Thomas Detwiler

Inter-Channel Crosstalk Cancellation for Nyquist-WDM Superchannel Applications

Superchannel WDM systems employ narrow channel spacing to achieve high spectral efficiency and increase channel capacity. Additionally, these systems attempt to avoid inter-channel interference (ICI) and inter-symbol interference (ISI), by creating and maintaining both spectral and temporal orthogonality. This in turn imposes a strong requirement on the spectral amplitude and phase of the received signals. For Nyquist-WDM systems, the temporal shapes are Nyquist pulses, requiring uniform spectral density with flat phase. In practice, these requirements are only partially achieved, resulting in non-ideal Nyquist systems with inter-channel interference (ICI). We propose and demonstrate two joint ICI cancellation methods based on our new “super receiver” architecture, which jointly detects and demodulates multiple subchannels simultaneously. The maximum a posteriori (MAP) algorithm is most readily implemented for systems with channel spacing equal to the baud rate, and the adaptive linear equalizer is effective for all channel spacings. Simulation results show that both joint ICI cancellation schemes outperform conventional linear equalization, approaching the performance of an isolated single channel.

Passband Narrowing and Crosstalk Impairments in ROADM-Enabled 100G DWDM Networks

100G dense wavelength-division multiplexing networks with reconfigurable optical add-drop multiplexers (ROADMs) enable dynamically reconfigurable networks and are therefore part of the solution needed to meet increasing bandwidth and routing flexibility requirements for transport networks. ROADMs, in particular cascaded ROADMs, also incur penalties including those induced by passband narrowing, frequency drift, and imperfect isolation. As networks migrate toward denser wavelength assignments in search of higher capacity, these issues will become more challenging. Here, passband and in-band crosstalk effects for long-haul ROADM-enabled 112 Gb/s polarization division multiplexing quadrature phase shift keying systems are examined. We investigate the pulse format dependence for passband narrowing/frequency detuning. We also demonstrate a spectral weighting method to evaluate crosstalk penalties for widely varying crosstalk spectral content. Our experimental results demonstrate that the conventional crosstalk metric is insufficient. Finally, we demonstrate experimentally and in simulation a nonlinearity-enhanced crosstalk penalty that results from the nonlinear parametric interaction between the primary signal and crosstalk. We show that this penalty is more limiting than the nonlinear interactions between signal and amplified spontaneous emission noise.

System Performance Prediction With the Gaussian Noise Model in 100G PDM-QPSK Coherent Optical Networks

We demonstrate that the transmission BER, OSNR penalty, and system margin can be accurately predicted for multiple fiber types using the back-to-back response together with the Gaussian Noise model of nonlinear penalties. We first experimentally quantify the 1600 km link performance of SMF, MDF, and LAF fiber types in a coherent, WDM PDM-QPSK system at both 28 and 32 GBaud employing all-EDFA amplification and nearly identical span lengths to isolate fiber performance effects. We quantify the BER, OSNR transmission penalty, and link margin versus per-channel launch power in both linear and nonlinear transmission regimes. We demonstrate that the total system performance can be directly and accurately predicted using the fiber parameters α, D, and γ, the number spans and noise figures, and back-to-back performance of the transmitter-receiver pair. We also show that the system margins scale as (α |D|/γ2)1/3 as predicted by the Gaussian Noise model of nonlinear penalties in uncompensated systems.

Continuous Phase Modulation for Fiber-Optic Links

Advances in photonics, silicon electronics and digital signal processing (DSP) have converged to enable highly efficient transmission across fiber optic channels. Single wavelength data rates of 112 Gb/s are sought for wide deployment based on QPSK transmission, coherent detection, and digital demodulation. Here we examine continuous phase modulation (CPM) as a means to enhance performance and reach of coherent optical links. We quantify the robustness of the constant amplitude CPM format to spectral filtering and nonlinearities in comparison to QPSK. The challenges of generating and receiving the CPM waveform are considered and a novel CPM transmitter architecture is proposed.

