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Fractals improve imaging performance

18 Jan 2007

A photon sieve containing pinholes that follow a fractal pattern produces high quality images. optics.org talks to Walter Furlan to find out more about fractal photon sieves.

A fractal photon sieve (FPS) offering improved focusing and image-forming properties is set to make an impact on applications ranging from X-ray microscopy, THz tomography and astronomy. That's according to researchers in Spain who say their FPS has an extended depth of field and reduced chromatic aberration compared with Fresnel zone plates. (Optics Express 14 11958)

Just like Fresnel and fractal zone plates, a fractal photon sieve is a diffractive lens. A Fresnel zone plate consists of a series of periodic concentric circles whereas a fractal zone plate comprises circles following a fractal pattern. In a FPS, the continuous transparent circular regions of a zone plate are replaced by a high density of discrete holes.

"Compared with a Fresnel zone plate, we demonstrated that a FPS has an extended depth of field and a reduced chromatic aberration," Walter Furlan of the University of Valencia told optics.org. "Compared with a fractal zone plate, the distribution of the holes is a degree of freedom that can be exploited to perform apodization, particularly to nearly suppress the higher order foci."

According the Furlan, there are also advantages when it comes to fabricating FPSs. "FPSs can be manufactured using the same technology as conventional zone plates," he explained. "However, an FPS can be constructed from a single structure without any supporting substrate. To test our proposal, we simply printed the diffractive lenses using a conventional laser printer and then we reduced them photographically onto 35 mm slides".

Furlan and colleagues designed their FPSs to work in the visible using white light illumination. They compared the performance of an FPS and a Fresnel photon sieve using a chart containing letters decreasing in size, such as those used by an optician. Images recorded using the FPS were judged to be "considerably better".

FPSs can also be designed to operate at short (extreme UV) wavelengths right through to long (microwave and THz) wavelengths.

"A broad range of polychromatic imaging applications where conventional Fresnel zone plates are used could benefit," said Furlan. "This includes X-ray microscopy where narrowband sources are limited; imaging and tomography using THz for an efficient projection of line spots onto the object; and also astronomy where diffractive lenses can be implemented in large aperture telescopes."

Furlan concludes by saying that his team, composed of researchers from both the University of Valencia and the Polytechnical University of Valencia, is currently developing new designs and applications of both FPSs and fractal zone plates as image forming devices. "We also plan to commercialise the technology in the near future," he revealed.

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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