Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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INTERLACED ARRAY OF PIANO MEMS MICROMIRRORS
CROSS-REFERENCE TO RELATED APPLICATIONS
[01] The present application claims priority from United States Patent
Application
No 10/850,407 filed May 21, 2004. The present application also claims priority
from
United States Patent Application No. 60/558,563 filed April 2, 2004.
TECHNICAL FIELD
[02] The present invention relates to a micro-electro-mechanical (MEMs)
mirror
device for use in an optical switch, and in particular to an interlaced arrays
of MEMs
mirrors providing minimal electrical cross talk between mirrors.
BACKGROUND OF THE INVENTION
[03] Conventional MEMs mirrors for use in optical switches, such as the one
disclosed in United States Patent No. 6,535,319 issued March 18, 2003 to
Buzzetta et al,
to redirect beams of light to one of a plurality of output ports include an
electro-statically
controlled mirror pivotable about a single axis. Tilting MEMs mirrors, such as
the ones
disclosed in United States Patent No 6,491,404 issued December 10, 2002 in the
name of
Edward Hill, and United States Patent Publication No. 2003/0052569, published
March 20,
2003 in the name of Dhuler et al, comprise a mirror pivotable about a central
longitudinal
axis, and a pair of electrodes, one on each side of the central longitudinal
axis for actuating
the mirror. The Dhuler et al reference discloses the positioning of electrodes
at an angle to
the mirrored platform to improve the relationship between the force applied to
the mirror
and the gap between the mirror and the electrodes. The MEMs mirror device,
disclosed in
the aforementioned Hill patent, is illustrated in Figure 1, and includes a
rectangular planar
surface 2 pivotally mounted by torsional hinges 4 and 5 to anchor posts 7 and
8,
respectively, above a substrate 9. The torsional hinges may take the form of
serpentine
hinges, which are disclosed in United States Patent No 6,327,855 issued
December 11,
2001 in the name of Hill et al, and in United States Patent Publication No.
2002/0126455
published September 12, 2002 in the name of Robert Wood. In order to position
conventional MEMs mirror devices in close proximity, i.e. with a high fill
factor, fill
factor=width/pitch, they must be positioned with their axes of rotation
parallel to each
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other. Unfortunately, this mirror construction restraint greatly restricts
other design
choices that have to be made in building the overall switch.
[04] When using a conventional MEMs arrangement, the mirror 1 positioned on
the planar surface 2 can be rotated through positive and negative angles, e.g.
2 , by
attracting one side 10a or the other side 10b of the planar surface 2 to the
substrate 6.
Unfortunately, when the device is switched between ports at the extremes of
the devices
rotational path, the intermediate ports receive light for fractions of a
millisecond as the
mirror 1 sweeps the optical beam past these ports, thereby causing undesirable
optical
transient or dynamic cross-talk.
[05] One solution to the problem of dynamic cross-talk is to initially or
simultaneously rotate the mirror about a second axis, thereby avoiding the
intermediate
ports. An example of a MEMs mirror device pivotable about two axes is
illustrated in
Figure 2, and includes a mirror platform 1 I pivotally mounted by a first pair
of torsion
springs 12 and 13 to an external gimbal ring 14, which is in turn pivotally
mounted to a
substrate 16 by a second pair of torsion springs 17 and 18. Examples of
external gimbal
devices are disclosed in United States Patents Nos. 6,529,652 issued March 4,
2003 to
Brenner, and 6,454,421 issued September 24, 2002 to Yu et al. Unfortunately,
an external
gimbal ring greatly limits the number of mirrors that can be arranged in a
given area and
the relative proximity thereof, i.e. the fill factor. Moreover, the external
gimbal ring may
cause unwanted reflections from light reflecting off the support frame. These
references
also require at least four electrodes to actuate each mirror.
[06] Another proposed solution to the problem uses high fill factor
mirrors, such as the
ones disclosed in United States Patent No. 6,533,947 issued March 18, 2003 to
Nasiri et al,
which include hinges hidden beneath the mirror platform. Unfortunately, these
types of mirror
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. =
devices require costly multi-step fabrication processes, which increase costs
and result in low
yields, and rely on four different pairs of electrodes for actuation.
