BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding the invention shall now be explained in more detail in the following description with reference to the figures. It is understood that the invention is not limited to this exemplary embodiment and that specified features can also expediently be combined and/or modified without departing from the scope of the present invention. In the figures:
FIG. 1 schematically depicts the writing path of a coaxial holographic storage system,
FIG. 2 schematically depicts the reading path of a coaxial holographic storage system,
FIG. 3 shows the detector image of a reference beam interfering with a reconstructed object beam with a single pixel,
FIG. 4 shows an enlarged section of FIG. 3,
FIG. 5 shows a cut through the simulated intensity distribution of the reconstructed object beam,
FIG. 6 shows a cut through the simulated intensity distribution of the detector image of the reference beam interfering with the reconstructed object beam, and
FIG. 7 illustrates a numerical simulation of the detector image of a full SLM data page written and read in accordance with the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 shows a simplified configuration of a coaxial holographic storage system 1 according to the invention during writing. A reference beam 2 is superimposed on-axis to an object beam 3 by a beam splitter 4. The reference beam 2 and the object beam 3 are preferably generated by a single light source 20, e.g. with the help of a collimator lens 21, a beam splitter 22 and two steering mirrors 23, 24. Both beams 2, 3 pass a common objective lens 5 and are focused into or nearby a holographic storage medium 6. In the figure the light 7 coming from a single pixel 8 of a spatial light modulator 9 for writing a data page is highlighted. The wavefront coming from the pixel 8 has a phase shift of π with respect to the reference beam 2. In practice, a phase shift of exactly π is difficult to realize. It is, however, sufficient if the phase shift is close to π. In a coaxial setup, where the wavefronts of the object beam and the reference beam are exactly parallel, the phase shift is automatically the same for all pixels. The correct phase shift is obtained by adjusting the relative path lengths of object beam 3 and the reference beam 2, or by providing a phase shifting or switching element 13 in one of the beam paths. Examples for a phase shifting element 13 or a phase switching element 13 are a liquid crystal element or a switchable mirror, respectively. Of course, other methods for generating a phase shift may likewise be used. Advantageously, a phase shift adjusting element 14 is provided for actively adjusting the phase shift. The interference pattern resulting from the superposition of the object beam 3 and the reference beam 2 is stored in the holographic storage medium 6 as usual.
During reading the reference beam 2 is focused into or nearby the holographic storage medium 6 in the same way as during writing. In the figure, the holographic storage medium 6 is a transmission type storage medium. Of course, the invention is likewise applicable to a reflection type storage medium. The hologram stored in the holographic storage medium 6 diffracts the light in such way that the reconstructed object beam 10 has all the properties of the object beam 3, including the phase. Since the phase of the object beam 3 was adjusted to be shifted by π with respect to the reference beam 2, the phase shift of the reconstructed object beam 10 with respect to the reference beam 2 is also π. By destructive interference the reconstructed object beam 10 cancels at least partly the light of the reference beam 2. The image of the reference beam 2 on a detector 11 obtained by a further objective lens 12 shows a dark region at the position corresponding to the SLM pixel 8 that was switched on during writing.
The physical properties of the interference simplify the detection of the dark pixel in the image of the reference beam 2. The cancellation C of the light by destructive interference calculates to:
where Iobj and Iref are the intensities of the reconstructed object beam 10 and the reference beam 2, respectively. Therefore, the weak light of the reconstructed object beam 10, which is determined by the diffraction efficiency of the hologram, is able to cancel a quite large amount of the reference beam 2. For example, a reconstructed object beam 10 with 6% of the intensity of the reference beam 2 cancels 47% of the light of the reference beam 2. A reconstructed object beam 10 of 1% cancels about 20%.
To prove the method and especially the amount of light cancellation, the reading of the hologram with a single bright pixel was simulated numerically. The result is shown in FIGS. 3 to 6. The light of a relatively weak reconstructed object beam 10 with 6% of the intensity of the reference beam 2 partly cancels the light of the reference beam 2. FIG. 3 shows the image of the reference beam 2 on the detector 11. FIG. 4 shows an enlarged section of FIG. 3. The dark pixel resulting from the interference is clearly visible. The plot in FIG. 5 shows a cut through the simulated intensity distribution of the reconstructed object beam 10. The intensity and coordinates are given in arbitrary units. The peak height is I=1.9e−7. The plot in FIG. 6 shows a cut through the simulated intensity distribution of the image of the reference beam 2 superimposed by the reconstructed object beam 10 on the detector 11. Again, the intensity is given in arbitrary units. The height of the plateau is I=3.1e−6. As can be seen, the reconstructed object beam 10 with its intensity of 6% of the intensity of the reference beam 2 cancels about 47% of the reference beam 2 at the pixel position.
In FIG. 7 the numerical simulation of the detector image of a full SLM data page written and read as proposed by the invention is illustrated. The beam propagation technique was used for simulating the full process of writing and reading of the hologram. In the simulation, the complete setup described above was implemented. The wavelength of the object beam 3 and the reference beam 2 was λ=405 nm, the focal length and the numerical aperture of the objective lens 5 were fobjective=4.5 mm and NA=0.6, respectively. The data page was a typical datapage with 128×128 pixels and evenly distributed marks for synchronization. As can be seen, the method works well for a whole data page. All pixels switched on during writing partly cancel the reference beam during reading. Accordingly, the data page appears in form of an inverted image on the detector 11.
Patent Application
20080055605
