Optical pre-amplifier forms an important component of a free space optical (FSO) communication system for improving the receiver sensitivity. The use of an optical pre-amplifier results into amplified spontaneous emissions (ASE) which dominates the receiver thermal and shot noises. The square law photodetector at the receiver in a FSO system necessitates the consideration of Chi-square statistics for the decision variable contrary to the Gaussian approximation that is widely used in literature. In this paper, we evaluate the bit error rate performance of a FSO system assuming non-return-to-zero on-off keying modulation in the presence of ASE noise under the combined effect of very strong turbulence (modeled by negative exponential distribution) and pointing errors. A further insight into the system is provided by performing the diversity analysis. © 2016 IEEE.