Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Patent
1998-12-28
2000-05-16
Henry, Jon
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
359 16, 359 19, 359569, 359577, 430 1, G02B 532, G02B 518, G02B 104, G03H 100
Patent
active
060645008
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to optical devices for producing non-fringing destructive interference of light, and to a method of making and using the same.
BACKGROUND TO THE INVENTION
Light moves through space as an electromagnetic wave. The wave can be envisioned as a series of peaks and troughs moving continuously along a given path at a given frequency. Interference occurs when two waves pass through the same region of space at the same time. Interference between waves can be both constructive and destructive. Constructive interference occurs when the peaks (and troughs) of two waves meet each other at the same time and overlap. These waves are said to be in phase and when this happens the amplitude of the waves at the point of overlap is increased.
Destructive interference occurs when the peas of one light wave meet and overlap with the troughs of a second light wave. When the peaks and troughs meet each other they cancel and the wave is said to be phase cancelled. A perfectly phase cancelled wave has no electromagnetic energy.
Both constructive and destructive interference of light can be demonstrated by a double split experiment whereby light from a single source falls on a screen containing two closely spaced slits. If a viewing screen is placed behind the first screen, a series of bright and dark lines will be seen an the viewing screen. This series of lines is called an interference pattern.
The bright lines of an interference pattern are areas of constructive interference, and the dark lines are areas of destructive interference. The pattern is generated as waves of a particular wavelength enter the two slits. The waves spread out in all directions after passing through the slits so as to interfere with each other. If a wave from each slit reaches the center of the viewing screen, and these waves travel the same distance before they hit the screen, they will be in phase and a bright spot indicating constructive interference will occur at the center of the viewing screen. There will also be constructive interference at each point the paths of two light rays differ by one wavelength or multiples of one wavelength. However, if one ray travels an extra distance of one-half a wavelength or some multiple of a half wavelength, the two waves will be exactly out of phase when they reach the screen, and so a dark band will appear in the interference pattern indicating destructive interference. Thus, you get a series of bright and dark lines in the interference pattern called "fringes".
The double slit experiment is one method of producing destructive interference. However, only a small portion of the source light is cancelled. Another method of producing destructive interference of light has been accomplished by using a beam splitter, mirrors and a laser. This type of device is often referred to as an interferometer.
An interferometer works on the following principle. A laser is used in conjunction with a beam splitter to cause the laser beam to split in two, with a certain percentage of light taking one path and a certain percentage of light taking another path. The path of one of the split beams can be delayed by using amovable mirror such that the beam can be reflected back parallel with the unreflected beam by variable path lengths which can differ by fractions of a wavelength. The degree of cancellation depends on the "coherence length" of the laser and the narrowness of the chromatic line. For these reasons, a laser of extremely high quality is required to produce a significant degree of cancellation. However, no laser produces purely monochromatic light and a fringe is produced regardless of the degree of cancellation. In order to produce a perfectly phase-cancelled non-fringing collinear beam, destructive interference must occur over all incident wavelengths and phases of the entire bandwidth of the incident light source, all of the light rays emitted by the source must be parallel, each photon in the beam must be paired with another photon having the exact same wavelength, and
REFERENCES:
patent: 4550973 (1985-11-01), Hufnagel
patent: 5071210 (1991-12-01), Arnold et al.
patent: 5243583 (1993-09-01), Ohuchida et al.
patent: 5420947 (1995-05-01), Li et al.
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