Methods of making planar lightwave guides

A method of making a planar lightwave-guide with light-conducting core zones by precipitation out of a gaseous phase a thin, glass-like strata onto a substrate in accordance with a controlled schedule so as to allow a predetermined refractive index curve to be obtained to thereby form a light-conducting core zone and sheathing layer adjacent to the core zone. The improvement is characterized in that the precipitation out of the gaseous phase is obtained by application of a heterogeneous reaction and in that initially a first layer with a doping medium which decreases the refractive index is applied to the substrate and thereafter a masking layer is applied on top of the first layer to act as a diffusion barrier with respect to the doping medium. The masking layer is structured by conventional masking technology in accordance with a desired strip or band pattern. Thereafter, the substrate provided with these layers is heated to a degree sufficient to diffuse the doping medium out of the unmasked regions of the first layer and thus forming at least a portion of the core zone of the light-conducting regions. Further layers are then applied in which the refractive indices are controlled to complete the lightwave-guide.

BACKGROUND OF THE INVENTION 
This invention relates to methods of making planar lightwave guides with 
light conducting regions by the precipitation out of a gaseous phase of 
thin, glass-like strata on a substrate in accordance with a controlled 
schedule, thereby obtaining a predetermined refractive index curve, so as 
to result in the formation of a light-conducting core zone and sheathing 
zones adjacent to the core zone. 
Planar lightwave guides are used in optical communication systems as 
coupling elements for optical wave conductors. Depending on the chosen 
arrangement, these coupling elements serve the purpose of signal branching 
and signal mixing, i.e. they serve as demultiplexer/multiplexer elements. 
A known method for making these wave guides is the CVD process in which 
SiCl.sub.4 of high purity is mixed with a few per cent of TiCl.sub.4 and 
caused to react with oxygen in an open flame. The glass particles which 
are produced by flame hydrolysis are deposited on a substrate. During the 
deposition process, the burner is continuously reciprocated so that 
several layers are formed. The refractive index is controlled by the 
TiCl.sub.4 current. Thereafter the substrate with the porous glass layers 
is heated in order that the individual layers will consolidate (Kawachi et 
al., Electronics Letters 1983, Vol. 19, No. 15, page 583). 
The layer system is then covered with a silicon mask and guide grooves for 
the accommodation of wave conductors as well as light conducting strips, 
are produced by targeted etching (Yamada et al., Electronics Letters 1984, 
Vol. 20, No. 8, Page 313). 
These known planar wave guides have the disadvantage that the refractive 
index profile through the deposited layers can be predetermined in only 
one direction, namely in the direction normal to the substrate. After 
etching, the light-conducting strip has a substantially rectangular 
cross-sectional configuration and the profile of the light-conducting core 
is not laterally adapted which gives rise to considerable losses due to 
dampening. Another drawback resides in that only relatively thick layers 
can be produced so that no finely graded refractive index profile can be 
obtained. 
A method is known from European Patent Application EP-0052901 whereby 
coupling elements are made with light-conducting strips which are round in 
cross section. To this end, grooves having a semicircular cross sectional 
configuration are formed by etching or mechanically in the substrate glass 
plate in accordance with a predetermined pattern. In the next step, 
glass-like layers are precipitated out of the gaseous phase on to the 
glass plate and in these grooves by application of a CVD process. With 
increasing layer thickness, increasingly more doping material is deposited 
jointly with the quartz glass. This is continued until the grooves are 
completely filled by these layers. The same is applied to another 
substrate provided with the corresponding mirror-image groove pattern. 
Then both substrates plates are polished and joined so that the grooves 
with the glass-like layers coincide. Whilst these strip conductors have a 
circular cross section with a radially outwardly decreasing refractive 
index, their manufacture is not without problems. 
The production process and particularly the polishing operation are very 
expensive. The grooves must coincide precisely and neither impurities nor 
air gaps may remain at the seam or junction between the substrate plates 
in the region of the light-conducting layers. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method for making 
planar waveguides in which the afore-described disadvantages appertaining 
to the state-of-the-art are avoided. The method according to the invention 
is substantially simpler whilst at the same time the resulting planar wave 
guide is distinguished from the known planar wave guides by lower losses. 
The aforementioned object is realized in accordance with one embodiment of 
the invention by a method which is characterized in that precipitation out 
of a material in a gaseous phase is applied by means of a heterogeneous 
reaction. A first layer with a doping medium which lowers the refractive 
index is applied to the substrate. Thereafter, a covering layer, by way of 
diffusion barrier for the said doping medium is applied to the first 
layer. The covering layer is structured by conventional masking technology 
in accordance with the desired strip pattern. Subsequently, the substrate 
provided with the layers is heated so that the doping medium diffuses out 
of the uncovered regions, thus forming the core zone, or a part of a core 
zone, of the light-conducting regions. Thereafter, further layers are 
applied in which the refractive indices are controlled so as to complete 
the lightwave guide. 
