While high-performance transmitter and receiver components are available at near-infrared wavelengths near 1.5 microns, atmospheric absorption, scattering and turbulence are less severe at mid-infrared wavelengths near 3.8 microns. We have developed a link architecture using 1.5-micron transmitter and receivers, in conjunction with nonlinear wavelength converters to shift the modulated signal to 3.8 microns prior to atmospheric propagation, and shift the received signal back to 1.5 microns for reception. Our 2.5-Gbit/s link has used quaternary phase-shift keying and coherent detection, with wavelength conversion performed by optical parametric amplifiers fabricated in periodically poled lithium niobate.
We have studied the use of adaptive optics to mitigate turbulence-induced phase fluctuations in links employing coherent (synchronous) detection. In ongoing work, we are developing adaptive optics algorithms that do not require wavefront sensors, because wavefront sensors with high sensitivity are not available for all wavelength bands of interest.
In previous work, we have developed electronic detection and signal processing algorithms to migtigate turbulence-induced amplitude fluctuations in links employing direct detection. These are based on joint temporal and spatial distributions of intensity fluctuations that are derived from physical models. We have studied both temporal-domain and spatial-domain techniques. In the temporal domain, we have derived performance bounds for various error-correction coding techniques. We have developed an optimal maximum-likelihood sequence detector (MLSD) that jointly detects the data and estimates the instantaneous fading state, and have derived a reduced-complexity implementation of MLSD based on a single-step Markov model of fading correlation. We have also studied pilot-symbol assisted modulation as a low-complexity means to estimate the instantaneous fading state, improving detection efficiency. We have experimentally demonstrated various temporal-domain detection techniques on outdoor free-space links. In the spatial domain, we have evaluated the use of spatial diversity reception, in which signals obtained at multiple receivers are processed using various optimal or suboptimal techniques.
X. Zhu and J. M. Kahn, "Markov Chain Model in Maximum-Likelihood Sequence Detection for Free-Space Optical Communication through Atmospheric Turbulence Channels", IEEE Trans. on Commun., vol. 51, no. 3, pp. 509-516, March 2003. PDF
X. Zhu and J. M. Kahn, "Performance Bounds for Coded Free-Space Optical Communications through Atmospheric Turbulence Channels", IEEE Trans. on Commun., vol. 51, no. 8, pp. 1233-1239, August 2003. PDF
X. Zhu and J. M. Kahn, "Pilot Symbol-Assisted Modulation for Correlated Turbulent Free-Space Optical Channels", Proc. of SPIE Intl. Symp. on Optical Science and Technol., San Diego, CA, July 29-August 3, 2001. The version published in the conference proceedings contains some minor typographical errors that are corrected in this version. PDF
X. Zhu, J. M. Kahn and J. Wang, "Mitigation of Turbulence-Induced Scintillation Noise in Free-Space Optical Links using Temporal-Domain Detection Techniques", IEEE Photon. Technol. Lett., vol. 15, no. 4, pp. 623-625, April 2003. PDF
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Last modified: June 18, 2008.