Abstract:
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.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to fiber optics and, more particularly, to a cladding configuration for increasing the efficiency of a multiclad optical fiber.  
         [0003]     2. Background Art  
         [0004]     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.  
         [0005]     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.  
         [0006]     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 in  FIG. 1  of the prior art, wherein the optical fiber is shown at  10  and the side-pumping input fiber is shown at  11 . The side-pumping fiber  11  is the pump source for the optical fiber  10 . The optical fiber  10  has a doped core  12 , a first cladding  13 , and a second cladding  14 . The second cladding  14  defines the outer periphery of the optical fiber  10 . A portion of the second cladding  14  is removed so as to expose the first cladding  13  of the optical fiber  10 . The side-pumping input fiber  11  has a core  15  and a first cladding  16 . A portion of the first cladding  16  of the side-pumping input fiber  11  is removed such that the first cladding  15  is exposed. Accordingly, the optical fiber  10  and the side-pumping input fiber  11  are interconnected by the exposed portions of the first cladding  13  of the optical fiber  10  and the core  15  of the side-pumping input fiber  11  being coplanar. An affixing material (not visible) may bond the optical fiber  10  to the side-pumping input fiber  11 . The indexes of refraction are such that light from the side-pumping input fiber  11  is coupled into the optical fiber  10  to potentially be absorbed by the doped core  12 . The interface surface between the pump source (i.e., the fiber  11 ) and the optical fiber  10  can thus be adjusted, so as to maximize the amount of the light from the pump source reaching the optical fiber  10 , and thus improving the coupling efficiency therebetween.  
         [0007]     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  
       [0008]     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.  
         [0009]     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.  
         [0010]     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.  
         [0011]     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.  
         [0012]     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.  
         [0013]     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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof and in which:  
         [0015]      FIG. 1  is a longitudinal cross-section view of a coupling configuration of the prior art between an optical fiber and a power input;  
         [0016]      FIG. 2  is a cross-section view of an optical fiber system in accordance with the present invention;  
         [0017]      FIG. 3  is a top plan view, fragmented, of the optical fiber system in a coupling configuration of the present invention with a power source;  
         [0018]      FIG. 4  is a top plan view, fragmented, of the optical fiber system in an alternative coupling configuration of the present invention with a power source;  
         [0019]      FIG. 5  is a perspective view, partly cross-sectioned, of an arrangement producing the optical fiber system of the present invention;  
         [0020]      FIG. 6  is a perspective view, partly cross-sectioned, of an alternative arrangement producing the optical fiber system of the present invention;  
         [0021]      FIG. 7A  is a cross-section view of the optical fiber system in accordance with a second embodiment of the present invention;  
         [0022]      FIG. 7B  is a cross-section view of the optical fiber system in accordance with a third embodiment of the present invention;  
         [0023]      FIG. 7C  is a cross-section view of the optical fiber system in accordance with a fourth embodiment of the present invention;  
         [0024]      FIG. 8  is a cross-section view of the optical fiber system in accordance with a fifth embodiment of the present invention;  
         [0025]      FIG. 9  is a schematic longitudinal cross-section of the optical fiber system of  FIG. 8 , in a coupling configuration with a power input;  
         [0026]      FIG. 10  is a top plan view of the optical fiber system in accordance with a sixth embodiment of the present invention, in a coupling configuration with a power input;  
         [0027]      FIG. 11  is a cross-section view taken along cross-section line XI-XI of  FIG. 10 ;  
         [0028]      FIG. 12  is a top plan view of the optical fiber system in accordance with a seventh embodiment of the present invention, in a coupling configuration with a power input;  
         [0029]      FIG. 13  is a cross-section view taken along cross-section line XIII-XIII of  FIG. 12 ;  
         [0030]      FIG. 14  is a top plan view of the optical fiber system in accordance with an eighth embodiment of the present invention, in a coupling configuration with a power input;  
         [0031]      FIG. 15  is a cross-section view taken along cross-section line XV-XV of  FIG. 14 ; and  
         [0032]      FIG. 16  is a schematic top plan view of power input arrays to be used with the optical fiber system of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]     Referring now to the drawings, and more particularly to  FIG. 2 , an optical fiber system in accordance with the present invention is generally shown at  20 . The optical fiber system  20  has contour fibers  21 A and  21 B, and one optical fiber  22 , having portions  22 A,  22 B and  22 C. The portions  22 A,  22 B and  22 C of the optical fiber  22  are cross-sections at various longitudinal positions of the optical fiber  22 . As will be described hereinafter, the optical fiber  22  is arranged such that portions thereof are side by side. For instance, as shown in  FIGS. 5 and 6 , cylindrical and annular arrangements are shown forming the optical fiber system  20 .  
