U.S. patent application number 10/118688 was filed with the patent office on 2002-12-05 for micromachined optomechanical switching cell with parallel plate actuator and on-chip power monitoring.
Invention is credited to Fan, Li, Husain, Anis.
Application Number | 20020181852 10/118688 |
Document ID | / |
Family ID | 26834300 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020181852 |
Kind Code |
A1 |
Husain, Anis ; et
al. |
December 5, 2002 |
Micromachined optomechanical switching cell with parallel plate
actuator and on-chip power monitoring
Abstract
A number of micromachined optomechanical switching cells and
matrix switches including such switching cells are disclosed
herein. One optomechanical switching cell of the present invention
includes a parallel plate actuator positioned on a substrate. A
mirror coupled to the actuator is disposed to selectively redirect
an incident optical beam. The present invention also contemplates
an optomechanical matrix switch including a substrate and a
plurality of optomechanical switching cells coupled thereto. The
matrix switch further includes an arrangement for monitoring the
optical power incident upon, and output by, the matrix switch.
Inventors: |
Husain, Anis; (San Diego,
CA) ; Fan, Li; (San Diego, CA) |
Correspondence
Address: |
OMM, INC.
9410 CARROLL PARK DRIVE
SAN DIEGO
CA
92121
US
|
Family ID: |
26834300 |
Appl. No.: |
10/118688 |
Filed: |
April 8, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10118688 |
Apr 8, 2002 |
|
|
|
09483276 |
Jan 13, 2000 |
|
|
|
6453083 |
|
|
|
|
60136438 |
May 28, 1999 |
|
|
|
Current U.S.
Class: |
385/18 |
Current CPC
Class: |
H04Q 2011/0049 20130101;
H04Q 2011/003 20130101; G02B 26/085 20130101; G02B 6/357 20130101;
G02B 6/3586 20130101; G02B 6/352 20130101; G02B 6/3576 20130101;
H04Q 11/0005 20130101; G02B 6/3514 20130101; G02B 6/3546 20130101;
G02B 26/0833 20130101 |
Class at
Publication: |
385/18 |
International
Class: |
G02B 006/35 |
Claims
What is claimed is:
1. An optomechanical switching cell, comprising: a parallel plate
actuator positioned on a substrate; and a mirror coupled to said
actuator.
2. An optomechanical matrix switch including both a first
optomechanical switching cell and a second optomechanical switching
cell according to claim 1.
3. An optomechanical matrix switch, comprising: a substrate; a
plurality of optomechanical switching cells coupled to said
substrate, each of said plurality of optomechanical switching cells
including a mirror and an actuator; and means, disposed upon the
substrate, for monitoring the optical power incident upon the
matrix switch.
4. The optomechanical matrix switch of claim 3, wherein said means
includes a first beam splitter and a first photodetector.
5. The optomechanical matrix switch of claim 4, wherein said first
beam splitter splits a beam in a plane that is substantially
parallel to said substrate.
6. The optomechanical matrix switch of claim 5, wherein said first
beam splitter splits a beam in a plane that is substantially
perpendicular to said substrate.
7. The optomechanical matrix switch of claim 4, wherein said means
includes a second beam splitter and a second photodetector.
8. The optomechanical matrix switch of claim 3 further including
means, disposed upon the substrate, for monitoring the optical
power output by the matrix switch.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. provisional
application No. 60/134,438, entitled, "ASSEMBLY AND PACKAGING OF
MICROMACHINED OPTICAL SWITCHES", filed on May 28, 1999, and which
is incorporated herein in its entirety including any drawings.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates generally to the field of optical
switching. More particularly, the invention relates to the design,
fabrication, assembly and packaging of micro electro mechanical
systems (MEMS) technology optomechanical switching cells, and
N.times.M matrix switches composed thereof.
[0004] 2. Discussion of the Related Art
[0005] There are many different types of optical switches. In terms
of the switching mechanism, optical switches can be divided into
two general categories. The first general category of optical
switches employs a change of refractive index to perform optical
switching. This first general category can be termed "electrooptic
switches." Actually, the refractive index change can be induced by
electro-optic, thermal-optic, acousto-optic, or free-carrier
effects. In the last of these examples, free carriers are generated
by an electric charge introduced into a device, thereby causing a
change in the material's dipoles, which in turn changes the
material's index of refraction. Heretofore, the general category of
electro-optic switches was generally employed in the case of
coupled optical waveguides.
[0006] The second general category of optical switches employs
physical motion of one, or more, optical elements to perform
optical switching. In this way, a spatial displacement of a
reflected beam is affected. This second general category can be
termed "optomechanical switches."
[0007] Optomechanical switches offer many advantages over
electro-optic switches. Optomechanical switches have both lower
insertion loss and lower crosstalk compared to electro-optic
switches. Further, optomechanical switches have a high isolation
between their ON and OFF states. Furthermore, optomechanical
switches are bidirectional, and are independent of optical
wavelength, polarization, and data modulation format. An
optomechanical switch can be implemented either in a free-space
approach or in a waveguide (e.g., optical fiber) approach. The
free-space approach is more scalable, and offers lower coupling
loss compared to the waveguide approach.
[0008] Macro-scale optomechanical switches employing external
actuators are currently available. For example, conventional
optomechanical switches are available from JDS, DiCon, AMP, and
Hewlett Packard. However, one problem with this macro-scale
optomechanical switch technology is that macro-scale optomechanical
switches are bulky. Another problem with this technology is that
macro-scale optomechanical switches require extensive manual
assembly. Another problem with this technology is that the
switching speed of macro-scale optomechanical switches is slow. For
instance, the switching times for the currently commercially
available optomechanical switches range from 10 milliseconds to
several hundred milliseconds. An even more serious problem is that
their switching times often depends on their specific switching
path (i.e., how far is the distance from the next output port from
the current output port). This variation of switching time as a
function of spatial displacement is highly undesirable from a
systems integration point of view. Therefore, what is needed is a
solution that requires less bulk and less manual assembly, while
simultaneously providing faster and more consistent switching
speed.
