Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Jun 1-~ 98 01:51p Karl Hormann, Es9. 617-491-8877 p.2
Attorney Docket 980267
Thermo-Optical Switch
The invention relates to a thermo-optical switch having a layer
structure on a substrate and containing, in a waveguide layer, a directional
coupler waveguide structure and, above the waveguide layer, a heating
electrode configured to complement the form of the coupler structure.
For the transmission of broadband optical signals without prior
conversion into electrical signals, it is necessary to utilize cross-connects
which may be switched to a state of optical transparency. Such optically
transparent switches contain, among others, spatial switches for directing
incoming optical signals to selected output fibers. The spatial switches must
satisfy the following requirements: low cross-talk, low coupling attenuation,
independence of signal polarization, low electric switching power, response
times < 10 ms. high integration density, low production cysts.
In recent years, thermo-optical switches have been developed on a
polymer basis because the properties of polymeric waveguides give rise to
the expectation that the above-mentioned requirements may be realized with
them by way of selective structuring. Thus, polymers have a large thermo-
optical coefficient, i.e. a change in temperature causes a large change of
their
refractive index, combined with low thermal conductivity. These properties
lead to a low switching power far a thermo-optical switch which is below that
of a comparable SiOz switch by a factor of about 100. Since polymers display
very low birefringence they can be used for the fabrication of components
which are independent of polarization. Switching times are in the range of
milliseconds, 1 to 10 ms being typical.
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Moreover, the use of polymer waveguides makes it possible to
fabricate spatial switches by relatively simple processes which are well-known
from the fabrication of microelectronic components. In addition, polymer
technology makes it possible to integrate on a single substrate, as hybrid
technology, a plurality of optical components, such as, for instance, Ill-V-
lasers, photo diodes with polymer waveguides, networks and switches. Thus,
components with complex functions may be fabricated in a cost-efficient
manner.
Proceeding from the above-mentioned state of knowledge, solutions
have been sought in recent years, to utilize as many of the above-mentioned
advantages of polymers for optical elements as possible. Since the neces-
sary switching power and switching time of thermo-optical elements are
primarily dependent upon their thermal properties, i.e., their thermal
conductivity, thermo-optical coefficient and the heat capacity of waveguide
layer, buffer layers and substrate material, as well as upon the shape and
size (dimensioning) of the waveguides and heating electrode, there are
known in the art many thermo-optical elements differing in their concrete
structure for optimally realizing defined functions.
In Journal of Lightwave Technology, Vol. 7, No. 3 (1989), pp. 449-453,
there is described a planar thermo-optical switch in which a polymeric
waveguide layer made of polyurethane is arranged upon a PMMA (polymethyl
methacrylate} substrate, with a PMMA buffer layer superimposed thereon on
which is provided a silver strip conducting electrode as a heating element. At
a switching power of 100 mW, typical switching times are 12 ms for on-off
switching and 60 ms for off-on switching.
In most thermo-optical switches based on polymer, the waveguides are
formed in strips which leads to reduced switching times and switching power.
Thus, a digital optical switch {DOS) which is independent of polarization is
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described in SP1E, Vol. 1560, Nonlinear Optical Properties of Organic
Materials IV (1991 ), pp. 42B-433 in which a gold strip electrode is provided
on
one of the two output branches of a symmetrically structured Y-junction.
When a heating voltage is applied tv the electrode it realizes an asymmetric
effect upon the described switch. The change of the refractive index of the
amorphous polymeric material of the waveguide is generally isotropic and,
therefore, independent of any polarization as regards fight propagating
through the structure. The monomodal waveguide made of DANS polymer
arranged upon a glass substrate was fabricated by photo bleaching the non-
waveguiding areas of the waveguide layer with UV irradiation and is covered
by a buffer layer. The switching times of this arrangement are in the
millisecond range.
In Proc. 21st Eur. Conf. on Opt. Comm. (ECOC '95 - Brussels), pp.
1063-1066, there is also described a Y-shaped waveguide in a polymer-
based digital optical switch. In this case the waveguide structure has been
realized by photo lithography followed by dry-etching of moats in a silicon
substrate, followed by thermal oxidation in water vapor and, thus, creation of
a Si02 buffer layer, spinning of CYCLOTENE~ polymer thereon and covering
the polymer layer by a further Si02 buffer layer. A titanium thin film
electrode
is divided and positioned above the two output branches. At a switching
power of between 130 mW and 230 mW the extinction coefficient in the
heated branch is better than 20 dB. The optical power is then fed wholly
through the other -unheated- branch. But even in this technical solution the
requisite switching power and switching times are still too high.
In European Patent EP 0,642,052 there is described a polymer-based
digital optical switch in a layer structure of a substrate, lower buffer
layer,
waveguide layer, upper buffer layer and heating element with a Y-shaped
waveguide structure, wherein the refractive indices of the two buffer layers
are smaller than the refractive index of the waveguide layer. Moreover, the
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refractive index of the buffer layer adjacent to the heating element is
smaller
than that of the lower buffer layer. Ranges covering the contrast of the
refractive indices have been disclosed in accordance with parameters desired
(optical loss, switching power) for realizing a predetermined function, and
the
dimensions of the output branches of the waveguide are structured
symmetrically or asymmetrically, and the heating elements are also arranged
symmetrically (at both output branches) or asymmetrically (at one output
branch only). White precise current control is not required for the described
arrangement it requires a higher switching power and leads to cross-talk of
not more than about -20 dB.
