U.S. patent application number 10/562391 was filed with the patent office on 2006-07-13 for y-branch-based thermo-optic digital optical switches and variable optical attenuators with non-uniform heating.
Invention is credited to Louay Eldada.
Application Number | 20060153499 10/562391 |
Document ID | / |
Family ID | 34572722 |
Filed Date | 2006-07-13 |
United States Patent
Application |
20060153499 |
Kind Code |
A1 |
Eldada; Louay |
July 13, 2006 |
Y-branch-based thermo-optic digital optical switches and variable
optical attenuators with non-uniform heating
Abstract
The present invention is concerned with 1.times.2 thermo-optic
digital optical switches known in the art as "Y-branch digital
optical switches" and variable optical attenuators.
Inventors: |
Eldada; Louay; (Lexington,
MA) |
Correspondence
Address: |
Lois A Santopietro;E I Du Pont De Nemours and Company
Legal Patent Records Center
4417 Lancaster Pike
Wilmington
DE
19805
US
|
Family ID: |
34572722 |
Appl. No.: |
10/562391 |
Filed: |
July 1, 2004 |
PCT Filed: |
July 1, 2004 |
PCT NO: |
PCT/US04/21682 |
371 Date: |
December 22, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60484364 |
Jul 2, 2003 |
|
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Current U.S.
Class: |
385/45 ; 385/16;
385/22; 385/39; 385/4; 385/40; 385/42; 385/43; 385/5; 385/50 |
Current CPC
Class: |
G02F 1/3137 20130101;
G02F 1/0147 20130101; G02B 6/125 20130101; G02F 2202/022 20130101;
G02F 2201/122 20130101; G02F 2203/48 20130101 |
Class at
Publication: |
385/045 ;
385/016; 385/022; 385/039; 385/042; 385/043; 385/040; 385/050;
385/004; 385/005 |
International
Class: |
G02B 6/26 20060101
G02B006/26; G02F 1/295 20060101 G02F001/295; G02B 6/42 20060101
G02B006/42 |
Claims
1. A 1.times.2 planar optical waveguide signal splitter in the form
of a Y-branch comprising a trunk and two branches conjoined thereto
to form a vertex, said branches diverging from one another, each of
said branches having a surface, at least one of said branches being
provided with a heating means, said heating means being disposed
with respect to said at least one of said branches such that upon
activation of said heating means, a spatially non-uniform heat flux
will be incident upon said at least one of said branches.
2. The 1.times.2 planar optical waveguide signal splitter of claim
1 wherein each of said branches further comprises an outer edge and
wherein further said spatially non-uniform heat flux will be
incident preponderantly on said outer edge of said at least one of
said branches.
3. The 1.times.2 planar optical waveguide signal splitter of claim
1 wherein said vertex is characterized by an angle of
0.05-4.degree..
4. The 1.times.2 planar optical waveguide signal splitter of claim
3 wherein said vertex is characterized by an angle of
0.4-1.degree..
5. The 1.times.2 planar optical waveguide signal splitter of claim
1 wherein said heating means is of uniform cross-section.
6. The 1.times.2 planar optical waveguide signal splitter of claim
1 wherein said heating means is of non-uniform cross-section.
7. The 1.times.2 planar optical waveguide signal splitter of claim
1 further comprising a polymeric core.
8. The 1.times.2 planar optical waveguide signal splitter of claim
7 wherein said polymeric core comprises a polymer selected from the
group consisting of polyacrylates, polyfluoroacrylates,
polychloroacrylates, polymethacrylates, and polycarbonates.
9. The 1.times.2 planar optical waveguide signal splitter of claim
8 wherein the polymer is a polyfluoroacrylate.
10. The 1.times.2 planar optical waveguide signal splitter of claim
1 wherein said heating means is an electrical resistance
heater.
11. The 1.times.2 planar optical waveguide signal splitter of claim
10 wherein said electrical resistance heater is of non-uniform
cross-section.
12. The 1.times.2 planar optical waveguide signal splitter of claim
11 wherein said cross-section has a minimum area, said heater being
disposed such that the distance between said vertex and said
minimum area is a minimum.
