Figure 1: Recording a hologram, or index modulation

Holographic Data Storage:

New Technology Meets Exponential Growth Needs

Liz Murphy
Vice President of Marketing
InPhase Technologies
2000 Pike Road
Longmont, Colo.

The pressures on today’s data storage technologies are tremendous. According to a study at University of California , Berkeley , more new information is predicted to be stored in the next several years, than in all of previous recorded human history combined .

High-resolution satellite imagery, high-definition video, and full body scans are just some of the applications that are driving this exponential growth in data storage requirements. Add the compliance regulations in many industries that require long data archive retention periods, and that specify the manner in which data must be protected, accessed, and archived, and the scope of the data storage problems becomes enormous. How will the petabytes of data be stored and protected?

Even as storage needs are increasing exponentially, technology improvements in current magnetic and optical data storage systems are saturating. Physical barriers that limit their theoretically achievable storage densities and data transfer rates frustrate advances. Holographic Data Storage (HDS) is a technology that makes possible storage densities that exceed the barriers of traditional magnetic and optical recording. HDS has the capability to meet and exceed the expected storage demands well into the 21st century.

A series of innovations that solve the fundamental problems associated with the development of high-density data storage as implemented via holographic technology have been developed in the past several years. These include the creation of a commercial holographic storage material, and development of key holographic recording techniques. The National Technology Alliance funded the development of the photopolymer recording material, the automation of media manufacturing, and the development of system electronics, data formats, and electronic and logical interfaces in the holographic drive to be used by the National Geospatial-Intelligence Agency (NGA) .

Recording Data

Unlike conventional storage technologies that record and recover data bit by bit, holography allows a million bits of data to be written and read out simultaneously. The primary advantage of holographic storage comes from using the volume of the media and not just the surface to store information.

Light from a single laser beam is split into two beams--the signal beam, which carries the data, and the reference beam. The data are encoded onto the signal beam by a spatial light modulator that translates the bit stream of 0’s and 1’s into an optical “checkerboard” pattern of light and dark pixels. The data are arranged in an array or page of ~1 million bits. The hologram is recorded in the storage medium at the point of intersection of the reference beam and the data-carrying signal beam. See Figure 1. By varying the reference beam angle, wavelength, or media position, multiple holograms are recorded in the same volume of the medium, vastly increasing the data density.

Figure 2: Reading data via a reconstructed data page

Reading Data

In order to read the data, a reference beam with characteristics identical to that used to store the data diffracts off the index modulation (hologram), reconstructing the stored data page. The reconstructed data page is imaged onto a detector that reads all of the one million bits on the data in parallel. This parallel recording and read out of data provides holography with its very fast transfer rates. See Figure 2.

InPhase’s Tapestry Media

The major challenge to implementing holographic storage had been the development of a suitable storage medium. InPhase solved this issue by developing a new class of photopolymer materials that satisfy the stringent criteria for a commercial viability. The material offers high dynamic range, high photosensitivity, dimensional stability, optical clarity, manufacturability, nondestructive readout, thickness, and environmental and thermal stability.

Typical photopolymers use a single chemistry for bonding molecules together to form the media and to perform the recording. The InPhase polymer system utilizes materials that use two distinct chemistries that are independent yet compatible. One chemistry is used to form the media and to control the mechanical, manufacturing, and archive life parameters. The second chemistry is used during the recording process. These two chemistries do not interact or interfere with each other, thus enabling high dynamic range with extremely good dimensional stability during recording. See Figure 3.

Figure 3 - Storage media with two distinct chemistries, independent and compatible
In addition to developing a new class of materials, InPhase Technologies also developed the ZeroWave manufacturing processes, which enables the cost-effective fabrication of optically flat media and makes the media price-competitive for mass consumption. Hitachi Maxell, Ltd. ( Tokyo), a key investor and development partner of InPhase, has developed a new light-tight cartridge suitable for the photopolymer material and is also developing high-volume manufacturing processes.

The Holographic Recorder and Player

The next step in commercialization is the development of a drive system that records and reads the data. In October of 2004, InPhase built the world’s first integrated holographic drive prototype that includes each of the primary functions of channel, servo, mechanical, and optical sub-systems. The completion of the prototype was enabled by InPhase’s development of key recording techniques and by the availability of critical components. In the past, high-quality lasers were costly and unreliable. However, the 407 nm blue lasers used in other optical devices meet the requirements. CMOS active pixel sensor arrays used in digital cameras are also available, as are digital micromirrors, and ferroelectric modulators used in digital TVs and projectors. These components are commercially available, or in some instances are being customized in cooperation with industry partners. Displaytech ( Longmont, Colo.) is InPhase’s partner for the spatial light modulator and the two companies have formed a joint development partnership funded by a grant from the Advanced Technology Program in the Department of Commerce, for the development of a modulator to be used in a 1.6 terabyte holographic recording device.

Figure 4 - The prototype system records and reads data using a laser.
The first holographic drive prototype is based on write once read many (WORM) media, with a 50-year life and is an ideal fit for long-term archive applications. The system records and reads data using a 407-nm laser into 1.5-mm thick photopolymer recording material which is in a 130-mm disk format. See Figure 4. The media cartridge is loaded and unloaded automatically using a loader mechanism developed for InPhase by Alps Electronics ( Tokyo), an InPhase investor and development partner.

The prototype records 1.3 million bits of data in one page and 80 to 130 pages of data are recorded in a single location in the disk. See Figure 5. Each data page is recorded at an unique angular address separated by .1 degrees. A collection of data pages is referred to as a book and additional data density is achieved by significantly overlapping the books of data. This polytopic recording method invented by InPhase has demonstrated a capacity of one hundred gigabits per inch squared. The average exposure time per page is under 10 milliseconds achieving a 6MB/s transfer rate at density.

Figure 5 - Example of a 1.3-megabit data page.
The prototype system serves as the mile marker on the path to commercialization of holographic storage. The InPhase family of holographic drives and media will range in capacity from a minimum of 200 gigabytes to 1.6 terabytes with transfer rates from 20MB to 120 MB per second. The first generation Tapestry drive product will come to market with a minimum capacity of 200GB and a 20 megabytes-per-second transfer rate.

Holographic technologies will enable the highest density removable media with transfer rates much faster than other optical devices. Ultimately, the technology will be integrated into a multitude of products ranging from enterprise solutions to consumer devices. The long-term vision is that holographic technology will take on a prominent role in the storage landscape, addressing the highest demands in human history.

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