Low switching power is required in a thermo-optical switch described in
1EEE Photonics Technology Letters, Vol. 5, July 1993, pp. 782-784. The
switch is provided with a Mach-Zehnder-interferometer above the two
waveguides of which there are arranged thin-film heating elements. While
this optical switch does realize low cross-talk as well, its overall length is
about thrice that of a conventional directional coupler.
For a different system of waveguide material there is described in
Electronics Letters, 29th October 1981, Vol. 17, No. 22, pp. 842-843, a
thermo-optically induced waveguide based upon LiNb03:Ti in which a nickel-
chromium electrode is arranged on one section of the waveguide_ When a
voltage is applied to the electrode the refractive index of the area of the
waveguide below the electrode changes thus deflecting the fed-in light.
Also, directional coupler switches with alternating D(3 are known, as
described, for instance, in IEEE Journal of Quantum Electronics, Vol. QE-12,
No_ 7, pp. 396-401, July 1976. In this case, several electrode sections are
arranged upon parallel waveguides made from the already mentioned LINb03
material. fn concrete switching conditions the electrodes generate in the
corresponding waveguide sections below them, based upon the electro-
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optical effect, a difference in the propagation velocities of the light at
respective alternating signs. If the interactive length between the two
waveguides is greater than the coupling length the desired switching state
(cross-over or throughput state) may be set by way of the switching power.
A directional coupler with alternating D~i is also described in Patent
Abstracts of Japan, Vol. 12, No. 192 (P-712), 4 June '1988 #JP 62.297,827
(Fujitsu Ltd.), in which in a first variant also several (here two) electrode
sections are arranged congruently on each of two parallel waveguides, and in
a second variant the electrode configuration is structured in a step-like
manner whereby each of the horizontal electrode sections covers about half
of the coupling length of the waveguides and wherein the areas of the
waveguides covered by the electrode sections are displaced relative to each
other. With such a directional coupler it possible, proceeding from a cross-
over state which is for technical reasons is bad in terms of realizing a cross-
over/throughput switching function, only to switch in the throughput direction
(symmetric switch).
In connection with testing and calculating the coupling properties of
directional couplers formed by strips of a dielectric material on a LiNb03
substrate and metal films between those strips, there are described in
APPLIED OPTICS, Vol. 17, No. 5, 1 March 1978, pp. 769-773 different
possibilities of the placement of the two waveguides in which the coupling
coefficient is never constant.
The state of the art upon which the invention is built may be taken from
several publications all of which describe the same subject: OFC '95,
Postdeadline Papers, PD 17-1, 1995; MICRO SYSTEM Technologies '94, 4th
Int. Conf. on Micro, Electro, Opto, Mechanical Systems and Components,
Berlin, October 19-21, 1994, VDE-Verlag GmbH., pp. 1097-1100;
Jahresbericht 1994 des Heinrich-Hertz-lnstituts fttr Nachrichtentechnik Berlin
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GmbH., pp. 54-55; SPIE Proceedings Series Vol. 2449, 1994, pp. 281-292
may be mentioned. In the last-mentioned publications there is described a
thermo-optical tuneable {4x4) switching field fabricated in polymer technology
in an integrated optical form, the basic element of which is a thermo-optical
controlled switch of the kind mentioned above, structured as a 2x2 directional
coupler.
This 2x2 directional coupler is provided with two symmetrically
arranged waveguides the center portions of which are spaced closely to each
other so that under controlled conditions there will be cross-talk of light
from
one waveguide into the other one. When a voltage is applied to it, the
electrode which is positioned over one waveguide only will heat this
waveguide somewhat so that its refractive index changes affecting a transfer
of light from one of the waveguides into the other one. The heat generated in
the heating electrode diffuses through the upper buffer layer, the waveguide
layer and the lower buffer layer into the silicon substrate which acts as a
heat
sink. Owing to the negative temperature coefficient of the waveguide, this
leads to a change of the refractive index in the waveguide and of the propa-
gation coefficient of the waveguide. As has already been mentioned, the
effect of the thermo-optically induced phase shift in waveguides is used for
switching in Mach-Zehnder or directional coupling structures. The
asymmetric coupler is very short and consumes little power. The extinction
ration in the initial cross-over state is set by the selection of an
appropriate
coupling length; subsequent setting is not possible. Process-conditioned
fabrication tolerances limit the extinction ratio in the cross-over state to
typically -25 dB which results in minimal cross-talk of only -21.5 dB in the
4x4
matrix. fn different coupling elements with an electrode length of 3 mm
extinction ratios were measured between 20 dB (cross) and 32 dB (straight),
at a power consumption of 30 to 40 mW. The switching times were stated to
be less than 1 ms. The coupler is structured to be in its cross-over state
when the electrode is not heated, i.e. light coupled into one of the input
gates
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is coupled from the input waveguide into the parallel adjacent waveguide and
exits at the output thereof. When the electrode is heated, the light will exit
at
the output gate of the same waveguide ("bar").
