KR20090110236A - Holographiic storage system with reduced noise - Google Patents

Abstract

PURPOSE: A holographic storage system is provided to reduce the diffraction noise caused by reference beam on a detector surface as the reference beam is not transmitted toward the detector. CONSTITUTION: A holographic storage system includes two or more reference beam(7',7") and object beam(8) or re-organized object beam(21) which are coaxially arranged to read or record data on a transmissive holographic storage medium(19). The reference beam is arranged on a circle around the object beam or re-organized object beam on a fourier plane of an apparatus(1) behind the holographic storage medium.

Description

Holographic storage system with reduced noise {HOLOGRAPHIIC STORAGE SYSTEM WITH REDUCED NOISE}

The present invention relates to an apparatus for reading from and / or writing to a holographic storage medium, and more particularly to reading from and / or writing to a holographic storage medium having two or more reference beams with an improved signal to noise ratio. It relates to a coaxial type device for.

By recording the interference pattern generated by the superposition of two coherent laser beams, digital data is stored in the holographic data storage, where one beam, called the "object beam", is space It is modulated by an optical modulator and carries the information to be recorded. The second beam acts as a reference beam. The interference pattern results in a deformation of the special properties of the storage material, which properties depend on the local intensity of the interference pattern. Reading of the recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. The result is a reconstruction of the recorded object beam.

One advantage of holographic data storage is that the data capacity is increased. Compared with conventional optical storage media, the volume of holographic storage media, rather than several layers, is used to store information. One further advantage of holographic data storage is the possibility of storing multiple data in the same volume, for example by changing the angle between two beams or by using shift multiplexing. Moreover, instead of storing a single bit, the data is stored as a data page. In general, a data page consists of a matrix of light-dark-patterns, a two-dimensional binary array or an array of gray values that code a number of bits. This allows to achieve increased data rates in addition to increased storage density. The data page is imprinted onto the object beam by a spatial light modulator (SLM) and detected through a detector array.

In WO2006 / 003077, a 12f reflective coaxial holographic storage device with three confocal arranged Fourier planes is shown. In such a device, the object beam and the reference beam are coupled in and out in the first and third Fourier planes, respectively. The reference beam is a small spot in these planes. More specifically, the reference beam forms a diffraction pattern similar to the Airy pattern. Such a device is a so-called common aperture device because the object beam and the reference beam fill the same area of the aperture in the object plane and the image plane. The beams fill the entire aperture of the objective lens. The disclosed apparatus allows for applying shift multiplexing, reference scanning multiplexing, phase coded multiplexing, or a combination of these multiplexing schemes. The reference beam is a pair (or pairs) of semiconical beams. The pair or pairs of tips of the semiconical reference beam form two lines along the diameter in the Fourier plane of the object beam.

In theory, for infinite holograms, the shift selectivity curve is a sinc (x) like function. See, eg, G. Barbastathis et al., "Shift multiplexing with spherical reference waves", Appl. Opt. See pages 35, 2403-2417. At the so-called Bragg distance, the diffraction efficiency of the shifted hologram is zero. In WO2006 / 003077, the distance between the vertices of a reference beam along two lines corresponds to these Bragg distances. In theory, cross-hologram crosstalk between multiplexed holograms can be ignored at Bragg distance. Assuming holograms of infinite diameter, there are selective and non-selective directions for shift multiplexing. See again G.Barbastathis et al. An optional direction is the direction of the line formed by the vertices of the reference beam. In the so-called non-selective direction orthogonal to the selective direction in the plane of the hologram, the shift distance is infinite. In actual storage systems, however, the volume of the hologram is finite. In practice, the order of the magnitude is about (0.4-0.6) x (0.4-0.6) x (0.2-0.6) mm ³ . Detailed research has shown that there is a large mismatch between the shift selectivity curves of infinite and finite holograms. In the case of finite volume holograms, no Bragg distance exists. 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. With finite volume holograms, the magnitude of the selectivity is similar in both directions. See, eg, Z. Karpati et al., "Selectivity and tolerance calculations with half-cone reference beam in volume holographic storage", J. Mod. See Opt., Vol. 53 (2006), 2067-2088. The presence of selectivity in both directions allows two-dimensional multiplexing. The problem is that the crosstalk between holograms is too high in the non-selective direction. This limits the achievable number of multiplexed holograms in this direction and consequently limits the total capacity of the hologram storage medium.

