Environmentally protected integrated optical component and its production process

An environmentally protected integrated optical component and its production process is disclosed. The component has at least one cavity (26a) isolated from the environment formed in at least one light guide layer (16, 18), with the cavity being filled with a fluid or a polymer (27) having a refractive index which is different or which can be made different from the refractive index of the layer. In particular, the cavity is filled with air. The component can be a splitting plate, a mirror, a grating, a microguide or a lens.

DESCRIPTION 
The invention relates to an active or passive optical component for 
integrated optics, as well as to its production process. It can more 
particularly be used in the field of processing radar signals in real 
time, e.g. in correlators, spectrum analyzers or interferometers, in the 
field of optical telecommunications, e.g. for multiplexing or 
demultiplexing light signals and in the field of optical fibre sensors. 
The integrated optical component according to the invention can be a 
mirror, a beam splitting plate, a diffraction grating, a focusing lens, a 
microguide and all integrated optical components using diffraction 
gratings, such as e.g. optical couplers. These optical couplers can in 
particular be used in polarization seperators and polarization converters. 
In general terms, it is very difficult to produce integrated optical 
components in light guides to the extent that the effective index 
variations which affect the guided light signals are relatively small. 
This makes it necessary to work in geometrical conditions of a special 
nature or to define new types of components adapted to particular problems 
to be solved. 
In a complete integrated optical system, use is very often made of a beam 
splitting plate. An example of the production of a splitting plate in 
integrated optics such as described in FR-A-2 613 826 filed by the present 
Applicant is shown in FIG. 1. Part a of FIG. 1 is a sectional view of the 
splitting plate and part b a plan view. 
The light guide in which the splitting plate is formed is constituted (part 
a) by a silicon substrate 2, above which is placed an undoped silica 
buffer layer 3, followed by a Si.sub.3 N.sub.4 guide layer 4 with a 
refractive index higher than that of the layer 3. An upper, undoped silica 
layer 5 with a refractive index lower than that of the guide layer 4 
completes the structure of the guide. The splitting plate 6 consists of a 
trench made in the upper layer 5, whose depth, in the present 
representation, is equal to the thickness of the upper layer 5. 
Two different guide structures can be defined in this embodiment, namely a 
Si/SiO.sub.2 /Si.sub.3 N.sub.4 /SiO.sub.2 structure (1) of effective index 
N1eff and a Si/SiO.sub.2 /Si.sub.3 N.sub.4 /air structure with an 
effective index N2eff. The effective index of a structure is dependent on 
the refractive index of the layers forming it and of their thickness. 
In part b of FIG. 1, I represents the incident light beam, R the reflected 
light beam, T the light beam transmitted by the plate 6 and N the 
perpendicular to the plate 6. These beams are carried by the guide layer 
4. 
It is only possible to have a high reflection coefficient R at the 
interface of the structures (1) and (2) if the angle of incidence A is 
close to or higher than the limit angle Al. The total reflection limit 
angle Al, such as can be calculated in a plane wave model is defined by 
the equation (1): 
EQU Al=Arc sin N2eff/N1eff (1) 
The greater the variation of the effective index N1eff-N2eff, the more the 
angle Al moves away from n/2. 
In the embodiment of FIG. 1 and for a wavelength of the incident beam of 
800 nm and a thickness of the guide layer 4 of 160 nm, N1eff is obtained 
close to 1.69 and N2eff close to 1.65 and consequently Al is approximately 
77.degree.. 
On replacing air by MgF.sub.2, which has a refractive index of 1.38 and 
which is lower than that of undoped silica which is approximately 1.45, an 
effective index N2eff of 1.68 is obtained and consequently an angle Al of 
approximately 84.degree.. This replacement of air by MgF.sub.2 is known 
from the aforementioned document. 
The use of air as the upper layer of the light guide and with a refractive 
index of 1 makes it possible to obtain high effective index variations and 
therefore makes it possible to work at low incidence angles. The interest 
of working with the minimum limit angles Al permits a better separation of 
the transmitted and reflected beams increasing the angles separating them. 
Unfortunately, such a splitting plate using as the upper layer air is not 
protected from all external pollution. 
These problems also exist for other guide structures, i.e. for materials 
other than those given hereinbefore, as well as for trenches 6 extended in 
to the guide layer 4 or even into the buffer layer 3. 
Thus, these disadvantages also exist for diffraction gratings which are 
only constituted by a succession of equidistant splitting plates, which 
are components widely used in integrated optics, as well as for mirrors 
and focussing lenses. 
At present few solid materials are known which have a refractive index 
below that of silica and which would make it possible to replace air in 
the guiding structure, which considerably limits the production of 
integrated optical components in said material. 
At present, only MgF.sub.2 is known for replacing air in the guide 
structures 1 and 2. Unfortunately the production of this material is not 
easy because it cracks as soon as its thickness exceeds 200 to 300 nm and 
the effective index differences induced by it are generally inadequate. 
It is also not possible to produce monomode structures compatible both with 
laser diodes and with optical fibres. Thus, laser diodes use guiding 
structures having very high refractive index variations between the core 
and the surrounding medium, whereas optical fibres have low refractive 
index variations of approximately 4.10.sup.-3 to 10.sup.-2. Consequently, 
the confinement of the light is very different, namely 1 micrometer or 
less (in the junction direction) or the laser diode and 5 to 9 micrometers 
for the optical fibres. 
