Mitigation of weak and strong turbulence-induced fading in optical homodyne RZ-QPSK via delay-diversity transmission

摘要

Delay-diversity transmission employing two orthogonal polarizations in turbulent media has been shown to be effective in combating fading. The technique employs simultaneous transmission of two orthogonal polarizations carrying the same information but delayed relative to each other by an amount (T_d) equal to or greater than the turbulence correlation time (τ_c). At the receiver, the polarization signals are detected separately with T_d compensated on the delayed polarization, resynchronizing the two signals, and they are then combined. Because T_d is comparable to τ_c, scintillation suffered by the two resynchronized polarization signals is essentially independent in a statistical sense. As a result, the average bit-error-rate (BER) of the polarization-combined signal is less than the average BER of either one of the polarization signals. The signal-to-noise ratio (SNR) is effectively improved. We describe here an experiment and test results of fading mitigation via polarization delay-diversity reception in homodyne optical RZ-QPSK transmission at 1.55 μm through a tabletop wind tunnel and phase plates. The wind tunnel and the phase plates produce, respectively, weak and strong turbulence with scintillation indexes less than and greater than one. The turbulence correlation time for the wind tunnel is less than 1 ms. The homodyne detected optical RZ-QPSK signals in the two orthogonal polarizations were captured and stored for off-line processing. Test results show that the computed BER of the polarization-combined signal is lower than the BER of either polarization signals. As a result, an equivalent signal-to-noise ratio improvement of at least 2 dB was deduced.