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A fusion splice including a first optical fiber having a first MFD and a first MFD expansion rate. The splice further includes a second fiber having a second MFD and a second MFD expansion rate, wherein the second MFD is lower than the first MFD. The second fiber comprises a core, a cladding radiall

A fusion splice including a first optical fiber having a first MFD and a first MFD expansion rate. The splice further includes a second fiber having a second MFD and a second MFD expansion rate, wherein the second MFD is lower than the first MFD. The second fiber comprises a core, a cladding radially surrounding the core, and a zone of high-concentration of fluorine between the core and the cladding. The rate of MFD expansion of the first fiber is less than the rate of MFD expansion of the second fiber during the fusion splicing operation.

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1. A fusion splice comprising: a) a first optical fiber having a first MFD and a first rate of MFD thermal expansion; b) a second fiber having a second MFD and a second rate of MFD thermal expansion, wherein the second MFD is lower than the first MFD; c) the second fiber comprising i) a cor

1. A fusion splice comprising: a) a first optical fiber having a first MFD and a first rate of MFD thermal expansion; b) a second fiber having a second MFD and a second rate of MFD thermal expansion, wherein the second MFD is lower than the first MFD; c) the second fiber comprising i) a core,ii) a cladding, radially surrounding the core,iii) a zone across the boundary of the core and the cladding having a high concentration of fluorine, andiv) at least one diffusion barrier layer;wherein the first rate of MFD thermal expansion of the first fiber is less than the second rate of MFD thermal expansion of the second fiber during fusion splicing.2. The fusion splice of claim 1, wherein the core of the second fiber is erbium doped.3. The fusion splice of claim 1, wherein the maximum concentration of fluorine in the zone is between 0.5 to 6 mol %.4. The fusion splice of claim 1, wherein the core is Al doped.5. The fusion splice of claim 1, wherein the core is Al and La doped.6. The fusion splice of claim 1, wherein the first fiber has a lesser concentration of index-raising dopants in the core than the second fiber.7. The fusion splice of claim 1, wherein the first fiber has a lower diffusivity of index-raising dopants in the core than the second fiber.8. The fusion splice of claim 1, wherein the second rate of MFD change of the second fiber is controlled by the amount of fluorine in the zone.9. The fusion splice of claim 1, wherein the first fiber also includes a core, a cladding, and a zone of high-concentration of fluorine intermediate the core and the cladding, wherein the rate of MFD change of the first fiber is controlled by the amount of fluorine and the amount of index raising dopants.10. A broadband amplifier including the fusion splice of claim 1, wherein the first and the second fibers provide amplification for different wavelengths.11. A broadband amplifier including the fusion splice of claim 1, wherein only the second fiber provides amplification.12. The broadband amplifier of claim 11, wherein the first fiber is a pump laser combiner.13. The broadband amplifier of claim 11, wherein the first fiber is a pump laser pigtail.14. A telecommunication system including the broadband amplifier of claim 10.15. A telecommunication system including the broadband amplifier of claim 11.16. A method for fusion splicing a first and a second optical fiber, the method comprising the steps of: a) providing a first fiber having a first MFD and a first MFD expansion rate upon heating; b) providing a second fiber having a second MFD and a second MFD expansion rate, wherein the second MN) is less than the first MFD, and the second MFD expansion rate is greater than the first MFD expansion rate; c) wherein the second fiber comprises i) a coreii) a cladding radially surrounding the core;iii) a high concentration F-ring intermediate the core and the cladding, wherein the average concentration of F in the ring is higher than that of the center of the core or the outer edge of the cladding, andiv) at least one diffusion barrier layer,wherein the F-ring reduces the rate of MFD expansion of the second fiber when compared to a similar fiber without the F-ring;d) fusing the first and second fibers by applying heat to the end faces of each fiber and bringing them into close contact and optical alignment, while matching the MFD of the first and second fibers.17. The method of claim 16 wherein the MFD of both the first and the second fiber are monitored during the step of fusing, wherein the step of applying heat is controlled to achieve MFD matching.18. The method of claim 16, wherein the rates of MFD expansion of the first and second fiber are known, and the step of applying heat includes applying heat at a predetermined time and temperature that results in the intersection of the MFD expansion curves.19. A fusion splice comprising: a) a first optical fiber having a first MFD and a first rate of MFD thermal expansion; b) a secon d fiber having a second MFD and a second rate of MFD thermal expansion, wherein the second MFD is lower than the first MFD; c) the second fiber comprising: i) a coreii) a cladding, radially surrounding the core,iii) a zone across the boundary of the core and the cladding having a high concentration of fluorine, andiv) at least one diffusion barrier layer; d) wherein the first rate of MFD thermal expansion of the first fiber is less than the second rate of ME) thermal expansion of the second fiber controlled by the amount of fluorine in the zone.