To obtain improved selectivity in the unpublished European patent application EP 06122233.7, reading from and / or writing to a reflective-holographic storage medium having at least three reference beams and a coaxial arrangement of one object beam or a reconstructed object beam An apparatus for doing this is described. In such a device, the reference beam is arranged on a circle or ellipse around the object beam in the Fourier plane of the device. To separate the reconstructed object beam from the reflected reference beam, an outcoupling filter is used, which blocks the reflected reference beam and passes the reconstructed object beam through the central aperture.

The main advantage of various coaxial holographic storage systems is that they are insensitive to environmental disturbances, since the object beam and the reference beam propagate along the same optical path. The use of reflective holographic storage media allows to reduce the size of the system compared to the size of the system for the transmissive holographic storage medium since all optical elements are arranged on the same side of the holographic storage medium. Moreover, no additional hardware is needed for shift multiplexing. Accurate movement of the holographic storage medium is sufficient, which can be easily realized by rotating the holographic storage medium.

However, a great challenge for reflective coaxial systems is to increase the signal-to-noise ratio (SNR) by attenuation of different noise propagating on the same axis as the reconstructed object beam. Due to the small diffraction efficiency of the multiplexed hologram, the amount of attenuation required is about 10 ⁻⁴ or 10 ⁻⁵ .

It is an object of the present invention to propose an apparatus for reading from and / or writing to a holographic storage medium in a coaxial arrangement of two or more reference beams and an object beam or a reconstructed object beam, having 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 transmissive holographic storage medium in a coaxial arrangement of two or more reference beams and an object beam or a reconstructed object beam, the reference beam being alone Arranged on a circle around the object beam or the reconstructed object beam in the Fourier plane of the device behind the graphics storage medium, where the mirror is located at or near the Fourier plane, and the mirror does not reflect the object beam or reconstruction Designed to reflect the object beam.

For common aperture systems, it has been found that the reflection of the object beam is important to increase the signal to noise ratio, or to reduce the bit error rate (BER) or symbol error rate (SER). Therefore, it is desirable to maintain a reflective system. However, the reflected reference beam is scattered and diffracted at the surface of the optical component of the system, which results in noise and the need to filter the reflected reference beam.

The present invention solves this problem by using a transmissive 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 beam is output in the Fourier plane immediately after forming the hologram or after reconstructing the object beam. Since the reference beam does not propagate towards the detector, the diffraction noise caused by the reference beam on the detector surface is reduced. At the same time, a relatively simple mirror is located behind the transmissive holographic storage medium. Therefore, system size does not vary significantly compared to reflective systems, where the entire optical element is located on the same side of the holographic storage medium.

Advantageously, the mirror is a circular mirror having a diameter smaller than the diameter of the circle in which the reference beam is arranged. Since the reference beam is arranged around the object beam or the reconstructed object beam, this is sufficient to reduce the diameter of the mirror to the point where the reference beam is no longer reflected. This is preferably accomplished by providing a transparent or absorbent substrate having a reflective coating in the appropriate circular region.

Preferably, 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 to the point where it no longer reflects the higher Fourier component of the object beam or the reconstructed object beam. In this case, the mirror is the reflective equivalent of the pinhole Fourier filter.

Alternatively, the mirror has a dimension larger than the diameter of the circle in which the reference beam is arranged. In this case, the non-reflective area is located at least in the position of the reference beam on the mirror. The non-reflective area is preferably an area without a reflective coating, which can be easily produced or can be a diffractive or refractive structure. In the latter case, the structure allows the reference beam to be directed in the desired direction, for example to control the intensity of the reference beam.