In addition, to connect these elements to the same waveguide, it would be 
necessary for the latter to be both a low index variation guide and a high 
index variation guide as a function of the type of element to be 
connected. However, this is not possible with a single guide structure and 
can only be obtained with two coupled guide structures. 
Unfortunately it is very difficult to exchange light energy between two 
light guides having very different guided mode profiles and the efficiency 
levels obtained are very low. Thus, this energy transfer can be realised 
in known manner by an end-to-end coupling in which the integral of the 
superimpositions between the light amplitudes of the two guided modes is 
very low (consequently low efficiency) or by a co-directional coupling, 
but then the propagation constants (in particular the phase velocity) of 
the guided modes in the two light guides differ very significantly, which 
does not permit the coupling. In a co-directional coupling, the two guide 
structures are stacked. 
It is known to improve the co-directional coupling of two guide structures 
by using a periodic structure of the grating type, which ensures the 
adaptation of the propagation velocity between the modes to be coupled. 
Such a coupling is in particular described in FR-A-2 579 044 filed in the 
name of the Applicant. The disadvantage of this method is that it leads to 
a waveguide sensitivity which can be very prejudicial during its use in a 
real optical system. 
The invention relates to an integrated optical component and its production 
process making it possible to obviate the aforementioned disadvantages. In 
particular, said optical component is protected against the environment, 
whilst ensuring high effective index variations. 
Moreover, when said component is a grating, it can be used for an effective 
co-directional coupling of two stacked optical guides having widely 
different guided mode profiles. 
Moreover, the component according to the invention can be produced from a 
much larger number of materials than those of the prior art and its 
production is much less critical. 
More specifically, the invention relates to an environmentally protected 
integrated optical component having at least one closed cavity isolated 
from the environment and formed in at least one layer of a light guide, 
said cavity being filled with a material having a refractive index which 
is different or which can be made different from the refractive index of 
the said layer. 
The isolation of the components from the external medium makes it possible 
to produce other technological stages independent of the existence of 
these components. 
The filling material enclosed in the cavity permits a local modification of 
the effective index of the light guide greater than that obtained in the 
prior art components. Moreover, this local modification of the effective 
index is solely fixed by the cavity filling material and is consequently 
not subject to the polluting disturbances due to the environment. The 
filling material of the cavity is in particular an organic polymer or 
fluid. The fluid used can be a gas or a liquid and is refractive index can 
be higher or lower than that of the layer containing it. 
The use of a material with a refractive index lower than that of the layer 
containing it has the advantage of not modifying the number of guided 
modes of the initial structure of the light guide, which means that a 
monomode structure remains monomode compared with the cavity. 
Moreover, due to the fact that the index difference created by the cavity 
is negative, light only exists in an evanescent form in components of the 
grating or coupler type. This ensures that a good reproducibility of the 
performances sought as a result of a reduction of the sensitivity to the 
thickness of the cavity. 
However, if it is wished that the initially monomode structure becomes 
multimode compared with the cavity, although the passage from a monomode 
to a multimode structure can cause parasitic intermode coupling problems, 
it is possible to use a material with a refractive index higher than that 
of the layer containing the filling material. 
The gas which can be used is a neutral or inert gas such as e.g. argon, 
neon, helium, nitrogen, or air or vacuum can be used. All these gases have 
a refractive index lower than that of silica, equal to or close to 1. 
Gases also have the advantage of not causing long term chemical reactions 
with the materials of the integrated structure and therefore do not modify 
the sought physical properties for the structure. 
Advantageously air is used as the filling material. Its refractive index of 
1 makes it possible to associate it with a large number of materails and 
ensure a high effective index difference. 
It is also possible to use as the filling material gases or vapours having 
particularly absorption lines, which can e.g. be optically saturated (the 
excitation of the energy levels corresponding to said absorption lines), 
such as helium, neon, rubidium or sodium vapours. The integrated optical 
component according to the invention makes it possible to use liquid, 
which was not possible in the prior art components. Thus, it increases the 
number of possibilities of components to be produced. 
Thus, if it is wished to have a given index value, it is much easier to 
find it among liquids than among solids. Moreover, it is possible to use 
colouring agents, which are liquids with a very large absorption band. It 
is therefore possible to produce waveguide-selective absorbers or optical 
filters. 
The use of a liquid in particular makes it possible to produce new 
components of the optical valve, modulator or deflector type. This is 
particularly the case when using electrically controllable liquids such as 
electrolytes or liquid crystals and in particular nematic or C smectic 
phase liquid crystals. 
It is also possible to produce components with a non-linear response by in 
particular using CS.sub.2. This liquid material has a refractive index 
(1.65) higher than that of silica. It is also possible to use organic 
polymers (e.g. polyimide, PMMA) with a refractive index between 1.45 and 
1.7, which is also higher than that of silica. CS.sub.2 has the advantage 
of an optically modifiable refractive index. Its index can be modified by 
the action of a light beam, which can be a guided beam or an external 
supplementary beam supplied to the structure through the upper layers 
(which must be transparent to the excitation wavelength) or through the 
lower layers (which must then be transparent to the excitation 
wavelength). 