13. A method for splitting an optical signal, the method
comprising: (a) disposing in the propagation path of a propagating
optical signal a 1.times.2 planar optical waveguide signal splitter
in the form of a Y-branch comprising a trunk and two branches
conjoined thereto to form a vertex said branches diverging from one
another, at least one of said branches being provided with a
heating means, said heating means being disposed with respect to
said at least one of said branches such that upon activation of
said heating means, a spatially non-uniform heat flux will be
incident upon said at least one of said branches; and (b)
energizing said heating means to effect the imposition of a
spatially non uniform heat-flux upon the surface of said at least
one of said branches in order to effect a rise in the temperature
of said at least one of said branches an amount sufficient to cause
a change in the relative intensity of the propagating optical
signal in the two said branches.
14. The method of claim 13 wherein each of said branches further
comprises an outer edge and wherein further said spatially
non-uniform heat flux is imposed preponderantly on said outer edge
of said at least one of said branches.
15. The method of claim 13 wherein said heating means is of
non-uniform cross-section.
16. The method of claim 13 wherein said vertex is characterized by
an angle of 0.05-4.degree..
17. The method of claim 16 wherein said vertex is characterized by
an angle of 0.4-1.degree..
18. The method of claim 13 wherein said rise in temperature is
sufficient to effect a digital optical switching function.
19. The method of claim 13 wherein said rise in temperature is
insufficient to effect a digital optical switching function, so
that said 1.times.2 planar optical waveguide signal splitter serves
as a variable optical attenuator.
20. The method of claim 13 wherein said 1.times.2 planar optical
waveguide signal splitter further comprises a polymeric core.
21. The method of claim 16 wherein said polymeric core comprises a
polymer selected from the group consisting of polyacrylates,
polyfluoroacrylates, polymethacrylates, and polycarbonates.
22. The method of claim 21 wherein the polymer is a
polyfluoroacrylate.
23. The method of claim 13 wherein said heating means is an
electrical resistance heater.
24. The method of claim 23 wherein said electrical resistance
heater is of non-uniform cross-section.
25. The method of claim 23 wherein the highest heat flux is imposed
at a minimum distance from said vertex.
26. A digital optical spatial switch comprising the 1.times.2
planar optical waveguide signal splitter of claim 1.
27. A variable optical attenuator comprising the 1.times.2 planar
optical waveguide signal splitter of claim 1.
Description
FIELD OF THE INVENTION
[0001] The present invention is concerned with optical
communications technology. More specifically, the present invention
is concerned with 1.times.2 thermo-optic digital optical switches
known in the art as "Y-branch digital optical switches" and
variable optical attenuators.
TECHNICAL BACKGROUND
[0002] Digital optical spatial switches (DOS) are well known in the
art. One class of such switches is the 1.times.2 digital optical
switch known as a "Y-branch digital optical switch" (Y-branch DOS)
wherein light input into the "base" or trunk of the Y is directed
through one or the other of the output branches by virtue of
changes effected in the refractive index of one or both of the
output branches. The switch can also be operated in reverse,
whereby one or the other "upper branches" of the Y can be selected
as an input channel with the base of the Y being the output
channel. Y-branches are a fundamental building block of optical
circuitry, and may be employed singly, or in various combinations
to form more complex switching and coupling devices.
[0003] The Y-branch DOS has received wide commercial acceptance
primarily because of its robustness to variations in critical
parameters such as electrical power applied, polarization,
wavelength, temperature, and to a large extent, even device
geometrical variations. Typically a Y-branch DOS is designed such
that two waveguide branches intersect to define a Y-shape structure
with a very small angle at the intersection of the branches. The
composition of the waveguide structure may include a wide variety
of materials such as lithium niobate, semiconductors, silica, or
polymers. A Y-branch DOS performs its switching function by
adiabatically changing (i.e. slowly varying, as opposed to abruptly
altering) the light propagation direction in one of the output
waveguides.
[0004] Specifically, switching in a Y-branch DOS is achieved by
forcing a refractive index change in one waveguide branch with
respect to the other. The change in refractive index may be induced
by applying for example voltage and/or current to selected sections
of the structure. Of particular significance among the
characteristics of a Y-branch DOS is its step-like response to
applied voltage or current, which allow the light to remain in a
higher index branch, notwithstanding an increase in the applied
voltage or current beyond the switching threshold. When a Y-branch
DOS operates above the switching threshold, variations in
polarization and wavelength do not impact significantly the
switching capacity of the Y-branch DOS.
[0005] One persistent problem presented to the designer by the
Y-branches of the art is footprint. In order to effect the
adiabatic transfer of energy, known in the art as adiabatic modal
transfer (AMT), of the propagating wave into the single output
channel selected, it is necessary to maintain a separation of the
two output branches of no greater than ca. 30 times the wavelength
of the propagating signal for a silica-fiber-level of refractive
index difference in the waveguide of ca. 0.5% of the base index.