[07] An object of the present invention is to overcome the shortcomings of
the prior art by
providing two interlaced arrays of MEMs mirror devices, the mirrors in each
array being
pivotable about different parrallel axes, thereby increasing the spacing
between electrodes and
minimizing electrical cross-talk.
SUMMARY OF THE INVENTION
[08] Accordingly, the present invention relates to a micro-electro-
mechanical device A
micro-electro-mechanical device mounted on a substrate comprising:
[09] an array of first pivoting members pivotally mounted on said substrate
about a first
lateral axis, each of said first pivoting members including a first and a
second supporting region
on opposite sides of said first lateral axis;
[10] an array o f s econd p ivoting m embers p ivotally mounted o n s aid s
ubstrate about a
second lateral axis parallel to the first lateral axis, each of said second
pivoting members
including a first supporting region interleaved with the first supporting
regions of the first array
of pivoting members between said first and second lateral axes, and a second
supporting region
on an opposite side of the second lateral axis; and
[11] a first electrode beneath each of the second supporting region for
pivoting the first
and second pivoting members about the first axis from a rest position.
BRIEF DESCRIPTION OF THE DRAWINGS
[12] The invention will be described in greater detail with reference to
the accompanying
drawings which represent preferred embodiments thereof, wherein:
[13] Figure 1 is an isometric view of a conventional tilting MEMs mirror
device;
[14] Figure 2 is a plan view of a pair of conventional external gimbal ring
MEMs mirror
devices;
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,
[15] Figure 3 is an isometric view of a plurality of Piano-MEMs mirror
devices;
[16] Figure 4 is an isometric view of a hinge structure of the mirror
devices of Fig. 3;
[17] Figure 5 is an isometric view of an electrode structure of the mirror
devices of Fig. 3;
[18] Figure 6 is an isometric view of a plurality of Piano-MEMs mirror
devices according
to an alternative embodiment of the present invention with electrode shields,
light redirecting
cusps, and a raised ground plane;
[19] Figure 7 is an isometric view of a plurality of Piano-MEMs mirror
devices according
to an alternative embodiment of the present invention with electrode shields;
[20] Figure 8 is a plan view of a pair of internal gimbal ring MEMs mirror
devices
according to the present invention;
[21] Figure 9 is an isometric view of an internal gimbal ring MEMs mirror
device
according to the present invention;
[22] Figure 10 is an isometric view of an alternative embodiment of the
internal gimbal
ring MEMs mirror devices according to the present invention;
[23] Figure lla is an isometric view of a hinge structure of the mirror
devices of Fig. 9;
[24] Figure 1 lb is an isometric view of an alternative hinge structure of
the mirror devices
of Fig. 9;
[25] Figure 12 is an isometric view of an electrode structure of the mirror
devices of Figs.
9 and 10;
[26] Figure 13 is a graph of Voltage vs Time provided by the electrode
structure of Fig.
11;
[27] Figure 14 is an isometric view of internal gimbal ring MEMs mirror
devices utilizing
a three electrode arrangement according to the present invention;
[28] Figure 15 is an isometric view of the three electrode arrangement of
Figure 14;
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[29] Figure 16 is a plan view of an ideal placement of the three electrodes
of Figures 14
and 15 relative to the pivoting platform;
[30] Figure 17 is a plan view of a possible misalignment of the three
electrodes of Figures
14 and 15;
[31] Figure 18 is a plan view of another possible misalignment of the three
electrodes of
Figures 14 and 15;
[32] Figure 19 is a graph of Voltage vs Time for the three electrodes of
Figure 14 and 15;
[33] Figure 20 is an isometric view of another embodiment of the present
invention with
an offset section on the pivoting member;
[34] Figure 21 is a plan view of the embodiment of Figure 20;
[35] Figure 22 is an end view of the embodiment of Figures 20 and 21;
[36] Figure 23 is a top view of a pair of adjacent interlaced mirrors of
Figs. 20 to 22 with
an alternative reflective cusp;
[37] Figure 24 is a cross-sectional view taken along line A-A from Fig. 23;
[38] Figure 25 is a cross-sectional view taken along line B-B from Fig. 23;
[39] Figure 26 is a top view of an alternative embodiment of interlaced
mirror arrays;
[40] Figure 27 is a side view of the embodiment of Fig. 26;
[41] Figure 28 is a schematic diagram of a wavelength switch utilizing the
mirror devices
of the present invention;
[42] Figure 29 is a schematic diagram of an input/output assembly for the
wavelength
switch of Fig 28; and
[43] Figure 30 is a schematic diagram of an alternative embodiment of an
input assembly
for the wavelength switch of Fig. 28.
