Holographic storage system with reduced noise

An apparatus for reading from and/or writing to transmission type holographic storage media is proposed, and more specifically a coaxial type apparatus for reading from and/or writing to transmission type holographic storage media with two or more reference beams, which has an improved Signal to Noise Ratio. The apparatus has a coaxial arrangement of two or more reference beams and an object beam or a reconstructed object beam. The reference beams are arranged on a circle around the object beam or the reconstructed object beam in a Fourier plane of the apparatus behind the holographic storage medium. A mirror is located in or close to this Fourier plane, which is designed such that it reflects the object beam or the reconstructed object beam without reflecting the reference beams.

This application claims the benefit, under 35 U.S.C. §119 of EP Patent Application 08154708.5, filed 17 Apr. 2008.

FIELD OF THE INVENTION

The present invention relates to an apparatus for reading from and/or writing to holographic storage media, and more specifically to a coaxial type apparatus for reading from and/or writing to holographic storage media with two or more reference beams, which has an improved Signal to Noise Ratio.

BACKGROUND OF THE INVENTION

In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.

One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a few layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array.

In WO2006/003077 a 12f reflection type coaxial holographic storage arrangement with three confocally arranged Fourier planes is shown. In this arrangement the object beam and the reference beams are coupled in and out at the first and third Fourier planes, respectively. The reference beams are small spots in these planes. More precisely, they form diffraction patterns, similar to the Airy pattern. This arrangement is a so-called common aperture arrangement, because at the object plane and the image plane the object beam and the reference beams fill out the same area of the aperture. The beams fill out the entire aperture of the objectives. The disclosed arrangement allows to apply shift multiplexing, reference scanning multiplexing, phase coded multiplexing, or a combination of these multiplexing schemes. The reference beams are a pair (or pairs of) half cone shaped beams. The tips of the pair or pairs of half cone shaped reference beams form two lines along a diameter at the Fourier planes of the object beam.

Theoretically, for infinite holograms the shift selectivity curve is a sinc(x) like function. See, for example, G. Barbastathis et al.: “Shift multiplexing with spherical reference waves”, Appl. Opt. 35, pp 2403-2417. At the so-called Bragg distances the diffraction efficiencies of the shifted hologram are zero. In WO2006/003077 the distances between the tips of the reference beams along the two lines correspond to these Bragg distances. The interhologram crosstalk between the multiplexed holograms in theory is negligible at the Bragg distances. Assuming infinite diameter holograms there are selective and non-selective directions for the shift multiplexing. See again the article of G. Barbastathis et al. The selective direction is the direction of the line formed by the tips of the reference beams. In the so-called non-selective direction, which is orthogonal to the selective direction in the plane of the holograms, the shift distance is infinite. However, in a real storage system the volume of the hologram is finite. Practically, the order of magnitude of the hologram volume is about (0.4-0.6)×(0.4-0.6)×(0.2-0.6) mm3. Detailed investigations have shown that there are large discrepancies between the shift selectivity curves of infinite and finite holograms. There are no Bragg distances in case of finite volume holograms. See Z. Karpati et al.: “Shift Selectivity Calculation for Finite Volume Holograms with Half-Cone Reference Beams”, Jap. J. Appl. Phys., Vol. 45 (2006), pp 1288-1289. Using finite volume holograms the order of magnitude of the selectivity is similar in both directions. See, for example, Z. Karpati et al.: “Selectivity and tolerance calculations with half-cone reference beam in volume holographic storage”, J. Mod. Opt., Vol. 53 (2006), pp 2067-2088. The presence of selectivity in both directions allows two-dimensional multiplexing. A problem is that the interhologram cross-talk is too high in the non-selective direction. This limits the achievable number of multiplexed holograms in this direction, and as a consequence limits the total capacity of the holographic storage medium.

In order to obtain an improved selectivity, in the not yet published European Patent Application EP 06122233.7 an apparatus for reading from and/or writing to a reflection-type holographic storage medium with a coaxial arrangement of three or more reference beams and an object beam or a reconstructed object beam is described. In this apparatus the reference beams are arranged on a circle or an ellipse around the object beam in a Fourier plane of the apparatus. In order to separate a reconstructed object beam from the reflected reference beams an outcoupling filter is used, which blocks the reflected reference beams and passes the reconstructed object beam through a central aperture.

The main advantage of the various coaxial holographic storage systems is their insensitivity against environmental disturbances, because the object and reference beams propagate along the same optical path. Use of a reflection type holographic storage medium allows to reduce the size of the system compared to the size of a system for a transmission type holographic storage medium, as all optical elements are arranged on the same side of the holographic storage medium. Furthermore, no additional hardware is necessary for shift multiplexing. A precise movement of the holographic storage medium is sufficient, which can easily be realized by rotating the holographic storage medium.

