Nondestructive Measurement of Groove Depth of Optical Disks
Original Publication Date: 1987-Apr-01
Included in the Prior Art Database: 2005-Feb-01
Optical disks commonly use a grooved substrate to provide the track- following and focussing information to the file's servo circuitry. The depth of these grooves is critical to the proper performance of the file. Astigmatism in the reflected beam is used to measure the groove depth. A collimated laser beam is incident at the polarizing beam splitter (PBS) 1 in the drawing. The polarization of this beam is such that it propagates through the PBS, the quarter-wave plate 2, and and is then brought to a focus on the media surface 4 by lens 3. The reflected beam then travels back through lens 3 and the quarter-wave plate 2. The polarization of this beam has been rotated by the two passes through the quarter-wave plate 2, and the beam is now directed to the amplitude beamsplitter (ABS) 5.