High density optical recording systems which may be used for recording and playing back information are well known in the art. Current practice in the optical recording field can be broadly distinguished into three categories, the classification being largely dependent on the physical and chemical characteristics of the information carrying media. Common to all three categories is the ability of each system to have the information stored on the media to at least be "read" or "played back" by irradiating selected areas of the media with a highly focused source of light, usually a laser. Changes in the transmission or reflection of the laser beam caused by the presence or absence of recorded information on the media is detected by suitable optical sensors. The three categories are differentiated by the media's response to radiation. "Read Only" as, for example, in Compact Disk technology, wherein all the information on the disk is pre-recorded. "Write-Once, Read (Mainly)" (WORM), in which information can be written on the media by the laser, read back by a laser at reduced power levels, but cannot be erased for correction or re-use. The third consists of media which can be written upon and read back by laser, but can also be erased (by the laser), in the presence of a suitable magnetic field, and which then can be reused.
This invention, relates generally to the "erasable" optical recording systems, known in the art as magneto-optical recording systems, and in which the media is described as magneto-optic media.
A magneto-optic recording medium is a magnetic material which causes the polarization angle of laser light to be changed when it is reflected from or transmitted through a recorded spot. Preparatory to the initial recording operation, the magneto-optic media is vertically magnetized in the same direction across the entire recording surface. During recording, the direction of magnetization of the media is reversed at the point of incidence of the laser beam due to the presence of a vertical magnetic field of a given strength and polarity and heating induced by a laser. Illumination from a playback laser beam of lower power reflected from this area will show a polarization angle change, which is detectable by suitable optical sensors. The erasing process restores the original magnetic direction which existed prior to recording by the use of a vertical magnetic field of polarity opposite to that used for recording when accompanied by heating from the laser beam. Traditionally, an electromagnet in which the current can be adjusted and reversed, (thereby changing the direction and/or intensity of the induced magnetic field) has been the preferred source for generating the magnetic bias for both the writing and the erasing. The electromagnet (in some cases, a reversible permanent magnet) is also sometimes referred to as the bias magnet.
Deguchi et al, in U.S. Pat. No. 4,497,006, schematically describe in their disclosure an electromagnet surrounding the objective lens of their recording/playback apparatus which provides the bias magnetization required for magneto-optic operations. Ota et al, in U.S. Pat. No. 4,477,852, discloses a pair of bias electromagnets adjacent the objective lens in conjunction with a pair of lasers for performing the erase and re-write functions in a magneto-optic recording and reproducing system. U.S. Pat. No. 4,466,004 also describes an electromagnet surrounding the objective lens, and records information by varying the field strength of the electromagnet in the areas of the media heated by a continuous laser beam. In the previous two examples, writing was accomplished by varying the intensity of the laser beam.
U.S. Pat. No. 4,610,009, assigned to the Xerox Corporation, describes a system wherein plural lasers operate through a single optical path and cooperate with bi-polar magnetic field producing means located on the opposite side of the magneto-optic medium. This system also includes a bias electromagnet essentially concentric with the focusing electromagnet of the objective lens. The deficiencies of these examples in providing an essentially vertical magnetic field coincident with the laser beam is addressed in U.S. Pat. No. 4,340,914 issued to Hanaoka and assigned to Olympus Optical. This patent discloses an auxiliary magnetic pole piece for focusing a magnetic flux from the magnetic field generating device (bias magnet) into at least the heated portion and which is located on the side of the media opposite the laser beam.
The teachings of U.S. Pat. No. 4,701,894, assigned to RCA, further address the problem of providing a vertical magnetic field at the point of incidence (with the laser beam) which rely on an electromagnet located around the objective lens, or, in some cases, on the side of the media opposite the laser beam. As discussed in this patent, given the very close proximity between the objective lens of the record/playback optics, there is very little space on the recording side of the media for an electromagnet capable of providing a vertical magnetic field at the point of incidence. This patent discloses a bias electromagnet configured in the shape of a letter "E", in which the top and bottom legs of the "E" extend radially from one edge of the media towards the center (when the media is in a disk configuration, the center leg of the "E" likewise extends from one edge towards the center, but on the opposite side of the media from the top and bottom legs). Current carrying coils are wound about each of the two segments of the vertical portion of the "E" to magnetize the electromagnet. Attached to the housing of the objective lens is a ring or cap of magnetic material which has a small hole in it to allow passage of the laser beam. Magnetic coupling of this cap to the top and bottom legs of the "E" is accomplished by a pair of pole pieces, in effect, filling in the gap between the legs and the cap. The pole pieces, the cap, and the objective lens can move radially in-between the top and bottom of the "E", with the return path of the magnetic flux being provided by the center leg which is located underneath the lens and the media.
When the magneto-optic media is provided on a thin tape or on a thin disk, as described in this patent, the magnetic elements can conveniently be placed on the side of the media opposite the side upon which the laser impinges. However, when this opposite side location is not physically accessible, for example, when the magneto-optic media is supported on the surface of a drum, or is supported on a very thick substrate, these magnetic elements must be located on the same side of the media as the optics are.
It is an object of this invention to provide a device and method of producing the necessary vertical bias magnetic field, essentially in concurrence with the point of incidence of the laser beam upon the media, in which access to the opposite side of the media is denied. It is a further object of this invention to provide the necessary vertical bias magnetic field for systems in which extensive cross-track movement of the optic system is required. It is an additional object of this invention to provide the necessary vertical magnetic field with a reduced number of non-vertical components of the bias magnetic field, and with greatly reduced power requirements for providing the bias magnetic field.