Optical disk

  1. Measurement principle
    Optical disks are widely used as a storage medium in multimedia. The existence of birefringence in a disk substrate reduces the reading accuracy of the signals written on it. Therefore, precise measurement of disk birefringence has become an essential technique to improve the reliability of optical recording. The birefringence of a disk is generally determined by the orientation direction of the polymer in the substrate and the internal stress generated during molding. The birefringence is measured not only when the light is incident perpendicular to the disk, but also when the light is incident at an angle. The birefringence of a disk is controlled by the birefringence measurements taken at perpendicular and oblique incidence, and the refractive index ellipsoid calculated from these measurements. On the other hand, optical disks are required to have a large capacity and high speed. For this reason, a method of rotating the disk at high speed has been devised. However, when a disk is rotated at high speed, stress is expected to be generated inside the disk due to the rotation. This stress may cause birefringence, which may degrade the accuracy of signal detection. Uniopt offers two types of birefringence measuring instruments: the DRES-10A, which measures the birefringence of discs with high accuracy, and the DRES-30, an ultra high-speed type that can measure birefringence caused by high-speed rotation. The interferometer parts are [ABR-10A] and [ABR-30], respectively. Both models can measure birefringence by grazing incidence light and obtain the refractive index ellipsoid.

  2. Measurement example
    • CD-R substrate measurement example
      cdr

      This is an example of measurement for two disks (120 mm diameter, 1.1 mm thickness) under different conditions. The length of the red line segment represents the magnitude of birefringence, and the direction of the line segment represents the orientation of the birefringence axis. The measurement point is the center of the line segment.

    • DVD substrate measurement example
    dvd

    This is an example of measurement for three DVD disc substrates of 0.6 mm thickness. The length of the red line segment represents the magnitude of birefringence, and the direction of the line segment represents the orientation of the birefringence axis. The measurement point is the center of the line segment.

    • MO disk substrate Measurement example at high speed rotation
mo

This is an example of MO disk measurement. It can be seen that the birefringence distribution changes between the stationary state (0 rpm) and the rotating state (3000 rpm). The length of the red line segment indicates the magnitude of birefringence, and the direction of the line segment indicates the orientation of the birefringence axis. The measurement point is the center of the line segment.