Fractional cladding for optical fibers

An optical fiber system comprises an optical fiber having a doped core and a first cladding about the doped core. The optical fiber has a first longitudinal portion and a second longitudinal portion, and is arranged such that the first longitudinal portion and the second longitudinal portion are longitudinally side by side. The first cladding of the first longitudinal portion is adjacent to the first cladding of the second longitudinal portion such that light propagating in the first cladding can move laterally from the first longitudinal portion to the second longitudinal portion to increase the amount of light reaching the doped core. The optical fiber is adapted to be coupled to a power input and has an output end for outputting light emitted by the doped core. The second fractional cladding about the first cladding conceals light in the first cladding.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to fiber optics and, more particularly, to a cladding configuration for increasing the efficiency of a multiclad optical fiber.

2. Background Art

There is a demand for fiber optics of increased output power. Amongst the solutions for obtaining fiber optics of increased output power, the input pump power (for example, a pump from a laser diode) can be increased. However, the coupling of the input power into the optical fiber is subjected to losses of light as the pump width typically increases with the output power, and coupling efficiencies then limit the upgrading of the power input. Pumping also increases in cost with lower coupling efficiencies.

Another solution to increasing the output power of an optical fiber system is to increase the coupling efficiency between the input power and the optical fiber. The pump source is positioned at an input end of an optical fiber. The diameter of the optical fiber is a limitation to the coupling efficiency. Hence, various configurations have been provided to overcome this limitation and thereby increase the input pump power in optical fibers. U.S. Pat. No. 5,268,978, issued to Po et al. on Dec. 7, 1993, discloses an optical fiber laser and geometric coupler. More precisely, the coupling efficiency between a light source and an output optical fiber is increased by providing coupling means and a cylindrical lens therebetween. The coupling means include a plurality of input optical fibers having respective input ends, each associated with a light-emitting facet of the light source. Each of these input optical fibers has an output end. A cylindrical lens is positioned between the output ends of the plurality of input optical fibers and the output optical fiber to focus light emerging from the facets onto the input end of the output optical fiber.

It is also known to increase the coupling surface between the power input and the optical fiber. For instance, U.S. Pat. No. 4,815,079, issued to Snitzer et al. on Mar. 21, 1989, describes a fiber-optic arrangement wherein a side-pumping input fiber is coupled longitudinally to an optical fiber so as to increase the coupling surface between the power input and the optical fiber. This is generally illustrated inFIG. 1of the prior art, wherein the optical fiber is shown at10and the side-pumping input fiber is shown at11. The side-pumping fiber11is the pump source for the optical fiber10. The optical fiber10has a doped core12, a first cladding13, and a second cladding14. The second cladding14defines the outer periphery of the optical fiber10. A portion of the second cladding14is removed so as to expose the first cladding13of the optical fiber10. The side-pumping input fiber11has a core15and a first cladding16. A portion of the first cladding16of the side-pumping input fiber11is removed such that the first cladding15is exposed. Accordingly, the optical fiber10and the side-pumping input fiber11are interconnected by the exposed portions of the first cladding13of the optical fiber10and the core15of the side-pumping input fiber11being coplanar. An affixing material (not visible) may bond the optical fiber10to the side-pumping input fiber11. The indexes of refraction are such that light from the side-pumping input fiber11is coupled into the optical fiber10to potentially be absorbed by the doped core12. The interface surface between the pump source (i.e., the fiber11) and the optical fiber10can thus be adjusted, so as to maximize the amount of the light from the pump source reaching the optical fiber10, and thus improving the coupling efficiency therebetween.

Although the coupling efficiency between pump source and optical fiber has improved as a result of novel configurations such as the ones described above, other configurations providing further coupling efficiency improvements and doped core absorption efficiency are desirable particularly for taking advantage of still higher power pump sources.

SUMMARY OF INVENTION

It is therefore an aim of the present invention to provide a novel optical fiber configuration for improving the coupling efficiency of high-power pump source or sources into an optical fiber.

It is a further aim of the present invention to provide a novel optical fiber configuration for improving and adjusting the absorption efficiency of a doped core fiber of an optical fiber.

It is a still further aim of the present invention to provide fiber optics designs that allow adjustment of a length and a width of contact between the pump source and the optical fiber.

It is a still further aim of the present invention that the optical fiber configuration includes an increase in interface surface between a power input and an optical fiber.

Therefore, in accordance with the present invention, an optical fiber system comprising an optical fiber having a doped core and a first cladding about the doped core, the optical fiber having a first longitudinal portion and a second longitudinal portion, the optical fiber being arranged such that the first longitudinal portion and the second longitudinal portion are longitudinally side by side with a portion of the first cladding of the first longitudinal portion being adjacent to a portion of the first cladding of the second longitudinal portion such that light propagating in the first cladding can move laterally from the first longitudinal portion to the second longitudinal portion to increase the amount of light reaching the doped core, the optical fiber adapted to be coupled to a power input to receive a light input and having an output end for outputting light emitted by the doped core; and a second fractional cladding about the first cladding to conceal light in the first cladding.

