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What is claimed is:
1. A microelectromechanical photonic switching array, comprising:
a plurality of first optical waveguides;
a plurality of second optical waveguides;
insulative cladding between the first and second pluralities of waveguides,
said insulative cladding having at least one patterned opening therein and
having a lower refractive index than the refractive indices of the
waveguides of said first and second pluralities, wherein each one of said
first waveguides is positioned with predetermined first waveguide portions
on one side of said at least one opening and each one of said second
waveguides is positioned with predetermined second waveguide portions
substantially parallel to respective ones of said predetermined first
waveguide portions on an opposing side of said at least one opening; and
means for moving a selected one of said predetermined second waveguide
portions closer to a respective one of said predetermined first waveguide
portions in response to an actuation force.
2. The switching array of claim 1, further including a silicon substrate
supporting said first and second pluralities of waveguides.
3. The switching array of claim 1, wherein one of said first and second
pluralities of waveguides comprises serpentine-shaped waveguides.
4. The switching array of claim 1, wherein one of said first and second
pluralities of waveguides comprises substantially straight waveguides and
the other of said pluralities of first and second waveguides comprises
waveguides having bends therein of about ninety degrees.
5. The switching array of claim 4, wherein each respective bend of about
ninety degrees includes a reflective joint at about a forty-five degree
angle with respect to each of the waveguide portions of said respective
bend.
6. The switching array of claim 1, wherein said means for moving comprises:
a plurality of first electrodes, each of said first electrodes being
situated on a respective portion of said one side of said at least one
opening;
an insulator layer situated between each of said first electrodes and each
respective portion of said at least one opening; and
a plurality of second electrodes, each of said second electrodes being
situated on a respective portion of said opposing side of said at least
one opening.
7. The switching array of claim 6, wherein the waveguides of said first and
second pluralities are comprised of a polyimide material and wherein said
first and second electrodes are comprised of a material which is at least
partially transparent.
8. The switching array of claim 7, wherein said insulator layer comprises
silicon dioxide.
9. The switching array of claim 1, wherein said means for moving comprises:
a first cladding layer facing the plurality of said first waveguides and
said insulative cladding, said first cladding layer having a lower
refractive index than that of the waveguides of said first plurality;
a second cladding layer facing the plurality of said second waveguides and
said insulative cladding, said second cladding layer having a lower
refractive index than the waveguides of said second plurality;
a plurality of first electrodes, each of said first electrodes being
situated over a portion of said second cladding layer situated above a
respective portion of said at least one opening;
a plurality of piezoelectric strip lines, each of said piezoelectric strip
lines being situated between a respective first electrode and said first
cladding layer; and
a plurality of second electrodes, each of said second electrodes being
situated between a respective portion of said second cladding layer and a
respective portion of said at least one opening.
10. The switching array of claim 9, wherein the waveguides of said first
and second pluralities are comprised of a polyimide material, said second
electrodes are comprised of a material which is at least partially
transparent, and said first electrodes are selected from the group
consisting of of aluminum, platinum, and chromium.
11. The switching array of claim 10, wherein said first and second cladding
layers are comprised of a polyimide and wherein said piezoelectric strip
lines are comprised of lead zirconate titanate.
12. A microelectromechanical photonic switching array, comprising:
a plurality of first optical waveguides;
a plurality of second optical waveguides, each of said second waveguides
intersecting respective portions of each of said first waveguides at
respective intersection points;
a plurality of waveguide couplers, each of said waveguide couplers having
one waveguide coupler portion situated substantially parallel to a first
coupler portion of a respective one of said first waveguides and another
waveguide coupler portion situated substantially parallel to a second
coupler portion of a respective one of said second waveguides, said first
and second coupler portions forming cross-over portions;
means for distancing said waveguide couplers and the first and second
pluralities of waveguides, said means for distancing including at least
one patterned opening therein, wherein each one of said cross-over
portions is situated on one side of said at least one opening and each one
of said waveguide couplers is situated on an opposing side of said at
least one opening; and
means for moving a predetermined one of said plurality of waveguide
couplers closer to a respective intersection point in response to a force
of the group consisting of electrostatically-generated and
piezoelectrically-generated forces.
