Fabrication of silica-based fiber doped with tungsten nanoparticles for second harmonic generation

• We have addressed all the comments listed by reviewers. • Attached is a file called Answer to Reviewers Comments in which we listed how we address all the comments. • We also attached the revised manuscript which we have revised accordingly. A novel silica-based fiber doped incorporating sub-stoichiometric tungsten oxide nanoparticles WO (3−x) was developed to investigate second-order nonlinear optical response, with an emphasis on second harmonic generation (SHG). WO (3−x) nanoparticles were synthesized and incorporated into the silica preform core using a modified chemical vapor deposition (MCVD) process combined with solution doping and subsequent drawn into a fiber. Under 1033 nm femtosecond excitation, both the doped preform and the corresponding fiber exhibited a distinct SHG emission band centered at 517 nm consistent with frequency doubling of the fundamental wavelength. The SHG signal increased with pump power following a predominantly quadratic dependence, confirming a second-order nonlinear mechanism in the WO (3−x) doped silica host. From the measured SHG output, a lower bound for the effective nonlinear coefficient was estimated to be d eff ≥ 2.2 × 10 −5 pm/V. These findings demonstrate the feasibility of WO (3−x) nanoparticle doped silica fibers as a second-order nonlinear processes and guided wave frequency conversion.