Theory-43 :- MOD Drive ( Mechanism of Magneto‐optical(MO) disk)

1. Data READ/WRITE Principle

WRITE process:

Magneto‐optical (MO) disk drive reads and writes data on the MO disk in principle of "Magneto‐optical effect”. It employs laser to read data on the disk, while it needs magnetic field to write data in addition to laser.

MO disk drive is so designed that an inserted disk will be exposed to a magnet on the label side and to the light (laser beam) on the opposite side.

This MO film has a lot of expressly micro magnetic domains vertically aligned on the disk surface.

The magnetic domain, though small‐sized, has a powerful magnetism enough not to be affected by magnetic field available in general environments.

On the other hand, the magnetic domain has a characteristic that its magnetic force will decrease according to the temperature increase. Once the temperature rises to a certain level, the magnetic domain loses its characteristic as a magnet. This temperature ranges, 300 while it varies with the MO film's composition.

This temperature is called the "Curie temperature."

• Magnet
• Magneto‐optical disk rotation direction
• Laser beam

 

The reason why laser beam is utilized is

1. It has sufficient energy to instantaneously achieve a temperature rise up to Curie temperature.
2. It allows so fine one‐micron diameter spot to be heated by means of an optical lens.

The magnetic domain which reaches the Curie temperature loses its own magnetism and it is easily affected by outer magnetic field. Exposing the heated domain into outer magnetic field generated by magnet allows its polarity to change according to the field. As a consequence, laser beam and magnet can make magnetic domain on MO recording film those size and magnetic polarity are controlled by laser beam and magnet respectively.

 

READ Process:

The laser beam is used to read the magnetic polarity of the domain. In the case, the beam has so small energy that the magnetization of the domain is not diminished. Reflective light coming from the domain is used to read data and it has a characteristic of polarization. The reflective light has a polarization angle (Keer rotation angle) which has been changed delicately according to the direction of magnetization of the domain. This is called the Keer effect. Since the reflective surface of the domain has two types of magnetization, pole S and N, we can detect two types of polarization angle. Moreover, those two types can be assigned to digital data. For instance, pole S is assigned as 1 and pole N is assigned as 0.

 

2. Construction of a MO Disk

Look at a sectional view of MO disk. Thickness of the disk is mostly dominated by poly carbonate. MO disk never distort while laser beam are passing it through. Wherever it passes, the substrate (poly‐carbonate plate) should allow the beam to be transmitted completely. MO disk should be neither distorted by heat nor cracked by the influence of impact. It is poly carbonate resin that meets such requirement. And it is the reason why the resin is applied to MO disk substrate. Onto this poly‐carbonate resin substrate, a few films are formed, thereby composing MO disk. Those films are functioning differently one another.

[Sectional view of Magneto‐optical Disk]

0.3 micron thickness

Protective film (ultra‐violet hardened resin)

To protect recording film(several micron thickness)

Reflective film

To improve read efficiency

2nd dielectric film

To protect magneto‐optical film, to insulate heat, etc.

Magneto‐optical film

Recording film

1st dielectric film

To protect magneto‐optical film, to improve read efficiency, etc.

Poly carbonate resin

Transparent plate (1.2mm thickness)

Protective film (UV‐hardened resin)

To protect Poly carbonate surface (several micron thickness) Incidence of Laser Beam

 

1st dielectric film plays a role of a protective film to protect the MO film while enhancing the rotation of polarization angle so that the sensor can detect more readily (referred to as "Keer effect enhancement"). MO film is the recording film which plays a leading role on MO disk. It is made of alloy including a few kind of metal element, such as Tb (terbium), Fe (iron) and Co(cobalt). It satisfies two requirements; one is the  reluctance to change the direction of a magnetization and the other is to arrange the orientation of the magnetization in a fixed direction.

2nd dielectric film also protects the MO film while playing a role of enhancing Keer rotation angle. In addition, it has a thermal insulation effect.

This insulation property prevents heat from escaping into the reflective film while MO film is heated. Thus, the thermal insulation is utilized for an efficient use of laser beam power.

Reflective film is used to reflect laser beam to read data. Since the film reflects light, MO disk is looked as bright as rainbow colors.

These films are produced on the poly carbonate substrate through the production process called "spattering." To protect these films against possible scratch and corrosion, additional protective films are coated over the films and also coated over the poly carbonate plate opposite side.

3 Re writable Optical Disk/Phase‐change Optical Disk

Using 1.2 micron track pitch and zone constant angular velocity (ZCAV), phase‐change optical disk realizes 650 megabytes recording capacity that is identical with that of CD‐ROM.

In addition, phase‐change optical disk is capable of repeating READ/WRITE so that you can store and make effective use of large‐capacity data, such as images, animation, databases, etc.

Phase‐change Recording System (READ/WRITE with Laser Beam)

The phase‐change optical disk employs a recording system different from that of CD‐R(record able CD) or of MO disk. In case of writing data, intensive laser beam is radiated onto the recording film of the disk, thereby it causes the recording film's phase change (alternation of material's status: crystalline phase and amorphous phase). If the recording film' phase changes, the reflectance of the film changes. And this phase‐change is utilized to write data. To read data, a lower power intensity of laser beam than that in the WRITE process are radiated to detect reflectance of the recording film. Since the recording film can be subjected to a number of phase‐changes, the data can be rewritten for number of times (for more than 500 thousand overwrite cycles).

1. Phase‐change Recording System
2. Anneal
3. Melt & quench
4. Amorphous phase
5. Crystalline phase
6. Disk rotation direction
7. Laser beam
8. Laser power
9. Recording level
10. Erase level
11. Read level

 

Direct Overwrite (simultaneously processing ERASE and WRITE at a time)

The phase‐change optical disk allows for direct OVERWRITE, or WRITE data at the ERASE process. This system, therefore, realizes high speed WRITE process.

 

Highly Durable Recording Film

Special dielectric films on the surface of phase‐change disk are helpful to increase recording efficiency while upgrading the recording film's strength. As a result, the disk achieves an excellent durability and longevity.

1. Sectional View of Phase‐change Optical Disk
2. Protect film
3. Effective film
4. 2nd dielectric film
5. Recording film
6. 1st dielectric film
7. Polycarbonate resin
8. Laser beam