However, a big challenge for the reflection type coaxial systems is to increase the Signal to Noise Ratio (SNR) by attenuation of the different noises propagating on the same axis as the reconstructed object beam. Because of the small diffraction efficiency of the multiplexed holograms, the order of magnitude of the required attenuation is about 10−4or 10−5.

SUMMARY OF THE INVENTION

It is an object of the invention to propose an apparatus for reading from and/or writing to a holographic storage medium with a coaxial arrangement of two or more reference beams and an object beam or a reconstructed object beam, which has an improved Signal to Noise Ratio.

According to the invention, this object is achieved by an apparatus for reading from and/or writing to a transmission type holographic storage medium, with a coaxial arrangement of two or more reference beams and an object beam or a reconstructed object beam, the reference beams being arranged on a circle around the object beam or the reconstructed object beam in a Fourier plane of the apparatus behind the holographic storage medium, in which a mirror is located in or close to the Fourier plane, the mirror being designed such that it reflects the object beam or the reconstructed object beam without reflecting the reference beams.

It has been found that for the common aperture system the reflection of the object beam is important for increasing the Signal to Noise Ratio, or for decreasing the Bit Error Rate (BER) or the Symbol Error Rate (SER). Therefore, it is desirable to maintain a reflection type system. However, the reflected reference beams are scattered and diffracted at the surfaces of the optical components of the system, which leads to noise and the necessity to filter out the reflected reference beams.

The invention overcomes this problem by using a transmission type holographic storage medium in combination with a specially designed mirror in the Fourier plane behind the holographic storage medium. Only the object beam or the reconstructed object beam is reflected back into the system towards the detector. The reference beams are coupled out at the Fourier plane, directly after forming a hologram or after reconstructing the object beam. As the reference beams do not propagate towards the detector, the diffraction noise caused by the reference beam on the detector surface is reduced. At the same time, only a relatively simple mirror is placed behind the transmission type holographic storage medium. Therefore, the system size does not change significantly in comparison to a reflection type system, where the entire optical elements are located at the same side of the holographic storage medium.

Advantageously, the mirror is a circular mirror with a diameter smaller than the diameter of the circle on which the reference beams are arranged. As the reference beams are arranged around the object beam or the reconstructed object beam, it is sufficient to reduce the diameter of the mirror up to the point where the reference beams are no longer reflected. This is preferably achieved by providing a transparent or absorptive substrate with a reflective coating in the appropriate circular area.

Favorably, the mirror acts as a Fourier filter for the object beam or the reconstructed object beam. This is possible by further reducing the diameter of the mirror up to the point where it does no longer reflect the higher Fourier components of the object beam or the reconstructed object beam. In this case the mirror is the reflective equivalent of a pinhole Fourier filter.

Alternatively, the mirror has dimensions larger than the diameter of the circle on which the reference beams are arranged. In this case non-reflective areas are located at least at the locations of the reference beams on the mirror.

The non-reflective areas are preferably areas without a reflective coating, which can be easily manufactured, or diffractive or refractive structures. In the latter cases the structures allow to direct the reference beams into desired directions, e.g. to control the intensity of the reference beams.

Preferably, the mirror is mechanically or electronically coupled to an objective lens. The apparatus has an objective lens for focusing the object beam and the reference beams into the holographic storage medium. This objective lens is generally movable for focusing and tracking, which means that positions of the object beam or the reconstructed object beam and the reference beams changes. Coupling of the mirror to the objective lens ensures the correct positioning of the object beam or the reconstructed object beam and the reference beams on the mirror.

Advantageously, the apparatus has four reference beams. Numerical simulations have shown that this is the optimal number of reference beams.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A simplified setup of a known coaxial reflection type holographic storage system1is illustrated inFIG. 1. For simplicity, an integrated servo system has been omitted in the figure. In the example the holographic storage system is a 12f system. A laser beam3emitted by a laser2is expanded by an optional beam expander4and divided into a reference beam7and an object beam8by a polarizing beam splitter (PBS) cube6. A half wave plate5is located between the beam expander4and the PBS cube6. The laser2emits a linearly polarized laser beam3. By rotating the half wave plate5the polarization direction of the laser beam3can be rotated into an arbitrary direction. The PBS cube6divides the laser beam3into two orthogonal polarized (P and S polarized) laser beams7,8. The rotation of the half wave plate5allows to control the beam intensity ratio of the P and S polarized beams, or in other words the intensity ratio in the object arm and reference arm. For optimizing the readout diffraction efficiency it is desirable to optimize the intensity ratio during recording. The object beam8is directed onto a reflection type SLM9by the PBS cube6. The reflection type SLM9not only imprints a data page on the object beam8, but also changes the direction of polarization of the object beam8. In this regard the reflection type SLM9acts as a half wave plate. After reflection from the SLM9, the object beam8passes through the PBS cube6and is combined with the reference beam7. In the optical path of the reference beam7there are a quarter wave plate10and a reflection type diffraction beam generator11. The beam generator11reflects two or more well-defined diffraction orders, which are circularly shaped tilted plane waves propagating in well-defined directions. Numerical simulations have shown that the optimal number of reference beams is four. Therefore, the beam generator11preferably generates four reference beams7′,7″, two of which are illustrated in the figure. Due to the practical realization of the beam generator11also the zero-order diffraction beam appears, though with low diffraction efficiency. This beam is suppressed in the further part of the optical system.

