Optical fiber diffusion tip for uniform illumination

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FIELD OF THE INVENTION

This invention relates to optical fiber assemblies for transmission and diffusion of laser radiation and, more particularly, to an optical fiber diffusion tip for producing a substantially uniform cylindrical radiation pattern over a prescribed axial length. The diffusion tip is particularly useful in laser balloon catheters utilized in coronary angioplasty, but is not limited to such use.

BACKGROUND OF THE INVENTION

Balloon angioplasty has been utilized for a number of years to treat coronary arteries narrowed by plaque deposits. A catheter having an inflatable balloon secured to its distal end is advanced through an artery to a narrowed region. The balloon is then inflated with a fluid from an external source, causing the narrowed region of the artery to be expanded. The balloon is then deflated and withdrawn. A serious problem associated with balloon angioplasty has been the occurrence in up to 30% of the cases of so called restenosis, either immediately after the procedure or within six months. Immediate restenosis, also known as abrupt reclosure, results from flaps or segments of plaque and plaque-ridden tissue which are formed during balloon angioplasty and which can block the artery. Such blockage of the artery requires emergency surgery and often results in death Furthermore, a surgical team is required to stand by during the balloon angioplasty procedure. Restenosis at a later time results from causes that are not totally known. Thrombus formation is believed to play an important part. Often repeat balloon angioplasty or surgery is required, and another episode of restenosis may occur.

A technique which has shown great promise for overcoming the problem of restenosis is the simultaneous application of heat and pressure to a plaque narrowed region of the artery. The technique is described by John F. Hiehle, Jr. et al in "Nd YAG Laser Fusion of Human Atheromatous Plaque-Arterial Wall Separations in Vitro", Am. J. Cardiology, Vol. 56, Dec. 1, 1985, pages 953-957 and by J. Richard Spears in U.S. Pat. No. 4,799,479 issued Jan. 24, 1989. In accordance with this technique, a catheter having an inflatable balloon at its distal end is advanced to a narrowed region of an artery and the balloon is inflated, as in the case of balloon angioplasty. However, in distinction to balloon angioplasty, sufficient heat is applied through the wall of the balloon to fuse the surrounding tissue and thereby eliminate the flaps which can later block the artery. One advantageous means of heating the surrounding tissue is by directing laser radiation through an optical fiber carried by the catheter and terminating within the balloon. The laser radiation is then directed through the balloon wall to cause heating of the surrounding tissue.

It has been found desirable to apply radiation which penetrates into the surrounding plaque and plaque-ridden tissue and the artery wall and heats that region by radiant heating, in distinction to conductive heating by the balloon. Furthermore, it has been found desirable to apply such radiation at a power level of 20-40 watts for times on the order of about 20 seconds. In applying laser radiation at relatively high power levels, it is important to provide a relatively uniform cylindrical radiation pattern over the length of the balloon. Otherwise, hot spots can produce localized burning of tissue, and cold spots can leave potentially dangerous tissue flaps unfused.

Prior art techniques have been disclosed for directing laser radiation outwardly from the tip of an optical fiber. A tapered optical fiber surrounded with a diffusing medium for laser radiation treatment of tumors is disclosed in U.K Patent Application No. 2,154,761, published Sep. 11, 1985. An optical fiber surrounded with a scattering medium for producing a cylindrical pattern of light at the tip of an optical fiber is disclosed in U.S. Pat. No. 4,660,925, issued Apr. 28, 1987 to McCaughan, Jr. A technique for roughening the surface of an optical fiber tip to cause wide angle radiation of laser energy is disclosed by H. Fujii et al in "Light Scattering Properties of a Rough-Ended Optical Fiber", Optics and Laser Technology, February 1984, pages 40-44.

The aforementioned Spears Patent No. 4,799,479, discloses an optical fiber that extends through a catheter and terminates in a light-disseminating tip located within an inflatable balloon. Spears teaches that the light-disseminating tip can be provided by removing the cladding from the fiber tip and roughening the fiber core surface. Spears also states that the light-disseminating tip can be made of an unspecified material which scatters laser energy.

U.S. Pat. No. 4,422,719 issued Dec. 27, 1983 to Orcutt discloses an optical illumination system including a transparent core surrounded by an unbonded sleeve. The surface of the core can have cuts or discontinuities to deflect light through the sleeve. In other embodiments, light is deflected through the sleeve by air bubbles in the core or by reflective particles embedded in the material of the sleeve.

U.S. Pat. No. 4,585,298 issued Apr. 29, 1986 to Mori discloses a photoradiator that is coupled to the end of an optical fiber. The photoradiator includes a transparent, light conducting member having a plurality of annular light-radiating strips on its outer surface. The strips have a higher refractive index than the light conducting portion so that light is coupled out of the photoradiator at the high refractive index strips. U.S. Pat. No. 4,195,907 issued Apr. 1, 1980 Zamja et al discloses the use of bubble-containing fibers. U.S. Pat. No. 4,466,697 issued Aug. 21, 1984 to Daniel discloses fibers having light-scattering particles in the core. The Zamja et al and Daniel patents illustrate techniques for emitting light from the sidewall of an optical fiber by providing scattering centers in the core of the optical fiber.

