An improved method for fabricating rare earth doped optical fiber using a new codopant, comprising the following steps: a. deposition of pure SiO 2 or P 2 O 5 -F doped synthetic cladding within a silica glass substrate tube to obtain matched or depressed clad type structure, b. forming a core by depositing unsintered particulate layer comprising at least one of GeO 2 and P 2 O 5 at a surface of the tube at a temperature in the range of 1200-1300° C, the unsintered particulate layer being deposited as a porous soot layer; c. maintaining GeO 2 concentrations between 3.0 and 15.0 mol% in the core layer; d. maintaining P 2 O 5 concentrations between 0.5 and 1.5 mol% in the core, when added with GeO 2; e. soaking the tube containing the porous soot layer into a solution containing RE salt in the concentration range of 0.002 M to 0.3 M with metal salt in the concentration range 0.15 to 2.0; f. drying the soaked core layer under dry N 2 or any inert gas through the tube for 10 to 30 minutes; g. heating the tube gradually in presence of O 2 and He in the temperature range 700-1100° C to effect oxidation; h. dehydrating the core layer at a temperature in the range of 800-1200° C and in presence of Cl 2 and O 2; i. sintering the core layer in presence of a mixture of O 2 and He in the temperature range of 1200 to 1850° C; j. collapsing the tube at a temperature in the range of 2000-2300° C to obtain a preform; k.jacketing the preform with silica tubes of suitable dimensions; l. drawing fibers from the perform; the said method being characterized in the use of Barium Oxide (BaO) from the said metal salt as a codopant to control RE incorporation while fabricating the RE doped optical fiber and provide smooth core clad interface therein. a. deposition of pure SiO 2 or P 2 O 5 -F doped synthetic cladding within a silica glass substrate tube to obtain matched or depressed clad type structure, b. forming a core by depositing unsintered particulate layer comprising at least one of GeO 2 and P 2 O 5 at a surface of the tube at a temperature in the range of 1200-1300° C, the unsintered particulate layer being deposited as a porous soot layer; c. maintaining GeO 2 concentrations between 3.0 and 15.0 mol% in the core layer; d. maintaining P 2 O 5 concentrations between 0.5 and 1.5 mol% in the core, when added with GeO 2; e. soaking the tube containing the porous soot layer into a solution containing RE salt in the concentration range of 0.002 M to 0.3 M with metal salt in the concentration range 0.15 to 2.0; f. drying the soaked core layer under dry N 2 or any inert gas through the tube for 10 to 30 minutes; g. heating the tube gradually in presence of O 2 and He in the temperature range 700-1100° C to effect oxidation; h. dehydrating the core layer at a temperature in the range of 800-1200° C and in presence of Cl 2 and O 2; i. sintering the core layer in presence of a mixture of O 2 and He in the temperature range of 1200 to 1850° C; j. collapsing the tube at a temperature in the range of 2000-2300° C to obtain a preform; k.jacketing the preform with silica tubes of suitable dimensions; l. drawing fibers from the perform; the said method being characterized in the use of Barium Oxide (BaO) from the said metal salt as a codopant to control RE incorporation while fabricating the RE doped optical fiber and provide smooth core clad interface therein.