Bell Laboratories:
Photopolymer Recording Media
Holography enables storage densities that can far surpass the super-paramagnetic and diffraction limits of traditional magnetic and optical recording. Holography can break through these density limits because it goes beyond the two-dimensional approaches of conventional storage technologies to write data in three dimensions. In addition, unlike conventional technologies which record data bit by bit, holography allows a million bits of data to be written and read out in single flashes of light, enabling data transfer rates as high as a billion bits per second. This would be comparable to a speed fast enough to transfer a DVD movie in about 30 seconds. A powerful combination of high storage densities and rapid data transfer rates makes it possible for holography to become a compelling choice for next-generation storage needs.

In holographic data storage, light from a coherent laser source is split into two beams, signal (data-carrying) and reference beams. Digital data to be stored are "encoded" onto the signal beam via a spatial light modulator. The data or strings of bits are first arranged into pages or large arrays. The 0's and 1's of the data pages are translated into pixels of the spatial light modulator that either block or transmit light. The light of the signal beam traverses through the modulator and is therefore encoded with the "checkerboard" pattern of the data page. This encoded beam then interferes with the reference beam through the volume of a photosensitive recording medium, storing the digital data pages.

The interference pattern induces modulations in the refractive index of the recording material yielding diffractive volume gratings. The reference beam is used during readout to diffract off of the recorded gratings, reconstructing the stored array of bits. The reconstructed array is projected onto a pixilated detector that reads the data in parallel. This parallel readout of data provides holography with its fast data transfer rates.

The readout of data depends sensitively upon the characteristics of the reference beam. By varying the reference beam, for example by changing its angle of incidence or wavelength, many different data pages can be recorded in the same volume of material and read out by applying a reference beam identical to that used during writing. This process of multiplexing data yields the enormous storage capacity of holography.

In the past, the realization of holographic data storage has been frustrated by the lack of availability of suitable system components, the complexity of holographic multiplexing strategies, and perhaps most importantly, the absence of recording materials that satisfied the stringent requirements of holographic data storage.

Recently the development of practical components for holographic systems has rekindled interest in this technology. While the development of the needed components has been accomplished largely in fields outside the storage industry, the volume of these markets is expected to lead to low-cost, reliable components for holographic data storage. Frequency-doubled, diode-pumped Nd:YAG green lasers, used in the medical, cable TV, and printing industries, are attractive recording sources due to their small size, ruggedness, and low cost. Digital micro-mirror devices appearing in new types of displays are ideal spatial light modulators with their large numbers of pixels (~ 1 million), fast frame rates (2000 Hz), and high optical contrast. The CMOS active pixel detector arrays emerging in digital photography exhibit the rapid access and data transfer properties required for holography.

At Bell Labs, we invented a multiplexing geometry that yielded a simple, easily-implement able architecture for holographic storage systems. Spurred by this development, we focused on the long-standing problem of the lack of suitable storage materials and invented new high-performance recording media with demonstrated high density data storage capabilities. Our work serves as the foundation for a practically realizable, high capacity storage system with fast transfer rates and low-cost, removable recording media.

The methods used to overlap or multiplex holograms determine the complexity and architecture of the recording system. In the past, multiplexing methods have required large optical systems and moving optical parts. We have developed a method known as correlation multiplexing where an optically complex reference beam, created by a fixed set of optics, encodes the position of the hologram in the recording medium. Large numbers of holograms can therefore be multiplexed in essentially the same volume of the recording medium through only micron-size spatial translations of the medium relative to the reference beam. This "fixed optics" method enables construction of a simple holographic storage system based on a spinning disk architecture used throughout much of the storage industry.

One of the major challenges in the area of holographic data storage has been the development of suitable storage materials. Holographic media must satisfy stringent criteria, including high dynamic range, high photosensitivity, dimensional stability, optical clarity and flatness, nondestructive readout, millimeter thickness, and environmental and thermal stability.

To meet the needs of high-density holographic data storage, researchers at Bell Laboratories have designed a new type photopolymer, a "two-chemistry" system, which yields high response, high photosensitivity media in millimeter-thick, optically flat formats. The media exhibit the some of the highest dynamic range of any holographic material and currently represent one of the few recording systems appropriate for high density digital holographic storage applications.

The media are fabricated from mixtures of two independently polymerizable yet compatible chemical systems. Recording disks are formed by an in-situ polymerization of one of the components to form the matrix or support of the medium. The other component, which is photosensitive, remains unreacted and dissolved in this matrix. Recording of holograms occurs through a spatial pattern of polymerization of the photosensitive species that mimics the optical interference pattern generated during holographic writing The concentration gradient that results from this patterned polymerization leads to diffusion of the unpolymerized species which creates a refractive index modulation that is determined by the difference between the refractive indices of the photosensitive component and the matrix. Our approach allows us flexibility in tailoring the media to the particular needs of high density holographic data storage.

In these materials, a storage densities of 31.5 channel Gbits/in2 (a density that would yield ~45 Gbytes on a 5 " disk) have been demonstrated by recording and retrieving >3000 digital data pages. Newer "two-chemistry" materials we have developed have the capability to store densities at least five times higher. With these photopolymer materials meeting the critical performance requirements for holographic mass storage, we believe they have removed much of the risk associated with the development of holographic technology.

The substantial advances in recording media, recording methods, and the demonstrated densities of greater than thirty channel gigabits/pairs coupled with the recent commercial availability of system components remove many of the obstacles that previously prevented the practical consideration of holographic data storage and greatly enhance the prospects for holography to become a next-generation storage technology.

Bell Labs has recently entered into an agreement with Imation Corporation, one of the leading data storage companies, to jointly further develop our work in holographic data storage. We are currently working with the Lucent Technologies New Ventures Group to explore avenues that would lead to commercialization of the technology