13. The switching array of claim 12, wherein each of said waveguide
couplers includes a bend of approximately ninety degrees and each of said
bends includes a reflective joint at about a forty-five degree angle with
respect to each of the waveguide portions of said respective bend.
14. The switching array of claim 12, wherein said means for moving
comprises:
a plurality of first electrodes, each of said first electrodes being
situated on a respective portion of said one side of said at least one
opening; and
a plurality of second electrodes, each of said second electrodes being
situated on a respective portion of said opposing side of said at least
one opening.
15. The switching array of claim 14, further including an insulator layer
situated between each of said first electrodes and each one of said
respective portions of said one side of said at least one opening.
16. The switching array of claim 15, wherein said waveguide couplers and
the waveguides of said first and second pluralities are comprised of a
polyimide material and wherein said first and second electrodes are
comprised of material which is at least partially transparent.
17. The switching array of claim 16, wherein said insulator layer comprises
silicon dioxide.
18. The switching array of claim 12, wherein said means for moving
comprises:
a first cladding layer facing said first and second waveguides and said at
least one opening, said first cladding layer having a lower refractive
index than that of the waveguides of said first and second pluralities;
a plurality of movable plates, each of said movable plates facing a
respective waveguide coupler and said at least one opening and having a
lower refractive index than that of said respective waveguide coupler;
a plurality of first electrodes, each of said first electrodes being
situated over a portion of a respective one of said movable plates
situated above a respective portion of said at least one opening; and
a plurality of second electrodes, each of said second electrodes being
situated between said first cladding layer and a respective portion of
said at least one opening.
19. The switching array of claim 18, further including a plurality of
piezoelectric strip lines, each of said piezoelectric strip lines being
situated between a respective one of said first electrodes and a
respective one of said movable plates.
20. The switching array of claim 19, wherein each of said first, second,
and waveguide couplers comprises a polyimide material, said first
electrode comprises aluminum, and said second electrode comprises material
which is at least partially transparent.
21. The switching array of claim 20, wherein said piezoelectric strip lines
comprise lead zirconate titanate. |
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Claims |
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Description |
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CROSS-REFERENCE TO RELATED APPLICATION
This application is related to the following co-pending application which
is commonly assigned and is incorporated herein by reference: Ghezzo et
al. "Microelectromechanical Photonic Switch", Ser. No. 08/144,165 now
pending. filed concurrently herewith.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to photonic switching arrays, and, more
particularly, to a switching array based on microelectromechanical motion
of overlapping polymer waveguides.
2. Description of the Related Art
One conventional photonic switch uses the Mach-Zehnder device architecture
with non-linear optical crystals. Although eight-by-eight arrays have been
fabricated, this type of switch is expensive to manufacture and difficult
to fit on standard electro-optic packages. Commercially available
non-linear optical crystal switches that fit on a single substrate are
limited to two-by-two arrays. A larger array is yield-limited by its size,
which is determined by the electro-optic coefficient of the non-linear
crystal, usually LiNbO.sub.3, and by the applied voltage, which must
comply with the material dielectric strength.
Another method of photonic switching is to convert the optical signal to an
electrical signal, reconfigure the input-to-output channel assignment
electronically, and convert the electrical signal back to an optical
signal. This complicated procedure adds significant overhead to the signal
propagation introduced by the decoder and modulator circuitry, thus
increasing the time delay and the power consumption. This method is also
limited by the bandwidth restrictions of the opto-electronic electronic
converter which downgrade the optical network capabilities.
Electromechanical deflection of reflective surfaces has been used for
waveguide photonic switching by means of microcantilevers or microbridges,
as described in R. Watts et al., "Electromechanical Optical Switching and
Modulation in Micromachined Silicon-on-Insulator Waveguides," 1991 IEEE
International SOI (silicon-on-insulator) Conference Proceedings, pp.
62-63. A voltage supplies electrostatic attraction resulting in a
deflection of the microcantilever or microbridge. When this technique is
used in free space, alignment and vibration problems can occur.
