Wavelength dependence of femtosecond laser ablation threshold of corneal stroma.

We measure the surface ablation threshold fluence of porcine corneal stroma for 100 fs laser pulses, with wavelengths between 800 nm and 1450 nm, generated by a Ti:sapphire-pumped optical parametric amplifier. The ablation threshold was found to vary only slightly within this wavelength range, between 1.5 and 2.2 J/cm(2). The data suggest a rapid increase of the ablation threshold for wavelengths up to about 1000 nm, followed by a plateau for longer wavelengths. This behavior is partly confirmed by a simple theoretical model of the ablation process. The influence of the wavelength on the physics of surface ablation is discussed on the basis of the model.

Temporal characterization of short pulses stretched by fiber Bragg gratings.

Short femtosecond pulses at 1054 nm are stretched up to 400 ps by long broadband chirped-fiber Bragg gratings. The temporal profile of the reflected pulses is measured with a cross-correlation scheme. The advantage of this technique over the usual spectral measurements is its high sensitivity, since the temporal profile of reflected pulses is revealed to be strongly affected by the presence of defects in the grating. We show that this method is a good test of the quality of fiber Bragg gratings.

Spectral characterization of second harmonic chi((2)) cascading phenomena.

The second harmonic generation in a thin ss-barium borate crystal is used to measure chi ((2)) cascading phenomena in the spectral domain. The harmonic generation is induced by two pulses produced by spectrally filtering a femtosecond pulse and centered at the wavelength lambda -Delta lambda and lambda +Delta lambda. New spectral components appear in spectral density of both the fundamental and harmonic pulses. High order cascading phenomena are evidenced. In good agreement with theoretical predictions, for large phase mismatch the evolution of the spectra demonstrates the competition between cascaded chi(2) and chi(3) phenomena.