For 1.5-micron radiation, this means that the separation between
the two branches must be maintained at a distance on the order of
45 micrometers or less until the energy transfer is complete. This
requirement in turn necessitates very small vertex angles on the
order of 0.1 to 0.3 degrees and device lengths up to 30 mm.
Controlled fabrication of such devices is quite difficult and error
prone. Furthermore, the large footprint of such devices greatly
limits their applicability in integrated optical circuitry.
[0006] One approach to addressing these problems is provided by
Okayama et al, J. Lightwave Tech. 11 (2), 379-387 (1983), in which
a two angle shaped Y-branch DOS wherein the output waveguides
initially diverge by an angle of ca. 2.degree. and then undergo a
bend to a smaller extrapolated angle of divergence of ca.
0.3.degree..
[0007] Several methods are known in the art for effecting the
desired change in refractive index. These involve the electro-optic
effect, the stress-optic effect and the thermo-optic effect. In a
typical Y-branch thermo-optic DOS known in the art, the two upper
branches of the "Y" are provided with a heating means, typically a
thin layer of metal deposited thereupon, which heating means when
activated induces a shift in the refractive index of the
corresponding branch, thereby effecting a coupling of power input
to the base of the "Y" to one or the other branches. By turning on
the heating of one branch and turning off the heating of the other
branch, switching of incoming optical signals can be effected.
[0008] Both polymeric and glass Y-branches are known. Because of
the much larger temperature dependence of the refractive index of
polymers, polymers are preferred for use in thermo-optic digital
optical switches.
[0009] Hida et al, IEEE Photonics Technology Letters 5 (7), 782-784
(1997), disclose polymeric 2.times.2 thermo-optic switches
consisting of two coupled Y-branches fabricated from deuterated and
fluoro-deuterated methacrylate polymers. The method of fabrication
involves spin coating polymer solutions onto a silicon substrate
followed by forming the Y-shaped components by conventional
photolithography, the core ridges being subsequently formed by
reactive ion etching. Chromium thin-film strip heaters were formed
on the upper Y-branches by electron beam evaporation and wet
etching. Separation of the arms was 250 micrometers. The Cr heater
strips were 5 mm long and 50 micrometers wide.
[0010] Eldada et al, Proc. SPIE, vol. 3950, pp. 78-89 (2000),
discloses 1.times.2 optical switches fabricated from polymeric
materials which Y-branches exhibit 0.1 dB insertion loss for vertex
angles of less than 2.degree.. The direct photolithographic
fabrication method using halogenated acrylates as practiced therein
is disclosed to enable sharp profiles of the components and the
removal of residue even at the vertex of relatively small angle
Y-branches.
[0011] Lackritz et al, U.S. Pat. No. 6,236,774B1, discloses
thermo-optic switches employing cross-linked polymeric waveguides
operated above T.sub.g. Disclosed are metallic heaters
substantially rectangular in shape disposed upon a polymeric
optical waveguide surface, the long side of said rectangular heater
being positioned at a slight angle to the direction of propagation
in the waveguide. Said heaters are positioned to be in uniform
thermal contact with waveguide material over the entire area of the
heater. It is disclosed that the temperature, and therefore the
refractive index, of the polymer waveguide material will depend
upon the distance of any point therein from the heater, those
regions closest to the heater experiencing greater temperature than
those further away.
[0012] He et al, U.S. Pat. No. 6,526,193 B1, discloses the
electro-optic effect in a Y-branch digital optical switch having
curved output waveguides provided with curved electrodes to provide
a shorter device than achievable in the earlier art which employed
diverging straight waveguide sections. The curvature of the output
waveguides provides a continuously increasing angle of
divergence.
[0013] Lee et al, U.S. Pat. No. 5,623,566, is drawn to thermally
induced guides in silicon optical benches. Disclosed in FIG. 2
thereof is the temperature profile through the various optical
materials employed therein as a result of localized heating applied
thereto.
[0014] Moosburger et al, Proc. 21.sup.st Eur. Conf. on Opt. Comm,
pp. 1063-1067 (1995) disclose Y-branches with "near perfect"
vertices having an angle of 0.12.degree. fabricated from
silica-clad polymeric waveguides having cores of ca. 9 micrometers.
The upper branches of the Y were coated with Ti thin film heaters.
27 dB cross-talk suppression was achieved between output branches
with heater power of ca. 180 mW. Moosburger expressly teaches that
blunted vertices induce losses and reduce the crosstalk suppression
between output waveguides.
