Preparation of frequency-doubling fibers under UV excitation

Germanium-doped silica fibers were prepared for frequency doubling of 1.06-microm wavelength radiation under the simultaneous presence of the first, second, and fourth harmonics of the laser light. It was found that the preparation rate for frequency doubling was dramatically increased by the presence of the fourth harmonic. The experiments also showed that a metastable excited state plays a significant role in the process.