The tracking and focus two-axis device
The objective lens is contained inside two separate coils: one for focus and the other for tracking control, placed at right angles to each other. The coils are placed inside a magnetic field produced by a permanent magnet as shown in Figure 20.17. This assembly, known as a ,u2-axis actuator will move the lens in the horizontal and vertical directions. Current through the focus coils will move the lens vertically to achieve focus and current through the track coil will move the lens horizontally (lateral movement) for tracking purposes.
Compatible pickup
The fact that DVD-video players could also play audio CDs has been and remains a major selling point. This is also true for the new generation blue-laser-based playback machines. HD DVD playback systems must now
be able to read not only HD DVD with a blue laser, it must also be able to read a DVD and a CD. Similarly for the BD playback systems, namely BD/DVD/ CD. Therefore, an HD DVD/DVD/CD or BD/DVD/CD compatible pickup requires three different wavelength beam sources and three different NAs to meet the requirements of each disc standard. The obvious solution is to have three switchable pickup units, one for each application. The size of such a pickup system becomes large and in most cases impractical. Other tech- niques include a multi-beam laser unit in which three LD chips of blue, red and infra-red are mounted in one package. Each one of the three wavelengths can be emitted by applying a different voltage to the appropriate LD chip.