Fiber lasers have small size, high conversion rates and excellent thermal properties. On the other hand they generally produce smaller output intensity than semiconductor lasers. Recent experiments reported that a small number of fiber lasers can be synchronized simply by coupling them with an optical waveguide coupler near the output end. As a result the output peak intensity increases as a square of the number of coupled lasers, and well focused beams of high intensities can be produced, while preserving all other properties of fiber lasers. Synchronous behavior arises spontaneously, at constant pumping levels and without any active control. We investigate synchronization properties using a theoretical model, based on an iterated map with a particular symmetry. Our model captures key qualitative features seen in the experiments. We illustrate our results in a simple fiber laser configuration.