WO2001095014A1

WO2001095014A1 - Optical mirror system with multi-axis rotational control

Info

Publication number: WO2001095014A1
Application number: PCT/US2001/040885
Authority: WO
Inventors: Paul Merritt Hagelin
Original assignee: C Speed Corporation
Priority date: 2000-06-09
Filing date: 2001-06-07
Publication date: 2001-12-13
Also published as: AU2001267050A1

Abstract

An optical mirror system (100) comprising a mirror (103) having outer and inner frames (104 and 102) wherein the inner frame is connected to the mirror and the outer frame is connected to the inner frame by suspension beams and flexures.

Description

OPTICAL MIRROR SYSTEM WITH MULTI-AXIS ROTATIONAL CONTROL

FIELD OF THE INVENTION

The present invention relates generally to a Microelectromechanical System (MEMS) fabricated optical mirror system that is capable of being tilted on two orthogonal axes, by means of electrostatic actuation. Particular application to the use of these mirrors in the deflection of optical space beams is emphasized.

BACKGROUND OF THE INVENTION

Fiber optic communication systems currently employ switching systems to route signals at central office switching centers. These electro-optic systems rely on converting the light output from each "incoming" fiber into electrical form, extracting the data content in the resultant electrical signal, then utilizing conventional electrical switches to route the data content to a modulatable optical source that is coupled to a " destination" optical fiber. This detection, switching, and remodulation process is expensive, complex, power consuming, and subject to component failure.

Alternate "All Optical" switching systems, employing mechanically actuated bulk optic and MEMS fabricated devices, currently exist. These devices utilize electromagnetic, piezoelectric and electrostatic actuators to physically move prisms, mirrors and portions of optical fibers to affect switching of signals between optical fibers.

In addition, fiber-to-fiber switches employing Grating Waveguides, Rowland Circle Gratings, and planar gratings, permit dedicated switching based on optical wavelength.

Cascaded binary tree configurations, employing switchable optical couplers using electrostatically variable index material, (Lithium Niobate and polymers), as well as Mach Zender interferometers utilizing thermoelectric heaters to affect unbalance, are also currently state of the art.

Many of the MEMS switches employ a space-beam deflection system similar to the electrical "Cross Bar" switch common in telephone system. This approach requires that the number of mirrors for a given input/output port count be determined by the square of the port count figure. The overwhelming number of mirrors dictated by this high port-count switching approach exceeds that which can be produced with any realistic process yield, and survive any reasonable operating period.

Except for some of the MEMS electrostatically actuated devices, none of the above methods of optical switching meets the requirements currently being specified for high fiber port count, (up to 1024 by 1024) Optical Cross Connect switches. Problems of cost, reliability, insertion loss, polarization sensitivity, isolation, wavelength dependence, power consumption, and in some instances, switching speed, either individually or collectively mitigate against their use. Accordingly, what is needed is a system and method for overcoming the above-identified issues under the constraint of a simple CMOS-compatible fabrication process.

An optical mirror system design is desired that has high-resolution 2-D scanning capability and deflection capability, made with a surface-micromachining process. In order to achieve high-resolution, large mirror size and rotation angles are necessary.

The present invention addresses such a need.

SUMMARY OF THE INVENTION

An optical mirror system is disclosed. The system includes a mirror and a first frame member coupled to and surrounding the mirror via a first plurality of suspension beams. The system also includes a first plurality of actuators coupled to the first frame member, and a second frame member coupled to and surrounding the first frame member via a second plurality of suspension beams. The mirror rotates about a first axis relative to the first plurality of suspension beams. Finally, the system includes a second plurality of actuators coupled to the second frame member.

A key feature of the present invention is the incorporation of a comb drive, an electrostatic actuator with interdigitated electrodes, to provide actuation to the mirror. This comb drive facilitates low- voltage and high-force actuation. The design also features self- assembly using bimorphs. The bimorph separates the two halves of the comb drive. Electrostatic force pulls the halves together.

In a method and system in accordance with the present invention, the comb (preferably fabricated from a layer of polyciystalline silicon at least 4μm thick) enables large forces (one or two orders of magnitude larger than conventional micromachined mirrors) while maintaining low actuation voltage and high resonant frequencies. In addition, the actuator is built into the frame that supports the mirror. Putting the actuator in the frame also provides great flexibility in the mirror design. Furthermore, the actuator can be situated in the frame surrounding the mirror. This can reduce the overall size of the mirror.

The other applications for such mirrors are, for example, optical add-drop multiplexers, wavelength routers, free-space optical interconnects, chip-level optical I/O, optical scanning displays, optical scanner (bar-codes, micro cameras), optical storage read/write heads, laser printers, medical replacement for glasses (incorporated with adaptive optics), medical diagnostic equipment, optical scanning for security applications. Integration of an optical scanning mirror with optical detectors can reduce the cost of imaging applications by reducing the number of detectors needed to capture 1-D or 2-D optical information.

