We introduce a computationally efficient frequency-domain implementation of a fractionally spaced block-least-mean-square (LMS) or block-constant-modulus-algorithm (CMA) equalizer. The proposed implementation is less complex than its time-domain counterpart. Besides, it lends itself naturally to high-throughput parallel architectures thanks to block-processing based on fast Fourier transforms. The resulting equalizer may be used for adaptive polarization-mode dispersion (PMD) equalization, or for joint adaptive compensation of chromatic dispersion (CD) and PMD in digital coherent receivers.

In this paper, we present an original work on subwavelength optical switching performed over a coherent multiband orthogonal frequency-division multiplexing (MB-OFDM) super-channel operating at 100 Gbps. After having demonstrated that dual-polarization MB-OFDM (DP-MB-OFDM) is as efficient as single-carrier dual-polarization quaternary phase shift keying (DP-QPSK) technology to transport 100 Gbps data-rate over a 10 × 100-km G.652 fiber-based transmission line, we show that optical add-drop of OFDM sub-bands as narrow as 8 GHz inside a 100 Gbps DP-MB-OFDM signal constituted of four sub-bands is feasible in the middle of this 1000-km transmission line. The flexible optical add-drop multiplexer (FOADM) implemented here is constituted by the association of an ultra-narrow pass-band and stopband optical filter. The design and realization of such ultra-selective optical filters is presented, while the impact of their physical features over the quality of transmission is discussed. To prove that several add-drop multiplexers can be cascaded, our FOADM is introduced into a G.652 fiber-based recirculating loop and the impact of the cumulated filtering transfer function as well as the crosstalk inside the OADM are investigated. A typical use case for the introduction of such FOADM into long-haul transport networks is given, and the capital expenditure (CAPEX) cost advantage for the multi-layer transport networks is highlighted. By the proof of concept delivered here, combination of super-channel and subwavelength optical switching pushes network flexibility far away of what is today proposed by system vendors, opening new horizons for an optimized use of multi-layer transport networks.

We propose a constrained constant modulus algorithm (CMA) for solving the singularity issue of CMA equalizers in polarization-division-multiplexed coherent receivers. Switching to the standard CMA is based on mean square error monitoring over the two output streams.

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