Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2002-01-09
2004-10-26
Lee, John D. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S031000, C385S039000, C385S043000, C385S124000
Reexamination Certificate
active
06810184
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a device and method to diffuse light from the distal end of an optical fiber.
2. Information Disclosure Statement
The radiation emitted by a laser beam source can be coupled into an optical fiber of suitable dimensions and optical properties wherein the light can be transported with no significant losses over very long distances. Today's state of the art fibers have found broad application in the fields of telecommunication, optical inspection, medical therapy, laser applications and many more. The fabrication processes are well understood and optical fibers are manufactured in large quantities, having high quality and long lifetimes.
Optical fibers rely on total internal reflection at the interface between the fiber core and the surrounding cladding to contain the light within the core of the fiber. The light guiding effect occurs in optical fibers where cores are much larger than the wavelength of the incident light. For light guiding to occur, the refractive index of the fiber cladding must be lower than the refractive index of the fiber core. A light ray incident to the fiber core's end under an angle sufficiently small relative to the fiber axis can enter the fiber and is refracted according Snell's law into a certain angle. It then hits the interface between fiber core and cladding and is, assuming the angle of incidence to the surface is sufficiently large, totally reflected back into the core. If no bends occur that exceed a critical curvature, the light cannot leave the fiber core and is thus guided through the fiber until it reaches the end.
There are a number of medical and technical applications that require some means to diffuse the light from the distal end of an optical fiber. Photodynamic therapy, deposition of thermal energy, special illumination and irradiation are prime uses. Diffusion sites on optical fiber have also found uses as optical sensors. Most often, special fiber optic tips are prepared to realize these functions.
There are several general methods for producing diffusing tips for optical fiber. The simplest is removing a section of cladding from an optical fiber then coating the bare core with a layer of optical scattering materials. Another would be to physically change the core or cladding by roughing their surfaces. Light scattering elements may also be introduced into the core or cladding to enhance scattering at desired locations, but this typically must be done by adding scattering materials at the time the fibers is drawn or by causing physical defects in the core or cladding post production. Any mechanical processes used to manufacture such diffusion sites inherently weaken the optical fiber.
The most common means to achieve light distribution from optical fibers are specially fabricated diffusion tips that are joined to the distal end of an optical fiber. These diffusers are frequently threaded onto the end of the optical fiber or glued on with an optical adhesive. These techniques require skilled persons to fabricate and assemble these probes. Misalignment or mishandling of the fiber end during manufacturing or use gives rise to concerns about whether the mechanical reliability and strength of the fiber has been compromised in the critical distal end. This is especially true with small fiber diameters. A selection of patents is presented to demonstrate the details associated with cylindrical diffusers.
U.S. Pat. No. 5,074,632 discloses a fiber optic cylindrical diffuser which includes a fiber with a jacket stripped core tip, a thin layer of scattering medium coated on the bare core tip and a sleeve member that encloses the fiber tip without touching the scattering medium. The scattering medium is a filled optical adhesive and the sleeve member threads onto the fiber jacket.
U.S. Pat. No. 5,337,381 discloses a cylindrical light diffuser where an exposed fiber core end has a conical shape. The core end is enclosed by a sleeve having a conical end, which is filled with a light diffusing polymeric material, and threaded onto the fiber sheath. Also disclosed is shaping of the exposed core into a stepped conical or undulating shape to permit greater uniformity in near and far field illumination.
U.S. Pat. No. 5,363,458 discloses a cylindrical diffuser which improves on U.S. Pat. No. 5,337,381 by including rings of prescribed indices of refraction about the unclad distal end of the fiber. One or more rings of low index of refraction material permit the tailoring of the emission profile.
U.S. Pat. No. 5,373,571 discloses a fiber optic diffusing tip having an unclad stripped terminal end of the optical fiber that is inwardly tapered towards its end such that light scattering epoxy media disposed between a glass tube and the unclad end provides a predictable light distribution. A mirrored end face is disclosed to reduce hot spots. Scattering particles size is used to center the core in the tube.
U.S. Pat. No. 5,431,647 describes a fiber optic diffuser comprising a transparent resin cylindrical cap and a polyester diffusing sleeve disposed about an exposed core. The diffuser is internally threaded and secured to a buffer layer surrounding the fiber with that aid of a wicking adhesive. A reflector is incorporated to reflect light rays exiting the end the core. The described diffuser would be expensive to manufacture due to its complex design. Due to the cap structure, the length of the diffusion area is limited.
U.S. Pat. No. 5,754,717 discloses a diffusing tip surrounding having an inner core and an outer covering, where the interior surface of the outer covering is modified such that light transmitting down the fiber is removed from the core upon encountering the modifications. The core material is preferably transparent silicone and the covering material a fluoropolymer. A scattering portion prevents formation of a hot spot and the distal end of the tip. The index of refraction of silicone is temperature sensitive, decreasing as the temperature increases.
Cylindrical diffusers have technological limits. Optical fibers are weakened by any mechanical processing. Cylinder diffusers are often inflexible, having glass or plastic outer shells. The greatest hindrance may be the materials used, such as epoxy adhesives, plastics shells and connectors, and silicone fillers that may limit the power the device can support and maintain. Under high laser intensities, hard plastics such as polymethylmethacrylate resin and polystyrene resin, and some softer plastics, like polyethylene resin, experience a blackening phenomenon due to the generation of free carbon that deteriorates light irradiating performance.
Closely related to the cylindrical diffusers are spherical diffusers, which produce illumination essentially in a spherical pattern.
U.S. Pat. No. 4,693,556 discloses an optical radiator that produces a spherical pattern of light. A short exposed core from an optical fiber is dipped into a scattering medium composed of powdered quartz and an optical adhesive. The medium is shaped into a spherical pattern then cured. The diffuser may have a uniform output, but would be impractical to manufacture on a large scale.
U.S. Pat. No. 5,429,635 discloses a fiber optic diffuser for photodynamic therapy comprising a core with an exposed distal end having a conical configuration, which is covered by a cap comprised of polycarbonate and a light scattering material, having a cavity filled with air or a substantially transparent material having a low index of refraction surrounding the fiber core. The cap is threadably attached to a buffer layer on the fiber with a wicking adhesive. This would be difficult to manufacture and the materials would limit the application environments.
A newer class of optical diffusers involves the use of elastomers.
U.S. Pat. No. 5,269,777 discloses a diffusion tip for an optical fiber comprising a silicone core abutted to the end of a conventional optical fiber, an outer layer of silicone plus a scatterer, and a final “cladding” of plastic tubing
B J Associates
Ceram Optec Industries Inc.
Lee John D.
Skutnik Bolesh J.
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