WO2006064427A1

WO2006064427A1 - Holographic device

Info

Publication number: WO2006064427A1
Application number: PCT/IB2005/054130
Authority: WO
Inventors: Teunis Tukker; Gert 't Hooft; Frank Schuurmans
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-12-15
Filing date: 2005-12-08
Publication date: 2006-06-22
Also published as: TW200630762A

Abstract

An optical holographic device comprises means for receiving a holographic medium (101, 203), a pixelated element (103, 201) and a lens (102, 202) between the pixelated element and the receiving means. It further comprises a field flattener (104, 204) between the lens and the pixelated element. The field flattener is arranged in such a way that it compensates for the curvature aberrations of the lens.

Description

Holographic device

FIELD OF THE INVENTION

The present invention relates to an optical holographic device for recording in and/or reading out a data page from a holographic medium.

BACKGROUND OF THE INVENTION

An optical device capable of recording on and/or reading from a holographic medium is known from William L. Wilson, K. Curtis, M. Tackitt, A. Hill, A. Hale, M. Schilling, C. Boyd, S. Campbell, L. Dhar and A. Harris, "High density, high performance optical data storage via volume holography : viability at last?" in Optical and Quantum Electronics 32: 393-404, 2000, Kluver Academic Publishers.

During recording of a data page in a holographic medium, the data page is encoded in a spatial light modulator, and a signal beam that passes through the spatial light modulator is imaged on the holographic medium by means of a lens. Interference with a reference beam in the holographic medium causes the data page to be recorded in said holographic medium. During reading of a data page from the holographic medium, the reference beam is diffracted by the data page, and the resulting beam is imaged on a detector array by means of a lens. As a consequence, an optical holographic device comprises means for receiving a holographic medium, a pixilated element and a lens between said pixilated element and said receiving means. The pixilated element is either the spatial light modulator or the detector array.

In the holographic device mentioned hereinbefore, the lens is a photographic lens. Such a lens is described, for instance, in patent US 3,948,584. Such a lens is diffraction limited, which is needed in the optical holographic device. However, such a photographic lens is bulky and expensive.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a holographic device which is less bulky and less expensive. To this end, the invention proposes an optical holographic device comprising means for receiving a holographic medium, a pixilated element, a lens between said pixilated element and said receiving means and a field flattener between the lens and the pixilated element, said field flattener being arranged in such a way that it compensates for the curvature aberrations of the lens. Although a photographic lens is used in the prior art, much of the performance of said photographic lens is not needed. Actually, a photographic lens is designed to be aberration-free, whatever the wavelength of the imaged beam. In a holographic device however, the bandwidth used is narrow. Moreover, a photographic lens is able to image a point, whatever the distance between this point and the photographic lens. In a holographic device however, the distance between the points to be imaged and the lens is fixed. According to the invention, the photographic lens is replaced by a single lens and a field flattener. Hence, only two elements are needed, instead of at least five when a photographic lens is used. This makes the holographic device less bulky and less expensive.

Advantageously, said lens is aspheric. An objective lens that is manufactured with the same technology and equipment as objective lenses used in optical scanning devices such as CD or DVD or BD devices, may be used.

Preferably, said field flattener is spherical and, more preferably, it is plano-concave. These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which :

- Fig. Ia shows a holographic read-out device in accordance with the invention and Fig. Ib shows the same device without a field flattener;

- Fig. 2a shows a holographic recording device in accordance with the invention and Fig. 2b shows the same device without a field flattener.

DETAILED DESCRIPTION OF THE INVENTION Fig. Ia shows a holographic read-out device in accordance with the invention, and