Micromachining has recently been used for fabrication of diffraction
gratings for spectral analysis and optical modulator switches because of
the high resolution sculpting capability of this technique, as described
in O. Solgaard et al., "Deformable grating optical modulator," Optics
Letters, vol. 17, no. 9, 688-90 (May 1, 1992). Other approaches include a
monolithic four-by-four photonic crossbar switch that has been fabricated
for avionic systems using rib waveguides with etched facets and turning
mirrors, and a multimode two-by-two optical switch in which micromachined
pivoting silicon moving mirrors selectively direct optical beams from
input fibers to output fibers. Vibration and alignment difficulties reduce
the effectiveness of these techniques.
Polymeric waveguide technology has been used with ferroelectric liquid
crystals to develop a six-by-six matrix switching array and provide guided
wave connectivity to a multi-element spatial light modulator. The maximum
operating temperature of this type of switch is about 60.degree. C.,
however, which is too low for aerospace, military, and automotive
applications.
Aforementioned Ghezzo et al., Ser. No. 08/144,165, now pending discloses a
switch which uses microelectromechanical motion of overlapping polymer
waveguides. The principle of operation is based on modulation of optical
energy transfer between overlapping polyimide waveguides which determines
whether the incoming light remains in the initial waveguide or is
partially transferred to the adjacent waveguide. This transfer depends on
the mutual separation between waveguides, which is controlled by
electrostatic or piezoelectric actuation.
SUMMARY OF THE INVENTION
Accordingly, one object of the invention is to provide a photonic switching
array having a high tolerance to a wide range of ambient operating
conditions.
Briefly, in accordance with one embodiment of the invention, a
microelectromechanical photonic switching array comprises a plurality of
first waveguides and a plurality of second waveguides. Insulative cladding
is situated between the first and second pluralities of waveguides. The
insulative cladding has at least one patterned opening. Each one of the
first waveguides is positioned such that predetermined first waveguide
portions are on one side of the opening, and each one of the second
waveguides is positioned such that predetermined second waveguide portions
are substantially parallel to respective predetermined first waveguide
portions and situated on an opposing side of the opening. Means are
provided for moving a selected one of the predetermined second waveguide
portions closer to a respective one of the predetermined first waveguide
portions in response to an actuation signal.
In accordance with another embodiment of the invention, a
microelectromechanical photonic switching array comprises a plurality of
first waveguides and a plurality of second waveguides with each of the
second waveguides intersecting respective portions of each of the first
waveguides at a respective intersection point. A plurality of waveguide
couplers each have one waveguide coupler portion situated substantially
parallel to a first coupler portion of a respective one of the first
waveguides and another waveguide coupler portion situated substantially
parallel to a second coupler portion of a respective one of the second
waveguides. The first and second coupler portions form cross-over
portions. Distancing means, which are provided for keeping the waveguide
couplers and the first and second pluralities of waveguides a desired
distance apart, have at least one patterned opening. Each one of the
cross-over portions is situated on one side of the opening and each
respective one of the waveguide couplers is situated on an opposing side
of the opening. Means are provided for moving a predetermined waveguide
coupler closer to a respective intersection point in response to an
actuation signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention believed to be novel are set forth with
particularity in the appended claims. The invention itself, however, both
as to organization and method of operation, together with further objects
and advantages thereof, may best be understood by reference to the
following description taken in conjunction with the accompanying drawings,
where like numerals represent like components, in which:
FIGS. 1a and 1b are cross-sectional side views of one embodiment of a
photonic switch in an unactuated state and an actuated state,
respectively;
FIGS. 2a and 2b are cross-sectional side views of another embodiment of a
photonic switch in an unactuated state and an actuated state,
respectively;
FIGS. 3a and 3b are schematic top views of two embodiments of waveguides
and openings forming electromechanical photonic switching arrays;
FIG. 4a is another schematic top view of waveguides and an open region
forming an electromechanical photonic switching array; and
FIG. 4b is an isometric view of the switching array shown in FIG. 4a.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
FIGS. 1a and 1b are cross-sectional views of an embodiment of an
electrostatic switch used in the arrays (shown in FIGS. 3a-4b) of the
present invention in unactuated and actuated states, respectively.
