By applying recirculating frequency shifter technology to generate optical comb as neighboring channels, we have experimentally investigated the interchannel nonlinear tolerance of 256-Gbit/s polarization-division multiplexing return-to-zero 16-ary quadrature amplitude modulation (PDM-RZ-16QAM) in a co-propagation with 32-Gbaud on-off keying, binary phase-shift keying, quadrature phase-shift keying (QPSK) signals in 792-km large-area fiber transmission system. The results show that the experimental back-to-back optical signal-to-noise ratio requirement of 256-Gbit/s (32-GBaud) PDM-RZ-16QAM signal is shifted by 4.8-dB penalty in comparison with the theoretical limits. The cross-phase modulation (XPM) tolerance for 256-Gbit/s PDM-RZ-16QAM in 32-Gbaud phase-modulated neighboring channels is better than in 32-Gbaud intensity-modulated neighbors. Moreover, the performance of XPM tolerance for 256-Gbit/s PDM-RZ-16QAM in 32-Gbaud QPSK neighboring channels is the best among other kinds of modulation neighboring formats.
Based on recirculating frequency shifter, we generate 20 high-quality multicarrier optical combs with a tone-to-noise ratio >25 dB . We also experimentally demonstrate 2.56-Tb/s , polarization division multiplexing return-to-zero 16-ary quadrature amplitude modulation, coherent optical wavelength division multiplexing transmission over 800 km standard single-mode fiber with 25-GHz channel spacing, and the spectrum efficiency is 5.1 b/s/Hz .
The constant envelope characteristic of CPM signal is particularly interesting for use in fiber optic links since it can be
leveraged to avoid nonlinear phase modulation. Implementation complexity of CPM systems is generally higher than
their QPSK counterpart, partly due to the nontrivial task of generating the signal as well as the need to observe the
received signal over multiple symbol periods to make an optimal decision. Because of this complexity, the use of full-response
CPM systems is favorable for complexity since optimal reception is achieved with lower order, however,
partial response systems can achieve higher minimum distance. We analyze parameter selection for CPM transmission,
optimizing error performance and spectral efficiency in a tightly filtered reconfigurable optical add-drop multiplexor
(ROADM) application. We illustrate their impact on the normalized minimum Euclidean distance (as a proxy for error
performance). The impact of parameters on spectral efficiency is implicit on the choice of ROADM filters. The results
provide guidance for a suitable choice of CPM scheme for consideration in DWDM systems. Optimal parameters are
given for full and partial response systems for a variety of filter scenarios.
Recently efforts have been focused on ultra-high speed optical communication systems which can support 1Tb/s per
channel transmission. However, 1Tb/s over a single carrier requires either or both very high-level modulation format (i.e.
PDM-1024QAM) and high baud rate. Therefore, grouping a number of tightly spaced "sub-carriers", to form a terabit
channel has been considered and this has been refered to as a superchannel. Nyquist-WDM and Coherent Optical-
OFDM (CO-OFDM) are the two approaches to achieve ultra-high spectral efficiency in superchannel coherent optical
systems. In Nyquist-WDM systems, optical subcarriers are tightly packed at channel spacing near or equal to the baud
rate, potentially inducing strong inter-channel interference (ICI). The traditional way to mitigate the impact of ICI is by
applying aggressive optical filters to each channel; however this typically induces severe inter-symbol interference (ISI).
In this paper, we investigate receiver architectures for Nyquist-WDM superchannel coherent systems, and propose a new
"super receiver" architecture, which jointly detects and demodulates multiple channels simultaneously. Several joint
DSP algorithms are developed and tested through experimental and simulated data. The simulation results showed that
more than 5 dB ONSR gain was achieved comparing to conventional method at narrow channel spacing conditions.
We experimentally investigate the performance of WDM coherent polarization-division multiplexed-return to zero-quadrature
phase shift keying (PDM-RZ-QPSK) network in nonlinear transport regimes. Seven, 28 or 32-Gbaud PDM-RZ-
QPSK channels are employed on a 50-GHz grid and transmitted over 1600-km fiber on an all-EDFA recirculating
loop without any dispersion compensation module (DCM). The transmission link is configured entirely of either
standard single-mode fiber (AllWave), medium dispersion fiber (TrueWave REACH), or ultra-large area fiber (ULAF).
