GB2046467A - Holographic digital data recording - Google Patents

Abstract

Pages of digital data, e.g. 50 to 100 bits each, are recorded as holograms formed by modulating individual digit- beams of coherent light, combining the digit-beams and a reference beam and scanning the holograms relative to a film to record each hologram in a different position. The digit beams pass simultaneously, parallel to each other through a page composer where they are modulated. In order to keep the spatial frequency bandwidth of the holograms low with low variance in the light intensity distribution at the hologram plane, the digit beams (squares with crosses) are clustered around the reference beam axis (open square) so as to be irregularly arranged in two coordinates in all four quadrants and with no point symmetry between beam pairs. The Figure shows a 50-beam arrangement with digit-beams of cross- sectional dimension d at a minimum pitch of 2d and randomly located on a grid of points defined by the (x,y) coordinates 2(m+1 DIVIDED 2)d, 2(n+1 DIVIDED 2)d, where m.n are integers. The beams are preferably converged through a focus where a field stop delimits the radiation, allowing close hologram packing on the film. The scanning movement is compounded of continuous film movement and transverse deflection of the combined beams by an electro-optical device, which moves the image position stepwise, allowing use of a continuous wave laser as the coherent light source. <IMAGE>

Description

SPECIFICATION Digital data recording The present invention relates to holographic digital recording apparatus wherein a plurality of digitbeams of coherent light are individually modulated in accordance with bits of information being recorded and combined with a reference beam to form a hologram, a plurality of the holograms being recorded on film by relative scanning movement such that the holograms are recorded on areas individual thereto.

As is well known either amplitude or phase holograms can be employed and the choice therebetween will determine the form of modulation. It is convenient to refer to the group of bits generating a hologram as a page of data and the device wherein the beams are modulated is thus known as a page composer.

This invention is particularly concerned with the problems which arise when recording digital data, at high transfer rates, for instance for recording television signals in real time.

A television signal, coded in binary digits (bits), requires information transfer rates in the region of 108 bits/sec for the recording and replay of high quality signals in real time. Although conventional analogue recorders using magnetic tape can be adapted for direct digital recording there is a significant increase in tape consumption. Optical methods, on the other hand, offer a greater information storage density, and are attractive for archival storage.

Proposals have been made for read-only optical memories in computer applications, which are based on laser sources of light and holographic recording techniques and in which pages of binary data are assembled and recorded as Fourier transform holograms on a photographic plate. To take full advantage of their inherent properties, the holograms need to occupy at least 1 mm2 of medium, which means a large number of bits (e.g. 104) per hologram if an acceptable storage density is to be achieved. A substantial reduction has to be made in the number of bits per hologram and the hologram size, when the method is applied to television signals, because of the difficulty of constructing page composers and photodetector arrays capable of handling the high data-transfer rates involved.

One possible arrangement for a holographic recorder, using low-power laser sources, is disclosed in our British Patent Specification No 1,383,323. A wide-band holographic recorder is described by BARDOS, A.M., in Applied Optics, Vol 13, page 832, 1974, which, although not designed for television, is capable of writing at speeds greater than 108 bits/sec (viz 4 x 108 bits/sec) although the reading speed is an order of magnitude lower. Both of these arrangements are based on photographic film as the recording media, with a continuous reel-to-reel transport, but use mechanical devices for writing (and reading) rows of holograms.

The recorder described by Bardos uses a line of 128 beams in the page composer. Such an arrangement demands a high spatial frequency bandwidth (SFB) of the film and leads to problems with interferring beams formed from inter-modulation products in the holographic processing. Our prior specification 1,383,323 utilizes very small "pages" of 7 or 9 bits (each "page" merely being a digitization of one picture element) but arranges the beams in two regular rows on a pitch of h, the rows being spaced 2h and 3h respectively either side of the central axis. This is done to reduce the SFB to say less than 200 cycles/mm.

