U.S. patent application number 15/980768 was filed with the patent office on 2019-11-21 for ftir/tir optical switch using a moving waveguide.
The applicant listed for this patent is Mellanox Technologies, Ltd., Yissum Research Development Company of the Hebrew University LTD.. Invention is credited to Eran Aharon, Dan Mark Marom, Elad Mentovich.
Application Number | 20190353848 15/980768 |
Document ID | / |
Family ID | 68391918 |
Filed Date | 2019-11-21 |
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United States Patent
Application |
20190353848 |
Kind Code |
A1 |
Aharon; Eran ; et
al. |
November 21, 2019 |
FTIR/TIR optical switch using a moving waveguide
Abstract
An optical device includes a first waveguide having a
longitudinal axis and a first end facet inclined at a non-normal
angle to the longitudinal axis, and a second waveguide, which has a
second end facet and is fixed with the second end facet in
proximity to and parallel with the first end facet. An actuator is
coupled to move the first end facet of the first waveguide in a
direction transverse to the longitudinal axis between a first
position in which a distance between the first and second end
facets is less than 25 nm, and a second position in which the
distance between the first and second end facets is greater than
300 nm.
Inventors: |
Aharon; Eran; (Mevaseret
Zion, IL) ; Marom; Dan Mark; (Mevaseret Zion, IL)
; Mentovich; Elad; (Tel Aviv, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Yissum Research Development Company of the Hebrew University
LTD.
Mellanox Technologies, Ltd. |
Jerusalem
Yokneam |
|
IL
IL |
|
|
Family ID: |
68391918 |
Appl. No.: |
15/980768 |
Filed: |
May 16, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/3512 20130101;
G02B 6/3502 20130101; G02B 6/122 20130101; G02B 6/3508 20130101;
G02B 6/354 20130101; G02B 2006/12176 20130101; G02B 6/357 20130101;
G02B 2006/12178 20130101 |
International
Class: |
G02B 6/35 20060101
G02B006/35; G02B 6/122 20060101 G02B006/122 |
Claims
1. An optical device, comprising: a first waveguide having a
longitudinal axis and a first end facet inclined at a non-normal
angle to the longitudinal axis; a second waveguide, which has a
second end facet and is fixed with the second end facet in
proximity to and parallel with the first end facet; an actuator
coupled to move the first end facet of the first waveguide in a
direction transverse to the longitudinal axis between a first
position in which a distance between the first and second end
facets is less than 25 nm, and a second position in which the
distance between the first and second end facets is greater than
300 nm; and a third waveguide, which has a third end facet and is
fixed in a disposition such that when the first end facet is in the
second position, radiation propagating through the first waveguide
is reflected by total internal reflection (TIR) from the first end
facet through a side surface of the first waveguide and into the
third waveguide through the third end facet.
2. The device according to claim 1, wherein in the first position
the first end facet contacts the second end facet.
3. (canceled)
4. The device according to claim 1, wherein in the second position,
the side surface of the first waveguide contacts the third end
facet.
5. The device according to claim 4, wherein the second waveguide is
aligned along the longitudinal axis, and wherein the first and
second end facets are angled at 45.degree. relative to the
longitudinal axis.
6. The device according to claim 1, wherein the actuator comprises
a micro-electromechanical system (MEMS) mechanism.
7. The device according to claim 6, wherein the MEMS mechanism
comprises: a substrate; at least one electrode formed on the
substrate; a conductive cantilever beam formed on the substrate in
proximity to the at least one electrode, wherein the first
waveguide is mounted on the cantilever beam, and the cantilever
beam has a first end that is attached to the substrate and a second
end, in proximity to the first end facet of the first waveguide,
that is released from the substrate; and a controller coupled to
apply a varying electrical potential between the at least one
electrode and the cantilever beam so as to deflect the cantilever
beam between the first and second positions of the first
waveguide.
8. The device according to claim 7, wherein the at least one
electrode comprises a pair of electrodes, and the cantilever beam
is disposed between the electrodes.
9. The device according to claim 7, wherein the substrate comprises
a silicon-on-insulator (SOI) substrate, comprising: a silicon
substrate; an isolation layer formed on the silicon substrate,
wherein the isolation layer comprises a dielectric material; and an
actuation layer formed on the isolation layer, wherein the
actuation layer comprises silicon, which is doped for conducting
electricity, wherein the conductive cantilever beam is formed in
the actuation layer.
10. The device according to claim 9, wherein the dielectric
material comprises silicon dioxide.
11. The device according to claim 7, wherein the conductive
cantilever beam is configured to latch in first and second stable
beam configurations, such that in the first stable beam
configuration the first end facet is in the first position and in
the second stable beam configuration the first end facet is in the
second position, wherein the controller is coupled to apply a
varying electrical potential between the two electrodes and the
beam so as to bend the beam between the first and second stable
beam positions.
12-36. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to photonic devices,
and particularly to optical switches based on mechanical motion,
including specifically micro-electromechanical systems.
BACKGROUND
[0002] In optical communication systems, optical switches are used
to direct optical signals along desired transmission paths or to
route these optical signals around a fault in the transmission
path. The transmission path is typically an optical fiber, a form
of waveguide. There are many physical mechanisms for performing
optical switching from one or more input waveguides to one or more
output waveguides. Some key performance parameters of optical
switches include their transmission losses to desired output port,
cross-talk to undesired ports, switching time, physical size,
scalability and cost.
[0003] It has been suggested that frustrated total internal
reflection can be used in optical switching. For example, U.S. Pat.
No. 6,519,382 describes an optical switch for processing an optical
signal, where the switch includes an input waveguide having a
reflective surface, a first output waveguide coupled to the input
waveguide, and a second output waveguide. The second output
waveguide has a first position spaced apart from the reflective
surface of the input waveguide such that the reflective surface
totally internally reflects an optical signal toward the first
output waveguide. The second output waveguide has a second position
in proximal contact with the reflective surface to frustrate the
total internal reflection of the optical signal such that the
second output waveguide receives the optical signal.
SUMMARY
[0004] Embodiments of the present invention that are described
hereinbelow provide an improved optical switch.
[0005] There is therefore provided, in accordance with an
embodiment of the invention, an optical device, including a first
waveguide having a longitudinal axis and a first end facet inclined
at a non-normal angle to the longitudinal axis, and a second
waveguide, which has a second end facet and is fixed with the
second end facet in proximity to and parallel with the first end
facet. An actuator is coupled to move the first end facet of the
first waveguide in a direction transverse to the longitudinal axis
between a first position in which a distance between the first and
second end facets is less than 25 nm, and a second position in
which the distance between the first and second end facets is
greater than 300 nm.
[0006] In some embodiments, in the first position the first end
facet contacts the second end facet.
