Coherent amplification of femtosecond pulses with passive enhancement-cavities

We demonstrate a general technique for amplification of femtosecond pulses through coherent buildup in an external cavity. A net single-pulse energy gain of 42 to more than 70 times for 38 to 58 fs pulse durations, respectively, is achieved. This technique offers a flexible approach to enhance the energy of fs pulses while maintaining relatively high repetition rates.