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DETAILED DESCRIPTION
[44] An array of "Piano" MEMs mirror devices 21, 22 and 23, which pivot
about a single
axis of rotation 03, above a substrate 25, is illustrated in Figures 3, 4 and
5. Each mirror device
21, 22 and 23 includes a pivoting member or platform 26 defined by first and
second
substantially-rectangular planar supporting regions 27 and 28 joined by a
relatively-thin
substantially-rectangular brace 29 extending therebetween. Typically, each
planar surface is
coated with a reflective coating, e.g. gold, for simultaneously reflecting a
pair of sub-beams of
light traveling along parallel paths, as will be hereinafter discussed. Each
brace 29 acts like a
lever and is pivotally mounted to anchor posts 30 and 31 via first and second
torsional hinges 32
and 33, respectively. The anchor posts 30 and 31 extend upwardly from the
substrate 25. The
ends of the first torsional hinge 32 are connected to the anchor post 30 and
the brace 29 along the
axis Oy. Similarly, the ends of the second torsional hinge 32 are connected to
the anchor post 31
and the brace 29 along the axis 0y. Preferably, each of the first and second
torsional hinges 32
and 33 comprises a serpentine hinge, which are considerably more robust than
conventional
torsional beam hinges. The serpentine hinge is effectively longer than a
normal torsional hinge,
which spans the same distance, thereby providing greater deflection and
strength, without
requiring the space that would be needed to extend a normal full-length
torsional hinge.
[45] With p articular r eference t o F igure 5, e ach p latform 2 6 i s r
otated b y t he s elective
activation of a first electrode 36, which electro-statically attracts the
first planar section 27
theretowards or by the selective activation of a second electrode 37, which
electro-statically
attracts the second planar section 28 theretowards. A gap 38 is provided
between the first and
second electrodes 36 and 37 for receiving the anchor posts 31, which extend
from the substrate
25 to adjacent the platforms 26.
[46] In the disclosed open loop configuration, the angular position of the
platforms 26 depend
non-linearly on the voltage applied by the electrodes 36 (or 37), i.e. as the
applied voltage is
increased linearly, the incremental change in angular platform position is
greater as the voltage
increases. Accordingly, there is a maximum voltage, i.e. an angular platform
position, at which
the platform angular position becomes unstable and will uncontrollably tilt
until hitting part of
the lower structure, e.g. the electrode 36. This maximum voltage sets the
range of angular
motion that the platform 26 can travel. The instability in the platform's
angular position is a
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result of the distance between the platform 26 and the electrode 36 (the hot
electrode) decreasing
more rapidly at the outer free ends of the platform 26 than at the inner
sections, nearer the pivot
axis or As a result, the force per unit length along the platform 26 increases
more rapidly at the
outer free ends of the platform 26 than the inner sections. To increase the
platform's range of
angular motion, the field strength, i.e. the force per unit area, that is
sensed at the outer free ends
of the platform 26 must be reduced. With reference to Figures 5, this is
accomplished by
providing the electrodes 36 and 37 with a two-step configuration. Upper steps
36a and 37a are
positioned proximate the inner end of the platform 26, i.e. the Y axis, while
lower steps 36b and
37b are positioned under the outer free ends of the platform 26, thereby
making the gap between
the platforms 26 and the electrodes 36 and 37 greater at the outer free end
than the inner end.
The area of the lower steps 36b and 37b can also be made smaller, thereby
reducing the force per
unit area sensed by the outer free end of the platform 26. Multi-step
electrodes, e.g. three or
more can also provide a more even distribution of force.
[47] A consequence of closely packed micro-mirrors is that the actuation of
a single
mirror will impart a torque, i.e. an angular rotation, onto adjacent mirrors
as a result of fringing
electric fields. In an effort to minimize this cross-talk, electrode grounding
shields 41 are
positioned on the substrate 25 around or on either side of the first and
second electrodes 36 and
37 forming electrode cavities, which are electrically isolated from each
other. Figure 5 illustrates
C-shaped grounding shields 41, which include lateral portions 41a for
partially surrounding the
first and second electrodes 36 and 37. The grounding shields 41 are kept at
ground potential, i.e.
the same as the mirrored platforms 26, while one of the first and second
electrodes is held at an
activation voltage, e.g. 100 Volts.
