Consumer Electronics

READOUT FROM THE DISC

The disc is optically scanned in the player. This is done by AlGaAs semiconductor laser Fig. 14.4 shows the optical part of the pickup. The light from the laser La (wavelength 800 nm) is focused through the lenses L₂ and L_l onto the reflecting layer of the disc. The diameter of the light spot S, Fig. 14.5, is about 1 μm. When the light falls on an interval between two pits, the light is almost totally reflected and reaches the four photodiodes D₁ to D₄ via the half-silvered mirror M. When the spot lands on a pit—the depth of a pit is about 1/4 of the wavelength in the transparent substrate material—interference causes less light to be reflected and an appreciably smaller amount reaches the photodiodes. When the output signals ...