Semiconductor device comprising a directional coupler for the TE and TM components

An integrated semiconductor device including a directional optoelectronic coupler, which coupler comprises two parallel single-mode rectilinear optical waveguides over a total length D, separated by a small distance d, which coupler also comprises four electrodes of the same conductivity type, two on each waveguide, and at least one other electrode of the opposite conductivity type, characterized in that the coupler has an operation which is independent of the transverse electrical and transverse magnetic radiation components, TE and TM, respectively, present in random proportions in the incoming signal, under the condition in which the structural parameters of the coupler render it possible to ensure that the following relations are true: ##EQU1## In which relations: Z is the dimension of each electrode on the waveguides, PA0 .phi.TM and .phi.TE are the phase mismatches provoked by the refractive-index changes in the waveguides under the influence of an electric field, PA0 L.sub.CTE and L.sub.CTM are the coupling lengths for TE and TM, respectively, PA0 m, n, p are random integer numbers, PA0 .epsilon.=.+-.1.

BACKGROUND OF THE INVENTION 
The invention relates to an integrated semiconductor device including a 
directional optoelectronic coupler, which coupler comprises two parallel 
single-mode rectilinear optical waveguides over a total length D, 
separated by a small so-called coupling distance d, of which waveguides at 
least one receives a radiation signal at its so-called input end, the 
coupler further comprising four electrodes of the same conductivity type 
disposed two by two on each waveguide and at least one other electrode of 
the opposite conductivity type to permit of the creation of an electric 
field in the waveguides and, depending on the value of the said electric 
field, of the passage of the radiation into the waveguide opposed to the 
input waveguide, which corresponds to the so-called crossover state of the 
coupler, or of its propagation in the extended direction of entrance, 
which corresponds to the so-called straight-through state of the coupler. 
The invention finds its application in the realisation of switching 
matrices used in the field of telecommunications. 
A directional coupler formed by two parallel strip waveguides is known from 
the publication entitled "Switched Directional Couplers with Alternating 
.DELTA..sub..beta. " by H. KOGELNIK et al. in "IEEE Journal of Quantum 
Electronic, VOL.-QE-12, no. 7, July 1976". This document discloses that 
such a coupler is characterized by its interaction length and its coupling 
coefficient. The coupling length, which is inversely proportional to the 
coupling coefficient, indicates the minimum length necessary to achieve 
that the radiation passes completely from one waveguide into the other. 
The crossover is complete when the propagation constants of the waveguides 
are the same and when the interaction length is an odd multiple of the 
coupling length. In order to minimize the demands on the dimensions of the 
device, the cited publication proposes to place four electrodes on the 
waveguides, two on each waveguide, in order to vary the propagation 
constant in each of the waveguides in order to, on the one hand command 
the crossover by the application of a voltage to the electrodes and, on 
the other hand, obtain the crossover in a wide spectrum of interaction 
lengths of the device by simply controlling the voltages applied to the 
electrodes. 
The major and inhibitive shortcoming of the known device is that it is 
strictly limited to use with beams which have only one of the two 
polarizations TE or TM, TE being the so-called transverse electrical 
component, i.e. of which the diagrammatic representation is a vector 
parallel to the plane of the substrate on which the waveguides are 
integrated and at the same time perpendicular to the direction of 
propagation, and TM being the so-called transverse magnetic component, of 
which the diagrammatic representation is a vector which is simultaneously 
perpendicular to the plane of the substrate and to the direction of 
propagation of the radiation. 
The shortcoming inherent in the known device is on the one hand the result 
of the fact that the coupling length is different for each of the 
polarizations TE and TM and on the other hand that the phase mismatch 
induced by the refractive-index change in the waveguides under the 
influence of an electric field depends to a very high degree on the 
initial polarization condition of the beam which is propagated in the 
waveguides, and also on the orientation of the waveguides on the 
substrate, at least when the waveguides are realised in III-V material. 
Now it is known that the optical fibres usually employed in 
telecommunications never maintain a polarization condition as initially 
given over a very long distance. The beam which arrives at an 
optoelectronic integrated device, therefore, is usually in a random 
polarization condition. 
Under these circumstances the known device cannot be used. 
On the other hand, the III-V materials are now the materials of the future 
for realising optical waveguides in the field of telecommunications on the 
ground that they are semiconductors, unlike, for example, lithium niobate, 
and that they therefore permit of the realisation of optoelectronic 
components, or indeed purely electronic components, in manufacturing 
synergy, integrated on the same substrate. 
