Optical waveguides – Optical fiber bundle
Reexamination Certificate
1999-04-23
2002-02-12
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Optical fiber bundle
C385S116000, C359S618000
Reexamination Certificate
active
06347173
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical coherence reduction method and device, an illuminating method and system and an optical fiber bundle.
2. Description of the Related Art
Heretofore, for a light source for illumination used for a luminaire for a projection-type liquid crystal display, measuring device and others, incoherent light source such as a lamp and a light emitting diode (LED) have been used because of various reasons such as costs and simplicity.
A laser beam emitted from a laser such as a solid state laser, a gas laser or a semiconductor laser has been used for illumination. A laser beam is excellent in directivity, is simultaneously provided with high luminous intensity and is a light beam high in coherence. However, a technically most difficult problem is “speckle” (i.e. speckle noise) caused by high coherence.
For example, a semiconductor laser is a light source, the photoelectric conversion coefficient of which is very high and which emits a laser beam excellent in directivity. However, a semiconductor laser has rarely been used for a light source for illumination because of a speckle problem caused by high coherence.
In the 1970s, research on a display using a laser beam (hereinafter called a laser display) was conducted in various places. However, one of the problems which prevented practicality was the generation of speckle in addition to problems such as the shortage of output from a light source and a modulating method.
Recently, there has been rapid progress in the technical development of elements which are key components of a laser display, such as: a high output laser using the conversion of wavelength by a solid state laser; a semiconductor laser which can oscillate beams in three primary colors of red (R), green (G) and blue (B); a spatial light modulator (a light bulb) using liquid crystal and a micro machine; and other elements.
When N pieces of speckle patterns incoherent mutually, (that is, which do not interfere with one another and are not correlated) are overlapped, the sum is equivalent to the sum of the intensity of each speckle pattern and the contrast of speckles is reduced by up to 1/·N.
Therefore, if N pieces of optical fiber are bundled and the length of each optical fiber is differentiated by a quantity in which coherence is lost, the interference between the optical fibers can be ignored. The resulting speckle is equivalent to the overlap of the intensity of speckle patterns I
1
, I
2
. . . , I
N
caused by each optical fiber. Therefore, the contrast of a speckle is reduced by equalization. That is, if N pieces of speckle patterns which are not correlated and are equal in intensity are overlapped, the contrast becomes 1/·N.
A speckle (pattern) has also been a large problem in the field of a semiconductor exposure system and measures against it have been taken. The problem exists because an excimer laser has been adopted as a short wave-length light source to enhance resolution.
For example, in exposure processing related to a semiconductor device, a coherence reduction method is proposed that provides for a flying eye lens
20
composed of elements different in length, as shown in
FIG. 7
, used for the control of coherence. That is, a measure against speckle and a lens
21
is arranged in a position apart from flying eye lens
20
by distance f between the end face on the side of outgoing radiation of the flying eye lens
20
and a mask
22
. (Refer to Lighting Optical System written by Messrs. Shibuya and Uehara and Japanese published examined patent application No. Sho 60-230629.)
However, according to this method, because the length of each element of the flying eye lens
20
is extended and the size of a lighting area from each element is different (as shown in FIG.
7
), problems result, including a reduction in efficiency.
It is proposed in Japanese Patent Application No. Sho 63-22131 to realize a similar effect using a prism
23
shown in FIG.
8
. However, as for the above method, the coherence reduction effect is insufficient and the optical loss is large.
In principle, a similar effect can be obtained using the dispersion of a refractive index. However, obtaining a sufficient effect by using the normal dispersion of a refractive index has been hampered by a problem in that an element becomes huge to reduce coherence.
In addition, other coherence control methods have been proposed. However, according to any of these methods, a speckle caused between an illuminating body and the naked eye cannot be sufficiently reduced in a display, a microscope, etc. Further, to remove a speckle, coherence control more severe than in a projection exposure system according to lithography and others is required.
That is, as shown in
FIG. 9
, the image
27
of an object
24
illuminated by an illuminating beam a, is formed on a screen
26
via a lens
25
. When a phase is disturbed at random because of the rough surface of the object
24
or the state of the optical surface of the lens
25
in the case illuminating beam a is coherent light, a speckle is included in the image
27
on the screen
26
.
Further, as schematically shown in
FIG. 10
, this means imaging the image on a screen
32
of an object
30
via a lens
31
on a retina
34
in an eye ball
33
, that the image of the object on the screen via the lens may be observed with eyes. That is, in this process, the random displacement of a phase is caused on an optical path by the confusion of a beam on the screen
32
and the eye-ball
33
and a speckle is also caused in an imaging process. If spatial coherence operates on the plane of an image even if a speckle is not superposed on the image on the screen
32
, a secondary speckle is caused on the retina
34
or on the retina of an observer
28
shown in FIG.
9
.
The effect upon a speckle caused on the naked eye cannot be reduced by much even if these methods are used, because methods, such as the fluctuation of a mirror and a rotary diffusion plate, used in a projection exposure system based upon lithography does not reduce coherence but only moves and averages a speckle. To apply these methods to a display and others, vibration of a screen is required so that positional relationship between an illuminating body, such as a screen and an eye, is varied. (Refer to pp. 1290 to 1294 of “Speckle-free rear-projection screen using two close screens in slow relative motion”, Vol. 66, No. 11 of Journal of Optical Society of America written by Epic G. Rawson, Antonio B. Nafarrate, Robert E. Norton, Joseph W. Goodman and published in November, 1976.) However, the above method is very inconvenient for practical purposes.
Heretofore, an optical fiber has been developed mainly for communication and for its material. Conventionally, a glass fiber mainly composed of quartz has been mainly used. To avoid mode dispersion, a single-mode optical fiber has been mainly developed.
As for a glass fiber, dispersion increases in a visible short wave-length region and its transmissivity is deteriorated. Therefore, the application of an optical fiber to visible light has been limited to an illuminating multimode optical fiber bundle which does not require transmission over a long distance. Particularly, because the intensity distribution of an outgoing beam is uniform if a multimode optical fiber is used, it is also a large merit that a complicated optical system such as a flying eye lens is not required.
Recently, a plastic multimode optical fiber has been developed and has attracted attention. (Refer to pp. 4261 to 4266 of “Graded-index polymer optical fiber for high-speed data communication”, Vol. 33, No. 19 of Applied Optics written by Takaaki Ishigure, Eisuke Nihei and Yasuhiro Koike and published on Jul. 1, 1994.) Compared with glass fiber, plastic multimode optical fiber is low-priced, light, and shows the largest transmission efficiency in a visible region. Further, its multimode dispersion is also very large, compared with that of formal glass fiber.
Recently, a hollow waveguide for transmit
Imai Yutaka
Suganuma Hiroshi
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