Compact femtosecond lasers based on novel multipass cavities

This paper provides a comprehensive description of the design of compact femtosecond solid-state lasers that are based on novel multipass cavity (MPC) configurations to extend the resonator length. Of special importance are the q-preserving MPCs, which leave invariant the original spotsize distribution and Kerr lens mode-locking point of the short cavity. The general design guidelines of q-preserving MPCs are first reviewed and a novel configuration is proposed for the case where the MPC consists of notch mirrors. A class of non-q-preserving compact cavities is also analyzed and conditions needed to minimize the deviation from the q-preserving configuration are discussed. The design and performance of a q-preserving and a non-q-preserving mode-locked Ti:Al₂O₃ laser are then described as examples. These compact oscillators measuring only 30 cm × 45 cm could produce pulses as short as 19 fs at a repetition rate of around 31 MHz. Up to ∼3.6 nJ of pulse energy could be obtained with only ∼1.5 W of pump power. Finally, two-mirror MPC geometries are examined to investigate the limits of compactness and energy scaling.