Double-core active fiber optical amplifier having a wide-band signal wavelength

Abstract

The invention relates to an optical amplifier, in particular for optical fiber telecommunication lines ( 1 ), operating with a transmission signal in a predetermined wavelength range, which amplifier comprises a fluorescent active optical fiber ( 6 ) doped with erbium, having two cores ( 11 and 12, 101 and 102 ), one ( 11, 101 ) of which is connected to a fiber ( 4 ) in which a transmission signal to be amplified and a luminous pumping energy are multiplexed, and to an outgoing fiber adapted to transmit the amplified signal, whereas the second core ( 12, 102 ) is optically coupled to the first core and is capable of absorbing the spontaneous erbium emission which would constitute a noise source, allowing a signal to be amplified in a wavelength range substantially corresponding to the tolerance range of the commercially available signal laser emitters.

We claim:  
     
       1. An optical amplifier having a wide signal band, in particular for optical fiber telecommunication lines ( 1 ) operating with a transmission signal in a predetermined wavelength band, comprising a dichroic coupler adapted to multiplex a transmission signal and luminous pumping energy in a sole outgoing fiber and an active optical fiber containing a fluorescent doping substance, connected to the fiber coming out of the dichroic coupler and to a telecommunication line fiber adapted to receive and transmit an amplified signal, characterised in that the active optical fiber ( 6 ) is completely formed by two cores ( 11  and  12 ,  101  and  102 ) in a common cladding ( 13 ,  103 ), where the former core ( 11 ,  101 ) is an active or amplifying core containing a fluorescent laser emission dopant in a wavelength range including the wavelength band of the transmission signal and is optically connected at one end to said fiber coming out of the dichroic coupler and, at the other end, to said telecommunication line fiber, whereas the other core ( 12 ,  102 ) of the active optical fiber is an attenuating core and contains a substance adapted to absorb luminous energy and is cut at the ends, the two cores being optically coupled to each other in a wavelength band comprised in the range of the laser emission wavelengths of the first core and different from the transmission signal band. 
     
     
       2. An optical amplifier of a wide signal band according to  claim 1 , characterised in that the second core ( 12 ,  102 ) in the active optical fiber ( 6 ) contains a dopant having a high luminous absorption in the laser emission range of the dopant of the first core. 
     
     
       3. An optical amplifier of a wide signal band according to  claim 2 , characterised in that the dopant having a high luminous absorption in the second core ( 12 ,  102 ) consists of the same fluorescent substance as that present in the first core ( 11 ,  101 ). 
     
     
       4. An optical amplifier of a wide signal band according to  claim 1 , characterised in that the doping substance in the second core ( 12 ,  102 ) is a substance having a high luminous absorption over the whole spectrum, selected from titanium, vanadium chromium or iron, which are at least partly present in their lowest valence state. 
     
     
       5. An optical amplifier of a wide signal band according to  claim 1 , characterised in that the content of dopant having a high luminous absorption in the second core ( 12 ,  102 ) and the coupling characteristics of the fiber cores ( 11  and  12 ,  101  and  102 ) are so related that they give rise in the second core to an attenuation length lower than 1/10 of the beat length between the coupled cores in the selected core-coupling band. 
     
     
       6. An optical amplifier of a wide signal band according to  claim 5 , characterised in that the two cores ( 11 ,  12 ;  101 , 102 ) are optically coupled to each other in the wavelength range between 1530 and 1540 nm. 
     
     
       7. An optical amplifier of a wide signal band according to  claim 1 , characterised in that the fluorescent doping substance present in the first core ( 11 ,  101 ) is erbium. 
     
     
       8. An optical amplifier of a wide signal band according to  claim 1 , characterised in that the first core ( 11 ,  101 ) is disposed in coaxial relation with the outer fiber surface, in alignment with the core of the fiber coming out of the dichroic coupler and with the core of the telecommunication line fiber ( 1 ) to which the amplifier is connected, whereas the second core ( 12 ,  102 ), at the ends thereof, faces the cladding of said fibres. 
     
     
       9. An optical amplifier of a wide signal band according to  claim 1 , characterised in that at least the active core ( 11 ,  101 ) of the two fiber cores ( 11  and  12 ,  101  and  102 ) is adapted to allow the luminous single-mode propagation at the transmission wavelength and at the pumping wavelength. 
     
