Wide angle light diffusing optical fiber tip

Details Wide angle light diffusing optical fiber tip Wide angle light diffusing optical fiber tip

2001-08-31

2004-12-07

Patel, Tulsidas C. (Department: 2839)

Optical waveguides

With optical coupler

Input/output coupler

Reexamination Certificate

active

06829411

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
The present invention relates to fiber optic illumination devices. In particular, illuminations device that are required to provide illumination over a large angular area compared to normal fiber optic output apertures.
Optical fibers are useful for conducting light from a light source to a remote site. Areas may be conveniently illuminated that are too small for the light source, contain explosion hazards, which are inconvenient to service and which are otherwise unsuitable for conventional lighting sources. Optical fibers are used extensively in medical applications, for microsurgery, endoscopic surgery, and photodynamic therapy. Additionally, optical fibers are used in industrial and commercial applications for inspections, as decoration, and for general illumination purposes. Optical fibers have also been used to conduct sunlight from the exterior of a building to interior rooms to provide natural lighting, and as indicator or pathway lighting in confined spaces.
A normal optical fiber is cylindrically shaped and relatively long compared to its diameter. Optical fibers are manufactured with a light transmitting material having a relatively high index of refraction. Most often, it is coated with a material having a lower index of refraction. The central material of the optical fiber is referred to as the core, and the coating material is known as the cladding. Light which is traveling within the core of the fiber will eventually impinge on the core/cladding interface. If the intersection angle is narrow enough, as determined by the ratio of the cladding index of refraction to the core index of refraction), the light will be entirely reflected back into the core. Light intersecting the interface at an angle which exceeds this “critical angle” escapes from the optical fiber through the cladding material. An optical fiber will only accept light within a certain angle range. Output light will exit the normal optical fiber in a relatively narrow cone shaped pattern. The included angle of the cone of exiting light is generally 60 degrees or less.
For many potential applications of an optical fiber for illumination purposes, the limitation of the output angle is a great disadvantage. In order to illuminate a desired area entirely, it is often necessary to space the fiber a distance from the area or illuminate the area with multiple fibers pointed in several different directions. Either solution is may be impractical or expensive. Accordingly, various methods have been utilized to overcome this disadvantage by increasing the illumination angle of light available from an optical fiber. One method is to simply roughen the surface of the core material. This causes the relative angle of incidence of the light at the core/cladding interface to change, and additional light to correspondingly leak out of the cladding. This method requires the use of a great length of fiber, otherwise a majority of the light still exits the optical fiber from the end in a normal manner. Another method illustrated in
FIG. 1
is to modify the optical fiber core
1
, but not the cladding
2
, to include bubbles
3
or other internal structural configuration which interfere with the light pathways, such as is disclosed in U.S. Pat. No. 4,466,697 to Daniel and U.S. Pat. No. 4,195,907 to Zamja et al., both herein incorporated by reference. The light traveling in the core
1
refracts and reflects off of the bubbles
3
, again altering the angle of incidence of the light at the core/cladding interface, and causing additional light to escape through the cladding
2
. However, it is difficult to control the density and size of the bubbles.
An alternative technique is to apply a material containing bubbles to the tip of the optical fiber. If the bubbles are homogenous, then the light scattering is greatest near the tip of the optical fiber. However, because there is less light to scatter, the intensity of the output reduces with the distance from the top of the optical fiber. Additionally, manufacturing this type of optical diffusing tip is expensive and inconsistent.
An additional alternative technique shown in
FIG. 2
is to mix a light scattering and/or reflecting medium
5
, usually a powder in a transparent carrier material, which is then applied to the tip of the optical fibers, or to an exposed portion of the core using various conventional methods. Such a technique and diffuser is shown in U.S. Pat. No. 5,269,777 to Doiron et al. Alternatively, a tip portion for attachment to the end of an optical fiber may be provided which consists of a light propagating material having inclusions distributed therein for interacting with the light exiting the optical fiber to produce a predetermined light distribution, such as is shown in U.S. Pat. No. 5,807,390 to Fuller et al. and herein incorporated by reference. As with the previously described techniques, these techniques suffer from the problems of reduced output pattern, control difficulties, expense, and size, as well as the difficulty of attaching the tip portion to the optical fiber end.
Several other techniques seek to utilize a separate optical element located near or on the optical fiber tip to refract light exiting the optical fiber tip. For example, as seen in
FIGS. 3 through 5
, disks
8
with holographic microlenses, frustoconic lenses
9
, and spherical lenses
11
such as shown in U.S. Pat. No. 5,784,508 to Turner, have been utilized in this manner. Each of these methods is limited in the amount of light dispersion which they are capable of providing (typically, no more than 120 degrees of total output angle from the optical fiber tip). Light exiting the optical fiber is refracted differently by the lens
9
depending upon the area of the lens which it impinges upon. The angular distribution of the light affected by the two areas of the lens
9
is tailored to overlap, yielding an even illumination level over the entire output of the lens. Additionally, the inclusion of separate optical elements add to the complexity and expense of manufacture due to the added optical element and the architecture required to support it.
As seen in
FIG. 6
, optical elements have even been manufactured directly into the end of the optical fibers by providing a faceted end surface
13
with a reflective coating. This results in the output light bending away from its normal illumination pattern, but the new patterns remain inconsistent.
Finally, as illustrated in
FIG. 7
, methods to disperse the output light from optical fibers have attempted to modify the end of the optical fibers in a “bullet” shape
14
configured to cause both reflection and refraction of the exiting light, yielding a nearly ideal output angle of 180 degrees or more near the point of the bullet shape
14
without resulting in light and dark areas. One such configuration is disclosed in U.S. Pat. No. 5,351,168 to Easley, herein incorporated by reference. However, the bullet shape of the optical fiber end is difficult to manufacture, and must be nearly perfect to achieve the desired illumination pattern, with any deviations resulting in patterns of light and dark rings.
BRIEF SUMMARY OF THE INVENTION
Briefly stated, the present invention provides a diffusing optical fiber tip yielding a homogenous output pattern having a total illumination angle of at least 180 degrees. The diffusing optical fiber tip has an outer diameter which is no greater than that of the optical fiber, and appears as a point source of illumination, having substantially the same output pattern when immersed in water as it does in air. The diffusing optical fiber tip is manufactured on the end of a typical acrylic optical fiber by causing longitudinal stresses in the fiber end, which are then relieved by forming axial cracks or inclusions in the fiber core at the optical fiber tip. As a result, light exiting the optical fiber must traverse a scrambled pathway caused by a complex interaction of reflections and refractions. The

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