The performance of multi-hop free space optics (FSO) is investigated using serial decode-and-forward (DF) relay transmission. While researching the optical channel fading model, a statistical model for the optical intensity fluctuation at the receiver due to the combined effects of strong turbulence-induced fading, misalignment error and pass loss is presented. For given weather and misalignment conditions, the end-to-end outage probability of serial DF multi-hop FSO is derived and a closed-form analytical expression is got. Numerical results show that the serial DF multi-hop transmission is a promising technology to increase the performance of FSO. The derived analytical expression shows excellent agreement with the simulation results.