In general, the so-called "4th-generation optical discs" are expected to reach 1Tbyte in 2011-2015. To reach this target, Holographic storage systems have to compete with other candidate technologies, including Multi-layer/2-beam absorption technology, Near-field recording and Super-RENS technology.
In this article, we focus on holographic storage solutions.
Holographic storage is a technology that can store information at high density inside photopolymers. Current storage techniques such as DVD use multiple layers to store information on the volume. Both techniques share an information density limit defined by the used wavelength. In the case of point-wise information retrieval, this is called diffraction (of the writing beams). The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface.
The photopolymer-based media, store the information following a procedure that resembles photography, but it uses laser light as a source. The biggest challenge here is media stability.
Several competing technologies have been proposed for application in holographic systems. Inphase Technologies uses the angle multiplexing technique, where overlapping holograms increase the volumetric capacity. Holographic Versatile Disc (HVD) adopts the Collinear multiplexing technique, where red and green laser are collimated into a single beam. HVD is backwards compatible with the Blu-Ray and DVD formats, and it is the only holographic approach that has been standardized by ECMA and ISO. Another approach is called Atomic Photon 3D, which uses multilayers providing 1TB of storage per surface.
Holographic Versatile Disc (HVD)
Holographic Versatile Disc (HVD) is an optical disc technology which would hold up to 3.9TB of information. It employs a technique known as collinear holography, whereby two lasers, one red and one green, are collimated into a single beam. The green laser reads data encoded as laser interference fringes from a holographic layer near the top of the disc while the red laser is used as the reference beam and to read servo information from a regular CD-style aluminum layer near the bottom. Servo information is used to monitor the position of the read head over the disc, similar to the head, track, and sector information on a conventional hard disk drive. On a CD or DVD, this servo information is interspersed amongst the data.
A dichroic mirror layer between the holographic data and the servo data, reflects the green laser while letting the red laser pass through. This prevents interference from refraction of the green laser off the servo data pits and is an advance over past holographic storage media, which either experienced too much interference, or lacked the servo data entirely, making them incompatible with current CD and DVD drive technology.
HVD supports more than 10 recording layers.
Japanese Optware and Maxell are supporting the HVD technology and have announced 100GB and 200GB HVD discs, although their commercialization has no yet been decided.
HVD is also the only holographic technology that has been included in the ECMA standards. (ECMA 377 HVD Recordable 200GB, ECMA 378 HVD-ROM 1200mm 100GB, ECMA 375). The technology is also backed by the HVD Alliance, which includes companies such as Fujifilm, CMC Magnetics, ITRI, Pulstec, LiteOn, Oerlikon and others.
InPhase is currently proposing its Tapestry 300r holographic drive which is capable of reading/writing 300GB discs.
The drive will offer a 20MBps (160 Mbps) transfer rate, and is expected to ship by the end of 2007.
As in the case of Optware's proposal (HVD), Tapesty uses two laser beams - a reference beam and a signal beam. In the Inphase recording device, the beam is split into two laser paths. The reference beam is guided directly on the disc, while the other passes through a spatial light modulator (SLM). The SLM creates the page array by translating the digital marks ('1s' and '0s') into light and dark pixels and creating the "checkerboard" pattern of the data page, which is finally recorded on the photosensitive recording polymer of the disc.
The basic advantage of the Tapesty system is the fact that millions of bits (data) can be recorded and read in pages by a single flash of light (laser). As a result, Tapesty outruns optical discs in terms of data transfer rates.
According to the company, Tapestry's target markets include professional video/rich media, regulatory compliance, and data archive applications that value high capacity, fast transfer rate and long media archival life. Specific application areas include security, geospatial imagery, broadcast, medical, and digital video in business and entertainment.
Together with its media partner Maxell, Inphase plans to offer the Tapestry 300r drive at an $18K price tag, with the media to cost around $120.
2photon 3D - Call/Recall
Call/Recall supports a patented 2-photon recorded 3D optical storage technology that can provide 40 times the capacity of Blu-ray.
Call/Recall's optical storage technology utilizes a 2-photon recording process to record bits in a three-dimensional volume on a disk. Multiple layers of information can be stored within the 3D volume with less than a 10-micron layer of separation, effectively allowing the equivalent of 250 conventional DVD layers to be put onto one DVD with projected layer densities reaching High-Def formats allowing a roadmap to increase capacities by 5X.
The company also claims that another key benefit of the new technology is its use of commercially available, off-the-shelf components. This approach allows optical hardware manufacturers to extend the roadmap of their existing technologies, such as DVD and Blu-ray Discs, while maintaining backward compatibility with their installed base.
The company is currently recording a 1TB disk and has established a product roadmap delivering solutions capable of storing multiple terabytes of information per disk. The versatile 2-photon 3D technology can be applied to solutions such as a 100+ terabyte optical libraries using DVD-size disks for enterprise data storage.
Call/Recall expects to offer a 2-photon 3D holographic drive at around $500.
Atomic 3D Holographic
Colossal Storage Corp. proposes the Atomic 3D Holographic technology and the Spintronic data nanotechnology. The technology stores data by controlling the direction of electrons/bits.
The company has developed a rewritable 3-D volume holographic removable disk. Unlike magnetic media, which only stores data on the surface of the disk drive, holographic optical disk drives use two or more laser beams that work with one another to read and write data throughout the disk media.
Colossal's solutions promise to offer 100+TB capacities, near zero read and write response times and 100-plus year lifespan.
Mempile was able to create a media with about 200 virtual (i.e., created by the laser) layers, five microns apart, each containing approximately 5 GB of data. The company calls this media "TeraDisc". This polymer acrylic disc allows for the focusing of a red laser on any of up to 200 virtual layers per 1.2 mm disc.
The process that Mempile uses to write and read data is called two-photon absorption and is nonlinear in nature. When the laser beam is focused to a small radius on the disc, it is very easy for the photons to excite the polymer molecules (chromophores). Nonlinear optics is required in a 200-layer disc, because linear optics would cause some of the light to be absorbed by the layers above the intended one resulting in errors and loss of signal.
Although current prototypes are still in the 600~800GB per media range, Mempile claims that further optimization will enable it to reach its goal of 1 TB per 1.2mm disc in the very near future.
The first beta version of the TeraDisc will be available in 12-18 months and the initial market price for the drive from the CE industry is expected to be in the range of $3,000 - 4,000 U.S. dollars. Following that, when the company comes out with its solution geared towards the consumer home, it will then have the potential to drop to prices as low as $400 U.S. dollars.
Analyzing the various technologies, we can note that Blue laser technologies, which targets today's consumer market, has been selected by the entertainment and VOD industries. While it is cost-effective for archiving, it has suffered from a very slow adoption rate, especially from the IT market. Tapestry technology is still in beta phase and has had virtually no market exposure. Its greatest drawback is probably that there are few OEM and technology partners backing it at this stage. Finally, HVD appears to be a very promising technology, but the question is when will it be available, and also at what cost.