Abstract: A bidirectional optical wavelength multiplexing-demultiplexing device comprises a self-focusing lens having an index gradient and situated between the transmission fibre and filtering means and of which the length is such that the spatial separation between the outgoing wavelengths and the return wavelength is maximum, which permits decoupling of the return channel from the outgoing channels.

Abstract: The invention relates to support systems for flexible information-carrying discs. More particularly, the invention relates to an air bearing support plate (1) for a flexible disc (3) of which the sliding surface (2) is free in a preponderant sector from any profile irregularity (6) capable of affecting the flatness of the annular band of the disc (3) containing the information. The invention is applicable in particular to the optical reading by reflection or transmission of information carried by a flexible disc.

Abstract: The invention relates to the exact focussing of a light beam on a track carried by an optically readable information support. The object of the invention is to provide in an optical reading device a dichotomic focussing error detection device of which the two states correspond to the absence and to the presence of the reading signal. In the absence of the reading signal, the lens of the optical reading device is moved at a uniform speed between two limit positions by means of a triangular control signal, the position control loop of the lens being open. In the presence of the reading signal, the lens is no longer controlled by the triangular control signal, but instead by an error signal emanating from the position control loop which is then closed.

Abstract: The invention relates to the focusing of an optical system by variation of the radius of curvature of a reflecting element, comprising two prepolarized thin plates of piezoelectric material which are joined together and each of which carries an electrode, the outer surface of one of the two thin plates being made reflective. An electrical signal characteristic of the correction to be made is applied between the two electrodes and produces a variation in the radius of curvature of the thin plates. This reflecting element is used in an optical system of the Cassegrin type or as a reflecting mirror.

Abstract: The invention relates to methods for controlling the starting and stopping of the reading of recording supports read in appropriate readers. The method according to the invention comprises providing a zone free from information at least at the beginning of the track of such a support. The reading head of the reader advances rapidly to that zone and, after having detected the presence thereof, continues its advance at a normal speed.