Coherent operation of a monolithic two dimensional array of laser diodes in an external cavity

There has been interest recently in the development of schemes for the coherent combination of the outputs from arrays of diode lasers so as to produce a high brightness output in the far-field. There have been successful demonstrations of both on-chip and external cavity techniques for generating coherent output from laser diode arrays. However up until now the sources used for external cavity demonstrations have been restricted to linear arrays of laser diode emitters. Here we report the first demonstration of coherent combination of a monolithic two dimensional array of laser diodes in an external cavity. The source used for this experiment was a 2-element by 5-element section of an incoherent monolithic array of Fabry-Perot lasers, that had etched 45-degree micromirrors to direct the laser output normal to the wafer surface. The device was fabricated at TRW and provided by the US Air Force. The detailed structure of this device will be described at the meeting. The front facets of the lasers were antireflection coated (reflectivity of about 3%) and the back facets were coated with a gold film to increase their reflectivity. Due to thermal limitations the array could only be operated in pulsed mode, typical pulse widths were one microsecond at 0.1% duty cycle.