Apparatus for direct-to-digital spatially-heterodyned holography

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of holography. More particularly, the present invention relates to a direct-to-digital hologram acquisition and replay system (i.e., no film, no plates). In a preferred implementation of the present invention, the hologram acquisition is based on a charge coupled device (CCD) camera. The present invention thus relates to a holographic system.

2. Discussion of the Related Art

Traditional methods of holography have used film or holographic plates (glass plates with a photographic emulsion optimized for holography) to record the hologram..sup.(1) Replay has only been possible using lasers (or in some cases white light) and the original recorded hologram or a duplicate of it, in an analog method. These analog methods are slow, cumbersome, and expensive..sup.(4) There is also no way to reduce them to electronic signals that can be transmitted and replayed at another location. It is always necessary to send hard copy. Worse still, the time delay involved in processing the film prevents the use of holography and its variants in many situations. Even if the expense of the classical holographic system itself was tolerable, the time delay and low throughput caused by the necessity of processing the film, introduces expenses associated with the delay that are absolutely intolerable (e.g., a tire manufacturer cannot wait 45 minutes, or even two minutes, to know that a particular tire has a flaw in it).

Referring to FIG. 1, a classical side-band holography system recordation geometry is shown..sup.(2-3) Light from a laser 110 is expended by a beam expander 120. After passing through a lens 130, the light is split into two components by a beamsplitter 140. The beamsplitter 140 can be, for example, 90% reflective. The reflected beam constituting an object beam 150 travels toward and is reflected by a mirror 160. The object beam 150 then travels toward an object 170. The object beam 150 is then incident upon a holographic plate 190.

Meanwhile, that portion of the light from lens 130 that is transmitted through the beamsplitter 140 constitutes a reference beam 180 that travels toward and is reflected by a mirror 200. The reflected reference beam is then incident upon the holographic plate 190.

More recently, holographic interferometry has been developed, albeit also as an analog method..sup.(5) This has included the development of focused holography..sup.(6-7)

Within this-application several publications are referenced by superscripts composed of arabic numerals within parentheses. Full citations for these, and other, publications may be found at the end of the specification immediately preceding the claims. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference into the present application for the purposes of indicating the background of the present invention and illustrating the state of the art.

SUMMARY OF THE INVENTION

Therefore, there is a particular need for a method for 1) recording holograms directly to a CCD (charged coupled device) camera or any other suitable video camera with a digital computer interface and then 2) storing the holograms to a digital storage medium (e.g., RAM, hard drive, tape, recordable CD, etc.). Significant features of an apparatus for implementing this method include the use of a very small angle between the reference beam and object beam and focusing the hologram on the image plane to simplify the image. Additionally, the invention includes 1) a method of displaying the hologram phase or amplitude on a two-dimensional display and 2) a method of replaying the holograms completely using an optically active crystal and lasers. In contrast, the prior art does not include a description of how to electronically (digitally) record an optical hologram, much less replay, or broadcast an optical hologram.

The improvements disclosed herein allow for higher quality, lower-noise digital hologram acquisition and replay. The improvements make use of variations in the geometry and optical components to allow the acquisition and analysis of high resolution holograms. In addition, improvements to the replay system have been made that allow writing of a digital grating (hologram) to a photorefractive crystal, and then the replay of that grating or hologram with a single laser beam.

One embodiment of the invention is based on an apparatus to record an off-axis hologram, comprising: a laser; an illumination beamsplitter optically coupled to said laser, an objective lens optically coupled to said illumination beamsplitter; an object optically coupled to said objective lens; a reference beamsplitter coupled to said laser; a reference mirror optically coupled to said reference beamsplitter; a beam combiner optically coupled to both said reference beamsplitter and said illumination beamsplitter; and a digital recorder optically coupled to said beam combiner, wherein a reference beam and an object beam are combined at a focal plane of said digital recorder to form an off-axis hologram, and said object beam and said reference beam constitute a plurality of substantially simultaneous reference and object waves. Another embodiment of the invention is based on a method of recording an off-axis hologram, comprising: splitting a laser beam into an object beam and a reference beam; reflecting said reference beam from a reference beam mirror; reflecting said object beam from an illumination beamsplitter; passing said object beam through an objective lens; reflecting said object beam from an object; focusing said reference beam and said object beam at a focal plane of a digital recorder to form an off-axis hologram; digitally recording said off-axis hologram; and transforming said off-axis hologram in accordance with a Fourier transform to obtain a set of results.

Another embodiment of the invention is based on an apparatus to write an off-axis hologram, comprising: a laser; a spatial light modulator optically coupled to said l