Mitigation of scintillation effects using incoherent light source for free-space optical communication systems

Abstract

Free-space optical communications (FSOCs) have long been considered as a promising technology to deliver high-capacity wireless signals over atmospheric channel. It combines the wireless communication with optical transmission technology. Thus, high-speed (e.g., >10 Gb/s) data can be carried over free-space channel without exhausting RF resources, nor deploying fiber-optic cables. Major obstacle to wide spread use of this attractive technology is that the performance of FSOC systems is severely affected by weather conditions. Light signal is attenuated significantly when passing through fog, cloud, or heavy rain. Even in the clear day, the atmospheric turbulence gives rise to adverse effects, such as scintillation, beam wandering, fluctuation of angle-of-arrival, and beam spreading. Scintillation produces random fluctuations of light intensity at the receiver, which in turn, deteriorates the bit-error rate performance. Several techniques have been proposed to reduce the scintillation effect including aperture averaging, adaptive optics, diversity, and partially coherent beam (PCB). Aperture averaging employs a large aperture to average out the intensity fluctuations of the spatially distributed light signal. This technique becomes less effective as atmospheric turbulence gets stronger. Also, it requires a large beam-collecting optics, which increases the size and weight of the receiver. Adaptive optics and diversity (for example MIMO technique) are both considered the most powerful way to combat atmospheric turbulence effect

however, the cost of these techniques is prohibitively high. For example, expensive wavefront sensors and deformable mirror is needed to implement the adaptive optics. Multiple transmitters and receivers are required for the diversity technique. PCB can be employed to reduce the scintillation effect since scintillation can be understood as intensity fluctuations caused by multi-path interference. PCB can be realized in two independent dimensions

space and time. Most of previous works on PCB focus on the spatial PCB using coherent light. In this thesis, I propose to use incoherent light to mitigate the effect of turbulence. PCB implemented by using optical frequency comb has been proposed and demonstrated. However, this scheme requires long fiber and high-power optical amplifier to facilitate the fiber nonlinearities. Spectrum-sliced incoherent light (SSIL) has been employed to generate multiple-wavelength light sources for wavelength-division-multiplexed (WDM) passive optical networks. This WDM light source can be implemented by using an optical amplifier and arrayed waveguide grating (AWG). Thus, multiple-wavelength signals can be generated simultaneously in a cost-effective manner. I propose to use this SSIL source for FSOC systems. Fully incoherent nature of light in time helps to mitigate the scintillation effect especially when the scintillation is strong. Multiple-wavelength generation would be used to increase the capacity of FSOC systems though WDM. To evaluate the proposed scheme, I develop a computer simulation program for the FSOC system. I model the atmospheric channel by using a series of phase screens and solve the stochastic wave equation through the split-step Fourier method. The distance between phase screens are optimized for the computation time and accuracy. The coupling efficiency of distorted optical beam to single-mode fiber at the receiver is also estimated accurately. The verification method of the simulation is given in each part to validate the developed simulation program. I first evaluate the performance of the proposed scheme using the scintillation index (SI). The comparison of SI between SSIL and fully coherent beam is given in several turbulence conditions. From SI measurement, it is confirmed that the SI reduction of the proposed scheme is more effective when the atmospheric turbulence is strong. For example, when ${\sigma_R}^2$ =7.095, SSIL source has SI of 2.01 for $B_o$/$B_e$=30 GHz/1 GHz (optical bandwidth/ electrical bandwidth). However, the coherent source exhibits the SI of 2.4 for the same channel condition. In addition, I also compare the bit-error ratio (BER) performance between the two light sources for 1-Gb/s OOK signal over 2-km free-space link. In a weak turbulence condition, the fully coherent light source has better performance than SSIL source. This is because of the relative intensity noise inherent in SSIL. However, as the turbulence gets stronger (e.g., turbulence strength of ${C_n}^2$=$10^{-13}$ $m^P{-2/3}$) , the SSIL source outperforms the fully coherent light. For example, we achieve a sensitivity improvement of 0.81 dB when ${C_n}^2$ is as strong as $10^{-13}$ $m^P{-2/3}$. These results indicate that SSIL source could be used for cost-effective implementation of WDM FSOC systems in the presence of atmospheric turbulence.