Distribution of intensity at various cross-sections of an astigmatic focus-error detection system
This example shows the effect of disk defocus on the distribution of light intensity arriving at the photodetector chip of a laser disk drive.
The basic elements of an astigmatic focus-error detection system used in many laser disk drives are shown in the above diagram. The beam from a semiconductor diode laser is focused on the disk surface through a high-NA objective lens. The reflected beam is directed towards an astigmat and a quadrant photodetector. The various combinations of the four detector signals yield not only the magnitude and sign of the disk defocus, but also (in the case of grooved disks) they produce a push-pull track-error signal.
The pictures below show the intensity distribution of the beam at the entrance pupil of the astigmat (left images), and in the plane of the detector (right images). Two pictures corresponding to two different amounts of disk defocus are shown in each case.
the detector (right) for +10 waves of defocus.
the detector (right) for -5 waves of defocus.
DIFFRACT enables you to scan the beam in the focusing direction to compute the focus-error signal; move across tracks to find the track-error signal; determine the tolerance of servo signals to tilt, decenter, aberrations of the beam, birefringence of the substrate, and many other sources of error.
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