Holographic method records submicron pixels

To make a holographic memory system inexpensive enough to be commercially competitive, it is important to be able to record pixels less than a micron in size. Researchers at the California Institute of Technology (Pasadena, CA) have recorded submicron pixels in lithium niobate doped with iron. The high storage density is a result of both the wide recording bandwidth of the material and the researchers' use of the phase-conjugate readout method. They began with a phase-conjugate memory module that consisted of a spatial light modulator (SLM), a pixel-matched detector array, a photorefractive crystal, and an array of laser diodes. A signal beam passes through the SLM and interferes with a plane-wave reference beam to record the pattern. A reference beam reflecting off a mirror on the opposite side of the crystal propagates in the opposite direction of the recording reference beam to retrieve the information. By having multiple laser sources at slightly different angles, several holograms can be recorded in the same crystal. The researchers used a resolution mask with pixels ranging from 2 to 0.2 µm² and recorded a strong image.