The object of the present invention is to provide apparatus which enables a reasonably low SFB to be maintained when using larger numbers of bits per page, e.g. 50 to 100 or more bits per page.

The present invention is characterised in that the beams of light in the page composer are clustered irregularly about a central axis of the reference beam, so that in each of four quadrants about the axis there are at least ten beams distributed irregularly in two dimensions and there are no pairs of beams related by point symmetry through the axis.

The reference beam does not have to pass through the page composer but may join the axis downstream of the page composer.

Although the term light is used for simplicity, the radiation employed can be outside the visible spectrum. E.g. ultra violet light maybe employed.

All the beams are preferably converged through a focus before being refocused on to the film. A field stop at the focus can then delineate the boundary of the light forming the hologram. The holograms can therefore be packed close together without producing inter-hologram cross-talk.

The scanning movement is preferably compounded of continuous film movement and transverse deflection of the combined beams by an electrooptical device. Apart from the advantage of having no mechanical moving parts apart from the film transport mechanism, the image position can be moved stepwise with an electro-optical device. A simple continuous-wave laser can therefore be used as the light source whereas in the abovementioned prior art systems using mechanical scanning (vibrating or rotating mirror) the laser has to be pulsed to form images only on the correct film areas, without smearing from one image to the next.

The holograms recorded on the film can be about 50 x 50 lim arranged in rows across the film of 128 holograms. This is roughly equivalent to storing one television line per row and for such an application and with guard bands of 10 um left between rows, a film speed of 0.94 m/s is needed to record 108 bits/s in real time.

In the replay process, the recorded information is recovered by scanning the record with a single iaser-beam focused to approximately the same size as the individual holograms. On the far side of the record, a diffraction pattern appears as the beam intercepts a hologram which is a replica of the digit-beam illumination pattern existing at the time of recording that particular hologram. The reconstructed beams in the diffraction pattern are intercepted by a spatial array of photodetectors with a geometrical disposition similar to that of the beams in the page composer. From the photodetector outputs, a decision is made as to the absence or presence of a digit-beam at any instant and thus the original digital signal is reconstructed.

The invention will now be described in more detail, by way of example, with reference to the drawings, in which: Figure 1 is a side view of one form of recording apparatus; Figure 2 is a side view of one form of replay apparatus; and Figure 3 illustrates one particular page array.

Figure 1 refers to recording. The linearly polarised output-beam from a low-power, continuous-wave laser 10 passes through polarisation switch 12 and is split by means of a partially reflecting mirror 14 into two paths. In one path S, the beam is redirected parallel to the reference beam R by a mirror 16 and expanded and re-collimated by a lens system 18 before impinging on an electro-optic page-composer 20. The latter forms, by means of afocally matched arrays of small lenses 21 and 22 or aperture masks, an array of collimated beams. The cross-section of the output array is an illumination pattern which conforms to a prescribed geometrical arrangement which will be described in more detail below. Each beam passes through an electro-optic modulator or switch.

In the example shown, an electro-optic ceramic material PLZT (polycrystalline lead zirconium titanate doped with lathanum) is used for the modulator in the form of a thin plate 24. The individual beams are focused on the surface of the plate at which points small pairs of metal electrodes have been deposited. A voltage applied to an electrode-pair generates a transverse electric field in the gap between the electrodes through which the beam passes. The field produces birefringence and, by correct adjustment of the voltage, the polarisation direction of the incident light can be rotated through 900. Thus by connecting each pair of electrodes to electronic switches controlled by the incoming digital television signal, via buffer stores, the polarisation states of the emerging beams can be set correspondingly.When a complete page has been composed, its hologram is exposed at a selected location on the recording film and the digit-beam states are reset for the next page, and so on.

