Patent ID: 7206106

Claim:
A holographic ROM reader comprising: an objective lens for focusing light irradiated on a storage medium from a light source and then diffracted by the storage medium; a pinhole plate provided with a pinhole having a size that allows part of the light, which is focused by the objective lens, to pass through the pinhole; a transmissive diffraction grating, embedded into the pinhole of the pinhole plate, for diffracting the light focused by the objective lens to produce 0 th order diffracted light, +1 st order diffracted light and −1 st order diffracted light therethrough; a photo detector for detecting the amounts of the 0 th order diffracted light, the +1 st order diffracted light and the −1 st order diffracted light; and a servo controller for comparing an amount of light of an overlapped area of the 0th-order diffracted light and the +1 st order diffracted light with an amount of light of an overlapped area of the 0 th order diffracted light and the −1 st order diffracted light, and performing servo control to pass the light through a center of the transmissive diffraction grating if the amounts of light are not identical to each other.