optical storage

What is optical storage?

Optical storage is any storage type in which data is written and read with a laser. Typically, data is written to optical media such as compact discs (CDs) and digital versatile discs (DVDs). At one time, optical discs were considered a potential replacement for hard disk drives (HDDs) in computing systems, but their lack of growth in capacity compared to both HDDs and later flash-based solid-state drives (SSDs) has relegated optical storage use mostly to long-term archiving and data backup.

Although optical media is more durable and less vulnerable to environmental conditions than tape, HDDs and SSDs, optical discs are slower than the typical HDD and significantly slower than the SSD and offer lower storage capacities than either. Blu-ray disks are currently the fastest optical media on the market and provide much more capacity than CDs and DVDs, but they still lag behind HDDs and SSDs.

What are common examples of optical media?

Over the years, optical media have taken a variety of forms, including LaserDisc (LD), HD-DVD, write-once, read-many (WORM) optical cartridges and several others. From this mix, three formats have emerged as standards in today's optical storage market:

• Compact disk (CD). The CD represents the first generation of commercial optical storage. After its introduction, it quickly replaced both vinyl records and cassette tapes as the audio medium of choice. Originally, CDs were available only as prerecorded read-only disks, but it wasn't long before they became available as recordable discs and rewritable discs that could be used for data storage. The CD can hold up to up 700 megabytes (MBs) of data.
• Digital versatile disc (DVD). Also referred to as the digital video disc, the DVD started out as a read-only medium similar to CDs but with the ability to hold enough data to store a full-length movie. A single-layer DVD can hold 4.7 gigabytes (GB) of data, and a double-layer disc can hold 8.5 GB. Not long after DVDs were introduced, recordable and rewritable discs became available for data storage.
• Blu-ray. The Blu-ray disk has emerged as clear leader in today's optical storage market. Unlike CDs and DVDs, which use a red laser to read and write data, a Blu-ray disk uses a blue laser, which dramatically increases capacities and data transfer rates over CDs and DVDs. Today's Blu-ray discs can store up to 128 GB of data and are available as read-only disks that can hold prerecorded high-definition feature films as well as recordable and rewritable disks for data storage.

The standard size for all three formats is the same: 120 mm (4.7 inches) in diameter and 1.2 mm (0.05 inches) thick. This standard makes it possible for Blu-ray drives to support DVDs and CDs and for DVD drives to support CDs. That said, optical drives are compatible only with earlier formats and not the other way around. CD drives cannot run DVD or Blu-ray discs, and DVD drives cannot run Blu-ray discs.

History of optical media

The first method for storing data using light on a hard medium was invented by James T. Russell in the late 1960s. Russell's initial creation bore little resemblance to current optical storage technology.

His invention used micron-sized dots of light and dark to indicate the presence or absence of a digital bit. This pattern was read by passing light through the otherwise transparent medium it was encoded on.

During the 1970s, Russel continued to refine his design as the CD format began to take shape, but it wasn't until 1982 that the CD received serious recognition when Philips and Sony introduced the world's first commercial CD player. In contrast to Russell's original design, these modern discs use a laser to create pits on a reflective surface such as aluminum foil, which lies beneath a hard transparent plastic covering. The size of the pit is determined by the wavelength of the laser light.

Because the blue light laser used with Blu-ray discs has a shorter wavelength than red laser light, 25 GBs of storage can be encoded on a single layer of a 12 cm Blu-ray disc. In comparison, a CD can hold only 700 MB, and a single-layer DVD can store only 4.7 GB, making Blu-ray the current standard in optical disc storage.

Although optical storage capacity remains lower than HDDs or SSDs, research and development efforts toward greater capacities continue. In 2016, Sony announced a Blu-ray disc that could hold up to 3.3 terabytes (TB) of data. 

Since then, there has been little indication whether Sony still plans to deliver on its promise, but the company now offers a quad-layer Blu-ray disc that can store 128 GB data. In the meantime, the Blu-ray Disc Association is throwing its full weight behind Ultra HD Blu-ray, which can store up to 100 GB of data.

Advantages and disadvantages of optical storage

One of the biggest advantages of optical storage over other storage media is durability. Optical discs are not vulnerable to data loss due to power failure like volatile memory, and they're not as subject to wear as non-volatile memory (NVM) such as HDDs and flash SSDs. Optical discs are also much sturdier than magnetic tape, which is the leading archival storage medium.

Another advantage of the optical disc is that the storage medium is inexpensive to manufacture, although costs can vary depending on the type of discs and how they're used. Discs that contain prerecorded material such as audio CDs or Blu-ray movies are made up primarily of aluminum foil and plastic. Manufacturers can easily bulk-produce these kind of discs by using a die and stamp technique that presses the tiny pits into the reflective foil medium in an assembly line process.

Optical discs for data storage have different requirements. Unlike prerecorded discs that are written to only once, optical discs for storage typically need to be rewritable, which requires different recording material. Rewritable discs cannot use a low-cost reflective foil layer like prerecorded discs. Instead, they must use a more expensive layer of phase-change material that enables the data to be erased and written over multiple times.

Despite these difference, optical discs for storage are still made up mostly of plastic and can be produced in bulk, making them cheaper to manufacture compared to other storage media.

The biggest disadvantage of optical storage is disk capacity. The latest 12-cm Blu-ray discs top out at 128 GB, far below what is now possible with either HDDs or SSDs on a per-centimeter basis. At the same time, the rise of internet streaming and Universal Serial Bus (USB) flash drives has also diminished the reliance on optical discs.

However, the future of optical storage might not lie with plastic disks, but with quartz crystal, at least for archival or write-once data. Microsoft's Project Silica is actively working on a storage technology that uses ultrafast laser optics to store data in silica glass. 

As their first proof of concept, the project stored the 1978 Superman movie in a piece of quartz glass that's 2 mm thick and 75 mm square in area. Researchers believe that they'll eventually be able to store 360 TB of data on a piece of quartz about the size of a DVD.