International Journal of Engineering & Technology, 7 (4) (2018) 3747-3751 International Journal of Engineering & Technology Website: www.sciencepubco.com/index.php/IJET doi: 10.14419/ijet.v7i4.19109 Research paper Quality Factor and SNR Compensation of Free Space Optical Communication Link Using Different Modulators Ali Mehdi Hammadi 1 , Ali J. Al-Askery 2* and Entidhar Mhawes Zghair 3 1 Electrical Engineering Technical College , Middle Technical University, Baghdad, Iraq 2 Electrical Engineering Technical College , Middle Technical University, Baghdad, Iraq 3 Technical Instructors Training Institute, Middle Technical University, Baghdad, Iraq * a.al-askery@eetc.mtu.edu.iq Abstract Free space optical (FSO) systems introduce the best solution for the broadband network requirements with a cost reduction compared to the optical wire communication systems. This paper, studies the signal to noise ratio (SNR) and quality factor (Q-factor) for using two types of modulation formats and compares their performance. The analysis are performed for Mach-Zehnder modulator (MZM) and electro-absorption modulator (EAM) with distance range (1-4) km of FSO by using avalanche photo-diodes (APD) receivers. The simulation results are obtained based on the Optisystem 7.0 with optical high data rate of 10 Gbs for this communication system. The simula- tion results have shown that MZM gives better performance compared to EAM for different ranges and for the selected beam divergence value. Keywords: Electro-absorption modulator;Free space optical systems;Mach-Zehnder modulator 1. Introduction Free space optic (FSO) technology requires direct line of sight be- tween the transmitter and the receiver. There are several factors that affects the link quality such as the physical obstruction and weather conditions that limit the range of FSOs. Particularly, in the case of fog, the tiny size drops with high density diffuse the light at the oper- ating frequencies. In addition, the scintillation effect as consequence of atmospheric turbulence, give rise to variations in the optical index of the air in a localized temporary way. In consequences to this phenomenon, the power of the received signal at the detector varies and generate errors to the interpretation of the transmitted signal [1]. Utilizing multiple transmitters, each taking a slightly different route through the atmosphere enhances the transmission and helps smooth out disturbances [2, 3]. The FSO communication system is very low cost compared to other telecommunication systems, however, it is vulnerable to interference and jamming due to its narrow beam transmission [4]. The performance of FSO system is related to the transmission elements such as, modulation formats, type of Laser, transmission window, and atmospheric attenuation that is, signal degradation, signal absorption, scattering and scintillation [5]. In [6], two types of modulators namely Mach-Zehnder modulator (MZM) and electro-absorption modulator (EAM) are compared un- der clear weather conditions at a rate of 10 Gbps that have been simulated through FSO utilizing OPTISYSTEM. As a result, the EAM have achieved better signal to noise ratio (SNR) and Q-factor compared to the MZM system. On the other hand, dense wavelength division multiplexing (DWDM) have been utilized to enhance the data rate in optical fiber systems. In [7], system analysis of DWDM system utilizing erbium doped fiber amplifier (EDFA) is investigated at different fiber lengths for the return-to-zero (RZ) and non return-to-zero (NRZ). The results have shown that the BER will increase and the Q-factor decrease as the fiber length increase and vice versa. Designing FSO systems requires the consideration of these atmo- spheric attenuation in addition to the link distance and link misalign- ment. Other factors such as, data rate, sun radiation, selecting the wave-length, and the line-of-sight (LOS) [8]. Unlike radio frequency (RF) systems that requires Fresnel zone clearance, the clearance of the FSO systems between the beam center and any obstructions equals the beam radius. In addition, the alignment of the laser com- munication systems is crucial compared to the RF systems due to their narrow beam [9]. The power exceeds the receiver sensitivity is called link margin, M link (dB) and can be calculated using [10] M link = P e - S r - A geo - A atmo - A sc - A sys . (1) The total emitter power P e is measured in dBm, while S r represents the sensitivity of the receiver, the link geometrical attenuation is A geo , the absorption and scattering attenuation is A atmo , the atmospheric turbulence attenuation is A sc , while the rest of the system based attenuation is represented as A sys , these quantities are measured in dB. The successful installation of the FSO to achieve optimum trans- mission requires accurate estimation to the weather conditions, sur- rounding obstacles, path type, and transceiver alignment [11]. The accurate alignment is a crucial issue in the design of the FSO due to the very narrow beam widths which might result in a sever reduction in the signal [12]. In addition, weather conditions such as, wind and Copyright © 2016 Author. 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