[48] Trace lines 36c and 37c electrically connect the electrodes 36 and 37,
respectively, to a
voltage supply (not shown). Since the trace lines 36c and 37c also act as a
hot electrode, i.e.
contributing to the total torque applied to the platform 26, covering the
traces 36c and 37c with a
ground plane 43 (Figure 6) also reduces the force applied to the outer free
end of the platform 26.
[49] Figure 6 also illustrates an alternative configuration for the electrode
36, in which the two
step hot electrode 36 is sunken slightly below a surrounding grounded metallic
surface, which is
a continuation of the ground plane 43. A small vertical step 44 between the
hot electrode 36 and
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the surrounding ground plane 43 is a dielectric surface that isolates the hot
electrode 36 from the
surrounding ground plane 43. This arrangement reduces the angular drift of the
platform 26,
which is caused by a build up of electrostatic charges on exposed dielectric
or insulating
surfaces. The electric field generated by these electrostatic charges perturbs
the electric field
generated by the applied voltage from the electrodes 36 and 37, thereby
causing the angular
position of the platform 26 to drift over time. The present arrangement limits
the exposed
dielectric to the small vertical surface 44, which generates e lectrostatic
field lines that do not
significantly affect the field lines between the hot electrodes 36 and 37 and
the ground plane 43.
To further reduce the angular drift of the platform 26, the vertical surface
44 can be under cut
beneath the ground plane 43 at a slight negative angle ensuring that the gap
between the hot
electrode 36 and the ground plane 43 is substantially zero. The ground plane
43 could also be
positioned slightly below the hot electrodes 36 and 37 to create the vertical
step.
[50] Since the MEMs mirror devices 21, 22 and 23 are for use in optical
devices, i.e.
wavelength blockers and multiple wavelength switches (see Figure 23), which
include a grating
for dispersing the light into spectral wavelength component channels, it is an
important
performance requirement that the spectral response has a high rejection of
light between the
selected wavelength channels. Unfortunately, in conventional MEMs devices,
light passes
between the mirrors and is reflected off the substrate back into the optical
device, thereby leading
to a deterioration in the isolation between the wavelength channels.
Accordingly, the present
invention provides back reflection cusps 50, defined by angled, curved or
concave reflecting
surfaces intersecting along a ridge, extending longitudinally below the gap
between the platforms
26, for scattering any light passing between the mirrored platforms 26 in a
direction substantially
parallel to the surface of the platforms 26.
[51] To further eliminate cross-talk between adjacent electrodes,
additional platform
shields 42 (Figure 7) can be added to the underside of the planar supporting
regions 27 and 28,
outside or inside of the electrode shields 41. Typically, in the rest
position, the two different sets
of shields 41 and 42 do not overlap; however, as the platform 26 tilts the
platform shields 42
begin to overlap the grounding shielding 41. The added protection provided by
overlapping
shielding is particularly advantageous, when the tilt angle of the platform 26
is proportional to
the voltage applied to the electrode 36 (or 37), such as in open loop
configurations. Accordingly,
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the greater the tilt angle, the greater the required voltage, and the greater
the amount of potential
cross-talk, but consequently the greater the amount of shielding provided by
the overlapping
ground and platform shields 41 and 42, respectively. Back reflection cusps 50
are also provided
for reasons hereinbefore discussed. A single structure 50 between adjacent
electrodes can
replace the pair of adjacent shields 41.
[52] With reference to Figures 8, a pair of internal gimbal
ring MEMs mirror devices 131
and 132 are illustrated mounted adjacent each other on a substrate 133. The
present invention
enables mirrors 134 and 135 to be positioned relatively close together, i.e.
with a high fill factor,
while still providing the two degrees of motion provided by the more
complicated prior art.
