Phase control algorithms and filamentation of ultrashort laser pulses in a scattering medium

The scattering nature of biological systems restricts the non-linear propagation of coherent light in media of biological interest. Using a spatial phase shaping technique, we optimize the filamentation of femtosecond laser pulses in an ethanol solution containing a fluorescent dye, while the laser beam has first traveled through 1 cm of scattering aqueous solution. Following three-photon absorption process, the fluorescence signal of the Coumarin 440 dye is used as a control parameter by genetic and partitioning algorithms. Applied to the incident wavefront, the partitioning algorithm successfully enhances the dye fluorescence signal in a medium with a scattering coefficient (\(\mu _{\text {s}}\)) varying from 1.24 to \(3.71\hbox { cm}^{-1}\) and gives rise to the production of a single laser filament several millimeters inside the observation cell that contains an ethanol solution. Interestingly, the optimal enhancement for the highest scattering medium is obtained with the partitioning algorithm at the lowest resolution. On the other hand, no significant enhancement is obtained when applying the genetic algorithm.