PMD monitoring of polarization division multiplex transmission system using RF tone harmonics

We propose and demonstrate a novel technique for monitoring the first order polarization-mode-dispersion (PMD) effects on polarization division multiplex (PDM) quadrature phase shift keying (QPSK) signals. This technique uses the radio frequency (RF) tone at first harmonic and at half the data rate (half tone) of the detected electric spectrum. The electric spectrum is generated in a tap-line using square detection. The RF tones are usually absent after coherent detection of the PDM-QPSK signals. The technique uses fiber Brag grating (FBG) optical notch filter in the tap-line to remove one of the sidebands of the optical spectrum, and thus preventing the beating the sideband terms which normally cancels the RF clock tones. We focus on monitoring the effect of PMD on the regenerated RF tones by comparing their relative amplitude slopes. The simulated technique is simple and do not require modification at the transmitter end. It is shown that for PDM-QPSK data, the monitoring window is increased covering the whole bit period with improved accuracy.