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Abstract

The generation of ultrashort pulses with adjustable repetition rates from passively modelocked fiber lasers is demonstrated. By inserting semiconductor saturable absorbers with life-times of the order of 10 nsec into fiber lasers with cavity round-trip times of the order of 100 nsec, passive harmonic modelocking is obtained, leading to the stable generation of pulses at integer multiples of the fundamental cavity frequency. For polarization states that allow for optical limiting of the lasers, pulses are obtained in a frequency range between 20 and 500 MHz., where different repetition rates can be simply selected by changing the pump power level to the cavity. The pulse jitter within one cavity round-trip time is measured to be between 300 psec and 50 psec, where side bands in the frequency domain may be suppressed by up to 70 dB. The saturable absorber acts to both stabilize the cavity repetition rate and to initiate passive modelocking, where nonlinear-polarization evolution is used to sustain modelocking and to suppress amplitude fluctuations of the pulses by providing for optical limiting. Practical embodiments can be obtained by employing environmentally stable cavity designs in polarization preserving single-clad and double clad fibers, where the latter fiber designs allow the pumping of the fiber lasers with broad area-diode array lasers.

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