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What are micro-mirrors?

Scientific American http://www.sciam.com
Micro-mirror technology could increase the speed at which we communicate.
Micro-mirrors are mirrors that have been "shrunk" down to the microscopic world. Lately, the design and fabrication of micro-mirrors has received much more attention than in the past. This is due to their ingenious application to the field of fiber optics. In any optical network, be it phone lines, small office networks, or large university networks, most information is sent through optical fibres as beams of light that are either on or off, depending upon whether the piece of information is a 1 or a 0 in binary form. But, when the information needs to be re-routed (to a personal computer, for example) the information transmission process becomes a little more complicated. A switching station takes the incoming light beam, converts it into a digital signal, sends it to the fibre that is going towards the PC, then converts it back into a light beam, as shown below.

This is all well and good, but what exactly does it have to do with micro-mirrors? Well, wouldn't it be easier if, instead of having to convert the signal twice, one could simply have the signal reflect off something, automatically sending it to its correct destination? That's where micro-mirrors come in. By focussing the incoming light beam onto a micro-mirror, the beam will reflect off of it and be sent into another fibre. If, however, the micro-mirror has been actuated and is thus in an upright or slanted position, the incoming beam will be sent to a different outgoing fibre. By utilizing many of these micro-mirrors in sequence, all the incoming signals can be re-routed to their correct destination, without ever having to be converted into a digital signal! This means a huge saving of time, which corresponds to a higher network data transmission speed.

How do you make micro-mirrors?

The fabrication method for these micro-mirrors is exactly the same as for a cantilever structure, which can be found on the mems page, except after the process is completed, a reflective layer, such as aluminum, must be placed on top of the beam.

How are the micro-mirrors moved?

Running an electric current around the edge of the micro-mirror while it is in a magnetic field produces a force (either up or down, depending upon the direction of the current and magnetic field) known as a Lorentz, or electro-magnetic, force. By rapidly changing the direction of the current while holding the magnetic field constant, the micro-mirror can be made to pulse up and down thousands of times per second. Click here to see a movie.

Although this is the most popular form of actuation, it is not the only one. Piezoelectric materials can be implanted into the arms of the cantilever so that when current is run through them, they will force the beam, and the micro-mirror, to deflect upwards. Electrostatic actuation is one more method that researchers are looking into. Since positive and negative charges attract each other (and like charges repel), if a cantilever can be made to keep a positive charge while placing an alternating positive-negative charge above it, then by electrostatics, the cantilever will resonate up and down.

What are some other applications of micro-mirrors?

Since micro-mirrors have had such great success in the field of optical switching, researchers are now looking for other applications for them. For example, by adding another slender arm to the opposite side of the cantilever structure, the mirror can be made into what's known as a torsional mirror. This torsional mirror can be actuated in two different planes, that is, both up and down, and side to side. This added dimension allows incoming light beams to be reflected onto any one of four different outgoing fibres, thus doubling the switching capacity of a single micro-mirror.

Texas Instruments has incorporated micro-mirrors into its new Digital Micro-mirror Device (DMD), an important part of its new Digital Light Processing (DLP) image projector. By using micro-mirrors, sharper images with much higher resolution can be projected onto a screen by a projector that weighs less than three lbs. An in-depth look at this device can be found here: http://www.dlp.com/dlp/default.asp.
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  2001 SMA/MEMS Research Group 
 Last modified: Aug 17, 2001