The pulse broadening and communication capacity in atmospheric turbulent and disperse channels are investigated. Models on pulse broadening are proposed, and the simulated results for a typical atmospheric condition and configuration are provided. The results show that the capacity of the system is affected mainly by atmospheric dispersion rather than atmospheric turbulence. The received pulse width can be minimized, and the system-achievable capacity can be maximized by selecting the optimum input pulse width. The proposed method can be used to predict the achievable capacity of a free space laser communication system in turbulent and disperse channels.