Coherent beam combining and noise analysis of a colliding pulse modelocked VECSEL

Abstract

Optically-pumped SESAM-modelocked semiconductor disk lasers have become interesting ultrafast lasers with gigahertz pulse repetition rates, high average power and adjustable lasing wavelength. It is well established that colliding pulse modelocking (CPM) can generate both shorter pulses and improved stability. These improvements however typically come at the expense of a more complex ring cavity and two output beams. So far similar modelocking results have been obtained with CPM vertical external-cavity surface-emitting lasers (VECSELs) and with SESAM-modelocked VECSELs or modelocked integrated external-cavity surface-emitting lasers (MIXSELs) in a linear cavity. However coherent beam combining of the two output beams of a CPM VECSEL could result in a significantly higher peak power. This is interesting for example for applications in biomedical microscopy and frequency metrology. Here we demonstrate with a more detailed noise analysis that for both output beams of a CPM VECSEL the pulse repetition rates and the carrier envelope offset frequencies are locked to each other. In contrast to standard SESAM-modelocked VECSELs in a linear cavity, we only have been able to actively stabilize the pulse repetition rate of the CPM VECSEL by cavity length control and not by pump-power control. Furthermore, a first coherent beam combining experiment of the two output beams is demonstrated.

Optimizing the Cavity-Arm Ratio of V-Shaped Semiconductor Disk Lasers

“…Typically, the achievable pulse peak power is limited by multi-pulse instabilities that arise at larger pump powers due to the relatively short upper state lifetimes of semiconductor gain media [21]. One approach to overcome this issue is to construct elaborate multi-pass geometries [7,[22][23][24], which prevent the built-up of an excess inversion in the gain chip and thus the formation of additional pulses. The complex interplay between the gain medium dynamics and the laser cavity setup yields a fundamental mode-locking stability boundary, which characteristically depends on the geometric cavity configuration.…”