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This PDF file contains the front matter associated with SPIE Proceedings Volume 8284, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.
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The capacity of optical transmission systems has increased dramatically since their first deployments in the mid
1970s . However, studies show that the theoretical capacity limit of single-mode fiber is about to be reached, and
space-division multiplexing has been proposed to overcome this limit. With the high levels of integration needed
for economic deployment, space-division multiplexing may exhibit large crosstalk between the supported fiber
modes. We propose to use coherent multiple-input multiple-output (MIMO) digital signal processing (DSP), a
technique widely used in wireless communication, to compensate crosstalk present in spatial multiplexing over
fibers. According to MIMO theory, crosstalk in multi-mode transmission systems can be completely reversed
if the crosstalk is described by a unitary transformation. For optical fibers this is fulfilled if all available fiber
modes can be selectively excited and if all the modes are coherently detected at the end of the fiber, provided
that mode-dependent loss is negligible. We successfully applied the technique to demonstrate the transmission
of six independent mode-multiplexed 20-Gbaud QPSK signals over a single, optically amplified span of 137-km few-mode fiber (FMF). Further, in a multi-span experiment, we reach 1200 km by transmitting over a
3-core coupled-core fiber (CCF). Details for both experiments will be presented, including the description of the
supported polarization- and spatial modes of the fiber, the mode multiplexers used to launch and detect the
modes, and the MIMO DSP algorithm used to recover the channels.
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Steadily increasing data traffic gives rise to increasing capacity requirements in optical communication networks. It is
well understood that systems with higher symbol rates and/or multi-level modulation formats generally demand higher
optical signal-to-noise ratio (OSNR) at the receiver to achieve acceptable system performance. In terms of the optical
fiber medium, higher OSNR can be attained by lowering fiber loss and reducing fiber nonlinearity. We review several
recent experimental investigations of 112 Gb/s PM-QPSK transmission with reach-length results enabled by the use of
optical fibers with ultra-low loss and very large effective area.
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Advanced Signal Processing: Joint Session with Conference 8282
We present a novel idea of multi-input injection locking phenomenon in single mode Fabry-Pérot Laser diode (SMFP-LD).
The key principle of multi-input injection locking is the proper power management of input beams for injection
locking one of the side modes of Fabry-Pérot laser diode (FP-LD) and the suppression of the dominant mode of FP-LD.
The multi-input injection locking (MIL) principle is extended to the combinational multi-input injection locking (CMIL)
by which the dominant mode of SMFP-LD will be suppressed with certain combination of input beams. The proposed
idea can be implemented for realizing logic gates, decision making circuit and others. MIL and CMIL are explained
theoretically and verified by realizing some of the logic gates. Experimental results for logic gates are presented with the
input data rates of 10 Gbps.
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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.
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Coding and Modulation Format: Joint Session with Conference 8283
In the near-field regime, the number of spatial modes that a free-space communication system can efficiently use
is given by the product of the Fresnel numbers of the transmit and receive apertures. It can be advantageous
to decompose the field into modes that have rotational symmetry or definite orbital angular momentum (OAM
modes). A key challenge to using OAM modes as parallel channels in a practical communication system is
efficient multiplexing of single-spatial-mode transmitters to the orthogonal OAM modes, and demultiplexing the
combined beam into single-spatial-mode receiver arrays. Previous approaches have utilized modes of different
OAM, but ignored the radial coordinate, leading to inefficient use of the Fresnel number. We identify a method,
using lenses and holographic phase plates, to efficiently and reversibly convert concentric Laguerre-Gauss OAM
beams into an array of separated Gaussian beams.
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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.
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High-Capacity Coherent Optical Technologies: Joint Session with Conference 8282
We describe a innovative OFDM scheme based on orthogonal chirped subcarriers, that corresponds to the fractional
Fourier transform (FrFT) of the input signal. The FrFT can be electronically implemented with a complexity equivalent
to the conventional fast Fourier transform (FFT); on the other hand, the planar device that implements the FrFT in the
optical domain is similar to the passive arrayed waveguide grating (AWG) device that performs the FFT.
We analyze the spectral efficiency, the peak-to-average power ratio (PAPR) and the frequency offset sensitivity of a
FrFT-based optical OFDM system, and make an accurate comparison with the standard FFT-based implementation.
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Wolfgang Freude, René Schmogrow, David Hillerkuss, Joachim Meyer, Michael Dreschmann, Bernd Nebendahl, Michael Huebner, Juergen Becker, Christian Koos, et al.