[0009] Meanwhile, a number of different micromachining technologies
have been developing. Micromachining offers many advantages for
building optomechanical switches.
[0010] Micro electro mechanical systems (MEMS) technology is a
micromachining technique that uses a batch processing technique.
Micro electro mechanical systems technology is similar to
semiconductor electronics fabrication except that the resulting
devices possess mechanical functionality, as well as electronic
and/or optical functionality.
[0011] Micro electro mechanical systems technology is currently
used to fabricate movable microstructures and microactuators. The
use of micro electro mechanical systems technology to fabricate
optomechanical switches can significantly reduce the size, weight,
and cost of the resulting optomechanical switches.
[0012] Micro electro mechanical systems technology includes
bulk-micromachining and surface-micromachining techniques. Both
bulk-micromachining and surface-micromachining have been applied to
fabricate fiber optic switches.
[0013] Many optomechanical switches employ movable micromirrors.
Although there are many possible configurations for the
micromirrors, vertical micromirrors (i.e., the mirror surface is
perpendicular to the substrate) offer many advantages from the
architecture and packaging point of view. Using vertical
micromirrors, a simple matrix switch with a regular two-dimensional
array of switching cells can be realized. In more detail, the input
and output fibers can be arranged in the same plane as the matrix
substrate. Further, packaging is greatly simplified in this
configuration.
[0014] Most of the vertical micromirrors reported in the literature
have been fabricated by one of five methods. The first method is
anisotropic chemical etching of (110) silicon wafer (using, e.g.,
KOH solution). The second method is deep reactive ion etching
(DRIE). The third method is electroplating or the LIGA process. The
fourth method is flip-up micromirrors with surface-micromachined
microhinges. The fifth method is torsion mirrors.
[0015] Referring to the first method, anisotropic etching of (110)
silicon substrate can produce an atomically smooth micromirror
surface. However, a problem with the anisotropic etching method is
that monolithic integration of the micromirrors with the
microactuators is difficult. In an attempt to address this problem,
external bulk actuators have been used. In another approach to
addressing this problem, the micromirror substrate is simply glued
to a micro flap actuator. However, this is not a manufacturable
process. Therefore, what is also needed is a solution that
facilitates integration of the micromirrors with the microactuators
while simultaneously yielding a manufacturable process.
[0016] Referring to the second method, direct reactive ion etching
can produce vertical micromirrors with straight sidewalls (with an
aspect ratio of approximately 50:1). However, a problem with the
direct reactive ion etching method is that the surface of the
etched sidewalls tend to be rough. The Bosch DRIE process produces
a periodic corrugation on the sidewalls due to alternating
etching/coating process. The actuators of DRIE mirrors are usually
limited to comb drive actuators, which have a limited travel
distance. Therefore, what is also needed is a solution that
provides a smooth mirror surface while simultaneously providing a
large travel distance.
[0017] Referring to the third method, a problem with electroplated
micromirrors is that they often may not have straight or vertical
sidewalls. The LIGA process can produce high quality micromirrors,
however, it requires expensive X-ray lithography. Further,
integration with the actuators is a difficult issue for LIGA
micromirrors. Therefore, what is also needed is a solution that
provides an economical straight mirror surface while simultaneously
facilitating the integration of the micromirrors with the
microactuators.
[0018] Referring to both the fourth and fifth methods, the
microhinged mirrors and torsion micromirrors are usually made of
polysilicon plates. However, chemical-mechanical polishing (CMP) or
other process is usually required to smooth the resulting mirror
surface. This reduces the efficiency of the manufacturing process
by significantly increasing the number of process steps. In
addition, control of the mirror angle to within 0.5.degree. as
required by large matrix switches is difficult to achieve with
microhinged mirrors and torsion micromirrors. Therefore, what is
also needed is a solution that provides manufacturing efficiency
while simultaneously providing the required control of the mirror
angle.
[0019] Heretofore, the requirements of less bulk, less manual
assembly, faster and more consistent switching speed, integration
with actuators, smoothness and straightness of the mirror surface,
sufficient mirror travel distance, economy, manufacturing
efficiency, and control of the mirror angle referred to above have
not been fully met. What is needed is a solution that
simultaneously addresses all of these requirements.
SUMMARY OF THE INVENTION
[0020] A primary object of the invention is to provide an approach
to integrating optomechanical switching cell micromirrors and
microactuators that can be implemented on an optomechanical
switching matrix scale, or even on a wafer scale. Another primary
object of the invention is to provide an approach to
self-assembling optomechanical switching cell micromirrors and/or
microactuators. Another primary object of the invention is to
provide an approach to making optimechanical switching cell
micromirrors tilt-insensitive. Another primary object of the
invention is to provide a microactuated optomechanical switching
cell. Another primary object of the invention is to provide an
optomechanical matrix switch architecture for uniform fiber
coupling loss. Another primary object of the invention is to
provide input/output power monitoring for an optomechanical matrix
switch. Another primary object of the invention is to provide an
optomechanical matrix switch with integrated microlenses. Another
primary object of the invention is to provide an optomechanical
matrix switch with integrated wavelength division multiplexers
and/or demultiplexers. Another primary object of the invention is
to provide on-chip hermetic sealing for an optomechanical matrix
switch. Another primary object of the invention is to provide an
approach to aligning optomechanical matrix switches with optical
fiber ribbons.