Aforementioned, Ghezzo et al., Ser. No. 08/144,165, now pending, describes
several embodiments, for fabricating an electrostatic switch. A substrate
1, which is preferably a portion of a silicon wafer about 500 .mu.m thick
passivated with a 5000 .ANG. thick silicon dioxide layer 40, provides
mechanical support and allows integration of the switch. Using a silicon
wafer as the substrate allows for convenient integration of switch
electrodes 5 and 6 to other integrated circuitry which is easily
fabricated on the semiconductor substrate. Substrate 1 can include similar
switches, for example in an array geometry, and can further include
electronic controls fabricated in the silicon prior to the photonic switch
fabrication.
A first cladding layer 2, about 10 .mu.m thick is sputtered or laminated
over the silicon dioxide-covered substrate. A first waveguide 3 having a
higher refractive index than the refractive index of the first cladding
layer is applied and patterned over first cladding layer 2. First
waveguide 3 is about 10 .mu.m thick and is designed for multi-mode light
transmission.
Both first cladding layer 2 and first waveguide 3 preferably include
polyimide materials which are chosen according to their refractive indices
and optical transmissivities per unit length. Waveguides need a higher
refractive index than cladding to generate total internal reflections at
the waveguide boundaries and thus eliminate external light loss. The
refractive index change is easily accomplished by a slight modification of
the polyimide composition. For example decreasing the fluorine content in
a polyimide results in a higher index of refraction.
A first electrode 5 is formed over portions of first waveguide 3 and over
adjacent regions of first cladding layer 2. The first electrode is about
400 .ANG. thick and preferably comprises an at least partially transparent
material such as antimony tin oxide (ATO) or indium tin oxide (ITO). A
thin, at least partially transparent, insulator layer 7 is applied over
first electrode 5 for electrical isolation during mechanical contact
between first electrode 5 and a second electrode 6. In one embodiment,
thin insulator layer 7 comprises silicon dioxide having a thickness of
approximately 400-1000 .ANG.. The thin insulator layer is also useful as
an etch stop during formation of an air gap 8.
An insulative cladding layer 9 is next applied over insulator layer 7 and
first waveguide 3. In this embodiment, the cladding material may comprise
a polyimide having a thickness of 15 .mu.m, for example.
A well 10, which provides an air gap 8, is formed in insulative cladding
layer 9 preferably by using excimer laser ablation. The well is then
refilled with a self-aligned sacrificial layer (not shown) of
electroplated copper, for example. Ghezzo et al., "Micromechanical Moving
Structures Including Multiple Contact Switching System, and Micromachining
Methods. Therefor," U.S. application Ser. No. 08/000,172 now pending filed
Jan. 4, 1993, discloses a method of applying a sacrificial layer of copper
and is herein incorporated by reference.
Second electrode 6 is formed over insulative cladding layer 9 and the
sacrificial layer by film deposition and patterning. In one embodiment,
the second electrode comprises ITO or ATO having a thickness of 400 .ANG..
The first and second electrodes, as well as insulator layer 7, must be at
least partially transparent because they are, in this embodiment,
positioned between the waveguides to reduce their maximum separation and
consequently increase the electrostatic force. Transparency is required to
allow light transfer between the waveguides when the electrodes are in
mechanical contact.
A second waveguide 11 is formed over second electrode 6 by film deposition
and patterning. The second waveguide may comprise a polyimide with a
similar index of refraction as first waveguide 3 and a thickness of 10
.mu.m. The second waveguide is positioned so that a portion is on one side
of gap 8, while a portion of the first waveguide is situated on an
opposite side of gap 8. The first and second waveguide portions on
opposite sides of gap 8 are substantially parallel in that at least one of
the two waveguides is movable in the direction of the other waveguide and
capable of transferring light.
Neither second electrode 6 nor second waveguide 11 completely covers the
sacrificial layer (not shown) which fills well 10. After the second
waveguide is applied, the sacrificial layer is laterally removed by
applying a selective etch which etches the sacrificial layer but not the
electrodes or waveguides, thereby forming the air gap. The use of a
sacrificial layer, although preferred, is not required for this invention.
