We describe the design and mode-locked operation of compact femtosecond Ti<sup>3+</sup>:Al<sub>2</sub>O<sub>3</sub> lasers based on novel multi-pass cavity (MPC) configurations. We first review the design guidelines of q-preserving MPCs which leave invariant the original spotsize distribution of the short cavity. The design and performance of a q-preserving and a non-q-preserving MPC Ti<sup>3+</sup>:Al<sub>2</sub>O<sub>3</sub> laser are then described as examples. In the case of the non-q-preserving MPC, conditions needed to minimize the deviation from the q-preserving configuration are discussed. Both resonators are compact (30 cm x 45 cm) and operate around 31 MHz. With only 1.5 W of pump power, pulses as short as 19 fsec and with energies up to ~3.6 nJ were produced.

© 2004 Optical Society of America

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