A device in accordance with the present invention meets the requirements for a directly scalable, high port count optical switch, utilizing a unique two mirror per optical I/O port configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a conceptual drawing of an optical mirror system in accordance with the present invention.

Figure la is a top view showing bimorph in an optical mirror system in accordance with the present invention.

Figure lb is a side view of bimorph detail of an assembled optical mirror system in accordance with the present invention.

Figure lc is a side view of bimorph detail of an unassembled optical mirror system in accordance with the present invention.

Figure 2a is a first cross section of torsion detail of the inner frame of an unassembled optical mirror system in accordance with the present invention.

Figure 2b is a second cross section of torsion detail of the inner frame of an assembled optical mirror system in accordance with the present invention, having the electrodes assembled.

Figure 2c is a third cross section of torsion detail of the inner frame of an optical mirror system in accordance with the present invention, showing mirror tilt.

Figure 3 illustrates a top view of an actuator in accordance with the present invention. Figure 4 illustrates a side view of the actuator of Figure 3.

Figure 5 illustrates an optical mirror system coupled to a substrate via anchors.

DETAILED DESCRIPTION

The present invention relates to an optical mirror and more particularly to an optical mirror system with a multi-axis rotational control. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein.

In a method and system in accordance with the present invention, the comb enables large forces while maintaining low actuation voltage and high resonant frequencies. In addition, the actuator is built-in to the frame that supports the mirror. Putting the actuator in the frame provides great flexibility in the mirror design, which can reduce the overall size of the mirror system.

A system and method in accordance with the present invention utilizes a comb-drive in tilt-up optical mirror system. In a preferred embodiment, the comb-drive can be self- assembled, and the mirror can be made of thin polysilicon and supported by a stiff frame. This lowers the mass of the mirror, enhancing its resonant frequency.

Figure 1 is a conceptual drawing of an optical mirror system 100 in accordance with the present invention. The optical mirror system 100 has an outer frame 104, with a first plurality of actuators 110 and 112, and an inner frame 102, with a second plurality of comb actuators 106 and 108. The inner frame 102 surrounds the mirror 103, and the outer frame 104 surrounds the inner frame 102. The inner frame 102 and outer frame 104 are coupled together by a plurality of suspension beams 122, 124, 126, and 128 (I added 126 and 128 to the figure). In a preferred embodiment, the suspension beams are flexure beams. Both frames are coupled to the first and second plurality of actuators with bimorph flexures 134-140. The optical mirror system is fabricated flat on the surface of the chip. Upon release, the bimorph separates the bottom edge of the actuator from the bottom edge of the frame. When voltage is applied between a frame and the actuator, the actuator is pulled back to the frame. The actuation is stable and can achieve analog positioning control.

Figure la is a top view showing a bimorph in an optical mirror system in accordance with the present invention. As is seen, the frame 102 is coupled to one alf of the comb drive 208 and the bimorph 134. The other half of the comb drive 106 (corrected call-out) is coupled through a plurality of flexure beams 114, 116 to the mirror 103. A first plurality of flexure beams 210 are coupled to the bimorph 134 and the actuator half 106.

Figure lb is a side view of detail of an assembled bimorph 134 in accordance with the present invention. Figure lc is a side view of detail of an unassembled bimorph 134 in accordance with the present invention. Referring back to Figure 1 , the first plurality of actuators 106 and 108 connect to the mirror 103 through torsion flexure beams 114, 116, 118 and 120. Short, thin torsion beams are desired to minimize sagging that can reduce the angular range of the mirror. When actuator 106 is activated, the position of actuator 108 does not change appreciably because the bimorph flexures are stiff in relation to torsion flexures 116- 120. The differential height between actuators 106 and 108 cause the mirror to rotate. The actuators 110 and 112 in the outer frame 104 connect to the inner frame 102 in the same manner. Bi-directional actuation on orthogonal axes is possible with this method.

Electrical paths (not shown) are routed to frame 102 with use conductors and insulators. If the torsion beams are electrically conductive, they can be used to route the driving signals between the frames 102 and 104. A total of five electrical connections are made to actuators 106, 108, 110 and 112 and to frame 102 and 104 to perform the bidirectional actuation. The frame can be connected to electrical ground. If bi-directional actuation is not necessary, two actuators can be used to perform unidirectional actuation on two rotational axes, requiring a total of three electrical connections.

Figure 2a (corrected call-outs) is a cross-section of detail of the flexure beam of an unassembled optical mirror system in accordance with the present invention. Figure 2b is a cross-section of torsion detail of the flexure beam of an assembled optical mirror system in accordance with the present invention. Figure 2c is a cross-section of torsion detail showing mirror rotation in response to activation of an actuator.

