# US 6,191,875 B1 · Process for holography using reference beam having correlated phase content

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- Number: US 6,191,875 B1
- Document ID: US6191875B1
- Status: Granted
- Kind: B1
- Application number: 09/363,336
- Filed: 1999-07-29
- Granted: 2001-02-20
- Earliest claimed: 1999-07-29
- Assignees: Lucent Technologies Inc.
- Inventors: Kevin Richard Curtis; Peter B. Littlewood; Partha Pratim Mitra; William Larry Wilson
- Web page: https://patentgrep.com/patent/US6191875B1

## CPC

- G11C 13/042: {using information stored in the form of interference pattern}
- G03H 2001/2675: {Phase code multiplexing, wherein the sub-holograms are multiplexed according to spatial modulation of the reference beam}
- G11C: STATIC STORES (semiconductor memory devices H10B)
- G03H: HOLOGRAPHIC PROCESSES OR APPARATUS (holograms, e.g. point holograms, used as ordinary optical elements G02B5/32; producing stereoscopic or other three-dimensional effects G02B30/00; diffraction-grating systems G02B27/44; systems using moiré fringes G02B27/60; optical logic elements G02F3/00; stereo-photography G03B35/00; photosensitive materials or processes for photographic purposes G03C; {stereo-photographic or similar processes G03C9/00}; apparatus for processing exposed photographic materials G03D; analogue computers performing mathematical operations with the aid of optical elements G06E3/00; authentication by radiation, of concealed information carried by holograms or diffraction gratings G06K19/16; holographic storage G11B7/0065, G11C13/04; {stereoscopic or other three dimensional effects in television systems H04N13/00})

## Abstract

An improved process for holography is provided, in which the reference beam used for storage and readout has a correlated phase content. In particular, it was found that reference beams having random phase content limited the attainable storage density and the accuracy of readout of stored holograms due to uncontrolled fluctuations in the correlation selectivity. By contrast, a reference beam having correlated phase content provides accurate shift selectivity, the ability to tailor the reference beam spectrum to minimize crosstalk between neighboring holograms, and mitigation of limitations inherent in the holographic system or holographic medium. Improved storage density and readout accuracy are thereby attained.

## Claims

### Claim 1

A process for holography, comprising the steps of:
(a) directing a reference beam at a holographic medium;
(b) providing relative movement between the reference beam and the medium; and
(c) repeating steps (a) and (b) at least once,
wherein the reference beam contains the same correlated phase content for all repetitions of steps (a) and (b).

### Claim 2 (depends on 1)

The process of claim 1, wherein the correlated phase content is provided by passing the beam through a linearly chirped phase mask.

### Claim 3 (depends on 1)

The process of claim 1, wherein the correlated phase content is provided by passing the beam through a mask having pixels of varying phase values.

### Claim 4 (depends on 1)

The process of claim 1, wherein the correlated phase content is provided by passing the beam through an asymmetric phase mask.

### Claim 5 (depends on 1)

The process of claim 1, wherein the correlated phase content is provided by passing the beam through a cylindrical lens and a phase mask selected from an asymmetric mask and a linearly chirped mask.

### Claim 6 (depends on 1)

The process of claim 1, wherein the correlated phase content is provided by at least one of a point source array and a lenslet array.

### Claim 7 (depends on 1)

The process of claim 1, wherein the correlated phase content is provided by filtering the beam optically in the Fourier plane.

### Claim 8 (depends on 1)

The process of claim 1, further comprising the step of directing an object beam at the medium such that the object beam and the reference beam intersect on or within the medium.

### Claim 9 (depends on 1)

The process of claim 1, wherein the holography comprises phase correlation multiplexing.