1. Field of Invention
This invention relates generally to optical systems for the storage and retrieval of information and, more particularly, to the read/write head of the magneto-optical information storage system which directs radiation to the storage medium and then directs radiation resulting from the interaction with the medium to the radiation detectors.
2. Description of the Related Art
The optical storage systems, at present can generally be placed into one of two categories, the categories determined by the optical property used to identify different logical states on the storage medium. The first optical storage system can be referred to a differential absorption (or reflection) of a radiation beam impinging on the storage medium surface. In the differential absorption optical systems, each logical states are associated with changes in the intensity of a beam of radiation interacting with the storage medium. In the second category of optical storage systems, changes in the rotation of plane polarized beam of radiation are used to identify optical states. The present invention is directed to the second or the magneto-optical storage and retrieval systems and provides a technique for determining selected parameters in the optical path to enhance the identification of the two orientations of magnetic regions, orientations which encode the data stored on the disk.
Referring to FIG. 1, the implementation of the read/write head in a magneto-optical information storage system, the system relying on differential rotation of the planar polarization of a optical radiation caused by the interaction of the optical radiation with the storage surface, is shown. This type of storage system relies on the Kerr effect wherein the rotation of a plane of polarization is different when a magnetic material has a magnetic orientation parallel to or a magnetic orientation anti-parallel to the direction of the radiation interacting with the magnetic material, i.e., the differential change in polarization of a reflected beam depends upon the orientation of the magnetization of the local domain with which the radiation interacts. As with the implementation for detecting a change in reflected light amplitude, the radiation from a light source 10 is collimated by lens 11 and one plane of polarization is selected by passing the collimated beam through the partial beam splitter 12'. Because linearly polarized radiation can be considered to be comprised of two circularly polarized radiation components, the interaction with the magnetic layer forming a portion of storage medium 15 effects the two circularly polarized components differently. As a result, after interaction with the storage material, the reflected radiation is not linearly polarized parallel to the applied radiation, but an elliptical polarization of the reflected radiation results in a rotation of the reflected linear polarization due to the circular dichroism and the circular birefringence of the storage media. The reflected radiation is recollimated by objective lens 14. The recollimated beam is applied to beam splitter 12 and the components of the radiation beam orthogonal to the plane of polarization of the radiation impinging on storage medium, i.e., the components induced by the interaction, are reflected by the beam splitter 12. Some of the light with polarization parallel to the impinging radiation can also be reflected from the magneto-optic region. The radiation reflected by the beam splitter 12 is transmitted through a quarter wave plate 16A and a half wave plate 16B to correct for ellipticity introduced into the radiation beam. The polarization beam splitter 17 divides the radiation reflected from beam splitter 12 into radiation components which have been rotated by the interaction with the storage material. Each detector 18 and 19 receives a component resulting from one orientation of the magnetic regions of the storage medium interacting with the impinging radiation beam. The differential amplifier 20 is used to enhance the detectability of the small signals, the rotation due to the Kerr effect typically being less than 2.degree. relative to reflected radiation which had not been subjected to differential interaction of the circularly polarized components with the optical storage material and to cancel the large DC component of the two radiation components.
In the optical storage systems using a magneto-optical storage medium, a need has been felt for a technique of determining how to optimize the parameters of the system in order to achieve the most detectable signal. In the article by W. A. Challener and T. A. Rinehart, "Jones Matrix Analysis of Magnetooptical Media and Read-Back Systems", Appl. Opt. 26, 3974 (1987), part of the problem of a differential detection system was addressed. In that article, the substrate birefringence and the wave plate tolerances were studied. However, the DC offset in the differential signal was not considered and a range of "ideal" wave plates was found, each with a sensitivity to the optical path birefringence. Therefore, the need has remained for generally applicable technique for identifying the parameters which would permit optimization of the detection of the state of the region of the storage system to which radiation was being applied.