Preferably, the mirror is mechanically or electronically coupled to the objective lens. The apparatus has an objective lens that focuses the object beam and the reference beam into a holographic storage medium. Such objective lenses are generally movable for focusing and tracking, meaning that the position of the object beam or the reconstructed object beam and the reference beam changes. The combination of the objective lens and the mirror ensures accurate positioning of the object beam or the reconstructed object beam and the reference beam onto the mirror.

Advantageously, the device has four reference beams. Mathematical simulations show that this is the optimal number of reference beams.

To better understand the present invention will now be described in more detail in the following description with reference to the drawings. It is understood that the invention is not limited to this exemplary embodiment, and that particular features may also be conveniently combined and / or modified without departing from the scope of the invention.

The present invention solves this problem by using a transmissive 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 beam is output in the Fourier plane immediately after forming the hologram or after reconstructing the object beam. Since the reference beam does not propagate towards the detector, the diffraction noise caused by the reference beam on the detector surface is reduced. At the same time, a relatively simple mirror is located behind the transmissive holographic storage medium. Therefore, system size does not vary significantly compared to reflective systems, where the entire optical element is located on the same side of the holographic storage medium.

A schematic configuration of a known coaxial reflective holographic storage system 1 is shown in FIG. 1. For simplicity, the integrated server system is omitted in the drawings. In an example, the holographic storage system is a 12f system. The laser beam 3 emitted by the laser 2 is expanded by the optical beam expander 4 and separated by the polarizing beam splitter (PBS) cube 6 into the reference beam 7 and the object beam 8. do. Half-wave plate 5 is located between beam expander 4 and PBS cube 6. The laser 2 emits a linearly polarized laser beam 3. By rotating the half-wave plate 5, the polarization direction of the laser beam 3 can be rotated in any direction. The PBS cube 6 separates the laser beam 3 into orthogonally polarized (P and S polarized) laser beams 7, 8. Rotation of the half-wave plate 5 allows to control the beam intensity ratios of the P and S polarized beams, or in other words the intensity ratios at the object and reference arms. In order to optimize the readout diffraction efficiency, it is desirable to optimize the intensity ratio during recording. The object beam 8 is directed onto the reflective SLM 9 by the PBS cube 6. The reflective SLM 9 not only imprints the data page on the object beam 8 but also changes the polarization direction of the object beam 8. In this respect, the reflective SLM 9 serves as a half-wave plate. After reflection from the SLM 9, the object beam 8 passes through the PBS cube 6 and is combined with the reference beam 7. In the optical path of the reference beam 7 is a quarter wave plate 10 and a reflective diffraction beam generator 11. The beam generator 11 reflects two or more well-defined diffraction orders, which are inclined plane waves of circular form propagating in a well-defined direction. Mathematical simulations showed that the optimal number of reference beams was four. Therefore, the beam generator 11 preferably produces four reference beams 7 ', 7 ", of which two reference beams are shown in the figure. Due to the practical realization of the beam generator 11, the zero- The differential diffraction beam appears even though it has a low diffraction efficiency, which beam is suppressed in an additional part of the optical system.

As described above, the object beam 8 and the reference beams 7 ′, 7 ″ are coupled to the main coaxial arrangement by a PBS cube 6. Following this PBS cube 6, the first long focal length There is an objective 12. In this case, a long focal length means that the focal length is long enough to position additional optical components between the lens and the focal point without too much aberration. The objective lens of has the advantage that the manufacturing is simple and requires less optical elements.Furthermore, the diameter of the Fourier plane of the long focal objective lens is large, which is a filter located in the Fourier plane because the manufacturing tolerance is reduced. This first objective lens 12 produces a Fourier transform of the SLM 8 in the rear focal plane, which is the first Fourier plane of the 12f system and the Fourier plane of the SLM 8. First generation Lens 12 and also focuses the multiple reference beams (7 ', 7 ") in the first Fourier plane. The in-coupling filter 13 is located in this first Fourier plane, which low pass filters the object beam 8 and does not rotate the polarization of the zero-order component of the reference beam 7 ', 7 ". It is designed to rotate the polarization of N. The in-coupling filter 13 will be described in more detail below with reference to FIGS.

