion of the index of refraction near excitonic 
transitions which are coupled to the electron-hole energies of the the 
well. However, the principal interest in these devices has been in the 
electro-absorptive effect or the change in absorption as a function of an 
electric field. Such devices have been investigated for use in external 
modulators for lasers used in transmitting data at high rates. The light 
generated by the laser is injected into the multiple-quantum-well material 
which is selected such that the light is absorbed in the "OFF" state and 
passed through in the "ON" state. One disadvantage of such a device is 
that there is still a great deal of absorption in the "ON" state so that 
the efficiency of the device is not favorable. In addition, the switching 
time of such devices is limited by the life time of the excited carriers. 
There remains a need therefore for an optical coupler small in size 
suitable for large scale integration. 
There is a concurrent need for such an optical coupler which operates at 
low voltages and with low energy consumption. 
There is also a need for such an optical coupler having a very fast 
switching time. 
There is a further need for such an optical coupler with improved 
separation between the on and off states. 
There is yet another need for such an optical coupler with the above 
characteristics which can be used either as a light switch or an 
attenuator. 
There is an additional need for such an attenuator exhibiting a reduction 
in residual absorption in the "ON" state. 
SUMMARY OF THE INVENTION 
These and other needs are satisfied by the invention which is directed to 
an optical coupler comprising a plurality of layers of semiconductor 
materials. These layers of semiconductor materials include a first 
waveguide layer having an index of refraction n.sub.1, a second waveguide 
layer made of an electro-optically active multiple-quantum-well structure 
having an index of refraction n.sub.2, and a coupling layer between the 
waveguide layers having an index of refraction N. Confinement layers 
adjacent the waveguide layers have indices of refraction n.sub.3. The 
coupler includes means for applying an electric field across the plurality 
of layers of semiconductor material, which in the preferred form comprises 
a metallic contact layer coextensive with the waveguide layers, and a 
substrate on which the layers of semiconductor materials are grown. The 
indices of refraction n.sub.1, N, and n.sub.3 of the first waveguide, the 
coupling layer and the confinement layers respectively are relatively 
unaffected by the electric field when compared to the index of refraction 
n.sub.2 of the multiple-quantum-well structure which varies substantially 
with the electric field between a first value in the absence of the 
electric field and a second value in the presence of the field. One of 
these values of n.sub.2 is selected to be substantially equal to the index 
of refraction n.sub.1 of the first waveguide. The index of refraction of 
the coupling layer, N, is selected to be less than n.sub.1 and both values 
of n.sub.2, while n.sub.3, the index of refraction of the confinement 
layers, is the lowest of all. When n.sub.1 and n.sub.2 are not equal, a 
parallel propagation condition exists in which light injected into one 
waveguide essentially remains in that waveguide. When n.sub.1 equals 
n.sub.2, a cross-propagation condition is established and light injected 
into the one waveguide is switched to the other waveguide. The layers of 
the coupler extend a distance, L, in the direction of propagation of light 
injected into one of the waveguides, selected to effect maximum coupling 
between the waveguides when n.sub.2 equals n.sub.1. This optimum length 
for the device changes with variations in the absorption coefficients of 
the waveguide materials. 
In the preferred form of the invention, the semiconductor materials are 
selected such that the index of refraction n.sub.2 of the 
multiple-quantum-well structure of the second waveguide is equal to the 
index of refraction n.sub.1 of the first waveguide in the presence of the 
electric field. 
The electro-optic coupler of the invention is useful as a light switch and 
as an attenuator. When used as an attenuator, or light modulator, light is 
injected into the first waveguide. The light remains in the first 
waveguide in the "ON" state in which n.sub.1 and n.sub.2 are not equal. In 
the "OFF" state, n.sub.1, equals n.sub.2, and light is switched to the 
multiple-quantum-well structure where it is dissipated. For this 
application, the materials are selected such that the device operates for 
the selected light wavelength closer to the excitonic transition energy 
where the absorption coefficient is higher to effect better dissipation of 
the switched light.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The electro-optic coupler of the invention makes use of the unusually 
strong dispersion of the index of refraction of quantum-well material 
structures near and below the excitonic transition energy, and a vertical 
structure rather than the planar structure typically used to date in such 
devices. 
