Achromatic overclad fiber optic coupler

Abstract

An achromatic fiber optic coupler of the type wherein a plurality of single-mode optical fibers, each having a core and a cladding, are fused together along a portion of the lengths thereof to form a coupling region. The propagation constants of the fibers are preferably equal; however if the fiber claddings have different refractive indices, the lowest cladding refractive index is n 2 . A matrix glass body of refractive index n 3 surrounds the coupling region, n 3 being lower than n 2 by such an amount that the value of Δ 2-3 is less than 0.125%, wherein Δ 2-3 equals (n 2 2- n 3 2 )2n 2 2 .

What is claimed is: 
     
       1. .[.A.]. .Iadd.An achromatic .Iaddend.coupler comprising a body of matrix glass, and   a plurality of optical waveguide paths extending through said body, each of said paths comprising a core region surrounded by a cladding region of refractive index less than that of said core region, the lowest refractive index of the cladding regions of said paths being n 2 ,   at least a portion of one of said optical waveguide paths being disposed in close proximity to another of said paths to form a coupling region,   the refractive index of at least that region of said body adjacent said paths being n 3 , where n 3  is lower than n 2  by such an amount that the value of Δ 2-3  is less than .[.0.125%.]. .Iadd.0.07%.Iaddend., wherein Δ 2-3  equals (n 2   2  -n 3   2 )/2n 2   2 .   
     
     
       2. A coupler in accordance with claim 1 wherein said waveguide paths comprise optical fibers. 
     
     
       3. A coupler in accordance with claim 2 wherein said matrix glass is a cylindrically-shaped body through which said fibers longitudinally extend, said body having first and second opposed ends and a midregion, the diameter of the central portion of said midregion and the diameters of said optical fibers in said central portion of said midregion being smaller than the diameters thereof at the ends of said body. 
     
     
       4. .[.A.]. .Iadd.An achromatic .Iaddend.fiber optic coupler comprising an elongated body of matrix glass, said body having first and second opposed ends and a midregion,   a plurality of optical fibers extending longitudinally through said body, each of said fibers comprising a core surrounded by a cladding of refractive index less than that of said core, the lowest refractive index of the claddings of said plurality of optical fibers being n 2 ,   the refractive index .Iadd.of at least .Iaddend.that region of said body adjacent said fibers being n 3 , where n 3  is lower than n 2  by such an amount that the value of Δ 2-3  is less than .[.0.125%.]. .Iadd.0.07%.Iaddend., wherein Δ 2-3  equals (n 2   2  -n 3   2 )/2n 2   2 ,   the diameter of the central portion of said midregion and the diameters of said optical fibers in said central portion of said midregion being smaller than the diameters thereof at the ends of said body, whereby a portion of the optical power propagating in one of said fibers couples to the other of said fibers.   
     
     
       5. A fiber optic coupler in accordance with claim 4 wherein M optical fibers extend from said first end of said body and N optical fibers extend from said second end of said body, wherein M≧1 and N≧2. .[.6. A fiber optic coupler in accordance with claim 4 wherein the cladding refractive index n 2  ' of at least one of said fibers is greater than n 2  by an amount such that Δ clads  is no greater than 0.03%, wherein Δ clads  is (n 2   2  -n 2   
     
     
        ' 2 )/2n 2   2 . 7. A fiber optic coupler in accordance with claim 4 wherein said matrix glass comprises SiO 2  doped with up to 2.8 wt. % 
     
     
        B 2  O 3 . 8. A coupler in accordance with claim 4 wherein said 
     
     
        matrix glass is a cylindrically-shaped body. 9. A fiber coupler in accordance with claim 8 wherein M optical fibers extend from said first end of said body and N optical fibers extend from said second end of said 
     
     
        body, wherein M≧1 and N≧2. 10. A fiber optic coupler in accordance with claim 8, wherein the cladding refractive index n 2  ' of at least one of said fibers is greater than n 2  by an amount such that Δ clads  is no greater than 0.03%, wherein Δ clads   
     
     
        is (n 2   2  -n 2   '2 )/2n 2   2 . 11. A fiber optic coupler in accordance with claim 8 wherein said matrix glass comprises 
     