[0007] In some embodiments, the device includes a third waveguide,
which has a third end facet and is fixed in a disposition such that
when the first end facet is in the second position, radiation
propagating through the first waveguide is reflected from the first
end facet through a side surface of the first waveguide and into
the third waveguide through the third end facet. Typically, in the
second position, the side surface of the first waveguide contacts
the third end facet. In a disclosed embodiment, the second
waveguide is aligned along the longitudinal axis, and wherein the
first and second end facets are angled at 45.degree. relative to
the longitudinal axis.
[0008] In some embodiments, the actuator includes a
micro-electromechanical system (MEMS) mechanism. In one such
embodiment, the MEMS mechanism includes a substrate, with at least
one electrode formed on the substrate. A conductive cantilever beam
is formed on the substrate in proximity to the at least one
electrode, wherein the first waveguide is mounted on the cantilever
beam, and the cantilever beam has a first end that is attached to
the substrate and a second end, in proximity to the first end facet
of the first waveguide, that is released from the substrate. A
controller is coupled to apply a varying electrical potential
between the at least one electrode and the cantilever beam so as to
deflect the cantilever beam between the first and second positions
of the first waveguide.
[0009] In one embodiment, the at least one electrode includes a
pair of electrodes, and the cantilever beam is disposed between the
electrodes.
[0010] Additionally or alternatively, the substrate includes a
silicon-on-insulator (SOI) substrate, including a silicon
substrate, an isolation layer formed on the silicon substrate,
wherein the isolation layer includes a dielectric material, and an
actuation layer formed on the isolation layer. The actuation layer
includes silicon, which is doped for conducting electricity, and
the conductive cantilever beam is formed in the actuation layer. In
a disclosed embodiment, the dielectric material includes silicon
dioxide.
[0011] Further additionally or alternatively, the conductive
cantilever beam is configured to latch in first and second stable
beam configurations, such that in the first stable beam
configuration the first end facet is in the first position and in
the second stable beam configuration the first end facet is in the
second position. The controller is coupled to apply a varying
electrical potential between the two electrodes and the beam so as
to bend the beam between the first and second stable beam
positions.
[0012] There is also provided, in accordance with an embodiment of
the invention, an optical switching apparatus including a plurality
of optical switches. Each switch includes a first waveguide having
a longitudinal axis and a first end facet inclined at a non-normal
angle to the longitudinal axis; a second waveguide, which has a
second end facet and is fixed with the second end facet in
proximity to and parallel with the first end facet; and a third
waveguide, which has a third end facet and is fixed in a
disposition such that when the first end facet is in the second
position, radiation propagating through the first waveguide is
reflected from the first end facet through a side surface of the
first waveguide and into the third waveguide through the third end
facet. An actuator is coupled to move the first end facet of the
first waveguide in a direction transverse to the longitudinal axis
between a first position in which a distance between the first and
second end facets is less than 25 nm, and a second position in
which the distance between the first and second end facets is
greater than 300 nm. A waveguide inter-connection having multiple
legs is connected to respective waveguides of the optical switches,
so that actuation of the optical switches interconnects different
ones of the waveguides via the waveguide inter-connection.
[0013] In one embodiment, the plurality of optical switches include
a first switch, a second switch, a third switch, and a fourth
switch, wherein the second waveguide of the first switch is
connected to the second waveguide of the second switch, and the
second waveguide of the third switch is connected to the second
waveguide of the fourth switch, and wherein the waveguide
cross-connector connects the third waveguide of the first switch to
the third waveguide of the third switch and connects the third
waveguide of the second switch to the third waveguide of the fourth
switch.
[0014] In another embodiment, the plurality of optical switches
include at least a first switch and a second switch, wherein the
waveguide cross-connector connects the second waveguide of the
first switch to the second waveguide of the second switch, and
wherein the third waveguide of the first switch is connected to the
third waveguide of the second switch.
[0015] In some embodiments, the waveguide inter-connection includes
interconnect waveguides, which are routed so as to cross each other
at right angles. In one embodiment, all of the interconnect
waveguides in the waveguide inter-connection cross one another only
at right angles.
[0016] There is additionally provided, in accordance with an
embodiment of the invention, a method for manufacturing an optical
device. The method includes fixing a first waveguide, having a
longitudinal axis and a first end facet inclined at a non-normal
angle to the longitudinal axis, to an actuator, which is configured
to move the first end facet of the first waveguide in a direction
transverse to the longitudinal axis. A second waveguide, which has
a second end facet, is fixed so that the second end facet is in
proximity to and parallel with the first end facet. The actuator is
driven to move between a first position in which a distance between
the first and second end facets is less than 25 nm, and a second
position in which the distance between the first and second end
facets is greater than 300 nm.
[0017] There is further provided, in accordance with an embodiment
of the invention, a method for switching an optical signal. The
method includes introducing an optical signal into a first
waveguide having a longitudinal axis and a first end facet inclined
at a non-normal angle to the longitudinal axis. A second end facet
of a second waveguide is placed in proximity to and parallel with
the first end facet. The optical signal is switched between the
first and second waveguides by moving at least one of the first and
second end facets in a direction transverse to the longitudinal
axis between a first position in which the optical signal is
reflected from the first end facet due to total internal reflection
(TIR) and a second position in which the optical signal propagates
into the second waveguide by frustrated total internal reflection
(FTIR).
[0018] The present invention will be more fully understood from the
following detailed description of the embodiments thereof, taken
together with the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic top view of an optical switch, in
accordance with an embodiment of the invention;
[0020] FIG. 2 is a schematic perspective view of the optical switch
of FIG. 1, in accordance with an embodiment of the invention;
[0021] FIG. 3 is a schematic cross-section of the optical switch of
FIG. 1, in accordance with an embodiment of the invention;
[0022] FIGS. 4A-B are schematic top views of the optical switch of
FIG. 1 in two different switching states, in accordance with an
embodiment of the invention;
[0023] FIG. 5 is a schematic top view of an optical bypass-exchange
switch based on four 1.times.2 optical switches, in accordance with
an embodiment of the invention;
[0024] FIGS. 6A-B are schematic top views of the bypass-exchange
switch of FIG. 5 in different switching states, in accordance with
an embodiment of the invention;
[0025] FIG. 7 is a schematic top view of an optical crossbar switch
based on two optical 1.times.2 switches, in accordance with an
embodiment of the invention;
[0026] FIGS. 8A-B are schematic top views of the crossbar switch of
FIG. 7 in different switching states, in accordance with an
embodiment of the invention;
[0027] FIG. 9 is a schematic top view of a latching actuator, in
accordance with an embodiment of the invention;
[0028] FIGS. 10A-B are schematic top views of an optical latching
switch in different switching states, in accordance with an
embodiment of the invention;
[0029] FIGS. 11A-M and FIGS. 11O-V are schematic sectional views
showing stages in a possible process flow of the fabrication of an
optical switch, while FIG. 11N is a schematic top view showing a
detail of the one of the stages, in accordance with an embodiment
of the invention;
[0030] FIGS. 12A-B are schematic top views of two 4.times.4 optical
bypass-exchange switches, one with some oblique and the other with
all right-angle waveguide crossings, in accordance with embodiments
of the invention; and
[0031] FIGS. 13A-B are schematic top views of two 8.times.8 optical
bypass-exchange switches, one with some oblique and the other with
all right-angle waveguide crossings, in accordance with further
embodiments of the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
Overview
[0032] Despite the widespread use of fiberoptic communications,
there is an unmet need for optical switches that are fast, small,
reliable and inexpensive. Embodiments of the present invention that
are described herein address this need by providing an optical
switch utilizing frustrated total internal reflection (FTIR).
