Intensity modulated atmospheric laser communication requires the uniform and flat far-field intensity distribution and intensity uniformity will be improved due to the degradation of its spatial coherence. An experimental study of a D-shaped multi-mode fiber output optical field intensity distribution characteristics is presented, compared with the output optical field intensity distribution from the laser through the cylindrical multi-mode optical fiber and not via an optical fiber. Experimental results show that using a D-shaped multi-mode fiber as the mode mixer, the output beam has a low degree of coherence and good uniformity.