Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-02-07
2002-11-12
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S290000, C385S018000
Reexamination Certificate
active
06480320
ABSTRACT:
TECHNICAL FIELD
This invention relates to microelectromechanical mirrors and mirror arrays, and a method for manufacturing the same.
BACKGROUND
As the internet has grown, so too has the strain on the telecommunications infrastructure. As more and more information is transmitted across the Internet, and the demand for information rich content like streaming video has grown, telecommunication providers have struggled to provide the necessary bandwidths and data rates necessary to carry the requisite data. To that end, telecommunications providers have looked to carrying more and more data on fiber optic networks, and to extending the reach of their fiber optic networks beyond the network backbone.
One limitation of fiber optic networks as currently implemented is their inability to directly switch optically encoded packets of data from a fiber on a source network or network node to a fiber on a destination network or network node. Instead, the optically encoded data is dropped from the source network fiber, converted to electrically encoded data, switched to the destination network using conventional electronic switches, converted back into optically encoded data, and injected into the destination network fiber.
Micromachined mirror arrays offer the ability to directly switch optically encoded data in devices, known as all-optical cross connect switches, from a source fiber on a source network to a destination fiber on a destination network without having to convert the data from optical to electronic and back again. For such mirror arrays to be commercially useful, they must be able to cross connect approximately 1000 input fibers with an equal number of output fibers in a compact volume. This can be achieved with mirrors that can be densely packed together and that are rotatable by relatively large angles (~5°) in an arbitrary angular direction.
Recent developments in the field of microelectomechanical systems (MEMS) allow for the bulk production of microelectromechanical mirrors and mirror arrays that can be used in all-optical cross connect switches. MEMS-based mirrors and mirror arrays can be inexpensively designed and produced using conventional tools developed for the design and production of integrated circuits (IC's). Such tools include computer aided design (CAD), photolithography, bulk and surface micromachining, wet and dry isotropic and anisotropic etching, and batch processing. In addition, deep reactive ion etching methods (DRIE) allow silicon devices to be produced having high aspect ratios (~20:1) that rival those that can be achieved using the prohibitively expensive lithography, electroplating and molding process (LIGA) which requires access to a synchrotron radiation source. (LIGA is an acronym for the German lithographie, galvanoformung und abformung).
A number of microelectromechanical mirror arrays have already been built using MEMS production processes and techniques. These arrays have designs that fall into approximately three design categories, each of which suffers from one or more limitations that make them sub-optimal for use in an all-optical cross connect switch.
The first and simplest design is illustrated by U.S. Pat. No. 5,960,132 to Lin. In this design, a reflective panel is hinged to a reference base and is electrostatically rotated about the hinge. Since the panel's freedom of motion is limited to rotation about the hinge, light incident on the panel cannot be reflected in an arbitrary angular direction (d&thgr;, d&phgr;) but only along an arc defined by a single angle, i.e., d&thgr; or d&phgr;. As a result, light incident from a source fiber cannot be directed to an arbitrary output fiber but only to those output fibers located along the defined arc. Consequently, Lin's system requires large and costly system redundancies to connect a plurality of input fibers to a plurality of output fibers. These redundancies can be in either the number of output fibers or in the number of mirrors. In Lin, the redundancy is in the number of mirrors, where N
2
mirrors are used to connect N input fibers to N output fibers. An optimal system would only require N mirrors to make the N input to N output possible fiber interconnections.
A more sophisticated design is illustrated in U.S. Pat. No. 6,044,705 to Neukermans et al which is hereby incorporated by reference. In Neukermans, a gimbal is mounted on a first hinge connected to a reference surface, while a mirror is mounted on a second hinge connected to the gimbal. The first and second hinges are orthogonal to each other and allow the mirror to be rotated in an arbitrarily angular direction (d&thgr;, d&phgr;). The gimbal is electrostatically rotated about the first hinge by applying a potential between it and electrodes located on the reference surface. The mirror is electromagnetically rotated about the second hinge by injecting a current in a conductive coil wrapped around the mirror perimeter. The current flow through the coil generates a small magnetic moment which couples to a permanent magnetic field established across the plane of the mirror (e.g. with bar magnets), and causes the mirror to rotate. While Neukermans use of a gimbal thus allows the mirrored surface to rotate in an arbitrary angular direction, it also makes the system more mechanically and electrically complex than it needs to be. The mechanical complexity increases the sensitivity of the system to mechanical vibrations, while the electrical complexity increases the intricacy of the electrostatic and electromagnetic actuators. Both complexities increase the cost of producing the system. Additionally, Neukermans electromagnetic actuator coil occupies a large amount of the surface of the device, thus reducing the mirrored surface area and the mirror density.
A third mirror design is illustrated in U.S. Pat. No. 6,040,953 to Michalicek. In Michalicek, a mirror is mounted on a central post anchored to a locking pin joint that is carved into a reference surface. The post can be electrostatically actuated to freely rotate about the pin joint in an arbitrary direction. However, because the post is not mechanically attached to the pin joint with flexures, it can only be stably rotated in directions where the mirrored surface can be supported by a landing pad provided for that purpose. The mirror can therefore only be rotated and held in a fixed number of stable positions. In Michalicek's preferred embodiment, the mirror can only be rotated to and held in two stable positions.
BRIEF SUMMARY OF THE INVENTION
A freely rotatable, microelectromechanical mirror is disclosed. In one embodiment, the mirror can be used as a single switching element in an array of switching elements in an all-optical cross connect switch. The disclosed mirror includes a mirrored surface that can be mounted onto the top surface of a cylindrically shaped support post. The bottom surface of the support post can be mounted onto a freely movable, arbitrarily rotatable plate that is suspended in an actuation layer.
The actuation layer includes a freely movable plate suspended from and flexibly connected to a plurality of actuators that are themselves suspended from and flexibly connected to a support structure. In one embodiment, the support structure is a support frame held above a reference surface. In another embodiment the support structure is a plurality of support posts extending from the reference surface. The actuators can be actuated by any actuation means including electrostatic, electromagnetic, piezoelectric, and thermal actuation means. In one embodiment, the actuators are electrostatically actuated and the freely movable plate is suspended from and flexibly connected to three or more actuators that are distributed symmetrically about it. For example, in one preferred embodiment, the plate is suspended from and flexibly connected to four electrostatic actuators distributed about the plate at 90 degree intervals.
Each actuator is connected to the support structure by a plurality of actuator flexures that define a direction of motion in which th
Epps Georgia
Fish & Richardson P.C.
Spector David N.
Transparent Optical, Inc.
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