After passing through the in-coupling filter 13, the object beam 8 and the reference beams 7 ′, 7 ″ pass through the second PBS cube 14. The zero-order component of the reference beam has a different beam ( Because it is orthogonal to 7 ', 7 ", 8, the second PBS cube 14 transmits the low pass filtered object beam 8 and the diffracted reference beam 7', 7", but the reference from the optical system Reflects the zero-order component of the beam The second long focal length objective lens 15 behind the PBS cube 14 reconverts the SLM image onto the intermediate object plane 16 and focuses the focused reference beam 7 ', 7 ") to generate a circular inclined plane wave again. The high NA Fourier objective lens 17 converts the SLM image onto the mirror layer 20 of the holographic storage medium 19 located in the second Fourier plane 30. The position of the high NA Fourier objective lens 17 is adjusted by the actuator 31, which is controlled by the servo circuit 32. During recording, the object beam 8 interferes with the reference beams reflected by the reference beams 7 ′, 7 ″ and the mirror layer 20 directly in the hologram layer 29 of the holographic storage medium 19. During readout, the reconstructed object beam 21 is reconverted onto the intermediate image plane 16 by a high NA Fourier objective lens 17. For better clarity, as in the figure, the reconstructed object beam 21 corresponds to the object beam 8, and reference numerals for the reconstructed object beam 21 are shown behind the PBS cube 14. The quarter wave plate 18 is a high NA Fourier objective lens 17. ) And the holographic storage medium 19. When the beam passes through this quarter-wave plate 18 twice, the polarization direction of the reconstructed object beam 21 is Orthogonal to the polarization direction: The reconstructed object beam 21 is Fourier transformed again by the second long focal length objective lens 15. Due to the rotation of the polarization, P BS cube 14 reflects the reconstructed object beam 21 onto the out-coupling filter 22, which is located in the third Fourier plane of the 12f system. Reference numeral 22 blocks the reflected reference beam so that only the reconstructed object beam 21 is imaged onto the detector array 24 by the third long focal length objective lens 23.

2 shows a cross-sectional view of the in-coupling filter 13 located in the first Fourier plane of the 12f optical system. The filter comprises a beam block 130, for example a thin black metal plate or a transparent substrate with a reflective or absorbing layer, the beam block having a diameter D ₃ for the object beam 8 and the zero-order reference beam. Having a central aperture 132 and a hole 131 having a diameter d for the reference beams 7 ', 7 ". A ring-shaped half-wave plate 133 is arranged on the beam block 130 The ring-shaped half-wave plate 133 has a central aperture 134 having a diameter D _2. The zero-order components of the object beam 8 and the reference beam are such central aperture 134 without any deformation. And also through the central aperture 132 of the beam block 130. The lower aperture for the object beam 8 because the central aperture 132 cuts the higher Fourier component of the object beam 8. Acts as a pass filter. The remaining reference beams 7 ', 7 "pass through a half-wave plate 133, which half-wave plate 133 passes through these beams 7', 7". The polarization directions of the object beam 8 and the reference beams 7 ', 7 "are orthogonal. The half-wave plate 133 in the ring rotates the polarization direction of the diffracted reference beams 7 ', 7 ", while the low energy zero-order component of the reference beam maintains the polarization direction. A hole 131 is arranged for the diffracted reference beams 7 ′, 7 ″ on a ring having a diameter D1 around the central aperture 132. Thus, the filter 13 in the first Fourier plane transmits the zero order components of the reference beam as well as the diffracted reference beams 7 ′, 7 ″, and also the low pass filtered object beam 8. Due to the ring-shaped half-wave plate 133, the polarization direction of the zero-order component of the reference beam is orthogonal to the polarization directions of the other beams 7 ', 7 ", 8. Therefore, the PBS cube 14 behind the filter 13 transmits the low pass filtered object beam 8 and the diffracted reference beams 7 ', 7 ", while rejecting the zero-order component of the reference beam from the optical system. In the figure, the central aperture 132 is circular, which best fits the circular aperture of the lens of the optical setup, but the aperture 132 is also for example a reference beam 7 '. , 7 ") may be elliptical when the vertices are arranged on the elliptical. Moreover, the aperture can also have a square or rectangular shape, which better fits the diffraction image of the SLM 9 with square or rectangular pixels. The aperture 131 for the reference beams 7 ', 7 "may be a switchable aperture. This is advantageous for a special multiplexing scheme.