[0021] In accordance with these objects, there is a particular need
for the invention. Thus, it is rendered possible to simultaneously
satisfy the above-discussed requirements of less bulk, less manual
assembly, faster and more consistent switching speed, integration
with actuators, smoothness and straightness of the mirror surface,
sufficient mirror travel distance, economy, manufacturing
efficiency, and control of the mirror angle, which, in the case of
the prior art, are mutual contradicting and cannot be
simultaneously satisfied.
[0022] A first aspect of the invention is implemented in an
embodiment that is based on a method of making an optomechanical
matrix switch, comprising: joining a plurality of mirrors on a
carrier to said plurality of actuators on a substrate; and removing
said carrier from said plurality of mirrors so as to form a
plurality of optomechanical switching cells on said substrate. A
second aspect of the invention is implemented in an embodiment that
is based on a method of making an optomechanical matrix switch,
comprising: positioning a plurality of mirrors adjacent a plurality
of actuators on a substrate; joining said plurality of mirrors to
said plurality of actuators so as to form a plurality of
optomechanical switching cells. A third aspect of the invention is
implemented in an embodiment that is based on an optomechanical
switching cell, comprising a tilt-insensitive mirror. A fourth
aspect of the invention is implemented in an embodiment that is
based on an optomechanical switching cell, comprising: an actuator
positioned on a substrate; and a mirror coupled to said actuator. A
fifth aspect of the invention is implemented in an embodiment that
is based on an optomechanical matrix switch, comprising: a
substrate; a plurality of optomechanical switching cells coupled to
said substrate, each of said plurality of optomechanical switching
cells coupled to said substrate, each of such plurality of
optomechanical switching cells including a mirror and an actuator;
and a switch architecture for uniform fiber coupling loss. A sixth
aspect of the invention is implemented in an embodiment that is
based on an optomechanical matrix switch, comprising: a substrate;
a plurality of optomechanical switching cells coupled to said
substrate, each of said plurality of optomechanical switching cells
including a mirror and an actuator; and a means for input/output
power monitoring. A seventh aspect of the invention is implemented
in an embodiment that is based on an optomechanical matrix switch,
comprising: a substrate; a plurality of optomechanical switching
cells coupled to said substrate, each of said plurality of
optomechanical switching cells including a mirror and an actuator;
and a plurality of integrated microlenses coupled to said
substrate. An eighth aspect of the invention is implemented in an
embodiment that is based on an optomechanical matrix switch,
comprising: a substrate; a plurality of optomechanical switching
cells connected to said substrate, each of said plurality of
optomechanical switching cells including a mirror and an actuator;
and a plurality of integrated wavelength division devices coupled
to said substrate. A ninth aspect of the invention is implemented
in an embodiment that is based on an optomechanical matrix switch,
comprising: a substrate; a plurality of optomechanical switching
cells coupled to said substrate, each of said optomechanical
switching cells including a mirror and an actuator; and a hermetic
seal coupled to said substrate, said hermetic seal providing a
substantially gas tight isolation of said plurality of
optomechanical switching cells. A tenth aspect of the invention is
implemented in an embodiment that is based on a method of aligning
an optomechanical matrix switch with an optical waveguide,
comprising: providing an optomechanical matrix switch on a
positioning stage; providing an optical waveguide on a substrate;
and positioning said optomechanical matrix switch by moving said
positioning stage relative to said substrate.
[0023] These, and other, objects and aspects of the invention will
be better appreciated and understood when considered in conjunction
with the following description and the accompanying drawings. It
should be understood, however, that the following description,
while indicating preferred embodiments of the invention and
numerous specific details thereof, is given by way of illustration
and not of limitation. Many changes and modifications may be made
within the scope of the invention without departing from the spirit
thereof, and the invention includes all such modifications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] A clear conception of the advantages and features
constituting the invention, and of the components and operation of
model systems provided with the invention, will. become more
readily apparent by referring to the exemplary, and therefore
nonlimiting, embodiments illustrated in the drawings accompanying
and forming a part of this specification, wherein like reference
characters designate the same parts. It should be noted that the
features illustrated in the drawings are not necessarily drawn to
scale.
[0025] FIGS. 1A-1B illustrate schematic perspective views of a
wafer scale assembly method, representing an embodiment of the
invention.
[0026] FIGS. 2A-2B illustrate schematic perspective views of a
mirror fabrication method, representing an embodiment of the
invention.
[0027] FIGS. 3A-3B illustrate schematic perspective views of two
different types of mirrors, representing embodiments of the
invention.
[0028] FIG. 4 illustrates a schematic perspective view of a wafer
scale assembly method, representing an embodiment of the
invention.
[0029] FIGS. 5A-5C illustrate schematic views of a matrix of flat
mirrors, representing an embodiment of the invention.
[0030] FIGS. 6A-6C illustrate schematic views of a matrix of
two-dimensional (2D) retro-reflectors, representing an embodiment
of the invention.
[0031] FIG. 7 illustrates a schematic side view of a thermal
actuator based switch, representing an embodiment of the
invention.
[0032] FIG. 8 illustrates a schematic side view of a parallel plate
based switch, representing an embodiment of the invention.
[0033] FIGS. 9A-9B illustrate schematic views of a matrix
architecture with uniform optical coupling loss, representing an
embodiment of the invention.
[0034] FIGS. 10A-10C illustrate schematic views of two types of
power monitoring capable matrixes, representing two embodiments of
the invention.