In a method and system in accordance with the present invention, the comb drive enables large forces while maintaining low actuation voltage and high resonant frequencies. In addition, the actuator is built-in to the frame that supports the mirror. Figure 3 illustrates a top view of an actuator in accordance with the present invention. Figure 4 illustrates a side view of the actuator of Figure 3. In this embodiment, there are three portions to the actuator system 105, first end portion 302, second end portion 304 and a middle portion 306. The end portions 302 and 304 engage the middle portion 306 through interdigitated teeth 308 and 310. In this embodiment, each of the end portions 302 and 304 comprise three electrically isolated actuators 302a-302c and 304a-304c, respectively. Accordingly, if actuators 302a and 304a are activated, the middle portion 306 pulls "up". If actuators 302b and 304b are activated, the middle portion 306 holds the position shown, and if actuators 302c and 304c are activated, the middle portion 306 pulls down. Accordingly, the actuator system 105 can move the mirror in various ways dependent upon the voltages applied to the motors 302a-302c and 304a-304c. Although this actuator system 105 has been described in the context of a three position (up, down and hold position) system, one of ordinary skill in the art readily recognizes that a two position (up or down), (hold position or down), (hold position or up) could be provided by using two actuators rather than the three actuators in the system disclosed herein. The system 105 could be operated in a rotational mode by activating electrodes on a diagonal, such as 304a and 302c. Furthermore, this system could utilize a parallel plate drive actuator and its use would be within the spirit and scope of the present invention.

In a preferred embodiment as shown in Figure 5, the frame 104 can be attached to actuator 501 that is connected to the substrate through flexures 503 and anchors 504. The substrate can have actuators thereon used to elevate the mirror above the substrate, or to rotate the mirror. The electrostatic actuators can be uni-directional or bi-directional and the actuators could be parallel plate or comb drive. Further, the actuators are not limited to electrostatic technology. Finally, the electrodes in the electrostatic actuators can be separated by a bimorph, by a sacrifical layer or by any combination thereof.

In another preferred embodiment, the outer frame of the mirror is separated from the substrate by a self-assembly mechanism that provides clearance for mirror rotation. This self- assembly mechanism can be powered by bimorphs or other forms of actuation, such as electrostatic. Putting the actuator in the frame also provides great flexibility in the mirror design. Furthermore, the actuator can be situated in the frame surrounding the mirror. This can reduce the overall size of the mirror.

A mirror in accordance with the present invention can be operated in a bi-directional mode, and electrodes on each axis can be used to perform bi-directional motion. In conventional designs, one electrode is dedicated to each direction of rotation on each axis which can add to the overall cost. The number of mechanical connections to the mirror enables more independent electrical interconnects, which can be used for other features, such as active shaping of the mirror surface. Another advantage of an optical mirror in accordance with the present invention is that, due to the distances between the actuators (on opposite sides of the mirror), there effectively is no electrical cross-talk.

Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. For example, although it is disclosed in the preferred embodiment that the mirror rotates in a first and a second direction, the mirror can rotate in a plurality of directions (i.e. twisting motion) dependent upon the electrostatic forces applied thereto. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.

Claims

CLAIMSWhat is claimed is:

1. An optical mirror system comprising: a mirror; a first frame member coupled to and surrounding the mirror via a first plurality of suspension beams; a first plurality of actuators coupled to the first frame member; a second frame member coupled to and surrounding the first frame member via a second plurality of suspension beams, wherein the mirror rotates about a first axis relative to the first plurality of suspension beams; and a second plurality of actuators coupled to the second frame member.

2. The optical mirror system of claim 1 in which the first frame member comprises an inner frame member and second frame member comprises an outer frame member.

3. The optical mirror system of claim 1 wherein the first axis is substantially colinear to the first plurality of suspension beams.

4. The optical mirror system of claim 1 in which each of the first and second plurality of actuators includes a bimorph flexure.

5. The mirror system of claim 1 wherein the first plurality of actuators rotates the mirror about the first axis which is substantially parallel to the first plurality of beams and the second plurality of actuators initiates a motion which has a second axis which is different than the first axis.

6. The mirror system of claim 5 wherein the first axis is substantially colinear to the first plurality of flexure beams.

7. An optical mirror system comprising: a mirror; a first frame member coupled to and surrounding the mirror via a first plurality of flexure beams, wherein the mirror rotates about an axis relative to the first plurality of suspension beams; and a plurality of actuators coupled to the first frame member.

8. The mirror system of claim 7 wherein the first axis is substantially colinear to the first plurality of suspension beams.

9. The mirror system of claim 7 in which each of the plurality of actuators includes a bimorph flexure.