Transporting shaped pulses through an optical fiber in the nonlinear regime for selective excitation of two-photon transitions

Ultrashort pulses with shaped spectral phase, amplitude and polarization have numerous applications. Many of them involve samples that are difficult to access and therefore would benefit from the possibility of transporting the pulses from the laser setup to the sample through an optical fiber. This can be achieved by a photonic crystal fiber but still only a small class of them is commercially available and they are expensive. On the other hand, standard step-index fibers distort the pulses because of dispersion and nonlinear effects. For pulse energies in the order of 0.01 nJ nonlinear effects are negligible and by compensating dispersion (and residual birefringence) it was possible to obtain phase, amplitude and polarization shaped pulses after transmission through the fiber [1]. However, for higher energies self-phase modulation causes strong spectral narrowing (see Fig 1(a)).