Note: Descriptions are shown in the official language in which they were submitted.
CA 02291405 1999-12-02
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Optical Waveguide Having a Hybrid Core
Field of the Invention
This invention relates to optical waveguides, and more particularly, an aspect
of this
invention relates to an optical waveguide having a hybrid core, a portion of
which is an
organic material, and an adjacent contiguous portion being an inorganic
material.
For many year now, waveguides in the form of optical fibers have received
widespread
1 o interest for information and data transfer. Fiber-guided modulated light
beams are useful
in many applications, for example, telecommunications, computer link-ups, and
automotive controls. Advantageously, fiber optic linkages have a greater
information
carrying capacity as compared to metal wires carrying electrical signals.
Furthermore,
fiber optics are less likely to suffer from external interference, such as
electromagnetic
radiation.
Typically, optical fibers comprise a light-carrying core, for example an
inorganic glass
such as fused silica or a polymer such as polymethyl methacrylate, and a
cladding
material having a lower refractive index than the core. The cladding material
serves to
2o confine the light energy within the core and thereby allows propagation of
light by a
phenomenon generally known as "total internal reflection."
Characteristically, glass optical fiber cores have very low optical loss and
are generally
preferred over polymer waveguides for long distance applications.
As of late, monolithic waveguiding devices have gained popularity. These
devices tend to
be compact and cost effective to manufacture. Such devices are described by
the
applicant in United States Patent 5,470,692 entitled Integrated optic
components issued
November 28, 1995. In the '692 patent an integrated optic component comprises
a
3o substrate carrying a layer of polymeric material. The component may be
poled so as to be
an active component and may be in the form of a ridge guide.
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Many monolithic devices having, for example polymer waveguides disposed
therein
provide a single guided mode, similar to single mode optical fibre. Another
class of
monolithic waveguiding devices are comprised of waveguides disposed in glass
wherein
an ion diffused region or a reactive-ion-etched structure overcoated with a
cladding can
serve as a waveguide core.
Polymer waveguides disposed on a substrate offer some advantages over
inorganic glass
such as silica, in some respects, however, low levels of signal loss i.e. high
transparency
of inorganic glass is desirable and preferred to polymer. Polymer waveguides
are noted
for low transparency, i.e. significant loss; polymer waveguides have a high co-
efficient of
expansion and, associated with that a high (negative) thermo-optic co-
efficient, and a low
thermal conductivity. In contrast, inrganic glass has a high transparency, a
high thermal
conductivity, and a low (positive) thermo-optic coefficient.
This invention utilizes these differences in the two materials in a
synergistic manner by
providing a inorganic glass/polymer hybrid core structure that highly
advantageous.
Since polymer waveguides are suitable as active device, they are also useful
in optical
zo switches.
It is an object of this invention, to provide a waveguide that uses the
beneficial
characteristics of inorganic glass such as silica, and as well the beneficial
characteristics
of polymer waveguides, while minimizing the unwanted characteristics of these
materials.
For example, it is desired to have a optical waveguide with an active region
which is
highly thermo-optic active, so that it may be switched, attenuated, or
modulated with low
power. Notwithstanding, it is desired to have an optical waveguide that under
normal
transmission is highly transparent, i.e. has little signal power loss. Yet
still further, it is
3o desired to have a waveguide having an athermal or relatively temperature
stable effective
index, at least in particular regions. Yet still further, it is desired to
have a waveguide
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wherein the refractive index can be changed relatively efficiently and
significantly with
minimal power. And yet still further, it is desired to have a waveguide with
two different
regions, having guided light transmitting cores that have relatively different
refractive
indices, yet that can be modified by the application of a suitable energy, to
lessen or
obviate the refractive index difference between the two regions. The latter
being
significantly useful in optical switching applications and in in-line Bragg
grating
applications. .
to It is an object of this invention, to provide such a waveguide for a host
of optical
applications and devices.