The described switching arrangements are processed under dust-free
conditions. To this end, a silicon substrate, which also serves as a heat
sink,
is covered with an SiOz passivation layer by thermal oxidation. Thereafter,
the PMMA waveguide layer and a further passivation layer made of Teflon~
are applied in succession by a spin process. The PMMA is doped with a
photo initiator molecule in which a photochemical process is released (light
induced photo locking) under intense exposure to UV radiation, which leads
to an increase of the refractive index of the waveguide layer. The integrated
optical light waveguides of a width of a few pm are defined by localized UV
exposure through a photo mask. The refractive index and the difference in
refractive indices between the exposed and unexposed areas may be set
very precisely in a wide range by selecting appropriate mixture ratios of
photo
initiator and PMMA and by varying the dose of exposure. In a subsequent
process step the remaining photo initiator molecules are heated out of the
unexposed areas of the waveguide layer, and the waveguide structures will
thus be fixed. As a fins! step, an aluminum-gold layer is vapor deposited from
which the micro heating electrodes are etched by wet chemical action.
Aside from the electrode configuration which is asymmetric relative to
the optical axis described in respect of a concrete switch, a symmetric
electrode configuration has also been mentioned in which a unitary strip-
shaped heating electrode is arranged symmetrically relative to the optical
axis
of the directional coupler. Hence, both waveguides are influenced identically
by the heating electrode so that simultaneous coupling may occur between
the overlapping modal ends. Therefore, a transfer of 100% of the optical
power from one waveguide into the other one of the symmetric waveguide
configuration and, accordingly, a high extinction ratio may in principle be
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achieved. However, the switching power required for operating the
symmetrical switch is too high (for polymer waveguides several hundred mW
per switch), so that they have not been used in practice. Even the iast-
mentioned arrangements suffer from excessive power consumption and
cross-talk.
Therefore, it the task of the invention to provide a thermo-optical switch
the power consumption and cross-talk of which are lower than in prior art
device and the fabrication of which is no more complex than that of known
thermo-optical switches.
The task is solved in that in a thermo-optical switch of the kind
mentioned above two waveguides disposed closely adjacent each other
along their interactive lengths are covered across at least same of their
width
by at least one pair of lamellate electrode arms of a heating electrode
connected to each other by a common web, the electrode arms being of
similar geometric shape as the waveguides positioned below them, and that
means are provided for electrically energizing at least one electrode arm of
an
electrode arm pair, and that further means are provided for changing andlor
setting the thermal andlor geometric symmetrylasymmetry of the refractive
indices in the two waveguides which extend closely adjacent each other over
their interactive length.
The task is also solved by the fact that in a thermo-optical switch of the
kind referred to above, two waveguides extending closely adjacent each other
are covered over at least part of their width by a pair of lamellate electrode
arms each pair of which is connected by a common web, whereby the two
webs are thermically and electrically insulated from each other and arranged
symmetrically to each ether, the electrode arms being of a similar geometric
form as the waveguides positioned below them, that means are provided for
electrically energizing at least one electrode arm of an electrode arm pair,
and
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further means for changing andlor setting the thermal and/or geometric
symmetrylasymmetry of the refractive indices in the two waveguides
extending closely adjacent each other over their interactive length.
It has been found that the geometric as well as the material-specific
asymmetry of the switch in accordance with the invention relative to its
optical
axis exert a profound influence upon its switching action.
For that reason, there are provided embodiments for influencing the
symmetry/asymmetry of the optical switch in a particular manner.
Thus, for influencing the switching action the electrode arms of an
electrode arm pair may be arranged in a displaced manner or the inner
margins of the electrode arms of an electrode arm pair may be arranged
t 5 congruently relative to the inner margins of the waveguides positioned
below
them.
In another embodiment the waveguides which extend closely adjacent
each other along their interactive length are disposed in parallel
relationship.
In further embodiments, a pair of lamillate electrode arms is connected
by a common web {U-shaped) and means are provided for electrically
energizing at least one or both electrode arms of an electrode arm pair,
whereby in the case of two arms one of them may be energized by a constant
bias voltage or, alternatively, only one electrode arm is electrically
energized,
or two pairs of lamelfate electrode arms are arranged symmetrically relative
to
a common web (H-shaped) and one electrode arm of each electrode arm pair
is simultaneously electrically energized with the mirror-symmetrically
arranged
electrode arm of the other electrode arm pair in order to influence the
thermal
symmetrylasymmetry.