As indicated before, the object beam8and the reference beams7′,7″ are coupled into the main coaxial arrangement by the PBS cube6. Following this PBS cube6there is a first long focal length objective12. Long focal length in this case means that the focal length is long enough to place additional optical components between the lens and the focus without having too much aberrations. Long focal length objectives have the advantage that their fabrication is simple and requires less optical elements. In addition, the diameter of the Fourier plane of a long focal objective is large, which simplifies the fabrication of filters placed into the Fourier plane as the fabrication tolerances are reduced. This first objective12generates the Fourier transform of the SLM8at its back focal plane, which is the first Fourier plane of the 12f system and the Fourier plane of the SLM8. The first objective12also focuses the multiple reference beams7′,7″ into the first Fourier plane. Located in this first Fourier plane is an in-coupling filter13, which is designed such that it low-pass filters the object beam8and rotates the polarization of the reference beams7′,7″ without rotating the polarization of the zero order component of the reference beam. The in-coupling filter13will be explained below in more detail with reference toFIGS. 2 and 3.

After passing the in-coupling filter13the object beam8and the reference beams7′,7″ pass through a second PBS cube14. As the zero order component of the reference beam is orthogonal to the other beams7′,7″,8, the second PBS cube14transmits the low-pass filtered object beam8and the diffracted reference beams7′,7″, but reflects the zero order component of the reference beam out of the optical system. A second long focal length objective15after the PBS cube14retransforms the SLM image onto an intermediate object plane16and generates again circularly shaped tilted plane waves from the focused reference beams7′,7″. A high NA Fourier objective17transforms the SLM image onto a mirror layer20of a holographic storage medium19located in a second Fourier plane30. The position of the high NA Fourier objective17is adjusted with an actuator31, which is controlled by a servo circuit32. During writing the object beam8interferes within a hologram layer29of the holographic storage medium19with the direct reference beams7′,7″ and the reference beams reflected by the mirror layer20. During reading a reconstructed object beam21is retransformed by the high NA Fourier objective17onto the intermediate image plane16. For better clarity, as in the figure the reconstructed object beam21coincides with the object beam8, the reference numeral for the reconstructed object beam21is drawn behind the PBS cube14. A quarter wave plate18is located between the high NA Fourier objective17and the holographic storage medium19. As the beams pass through this quarter wave plate18twice, the polarization direction of the reconstructed object beam21is orthogonal to the polarization direction of the original object beam8. The reconstructed object beam21is again Fourier transformed by the second long focal length objective15. Due to the rotation of the polarization, the PBS cube14reflects the reconstructed object beam21onto an out-coupling filter22, which is located in a third Fourier plane of the 12f system. The out-coupling filter22blocks the reflected reference beams, thus only the reconstructed object beam21is imaged onto a detector array24by a third long focal length objective23.

FIG. 2shows a cross sectional view of the in-coupling filter13located in the first Fourier plane of the 12f optical system. It includes a beam block130, e.g. a thin black metal plate or a transparent substrate with a reflective or absorbent layer, with a central aperture132with a diameter D3for the object beam8and the zero order reference beam, and holes131with a diameter d for the reference beams7′,7″. A ring type half wave plate133is arranged on the beam block130. The ring type half wave plate133has a central aperture134with a diameter D2. The object beam8and the zero order component of the reference beam pass through this central aperture134without any modification, and also pass through the central aperture132of the beam block130. The central aperture132acts as a low-pass filter for the object beam8, because it cuts the higher Fourier components of the object beam8. The remaining reference beams7′,71″ pass through the half wave plate133, which rotates the direction of polarization of these beams7′,7″. Before the first Fourier plane the directions of polarization of the object beam8and the reference beams7′,7″ are orthogonal. The ring type half wave plate133rotates the direction of polarization of the diffracted reference beam7′,7″, while the low energy zero order component of the reference beam conserves its direction of polarization. Arranged on a ring with the diameter D1around the central aperture132of the beam block130there are holes131for the diffracted reference beams7′,7″. Thus the filter13in the first Fourier plane transmits the diffracted reference beams7′,7″ as well as the zero order component of the reference beam, and also transmits the low-pass filtered object beam8. Because of the ring type half wave plate133the direction of polarization of the zero order component of the reference beam is orthogonal to the direction of polarization of the other beams7′,7″,8. Therefore, the PBS cube14after the filter13transmits the low-pass filtered object beam8and the diffracted reference beams7′,7″, whereas it reflects the zero order component of the reference beam out of the optical system. In the figure the central aperture132is circular, which fits best to the circular apertures of the lenses of the optical setup. However, the aperture132may also be elliptical, e.g. when the tips of the reference beams7′,7″ are arranged on an ellipse. Furthermore, the aperture may also have a square or rectangular shape, which fits better to the diffraction image of the SLM9with its square or rectangular pixels. The apertures131for the reference beams7′,7″ may be switchable apertures. This is advantageous for special multiplexing schemes.