Further details of such a device are given in Waterworth P and Chilton G A A, Integrated electrooptic modulator arrays. Opto-electronics, 1972,4, pp 339-340. It is arranged that the normal beam polarizations are at 90 to the reference beam. An digit-beam in this state is effectively "off" and does not participate in hologram formation. Each 1-bit causes a voltage (e.g. 200-250v) to be applied across the corresponding electrodes to rotate the polarization vector into alignment with that of the reference beam.

The beam transmitted by the partially reflecting mirror 14 forms the reference-beam Rand, by means of a mirror 26 and a small prism 28 located on the central axis is recombined with the signal-beam array just prior to the hologram-lens 30. From this point on, the reference-beam propagates along the optical axis of the system with the digit-beams clustered around it.

At the focus of the hologram-lens, where the beams all superpose, is placed a small aperture in an opaque screen 32. This aperture acts as a field-stop and rigidly delineates the boundary of the hologramforming radiation. The field-stop prevents mutual interference between neighbouring holograms in the recorded array due to overlap of diffraction sidelobes or wavefront perturbations in the pagecomposer. In this sense, the aperture acts also as a "clean up" stop.

Two lenses 34 and 36 form a simple relay system whereby the field-stop aperture is re-imaged on to recording film Fwhich is being continuously transported over a capstan drum 38. In the region between the lenses 34 and 36, where the digit-beams are collimated, is placed a single-axis, wide-aperture (20 mm x 20 mm), digitally controlled light-deflector 40. The single deflection axis is transverse to the direction of film transport.

By way of example, a suitable deflector would be a solid-state type using electro-optic polarisation switches and Wollaston prisms as described by NELSON, T J in Bell Syst. Tech. J 1961, Vol 43, pages 821 to 845. If there are N stages in the deflector, there are 2N discrete output positions at which holograms can be recorded. For rows of 128 holograms N has to be 7. The positions are selected by applying the appropriate voltages to the polarisation switches.

The voltages are applied via fast electronic switches which are synchronised with those used in the page composer.

In more detail, the digital light-deflector comprises a number of deflection stages in tandem, each stage consisting of a Wollaston prism preceded by an electro-optic polarisation switch. The prism deviates the incident digit-beams through a fixed angle, +B, the sign depending on the direction of polarisation.

This direction can be rotated through 90 by applying a suitable voltage (the half-wave voltage) to the electro-optic switch. If the deviation angle of each stage is made precisely twice that of the preceding stage, a deflector with 2N angular output positions can be constructed, where N is the number of stages.

Regular scanning is achieved by driving each stage with a square-wave voltage whose period is proportional to its angle of deviation.

Apart from the advantage of no mechanical movements, the digital light-reflector 40 avoids the need to use a pulsed laser source because there is no relative movement in the scanning direction between the radiation and the recording film.

Figure 2 refers to an arrangement for recovering the recorded information (replay). A separate lowpower laser source 42 is used, which need not be linearly polarised, and the output beam is slightly expanded and collimated by a lense system 44. An acousto-optic deflector 46, driven by an r.f. sweep frequency generator, causes the laser beam to scan repeatedly through a suitable angular range. A lens 48 near the deflector focuses the beam on the film-record F so that a complete row of holograms is interrogated for each scan. The reconstructed page of digit-beams, formed as the scanning beam coin cides with each hologram in turn, comprises a bundle of light beams diverging from a lens 50. The beams are intercepted by an optical probe 52.This includes a bundle of monofilament light guides 54 of which the input ends are formed into a planar array replicating the page geometry, while each of the other ends is optically coupled to a discrete photodetector 56. The output signals from the photoderectors are amplified and then fed into levelcomparator circuits (not shown) from which the original data stream is reconstituted.

The lens 50 is a small field-lens placed close to the film-record in order to avoid a lateral shift of the reconstructed digit-beams, while scanning over a finite range. By this means the digit-beam array is held in register with the optical detector probe 52.