In this case, as well as in the case of an electrically controlled filling 
material, when the material is not controlled it can have the same 
refractive index as the layer or layers containing it. 
The values of the refractive indices referred to during the description 
correspond to a wavelength of 800 nm. 
The optical component according to the invention can be a mirror. In this 
case, the cavity advantageously extends from the upper layer to the lower 
layer of the light guide. The optical component according to the invention 
can also be a beam splitting plate or a focussing lens. The dimensions and 
shape of the cavity define the optical properties of the component. 
For both a lens and a splitting plate, the cavity is located in at least 
one of the layers of the guide structure. The optical component can also 
be a microguide intended for the lateral confinement of light. 
When the optical component is a diffraction grating, the latter has several 
cavities preferably arranged parallel to one another and each filled with 
a material with a different index. These cavities constitute the lines of 
the grating and are formed in any random one of the layers of the guide 
structure. 
The latter type of component can advantageously be used for coupling two 
superimposed light guides with a satisfactory energy transfer efficiency. 
The coupling force of a grating is proportional to the difference of the 
refractive index between the fluid material more particularly contained in 
the cavities and the material in which said cavities are formed. Moreover, 
the greater the said index difference, the smaller the number of lines. 
A diffraction grating with a high coupling coefficient according to the 
invention makes it possible to couple two superimposed light guides with a 
very different guided mode profile, a first guide consisting of a first 
upper layer and a first lower layer, positioned on either side of a first 
guide layer with a refractive index higher than that of the first upper 
and lower layers and a second guide consisting of a second upper layer and 
a second lower layer positioned on either side of a second guide layer 
with a refractive index higher than that of the second upper and lower 
layers, the first upper layer and the second lower layer constituting one 
and the same coupling layer and the grating is located in the coupling 
layer. 
A diffraction grating according to the invention also makes it possible to 
couple two guides with a very different guided mode profile having a 
first, a second, a third and a fourth stacked layers, the second and third 
layers constituting the guide layers respectively of the first and second 
guides and having refractive indices higher than those of the first and 
fourth layers, the refractive index of the second layer also being 
different from that of the third layer, said third layer having cavities 
filled with a material with a different refractive index or whose 
refractive index can be made different from that of the third layer. 
For a guide structure with a low refractive step index, the typical 
refractive index variation between the guide layer and the adjacent layers 
is 5.10.sup.-3 to 2.10.sup.-2 and for a structure with a high step index 
said index variation is typically between 10.sup.-1 and 5.10.sup.-1. 
A guide with a high refractive index variation is e.g. constituted by upper 
and lower undoped silica layers with an index of 1.45 or which are doped 
with phosphorus and/or boron with an index of 1.46 and a silicon nitride 
guide layer with an index of 2.01 or a silicon oxynitride layer (SiO.sub.x 
N.sub.y with 0&lt;x&lt;2 and 0&lt;y&lt;4/3) with an index of 1.45 to 2, of alumina 
with an index of 1.65 or organic materials such as PMMA (polymethyl 
methacrylate) and polyimides with an index between 1.45 and 1.7. 
A light guide with a low refractive index variation is in particular 
constituted by upper and lower layers made from undoped silica or silica 
doped with fluorine and/or boron and a guide layer of silica doped with 
germanium, titanium, nitrogen or phosphorus. 
The doping of the silica by boron or fluorine reduces its refractive index, 
whereas doping by germanium, phosphorus, nitrogen or titanium increases 
the refractive index of the silica. 
The production of a diffraction grating with a very high coupling 
coefficient according to the invention and therefore a small number of 
lines or grooves (5 to 25) permits a coupling between the high index 
variation optical guide and the low index variation optical guide with a 
considerably reduced wavelength sensitivity compared with those of the 
prior art and in all cases compatible with the light sources, whereof the 
emission spectrum has widths of 10 to 20nm. 
Advantageously, the gratings can work in the reflective mode, which 
represents the advantage of making them relatively insensitive to 
technological errors (grating spacings, width of the cavities and 
effective indices of the guided modes). Moreover, the splitting plates and 
mirrors according to the invention advantageously operate in the vicinity 
of the limit total reflection angle Al. 
The invention also relates to a process for producing an optical component 
as defined hereinbefore. 
According to a first embodiment the process comprises the following stages: 
deposition of at least one first layer on a support, 
production of at least one trench in said first layer, whose height/width 
ratio is .gtoreq.0.5 and 
chemical vapour phase deposition (CVD) of a second layer on the first layer 
etched in this way, so as to form an air-filled sealed cavity in the 
trench. 
The second layer can be deposited by low pressure CVD (LPCVD) or by 
plasma-assisted CVD (PECVD). 
The inventors have found that this procedure of the isotropic deposition of 
a material layer necessarily lead to the formation of a bubble of gas, 
generally air, in the trench when the height/width ratio was at least 0.5. 
The exact composition of the air is obviously fixed by that of the gaseous 
atmosphere in the deposition enclosure. The volume and shape of the air 
bubble are dependent of the height/width ratio and the properties of the 
first layer. 
