Optical fibre amplifier with fluorescent additive and method of amplifying signals using same

Abstract

An optical fibre for use in fibre lasers has the lasing additive eg Er3+, concentrated in center of the core. Preferably the core has an inner region which contains the additive and an outer region which is dopant free. The concentration of the dopant reduces the pump threshold for a laser and improves the gain performance for a given pump power. The fibre is conveniently made in MCVD. The use of Al2O3 in the inner zone appears to reduce loss of dopant during tube collapse.

We claim: 
     
       1. An optical fibre suitable for forming fluorescent devices, said optical fibre having a cladding, and a core having a longitudinal core axis, said core containing a fluorescent dopant, characterized in that the concentration of the dopant is substantially greater on the core axis than at the interface between the gore and the cladding. 
     
     
       2. A fibre according to claim 1 wherein the core comprises an inner region and an outer region surrounding the inner region wherein substantially all of the dopant is contained in the inner region. 
     
     
       3. A fibre according to claim 2, wherein the inner region constitutes not more than a quarter of the total cross sectional area of the core. 
     
     
       4. A fibre according to claim 3, wherein the inner region constitutes 5% to 15% of the cross sectional area of the core. 
     
     
       5. A fibre according to claim 2, wherein the dopant is substantially uniformly distributed in the inner region of the core. 
     
     
       6. A fibre according to claim 1, which is monomode et signal wave length. 
     
     
       7. A fibre according to claim 1, wherein the dopant is capable of lasing transition. 
     
     
       8. A fibre according to claim 7, wherein the dopant is a three level lasing element. 
     
     
       9. A fibre according to claim 1, wherein the dopant is a rare earth element. 
     
     
       10. A fibre according to claim 9, wherein the dopant is selected from Er, Nd, Pr, Ho, Yb and Tm. 
     
     
       11. A fibre according to claim 10, wherein the dopant is Er 3+ . 
     
     
       12. An optical fibre which comprises (a) a cladding region formed of a first glass composition having a first refractive index;   (b) an outer core region surrounded by said cladding region and formed of a second glass composition having a second refractive index said second refractive index being higher than said first refractive index;   (c) an inner core region surrounded by said outer core region and formed of a third glass composition having a refractive index substantially equal to said second refractive index wherein said third glass composition contains a dopant which is capable of undergoing lasing transitions wherein the concentration of said dopant is 0.001 to 10% by weight based on the total third glass composition.   
     
     
       13. An optical fibre which comprises (a) a cladding region formed of a first glass composition having a first refractive index;   (b) an outer core region surrounded by said cladding region and formed of a second glass composition having a second refractive index said second refractive index being higher than said first refractive index;   (c) an inner core region surrounded by said outer core region and formed of a third glass composition having a refractive index substantially equal to said second refractive index wherein said third glass composition contains a dopant which provides a rare earth element in a form capable of undergoing lasing transitions wherein the concentration of said dopant is such as to provide 0.001 to 10% by weight of said rare earth element based on the total third glass composition.   
     
     
       14. An optical fibre which comprises (a) a cladding region formed of a first glass composition having a first refractive index;   (b) an outer core region surrounded by said cladding region and formed of a second glass composition having a second refractive index said second refractive index being higher than said first refractive index;   (c) an inner core region surrounded by said outer core region and formed of a third glass composition having a refractive index substantially equal to said second refractive index wherein said third glass composition contains a dopant which provides an element selected from Er, Nd, Pr, Ho, Yb and Tm in a form capable of undergoing lasing transitions wherein the concentration of said dopant is such as to provide 0.001 to 10% by weight of said element based on the total third glass composition.   
     
     
       15. An optical fibre which comprises (a) a cladding region consisting essentially of SiO 2  ;   (b) an outer core region surrounded by said cladding region and consisting essentially of SiO 2  and an additive which increases the refractive index of the SiO 2  ;   (c) an inner core region surrounded by said outer core region and consisting essentially of SiO 2 , an additive which increases the refractive index of the SiO 2  so that the refractive indices of the inner and outer core regions are substantially equal, and a dopant which is capable of undergoing lasing transitions wherein the mole ratio of Si to dopant is between 1:10 -4  and 1:10.   
     
     
       16. An optical fibre which comprises (a) a cladding region consisting essentially of SiO 2  ;   (b) an outer core region surrounded by said cladding region and having a refractive index greater than said cladding region, said outer core region consisting essentially of SiO 2  and GeO 2  ;   (c) an inner core region surrounded by said outer core region and having a refractive index substantially equal to said outer core region, said inner core region consisting essentially of SiO 2 , Al 2  O 3  and a dopant capable of undergoing lasing transitions.   
     
     
       17. An optical fibre according to claim 16, wherein the mole ratio of Si to dopant in the inner core region is between 1:10 -4  and 1:10. 
     
     
       18. An optical fibre according to claim 16, wherein the dopant is an element selected from Er, Nd, Pr, Ho, Yb and Tm. 
     
     
       19. An optical fibre according to claim 18, wherein the concentration of the dopant is between 0.001 and 10% based on the inner core region. 
     
     
       20. An optical fibre according to claim 19, wherein the concentration is between 0.1 and 10% by weight. 
     
     
       21. An optical fibre according to claim 15, wherein the dopant is a rare earth element. .Iadd. 
     
     
       22.  A method of amplifying an optical signal, the method comprising: launching said optical signal into a core of an optical fibre, the optical fibre being single mode at a wavelength of the optical signal, said core having a doped region consisting essentially of silica, alumina and a fluorescent additive, the fluorescent additive being erbium;   simultaneously launching pump radiation into said core whereby photons of said pump radiation are absorbed by said fluorescent additive to produce a population inversion which sustains stimulated emission of signal photons so as to amplify said optical signal..Iaddend..Iadd.23. A method of amplifying an optical signal, the method comprising:   launching said optical signal into a core of an optical fibre, the optical fibre being single mode at a wavelength of the optical signal, said core consisting essentially of silica, alumina, germania, and a fluorescent additive, the fluorescent additive being erbium;   simultaneously launching pump radiation into said core whereby photons of said pump radiation are absorbed by said fluorescent additive to produce a population inversion which sustains stimulated emission of signal photons so as to amplify said optical signal..Iaddend..Iadd.24. An optical amplifier adapted for the amplification of an optical signal, said amplifier comprising:   a fibre having a core, the fibre being single mode at a wavelength of the optical signal, said core having a doped region consisting essentially of silica, alumina and a fluorescent additive, the fluorescent additive being erbium;   an input connected to the fibre, said input being adapted for launching an optical signal into the fibre for amplification;   a pump optically connected to said fibre for providing pump radiation into said core, said pump radiation being adapted to interact with the fluorescent additive so as to produce a population inversion capable of sustaining stimulated emission of signal photons, said stimulated emission constituting amplification of said optical signal..Iaddend..Iadd.25. An optical amplifier adapted for the amplification of an optical signal, said amplifier comprising:   a fibre having a core, the fibre being single mode at a wavelength of the optical signal, said core consisting essentially of silica, alumina, germania, and a fluorescent additive, the fluorescent additive being erbium;   an input connected to the fibre, said input being adapted for launching an optical signal into the fibre for amplification;   a pump optically connected to said fibre for providing pump radiation into said core, said pump radiation being adapted to interact with the fluorescent additive so as to produce a population inversion capable of sustaining stimulated emission of signal photons, said stimulated emission constituting amplification of said optical signal..Iaddend.