We propose and experimentally validate a blind phase recovery algorithm based on tracking low-frequency components of the phase noise, which we call "filtered carrier-phase estimation (F-CPE)." Tracking only the low-frequency components allows F-CPE to reduce the computational complexity by using a frequency-domain equalizer and to simplify the partitioning of a 16 quadrature amplitude modulation (16QAM) constellation. Further, this approach eliminates cycle slips by suppressing the impact of amplified spontaneous emission on phase noise estimation. The experimental results demonstrate cycle-slip-free operation for 15 and 32 GBd 16QAM signals. Additionally, the proposed method showed similar or better sensitivity compared with the blind-phase-search algorithm, near standard forward error correction thresholds of modern wavelength division multiplexing systems.

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Coherent detection and digital signal processing techniques have driven a remarkable development in optical transport technologies, enabling channels at 100 Gb/s to be transmitted over thousands of kilometers. Future optical communications systems will achieve even higher data rates (> Gb/s) through the deployment of superchannels, a designation for subcarrier multiplexing in the optical domain. In addition, a software-defined configuration of modulation format, transmission rate and coding scheme will enable advanced features such as automatic bandwidth provisioning and optimized spectrum allocation. However, compared with the wireless environment, optical systems are still very primitive in terms of intelligence, because their installation and operation require highly skilled manpower. The solution to this problem are adaptive optical transceivers, able to sense the channel conditions and to adapt their operation parameters to extend reach and reduce power consumption. In this paper we review a set of enabling concepts and algorithms of an adaptive optical transceiver, and discuss the challenges for its successful implementation.

We propose and experimentally demonstrate an all-optical digital-to-analog converter based on cross-phase modulation with temporal integration. The scheme is robust for driving signal noise due to the low-pass filtering feature of the temporal integrator. The proof-of-concept experiment demonstrates the generation of pulse-amplitude modulation (PAM) sequences up to eight levels. The performance of random PAM 2 and PAM 4 signals with different optical signal-to-noise ratios of the binary driving signal is also investigated. The scheme is scalable for high-speed operation with an appropriate dispersion profile of the nonlinear medium.

We propose occupying the guard bands in closely spaced WDM systems with redundant signal spectral components to increase tolerance to frequency misalignment and channel shaping from multiplexing elements. By cyclically repeating the spectrum of a modulated signal, we show improved tolerance to impairments due to add/drop multiplexing with a commercial wavelength selective switch in systems using 5%-20% guard bands on a 50 GHz DWDM grid.

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We propose a set of techniques to enable hitless rate switching for reconfigurable optical systems and validate, by computer simulations, their applicability to NRZ, RZ, and Nyquist pulse shapes. The polarization-multiplexed quadrature phase-shift keying (PM-QPSK) and 16 quadrature amplitude modulation (16QAM) modulation formats are investigated. Error transients that appear during lower-to-higher rate switching are avoided by sufficiently long equalizer training periods. The robustness of the proposed scheme to polarization mode dispersion (PMD) and signal phase noise is also demonstrated.

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