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Experimental Evaluation of Performance of Underwater Free Space Optical Link in the Presence of Air Bubbles

Free space optics (FSO) has become a potential candidate for underwater communications due to its tremendous advantages such as large bandwidth, security and high data rates than that of acoustic and radio waves at moderate distances. When an optical beam interacts with water molecules, it suffers from absorption, scattering and attenuation phenomenon that further results into the poor reception of the signal. The suspended particulate matter such as air bubbles severely degrade the system performance. The air bubbles in water spread the optical beam into spatial and temporal domain, distort the signal amplitude and hence deviate the optical beam from the desired path. So, an underwater FSO link was established to study its performance in the presence of different sized air bubbles. The attenuation constant, relative intensity with respect to forward scattering angle and randomness caused by this impairment have been evaluated experimentally. The attenuation constant rises in the presence of air bubble populations and relative intensity tends to decrease with increasing forward scattering angles. Further the increase in randomness of received intensity is evaluated and compared for two different bubble populations.