SUMMARY OF THE INVENTION 
The present invention, therefore, has for its object to provide a 
directional coupler which is independent of the initial polarization 
condition of the incoming beam. The result envisaged by the invention is 
achieved when, by the application of a first electric field value, the two 
components TE and TM emerge jointly through the waveguide opposite to the 
input waveguide (crossover state) or emerge jointly through the extension 
of the input waveguide (straight-through state) with a minimum loss. 
Another object of the invention is to provide a coupler realised in a III-V 
semiconductor material. 
Another object of the invention is to provide an optoelectronic switch 
which has the same performance characteristics as the coupler. 
Another object of the invention is to provide a matrix realised by means of 
the said switch. 
According to the invention, these problems are resolved and these objects 
are achieved by means of a device as described in the heading of claim 1 
and further characterized in that the coupler has an operation which is 
independent of the transverse electrical and transverse magnetic radiation 
components, TE and TM respectively, present in random proportions in the 
incoming signal, under the conditions in which the structural parameters 
of the coupler render it possible to ensure that the following relations 
are true: 
##EQU2## 
In which relations: Z is the dimension of the electrodes on the 
waveguides, 
.phi..sub.TM and .phi..sub.TE are the phase mismatches provoked by the 
refractive-index changes in the waveguides under the influence of an 
electric field for the TM component and the TE component, respectively, 
L.sub.CTE and L.sub.CTM are the lengths necessary for the given 
polarization components, TE or TM, respectively, introduced into a 
waveguide to pass into the other waveguide, called hereinafter coupling 
length for TE and coupling length for TM, 
m, n, p are random integer numbers, 
.epsilon.=.+-.1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an integrated semiconductor device realised on a substrate 10, 
comprising at least a first input waveguide G'.sub.1, and an optical 
second input waveguide G'.sub.2, two output waveguides among which a first 
output waveguide called G".sub.1 and a second output waveguide called 
G".sub.2, and a directional optoelectronic coupler 11. 
The optoelectronic coupler 11 comprises two rectilinear optical waveguide 
portions G.sub.1 and G.sub.2, which are parallel and have the same lengths 
D, of which the first is so arranged as to connect the first input 
waveguide G'.sub.1 to the first output waveguide G".sub.1, and the second 
G.sub.2 is connected at one end to the second output waveguide G".sub.2 
and at the other end to the second input waveguide G'.sub.2, if the latter 
exists. 
Two identical electrodes E'.sub.1 and E".sub.1 are disposed at the surface 
of the first portion of the rectilinear waveguide G.sub.1. Opposite these 
electrodes, on the second portion of the rectilinear waveguide G.sub.2, 
there are arranged two other identical electrodes E'.sub.2 and E".sub.2. 
The electrodes have a length Z. The four electrodes E'.sub.1, E".sub.1, 
E'.sub.2, E".sub.2 are of the same conductivity type, for example p. 
The device further comprises one or several electrodes of the opposing 
conductivity type n to render possible the creation of an electric field 
in the waveguides by the application of a voltage between the p and n 
electrodes. 
In order to have the coupler shown in FIG. 1 function, a first voltage is 
applied between the electrodes n and p to obtain the so-called 
straight-through or parallel state, and a second voltage to obtain the 
so-called crossover state. 
Only two electrodes among those which are realised in the surface of the 
waveguides will be used, the other two are only present for the sake of 
symmetry. One single electrode will be used for each waveguide, each 
electrode being chosen so as not to be directly opposite the other 
electrode used. These two electrodes of the same conductivity type must be 
electrically interconnected for the application of the potential between 
them and the electrodes of the second conductivity type. 
In order to have the device function, the choice between the pair E'.sub.1, 
E".sub.2 and the pair E".sub.1, E'.sub.2 is free. The electrodes of the 
second conductivity type are preferable disposed in the device in order to 
produce the desired electric field. Examples of embodiments will be given 
below. 
The object of the invention is therefore to make possible the use of such a 
coupler, not as it is known in the present art, with the input of a signal 
comprising a single component, TE or TM, but with the input of a signal 
comprising the two components, TE and TM, in random proportions. According 
to the invention, it should be possible to recuperate the total power 
input at the output of the coupler at one or the other waveguide, 
depending on whether the straight-through or the crossover state obtains. 
The objects of the invention are achieved if and only if the parameters 
defining the structure of the coupler render it possible to ensure that 
the relations, which will be given below, are true. 