[531 With further reference to Figure 9, a first torsion hinge
137, preferably in the form of
a rectangular beam, is fixed, proximate the middle thereof, to the substrate
133 via a central
anchor post 138. The supporting structure for the mirror device of the present
invention is based
on a single anchor post 138, rather than the dual anchor points required in
the aforementioned
external gimbal ring devices. The first torsion hinge 137 provides for
rotation Oy about a first
axis Y, and may also include a serpentine hinge 140, as illustrated in mirror
device 131, or any
other torsional hinge known in the art. Opposite sides of an internal gimbal
ring 139 are
connected to opposite ends of the first torsion hinge 137, whereby the first
torsion hinge 137
bisects the internal gimbal ring 139. The internal gimbal ring 139 is
preferably not flexible, but
can t ake v arious geometric forms, although r ectangular o r circular frames
w ould b e t he m ost
convenient to fabricate and use. Spring arms 141 and 142, which define a
second torsion hinge,
extend outwardly from opposite sides of the internal gimbal ring 139
perpendicular to the first
torsion hinge 137. Each of the spring arms may also include a serpentine hinge
as hereinbefore
described. The second torsion hinge provides for rotation O. about a second
axis X, which is
perpendicular to the first axis Y, but still substantially in the same plane
as the mirrors 134 and
135. A generally rectangular platform 143, for supporting one of the mirrors
134 or 135, is
mounted on the ends of the spring arms 141 and 142. Preferably, the platform
143 is comprised
of a pair of rectangular planar surfaces 144 and 145 joined together by a pair
of elongated braces
147 and 148, which extend on either side of the internal gimbal ring 139
parallel with the spring
arms 141 and 142.
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[54] Fabrication of the preferred embodiment illustrated in Figures 8 and
9, is simplified
by having all of the structural elements, i.e. the first torsional hinge 137,
the gimbal ring 139, the
spring arms 141 and 142, and the first and second planar surfaces 144 and 145,
in the same upper
substrate layer and having coplanar upper surfaces, whereby the same b asic
process steps are
used as are used to fabricate the MEMs device illustrated in Figure 1. In
particular, a single
photolithographic step is used to identify the structural elements, followed
by a deep reactive ion
etching (DRIE) step used to remove the unwanted portions of the upper
substrate. Finally the
moveable elements in the upper substrate are released from the lower substrate
by removal of a
sacrificial layer therebetween.
[55] Figures 10 and 11 a illustrate an array of internal gimbal ring MEMs
mirror devices
201 utilizing a first pair of serpentine torsional hinges 202 for pivoting a
rectangular platform
203, including first and second planar supporting regions 203a and 203b, about
a first axis of
rotation ox, and a second pair of serpentine torsional hinges 204 for rotating
the platform 203
about a second axis of rotation 05, above a base substrate 205. The first and
second planar
supporting r egions are j oined by a p air o f e longated b races 2 00. T he
first p air o f s erpentine
torsional hinges 202 extend from a single anchor post 206, which extends
upwardly from the
base substrate 205 through the center of the platform 203, i.e. at the
intersection of the minor and
major axes thereof. Outer ends of the first pair of torsional serpentine
torsional hinges 202 are
connected to a rectangular gimbal ring 208, which surrounds the first pair of
serpentine hinges
202, at points along the minor axes (0y) of the platform 203. The second pair
of serpentine
torsional h inges 2 04 extend from o pposite s ides o f t he gimbal r ing 2 08
i nto c ontact w ith t he
platform 203, at points along the major axis (Ox) of the platform 203.
[56] An alternative hinge structure for the mirror array 201, illustrated
in Figure 11b,
includes the first pair of serpentine torsional hinges 202 extending from
anchor post 206 enabling
the platform 203 to rotate about the y axis; however, only a single serpentine
torsional hinge 204
is provided enabling rotation about the x axis. Accordingly, a C-shaped gimbal
ring 209 extends
from the ends of the hinges 202, partially surrounding the hinges 202, into
contact with the end
of the single hinge 204.
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[57] To provide a full range of motion for the platform 143 or 203, a set
of four two-step
electrodes 211, 212, 213 and 214 are provided (See Fig. 12); however, for the
present invention
only the first, second and third electrodes 211, 212 and 213 are required to
roll the mirrors out of
alignment w ith any intermediate output ports and then back into alignment
with a designated
output port. As in Figure 5, each of the electrodes 211, 212, 213 and 214
include an upper step
211a, 212a, 213a, and 214a, and a lower step 211b, 212b, 213b, 214b,
respectively, for reasons
discussed hereinbefore.