Referring to FIG. 1, the coupler 1 comprises a plurality of layers of 
semiconductor material grown on a substrate 3. These layers include a 
first waveguide layer 5, a second waveguide layer 7, a coupling layer 9 
between the first and second waveguide layers, a first confinement layer 
11 between the substrate and the second waveguide layer 7, and a second 
confinement layer 13 over the first waveguide layer 5. A metalization 
contact layer 15 is coextensive with the second confinement layer 13. An 
electric field is applied to the device by a voltage source 17 connected 
to the contact layer 15 and the substrate 3, and controlled by a switch 
19. 
The first waveguide 5 is made of a bulk semiconductor material having an 
index of refraction n.sub.1. The second waveguide 7 is a 
multiple-quantum-well structure having an index of refraction n.sub.2. The 
coupling layer 9 has an index of refraction N, while the confinement 
layers 11 and 13 have an index of refraction n.sub.3. The index of 
refraction n.sub.3 of the confinement layers 11 and 13 is much lower that 
the indices of refraction n.sub.2 and n.sub.1 to confine light to the 
waveguides 7 and 5 respectively. The index of refraction n.sub.2 is 
variable between a first value in the absence of the electric field E and 
a second value in the presence of the field. One of these values of 
n.sub.2 is substantially equal to n.sub.1, the index of refraction of the 
first waveguide 5 which is comparatively unaffected by the field E. The 
index of refraction N of the coupling layer is less than n.sub.1 and both 
values of n.sub.2, but greater than n.sub.3, and together with n.sub.3 is 
comparatively unaffected by the field E. 
Light injected into the waveguide 5, as indicated by the arrow labeled 
"input" in FIG. 1, is resonant coupled to the second waveguide 7 through 
the coupling layer 9 and emerges from the second waveguide 7 as indicated 
by the arrow marked "cross", when n.sub.2 is substantially equal to 
n.sub.1. This is referred to as the cross propagation condition. When 
n.sub.2 is not equal to n.sub.1 there is a phase shift between light in 
the two waveguides, and a parallel propagation condition exists with the 
light passing through and emerging from the first waveguide as indicated 
by the arrow marked "parallel" in FIG. 1. 
The second waveguide 7 which is a multiple-quantum-well structure is 
illustrated in FIG. 2. This structure comprises a superlattice of 
alternating quantum-well layers 21 and barrier layers 23. These layers 21 
and 23 are very thin; on the order of a few nanometers, and thus FIG. 2 is 
greatly enlarged. 
As mentioned previously, such quantum-well materials exhibit an unusually 
strong dispersion of the index of refraction near the excitonic transition 
energy. This phenomenon is illustrated in FIG. 3 which is a plot of light 
energy versus the index of refraction. The solid line 25 represents the 
estimated index of refraction for an exemplary quantum well material in 
the absence of an electric field. The dotted line 27 represents the 
estimated index of refraction of this superlattice material for an applied 
field of 100 KV/cm. As can be seen from the plot, there is a range of 
light energies for this material just below the exciton transition energy, 
represented by the sharp drop in the trace 25 at 29, in which there is a 
substantial differential, .DELTA.n, in the index of fraction in the 
presence and absence of the electric field. For instance, for light energy 
of 1.54 eV, the exemplary material has an index of refraction of about 
3.41 in the absence of an electric field, and about 3.38 in the presence 
of the 100 KV/cm field. Such a change in the index of refraction with an 
electric field, dn/dE, is a figure of merit which is used to determine the 
magnitude of the electric field required to effect switching: the larger 
dn/dE is, the smaller the voltage required for a given geometric design. 
