Research Article Optical Spatial Filter to Suppress Beam Wander and Spatial Noise Induced by Atmospheric Turbulence in Free-Space Optical Communications Ucuk Darusalam, Purnomo Sidi Priambodo, and Eko Tjipto Rahardjo Department of Electrical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus Baru UI, Depok 16424, Indonesia Correspondence should be addressed to Ucuk Darusalam; ucuk.darusalam@gmail.com Received 20 November 2014; Revised 5 April 2015; Accepted 27 April 2015 Academic Editor: Mikhail Noginov Copyright © 2015 Ucuk Darusalam et al. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We propose an optical spatial ilter (OSF) method to suppress beam wander and spatial noise efects. Signal from random displacements of the focus spot around the optical axis within the constricted area is collected. his method advantageously suppresses luctuations in signal intensity. he OSF consists of a pinhole and cone relector. he pinhole produces Fresnel difraction on the focus spot. he cone relector provides directed relectance onto the pinhole for random focus spot displacements due to beam wander. he calculations of signal power are based on luctuations of signal intensity that are minimized by the circular aperture function of the pinhole and the cosine of the relectance angle from the cone relector. he method is applied to free-space optical communications at a wavelength of 1.55 m with an atmospheric chamber to provide optical propagation media. Based on calculations, the beam wander angles that can be received by the OSF are from 14.0 ∘ to 28.0 ∘ . Moreover, based on experiment, the OSF with a pinhole diameter of 20.0 m and cone relector diameter of 1.5 mm produces signal power of −15.3 dBm. Both calculations and experiment show that the OSF enhances the received signal power in the presence of turbulence. 1. Introduction Free-space optical (FSO) communications are a class of prospective telecommunication technologies capable of sup- porting satellite-, terrestrial-, high altitude-, and mobile terminal-platforms that have been developed worldwide, recently [1–4]. However, atmospheric turbulence can cause severe problems that degrade FSO performance, such as decreased signal-to-noise ratio (SNR) and increased bit- error-rate (BER) [5, 6]. Turbulence causes optical propaga- tion phenomena such as difraction, scattering, absorption, and beam spreading that attenuate or difuse the FSO signal through stochastic processes. he FSO experiences spatial noise or random movement of hot spots around the center of the focus spot in the receiver plane [7]. Turbulence also produces beam wander that luctuates the angle-of-arrival on the receiver lens, leading to random movements of the focus spot [8]. Beam wander and spatial noise are turbulence efects on optical propagation that become major factors in luctuating signal intensity. Several methods have been developed to overcome tur- bulence efects. hese are spatial-diversity [9], time-diversity [10], cooperative diversity [11], optical ampliication [12], photo sensor design [13], and adaptive optics [14]. he afore- mentioned methods have several common drawbacks, such as a high complexity and decreased bit-rate-capacity process- ing. For example, spatial-diversity involves transmission or reception of multiple beams to produce high signal intensity with minimum noise and requires complex electronics to achieve equal gain combining. Time-diversity uses a complex of decoding techniques to receive redundant signals over a period of time. hus, spatial- and time-diversity use complex signal processing that risk decreasing the bit-rate-capacity if signal intensity luctuations are not minimized beforehand. However, since these methods use direct- or iber-detection to retrieve signals, optics suppressing beam wander and spatial noise may be implemented before the receiver plane [15] to minimize signal intensity luctuations. Beam wander and spatial noise cannot be treated as separate and independent processes, since they both occur Hindawi Publishing Corporation Advances in Optical Technologies Volume 2015, Article ID 594628, 6 pages http://dx.doi.org/10.1155/2015/594628