[0015] Diemeer, Optical Materials 9, 192-200 (1998) provides a
thoroughgoing analysis of the thermal transport and physical
aspects of thermo-optic switching in polymeric vs. silica
thermo-optic digital optical switches. For polymers in general, and
polycarbonate and polymethylmethacrylate in particular, it is shown
that switching power lies in the range of 50-100 mW, and that a
temperature rise of ca. 10.degree. C. in the waveguide core is
necessary to achieve a minimum refractive index dfference of ca.
0.001.
SUMMARY OF THE INVENTION
[0016] The present invention provides for a 1.times.2 planar
optical waveguide signal splitter in the form of a Y-branch
comprising a trunk and two branches conjoined thereto to form a
vertex, said branches diverging from one another, each of said
branches having a surface, at least one of said branches being
provided with a heating means, said heating means being disposed
with respect to the at least one of said branches such that upon
activation of said heating means, a spatially non-uniform heat flux
will be incident upon said at least one of said branches.
[0017] Further provided in the present invention is a method for
effecting an optical switching function, the method comprising:
[0018] (a) disposing in the propagation path of a propagating
optical signal a 1.times.2 planar optical waveguide signal splitter
in the form of a Y-branch comprising a trunk and two branches
conjoined thereto to form a vertex, said branches diverging from
one another, at least one of said branches being provided with a
heating means, said heating means being disposed with respect to
the at least one of said branches such that upon activation of said
heating means, a spatially non-uniform heat flux will be incident
upon said at least one of said branches;
[0019] (b) energizing said heating means to effect the imposition
of a spatially non uniform heat-flux upon the surface of said at
least one of said branches in order to raise the temperature of
said at least one of said branches an amount sufficient to cause a
change in the relative intensity of the propagating optical signal
in the two said branches.
BRIEF DESCRIPTION OF THE FIGURES
[0020] FIGS. 1a-1c depict some, but not all, designs for Y-branches
which are suitable for the practice of the present invention.
[0021] FIGS. 2a and 2b depict schematically one of many possible
embodiments of a Y-branch of the art and one of many possible
embodiments of a Y-branch of the present invention,
respectively.
[0022] FIGS. 3a and 3b depict schematically a 112 Y-branch
8.times.8 optical switching device of the art and a 112 Y-branch
8.times.8 optical switching device of the present invention,
respectively.
[0023] FIGS. 4a and 4b depict two of many possible embodiments of
the present invention wherein the heaters are of substantially
uniform cross-section.
[0024] FIGS. 5a and 5b depict two of many possible specific
embodiments of the most preferred embodiment of the present
invention wherein the heaters are of non-uniform cross-section and
the heaters are disposed along the outer edge of the branch.
[0025] FIG. 6 is a graph of digital optical switch
characteristics.
DETAILED DESCRIPTION OF THE INVENTION
[0026] The teachings of the art are clearly directed at employing
the thermo-optic effect to effect switching by the imposition of an
essentially spatially uniform heat flux to the surface of a planar
optical waveguide, the heat thereby imposed inducing an increase in
the temperature of the waveguide, thereby in turn causing a change
in the refractive index, thereby in turn a change in the relative
intensity of the optical signals propagating in the two branches of
the Y. In particular, as taught in the art, when a certain
temperature threshold is passed, the change in refractive index in
the heated branch will be sufficient to effect an essentially
complete shift of the optical signal into one or the other branch.
In the practice of the present invention the cross-talk suppression
between the two branches when the Y-branch of the invention is in
the digital switching mode is at least 15 dB, preferably at least
20 dB, most preferably at least 25 dB.
[0027] Which branch is turned "on" and which "off" upon the
application of heating depends upon the thermo-optic coefficient,
dn/dT. In the case of inorganic glasses, most notably silica, dn/dT
is positive, such that heating in the silica Y-branch results in
switching of the propagating signal into the heated branch. Organic
polymers exhibit a negative dn/dT so that heating in a polymeric
Y-branch results in switching of the propagating signal into the
unheated branch.
[0028] It is known in the art that the absolute magnitude of dn/dT
for organic polymers exceeds that for inorganic glasses by ca. an
order of magnitude. For this reason, organic polymers are highly
preferred in thermo-optic switches. The description of the present
invention is directed to the preferred organic polymer embodiments
hereof. However, also contemplated within the scope of the present
invention are Y-branches made of inorganic materials such as
semiconductors and inorganic glasses, especially silica. One of
skill in the art will appreciate that the same discussion will be
Y-branches fabricated from inorganic materials keeping in mind the
difference in sign and magnitude of dn/dT.
