Abstract: According to the present invention, recording laser beam is shot from a semiconductor laser to form a mark set which comprises a pair of marks on the track of an optical recording medium in the track width direction thereof and first information which has been coded corresponding to the interval between the paired marks in the mark set is recorded on the track of the optical recording medium. At the same time, the plural mark sets are formed on the track of the optical recording medium in the track length direction thereof and second information which has been coded corresponding to the interval between these mark sets is recorded on the track of the optical recording medium. The track of the optical recording medium is scanned in the track width direction thereof with reproducing laser beam shot from a semiconductor laser and .+-. first-order diffraction beam of an optical interference pattern which is created from the paired marks in the mark set thus scanned is detected to reproduce the first information.

Abstract: An erasable optical information recording medium has a transparent substrate, and a recording layer supported by the substrate. The recording layer includes a thin film of at least one polydiacetylene having side chains. The polydiacetylene is capable of undergoing a reversible planar-nonplanar transition of the main chain structure of the polydiacetylene due to a conformational change of the side chains produced by irradiation of light. When the recording layer is irradiated with light, the reversible planar-nonplanar transition occurs at the irradiated portions to cause a corresponding change of optical properties of the recording layer, thereby recording information thereinto and erasing information therefrom.

Abstract: An optical recording medium such as an optical disk, which is capable of reversibly repeating a recording/erasing cycle. This optical recording medium comprises a transparent substrate and recording layer formed on the substrate. This recording layer comprise a liquid crystalline high polymer having side chains and a dyestuff having a recording-light absorbing property. The side chain may preferably comprises mesogen group, a monocyclic or polycyclic compound such as phenyl, benzyl, naphthyl or anthryl group, and alkyl or aryl group having 1 to 18 carbon atoms.