         [0034]     The optical fiber  22  has a doped core  23 , a first cladding  24  and a second cladding  25 . The second cladding  25  covers a pair of opposed surfaces of the first cladding  24 , whereby it is referred to as fractional. This configuration allows for side-by-side portions of the optical fiber  22  (i.e., portions  22 A and  22 B, or portions  22 B and  22 C) to have longitudinal portions of the first cladding  24  coplanar (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 fiber  22  is a typical optical fiber, wherein the index of refraction increases from the fractional cladding  25  to the first cladding  24 , and from the first cladding  24  to the doped core  23 , whereby light will be guided toward the doped core  23  so as to maximize and/or optimize the amount of light absorbed by the doped core  23 .  
         [0035]     Returning to  FIG. 2 , the contour fibers  21 A and  21 B are shown both having a core  24 ′ (which can be single mode or multimode) and a cladding  25 ′. The cladding  25 ′ covers three of the four faces of the core  24 ′, such that, in the optical fiber system  20 , the first cladding  24  and the core  24 ′ are concealed by the fractional cladding  25  and  25 ′. The core  24 ′ is preferably of the same material, with the same index of refraction as the first cladding  24 , whereas the fractional cladding  25 ′ is preferably of the same material and has the same index of refraction as the fractional cladding  25 . 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 cladding  25  of the optical fiber  22 , to reflect/guide light of the optical fiber  22  projected thereon.  
         [0036]     Referring to  FIG. 3 , coupling means  30  is shown mounted to the optical fiber system  20 . More specifically, the coupling means  30  is illustrated as a triangular base prism, positioned so as to longitudinally overlap the portions  22 A,  22 B and  22 C of the optical fiber  22 . The prism has a surface  31  being shaped as an elongated rectangle. Therefore, a bar of lights/lasers can be coupled to the surface  31 , so as to transmit pump power to the optical fiber system  20  via the coupling means  30 . It is observed that, with the above-described coupling configuration, the coupling surface between the power input (via the coupling means  30 ) and the optical fiber system  20  can 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 system  20 .  
         [0037]     The coupling means  30  can be mounted directly onto the second cladding  25  and cladding  25 ′. Alternatively, a portion (not shown) of the second cladding  25  and cladding  25 ′ may be removed from the optical fiber  22  and contour fibers  21 , respectively, such that the coupling means  30  directly contacts the first cladding  24  and core  24 ′. In either case, the indexes of refraction must be chosen to maximize the amount of light from the power input pumped in the first cladding  24  and core  24 ′ to increase the amount of light absorbed by the doped core  23 .  
         [0038]     Referring to  FIG. 4 , coupling means  40  are shown mounted to the optical fiber system  20 . However, as opposed to the embodiment of  FIG. 3 , the coupling means  40  are mounted to lateral portions of the contour fibers  21 A and  21 B. The coupling means  40  can be mounted directly to the cladding  25 ′ or, alternatively, to the core  24 ′ (not illustrated in  FIG. 2 ).  
         [0039]     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 core  23  to be absorbed thereby. Yet, the optical fiber  22  has a simple cross-section (e.g., square, as illustrated in  FIG. 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 core  23 . Such optical fibers with more complex cross-sections can also be used with the optical fiber system  20  (although not shown).  
         [0040]     Referring to  FIG. 5 , an arrangement of the optical fiber  22  in accordance with the optical fiber system  20  is shown at  50 . In this arrangement, the optical fiber  22  is rolled onto a cylinder  51 , so as to form a three-dimensional spiral. A portion of the optical fiber  22  has been removed to illustrate the cross-section. The contour fibers  21 A and  21 B are also shown in  FIG. 5 , preventing the light from being transmitted out of the optical fiber system  20 . The power input may be mounted to the optical fiber configuration  20  according to the embodiments of  FIG. 3  or  FIG. 4 , or may be coupled in any other suitable way. For instance, a free end of the optical fiber  22  or of the contour fibers  21 A and/or  21 B can be coupled to a power input. Obviously, the optical fiber  22  is connected to an output downstream of the spiral. Moreover, contour fibers  21 A and  21 B can be made of many sections in order to increase the number of pump inputs  
         [0041]     Referring to  FIG. 6 , another arrangement of the optical fiber  22  in accordance with the optical fiber system  20  is shown at  60 . In this arrangement, the optical fiber  22  is spiraled on a surface to form a two-dimensional spiral (i.e., a disk). Once more, a portion of the optical fiber  22  has 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 cladding  24  to be side by side.  