Figure 3 shows a plan view of the beam block 130 of the in-coupling filter 13 for the case of four reference beams 7 ', 7 ". Holes for the reference beams 7', 7". 131 is arranged on a circle having a diameter D ₁ . The diameter of the central aperture 132 is D ₃ . The difference in diameters D ₁ to D ₃ is about 40 to 100 μm. The diameter d of the holes 131 for the reference beams 7 ', 7 "is about 10 to 100 [mu] m. Of course, the number of reference beams 7', 7" is four reference beams 7 ', 7 ").

4 shows a schematic configuration of a coaxial reflective holographic storage system 1 according to the invention. System 1 is essentially the same as the system of FIG. However, instead of the reflective holographic storage medium 19 with the reflective layer 20, a transmissive holographic storage medium 19 with a transparent substrate 25 is used. The high NA Fourier objective lens 17 converts the SLM image onto the second Fourier plane 30 via the transmissive holographic storage medium 19. A specially shaped mirror 27 arranged on the transparent substrate 26 is located in this second Fourier plane 30. The mirror 27 has a circular shape with a diameter essentially the same as the diameter of the low pass filtered object beam 8. This means that the mirror 27 is a reflective equivalent of the Fourier filter aperture. The mirror 27 reflects the object beam 8, but the focused reference beams 7 ', 7 "leave the system near this mirror 27 through the transparent substrate 26. Similarly, the transparent substrate It is possible to use an absorbing substrate instead of 26. It is necessary to ensure that the reference beams 7 ', 7 "are not reflected back to the system. When the high NA Fourier objective lens 17 is moved by the actuator 31, the mirror 27 needs to follow this movement, ie the high NA Fourier objective lens 17 and the mirror 27 with respect to each other. It is fixed. This is accomplished by mechanically coupling the mirror 27 and the high NA Fourier objective 17, or by providing an additional actuator 33 to the mirror 27. This additional actuator 33 is controlled by the servo circuit 32 of the high NA Fourier objective lens 17 or by the additional servo circuit 34. The additional servo circuit 34 preferably uses transmitted reference beams 7 ′, 7 ″ to control the position of the mirror 27.

An enlarged side view of the object beam 8 and the reference beams 7 ', 7 "at the position of the holographic storage medium 19 is shown in Fig. 5. During the recording of the hologram, the direct and reflected object beams 8 Overlaps the directly focused reference beams 7 ′, 7 ″, and forms an interference pattern (hologram) in the storage material 29.

During recording, only the reference beams 7 ′, 7 ″ illuminate the hologram. The reconstructed direct and reflected object beam 21 is reconverted onto the intermediate image plane 16 by the high NA Fourier objective lens 17. Since the focused reference beams 7 ', 7 "are not reflected by the mirror 27, there are no reflected reference beams.

A plan view of the circular mirror 27 is shown in FIG. 6. The mirror 27 is arranged on the transparent or absorbent substrate 26. The small circular area 28 represents the position of the four reference beams 7 ', 7 "on the substrate 26. As can be seen, the reference beams 7', 7" do not impinge on the mirror 27. Do not.

