Variable focal length achromatic lens system comprising a diffractive lens and a refractive lens
A variable focal length achromatic lens includes a flat liquid crystal diffractive lens and a pressure-controlled fluidic refractive lens. The diffractive lens is composed of a flat binary Fresnel zone structure and a thin liquid crystal layer, producing high efficiency and millisecond switching times while applying a low ac voltage input. The focusing power of the diffractive lens is adjusted by electrically modifying the sub-zones and re-establishing phase wrapping points. The refractive lens includes a fluid chamber with a flat glass surface and an opposing elastic polydimethylsiloxane (PDMS) membrane surface. Inserting fluid volume through a pump system into the clear aperture region alters the membrane curvature and adjusts the refractive lens' focal position. Primary chromatic aberration is remarkably reduced through the coupling of the fluidic and diffractive lenses at a number of selected focal lengths. Potential applications include miniature color imaging systems, medical and ophthalmic devices, or any design that utilizes variable focal length achromats.
1. An optical system comprising:
two lenses wherein a focal length of each of the two lenses is adjustable to alter a focal length of the system, so that the system is substantially achromatic over a range of values of focal length of the system, wherein the two lenses include a diffractive lens and a refractive lens; and
at least one device for adjusting the focal length of each of the two lenses.
2. The system of claim 1 , wherein the diffractive lens comprises a Fresnel zone plate having portions that form a pattern and a liquid crystal material, and the at least one device alters a refractive index of the liquid crystal material and electrically modifies subzones of the Fresnel zone plate and re-establishes phase wrapping points of the Fresnel zone plate in order to adjust the focal length of the diffractive lens.
3. The system of claim 2 , wherein the at least one device comprises a power supply and an electrode adjacent to the liquid crystal material, so that when voltages are applied by the power supply to the pattern to shunt some of the portions of the Fresnel zone plate, the focal length of the diffractive lens is altered.
4. The system of claim 1 , wherein the refractive lens includes a fluidic lens, the at least one device comprising means for controlling an amount of fluid in the fluidic lens to alter its focal length.
5. The system of claim 4 , wherein the fluid in the fluidic lens has an Abbe number in a range from about 5 to 20.
6. The system of claim 5 , wherein the fluid in the fluidic lens includes methanol.
7. The system of claim 1 wherein the fluid in the fluidic lens and the focal length of the fluidic lens are such that the fluidic lens compensates for the primary chromatic aberration of the diffractive lens.
8. The system of claim 1 , wherein the system is substantially achromatic for red and blue light over a range of focal lengths in excess of 10 mm.
9. The system of claim 6 , wherein the system is substantially achromatic for red and blue light over a range of focal lengths of the system from about 40 to about 200 mm.
10. A method for making an optical system comprising:
providing a diffractive lens which comprises a Fresnel zone plate having portions that form a pattern, a liquid crystal material and an electrode adjacent to the liquid crystal material;
applying voltages to the electrode and the portions of the Fresnel zone plate to select a focal length of the diffractive lens;
selecting a fluid with an Abbe number;
adjusting amount of said fluid in a fluidic lens to select a radius of curvature of the fluidic lens, wherein when the fluidic lens is placed adjacent to the diffractive lens, the fluidic lens compensates for the primary chromatic aberration of the diffractive lens over a range of focal lengths of the optical system in excess of 10 mm.
11. The method of claim 10 , wherein the system is substantially achromatic for red and blue light over a range of focal lengths of the system from about 40 to about 200 mm.