Phased beam projection from tiled apertures in the presence of turbulence and thermal blooming

The individual phases of a multi-beamlet laser source can be manipulated by exploiting high-bandwidth phase loops to correct for aberrations induced within the optical beamlet trains. With the current state of the art in phasing technology, this phasing of the beamlet trains is successfully accomplished up to a common aperture sharing element or on a point-source target; however, in the presence of an extended target, rough surface scattering through laser-target interaction adds the additional constraints of speckle and depolarizing effects. In particular, speckle phenomena and atmospheric effects create unobservable modes in the beam control system. One such unobservable mode is termed stair mode and is appropriately identified by a stair-step pattern of piston phase across the individual subapertures that comprise a tiled aperture. This paper investigates the effects of turbulence and thermal blooming on phased beam projection from tiled apertures using wave-optics simulations. To represent different array fill factors in the source plane, both seven and 19 element hexagonal close-packed tiled apertures are used in the simulations along with both Gaussian and flat-top outgoing beamlets. Peak Strehl ratio and power in the bucket are calculated in the target plane over multiple random realizations that are then averaged. This is done for all simulation setups with and without the presence of stair mode.