FIG. 3shows the top view of the beam block130of the in-coupling filter13for the case of four reference beams7′,7″. The holes131for the reference beams7′,7″ are arranged on a circle with the diameter D1. The diameter of the central aperture132is D3. The difference of the diameters (D1-D3) is about 40-100 μm. The diameter d of the holes131for the reference beams7′,7″ is about 10-100 μm. Of course the number of reference beams7′,7″ is not limited to four reference beams7′,7″.

FIG. 4depicts a simplified setup of a coaxial reflection type holographic storage system1according to the invention. The system1is essentially the same as the system ofFIG. 1. However, instead of a reflection type holographic storage medium19with a reflective layer20a transmission type holographic storage medium19with a transparent substrate25is used. The high NA Fourier objective17transforms the SLM image through the transmission type holographic storage medium19onto the second Fourier plane30. Located in this second Fourier plane30is a specially shaped mirror27arranged on a transparent substrate26. The mirror27has a circular shape with a diameter essentially equal to the diameter of the low pass filtered object beam8. This means that the mirror27is the reflective equivalent of a Fourier filter aperture. The mirror27reflects the object beam8, but the focused reference beams7′,7″ leave the system near this mirror27through the transparent substrate26. It is likewise possible to use an absorptive substrate instead of the transparent substrate26. It just needs to be ensured that the reference beams7′,7″ are not reflected back into the system. As the high NA Fourier objective17is moved by the actuator31, the mirror27needs to follow this movement, i.e. the high NA Fourier objective17and the mirror27are fixed relative to each other. This is achieved by mechanically coupling the mirror27and the high NA Fourier objective17, or by providing the mirror27with an additional actuator33. This additional actuator33is either controlled by the servo circuit32of the high NA Fourier objective17, or by an additional servo circuit34. The additional servo circuit34preferably uses the transmitted reference beams7′,7″ for controlling the position of the mirror27.

An enlarged side view of the object beam8and the reference beams7′,7″ at the position of the holographic storage medium19is shown inFIG. 5. During writing of a hologram the direct and reflected object beams8overlap with the direct focused reference beams7′,7″ and form an interference pattern (hologram) in the storage material29.

During reading, only the reference beams7′,7″ illuminate the hologram. The reconstructed direct and reflected object beams21are retransformed by the high NA Fourier objective17onto the intermediate image plane16. No reflected reference beams are present, as the focused reference beams7′,7″ are not reflected by the mirror27.

A top view of the circular mirror27is depicted inFIG. 6. The mirror27is arranged on a transparent or absorbing substrate26. The small circular areas28indicate the positions of the four reference beams7′,7″ on the substrate26. As can be seen, the reference beams7′,7″ do not impinge on the mirror27.

Instead of the small circular mirror27it is likewise possible to use a larger mirror with holes for transmitting the reference beams7′,7″. This is depicted inFIG. 7. The whole substrate26except for the locations28of the reference beams7′,7″ is provided with a reflective coating. In this case, however, the mirror27does not act as a Fourier filter for the object beam8. In addition, instead of coupling the reference beams7′,7″ out of the system by letting them pass through the substrate26, the reference beams7′,7″ can also be coupled out using diffractive or refractive structures at the position locations28of the reference beams7′,7″, e.g. gratings or prisms.

In order to avoid the coupling of the high NA Fourier objective17and the mirror27, it is possible to increase the size of the mirror27. This is depicted inFIG. 8. If the distance between the object beam8or the reconstructed object beam21and the reference beams7′,7″ is not too small, a slight movement of the high NA Fourier objective17does not need to be compensated. The object beam8or the reconstructed object beam21remain on the mirror27, while the reference beams7′,7″ still do not impinge on the mirror27.

FIG. 9shows a further improved-solution for the mirror27, which allows for a larger displacement of the high NA Fourier objective17. In this case the mirror is further enlarged, but has special cut-outs for the reference beams7′,7″.