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While compared to the energization of a single electrode arm twice as
many energizing sources and higher switching power are required for the
simultaneous energization of both electrode arms where a pair of laminate
electrode arms are connected by a common web, because the bias voltage
applied to the other electrode arm must first be compensated before the same
effect can be achieved as when only one electrode arm is energized, it
enhances the flexibility in the design of the solution in accordance with the
invention.
l0 When a low voltage is applied to one arm of the heating electrode the
symmetric behavior of the coupler in accordance with the invention
dominates; because of the high thermal conductivity in the split metal
electrode the heat is transferred from the energized electrode arm to the non-
energized arm of the divided electrode.
In order further to improve this symmetric action, an electrode arm -as
has already been mentioned-, in addition to the small vertical displacement of
the electrode arms relative to the waveguides occurring during fabrication of
the multi-layer structure, one electrode arm is selectively displaced by a
small
distance relative to the waveguide below it, i.e. asymmetrically relative to
the
optical axis of the switch in accordance with the invention.
It has been found that this non-congruent alignment of the electrode
arms relative to the two waveguides constitutes the dominant asymmetry.
Further ways of attaining asymmetry in the switch structure relative to its
optical axis have already been mentioned and will be explained hereinafter.
If the initial operating point of such a switch in accordance with the
invention is disposed close to the cross-over state only low heating power is
required to attain the cross-over state: The rise in the temperature in both
arms of the split electrode is very small. as is the difference in temperature
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between the two electrode arms. This may be ascribed to the great thermal
conductivity of the metal electrode. if a voltage is applied to the electrode
arm which because of the displacement relative to the waveguide does not
fully cover the waveguide, the temperature in this arm will be slightly higher
than in the non-energized arm. However, because of the more favorable
geometric position of the other, i.e. the non-energized, electrode arm
relative
to the waveguide below it, the rise in temperature is the same in both
waveguides so that the velocity of light propagation is the same in both
waveguides. This, of course, is in conformity with the action of the symmetric
directional coupler- The more the heat is increased the more prominent is the
function of the directional coupler as an asymmetric one. This is so because
of the increasing temperature between the energized and the non-energized
electrode arms.
If the initial operating point is further removed from the cross-over state
greater heating power is required to attain the cross-over state. If the
heating
power is very large the difference in temperature between the electrode arms
will become large as well as only one arm is being energized. Accordingly,
the velocity of light propagation in the corresponding waveguides below the
energized and non-energized electrode arms is also different. In these
circumstances the switch operates as an asymmetric directional coupler and
cannot reach the first cross-over point.
If two pairs of lamillate electrode arms are arranged symmetrically
relative to their common web (H-shaped electrode configuration) and if one
electrode arm of one electrode arm pair is electrically energized
simultaneously with the mirror-symmetrically disposed electrode arm of the
other pair of electrode arms, opposing temperature gradients will be created
in the two pairs of electrodes. Because of the thermo-optical effect these
opposing temperature gradients cause the velocities of light propagation in
those waveguide sections arranged mirror symmetrically relative to their
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common web below the electrode arms to be different by alternating values of
Ll~i. Energization may be accomplished by only one source. In That case the
same power is fed to the mirror-symmetrically energized electrode arms by
current flowing through one electrode arm as well as through the other mirror-
s symmetrically disposed electrode arm. Energization is also possible with two
sources which may feed different powers to the corresponding electrode
arms. In the first-mentioned situation the ~~i in the sections of the thermo-
optical switch. If two sources are used the values of ~J3 in both section may
be set separately and, hence, at different levels. The desired switching
action of the thermo-optical switch in accordance with the invention may be
further supplemented by the embodiment just described.
This effect is even more advantageous if the width two waveguides
which extend closely adjacent each other over their interactive length is at
least partially covered by two pairs of (amellate electrode arms which in each
pair are connected to each other by a common web, the two webs being
thermally and electrically insulated from each other and arranged in symmetry
to each other (double-U-shaped electrode configuration), since heat
exchange is then restricted to one electrode pair and is not interacting. In
such an arrangement, the webs may be disposed in the same plane either
facing each other, or they may be rotated by 780°.
Other embodiments which are either independent of each other or
which may be combined relate to further means for setting the thermal and/or
geometric symmetry/asymmetry of the refractive indices in the two
waveguides which extend closely adjacent each other, and relate to a flexible
structuring of the electrode arms and waveguides.
Thus, these means are
- electrode arms of different widths;
- electrode arms of different thicknesses;
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- electrode arms made from different materials;
- waveguides with different refractive indices;
- waveguides of different widths.
It is possible by way of the variable placement of the two waveguides
in the thermo-optical switch, i.e. the coupling coefficient is not constant in
the
range of the two closely adjacent waveguides, to reduce the wavelength
dependency of the operating point and thus to increase the broadband state
of the switch in accordance with the invention. Waveguides of many different
shapes may be selected, for instance, two straight guides at a predetermined
angle, a straight and a curved waveguide, or two curved waveguides.
1n another embodiment there is provided on a substrate (S) a lower
buffer layer (uP), a polymer waveguide (WL1 and WL2) containing layer (W)
thereon and an upper buffer layer (oP) thereon with the heating electrode {E)
is arranged in a formed in a manner to cover the polymer waveguides {WL1
and WL2).
