Abstract

In recent few years, femtosecond fiber lasers have been developed with ever increasing performances in terms of pulse energy. The energy produced in a mode-locked fibre oscillator has been increased from some nanojoules to several hundreds of nanojoules. This breakthrough results from the emergence of low-nonlinearity Yb-doped large mode area (LMA) fibers and progress in development of high contrast semiconductor saturable absorber mirrors (SESAM). As the first reported laser based on this concept, an Yb-doped fiber laser generating 0.9 W of average output power in 500-fs soliton pulses was demonstrated [1]. More recently, a significant energy scaling (up to more than 0.25 μJ) in femtosecond pulses has been demonstrated thanks to the use of these new fiber designs in purely-normal dispersion laser configurations [2,3]. Further energy scaling should be possible by incorporation of one or more segments possessing large positive dispersion to achieve the so-called chirped-pulse oscillator configuration [4]. In this contribution, we report on generation of high-energy femtosecond pulses from a mode-locked chirped-pulse oscillator featuring large-mode-area fibers. The laser is based on a 1.3 m long LMA micro structure fiber featuring a 40 μm Yb-doped core and a fast saturable absorber mirror for passive mode-locking. The total cavity dispersion is controlled by insertion of a passive LMA microstructure fiber with a core diameter of 25 μm. Preliminary results are obtained using a 3 m long LMA passive fiber leading to a total cavity dispersion of 0.09 ps². In this configuration, we obtain self-starting mode-locking for an average output power of 500 mW at a repetition rate of 33 MHz. The single-pulse operation is maintained up to an average output power as high as 3 W, which results in an energy per pulse of 91 nJ.

© 2009 IEEE