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

Doctoral thesis, 2012

To increase the data rates of fiber-optic communication systems, modulation formats with higher spectral efficiency and/or higher power efficiency than binary formats are of great interest. These formats require coherent detection and this thesis has been dedicated to investigating 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 a polarization-multiplexed pilot tone. The optical signal-to-noise ratio requirement of selfhomodyne systems has been quantified and compared with intradyne systems. Using both experiments and numerical simulations, we also demonstrated the unique ability of selfhomodyne systems to compensate for nonlinear distortion due to the Kerr effect and a scheme 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 than polarization-multiplexed quadrature phase shift keying (PM-QPSK) and polarizationmultiplexed 16-ary quadrature amplitude modulation (PM-16-QAM), can be obtained by optimizing 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 modulation format in four dimensions. We were the first to generate PS-QPSK experimentally and we also demonstrated an algorithm for equalization and polarization demultiplexing. In addition, we investigated single channel and WDM transmission with both numerical simulations 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 power efficiency. We performed a numerical study of 128-SP-QAM and found that the transmission distance can be increased with more than 40% compared to PM-16-QAM, that the tolerance to laser phase noise is similar, and that differential data encoding can be used without joint phase estimation. Finally, we demonstrate that a relative amplitude scaling between the symbols with even and the symbols with odd parity in Gray-coded polarization-multiplexed NPSK yields a new class of modulation formats, with higher asymptotic power efficiency than the original constellations. A 16-level format with 0.44 dB gain over PM-QPSK is obtained for an amplitude scaling equal to the golden ratio.