Optical switches and switch networks are examples of optical
devices in accordance with various embodiments of the
invention.
[0033] FTIR is based on total internal reflection (TIR), wherein a
beam of light propagating in a material with a high refractive
index n.sub.1 (for example, glass) encounters an interface to a
material with a low refractive index n.sub.2 (for example, air) at
an angle of incidence that is higher than the critical angle
.theta.. (The critical angle .theta. is
.theta.=arcsin(n.sub.2/n.sub.1)). At TIR, all of the optical energy
is reflected from the interface between the two materials back to
the high refractive index material. If a body having a sufficiently
high refractive index is brought close to the high/low refractive
index interface from the low-index side, however, some of the light
will "leak" into this body. This phenomenon is known as frustrated
total internal reflection (FTIR). For any appreciable amount of
light to leak into the body, the distance to the interface needs to
be within a fraction of the wavelength of the light, with the
portion of the leakage increasing with diminishing distance.
[0034] In the disclosed embodiments, the optical switch includes a
first waveguide and a second waveguide with non-normal end facets
arranged so that the two end facets are parallel to each other and
in close proximity. An actuator is coupled to move the end of the
first waveguide in a direction transverse to the longitudinal axis
of the waveguide between two positions:
[0035] 1) A first position in which the end facet of the first
waveguide is brought to within a distance of typically 25 nm or
less from the angle-matched end facet of the second waveguide, and
possibly into contact with this end facet. This proximity of the
two end facets enables an optical signal propagating in the first
waveguide to propagate with minimal reflection losses into the
second waveguide due to FTIR.
[0036] 2) A second position in which the end facet of the first
waveguide is moved away from the end facet of the second waveguide,
typically to a distance of 300 nm or more, or possibly more than
1000 nm. At this second position, light propagating in the first
waveguide strikes the end facet of the first waveguide at an angle
larger that the critical angle and experiences TIR, radiating (or
emitting) out of the first waveguide via its sidewall at an angle
that is efficiently collected by a suitably positioned third
waveguide.
[0037] Between the first and second positions the first waveguide
is in a neutral position. This is a position where the actuator is
not activated and does not bend the first waveguide in any
direction.
[0038] Thus, shifting the first waveguide by a small distance,
typically a few hundreds of nanometers, between the first and
second positions, the optical signal propagating in the first
waveguide is either coupled to the second waveguide or to the third
waveguide. This describes a 1.times.2 optical switch. The same
mechanism can be operated in reverse to couple selectively optical
signals from the second or third waveguides into the first
waveguide, as well as assembled into a 2.times.1 optical switch.
This scheme is particularly well-suited for actuation by a MEMS
mechanism, although other sorts of actuators that are known in the
art may alternatively be used.
Basic Switch Configurations
[0039] FIG. 1 is a schematic top view of an optical switch 20, in
accordance with an embodiment of the invention. Optical switch 20
comprises a first waveguide 21 with a first end facet 22, a second
waveguide 23, parallel to and slightly laterally shifted with
respect to first waveguide 21, and with a second end facet 24, and
a third waveguide 25, perpendicular to first waveguide 21, and with
a third end facet 26. End facets 22, 24, and 26 are planar. In an
alternative embodiment end facets 22, 24, and 26 may be slightly
convex with a typical radius of curvature of 1 mm, so as to reduce
the contact area when the end facets touch each other or another
optical surface. A reduced contact area helps to reduce stiction
forces between the touching surfaces.
[0040] Waveguides 21, 23, and 25 confine light to their core
geometry and material, and may in general be either single-mode or
multi-mode waveguides or combinations thereof. In the present
embodiment, the waveguides are made of silicon in a strip
waveguides design, with a height of 3 .mu.m and a width of 2.8
.mu.m and positioned on top of an silicon oxide layer with air
cladding around the other surfaces, making them multi-mode
waveguides. In alternative embodiments, other dimensions and core
and cladding materials may be used. Waveguides 21, 23, and 25 are
etched into a silicon waveguide layer 33, which is transparent over
a typical wavelength band of 1200-1600 nm used in, for example,
optical communications. First and second end facets 22 and 24,
respectively, are parallel to each other and in this embodiment are
inclined at an angle of 45.degree. with respect to the common
longitudinal axis of first and second waveguides 21 and 23, which
runs in the horizontal direction in FIG. 1. Third end facet 26 is
perpendicular to the longitudinal axis of third waveguide 25, and
the third waveguide is perpendicular to first waveguide 21 due to
the choice of the end facet angle of 45.degree. for first and
second end facets 22 and 24, respectively.
[0041] In an alternative embodiment (not shown in the figures), an
angle that is different from 45.degree. may be used for end facets
22 and 24, respectively, as long as the condition for TIR is
satisfied. Choosing an angle different from 45.degree. will change
the direction of the totally internally reflected radiation exiting
from first waveguide 21. Consequently, the angle of third end facet
26 with respect to third waveguide 25, as well as the angle of the
third waveguide with respect to the first waveguide should be
respectively modified.
[0042] First waveguide 21 is attached to an actuator 30, which
bends the first waveguide in the plane of switch 20 so that first
end facet 22 shifts in direction transverse to the longitudinal
axis, from a neutral position (where the first waveguide is not
bent) either to a first position on one side or to a second
position on the opposite side. Both positions reduce the distance
between the tip of the first waveguide and the targeted output
waveguide to a sub-wavelength distance, and desirably coming into
direct contact with the target waveguide. The distance to the
undesired output waveguide should be sufficiently large so that
very little light couples to the unintended waveguide (a criterion
of -25 dB or lower or higher can be employed, as defined by the
target application). In the embodiment shown in FIG. 1 comprising a
silicon waveguide, the distance to the undesired waveguide is 600
nm, and can be larger to ensure good isolation (i.e. low
crosstalk).