The layer which is provided with the doping medium which lowers refractive 
index may also show a predetermined refractive index curve. 
Due to diffusing out of the doping medium, for which fluorine is preferably 
used, a semicylindrical zone with a refractive index which increases 
toward the cylinder axis is produced in the unmasked regions of the first 
layer. Fluorine or any other doping medium may be used which has the 
effect of lowering the refractive index and may be diffused out. The 
semicylindrical core zone is then complemented by further layers, which 
may potentially also be doped by fluorine in such a way as to result in 
the formation of a low-dampening light-conducting zone. 
Preferably a non-isothermic plasma CVD process is used for the 
precipitation of the glass-like layers. 
In accordance with a second embodiment of the invention, a monomode light 
wave guide for transmitting polarized light is produced by forming two 
narrow ribs directly adjacent to the core zone which ribs are flanked by 
films of silicon dioxide. The structure is then covered by films of 
silicon dioxide doped with fluorine. 
The process for precipitating out of material in the gaseous phase, which 
is already known from EP-0017296, incorporated herein by reference is 
herein understood to be a process which operates with a socalled "cold" 
plasma in which only electrons have a high kinetic energy. With such a 
plasma, even gas mixtures which are not thermally reactive can be caused 
to react. With this non-isothermic, PCVD process, it is possible at fairly 
low temperatures, to precipitate glass-like layers directly out of a 
gaseous phase so that subsequent heat application for vitrification may be 
dispensed with. A further advantage resides in that for a precipitation at 
low temperature, that is to say between room temperature and 300.degree. 
C., any potentially existing difference in the thermal expansion 
coefficients of the glass plate material and the layers deposited thereon 
will not have a noticeable adverse effect. 
Upon further study of the specification and appended claims, further 
objects and advantages of this invention will become apparent to those 
skilled in the art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the first embodiment of the invention illustrated by FIGS. 
1 and 2, a planar waveguide according to the invention is shown in section 
with a substrate plate 2 and light-conducting regions 1. A first layer 3, 
which is provided with a doping medium, such as fluorine, is applied to 
the substrate 2 and subsequently selectively covered over by a masking 
layer 4. The masking layer 4 works as a diffusion barrier with respect to 
the doping medium contained in layer 3. The layer 3 has a thickness in the 
range of 10-60 .mu.. The masking layer 4 has a thickness in the range of 
3-10 .mu.. By application of heat, the doping medium is subsequently 
caused to diffuse out of the unmasked regions 3a of layer 3 so that a core 
zone 3b indicated approximately by the dotted line 6, will be formed in 
which the refractive index increases continuously towards the axis of the 
core zone which is approximately semi-cylindrical in shape. The thickness 
of layer 3 is selected so as to ensure that between line 6 and substrate 2 
there is preserved a sufficiently large area 3a of layer 3, complete with 
doping medium, in order to isolate wave energy from the substrate so that 
the substrate does not participate in light-conduction. Thereafter, a 
further layer 5 is applied, in which the refractive index curve is 
selected such as to complement the core zone 3b and form a sheathing zone. 
The refractive index curve through layer 3 and 5 in the X-direction along 
line II--II in FIG. 1 is illustrated in FIG. 2. 
For example, for making a monomodal fiber, a core zone of about 2-8 .mu.m 
will be formed. The fluorine (or boron) contained in layer 3 will be 
allowed to diffuse out for about one to two minutes at a temperature in 
the range of 2000.degree.-2200.degree. C., whereafter the final layer 3 
will be applied in a known manner. 
The masking layer 4, doped for example with B.sub.2 O.sub.3, acts as a 
diffusion barrier for the doping medium in the first layer 3 therebeneath. 
This ensures that it is only in the unmasked regions 3b of the first layer 
3 that a refractive index gradient can form by diffusion out of the doping 
medium. 
The refractive index profile and the dimensions of the core zone 3b are 
adjusted by the width of the unmasked regions 3b of the first layer 3 and 
the temperature and length of time of the diffusing out process in such a 
way that either a monomodal wave guide or a multimodal wave guide is 
produced. 
The refractive indices of the remaining layers and the numerical aperture 
are also adapted to the given purpose of application of the device. This 
is accomplished by the differences between the fractive indices (numerical 
aperture), and by the geometry and dimensions of the core zone 5 which are 
about 5-10 .mu. for mono-mode wave guides. 