[0029] For the purpose of succinctness, the discussion herein will
treat the embodiment of the present invention in which an optical
signal is input at the bottom of the Y and is then switched to one
or the other of the branches in the upper part of the Y according
to the present invention, the upper branches serving as alternative
output branches. One of skill in the art will appreciate that the
direction of propagation of the optical signal can be reversed
without alteration of the central aspect of the present invention,
namely the use of a spatially non-uniform heating profile to effect
the necessary temperature change for switching to occur.
[0030] In the reverse embodiment, light is input into one or the
other upper branches of the Y, and the thermo-optic effect as
described herein is employed to determine which branch of the Y
will permit propagation of its input signal into the output trunk
at the bottom of the Y.
[0031] The present invention further encompasses those embodiments
in which neither output branch is sufficiently heated to effect
complete switching of the incident optical power to one or the
other output branches, but instead achieves a continuously variable
intermediate state in which the ratio of optical power propagating
in the two branches may be varied continuously between the extremes
of "on" and "off," thereby providing not a digital optical switch
but rather a variable optical attenuator.
[0032] The present invention is equally applicable to Y-branches of
many varying designs. Suitable Y-branch designs include but are not
limited to Y-branches having straight output waveguides diverging
at a fixed angle from a well-defined vertex having an angle,
.alpha., as shown in FIG. 1a; Y-branches having outwardly curved
output branches as shown in FIG. 1b; Y-branches having a blunted
vertex, FIG. 1c, wherein the output waveguides initially diverge in
a first straight section from a vertex at an angle, .beta., and
then undergo a slight bend, which may be on the inside edge only as
shown, or on both inside and outside edges, to form second straight
section such that the second straight sections of the output
waveguides diverge with a smaller angle, .gamma.. In preferred
embodiments, .alpha. is 0.05-0.4 preferably 0.1-0.2.degree., .beta.
is 0.2-4.0 preferably 0.4-1.0.degree., and .gamma. is 0.05-0.4
preferably 0.1-0.2.degree.. In still another embodiment, the output
waveguides may be curved, with the angle of divergence in one
embodiment decreasing essentially logarithmically with distance
from the vertex. In a limiting embodiment of the Y-branch depicted
in FIG. 1c, .beta. may be 180.degree..
[0033] The Y-branches suitable for the practice of the invention
may be prepared according to any of the well-known processes in the
art. Particularly beneficial is the direct photolithographic
process applied to a suitably transparent photoresistive polymer,
as described in L. Eldada, Opt. Eng. 40, 1165 (2001). Because the
photoresist polymer itself is utilized to form the waveguide, no
reactive ion etching step is required as in the other processes of
the art. Reactive ion etching is known to cause striations in the
waveguide walls which lead to an aggravation of scattering loss and
polarization dependent loss (PDL). Scattering loss and PDL are
minimized by employing direct photolithography. The resultant
waveguide according to the process taught by Eldada is a graded
index polymeric waveguide which can reduce scattering loss and PDL
in the propagating signal.
[0034] The inventor hereof has determined surprisingly that when a
spatially non-uniform heat flux is imposed upon the surface of one
or the other of the output branches according to the present
invention, AMT may be achieved over a shorter distance than is
achievable according to the teachings of the art. An embodiment of
the known art which represents typical practice is shown in FIG. 2a
wherein a Y-branch intended for use at 1.55 micrometers incident
light undergoes spatially uniform heating to achieve switching. In
order to maintain the necessary separation between the branches at
less than 45 micrometers for a length sufficient to effect complete
AMT requires a heated length of ca. 13 mm for the output branches.
By contrast, according to the present invention, the heated length
of the output branches may be in the range of as little as 1-3 mm.
Several important benefits accrue therefrom. One benefit is reduced
footprint as illustrated in FIGS. 2 and 3.
[0035] FIG. 2a depicts a Y-branch typical of that in the art. The
heater length alone is 13 mm. The much smaller Y-branch of the
invention depicted in FIG. 2b has a heater length of only 2.2 mm.
FIG. 3 provides a real-world illustration of the improvement
derived from the shorter footprint. FIG. 3a depicts a design for a
112 switch 8.times.8 switch array utilizing the Y-branch of the
art. FIG. 3b depicts the same architecture but utilizing the
Y-branch of the present invention, with a footprint half the size
of that of the art.