     
       10. An optical amplifier according to  claim 1 , characterised in that the active fiber ( 6 ) is longer than half the beat distance of its two coupled cores ( 11  and  12 ,  101  and  102 ) in the selected optical core-coupling band. 
     
     
       11. A double-core active optical fiber ( 6 ) comprising fluorescent doping substances, particularly for use in optical fiber amplifiers for optical telecommunication lines, characterised in that it has two optically coupled cores ( 11  and  12 ,  101  and  102 ) uniformly spaced in a common cladding in which the former ( 11 ,  101 ) is an amplifying core and contains a fluorescent doping substance having a stimulated emission in a wavelength range and is adapted to be connected to an optical fiber carrying a telecommunication signal and luminous pumping energy multiplexed in the same fiber, and the second core ( 12 ,  102 ) is an attenuating core and contains a doping substance having high luminous absorption, the maximum optical coupling of the two cores occurring in a wavelength band comprised within said stimulated emission range of the first core and being different from that of the telecommunication signal. 
     
     
       12. A double-core optical fiber comprising fluorescent doping substances according to  claim 11 , characterised in that the dopant having high luminous absorption in the second core ( 12 ,  102 ) consists of the same fluorescent substance present in the first core ( 11 ,  101 ). 
     
     
       13. An optical fiber according to  claim 11 , characterised in that the doping substance in the second core ( 12 ,  102 ) is a substance having high luminous absorption over the whole spectrum, selected from titanium, vanadium, chromium or iron, which are at least partly present in their lowest valence state. 
     
     
       14. An optical fiber according to  claim 11 , characterised in that the content of dopant having high luminous absorption in the second core ( 12 ,  102 ) and the coupling characteristics of the fiber cores are so related that they give rise in the second core ( 12 ,  102 ) to an attenuation length lower than 1/10 of the beat length between the coupled cores in the selected core-coupling band. 
     
     
       15. An optical fiber according to  claim 11 , characterised in that the fluorescent doping substance present in the first core ( 11 ,  101 ) is a wave-hearth dopant. 
     
     
       16. An optical fibre structure as claimed in  claim 15  wherein said amplifying core is erbium doped. 
     
     
       17. An optical fibre structure as claimed in  claim 16  wherein said amplifying core contains from 10 to 1000 ppm Er 2 O 3 . 
     
     
       18. An optical fiber according to  claim 16 , characterised in that the two cores ( 11  and  12 ) are optically coupled to each other between 1530 and 1540 nm. 
     
     
       19. An optical fibre structure as claimed in  claim 1  or  11 , wherein said attenuating core contains more than 5000, preferably about 10,000 ppm Er 2 O 3 . 
     
     
       20. An optical fibre structure as claimed in  claim 1  or  11  wherein the diameter of amplifying core is equal to that of the attenuating core. 
     
     
       21. An optical fibre structure as claimed in  claim 1  or  11 , wherein the diameter of the amplifying core ( 101 ) is less than the diameter of the attenuating core ( 102 ). 
     
     
       22. An optical fiber according to  claim 11 , characterised in that the first core ( 11 ,  101 ) is disposed in coaxial relation with the outer fiber surface. 
     
     
       23. An optical fiber according to  claim 11 , characterised in that at least one of the two fiber cores ( 11  and  12 ,  101  and  102 ) is adapted to allow the luminous single-mode propagation at the transmission wavelength and at the pumping wavelength. 
     
     
       24. A method of manufacturing an optical fibre structure as claimed in  claim 1  or  11  comprising inserting respective core rods into ultrasonically bored holes in a cladding rod to form a preform. 
     
     
       25. A method of manufacturing an optical fibre structure as claimed in  claim 1  or  11  comprising fabricating a preform comprising cladding glass surrounding glass for the amplifying core and inserting a core rod for the attenuating core into an ultrasonically bored hole in the cladding glass of the preform. 
     
     
       26. An optical amplifier for amplifying signals at a plurality of wavelengths throughout a predetermined spectral window comprising: 
       
         a length of active optical waveguide pumpable at a pump wavelength for amplifying said signals, when said signals are passed through said waveguide, in accordance with a gain spectrum which includes a peak;  
       
       
         and an optical filter coupled to said length the waveguide intermediate its ends and operative to reduce gains at wavelengths in a band within said window which includes the peak wavelength of the gain spectrum whereby the gain throughout the window is substantially constant for all signals at wavelengths within the window. 
       