Any method of applying the second electrode over a selected portion of
insulative cladding 9 and well 10 which retains the spatial integrity of
an opening in the well is appropriate.
Well 10 is preferably not any thicker than required to isolate light in an
unactuated state and provide adequate light transfer in an actuated state
because waveguide motion is preferably limited to several micrometers in
order to use electrostatic actuation voltages compatible with conventional
integrated circuit technology. Additionally, limiting the deflection
distance of the second waveguide leads to short mechanical switching time
(preferably less than 100 .mu.sec).
A second cladding layer 12, formed by lamination or spin coating, for
example, completes the switch structure by sealing second waveguide 11
with a polyimide of a lower refractive index. Second cladding layer 12 may
comprise a polyimide having a thickness of 10-15 .mu.m, for example. Each
of the materials used in the photonic switch must be stable at the
operating temperatures of the device at which the switch will be used.
Typically the ambient temperatures range from -55.degree. C. to
125.degree. C.
When the voltage between the first and second electrodes is zero, the first
and second waveguides are separated by air gap 8 and by insulative
cladding 9, leaving them optically and mechanically isolated. In this
position the structure is in equilibrium, and thus can remain in
equilibrium indefinitely without a holding force. When voltage is applied
across the first and second electrodes, second waveguide 11 is pulled
downwards electrostatically by the mutual attraction of electrodes 5 and 6
towards first waveguide 3, as shown in FIG. 1b, making mechanical contact
through the second electrode and insulator layer 7 covering the first
electrode over most of the air gap length. Hence the evanescent wave of
the light-carrying waveguide penetrates into the adjacent waveguide,
creating a partial optical energy transfer.
Although the switch is shown in this embodiment as being formed and
positioned in a vertical orientation, it matters neither how the switch is
manufactured nor whether the orientation of the switch in a device is
vertical or horizontal. Because the mass of the switch is small, gravity
does not have much effect on electrode motion, and one or both electrodes
(and thus both waveguides) tend to draw together in response to an
electrostatic force no matter what the orientation of the switch.
Many modifications can be made to the embodiments discussed thus far. For
example, instead of first and second electrodes 5 and 6, respectively,
being applied over or otherwise touching first and second waveguides 3 and
11, respectively, the electrodes can be laterally situated on opposite
sides of respective waveguides and not come between a respective waveguide
and air gap 8. Several advantages of such embodiment are that the
waveguides can come into actual mechanical contact and that the electrodes
do not need to be transparent.
FIGS. 2a and 2b are cross-sectional views of an embodiment of a
piezoelectric switch usable in an array (shown in FIGS. 3a-4b) of the
present invention in an unactuated state and an actuated state,
respectively. This embodiment differs from that of FIGS. 1a-1b in that one
of the electrodes, a first piezoelectric electrode 14, is situated over a
piezoelectric strip line 13 and second cladding layer 12 rather than
between first waveguide 3 and insulative cladding layer 9, as shown for
first electrode 5 in FIG. 1a.
Piezoelectric strip line 13, which is preferably 1-2 .mu.m thick, changes
length in response to a perpendicularly directed electric field.
Piezoelectric strip line 13 must overlie second cladding layer 12 to
produce a lateral stress between these layers and consequently bend the
composite beam to relieve this stress, thereby generating a vertical
deflection. To apply the electric field across the piezoelectric strip
line, the electrodes must be positioned on opposite sides of the strip
line. Since first piezoelectric electrode 14 is not between the optically
active layers, the first piezoelectric layer does not need to be
transparent and can include materials such as aluminum, copper, gold,
platinum, or chromium, for example. In one embodiment, the piezoelectric
electrode has a thickness of about 1000 .ANG.. Piezoelectric strip line 13
can be patterned from a deposited or sputtered layer of gelatin solution
containing a piezoelectric material such as lead zirconate titanate (PZT),
for example, which has a large transversal piezoelectric coefficient.