[0035] FIGS. 11A-11C illustrate schematic views of a matrix having
switches with integrated microlenses, representing two embodiments
of the invention.
[0036] FIG. 12 illustrates a schematic top view of a matrix having
switches and microlenses, representing an embodiment of the
invention.
[0037] FIG. 13 illustrates a schematic top view of a matrix having
switches with wave division multiplexing functionality,
representing an embodiment of the invention.
[0038] FIG. 14 illustrates a schematic perspective view of a matrix
with on-chip hermetic sealing, representing an embodiment of the
invention.
[0039] FIG. 15 illustrates a schematic top view of a matrix with
switches mounted on a positioning stage, representing an embodiment
of the invention.
[0040] FIG. 16 illustrates a schematic side view of a matrix with
switches mounted on a positioning stage.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0041] The invention and the various features and advantageous
details thereof are explained more fully with reference to the
nonlimiting embodiments that are illustrated in the accompanying
drawings and detailed in the following description. Descriptions of
well known components and processing techniques are omitted so as
not to unnecessarily obscure the invention in detail.
[0042] The below-referenced U.S. Patent Application discloses
micromachined optomechanical switches. The entire contents of U.S.
Ser. No. 09/093,644 are hereby expressly incorporated by reference
into the present application as if fully set forth herein.
[0043] The context of the invention is communication systems,
and/or computing systems, and/or any other systems where optical
switching can be implemented. The invention can also utilize data
processing methods that transform the optical signals so as to
actuate interconnected discrete hardware elements, such as, for
example, one or more of the optomechanical switching cells and/or
one or more of the optomechanical matrix switches and/or one or
more of the positioning stages.
[0044] Referring to the drawings, a detailed description of
preferred embodiments of the invention is provided with respect to
FIGS. 1A through 16. The most critical parameters for the micro
electro mechanical systems optomechanical switching cells and
matrix 11. switches are the smoothness and reflectivity of the
micromirrors; the angular variation of the micromirrors when they
direct the input beams to the output ports (e.g., fibers); and the
actuation mechanism. The quality and angular variation of the
micromirror depends on the fabrication technique for the
micromirror and how it is integrated with the actuator.
[0045] A. Wafer-Scale Mirror Attachment
[0046] The invention includes a wafer-scale, batch processing
technique for fabricating high-quality mirrors for micro electro
mechanical systems optical switches. This aspect of the invention
is based on fabricating the micromirrors and the actuators
separately on two different wafers, and then bonding the two wafers
together with each individual micromirror bonded to an individual
microactuator.
[0047] Referring to FIGS. 1A-1B, a micromirror wafer 110 includes a
plurality of micromirrors 120 positioned on a substrate 125. An
actuator wafer 130 includes a plurality of actuators 140 positioned
on a carrier 145. The actuator wafer 130 and the plurality of
actuators can be fabricated using MEMS technology. The micromirror
wafer 110 is depicted being lowered toward the actuator wafer 130
in FIG. 1A, but the two wafers can be brought together in any
orientation.
[0048] After the two wafers are brought together, the plurality of
micromirrors 120 are joined to the plurality of actuators 140. The
substrate of the micromirror wafer 110 (which can be termed a
carrier) is then removed from the micromirrors. This leaves a
plurality of optomechanical switching cells 150 arranged on the
substrate.
[0049] This approach, particularly when implemented at a wafer
scale, has at least two important advantages. First, the
micromirrors and the actuators are independently optimized. High
quality micromirrors and efficient actuators are thereby achieved
simultaneously in a single matrix switch. Second, more accurate
control of the micromirror angles is achieved. The variation in the
angle of the micromirrors is determined by the ratio of the
thickness variation of the bonding layer to the maximum dimension
of the wafer normal to the micromirror. For example, with bonded
mirrors, the variation of the bonding layer (glue) thickness across
the mirror (or wafer for batch process) will cause the mirrors to
tilt to various degrees. In the batch process, the mirrors are put
on all at once. In the alternative (when they are put on one by
one), the tolerance of alignment is determined by the size of the
mirrors. When the mirrors are attached on a wafer scale, the
tolerance is determined by the wafer size and not the micro-mirror.
When the mirrors are bonded one at a time, it usually results in a
large variation of mirror angles. Using the wafer-scale bonding,
the baseline is extended to the entire wafer. For example, the
angular variation is reduced to 0.003.degree. when micromirrors on
4-inch wafers are bonded directly, assuming the bonding layer
thickness variation is 10 um. This is to be compared to the angular
variation of 3.degree. when a single mirror with 100-um-wide base
is bonded to the actuator.
[0050] The micromirrors can be fabricated by anisotropic etching of
(110) silicon wafer using KOH, or by other wet or dry etching
techniques. The micromirrors can also be fabricated on non-silicon
wafers such as quartz. In more detail, the micromirrors can be
dry-etched on quartz wafer.
[0051] The micro electro mechanical systems chips will need to be
handled with extreme care, particularly at the dicing stage. The
micro electro mechanical systems devices cannot be released before
dicing, thus releasing cannot be done as a wafer-scale processing
step. This means that after fabrication, the actuators and moveable
structures are held rigid by the dielectric materials deposited in
between the structural layers (polysilicon). After the micro
electro mechanical systems chips are diced, the dielectric layers
are selectively removed (this process is called releasing). Then
the structures are free to move or be assembled into 3D
structures.
[0052] Without limiting the invention, the actuator can take the
form of electrostatic torsion plates. Examples of other actuators
include thermal actuators, bimorph actuators, electromagnetic
actuators, torsion plate actuators with permalloy, and actuators
based on stress-induced bending and/or electrostatic force.
