The CD Optical Pickup

Philips and Sony shipped the compact disc in 1982, and the pickup inside it is the direct descendant of the laser diode in the previous batch. An AlGaAs diode at 780 nanometres, a lens of numerical aperture 0.45, and a spot about two micrometres across scanning a spiral of pits 1.6 micrometres apart. Nothing touches the disc. Two things about it are cleverer than they look. First, a pit is not read as a shadow. It is a quarter of a wavelength deep INSIDE the polycarbonate, so when the spot straddles a pit edge, half the beam travels an extra half wavelength, and the two halves cancel on the way back. The disc is an interferometer, and the read signal is a destructive interference null. Second, the lens has to stay within about a micrometre of focus while the disc wobbles axially by hundreds of micrometres. It does that by measuring its own error: a cylindrical lens in the return path makes the spot on a four-quadrant detector an ellipse whose orientation FLIPS through focus, so the difference of the diagonal sums is a signed error signal. That S-shaped curve drives a voice-coil servo — the same actuator as blueprint 6, doing the same job in a different axis. MEASURABLE, AND PARTLY BUILDABLE. A salvaged pickup plus the laser-diode driver from the previous batch puts the focus-error S-curve on a scope. And the classic measurement needs no pickup at all: a compact disc is a reflection grating with a 1.6 micrometre pitch, so a laser pointer and a ruler measure the track spacing to within a few percent.