Abstract: A mask is used to remove substantially all of the first order diffraction components of a reflected light beam which are generated as the beam crosses grooved data tracks in an optical media. The mask may comprise a transparent substrate with an opaque layer having a pair of circular apertures or a bow-tie shaped aperture. The apertures are located outside the regions of interference between the diffracted components and the undiffracted component. The layer may be made of a reflective material for reflecting diffracted components of the reflected beam to a tracking error sensor. The layer may be made of a holographic material for deflecting the diffracted components to a tracking error sensor.

Abstract: An apparatus is described for recording/reproducing to/from an optical record carrier by means of a laser light beam. The record carrier has pre-recorded sarvo marks aligned in a track. In order to get a focus error signal, the apparatus measures phase difference between signals from two photo sensor elements which receive the laser light beam through the record carrier. A circuit for measuring the phase difference has a first and a second phase shift circuits, RS flip-flops and a focus error converter which has a shift register and a low-pass filter. The signals from the photo sensor elements are fed to the first and second phase shift circuits, respectively. The second phase shift circuit is composed of a plurality of delay elements, output of which are fed to the S inputs of the RS flip-flops. Data at the S inputs are latched in response to an output signal of the first phase shift circuit which is fed to R inputs of the RS flip-flops.