Holographic storage isn't new. Already, it has found its way into specialized areas that require massive amounts of storage and aren't so strangled by cost restrictions. With dropping device costs and shrinking system size, it's set to take on a broader range of applications.
Another reason for this rosy outlook is that holographic storage, as well as projection systems, can now take advantage of ferroelectric liquid-crystal (FLC) technology. Displaytech's SLM-1216-1 module targets both of these areas, and InPhase Technologies will use it in its latest holographic storage units (Fig. 1).
The optics and the spatial light modulator (SLM), like the SLM1216-1, represent the key components in these systems (Fig. 2). InPhase Technologies combines them with a rotating disk made of the holographic recording media. When writing information, a coherent laser light source is split to form a reference beam and a signal beam, which are then recombined at the storage medium.
The storage medium changes state based on the two beams, just like other read/write optical media. The changed media state is retained so it can be read later using only the reference beam. The detector array then reads the light that shines through the media.
SMOKE AND MIRRORS
Maybe polarized light and mirrors would be a better description, because this isn't magic. The SLM1216-1 (see table), which operates like reflective LCD projectors, is just well-applied science and engineering.
The module's rectangular array of 1200 by 1200 pixels is used to write an approximately 175-kbyte block of data in one step. Compare this to the typical hard-disk head that writes a single bit, and it's easy to see why optical storage can hit impressive speed goals and not just high-density goals.
FLC tends to be about 100 times faster than the LCDs in laptops and desktops. The module's 1.1kHz frame rate can deliver a theoretical throughput of 188 Mbytes/s. A single x1 PCI Express lane can manage this kind of bandwidth.
Density drives holographic storage. The big hurdle, though, has been the cost and complexity of the support system, especially the SLM. Displaytech cuts costs and improves reliability by combining the FLC on the same chip as the control electronics.
The SLM-1216-1 features two interface ports. The four-wire serial peripheral interface (SPI) port handles the control interface. Almost any microcontroller can manage this load speed interface, though cranking out 16-bit data at speeds up to 120 MHz for data transfer may stress some MCUs.
Holographic storage isn't the only bastion for this technology. Small projected displays would make a nice fit. Also, its small size would be useful in applications like head-mounted displays. FLC's fast response time eliminates the motion smearing and screen-door effects often seen on LCD displays and project systems as well.