This letter describes an optimized vapor phase doping technique using modified chemical vapor deposition system to fabricate rare-earth doped optical fibers for a high power laser. The process comprises deposition of aluminium oxide and ytterbium oxide in vapor phase simultaneously in combination with silica during formation of the core layer. The process parameters have been judiciously controlled to deliver aluminium chloride and rare-earth-chelate compounds to the reaction zone without decomposition and/or condensation of the precursor materials prior to the reaction zone. The standardization of the process parameters resulted in a good repeatability with a very low (< 1%) variation of dopant concentrations throughout the length of the preform. The fabricated fibers exhibit good optical properties with lasing efficiency of 76% at 1.06 µm.

The paper demonstrates a standardized process of vapor phase doping to fabricate large core Yb-doped preforms with longer useful length in reproducible manner. The optimization of the process led to successful achievement of Yb-doped core thickness of 4.5 mm (in 14.8 mm of preform diameter) by depositing up to 30 number of core layers with controlled amount of generated precursor vapors. The influence of the process parameters was studied rigorously to enhance the useful preform length up to 380 mm. A combination of Yb and Al in different proportions was doped into the core with uniform dopant concentration along the length by adjusting few process parameters efficiently. The Al 2 O 3 concentration up to the level of 17.8 mol% has been achieved successfully which resulted in NA of 0.31. This is the highest ever doping of Al in passive fibers by any modified chemical vapor deposition process. The Yb 2 O 3 content in the active fibers is as high as 0.47 mol%.

We present the characteristics of an ytterbium-doped alumino-silicate fiber, fabricated through vapor phase doping of aluminum and ytterbium in the core, along with silica and in conjunction with the modified chemical vapor deposition process. The vapor phase doping of rare-earths provides the opportunity to fabricate large core active fibers with a uniform distribution of dopants. The fibers fabricated exhibited low OH− content, negligible center dip and good optical properties. Lasing performance was tested up to output power of 105 W, with a slope efficiency of 77% with respect to launched pump power. The linear variation of the laser power with a pump shows its potentiality for further power scaling.

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The synthesis of rare earth‐doped aluminosilicate soot particles through vapor phase doping technique has been systematically investigated to achieve better control of rare earth homogeneity in silica glass. The experiments were designed to establish the correlation of process conditions with the physicochemical properties of the formed soot particles. Attained results depict that the characteristics of soot particles are much dependent on vapor phase compositions as well as the deposition temperature. Doping of alumina/rare earth oxide into silica matrix lowers the viscosity of the glass material at a level where size of soot particles becomes independent of deposition temperature. But dopant concentration in the formed soot samples moderately depends on the main burner temperature and by altering it appropriately one can eliminate soot layer sintering step. The paper also reports on the optimum conditions which are necessary to produce rare earth‐doped preforms of better homogeneity and thus, assist in achieving greater control over the process technology of vapor phase doping. As per the authors’ knowledge, this investigation presents completely new findings which have not been reported by any other research group earlier.