Tapered monolithic mode-locked laser diode with 200pJ pulse energy for space applications

Abstract

European Space Agency (ESA) considers Mode-Locked Semi-Conductor Lasers as a promising technology for precision metrology systems in space such as High Accuracy Absolute Long Distance Measurement. We report our progress towards challenging ESA requirements: picosecond pulse duration, pulse energy 200 pJ, Pulse Repetition Frequency (PRF) 1-3 GHz, PRF stability &lt; 5·10^-9 at 1 second and PRF tunability 20 MHz. The laser should have small power consumption, be compact and robust against launch vibrations. We have reported in the past two such mode-locked (ML) laser diodes, each reaching only 90 pJ pulse energies: (<i>i</i>) very long (13.5mm) monolithic tapered laser and (<i>ii</i>) inverse bow-tie external cavity (EC) laser. The subject of the present communication is a novel passively mode-locked monolithic tapered laser achieving 201 pJ pulses. Large optical cavity with 2QWs heterostructure provides a low internal loss (~1 cm^-1) together with high quantum efficiency (&lt; 90 %) and low series resistance. To reach high energy output pulses, the tapered gain section gets a low (&lt; 0.1 %) reflectivity dielectric coating. For passive mode-locking at fundamental cavity frequency, the saturable electroabsorber section is located at the back side of the gain chip with a high reflectivity coating (&lt; 95 %). The monolithic cavity is made 13.5mm long by introducing an intermediate section for PRF tuning around 3 GHz. We reached passive ML at 2.9 GHz PRF with pulse energy of 201 pJ, compressed pulse width of 2.6 ps and electric power consumption of 8.2 W. PRF can be continuously tuned by 9.8 MHz. Active current modulation for hybrid ML resulted in PRF relative stability at 9.16·10^-10 level on 1s intervals, while with a phase lock loop (PLL) acting on the DC gain section current we reached PRF stability of 1.15·10^-10 on 1 s measurement interval.