FIGS. 3a and 3b are schematic top views of first and second waveguides 3
and 11 and at least one opening 18 forming electromechanical photonic
switching arrays. Each individual switch of an array operates as described
with respect to the embodiments of FIGS. 1a-2b. The one or more openings
18 provide the air gaps 8 needed by the switches. The openings need not be
in the shapes shown since any shape or size of opening which leaves a
sufficient region for light transfer to occur is acceptable. Cross-over
portions 25 are those portions of the switch where predetermined portions
of first waveguides 3 are substantially parallel on one side of an opening
18 opposite predetermined portions of second waveguides 11.
The electrodes and the second cladding layer shown in FIGS. 1a-2b are
omitted in FIGS. 3a-3b for clarity. Either an electrostatic or
piezoelectric switch can be used with any of the arrays of the present
invention, and the positions of the electrodes can either be adjacent
respective waveguides or in other locations. Insulator layer 7 (shown in
FIGS. 1a-1b) is present in an electrostatic switching array. The insulator
layer can either be a sheet of insulative material, such as silicon
dioxide, covering each of the first electrodes or a plurality of
individual insulative material patches, with each of the patches covering
individual electrodes at least over the mechanical contact areas.
In the embodiments of FIGS. 3a-3b, any input channel 20 can be connected to
any output channel 22 so that the activated switches do not interfere with
the optical paths of other channels. Because waveguide bends create losses
(typically at least 0.3-0.5 dB per bend), the number of bends is
preferably minimized. In each of FIGS. 3a-3b, example light paths are
illustrated with phantom lines. Light path 41 represents the light
entering first waveguides 3. If no switch is activated in a first
waveguide, the light path remains in that waveguide. If a switch is
activated, some of the light is coupled from the first waveguide to a
respective second waveguide and follows light path 42 to an output channel
22, whereas the uncoupled portion of the light remains in the first
waveguide along light path 44.
In the embodiment shown in FIG. 3a, first waveguides 3 are jogged and run
left to fight at a 45 degree angle, whereas second waveguides 11 have a
serpentine pattern running left to fight. Input light 21 is sent at input
channel 20 through the first waveguides, and, when appropriate switches
are activated, output light 23 is coupled out of the second waveguides
through output channels 22. For a four-by-four switching matrix, the
longest optical path results in a total of 21 waveguide bends, giving a
worst-case optical transfer loss of >10 dB.
The array shown in the embodiment of FIG. 3b achieves a reduced number of
waveguide bends by using right angle bends and by altering the switch
geometry slightly. In this design, first waveguides 3 have a switch every
second waveguide bend, and there are no bends on second waveguides 11,
resulting in a worst-case total of 7 waveguide bends, or a worst-case
insertion loss of 3-4 dB due to bends alone. Because second waveguides 11
have no bends, losses are minimized once the light is coupled. The
disadvantage is the modified switch geometry, which requires higher
switching voltages because of the larger restoring spring constant. In a
preferred embodiment, each right angle bend has a 45 degree reflecting
joint 4, preferably with a mirror (not shown), to minimize scattering
light loss at the bend.
The embodiment shown in FIGS. 4a-4b uses another modification of the
optical switch to control coupling between a Cartesian grid of straight
waveguides. The electrodes shown in FIGS. 1a-2b are omitted in FIGS. 4a-4b
for clarity. Either an electrostatic or piezoelectric switch can be used
with any of the arrays of the present invention, and the positions of the
electrodes can either be adjacent respective waveguides, as shown in FIGS.
1a-1b, or in other locations, as shown in FIG. 2k.
The individual switches operate in a similar manner as described with
respect to FIGS. 1a -2b, except that first waveguides 3a and second
waveguides 11a are situated on the same plane and coupling of light from a
predetermined first waveguide to a predetermined second waveguide is
achieved by using a movable waveguide coupler 26 which comprises a
polyimide material similar to that of the first and second waveguides and
preferably has a right angle bend with a 45 degree reflecting joint,
preferably with a mirror 26a.
An open region 18 has one or more openings which provide the air gaps
needed by the switches. In the embodiment of FIGS. 4a-4b, one large
opening is used with insulative cladding between the waveguide couplers
and first and second waveguides forming pedestals 30, comprising a
polyimide, for example, for supporting movable plates 28 which hold
waveguide couplers 26 in place above the first and second waveguides and
provide the appropriate distance between the waveguide couplers and the
first and second waveguides. The pedestals can be formed, for example, by
coating he substrate with a layer of polyimide material and etching the
material except in the area where pedestals are desired.
