Objectives

• The student will identify the typical physical components of optical media.
• The student will describe the size and composition of data in optical formats.
• The student will describe the relationship of iridescence to nanostructures.
• Given a track format and medium, the student will estimate the data capacity of the medium.

Examples

Examples of optical media include:

• Video Laser Disks
• CD-ROM (Compact Disk - Read Only Memory)
• DVD (Digital Video Disk)

Optical media use variations in surface reflectance to encode the 0s and 1s of binary data.

The first popular use of optical storage began in the 1970’s with release of the Pioneer® video laser disk. The disks were about 30 cm in diameter. Despite slow public acceptance of the videodisk, soon the 13 cm audio CD-ROM was released. The audio CD was accepted almost immediately and soon replaced the LP record as the most popular media for distribution of music. The use of CD-ROM for data storage followed soon after. Data CD-ROMs can store up to 650 megabytes of data or 75 minutes of audio information.

Optical media systems read the differences in laser light reflected from a series of holes or pits to represent the 0s and 1s of binary data. The use of phase shifts to detect minor changes in material surface is based on a technique developed in 1934 by Nobel Prize winner F. Zernike.

CD-ROM

CD-ROM store data in patterns of small pits arranged in over 22,000 tracks accross the surface of the disk.

CD-ROM drives use a laser from below the disk to read the data from the disk. The presence of a passing pit, or space, effects the reflection of the laser from the surface of the CD-ROM. The changes are captured by the lens, converted into electrical signals and translated into data. The data can represent audio, video or computer readable files.

Due to the small size of the pits representing the data (0.5 – 1 m m ), any scratches or other material on the bottom surface of the CD-ROM can interfere with the laser and cause problems reading data from the disk. Occasionally, cleaning or polishing the bottom surface can make it possible to read a skipping CD-ROM.

SPM Image of Stamper

Iridescence

The small size of the data is responsible for another identifying characteristic of the CD-ROM. The pits are close in size to the wavelengths of visible light. The interference of the light with the pits diffracts the light, creating the familiar "rainbow" pattern on the surface of the CD-ROM.

The pits are approximately 1/4 the wavelength, or 0.5  mm high.

Pattern w/Link to diffraction module

Movement

Like their mechanical and magnetic counterparts, optical media require movement to retrieve data.

For example, the CD-ROM spins at up to 1.2 meters per second while a laser follows the data tracks and a sensor reacts to the light reflected from the pattern of pits and spaces that represent the data.

Production of CD

Most commercial CD-ROMs are produced using a press, or mold, to make multiple copies from a single master. The recording process involves creating a master optical disk, usually using optical glass as the substrate, or backing. The glass is coated with a thin photographic coating, 0.1 – 0.2 m m thick. An argon laser   with a wavelength of 400-500 Nm is used to expose a spiral track onto the photographic coating with the data represented as a series of pits in untouched media. The disk is then developed to produce a photographic master disk with microscopic patterns of data on the surface of the disk    

SPM image of CD-ROM stamper

The master is electroplated with a 0.11-0.13  mm layer of nickel then used to produce a master "Father" disk. The positive "Father" disk is used to make negative "Mother" disks that are then used to create many production "stampers".

The "stampers" are used to press the data pattern onto a blank disk composed of polycarbonate or other plastic. The pressed disks are then coated with a 0.11 – 0.13 m m layer of aluminum to make the surface reflective. The CD "sandwich" is finished with a clear, 30 m m thick top layer of lacquer and printed label.

Though the acrylic material is relatively soft, the CD-ROM stampers still wear during production. The critical tolerances and minute scale of the pits and holes, 0.1 - 0.2 m m, make the SPM an ideal tool to measure wear to the extrusions on the CD-ROM production stampers.

Recordable

Two variations in recordable optical media are common today. Write-Once-Read-Many times, or WORM drives, use a medium that can change state only once during the recording process. A laser is used to melt small holes representing the data into the surface of the medium. The holes do not reflect the laser light while the backing material does, permitting a sensor to "read" the pattern from the reflected light as the CD spins past the head mechanism. Recordable optical media can change state more than once.

It is important to note that recordable optical media are potentially less stable and potentially have a shorter life span than pressed CDs. Recordable CDs are sensitive to exposure to UV light sources such as fluorescent lights or sunlight. As the medium deteriorates, it can loose information in several ways. The backing can oxidize and become less reflective, eventually not reflecting enough light to be correctly read by the sensor. The edges of the pit may also deteriorate and enlarge, eventually merging with adjacent pits. Both of these problems can reach a point where the data can no longer be read accurately and the recorded information will be lost.

Magn-OPT

An interesting variation in data storage combines magnetic media with optical recording and reading. Magneto-optical media take advantage of a minute shift in the rotation of light, related to the polarity of the bit, when it is reflected from a magnetic field, known as the Kerr effect. The underlying principle of the magneto-optic drive is the ability of the sensors in the drive to detect the shift in laser light reflected from magnetic bits on the surface of the disk.

Magneto-optical, or phase change, drives use a laser to liquefy a small area in the metal coating of the media. A magnetic field then realigns the coating to reflect light differently from the backing material. To decode the data, the laser "reads" the pattern of reflections to reproduce the data that has been encoded on the disk.

Reading-OPT

Reading optical data requires a relatively complex collection of lasers, lenses, prisms, and electronics that are significantly more massive than the magnetic pick-up head. The storage capacity of optical media, such as CD-ROM, is determined by several factors including:

• mechanical accuracy of the laser and tracking mechanisms
• wavelength of the laser light used to read the data

The mechanical accuracy directly effects the data track width. As the accuracy of the laser and optics increases, data tracks can theoretically be placed closer together, increasing the storage capacity of the media.

Reading Laser

The most common laser used to read data in optical media has a wavelength of X, in the red range of the visible spectrum. However, a shorter wavelength, such as a blue light, can react to smaller data pits than red laser light. Recently, a Japanese company developed blue lasers with a wave length of X. The increased responsiveness of the shorter wavelength and the ability to use smaller data pits are technological tools which may result in increased storage capacity for optical media in the future.

DVD

The recently introduced optical storage medium, Digital Video Disc or DVD, is designed to store audio, computer data, and video. A DVD can store up to two hours of uncompressed digital video, or eight hours of compressed digital video with 32 audio tracks. Dual layer discs are also available offering twice the capacity of the single discs.

DVDs are made of two platters similar to two CD-ROMS that are glued together. Each side can have dual data layers. Each layer can store about 4.5 gigabytes of data, equal to about seven conventional CD-ROMs. A double-sided, double-layer DVD can store almost 16 gigabytes, the capacity of about 28 CD-ROMs.

Like the CD-ROM, the DVD uses small 0.5 – 1 m m-sized   pits to represent the data. Any scratches or other material on the bottom surface can interfere with the laser and cause problems reading data from the disk.

The techniques DVD drives use to increase data capacity include:

• Use of a smaller wavelength laser
• Smaller pit size
• Closer track spacing
• Larger data area
• More efficient error correction
• More efficient compression techniques