Coherent operation of 2×2 vertical cavity laser array by resonance tuning

Coherently coupled vertical cavity surface emitting laser (VCSEL) arrays are of interest for high brightness operation [1], whereby multiple laser elements are coherently combined into a narrow divergence single radiation mode. 2×2 VCSEL element arrays have been pursued in the past [2, 3], but there has always been an issue with the low yield of coherently coupled arrays. Recently resonance tuning has been explored to achieve coherence in 1×2 VCSEL arrays [4]. By independent current injection into each array element, the emission wavelength of each element can be tuned due to the combined effect of the thermal and electronic dependence of the refractive index [5], enabling coherent coupling in the array [4]. This work demonstrates the next extension of resonance tuning by achieving coherent operation of an initially incoherent 2×2 array.