Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
1999-02-23
2002-11-05
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C356S003000, C356S004050, C356S614000, C250S201900
Reexamination Certificate
active
06476943
ABSTRACT:
1. BACKGROUND OF THE INVENTION
1.1 Field of the Invention
The present invention relates generally to an optical element (OE) which can serve as an optical analogue to digital converter (OADC) and/or an optical digital to analogue converter (ODAC), and uses therefor. The OE can be usefully employed in a variety of applications to transform analogue information present in a light wave front into digital light signals and/or to transform digital information into analogue information in the form of the physical parameters of a light wave front.
1.2 Description of the Related Art
A discussion of art related to the present invention follows.
1.2.1 Interferometry
An interferometer is an instrument used to make measurements of beams of light. Interferometers measure properties such as length, surface irregularities, and index of refraction. The interferometer operates by dividing a beam of light into multiple beams traveling unequal paths. When these beams interfere with each other, an interference pattern is formed.
The interference pattern appears as a series of light and dark bands referred to as interference fringes. Information derived from fringe measurements can be used to make precise wavelength determinations to measure minute distances and thickness, to study spectrum lines, and to determine refractive indices of transparent materials.
A classic example of an interferometer is the Twyman-Green interferometer (a modification of the Michelson interferometer), which is used for testing optical elements such as lenses and prisms. The Twyman-Green interferometer uses a point source of monochromatic light at the focus of a quality lens. When the light is directed toward a perfect prism, it returns to a viewing point exactly as it was from the source, resulting in a uniform field of illumination; however, imperfections in the prism glass distort the wave front. Similarly, when the light is directed toward a lens backed by a convex mirror, it passes through the lens, strikes the mirror, and retraces its path through the lens to a viewing point. Imperfections in the lens result in visually observable fringe distortions.
Interferometers have the capacity to transform a light wave front, which varies according to the radius of the curvature of the wave front, into a series of light rings. The direction vector of the distribution of the wave front is, in effect, coded to the period and inclination of the stripes of the interferometer picture. See, for example, Max Born et al.,
Principles of Optics,
(1968). The rings of the interferometer pattern are strictly related to the physical parameters of the incoming wave front. However, to the inventor's knowledge, no one to date has used an interferometer to transform the information contained in a wave front into an optical digital image, such as an image consisting of a series of light spots.
1.2.2 Holography
Holography is a means of creating a unique photographic recording called a hologram. The recording appears to the naked eye as an unrecognizable pattern of stripes and whorls. However, when the hologram is illuminated by coherent light (e.g., a laser beam), the hologram projects a three-dimensional image of the recorded object.
While ordinary photographs record variations in the intensity (and sometimes color) of light reflected and scattered from an object, a hologram is a recording of both the intensity and phase of the light from the object. Holography depends on the effects of interferometry and diffraction to configure three-dimensional images.
A standard hologram is recorded as follows: A beam of coherent laser light is directed onto an object from a coherent light source. The beam is reflected, scattered, and diffracted by the physical features of the object and impacts on a photographic plate (this beam is referred to as the object beam). Simultaneously, part of the laser beam is split off as a reference beam and is reflected by a mirror onto the photographic plate.
The two parts of the laser beam-the reference and the object beams-meet on the plate and are recorded as interference fringes on the hologram. This pattern of fringes contains an optical record of the phase and amplitude of the light being reflected from the object.
To reconstruct the phase fronts of the object beam, the hologram must be illuminated by a beam which is similar to the beam used to construct the hologram. When the coherent light of the laser beam illuminates the hologram, most of the light from the laser passes through the film as a central beam. The fringes on the hologram act as a diffraction grating, bending or diffracting the remaining light to reverse the original condition of the coherent light waves that configured the hologram.
On the light source side of the hologram, a visually observable virtual image is formed. On the other side, a real image that can be photographed is formed. Both reconstituted images have a three-dimensional character because the hologram is a recording of both amplitude information and phase information. The phase information provides the three-dimensional characteristic of the image because it contains information regarding the contours of the object.
A common example of a hologram is the white-light hologram commonly used on a German visa. Such holograms permit an observer to view multiple images (typically about 10 images) depending on the angle from which the observer views the hologram.
Holograms are also commonly impressed on documents, such as credit cards, as a security measure, to display the control panels of aircraft on their windshields, and are used as an archival method for storage of experimental results, and to detect minute distortions in three-dimensional objects. See Laufer, Introduction to
Optics and Lasers in Engineering,
p. 204 (1996) (the entire disclosure of which is incorporated herein. by reference).
The present invention employs an OE, such as a specially recorded hologram, to transform varied incoming light into an output digital light code which can be visually observed and/or read by a photodetector and interpreted by a computer processor.
As will be discussed in greater detail below, the present invention also employs a variety of elements known in the art which can mimic the effects of a hologram, including for example, spatial light modulators and kinoforms. A kinoform is essentially a complex lens which operates on the phase of the incident light. The phase modulation of an object wave front is recorded as a surface-relief profile.
1.2.3 Use of Lasers in Distance Measurement
Lasers have been applied in a variety of ways to measure distance. Typical methods include interferometry, laser Doppler displacement, beam modulation telemetry, and pulse time of light.
Laser interferometers typically provide measurement of displacement from a starting position rather than a measurement of position. The instrument reading is typically set to zero as the initial position of the moving part, and the motion is then measured relative to this zero position. See Ready,
Industrial Applications of Lasers,
page 260 (1997) (the entire text of which is incorporated herein by reference).
Laser interferometry or distance measurement must be used in a controlled environment. Accordingly typical applications include setup of work holding fixtures for the production of aircraft engine components, checking out of the motion of machine tools, positioning operations, rack setup on boring mills, building vibration measurement, and measurement of strain in the earth. See Id. at pages 268 to 269.
Laser Doppler displacement distance measurement takes advantage of the Doppler shift of laser frequency effected when a stabilized laser is reflected from a moving surface. This frequency shift can be measured and converted to a measurement of surface displacement, i.e., the difference between a start position and position of an object.
Neither interferometric nor Doppler displacement methods can be used to measure large distances in uncontrolled environments. In particular, fluctuations of the density of the atmosph
Assaf Fayez
Knobbe Martens Olsen & Bear LLP
Spyrou Cassandra
Virtual Pro Inc.
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