Time division multiplexed array development by using small size glass-fiber reinforced polymer based fiber optic acoustic sensors

Fiber optic systems are well suited choice for underwater acoustic measurements with respect to conventional piezoelectric ceramic based systems. Main advantages of fiber optic systems are low cost, small size, light weight; high sensitivity and most importantly they are electromagnetic interference (EMI) free. Fiber optic interferometric acoustic systems have seen extensive development for military applications such as horbour protection sonars, towed array sonars, flank array sonars. and etc. Recently, due to the success in the military area, fiber optic interferometric acoustic sensors are also being used in commercial seabed sensing applications for oil/gas exploration and seismic research. There are many techniques for sensing acoustic pressure like FBG (Fiber Bragg Grating) based and mandrel based Mach-Zehnder interferometer systems. However, because of its high responsitivity, mandrel based Mach-Zehnder interferometer system has been preffered. In Mach-Zehnder interferometer system, laser light is split into the sensing and the reference arm. Some part of the laser light pass through fiber wounded around reference mandrel, remaining pass through fiber wounded acoustic pressure sensitive sensor arm. Pahse difference between two arms gives information about acoustic wave. In this thesis, A full-detailed design has been done considering sensor mandrel material type. Glass Fiber Reinforced Polymer (GFRP) has been chosen because of its strength, durability and sensitivity. To the best of our knowledge there is no detailed work based Glass Fiber Reinforced Polymer based fiber optic acoustic system. The results of these studies, which were carried out for the first time, are promising It is important to demodulate phase difference between reference and sensor arms. To decrease environmental effects and prevent phase fading, PGC (Phase Generated Carrier) method has been applied for the demodulation of the acoustic signal. As a result, we have demonstrated an high sensitive fiber optic underwater acoustic sensors. The pressure sensitivity of sensors are are around -150 dB re rad/µPa. System noise level is both analyzed and measured and it is better than 50 dB re µP a/√ Hz at low frequencies. Finally, main advantage of fiber optic systems is that it is easy to increase number of sensors. For this reason, considerable efforts has been put for the development of various techniques. Some of the most recent techniques are frequency division multiplexing (FDM), wavelength division multiplexing (WDM) and time division multiplexing (TDM). We demonstrate a time division multiplexed sensor array because of advantages on cost and small dimension.