[0036] A further benefit of the present invention is that because
the heated length can be shorter, the vertex angle can be as large
as 10-15.degree., ca. 10 times the angles taught in the art. Actual
practice in fabricating Y-branches according to the teachings of
the art normally results in the creation of un-etched material at
the vertex, which greatly aggravates PDL in the propagated signal.
As emphasized in Mossburger et al, op.cit., having a "perfect"
vertex is critical to performance. With the larger vertex angles of
the present invention, it is much easier to achieve sharp
definition of the vertex and control of the shape with concomitant
reduction in PDL.
[0037] One particularly surprising aspect of the present invention
is that contrary to the teachings of Moosburger, a blunted vertex
such as that depicted in FIG. 1c can result in reduced PDL with no
significant light loss.
[0038] One of skill in the art will appreciate that the heating
profile of choice will depend upon many factors including the
specific choice of materials from which the waveguide is
fabricated, the architecture of the waveguide, whether or not there
is an intervening layer of cladding between heater and core, the
wavelength of the propagating signal, the rapidity of switching
desired, whether or not it is desired to employ the device partly
or exclusively as a variable optical attenuator, and so forth.
[0039] It is desirable in the practice of the invention that the
heating profile be a smoothly varying function of position on the
surface of the waveguide being heated. Sharp discontinuities in
heat flux must be avoided in order to avoid sudden changes in
refractive index in the waveguide, which can result in mode
matching losses.
[0040] In the practice of the present invention it has been found
that power levels of 10 to 50 mW are effective in causing switching
in polymers characterized by dn/dT in the range of -2 to
-5.times.10.sup.-4/.degree. C. Examples of such polymers include
but are not limited to polyacrylates, polyfluoroacrylates, and
polychloroacrylates. Power levels of 30-150 mW are found to be
effective with polymers characterized by dn/dT of -0.5 to less than
-2.times.10.sup.-4/.degree. C in absolute value. Examples of such
polymers include but are not limited to polycarbonate and
polymethylmethacrylate. In a typical embodiment of the present
invention, the entire waveguide is made from the same polymeric
system.
[0041] In the practice of the present invention, temperature
increases in the core may range from 10 to 100.degree. C. The high
temperatures are generally narrowly localized, and are beneficially
achieved by use of a heater having a relatively narrow "neck" area.
As will be understood by one of skill in the art, resistance in a
metallic conductor increases inversely with cross-sectional
area.
Thus for a given power input to the heater, the localized heating
in the narrow cross-section will be highest.
[0042] The present invention is operable with any convenient
heating means such as is known in the art. This can include
induction heating, radiative heating, and electrical resistive
heating. From the standpoint of simplicity of implementation,
electrical resistive heating is preferred. Electrical resistive
heating may be accomplished according to means wellknown in the
art. In one approach, a thin metal strip is sputter coated onto the
waveguide using a mask to prepare the heater shape desired. When
the heater is not disposed on the surface of the output branch over
its entire length, it is deposited on top of that part of the
overclad on the chip where there is no core underneath.
[0043] Alternatively, the heater may be formed by evaporation or
any other metal deposition process.
[0044] Suitable metals for electrical resistive heating according
to the present invention include but are not limited to chromium,
titanium, aluminum, nickel, gold, platinum. Preferred are chromium,
titanium, nickel, and gold. The spatially non-uniform heating
according to the present invention may be accomplished by applying
a thin film heating strip of variable cross-sectional area onto the
waveguide surface at any convenient location thereupon
[0045] The present invention places no specific limits on the
manner in which spatially non-uniform heating may be accomplished.
In one embodiment of the present invention, spatially non-uniform
heating is beneficially accomplished by employing a heating means
of uniform design which is disposed with respect to the waveguide
in a manner which results in the imposition of a non-uniform heat
flux onto the waveguide surface. Illustrative of this embodiment
are the configurations shown in FIG. 4. One of skill in the art
will appreciate that many other embodiments of the same generic
nature. Such embodiments include but are not limited to rectangular
heaters with curved waveguides, rectangular heaters with straight
waveguides, curved heaters with straight waveguides, and curved or
rectangular heaters with straight waveguides wherein the vertex of
said Y-branch is blunted.
[0046] FIG. 4 depicts an embodiment of the invention wherein the
output waveguide is curved, the heater is uniform in design, and
the heat-flux delivered to the waveguide surface is a smoothly
continuous function of the proximity of the heater to the waveguide
surface. In this case, the greatest amount of heating occurs at the
point farthest from the vertex. Two slightly different embodiments
are depicted, differing by the slightly different shapes of the
heaters.