Multiple-quantum-well structures exhibit a change in the index of 
refraction, dn/dE, which is up to one to two orders of magnitude higher 
than materials conventionally used in such devices. Hence, the voltage 
required for switching is reduced by this factor. For example, the 
required phase change can be accomplished in multiple-quantum-well 
structures with 1/100th of the voltage normally used with a GaAs switch. 
Another advantage of the electro-optic coupler 1 of the invention is that 
the change in the index of refraction in the second waveguide 7 is the 
result of virtual optical transitions which take place in the 
quantum-wells at wavelengths slightly longer (lower energy light) than 
that at which excitonic absorption occurs. The response time of optical 
processes based on virtual transitions is extremely short (about 100 fs). 
Thus, the intrinsic switching time of the electro-optic coupler 1 is based 
only on the speed of the driving signal, i.e., by the RC time constant of 
the device and by the rise time of the switching voltage. 
While a uniform electric field is applied to the first waveguide layer 5 
and the coupling layer 9 , as well as the multiple-quantum-well layer 7 in 
the electro-optic coupler 1, the lower voltage applied to effect the dn/dE 
required for switching does not appreciably change the indices of 
refraction n.sub.1 or N. Thus, the dashed lines 31 in FIG. 3 representing 
exemplary indices of refraction for the first waveguide 5, do not shift 
appreciably in response to the electric field. It can be appreciated, 
therefore, that a material can be selected for the first waveguide which 
has an index of refraction of either about 3.41 or 3.38. In the first 
instance, light would be switched in the absence of the electric field, 
and in the latter case only in the presence of the electric field. It is 
preferable, that the light be switched in the presence of the electric 
field, since the multiple-quantum-well structure has a sizeable absorption 
coefficient, which is lower, however, in the presence of the electric 
field. It is also for this reason that it is preferable to inject the 
light into the first waveguide rather than the quantum well structure of 
the second waveguide so that there is better efficiency under parallel 
propagation conditions. 
Another figure of merit in the performance of the electro-optical coupler 1 
is related to the index of refraction, N, and the width, D, of the 
coupling material between the waveguides. Within limits, the narrower D 
and/or the smaller the difference between N and n.sub.1 and n.sub.2, the 
shorter is the length L of the waveguides required for cross-coupling. In 
turn, a shorter device length means a decreased device capacitance which, 
as previously discussed, increases the switching speed. It also reduces 
the energy required per switching cycle. 
FIG. 4 illustrates the relation of the coupling length L for maximal cross 
propagation to waveguide separation calculated for different indices of 
refraction of the coupling layer 9. The refractive index in both 
waveguides 5 and 7 is assumed to be n=3.38. It can be seen that for a 
width, D, of the coupling layer of 0.5 .mu.m and an N of 3.355, that the 
length, L, required for resonant coupling is about 80 .mu.m. This is well 
over an order of magnitude less than the length required in planar 
switches where fabrication techniques limit D to about 3 mm. The 0.5 .mu.m 
thickness of the coupling layer 9 is easily achieved with techniques such 
as metal-organic chemical vapor deposition (MOCVD). 
FIG. 5 illustrates the coupling length, L, for maximal cross propagation as 
a function of the refractive index, N, of the coupling layer 9 for 
different values of the coupling layer thickness D. The refractive index 
of the first and second waveguides 5 and 7 is n=3.38. 
FIG. 6 illustrates the power coupled to the outputs of the waveguides, with 
light input to the first waveguide 5, as a function of switch length, SL, 
for cross propagation (n.sub.1 =n.sub.2). Switch length SL takes into 
account variations in the maximal coupling length attributable to 
differences in the absorption coefficients of the waveguides. Traces 33, 
35, 37, 39 and 40 correspond to absorption coefficients (.beta..sub.2) in 
the second waveguide, 7, of 10.sup.-3, 10.sup.-2, 2.times.10.sup.-2, 
5.times.10.sup.-2, and 10.sup.-1 .mu.m.sup.-1 respectively. The absorption 
coefficient (.beta.1) in the first waveguide 5 is 10.sup.-3 .mu.m.sup.-1. 