We sweep the launch power of the center channel and side channels together to measure the nonlinear effects of self-phase
modulation (SPM), cross-phase modulation (XPM), and cross-polarization modulation (XPolM) on the center
channel's BER performance. Furthermore, for all link configurations, we employ three different carrier phase recovery
methods in the demodulation routine - Viterbi-Viterbi, Viterbi-Viterbi with a minimum mean-squared error (MMSE)
filter, and the Optametra/Tektronix Weiner filter - to ascertain their relative performance in the presence of nonlinear
effects.
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.
Polarization multiplexing is an integral technique for generating spectrally efficient 100 Gb/s and higher optical links.
Post coherent detection DSP-based polarization demultiplexing of QPSK links is commonly performed after timing
recovery. We propose and demonstrate a method of asynchronous blind source separation using the constant modulus
algorithm (CMA) on the asynchronously sampled signal to initially separate energy from arbitrarily aligned polarization
states. This method lends well to implementation as it allows for an open-loop sampling frequency for analog-to-digital
conversion at less than twice the symbol rate. We show that the performance of subsequent receiver functions is
enhanced by the initial pol demux operation. CMA singularity behavior is avoided through tap settling constraints. The
method is applicable to QPSK transmissions and many other modulation formats as well, including general QAM
signals, offset-QPSK, and CPM, or a combination thereof. We present the architecture and its performance under
several different formats and link conditions. Comparisons of complexity and performance are drawn between the
proposed architecture and conventional receivers.
Reliable simulations of high-speed fiber optic links are necessary to understand, design, and deploy fiber networks.
Laboratory experiments cannot explore all possible component variations and fiber environments that are found in
today's deployed systems. Simulations typically depict relative penalties compared to a reference link. However,
absolute performance metrics are required to assess actual deployment configurations. Here we detail the efforts within
the Georgia Tech 100G Consortium towards achieving high absolute accuracy between simulation and experimental
performance with a goal of ±0.25 dB for back-to-back configuration, and ±0.5 dB for transmission over multiple spans
with different dispersion maps. We measure all possible component parameters including fiber length, loss, and
dispersion for use in simulation. We also validate experimental methods of performance evaluation including OSNR
assessment and DSP-based demodulation. We investigate a wide range of parameters including modulator chirp,
polarization state, polarization dependent loss, transmit spectrum, laser linewidth, and fiber nonlinearity. We evaluate 56
Gb/s (single-polarization) and 112 Gb/s (dual-polarization) DQPSK and coherent QPSK within a 50 GHz DWDM
environment with 10 Gb/s OOK adjacent channels for worst-case XPM effects. We demonstrate good simulation
accuracy within linear and some nonlinear regimes for a wide range of OSNR in both back-to-back configuration and up
to eight spans, over a range of launch powers. This allows us to explore a wide range of environments not available in
the lab, including different fiber types, ROADM passbands, and levels of crosstalk. Continued exploration is required to
validate robustness over various demodulation algorithms.
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 receiver's
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.
This paper investigates DQPSK transport using both simulation and experimental results from our 100G testbed. We
examine 56 Gb/s single polarization (single-pol) RZ-DQPSK and 112 Gb/s polarization multiplexing (POL-MUX) RZDQPSK
with 12 Gb/s OOK aggressor channels and a variety of dispersion management maps using AllWave® zero
water peak (ZWP) fiber. Although a number of studies of 40 Gb/s line rates within 10 Gb/s networks have been reported,
there has been little with respect to 28 Gbaud DQPSK formats. We quantify the OSNR penalty due to nonlinearities of
these hybrid optical links. Using a nominal span loss of 22 dB and different span lengths while keeping the dispersion
compensation per span constant and the loss per span constant allows a direct examination of the impact of the residual
dispersion per span (RDPS) on the nonlinear penalty in the DQPSK channel. We vary compensation from 90% - 110%
(of total dispersion) across 8 spans (-119 ps/nm - +153 ps/nm). We report the required OSNR to achieve a non-FEC
BER of 10-4 versus RDPS for both single- and dual-polarization (dual-pol) RZ-DQPSK. Experimental data is validated
against RSoft OptSim simulations.
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