Optics: measuring and testing – Of light reflection – With diffusion
Reexamination Certificate
1999-03-31
2001-01-09
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Of light reflection
With diffusion
Reexamination Certificate
active
06172760
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to holography and, more particularly, to a simple optical apparatus for simultaneously measuring in real time the spectral and angular selectivities, the wavelength of maximum diffraction efficiency, and the direction of the grating vector of a reflection-type holographic optical element.
2. Description of the Related Art
A holographic optical element (HOE) is a hologram that is used to control transmitted light beams, rather than to display images. The angular and spectral selectivities of the HOE represent the deviation of incident angle satisfying the Bragg condition and the deviation of wavelength in the incident angle, respectively. Angular and spectral selectivities are measured, since reflection-type HOEs are used primarily as filters, beam combiner, and image display screens.
A conventional method for measuring angular and spectral selectivities involves measuring the diffraction efficiency for each wavelength or each incident angle by means of an instrument such as a monochrometer, a photodetector, or spectrophotometer. Conventional methods for measuring the angular and spectral selectivities:
(a) require a high-precision rotational or translational stage to control the angle of incidence of the beam incident on the HOE;
(b) require one detector to measure the spectral selectivity and another detector to measure the angular selectivity, because the position of the detector is fixed when the spectral selectivity is measured;
(c) cannot be used to tune an HOE in real-time, since the HOE must be rotated or repositioned after a measurement at a certain position or wavelength, both because the position or wavelength of an incident beam must be varied to measure the angular or spectral selectivity, respectively, and because whether HOE was tuned should be measured while swelling or shrinking its activity by wetting or heating for tuning HOE;
(d) are prone to error, since the beam on which measurements are made is not a collimated beam; and
(e) cannot readily be employed when several grating vectors are present in one HOE.
SUMMARY OF THE INVENTION
Measurement of the characteristics of a reflection-type HOE according to the present invention is accomplished by means of a simple optical apparatus comprising a multi-wavelength laser, a beam diffuser, a wide-angle objective lens, a diffusive plate, and an optical imaging device, such as a charge coupled device. The apparatus of the present invention enables simultaneous measurement in real time of the angular and spectral selectivities and the direction of the grating vectors of the reflection-type HOE from analysis of the intensity distribution of the transmitted beam when a diverging diffusing beam is incident on the HOE. The measurement employs a diverging or diffusive beam having an angle of divergence greater than the incident angle satisfying the Bragg condition of the HOE.
REFERENCES:
J. M. Heaton, et al., Applied Optics, vol. 24, No. 18, pp. 2931-2936, “Wavelength and Angular Selectivity of High Diffraction Efficiency Reflection Holograms in Silver Halide Photographic Emulsion”, Sep. 15, 1985.
Herwig Kogelnik, The Bell System Technical Journal, vol. 48, No. 9, pp. 2909-2947, “Coupled Wave Theory for Thick Hologram Gratings”, Nov. 1969.
Choi Yong Jin
Jeon Hyung Wook
Smirnov Vadim V.
Son Jung Young
Font Frank G.
Korea Institute of Science and Technology
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Stafira Michael P.
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