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.
Highlights
Colliding pulse modelocking (CPM) was first introduced with passively modelocked dye lasers [1]
It is well established that colliding pulse modelocking (CPM) can generate both shorter pulses and improved stability
Similar modelocking results have been obtained with CPM vertical external-cavity surfaceemitting lasers (VECSELs) and with SESAM-modelocked VECSELs or modelocked integrated external-cavity surface-emitting lasers (MIXSELs) in a linear cavity
Summary
Colliding pulse modelocking (CPM) was first introduced with passively modelocked dye lasers [1]. The pulse duration, measured with a second-harmonic autocorrelation, is for both beams 780 fs [Fig. 2(b)] and the pulse repetition frequencies are identical with 1.668 GHz, shown for the full span of the microwave spectrum analyzer (MSA) of 13 GHz with a resolution bandwidth (RBW) of 30 kHz [Fig. 2(c)] and magnified around the first harmonic in a span of 10 MHz with a RBW of 1 kHz [Fig. 2(d)] The goal of this experiment was not to obtain a new record performance of CPM VECSELs but to study the coherence of the two output beams and investigate if coherent beam combining is possible. Without CPM we stabilized the pulse repetition frequency of a standard SESAM-modelocked VECSEL via both cavity length and pump-power control and obtained the same stabilization performance. The large drifts of the CEO frequencies can be explained be the absence of any housing or shielding for the laser setup
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