Optimal cross propagation is obtained for a switch length of 77 .mu.m for 
a material with an absorption coefficient of 10.sup.-3 .mu.m.sup.-1. 
FIG. 7 illustrates, for light input to the first waveguide 5, power coupled 
to the outputs of the waveguides as a function of the switch length, SL, 
for the parallel propagation condition. The value of 
.DELTA.n/n=3.05.times.10.sup.-3 is optimal for switch length SL=77 .mu.m. 
The traces 33', 35', 37', 39' and 40' represent the values of the 
absorption coefficient .beta..sub.2 of the second waveguide 7 
corresponding to those identified for the traces 33, 35, 37 and 39 
respectively in FIG. 6. 
FIG. 8 illustrates output power for the two waveguides as a function of the 
relative changes in the index of refraction of the multiple-quantum-well 
structure of the second waveguide 7. The curves 33, 35, 37, 39 and 40 
represent the values of the absorption coefficient .beta..sub.2 of the 
second waveguide 7 covered in FIG. 6. 
The optical coupler 1 shown in FIG. 1 is particularly suitable for use as a 
light switch. Light injected into the first waveguide 5 is cross 
propagated to the second waveguide 7, preferably in the presence of an 
electric field. In the absence of an electric field, the light is output 
from the first waveguide. Although not shown, the light output from the 
first and second waveguides of the switch is injected into connecting 
waveguides for transmission to other components. The waveguide connected 
to receive light from the second waveguide 7 of the switch would 
preferably not be made of multiple-quantum-well material but would be 
constructed from another suitable material with an absorption coefficient 
considerably lower than that of the multiple-quantum-well structure. 
The electro-optical coupler of the invention can also be used as a light 
attenuator or intensity modulator. An exemplary coupler 41 adapted for 
such use is shown in FIG. 9. This device comprises semiconductor layers 
grown on a substrate 43 and includes a first wave guide layer 45 and a 
second waveguide layer 47 on top of the first waveguide layer with a layer 
49 between which serves as a coupling layer where the second waveguide 
layer is coextensive with the first waveguide layer, and serves as a 
confinement layer for the first waveguide beyond the boundaries of the 
second waveguide layer. Additional confinement layers 51 and 53 are 
provided between the first waveguide 45 and the substrate 43 and above the 
second waveguide layer 47 respectively. As in the case of the switch of 
FIG. 1, the second waveguide 47 is made of multiple-quantum-well structure 
having an index of refraction n.sub.2 which varies substantially with an 
electric field E generated by a voltage applied across a metallization 
layer 55 on the confinement layer 53, and the substrate 43. Also as with 
the switch 1, the index of refraction n.sub.1 of the first waveguide 45 is 
comparatively unaffected by the electric field and is selected to be 
substantially equal to one of the values of n.sub.2, preferably the value 
of n.sub.2 in the presence of the field E. 
Light is applied to the first waveguide 45 as indicated by the arrow marked 
"input" in FIG. 8. In the parallel propagation condition, that is with 
n.sub.2 not equal to n.sub.1, the attenuator is "ON" and light is output 
from the first waveguide as indicated by the arrow marked "ON/OFF". Under 
cross-propagation conditions, light input to the first waveguide 45 is 
switched to the second waveguide 47 where it is dissipated through 
absorption in the multiple-quantum-well structure. 
FIG. 10 illustrates attenuation by the attenuator 41 in the "ON" state as a 
function of the length of the second waveguide for multiple-quantum-well 
absorption coefficients .beta..sub.2 of 10.sup.-3, 3.times.10.sup.-2, 
4.times.10.sup.-2 and 5.times.10.sup.-2 .mu.m.sup.-1 as represented by 
traces 57, 59, 61 and 63 respectively. Attenuation in the "OFF" state is 
shown by the traces 57', 59', 61' and 63' for the corresponding values of 
.beta..sub.2. 