Recent advances in electronic data processing allow constructing reconfigurable optical transmitters and receivers, where
modulation formats and symbol rates are set by software-controlled field programmable gate arrays (FPGA). We report
on such a real-time optical transmitter for 8 modulation formats, which can be swapped in 5 ns without data loss. With
single-polarization 64QAM symbols generated at 28 GBd, we transmit data at 168 Gbit/s in real time. A similar arrangement
defines a single-polarization orthogonal frequency division multiplexing (OFDM) transmitter for a data rate
of 101.5 Gbit/s, where 58 subcarriers are encoded with 16QAM data. With a different software setup, the FPGA realizes
an optical 56 Gbit/s transmitter for sinc-shaped so-called Nyquist pulses, the spectrum of which is rectangular having the
minimum theoretically achievable bandwidth (suitable for Nyquist wavelength division multiplexing, N-WDM). For
terabit OFDM reception, optical pre-processing is required to demultiplex high-bitrate signals down to lower-bitrate tributaries,
which then can be processed electronically. We discuss a 10.8 Tbit/s (26 Tbit/s) receiver employing an all-optical
fast Fourier transform to demultiplex 75 (325) optical subcarriers modulated with 16QAM-formated symbols at a
rate of 18 GBd (10 GBd). Groups with any number of subcarriers can be selected with a simple hardware reconfiguration
step.
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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.
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The performance of polarization multiplexed, quadrature phase shift keying (PM QPSK) and polarization multiplexed
16-ary quadrature amplitude modulation (PM 16-QAM) is considered with an emphasis on the signal processing
algorithms that compensate transmission impairments and implement key receiver functions.
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Next-Generation Devices and Components: Joint Session with Conferences 8282 and 8283
We realized high-speed and low driving voltage InP-based Mach Zehnder modulators with an npin high-mesa waveguide
structure and traveling-wave electrodes. A full C-band 40-Gbit/s DPSK signal was successfully generated using a
compact tunable wavelength transmitter module, which incorporated a tunable DFB laser array as a wavelength tunable
laser and an InP MZM in one package. The MZM had a low driving voltage of 3 Vpp in a push-pull driving
configuration. We have demonstrated 112 Gbit/s RZ-DP-QPSK modulation using two InP MZM modules. One
modulator was used as a pulse carver in a push-pull configuration, and the other was used as an IQ modulator in a dual-drive
configuration. Since we used an RZ pulse carver to remove the transient region, we obtained clear eye openings
without any amplitude ripple. We also demonstrated 50 Gbit/s (12.5 GSymbol/s x 4) 16QAM signal generation
employing a novel dual-drive modulation method consisting of a single MZM. We utilized the electro-absorption
characteristics of an InP semiconductor to adjust the QPSK amplitude. We confirmed that a single MZM operated as a
DPSK, a QPSK, and a 16QAM modulator with the same device. We believe these modulators to be suitable for next
generation optical transmission systems.
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A study is presented of the fiber properties needed to achieve 10-mode multiplexing transmission. A combination of
MIMO processing with optical LP mode separation is proposed to prevent the need for massive MIMO computation. The
impact of mode crosstalk, differential mode delay, and the mode dependent loss of the few-mode fibers on mode
multiplexing are discussed.
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Spatial division multiplexing has been proposed for fiber capacity increase as an alternative to enhanced bandwidth
efficiency by higher order modulation or wider wavelength bands. The commercial viability of this approach will depend
strongly on the feasibility of cost and energy efficient optical amplification. Lumped erbium doped fiber and distributed
Raman amplifiers are the most promising candidates. Different approaches have been analyzed for the realization of
preamplifier and booster stages with different active fiber types. The multi mode fiber type was found to possess more
potential for cost and energy efficiency than the multi core fiber type.
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In response to ever growing data traffic, significant efforts are being made to increase optical network capacity.
One promising candidate is mode-division multiplexing (MDM) in few-mode fibers, which uses space as a new
information-bearing dimension. A fundamental element for MDM is a modal transformer. Modal transformation
can be implemented in a free-space basis by using multi-region phase plates. In this work we present the design,
fabrication and characterization of monolithic binary phase plates by highly-uniform Ag+/Na+ ion-exchange in
glass. Diffracted optical field intensities have been measured and high quality mode transformation has been
confirmed.
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Space-Division Multiplexed Optical Transmission II
The limitations of crosstalk and core-to-core distance in step-index multi-core fibers (SI-MCFs) are clarified for long-haul
transmission, and the low-crosstalk MCF structures of trench-assisted MCFs (TA-MCFs) are investigated for
realizing large effective area (Aeff) and high core density, simultaneously, with a limited cladding diameter. It is shown
that the crosstalk between neighboring cores in TA-MCFs can be greatly suppressed even if the Aeff and the cutoff
wavelength are fixed compared with SI-MCFs. In addition, the possibility of MCFs with heterogeneous core
arrangement is considered for transmission fibers and low-crosstalk heterogeneous MCFs with bending radius
insensitive characteristics are investigated.
