Picosecond surface-emitting semiconductor laser with > 200 mW average power

Abstract

A passively modelocked diode-pumped surface-emitting semiconductorlaser at 950nm with a 2GHz repetition rate is reported. Compared tothe first device of this kind, which the authors recently reported, agreatly improved average output power of 213mW and a reduced pulseduration of 3.2ps are achieved. The device consists of an opticallypumped semiconductor gain structure and a semiconductor saturableabsorber mirror (SESAM) in an external cavity. Introduction: Pulsed laser sources with multi-GHz repetition rates andaverage powers > 100mW are required for applications such as opticalclocking of integrated circuits, optical testing of semiconductor elec-tronics and telecommunication components, particle accelerators, andanalogue-to-digital conversion. Edge-emitting semiconductor lasershave been passively modelocked for quite some time [1] and ultra-shortpulses [2] and extremely high repetition rates [3] have been reported.The average and peak power is limited in these devices, because stablemodelocking requires fundamental transverse mode operation, whichrestricts the size of the facet. With optically pumped vertical-external-cavity surface-emitting semiconductor lasers (VECSELs [4]) these con-straints are eliminated, and much higher powers are possible withaccordingly increased mode areas, while diffraction-limited output isenforced by the external cavity. Pulsed operation has been achieved withsynchronous pumping [5, 6] and with active modelocking [7]. Recently[8], we demonstrated the first VECSEL which is continuously pumpedand passively modelocked with a semiconductor saturable absorber mir-ror (SESAM [9, 10]). With a new gain structure optimised for a lowthermal impedance and a smooth gain spectrum we have obtained tentimes more average power and six times shorter pulse duration. Gain structure: In the previously described gain structure [8], the gener-ated heat is removed through the GaAs substrate, which introduces asignificant thermal impedance. We have eliminated this problem by fab-ricating a structure where the active region is separated from the copperheat sink only by a semiconductor Bragg mirror. This is achieved by epi-taxial lift-off (ELO): first, we grew the gain structure in reverse order ona GaAs substrate with three intermediate etch-stop layers. We used afluxless indium soldering process similar to the one published by So etal . [11] to solder a dice to a copper heat sink and finally removed theGaAs substrate by etching. The additional effort of the epitaxial lift-off is justified by two impor-tant improvements. First, the reduced thickness of the semiconductormaterial leads to a greatly reduced thermal impedance, and the nearlyone-dimensional heat flow into the heat sink makes the device power-scalable: the output power can, for example, be doubled by applyingtwice the pump power to twice the mode area without raising the tem-perature of the gain structure, as long as the heat sink is able to removethe additional power. The second improvement is that we eliminate thepreviously observed etalon effect from the interference of reflectedwaves from the Bragg mirror and the soldered surface. This effect lim-ited the bandwidth and disturbed the modelocking in the previousdevice. The gain structure consists of a bottom mirror, an active region, andan anti-reflective section grown by metal-organic chemical vapour depo-sition (MOCVD). For the bottom mirror we used seven pairs of GaAs/AlAs and 23 pairs of Al