     
        SiO 2  doped with up to 2.8 wt. % B 2  O 3 . 12. In an optical coupler of the type comprising at least two adjacent waveguide paths that are elongated and in close proximity to one another in a narrowed coupling region to induce coupling between said waveguide paths, each of said waveguide paths comprising a core region surrounded by a cladding region of refractive index less than that of said core region, the lowest refractive index of the cladding regions of said waveguides being n 2 , said coupling region being surrounded by matrix material of refractive index n 3 , the improvement wherein n 3  is lower than n 2  by such an amount that the value of Δ 2-3  is less than .[.0.125%.]. .Iadd.0.045.Iaddend., wherein Δ 2-3  equals (n 2   2   
     
     
        -n 3   2 )/2n 2   2 . 13. A coupler in accordance with claim 12 
     
     
        wherein said waveguide paths comprise optical fibers. 14. A coupler in accordance with claim 13 wherein said matrix glass is an elongated body through which said fibers longitudinally extend, said body having a first and second opposed ends and a midregion, the diameter of the central portion of said midregion and the diameters of said optical fibers in said central portion of said midregion being smaller than the diameters thereof 
     
     
        at the ends of said body. 15. A coupler in accordance with claim 14 
     
     
        wherein said body is cylindrically-shaped. .Iadd.16.  A coupler in accordance with claim 1 wherein Δ 2-3  is less than 0.045%. .Iaddend. .Iadd.17. A coupler in accordance with claim 5 wherein Δ 2-3  is less than 0.045%. .Iaddend. .Iadd.18. A coupler comprising a body of matrix glass, and   a plurality of optical fibers extending through said body, each of said optical fibers comprising a core surrounded by cladding layer of refractive index less than that of said core, the lowest refractive index of the cladding layers of said optical fibers being n 2 ,   at least a portion of one of said optical fibers being disposed in close proximity to another of said optical fibers to form a coupling region,   the refractive index of at least that region of said body adjacent said paths being n 3 , where n 3  is lower than n 2  by such an amount that the value of Δ 2-3  is less than 0.045%, wherein Δ 2-3  equals (n 2   2  -n 3   2 )/2n 2   2 . .Iaddend. .Iadd.19. An achromatic coupler comprising   a body of matrix glass, and   a plurality of optical fibers extending through said body, each of said fibers comprising a core surrounded by a cladding layer of refractive index less than that of said core, one of said fibers extending from both ends of said body and N fibers extending from only one end of said body, wherein N≧3,   said N fibers being equally spaced around said one fiber in a coupling region where a portion of each of said N fibers is disposed in close proximity to a portion of said one fiber,   the refractive index of the cladding layers of said N fibers being different from the refractive index of the cladding layer of said one fiber, the refractive index of the cladding layers of said N fibers being n 2  if the refractive index of the cladding layer of said one fiber is n 2  ' and the refractive index of the cladding layers of said N fibers being n 2  ' if the refractive index of the cladding layer of said one fiber is n 2 , n 2  being lower than n 2  ',   the refractive index of at least that region of said body adjacent said paths being n 3 , where n 3  is lower than n 2  by such an amount that the value of Δ 2-3  is less than 0.125%, wherein Δ 2-3  equals (n 2   2  -n 3   2 )/2 2   2 . .Iaddend. .Iadd.20. An achromatic fiber optic coupler comprising   an elongated body of matrix glass, said body having first and second opposed ends and a midregion,   a plurality of optical fibers extending longitudinally through said body, each of said fibers comprising a core surrounded by a cladding of refractive index less than that of said core, the lowest refractive index of the claddings of said plurality of optical fibers being n 2 ,   the cladding refractive index n 2  ' of at least one of said fibers is greater than n 2  by an amount such that Δ clads  is no greater than 0.03%, wherein Δ clads  is (n 2   2  -n 2   '2 )/2n 2   2 ,   the refractive index of at least that region of said body adjacent said fibers being n 3 , where n 3  is lower than n 2  by such an amount that the value of Δ 2-3  is less than 0.125%, wherein Δ 2-3  equals (n 2   2  -n 3   2 )/2n 2   2 ,   the diameter of the central portion of said midregion and the diameters of said optical fibers in said central portion of said midregion being smaller than the diameters thereof at the ends of said body, whereby a portion of the optical power propagating in one of said fibers couples to the other of said fibers. .Iaddend.