It will be assumed that those skilled in the art of optoelectronics know 
that for a power input P.sub.a at, for example, the input end of the 
waveguide G.sub.1 through input portion G'.sub.1 the following holds: 
EQU P.sub.a =(1-2B.sup.2).sup.2 (1) 
##EQU3## 
EQU and K=.pi./2L.sub.c (3) 
L.sub.c being the coupling length of any of the components. and also 
EQU .delta.=(.pi./2)(.phi./Z) (4) 
.phi. being the total phase mismatch along the length D, 
Z being the length of one electrode arranged on one of the waveguides of 
the coupler. 
For the two components TE and TM to remain jointly in a single waveguide, 
it is necessary that: 
in the crossover state, the power P.sub.a of TE is equal to the power 
P.sub.a of TM and equal to 0, which is written as: 
EQU P.sub.aTE =P.sub.aTM =0 (5) 
in the straight-through state, the power P.sub.a of TE is equal to the 
power P.sub.a of TM and equal to 1, which written as: 
EQU P.sub.aTE =P.sub.aTM =1 (6) 
The phase mismatch is proportional to the distance travelled in a 
waveguide, which can be expressed by: 
EQU .phi.=.epsilon.D (7) 
in which is a proportionality coefficient. 
In the following description, the electrodes E'.sub.1 and E".sub.1 on the 
one hand, and E'.sub.2 and E".sub.2 on the other hand are very close to 
one another, with a small space in between just sufficient to avoid 
short-circuits. The dimension of this small space is negligible in 
relation to the length D. In this way it is achieved that a first 
condition for the operation of the device according to the invention is 
resolved: 
EQU a) Z=D/2 (8) 
in which Z is the length of each of the electrodes E'.sub.1, E".sub.1, 
E'.sub.2, E".sub.2. 
To establish the relations necessary for the operation of the device it is 
stipulated: 
EQU k.sub.1 =L.sub.CTM /L.sub.CTE (9) 
EQU k.sub.2 =.phi..sub.TE /.phi..sub.TM (10) 
.phi..sub.TM and .phi..sub.TE are the phase mismatches generated by the 
refractiveindex changes in the waveguides under the influence of an 
electric field for the TM component and for the TE component, 
respectively. These are the structural quantities which can be measured by 
methods known to those skilled in the art. 
L.sub.CTE and L.sub.CTM are the lengths necessary for the given 
polarization component, TE or TM, respectively, introduced into a 
waveguide to pass into the other waveguide, to be called hereinafter 
coupling length for TE and coupling length for TM. L.sub.CTE and L.sub.CTM 
are structural quantities which can be measured by methods known to those 
skilled in the art. 
Starting from equation (2) one can now write for the component TE: 
##EQU4## 
and for the component TM: 
It has already been explained that these quantities must be equal for TE 
and TM in the case in which the coupler is in the straight-through state 
(parallel), i.e. P.sub.a =1, and in the case in which the coupler is in 
the crossover state, i.e. P.sub.a =0. This means that the following 
equations must be solved: first in the straight-through (parallel) state 
EQU P.sub.a =1 
EQU B=0. 
For the component TE: 
EQU .alpha..sup.2 +4.phi..sub.1.sup.2 =16m.sub.o.sup.2 (13) 
in which m.sub.o is an integer and for the component TM: 
EQU (.alpha./k.sub.1).sup.2 +4(.phi..sub.1 /k.sub.1).sup.2 =16 (14) 
in which n.sub.o is an integer. 
Then in the crossover state: 
EQU P.sub.a =0 
EQU B=1/2 
for the component TE: 
##EQU5## 
and for the component TM: 
EQU (.alpha./k.sub.1).sup.2 /[(.alpha./k.sub.1).sup.2 +4(.phi..sub.2 
/k.sub.2).sup.2 ]sin.sup.2) 
In the above equations, .phi..sub.1 and .phi..sub.2 are the respective 
phase mismatches necessary for putting the coupler in the straight-through 
(or parallel) state and in the crossover state for the component TE. 
A whole family of parameters k.sub.1, k.sub.2 and .alpha. exists for which 
the equations (13) and (14) have solutions for .phi..sub.1 and 
.phi..sub.2. This family of parameters can be determined by calculation in 
a simple manner by all those skilled in the art. 