[0047] One of skill in the art will appreciate that any of the
Y-branch designs depicted in FIGS. 1a-1c, and the others as
described hereinabove, can be substituted for the Y-branch depicted
in FIGS. 4a and 4b with no loss of effectiveness in the practice of
the present invention.
[0048] In a preferred embodiment of the present invention, a heater
wherein the cross-sectional area thereof is not constant is
disposed along the length of the output waveguides. In this
embodiment, a non-uniform heat flux is imposed upon the waveguide
surface by virtue of the higher temperatures realized in the
portions of the heater having smaller cross-sectional areas. In a
more preferred embodiment, the heater is in the shape of a bow-tie
wherein a rectangular portion gives way at each end in the longer
dimension to a triangular portion, the two triangular portions
being joined at their truncated apexes. In this embodiment, the
heat flux incident upon the surface of the waveguide increases
continuously along the long dimension of the heater as the
cross-sectional area narrows until it reaches a peak at the
narrowest point, and then continuously decreases with increasing
distance from the narrowest point along the long dimension of the
waveguide. In the most preferred embodiment hereof, the narrowest
portion of the heater is disposed in close proximity to the vertex,
thereby subjecting the region of the vertex to the highest
temperature. Because of the continuous nature of the effect herein
realized, one of skill in the art will understand that a small
positional deviation of the narrowest point on the heater from the
closest point of approach to the vertex will have little effect on
the practive of the present invention.
[0049] The most preferred embodiment is depicted in FIGS. 5a and 5b
wherein two slightly different bow-tie designs are placed along the
straight output waveguides wherein the Y-branch depicted is that in
FIG. 1c wherein the angle .beta. is 0.2-4.0.degree. preferably
0.4-1.0.degree..
[0050] One of skill in the art will appreciate that a further
embodiment will encompass both spatial separation of the heater and
the waveguide and a heater of non-uniform cross-section. There is
no limit according to the present invention to the possible heater
designs, the number of ways the heater can be disposed with respect
to the waveguide, or the combinations thereof with each other in
order to practice the present invention.
[0051] One of skill in the art will further appreciate that the
requisite heating profile of the invention may be obtained by
employing a plurality of individually uniform heaters along the
length of the output branch, at least two of said heaters being
heated to different temperatures. This embodiment is however less
preferred because of the multiplicity of wires and controllers
which would be required for its implementation.
[0052] Placement of the heaters according to the present invention
may have a significant impact on the operability of the present
invention. If a heater is positioned on the top surface of an
output branch, it will be in very close proximity to the other
output branch, and it is highly likely that some undesirable degree
of heating will occur in that output branch which is not intended
to be heated. For this reason it is highly desirable that the heat
flux from the heaters be directed to the outside edge of the
respective branches in order to place as much as possible of the
thermally insulating waveguide between the heated surface and the
adjacent waveguide. Thus lateral positioning of the heater is an
important consideration.
[0053] The Y-branch of the invention may be beneficially employed
not only as a digital optical spatial switch but as a variable
optical attenuator (VOA). This is accomplished by heating the
output branches to temperatures below the threshold temperature for
switching. In the operation of the VOA according to the present
invention, the degree of transfer of power from one branch to the
other is continuously varied by continuously varying the heat input
until the stage at which essentially all the optical power is
transferred to one output waveguide, after which further heating
has no effect--the digital switching region. When the Y-branch is
operated as a VOA, a first arm is heated to achieve attenuation up
to 3 dB or 50% (in polymer, said first arm is not the output arm),
and the second arm is heated to achieve attenuation above 3 dB (in
polymer, said second arm is the output arm). The heat may then be
subject to small adjustments to alter the relative intensity of
propagation in the two branches.
EXAMPLES
Example 1
[0054] In this Example, the following terms are employed:
[0055] The composition designated B3 was prepared by combining 94%
by weight of ethoxylated perfluoropolyether diacrylate (MW1100), 4%
by weight of di-trimethylolpropane tetraacrylate, and 2% by weight
of Darocur 1173, a photoinitiator available from Ciba-Geigy.
[0056] The composition designated BF3 was prepared by combining 98%
by weight of ethoxylated perfluoropolyether diacrylate (MW1100) and
2% by weight of Darocur 1173.
