In this paper, by using a simple mathematical inequality, we derive a $ new upper bound fkr the capacity
of$ free space optical channel in coherent case. Then, by applying general fading distribution, we obtain
an upper bound for mutual information in non-coherent case. Finally, we derive the corresponding
optimal input distributions for both coherent and non-coherent cases, compare the results with previous
works numerically and illustrate that our results subsume some of previous results in special cases.