Self Q-switching and mode locking in broad area lasers subject to transverse mode selective feedback

Numerical and experimental results of output dynamics investigations of AR-coated broad area lasers (BALs) above laser threshold are presented. The BALs are subject to feedback from a free-space external Fourier-optical 4 f -setup with a spatial reflective filter in the Fourier-plane for transverse mode selection. It is shown theoretically and experimentally that under certain pump current conditions the BALs are operating in a repetitive self-pulsation mode. Pulse duration is approx. 1 ns at repetition rates of 200 to 500 MHz. Using the same setup active mode-locking of a BAL is achieved experimentally. Pulse durations of 103 ps are obtained. The Gaussian-like fundamental and higher order transverse modes up to mode No. 4 can be adjusted while the laser is operating in a mode-locked state. Experimentally, the simultaneous combination of mode-locking, transverse mode selection, and pulse shaping of a BAL in a modified 4 f -setup implementing a spectral filter is investigated. Employing an optimized spectral sinc-like function as amplitude and phase filter the mode-locked BAL emits nearly square-shaped pulses with a pulse duration of 705 ps, while running close to the Gaussian-like transverse mode.