Third-harmonic generation of 1.319 μm Nd:YAG laser pulses in circular-core GeO2-doped silica single-mode fibers is reported. The experimental results have shown that the blue-violet light generated at 439 nm is less than 0.2 nm wide and its intensity depends strongly upon GeO2-doping concentration for fixed fiber length and pump power. It has been observed that for GeO2-doping concentrations greater than 9 mol %, the third-harmonic signal was the unique visible wavelength line exiting the fiber span throughout the spectral region of 400–800 nm. The frequency tripling process started immediately with fiber pump light illumination, i.e., no previous long-time preparation mechanism was required in order to have appreciable third-harmonic generation.

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