« PreviousContinue »
1*/78 OR 4,073.571
United States Patent 
Grinberg et al.
[ii] 4,073,571  Feb. 14, 1978
 CIRCULARLY POLARIZED LIGHT SOURCE
 Inventors: Jan Grinberg, Los Angeles; Leroy J.
Miller, Canoga Park, both of Calif.
 Assignee: Hughes Aircraft Company, Culver City, Calif.
 Appl. No.: 683,557
 Filed: May 5,1976
 Int. CI.* G02F 1/13
 U.S. CI 350/337; 350/147
 Field of Search 350/147, 154, 157, 158,
 References Cited
U.S. PATENT DOCUMENTS 3,669,525 6/1972 Adams et al 350/158
Primary Examiner—Edward S. Bauer
Attorney, Agent, or Firm—Donald C. Keaveney; W. H.
There is disclosed a circularly polarized light source
including a circular light polarizer using cholesteric liquid crystal material in an arrangement such that substantially 100% of the unpolarized light input may be utilized in a circularly polarized output in either a narrow band or wide band configuration. The circular polarizer itself comprises one or a plurality of liquid crystal cells having liquid crystal material of a predetermined ratio by weight of cholesteric to nematic liquid crystal type in each cell. The ratio of the types of materials determines a central wavelength to which each cell is tuned in its polarizing action. If a single cell is used, light of a predetermined bandwidth around its central wavelength is polarized. If a plurality of cells are stacked in a sandwich arrangement wherein each cell is tuned to a different wavelength selected in such a sequentially stepped fashion that the bandwidths for the cells form a continuous spectrum, then there is formed a wideband polarizer which can readily encompass the entire visible wavelength region of the spectrum.
7 Claims, 3 Drawing Figures
U.S. Patent Feb. 14, 1978 Sheet 1 of 2 4,073,571
U.S. Patent Feb. 14, 1978 Sheet 2 of 2 4,073,571
CIRCULARLY POLARIZED LIGHT SOURCE
BACKGROUND OF THE INVENTION
Currently used arrangements for producing circu- 5 larly polarized light are described, for example, at page 42 of a textbook entitled "Introduction to Modern Optics" by Grant R. Fowles, published by Holt, Rhinehart & Winston in 1968. As shown therein the arrangement comprises a linear polarizer having its transmission axis 10 positioned at a 45° angle to the orthogonal fast and slow axes of a quarter waveplate through which the light passes after passing through the linear polarizing crystal.
Such a circular polarizer has two major drawbacks. 15 First, the extinction ratio for such devices can have an acceptably high value only for a narrow bandwidth of light wavelengths. By the "extinction ratio" is here meant the ratio of the light intensity transmitted by two sequentially positioned polarizers that each pass polar- 20 ization of the same circular polarization handedness sense to the light intensity transmitted by two such polarizers which respectively pass circular polarization handedness senses which are opposite. Outside of the narrow bandwidth for which known circular polarizers 25 have a high extinction ratio, the ratio drops fast. This characteristic is intrinsic to the presently available devices because, as noted, they are constructed from a linear polarizer and a quarter waveplate. Naturally, the waveplate can be a quarter waveplate only for one 30 wavelength. For other wavelengths the plate retardation in terms of the wavelength changes proportionally to the reciprocal of the wavelength. Thus, the light instead of being circularly polarized is elliptically polarized. Circular polarizers for the visible wavelength 35 region have extinction ratios at the two extremes of the visible region of 5 or 6 at 400 nm and of 6 or 7 at 700 nm compared to 1000 at 550 nm.
The second major drawback of known absorption type circular polarizers is that they absorb between 40 60% and 80% of the incident unpolarized light. This is a problem not only because the efficiency of the resulting light source is thereby low, but also because the dissipated power in the polarizer, in the case of a high intensity beam, damages the polarizer. 45
In view of these problems, systems such as that disclosed, for example, in U.S. Pat. No. 2,958,258 to D. H. Kelly or U.S. Pat. No. 3,893,758 to Hunzinger et al, have used linearly polarized light rather than circularly polarized light in high intensity projection systems re- 50 quiring polarized light sources. Such systems may, for example, use a wideband linear polarizer of the type shown in U.S. Pat. No. 3,403,731, issued to S. M. MacNeille, in order to obtain the wideband or white polarized light source needed for the system. Although the 55 MacNeille device solves the problem of providing a wideband rather than a narrow band polarizer for linearly polarized light, it does not purport to be useful for providing circularly polarized light which may often be more desirable than linearly polarized light for various 60 system requirements.
There have been proposals in the literature to use alternately left-handed and right-handed cascaded cholesteric liquid crystal cells to form narrow band or notch color filters. Such a proposal was made, for exam- 65 pie, by James Adams et al in an article entitled "Cholesteric Films as Optical Filters" which appeared in Vol. 42, No. 10, of the September 1971 issue of the Journal of
Applied Physics beginning at page 4096. See also an article by Sato et al entitled "Liquid Crystal Color Light Valve" appearing in the February 1974 issue of the IEEE Transactions on Electron Devices at page 171. These devices utilize the polarizing action of liquid crystal materials. However, they do not form wideband circular polarizers which will take a source of unpolarized white light and transform it into a beam of circularly polarized white light. Furthermore, even at narrow bandwidths these devices inherently use only 50% of the incident unpolarized light since their function by definition is to transmit 50% of the incident light as circularly polarized light in a first handedness sense (left-handed or right-handed as the case may be) and to reflect the other 50% of the incident unpolarized light as circularly polarized light of the opposite handedness sense.
It is an object of the present invention to overcome the foregoing shortcomings of the prior art. and to provide a circularly polarized light source using one or more cholesteric liquid crystal cell means to achieve 100% nominal efficiency in the use of the incident light at any bandwidth rather the 50% shown in the prior art and/or to achieve a wideband circular polarizer capable of producing an output beam of white circularly polarized light.
SUMMARY OF THE INVENTION
This is achieved by providing a circular polarizer comprising one or more cholesteric liquid crystal cells with mirror means arranged to produce two transits of light through the polarizer, the first being that of the unpolarized light and the second being that of the light first reflected from the polarizer but with its circular polarization handedness sense reversed. Wideband characteristics are achieved by providing a group of stacked polarizer cells each tuned to a different central polarizing wavelength such that the totality of their bandwidths covers the entire while light spectrum.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, objects and advantages will be more readily understood from the following detailed description taken in conjunction with the attached drawing wherein like reference characters refer to like parts throughout and in which:
FIG. 1 is an optical schematic view showing a circularly polarized light source system in accordance with the present invention.
FIG. 2 is an enlarged prospective view partially broken away of one of the liquid crystal cell members of the wideband circular polarizer device shown as a part of the system of FIG. 1.
FIG. 3 is a graph showing central polarizing wavelength Xq as a function of composition ratio for one exemplary liquid crystal mixture which may be used in the cells shown in FIGS. 1 and 2.
DETAILED DESCRIPTION OF THE
There is shown in FIG. 1 a circularly polarized light source which includes a wideband circular polarizer 14 using a multilayer cholesteric liquid crystal sandwich consisting of the liquid crystal cells 14a, 146, 14c, 14a", 14e, 14/and 14g. It will, of course, be understood that seven cells are shown by way of example only and that as many cells are appropriate to a given application may be used. The details of the exemplary individual cell 14a