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A simple optical technique is described for the measurement of complex mode amplitudes in multimode fibers. The
technique used in this experiment involves launching a laser beam into the core of low moded optical fiber, exciting a
mixture of normal modes. The emerging mode patterns are stored in microcomputer. The complex modes are then
extracted from these recoded intensities with help of modified version of Gerchberg-Saxton algorithm. Many
experimental results are recorded in photographic form and comparison with computer analyzed pattern is made. The
evaluation of complex mode amplitudes in multimode fiber can yield useful information about the mode filling of such
fiber.
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We present techniques for modeling the physics and systems-level characteristics of integrated IQ-transmitters for 100G+ applications and emphasize important design aspects. Using time-and-frequency-domain modeling (TFDM) of Photonic Integrated Circuits (PIC), we present a detailed IQ-transmitter model based on the physics and setup of active and passive subcomponents. With this, we link characteristics of subcomponents (bending loss of waveguides, phase changes in MMI couplers, sweep-out time of EAMs) to systems-level characteristics of the integrated IQ-transmitter (extinction ratio, modulation bandwidth, chirp). Further, a behavioral transmitter model is introduced and utilized to assess electrical driving requirements (allowed jitter, noise, synchronization offset).
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Novel closed form formulae are derived to study crosstalk degradation due to stimulated Raman scattering
(SRS) in WDM systems employing multi-span bi-directional pumped distributed Raman amplifier (DRA). The
formulae are used to evaluate the crosstalk performance of differential phase-shift keying (DPSK) and ON-OFF keying (OOK) modulation format which are widely used in optical communication. SRS crosstalk is
further evaluated for different data rates and pulse shapes prevalent in optical data transmission. Next, crosstalk
is calculated for different pumping schemes by taking into account the launched power of bi-directional
pumped DRA. The study shows that minimum SRS crosstalk can be achieved for 40 Gb/s RZ-DPSK signal
with 33.3% duty cycle in WDM system employing backward pumped DRA.
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Recent numerical and experimental studies have shown that coherent transmission with advanced modulation
formats i.e. DP-QPSK and QAM are the promising candidates for next-generation systems with data rates
of 100Gbit/s and above. Coherent detection is considered efficient along with digital signal processing (DSP)
to compensate many linear effects in fiber propagation i.e. chromatic dispersion (CD) and polarization-mode
dispersion (PMD). Despite of fiber non-linearities (NL), which are the major limiting factors, next-generation
optical systems are employing higher order modulation formats in order to fulfil the ever increasing demand
of capacity requirements. However, the channel capacity is limited at higher signal input powers because the
system is operating in the non-linear regime. Due to this phenomenon the compensation of non-linearities is
a topic of great interest and research these days, especially for long-haul fiber transmission. Digital backward
propagation (DBP) algorithm has emerged as a promising and potentially capable candidate, which can jointly
compensate fiber dispersion and non-linearities along with the coherent receiver. In this paper we give a
detailed overview on the advancements in DBP algorithm based on different types of mathematical models i.e.
Wiener (Asymmetric Method) and Wiener Hammerstein models (Symmetric Method). We also discuss the
importance of optimized step-size selection, i.e. constant step-size and logarithmic step-size based split step
Fourier methods, for simplified and computationally efficient implementation of DBP algorithm. Moreover, by
means of numerical investigations we refer to recent system investigations to further improve the performance
of DBP algorithm.
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Nonlinear distortion is one of the major obstacles in DWDM systems with enhanced spectral efficiencies. In this paper
several approaches to address the issue of nonlinear impairments by means of digital signal processing are discussed.
Firstly, implementation-efficient and novel intra-channel nonlinear compensation schemes are proposed; one is based on
digital pre-distortion at the transmitter end and the other is based on digital back-propagation at the receiver end. The
virtues of the two approaches and implications to various applications are discussed; the pre-distortion technique is in
particular advantageous with QPSK modulation format; on the other hand, the improved version of digital back-propagation
is attractive in transceivers with adaptive or variable modulation/demodulation. Second, digital signal
processing algorithms to counteract inter-channel nonlinearities, namely cross-phase modulation, are discussed;
nonlinear polarization crosstalk canceller (NPCC) is proposed for mitigating the impact of nonlinear-induced fast
polarization crosstalk in dual-polarization systems (in the speed beyond MHz), which is too fast to be tracked by
ordinary polarization demultiplexing algorithms; improvement to the carrier phase recovery circuit and its combination
with NPCC are even more useful for further performance improvement. Numerical and experimental data are introduced
to support the above discussions.