Moreover, it is possible to modify the shape and volume of said air bubble 
by carrying out surface treatments of the deposited layers. In particular, 
it is possible to carry out a heat treatment of the first and second 
layers in such a way as to make them flow. This flow is in particular 
possible with silica layers advatageously doped with boron, phosphorus or 
both of them, so as to lower the treatment temperature compared with the 
flow temperature of the undoped silica. 
As it is relatively easy to carry out etching processes, whose height/width 
ratio is approximately 1 to 5, it is found that the creation of said air 
bubble is not absolutely critical. 
According to a second embodiment, the process for the production of the 
optical component according to the invention comprises the following 
stages: 
(a) deposition of at least one first layer on a support, 
(b) production of at least one trench in said first layer, 
(c) deposition of an intercalated layer in the trench and above the latter, 
which can be selectively eliminated with respect to the support, the first 
layer and a second layer, 
(d) deposition on the structure obtained in (c) of the second layer and 
(e) elimination of the intercalated material thus forming a sealed cavity 
filled with air in the trench. 
This second embodiment has a much more general application than the first 
because, unlike the first embodiment, it makes it possible to produce an 
air bubble, whose height/width ratio is below 0.5 and is e.g. between 0.1 
and 0.3. 
Moreover, said second method is much more reproducible with regards to the 
shape of the bubbles. 
According to a third embodiment, the process for the production of the 
optical component according to the invention comprises the following 
stages: 
(A) deposition of a first layer on a support, 
(B) deposition of an intercalated material layer on the first layer and 
which can be selectively etched with respect to said first layer, a second 
layer and the support, 
(C) etching said intercalated layer so as to form at least one intercalated 
material stud, 
(D) deposition on the structure obtained in (C) of the second layer and 
(E) elimination of the intercalated material stud, thus forming a sealed 
cavity filled with air at the location of the stud. 
The three embodiments are applicable to different types of optical 
components as referred to hereinbefore. 
As a function of the envisaged application, it is possible to replace said 
air bubble by a liquid or another gas, provided that the latter has a 
refractive index which is different or which can be made different from 
that of the first and second layers, by injection after the deposition of 
the second layer. 
According to the invention, the injected liquid can be a liquid monomer 
polymerizable by heating or irradiation such as methyl methacrylate, which 
is then polymerized.

FIGS. 2 and 3A diagrammatically show the production of a beam splitting 
plate according to the invention. It is possible to see a monocrystalline 
silicon substrate 14 on which has been formed an 8 to 12 micrometer thick, 
not intentionally doped silicon oxide layer 16, e.g. by high pressure 
thermal oxidation of the substrate under an oxygen atmosphere between 
800.degree. and 1200.degree. C. The refractive index of this oxide layer 
16 is approximately 1.45. 
By plasma-assisted or plasma-unassisted chemical vapour phase deposition is 
deposited the silicon oxide guide layer 18, which is doped with 
phosphorus, germanium, nitrogen or titanium and has a thickness of 3.2 to 
10 micrometers. Phosphorus doping is ensured by 10.sup.21 to 10.sup.22 
atoms/cm.sup.3. The refractive index of this layer 18 is approximately 
1.46. 
By conventional photolithography processes is formed a photosensitive resin 
mask 19 (part a, FIG. 2) having an opening 21 fixing the location of the 
splitting plate and its dimensions. With the aid of said mask an 
anisotropic etching takes place of the layer 18, thus forming a trench 22 
in the area of layer 18 facing the mask opening 21. This etching is e.g. a 
reactive ionic etching (RIE) using CHF.sub.3 or CF.sub.4 as the etching 
agent. 
It can be carried out over all or part of the thickness of the layer 18 or, 
as shown in FIG. 3A, it is possible to said etching to also reach the 
lower layer 16 of the guide, but without extending to the substrate 14. 
The thus formed trench carries the reference 22a. 
The height of the trench is designated h and its width is designated w. 
According to this method, the h/w ratio is at least 0.5. 
Preferably, a splitting plate has a reflection coefficient between 30 and 
70%. Fundamentally it is sufficient if h is of the same order of magnitude 
as half the spatial extension (or spatial width of the guided mode). 
In FIG. 2, h can be 3.4 micrometers and w 2.5 micrometers, whilst in FIG. 
3A h can be 5 micrometers or the same value of w. 
After eliminating the etching mask 19 by an oxygen plasma, deposition tkaes 
place of a 1 to 6 micrometers thick, not intentionally doped silicon oxide 
layer 24, or which optionally contains a doping agent reducing the 
refractive index of the oxide, such as boron or fluorine, with 
concentrations of 10.sup.21 to 10.sup.22 atoms/cm.sup.3. 
This layer is deposited by plasma-assisted chemical vapour phase deposition 
(PECVD) at 300.degree. to 500.degree. C. by thermal decomposition of 
silane and oxygen or by low pressure chemical vapour phase deposition (130 
Pa) with the same gases (LPCVD). This layer 24 constitutes the upper layer 
of the light guide and provides a protection against the environment of 
the splitting plate. 