In-Band Crosstalk Transmission Penalties on 112-Gb/s PDM-QPSK Optical Links

We demonstrate a method of quantifying the penalty of in-band crosstalk that accounts for the varying spectral content. Experimentally evaluated penalties for widely varying crosstalk spectral content are shown to be readily predetermined based on the crosstalk spectral content and power within a single 112 Gb/s polarization-division multiplexing quadrature phase-shift keying (PDM-QPSK) link. Optical signal-to-noise ratio (OSNR) penalties predicted using a weighted crosstalk correlated with experimental results to within 1 dB for 8 spans AllWave [standard single-mode fiber (SSMF)] or TrueWave [nonzero dispersion fiber (NZDF)] links with different dispersion maps. Moreover, we identify a nonlinearity-enhanced crosstalk effect that is evident in the nonlinear transport regime and induce additional crosstalk-induced ONSR penalty.

Scaling 112 Gb/s Optical Networks With the Nonlinear Threshold Metric

A nonlinear penalty threshold that contains the total nonlinear phase shift is examined as a criterion for scaling fiber optic links. We investigate 112 Gb/s PDM-QPSK hybrid optical networks consisting of either TrueWave (G.655) or AllWave (G.652) fiber. We establish experimental and simulation environments with robust absolute matching in both linear and nonlinear regimes. We identify both XPM and SPM penalties in 0% and 100% inline dispersion compensation schemes for the two different fiber types. Both experimental and simulation results reveal that 0% inline-compensated links operate with larger nonlinear threshold for increasing span count and readily yield simple design rules.

Avoiding fiber nonlinearities by choice of modulation format

Nonlinear refraction in fiber optic links is a capacity limiting mechanism, whereby the phase of each propagating signal is modulated by intensity variations of signals in nearby channels. The transition to coherent detection enables a wide variety of modulation formats to be considered. Indeed, the choice of modulation format plays a primary role in determining the degree of amplitude variation in the channel as well as the robustness to the phase noise impairment that nonlinearities induce. On one hand, constant envelope formats (or nearly-constant) avoid fluctuations in the signal and produce lower nonlinearity-based impairments. Alternatively, star-QAM modulation formats enhance the receivers robustness to phase noise. Using simulated and experimental results we demonstrate the effectiveness of each format in avoiding fiber nonlinearity effects for both standard fiber (17ps/nm-km) and NZDF (5 ps/nm-km). We show sensitivity of several formats to nonlinear phase modulation from adjacent channels. We show the interaction between dispersion and constant envelope formats that guides the applications in which constant envelope formats, such as continuous phase modulation (CPM) provide gain over non-constant formats, such as QPSK. Consideration is made to scaling to 100 Gb/s and beyond in practical implementations.

Offset QPSK receiver implementation in 112 Gb/s coherent optical networks

We demonstrate Offset-QPSK transmitter and receiver architectures for 112 Gb/s coherent optical networks, highlighting similarities and differences to QPSK implementations. Experimentally we demonstrate that O-QPSK exhibits an enhanced immunity to nonlinearities when transmitted over TrueWave® fiber links.

Scaling 100G QPSK links for reliable network development

Nonlinearities are a performance limitation in coherent optical links, and efforts have been made to understand the tradeoffs between launch power and the penalties related to nonlinearities. Using both simulation and experimental results from our 100G testbed we investigate the use of a nonlinear phase criterion that quantifies the total nonlinear phase accumulation within a 112 Gb/s PDM-QPSK link. We examine the nonlinear effects of self-phase (SPM) and cross-phase modulation (XPM) on a 112 Gb/s PM-QPSK channel propagating between four 10 Gb/s OOK aggressor channels on a 50 GHz grid and quantify the launch power and span count scaling behavior. In order to assess the applicability of a nonlinear phase criterion on real-world links, we determine the launch power that yields a 1.5 dB OSNR penalty at a BER of 10-3 for each configuration. This launch power then allows the identification of a Nonlinear Threshold Power (number of spans times launch power) that fully incorporates the increasing nonlinear penalties with further transmission distance. This metric allows for the determination of a set of engineering rules for deployment of 100 Gb/s PDM-QPSK in linear links with arbitrary number of spans and span distances. We find that this nonlinear threshold is constant in dispersion-compensated links. These experimental results are validated with simulations.

Offset QPSK for 112 Gb/s coherent optical links

Offset QPSK is a linear modulation format exhibiting nearly constant envelop and hence may avoid nonlinear refraction effects and thus enable higher launch power and a commensurate increase in reach. We experimentally demonstrate >2dB advantage for OQPSK compared to QPSK for multi span coherent optical links using TrueWave® RS fiber.