Accordingly, first, second and third voltages can be established
between the platform 143 or 203 and the first electrode 211, the second
electrode 212 and the
third electrode 213, respectively. Initially, the first and second electrodes
211 and 212 are
activated to rotate the platform 143 or 203 about ox. Subsequently, the first
voltage is gradually
lowered to zero, while the third voltage is gradually increased until it is
equivalent to the second
voltage ( See Fig 1 3). To m inimize u nwanted e ffected caused b y r inging,
i .e. v ibration o f t he
mirrors caused by an abrupt start or stop, the first, second and third
voltages are increased and
decreased gradually, e.g. exponentially, as evidenced in Figure 13, which
illustrates the voltages
curves for the various electrodes (first, second and third) over the actuation
time of the mirror
device. Various mirror tilting patterns can be designed based on the desired
characteristics, e.g.
attenuation, of the light.
[58] An improved electrode configuration is illustrated in Figures 14 and
15, in which a
first two-step Oy electrode 236 includes an upper U-shaped step 236a, and a
lower rectangular
step 236b. The arms of the U-shaped step 236a extend from the lower step 236b
on opposite
sides of the second hinge 204 beneath the first planar supporting region 203a.
Similarly, a
second two-step Oy electrode 237 includes an upper U-shaped step 237a, and a
lower rectangular
step 237b. The arms of the U-shaped step 237a extend from the lower
rectangular step 237b on
opposite sides of the second hinge 204 beneath the second planar supporting
region 203b. A
single two-step 09õ electrode 238 includes an upper U-shaped step 238a, and
lower rectangular
steps 238b extending from each arm of the upper U-shaped step. The single 0õ
electrode 238
extends from adjacent the first Oy electrode 236 to adjacent the second Oy
electrode 237 across the
gap therebetween, and beneath one side of both the first and second planar
supporting regions
203a and 203b. The lower steps 238b provide a larger gap between the outer
free ends of the
platform 203, when the platform is tilted towards the first or the second Oy
electrode 236 or 237.
The arms of the upper U-shaped step 238b extend on opposite sides of the first
pair of hinges
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202. The arms of the U-shaped step 238a are three to five times wider than the
arms of the U-
shaped step 236a or 237a. Multi-step electrodes are also possible to further
spread the
application of force over the length of the platform 203. Actuation of the
electrodes is controlled
by an electrode control 240, as will be discussed hereinafter with reference
to Figure 19.
[59] An unfortunate consequence of relying on only three electrodes is that
a slight
misalignment in positioning the platform 203 over the first and second Ay
electrodes 236 and
237 can result in an imbalance that can not be corrected for using the single
ox electrode 238.
Figure 16 illustrates the ideal case, in which the longitudinal axis of the
first electrode 236 is
aligned with t he longitudinal a xis X o f t he p latform 2 03. H owever, F
igure 1 7 i llustrates t he
results of a mask misalignment during fabrication, in which the longitudinal
axis X of the
platform 203 has a +6.x misalignment relative to the electrode axis.
Accordingly, actuation of
the first 0), electrode 236 would introduce an undesirable tilt in the
platform 203 towards the
bottom left hand corner, which could not be compensated by the single ox e
lectrode 238. I n
Figure 18, the illustrated mask misalignment, in which the longitudinal axis X
of the platform
203 has a ¨Ax misalignment relative to the electrode axis. In this case,
actuation of the first Oy
electrode 236 would introduce an undesirable tilt in the platform 203 towards
the top left hand
corner. However, this tilt can be compensated for by applying a voltage to the
single ox electrode
238. Accordingly, the solution to the problem of mask misalignment is to
introduce an
intentional or predetermined ¨Ax misalignment, which would cancel or at least
minimize any
+Ax misalignment and which could be compensated for by the single ox electrode
238.
[60] Figure 19 illustrates an electrode voltage vs time graph, detailing
the voltages of the
three electrodes 236, 237 and 238 as the platform 203 is switched from one
position to another
by an electrode control, i.e. from reflecting light from one port to another,
without traveling
directly, i.e. without reflecting light into any intermediate ports. To
prevent undesirable
"ringing" of the platform 203, the voltage VyR of the first Oy electrode 236
is gradually
decreased as the voltage Vx of the single ex electrode 238 is increased. As
the voltage VyR
decreases to zero, the voltage VyL of the second Oy electrode 237 gradually
increases. As the
voltage VyL reaches its set amount to maintain the platform in the desired
position, the voltage
Vx is decreased to a minimum amount, assuming no compensation voltages are
required.