An analytical description is given below of a particular preferred 
embodiment according to which the condition is fulfilled: 
EQU k.sub.1 =k.sub.2 =k (17) 
that is to say, the condition: 
EQU b) .phi..sub.TE .multidot.L.sub.CTE =.phi..sub.TM .multidot.L.sub.CTM(18) 
In physical terms, these conditions can be interpreted as follows: the 
cumulative phase mismatch over a length equal to the coupling length is 
independent of the polarization. 
To simplify the calculations, new relations are introduced. It is 
stipulated: 
EQU .phi..sub.C =.delta./K=.phi..multidot.L.sub.C /Z 
According to the starting hypothesis, .phi..sub.C is independent of the 
polarization, which means that: 
EQU .phi..sub.CTE =.phi..sub.CTM =.phi..sub.C (19) 
The equations (11) and (12) are then rewritten: 
##EQU6## 
From this it follows that the condition P.sub.aTE =P.sub.aTM is fulfilled 
if: 
##EQU7## 
in which m1 is an integer and .epsilon.=.+-.1. The equation (22) can be 
put in the form: 
##EQU8## 
This condition obviously depends on .phi..sub.C. In the case of the 
invention, it is the object only to have B.sub.TE =B.sub.TM in the 
straight-through and crossover states. 
Study of the Straight-Through (or Parallel) State 
The straight-through (or parallel) state is defined by a certain value of 
.phi..sub.C which will be written as .phi..sub.C1 and which is such that: 
EQU B.sup.2.sub.TE =B.sup.2.sub.TM =0 
which entails, starting from the equations (20) and (21) that: 
##EQU9## 
in which n is an integer. From the equation (23) follows, to achieve that 
B.sub.TE =B.sub.TM 
##EQU10## 
in which m is an integer The relation between the equation (26) and the 
equation (25) means that: 
EQU (1-.epsilon.k)=m/n (27) 
in which m and n are integers. The following condition necessary for the 
invention can be derived: 
EQU c) k=(1-m/n)/.epsilon. (28) 
in which m and n are integers and .epsilon.=.+-.1 
The equation (28) shows that, if k.sub.1 has the indicated form, it is 
possible to achieve simultaneously the straight-through (or parallel) 
states for the two components TE and TM, whatever the values of the other 
parameters may be. 
Study of the Crossover State 
The crossover state is defined by a certain value of .phi..sub.C which will 
be written as .phi..sub.C2 and which is such that: 
EQU B.sub.TE.sup.2 =B.sub.TM.sup.2 =1/2 
from which follows, starting from the equations (20) and (21) that: 
##EQU11## 
From the equation (23) follows: 
EQU (1-.epsilon.k)(D/L.sub.CTM).sup.2 [1/(m.sub.2.sup.2 .multidot.16)] 
Sin.sup.2 {m.sub.2 .multidot..pi./(1-.epsilon.k)=1/2 (30) 
in which m2 is an integer. from which follows that: 
EQU (D/L.sub.CTM).sup.2 =8m.sup.2 /(1-.epsilon.k)[1/Sin.sup.2 
{m(.pi./1-.epsilon.k)} (31) 
in which m2 is an integer and .epsilon.=.+-.1. From this can be derived the 
fourth condition to be fulfilled for the invention: 
##EQU12## 
in which p is an integer and .epsilon.=.+-.1. 
Thus we have a set of permitted values given by the equation (32) for the 
ratio D/L.sub.CTM. 
It will be noted that there is only a solution if (m.sub.2 
.pi.)/(1-.epsilon.k) is not a multiple of .pi., that is to say if m2 is 
not a multiple of 1-.epsilon.k. 
Conclusion of this Preliminary Study 
It is possible to construct a coupler independent of the polarization in 
the two extreme states, straight-through and crossover, in all cases 
provided the following conditions are fulfilled: 
##EQU13## 
in which m, n are random integers, p is an integer .epsilon.=.+-.1 All 
these quantities can be measured by conventional techniques known to those 
skilled in the art. 
EXAMPLE I 
The invention will preferably be realised by choosing 
EQU k=2 (33) 
(see equations 9, 10, 17). From which follows that the condition c) is 
expressed by 
EQU L.sub.CTM =2L.sub.CTE (34) 
In this case 
EQU m=3,n=1,.epsilon.=-1 (35) 
The result is that the condition d) is expressed by 
##EQU14## 
or 
EQU D/L.sub.CTM =1,089p (36) 
The preferred choice will be 
EQU p=2 (37) 
from which follows the condition 
EQU D/L.sub.CTM .apprxeq.2,177 (38) 
Other cases (k=3, etc.) will not be discussed since those skilled in the 
art can easily implement them on the model of the example I described 
above. 