[0043] Optical switch 20 is be implemented by a stack of three
silicon layers:
[0044] 1) A substrate 39 (also called a handle) is a thick layer
(typically 400-1000 .mu.m thick), whose primary purpose is to carry
the top layers above it and provide mechanical rigidity.
[0045] 2) An actuation layer 34 is of intermediate thickness
(typically 3-100 .mu.m) and provides mechanical actuation and
electrical addressability.
[0046] 3) A waveguide layer 33 is relatively thin (typically 0.1-10
.mu.m) and provides optical waveguiding.
[0047] Actuator 30 comprises a conductive silicon cantilever beam
32, which is etched into actuation layer 34 using methods known in
MEMS technology. Further details of layers 33, 34, and 39 are shown
in FIGS. 2-3. Silicon cantilever beam 32 is hidden by first
waveguide 21 in the top view of FIG. 1, but is shown in FIGS. 2-3.
Electrodes 36 and 38 are formed in actuation layer 34 on two sides
of cantilever beam 32, and are isolated from the substrate 39 by an
isolation layer 40 and from other parts of actuation layer 34 by
etched trenches 41. Isolation layer 40 is typically SiO.sub.2 with
a thickness of 1.5 .mu.m, but other isolation materials and
thicknesses commonly used in semiconductor and MEMS technology may
be used.
[0048] A controller 42 is coupled to a first voltage source 43 and
a second voltage source 45, which in turn are coupled to electrodes
36 and 38. Cantilever beam 32 is coupled to a ground potential 46
via the part of actuating layer 34 that is not connected to
electrodes 36 and 38. By applying suitable potentials between
cantilever beam 32 and electrodes 36 and 38, controller 42 causes
cantilever beam 32 to bend, and with it first waveguide 21, and
first end facet 22 is laterally translated to either the first or
the second position. For example, applying a voltage (positive or
negative) to electrode 36 while simultaneously grounding (bringing
to zero potential) both cantilever beam 32 and opposite electrode
38, will cause the cantilever beam to bend towards electrode 36 due
to the attractive force between potential differences of the two. A
typical voltage range for bending cantilever beam 32 to either the
first or the second position is 10-45 V (depending strongly on the
physical and geometrical parameters of actuator 30).
[0049] In an alternative embodiment (not shown in the figures),
cantilever beam 32 may be coupled, instead of to ground potential
46, through a separate voltage source to controller 42, so that the
potential of the cantilever beam, as well as the potentials of
electrodes 36 and 38, may be independently controlled by the
controller.
[0050] FIG. 2 is a schematic perspective view of optical switch 20,
in accordance with an embodiment of the invention. Conducting
silicon cantilever beam 32 is now visible under first waveguide 21.
The left end of cantilever beam 32 (in the view shown in FIGS. 1
and 2) is attached to actuation layer 34, while the remainder of
the cantilever, including the right end, in proximity to end facet
22 of first waveguide 21, is free-standing (i.e., released from
isolation layer 40 and the rest of actuation layer 34) and thus
free to bend towards either electrode 36 or 38. For the sake of
clarity, controller 42, as well as voltage sources 43 and 45, have
has been omitted from this figure. A line 44 denotes the location
for a schematic cross-section of optical switch 20 shown in FIG.
3.
[0051] FIG. 3 is a schematic cross-section of optical switch 20, in
accordance with an embodiment of the invention. The cross-section
is taken across first waveguide 21 and cantilever beam 32, as shown
by line 44 in FIG. 2.
[0052] All three silicon layers (waveguide layer 33, actuation
layer 34, and substrate 39) are visible in FIG. 3. Actuation layer
34 is typically doped for increased electrical conductivity. For
electrical isolation between the layers, isolation layer 40 is
disposed between substrate 39 and actuation layer 34, while an
optical isolation layer 52 is disposed between waveguide layer 33
and actuation layer 34. Electrical isolation layers are typically
formed in SiO.sub.2. Cantilever beam 32 and a trough 50 between
electrodes 36 and 38 have been etched through actuation layer 34
using methods known in semiconductor and MEMS technologies, such as
photolithographic methods and etching. First waveguide 21 is
attached to and isolated from cantilever beam 32 by optical
isolation layer 52 in order to prevent optical losses, or
preventing the confined optical mode from leaking into the
cantilever beam. Optical isolation layer 52 is fabricated of
SiO.sub.2 or other suitable dielectric.
[0053] FIGS. 4A-B are schematic top views of optical switch in two
different switching states, illustrating the operation of the
switch, in accordance with an embodiment of the invention.
[0054] In FIG. 4A, first waveguide 21 is in the first position, and
first and second end facets 22 and 24, respectively, are very close
to each other (typically less than 25 nm) and desirably in contact.
As depicted in FIGS. 4A-B, the end facet angles of first and second
end facets 22 and 24, respectively, are 45.degree.. A side wall 60
of first waveguide 21 is typically within a few hundred to a few
thousand nanometers from third end facet 26. An optical signal
propagating in first waveguide 21 continues with minimal losses
into second waveguide 23 through first and second end facets 22 and
24, respectively, due to FTIR. The propagating signal is shown by
arrows 62.
[0055] In FIG. 4B, first waveguide 21 is in the second position.
First and second end facets 22 and 24 are separated from each other
by at least a few hundred nanometers. Side wall 60 and third end
facet 26 are very close to each other (typically less than 100 nm),
and desirably in contact with each other. The optical signal
propagating in first waveguide 21 is totally reflected by first end
facet 22 due to TIR and is emitted through side wall 60, entering
third waveguide 25 through third end facet 26, as shown by arrows
64. Due to TIR, no optical signal is transmitted to second
waveguide 23.
[0056] Based on the inventors' calculations, a cross-talk ratio
lower than -40 dB is achieved for a translation of 300 nm from the
neutral position towards the first position, when the gap between
first and second end facets 22 and 24 is very small, typically 5 nm
or less. A similar cross-talk ratio is achieved for a translation
of 300 nm towards the second position.
[0057] FIGS. 4A-B describe a 1.times.2 optical switch. The same
mechanism can be operated in reverse to couple selectively optical
signals from the second and third waveguides into the first
waveguide, as well as assembled into a 2.times.1 optical
switch.
[0058] Although the embodiments described above relate to a
particular MEMS device configuration and geometry, the principles
of the present invention may similarly be applied, mutatis
mutandis, in other configurations and geometries using other
technologies. For example, an optical switch in accordance with
these principles may controllably couple optical signals between
first and second waveguides 21 and 23 without necessarily including
third waveguide 25 in the location that is shown in the figures. As
another example, other sorts of actuators, including actuators
using only a single electrode, or other means of moving the end
facets, may be used.
