Continuously tunable femtosecond delay-line based on liquid crystal cells

We introduce a new device for group and phase delay steering of femtosecond pulse trains that makes use of cascaded, electrically driven, nematic liquid-crystal cells. Based on this approach we demonstrate a continuously tunable optical delay line. The simple collinear implementation with no moving parts enables to shape the achievable temporal range with sub-femtosecond accuracy. By appropriately choosing the bias voltages applied to the cascaded cells, the imparted group delay can be made either positive or negative and precisely adjusted. Moreover, independent control of the group delay and the phase of femtosecond pulses is demonstrated.