Fiber-Optic Systems with Coherent Detection and Four-Dimensional Modulation

Abstract: To increase the data rates of fiber-optic communication systems, modulation formats withhigher spectral efficiency and/or higher power efficiency than binary formats are of greatinterest. These formats require coherent detection and this thesis has been dedicated toinvestigating both novel modulation formats and two different types of coherent systems.Apart from intradyne systems with a free-running local oscillator laser in the receiver,we have investigated self-homodyne coherent detection, in which the phase reference is apolarization-multiplexed pilot tone. The optical signal-to-noise ratio requirement of selfhomodynesystems has been quantified and compared with intradyne systems. Using bothexperiments and numerical simulations, we also demonstrated the unique ability of selfhomodynesystems to compensate for nonlinear distortion due to the Kerr effect and ascheme to increase the spectral efficiency in WDM transmission.The thesis also explores power-efficient modulation formats in intradyne systems.Modulation formats with higher power efficiency and comparable spectral efficiency thanpolarization-multiplexed quadrature phase shift keying (PM-QPSK) and polarizationmultiplexed16-ary quadrature amplitude modulation (PM-16-QAM), can be obtained byoptimizing constellations in the four-dimensional (4-D) signal space of the optical carrier.Polarization-switched QPSK (PS-QPSK) recently emerged as the most power-efficient modulationformat in four dimensions. We were the first to generate PS-QPSK experimentallyand we also demonstrated an algorithm for equalization and polarization demultiplexing.In addition, we investigated single channel and WDM transmission with both numericalsimulations and a loop experiment, and proposed a scheme for differential coding.Another interesting 4-D constellation is set-partitioning 128 PM-16-QAM (128-SPQAM),exhibiting almost the same spectral efficiency as PM-16-QAM and higher powerefficiency. We performed a numerical study of 128-SP-QAM and found that the transmissiondistance can be increased with more than 40% compared to PM-16-QAM, that thetolerance to laser phase noise is similar, and that differential data encoding can be usedwithout joint phase estimation.Finally, we demonstrate that a relative amplitude scaling between the symbols witheven and the symbols with odd parity in Gray-coded polarization-multiplexed NPSK yieldsa new class of modulation formats, with higher asymptotic power efficiency than the originalconstellations. A 16-level format with 0.44 dB gain over PM-QPSK is obtained for anamplitude scaling equal to the golden ratio.