Noise Correlations in Phase-Locked VECSEL Arrays

Phase-locking of laser arrays was widely investigated in solid state Nd:YAG lasers [1]–[4] and CO 2 gas lasers [5]. Intracavity phase-locking of the laser array provides a methodology of high-quality beam shaping for medical, analytical, and industrial applications. Moreover, this kind of many-oscillator system is of interest for optimization problems and optical computations. Above mentioned lasers obey class-B laser dynamics due to the large values of their carrier lifetime. These dynamical classes are characterized by strong intensity oscillation and strong response to any system perturbations and noise even after reaching the system's steady state. Semiconductor active elements, such as VCSEL's hetero-structures (vertical cavity surface emitting laser), have an ns carrier lifetime. External cavity VCSEL was shown to be a very low noise laser (RIN∼−140dB/Hz) exhibiting quiet behavior without relaxation oscillations thanks to its low-loss long cavity [6]–[8]. Thus using such a structure as an active medium inside a degenerate cavity provides a class-A laser array. Here we present experimental results of VECSEL laser array development in a degenerate cavity and investigation of the noise characteristics of the free-running (Fig. 1.(b) top) and phase-locked regimes (Fig. 1(b) bottom). The experimental setup is presented in Fig. 1(a). The laser array is created using a metal mask placed very close to the output coupler (holes diameter = $200\mu\mathrm{m}$ , center-to-center separation = $250\mu\mathrm{m}$ ). Phase-locking is achieved using injection locking between the different lasers created by diffraction from the mask. Diffracted light of each laser creates a coupling channel with its neighbors after reflection on the mirror. Noise correlation between the coupled lasers was investigated for different sets of laser parameters in a wide range of noise frequencies. The influence of phase-locking on the intensity noise correlation between neighboring lasers (Fig. 1. (c)) and spectral synchronization up to single frequency operation was clearly identified.