Due to the growing demand of bandwidth in optical communication systems, the step towards 40 Gbit/s is inevitable. This step is limited mainly by the polarization mode dispersion of the fiber infrastructure. To extend the usability of the infrastructure it is necessary to employ PMD-compensation. The On/Off-keying modulation format in conjunction with direct detection is state of the art in high bitrate optical communication systems. But as a drawback, direct detection only provides an output signal which is proportional to the square of the absolute value of the electrical field and therefore transforms linear effects such as PMD into the nonlinear domain, which makes linear compensation schemes less effective. Coherent detection on the other hand delivers amplitude, phase and polarization information of the field and thus enables advanced PMD-compensation in the electrical domain. In our work, we employ optical coherent detection to receive two orthogonal components of the complex valued electrical field of an On/Off-keying modulated optical carrier. This single input multiple output system delivers us up to four output signals, i.e. real and imaginary part of the two detected polarization planes, which can be fed to feed forward equalizers or other electronic processing methods for an effective compensation of signal distortions caused by PMD. The required feed forward equalizer settings and their performance are presented.
Polarization mode dispersion (PMD) is one of the major limitations for optical transmission systems at 10 Gb/s and beyond. While first- or second-order PMD compensators (PMDC) can be driven with a feedback signal, more complex broadband PMDCs have to be set feed forward. An exact knowledge of the fiber's PMD characteristics - e.g. the PMD vector - is needed for the feed forward setting. Since PMD changes with time, real-time PMD measurement without data traffic interruption is necessary. Some recently published frequency- and time-domain methods meet these conditions. In this publication we are going to examine and compare different on-line measurement methods. Using numerical simulations, the performance of the measurement methods is assessed in terms of the accuracy of the PMD vector measurement and the qualification as feed forward control signal for setting a PMDC. The measurements exhibit an inherent inaccuracy if the signal is launched close to one of the principal states of polarization (PSP). Although these combinations of PSP and signal polarization result in inaccurate PMD vector measurements, the transmitted signal is not degraded by first order PMD. Consequently, the accuracy of the PMD vector measurement is a bad figure of merit for the performance of a system including a feed-forward set PMDC. Furthermore, due to the averaging over the signal bandwidth, the measured PMD vector is a better control variable for a PMDC than the analytically calculated PMD vector if second order PMD is considered.
As is well known, chromatic dispersion (CD) and nonlinear effect such as self-phase modulation (SPM),
cross-phase-modulation (XPM), and four-wave-mixing (FWM), as well as their impairments interaction
with each other are recognized as a limiting impairment for a high bit rates optical systems. With the advent
of the 40 Gb/s, it is necessary to study transmission performance, which clearly depends on the modulation
format and the system design.
A numerical comparison of non-return-to-zero (NRZ), return-to-zero (RZ) and differential phase shift
keying (DPSK) formats is made at a bit rate of 40 Gbit/s for single-channel and WDM systems with
different compensation method in attempt to find the optimum modulation format. The transmitter under
consideration used a 1.5 um DFB-laser externally modulated by a MZM modulator with modulation format
(NRZ, RZ, DPSK), 64 PRBS data. At the receiver end an optical filtering using gaussian, fabry-perot and
rectangular filter is used. Between the compensation method the symmetrical design leaves the best results
in comparison to pre- and post-compensation. The impact of SPM can be reduced considerably by the
symmetrical design. NRZ shows a best toleranz again chromatic dispersion and DPSK a best toleranz to
This paper presents different approaches to enable high-speed
transmission of 10 Gbit/s and beyond on polarization mode
dispersion (PMD) limited fibers. An introduction to the phenomenon
of PMD and its impact on system performance is presented. An
overview of common optical PMD mitigation methods and their basic
concept is given, including the problem of multi-channel PMD
compensation schemes for WDM systems. Furthermore alternative
methods like polarization scrambling, forward error correction and
electrical mitigation are considered. Bit error rate (BER)
determined by error counting is used as quantification for PMD
induced outages and a comparison to eye opening penalty (EOP)
based performance evaluation is given. The performance of basic
PMD mitigation schemes is compared by using the EOP and the BER
based outage criterion.
Chromatic dispersion and polarization mode dispersion (PMD) are the most relevant disturbances in optical high speed transmission systems. The influence of chromatic dispersion can be overcome by different methods such as dispersion compensating fibers or fiber gratings. Because of its stochastic behavior the struggle against
PMD is much more troublesome and essential for bit rates above 10 Gbit/s. The paper describes the influence of polarization mode dispersion derived from basic principles of transmission. It gives some physical understanding and simple derivation of the characteristic equations of PMD. One distinguishes between first and second order PMD. An optical fiber link showing first order PMD only can be modelled by a waveguide with time varying birefringence. If the birefringence additionally is frequency dependent the fiber will show higher order polarization mode dispersion. PMD compensators can be divided in a similar manner: compensators considering first order PMD only and compensators including PMD of higher order. Compensators operating in the optical domain or in the electrical
domain have been proposed.