Summary of the Invention
This invention is not limited to waveguides having a core of a particular
shape, however
this invention is related to a waveguide having a core having a hybrid of
materials having
different optical properties contiguously disposed one beside the other.
In a preferred embodiment the contiguous cores consisting of dissimilar
materials having
dissimilar optical properties have a substantially similar mode field
diameter.
In accordance with the invention there is provided, an optical device, for
single moded
guiding of light comprising a waveguide having a core and a cladding about the
core, for
substantially confining and guiding light within the core, the cladding having
a lower
refractive index than the core, the core having a plurality of contiguous
sections of
different material, a first section of the core being comprised of inorganic
glass, a second
section of the core being comprised of a polymer.
In accordance with the invention, there is further provided an optical
waveguide having a
3o core and a cladding surrounding the core, for substantially confining and
guiding light
within at least a length of the core, the cladding having a lower refractive
index than the
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core, the length of the core having a plurality of contiguous sections of
different material,
a first section of the core being substantially inorganic material, a second
section of the
core being a an organic material, the thermo-optic coefficients of the first
and second
core sections being substantially different.
In accordance with another aspect of the invention, there is provided, an'
optical device
comprising:
a first planar optical waveguide having at least a longitudinal portion having
inorganic
glass core;
a second planar optical waveguide having at least a longitudinal portion
having polymer
core, the portion of the first planar waveguide and the portion of the second
planar
waveguide being disposed such that coupling of light from the first planar
waveguide and
the second planar waveguide occurs; and,
means for relatively varying the refractive index difference between the
inorganic glass
core and the polymer core. In this aspect of the invention, controlled
coupling of light
can be accomplished between a inorganic glass portion of a planar waveguide
core and an
adjacent polymer portion of a planar waveguide core by controlling the
refractive index
of the polymer portion. In yet another embodiment two substantially inorganic
glass
waveguides disposed adjacent each other and having polymer grafted portions in
close
2o proximity wherein at least one portion is controllable with heat and can be
used as a
switch
In accordance with another aspect of the invention there is provided a method
of making
a planar optical device, comprising the steps of:
providing a waveguide having core of inorganic glass or other non-polymer
material
having a substantially different thermo-optic coefficient than a polymer;
3o disposing within a region, contiguous with the inorganic glass core, having
predetermined waveguiding boundaries, the polymer to provide a polymer core
portion.
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In accordance with a broad aspect of the invention, there is provided, an
optical device
having a waveguide therein, the waveguide having a core of inorganic glass and
having a
lesser portion of the core being a grafted section of polymer having a
substantially
different thermo-optic coefficient than the inorganic glass.
Typically, when designing optical components it has been desired to provide
materials
for example within a longitudinal core section wherein the material are
matched as
closely as possible with respect to refractive index at a certain temperature
and
to mismatched as widely as possible at a different temperature. With regard to
aspects of
this invention, materials having substantially different thermo-optic co-
efficients are
utilized to provide useful optical devices.
Brief Description of the Drawings
Exemplary embodiments of the invention will now be described in conjunction
with the
drawings, in which:
Fig. 1 is an isometric view of a two layered planar structure on a flat
substrate serving as
2o a base for fabricating a waveguide device;
Fig. 2 is an isometric view of the device shown in Fig. 1 including a metal
mask disposed
atop an upper layer for use in providing grafted parts;
Fig. 3 is an isometric view of the device shown in Fig. 2 wherein three
grafting parts are
shown after removing unmasked material around the parts;
Fig. 4 is an isometric view of the device shown in Fig. 3 including an
additional spin
coated layer;
Fig. 5 is an isometric view of the device shown in Fig. S having a mask upon
the grafting
parts and adjacent polymer material for form a longitudinal core section;
Fig. 6 is an isometric view of the device shown in Fig. 5, wherein the
composite core
3o with grafted sections are shown after removal of the unmasked surrounding
material,
awaiting a final upper cladding layer to be spin-coated thereon;
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Fig. 7 is an isometric view of a waveguide device having a grafted core
surrounded by a
cladding;
Figs 8 through 11 are isometric views of a grafting process for providing
polymer core
sections into a silica core waveguide;
Figs. 12a through 12d illustrate different etch back states;
Fig. 13 is a cross sectional view from a prior art patent EP 0707113A1 in the
name of
Bosc et al assigned to France Telecom wherein a planar waveguide is disclosed
having
silica core and a polymer cladding region.
