In this paper, we design and analyze the dense wavelength division multiplexing (DWDM) in the multiple-input multiple-output (MIMO) free-space optical (FSO) communications system using the digital-pulse position modulation (DPPM) and on-off keying (OOK). We show the effects noises, interchannel crosstalk, and atmospheric turbulence for the proposed system in the weak and strong turbulence with the modulation techniques. DWDM channels suffer from the inter-channel crosstalk while the communication performance in an obviously limited by the MIMO-FSO caused by atmospheric scintillation. All Impairments, in addition to noises of the optical amplification which is emerged the amplified spontaneous emission and are likely to be problematic, particularly in the upstream direction, which are investigated, the results obtained a high enough crosstalk ratio can achieve access to high-capacity in a system network safe and human. DPPM is a more power efficient modulation and lower power penalty 0.2 ~ 0.3 dB from the OOK modulation.