Dispersion-shifted monomode optical fiber

Abstract

Dispersion-shifted monomode optical fibers have an effective mode surface area greater than 65 μm 2 by optimization of the geometrical characteristics that characterize the fibers.

We claim:  
     
       1. A monomode optical fibers with substantially zero chromatic dispersion in the vicinity of 1.55 μm, comprising an optical core having: 
       a central portion of index varying with distance from the axis of said optical fiber between a minimum index n s +hαn,  n s   +Δn,  with −1<h<0 with Δn strictly positive, and a maximum index n s ;  
       a layer surrounding said central portion, of index that varies with distance from the axis of said fiber over the range n s  to n s +Δn;  
       and optical cladding surrounding said layer and having an index substantially equal to n s , said profile being defined by the following geometrical parameters:  
       a: total radius of the core measured at said layer;  
       y: ratio of the radius of said central portion to a, where 0<y<1;  
       said fiber being characterized in that a and Δn are determined so that the chromatic dispersion of said fiber is substantially zero at 1.55 μm and the cutoff wavelength λ c  of said fiber is such that 1.4 μm<λ c <1.55 μm, y being an arbitrary value in the range 0 to 1, and h being selected so that the following relationships are satisfied:                  1   )                       h   1          (   y   )         ≦   h   ≦         h   2          (   y   )              where:                   {               h   1     (   y   )     =       -   6.56     +     14.96      y     -     8.71        y   2            and                       h   2     =       -   6.2     +     16.76      y     -     11.61        y   2                                 2   )                -     2.33      y     +   0.6     <   h   <     -     0.2   .                             
     
     
       2. An optical fiber according to claim  1 , characterized in that a lies in the range 2.5 μm to 4 μm, and Δn lies in the range 12×10 −3  to 20×10 −3 . 
     
     
       3. A dispersion- shifted monomode optical fiber comprising a core and a cladding surrounding said core, said core including at least two layers, at least one of which has a depressed refractive index with respect to the other layers in said core, said fiber having an effective mode surface area greater than  65  μm   2    and having low curvature losses.    
     
     
       4. An optical fiber according to claim  3 , wherein said fiber has an effective mode surface area of greater than  70  μm 2   .    
     
     
       5. An optical fiber according to claim  3 , wherein said curvature losses do not exceed  0 . 005  dB/m when a radius of curvature of the fiber is  30  mm.  
     
     
       6. An optical fiber according to claim  3 , wherein said core includes a central portion and at least one first layer surrounding said central portion, said central portion having a refractive index varying between minimum and maximum levels and having said minimum level adjacent said first layer.  
     
     
       7. An optical fiber according to claim  6 , wherein a refractive index profile of said central portion is trapezium- shaped.    
     
     
       8. An optical fiber according to claim  6 , wherein said refractive index of said cladding is substantially equal to said minimum level.  
     
     
       9. An optical fiber according to claim  6 , wherein said core further includes a second layer between said first layer and said cladding.  
     
     
       10. An optical fiber according to claim  9 , wherein said second layer has a refractive index between said minimum and maximum levels.  
     
     
       11. An optical fiber according to claim  10 , wherein said refractive index of said cladding is less than the refractive index of said second layer.  
     
     
       12. An optical fiber according to claim  3 , wherein said core includes a central portion and at least one first layer surrounding said central portion, said central portion having a refractive index varying between minimum and maximum levels and having said maximum level adjacent said first layer.  
     
     
       13. An optical fiber according to claim  12 , wherein said refractive index of said cladding is greater than said minimum level.  
     
     
       14. An optical fiber according to claim  12 , wherein said refractive index of said cladding is substantially equal to said maximum level.  
     
     
       15. An optical fiber according to claim  12 , wherein said refractive index of said first layer varies between said maximum value and a higher value.  
     
     
       16. An optical fiber according to claim  15 , wherein said refractive index of said cladding is less than a refractive index of at least part of said first layer.  
     
     
       17. An optical fiber according to claim  12 , wherein said refractive index of said cladding is less than a refractive index of at least part of said first layer.