Fig. 14 is an isometric view of a hybrid core of an optical waveguide in
accordance with
1 o an embodiment of the invention;
Fig. 15 is an isometric view of an alternative hybrid core of an optical
waveguide in
accordance with an embodiment of the invention;
Fig. 16 is a cross sectional side view of a hybrid core having grafted polymer
regions
forming a grating;
Fig. 17 is a an optical coupler;
Fig. 18 is a table illustrating properties of photonic materials;
Fig. 19 is a top view of a 1 x 2 optical switch;
Fig. 20 is a cross sectional view of the 1 x 2 optical switch shown in Fig:
19;
Fig. 21 is a top view of an alternative embodiment of a 1 x 2 optical switch;
Fig. 22 is a cross sectional end view of the 1 x 2 optical switch shown in
Fig. 2l;and,
Figs. 23 and 24 are top and cross sectional views of a Mach Zehnder
interferometer
having hybrid sections.
Detailed Description
The grafting of planar polymeric waveguides is known and is described in a
publication
entitled Novel "serially grafted" connection between functional and passive
polymer
waveguides, by Watanabe et al Appl. Phys. Lett. 65 (10), 5 Sept. 1994, pp.
1205-1207
The process steps required to create inlay-structures are shown in the figures
and begin
3o with spin-coating a lower cladding layer 12 onto a silicon substrate 10
followed by
coating the core polymer 14 as is shown in Fig. 1. This core layer is used to
create parts
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to be grafted. Fig. 2 illustrates the application metal structures 16 used as
a mask for the
grafting parts and realized onto lift off rersist by evaporation of a metal
layer, resist
spinning and definition by photo-lithography. The grafting parts 18 are shown
in Fig. 3
after reactive ion etching (RIE) to remove the unmasked core layer material.
After a lift-
s off step to remove the metal mask, a second core layer 20 is spin coated.
The remaining
portion of the waveguide core is formed by this layer. Conventional etch-back
planarization is performed to reach a flat surface. A planarization layer is
spin-coated
onto the second core layer 20 and then etched back until the preferred
waveguide height
is reached. The topography of the upper surface of the planarization layer is
transferred
to to the underlying layer. In this manner a polymer stack with grafted parts
and a flat
surface is reached as shown in Fig. 4. After this, another metal structure 22
is defined
onto lift-off resist, by evaporation of a metal layer, resist spinning and
definition by
photo-lithography with the final waveguide pattern as shown in Fig. 5. Fig. 6
illustrates
the waveguide consisting of grafted parts 18, 19 after reactive ion etching.
Fig. 7
15 illustrates the polymer stack after a final spin-coat 24 of upper cladding
is applied.
Although optical devices made of two different polymer cores such as the
grafted cores
described heretofore are useful in certain optical applications, it is
believed that this
structure can greatly be improved upon.