Instead of the small circular mirror 27, it is likewise possible to use larger mirrors with holes for transmitting the reference beams 7 ', 7 ". This is shown in Figure 7. Reference beams 7', 7". Except for location 28, the entire substrate 26 has a reflective coating. In this case, however, the mirror 27 does not serve as a Fourier filter for the object beam 8. Furthermore, instead of allowing the reference beam to pass through the substrate 26 so that the reference beams 7 ', 7 "are output from the system, the reference beams 7', 7" are also referred to as reference beams 7 ', 7 May be output using a diffractive or refracting structure, such as a grating or prism, at location location 28 of "

In order to avoid coupling the high NA Fourier objective lens 17 and the mirror 27, it is possible to increase the size of the mirror 27. This is shown in FIG. If the distance between the object beam 8 or the reconstructed object beam 21 and the reference beams 7 ', 7 "is not too small, slight movement of the high NA Fourier objective lens 17 need not be compensated. The object beam 8 or the reconstructed object beam 21 remains on the mirror 27, while the reference beams 7 ′, 7 ″ still do not hit on the mirror 27.

9 shows a further improved solution for the mirror 27, which allows for greater displacement of the high NA Fourier objective lens 17. In this case, the mirror is further enlarged but has a specific cross section with respect to the reference beams 7 ', 7 ".

As noted above, the present invention relates to an apparatus for reading from and / or writing to a holographic storage medium, wherein the invention relates to reading and / or from a holographic storage medium having two or more reference beams with an improved signal to noise ratio. It is used for the coaxial type apparatus for recording on this.

1 is a schematic diagram illustrating a known coaxial reflective holographic storage system configuration.

2 shows a cross-sectional view of an in-coupling filter with a half-wave plate in the form of a ring.

3 is a plan view of the in-coupling filter in the case of four reference beams.

4 is a schematic diagram illustrating a configuration of a coaxial reflective holographic storage system in accordance with the present invention.

5 is an enlarged side view of the object beam and the reference beam at the location of the holographic storage medium.

6 is a plan view of a circular mirror located in a Fourier plane behind the holographic storage medium.

FIG. 7 illustrates an alternative solution to a mirror located in the Fourier plane behind the holographic storage medium.

8 shows a further solution for the mirror located in the Fourier plane behind the holographic storage medium.

9 illustrates another solution for a mirror located in a Fourier plane behind a holographic storage medium.

Claims (8)

Apparatus for reading from and / or writing to a transmissive holographic storage medium 19 having a coaxial arrangement of at least two reference beams 7 ′, 7 ″ and an object beam 8 or a reconstructed object beam 21. As reference, the reference beams 7 ′, 7 ″ are arranged on a circle around the object beam 8 or the reconstructed object beam 21 in the Fourier plane of the device 1 behind the holographic storage medium 19. Device for reading from and / or writing to a transmissive holographic storage medium 19 The mirror 27 is located at or near the Fourier plane, and the mirror 27 reflects the object beam 8 or the reconstructed object beam 21 without reflecting the reference beams 7 ′, 7 ″. Designed for reading from and / or writing to a transmissive holographic storage medium. 2. The reading and / or writing from a transmissive holographic storage medium according to claim 1, wherein the mirror 27 is a circular mirror having a diameter smaller than the diameter of the circle on which the reference beams 7 ', 7 "are arranged. Device for 3. Apparatus according to claim 2, wherein the mirror (27) is a Fourier filter for an object beam (8) or a reconstructed object beam (21). 4. Apparatus according to claim 2 or 3, wherein the mirror (27) is arranged on a transparent or absorbent substrate (26). 2. The mirror 27 according to claim 1, wherein the mirror 27 has a dimension larger than the diameter of the circle in which the reference beams 7 ', 7 "are arranged, and the non-reflective area is at least on the mirror 27 7 ', 7 "), an apparatus for reading from and / or writing to a transmissive holographic storage medium. 6. The apparatus of claim 5, wherein the non-reflective region is a region that does not have a reflective coating or a diffractive or refractive structure. 6. Apparatus according to any one of the preceding claims, wherein the mirror (27) is mechanically or electrically coupled to the objective lens (17). 8. Device according to any of the preceding claims, wherein the device has four reference beams (7 ', 7 ").

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