A simple method for determination of nonlinear propagation regimes in gases

A simple method to evaluate the nonlinear propagation regimes in gases is demonstrated. The principle is to focus ultrashort laser pulses into a gas cell, vary the input pulse power and measure the transmission through a pinhole placed at the output. The resulting transmission curve yields an intuitive signature of various nonlinear propagation regimes. Going from low powers to higher, one first observes a brief decrease in the transmission due to nonlinear moving focus. Then, a sharp rise occurs, indicating the starting of the filamentation process. When the power increases further, the transmission saturates and eventually decreases due to the beginning of multi-filamentation. As a result, this method gives a quick and sensitive measurement of pulse energies required to have single and multiple filaments.