Univ. Paderborn, Germany
  CeLight Israel
  Photline, France
  IPAG, Germany
  Univ. Duisburg-Essen, Germany

Key Components for Synchronous Optical Quadrature Phase Shift Keying Transmission
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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.

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