Enhanced spectral broadening and spectral distortions of short laser pulses in holey fibers

The influence of the structure of the holey-fiber cladding on the spectral broadening of femtosecond laser pulses is experimentally studied. These experiments demonstrate that the spectral broadening of 70-fs pulses of 800 nm Ti:sapphire laser radiation transmitted through 2- and 3- micrometers -pitch holey fibers can be enhanced by a factor of about 1.5 by increasing the air-filling fraction of the fiber cladding from 16 up to 65%. The main physical factors influencing the phase distribution and coherence properties of broadly spanning frequency combs produced with the use of holey fibers are considered. Phase shifts and spectral distortions arising due to dispersion effects, modulation instabilities, and shock waves of pulse envelopes are explored, and general recipes to reduce the influence of these effects by means of fiber optics are discussed.