In advantageous embodiments thereof
- the refractive index of the lower bufFer layer is only insignificantly lower
than
the refractive index of the waveguides and the difference between the
refractive indices is about .005;
- the lower buffer layer consists of two partial layers, the layer adjacent
the
substrate being of a much tower index of refraction than the waveguides;
- the difference between the refractive indices between the waveguides and
the upper buffer layer is about .2.
The last-mentioned embodiment with its advantageous features, i.e. a
thermo-optical switch made of polymer, makes use of the mentioned and
3o acclaimed possibilities derived from the polymer as waveguide material;
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Owing to the variability of the geometric, material-specific and electrical
parameters of the heating electrode and of the waveguides and different
possibilities of energizing the heating electrode, it is possible, in
accordance
with the invention to affect the symmetrylasymmetry of the TO directional
coupler switch in a particular way to provide the parameters necessary for the
switch to function as a symmetric or asymmetric one.
!t has been found to be particularly advantageous subsequently to
change the parameters of the switch, i.e. after its fabrication, by energizing
the two electrode arms, or one electrode arm only, in a particular manner in
order to conform them to desired operating conditions.
The symmetric and asymmetric properties, as the case may be, of the
switch in accordance with the invention may be attained in a simple manner
by changing the electrical resistance of the electrode arms over their widths
or thickness, or by their material.
In order to reduce the fiber-chip coupling losses in polymeric thermo-
optical switches the refractive index of the lower buffer layer is only a
little
lower than the refractive index of the waveguide, and the difference between
the two refractive indices is about .005. Since the difference between the two
refractive indices is so small the lower buffer layer must be very thick for
operating the switch at large wavelengths. For that reason, the lower buffer
layer is made up of two partial layers, with the partial layer adjacent to the
substrate having a much smaller refractive index than the waveguide layer.
In order further to reduce the switching power, the upper buffer layer should
be structured as thin as possible. For that reason, a further embodiment of
the invention - which has already been mentioned - provides for a difference
between the refractive indices between the waveguide layer and the upper
buffer layer of about .2.
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76904-10
The thermo-optical switch in accordance with the
invention is characterized by extremely low polarization
independent cross-talk as well as low power consumption.
Its function mode and compactness are such as to render the
switch suitable as a basic element in large switching
matrices. The fact that desired parameters of individual
switching elements in a matrix may subsequently be
individually set by way of variable switching of the
electrode arms is particularly advantageous.
A broad aspect of the invention provides a thermo-
optical switch, comprising: a substrate; a waveguide layer
supported by the substrate; first and second waveguides in
the wave guide layer, the waveguides being of predetermined
width and refractive indices of a predetermined state of at
least one of thermal and geometric symmetry and having
interactive sections of predetermined length extending
closely adjacent to each other; at least one heating
electrode comprising first and second interconnected
lamellate arms respectively disposed in superposition over
the length and at least part of the width of the interactive
sections of the first and second waveguides, the first and
second arms being of a configuration substantially similar
to the first and second waveguides and provided with free
ends extending in the same direction; means for electrically
energizing at least one of the electrode arms thereby to
adjust the state of symmetry of the refractive indices,
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76904-10
Further characteristics and useful structures of the embodiment will be
apparent from the following embodiments which will be explained in detail
with reference to the drawings, in which:
Fig. 1 is a schematic presentation in top elevation of a multiple layer
structure of the polymer based switch in accordance with the
invention, with a pair of lamellate electrode arms;
Fig. 2 is a cross-section along line A-A of the multiple layer structure
shown in Fig. 1;
Fig. schematically depicts the coupling of the light
3 propagating in
waveguides WL1 and WL2;
Fig. 4 schematically depicts a parallel arrangement of
the waveguides
WL1 and WL2 along their interactive lengths L;
Fig. 5 depicts the dependency of light coupling from the
interactive
length L of both parallel waveguides WL1 and WL2;
Fig, 6 schematically depicts a cross-section corresponding
to Fig. 2
having electrode arms E1 and E2 arranged congruently
with
respect to the inner margins of the waveguides WL1
and WL2
with a a symmetrical energization of the electrode
arms E1 and
E2;
b an asymmetric energization of the electrode arms E1 and E2;
Fig. 7 schematically depicts a cross-section corresponding to Fig. 2
having electrode arms E1 and E2 displaced relative to the inner
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margins of the waveguides WL1 and WL2 with
a electrode arm E1 being energized;
b electrode arm E2 being energized;
Fig. 8 depicts the switching action of a polymer based switch in
accordance with the invention including a structure and
energized according to Fig. 7a;
Fig. 9 depicts the switching action of a polymer base switch in
accordance with the invention including a structure and
energized according to Fig. 7b;
Fig. 10 depicts the switching action of a polymer based switch in
accordance with the invention with inner margins of electrode
arms E1 and E2 being displaced relative tv inner margins of
waveguides WL1 and WL2 with simultaneous energization of
both electrode arms, the switching power at electrode arm E2
being constant;
Fig. 11 similar to Fig. 10 but with the switching power at electrode arms
E1 being constant;
Fig. 12 is a schematic presentation in top elevation of the layer
structure of the polymer based switch in accordance with the
invention having two pairs of lamellate electrode arms (E'1, E'2
and E"1 and E"2) arranged symmetrically on a common web
(G);
Fig. 13 is a schematic presentation according to Fig. 12 but with a
divided and electrically and thermically insulated web with a web
portion G' and a web portion G".
