Optical Data Storage Systems

It may sound strange, but the first optical recording technology was invented about 5000 years ago, when the Egyptians started using hieroglyphic and alphabetic writing systems.

Writing allowed storing, retrieval, and duplication of data without errors (except for misspellings and misunderstandings). Today's meaning of optical data storage refers to storage systems that use light for recording and retrieval of information.

Photography was the first example of optical data recording in the modern sense. The first photographs were developed about 200 years ago and represented an analog optical storage, which has a limited application for information storage since it is difficult to interface analog data with machines, plus there is a continuous degradation of data at every step of reading and writing. But, light can be easily used for digital information recording, especially since the invention of the lasers.

Optical recording was for a long time, and is still, considered a future replacement for magnetic recording. Optical recording systems potentially have much greater reliability than magnetic recording systems since there is a much larger distance between the read/write element and the moving media. Therefore, there is no wear associated with repeated use of the optical systems. However, there are other possible souses of trouble: the life and stability of the laser, mechanical damage to the relatively soft and exposed-to-the-environment media, mechanical damage due to shock and vibration, etc. Another advantage of the optical recording systems over the best performing magnetic recording systems - hard drives - is their removability.

The main disadvantages of optical storage when compared to magnetic is slower random data access. This partially comes from the design of the relatively large (and heavy) optical heads. Moving 100 grams over the disks at the high acceleration and speed needed to match the 5 to 10 ms average access time for magnetic hard disk drives is a real challenge, since the effective weight of the moving parts of a hard drive's head-stack assembly (actuator arm with suspensions and sliders) is on the order of only a few grams. Plus, unlike the hard disk, an optical disk is usually removable, which limits rotational velocities and, correspondingly, limits access time. Increasing rpm causes the relatively loosely fixed CD-ROM disk to vibrate significantly compare to a stiff, fixed, and balanced hard disk.

Optical drives of all kinds operate on the same principle of detecting variations in the optical properties of the media surface. CD and DVD drives detect changes in the light intensity, MO drives - changes in the light polarization. All optical storage systems work with reflected light.

A convenient way to define tracks on the optical disks is by using pre-grooves. These are created by etching, stamping, or molding the substrate. The bottoms of the grooves are used as a storage medium, and the grooves are separated by lands. But, the lands could be also used as a storage medium, instead of or together with the grooves. The groove depth is based on the laser wave-length and, typically, equals 1/8 of the wavelength of the laser beam.

Another way to define tracks and provide servo information for the drive's electronics is to use a so-called sampled servo, where the tracks are defined by occasional marks placed on the substrate at regular intervals. The marks define the outer limits of the track and help to position the laser spot on the track.

It is usually desired in optical recording to achieve the smallest laser beam spot possible, since the spot size is a measure of the bit size, which defines the areal density of the storage system. The spot size also affects the resolution of the system (smaller is better). The spot size of the optical system is given by the following expression:

Spot (Beam) diameter ~ wavelength of light / Numerical Aperture

The numerical aperture (exactly speaking, its square) is a measure of the light-gathering power of the system. Current practice is to use lenses with numerical aperture of about 0.5 to 0.6.

The wavelength of the currently used GaAs lasers is in the range of 670-840 nm, which yields a minimum laser spot size of about 1 mm. These lasers are being replaced by those in the blue end of the spectra.

The depth of focus of a lens indicates how much the lens can be moved away from the perfect focus and still maintain an acceptable focus. This value could be defined as follows:

Depth of focus ~ wavelength of light / (Numerical Aperture)²

For the current optical storage systems, the depth of focus is about 1 mm.