Further in accordance with the present invention, there is provided an optical fiber system comprising an optical fiber having a doped core, a first cladding about the doped core, and a second cladding partially covering the first cladding such that the first cladding is exposed longitudinally, the optical fiber having at least a first longitudinal portion and a second longitudinal portion, the optical fiber being arranged such that the first longitudinal portion and the second longitudinal portion are longitudinally side by side with an exposed portion of the first cladding of the first longitudinal portion being adjacent to an exposed portion of the first cladding of the second longitudinal portion such that light propagating in the first cladding can move laterally from the first longitudinal portion to the second longitudinal portion to increase the amount of light reaching the doped core, the optical fiber adapted to be coupled to a power input to receive a light input and having an output end for outputting light emitted by the doped core; and at least one contour fiber having an index of refraction as a function of the optical fiber, the at least one contour fiber covering further exposed portions of the first cladding of the doped core fiber to conceal light in the first cladding.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, and more particularly toFIG. 2, an optical fiber system in accordance with the present invention is generally shown at20. The optical fiber system20has contour fibers21A and21B, and one optical fiber22, having portions22A,22B and22C. The portions22A,22B and22C of the optical fiber22are cross-sections at various longitudinal positions of the optical fiber22. As will be described hereinafter, the optical fiber22is arranged such that portions thereof are side by side. For instance, as shown inFIGS. 5 and 6, cylindrical and annular arrangements are shown forming the optical fiber system20.

The optical fiber22has a doped core23, a first cladding24and a second cladding25. The second cladding25covers a pair of opposed surfaces of the first cladding24, whereby it is referred to as fractional. This configuration allows for side-by-side portions of the optical fiber22(i.e., portions22A and22B, or portions22B and22C) to have longitudinal portions of the first cladding24coplanar (although the side-by-side optical fiber portions are shown separated throughout most of the figures to better illustrate the cross-sections of the optical fiber, they are in fact in contact). The optical fiber22is a typical optical fiber, wherein the index of refraction increases from the fractional cladding25to the first cladding24, and from the first cladding24to the doped core23, whereby light will be guided toward the doped core23so as to maximize and/or optimize the amount of light absorbed by the doped core23.

Returning toFIG. 2, the contour fibers21A and21B are shown both having a core24′ (which can be single mode or multimode) and a cladding25′. The cladding25′ covers three of the four faces of the core24′, such that, in the optical fiber system20, the first cladding24and the core24′ are concealed by the fractional cladding25and25′. The core24′ is preferably of the same material, with the same index of refraction as the first cladding24, whereas the fractional cladding25′ is preferably of the same material and has the same index of refraction as the fractional cladding25. It is pointed out that the contour fiber could simply be a cladding having an index of refraction at most equal to the index of refraction of the fractional cladding25of the optical fiber22, to reflect/guide light of the optical fiber22projected thereon.

Referring toFIG. 3, coupling means30is shown mounted to the optical fiber system20. More specifically, the coupling means30is illustrated as a triangular base prism, positioned so as to longitudinally overlap the portions22A,22B and22C of the optical fiber22. The prism has a surface31being shaped as an elongated rectangle. Therefore, a bar of lights/lasers can be coupled to the surface31, so as to transmit pump power to the optical fiber system20via the coupling means30. It is observed that, with the above-described coupling configuration, the coupling surface between the power input (via the coupling means30) and the optical fiber system20can be substantially the same as the output surface of the power input (not shown). Therefore, it is not essential to have optical elements that will have the light input from the power input converge into the optical fiber system20.

The coupling means30can be mounted directly onto the second cladding25and cladding25′. Alternatively, a portion (not shown) of the second cladding25and cladding25′ may be removed from the optical fiber22and contour fibers21, respectively, such that the coupling means30directly contacts the first cladding24and core24′. In either case, the indexes of refraction must be chosen to maximize the amount of light from the power input pumped in the first cladding24and core24′ to increase the amount of light absorbed by the doped core23.

Referring toFIG. 4, coupling means40are shown mounted to the optical fiber system20. However, as opposed to the embodiment ofFIG. 3, the coupling means40are mounted to lateral portions of the contour fibers21A and21B. The coupling means40can be mounted directly to the cladding25′ or, alternatively, to the core24′ (not illustrated inFIG. 2).

Light will therefore be coupled laterally and thus be transmitted from optical fiber portion to optical fiber portion, and is thus likely to cross the doped core23to be absorbed thereby. Yet, the optical fiber22has a simple cross-section (e.g., square, as illustrated inFIG. 2B), that involves relatively low costs in manufacturing. More complex cross-sections (e.g., hexagonal cross-section or cross-sections involving a nonconcentric doped core) have been provided to increase the probability that light crosses the doped core so as to maximize the amount of light absorbed by the doped core23. Such optical fibers with more complex cross-sections can also be used with the optical fiber system20(although not shown).

