A process, comprising: (i) forming, by Isothermal RF Plasma Inside Deposition (IRFPID), a down-doped glass layer on the inside of an IRFPID silica starting tube, wherein the IRFPID comprises: (i') placing the silica starting tube within a resonant coil of a plasma generator; (ii') introducing a down-dopant-containing chemical reactant into the silica starting tube; (iii') maintaining the internal pressure within the silica starting tube at less than atmospheric pressure; (iv') energizing the resonant coil to create an isothermal plasma within the silica starting tube; (v') heating the inner wall of the substrate tube; and (vi') depositing down-doped glass on the silica starting tube within only a narrow zone upstream of the created isothermal plasma without deposition occurring within the isothermal plasma and the narrow zone being no greater than 1% of the length of the silica starting tube, wherein the plasma generator has a power range of 2-20 kW, the down-dopant-containing chemical reactant is a fluorine-dopant-containing chemical reactant, the fluorine-dopant-containing chemical reactant is delivered to the inside of the silica starting tube in the range of 1 to 2000 cc per minute (cc/min); the internal pressure within the silica starting tube is between 0.1 to 50 Torr; the inner wall of the substrate tube is heated to a range between 1000 °C to 1600 °C; O 2 is delivered in the range of 50 to 15,000 cc/min; SiCl 4 is delivered in the range of 0 to 2000 cc/min; a traverse velocity of the isothermal plasma is more than 1 m/min; and a traverse length of the isothermal plasma is more than 0.1 m, and negative delta values for the down-doped glass layer is lower than -0.05%. (i) forming, by Isothermal RF Plasma Inside Deposition (IRFPID), a down-doped glass layer on the inside of an IRFPID silica starting tube, wherein the IRFPID comprises: (i') placing the silica starting tube within a resonant coil of a plasma generator; (ii') introducing a down-dopant-containing chemical reactant into the silica starting tube; (iii') maintaining the internal pressure within the silica starting tube at less than atmospheric pressure; (iv') energizing the resonant coil to create an isothermal plasma within the silica starting tube; (v') heating the inner wall of the substrate tube; and (vi') depositing down-doped glass on the silica starting tube within only a narrow zone upstream of the created isothermal plasma without deposition occurring within the isothermal plasma and the narrow zone being no greater than 1% of the length of the silica starting tube, wherein the plasma generator has a power range of 2-20 kW, the down-dopant-containing chemical reactant is a fluorine-dopant-containing chemical reactant, the fluorine-dopant-containing chemical reactant is delivered to the inside of the silica starting tube in the range of 1 to 2000 cc per minute (cc/min); the internal pressure within the silica starting tube is between 0.1 to 50 Torr; the inner wall of the substrate tube is heated to a range between 1000 °C to 1600 °C; O 2 is delivered in the range of 50 to 15,000 cc/min; SiCl 4 is delivered in the range of 0 to 2000 cc/min; a traverse velocity of the isothermal plasma is more than 1 m/min; and a traverse length of the isothermal plasma is more than 0.1 m, and (i') placing the silica starting tube within a resonant coil of a plasma generator; (ii') introducing a down-dopant-containing chemical reactant into the silica starting tube; (iii') maintaining the internal pressure within the silica starting tube at less than atmospheric pressure; (iv') energizing the resonant coil to create an isothermal plasma within the silica starting tube; (v') heating the inner wall of the substrate tube; and (vi') depositing down-doped glass on the silica starting tube within only a narrow zone upstream of the created isothermal plasma without deposition occurring within the isothermal plasma and the narrow zone being no greater than 1% of the length of the silica starting tube, wherein