Mitigation of scintillation in free-space optical communication using incoherent light

Abstract

Recently, free-space optical (FSO) communications have attracted a great deal of attention as an alternative means to transport high-speed wireless signals without using radio-frequency resources. Due to several-hundred-of-THz carrier frequency, the performance of FSO communication systems is highly affected by weather events and atmospheric turbulence. In particular, atmospheric turbulence gives rise to spatio-temporal variations of the refractive indexes of air, which, in turn, causes a time-varying magnitude fluctuations of light intensity collected by the receiver. This scintillation makes the received optical power fall below the receiver sensitivity, and as a result, would induce communication disruptions. In this thesis, I study through experiment the mitigation of scintillation by using an incoherent light source. Incoherent light sources are divided into temporally and spatially incoherent light sources. First, I study the mitigation of scintillation using temporally incoherent light source. Spectrum-sliced incoherent light (SSIL) was proposed to implement the wavelength-division multiplexed (WDM) passive optical networks in a cost-effective manner. Multiple wavelength-specific channels are generated simultaneously by using a broadband incoherent light source and an arrayed waveguide grating. However, large relative intensity noise (RIN) inherent in incoherent light degrades the receiver sensitivity of this light source. Gain-saturated reflective semiconductor optical amplifiers (RSOAs) can be used for both RIN suppression and data modulation. In this thesis, I apply this light source to FSO communications and show that this light source is effective in suppressing the scintillation, compared with coherent light sources. The mitigation of scintillation could be further enhanced by adding spatial incoherence to the temporally incoherent light source. For this purpose, I insert a single-mode-fiber to multi-mode fiber offset launcher at the output of the RSOA-based SSIL source. I analyze the spatial incoherence characteristics and the scintillation mitigation effect as the offset distance varies, and confirm that the proposed spatio-temporally incoherent light source further mitigates the scintillation effect. I believe that the proposed incoherent light source could be used to increase the capacity of FSO communication systems in a cost-effective manner due to its capability of mitigating the scintillation.