As shown in part b of FIG. 2 and in FIG. 3A, the deposition by PECVD or 
LPCVD of a silica layer necessarily leads, for h/w.gtoreq.0.5, to the 
formation of a cavity 26 or 26a containing air 27 located in the trench 
respectively 22,22a. The presence of this air bubble leads to a 
significant local modification of the effective index of the guided mode. 
For h/w&lt;0.5, it is very difficult to determine the presence or absence of 
an air bubble by this method. 
It is possible to improve the reproducibility of the shape of the bubble by 
bringing about a flow of the layers containing the cavity consisting of an 
oven treatment of the structure at between 900.degree. and 1100.degree. C. 
For an 800 nm guided light beam, the reflection coefficient R (FIG. 1b) of 
the thus formed splitting plate passes from 0 to 95% when h varies from 0 
to 5 micrometers, w being fixed at 2.5 micrometers. When h=3.4 
micrometers, the transmission and reflection coefficients of the plate are 
close to 50% and the limit reflection angle Al is close to 45.degree.. 
As shown in FIG. 3B, it is also possible to form the trench 22c over all or 
part of the upper guide layer 24. In this case, the etching of the layer 
24 is followed by the deposition of a 1 to 10 micrometers thick, undoped 
SiO.sub.2 layer 28 by PECVD or LPCVD under the same conditions as 
described in FIG. 2 for layer 24. Thus, an air-filled cavity 26e is 
formed, whose shape is fixed by the height/width ratio of the trench 22c. 
In the variant shown in FIG. 3B, the guide layer 17 is made from 
LPCVD-deposited silicon nitride and has a thickness of 100 to 400 nm. 
FIG. 4 diagrammatically shows an integrated mirror according to the 
invention. As previously, this mirror is formed in a waveguide of 
Si/SiO.sub.2 /+doped SiO.sub.2 /SiO.sub.2. The deposition techniques and 
the thicknesses of the layers are identical to those described relative to 
FIG. 2. 
Following the successive deposition of the layers 16, 18 and 24 (part a), 
using an appropriate mask (cf. FIG. 2a) anisotropic etching takes place of 
the stack of layers 16, 18 and 24. The depth of the trench 22b formed is 
designated h' and its width w'. This etching takes place over the entire 
thickness of the layers 24 and 18 and over all or part of the layer 16. 
The etching agents are e.g. CHF.sub.3 or CF.sub.4. 
This etching is followed by the deposition of a 1 to 10 micrometers thick, 
undoped silicon oxide layer 28 by PECVD or LPCVD under the same conditions 
as described in FIG. 2 for the layer 24. This leads to the formation of an 
air bubble 26b, whose shape is fixed by the ratio h'/w' of the trench 22b. 
The width w' of the trench 22b must exceed a minimum value wo, so as to 
avoid any recoupling of the guided mode in the guide structure. In part b 
of FIG. 4, I and R respectively represent the incident and reflected beams 
and N represents the perpendicular to the mirror defined by the trench 
22b. The angle A between the perpendicular and the instant beam must 
exceed the total reflection angle Al defined in the equation (2): 
EQU Al=Arc sin N2/N1eff (2) 
in which N1eff is the effective index of the guided mode and N2 represents 
the index of the air bubble, namely 1. 
In the represented example, N1eff is approximately 1.47 and Al 
approximately 43.degree.. Thus, A must exceed 43.degree.. 
The above formula (2) is applicable to the mirror and corresponds to the 
case where the guided mode only exists in the cavity containing the fluid. 
However, it can be used in place of formula (1) for beam splitting plates, 
as a function of the value of the sought reflection coefficient. 
When N1effsinA is &lt;N2, there is merely a splitting plate and not a mirror. 
If the SiO.sub.2 layer 18 doped with Ti, Ge, N.sub.2 or P is by a 165 nm 
thick Si.sub.3 N.sub.4 layer deposited by LPCVD by decomposition of silane 
and ammonia, an index N1eff of the guided mode close to 1.73 is obtained. 
Under these conditions, A must exceed 35.3.degree. in order to have a 
mirror. 
The evanescent wave associated with the total reflection on the mirror 22b 
has a penetration depth given by the function: 
##EQU1## 
This function has a value of 1/e when 
##EQU2## 
For N1eff=1.47 and A=45.degree., we obtain: exp(-2,2x)=1/e and therefore 
x=450 nm for .lambda.=800 nm. 
If the width w' greatly exceeds 450 nm and is e.g. e or 4 micrometers 
practically no light energy can clear the air bubble 26b, so that the thus 
formed mirror has high performance characteristics. If A increases, it is 
possible to reduce the width w'. 
To ensure that the evanescent wave associated with the total reflection 
does not reach the rear face 23 of the mirror, the expression: 
##EQU3## 
must greatly exceed w, with wm being the minimum value of w'. For 
N1eff=1.47, A=45.degree., N2=1 (air) and .mu.=800 nm, wm is approximately 
2 micrometers, which corresponds to an attenuation of the evanescent wave 
by a factor of 100. The more that A increases (moving towards .pi./2), the 
more readily the above condition can be satisfied with a low value wm. 
