Researchers announce real-time electronic polarization tracking in optical polarization-multiplexed QPSK transmission

March 21, 2007
MARCH 21, 2007 -- The synQPSK modulation scheme quadruples optical fiber capacity and allows for a purely electronic equalization of chromatic and polarization mode dispersions in optical fiber, say the researchers.

MARCH 21, 2007 -- The University of Paderborn in Germany, CeLight Israel, and Photline in France today announced what they claims is the first real-time electronic tracking of optical polarization changes in a polarization-multiplexed synchronous optical quadrature phase shift keying (search for QPSK) data transmission (Visit the synQPSK Consortium Web site for photographs and detailed measurement results.)

This milestone 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 chromatic and polarization mode dispersions in optical fiber.

In the University of Paderborn, the signal from a standard distributed-feedback laser was impressed with 2.8-Gbit/sec 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 Gbits/sec. 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 comprising two mixed polarizations was transformed into a polarization-separated one by a matrix multiplication in a field-programmable gate array, say the researchers. Finally, a phase-noise tolerant feed-forward 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/sec 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, note the researchers. The achieved bit error ratio was within the threshold of state-of-the-art forward error correction schemes, they claim.

A summary of the paper can be found here.

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, say the researchers. Those ingredients include real-time adaptive electronic polarization tracking and synchronous demodulation, transmission of four bit per symbol using QPSK, and polarization division multiplexed standard lasers. With suitable electronics for 40-Gbit/sec (10-Gbaud) operation on the horizon, including the possibility of a greater than tenfold polarization tracking speed increase, these properties make synQPSK a very attractive option for the cost-effective growth of optical transmission capacity, say the researchers.

Visit synQPSK Consortium