Principle of operation

The light emitted by the diodelaser (LD) passes a diffraction grating and subsequently the polarising beam splitter (BC). The orientation of the diodelaser axis is chosen in such a way that almost the entire light passes through the beam splitter. After leaving the lens (L1), the beam is collimated and is focused down to app. 0.6 µm by the lens (L2) which has a short focal length of 3-4 mm. The focus of the laser beam hits the underside of the CD-ROM where it is reflected back and the spot is imaged onto the position sensitive detector (PSD). A quarter wave plate changes the polarisation of the back travelling radiation in such a way that it is reflected at the polarising beam splitter. In case the beam does not hit the CD perpendicular the PSD notices a change in the position of the imaged spot and a control loop changes the orientation of the focusing lens. Once the laser beam hits a pit, the back reflected light will be reduced. This is interpreted as data information. The pit length or the time duration within the light level decides whether the data is interpreted as logically high or low. During the reading process the control loop makes sure that the contrast of the read signal is always maximised. This means the reading head follows the data track on the CD. Full details of all aspects are given in the comprehensive manual.
Within this course the data detection, the control loop for the reading laser head, the optical components and more details will be demonstrated at an open frame CD player with visual and electronic access to important sub assemblies. Electrical signals will be visualised and interpreted by means of an oscilloscope. The connection to a PC and with special software, all operations of the CD player shall be performed which are accessible via the regular software driver concept. Besides the aspects of optics, laser technique and electronics, this work-shop also focuses on the interaction with PC hard-and software.