It follows from a), b), c) and d) that, if k=2: 
EQU .phi..sub.TE =2.phi..sub.TM (39) 
and that 
EQU D/L.sub.CTE .apprxeq.4,354 (40) 
The total length D of the coupler 11, the distance d which separates the 
rectilinear portions of waveguides G.sub.1 and G.sub.2 in the coupler, the 
choice of materials, their doping and the structure of the optical 
waveguides (external stripn waveguides or others) are so many factors 
which those skilled in the art can vary in order to fulfil the conditions 
for realising this embodiment, these conditions being expressed now by the 
equations (8), (34), (39) and (40): 
##EQU15## 
In this example I, the directional coupler corresponds to the diagram of 
FIG. 1 as far as the plan view is concerned. It conforms to the diagram of 
FIG. 2a in its transversal cross-section taken on the line I--I, and to 
the diagram of FIG. 2b in its longitudinal section taken on the line 
II--II. 
In order to achieve b'), that is the desired proportion between L.sub.CTE 
and L.sub.CTM, those skilled in the art can influence the coupling 
distance d which separates the rectilinear portions of the waveguides 
G.sub.1 and G.sub.2 as well as the transversal dimension W of the 
waveguides and the height h of the strip, if the structure chosen includes 
strip waveguides. 
In order to achieve c'), that is the desired proportion between 
.phi..sub.TE and .phi..sub.TM, those skilled in the art can vary the 
choice of doping of the layers and the structure of the waveguides 
constituting the coupler 11, as well as the direction of the waveguides 
relative to the substrate. Especially the doping levels of the guiding 
layer and the layers forming the p-n structure governing the 
refractive-index change in relation to the voltage applied between the 
electrodes p-n make it possible to adjust the ratio between .pi..sub.TE 
and .pi..sub.TM. 
To fulfil the conditions a), b'), c'), d') in this example I in such a way 
as shown in cross-section in FIG. 2a, each rectilinear optical waveguide 
G.sub.1, G.sub.2 of the coupler 11 comprises in that order: 
A/ a substrate 10, which may be of a semi-isolating material III-V, such as 
GaAs or InP, or which may be of a III-V material of the n.sup.+ 
conductivity type, for example InP doped with at least 10.sup.17 
at/cm.sup.3, for example 10.sup.18 at/cm.sup.3. The substrate may also be 
made of silicon provided with, for example, layers for adapting the 
lattice parameters to III-V materials. 
B/ a first epitaxial layer 51 of indium phosphide InP of the n.sup.+ 
conductivity type, obtained by doping with 10.sup.17 at/cm.sup.3. If the 
substrate 10 is of a different structure, the layer 51 is indispensable 
and preferably has a thickness e1.apprxeq.1 .mu.m. 
C/ a second epitaxial layer 52, called the guiding layer, of a III-V 
material of the composition 
EQU Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y 
In this composition, there is a relation between the x and y concentrations 
known to those skilled in the art 
EQU x=0,23y 
The concentration is preferably chosen to be 
EQU y=0,6 
The guiding layer 52 of GaInAsP is also doped with 10.sup.17 at/cm.sup.3 to 
obtain the n.sup.+ conductivity type. The wavelength associated with the 
forbidden band energy is then .lambda..sub.gap =1,3 .mu.m, and the 
operating wavelength of the waveguides is in the chosen band for the 
telecommunication application .lambda..sub.0 =1,52 .mu.m. 
The guiding layer 52 preferably has a thickness e2.apprxeq.0,4 .mu.m. 
D/ a third layer 53 of indium phosphide InP of the n.sup.- conductivity 
type, obtained by doping with at most 10.sup.16 at/cm.sup.3. This layer 53 
will have a total thickness e3.apprxeq.0,5 .mu.m at the moment of its 
realisation. 
E/ a fourth layer 58 of indiumphosphide Ip of the p.sup.+ conductivity type 
obtained by doping with 3.10.sup.17 at/cm.sup.3, with the function of 
forming a p-n junction with the preceding layer 53. This fourth layer 54 
of the p.sup.+ type preferably has a thickness e4.apprxeq.1,05 .mu.m. 
As is shown in cross-section in the same FIG. 2, each waveguide comprises 
furthermore a ribbon or strip structure R with the object of delimiting 
the guiding region. 