Since the function of the second waveguide layer 47 in the attenuator 41 is 
to dissipate light under cross propagation conditions, a higher absorption 
coefficient is desired in this application. This can be achieved using the 
same material as for the light switch 1 by utilizing light of a higher 
energy. The higher energy light moves the operating point of the device 
closer to the excitonic transition energy which results in the higher 
absorption coefficient. This, however, also results in an increase in the 
modulator length needed for maximum cross-propagation as indicated by FIG. 
10. 
The various layers of semiconductor material are grown on the substrate by 
using for instance MOCVD. With such techniques, the thickness and 
uniformity of the layers of semiconductor materials can be closely 
controlled. This makes it possible to fabricate devices with a coupling 
layer having submicron thickness. As previously mentioned, this allows the 
length of the device to be shorter so that the device occupies less real 
estate and reduces the capacitance so that the device can operate faster. 
Techniques such as MOCVD also make it possible to build the 
multiple-quantum-well structures forming the second waveguides 7 and 47. 
The alternating barrier layers 23 and well layers 21 are on the order of 
only about 5 to 20 nm thick. Preferably, the wells are about 10 nm thick. 
The wider the wells are made, the lower the energy of the transitions and 
the closer the higher energy levels come to the operating point. On the 
other hand, the narrower the wells, the smaller the effect of the field 
and the smaller the range of wavelengths that can be used. For barrier 
thicknesses above about 5 nm, increasing thickness of the barrier results 
a decrease in the absolute value of the multiple quantum well structure. 
The semiconductor materials having the suitable electro-optic properties 
for making the light switches and attenuators of the invention can be 
selected from group III-group V compounds and their solid solutions. In 
general, a GaAs system or an InP system can be used. We have found that 
ternary compounds such as in particular, AlGaAs are particularly useful 
since they can be fine tuned to the particular properties desired. We have 
found for instance ternary Al.sub.x Ga.sub.1-x As useful as the material 
for the first waveguide and, with slightly different Al to Ga ratios, for 
the barriers in the multiple-quantum-well structures in which the wells 
are made of GaAs. Other examples of group III-group V systems which could 
be employed for making the electro-optic couplers of the invention are 
GaSb, GaAlAsSb and InGaAsP. In addition, group II-group VI compound 
semiconductor materials such as a CdS system, a CdSe system, a ZnS system 
or a ZnSe system could be used. 
By selecting semiconductor materials which have compatible crystal 
structures and lattice constants, the various layers can be easily grown 
on the substrate to the precise dimensions required, otherwise organic or 
other films would be needed between layers to grow the device. 
It should also be noted that the semiconductor materials used are undoped 
and hence highly resistive since an electric field and not charge carriers 
are used as the mechanism to induce the change in the refraction index of 
the multiple-quantum-well structure. As mentioned, this produces a device 
which operates much faster and consumes less power than devices which 
employ charge carriers. The substrate may be an n.sup.+ or p.sup.+ 
semiconductor material, but this is to make the substrate a conductor so 
that a uniform field can be generated across the multiple-quantum-well 
structure. Alternatively, the substrate could be an insulator and a 
metalization layer could be applied to the substrate to serve as the 
conductor, or the substrate could be removed and the metalization applied 
directly to the lower confinement layer. 
For single mode light, the total thickness of the coupler is between 1 and 
3 .mu.m with a width of about 1 to 5 .mu.m. 
EXAMPLE 1 
An exemplary electro-optic coupler in accordance with the invention adapted 
for use as a switch for 1.540 eV light is depicted in FIG. 1. The first 
waveguide 5 is a 1 .mu.m thick layer of ternary Al.sub.0.35 Ga.sub.0.65 As 
which is calculated to have a refractive index of 3.38 at room temperature 
for 1.540 eV light and an absorption coefficient .beta..sub.1 of 10.sup.-3 
.mu.m.sup.-1. The second waveguide 7 is a 1 .mu.m thick superlattice of 10 
nm wide GaAs quantum-wells 21 and 10 nm wide Al.sub.0.3 Ga.sub.0.7 As 
barriers 23 which have been estimated to have an index of refraction of 
3.38 in the presence of an applied field of 30 KV/m for 1.540 eV light and 
an index of refraction of about 3.41 in the absence of the field. The 
absorption coefficient .beta..sub.2 is about 10.sup.-3 .mu.m.sup.-1 in the 
presence of the 30 KV/m field and about 3.times.10.sup.-3 .mu.m.sup.-1 in 
its absence. 