Since during diffusing out of the refractive index lowering doping medium 
(which is preferably fluorine), the doping medium of the masking or 
covered layer 4 also diffuses out of proximate surface regions, increasing 
the possibility that a leaking wave conductor could be produced, which 
would give rise to greater losses. For this reason, a thin superficial 
layer of the unmasked regions of the layer 3b and the masking layer 4 is 
removed by etching, preferably after the diffusion process. 
In order to ensure the complete isolation of wave energy relative to the 
substrate 2, the layer 3 which is provided with a doping medium is applied 
to the substrate with a layer thickness such that after the diffusing out 
of the said doping medium from the unmasked regions, a correspondingly 
large doped region 3c and 3a is preserved between the thus formed core 
zone 3b and the substrate 2. This has the advantage that the substrate 
does not participate in conduction of light so that the substrate 2 need 
not consist of a highly pure material, this saving costs in the production 
of the planar wave guides. 
In operative glass fiber connections (not shown), guide grooves (not shown) 
are formed in front of the light-conducting strips 1 and in the substrate 
2 in which the wave conductors or guides which are to be coupled are 
inserted. 
With the planar light wave guides produced according to the invention 
absorption losses amount to significantly less than 0.2 dB/cm. 
Referring now to the second embodiment of the invention shown in FIGS. 3-6, 
wherein primed reference numerals are used to illustrate layers or regions 
analogous to the layer or regions in FIG. 1; it is seen in FIG. 3 that the 
core zone 3b' is formed by heating the waveguide to drive fluorine from 
the layer 3' to form a core zone having an arcuate boundary 6'. After the 
heating process, the masking area 4' which is comprised of SiO.sub.2 doped 
with B.sub.2 O.sub.3, of course remains. 
Referring now to FIG. 4, the masking area 4' has been etched away with 
C.sub.2 F.sub.4, CCl.sub.2 F.sub.2, SF.sub.6 except for two rib areas A 
and B positioned directly adjacent the core zone 3b'. The cross-section 
illustrated in FIG. 4 is accomplished by first masking the areas A and B 
with layer of photo resist (not shown) and then etching the exposed 
portion of mask 4' with C.sub.2 F.sub.4, CCl.sub.2 F.sub.2 or SF.sub.6. 
Referring now to FIG. 5, a thin film of silicon dioxide C is deposited over 
the structure shown in FIG. 4, i.e., over layer 3', narrow ribs or ridges 
A and B and core zone 3b'. The portion of the thin film of silicon dioxide 
C not covering narrow ridges A and B and core zone 3b' are then masked and 
the unmasked silicon dioxide is etched away with C.sub.2 F.sub.4, 
CCl.sub.2 F or SF.sub.6 over narrow ridges A and B and the core zone 3b', 
leaving the structure shown in FIG. 6. In FIG. 6, the silicon dioxide 
layer C extends laterally away from the narrow ribs or ridges A and B, 
leaving the core zone 3b' and narrow ridges A and B exposed. 
In a final step, a layer system D comprised of a plurality of layers is 
applied over the structure shown in FIG. 6 whereby the refractive index 
course of the layer system is selected so that the core zone 3b' is 
complemented. The area D is formed of layers of silicon dioxide doped with 
fluorine and completely covers the structure shown in FIG. 6 so as to 
produce the structure shown in FIG. 7. In this way, a planar monomode 
light wave guide is formed which is able to transmit light with a defined 
polarization direction. The said planar monomode light wave guide is also 
able to polarize the monomode light propagating in two orthogenal modes. 
Those half part of the light the plane of polarization of which is 
parallel to the substrate can leave the core zone by "tunnelling" through 
the two narrow strips A and B into the SiO.sub.2 layer C. The other half 
part of the light the plane of polarization of which is perpendicular to 
the substrate, and the said plane of polarization itself, remain due to 
stress birefringence caused by the narrow rips A and B. 
With respect to practicing both the first embodiment (FIGS. 1 and 2) and 
the second embodiment (FIGS. 3-7) of the instant invention, the process 
for depositing the first layer 3 and 3' is already known, for example from 
EP-0017296, and is here understood to be a process which operates with a 
so-called "cold" plasma in which only electrons have a high kinetic 
energy. With such a plasma, even gas mixtures which are not thermally 
reactive can be caused to react. With this non-isothermic PCVD process, it 
is possible at fairly low temperatures to precipitate glass-like layers 
directly out of a material in a gaseous phase, so that subsequent heat 
application for vitrification may be dispensed with. A further advantage 
of this approach resides in that for ar precipitation at low temperatures, 
that is to say temperatures between room temperature and about 300.degree. 
C., any potentially existing difference in the thermal expansion 
coefficients of the glass plate material and the layers deposited thereon 
will not have a noticeable adverse effect. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention, and without departing 
from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various usages and 
conditions.