Fig. Ib shows the same device without field flattener. Both devices comprise a holographic medium 101, a lens 102 and a pixilated detector array 103. The holographic device of Fig. Ia further comprises, between the lens 102 and the pixilated detector array 103, a field flattener 104. The holographic device further comprises, at the position of the holographic medium 101, means for receiving said holographic medium 101, which receiving means are not shown. These receiving means are, for example, a table on which the recording medium can be put. A table such as those conventionally used in CD or DVD players can be used for example. A read-out beam, represented by arrows in Fig. Ia and Ib, is diffracted by a data page recorded in the holographic medium 101. This leads to a flat wavefront between the holographic medium 101 and the lens 102. The lens 102 is a single lens, such as an aspherical lens. Such a lens has curvature aberrations, which means that a flat wavefront is imaged on a curved focal plane of the lens 102. This is illustrated in Fig. Ib, where the curved focal plane of the lens 102 is shown in dotted line. As a consequence, as can be shown from Fig. Ib, the pixels of the data page which are not located on the optical axis of the lens 102 are imaged as wide areas on the pixilated detector array 103. Hence, the image on the pixilated detector array 103 is blurred and the detection of the data page is difficult. This problem is solved in that a field flattener 104 is placed between the lens 102 and the pixilated detector array 103. The field flattener 104 is designed in such a way that it compensates for the curvature aberrations of the lens 102. In the example of Fig. Ia, the field flattener 104 is a plano-concave lens placed on top of the pixilated detector array 103. However, any field flattener may be placed between the lens 102 and the pixilated detector array 103, as soon as it compensates for the curvature aberrations of the lens 102. As can be seen from Fig. Ia, the data page is imaged as a flat wavefront on the pixilated detector array 103, which makes detection of said data page much easier.

Fig. 2a shows a holographic recording device in accordance with the invention, and Fig. 2b shows the same device without field flattener. Both devices comprise a pixilated spatial light modulator 201, a lens 202 and a holographic medium 203. The holographic device of Fig. 2a further comprises, between the pixilated spatial light modulator 201 and the lens 202, a field flattener 204. The holographic device further comprises, at the position of the holographic medium 203, means for receiving said holographic medium 203, which receiving means are not shown. These receiving means are, for example, a table on which the recording medium can be put. A table such as those conventionally used in CD or DVD players can be used for example.

A signal beam, represented by arrows in Fig. 2a and 2b, is diffracted by individual pixels of the pixilated spatial light modulator 201, which form a data page that has to be recorded in the holographic medium 203. This leads to diffraction beams which reach the lens 202, as can be seen from Fig. 2b. In order to record a diffraction- limited hologram in the holographic medium 203, flat wavefronts have to interfere with the reference signal in said holographic medium 203. Now, due to curvature aberrations in the lens 102, the wavefronts after the lens 102 are not flat, as can be seen in Fig. 2b This problem is solved in that a field flattener 204 is placed between the pixilated spatial light modulator 201 and the lens 202. The field flattener 204 is designed in such a way that it compensates for the curvature aberrations of the lens 102. In the example of Fig. 2a, the field flattener 104 is a plano-concave lens placed on top of the pixilated spatial light modulator 201. However, any field flattener may be placed between the pixilated spatial light modulator 201 and the lens 202, as soon as it compensates for the curvature aberrations of the lens 202. As can be seen from Fig. 2a, flat wavefronts are generated after the lens 202, which makes the holograms in the holographic medium 203 diffraction-limited.

Measurements have been carried out to show that the recorded holograms are diffraction- limited. The lens 202 is made of COC and has a center thickness of 3mm and a pupil diameter of 20 mm. Its surface sags are described with their radii and conies, namely : Rl=42.8mm, kl=-0.918, R2=-70.3mm and k2=-4.36. The field flattener 204 is a planoconcave lens with a radius of -26mm, made of BK7 glass, and has a thickness of 1 mm at the optical axis. The focal length of the system formed by lens 202 and field flattener 204 is 50mm. The distance between the lens 202 and the holographic medium 203 is 50mm. Holograms have been formed at a wavelength of 532nm, with a pixilated spatial light modulator 201 having 1024*1024 pixels, each having a size of lOμm. It has been measured that the mean wavefront error in this case is 7mλ, whereas the diffraction limit is about 70mλ. Hence, the holographic device of Fig. 2a is diffraction limited.

Any reference sign in the following claims should not be construed as limiting the claim. It will be obvious that the use of the verb "to comprise" and its conjugations does not exclude the presence of any other elements besides those defined in any claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

1 An optical holographic device comprising means for receiving a holographic medium (101, 203), a pixilated element (103, 201), a lens (102, 202) between said pixilated element and said receiving means and a field flattener (104, 204) between the lens and the pixilated element, said field flattener being arranged in such a way that it compensates for the curvature aberrations of the lens.

2 An optical holographic device as claimed in claim 1, wherein said lens is aspheric.

3 An optical holographic device as claimed in claim 1, wherein said field flattener is spherical.

4 An optical holographic device as claimed in claim 3, wherein said field flattener is plano-concave.

5 An optical holographic device as claimed in claim 1, wherein said pixilated element is a spatial light modulator (201). 6 An optical holographic device as claimed in claim 1, wherein said pixilated element is a detector array (103).