Abstract-We propose and experimentally demonstrate a nondestructive method to monitor chromatic dispersion (CD) and polarization-mode dispersion (PMD) in traffic-carrying wavelength-division-multiplexing optical systems. Coherent heterodyne detection is used to down convert the spectrum of digitally modulated signal from optical domain into radio-frequency (RF) domain. By analyzing group delay difference and polarization walkoff between different frequency components through proper RF signal processing, both CD and PMD can be precisely determined. Good agreement between experimental results and theoretical values has been obtained.Index Terms-Chromatic dispersion monitoring, coherent detection, heterodyne, optical communication, polarization-mode dispersion (PMD) monitoring, radio-frequency (RF) signal processing.
FIBER chromatic dispersion (CD) and polarization-mode dispersion (PMD) are two major sources of transmission performance degradation in high-speed long-distance optical communication systems [1].So far, several CD and PMD monitoring methods have been proposed. The measurements of eye-opening penalty, -factor, or BER have been proposed earlier to monitor CD [2], [3], but these electrical domain measurements are data rate dependent and are performed on per-channel basis. The phase-modulation-amplitude-modulation (AM) conversion method proposed in [4] and the AM pilot tones method proposed in [5] and [6] require nonstandard transmitters and receivers. The CD can also be determined by monitoring the power of the clock component after photodetection [7]. However, the measured clock power is influenced not only by CD but also by PMD, and these two effects cannot be separated. Sideband optical filtering technique was also proposed recently to monitor CD [8]. In this case, the measurement accuracy may depend on the optical filter's parameters such as shape, bandwidth, and detuning as well as the optical signal modulation format. In practice, the bandwidth and the shape of an optical filter is difficult to control especially when the required bandwidth is very narrow.As far as the PMD monitoring is concerned, the effect of PMD on an optical system can be evaluated by monitoring the degree of polarization of the optical signal [9], but the results may be sensitive to the variation of signal optical spectrum and the modulation bandwidth. Vestigial sideband optical filtering has been recently proposed to evaluate PMD through measuring the strength reduction of the beating signal between the optical . Although this method is insensitive to effect of CD, the absolute signal strength at the beating frequency depends not only on PMD, but also on the modulation format and the spectral distribution of the optical signal. Therefore, a quantitative and reliable evaluation of PMD might not be possible. Very recently, coherent frequency-selective polarimeter was proposed which has the ability to evaluate PMD [12]. In this method, the measurement of different frequency components within the signal bandwidth is accomplished b...