Polarization mode dispersion (PMD), especially in "old" fibers, is considered harmful for installation and upgrading of trunk lines. An optical PMD equalizer should have several or many differential group delay (DGD) sections with polarization transformers in between which can endlessly transform any input polarization into a principal state of the following DGD section. The sections must practically have fixed DGD's unless there is only one section. The small-signal baseband transfer function for PMD, higher order PMD, and the necessary number of sections as well as their control by the output signals of an electrical filter bank in the receiver are also discussed in this context. Several PMD equalizers have been realized and successfully tested in transmission systems with bit rates of 10, 20, and 40 Gb/s. The systems operated stably with well-opened eye diagrams for DGD's ranging between 0 and 1.7 bit durations. Best performance is obtained from a distributed PMD equalizer with one piece of polarization-maintaining fiber twisted by 64 stepper motors. The principle can also be realized in LiNbO 3 .

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We present simple analytical expressions which relate fiber grating group-delay ripples to system performance and allow the estimation of expected penalties. Experiments show excellent agreement with our theory. A WDM transmission experiment shows that the system performance can be drastically improved if the channels are slightly detuned in wavelength. In trod uc ti onDispersion-compensating fiber bragg gratings (DCFBGs) are a promising alternative to dispersion compensating fiber in order to compensate chromatic dispersion [I]. . They exhibit low loss, low nonlinear effect, the capability to compensate higher order dispersion together with compactness and potentially low costs. However, the group delay response of these gratings exhibit undesirable oscillations called ripples. These are caused by residual multiple reflections due to imperfections resulting from the manufacturing process and are easily measured [2]. Using simulations the influence of the ripples on the system performance has been addressed by several authors [3, 41. In this paper we present the first analytical model that relates the group-delay ripple to system penalty. The derived expressions are based on arbitrary group-delay characteristics and the basic signal parameters, i. e. the pulse shape. The validity of the model is confirmed by penalty measurements on narrow band and broadband DCFBGs. System designers can use this model to specify tolerable group-delay ripples.The model predicts a strong wavelength dependence of the penalty. This is also confirmed by a 320 Gbit/s transmission experiment over 90 km of standard singlemode fiber compensated with a single 30 nm DCFBG module.

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