Valery N. Rozental

Experimental Analysis of the Power Auto-Correlation-Based Chromatic Dispersion Estimation Method

We experimentally investigate the performance of the signal power autocorrelation-based method for chromatic dispersion (CD) estimation in a polarization-multiplexing quadrature phase-shift keying (PM-QPSK) 100G coherent optical system conveying optical channel transport unit level-4 (OTU4) frames. It is shown that the typical laboratory setup, where the signal components are generated from delayed versions of the same pseudorandom binary sequence (PRBS), is inadequate for experimental validation because of artifacts in the signal auto-correlation function. This problem is circumvented by the use of a commercial line card transporting independent data sequences. The algorithm is used to estimate accumulated CD values from 0 to 50 000 ps/nm under up to 80-ps differential group delay (DGD). We further evaluate its convergence properties in terms of the length of the sample vector required for correct CD estimation and address the hardware resource requirements. The CD-shifted version of the algorithm yielded a maximum estimation error of 186 ps/nm in all tested conditions.

Analysis of signal processing techniques for optical 112 Gb/s DP-QPSK receivers with experimental data

This paper evaluates several signal processing techniques for optical dual-polarization quadrature phase-shift keying (DP-QPSK) coherent receivers, using offline post-processing of experimental data. In addition to electronically separating the two polarization multiplexed signals, the investigated signal processing algorithms also compensate for chromatic dispersion (CD) and polarization mode dispersion (PMD). In particular, we investigate two distinct architectures: 1) Constant modulus algorithm (CMA) equalization followed by feedforward phase recovery. 2) Decision-directed least mean square (DD-LMS) equalization combined with decision-directed phase recovery. The interplay of both architectures with bulk dispersion compensators and carrier recovery algorithms is also analyzed.

Experimental analysis of singularity-avoidance techniques for CMA equalization in DP-QPSK 112-Gb/s optical systems

We experimentally investigate the singularity problem in DP-QPSK 112-Gb/s receivers using the CMA. Three algorithms are compared: Constrained, Two-Stage, and Multi-User. Although these algorithms have been individually evaluated, they have not been compared by extensive experiments. The transmission setup emulates amplifier noise; first-order PMD; and chromatic dispersion. It is shown that all algorithms effectively mitigate singularities. However, under certain conditions, the Multi-User and the Constrained algorithms--both used for system startup--outperformed the Two-Stage, which does not distinguish between system operation and startup. In light of its effectiveness and low computational complexity, we recommend the Constrained algorithm.

Hitless Rate Switching for Dynamically Reconfigurable Optical Systems

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.

Cognitive power management in 100G optical transponders

We propose a scheme for efficient power control in 100G optical transponders through reduction of the length of electronic chromatic dispersion compensation filters, according to the network traffic fluctuations. Transmission rate reduction is achieved by symbol retransmission, while maintaining system parameters unchanged, to avoid increased complexity. Power savings offered by the scheme are estimated.

Optoelectronic specifications of emerging coherent optical solutions for data center interconnect

Emerging short-reach data center interconnect is a scenario wherein the capacity has to be maximized over point- to-point optical links without intermediate optical amplification. Most of the transceiver solutions are based on 100G modules with direct detection modulation. Although these legacy solutions are cost-efficient in a short- term, they are not scalable in a long-term, when the capacity x distance product will become more and more stringent. This paper addresses coherent optical solutions for emerging data center interconnect, with optical transmission reach being limited to around unrepeated 100 km. The main advantage of coherent solutions, when compared to legacy direct detection technologies, is the inherently improved spectral efficiency (e.g. 400 Gb/s channels in a 50 GHz grid) and receiver sensitivity provided with high baudrate (>40 GBd) transceiver modules. In this paper, two technological options for single-carrier optical 400 Gb/s modules are exploited for high capacity links over short reach scenarios: 43 GBd polarization-division-multiplexed (PDM)-64QAM, suitable for a 50-GHz grid; and 64 GBd PDM-16QAM, suitable for a 75-GHz grid. These two solutions are compared in terms of capacity allocated in C band (∼4 THz bandwidth), when considering 50 GHz (80 channels at 400G, 32 Tb/s) and 75 GHz (53 channels with 21.2 Tb/s) grids and back-to-back requirements in terms of optoelectronics (digital-to-analog and analog-to-digital converters, modulators, receivers etc.).

Digital Nonlinear Compensation for Spectrally Sliced Optical Receivers With MIMO Reconstruction

We experimentally investigate, in a 20×56-GBd 400 G WDM transmission system, different approaches for nonlinear compensation in spectrally sliced optical receivers with MIMO-based signal reconstruction. Slice-by-slice nonlinear compensation shows comparable performance to inter-slice nonlinear compensation in a WDM system. In addition, an eight-stages compensation yields a 0.4-dB Q2 gain equivalent to 250-km distance increase, resulting in 2000-km transparent reach.

Limitations of the Power Auto-Correlation-Based Chromatic Dispersion Estimation Method in Dispersion-Managed Links

We propose to extend an existing chromatic dispersion (CD) estimation algorithm, based on the auto-correlation of the signal power waveform, by electronically adding CD, to overcome the limitations of the original proposal in dispersion-managed links.

Gradual Symbol Rate Switching for Synchronous Operation of Flexible Optical Transceivers

We propose and experimentally validate a novel synchronous symbol rate switching method for digitally shaped optical signals, based on synchronized transmit- and receive-side interpolators. We show that a gradually performed rate adjustment in small discrete steps allows the dynamic equalizer at the receiver to successfully track signal changes. The proposed technique may be exploited, e.g., to reduce power consumption during night-time. Furthermore, the released spectrum may be used to accommodate other short-living opportunistic demands, such as in data center traffic.

Digital-domain chromatic dispersion compensation for different pulse shapes: Practical considerations

We infer, by computer simulations, practical values of the overlap length for frequency-domain chromatic dispersion compensation in long and ultra-long-haul coherent optical systems. The overlap length has a direct impact on computational complexity and the receive-side DSP power consumption. The NRZ, RZ, and Nyquist-shaped signals are investigated. The presented results may be used as guidelines for trading off OSNR margin and computational complexity. In a sufficiently high OSNR scenario, these trade-offs may reduce computational complexity by 10-25%, depending on the particular filter implementation.