     
     
       27. The invention as set forth in  claim 26  wherein said optical filter comprises an optical band absorption filter. 
     
     
       28. An optical amplifier as set forth in  claim 26  wherein said length of active optical waveguide is continuous. 
     
     
       29. An optical amplifier as set forth in  claim 26  wherein said optical filter is operative to reduce gains at the peak wavelength more than it reduces gains at higher wavelengths adjacent thereto. 
     
     
       30. The invention set forth in  claim 27  wherein said optical band absorption filter is distributed along or continuous along the length of said waveguide. 
     
     
       31. The invention set forth in  claim 26  wherein said waveguide is a rare- earth - doped optical fiber.   
     
     
       32. A fiber optic filter structure comprising an optical fiber having a single- mode core surrounded by cladding material, and at least one light - attenuating light path in said cladding material uniformly spaced from said single - mode core, the propagation constants of said single - mode core and said light - attenuating light path being different at wavelengths except for at least one wavelength λ   f    the spacing between said single - mode core and said at least one light - attenuating light path being sufficiently small that light within a first band of wavelengths centered around λ   f    couples between said single - mode core and said light - attenuating light path, at least a portion of the light within said first band of wavelengths being absorbed in said light - attenuating light path.   
     
     
       33. A filter structure in accordance with  claim 32  wherein said single- mode core is a gain core containing active dopant ions that are capable of producing stimulated emission of light.   
     
     
       34. A filter structure in accordance with  claim 33  wherein said at least one light- attenuating light path is single - mode at wavelength λ   f   .   
     
     
       35. A filter structure in accordance with  claim 33  wherein said gain core is located at the longitudinal axis of said optical fiber. 
     
     
       36. A filter structure in accordance with  claim 35  wherein said at least one light- attenuating light path comprises a light - attenuating core that is laterally spaced from said gain core.   
     
     
       37. A filter structure in accordance with  claim 33  wherein the light absorbing properties of said at least one light- attenuating light path is such that essentially none of the light that couples to said light - attenuating light path couples back to said gain core.   
     
     
       38. A filter structure in accordance with  claim 33  wherein said dopant ions are capable of producing gain over a given band of wavelengths that extends from a given first wavelength to a given second wavelength, and wherein said light- attenuating light path comprises a first light - attenuating core having propagation characteristics such that a band of wavelengths including said given first wavelength couples thereto.   
     
     
       39. A filter structure in accordance with  claim 38  wherein said dopant ions are erbium ions, and wherein said given short wavelength is within the range of  1530  nm to  1540  nm. 
     
     
       40. A filter structure in accordance with  claim 33  wherein said at least one light- attenuating light path extends along the entire length of said gain core.   
     
     
       41. A filter structure in accordance with  claim 32  wherein said at least one light- attenuating light path is single - mode at wavelength λ   f   .   
     
     
       42. A filter structure in accordance with  claim 32  wherein said single- mode core is located at the longitudinal axis of said optical fiber.   
     
     
       43. A filter structure in accordance with  claim 42  wherein said at least one light- attenuating light path comprises a light - attenuating core that is laterally spaced from said single - mode core.   
     
     
       44. A filter structure in accordance with  claim 32  wherein the light absorbing properties of said at least one light- attenuating light path is such that essentially none of the light that couples to said light - attenuating light path couples back to said single - mode core.   
     
     
       45. A fiber optic amplifier structure comprising an optical fiber having a single- mode gain core doped with active dopant ions capable of producing stimulated emission of light within a predetermined range of wavelengths, said optical fiber further comprising light - attenuating coupling means, the coupling characteristics of said gain core and said coupling means being such that optical power in at least one wavelength band centred around at least one wavelength λ   f    within said predetermined range of wavelengths is selectively coupled and attenuated, while optical power at other wavelengths within said predetermined range of wavelengths remains substantially guided by said gain core.   
     
     
       46. The fiber optic amplifier structure of  claim 45  wherein said coupling means comprises an attenuating core uniformly spaced from said gain core, and wherein the propagation constants of said attenuating core and said gain core are different except at said at least one wavelength λ f   .   
     
     
       47. The fiber optic amplifier structure of  claim 46  wherein said attenuating core comprises light- absorbing means.   
     
     
       48. A fiber optic structure for amplifying optical signals, comprising an optical fiber having a single- mode gain core and a light - attenuating core uniformly spaced within a common cladding, said gain core containing active dopant ions that are capable of producing stimulated emission of light within a predetermined band of wavelengths, the optical characteristics of said cores being such that the propagation constants of said cores are different except for at least one wavelength λ   f    within said predetermined band of wavelengths, the spacing between said cores being sufficiently small that light within a band of wavelengths centered around λ   f    couples between said gain core and said light - attenuating core.   
     
