Tuning and Automatic Phase Adjustment in External Cavity Diode Laser BeamCombiners

摘要

We report quantitative measurements of self correcting phase shift produced inphaselocked beam combiners using multiple antireflection coated diode lasers in an external cavity. The major result is the demonstration that the primary automatic phase adjustment is produced by optical frequency changes acting on the difference in pathlengths to the individual diode lasers, and that secondary phase changes are produced by fluctuations of the internal optical power acting on any imbalance among the various diode lasers' saturation behaviors. This suggests that the worst condition for a beam combining external cavity using automatic phaselocking is a near perfect pathlength match of the multiple laser arms. Further, the dynamic instability of a two laser system is measured and shown to be at its worst as the cavity length mismatch approaches zero. A second demonstration is controlled tuning over a frequency range corresponding to the inverse of the cavity length difference by piezo control of the phase error. Finally, the advantage for automatic phaselocking of using geometrically increasing feed lengths for the various diode lasers in the cavity is modeled and demonstrated.