The principal feature of the hologram pagecomposer is the digit-beam arrangement in relation to the central reference-beam (i.e. the pageorganisation). In terms of the spatial-frequency bandwidth of the system, the wanted spectrum extends almost down to zero frequency, represented by the central reference-beam. Morever, the crosssection of the digit-beam illumination pattern is arranged so that the inter-modulation products, formed inherently in the holographic processing, give rise to unwanted digit-beams in the reconstruction which interleave with the wanted beams and can therefore be spatially isolated; the fall on the gaps between the fibres 54.

Figure 3 shows one particular arrangement for a full page of fifty digit-beams, represented by square with crosses therein. The digit-beams of dimension d have a minimum pitch of 2d, with their axes randomly located on a grid of points defined by the (x,y) coordinates 2(m+1/2)d, 2(n+)d, where m, n are non-zero integers. If a location in one quadrant is occupied, its twin position in the opposite quadrant, as reflected in the origin, is left unoccupied. The reference-beam, represented by a square with no cross therein, is located at the origin and its intensity is much greater than that of the digit-beams (which are all fifty split out of one half of the light from the beam splitter 14).The random spacing of beams in the array leads to a smaller variance in the light intensity distribution of the hologram plane, and reduces distortion due to the limited dynamic transfer characteristics of the recording film.

Using 50 to 100 bits per hologram, and a 7 or 8 stage digital light-deflector, the arrangements illustrated are suitable for recording digital television signals at transfer rates up to 108 bits/sec. On high resolution recording film, information packing densities approaching 107 bits/cm2 are believed to be feasible.

The system has a number of advantages, including the following: 1. The spatial-frequency bandwidth is low, typically less than 500 cycles/mm, which reduces problems due to environmental vibration and irregularities of film motion.

2. Only ordinary low-power laser sources are required operating in a continuous-wave mode.

3. There are no mechanically moving parts (apart from the film drive), and the system is potentially rugged and maintenance free.

4. Holograms can be closely packed in the array, with a minimum of cross-talk; their positions in the record are precisely determined by the optical design of the deflector, and their boundaries by the field-stop aperture.

5. The replay apparatus is simple, and the means for detection using fibre light-guides can be adapted to suit various arrangements of the digit-beams in the page. Naturally conventional servo techniques must be used to synchronise the film speed and scanning rate in the replay apparatus and to keep the scanning spot centred along the rows of holograms.

Claims (7)

1. Holographic digital recording apparatus wherein a plurality of digit-beams of coherent light are individually modulated in a page composer in accordance with bits of information being recorded and combined with a reference beam to form a page hologram, a plurality of the holograms being recorded on film by relative scanning movement such that the holograms are recorded on areas individual thereto, and wherein the beams of light in the page composer are clustered irregularly about a central axis of the reference beam, so that in each of four quadrants about the axis there are at least ten beams distributed irregularly in two dimensions and there are no pairs of beams related by point symmetry through the axis.

2. Apparatus according to claim 1, wherein the digit-beams of dimension dare arranged in crosssection on selected points of a grid of points defined by (x,y) coordinates 2 (m+)d, 2(n+1/2)d, where m,n are non-zero integers, where the central axis is on the orin of the coordinates and 2d is the minimum pitch between beams.

3. Apparatus according to claim 1 or 2, wherein the reference beam and a signal beam are provided from a source of coherent light and the signal beam is expanded to form simultaneously all of the digit-beams which are of substantially smaller intensity than the reference beam.

4. Apparatus according to claim 1,2 or 3, wherein the digit-beams and reference beam are converged through a focus before being refocused on to the film and wherein a field stop at the said focus delineates the boundary of the light forming the hologram.

5. Apparatus according to claim 1 to 4, wherein the scanning movement is compounded of continuous film movement and transverse deflection of the combined beams by an electro-optical device.

6. Apparatus according to claim 5, wherein the electro-optical device is digitally controlled to effect stepwise deflection.

7. Holographic digital recording apparatus substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 3 of the accompanying drawings.