Detectors & Imaging

HOLOGRAPHIC OPTICAL DATA STORAGE: Component tester allows flexible analysis

April 1, 1996

Researchers at the IBM Almaden Research Center (San Jose, CA) have developed a small demonstration system (DEMON) for testing any spatial light modulator (SLM) in combination with most charge-coupled-device (CCD) cameras, as well as allowing recording and retrieval of angularly multiplexed holograms.

Rick DeMeis

Holographic-data-storage demonstrator stores multiple holograms in a single crystal. A galvo scanner—the thin, vertical black box in the left foreground—uses a rotating mirror to angularly multiplex reference beams in the recording medium. The demonstrator measures 18 × 24 in., not including the laser, which is not shown.

As a partner in the Holographic Data Storage Systems consortium, IBM wanted to test combinations of various holographic data-storage components in a data-storage environment rather than individually in conventional test setups (see Laser Focus World, March 1996, p. 38). The performance of a data-storage system and the development of error-correction codes, for example, depend on the sum of all system characteristics and errors. Furthermore, testing is faster and more efficient if it is done in parallel with other consortium members on similar test equipment in place at their sites.

System design

The DEMON’s design is flexible enough to permit use of most Fourier lenses. The lens pair currently in use at the center allows direct error-free imaging of a SLM having a 1024 × 1024-pixel pattern on a 1-megapixel CCD camera because both devices have the same number of pixels. Most commercially available SLMs and CCD cameras, however, differ in pixel sizes. To match the pixel sizes between the modulators and cameras, the researchers built a low-distortion zoom lens with a range of 0.9X to 3.0X. Thus, for instance, the DEMON optical train can image 9-µm pixels from a CCD camera onto a 1-megapixel SLM whose pixel size can range from 8 to 27 µm.

The DEMON configuration currently in use stores digital data patterns in the holographic media via a video-graphics-array-format SLM. The holograms are angularly multiplexed and addressed by a standard scanner. A CCD camera (640 × 480 pixels) reads the stored holograms. This configuration can read up to 60 frames/s at 76 Kbit/frame for an overall data rate of 4.6 Mbit/s. While such performance is not impressive compared to current hard-disk drives, notes development-team member Hans Coufal, "it allows us to develop error-correction and modulation codes and to test components in a realistic environment." Thus, the consortium expects to demonstrate that holographic storage can be competitive with the most advanced disk drives.