The performance of a free-space optical (FSO) communication system is significantly affected by atmospheric turbulence (AT) and pointing errors (PEs) apart from the additive noise which follows the Gaussian distribution. Optical pre-amplifier is an essential component of FSO systems for improving the receiver sensitivity. However, optical pre-amplification results into amplified spontaneous emissions (ASEs) which dominate the receiver thermal and shot noises. The square law photodetection process 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 (BER) performance of a FSO system assuming non-return-to-zero on-off keying modulation in the presence of ASE noise under Gamma-Gamma (ΓΓ) AT and PEs. We also derive an expression of asymptotic BER for the considered FSO system for large values of signal-to-noise ratio in terms of simple elementary functions. A further insight into the system is provided by performing the diversity analysis. © 2017 IEEE.