Coherent combining of multiple beams with multi-dithering technique: 100KHz closed-loop compensation demonstration Ling Liu a , Dimitrios N. Loizos b Mikhail A. Vorontsov ac , Paul P. Sotiriadis b , Gert Cauwenberghs d a Intelligent Optics Laboratory, Institute for Systems Research, University of Maryland College Park, MD 20742, U.S.A b Department of Electrical and Computer Engineering, The Johns Hopkins University Baltimore, MD 21218, U.S.A c Computational and Information Sciences Directorate, Army Research Laboratory 2800 Powder Mill Road, Adelphi, MD 20783, U.S.A d Division of Biological Sciences, University of California San Diego, CA 92093, U.S.A Abstract We demonstrate the coherent combining of three beams with a phase-locking controller using VLSI multi- dithering technique. Three ber-coupled phase shifters are used to compensate phase distortions in the beam propagation path. The highest dither frequency in our system is 70MHz. The achieved closed-loop compen- sation bandwidth of three beamlets is up to 100KHz. Keywords: coherent beam combining, phase-locking, multi-dithering 1. Introduction Coherent beam combining is an important research area for laser communications and beam projection applications. The reported experimental demonstrations from other research groups are briey described as follows. In [13], a phase-compensating 70-mm-diameter aperture transceiver with a hexagonal closely-packed array of seven 23-mm-diameter ber collimator sub-apertures was demonstrated. The signal at the far eld receiver was maximized by modulating each sub-apertures phase through adjusting the pump current to its ampliers pump diode using multi-dithering control with lock-in amplier. The dither frequency is about 20KHz. The feedback signal was acquired from the photo detector at the target plane in the concave-mirror- converted far eld. In [4], the optical outputs from 48 polarization maintaining bers in an 88 ber array (only 48 were used) with 250m pitch were collimated through an 88 lenslet array with the same pitch. The 48 collimated micro-beams were coherently combined through modulating individual in-line phase modulators (piezo stretchers) using stochastic parallel gradient descent method. The update rate of the controller is about 8KHz iterations per second. The feedback signal was acquired from the photo detector at the target plane in convex-lens-converted far eld. In both systems, the compensation e/ects for the phase distortions along the propagation path were demonstrated. However, the speed of the phase-locking controller are not very fast in these two systems. As a part of the research e/orts for the conformal adaptive phase-locked ber collimator array [5], the coherent beam combining using multi-dithering technique is demonstrated in this paper. The coherent beam combining using stochastic parallel gradient descent techniques for the conformal optical system is presented separately in [6]. 2. Experimental Setup The real far eld distance is too large (up to a few kilometers) to do the experiments in the laboratory. A far eld conversion lens is used to simulate the far eld in our experiments. A picture of the experimental optical setup with a three-element conformal optical transmitter in the laboratory is given in gure 1. The red arrowed lines show the propagation paths of the three beamlets.