The writing process in magneto-optical recording

Abstract

An optical disc is a multilayered medium consisting of a thick glass substrate on which many layers of different materials are sputtered. Only one of these layers serves as a recording medium and it is initially magnetized uniformly in one direction. To write on this disc, the initial magnetic field is altered by focusing light from a laser source with high numerical aperture (NA) lens to heat the magnetic medium beyond its Curie point, the temperature at which the magnetic medium loses its magnetization. This domain with zero magnetization is subsequently magnetized in the reversed direction by using an induction magnet. All these processes take place while the disc is rotating at a very high speed with respect to the laser. It is well known that partial obscuration reduces the spot size and the depth of focus; and in the present case of optical recording both are highly desirable. Significant improvement in data densities and quality of the written spot is achieved by writing with an obscured pupil as shown here. In this paper, we examine optical imaging and transient heat transfer problems to study the writing process. The vector diffraction theory is employed to calculate energy distribution on the disc surface and then this energy distribution is used as a source for the heat transfer problem. Writing with lenses of various NA is examined and various optical effects such as fill factors and obscuration, mechanical effects such as the thermal properties of the disc material, and writing conditions are considered. Results pertinent to the design of the disc medium, the optical elements, the mechanical components, and the development of the write algorithm are discussed. The quality of the written spots and the resulting data densities during a typical write sequence is discussed.