The present invention relates generally to optical components and systems, and specifically to diffractive optical elements and optical phase elements.
Optical imaging systems using lenses or materials transmitting light in general have to deal with the problem of dispersion of the light. Dispersion is the change of light velocity with wavelength, and dispersion causes chromatic aberrations in the optical system by creating different refractive indices and different phase changes for different wavelengths.
The phase delay xcfx86 in light of wavelength xcex introduced by a thickness d of material of refractive index n is given by:                     φ        =                                            2              ⁢              π                        λ                    ⁢                      (                          n              -              1                        )                    ⁢          d                                              (          1          )                .            
Diffractive systems, for directing and focusing beams of light, are well-known in the art, as described, for example, by Francis Jenkins and Harvey White in Fundamentals of Optics, Fourth Edition, pp. 385-386 (1981), which is incorporated herein by reference. The theory of diffractive optical elements (DOEs) is further described, for example, by Nieuborg, et al., in an article entitled xe2x80x9cPolarization-Selective Diffractive Lenslet Arrays,xe2x80x9d published in European Optical Society Topical Meeting Digest Series, Vol. 5 (1995), which is also incorporated herein by reference. DOEs are generally highly dispersive.
Zone plates are a well-known type of DOE, typically comprising concentric rings having radii proportional to the square roots of the whole numbers and a phase retardation varying by xcfx80 between neighboring rings. When a beam of collimated light is incident on the zone plate, it will be diffracted to a focal point. Like other DOEs and computer-generated holograms known in the art, however, zone plates known in the art are highly chromatically dispersive.
It is an object of the present invention to provide methods for designing and producing diffractive optical elements having a desired wavelength dispersion.
It is a further object of the present invention to provide optical elements designed and/or produced in accordance with these methods.
In one aspect of the present invention, diffractive optical elements are designed and produced so as to be substantially achromatic, i.e., so that their dispersion is effectively minimized.
It is another object of the present invention to provide diffractive optical elements which compensate for chromatic aberrations. Thus, in one aspect of the present invention, diffractive optical elements are applied to a refractive element, such as a lens, to reduce or remove the chromatic aberrations present because of the dispersive properties of the refractive element.
It is a still further object of the present invention to provide an aberration-corrected, diffractive optical element and a method for producing same for a wideband light source.
In preferred embodiments of the present invention, a DOE comprises first and second gratings substantially in mutual registration. The gratings are made of respective first and second materials having respective first and second refractive indices. The dimensions of the gratings, in particular their thicknesses, are chosen so as to give a desired value of wavelength dispersion and/or phase retardation. Preferably, the refractive indices and the dimensions of the gratings are chosen so as to reduce or minimize chromatic aberrations produced by the DOE or by an optical system including the DOE. Such embodiments differ from DOEs known in the art, which typically are designed for a narrow wavelength range and are highly wavelength dispersive.
In some preferred embodiments of the present invention, a DOE is formed on a surface of a lens by etching the first grating into the surface of the lens, overlaying the grating with the second material having a different refractive index from that of the lens, and then etching the second grating into the second material, in registration with the first grating. Alternatively, the second material may first be overlaid on the surface of the lens, and then both the first and second gratings may be etched simultaneously. The DOE preferably corrects for the chromatic dispersion of the lens.
In other preferred embodiments of the present invention, the DOE is formed on a surface by overlaying the surface with a layer of the first optical material, overlaying the first optical material with a layer of the second optical material, and etching the gratings in registration into both optical materials.
In still other preferred embodiments of the present invention, a DOE is formed by etching the first grating into a first surface of a flat plate formed from two materials having different refractive indices, and the second grating is etched in registration with the first grating into the second, opposite surface of the flat plate. DOEs in accordance with these preferred embodiments may be designed to function as substantially dispersionless diffraction gratings or diffractive focusing elements.
In still other preferred embodiments of the present invention, a DOE is formed by combining in registration a plurality of DOEs, each DOE of the plurality being formed as described above.
In some preferred embodiments of the present invention, the DOE is generated by computer calculation and fabricated, alternatively or in combination, by a lithographic process or a plasma-etch process.
The principles of the present invention may thus be applied both in diffractive optical systems and in hybrid systems that mix diffractive and refractive optics, using any suitable methods of design and fabrication known in the art. Although the preferred embodiments described herein relate primarily to achromatization of such optical systems, the principles of the present invention may be applied more generally to control the chromatic response of such systems in substantially any desired manner, for example, to create a desired chromatic dispersion.
Although preferred embodiments are largely described herein with reference to optical wavelengths of radiation, it will be appreciated that similar embodiments of the present invention may generally be constructed utilizing materials operating at non-optical wavelengths such as ultraviolet, infrared, microwave and radio wavelengths.
Furthermore, although preferred embodiments are described herein with reference to two layers of gratings, it will be appreciated by those skilled in the art that similar preferred embodiments may generally be constructed using additional layers of gratings.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for producing a diffractive optical element, comprising forming first and second gratings, substantially in mutual registration, of respective at least first and second optical materials, such that for predetermined two or more wavelengths, the diffractive optical element has phase retardations in a desired mutual relation, thereby avoiding the generation of chromatic aberrations.
The invention further provides a diffractive optical element, comprising first and second optical materials in which respective first and second phase gratings are formed substantially in mutual registration, such that for predetermined wavelengths the diffractive optical element has desired phase retardations, whereby generation of chromatic aberration is avoided.
The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.