A process for fabrication of ytterbium (Yb) doped optical fiber through vapor phase doping technique, said process comprising the steps of: (i) depositing pure silica cladding layers inside a silica glass substrate tube at a main burner temperature in the range of 1900 to 1980°C using MCVD process; (ii) sublimating AlCl 3 and Yb(thd) 3 in their respective sublimator chamber at a temperature in the range of 100 to 170°C and 180 to 260°C respectively to obtain vapors of Al-precursors and Yb-precursors; (iii) introducing preheated Helium in the sublimator chamber of step (ii) at a flow rate in the range of 10 to 50 sccm for Al precursors and 100 to 300 sccm for Yb precursors; (iv) transporting Al and Yb precursors with Helium obtained in step (iii) to the substrate tube with the adjustment of temperature of a ribbon burner in the range of 180 - 370 °C, wherein the ribbon burner is fixed before the input end of the silica substrate tube; (v) passing O 2 gas into a SiCl 4 bubbler at a temperature in the range of 15 to 40°C and a flow rate in the range of 80 to 150 sccm to transport SiCl 4 -O 2 gas mixture to the substrate tube; (vi) mixing SiCl 4, O 2, Al precursors, Yb-precursors, and Helium in the substrate tube followed by concurrent oxidation to form SiO 2, Al 2 O 3 and Yb 2 O 3; (vii) depositing a sintered core layer comprising SiO 2 -Al 2 O 3 -Yb 2 O 3 with targeted Al 2 O 3 and Yb 2 O 3 concentrations to obtain a deposited tube; (viii) collapsing the deposited tube at a temperature in the range of 1900 to 2300°C to obtain a fabricated preform; and (ix) drawing fibers from the fabricated preform obtained in step (viii) to obtain ytterbium (Yb) doped optical fiber. (i) depositing pure silica cladding layers inside a silica glass substrate tube at a main burner temperature in the range of 1900 to 1980°C using MCVD process; (ii) sublimating AlCl 3 and Yb(thd) 3 in their respective sublimator chamber at a temperature in the range of 100 to 170°C and 180 to 260°C respectively to obtain vapors of Al-precursors and Yb-precursors; (iii) introducing preheated Helium in the sublimator chamber of step (ii) at a flow rate in the range of 10 to 50 sccm for Al precursors and 100 to 300 sccm for Yb precursors; (iv) transporting Al and Yb precursors with Helium obtained in step (iii) to the substrate tube with the adjustment of temperature of a ribbon burner in the range of 180 - 370 °C, wherein the ribbon burner is fixed before the input end of the silica substrate tube; (v) passing O 2 gas into a SiCl 4 bubbler at a temperature in the range of 15 to 40°C and a flow rate in the range of 80 to 150 sccm to transport SiCl 4 -O 2 gas mixture to the substrate tube; (vi) mixing SiCl 4, O 2, Al precursors, Yb-precursors, and Helium in the substrate tube followed by concurrent oxidation to form SiO 2, Al 2 O 3 and Yb 2 O 3; (vii) depositing a sintered core layer comprising SiO 2 -Al 2 O 3 -Yb 2 O 3 with targeted Al 2 O 3 and Yb 2 O 3 concentrations to obtain a deposited tube; (viii) collapsing the deposited tube at a temperature in the range of 1900 to 2300°C to obtain a fabricated preform; and (ix) drawing fibers from the fabricated preform obtained in step (viii) to obtain ytterbium (Yb) doped optical fiber.