Patent ID: 6851281

Claim:
A process of fabricating rare earth doped optical fibre, wherein an evaporation and diffusion from a deposited porous silica soot layer containing refractive index modifying dopants is controlled by presintering of a porous layer by backward pass along with flow of dopant halides, the process comprising the steps of: depositing P 2 O 5 and F doped synthetic cladding within a silica glass substrate tube to obtain matched or depressed clad type structure, depositing a porous silica soot layer containing refractive index modifiers in a desired proportion at a temperature in the range of 1400-1700° C. by a backward deposition to obtain a deposited unsintered core layer, presintering of the deposited particulate core layer by backward pass at a temperature in the range of 1000-1400° C. with flow of said dopant halides and refractive index modifiers, immersing said tube containing the porous silica soot layer into a solution containing RE salt of concentration in the range of 0.002M to 0.25 M with aluminium salt of concentration in the range of 0.0 M to 1.25 M for a period of 1 to 2 hours, drying the porous layer by flowing dry nitrogen through the tube, heating the tube stepwise in presence of oxygen at a temperature in the range of 600-11000° C., dehydrating the core layer of the tube at a temperature in the range of 800-1200° C. and in presence of Cl 2 , sintering the core layer in presence of a mixture of oxygen and helium at a temperature in the range of 1400 to 1900° C., collapsing the tube at a temperature in the range of 2000-2300° C. to obtain a preform, overcladding the preform with a silica tube, and drawing fibres from the preform.