Optical scanning apparatus for a multi-layer record carrier, including a focus control circuit

Claim 1

A focus control system for an optical scanning apparatus for scanning an optical record carrier having a plurality X of stacked information layers, where X≧2, the optical system comprising: an optical system which focuses a radiation beam to a focal point; a focus actuator which controls the optical system to selectively shift the focal point in a direction perpendicular to the information layers; a focus error detector which derives a focus error signal indicative of an amount of deviation of the focal point from a prescribed in-focus position relative to a first one of the information layers currently being scanned; a focus servo control circuit having first and second modes of operation, wherein: a) in the first mode of operation, the focus servo control circuit generates a first focus actuator drive signal in response to the focus error signal and supplies the first focus actuator drive signal to the focus actuator, to thereby shift the focal point in a direction and by a distance calculated to reduce the deviation of the focal point from the prescribed in-focus position relative to the first one of the information layers currently being scanned; and, b) in the second loop mode of operation, the focus servo control circuit generates a second focus actuator drive signal and supplies the second focus actuator drive signal to the focus actuator in order to shift the focal point from the first one of the information layers currently being scanned to a second one of the information layers; a first loop control circuit which generates a binary loop control signal for selectively changing the mode of operation of the focus servo control circuit, in response to a layer jump command signal, from the first mode of operation to the second mode of operation, and for selectively changing the mode of operation of the focus servo control circuit from the second mode of operation back to the first mode of operation in response to a loop close signal; a second loop control circuit for generating the loop close signal in response to a first prescribed characteristic of the focus error signal which is indicative of the focal point reaching the second one of the information layers; and, wherein the second loop control circuit includes a loop close inhibit circuit which inhibits the generation of the loop close signal in response to a second prescribed characteristic of the focus error signal which is indicative of the focal point being located between the first and the second information layers.