Multi-Switch Configurations
[0059] FIG. 5 is a schematic top view of an optical bypass-exchange
switch 100 based on four 1.times.2 optical switches, in accordance
with an embodiment of the invention.
[0060] Bypass-exchange switch 100 (or 2.times.2 switch) comprises
four optical 1.times.2 switches on a common substrate, with the
switches marked by dotted lines and labelled as 102, 104, 106, and
108. Each switch is substantially identical to optical switch 20 of
FIGS. 1-3, and the switches are configured pair-wise in a
back-to-back configuration. For clarity, the silicon layers, common
to all four switches, are not labelled. Similarly to optical switch
20, switches 102, 104, 106, and 108 comprise first, second, and
third waveguides according to Table 1, below.
TABLE-US-00001 TABLE 1 Labelling of waveguides in bypass-exchange
switch 100 first second third 1 .times. 2 switch waveguide
waveguide waveguide 102 112 116 120 104 122 126 130 106 132 136 140
108 142 146 150
[0061] Second waveguides 116 and 136 of switches 102 and 106,
respectively, are coupled to each other. Similarly, second
waveguides 126 and 146 of switches 104 and 108, respectively, are
coupled to each other. The third waveguides are coupled through a
waveguide cross-connector 152, marked by a dot-dot-dash line.
Waveguide cross-connector 152 has four rib waveguides extending out
from the cross-connector, from here onwards called legs, 191, 192,
193, and 194, coupled so that an optical signal entering leg 191
exits from leg 194 (and vice versa), and an optical signal entering
leg 192 exits from leg 193 (and vice versa). Thus, third waveguides
120 and 150 of switches 102 and 108, respectively, are coupled to
each other through legs 191 and 194, as are third waveguides 130
and 140 of switches 104 and 106 coupled to each other through legs
192 and 193, respectively.
[0062] Similarly to optical switch 20, each switch 102, 104, 106,
and 108 is actuated through electrodes formed in actuation layer
34. Electrodes 154, 156, and 158 actuate switches 102 and 104,
wherein electrode 158 is a common electrode for the two switches,
electrode 154 is dedicated to switch 102, and electrode 156 is
dedicated to switch 104. Similarly, electrodes 160, 162, and 164
actuate switches 106 and 108, wherein electrode 164 is a common
electrode for the two switches, electrode 160 is dedicated to
switch 106, and electrode 162 is dedicated to switch 108. The part
of actuation layer 34 that is not coupled to the electrodes is
coupled to a ground potential 166, causing the cantilever beams
under first waveguides 112, 114, 116, and 118 to be permanently at
the ground potential.
[0063] Electrodes 154, 156, 160, and 162 are all coupled to a first
voltage source 168 with an output voltage V.sub.1, and common
electrodes 158 and 164 are both coupled to a second voltage source
170 with an output voltage V.sub.2. Voltage sources 168 and 170 are
driven by a controller 172.
[0064] FIGS. 6A-B show schematically the functioning of
bypass-exchange switch 100, in accordance with an embodiment of the
invention.
[0065] The output voltages V.sub.1 and V.sub.2 of voltage sources
168 and 170, respectively, are driven by controller 172 so that one
of them is at a non-zero (positive or negative) voltage, while the
other one is at zero voltage. A non-zero voltage on an electrode is
indicated in FIGS. 6A-B by cross-hatching. For the sake of clarity,
voltage sources 168 and 170, ground 166, and controller 172 are not
shown in FIGS. 6A-B, and only electrodes 154, 156, 158, 160, 162,
and 164, as well as 1.times.2 switches 102, 104, 106, and 108 are
labelled.
[0066] In FIGS. 6A-B a first optical signal 180 and a second
optical signal 182, shown schematically as a solid white arrow and
a cross-hatched arrow, respectively, are shown both entering into
and exiting from bypass-exchange switch 100. In both of FIGS. 6a-b
first optical signal 180 enters first waveguide 112 of switch 102
and second optical signal 182 enters first waveguide 122 of switch
104.
[0067] In FIG. 6A, with reference to FIG. 5, voltage V.sub.1 of
voltage source 168 is non-zero and voltage V.sub.2 of voltage
source 170 is zero. This causes the potentials of electrodes 154,
156, 160, and 162 to be at a the non-zero voltage V.sub.1 (shown by
cross-hatching), and the potentials of electrodes 158 and 164 to be
at zero voltage V.sub.2. The end facets of the first and second
waveguides of each switch are in close proximity or in contact
(corresponding to FIG. 4A). First optical signal 180 propagates
from first waveguide 112 to second waveguide 116 (due to FTIR),
further to second waveguide 136, and (again due to FTIR) to first
waveguide 132, and exits from there. Similarly, second optical
signal 182 propagates from first waveguide 122 to second waveguide
126 (due to FTIR), further to second waveguide 146, and (again due
to FTIR) to first waveguide 142, and exits from there. This
describes a straight-through (bypass) state of bypass-exchange
switch 100.
[0068] In FIG. 6B, again with reference to FIG. 5, the voltages of
voltage sources 168 and 170 are interchanged by controller 172:
V.sub.1 is now zero and V.sub.2 is non-zero. Now the potentials of
electrodes 154, 156, 160, and 162 are at the zero voltage of
V.sub.1, and the potentials of electrodes 158 and 164 are at the
non-zero voltage of V.sub.2 (shown by cross-hatching). The end
facets of first and second waveguides of each switch are separated
(corresponding to FIG. 4B), and the signals propagate between the
first and third waveguides: First optical signal 180 propagates
from first waveguide 112 to third waveguide 120, through legs 191
and 194 of waveguide cross-connector 152 to third waveguide 150,
and further to first waveguide 142, exiting from there. Similarly,
second optical signal 182 propagates from first waveguide 122 to
third waveguide 130, through legs 192 and 193 of waveguide
cross-connector 152 to third waveguide 140, and further to first
waveguide 132, exiting from there. This describes a cross-state
(exchange) of bypass-exchange switch 100.
[0069] One bit of information provided to controller 172, that of
exchanging the values of voltages V.sub.1 and V.sub.2, is
sufficient to change the state of bypass-exchange switch 100 from
its straight-through state to cross-state and back.
[0070] The reversibility of the propagation of optical signals in
switches 102, 104, 106, and 108 and waveguide cross-connector 152
enables the reversing of optical signals 180 and 182.
[0071] In additional embodiments (not shown in the figures),
several bypass-exchange switches 100 may be coupled together for an
extension to larger switch fabrics.
[0072] FIG. 7 is a schematic top view of an optical crossbar switch
200 based on two 1.times.2 optical switches in a back-to-back
configuration, in accordance with an embodiment of the
invention.