U.S. Pat. No. 4,363,533 issued Dec. 14, 1982 to Stowe et al discloses a fiber optic device which can be utilized as an acoustic transducer. The transducer comprises an optical fiber having an intermediate cladding between an inner core and an outer core. The intermediate cladding has an index of refraction lower than that of the inner core. Light is coupled from the inner core to the outer core as a result of penetration of the evanescent field through the cladding. The coupling between cores varies in response to mechanical pressure applied to the device. U.S. Pat. No. 4,679,894 issued Jul. 14, 1987 to Pavlath discloses a fiber optic coupler wherein light is coupled between adjacent fibers by evanescent field coupling.

All known prior art light-disseminating or light-diffusing optical fiber tips have one or more disadvantages, including a lack of flexibility or power handling capability, a tendency to break, a nonuniform radiation pattern and difficulties in fabrication.

It is a general object of the present invention to provide an improved optical fiber diffusion tip.

It is another object of the present invention to provide an optical fiber diffusion tip having a substantially uniform cylindrical radiation pattern.

It is a further object of the present invention to provide an improved laser balloon catheter

It is yet another object of the present invention to provide an optical fiber diffusion tip that is small in diameter and highly flexible.

It is still another object of the present invention to provide an optical fiber diffusion tip capable of delivering high power laser radiation.

It is another object of the present invention to provide a laser balloon catheter which produces substantially uniform heating of tissue surrounding the balloon.

SUMMARY OF THE INVENTION

According to the present invention, these and other objects and advantages are achieved in a laser transmission and diffusion assembly comprising an optical fiber having one end adapted for coupling to a source of laser radiation, and a tip assembly connected to the other end of the optical fiber for directing laser radiation outwardly in a generally radial direction. The tip assembly includes a core, a cladding around the core and a jacket around the cladding. The jacket has an index of refraction that is higher than that of both the core and the cladding. The cladding has an index of refraction that is lower than that of the core and has a thickness selected to transmit a portion of the laser radiation that is carried through the optical fiber, such that laser radiation penetrates through the cladding and the jacket over the length of the tip assembly.

A cladding with an index of refraction lower than that of the core reflects laser radiation back to the core when the angle of incidence of the radiation is less than the numerical aperture of the fiber. The tip assembly of the present invention operates by providing a cladding having a thickness that is about the same as or slightly less than the penetration depth of the evanescent field in the cladding. At each axial location along the tip assembly, a portion of the laser radiation is coupled from the core through the cladding and into the jacket. As a result, light is emitted generally radially along the length of the tip assembly. The amount of laser radiation that is coupled through the cladding is a function of the difference between the index of refraction of the core and the index of refraction of the cladding, the thickness of the cladding, the wavelength of the laser radiation being transmitted, the index of refraction of the jacket and the bending of the tip assembly. In a preferred embodiment, the cladding has a thickness of about one micrometer for a wavelength of 1.06 micrometers. The thickness of the cladding can be varied along the length of the tip assembly to provide a desired radiation pattern. In particular the thickness of the cladding can be tapered along the length of the tip assembly to provide a substantially uniform cylindrical radiation pattern.

According to another aspect of the present invention, there is provided a laser balloon catheter comprising an elongated, flexible tube having a distal end and a proximal end, an inflatable balloon secured to the flexible tube at or near the distal end thereof, means for inflating and deflating the balloon, an optical fiber for carrying laser radiation through the flexible tube into the balloon, and tip assembly means located within the balloon and coupled to the optical fiber for directing laser radiation outwardly through a major portion of the balloon surface. The tip assembly means includes a core, a cladding around the core, and a jacket around the cladding. The cladding has an index of refraction that is lower than that of the core and has a thickness selected to transmit at least a portion of the laser radiation which penetrates from the core into the cladding. The jacket has an index of refraction that is higher than that of the core and the cladding, such that laser radiation carried through the optical fiber penetrates through the cladding and the jacket over the length of the tip assembly means.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention together with other and further objects, advantages and capabilities thereof, reference is made to the accompanying drawings which are incorporated herein by reference and in which:

FIG. 1 is a fragmented illustration of a laser balloon catheter incorporating an optical fiber diffusion tip in accordance with the present invention;

FIG. 2 is an enlarged cross-sectional view of the distal end of the laser balloon catheter taken along the lines 2--2 of FIG. 1;

FIG. 3 is an enlarged cross-sectional view of an optical fiber diffusion tip in accordance with the present invention;

FIG. 4 is an enlarged cross-sectional view of the optical fiber diffusion tip taken along the lines 4--4 of FIG. 3; and

FIG. 5A is an enlarged, partial cross-sectional view of a conventional optical fiber showing penetration of laser radiation into the cladding;

FIG. 5B is a graphic representation of laser intensity as a function of radial position in the conventional optical fiber of FIG. 5A;

FIG. 5C is an enlarged, partial cross-sectional view of the diffusion tip of the invention showing penetration of laser radiation through the cladding;

FIG. 5D is a graphic representation of laser intensity as a function of radial position in the diffusion tip of FIG. 5C;

FIGS. 6A-6D are graphic representations of relative laser intensity as a function of axial position along the diffusion tip for different cladding configurations.

DETAILED DESCRIPTION OF THE INVEN