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Timestamp: 2019-04-20 05:17:57+00:00

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The different dynamical regions of an optically-pumped SESAM mode-locked, long-cavity VECSEL system with a fundamental pulse repetition frequency of ~200 MHz are investigated. The output power, captured as 250 μs long time series using a sampling rate of 200 GSa/s, for each operating condition of the system, is analyzed to determine the dynamical state. A wavelength range of 985-995 nm and optical pump powers of 10 W-16.3 W is studied. The system produces high quality fundamental passive mode-locking (FML) over an extensive part of the parameter space, but the different dynamical regions outside of FML are the primary focus of this study. We report five types of output: CW emission, FML, mode-locking of a few modes, double pulsing, and, semi-stable 4th harmonic mode-locking. The high sampling rate of the oscilloscope, combined with the long duration of the time series analyzed, enables insight into how the structure and substructure of pulses vary systematically over thousands of round trips of the laser cavity. Higher average output power is obtained in regions characterized by semi-stable 4th harmonic mode-locking than observed for FML, raising whether such average powers might be achieved for FML. The observed dynamic transitions from fundamental mode-locking provide insights into instability challenges in developing a stable, widely tunable, low repetition rate, turn-key system; and to inform future modelling of the system.
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Fig. 1 Experimental setup for passive mode-locking using an optically pumped VCSEL semiconductor gain and SESAM with a pulse repetition rate of ~200 MHz with cavity round-trip length ~1.5 m. ROC- radius of curvature. BW – bandwidth.
Fig. 2 The average output power (W) for each pump laser diode current (18 A - 24 A) is plotted as a function of VECSEL output wavelength (nm) and classified into three regions (i) CW low power region < 0.16 W, (ii) Medium power region between 0.22 W and 0.95 W, and (iii) high-power region >1 W. Points enclosed within ellipses indicate wavelengths at which the average output power has an almost linear relationship to the pump laser diode current.
Fig. 3 Dynamical classification of the parameter space region. Each operating condition, defined by a manually tuned wavelength value and pump source drive current is represented with a colored circle, where, the color indicates the average output power as seen in Fig. 2 and calibrated by the color bar on the right of the figure.
Fig. 4 0.5 μs subset of the time series captured at 991 nm and 23 A, displayed in a space-time representation showing a single pulse over 100 round-trips. The horizontal axis represents the laser cavity roundtrip consisting of ~1020 consecutive data points captured at 5ps sampling time. One round-trip is 5.0981 ns. The vertical axis represents consecutive round-trip times captured by the oscilloscope and stacked one above the other to show the pulse evolvement over time.
Fig. 5 (a) A space-time plot, obtained at 986 nm and 21 A, showing a window of time displaying transient locking with additional pixel correction. Sample pulses captured at progressive round-trips are shown in (b)-(d): (b) prominent pulse sub peaks at 11300th round-trip, (c) 11430th round-trip, and, (d) 11490th round-trip.
Fig. 6 (a) Space-time plot for a 0.5μs subset of the time series captured at 985 nm, 23 A; (b) average inter-pulse durations shown for a sample of pulse train.
Fig. 7 (a) Space-time plot of 5000 round-trips showing window of destabilization of the first pulse followed by a window of destabilization of the second pulse with increasing number of round-trips. Each window of destabilization shows a systematic sequence of pulse broadening with sub-structure and pulse amplitude redistribution. Sample pulses show pulse broadening with sub structure for (b) the first pulse in the cavity while the second pulse appears stable, and (c) the second pulse destabilized with the first pulse stable.
Fig. 8 (a), (c) Space-time representation over for (a) 0-0.5 μs and (c) ~229.41- 229.92 μs from the time series captured at 990 nm, 23 A. Sample temporal trace from space-time representations shown on the left respectively for (b) the 1st and (c) 45000th round-trip.
Fig. 9 0-50 ns subset of time series captured at 986 nm and 23 A.
Fig. 10 0-0.5μs subset of time series captured at 986 nm, 23 A showing dynamics over 100 round-trips of the region where the pulse formation is in the initial stages.

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