The strip R is generally realised by engraving the upper layers 54 and 53 
by any method for engraving a III-V material known to those skilled in the 
art, but preferably by dry engraving, such as the RIE method, which makes 
it possible to obtain flat edges perpendicular to the layers. The 
engraving depth for the waveguides in the layer 52 will favourably be 
EQU h=1,2.mu.m 
It is important that on top of the guiding layer 52 remains a thickness of 
material 53 
EQU e'.apprxeq.0,35.mu.m. 
The transversal dimension w of the waveguide ribbons is favourably in the 
order of w.apprxeq.3 .mu.m to 5 .mu.m for transporting a single-mode wave. 
The differences in refractive-index caused by the differences in thickness 
in the region provided with ribbon and in the regions on either side of 
the ribbon cause the radiation to remain inside the guiding layer 52 and 
under the strip R, in other words, guide it. Owing to the differences in 
refractive-index caused by the differences in thickness, all this happens 
as if the "guided" portion were surrounded by two zones of lower 
reftactive-index serving as a confinement. FIG. 2 shows in cross-section 
the isoenergy lines which are the result for the waveguided luminous 
fluxes F. 
Finally, the rectilinear waveguide portions G.sub.1 and G.sub.2 are 
provided with a metal layer 55, for example of gold (Au), which is chosen 
on account of its low optical losses, with a thickness e5.apprxeq.0,3 
.mu.m to realise the contacts of the ptype on the upper layer 54 of the 
p-type. These contacts are arranged so as to form the electrodes E'1, E'2, 
E"1, E"2, as shown in FIG. 1 seen from above, with special precaution to 
provide an interspacing of approximately 5 .mu.m, in the 2-20 .mu.m range, 
between the electrodes E'1, E"1 and E'2, E"2 in order to avoid 
short-circuits. 
A favourable method for realising this structure is in fact to realise 
first the electrodes on the surface of the layer 54, then to engrave the 
layer 54 of the ptype and the upper portion of the layer 53 of the n.sup.- 
type, using the metal of the electrodes as an engraving mask. 
FIG. 2b, which is a layer taken on the line II--II, shows the device 
obtained by means of this manufacturing procedure. 
If the electric field necessary for the operation of the device is to be 
created, contacts of the n-type must furthermore be provided. 
If the substrate is of the ntype, these contacts may be realised on either 
of the surfaces of the substrate. If the substrate is of another type, an 
opening will be made in the layers up to the first layer 51 of the ntype, 
and a contact E.sub.0 of the ntype will be realised on this layer in this 
opening by any method known to those skilled in the art, for example by 
means of a metal stud, for example of gold/nickel (Au/Ni). 
The III-V semiconductor materials recommended for realising this device are 
particularly favourable for several reasons. 
Firstly, they allow of the synergy of the manufacture of different devices 
on a single substrate. 
Furthermore, they permit of less costly silicon (Si) substrates, provided 
that some method for adapting the lattice between the silicon substrate 
and the III-V device, of which is already known nowadays, is applied by 
those skilled in the art. 
Moreover, they are attuned to the wavelength range adapted to 
telecommunications. 
In this embodiment, the conditions a), b'), c') and d') are fulfilled with 
the structure described above for the coupling distance between waveguides 
d=.apprxeq.4,5 .mu.m the transversal dimension of the waveguides 
W=.apprxeq.3 to 5 .mu.m 
##EQU16## 
These latter data depend to a very important degree on the doping of the 
layers. 
In fact, when the electric field in the guiding layer 52 is increased, this 
layer is depleted, which reduces the doping and increases the 
refractive-index in the said guiding layer. This effect is independent of 
the polarization of the light beam. 
Two other effects also influence the change in refractive-index in the 
waveguides: the Pockels effect, which depends on the polarization and only 
influences the TE component; and the Kerr effect, which is independent of 
the polarization. 
In the present invention, the effect of the electric field on the 
quaternary layer is particularly interesting. It makes it possible to 
change the refractive-index and the phase mismatch. This is because the 
.phi..sub.TM /.phi..sub.TE ratio depends strongly on the doping of the 
quaternary layer. This ratio also depends on the thickness of the layers 
because, for a given voltage applied between the electrodes, the created 
electric field will vary if the distance between the layers p and n, on 
which the electrodes are disposed, varies. The most advantageous situation 
is the one in which is .phi..sub.TM is greatest. 
