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Real-time electronic polarization tracking enables optical polarization-multiplexed QPSK transmission
PADERBORN, Germany - On 21st March 2007, the Univ. Paderborn in Germany,
CeLight Israel and Photline in France announce the worldwide first real-time
electronic tracking of optical polarization changes in a polarization-multiplexed
synchronous optical quadrature phase shift keying data transmission. This
was achieved in the "synQPSK" consortium funded by the European Commission.
The synQPSK modulation scheme quadruples optical fiber capacity and allows
for a purely electronic equalization of fiber chromatic and polarization mode
dispersions.
In the Univ. Paderborn, the signal from a standard distributed-feedback laser
was impressed with 2.8 Gb/s data in two QPSK modulators from Photline, to
transport in-phase and quadrature data in two orthogonal polarizations at
a line rate of only 0.7 Gb/s. After transmission over 80 km of fiber, the
signals were received in a polarization diversity intradyne coherent optical
receiver with two integrated-optical 90° hybrids from CeLight and another
standard laser as a local oscillator. The detected four photocurrent signals
were proportional to in-phase and quadrature components of the received optical
field in two polarizations. After analog-to-digital conversion, the electronic
field vector composed of two mixed polarizations was transformed into a polarization-separated
one by a matrix multiplication in a field-programmable gate array. Finally,
a phase-noise tolerant feedforward scheme recovered the intermediate frequency
carrier in spite of its 2 MHz linewidth, and the four digitized data signals
were demodulated synchronously. Correlation of the four data streams before
and behind the decision circuits was performed to dynamically update the sixteen
matrix elements. Motorized fiber-loops causing endless polarization changes
at a speed of up to 50 rad/s made the received signal highly time-variable.
However, a polarization control time constant of 23.5 microseconds rendered
this harmless, and allowed successful data recovery. The achieved bit error
ratio was within the threshold of state-of-the-art forward error correction
schemes. Pictures and a video as well as detailed measurement results can
be downloaded from the synQPSK website.
While other researchers have reported only offline polarization acquisition,
or real-time data transmission without polarization multiplex, this is the
first time that all ingredients of an ultimate performance, bandwidth-efficient,
robust optical modulation scheme have been combined: real-time adaptive electronic
polarization tracking and synchronous demodulation, transmission of four bit
per symbol using QPSK and polarization division multiplex, standard lasers.
With suitable electronics for 40 Gb/s (10 Gbaud) operation on the horizon,
including the possibility of a >10fold polarization tracking speed increase,
these superb properties make synQPSK a very attractive solution for a cost-effective
growth of optical transmission capacity.
Detailed measurement results
synQPSK transmitter
Electrical part of synQPSK receiver
Endlessly rotating fiber loops generate polarization changes at speeds up
to 50 rad/s, which are being tracked by the electronic polarization control
in the QPSK receiver ( Download
video ).
Start Video.