[0073] Crossbar switch 200 comprises two 1.times.2 switches 202 and
204 on a common substrate, marked by dotted lines, each
substantially identical to optical switch 20 of FIG. 1. For
clarity, the silicon layers, common to both switches, are not
labelled. Similarly to optical switch 20, switches 202 and 204
comprise first, second, and third waveguides according to Table 2,
below.
TABLE-US-00002 TABLE 2 Labelling of waveguides in crossbar switch
200 first second third 1 .times. 2 switch waveguide waveguide
waveguide 202 212 216 220 204 222 226 230
[0074] A waveguide cross-connector 232, marked by a dot-dot-dash
line and similar to waveguide cross-connector 152, is coupled to
switches 202 and 204, with a first leg 234 of the cross-connector
coupled to second waveguide 216 and a second leg 236 coupled to
second waveguide 226. A third leg 238 and a fourth leg 240 end at
the edges of crossbar switch 200. Third waveguide 220 of switch 202
is coupled to third waveguide 230 of switch 204.
[0075] Similarly to optical switch 20, both switches 202 and 204
are actuated through electrodes 242, 244 and 246 formed in
actuation layer 34. Electrode 242 is dedicated to switch 202,
electrode 244 is dedicated to switch 204, and electrode 246 is a
common electrode for the two switches. The part of actuation layer
34 not coupled to the electrodes is coupled to a ground potential
247, causing the cantilever beams under first waveguides 212 and
222 to be permanently at the ground potential.
[0076] Electrodes 242 and 244 are both coupled to a first voltage
source 248, and common electrode 246 is coupled to a second voltage
source 250. Voltage sources 248 and 250 are driven by a controller
252.
[0077] FIGS. 8A-B show schematically the functioning of crossbar
switch 200, in accordance with an embodiment of the invention.
[0078] The output voltages V.sub.1 and V.sub.2 of voltage sources
248 and 250, respectively, are driven by controller 252 so that one
of them is at a non-zero (positive or negative) voltage, while the
other one is at zero voltage. A non-zero voltage on an electrode is
indicated in FIGS. 8A-B by cross-hatching of the electrode. For the
sake of clarity, voltage sources 248 and 250, ground 247, and
controller 252 are not shown in FIGS. 8A-B, and only electrodes
242, 244, and 246, 1.times.2 switches 202 and 204, and waveguide
cross-connector 232 with its legs 234, 236, 238, and 240 are
labelled.
[0079] In FIGS. 8A-B, a first optical signal 260 is shown
schematically as a solid white arrow and a second optical signal
262 is shown schematically as a cross-hatched arrow. First optical
signal 260 enters, with reference to FIG. 7, first waveguide 212 of
switch 202, and second optical signal 262 enters fourth leg 240 of
cross-connector 232.
[0080] In FIG. 8A, with reference to FIG. 7, voltage V.sub.1 of
voltage source 248 is non-zero and voltage V.sub.2 of voltage
source 250 is zero. Consequently, the potentials of electrodes 242
and 244 are at the non-zero voltage V.sub.1, and the potential of
electrode 246 is at the zero voltage V.sub.2. The end facets of the
first and second waveguides of both switches 202 and 204,
respectively, are in close proximity or contact (corresponding to
FIG. 4A). First optical signal 260, entering switch 202, propagates
from first waveguide 212 to second waveguide 216 (due to FTIR) and
further to first leg 234 of waveguide cross-connector 232, exiting
through third leg 238. Second optical signal 262, entering fourth
leg 240 of waveguide cross-connector 232, exits the cross-connector
through its second leg 236 and enters second waveguide 226 of
switch 204, propagates (due to FTIR) to first waveguide 222 and
exits from there.
[0081] In FIG. 8B, again with reference to FIG. 7, voltage source
248 voltage V.sub.1 is now zero and voltage source 250 voltage
V.sub.2 is non-zero. Consequently, the potentials of electrodes 242
and 244 are at the zero voltage V.sub.1, and the potential of
electrode 246 is at the non-zero voltage V.sub.2. The end facets of
the first and second waveguides of both switches 202 and 204,
respectively, are separated from each other (corresponding to FIG.
4B). First optical signal 260, entering switch 202, propagates now
(due to TIR) from first waveguide 212 to third waveguide 220,
continuing to third waveguide 230. Signal 260 further enters first
waveguide 222, and (due to TIR) propagates in the first waveguide
and exits from it at the edge of crossbar switch 200.
[0082] One bit of information, that of exchanging the values of
voltages V.sub.1 and V.sub.2, is sufficient for driver to drive
crossbar switch 200 between its two states.
[0083] In additional embodiments, several crossbar switches 200 may
be coupled together for an extension to larger switch fabrics, as
is known to those skilled in art.
Latching Switches
[0084] FIG. 9 is a schematic top view of a latching actuator 300,
in accordance with an embodiment of the invention.
[0085] Actuator 300 comprises a silicon beam 302 etched in
actuation layer 34 and clamped at its ends at two clamping points
304 and 306. Silicon beam 302 has two mechanically stable beam
configurations 302A and 302B, which are, respectively, convex to
the right and to the left. In any position between stable beam
configurations 302A and 302B beam 302 is under compressive stress.
In FIG. 9 silicon beam 302 is shown in its left stable beam
configuration 302B, with right stable beam configuration 302A shown
by dotted outlines.
[0086] Similarly to optical switch 20 of FIG. 1, electrodes 308 and
310 are defined by a trench 311 etched in actuation layer 34 down
to isolation layer 40 (seen in the bottom of trench 311 in the top
view). A first voltage source 312 and a second voltage source 314
are coupled to electrodes 308 and 310, respectively, and are driven
by a controller 316. Silicon beam 302 is coupled to a ground
potential 318 through the part of actuation layer 34 that is not
connected to electrodes 308 and 310.
[0087] FIGS. 10A-B are schematic top views of an optical latching
switch 400, in accordance with an embodiment of the invention.
Latching switch 400 incorporates latching actuator 300 described in
FIG. 9.
[0088] Latching switch 400 comprises a first waveguide 320, a
second waveguide 322, and a third waveguide 324. First and second
waveguides 320 and 322 have respective end facets 330 and 332 at
45.degree. angles, similarly to optical switch 20. Further,
similarly to optical switch 20, the end of third waveguide 324 that
is closest to first waveguide 320 is oriented perpendicularly to
the first waveguide, and has an end facet 334 at a 90.degree. angle
with respect to the longitudinal axis of the third waveguide.
[0089] For the sake of clarity, voltage sources 312 and 314, as
well as controller 316 are not shown in FIGS. 10A-B.
[0090] First waveguide 320 is integrally fixed to silicon beam 302
by optical isolation layer 52 (with reference to FIG. 3). Second
waveguide 322, however, is released from silicon beam 302 by
etching optical isolation layer 52 away during the fabrication
process from the area between the second waveguide and the silicon
beam. Consequently, a transverse movement of silicon beam 302 moves
only first waveguide 320, while second waveguide 322 remains
stationary. Third waveguide 324 is also stationary.
