Current digital cameras inserted into mobile phones have a lousy quality.

But now engineers at UC San Diego have made an ultrasmall yet seven times more powerful digital camera by folding up the telephoto lens.

The high resolution minicameras could be used for unmanned surveillance aircraft, cell phones and infrared night vision applications. "Our imager is about seven times more powerful than a conventional lens of the same depth," said Eric Tremblay, lead researcher and an electrical and computer engineering Ph.D. candidate at UCSD's Jacobs School of Engineering.

"This type of miniature camera is very promising for applications where you want high resolution images and a short exposure time. This describes what cell phone cameras want to be when they grow up," said Joseph Ford, a professor of electrical and computer engineering at the Jacobs School who leads the camera project within UCSD's Photonic Systems Integration Lab.

"Today's cell phone cameras are pretty good for wide angle shots, but because space constraints require short focal length lenses, when you zoom them in, they're terrible. They're blurry, dark, and low contrast."

The team replaced the common lens with a "folded" optical system similar to conventional mirror astronomical telescopes, like the Cassegrain telescope (image), developed in 1672. "The folding idea was new in 1672, but they were doing it with two separate mirrors. We cut all of our reflective surfaces out of a single component and quadrupled the number of folds," said Ford.

Traditional lens systems bend and focus the light passing through a series of mirrors and lenses, while the new system bends and focuses light reflected back and forth inside a single 5 mm thick optical crystal, with a 7 times more accuracy than in a traditional lens system of the same thickness. "When all is said and done, our camera will look a lot like a lens cap that can be focused and used as a regular camera," said Ford.

"Traditional camera lenses are typically made up of many different lens elements that work together to provide a sharp, high quality image. Here we did much the same thing, but the elements are folded on top of one another to reduce the thickness of the optic," said Tremblay.

"Our 'folded lens' is not technically a lens, since it is reflective. I am guilty of calling it a lens sometimes, but I'm trying to control myself. 'Imager,' or 'folded optic' are more accurate."

This technique increases the "effective focal length" (the zooming power) without increasing thickness. "The larger the number of folds in the imager, the more powerful it is," said Ford.

The engineers created a series of concentric, reflective surfaces that bend and focus the light on a disk of calcium fluoride (a transparent optical crystal). Two disks with 60 mm diameters are separated by 5 mm of fluoride crystal, so that the light bounces back-and-forth between the two reflective surfaces, following a predetermined zigzag path as it moves from the largest of the four concentric optic surfaces to the smallest and then to the CMOS light sensor.

A diamond tip was used for cutting all the surfaces onto a single fluoride disk. "You mount the optic once and the diamond machining tool cuts all the optical surfaces without having to adjust the setup," explained Tremblay. A 5 mm thick, 8-fold imager optimized to focus on objects 2.5 meters away proved to have the same resolution, color and image quality with a compact camera lens with a 38 mm focal length.

The team is working now on a variable-focus folded optical system with air between the reflective surfaces of the image, which could be very practical especially for infrared vision devices.

These systems also allow ultra-broad-spectrum imaging, which can be used for ultraviolet lithography, a novel technique for crowding more transistors onto silicon, developing more performant chips. "I think it's a good possibility. You can see how much smaller this has gotten already. It's going to keep shrinking", said Trembley about the possibility of having the new technique incorporated into cell phone cameras.