Experimental results of the combination of transverse mode selection and active mode-locking with anti-reflection-coated broad area lasers (BALs) are presented. The BALs are subject to feedback from a free-space external Fourier-optical 4 -setup with a reflective spatial frequency filter in the Fourier-plane for transverse mode selection. Driving the BALs with a high frequency modulated pump current above threshold active longitudinal mode-locking is achieved. Pulse durations as low as 88？ps are obtained, while the Gaussian-like fundamental or a higher order transverse mode up to mode number 5 is selected on purpose. Pulse duration and shape are nearly independent of the selected transverse mode. 1. Introduction Semiconductor lasers are widely used in many of today’s photonic applications, due to their advantages like reliability, small size, the opportunity for electrical pumping, and an excellent efficiency. However, catastrophic optical mirror damage (COMD) of the edge-emitting laser facets limits the output power of laser diodes. The use of wide active stripes in so-called broad-area lasers (BALs) circumvents this restriction. BALs with an emitter width of 50？μm up to 1？mm [1] are available. The large size of the active region and the high power lead to strong nonlinear optical coupling of the amplified wave and the active material and result in an uncontrolled temporally unstable emission of more than one transverse mode. The emission of BALs can be controlled by, for example, using a grating integrated in the semiconductor chip [2–4] or by external cavities [5–10]. The latter also influence the longitudinal mode emission and the stability of the laser. On the other hand feedback from external cavities can also destabilize the emission of laser diodes [11–15] leading to self-pulsation. Semiconductor lasers are advantageous for the generation of short optical pulses due to the large gain bandwidth of 30–45？nm. Mode-locking is the most common method applied to generate short pulses in semiconductor lasers. Two mode-locking techniques can be distinguished, that is, active and passive mode-locking [16]. In active mode-locking intracavity amplitude or phase modulators are used to force coupling of the longitudinal cavity modes. The advantage of active over passive mode-locking is a higher reproducibility of pulses with respect to pulse height and width [16]. Pulse widths in the picosecond range are reported by many groups investigating active or passive mode-locking of external cavity narrow stripe laser diodes [17–20]. However the power of available
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