     
       49. A fiber amplifier comprising 
         a gain optical fiber having a single - mode core containing dopant ions capable of producing stimulated emission of light within a predetermined band of wavelengths including a wavelength λ   s    when pumped with light of wavelength λ   p   , said gain fiber having input and output ends,    
       
         absorbing ion filtering means for attenuating light at at least some of the wavelengths within said predetermined band of wavelengths, said absorbing ion filtering means comprising unpumped gain ions,  
       
       
         means for introducing a signal of wavelength λ 
         s  
         into said gain fiber input end,  
       
       
         means introducing pump light of wavelength λ 
         p  
         into said gain fiber, and  
       
       
         means for preventing the excitation of said umpumped gain ions by light of wavelength λ 
         p 
         . 
       
     
     
       50. A fiber amplifier in accordance with  claim 41  wherein the radial distribution of said gain ions in said gain fiber extends beyond the mode field radius of light of wavelength λ p   , whereby those gain ions at radii greater than said mode field radius are unexcited by pump light and are free to absorb signal light.   
     
     
       51. A fiber amplifier comprising 
         a gain optical fiber having a single - mode core containing dopant ions capable of producing stimulated emission of light within a predetermined band of wavelengths including a wavelength λ   s    when pumped with light of wavelength λ   p   , said gain fiber having input and output ends,    
       
         filtering means for attenuating light at at least some of the wavelengths within said predetermined band of wavelengths, said filtering means containing ions that can be excited by light of wavelength λ 
         p 
         ,  
       
       
         means for introducing a signal of wavelength λ 
         s  
         into said gain fiber input end,  
       
       
         means introducing pump light of wavelength λ 
         p  
         into said gain fiber, and means for preventing the excitation of said filtering means by light of wavelength λ 
         p 
         . 
       
     
     
       52. A fiber amplifier comprising 
         a gain optical fiber having a single - mode core containing dopant ions capable of producing stimulated emission of light within a predetermined band of wavelengths including a wavelength λ   s    when pumped with light of wavelength λ   p   , said gain fiber having input and output ends, said dopant ions being selected from the group consisting of erbium, neodymium and praseodymium,    
       
         filtering means for attenuating light at least some of the wavelengths within said predetermined band of wavelengths, said filtering means containing a dopant selected from the group consisting of erbium, dysprosium, neodymium, ytterbium, samarium, praseodymium, thulium, vanadium and cadmium selenide,  
       
       
         means for introducing a signal of wavelength λ 
         s  
         into said gain fiber input end, and  
       
       
         means introducing pump light of wavelength λ 
         p  
         into said gain fiber. 
       
     
     
       53. A fiber amplifier in accordance with  claim 52  wherein said filtering means comprises an optical fiber containing said dopant ions. 
     
     
       54. An optical amplifier for amplifying signals at a plurality of wavelengths throughout a predetermined spectral window with a gain which is substantially constant throughout said window comprising: 
       
         a length of active optical waveguide pumpable at a pump wavelength for amplifying said signals at said plurality of wavelengths within said window, when said signals are passed through said waveguide, in accordance with a gain spectrum which is substantially constant except for wavelengths in a band within said window which have a greater gain than other wavelengths in said window;  
       
       
         a source of pumping energy coupled to said waveguide for pumping said waveguide, said pump wavelength being less than said wavelengths in said band;  
       
       
         and an optical filter disposed at a least one point along the length of the waveguide intermediate its ends and operative to reduce gains at wavelengths in said band within said window whereby the gain throughout the window is substantially constant for all signals at wavelengths within the window. 
       
     
     
       55. The invention as set forth in  claim 54  wherein said optical filter comprises an optical band rejection filter. 
     
     
       56. An optical amplifier as set forth in  claim 54  wherein said length of active optical waveguide is continuous. 
     
     
       57. An optical amplifier as set forth in  claim 54  wherein said optical filter is operative to reduce gains at wavelengths in the band more than it reduces gains at higher wavelengths adjacent thereto. 
     
     
       58. The invention set forth in  claim 55  wherein said optical band- rejection filter is distributed along or continuous along the length of said waveguide.   
     
     
       59. The invention set forth in  claim 54  wherein said waveguide is a rare- earth - doped optical fiber or planar waveguide.