Under these conditions the coupler is in the CROSSOVER STATE owing to the 
application of a voltage between the electrodes n and p of 
EQU V.sub.1 =-4,5volts. 
In this crossover state, illustrated in FIG. 4a, the two polarizations TE 
and TM emerge jointly through the second output waveguide G".sub.2 if a 
random polarization flux F1 is injected into the input waveguide G'.sub.1. 
It should be noted that the n-type electrode can be favourably connected to 
earth, while the voltage V.sub.1 can be applied to one of the sets of 
electrodes of the ptype E'1, E"2, or E"1, E'2. 
Under the above conditions, on the other hand, the coupler is in the 
STRAIGHT-THROUGH (OR ALLEL) STATE by the application of a voltage 
between the electrodes n and p of 
EQU V.sub.2 =-10,1volts. 
In this straight-through state, illustrated in FIG. 4b, the two 
polarizations TE and TM emerge jointly through the first output waveguide 
G".sub.1 if a random-polarization flux F1 is injected into the input 
waveguide G'.sub.1. The voltage V.sub.2 can be applied in the same way as 
the voltage V.sub.1. 
FIG. 5a illustrates the case in which the coupler is provided with a second 
input waveguide G'.sub.1. It can then function as a switch. When the 
voltage 
EQU V.sub.1 .apprxeq.-4,5Volts 
is applied between the electrodes, a flux F1 entering through the input 
waveguide G'.sub.1 exits through the opposing waveguide G".sub.2, and a 
flux F2 entering in the input waveguide G".sub.1 exits through the other 
waveguide G'.sub.2. The switch is in the CROSSOVER STATE. 
FIG. 5b illustrates the case in which the switch is in the STRAIGHT-THROUGH 
STATE through the application of the voltage 
EQU V.sub.2 .apprxeq.-10,1Volts 
between the electrodes. The flux F1 entering through the input waveguide 
G'.sub.1 continues its path through the rectilinear portion G.sub.1 and 
exits through the output waveguide G".sub.1. Similarly, the flux F2 enters 
through the input waveguide G'.sub.2, continues its path through the 
rectilinear portion G.sub.2 and exits through the output waveguide 
G".sub.2. 
The coupler functions as a switch through the application of the voltages 
V.sub.1 and V.sub.2 under exactly the same conditions as described above, 
i.e. between a set of electrodes of the ptype, for example E'1, E"2 or 
E'2, E"1, and an electrode of the ntype, possibly connected to earth. 
It should be noted that a single electrode of the ntype is sufficient for 
the operation of the device. In practice, four electrodes of the ntype as 
close as possible to the electrodes of the ptype will preferably be 
provided, as shown in FIGS. 4 and 5, in order to reduce the resistance and 
make the device symmetrical. 
FIGS. 6a and 6b illustrate the results which may be expected from a coupler 
or switch described in this example I. 
FIG. 6a depicts graphically the ratios P.sub.2 /P.sub.1 or P.sub.1 /P.sub.2 
in dB, P.sub.1 being the output power of the first waveguide of the 
coupler or switch 11 through G".sub.1, and P.sub.2 being the output power 
of the second waveguide of the coupler or switch 11 through G".sub.2, for 
an input power P.sub.0 =1 in the first input waveguide G'.sub.1, as a 
function of the percentage of the TE component in this input, under the 
conditions in which 
EQU .phi..sub.TM /.phi..sub.TE =0,5 and 
EQU L.sub.CTM /L.sub.CTE =2 
Among the curves of FIG. 6a, the curve A represents the ratio P.sub.2 
/P.sub.1 when the coupler is in the straight-through (or parallel) state. 
It is to be understood that the representation of the TE component on the 
abscissa indicates that, if the TE component accounts for 100% in the 
input signal with the power P.sub.0, the TM component is then 0%, and vice 
versa. The curve A shows that the ratio is in the order of -60 dB and that 
it depends very little both on the initial polarization state and on the 
ratio D/L.sub.CTE. 
The dotted curve B shows the ratio P.sub.1 /P.sub.2 in the case in which 
the coupler or switch is in the crossover state. This curve B shows that 
the ratio is in the order of -35 dB when the ratio D/L.sub.CTE =4,4. This 
curve, therefore, shows the conformity of the results obtained to the 
results calculated. 
The broken-line curves C and D show the ratio P.sub.1 /P.sub.2 in the case 
in which the coupler or switch is in the crossover state and when the 
D/L.sub.CTE ratio equals 4,0 and 4,8, respectively. The P.sub.1 /P.sub.2 
ratio is then -15 dB, which is still quite acceptable and even very good 
for a number of applications. 