The waveguides 5 and 7 are grown using MOCVD techniques on an n+ GaAs 
substrate with a 0.25 .mu.m confinement layer 11 made of Al.sub.0.8 
Ga.sub.0.2 As having an index of refraction n.sub.3 of 3.136 between the 
substrate 3 and the second waveguide layer 7. A second confinement layer 
13 with similar parameters covers the first waveguide layer. The coupling 
layer 9 is a 0.5 .mu.m thick layer of Al.sub.0.43 Ga.sub.0.57 As having an 
index of refraction of 3.355 with 1.540 eV light. The switch 1, which is 
designed for single mode light transmission, has a length of 77 .mu.m and 
a width of 2 .mu.m. FIGS. 6 and 7 show that with 1.540 eV light injected 
into the first waveguide, the output power of the first waveguide of this 
exemplary device is -17 dB below the input power for the cross-propagation 
condition and -1 dB for the parallel propagation condition. The 
corresponding power output for the second waveguide is -1 dB below the 
input power for the cross propagation condition and -17 dB for the 
parallel propagation condition. 
EXAMPLE 2 
FIG. 9 illustrates an example of an electro-optical coupler 41 of the 
invention adapted for use as an attenuator. The materials used for the 
first waveguide layer 5, the superlattice second waveguide layer 7, the 
coupling layer 9 and the confinement layers 11 and 13 are the same as in 
the case of the device of example 1. In this device, however, the second 
waveguide 7 is above the first waveguide layer 5 and the coupling layer 9 
so that the first waveguide layer can extend beyond the required 
dimensions of the device to serve also as a connecting waveguide, and the 
second waveguide 7 can be etched to the desired dimensions. The length of 
this device is 110 .mu.m. The difference in length is attributed to the 
function of this device to dissipate power cross propagated from the first 
waveguide. Using the higher absorption coefficient .sub.2 =10.sup.-1 
.mu.m.sup.-1, FIG. 10 shows that the length L for maximal cross 
propagation in 110 .mu.m. This increase in the absorption coefficient 
.beta..sub.2 can be obtained employing the same materials as in the switch 
of Example 1 by using light of the slightly higher energy level of about 
1.55 eV. Of course, alternatively, the material composition can be 
adjusted to provide the desired operation at the 1.540 eV energy level. 
Performance parameters of the devices 1 of Example 1 and device 41 of 
Example 2 are set forth in the following table. 
TABLE 
______________________________________ 
Device 1 
Device 41 
______________________________________ 
Length [.mu.m] 77 110 
Switching Voltage 
[V] 6 4 
Capacitance [fF] 5 7 
Switching time [ns] 0.25 0.36 
(for R = 50 K Ohms) 
Energy/Cycle [pJ] 0.08 0.055 
Power [.mu.W] 
for 16 MHz 1 1 
for 4 GHz 400 400 
Opt Transfer [%] 75-3.7 85-0.8 
Efficiency 
(max-min) 
Power Loss [dB] 1.3* 0.7* 
Power Isolation [dB] 13 20 
______________________________________ 
*without absorption. 
While specific embodiments of the invention have been described in detail, 
it will be appreciated by those skilled in the art that various 
modifications and alternatives to those details could be developed in 
light of the overall teachings of the disclosure. Accordingly, the 
particular arrangements disclosed are meant to be illustrative only and 
not limiting as to the scope of the invention which is to be given the 
full breadth of the appended claims and any and all equivalents thereof.