Free-space optical communication through atmospheric turbulence and optical modeling of MEMS corner cube retroreflectors.

摘要

This dissertation investigates spatial and temporal free-space optical communication techniques to combat atmospheric turbulence induced intensity fluctuations and the optical modeling of a very useful passive free-space optical transmission device, the MEMS corner cube retroreflector.; In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. We describe several communication techniques to mitigate turbulence-induced intensity fluctuations, i.e., signal fading. These techniques are applicable in the regime in which the receiver aperture is smaller than the correlation length of the fading, and the observation interval is shorter than the correlation time of the fading. We assume that the receiver has no knowledge of the instantaneous fading state. When the receiver knows only the marginal statistics of the fading, a symbol-by-symbol ML detector can be used to improve detection performance. Spatial diversity reception with multiple receivers can be used to overcome turbulence-induced fading. We describe the use of ML detection in spatial diversity reception to reduce the diversity gain penalty caused by correlation between the fading at different receivers. In the temporal domain, maximum-likelihood sequence detection (MLSD) can be employed, yielding a further performance improvement, but at the cost of very high complexity. We describe two reduced-complexity implementations of the MLSD, which make use of a single-step Markov chain model for the fading correlation in conjunction with per-survivor processing. Next, we also investigate the performance of using error-control coding and pilot-symbol assisted modulation schemes through atmospheric turbulence channels.; Micromachined corner cube retroreflectors (CCRs) can be employed as transmitters in free-space optical communication links. In this application, a CCR is illuminated by an unmodulated beam, and one mirror of the CCR is intentionally misaligned to modulate the intensity of the retroreflected beam. The low power consumption, small size, and ease of operation of a CCR makes it an attractive option for certain types of optical links. However curvature and misalignment of the micromachined mirrors can cause CCRs to perform far from theoretical limits. In the second part of this dissertation, we develop two methods to predict the optical performance of CCRs having ideal or non-ideal mirrors. We first introduce a discretized analysis method based on ray tracing and scalar diffraction theory. We then propose a simpler phase-shift model under the assumptions that the misalignment and surface non-flatness are small, and that they do not alter the optical topology of the CCR. These assumptions are satisfied by typical CCRs to be used in free-space optical links. Using our two methods, we determine tolerances on mirror curvature and misalignment for representative micromachined CCRs.