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This minimizes crosstalk
from neighboring pits[3].
D. The optical train -- three beam pick-up
The most common optical train in modern CD players is the three beam pick-up, depicted below[4].
The light is emitted by the laser diode and enters a diffraction grating. The grating converts the light into a
central peak plus side peaks. The main central peak and two side peaks are important in the tracking mechanis
The three beams go through a polarizing beam splitter. This only transmits polarizations parallel to the page.
The emerging light (now polarized parallel to the page) is then collimated.
The collimated light goes through a 1/4 wave plate. This converts it into circularly polarized light.
The circularly polarized light is then focused down onto the disk. If the light strikes "land" it is reflected back
into the objective lens. (If the light strikes the pit, now a bump, it is not reflected.)
The light then passes through the 1/4 wave plate again
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