We demonstrate a semiconductor based, both actively and passively mode-locked laser, generating ps scale pulses at low repetition rate of 70MHz and relatively high pulse energy (1nJ).

Developing mode locked lasers based on semiconductor gain media is a long-standing objective in laser research. A major hurdle for the realization of ultra-short pulses in such lasers is the short excited state lifetime in semiconductors (a few nanoseconds), which prevents gain accumulation in the medium for long periods. Thus, most mode locked semiconductor lasers to date operate with very high repetition rates (few - tens of GHz), and low pulse energies. Because of the operation in this 'low power', close to threshold regime, Kerr-lens cannot be used as the nonlinear mode-locking mechanism, hence mode-locking was achieved so far using fast saturable absorbers, which are limited in obtaining ultrashort pulses in the fs range. In addition, a short cavity makes it very difficult to include in the cavity additional optical elements that are common tools of mode-locking technology, such as dispersion compensation and additional foci for Kerr-media.

Recently, we demonstrated a novel method to control mode competition in a solid-state Ti:sapphire laser [Optics Express 20, 9991 (2012)], where we introduced a gain medium into the cavity at a position where the oscillation spectrum is spatially dispersed. Here, we exploit this concept to incorporate a high-power, broad-area semiconductor gain chip (InGaAs) within a similar intra-cavity shaper that dramatically reduces the local peak energy (by dispersion in both space and time), and allows much higher pump powers. To avoid the upper state lifetime limit we employ active mode-locking by synchronous pumping with ~3ns long current pulses at the repetition rate of the cavity. Consequently, a long cavity can be used, which allows the inclusion of dispersion compensation elements, and possibly a Kerr-medium to generate pulses in the femtosecond range. With this configuration we obtained ~40mW average output power of ps scale pulses (<60ps, limited by detection bandwidth) at 70MHz repetition rate.