Fibre optic couplers

Abstract

A fused fiber optic 1×4 or 2×4 coupler in which four optical fiber segments (A, B, C, D) extend longitudinally beside each other in a coupling region in which the fiber segments are at least partially fused together to form an assembly exhibiting a close packed cross section in which the fiber cores ( 11 ) are centered substantially at the corners of a four sided polygon having a pair or opposite internal acute angles substantially less than 90°.

We claim:  
     
       1. A fused fiber optic 1×4 or 2×4 coupler in which four optical fibre segments extend longitudinally beside each other in a coupling region in which the fibre segments are at least partially fused together to form an assembly exhibiting a close packed cross-section in which the fibre cores are centered substantially at the corners of a four-sided polygon having a pair of opposite internal acute angles substantially less than 90°. 
     
     
       2. A coupler according to  claim 1  wherein said opposite internal acute angles are about or a little less than 60°. 
     
     
       3. A coupler according to  claim 1  or  2  wherein said assembly is twisted in the coupling region. 
     
     
       4. A coupler according to  claim 3  wherein said coupler has been formed by drawing the fibre segments longitudinally while they are being at least partially fused together, and wherein said twist is applied prior to the fusion and drawing steps and has been at least partly applied as a dynamic helical twist. 
     
     
       5. A coupler according to  claim 1  wherein at least one of the two fibre segments on the shorter diagonal of the assembly has different propagation constants relative to the other two fibre segments. 
     
     
       6. A coupler according to  claim 5  wherein said one or both of the two fibre segments on the shorter diagonal is of a diameter smaller than that of the other fibre segments, which smaller diameters are not necessarily equal. 
     
     
       7. A coupler according to  claim 5  wherein both of the fibre segments on the shorter diagonal have different propagation constants relative to the other two fibre segments. 
     
     
       8. A coupler according to  claim 1  or  2  wherein said assembly is twisted in the coupling region with a twist of at least 45°. 
     
     
       9. A fused fibre optic 1×4 or 2×4 coupler in which four optical fibre segments extend longitudinally beside each other in a coupling region in which the fibre segments are at least partially fused together to form an assembly exhibiting a close packed cross-section in which the centre-to-centre spacings of each of the cores of two of the fibre segments from the other three cores are similar. 
     
     
       10. A coupler according to  claim 9  wherein at least one of the two fibre segments on the shorter diagonal of the assembly has different propagation constants relative to the other two fibre segments. 
     
     
       11. A coupler according to  claim 10  wherein both of the fibre segments on the shorter diagonal have different propagation constants relative to the other two fibre segments. 
     
     
       12. A coupler according to  claim 10  wherein said one or both of the two fibre segments on the shorter diagonal is of a diameter smaller than that of the other fibre segments, which smaller diameters are not necessarily equal. 
     
     
       13. A coupler according to  claim 9  wherein said assembly is twisted in the coupling region. 
     
     
       14. A coupler according to  claim 13  wherein said coupler has been formed by drawing the fibre segments longitudinally while they are being at least partially fused together, and wherein said twist is applied prior to the fusion and drawing steps and has been at least partly applied as a dynamic helical twist. 
     
     
       15. A coupler according to  claim 9  wherein said assembly is twisted in the coupling region with a twist of at least 45°. 
     
     
       16. A fused fibre optic 1×4 or 2×4 coupler in which four optical fibre segments extend longitudinally beside each other in a coupling region in which the fibre segments are at least partially fused together to form an assembly exhibiting a close packed cross-section in which two of the fibre segments are mutually adjacent and lie between the other two. 
     
     
       17. A coupler according to  claim 16  wherein at least one of the two fibre segments on the shorter diagonal of the assembly has different propagation constants relative to the other two fibre segments. 
     
     
       18. A coupler according to  claim 17  wherein both of the fibre segments on the shorter diagonal have different propagation constants relative to the other two fibre segments. 
     
     
       19. A coupler according to  claim 17  wherein said one or both of the two fibre segments on the shorter diagonal is of a diameter smaller than that of the other fibre segments, which smaller diameters are not necessarily equal. 
     
     
       20. A coupler according to  claim 16  wherein said assembly is twisted in the coupling region. 
     
     
       21. A coupler according to  claim 20  wherein said coupler has been formed by drawing the fibre segments longitudinally while they are being at least partially fused together, and wherein said twist is applied prior to the fusion and drawing steps and has been at least partly applied as a dynamic helical twist. 
     
     
       22. A coupler according to  claim 16  wherein said assembly is twisted in the coupling region with a twist of at least half a turn. 
     
     
       23. A method of forming a 1×4 or 2×4 fused fibre optic coupler comprising: 
       disposing four optical fibre segments so that they extend longitudinally beside each other in a cross-sectional array in which the fibre cores are centered substantially at the corners of a four-sided polygon having a pair of opposite internal acute angles substantially less than 90°;  
       while maintaining the fibre segments under tension, and with those segments on the shorter diagonal of the polygon preferably under greater tension than the other two segments, applying twist to the fibre segments to form a twisted assembly of the fibre segments;  
       heating at least a portion of the twisted assembly and at least partly fusing the fibre segments together while drawing the fibre segments longitudinally, whereby to form a coupling region in which the assembly is twisted.  
     
     
       24. A method according to  claim 23  wherein at least one of the two fibre segments on the shorter diagonal of the assembly has different propagation constants relative to the other two fibre segments. 
     
