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Sub-kHz RF-linewidth was demonstrated in two-section quantum-dash mode-locked lasers emitting near 1550 nm and 20 GHz repetition-rate using symmetric dual-loop and fiber delay. Compared to free-running lasing, RF-linewidth and timing-jitter reduced by ~100x and ~10x.

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We present a passive and robust mode-locking scheme for a Vertical External Cavity Surface Emitting Laser (VECSEL). We placed the semiconductor gain medium and the semiconductor saturable absorber mirror (SESAM) strategically in a ring cavity to provide a stable colliding pulse operation. With this cavity geometry, the two counter propagating pulses synchronize on the SESAM to saturate the absorber together. This minimizes the energy lost and creates a transient carrier grating due to the interference of the two beams. The interaction of the two counter-propagating pulses in the SESAM is shown to extend the range of the modelocking regime and to enable higher output power when compared to the conventional VECSEL cavity geometry. In this configuration, we demonstrate a pulse duration of 195fs with an average power of 225mW per output beam at a repetition rate of 2.2GHz, giving a peak power of 460W per beam. The remarkable robustness of the modelocking regime is discussed and a rigorous pulse characterization is presented.

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