This invention relates to the provision of an optical waveguide having a core
wherein a
region of the core is a polymer material and wherein an adjacent contiguous
region of the
core of the waveguide is inorganic glass preferably silica. Since silica is
highly
transparent, and less attenuating than polymer materials, it is preferable in
most instances
to manufacture devices wherein the core is substantially made of silica, and
wherein a
much smaller lesser portion is made of polymer. Furthermore, many of the
benefits of
polymer can be utilized by using only a small amount of polymer in these
devices. For
example in an active device such as an optical switch, the switching region
itself can be
realized with a small polymer grafted insert. In temperature stable devices,
where the
3o advantageous combination of combining a core of polymer with a core of
silica is
provided, the ratio of polymer to silica or glass is about 1:10, hence only a
small amount
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of polymer is required in many instances in smaller devices. Polymer silica
hybrid core
waveguides as described hereafter are particularly suitable in optical switch
or in-line
Bragg grating applications for a plurality of reasons. Since a polymer silica
core hybrid
waveguide can be provided wherein the refractive index is the same at ambient
temperature, or at a predetermined temperature, gratings can be manufactured
which are
substantially transmissive at a particular temperature and which are highly
reflective at
other higher temperatures for predetermined wavelengths of light. Hence such
an optical
waveguide would act as a reflective (or forward coupling) filter when heat is
applied and
would act as if the grating was absent when the heat was removed. Instead of
the
1o multiple polymer silica sections that are used in the gratings, a single
polymer section
would act as a wavelength insensitive reflector when heat is applied and would
act as if
the reflector was absent when the heat was removed.
Thus, practicable, useful active and passive optical devices can be made from
the
waveguides in accordance with this invention.
Figs. 1 through 7 as shown relate to the formation of a hybrid grafted core
section having
two different polymer materials adjacent one another forming the core of the
waveguide.
This process can be extended to yield a hybrid silica/polymer core in
accordance with this
2o invention. Referring now to Figs. 1 through 7 again, the initial base
layers 12 and 14 are
now made of silica; these layers can be created by flame hydrolysis deposition
(FHD)
process or a chemical vapour deposition (CVD) process; these layers precede
polymer
layers because they fabricated at temperatures well above the degradation
temperature for
polymers. Initially the lower silica cladding layer 12 is deposited onto the
silicon
substrate 10, followed by the silica core layer 14. This is illustrated in
Fig. 1 Channel
waveguide core sections will be etched out of the core layer by means of
reactive ion
etching (RIE) in CHF3, Ar gas mixtures using a Cr mask. This mask 16 is
created by Cr
layer sputtering onto the core layer followed by standard photolithographic
resist
patterning and wet chemical etching. Hence openings for the polymer channels
section to
3o be disposed are provided as is illustrated in Fig. 2. After RIE, the mask
is removed by a
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wet chemical etching process and the silica grafting parts 18 are ready for
polymer
overcoating as can be seen in Fig. 3 .
This is illustrated in Fig. 4 where a solution of cross-linkable polymer for
the core
sections has been spin-coated onto the wafer to embed the remaining silica
core sections
I 8 in the core polymer 20. Dependent on the polymer that is used, thermal or
photocuring
is used to make the polymer layer insoluble. Additional cured polymer layers
can be
deposited over this layer to further planarize the surface. The polymer
surface is then
etched down to the upper core surface using RIE with O2. A continuous Ti mask
pattern
22 for the hybrid channel waveguide is formed onto this surface by means of a
standard
photolithographic resist patterning followed by dry etching using RIE with
SF6.This is
shown in Fig. 5 The Ti is evaporated onto a photoresist layer that is
spincoated first onto
the surface. Fig. 6 shows the continuous hybrid channel 18+19 that is created
by polymer
etching using 02-RIE. The mask pattern is removed by a lift off procedure.
Finally a
polymer upper cladding layer 24 having a refractive index that is lower than
the refractive
index of the polymer core sections is spin-coated over the hybrid channel
waveguide
structure as illustrated in Fig. 7. After curing it forms an insoluble upper
cladding layer.
The final waveguide is formed of core sections of silica 18 and adjacent core
sections 19
of polymer.