First waveguides 3a and second waveguides 11a intersect at intersection
points 24. The term "intersect" is intended to encompass any situation in
which the waveguides intersect. In a is preferred embodiment, as shown
FIG. 4a, the first and second waveguides are coplanar waveguides
comprising the same layer of polyimide material in common. In other
embodiments, the intersection can result from the second waveguides
crossing over the first waveguides, or the first waveguides crossing over
the second waveguides, or some of the second waveguides crossing over the
first waveguides while others of the second waveguides are crossed over by
the first waveguides.
Each respective waveguide coupler 26 is situated on the side of the movable
plate facing the opening and the first and second waveguides. Each
respective waveguide coupler 26 is substantially parallel on one side of
an opening 18 opposite a respective predetermined portion of first
waveguides 3a and a respective predetermined portion of second waveguides
11a to form a respective cross-over portion of respective first and second
waveguides, shown as cross-over portion 34 in the cut-away of FIG. 4a.
When an electrostatic switch is used, first electrode 5 (shown in FIGS.
1a-2b) is situated on the same side of opening 18 as cross-over portion
34, while second electrode 6 (shown in FIGS. 1a-2b) is situated on the
same side of opening 18 as waveguide coupler 26. An electrode can be
between a respective waveguide and the opening, or adjacent to a
respective waveguide. When a piezoelectric switch is used, the placement
is similar except that the first piezoelectric electrode is placed on
movable plate 28 above the opening.
The movable plates comprise a flexible material such as a polyimide with
appropriate reinforcing layers to provide the desired mechanical support.
One fabrication method is to apply a sacrificial layer (not shown) of
material such as copper between the pedestals in a similar manner as
discussed with the sacrificial layer in the well of FIGS. 1a-2b. Then
waveguide couplers, respective electrodes, and the plate material can be
formed in the desired order. (The order of application of the electrodes
and waveguide couplers depends on whether the switch is electrostatic or
piezoelectric.) A selective etch can next be done to provide individual
plates from the plate material, and another etch can remove the
sacrificial layer. These etches must comply with the array design and
leave movable plates overhanging from the pedestals and forming flexible
cantilever beams.
By bringing the waveguide coupler in close proximity with the intersection
point of the two waveguides, optical coupling results. In FIG. 4a, example
light paths are illustrated with phantom lines. Light path 41 represents
the light entering first waveguides 3a. If no switch is activated in a
first waveguide, the light path remains in that waveguide. If a switch is
activated, some of the light is coupled from the first waveguide to a
respective waveguide coupler 26, followed by some of the light in the
respective waveguide coupler being coupled into a respective second
waveguide 11a and following light path 42 to a respective output channel
22. Any uncoupled portion of the light remains in the first waveguide
along light path 44.
An advantage of those over the embodiments of FIGS. 3a-3b is that it allows
a larger number of nodes to be present because, although a slightly larger
loss occurs at the active switching node, the loss at every inactive node
is negligible. The loss for each switching node in an active state is
approximately the same, instead of depending on the number of bends in a
waveguide. Additional advantages of this geometry are that straight
waveguides are used throughout, that a minimal (one bend, regardless of
optical path chosen) insertion loss is experienced across the entire
array, and that the geometry results in a simpler and more flexible
switch. A substantial portion of each metal plate can be used for a
respective electrode, thus increasing the switching efficiency and
reducing the required drive voltages. The disadvantages of this design are
that it requires two couplings per switch. The tradeoff then becomes a
balance between the crossover loss and the bending loss.
While only certain preferred features of the invention have been
illustrated and described herein, many modifications and changes will
occur to those skilled in the art. For example, although the description
has focussed primarily on electrostatic and piezoelectric waveguide
bending forces, any bending actuation force can be used, including
electromagnetic forces, thermal forces, fluid forces, and pneumatic
forces. It is, therefore, to be understood that the appended claims are
intended to cover all such modifications and changes as fall within the
true spirit of the invention.
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