The optical propagation of free-space optical (FSO) communications under the influence of atmospheric turbulence exhibits deterioration of signal spectral. By this nature, fiber-detection method that is implemented in the receiver system frequently, cannot overcome the turbulence effects. In order to improve the fiber-detection method, an optical spatial filter (OSF) that is composed of cone reflector, pinhole and multi-mode fiber is proposed as a detection method. It aims to increase intensity of signal spectral and signal power by suppressing temporal fluctuation. Thus performance improvement of FSO can be achieved by the OSF. The measurements of signal spectral from the OSF are compared to fiber-detection method. The OSF is intalled in rear focus spot of receiver lens before photodetector. The experiment results show significant improvement in suppressing temporal fluctuation in signal spectral from the OSF. (SNR) increases that are 36.5 dB, 37.5 dB, 37.7 dB, and 38.2 dB for DP1, DP2, DP3, and DP4, respectively. (BER) are produced more lower that are 10-9 to 10-12 by OSF. Those performances improvement is achieved higher by the OSF compared to fiber-detection method. The outcome of this work is recovering signal spectral that is deteriorated caused by turbulence effects modulation into optical propagation. Hence the OSF can be implemented for integrating FSO with optical fiber communication system in order to enhance bit-error-rate and signal-to-noise ratio performances.