Phase active diffractive optical method

Details Phase active diffractive optical method Phase active diffractive optical method

1997-03-04

2002-06-11

Henry, Jon (Department: 2872)

Optical: systems and elements

Diffraction

From grating

C359S009000, C349S201000

Reexamination Certificate

active

06404553

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to optical systems and classical optical components used in them, and more particularly, to a method utilizing a phase active diffractive optical system for emulating such components, and doing so in a dynamic manner.
Conventional refractive optical systems employ physical elements which have a particular geometric shape. Such elements include wedges or prisms, concave and convex lens, etc. As is well-known, an optical beam directed at one of these elements has both an amplitude and a phase component. These values are uniform along the wavefront of the beam as it travels through space, and these values are independent of the beam's wavelength. The beam impinges on the element and passes through or is reflected by it. The reflected or emergent beam has an amplitude and phase relationship which may now vary, along the beam wavefront, from that of the incident beam. This change in amplitude and phase is a function of both the shape of the element and its index of refraction. The index of refraction for an optical elementis given by the formula;
n=c/v
where v is the speed of light in a material and c is the speed of light in a vacuum. In a vacuum, the speed of light is approximately 186,000 miles/second (3.0×10
8
meters/second). Classically, mirrors, lens, prisms, and the other elements are made of a material (e.g., glass) which has a constant set of properties including an index of refraction. Thus, the effects of passing light through these geometrically shaped elements or reflecting the beam off of them tend to be uniform over the element and are readily determined. However, it has been a characteristic of many of these elements, almost from their inception and regardless of the material from which they are made, to suffer from various aberrations.
An additional problem with use of optical elements in various systems is weight. In optical systems used to achieve greater and greater ranges, the aperture for the system increases as a function of the desired range. There is a correlation in aperture increase and increase in weight. Also, for moving and/or unstable platforms such as helicopters or ships at sea, platform stability is also an important factor. There is a correlation between the required degree of stability and overall system weight. As a practical matter, the weight and cost of an optical system varies as a function of the cube of aperture size. Stabilization has an inverse relationship to aperture size. To reduce weight and lower system's cost, modifications can be made to the shape of the optical components. Typically, this involves altering the shape of the element as a function of 2&pgr; of the wavefront such as in a Fresnel lens. The disadvantage of this approach is that although it works, it does make the optical system frequency dependent.
Recently, work has been done using diffractive optics instead of the refractive optical elements discussed above. In optical diffraction, light waves are bent or spread apart as they pass through a diffractive element. The waves subsequently interfere or interact with each other so as to mutually reinforce themselves in one area and while weakening themselves in another. Diffractive devices include, for example, Fresnel zone plates, Gabor zone plates, etc. The functioning of these and other diffractive elements is based on Fourier optic principles. Further, it is a feature of these devices that they are fixed devices; i.e., their optical characteristics, for example, the focus of a zone plate, may not be interactively altered. In this regard, they are similar to conventional refractive optical devices.
Devices such as spatial light modulators have been developed by which the amplitude of an incident light beam is effected as the beam is transmitted through a diffractive element. In these devices, an aperture map is created to achieve a desired output from the diffractive element. For this purpose, the element is made from one of a group of electro-optical materials referred to as birefringent materials. These materials are capable of having more than one index of refraction. As such, the materials may be controlled to produce a refraction map, or refraction index profile, which covers a surface of the material and which includes multiple refraction indexes. Typically, a birefrin material has a uniform set of physical and chemical properties throughout the material. But, these properties can be externally controlled to produce localized variations in the material's refractivity. The control mechanism may be either electronic or optical.
One use of birefringent materials has been in optical calculators. Here, the amplitude of an incident beam of light is altered, at differing points along the wavefront, to produce a desired binary value. This is done by impinging the light beam onto the surface of the birefringent material. As noted, the surface of the material is mapped to produce a desired refraction profile or index gradient. This involves use of spatial light modulation, or SLM. If SLM is done electronically, transparent electrodes are spatially arranged along one side of the piece of birefringent material. The electrodes are then selectively energized when a light beam impinges on the material. The result is a series of localized channels through which relative portions of the wavefront pass from one side of the material to the other. The amplitude of the beam segment passing through the respective channels is changed according to the established refraction index for that channel. The calculator design is such that the index of refraction can be changed dynamically so the output amplitude, or binary value, of the transmitted beam, also changes dynamically.
SUMMARY OF THE INVENTION
Among the several objects of the present invention may be noted the provision of a method employing phase active diffractive optics in an optical system by which classical optic elements such as lens and prisms are readily emulated; the provision of such a method to emulate these optical system components in a dynamic manner; the provision of such a method to locally control the index of refraction in a birefringent material to manipulate an optical wavefront impinging upon and passing through the material; the provision of such a method usable with light occurring in the ultraviolet, visible, near infrared, or far infrared portions of the light spectrum; the provision of such a method which is usable with both partially and fully coherent light and monochromatic and polychromatic light; the provision of such a method for spatially controlling the phase and amplitude of incident light as the light passes through an object; the provision of such a method to control phase and amplitude both dependently and independently; the provision of such a method to synthesize any aperture function to provide an optical image for an object; the provision of such a method to map any aperture function required to form any coherent image with the map subsequently being realizable; the provision of such a method to utilize an optically addressable spatial light modulator (OASLM) capable of a resolution up to 70 lp/mm; the provision of such a method to control deflection angles of incident waves impinging on the SLM by controlling the ramping of the index of refraction across the aperture of the SLM; the provision of such a method by which total phase depth is obtainable with the SLM with a single wave; the provision of such a method to allow the SLM to function as a “blazed” diffraction grating; the provision of such a method which is incorporated in a low cost system that allows conventional geometric optical components to be emulated; and, the provision of such a method which involves minimum or no moving parts.
In accordance with the invention, generally stated, a method is provided for effecting the desired optical characteristics of an optical system. The method employs a phase active diffractive optics system. Incident light is directed onto a piece of birefringent material thr

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