What are optical disks and how do they work?

How do optical disks work?

Optical disks rely on a red or blue laser to record and read data. Most of today's optical disks are flat, circular and 12 centimeters (4.7 inches) in diameter. Data is stored on the disk as microscopic data pits and lands. The pits are etched into a reflective layer of recording material, and the lands are the flat, unindented areas surrounding the pits.

The type of material selected for the recording depends on how the disk is used. Prerecorded disks, such as those created for audio and video recordings, can use cheaper materials like aluminum foil. Write-once disks and rewritable disks require a more expensive layer of material to accommodate other types of digital data storage.

Data is written to an optical disk in a radial pattern starting near the center. An optical disk drive uses a laser beam to read the data from the disk as it is spinning. It distinguishes between the pits and lands based on how the light reflects off the recording material. The drive uses the differences in reflectivity to determine the 0 and 1 bits that represent the data.

Optical disk vs. magnetic storage media

When first introduced for commercial use, optical disks could hold much more data than similarly sized magnetic storage media. However, improvements in hard disk drive technology led to HDDs with much greater capacities on a per-centimeter basis than optical disks could support. Solid-state memory technologies also improved in capacity and endurance, while prices dropped steadily.

Optical disks have a significant advantage in durability over other storage types. They are less likely to degrade over time than magnetic tape, HDDs or solid-state drives. The data stored on them is relatively impervious to most environmental threats, such as power surges or magnetic disturbances. This makes optical disks well suited for prerecorded audio and video content and for backing up and archiving data, including cold storage.

Optical disk storage capacities

Here are the capacities of the three current optical disk technologies:

• A CD can store up to 700 megabytes (MB) of data.
• A single-layer DVD can hold 4.7 gigabytes of data, and a double-layer disk can hold 8.5 GB.
• A single-layer Blu-ray disk can store 25 GB of data, and a quad-layer Blu-ray disk can store up to 128 GB of data.

All three formats are available as disks that are 120 millimeters (4.7 inches) in diameter and 1.2 mm (0.05 inches) thick. The consistent size makes it possible for Blu-ray drives to support DVDs and CDs, and for DVD drives to support CDs. The compatibility works in only one direction, however. CD drives can't run DVDs or Blu-rays, and DVD drives can't run Blu-rays.

CDs, DVDs and Blu-ray disks can be obtained for read only, write once only and rewritable applications. Examples of available types include the following:

• CD-ROM (read only).
• CD-R (write once only).
• CD-RW (read/write and reuse).
• DVD-ROM (read only).
• DVD-R (write once only).
• DVD-RW (read/write and reuse).

Similarly, Blu-ray disks can be BD-ROM (read only), BD-R (write once only) and BD-RE (read/write and reuse).

Use cases for optical disk storage

Optical disks are used widely to store movies and TV series, music, and video games. They are also used for long-term archival storage and data backups, plus business and personal software apps, operating systems, and software updates and patches.

Academic institutions use optical disks for distributing educational content. The disks also convey business training materials, though direct downloads to end-user systems have superseded the optical disk distribution method more recently.

Optical disks remain popular for use in home entertainment systems and automobiles.

Pros and cons of optical disks

Convenience, durability and ease of use are primary advantages of optical disks. Here are some additional pros:

• They're cost-effective compared with other storage technologies.
• They're easily portable.
• They support read-only content and long-term archiving.
• They are compatible across different devices -- e.g., CD, DVD and Blu-ray drives.

Here are the optical disk cons:

• They have less capacity than current technologies like SSD and HDD.
• While generally durable, they can be scratched and broken.
• Read/write speeds are slower than SSDs and HDDs.
• They're not highly reusable; some are write once only.
• The use of plastic in disks introduces environmental concerns.

Advancements in technology such as those noted earlier suggest that current optical technologies might become obsolete in the next decade.

Optical disk manufacturers

Several companies currently manufacture optical disks. Here is a list:

• Bison Disc.
• CMC Magnetics.
• Falcon Technologies International.
• Fujifilm.
• Maxell.
• Resonac.
• Ritek USA.
• Singulus Technologies.
• Sony DADC.
• Verbatim.
• Vinpower Digital.