Fig. 8 to 11 show an alternative process that begins from silica channel
waveguides 34
including the upper silica cladding 3 (Fig. 8). Sections for the polymer core
are
provided by etching out the silica down to the lower silica cladding using a
metal mask
(Fig.9) to make grafting gaps in the silica core by RIE (Fig. 10). The gaps
are filled first
with the core polymer by spincoating and curing. This polymer is then etched
down by
RIE with 02 to the upper core interface. This process can be carried out
without the use
of a mask , because the silica is not etched in the RIE process for the
polymer. A
polymer cladding is applied thereafter (Fig.l 1).
Referring now to Figs 12a through 12d the etch back principle is illustrated.
To
successfully etch back the planarization material has to have the same etch
speed as the
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core or grafting material. The initial situation is a layer stack which is
built up to the
planarization layer as show in 12a. When the etch rate of the planarization
material vP is
larger than the etch rate of the core materials v~ a bump will remain as
illustrated in Fig.
12b. When the etch rate of the planarization material is smaller than the etch
rate of the
core material a dent can arise as shown in Fig. 12c. Preferably as shown in
Fig. 12d, vp =
v~.
Fig. 13 shows in a prior art European patent application EP 0797113A1 in the
name of
Bosc et al. a planar waveguide having silica core and a polymer cladding
region.
1o Although there are advantages to such a structure, in contrast the instant
invention
provides a planar optical waveguide that provides an entirely new class of
optical
devices.
This invention provides control of and within the core of a waveguide itself.
Hence by using these two very compatible materials having significantly
different
properties within a core of an optical waveguide, a host of new devices are
practicable;
devices which can route, switch, multiplex and modify channels or wavelengths
of light;
devices essential for optical communications. The core of the waveguide need
not be
confined to small dimension typically associated with single mode propagation
of light;
core dimensions may in fact be considerably larger, for example for use in
applications
such as multimode interference devices.
Referring now to Fig. 14, a core of an optical waveguide is shown, in
accordance with
this invention, having a polymer portion 12, grafted between two silica
sections 10. Of
course a suitable cladding is required (not shown) around the waveguide core
in Fig. 14
to ensure that light is confined within the core. Although the polymer portion
12 and
silica sections 10 are adjacent and contiguous to one another in a
longitudinal sense,
serially one portion after the other, this invention is not confined to
longitudinal
3o contiguous sections or portions of silica and polymer within a core of a
waveguide. For
instance, in Fig. 15 a core is shown having two contiguous portions of silica
40 and
to
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polymer 42, wherein there are no longitudinal abutting portions.
Alternatively, hybrid
cores shown in Fig. 14 and Fig. 15 can be provided where a portion of a silica
core is
grafted with polymer as shown in Fig. 16. Here, polymer sections 52 are
grafted into
voids created within the silica core 50 to achieve a grating. Of course these
grafted
sections can have polymers of different refractive indices to provide a
chirped grating, or
alternatively, the width of the sections 52 can be varied linearly or non-
linearly to achieve
a desired chirped effect. It should be noted that a cladding layer of polymer
or glass is
not shown. Gratings can be realized by creating comb like structure within a
silica
waveguide and spin coating polymer to fill in the voids, or alternatively, by
grafting in
polymer sections.
Another aspect of this invention is shown in Fig. 17, wherein a planar
waveguide having
a silica core, is disposed in a parallel adjacent manner with a planar
waveguide having a
polymer core. The cladding region surrounding these cores is not shown. Such a
device
can be used as an optical coupler. Although the polymer waveguide depicted
consists
entirely of polymer, it may be convenient to provide waveguide 62 only having
a portion
within and about the coupling region to consist of polymer, whereas an
adjacent
longitudinal portion on each side could be made of silica which is more
transparent and
has less unwanted loss.
Fig. 18 shows some basic properties of photonic material, including fused
silica, silicon,
polymer glass and polymer rubber. However, to assist the reader in
understanding this
specification, when glass is referred to without the word polymer preceding
it, as in
"polymer glass", glass shall refer to inorganic materials such as silica.