[0053] A biomorph actuator can be a beam composed of a sandwich of
two dissimilar materials. The two beams have different thermal
expansion coefficient for a thermal biomorph. By putting a voltage
or heat on this combination the two metals behave differently and
cause the beam to bend. Therefore, a temperature rise will cause
the beam to bend. Alternatively, the beams may consist of
piezoelectric materials with opposite orientation so that one beam
contracts while the other extends when a voltage is applied.
[0054] A thermal actuator is similar to a thermal bimorph, but
instead of two layers being attached to each other, two beams are
attached to each other. Again, a different thermal expansion
coefficient will cause the beams to buckle. The heat can be
provided by passing a current through beams that are conductive and
generate heat as resistive heaters.
[0055] In general, any microactuator having a displacement greater
than the outgoing beam diameter can be used. The optical beam is
usually expanded and collimated, and the beam diameters are
typically on the order of approximately 100 um to approximately
1000 um.
[0056] The micromirror wafer and the actuator wafer can be bonded
together by known bonding techniques: including fusion bonding,
eutectic bonding, anodic bonding, and epoxy bonding. Anodic bonding
is carried out between glass and doped silicon materials at
elevated temperature with high voltage. Fusion bonding is a
thermo-compression bonding process, where the treated mating
surfaces are brought in contact at elevated temperature and
pressure. Eutectic bonding is bonding formed between two mating
surface in presence of an eutectic interface material. The
interface material is heated to a temperature above eutectic
melting point of that interface material. For example, AuSn one
surface and Au on the other surface will be used for eutectic
bonding at 37.degree. degrees C. Epoxy bonding: Epoxy is
essentially like a glue. Ultraviolet (UV) sensitive epoxy can be
used and can be cured by exposing the epoxy to UV light. Different
types of bonding processes have different reaction temperatures and
different requirements on the interface (Si, glass, or metal
coated).
[0057] B. Self-Assembled Micromirror Blocks
[0058] The invention includes a optomechanical matrix switch based
on self-assembled mirror blocks. Referring to FIG. 2, a low-cost
method for fabricating high quality micromirrors is depicted.
Instead of wafer-scale fabrication, the micromirrors can be
obtained by dicing a section of square quartz rod 210. The section
of square quartz rod 210 can be fabricated at very low cost. It is
separated into a plurality of individual mirror blocks 220 by
dicing or cleaving the quartz rod. The section of square quartz rod
can be coated with gold before dicing to increase mirror
reflectivity.
[0059] Referring to FIG. 3, the shape and length of sections 310
and 320 that are diced from the rod can be made assymetric so that
only a gold-coated side 315 and 320 will face the optical
beams.
[0060] Referring to FIG. 4, the mirror blocks 410 can be dropped
into a fixture 420 with arrays of alignment grooves (not shown) and
holes 430. The fixture 420 is aligned with an actuator wafer 440.
The mirror blocks for the entire N.times.M switch array can then be
bonded to actuator wafer simultaneously. A weight 450 can be
applied to exert a force to assist in the bonding. The attachment
of micromirror blocks can also extended to fluidic self-assembly,
voltage assisted self-assembly, DNA-assisted self assembly,
magnetic assisted self-assembly, etc. A significant advantage of
this self-assembly process is that no individual mirror placement
or alignment is needed. A further advantage of this approach is
that very high quality mirrors can be obtained by low-cost
fabrication processes.
[0061] C. Tilt-Insensitive Micromirrors
[0062] The invention includes optomechanical matrix switches based
on tilt-insensitive mirrors. Because of the long optical path
between the micromirrors and the output fibers in large N.times.M
(e.g., N.times.N) matrix switches, one of the most critical
parameters for the micromirrors is tilt angle.
[0063] Referring to FIG. 5A, an optomechanical matrix switch 510 is
depicted. The switch 510 includes a plurality of optomechanical
switching cells 520. FIGS. 5B-5C illustrate sections taken along
line AA in FIG. 5A. Referring to FIG. 5B, in the ideal case, the
cells 520 do not exhibit mirror tilting. Referring to FIG. 5C, when
a tilt is present, it results in walk-off of the output optical
beams reflected from different micromirrors. This will result in
large variation of the output coupling efficiency 16. (insertion
loss). The invention can include the use of orthogonally arranged
mirror facets that will significantly reduce the tilt sensitivity.
Instead of using flat micromirrors as shown in FIGS. 5A-5C, a
two-dimensional (2D) retroreflector can be used to direct input
beams to the output fibers.
[0064] Referring to FIGS. 6A-6C, an embodiment of the
tilt-insensitive mirror invention is illustrated. Referring to FIG.
6A, a micro retroreflector 610 composes part of an optomechanical
switching cell 620. The principle of the operation of the
retroreflector 610 is shown in FIGS. 6A-6C. When the retroreflector
610 is titled as in FIG. 6C, the reflected beams are still parallel
to the input beam, and the walk-off problem is substantially
eliminated. The retroreflectors can be termed corner cubes. The
essential elements of a corner cube is two orthogonal mirrors.
[0065] D. Microactuated Optomechanical Switching Cells
[0066] Using the wafer scale mirror attachment and self assembly
fabrication methods described above, many different types of
switching cells can be realized. Some specific examples of
optomechanical switching cells suitable for N.times.M micro electro
mechanical systems (MEMS) switches follow.
EXAMPLE (1)
[0067] The CMOS (complementary metal-oxide-semiconductor)
transistor process is a low-cost commercial foundry process.
