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11/16/2005: "A possible software revolution for free space optics"

This could provide a real revolution for FSO.

Now, Pat Hanrahan and his team at Stanford University have figured out how to adjust the light rays after they have reached the camera. They inserted a sheet of 90,000 lenses, each just 125 micrometres across, between the camera's main lens and the image sensor. The angle of the light rays that strike each microlens is recorded, as well as the amount of light arriving along each ray.

Software can then be used to adjust these values for each microlens to reconstruct what the image would have looked like if it had been properly focused. That also means any part of the image can be refocused - not just the main subject.

Tracing the rays like this removes the conventional trade-off between the aperture size, which controls the amount of light that the camera takes in, and the depth of field. If light is low, a larger aperture will let enough light into the camera to form a clear image, but the laws of optics mean that a narrower slice of the world in front of the camera will appear in focus.


The major problem with most Free Space Optical systems is that the beam spreads; the more distant the connection, the wider it gets. Normally the way to handle that or to handle movement and alignment issues is to let the beam spread and take a loss in power, which usually limits the distance that the buildings can be separated. By using something like the lens array, you could do a sort of "beam-forming" for FSO links, increasing the effective distance, even for very low-power lasers, or even the LEDs found in commercial Gigabit Ethernet switches.

By allowing a larger aperture, the system can have more receive gain (more light collected at the receiver) and focused on the detector. This allows for a lower power transmitter to be used, reducing system cost.