Holographic ROM reader having servo control
View Patent ↗A holographic ROM reader having a servo control is disclosed. The holographic ROM reader includes an objective lens for focusing light reflected from a storage medium, a pinhole plate provided with a pinhole, a 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, the +1 st order and the −1 st order diffracted light, and a servo controller for comparing an amount of light of an overlapped area of the 0 th 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 to control the position of the objective lens to pass the light through a center of the diffraction grating if the amounts of light are not identical to each other.
1. 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.
2. The holographic ROM reader of claim 1 , wherein the transmissive diffraction grating has a land and a groove having the same width w and the same depth d , wherein the land is placed on the center portion of the diffraction grating and wherein the width w satisfies the following Equation,
w=p /2, where p is the pitch of the groove.
3. The holographic ROM reader of claim 2 , wherein a depth d of the groove satisfies the following Equation,
d=λ/ 2.
4. The holographic ROM reader of claim 2 , wherein the servo controller controls a position of the objective lens so that light can pass through the center of the transmissive diffraction grating.
5. The holographic ROM reader of claim 2 , wherein the servo controller controls a position of the pinhole plate so that the light can pass through the center of the transmissive diffraction grating.
6. A holographic ROM reader comprising:
an objective lens for focusing light irradiated onto 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 reflective diffraction grating, embedded into the pinhole of the pinhole plate, for diffracting the light that is focused by the objective lens to produce 0 th order diffracted light, +1 st order diffracted light and −1 st order diffracted light reflected therefrom;
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 0 th 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 reflective diffraction grating if the amounts of light are not identical to each other.
7. The holographic ROM reader of claim 6 , wherein the reflective diffraction grating has a land and a groove having the same width w and the same depth d , wherein the groove is placed on the center portion of the reflective diffraction grating and wherein the width w satisfies the following Equation,
w=p /2, where p is the pitch of the groove.
8. The holographic ROM reader of claim 7 , wherein the depth of the groove satisfies the following Equation,
d =λ/8.
9. The holographic ROM reader of claim 7 , wherein the servo controller controls a position of the objective lens so that the light can pass through the center of the reflective diffraction grating.
10. The holographic ROM reader of claim 7 , wherein the servo controller controls a position of the pinhole so that light can pass through the center of the reflective diffraction grating.
11. The holographic ROM reader of claim 6 , further comprising:
a parallel light lens for transforming the diffracted lights, which are reflected from the diffraction grating, into parallel light;
a λ/4 plate for transforming the parallel light into λ/4 parallel light; and
a condenser lens for condensing the λ/4 parallel light onto the photo detector.