         [0042]     Referring to  FIGS. 7A, 7B  and  7 C, optical fibers  72 ,  72 ′ and  72 ″, respectively, of alternative cross-sections are shown, to give optical fiber systems  70 ,  70 ′ and  70 ″. The optical fiber systems  70 ,  70 ′ and  70 ″ are likely to be more costly to produce than the system  20  because, for example, of the two different contour fibers (generally illustrated at  71 A,  71 B in  FIG. 7A ,  71 ′ in  FIG. 7B , and at  71 A″ and  71 B″ in  FIG. 7C ), and because of their more complex shapes. However, it is anticipated that the concave/convex coupling of the configurations  70  and  70 ″ of  FIGS. 7A and 7C , respectively, will improve the efficiency of respective fibers  72  and  72 ″ due to improved contact therebetween.  
         [0043]     Referring to  FIG. 8 , an optical fiber system in accordance with another embodiment of the present invention is shown at  80 . The optical fiber system  80  is similar to the optical fiber system  20  of  FIG. 2  in that it has the contour fiber  21 A and  21 B and the optical fiber  22  arranged, for instance, in a spiral to have fiber portions  22 A,  22 B and  22 C longitudinally adjacent to one another. Additionally, a pumping fiber  81  is positioned between the fiber portions  22 A and  22 B, and  22 B and  22 C. The pumping fiber  81  has a core  82  and a fractional cladding  83 . The core  82  has such properties, so as to enable light transmission therethrough from, for instance, the core portion  82  to the fiber portions  22 A and  22 B or to  22 B and  22 C. For instance, refractive index of core  82  and first cladding  24  are preferably of the same value. Moreover, the cladding  83  is preferably the same, or has the same properties, as the second cladding  25 , to conceal the light with the core  82 . The pumping fiber  81  is provided to couple input power to the optical fiber  22 . As shown in  FIG. 9 , the pumping fiber  81  can have a beveled end at  45  degrees, whereat light  91  will be coupled therein from a power input, herein laser diode  92 . An optical element  93  is provided to collimate light  91  so as to optimize the coupling of light into the pumping fiber  81 . According to the arrangement of the optical fiber system  80  (e.g., in a 3-D spiral as in  FIG. 5  or as a 2-D spiral according to  FIG. 6 ), it is anticipated that the light coupled into the pumping fiber  80  will have reached the optical fiber  22  after one revolution and will have then mostly been absorbed by the doped core  23 . Therefore, the pumping fiber  80  has a length generally equal to one turn of the spiral. This will make place for the embodiments of FIGS.  10  to  15 , wherein this length of pumping fiber  81  allows for a plurality of laser diodes to be coupled to the optical fiber systems.  
         [0044]     Referring to  FIGS. 10 and 11 , an optical fiber system in accordance with a further embodiment of the present invention is generally shown at  100 . The optical fiber system  100  has the optical fiber  22 , shown having four longitudinal portions, namely  22 A,  22 B,  22 C and  22 D, as well as the contour fibers  21 A and  21 B. Four pumping fibers  101 , each having a core  102  and a cladding  103 , are provided to couple light from laser diodes  104  ( FIG. 10 ) into the optical fiber  22 . In  FIG. 10 , the pumping fibers  101  and components thereof are affixed with a letter so as to be differentiated from one another. As mentioned previously, the pumping fibers  101  have a length generally equal to one revolution of the optical fiber  22 , so each of the pumping fibers  101  is shown having a leading beveled end  105  and a trailing end  106 . The leading ends  105  are opposite to the respective laser diodes  104 . The trailing ends  106  are cut just short of one revolution in the given arrangement of the optical fiber system  100  (e.g., according to the arrangements of  FIG. 5  or  6 ), whereby a subsequent pumping fiber  101  can be inserted between the optical fiber portions, to enable the leading beveled ends of the pumping fibers  101  to be aligned with the line/bar of laser diodes  104 . [ 0043 ] Referring to  FIGS. 12 and 13 , an optical fiber system in accordance with a further embodiment of the present invention is generally shown at  120 . The optical fiber system  120  has all the same components as the optical fiber system  100  of  FIG. 10 , with additionally a spacing fiber  121 . The optical fiber system  120  has the spacing fiber  121  so as to have the leading beveled ends  105  of the pumping fibers  101  each opposite one of the laser diodes  104 . The spacing fiber  121  has a core  122  and a cladding  123 , of suitable indexes of refraction for facilitating the coupling of light into the optical fiber  22 . Fiber  121  can have a geometry (width) such that each fiber  101  is facing an emitter of a pump bar with a regular and predetermined pitch.  
         [0045]     Referring to  FIGS. 14 and 15 , an optical fiber system in accordance with a further embodiment of the present invention is generally shown at  140 . The optical fiber system  140  has all the same components as the optical fiber system  100  of  FIG. 10 . However, the leading beveled ends  105  of the pumping fibers  101  are positioned to be opposite to an array of laser diodes  141 . Such an array is shown at  160  in  FIG. 16  and can have as many emitters as desired.