The invention relates to a record-carrier body in which a user can record information by means of optical radiation, which record-carrier body comprises a substrate and a recording layer on said substrate and has been provided with a preformed optically detectable relief structure of servo-track portions and sector addresses in which address information about associated recordable portions of the record-carrier body is contained in the form of optically detectable areas which alternate with intermediate areas, which areas in the sector addresses have another phase depth than the servo track portions. The invention also relates to an apparatus for recording a structure similar to the relief structure of sector addresses and servo-track portions of said record carrier body.
The record-carrier body may comprise a circular disc-shaped substrate carrying a recording layer in which an optically detectable change can be produced by means of a radiation beam of sufficiently high intensity. The servo track portions may comprise grooves recessed in the substrate surface or ridges on the substrate surface and the sector-address areas may comprise pits formed in the substrate surface or hills formed on this surface. The sector addresses are situated between successive servo-track portions, viewed in the track direction, thus forming one composite track. This track extends over the entire surface area of the recording layer and is preferably a spiral track, but alternatively it may comprise a multitude of quasi-concentric tracks.
When the sector addresses and servo-track portions are exposed to a radiation beam which has been focused to form a small radiation spot, this beam is split into a zero-order subbeam, first-order subbeams and higher-order subbeams. Herein, phase depth is to be understood to mean the phase difference between the zero-order subbeam and a first-order subbeam. This phase depth is determined by the geometries of the sector-address areas and the servo-track portions, inter alia by the depth or the height of these areas and track portions.
Such a record-carrier body is disclosed inter alia in U.S. Pat. No. 4,363,116. As described in this Patent Specification, the servo-track portions are used for detecting and correcting the radial position of a radiation spot formed on the recording layer by a radiation beam when information is being recorded by the user. This enables the requirements imposed on the drive and guide mechanisms by means of which the write beam and the record-carrier body are moved relative to each other to be less stringent, so that the write apparatus can be simpler and cheaper.
Preferably, the radial position of the radiation spot relative to a servo-track portion is detected by means of the "push-pull" or differential method. For this purpose use is made of two radiation-sensitive detectors which are arranged in the path of the radiation beam originating from the record-carrier body and which receive radially different portions of this beam. The difference between the output signals of the two detectors contains information about the radial position of the radiation spot relative to the servo-track portion. If these output signals are identical, the center of the radiation spot will coincide with the central axis of the servo-track portion. The differential tracking method may be used only if the servo grooves or servo ridges have such a depth or height that their phase depth is of the order of 90.degree..
The sector-address areas are read using the "central-aperture" or integral method. For this purpose the variation of the total intensity of the radiation issuing from the record-carrier body and traversing an objective system is detected by means of a single detector arranged on the optical axis or by means of the two detectors employed for tracking, whose output signals are then added to each other. For an optimum read-out of the sector addresses the areas therein should have a phase depth of the order of 180.degree..
It has been found that, in addition to the phase depth, the width, measured in a direction transverse to the track direction of the servo-track portions and the sector address areas, has a substantial influence on the amplitudes of the signals obtained during the recording of the user information. If the record-carrier body in accordance with U.S. Pat. No. 4,363,116, which is intended to be scanned by means of a radiation spot whose half-intensity value is approximately 800 nm, the servo-track portions and the sector-address areas have a width of approximately 600 nm, while the period, transverse to the track direction, of the track struture is approximately 1600 nm. If the radiation spot has a Gaussian intensity distribution the half-intensity value is equal to the distance between two points where the intensity is equal to 1/e.sup.2 of the intensity in the center of the radiation spot. By means of the known record carrier body it is possible to obtain a differential track signal whose signal amplitude is moderate.