A symmetrically structured electrode E divided into two portions may
be clearly seen in Fig. 1 as the uppermost layer of the polymer based thermo-
optical switch in accordance with the invention. The two electrode arms E1
and E2 are disposed congruently over two parallel waveguides WL1 and
WL2. At one end, the two electrode arms E1 and E2 are connected to each
1G
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other by a web G (U-shaped electrode configuration), the common web G
being here provided at the side of two input gates 1 and 2 of the switch TOS.
If the cross-section of the waveguide is designated as a flm x b um, a in this
embodiment <_ 5 pm and b ~ 10 pm, the spacing between the waveguides
WL1 and WL2 is about .5 a to 1.5 a, and the interactive length is several
millimeters.
For purposes of schematic presentations and measuring curves to be
determined under different operating conditions in the following figures, the
switch is of the following dimensions: a = 6.o pm, b = 5.0 um; the spacing of
the waveguides WL1 and WL2 measures 5.5 pm; the interactive length L =
4.5 mm. The spacing between gates 1 and 2 and gates 1' and 2' of the two
waveguides WL1 and WL2 measures 250 frm. The width of the two electrode
arms E1 and E2 is at least as large as the width of the two waveguides WL1
and WL2, in the present example it is 15 Nm. The overall length of the
thermo-optical switch in accordance with the invention in this example is less
than 10 mm.
In the crass-section AA' of the multiple layer structure of the switch in
accordance with the invention shown in Fig. 2, a lower buffer layer uP made
of SiOX and having a thickness of 6 lam and a refractive index of 1.475 is
arranged on a silicon substrate S having thickness of 400 Nm and a refractive
index of n = 3.5. On top of the buffer layer uP, there is provided the
waveguide Layer W consisting of 25% BDK; 75% PMMA and being of a
thickness of 5 lrm and refractive index of 1.5 and containing the waveguides
WL1 and WL2 of rectangular cross-section (5 pm x 6 pm) and a refractive
index of 1.505. The waveguide layer W is positioned adjacent to a further
buffer layer oP made of Teflon~ AF 13j00 (d = 2.5 pm, n = 1.3} upon which the
electrode arms E1 and E2 of a width of 15 pm of the aluminum-gold muiti-
layer electrode E is arranged congruently with respect to the two waveguides
WL1 and WL2. The individual layers may be fabricated by state of the art
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processes. The substrate may also be selected from one of the materials
listed hereafter: glass, polymeric material, ceramic or metal. Preferably, a
silicon plate is used as the substrate since silicon has a much higher thermal
conductivity than polymers and therefore functions particularly well as a heat
sink. Moreover, the end surface preparation is much simpler because of the
use of easily controllable cutting and polishing processes. Si has a very high
refractive index. For that reason the waveguide layer has to be optically
separated from the Si substrate by a lower buffer layer uP. Aside from the
above-mentioned SiOX, glass or polymeric material may also be used as a
material for this buffer layer. The latter has a much lower thermal
conductivity
than the other two mentioned materials and leads to a reduction in the
switching power of the thermo-optical switch. After the buffer layer uP has
dried, the waveguide layer W is deposited by spinning. The strip waveguides
WL1 and WL2 may be fabricated by various processes. Wet and dry etching
and photo-induced changes of the refractive index, such as photo bleaching
and photo locking may be mentioned as examples. Aside from the already
mentioned Teflon AF, the upper buffer layer oP may also be made of another
polymer material or glass or SIOx. In order to solve the task underlying the
present invention - reduced power consumption - the upper buffer layer is
structured as thin as possible and its refractive index is lower by about .2
than
the refractive index of the waveguide layer W. This rnay be ensured by the
selection of the layer materials. The process is terminated by vapor
deposition of an aluminum-gold layer of a thickness of ,22 Nm. The electrode
E provided with two electrode arms E1 and E2 is etched out of it by a wet
chemical process. The heat generated by electrical energization of the AIJAu
electrode E diffuses through the upper buffer layer oP, the waveguide layer W
and the lower buffer layer uP into the Si substrate S which also functions as
a
heat sink. The refractive index in the waveguide layer is lowered, and the
propagation constant of the waveguides is thus changed, because of the
negative temperature coefficient of the waveguide material (dnJdT = -140 x
10'~JK).
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In Fig. 3 which depicts the intensity distribution 1 of the light
propagating in waveguides WL1 and WL2 normal to the direction of
propagation is can be seen that a coupling region is formed over the
interactive length L between the two waveguides WL1 and WL2. The
coupling region may be varied by changing the refractive indices (by
energising the heating electrode and resulting temperature change in and,
therefore, change of refractive index of the waveguides). Cross and
throughput states may thus be set depending upon the coupling range.
