Holographic optical information recording/reproducing device
An optical information recording/reproducing device reproduces digital data recorded in a form of interference fringes produced by two coherent beams in a hologram disk, by projecting a coherent beam to the hologram disk and receiving a reproduction signal beam obtained by diffraction by means of a two-dimensional photodetector array. The holographic optical information recording/reproducing device includes a tunable coherent light source that emits the coherent beam, and a control section controls and optimizes a wavelength of the tunable coherent light source according to position information of the reproduction signal beam on the two-dimensional photodetector array. Thus, a holographic optical information recording/reproducing device is provided that is capable of maintaining a sufficient reproduction signal intensity and reproducing signals stably while undergoing a minimum of cross-talk, even in the case where the optimal reproduction wavelength varies due to a variance of media, a change of a temperature of a recording medium, etc.
1. A holographic optical information recording/reproducing device that projects a reference coherent beam to a recording medium and receives a reproduction signal beam obtained by diffraction of the reference coherent beam at the recording medium by means of a two-dimensional photodetector array so as to reproduce from the reproduction signal beam digital data recorded in a form of interference fringes produced by two coherent beams on the recording medium, the holographic optical information recording/reproducing device comprising:
a tunable coherent light source that emits the reference coherent beam; and
a control section that, when reproducing the digital data from the reproduction signal beam, reads a position information of the reproduction signal beam on the two-dimensional photodetector array, the position information varying in accordance with a wavelength of the tunable coherent light source, and controls a wavelength of the tunable coherent light source according to the position information.
2. The holographic optical information recording/reproducing device according to claim 1 , wherein at least one photoreceptor cell of the two-dimensional photodetector array is divided into not less than two regions, at least a part of the reproduction signal beam is made incident on the regions of the divided photoreceptor cell so as to be used as a servo-use beam, and the control section detects the position information according to a differential signal derived from signals obtained at the respective region by the servo-use beam.
3. The holographic optical information recording/reproducing device according to claim 2 , wherein recording is carded out so that the servo-use beam of the reproduction signal beam is in an ON state constantly.
4. The holographic optical information recording/reproducing device according to claim 2 , wherein recording is carried out so that the servo-use beam of the reproduction signal beam is in an ON state at a higher probability as compared with the other beam spots.
5. The holographic optical information recording/reproducing device according to claim 2 , wherein the divided photoreceptor cells are positioned at four corners of the two-dimensional photodetector array.
6. The holographic optical information recording/reproducing device according to claim 1 , further comprising:
an anamorphic optical system through which the coherent beam passes,
wherein the control section detects a position deviation of the coherent beam in a focusing direction and a wavelength deviation of the tunable coherent light source independently, according to changes in a reproduced image detected by the two-dimensional photodetector array.
7. The holographic optical information recording/reproducing device according to claim 1 , further comprising:
a beam splitter for dividing the coherent beam emitted from the tunable coherent light source into two beams that are a signal beam and a reference beam;
a spatial light modulator for modulating an intensity of the signal beam two-dimensionally;
an element for imparting a two-dimensional phase distribution to the signal beam on the spatial light modulator, the element having a greater coherence length in its peripheral region than in its central region; and
an optical system for crossing the signal beam and the reference beam on the recording medium.
8. The holographic optical information recording/reproducing device according to claim 7 , wherein the element for imparting a two-dimensional phase distribution includes cells that are arranged in a two-dimensional square grid and that have phase shifts of any one of 0, π/2, π, and 3π/2, and a phase difference between adjacent cells is either π/2 or 3π/2.
9. The holographic optical information recording/reproducing device according to claim 1 , further comprising a lens system for focusing diffracted light from the recording medium into the two-dimensional photodetector array, wherein the recording medium is disposed at a position different from a focus of the lens system.
10. The holographic optical information recording/reproducing device according to claim 1 , wherein the tunable coherent light source is a coherent light source utilizing a tunable semiconductor laser and a second-harmonic generation element.