The depth h' is not critical and it is merely necessary for it to be higher 
than a minimum value ho roughly equal to the spatial width of the guided 
mode. The spatial width of the guided mode represents the mid-height width 
of the profile of the light intensity of the wave propagating in the 
guide. 
In the same way as for the semi-reflecting plate and the mirror, it is 
possible to produce a diffraction grating in the manner shown e.g. in the 
FIG. 5. The waveguide is identical to that described with reference to 
FIGS. 2 and 3A. 
Following the deposition of the guide layer 18 on the lower layer 16 of the 
guide (part a), several parallel, equidistant trenches 30 are produced, w 
representing the width of the trenches 30 and p the spacing of the 
grating. The spacing of the grating p determines the the operating angle 
A, shown in part b of FIG. 5, which is the angle formed by the incident 
beam with the lines 30 of the grating. 
As previously, the deposition of the SiO.sub.2 layer 24 by LPCVD or PECVD 
on the guide layer 18 leads to the formation of bubbles and therefore to 
cavities 26c filled with air 27, located in each of the trenches 30. 
The depth of the trenches h and the index of the material filling these 
trenches, in this case air, determine the coupling coefficient K between 
the grating and the instant beam and therefore the coupling width 
Lc=.pi./2K of the mode guided with the grating. The creation of air 
bubbles 26c of index 1 increases the value of K and consequently decreases 
the coupling length Lc, which makes it possible to make the grating less 
sensitive to the wavelengths. 
In view of the fact that it is difficult to control the value of the 
coupling coefficient K, which is highly dependent on h and the shape of 
the bubbles 26c, the grating is made to operate systematically in 
reflection. This also makes it possible to make the grating less sensitive 
to technological errors, particularly with respect to the value of the 
spacing, the width of the trenches or the effective indices of the guided 
modes. Thus, to the extent that the interraction length of the grating 
with the instant wave L greatly exceeds 1/K, i.e. Lc, the value of the 
reflection in the case of a grating operating in reflection is independent 
of Lc and therefore K and in practice L=2Lc. 
According to the invention, it is possible to replace air by another gas, a 
liquid or a solid liquified by an appropriate heat treatment and thus to 
form a grating of components interconnected by channels. Thus, it is 
possible to render active the components described hereinbefore with 
reference to FIGS. 2 to 5. In particular, it is possible to replace air by 
C smectic or nematic liquid crystals, whose refractive index can be 
electrically controlled by the injection into the cavities of these liquid 
crystals. 
FIG. 6 shows two embodiments of active components, which are beam splitting 
plates, whereof it is possible to electrically control the reflection 
coefficient. For this purpose, the air initially present in the cavity 31 
formed in the trench 22a is replaced by an C smectic liquid crystal 32 and 
electrodes 34, 36 made from a conductive material such as aluminium are 
deposited on the light guide. 
As shown in part a, said electrodes 34 and 36 are placed on the layer 24, 
the electrode 34 being positioned facing the liquid crystal 32 or, as 
shown in FIG. 6, the electrode 34 is placed on the layer 24 facing the 
liquid crystal 32 and the electrode 36 on the rear face of the guide, i.e. 
on the substrate 14. The electrode 34 facing the liquid crystal is raised 
to a positive potential and the electrode 36 to ground. 
The dotted lines 33 in FIG. 6(a) symbolise the lines of the electric field 
applied to the optical component. 
FIG. 7 shows another embodiment of the components according to the 
invention and in particular the production of a splitting or 
semi-reflecting plate. After the formation of the trench 22 by partial 
etching of the layer 18, on the complete structure (part a) is deposited a 
layer 38 of a material which can be selectively etched with respect to the 
layer 18 and the upper layer 24 of the optical guide. In the case of 
silica layers 18 and 24, it is possible to use a metal coating and in 
particular a coating of aluminium or chromium. 
This coating 38 can be deposited by all known deposition methods and in 
particular by cathodic sputtering or electrolytic deposition for a metal 
layer. This layer has a thickness greater than that of the trench 22. For 
a height h of the latter of 3.4 micrometers, the layer 38 e.g. has a 
thickness of 10 micrometers. 
This layer is then selectively etched in such a way as to retain metal at 
least facing the trench 22, i.e. in and above the trench (it also being 
possible to retain metal on either side of the trench). Said etching 
operation is carried out with the aid of a not shown photolithogravure 
mask. The metal stud obtained is designated 38a (part b). 
It is also possible to directly produce the metal stud 38a filling the 
trench 22 by lift-off, when the trench has a width of .gtoreq.1.5 
micrometers. 
This is followed by the deposition of the silica layer 24. This deposition 
can be carried out by all known deposition methods and in particular by 
CVD using silane and oxygen as the deposition gases. 
As shown in part c of FIG. 7, this is followed by a selective etching of 
the metal stud 38a leading to its complete elimination. This elimination 
can be carried out by chemical etching in solution from a hole, e.g. 
produced in the layer 24 and connected to the metal. In the case of 
aluminium, use is made of a phosphoric acid solution and for chromium a 
cerium salt solution. 
This leads to a splitting plate equipped with an air-containing cavity 26d, 
whose shape is identical to that of the stud 38a. 