After considering
Fig. 18, and the text heretofore in this specification, it becomes readily
apparent that
providing waveguides hybrid cores offers significant advantages until now not
realized.
Turning now to Figs. 19 and 20 , a 1x2 digital optical switch (DOS) is shown,
having a
Y-shaped waveguide with a predominantly inorganic glass or fused silica
channel or
3o core 80. An upper branch of the Y-shaped waveguide is shown to have a
grafted
longitudinal section 82 of polymer, having a refractive index lower than the
silica channel
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80, in the absence of applied heat. A heater 84 is disposed on top of the
grafted polymer
section 82 and upon the application of a suitable heat for a suitable duration
the refractive
index of the polymer section 82 exceeds that of the silica channel 80;
Switching is
accomplished in this manner. It should be noted that the angle a between the
legs or
branches of the Y-shaped waveguide is very small, i.e., 0.1 °. The
significantly different
properties of silica and polymer are used in this instance to a great and
unexpected
advantage. The high thermo-optic coefficient and low thermal conductivity of
polymers
provide a simple and effective switching medium when coupled with silica in
this
manner. Furthermore, the high transmissivity of silica provides a switch that
is relatively
to efficient with little loss. Only the active region that is a small portion
of the switch shown
need be made of polymer. The operation of the DOS switch is based on the
evolution of
a waveguide mode arriving from a single-mode input waveguide at the bimodal
branch
point to the fundamental mode of the branching waveguide with the highest
refractive
index of the two branching waveguides. Thus, by relatively controlling the
refractive
index of the two polymer cores by relatively varying the temperature, the
light
propagating within the core 80 can be routed in one of two directions, or
alternatively can
be split into both polymer cores 82. Fig. 20 shows a cross section of the
switch of Fig.
19.
2o The required local temperature change in thermo-optic devices is generated
by a stripe
shaped electrodes that are deposited onto the waveguide cladding layer in
order to avoid
the interaction of light in the waveguide core with these electrodes that
would lead to
optical loss. The operation of various devices in accordance with this
invention is based
on inducing a difference in the core refractive indices of two or more close-
by channel
waveguides. The separation of the heater electrode from the channel core by
the cladding
layer and the fanning of the thermal field from the electrode makes this
difficult. By
using different core materials in the switching region, the refractive index
difference
between the two cores is now largely determined by the refractive index change
of the
polymer core alone, because the refractive index change of the silica core is
negligible
3o compared to that of the polymer core. An embodiment of the DOS is shown in
Fig. 21.
To let the switch operate as a thermo-optic switch, the grafted polymer branch
has a
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higher refractive index than the silica branch in order to have the light
switched to the
polymer branch in the unpowered state. By powering the heater electrode, the
refractive
index of the polymer branch can be brought below the value of the silica
branch. The
switch in Figs. 21 and 22 operates with less power than the switch having two
polymer
branches shown in Figs. 19-20.
Turning now to Figs 23 and 24 a Mach Zehnder interferometer (MZI), in
accordance with
an embodiment of this invention is shown. Two arms are shown branching from
and
converging to silica waveguides 40 and 41 respectively. Branch arms 42 and 43
are
optically coupled with branch arms 45 and 46 via grafted polymer core sections
47 and
48. The MZI only transmits light having a wavelength arriving in phase at the
combiner
section wherein two arms converge; therefore it can be used as a wavelength
filter. An
MZI having only a silica waveguide core would be temperature sensitive because
the
optical path length difference between the two branches changes due to the
thermo-optic
effect in silica. However, by grafting sections of polymer into the core the
device can be
made to be relatively temperature insensitive due to the opposite sign of the
thermo-optic
coefficient in polymer as compared with silica. The length of the polymer
grafted
sections can be relatively small because the thermo-optic effect in polymers
is much
larger in the absolute sense than in silica.
Of course, numerous other active and passive devices can be envisaged that
utilize the
basic principles of this invention.
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