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[61] When the size of the platform 203 is decreased, e.g. for small pitch
micro-mirrors in
the order of <100um, the electrodes 236, 237 and 238 must also be constructed
correspondingly
smaller. However, due to the fact that the electrodes necessarily become
thinner, while sharing
the same mirror section, stable tilt angles are difficult to achieve at high
resonant frequencies.
Accordingly, the size requirement of the electrode, and the required electrode
spacing become
the limiting factor in determining the maximum fill factor. An alternative
embodiment of a three
electrode configuration is illustrated in Figures 20 and 21, in which two
parallel arrays of
platforms 253 and 254 are made smaller than the original platforms 203 with
first and second
two-step 0), electrodes 256 and 257 positioned therebelow. A single ox
electrode 258 is
positioned below each offset section 259 and 260, which extend from the side
of the platforms
253 and 254, respectively, adjacent the mid-section thereof, i.e. the area of
first and second
hinges 251 and 252. This arrangement enables the single ox electrode 258 to be
separated from
the other two electrodes 256 and 257, and therefore, be larger in size, which
enables the
electrostatic torque to be increased for a common voltage. The added
separation between the
electrodes 256, 257 and 258 minimizes the angular instabilities, when the
single ox electrode 238
is actuated, and reduces the amount of electrical x-talk. Preferably, the two-
step ox electrodes
256 and 257 include the ground plane arrangement as disclosed in Figure 6 with
hot electrodes
sunken relative to a surrounding ground plane, and only a vertically extending
dielectric layer,
which provides a substantially zero-width vertical gap between the hot
electrode and the ground
plane.
[62] For high fill factor applications, a first planar section 253a of one
platform 253 from
the first array is positioned beside a second planar section 254b of an
adjacent platform 254 from
the second array, as in Figures 20 and 21, whereby adjacent mirrors have
offset x axes X1 and X2,
and every other platform pivots about the same laterally extending x axis.
Each or the platforms
253 and 254 are also rotatable about separate parallel longitudinal axes. Only
the relatively
closely disposed planar sections would require reflective material thereon,
and reflective cusps
260 would only be required therebelow (Figure 22).
[63] Substrate-mounted, grounded cross-talk shields 261 and platform
mounted cross-talk
shields 262 are provided to further minimize the amount of electrical cross-
talk between adjacent
mirrors. The platform mount cross-talk shields 262 are preferably mounted
outside of the
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Patent
platform mounted cross-talk shields 262 with enough spacing to enable rotation
about both the x
and the y axis; however, any combination for offsetting the shields 261 and
262 is possible.
[64] Figures 23 to 25 illustrate another embodiment of the reflective cusps
specific to the
interlaced mirrors of Figures 20 to 22. To ensure that the outer free ends of
the platforms 253
and 254 does not contact the reflective cusps 260, a portion of the reflective
cusp 260a is
positioned below the platforms 253 extending from an inner end proximate the
anchor post to
proximate the midway point of the first planar section 253a, and a portion of
the reflective cusp
260b is positioned below the platforms 254 extending from proximate the anchor
post to
proximate the midway point of the second planar section 254b. The portions of
the reflective
cusp 260a and 260b can extend any suitable length, whereby the ends thereof
are directly
adjacent each other providing overlapping protection. The portions of the
reflective cusps 260a
and 260b can have the same or different lengths.
[65] A simple alternative to the interlaced mirrors of Figures 20 to 25 are
illustrated in
Figures 26 and 27, in which a first array of 1-d Piano MEMs mirrors 273 pivot
about a first
lateral axis X1 via a single pair of torsional hinges 274, and a second array
of 1-d Piano MEMs
mirrors 275 pivot about a second lateral axis X2 via a single pair of
torsional hinges 276. A first
sections 273a and 275a of the platforms 273 and 275, respectively, are
preferably larger than a
second sections 273b and 275b, respectively, to enable the size of single
electrodes 277 and 278
to be wider and/or shorter than the size of the second sections 273b and 275b.
