Outage Probability Due to PMD in Coherent PDM QPSK Systems With Electronic Equalization

Polarization-division-multiplexed (PDM) quadrature phase-shift-keying (QPSK) coherent optical systems employ blind adaptive electronic equalizers for polarization-mode dispersion compensation. In this letter, we compare the performance of fractionally spaced, linear electronic equalizers, composed of four parallel finite impulse response (FIR) filters of various lengths, using the outage probability as a performance criterion. The constant modulus algorithm is applied for the adaptation of FIR filter coefficients. A parallel programming implementation of the multicanonical Monte Carlo method is adopted for the estimation of the tails of the outage probability distribution. It is shown that less than 20 complex, half-symbol-period-spaced taps per FIR filter suffice, in order to reduce the outage probability of PDM QPSK coherent optical systems to less than 10^-5 , for a mean differential group delay up to twice the symbol period.