This method makes it possible to use all deposition methods for 
constituting the layer 24. In addition, it is applicable to all values of 
h/w greater than 0.1, whereas the method described relative to FIG. 2 is 
only usable for h/w ratios greater than 0.5. 
FIG. 8 diagrammatically shows two superimposed guides coupled by a high 
coupling force diffraction grating 40 according to the invention. The 
guide structure with a high refractive index variation A is e.g. 
constituted by a lower layer 44 having a refractive index n1 deposited on 
a substrate 46, a guide layer 48 of refractive index n2 and an upper layer 
50 of refractive index n3, with the layer 48 intercalated between the 
layers 44 and 50, with n2 exceeding n1 and n3. 
The low index variation guide structure B is e.g. constituted by the layer 
50, a guide layer 52 of refractive index n4 and an upper layer 54 of 
refractive index n5 with the layer 52 intercalated between the layers 50 
and 54 and n4 being greater than n3 and n5. In addition, n4 must exceed n1 
and n2 must be greater than n4 (the order of superimposing the structures 
A and B obviously being reversible). 
For a silicon substrate 46, the layers 44,50 and 54 can be made from 
silica, which is either not intentionally doped, or which is doped with 
fluorine or boron. The layer 48 can be of silicon nitride, silicon 
oxynitride or alumina and the layer 52 can be of silica doped with 
phosphorus, germanium, nitrogen or titanium. The layers 48,50,52 and 54 
can be deposited by LPCVD or PECVD and the layer 44 can be formed by 
thermal oxidation of the substrate, so as to have good optical qualities. 
The structure shown in FIG. 8 has two very different propagation modes. For 
said structure to be able to function, it is also necessary for the 
thickness of the layer 48 to be less than a maximum value in order to be 
monomode, which is approximately 0.05 to 0.4 micrometer as a function of 
the wavelengths used for the silicon nitride layer and for the thickness 
of the layer 52 also to be below a maximum value to be monomode, 
approximately 1 to 8 micrometers as a function of the wavelengths used and 
the index variation must be between the different layers. 
In addition, the thickness e1 of the layer 44 must ensure the isolation of 
the two propagation modes of the guides A and B of the substrate 46. In 
addition, this thickness must exceed the penetration depth of the 
evanescent wave of the guided mode of the least confined nature 
propagating in the layer 52. Typically el exceeds 12 to 15 micrometers. 
The thickness of the layer 50 conditions the coupling force of the grating 
and can be 0 (FIG. 11). The coupling between the two guided modes 
decreases with the thickness of the layer 50. Beyond 3 micrometers for the 
silica layers and for the silicon nitride layer 58 with a thickness of 
approximately 50 nm, coupling no longer exists. The thickness of the layer 
54 is not critical. 
In particular, the layers 44,48,50,52 and 54 respectively have a thickness 
of 8 to 15 micrometers, 50 to 200 nm, 0.3 to 2 micrometers, 1 to 8 
micrometers and 2 to 10 micrometers. 
As previously, the spacing of the grating and the incidence angle on the 
grating are chosen so as to enable the grating 40 to operate in 
reflection, so as to prevent the light energy coupled from the high index 
variation structure to the low index variation structure from being able 
to return to the high index variation structure. 
The coupling between the structures A and B, via the grating, is possible 
when kA+kR=kB with .vertline.kA.vertline.=(2.pi./.lambda.)nA, 
.vertline.kB.vertline.=(2.pi./.lambda.)nB and 
.vertline.kR.vertline.=2.pi./p in which nA and nB are the effective 
indices of the guide structures A and B, .lambda. the wavelength of the 
incident light, p the grating spacing and kA, kB and kR wave vectors of 
the structures A,B and the grating. 
In certain special cases, it is possible to make the grating 40 function at 
an order different from +1 or -1, in order to increase its spacing p and 
thus facilitate its construction. This is only possible with a particular 
angle of incidence for which the orders lower than that used cannot exist. 
As shown in FIG. 8, the coupling grating 40 can be obtained by etching the 
layer 50 just prior to depositing the guide layer 52, so as to form 
parallel trenches 42, whose height/width ratio is at least equal to 0.5, 
followed by the deposition of the layer 52 by LPCVD or PECVD, as described 
relative to FIG. 2. 
The cavities 41 filled with air bubbles 27 formed during the deposition of 
the layer 52 and located in the trenches 42 ensure the coupling of the two 
guide structures. In practise there are 10 to 30 cavities 41 and the 
spacing of the grating is 0.3 to 3 micrometers. 
FIG. 9 shows another embodiment of the diffraction grating ensuring the 
coupling of the two guide structures. In this embodiment, on the etched 
layer 50 (part a) is effected the deposition of a metal layer which can be 
selectively etched with respect to the substrate 46 and the layers 
44,48,50,52 and possibly 54, when the metal is eliminated following the 
deposition of the layer 54. This layer is then etched so as to only 
maintain metal 38b facing the trenches 43 made in the layer 50. The 
etching of this metal layer is followed by the deposition of the layers 52 
and 54, as described hereinbefore relative to FIGS. 2 and 7. Obviously, 
layer 54 can also be deposited following the total elimination of the 
metal. 
