How Holographic Versatile Discs Operate

Holographic memory systems have existed for many years, offering much more storage space than traditional CDs and DVDs — even newer formats like Blu-ray. Their transfer speeds also outperform conventional discs. So, what’s kept holographic memory from becoming widespread until now?

Several obstacles have prevented holographic storage from entering mass markets, including cost and complexity. Until recently, these systems demanded an unreasonably precise manufacturing process. However, recent advancements have made the holographic versatile disc (HVD), created by Optware, a feasible choice for everyday consumers.

In this article, we'll explore how the HVD functions, its improvements over earlier holographic storage techniques, and how it compares to Blu-ray and HD-DVD.

Fundamentals of Holographic Memory

To grasp holographic memory, we first need to understand the concept of 'holography.' Holography is a technique for capturing light patterns to create three-dimensional representations. These captured light patterns are known as a hologram.

The creation of a hologram begins with a focused light source — a laser beam. This laser beam is split into two distinct beams: a reference beam, which stays mostly unchanged during the process, and an information beam, which passes through an object. As light interacts with the object, its composition shifts (refer to How Light Works for more details). Essentially, the information beam carries the image within its waveforms when it meets the object. When these beams converge, they produce a light interference pattern. Recording this light interference — such as in a photosensitive polymer layer on a disc — effectively captures the image’s light pattern.

To access the data stored in a hologram, you direct the reference beam onto the hologram. As it reflects off, it carries the light pattern of the stored image. The reconstruction beam is then sent to a CMOS sensor to regenerate the original image.

While we commonly associate holograms with capturing the image of objects, like the Death Star shown above, the holographic memory systems discussed here store digital rather than analog information. However, the principle is the same: instead of an information beam encountering a light pattern representing the Death Star, it meets a pattern of light and dark zones that signify ones and zeroes.

HVD provides several advantages over traditional storage methods. It can hold over 1 terabyte (TB) of data — 200 times more than a single-sided DVD and 20 times more than a current double-sided Blu-ray. This is partly because HVDs store holograms in overlapping patterns, whereas DVDs store information linearly. Additionally, HVDs have a thicker recording layer, allowing them to store data across almost the entire volume of the disc rather than just one thin layer.

One of the significant improvements over traditional memory systems is the HVD's transfer rate, which can reach up to 1 gigabyte (GB) per second — 40 times faster than a DVD. An HVD can store and retrieve an entire data page, around 60,000 bits of information, in a single pulse of light, while a DVD can only manage one bit of data per pulse.

Having grasped the basic principles behind HVD technology, let's now explore the structure of the Optware disc.

The Holographic Versatile Disc

Holographic memory has existed for over 40 years, but various factors made it challenging to introduce to the consumer market. Firstly, most systems direct both the reference and information beams into the recording medium from different angles. This requires highly sophisticated optical systems to align them precisely at the point where they must intersect. Another issue is the incompatibility with current storage media: Traditional holographic storage systems lacked servo data, as the beam carrying it could disrupt the holography process. Additionally, previous holographic memory discs were considerably thicker than standard CDs and DVDs.

Optware has made adjustments to its HVD that could make it more suitable for the consumer market. In the HVD system, the laser beams travel along the same axis and strike the recording medium at the same angle, a technique Optware refers to as the collinear method. This approach, according to Optware, requires a simpler optical system, resulting in a smaller optical pickup better suited for consumer use.

HVD also incorporates servo data. The servo beam in the HVD system uses a wavelength that doesn't affect the photosensitivity of the polymer recording medium. In the HVD test system, the servo data is carried by a red laser (650-nm wavelength). The size and thickness of an HVD are also comparable to those of CDs and DVDs.

The design of the disc places a thick recording layer between two substrates and includes a dichroic mirror that reflects the blue-green light containing the holography data, while allowing the red light to pass through to gather the servo information.

Since Optware's HVD system is still in the final stages of research and development, detailed technical information isn't yet available for public access. However, in the next section, we'll explore a simplified version of the system, covering the key components of the HVD.

The HVD System: Writing Data

A basic HVD system consists of these key components:

The process of writing data to an HVD begins by converting the information into binary data, which will be stored in the SLM. This data is transformed into ones and zeros, represented as opaque or translucent regions on a "page" — this is the image the information beam will pass through.

After the data page is created, the next step is to direct a laser beam into a beam splitter, which generates two identical beams. One of these beams is diverted from the SLM and becomes the reference beam, while the other beam is aimed at the SLM to become the information beam. As the information beam passes through the SLM, parts of the light are blocked by the opaque sections of the page, while the light passes through the translucent sections. In this way, the information beam transmits the image once it has passed through the SLM.

When the reference beam and the information beam converge on the same axis, they create a light interference pattern — the holography data. This combined beam carries the interference pattern to the photopolymer disc, where it is recorded as a hologram.

A memory system is only valuable if you can access the stored data. In the following section, we will explore how the HVD data retrieval process functions.

The HVD System: Reading Data

To retrieve data from an HVD, you need to extract the light pattern that was stored within the hologram.

In the HVD reading system, a laser projects a beam of light onto the hologram — a beam identical to the reference beam (Read System 1 in the image above). The hologram then diffracts this light in accordance with the unique interference pattern it holds. This diffraction recreates the image of the page data that originally formed the interference pattern. Once the reconstruction beam bounces off the disc (Read System 2), it travels to the CMOS sensor, which then reconstructs the page data.

Now, let's explore how HVD compares to other next-generation storage solutions.

How HVD Compares

While HVD aims to revolutionize data storage, other formats are competing to improve existing systems. Blu-ray and HD-DVD are two such formats, designed as the next-generation of digital storage. Both enhance current DVD technology to boost storage capacity. All three technologies target the high-definition video market, where both speed and capacity are essential. So, how does HVD measure up?

HVD is still in the final stages of development, and nothing is set in stone. However, the projected introductory price of about $120 per disc might be a significant barrier for consumers. That said, this price could be more palatable for businesses, which are the primary target for HVD developers. Both Optware and its competitors plan to highlight HVD’s vast storage capacity and transfer speed as perfect for archival purposes. Commercial systems could be available as soon as late 2006, with consumer devices expected to reach the market around 2010.