It may be concluded, therefore, that the device gives completely 
satisfactory results in the range 
EQU 4,0&gt;D/L.sub.CTE &gt;4,8 
Outside these values less good results are to be expected. These curves 
show those skilled in the art the way towards choosing the best conditions 
for a particular envisaged application. 
The curves of FIG. 6b graphically show the power P.sub.1 at the output of 
the first waveguide through G".sub.1 when the input power P.sub.0 is 
injected through G'.sub.1, as a function of the voltage V (volts) applied 
between the electrodes of the n and ptype, and under the following 
conditions 
##EQU17## 
The full-drawn curve E represents the variations of the TE component, while 
the broken-line curve F shows the variations of the TM component. 
These curves show that in the conditions described above, and for a voltage 
V.sub.1 =-4,5 volts, there is absolutely no radiation anymore in the 
extension of the first output waveguide G".sub.1, all radiation being 
present in the second output waveguide G".sub.2, for the TE polarization 
as well as for the TM polarization, and that for this value V.sub.1 the 
device is in the crossover state. 
These curves also show that for a value V.sub.1 =-10,1 volts the two 
components TE and TM emerge jointly through the first output waveguide 
G".sub.1 and that the device is in the straight-through (or parallel) 
state. 
Finally, these curves show that in the absence of any voltage applied 
between the electrodes n and p, i.e. for V=0 volts, only a part of the 
components TE and TM is waveguided by the first waveguide G'.sub.1, 
G.sub.1, G".sub.1. The rest of the signal is lost or passes through 
G".sub.2. 
EXAMPLE II 
The curves of FIGS. 6a and 6b show that the dimensions of the device 11 are 
not critical to the extent of rendering it difficult to realise. On the 
contrary. Example II also profits from the fact that the device gives good 
results in a range of dimensions which is wide enough to present a device 
of a particularly ingenious form, so that the realisation is made even 
easier. 
This example is illustrated by the FIGS. 3a and 3b. 
Since the length of the device 11 is 
EQU D=8mm, 
a small overhang of a few .mu.m is permitted on the length Z=D/2=4 mm of 
the electrodes. 
The device 11 is realised by the manufacturing process described above for 
example I. The metallizations of the electrodes E'1, E'2, E"1, E"2 serve 
as engraving masks for the waveguide strips up to the n.sup.- layer 53 in 
order to avoid short-circuits with the layer p. 
Thanks to this process, a cut G.sub.12 is realised in the strip of 
waveguide G.sub.1, so that two parts G.sub.10 and G.sub.11 are obtained 
separated no more than by approximately 5 .mu.m, as has been stated for 
example I, which cut cannot disturb the beam under these conditions. 
Similarly, a cut G.sub.22 is made in the strip of the waveguide G.sub.2, 
which leaves between these two cuts the passage for a strip section 
G.sub.32 covered by a metal layer 55, which connects the two electrodes 
E'.sub.1 and E".sub.2 in a simple manner. The signal is no longer 
disturbed by the portion G.sub.32 and a single electrical contact is then 
sufficient for applying the voltage to the electrode pair E'.sub.1, 
E".sub.2. The transversal dimension of the connection G.sub.32, E.sub.32 
will favourably be in the order of 3 to 5 .mu.m. 
Obviously, the connection between the electrodes can be realised between 
E'.sub.1, E"2, as desired, or, alternatively, between E".sub.1, E'.sub.2, 
the non-connected pair not being operational. 
The ends of the input waveguides G'.sub.1, G'.sub.2 and the output 
waveguides G".sub.1, G".sub.2 have been drawn in a general way in the 
various Figures. In fact, it is sufficient for these waveguides to be 
spaced apart by a distance much in excess of the coupling distance d at a 
short distance beyond the coupling length in order that the results 
expected of the coupler or switch are obtained. 
The switches according to the invention are particularly aimed at realising 
switching matrices with N inputs and N outputs, as disclosed in, for 
example, the publication "Photonic Switches and Switch Areas on 
LiNbO.sub.3" in "Optical and Quantum Electronics", 21, 1989 by A. 
SELVARAJAN and J. E. MIDWINTER, edited by CHAPMAN et al, pp. 1-15 with the 
advantage that it is not necessary to select one component, TE or TM, 
before input into the matrix in order to achieve the operation of the 
latter.