Fiber-Optic Systems with Coherent Detection and Four-Dimensional Modulation
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.
quadrature phase-shift keying (QPSK)
16-ary quadrature amplitude modulation