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We have investigated a new method to reduce the complexity of the digital backward propagation algorithm
(DBP). A logarithmic step-size based split-step Fourier method (SSFM) is investigated in this paper to compensate
fiber transmission impairments i.e. chromatic dispersion (CD) and non-linearities (NL) in dual-polarization
quadrature phase shift keying (DP-QPSK) system. The algorithm is numerically investigated for coherently-detected
multiple channel DP-QPSK system over 2000km (25 spans) standard single mode fiber (SMF-28)
with un-compensated transmission link. The algorithm is numerically evaluated for: (a) 20 channel 56Gbit/s
(14GBaud) with 25GHz channel spacing; (b) 10 channel 112Gbit/s (28GBaud) with 50GHz channel spacing
and (c) 5 channel 224Gbit/s (56GBaud) with 100GHz channel spacing. Each simulation configuration has
the bandwidth occupancy of 500GHz and a total transmission capacity of 1.12Tbit/s. The logarithmic DBP
algorithm (L-DBP) shows efficient results as compared to the conventional DBP method based on modified
SSFM (M-DBP). The results depict efficient mitigation of CD and NL, therefore improving the non-linear
threshold point (NLT) upto 4dB. Furthermore by implementing a low-pass-filter (LPF) in each SSFM step,
the required number of DBP stages to compensate fiber transmission impairments can be significantly reduced
(multi-span DBP) by 75% as compared to L-DBP and by 50% as compared to M-DBP. The results delineate
improved system performance of logarithmic step size based filtered DBP (FL-DBP) both in terms of efficiency
and complexity which will be helpful in future deployment of DBP algorithm with real-time signal processing
modules for non-linear compensation.
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We have numerically investigated the impact of non-linear impairments on the performance of 400Gbit/s DP-RZ-
QPSK transmission system over 1200km standard single mode fiber (SMF-28) having an average span
loss of 16dB and with no in-line optical dispersion compensation in the transmission link. Digital backward
propagation (DBP) algorithm based on split-step Fourier method (SSFM) is employed along with the coherent
receiver to compensate the fiber transmission impairments i.e. chromatic dispersion (CD) and non-linear (NL)
impairments. The system performance is monitored in terms of Q-value (calculated form BER) for various
signal input launch powers. We further quantify the impact of inter-channel non-linear impairments such
as cross-phase-modulation (XPM) and four-wave-mixing (FWM) on the performance of DBP algorithm by
investigating the multiple-channel transmission, i.e. 8x400Gbit/s DP-RZ-QPSK system. The results depict
efficient performance of DBP algorithm as compared to the system where only linear dispersion compensation
is implemented. This shows the promising impact of digital backward propagation algorithm on the high data-rate
transmission systems such as 400Gbit/s per single channel which is expected to be a possible data rate for
long-haul optical communication systems after 100Gb Ethernet in near future.
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Advanced Component Technologies: Joint Session with Conference 8283
Brian Robertson, Zichen Zhang, Haining Yang, Maura M. Redmond, Neil Collings, Jinsong Liu, Ruisheng Lin, Anna M. Jeziorska-Chapman, John R. Moore, et al.
To optimise the design of a wavelength selective switch based on a phase-only liquid crystal on silicon spatial light
modulator and mitigate crosstalk, we propose using a technique referred to as wavefront encoding that involves
purposefully building a wavefront error into the optical system. Experimental results taken at 674nm are presented that
show wavefront encoding based on defocus can reduce the worst case crosstalk by >10dB compared to a standard
Fourier transform set-up. In the case of the WSS we propose using wavefront encoding based on astigmatism.
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In the last decade, there has been increased interest in photonic technology for new satellite applications. One
critical issue is the high sensitivity to radiative environments of the Erbium Doped Fiber (EDF). It leads to
a radiation-induced absorption (RIA) that is not due to erbium content but mainly to the aluminium that
ensures the erbium inclusion in glass. As the radiation induced losses grow as an exponential function of fiber
length, the principal way so far to reduce EDFA degradation has consisted in increasing erbium concentration
using conventional doping techniques. However, this is limited by the quenching effect, which impacts the fiber
length needed to reach high gain, but also by the Aluminium-induced RIA. It has been recently proposed an
original nanoparticle (NP) doping approach, which allows codopant content decrease with reduced quenching
impact, while keeping EDF amplifying performances. A radiation-resistant amplifier can thus be designed as a
"quenching-free", heavily-erbium-doped amplifier with low RIA.
We demonstrate for the first time an aluminium-free EDF, exhibiting low quenching and low RIA. Despite the
lack of aluminium, using silica NPs allows an erbium concentration close to the one of standard EDFs (200 ppm).
This fiber is compared to a 1400 ppm Erbium-doped optical fiber with a strong aluminium concentration.
Whereas the two fibers exhibit similar initial optical gain (15 dB under saturation conditions), the NP doped
Al-free EDF shows only 2 dB gain reduction after a 600 Gy gamma deposit, while the Al/Er EDF incurs more
than 10 dB gain degradation.
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