Various types of micro electro mechanical systems actuators can be
made using the CMOS technology. The main issue for fabricating
optical matrix switches using CMOS is the difficulty of integrating
high quality vertical micromirrors with the microactuators.
Wafer-scale micromirror bonding techniques are particularly well
suited to making CMOS-based micro electro mechanical systems
optomechanical matrix switches. Two specific subexamples directed
to specific types of CMOS microactuators include a thermal actuator
and a parallel plate actuator.
[0068] Referring to FIG. 7, a CMOS thermal actuator includes a
cantilever beam 710 with materials of very different thermal
expansion coefficients. A first material 720 of higher thermal
expansion coefficient will shrink when the temperature is reduced,
and deflect the cantilever beam 710 upward, if the left edge of a
second material 730 (of lower expansion coefficient) is prevented
from sliding to the left by a structure (not shown).
[0069] Referring to FIG. 8, a parallel plate actuator can be
realized by undercutting a CMOS multilayer structure 810 with
selective etching. Parallel plate actuators are electrostatic force
between two parallel plates to move one moveable plate towards the
other fixed plate. By applying a bias between an upper CMOS layer
820 and a bottom 830 of an etched cavity 840, a suspended CMOS
plate 850 can be attracted downward.
[0070] An alternative CMOS embodiment is two plates that form a
wedge rather than a parallel structure (e.g., a >shape). In this
embodiment, assuming the lower plate is fixed, displacing the
radially supported actuator (upper plate) toward, or away from, the
lower plate will open and close the wedge. The main advantages of
the CMOS actuators include low cost, broad availability of CMOS
process, and monolithic integration with CMOS drive
electronics.
EXAMPLE (2)
[0071] Another example includes stress-induced electrostatic
gap-closing actuators with bonded vertical micromirror. A stress
can be used to generate an electric field using a piezoelectric
structure.
EXAMPLE (3)
[0072] Another example includes a torsion plate with bonded
vertical micromirror. The torsion plate includes a micromachined
plate that is mechanically hinged about a pivot axis to a
substrate.
EXAMPLE (4)
[0073] Another example includes a torsion plate with a permalloy
layer and bonded vertical micromirrors. Permalloy is a brand name
for any of a class of alloys of high magnetic permeability
containing from approximately 30 to approximately 90 percent, by
weight, of nickel. Thus, the torsion plate can be displaced with a
magnetic field.
EXAMPLE (5)
[0074] Another example includes a vertical mirrors on torsion plate
configured to move with a push-pull electrostatic force. Thus, the
torsion plate can be displaced with an electric field.
[0075] E. Matrix Switch Architecture for Uniform Fiber Coupling
Loss
[0076] Most of the volume of an optomechanical matrix switch is
composed of an array of free-space optical switches, an input fiber
array, and an output fiber array. Such arrangement, however, has
non-uniform optical insertion losses. In more detail, assuming the
ends of the fiber are coplanar, the optical path length is
different when each switching cell is activated (e.g., the optical
path length of input #1 to output #1 is less than that of input #1
to output #8).
[0077] Referring to FIGS. 9A-9B, the invention includes an
optomechanical matrix switch architecture that will have uniform
optical coupling loss, independent of which switch is activated. A
series of input fibers 910 are coupled to a substrate 920. An array
of optomechanical switching cells 930 is arranged on the substrate
920. A series of output fibers 940 are also connected to the
substrate 920. By staggering the input and output fibers with
increments equal to the size of the switch cell, an equal optical
path length is approximated. Thus, it can be appreciated that the
path length from input fiber #4 to output fiber #1 is approximately
equal to the path length from input fiber #2 to output fiber #1.
The staggered configuration depicted in FIG. 9A will result in a
more uniform optical-insertion loss.
[0078] Referring to FIG. 9B, an input fiber 950 can be provided
with a lens 960. Similarly, an output fiber 970 can be provided
with a lens 980.
[0079] F. Matrix Switch with On-Chip Input/Output Power
Monitoring
[0080] Monitoring of the input and output powers of an optical
matrix switch is very desirable for the application of the switch
in telecommunication networks. Power monitoring can be effected
with photodetector arrays. Because of the compact construction, the
micro electro mechanical systems optomechanical switch of the
invention offers unique advantages for integrating the
photodetector arrays on the switch chip for power monitoring. The
cost of adding this function to the switch is much lower for the
monolithic micro electro mechanical systems switches than for macro
scale optomechanical switches.
[0081] FIGS. 10A-10B illustrate two architectures of the micro
electro mechanical systems optical switch with input/output power
monitoring capabilities. Referring to FIG. 10A, a plurality of
vertical beamsplitters 1040 can be employed to deflect part of the
optical beams to an input photodetector array 1010 and an output
photodetector array 1020. The deflection in this embodiment is
coplanar with a substrate 1050.
[0082] Referring to FIGS. 10B-10C, in an alternative embodiment, a
plurality of 45.degree. beamsplitters 1060 can be employed to
reflect part of the optical beams out of the switch 20. plane to
the photodetector arrays 1070 above the micro electro mechanical
systems optical switch chip 1080. The deflection in this embodiment
is perpendicular to a substrate 1090.
[0083] In either embodiment, the beamsplitters should be almost
transparent (e.g., 1% reflection) to reduce the optical insertion
loss. The beamsplitters 1040 in FIG. 10(a) could be monolithically
fabricated with the micro electro mechanical systems chip using the
surface-micromachining microhinge technique. It is also possible to
mount high quality external beamsplitters on the chip. The
beamsplitters 1060 in FIGS. 10B-10C could be fabricated
monolithically, or attached to the packages of the photodetector
arrays so that the beamsplitter/photodetect- ors could be simply
dropped onto the micro electro mechanical systems switch chip.
