The multiple-input-multiple-output (MIMO) technique is one of the effective means to mitigate the effects of atmospheric scintillation and reduce the symbol error rate (SER) for free-space optical communications. Considering the combinatin of Alamouti space-time code and time topping ultra-wide band (TH-UWB) communications, a novel repetition space-time codes (RSTC) method for 2×2 free-space optical communications with pulse position modulation (PPM) is developed. In particular, two decoding methods of equal gain combining (EGC) maximum likelihood detection (MLD) and correlation matrix detection (CMD) are derived. When a quasi-static fading and weak turbulence channel model is considered, simulation is conducted. Simulation results show that whether the channel state information (CSI) is known or unknown, the coding system demonstrates more significant performance of the SER than the uncoding. In other words, transmitting diversity can be achieved while conveying the information only through the time delays of the modulated signals transmitted from the different antennas. CMD has almost the same effect of signal combing with maximal ratio combining (MRC). However, when the channel correlation increases, SER performance of the coding 2×2 system degrades significantly.