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Research

Abstract: Prof. Anderson is currently working on photonically producing true time delays for phased radar array antennas. She is also investigating optical cross-connects, optical code division multiple access, quality of signal monitoring in optical networks, and other optical devices. She is curious about ways to exploit the spatial coherence of lasers for high-speed communication and optical interconnection schemes, as well as for characterization of a laser's spatial mode structure. Earlier research topics include optical sensors, radiation hardness of laser diode, and diode laser structures.

The White cell

Adapting the White cell to true time delay
Adapting the White cell to optical switching
Advantages of White cell approach

Spatial Coherence
Older stuff (publications page)

Introduction:The White cell. Most of my work is based on the White cell, a simple system of three mirrors in which a single light beam makes multiple passes, and is refocused to a spot on each pass.

What is way cool about this is the spot pattern that develops. Here, on the left, is the spot pattern formed on Mirror A for two different beams. Notice how the spots for these two beams never overlap. You can extend this idea to a bunch of spots (right). The secret to the spot pattern is where the centers of curvature of the objective mirrors (B and C) are. A beam forming a spot at some particular spot on Mirror A form its next spot an equal and opposite distance from the center of curvature of the next mirror it visits.Top

Here is a photo of a spot pattern for a single beam.

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Adapting the White cell to true time delays: So, what we do is place a MEMS (micro-electro-mechanical systems) micromirror array in the plane of Mirror A, so we can switch beams between different White cells.
For true time delays, the White cells are of different lengths. Here is a simple design to illustrate the principle:

Normally, the mirrors on the MEMS are tipped to –10°, and the light bounces back and forth between Mirrors B and C. If a particular pixel is tipped to +10°, then a beam coming from C on that bounce is sent to E. It returns to the exact same next spot as if it had gone to B, but it took longer to get there. If the beam is sent to the long arm twice, it gets a delay of “2” units, the unit being how much longer it takes light to go to E compared to C. We call this the “linear cell” because the number of delays is proportional to the number of bounces.

Here is a photograph of the linear cell. This is looking down from the top. We used the Texas Instruments DLP (R) or DMD (R) chip. We bought a computer projector and took it apart! Engineering is great. Find out the whole story here. Top

This is the simplest design, but illustrates the general idea. If we add another spherical mirror somewhere, and let half the bounce go to Mirror C above, and the other half go to the new mirror, which is further away, we can have like a 1's place and an "m's" place if we are conting in base m. This is cool becuase now the number of delays goes as (m/2) squared. We have designs that go up to (m/8) to the power of eight, which we call the "octic cell." There is a long boring paper about it here.

Here is a photo of the current octic cell in progress- only six of the 10 arms are in place so far so right now it's really a quartic cell.The "MEMS" is the optical swtich (which is not in there yet), and the glass blocks effectively make some arms longer than others.

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Adapting the White cell to optical switching:When all you have is a hammer, everything looks like a nail. We can take the same geometry as the time delay device above, but adapt it to switching as follows. First, we make all the arms the same length. Then, we notice that the spot pattern forms in two rows, parallel to the centers of curvature of the spherical mirrors, below on the left. If we "misalign" one of the mirrors, then the spots will form in the wrong row,and the spot comes out in a different place. Every time we visit that misaligned mirror, the spot pattern slips another row. Eventually the beam comes out of the White cell at a new location. The exact location is controlled by how many times we go to the goofy mirror.

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Advantages of the White cell approach: The key reason we like the White cell so much is that you can have lots (hundreds or thousands) of light beams bouncing around in there simultaneously, so whether you want to delay them or switch them or whatever, you can do it to a large number of signals with the same hardware: a switch, and a handful of mirrors and lenses. Also, because we use free space (as opposed to fibers or waveguides), the loss is negligible. In the cross-connect, it turns out there that there are many different paths by which a given input can reach a selected output, which in turn means that if some of your swtiches fail, you don't have to replace the sswitch, just go around the bad ones. This makes the thing reliable.

More Information: For more information, please see publications and patents.

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last revised 8/10/2003