[0084] In either embodiment, with the unique micro electro
mechanical systems optical switch construction, it is also possible
to attach a photodetector array 1005 at the opposite end of the
input fibers to monitor possible failure micro electro mechanical
systems micromirrors. For normal switch operation, at least one of
the micromirrors in each column will be turned on. Therefore, no
photocurrent will be registered in the photodetector array 1005. By
combining information from the photodetector array 1005 with
information from the output power monitoring, it is possible to
identify failed micromirrors. A suitable photo-detector device can
be provided by a p-n junction, for example, In Ga As, or Silicon or
GaAs.
[0085] G. Optomechanical Matrix Switch with Integrated
Microlenses
[0086] The size of the micro electro mechanical systems optical
switch is limited by the maximum coupling distance between the
input and the output fibers. To facilitate optical alignment, fiber
collimators are employed for both input and output fibers. The
maximum coupling distance between the input and output collimators
determines the maximum size of the switch. For example, if the
maximum coupling distance is 2 cm, and the switch cell area is 1
mm.times.1 mm, then the largest switch that can be realized is
10.times.10. To increase the dimension of the switch, it is
therefore desirable to be able to extend the coupling distance
without sacrificing the coupling efficiency.
[0087] Referring to FIGS. 11A-11C, the invention includes
integrating microlenses on the micro electro mechanical systems
optomechanical switching cells 1110. This will extend the coupling
distance without sacrificing the coupling efficiency. Referring to
FIGS. 11B-11C, the microlenses can be directly formed on the
surface of the micromirror 1120. FIG. 11C illustrates a schematic
drawings of the micromirror 1120 with an integrated diffractive
microlens 1130. FIG. 11B illustrates a schematic drawing of the
micromirror 1120 with an integrated refractive microlens 1140. The
microlenses function as relay lenses to extend the coupling
distance while maintaining the same optical insertion loss. It is
noted that the microlens for each micromirror should be different
for uniform coupling efficiency.
[0088] The diffractive and refractive microlenses 1130 and 1140 can
be integrated with surface-micromachined micro electro mechanical
systems structures. A refractive lens can be integrated on a micro
electro mechanical systems flip up structure.
[0089] The integrated microlens acts as a relay lens. As the beam
is loosing collimation, these integrated microlenses bring the beam
back into collimation (parallel beams once again). Refractive
lenses can be made with grinding glass, or moulding glass, or
putting a gradient index in a cylinder of glass (GRIN). Diffractive
lenses can be made by moulding or etching indentations in a piece
of glass or other material according to a computer generated set of
masks.
[0090] Referring to FIG. 12, a plurality of microlenses 1210 can
also be integrated in between two of a plurality of micro electro
mechanical systems micromirrors 1220 to extend the coupling
distance. This is equivalent to stitching smaller micro electro
mechanical systems optical switches together to form a larger
dimension switch while maintaining almost the same optical
insertion loss.
[0091] In either the embodiment shown in FIGS. 11A-11C, or the
embodiment shown in FIG. 12, without the microlenses, due to
Gaussian optics, the collimated beam will diverge again after a
certain distance. This is known as throw distance. For large matrix
switches it is highly desirable for the beam to stay collimated
(otherwise it will become bigger than the switch and clipping loss
will occur). Having another lens to "help" it stay collimated is
the main purpose of integrating the lens onto the mirror
itself.
[0092] H. Matrix Switch with Integrated WDM Components
[0093] Referring to FIG. 13, the invention includes providing an
optomechanical matrix switch 1310 with one or more wavelength
division devices 1320. The wavelength division devices 1320 can
include wavelength division multiplexers and/or wavelength division
demultiplexers. The wavelength-division-multiplexing (WDM)
components can be integrated with the micro electro mechanical
systems optical switch to form more functional WDM micro electro
mechanical systems switches. One particular embodiment of such
device is shown in FIG. 13. Instead of micromirrors, the embodiment
depicted in FIG. 13 includes WDM components. For example, a first
WDM micromirror 1330 reflects wavelength .lambda.1 only when the
mirror is turned on; a second WDM micromirror 1340 reflects
.lambda.2 only when the WDM mirror is turned on; etc. Such a device
can perform selective WDM add-drop multiplexing as well as optical
switching. It is more powerful than combining discrete optical
switches and external WDM multiplexers and/or demultiplexers. This
means that the switch can be combined with WDM
multiplexers/demultiplexers to form wavelength-selective add/drop
filters that are programmable. It is better than combining a
separate switch and a separate WDM filter because the coupling loss
is reduced (there is no need to couple into fiber and then expand
the beam from the fiber again).
[0094] I. Matrix Switch with On-Chip Hermetic Sealing
[0095] Hermetic sealing is very important for the operation of
micro electro mechanical systems actuators and to reduce in-use
stiction. Conventional hermetic sealing is applied at the package
level.
[0096] The invention includes on-chip hermetic sealing. On-chip
hermetic sealing is very attractive for optical micro electro
mechanical systems devices. Since the micro electro mechanical
systems optomechanical devices are accessed by optical beams, the
micro electro mechanical systems optomechanical devices can
actually be sealed before dicing the chip.
[0097] Referring to FIG. 14, an optomechanical matrix switch 1410
with on-chip hermetic sealing feature is depicted. A sealing
structure 1420 (e.g., a transparent cap) is connected to a
substrate 1430.
[0098] In this way, the micro electro mechanical systems
optomechanical matrix switches can be fabricated by connecting the
mirrors to the actuators at a wafer scale. Then the constraining
structure that holds the actuators can be released. Then the
devices can be hermetically sealed with the transparent cap. After
sealing, the wafer can be treated as integrated circuit (IC)
wafers, and be diced.
