Compact ps-pulse laser source with free adjustable repetition rate and nJ pulse energy on microbench

A new compact picosecond light source is presented. It consists of a master oscillator, an ultra fast pulse picker element and integrated high-frequency electronics arranged on a micro bench with a footprint of 5 cm x 4 cm. The master oscillator is a 10 mm long laser consisting of 200 mu m long saturable absorber, 1500 mu m long gain, 8000 mu m long cavity, 200 mu m long DBR and 100 mu m long monitor sections. Pulses with a width of 7 - 10 ps (depending on output power) and a peak power of 1 W are generated by hybrid mode locking. Despite the 10 mm long cavity, the repetition rate is still 4.3 GHz, which is too high for many applications. In order to reduce the repetition rate to values between 1 kHz and 100 MHz, one has to pick single pulses out of the continuous pulse sequence generated by the mode-locked laser. For this purpose a semiconductor pulse picking element based on a ridge waveguide, which acts as an optical gate, and a tapered section for pulse amplification is developed. Selective pulse picking with a free choice of the repetition rate is achieved by driving the RW section by a high frequency GaN high-electron mobility transistor with low capacitances and high current density. If a current pulse with a width of about 200 ps (smaller than the spacing of the pulses) is injected, the RW section becomes transparent and an injected optical pulse can pass the RW section. This pulse is subsequently amplified by injecting short current pulses with a width of 2 ns and a peak current up to 20 A into the tapered section. Pulse energies in the nJ range at a frequency of 16 MHz are obtained.