[0091] In FIG. 10A, controller 316 has driven voltage V.sub.1 of
voltage source 312 and the potential of electrode 308 to a non-zero
value (positive or negative, shown by cross-hatching), while
voltage V.sub.2 of voltage source 314 is zero. Due to electrostatic
forces, silicon beam 302 is pulled to its left position 302B, which
brings end facets 330 and 332 to within 5 nm of each other
(possibly contacting each other). This is identical to the
situation described in FIG. 4A, and an optical signal can propagate
by FTIR from first waveguide 320 to second waveguide 322 and vice
versa. Due to the mechanical bistability of silicon beam 302, it
will remain in left position 302b even if voltage V.sub.1 is
brought to zero.
[0092] In FIG. 10B, controller 316 has driven voltage V.sub.1 of
voltage source 312 and the potential of electrode 308 to zero,
while driving voltage V.sub.2 of voltage source 314 and the
potential of electrode 310 to a non-zero value (shown by
cross-hatching). Due to electrostatic forces, silicon beam 302 is
now pulled to its right position 302A, which separates end facets
330 and 332 and brings first waveguide 320 close to or in contact
with third waveguide 324. This is identical to the situation
described in FIG. 4B, and an optical signal will propagate by TIR
from first waveguide 320 to third waveguide 324 and vice versa.
Again, similarly to FIG. 10A, bringing voltage V.sub.2 to zero
leaves silicon beam 302 in right position 302A due to its
mechanical bistability.
[0093] As shown in FIGS. 10A-B, optical latching switch 400 is
switched from one state to another by voltages V.sub.1 and V.sub.2.
However, once a state has been reached, no voltage is required in
order to keep optical latching switch 400 in that state.
Manufacturing Process
[0094] FIGS. 11A-M and O-V are schematic sectional views showing
stages in a process flow of the fabrication of optical switch 20,
in accordance with an embodiment of the invention. FIG. 11N is a
schematic top view showing a detail of the one of the stages.
[0095] Those layers in the process flow that will become layers of
optical switch 20 are throughout FIGS. 11A-V labelled with the same
labels that are used in describing optical switch 20 in FIGS. 1-3.
The process utilizes lithography, deposition, and etching processes
known to those skilled in the art.
[0096] FIG. 11A shows a cross section of a silicon-on-insulator
(SOI) wafer 500, which is the starting point of the process. SOI
wafer 500 comprises silicon substrate 39, isolation layer 40
(buried layer of silicon dioxide SiO.sub.2 or other suitable
dielectric), and an actuator layer 34. Actuation layer 34 is
typically 10-50 .mu.m thick silicon, and is doped for increased
electrical conductivity. Isolation layer 40 is typically 1-2 .mu.m
thick.
[0097] FIG. 11B shows top and bottom alignment marks 502 and 504,
respectively, etched onto front and back surfaces 503 and 505,
respectively, of SOI 500.
[0098] FIG. 11C shows the result after thermal oxidation of front
surface 503, which has formed optical isolation layer 52,
comprising SiO.sub.2. Optical isolation layer 52 is typically 0.5-1
.mu.m thick. The thermal oxidation process forms an oxide layer on
back surface 505, as well. This back surface oxide layer is not
relevant for the continuation of the process, however, and has been
left out for the sake of clarity.
[0099] FIG. 11D shows SOI wafer 500 after deposition of a
photoresist layer 506.
[0100] FIG. 11E shows openings 508 opened in photoresist 506 in a
photolithographic process. A typical width of openings 508 is 1-3
.mu.m, and a typical width of the narrowest remaining strips of
photoresist 506 is 2-3 .mu.m.
[0101] FIG. 11F shows trenches 510 etched through openings 508
using an oxide etch followed by a silicon etch, stopping at
isolation layer 40. Roughness of the sidewalls of trenches 510 is
not critical at this stage of the process.
[0102] FIG. 11G shows SOI wafer 500 after photoresist 506 has been
removed. At this stage SOI wafer 500 is cleaned and prepared for a
fusion bond.
[0103] FIG. 11H shows how a second SOI wafer 512 has been fusion
bonded to SOI wafer 500. Second SOI wafer 512 is in a "flipped
over" orientation as compared to SOI wafer 500. Second SOI wafer
512 comprises a second substrate 516, a second buried oxide layer
514, and waveguide layer 33. Second buried oxide layer 514 is
typically 0.5-1 .mu.m thick. Waveguide layer 33 comprises undoped
silicon, typically 3 .mu.m thick.
[0104] FIG. 11I shows the result after second substrate 516 has
been removed by grinding and a silicon etch that stops at second
buried oxide layer 514.
[0105] In FIG. 11J a window 518 has been opened through oxide 514
and waveguide layer 33 to expose top alignment mark 502. A rough
position of window 518 is established via bottom alignment mark
504.
[0106] FIG. 11K shows the deposition of a photoresist 520.
[0107] FIG. 11L shows the result after lithography steps for
defining a single-mode shallow ridge waveguide 532 and multimode
full trench first waveguide 21 (both shown in FIG. 11M). A first
photoresist pattern 522 and a first oxide pattern 524 (with a
critical dimension typically 3-4 .mu.m) will define shallow ridge
waveguide 532, and a second photoresist pattern 526 and a second
oxide pattern 528 will define first waveguide 21. The lithography
is performed using a dual exposure technique in the photoresist.
Alternatively, it may be performed based on a first photoresist and
oxide etch, followed by depositing and defining a second
photoresist layer. At this process step a critical dimension of 300
nm is required for defining the waveguide gap (shown in FIG. 11N),
corresponding to a 1:10 aspect ratio for a 3 .mu.m etch depth.
[0108] FIG. 11M shows the result after a dry silicon etch for
defining shallow ridge waveguide 532 and first waveguide 21.
[0109] FIG. 11N shows a top view of first waveguide 21, with a
typical width of 3 .mu.m. A gap 534 in first waveguide 21 has a
typical width of 300 nm. Similar gaps may be formed between first,
second and third waveguides 21, 23, 25 in their neutral positions
as manufactured.
[0110] In FIG. 11O, all exposed silicon has been thermally oxidized
to generate an oxide layer 536 in order to reduce the sidewall
roughness of the waveguides. Repeated oxidization and etching may
be used for further reducing the sidewall roughness.
[0111] FIG. 11P shows another deposition of a photoresist 538.
[0112] FIG. 11Q shows the result after patterning of photoresist
538 to open trenches 540 and etching of optical isolation layer 52
through the trenches in preparation of deposition of metal
electrodes.
