Generation of UV light from microstructured fibers pumped with femtosecond 800 nm oscillator

Summary form only given. We report the observation of the upconversion of the femtosecond 800 nm pulses from a commercial Ti:sapphire oscillator to the UV frequency range. It is unambiguously observed that the nonlinear mechanism is distinct from that of the supercontinuum generation and the UV generation occurs starting from the higher-order mode at the fundamental wavelength and yielding a very high-order mode in the UV. The efficiency of the UV generation was found to be strongly dependent on the polarization of the fundamental and varied between experiments. These findings strongly suggest that launching the fundamental light into a higher-order mode was critical for the process. The aforementioned threshold corresponds to the melting of the fiber tip after which the tip is no longer flat but rather works as a phase mask for the coupling of the input light. Such a mask decreases the efficiency of the fundamental mode excitation leading to the disappearance of the supercontinuum and increases the efficiency of higher-mode excitation which results in the UV generation through phase matching to even higher-order modes.