Fig. 4 depicts the arrangement of the waveguides WL1 and WL2 with
corresponding input and output gates 1 and 2 and 1' and 2', respectively.
Light propagating in the waveguides is coupled in and out within the
interactive length L in which the two waveguides WL1 and WL2 are placed in
parallel and closely adjacent each other.
The dependence of light overcoupling from the interactive length L of
the switch at fixed waveguide form and dimension and fixed spacing between
the waveguides WL1 and WL2 is shown in Fig. 5. It may be clearly seen from
the drawing, that the deviation from the interactive length L which at the
cross-over point equals the coupling length L~, may be varied by up to 25%
towards greater lengths so that the switch in accordance with the invention
functions as a symmetric switch, i_e. the first cress-over point may be
switched also. This permissible deviation within the desired symmetric
function is advantageous as regards the fabrication of the switch in
accordance with the invention_
Figs_ 6a and 6b are cross-sectional views, similar to Fig. 2, of a
polymer-based switch in accordance with the invention. In both structures,
the inner margins of the electrode arms E1 and E2 are arranged congruently
relative to the inner margins of the waveguide arms WL1 and WL2 positioned
below them. 1f, as shown in Fig. 6a, both electrode arms E1 and E2 are
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initially simultaneously energized with the same power, the switch will
function as a symmetric coupler with identical propagation constants of the
Sight in the waveguides WL1 and WL2, and it will switch into the cross-over
state. Once cress-over state has been reached, the switching power at the
electrode arm E2 will be further increased while at the same time the
switching power at the electrode arm E1 will remain constant. The
temperature difference between the two electrode arms E1 and E2 is thus
increased. The switch will, therefore, function as an asymmetric coupler and
changes ever to its throughput state.
Figs. 7a and 7b again depict a cross-section of a polymer-based switch
in accordance with the invention, similar to Fig. 2. In this presentation, the
inner margins of the electrode arms E1 and E2 are displaced relative to the
inner margins of the waveguides WL1 and WL2 positioned below them, and
only one electrode arm E1 (in Fig. 7a) or E2 (in Fig. 7b) is energized. Owing
to the good thermal conductivity of the heating electrode E the temperature of
both electrode arms E1 and E2 will be approximately equal when a low
switching power is initially applied.
The geometric asymmetry of the electrode arms E1 and E2 relative to
the optical axis of the switch results in a more favorable heat transfer from
electrode arm E2 to the underlying waveguide WL2 than from E1 ti WL1. In
this manner, an almost symmetrical effect is obtained in respect of the two
waveguides WL1 and WL2 so that at low switching power the propagation
constants of light in WL1 and WL2 are about equal and the switch functions
as a symmetric coupler and changes over to its cross-over state. if the
switching power at the electrode arm E1 is increased the difference of the
temperatures of the two electrode arms E1 and E2 will also increase. The
asymmetric switching effect will now dominate and the couple will change
over to a throughput state. It the electrode arm E2 is energized it will have
a
significantly higher temperature than the electrode arm E1. Because of this
CA 02241828 1998-06-19
Jun 19 98 01:57p Karl Hormann, Esc. 617-491-9877 p.22
temperature difference and the geometric asymmetry of the two electrode
arms E1 and E2 the switch can only function as an asymmetric coupler and
change over to its throughput state. In this arrangement, the cross-over state
cannot be attained.
The switching action of the switch in accordance with the invention in
which the inner margins of the electrode arms E1 and E2 are displaced
relative to the inner margins of the waveguides WL1 and WL2 -as shown in
Figs. 7a and 7b- differs depending upon the position where the heat is
generated (in E1 or E2, or simultaneously in E1 and E2). To present the
switching action of the switch in accordance with the invention light of
wavelength of ~ = 1.55 pm was coupled from a laser diode into the input 1 of
waveguide WL1 or into input 2 of waveguide WL2, and the optical output
power as a function of the heating power of the corresponding electrode arm
was measured for TE and TM polarization at the output 1' of the waveguide
WL1 (throughput) or at output 2' of waveguide WL2 {cross-over state)_ Since
only a very small dependency from the polarization of the lightwave could be
detected in ali switching states (typically ~ .5 dB), only the switching
curves
for TM polarization are being shown in the following figures.
Fig. 8 depicts the switching action of the thermo-optical switch in
accordance with the invention with electrode arms E1 and E2 being displaced
relative to waveguides WL1 and WL2 when only electrode arm E1 is
energized {according to Fig. 7a, see also the inserted image). When in a
powerless state the switch, at a point slightly above the cross-over state,
will
be at an extinction ratio of ~ 10 dB. If the electrode arm E1 is energized,
the
switch will first change to a cross-over state reaching an extinction ratio of
-42
dB at 3.1 mW for TM polarization. Cross-talk of < - 30 dB can be guaranteed
within an interval of t 13 % around this operating point. if the heating power
at electrode arm E2 is increased the switch will change to state with a
throughput extinction ratio -45 dB at 10.7 mW. In order again to guarantee
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cross-talk of < -30 dB in this switching state the heating power may deviate
by
about t S % in respect of the actual operating point. Switching time is less
than '! ms.
