Design of an optical pickup using double holographic lenses

An optical pickup system for optical disc drives utilizes optical elements for information signal detection, focus-error detection and track-error detection. We propose an optical pickup using double holographic lenses--replacing conventional optical elements for focus-error detection and track-error detection--to reduce the pickup size. There are two important requirements when using holograms for an optical pickup. One is to compensate for chromatic aberration caused by laser diode wavelength deviation, and the other is to obtain an appropriate beam spot size for focus-error detection. To meet these requirements, we used double holograms, where one has divergent power and the other convergent power. By optimizing the double hologram pattern, the optical system is achromatized. We observed the achromatic effect, 0.07 (lambda) (RMS) for a laser diode wavelength change of +/- 10 nm. The beam position error was less than +/- 1 micrometers . We reduced the NA of the system to give a relatively large spot beam and a large depth of focus.