Activation of the
single electrodes 277 and 278 attracts the first sections 273a and 275a,
respectively, thereby
pivoting the platforms 273 and 275 about the first lateral axes X1 and X2,
respectively. The
platforms 273 and 275 returns to the horizontal set position when the single
electrodes 277 and
278 are deactivated due to the resilient spring force inherent in the
platforms 273 and 275.
Since the hinges 274 and 274 are not as wide as the second sections 273b and
275b, the second
sections 273b and 275b form an array of closely packed reflective surfaces
between the first and
second lateral axes X1 and X2. Preferably, the electrodes 277 and 278 are two
step electrodes, as
above, and include the ground plane arrangement as disclosed in Figure 6 with
hot electrodes
sunken relative to a surrounding ground plane, and only a vertically extending
dielectric layer,
which provides a substantially zero-width vertical gap between the hot
electrode and the ground
plane.
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Patent
[66] With the aforementioned configuration, the mirror to mirror electrical
crosstalk can
be reduced even for relatively small mirror pitches, e.g. < 70 um, because the
closest adjacent
electrodes are separated by a mirror length in one direction, and by up to
half a mirror pitch in
the o ther d irection. T he v oltage-tilt a ngle a nd the r esonant frequency
characteristics a re also
improved for low pitches due to the increase electrode a rea and resultant
electrostatic torque.
Furthermore, the electrical pin-out is reduced to one connection per mirror.
[67] The "piano" MEMs mirror devices according to the present invention are
particularly
useful in a wavelength switch 301 illustrated in Figures 28, 29 and 30. In
operation, a beam of
light with a plurality of different wavelength channels is launched via an
input/output assembly
302, which comprises a plurality of input/output ports, e.g. first, second,
third and fourth
input/output p orts 3 03, 304, 3 05 and 3 06, r espectively. T he b eam i s d
irected to an e lement
having optical power, such as concave mirror 309, which redirects the beam to
a dispersive
element 311, e.g. a Bragg grating. The dispersive element separates the beam
into the distinct
wavelength channels (Xl, X2/ X3) / which are again directed to an element
having optical power,
e.g. the concave mirror 309. The concave mirror 309 redirects the various
wavelength channels
to an array of "piano" MEMs mirror devices 312 according to the present
invention, which are
independently controlled to direct the various wavelength channels back to
whichever
input/output port is desired. Wavelength channels designated for the same port
are reflected
back off the concave mirror 309 to the dispersive element 311 for
recombination and redirection
off t he concave m irror 309 t o t he d esired i nput/output p ort. T he c
oncave m irror 3 09 can b e
replaced by a single lens with other elements of the switch on either side
thereof or by a pair of
lenses with the dispersive element 311 therebetween.
[68] With particular reference to Figure 29, the input/output assembly 302
includes a
plurality of input/output fibers 313a to 313d with a corresponding collimating
lens 314a to 314d.
A single lens 316 is used to convert a spatial offset between the input/output
ports into an
angular offset. Figure 30 illustrates a preferred embodiment of the
input/output assembly, in
which the unwanted effects of polarization diversity are eliminated by the use
of a birefringent
crystal 317 and a waveplate 318. For incoming beams, the lens 316 directs each
beam through
the birefringent crystal 317, which separates the beam into two orthogonally
polarized sub-
beams (o and e). The half waveplate 318 is positioned in the path of one of
the sub-beams for
CA 02501012 2005-03-16
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Patent
rotating the polarization thereof by 900, so that both of the sub-beams have
the same polarization
for transmission into the remainder of the switch. Alternatively, the
waveplate 318 is a quarter
waveplate and rotates one of the sub-beams by 45 in one direction, while
another quarter
waveplate 319 rotates the other sub-beam by 45 in the opposite direction,
whereby both sub-
beams have the same polarization. For outgoing light, the polarization of one
(or both) of the
similarly polarized sub-beams are rotated by the waveplate(s) 318 (and 319),
so that the sub-
beams become orthogonally polarized. The orthogonally polarized sub-beams are
then
recombined by the birefringent c rystal 317 and o utput the appropriate
input/output port. The
micro-electro-mechanical devices according to the present invention are
particularly well suited
for use in switching devices with polarization diversity front ends, since
they provide a pair of
reflecting surfaces, i.e. one for each sub-beam
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