     
       25. A method according to  claim 24  wherein both of the fibre segments on the shorter diagonal have different propagation constants relative to the other two fibre segments. 
     
     
       26. A method according to  claim 24  wherein said one or both of the two fibre segments on the shorter diagonal is of a diameter smaller than that of the other fibre segments, which smaller diameters are not necessarily equal. 
     
     
       27. A method according to  claim 23  wherein said twist is at least partly applied as a dynamic helical twist. 
     
     
       28. A method according to  claim 23  or  27  wherein said fibres on the shorter axis of the parallelogram are slightly individually pre-twisted before effecting the aforesaid twisting of the assembly. 
     
     
       29. A method according to  claim 23  wherein said opposite internal acute angles are about or a little less than 60°. 
     
     
       30. A method according to  claim 23  wherein said twist is such as to result in a twist in the coupling region of the coupler, after formation and subsequent processing steps, of at least 45°. 
     
     
       31. A method according to  claim 23  wherein, when the manufactured assembly has cooled substantially to ambient temperatures in the coupling region, a slight further twist is applied to the assembly for fine tuning the coupler characteristics, such further twist being accumulative or opposite to twist already present in the assembly. 
     
     
       32. A fused fibre optic  1 × 4  or  2 × 4  coupler comprising 
       four optical fibre segments that extend longitudinally beside each other in a coupling region, the fibre segments being at least partially fused together to form an assembly in which the fibre cores are centered at the corners of a four-sided polygon, wherein coupling ratios for each of the fibres fall within  0 . 19  to  0 . 30  for wavelength bands of  1260  to  1360  nm and  1430  to  1580  nm. 
     
     
       33. The fused fibre optic  1 × 4  or  2 × 4  coupler of  claim 32 , wherein the fibre cores are centered at the corners of a four-sided polygon having a pair of opposite internal acute angles. 
     
     
       34. The fused fiber optic  1 × 4  or  2 × 4  coupler of  claim 33 , wherein the pair of opposite internal acute angles are substantially less than  90 °. 
     
     
       35. The fused fibre optic  1 × 4  or  2 × 4  coupler of  claim 34 , wherein the pair of opposite internal acute angles are about or a little less than  60 °. 
     
     
       36. The fused fibre optic  1 × 4  or  2 × 4  coupler of  claim 33 , wherein at least one of the two fibre segments on a shorter diagonal of the four-sided polygon has different propagation constants relative to the other two fibre segments. 
     
     
       37. The fused fibre optic  1 × 4  or  2 × 4  coupler of  claim 36 , wherein one or both of the two fibre segments on the shorter diagonal has a diameter smaller than that of the other fibre segments. 
     
     
       38. The fused fibre optic  1 × 4  or  2 × 4  coupler of  claim 32 , wherein coupling ratios for each of the fibres fall within  0 . 19  to  0 . 30  for wavelength band of  1250  to  1600  nm. 
     
     
       39. The fused fiber optic  1 × 4  or  2 × 4  coupler of  claim 32 , wherein the assembly is twisted in the coupling region. 
     
     
       40. The fused fibre optic  1 × 4  or  2 × 4  coupler of  claim 32 , wherein centre-to-centre spacings between each of the cores of two of the fibre segments and the other three cores are substantially similar. 
     
     
       41. The fused fibre optic  1 × 4  or  2 × 4  coupler of  claim 32 , wherein two of the fibre segments are in contact and positioned between the other two fibre segments. 
     
     
       42. A method of forming a  1 × 4  or  2 × 4  fused fibre optic coupler, comprising steps of: 
       disposing four optical fibre segments so that they extend longitudinally beside each other in an assembly in which the fiber cores are centered at the corners of a four-sided polygon; and  
       at least partially fusing the four optical fibre segments in a coupling region;  
       wherein, upon completion of the formation of the fibre optic coupler, the four optical fibre segments have coupling ratios for each of the fibres that fall within  0 . 19  to  0 . 30  for wavelength bands of  1260  to  1360  nm and  1430  to  1580  nm. 
     
     
       43. The method of  claim 42 , further comprising 
       twisting the fibre segments to from a twisted assembly prior to at least partially fusing the four optical fibre segments. 
     
     
       44. The method of  claim 43 , wherein the step of disposing four optical fibre segments comprises disposing four optical fibre segments so that they extend longitudinally beside each other in an assembly in which the fibre cores are centered at the corners of a four-sided polygon with a pair of opposite internal acute angles. 
     
     
       45. The method of  claim 44 , wherein the step of twisting the fibre segments comprises twisting the fibre segments while maintaining the fibre segments under tension. 
     
     
       46. The method of  claim 45 , wherein the step of twisting the fibre segments comprises twisting the fibre segments while maintaining the fibre segments under tension with greater tension maintained on those fibre segments on a shorter diagonal of the four- sided polygon. 
     
     
       47. The method of  claim 46 , further comprising twisting the two fibre segments on the shorter diagonal of the four-sided polygon prior to twisting the four fibre segments. 
     
     
       48. The method of  claim 43 , wherein at least partially fusing the four optical fibre segments comprises heating the four optical fibre segments in a coupling region. 
     
     
       49. The method of  claim 48 , further comprising twisting the four optical fibre segments after heating for fine tuning of the coupling characteristics. 
     
     
       50. The method of  claim 43 , wherein the step of twisting the fibre segments comprises twisting the fibre segments while maintaining the fibre segments under tension.