Optical disk development and history

The first optical disk, developed in the late 1960s by James T. Russell, stored data as light and dark micron-wide dots. Russell's optical storage system used a powerful backlight to read the dots through a transparent sheet of material on which the dots were encoded. His creation bears little resemblance to later CDs or DVDs. Russell used transparent foil as the medium, and then read the data by shining a light through it. Modern optical disks use a laser to read the light reflected back from the recording medium. In addition, Russell's system didn't spin as the data was read, so it could be any shape.

The modern CD and DVD are based on technology developed in 1969 by physicist Peter Kramer at Philips Research in the Netherlands. Kramer developed the method of encoding data on a reflective metallic foil that could be read using a small, low-powered red laser. The laser assembly read the dots and converted the data to an electrical signal, which was then converted to audio or visual output. His work went on to become the basis of all digital optical storage media, although initially, it was used only for analog video on the first LaserDisc.

In the 1970s, Philips teamed up with Sony in a joint consortium focused on optical storage. In 1979, they developed the first audio CD, which marked the beginning of digital optical storage for commercial use. However, the technology didn't get traction until Philips and Sony marketed the first commercial CD player in 1982. Since then, there has been a constant succession of optical disk formats, first in CD formats and then several DVD formats.

Five years after releasing the CD, Sony partnered with Denon to produce the first CD-ROM for storing all types of digital data, not just audio. The CD-ROM could hold 680 MB of data, which later increased to 700 MB. About 10 years later, Sony again teamed up with Philips, as well as Toshiba and Panasonic, to create the DVD, which increased data capacity to 4.7 GB.

It took another decade before the next generation of optical storage, the Blu-ray disk, surfaced. Instead of a red laser, Blu-ray technology uses a blue laser, which greatly increases capacities and data transfer rates. A consortium led by Sony developed the Blu-ray, which boasted storage of up to 25 GB. Toshiba did not participate this time, as it had developed and tried to market its own format, the HD-DVD. After a short format war, the Blu-ray emerged as the industry standard.

How optical storage devices are made

Optical disks are inexpensive to manufacture. All modern formats use the same basic sandwich-of-materials structure. A hard plastic substrate forms the base, and then a reflective layer -- typically aluminum foil for mass-produced disks -- is used to encode the digital data. Next, a layer of clear polycarbonate protects the foil and lets the laser beam pass through to the reflective layer.

Manufacturers can create prerecorded audio and video optical disks in bulk. They can also create software and computer game distribution disks in bulk, although internet streaming has reduced the need for such disks.

When producing prerecorded disks in bulk, manufacturers first build a glass master and use it to create a negative disk image made from nickel. They use this nickel image to stamp the digital pits into the reflective foil. This enables mass production at a scale not possible by encoding optical disks individually with a laser, as happens when a disk is written, or burned, using a computer.

Optical disks intended for digital data storage include different materials for the reflective layer, depending on whether the disk is write once or rewritable. A write-once optical disk includes an organic dye layer between the unwritten reflective foil and polycarbonate. Rewritable optical disks swap the aluminum foil for an alloy that is a phase-change material so that it can be erased and rewritten multiple times.

The next generation of optical disk technology

Folio Photonics is now developing a next-generation optical disk with expectations of a product being commercially available by 2026, according to the company's website. The technology, called Active, increases the number of optical layers to 16, with more expected, compared with the three on today's optical disks.

Folio plans to deliver a 1 terabyte disk for approximately $5 that can be packaged in a 10-disk cartridge holding up to 10 TB. Accessing the increased capacity requires a new optical drive. This cost per terabyte compares favorably with other existing high-capacity storage options like magnetic tape or HDD and SSD using flash memory.

Another advanced optical storage system on the horizon uses AIE-DDPR, or aggregation-induced emission dye-doped photoresist technology. It uses a light-sensitive material based on nanoscale technology that reacts to different wavelengths of light. The technology requires multiple lasers and can be built into disks with up to 100 layers of storage per disk. Storage capacity is targeted at 125 TB per disk initially; the technology can support capacities beyond that.

Both technologies promise a bright future for optical disk technology, providing vast improvements in archival storage and enterprise-grade storage.