We demonstrate a mode locked semiconductor laser with high pulse energy (>0.5nj) and high spatial brightness (single mode). Developing Mode-locked lasers based on semiconductor gain media is a long-standing objective in laser research. A major hurdle for realizing ultrafast pulses in semiconductor lasers is the short life time of the excited state (~1-2 nanoseconds) that prevents gain accumulation in the medium, which conversely limits the applicability to  short cavities with high repetition rates (>10GHz) and low pulse energies (picowatt range). Thus, traditional mechanisms that enable mode-locking operation such as Kerr-lensing and Saturable-absorber cannot be used as the main mechanism for mode-locking.

To circumvent the life-time limitation we employ synchronous pumping – we pump the laser with short, intense pulses of current, which are synchronized with the arrival of the optical pulse, completely evading the need for high-repetition rates.

In order to enhance the pulse energy and average power, we employ diodes with a broad active area. Normally, broad area diodes offer more optical power, but at the cost of decreasing the brightness of the lasing mode. To overcome this, we design our external cavity to be non-degenerate spatially and support only asingle spatial-mode. With the combination of the above techniques, we realized a laser with a repetition rate of 65MHz, pulse width of ~60-80ps and pulse energy of ~0.5nJ, while maintaining single-mode brightness. This results paves the way for other techniques in mode-locked lasers, such as passive mode-locking, that require sufficiently high intra-cavity pulse energies, thatwere so far unavailable in standard diode-laser configuration.