Design of broad-band PMD compensation filters

Abstract

Abstract-We describe a new design approach for broad-band PMD compensation filters. An efficient algorithm for minimization of the maximum differential group delay within a given frequency band is described.Index Terms-Fiber-optic communication systems, optical filter design, PMD compensation, polarization-mode dispersion. IN RECENT YEARS, several approaches for polarization-mode dispersion (PMD) compensation have been studied. These approaches were designed to compensate for the effects of first-order PMD [1], [2] as well as high-order PMD [3]- [7], [12]. Typically, high-order PMD compensators (PMDCs) are multistage devices that have many degrees of freedom. A robust control algorithm is essential for using these broad-band PMDCs in practical applications. One major concern in controlling broad-band PMDCs is the possibility that they will converge to a local compensation optimum, which can be far from the absolute optimum. To overcome this problem some authors studied "feed forward" methods in which the Jones matrix (JM) of the PMD medium is determined online and the following compensation filter is adjusted so that its JM becomes as close as possible to the inverse JM of the PMD medium [8], [9]. The JM of the composite system, PMD medium and PMDC, becomes approximately frequency independent and PMD is compensated. Recently, a synthesis algorithm for such an inverse filter, in the absence of polarization-dependent loss or gain (PDL/G) was proposed [5]. The algorithm required knowledge of the medium JM and was based on expanding the elements of the desired JM as Fourier series and using the coefficients of the expansion to iteratively solve for the parameters of cascaded interferometric filters. The number of the terms in the Fourier expansion determined the number of basic cells in the filter. In general, however, the use of a finite number of terms in the Fourier expansion (typically smaller than 20) may result in large local deviations of the approximated matrix from the desired matrix. These deviations may cause the synthesis algorithm to fail or to produce large spikes in the PMD of the compensated system. In this letter, we demonstrate this and propose a new approach for designing a broad-band PMD compensation filters. The approach is general and is guaranteed to produce a solution. Instead of trying to synthesize an inverse medium the approach is based on minimization of the maximum differential group delay (DGD) in the band of interest. The method can be used as a basis for control algorithms of broad-band PMDCs and is applicable in the presence of PDL with only minor modifications.Manuscript

PMD compensation techniques

“…Such structures are already well established for group velocity dispersion compensation, namely, lattice filters that have the form of a Mach-Zehnder interferometer (MZI) cascades consisting of phase tunable symmetric MZIs and asymmetric MZIs having a delay t typically less than a bit period. Different to chromatic dispersion compensation, PMD compensation has to be formulated in terms of two filter input ports rather than only one input port [30]. This is illustrated by Fig.…”

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