Femtosecond Laser Directly Writes Computer-Generated Holograms

Richard Gaughan

The unique characteristics of holograms are attributable to their ability to store both intensity and phase information. Traditional holograms are created by recording the interference pattern generated by a reference beam and by another beam reflected from the object being imaged. Computer-generated holograms, first described in the late 1960s, simplify the process because they do not require physical interference between two beams. Still, these holograms have their own manufacturing complexities in that they employ photomasking and etching techniques derived from the large-scale manufacture of integrated circuits.

Traditional computer-generated holograms are produced with lithographic techniques. Researchers at the Shanghai Institute of Optics and Fine Mechanics have developed an approach that employs femtosecond laser pulses to ablate hole patterns in a metal film.

Now researchers at Shanghai Institute of Optics and Fine Mechanics at the Chinese Academy of Sciences have demonstrated an alternate, single-step method for producing computer-generated holograms. Quan-Zhong Zhao and colleagues used 120-fs pulses of 800-nm radiation from a Spectra-Physics Spitfire Ti:sapphire laser to directly write a computer-generated hologram on an aluminum film on glass, without the need for masking or for other pre- or posttreatment of the substrate.

They focused the laser beam through a 1× microscope objective while the substrate was translated in the plane perpendicular to the incident radiation. With 1 mW of average power, the laser ablated clean, 10-µm-diameter holes without damaging the glass. As proof of principle, they constructed a 128 × 128 array of spots in a 3 × 3-mm region. The hologram, displaying the institute's acronym, SIOM, functioned as both a transmission and a reflection hologram.

Because femtosecond lasers are suitable for use with most materials, Zhao said, the technique opens the door to the wider fabrication of computer-generated holograms, and the group plans to improve diffraction efficiency by optimizing the writing parameters and the material conditions for substances suited to specific applications. For now, he is pleased that direct writing using a femtosecond laser simplifies the fabrication of holograms.