Fig. 9 depicts the switching action of a switch having the displaced
arrangement of the electrode arms E1 and E2 relative to the two waveguides
WL1 and WL2 shown in Fig. 7b and the inserted image, with only electrode
arm E2 being energized. As may be seen in Fig. 9, such an arrangement
does not reach the cress-ever point and cannot be operated as a symmetric
switch. The power needed for this switch to change over to the throughput
state is about 14 mW. In this arrangement of the electrode arms relative to
the waveguldes and the energization of electrode arm E2, the effect of the
asymmetric arrangement of the electrode arms E1 and E2 relative to the
waveguides WL1 and WL2 dominates, as may be expected.
If both electrode arms E1 and E2 are heated, the switching action
depicted in Fig. 10 will ensue with a switch structured as shown in the
inserted image: The electrode arm E2 is energized with a constant power PEZ
of 4.5 mW, and at the same time the heating power EE, of electrode arm E1 is
increased for measuring the switching action. In this manner it was found that
the effect of the power energizing the electrode arm E2 has to be
compensated by applying an qual power to the electrode arm E1 in order to
reach the cross-over point. The energization of both electrode arms thus
requires greater heating power.
Fig. 11 depicts the switching state of energizing the electrode arm E2
-as depicted in the inserted image- at a constant power P~z = 4.5 mW while
increasing the heating power PE, of the electrode arm E1 at the same time.
By comparison with Fig. 9 it may be seen that when energizing the electrode
arm E2 with only low heating power P~2 the switch structured as shown in Fig.
7a, b will change to the cross-over state since the electrode arm E2 is
already
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Jun 19 98 01:58p Karl Hormann, Es9. 617-491-8877 p.24
pre-heated. If the heating power at the electrode arm E2 is increased the
switch will quickly reach it throughput state because of the effective
additional
geometric asymmetry. The effect of pre-heating the electrode arm E1 is
equivalent to canceling the geometric asymmetry of the position of the
electrode arms E1 and E2 with respect to the waveguides WL1 and WL2.
Flexible structuring and fabrication of the thermo-optical switch in
accordance
with the invention is possible by knowledge of the switching action in
different
-but simultaneous- states of energization of the electrode arms E1 and E2.
Fig 12 is a schematic top elevation of a polymer-based thermo-optical
switch in accordance with the invention in which a pair of lamellate electrode
arms E'1, E'2 and E"1, E"2 are arranged symmetrically relative to a common
web G (H-shaped electrode configuration). In such an arrangement a ~(3
directional coupler may be realized by the simultaneous electric energization
of one electrode arm of an electrode arm pair with the mirror-symmetrically
arranged electrode arm of the other pair of electrode arms, i.e. E'1 and E"2
or
E'2 and E"1. This leads to the creation of a temperature gradient in the two
arms of each electrode arm pairs E'1 and E'2 or E"1 and E"2, the gradient in
one pair being opposite the gradient in the other pair. Because of the thermo-
optical effect different propagation velocities of the light may be set by the
two
arms E"1 and E'2 or E"1 and E"2 of an electrode pair in the sections of the
waveguides WL1 and WL2 positioned below the electrodes. The differences
may be of equal value but opposite signs or, if two energizing sources are
used, the ~~i may be different.
This embodiment makes it possible to enlarge the permissible
deviations of the interactive length L depicted in Fig. 5, in its dimension
toward greater interactive lengths as well as to smaller -but equal in terms
of
value- interactive lengths. Within the enlarged range of the wavelength L
about point L = L~, where L~ is the coupling length at the cross-over point,
the
switch functions as a symmetric switch and may change over into the first
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cross-over point. It may also be seen in Fig. 5 that where the interactive
length L differs from the coupling length L~, cross-talk deteriorates. The
embodiment shown in Fig. 12 and Fig. 13 makes it possible and guarantees
precise setting of cross-talk like a precisely matched value L = L~ by
combining the directional coupler arrangement having an alternating ~~3 with
the described variants for creating a geometric or material-specific asymmetry
of the switch in accordance with the invention relative to its optical axis.
The advantages already mentioned are also obtained in Fig. 13, which
in contrast to Fig. 12, depicts two pairs of lamellate electrode arms with the
arms of each pair being connected by a common web. The two electrode
arms E'1 and E'2 of one of the electrode pairs are connected to each other by
the web G' and the two electrode arms E"1 and E"2 of the other electrode arm
pair are connected to each other by the web G" (double-U-shaped electrode
configuration). The effect of the switch described in Fig. 12 is still further
enhanced because heat exchange between electrode arms E'1 and E'2 or
E"1 and En2 of an electrode pair is improved and restricted to the arms of an
electrode arm pair and interaction with the other electrode arm pair is
substantially reduced because of the insulation of the two webs G' and G".
25
24
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