Observation of non-exponential absorption of ultra-fast pulses in water

We observe non-exponential absorption as a function of path length for pulses propagating in pure water. Two types of pulses with differing time duration, bandwidth, and repetition rate were compared with simulated absorption predictions. Deviations from exponential behaviour occurred when the launched pulse had a temporal width of 60fs and a repetition rate of 1 kHz. Under these conditions we observe more than 2 orders of magnitude less absorption after propagation through 6 m of water compared to Beer's law prediction. No significant deviation was observed for launched pulses of varying bandwidth with temporal widths of 900fs and repetition rates of 80MHz.