[0057] The composition designated C3 was prepared by combining 91%
by weight of ethoxylated perfluoropolyether diacrylate (MW1100),
6.5% by weight of di-trimethylolpropane tetra-acrylate, 2% by
weight of Darocur1173, and 0.5% by weight of Darocur 4265 a
different photoinitiator available from Ciba Geigy.
[0058] The following fabrication process was performed twice, once
with mask designated P03, once with mask designated P05 (see FIG. 2
for structures of each mask).
[0059] A 6-inch oxidized silicon wafer (substrate) was cleaned with
KOH, then treated with (3-acryloxypropyl)trichlorosilane (Gelest).
Polymer waveguides were formed on the wafer using negative-tone
photosensitive monomers in the following way: 2 ml. of the BF-3
composition was deposited on the wafer which was then spun on a
CEE-100 spin-coater (Brewer Scientific) at 1000 rpm for 13 seconds
to form a 10-.mu.m-thick BF3 underclad layer. The coating so
prepared was cured with 350 nm UV light.(the Hg-i line) from a 1000
Waft mercury arc lamp in a nitrogen atmosphere and a pressure of
0.02 torr. A C3 core layer was deposited in a similar manner and
7-.mu.m.times.7-.mu.m-cross-section Y-branched waveguides were
patterned in it by shining 350 nm UV light (the Hg-i line) from a
1000 Watt mercury arc lamp through a dark-field photomask then
developing the unexposed region with ethyl acetate (HPLC grade,
Fisher Scientific Co.). In the Y-branches so prepared, shown
schematically in FIG. 2b, the initial angle of divergence of the
branches was 0.6.degree.; the branches curved outwards to an angle
of 1.5.degree. at a branch separation of 45 micrometers where the
heaters ended. 3 ml of the B3 composition was placed on the
patterned wafer and spin coated at 700 rpm for 14 second on the
CEE-100 to form a 17 micrometer-thick B3 overclad layer. The coated
layer so prepared was cured using the Hg-i line from the 1000 Watt
mercury arc lamp in a nitrogen atmosphere at atmospheric
pressure.
[0060] Heaters were formed by sputter-depositing onto the
polymer-coated wafer successive layers of Cr and Au that were at
respective thicknesses of 10/200 nanometers to form a two-layer
heater stack having an over-thickness of 210 nm]. Said heater stack
was patterned photolithographically using positive photoresist
(Type 1808 available from Shipley) and a clear-field photomask
followed by acid etching to form heaters and the base of
interconnects/wire bonding pads. An electroplating base of Ti/Au
was sputter-deposited on top of the heater stack and a positive
photoresist (Shipley SJR5740) applied thereto by spin-coating.
UV-exposure with a dark-field photomask exposed the base of
interconnects and wire bonding pads. Electroplating with gold was
then performed. Finally said positive photoresist was developed and
said electroplating base was acid etched, resulting in a wafer
populated with chips having thermo-optic Y-branch-based variable
optical attenuators (VOA's).
[0061] Each of wafers P03 and P05 was diced. P03 chips were 20-mm
long and P05 chips were 11-mm long. One specimen of each of the
so-prepared Y-branch thermo-optic devices was evaluated as a VOA.
The P03 device had the following performance characteristics:
insertion loss at minimum attenuation=1.0 dB,
polarization-dependent loss (PDL) at 15 dB attenuation=0.5 dB. The
P05 device had the following performance characteristics: insertion
loss at minimum attenuation=0.65 dB, polarization-dependent loss
(PDL) at 15 dB attenuation=0.2 dB.
Example 2
[0062] A second Y-branch specimen, prepared in a manner identical
to that in Example 1, was evaluated according to the following
protocol: The measurement was done in 2 steps:
[0063] 1. 1.55 micrometer wavelength light was coupled from a glass
optical fiber input into the Y-branch trunk and after traversing
the device the light was couple to a glass optical fiber at the
output of the `right` branch and was sent to a photodetector.
Electrical power was applied to the left branch heater and was
changed continuously from 50 mW to 0 mW, then electrical power was
applied to the right heater and was changed continuously from 0 mW
to 50 mW. The optical power attenuation measured at the
photodetector is indicated by the blue line in FIG. 6.
[0064] 2. Light was launched as in the preceding paragraph. but was
coupled to a photodetector at the output of the `left` branch
Electrical power was applied to the right heater and was changed
continuously from 50 mW to 0 mW, then electrical power was applied
to the left heater and was changed continuously from 0 mW to 50 mW.
The optical power attenuation measured at the photodetector is
indicated by the green line in FIG. 6.
* * * * *