| Nowadays, free space optical (FSO) systems are a promising approach for a variety of applications. FSO systems are used not only in military applications and inter-connection of network nodes, but also as a rapidly deployable communications system in disaster recovery situations. System capacity and transmission distance of FSO links are adversely affected by the presence of atmospheric turbulence. A variety of techniques have been proposed to mitigate the effects of scintillation. Amongst them, diversity in transmission and/or reception, and large aperture receivers are the two main methods widely used in commercial FSO systems. Diversity techniques are costly due to the multiplication of transmitters and receivers. The large aperture receiver reduces signal fluctuations by averaging the received waveform over the aperture area. The averaging factor is a function of wavelength, propagation path length and aperture diameter. The longer the path link (one of the main challenges in future FSO systems) the more the link suffers from higher scintillation index and the less benefit there is from the averaging factor. Therefore, other techniques are requested for long links.
Previously, we have proposed an efficient method using saturated optical amplifiers (OA) to mitigate the effects of turbulence in the FSO links with low scintillation indices. The gain of the OA in saturation adjusts to the variations of the intensity: giving less gain when the intensity is high and more gain when the intensity is low, reducing the scintillation effect. In this paper, we validate experimentally the effects of using saturated OA (EDFA and SOA in amplification and conversion schemes) in the severe atmosphere turbulence conditions where no data transmission is possible. The PDF of logic-one and logic-zero levels are measured without amplification and compared to those of systems using the saturated EDFA or SOA. We find significant reduction in scintillation index and great improvement in BER (up to 3-orders of magnitude). |
| Mohammad Abtahi received the Ph.D. degree from both Sharif University of Technology, Tehran and Institut National Polytechnique de Grenoble (INPG), Grenoble, France, in 2000. From Feb. 2001 to June 2005 he has been with Iran Telecom Research Center (ITRC) as a member of technical faculty (assistant professor). Currently, he is with Centre for Optics, Photonics ans Laser (COPL), Department of Electrical and Computer Engineering, Laval University, Canada. His research interests are wireless indoors infrared communication systems, photonic switches, free-space optics systems, optical CDMA Networks, specifically frequency-encoded CDMA and holographic CDMA. |