[0099] J. Alignment of Matrix Switch with Fiber Ribbons
[0100] One of the unique advantages of the invention is the
dramatically simplified optical packaging procedure. The input and
output fibers can be integrated monolithically with the micro
electro mechanical systems optical switching chip by etching
V-grooves for aligning the fibers. This will totally eliminate the
optical alignment step in packaging. However, some optical
alignment may be necessary because of the non-perfect angle of the
micromirrors.
[0101] The invention includes a micro electro mechanical systems
optomechanical matrix switch combined with fiber ribbons. By
employing fiber ribbons for the input and output fibers, the
optical alignment is greatly simplified. Instead of aligning 2N
individual fibers, there is only a need to align 2 fiber
ribbons.
[0102] Moreover, referring to FIG. 15, the invention also includes
combining a micro electro mechanical systems optomechanical matrix
switch 1510 with output fibers 1520 and input fibers 1530 located
in V-grooves provided on a fiber-package chip 1540. By mounting the
fiber ribbons on V-grooves on the same silicon (Si) substrate, all
fibers are automatically aligned. Thus, the only alignment that
needs to be effected, is alignment of the fiber-package chip 1540
to the chip upon which the micro electro mechanical systems
optomechanical matrix switch 1510 is located.
[0103] Still referring to FIG. 15, by placing the micro electro
mechanical systems chip on a 5-axis stage 1550 the micro electro
mechanical systems chip can be perfectly aligned to the
fiber-package chip.
[0104] Referring to FIG. 16, the mirror tilting angle is a critical
parameter. A fiber package chip 1610 includes a plurality of
optical fibers 1620 and a cylindrical lens 1630. A micro electro
mechanical systems matrix switch chip 1640 includes a plurality of
micro mirrors 1650. Even with perfect design, the mirror angle
might still deviate slightly from 90.degree.. With two degrees of
freedom in rotation, this non-ideal mirror angle can be corrected
during the packaging step. The tilt control restores the mirror to
perfect 90.degree. angle, while the rotation and linear
translations accurately position the micro electro mechanical
systems chip. Perfect alignment is represented in FIG. 16 by
arrowheads pointing in opposite directions on a single ray.
[0105] Practical Applications of the Invention
[0106] A practical application of the invention that has value
within the technological arts is in telecommunication networks,
optical instrumentation, and optical signal processing systems. In
telecommunication networks, the invention can be applied to network
restoration, reconfiguration, and dynamic bandwidth allocation. For
instance, the invention can be embodied in an optical crossbar
switch (N.times.M matrix switch) which is a general purpose switch
that is very useful for reconfiguring large telecommunication fiber
optic networks, restoration of services and dynamic allocation of
bandwidth. There are virtually innumerable uses for the invention,
all of which need not be detailed here.
[0107] Advantages of the Invention
[0108] An optomechanical switch made with micro electro mechanical
systems technology offers significant advantages over conventional
optomechanical switches for realizing optical crossbar switches.
Since the surface area (footprint) of a micro electro mechanical
systems fabricated switching cell is very small (e.g., from a few
hundred micrometers to a few millimeters), an entire N.times.M
switching matrix can be monolithically integrated on a single
substrate (e.g., a single silicon integrated circuit chip). This
significantly reduces the packaging cost of the switch. It also
enables the entire switch to be hermetically packaged, which is a
very important factor for the switch to satisfy the temperature and
humidity requirements such as those in the Bellcore standard.
[0109] The switching time can also be reduced because of their
higher resonant frequency. The resonant frequency is proportional
to the square root of the ratio of spring constant and mass. Switch
cells fabricated in accordance with the invention can be much
smaller (e.g., 10-100 smaller) physically than bulk mechanical
switches. Consequently, switch cells fabricated in accordance with
the invention can have smaller mass and, therefore, a higher
resonant frequency. The higher resonant frequency is directly
proportional to the speed of switching of the device. Furthermore,
an optomechanical switch made with micro electro mechanical systems
technology can be more rugged than the macro-scale switches because
the inertial forces are much smaller in the micro-scale
switches.
[0110] All the disclosed embodiments of the invention described
herein can be realized and practiced without undue experimentation.
Although the best mode of carrying out the invention contemplated
by the inventors is disclosed above, practice of the invention is
not limited thereto. Accordingly, it will be appreciated by those
skilled in the art that the invention may be practiced otherwise
than as specifically described herein.
[0111] For example, the individual components need not be formed in
the disclosed shapes, or assembled in the disclosed configuration,
but could be provided in virtually any shape, and assembled in
virtually any configuration. Further, the individual components
need not be fabricated from the disclosed materials, but could be
fabricated from virtually any suitable materials. Further, although
the N.times.M matrices are described herein as physically separate
modules, it is understood that the matrices may be integrated into
the apparatus with which they are associated. Furthermore, all the
disclosed elements and feature of each disclosed embodiment can be
combined with, or substituted for, the disclosed elements and
features of every other disclosed embodiment except where such
elements or features are mutually exclusive.
[0112] It is understood that various additions, modifications and
rearrangements of the features of the invention may be made without
deviating from the spirit and scope of the underlying inventive
concept. It is intended that the scope of the invention as defined
by the appended claims and their equivalents cover all such
additions, modifications, and rearrangements. The appended claims
are not to be interpreted as including means-plus-function
limitations, unless such a limitation is explicitly recited in a
given claim using the phrase "means-for." Expedient embodiments of
the invention are differentiated by the appended subclaims.
* * * * *