Apparatus and method for compensating wobble in holographic digital data storage system

1. A holographic digital data storage apparatus for detecting a wobbling in a disc shaped storage medium, the disc including therein one or more holographically generated patterns, the apparatus comprising:

a beam generator for projecting at least two wobble detection beams to the holographically generated patterns, the wobble detection beams having different diffraction efficiencies provided by respective characteristic angles, wherein each characteristic angle is an angle between an optical path of one wobble detection beam and that of its corresponding pattern forming beam used in forming one of the holographically generated patterns and a diffraction efficiency is given by a ratio of a power of a diffracted beam of one of the wobble detection beams to a power of said one of the wobble detection beams; and

means for finding at least a difference between the diffraction efficiencies to thereby determine the wobbling in the disc.

2. The apparatus of claim 1 , wherein the number of the holographically generated patterns is two and the number of the wobble detection beams is two.

3. The apparatus of claim 2 , wherein each of two holographically generated patterns is formed by two pattern forming beams, and wherein an optical path of one of the wobble detection beams is identical to that of one of pattern forming beams for the holographically generated patterns and an optical path of the other wobble detection beam is different from that of anyone of the pattern forming beams for the holographically generated patterns.

4. The apparatus of claim 2 , wherein said finding means includes:

a first power detector for detecting a power level of a diffracted beam of a first wobble detection beam;

a second power detector for detecting a power level of a diffracted beam of a second wobble detection beam; and

means for obtaining the diffraction efficiencies based on power levels of the wobble detection beams and the detected power levels to thereby determine the wobbling through the use of the obtained diffraction efficiencies and the characteristic angle.

5. The apparatus of claim 4 , wherein the diffraction efficiencies are calculated as follows:

D A =αI; and

D B =βI′,

wherein D A is the power level of the diffracted beam of the first wobble detection beam; D B is the power level of the diffracted beam of the second wobble detection beam; “α” and “β” are the diffraction efficiencies of the first and the second wobble detection beam, respectively; and I and I′ are power levels of the first and the second wobble detection beam, respectively.

6. A method for use in a holographic digital data storage system for detecting a wobbling in a disc shaped storage medium, comprising the steps of:

(a) generating one or more holographically generated patterns in the disc through the use of pattern forming beams;

(b) projecting at least two wobble detection beams having different diffraction efficiencies provided at respective characteristic angles, wherein each characteristic angle is an angle between an optical path of one wobble detection beam and that of its corresponding pattern forming beam and a diffraction efficiency is given by a ratio of a power of a diffracted beam of one of the wobble detection beams to that of said one of the wobble detection beams; and

(c) finding at least a difference between the diffraction efficiencies to thereby detect the wobble in the disc.

7. The method of claim 6 , wherein the number of the holographically generated patterns is two and the number of the wobble detection beams is two.

8. The method of claim 7 , wherein each of two holographically generated patterns is formed by two pattern forming beams, and wherein an optical path of one of the wobble detection beams is identical to that of one of pattern forming beams for the holographically generated patterns and an optical path of the other wobble detection beam is different from that of anyone of the pattern forming beams for the holographically generated patterns.

9. The method of claim 7 , wherein the finding step (c) includes the steps of:

(c1) detecting a power level of a diffracted beam of a first wobble detection beam;

(c2) detecting a power level of a diffracted beam of a second wobble detection beam;

(c3) obtaining the diffracted efficiencies based on power levels of the wobble detection beams and the detected power levels; and

(c4) determining the wobbling through the use of the obtained diffraction efficiencies and the characteristic angle.

10. The method of claim 9 , wherein the diffraction efficiencies are calculated as follows:

D A =αI; and

D B =βI′,

wherein D A is the power level of the diffracted beam of the first wobble detection beam; D B is the power level of the diffracted beam of the second wobble detection beam; “α” and “β” a re the diffraction efficiencies of the first and the second wobble detection beam, respectively; and I and I′ are power levels of the first and the second wobble detection beam, respectively.