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Nondestructive Measurement of Groove Depth of Optical Disks

IP.com Disclosure Number: IPCOM000039012D
Original Publication Date: 1987-Apr-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 38K

Publishing Venue

IBM

Related People

Karst, DL: AUTHOR [+2]

Abstract

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.

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Nondestructive Measurement of Groove Depth of Optical Disks

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. The ABS 5 evenly divides the beam, sending a portion to the phase detector 10 and the remainder to the astigmatic lens 6 and the quadrant detector 7. The quadrant detector is employed to determine the nominal focus position. A folding mirror 8 can be employed to make the device more compact. The beam reflected from the media 4 is astigmatic due to the groove structure. This astigmatism can be detected by the phase detector 10, and thereby the groove depth can be calculated. The phase detector 10 could be a Mach-Zehnder-type interferometer. Alternatively, a long focal length lens could be used, and the distance between the fo...