Referring toFIG. 5, an arrangement of the optical fiber22in accordance with the optical fiber system20is shown at50. In this arrangement, the optical fiber22is rolled onto a cylinder51, so as to form a three-dimensional spiral. A portion of the optical fiber22has been removed to illustrate the cross-section. The contour fibers21A and21B are also shown inFIG. 5, preventing the light from being transmitted out of the optical fiber system20. The power input may be mounted to the optical fiber configuration20according to the embodiments ofFIG. 3orFIG. 4, or may be coupled in any other suitable way. For instance, a free end of the optical fiber22or of the contour fibers21A and/or21B can be coupled to a power input. Obviously, the optical fiber22is connected to an output downstream of the spiral. Moreover, contour fibers21A and21B can be made of many sections in order to increase the number of pump inputs.

Referring toFIG. 6, another arrangement of the optical fiber22in accordance with the optical fiber system20is shown at60. In this arrangement, the optical fiber22is spiraled on a surface to form a two-dimensional spiral (i.e., a disk). Once more, a portion of the optical fiber22has been removed to illustrate the cross-section. Although the above-described arrangements are preferred, other arrangements can be used to cause exposed portions of the first cladding24to be side by side.

Referring toFIGS. 7A,7B and7C, optical fibers72,72′ and72″, respectively, of alternative cross-sections are shown, to give optical fiber systems70,70′ and70″. The optical fiber systems70,70′ and70″ are likely to be more costly to produce than the system20because, for example, of the two different contour fibers (generally illustrated at71A,71B inFIG. 7A,71′ inFIG. 7B, and at71A″ and71B″ inFIG. 7C), and because of their more complex shapes. However, it is anticipated that the concave/convex coupling of the configurations70and70″ ofFIGS. 7A and 7C, respectively, will improve the efficiency of respective fibers72and72″ due to improved contact therebetween.

Referring toFIG. 8, an optical fiber system in accordance with another embodiment of the present invention is shown at80. The optical fiber system80is similar to the optical fiber system20ofFIG. 2in that it has the contour fiber21A and21B and the optical fiber22arranged, for instance, in a spiral to have fiber portions22A,22B and22C longitudinally adjacent to one another. Additionally, a pumping fiber81is positioned between the fiber portions22A and22B, and22B and22C. The pumping fiber81has a core82and a fractional cladding83. The core82has such properties, so as to enable light transmission therethrough from, for instance, the core portion82to the fiber portions22A and22B or to22B and22C. For instance, refractive index of core82and first cladding24are preferably of the same value. Moreover, the cladding83is preferably the same, or has the same properties, as the second cladding25, to conceal the light with the core82. The pumping fiber81is provided to couple input power to the optical fiber22. As shown inFIG. 9, the pumping fiber81can have a beveled end at45degrees, whereat light91will be coupled therein from a power input, herein laser diode92. An optical element93is provided to collimate light91so as to optimize the coupling of light into the pumping fiber81. According to the arrangement of the optical fiber system80(e.g., in a 3-D spiral as inFIG. 5or as a 2-D spiral according toFIG. 6), it is anticipated that the light coupled into the pumping fiber80will have reached the optical fiber22after one revolution and will have then mostly been absorbed by the doped core23. Therefore, the pumping fiber80has a length generally equal to one turn of the spiral. This will make place for the embodiments ofFIGS. 10 to 15, wherein this length of pumping fiber81allows for a plurality of laser diodes to be coupled to the optical fiber systems.

Referring toFIGS. 10 and 11, an optical fiber system in accordance with a further embodiment of the present invention is generally shown at100. The optical fiber system100has the optical fiber22, shown having four longitudinal portions, namely22A,22B,22C and22D, as well as the contour fibers21A and21B. Four pumping fibers101, each having a core102and a cladding103, are provided to couple light from laser diodes104(FIG. 10) into the optical fiber22. InFIG. 10, the pumping fibers101and components thereof are affixed with a letter so as to be differentiated from one another. As mentioned previously, the pumping fibers101have a length generally equal to one revolution of the optical fiber22, so each of the pumping fibers101is shown having a leading beveled end105and a trailing end106. The leading ends105are opposite to the respective laser diodes104. The trailing ends106are cut just short of one revolution in the given arrangement of the optical fiber system100(e.g., according to the arrangements ofFIG. 5or6), whereby a subsequent pumping fiber101can be inserted between the optical fiber portions, to enable the leading beveled ends of the pumping fibers101to be aligned with the line/bar of laser diodes104.

Referring toFIGS. 12 and 13, an optical fiber system in accordance with a further embodiment of the present invention is generally shown at120. The optical fiber system120has all the same components as the optical fiber system100ofFIG. 10, with additionally a spacing fiber121. The optical fiber system120has the spacing fiber121so as to have the leading beveled ends105of the pumping fibers101each opposite one of the laser diodes104. The spacing fiber121has a core122and a cladding123, of suitable indexes of refraction for facilitating the coupling of light into the optical fiber22. Fiber121can have a geometry (width) such that each fiber101is facing an emitter of a pump bar with a regular and predetermined pitch.

Referring toFIGS. 14 and 15, an optical fiber system in accordance with a further embodiment of the present invention is generally shown at140. The optical fiber system140has all the same components as the optical fiber system100ofFIG. 10. However, the leading beveled ends105of the pumping fibers101are positioned to be opposite to an array of laser diodes141. Such an array is shown at160inFIG. 16and can have as many emitters as desired.