Fiber optic coupler exhibiting low nonadiabatic loss

Abstract

A fiber optic coupler comprising a plurality of single-mode optical fibers, each of which is tapered to form a small diameter section that extends in contiguous relationship with the small diameter sections of the other fibers to form a coupling region. Each of the fibers has a core surrounded by a cladding of refractive index lower than that of the core. At least one of the fibers has a refractive index pedestal of refractive index n i between said core and cladding, wherein n 1 >n i >n 2 , n 1 and n 2 being the refractive indices of the core and cladding, respectively.

We claim:  
     
       1. A fiber optic coupler comprising 
       a plurality of single-mode optical fibers, each having a biconical taper section, the taper sections of said fibers extending in contiguous relationship to form a coupling region, each of said fibers having a core surrounded by a cladding of refractive index lower than that of said core, at least one of said fibers having a refractive index pedestal having a maximum refractive index n i  between said core and cladding, wherein n 1 >n i >n 2 , n 1  being the maximum refractive index of the core of said at least one fiber and n 2  being the refractive index of the cladding of said at least one fiber, those portions of said fibers that are in said coupling region being surrounded by a medium having a refractive index n 3 , wherein n 3 <n 2 , wherein said at least one of said fibers has a Δ pedestal    and a pedestal radius below that corresponding to a cut - off wavelength limit, and wherein Δ   pedestal    equals  ( n   i   2   −n   2   2 ) / 2 n   i   2 .  
     
     
       2. A fiber optic coupler in accordance with  claim 1  wherein the maximum radius of pedestal is 25 μm. 
     
     
       3. A fiber optic coupler in accordance with  claim 1  wherein the radius of pedestal is between 20 and 25 μm. 
     
     
       4. A fiber optic coupler in accordance with  claim 1  wherein said at least one fiber contains chlorine, the amount of chlorine in the pedestal of said at least one fiber being greater than the amount of chlorine in the cladding thereof. 
     
     
       5. A fiber optic coupler in accordance with  claim 1  wherein the core of said at least one fiber contains silica and a refractive index-increasing dopant, and wherein said pedestal contains silica and a refractive index-increasing dopant. 
     
     
       6. A fiber optic coupler in accordance with  claim 5  wherein said core and said pedestal contain the same refractive index-increasing dopant. 
     
     
       7. A fiber optic coupler in accordance with  claim 5  wherein said core and said pedestal contain different refractive index-increasing dopants. 
     
     
       8. A fiber optic coupler in accordance with  claim 1  wherein the cladding of said at least one fiber contains a refractive index-increasing  index- decreasing  dopant in an amount sufficient to decrease its refractive index to a value lower than n i . 
     
     
       9. A fiber optic coupler in accordance with  claim 1  wherein said plurality of fibers are identical. 
     
     
       10. A fiber optic coupler in accordance with  claim 1  wherein each of said plurality of fibers has a refractive index pedestal between said core and cladding, the refractive index of the pedestal being greater than that of the cladding of the fiber and less than that of the core of the fiber. 
     
     
       11. A fiber optic coupler in accordance with  claim 1  wherein said at least one optical fiber is different from the remainder of said plurality of fibers. 
     
     
       12. A fiber optic coupler in accordance with  claim 1  wherein said medium comprises an elongated body of matrix glass having two end regions and a midregion, said optical fibers extending longitudinally within said body and being fused together along with the midregion of said body, the diameter of the central portion of said midregion being smaller than the diameters of said end regions, said central portion of said midregion constituting said coupling region. 
     
     
       13. A fiber optic coupler in accordance with  claim 1  wherein Δ pedestal  is less than 0.15%, wherein Δ pedestal  equals (n i   2 −n 2   2 )/n i   2 ( n   i   2   −n   2   2 ) / 2 n   i   2 . 
     
     
       14. A fiber optic coupler in accordance with  claim 1 , wherein said at least one of said fibers has a pedestal radius greater than  3  microns.