Finally, the metal 38b is eliminated with the aid of orthophosphoric acid 
for an aluminium layer and guide structures of Si/SiO.sub.2 /Si.sub.3 
N.sub.4 /SiO.sub.2 /+ doped SiO.sub.2 /SiO.sub.2 via at least one hole 
e.g. traversing the layers above the metal 38b. The space initially 
occupied by the metal 38b and in particular the trenches 43 is then filled 
with air 27 (part b of FIG. 9). 
The coupling grating can also be produced in the manner shown in FIG. 10, 
in which the deposition of the metal layer 38 takes place directly over 
the entire guide layer 48 (part a of FIG. 10). This is followed by a 
substantially anisotropic etching of the layer 38, so as to form parallel, 
equidistant metal studs 38c. For an aluminium layer, said anisotropic 
etching is carried out by the dry method using a chlorinated gas or by the 
wet method with orthophosphoric acid as the etching agent. 
Then, in successive manner, is carried out the deposition of the layers 50, 
52 and 54 (part b), as described hereinbefore. This is followed by the 
elimination of the etched metal studs 38c using the wet method and giving 
rise to air-filled cavities 45. 
The layers 52 and 54 can in this case be provided before or after the 
elimination of the metal studs. In order to be eliminated, said metal 
studs are interconnected in the layer 50, outside the useful zone of the 
component, at least one hole made in the layer 50 giving access to the 
metal 38. 
This method can cause certain difficulties for very small grating spacings 
below 1 micrometer with respect to the elimination of the metal studs 38c, 
which is not the case with the process described relative to FIG. 9. 
In the embodiment shown in FIG. 10, the layer 50 can have a zero thickness, 
as shown in FIG. 11. In this case, there is a direct deposition of the 
layer 52 on the metal studs 38c. Before or after the deposition of the 
layer 54, elimination takes place of the studs in the manner described 
hereinbefore. 
In the embodiment of FIG. 9, it is also possible to eliminate the layer 50. 
In this case, the metal layer is deposited on layer 48 in whcih have been 
produced the parallel trenches 42. 
FIGS. 5 and 8 to 11 show gratings with constant spacings and which 
consequently function with collimated incident beams. For uncollimated 
incident beams and therefore associated with source points, it is possible 
to use gratings identical to those shown associated with collimation 
optics of the integrated lens or mirror type according to the invention, 
or to use a grating, whose lines are eliptical or parabolic, or to use a 
variable spacing grating ensuring both the coupling and imaging functions. 
This makes it possible, in certain cases, to reduce the number of 
components of a complete optical circuit. 
As a result of the coupling grating according to the invention, it is 
possible to produce polarization separators or polarization converters. 
Thus, the low index variation guide structures are insensitive to the 
polarization of the incident beam, unlike in the case of high index 
variation guide structures. It is also very difficult to spatially 
separate or more simply select one of the transverse electric TE or 
transverse magnetic TM polarization of the guide mode in a low index 
variation structure. 
Conversely, this is possible by superimposing two guides, in the manner 
described hereinbefore and coupled by gratings according to the invention. 
It is also possible to produce a polarization converter for optical reading 
circuits associated with a magnetic reading/writing head, as described in 
FR-A-2 606 921. 
Obviously, the air contained in the cavities of the network can be replaced 
by another gas or liquid, whereof it is possible to electrically and/or 
optically modify the refractive index. 
Moreover, the production processes for a grating described hereinbefore can 
also be applied to the production of mirrors, splitting plates and lenses 
according to the invention. 
Finally, the components according to the invention can be made from 
materials other than those referred to hereinbefore and can in particular 
be of lithium tantalate or niobate. 
FIG. 12 shows a microguide according to the invention produced in a 
Si/SiO.sub.2 /Si.sub.3 N.sub.4 /SiO.sub.2 guide structure. This microguide 
has in its upper SiO.sub.2 layer 50, two air-filled cavities 26f, 26g. The 
index difference between the air and the silica of the layer 50 ensures a 
good lateral confinement of the light. These cavities are obtained by 
producing two parallel trenches 22d,22e in the layer 50 and then 
depositing an undoped SiO.sub.2 layer 28 by LPCVD or PECVD. 
It is also possible to ensure the lateral confinement of the light by 
producing two air-filled cavities in the lower layer 44 of the guide by 
the process described relative to FIGS. 9 or 10. The trenches can be 
formed in all or part of the layer 50 or 44, but not in the guide layer 
48. 
FIG. 13 shows another embodiment of an active component according to the 
invention of the beam splitting plate type, whereof it is possible to 
electrically control the reflection coefficient. This component is 
produced in a Si/SiO.sub.2 /+ doped SiO.sub.2 /SiO.sub.2 guide structure. 
This component has in its guide layer 18 a trench 22f filled with an 
organic polymer 56, whereof it is possible to modify the refractive index 
by applying to it a voltage via aluminium electrodes 58 and 60 parallel to 
the layers of the guiding structure and in particular the polymer 56. 
The upper electrode 58 is placed on the upper layer 24 of the guide facing 
the polymer 56. The lower electrode 60 is positioned in the layer 16, 
constituted in the present case for practical reasons by two superimposed 
layers 16a and 16b of the same composition.