[0113] In FIG. 11R a gold layer 542 has been deposited over the
entire wafer.
[0114] FIG. 11S shows bond pads 544 formed using lift-off of
photoresist 538.
[0115] FIG. 11T shows the result after substrate 39 has been
thinned to a typical thickness of 400 .mu.m.
[0116] In FIG. 11U trough 50 has been opened in substrate 39 by
backside DRIE (deep reactive-ion etching) in order to prepare for
the release of cantilever beam 32. Typical lateral dimensions of
trough 50 are 100 .mu.m.times.20 .mu.m.
[0117] In FIG. 11V all exposed oxide has been etched, typically by
vapor etch. Over-etch is not allowed, as this may jeopardize the
bond between cantilever beam 32 and first waveguide 21. A frame 548
shows the part of the sectional view of FIG. 11V that corresponds
to FIG. 3.
[0118] Processes similar to the one shown in FIGS. 11A-V can be
used for fabricating bypass-exchange switch 100, crossbar switch
200, and latching switch 400.
[0119] Alternatively, other fabrication methods that are known in
the art, such as surface micro-machining, may be used.
[0120] FIGS. 12A-B are schematic top views of 4.times.4 optical
bypass-exchange switches 600 and 700, with some oblique crossings
in switch 600 and all right-angle waveguide crossings in switch
700, in accordance with an embodiment of the invention.
[0121] Bypass-exchange switches 600 and 700 are extensions of
bypass-exchange switch 100 from a 2.times.2 switch to a 4.times.4
switch. Each of bypass-exchange switches 600 and 700 comprises
eight 1.times.2 optical switches 20, in a pairwise back-to-back
configuration. The embodiments shown in FIGS. 12A-B illustrate
routing of the waveguide interconnects, i.e., to the waveguides
coupling the switches to each other, in a manner that minimizes the
optical cross-talk between the waveguides. The internal details of
the switches have been omitted for the sake of simplicity.
[0122] Bypass-exchange switch 600 comprises four 1.times.2 switches
610, 620, 630, and 640 on the left side of the figure, coupled
back-to-back to four 1.times.2 switches 650, 660, 670, and 680 on
the right side of the figure. The interconnects comprise waveguides
612, 614, 622, 624, 632, 634, 642, and 644 according to Table 3,
below.
TABLE-US-00003 TABLE 3 Interconnects within bypass-exchange switch
600 1 .times. 2 switch on left Waveguide 1 .times. 2 switch on the
right 610 612 650 614 670 620 622 660 624 680 630 632 650 634 670
640 642 660 644 680
[0123] Bypass-exchange switch 700 comprises, similarly to
bypass-exchange switch 600, four 1.times.2 switches 710, 720, 730,
and 740 on the left side of the figure, coupled back-to-back to
four 1.times.2 switches 750, 760, 770, and 780 on the right side of
the figure. The interconnects comprise waveguides 712, 714, 722,
724, 732, 734, 742, and 744 according to Table 4, below.
TABLE-US-00004 TABLE 4 Interconnects within bypass-exchange switch
700 1 .times. 2 switch on left Waveguide 1 .times. 2 switch on the
right 710 712 750 714 770 720 722 760 724 780 730 732 750 734 770
740 742 760 744 780
[0124] The optical crosstalk between two crossing waveguides
reaches a minimum when the angle between the waveguides is
90.degree.. Several of the interconnects of bypass-exchange switch
600 (although not necessarily all) cross each other at an oblique
angle, i.e., at an angle different from 90.degree., which leads to
non-optimal crosstalk. Examples of such oblique crossings may be
seen, for instance, at the crossing of waveguides 614 and 622 and
at the crossing of waveguides 624 and 634.
[0125] In bypass-exchange switch 700, the switches on the left are
connected to the switches on the right in the same order as those
in bypass-exchange switch 600. However, as opposed to
bypass-exchange switch 600, all of the interconnects are routed in
such a way that all waveguide crossings form 90.degree. angles.
Such a routing minimizes the crosstalk between the
interconnects.
[0126] FIGS. 13A-B are schematic top views of 8.times.8 optical
bypass-exchange switches 800 and 900, with some oblique crossings
in switch 800 and all right-angle waveguide crossings in switch
900, in accordance with an embodiment of the invention.
[0127] Bypass-exchange switches 800 and 900 are extensions of
4.times.4 bypass-exchange switches 600 and 700, respectively, with
each switch 800 and 900 comprising eight 1.times.2 switches on the
left side of FIGS. 13A-B, respectively, and eight 1.times.2
switches on the right side in a back-to-back configuration. As in
FIGS. 12A-B, the details of the 1.times.2 switches have been
omitted for the sake of clarity. The switches and their
interconnects are listed in Tables 5-6, below.
TABLE-US-00005 TABLE 5 Interconnects within bypass-exchange switch
800 1 .times. 2 switch on left Waveguide 1 .times. 2 switch on the
right 802 804 850 806 874 808 810 856 812 880 814 816 862 818 886
820 822 868 824 892 826 828 850 830 874 832 834 856 836 880 838 840
862 842 886 844 846 868 848 892
TABLE-US-00006 TABLE 6 Interconnects within bypass-exchange switch
900 1 .times. 2 switch on left Waveguide 1 .times. 2 switch on the
right 902 904 950 906 974 908 910 956 912 980 914 916 962 918 986
920 922 968 924 992 926 928 950 930 974 932 934 956 936 980 938 940
962 942 986 944 946 968 948 992
[0128] As in bypass-exchange switch 600, several of the
interconnects of bypass-exchange switch 800 cross each other at an
oblique angle, which leads to non-optimal crosstalk. Examples of
such oblique crossings may be seen, for instance, at the crossing
of waveguides 806 and 828 and at the crossing of waveguides 846 and
842.
[0129] In bypass-exchange switch 900, the switches on the left are
connected to the switches on the right in the same order as those
in bypass-exchange switch 800. However, as opposed to
bypass-exchange switch 800, all of the interconnects are routed in
such a way that all waveguide crossings form 90.degree. angles.
Such a routing minimizes the crosstalk between the
interconnects.
[0130] Although 4.times.4 and 8.times.8 bypass-exchange switches
700 and 900, respectively, are shown as illustrations of
interconnect routing with minimal crosstalk, other embodiments may
comprise bypass-exchange switches with, for example, 10.times.10,
16.times.16, or 32.times.32 layouts.
[0131] It will thus be appreciated that the embodiments described
above are cited by way of example, and that the present invention
is not limited to what has been particularly shown and described
hereinabove. Rather, the scope of the present invention includes
both combinations and subcombinations of the various features
described hereinabove, as well as variations and modifications
thereof which would occur to persons skilled in the art upon
reading the foregoing description and which are not disclosed in
the prior art.
* * * * *