We describe the design trade-offs that are at stake when optimizing few-mode fibers (FMFs) that support a high number (≥ 6) of LP modes. We particularly detail the design of 6-LPmode fibers that ...allow to multiply the capacity by a tenfold factor (two modes being spatially non-degenerate and four modes being two times spatially degenerate). For low-differential-mode-groupdelay (low-DMGD) FMFs adapted to strongly-coupled modedivision-multiplexed systems, trench-assisted graded-index-core profiles can be optimized to have Max|DMGD| <;10 ps/km and undesired leaky LP modes appropriately cut off, while all guided LP modes show good robustness (Bend Losses <;10 dB/turn at 10 mm bend radius). Such low-DMGD FMFs being sensitive to process variability, we show how fiber concatenations can efficiently compensate for this issue and that values <;25 ps/km can realistically be reached. For weakly-coupled FMFs adapted to weakly-coupled mode-division-multiplexed systems, step-index-core profiles can be optimized to have large effective index differences, Δn eff , between the LP modes (Min|Δn eff | >1.0 × 10 -3 ) to limit mode coupling and A eff >~100 μm 2 to limit intra-mode non-linearity with good mode robustness. For such weakly-coupled FMFs, sensitivity to process variability is small and main characteristics do not significantly change when variations are within the manufacturing tolerances. We also briefly discuss experimental validations.
Capacity Limits of Optical Fiber Networks Essiambre, René-Jean; Kramer, Gerhard; Winzer, Peter J. ...
Journal of lightwave technology,
02/2010, Letnik:
28, Številka:
4
Journal Article
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We describe a method to estimate the capacity limit of fiber-optic communication systems (or ¿fiber channels¿) based on information theory. This paper is divided into two parts. Part 1 reviews ...fundamental concepts of digital communications and information theory. We treat digitization and modulation followed by information theory for channels both without and with memory. We provide explicit relationships between the commonly used signal-to-noise ratio and the optical signal-to-noise ratio. We further evaluate the performance of modulation constellations such as quadrature-amplitude modulation, combinations of amplitude-shift keying and phase-shift keying, exotic constellations, and concentric rings for an additive white Gaussian noise channel using coherent detection. Part 2 is devoted specifically to the "fiber channel.'' We review the physical phenomena present in transmission over optical fiber networks, including sources of noise, the need for optical filtering in optically-routed networks, and, most critically, the presence of fiber Kerr nonlinearity. We describe various transmission scenarios and impairment mitigation techniques, and define a fiber channel deemed to be the most relevant for communication over optically-routed networks. We proceed to evaluate a capacity limit estimate for this fiber channel using ring constellations. Several scenarios are considered, including uniform and optimized ring constellations, different fiber dispersion maps, and varying transmission distances. We further present evidences that point to the physical origin of the fiber capacity limitations and provide a comparison of recent record experiments with our capacity limit estimation.
Short range optical data links are experiencing bandwidth limitations making it very challenging to cope with the growing data transmission capacity demands. Parallel optics appears as a valid ...short-term solution. It is, however, not a viable solution in the long-term because of its complex optical packaging. Therefore, increasing effort is now put into the possibility of exploiting higher order modulation formats with increased spectral efficiency and reduced optical transceiver complexity. As these type of links are based on intensity modulation and direct detection, modulation formats relying on optical coherent detection can not be straight forwardly employed. As an alternative and more viable solution, this paper proposes the use of carrierless amplitude phase (CAP) in a novel multiband approach (MultiCAP) that achieves record spectral efficiency, increases tolerance towards dispersion and bandwidth limitations, and reduces the complexity of the transceiver. We report on numerical simulations and experimental demonstrations with capacity beyond 100 Gb/s transmission using a single externally modulated laser. In addition, an extensive comparison with conventional CAP is also provided. The reported experiment uses MultiCAP to achieve 102.4 Gb/s transmission, corresponding to a data payload of 95.2 Gb/s error free transmission by using a 7% forward error correction code. The signal is successfully recovered after 15 km of standard single mode fiber in a system limited by a 3 dB bandwidth of 14 GHz.
In this paper, numerical methods are suggested to compute the discrete and the continuous spectrum of a signal with respect to the Zakharov-Shabat system, a Lax operator underlying numerous ...integrable communication channels including the nonlinear Schrödinger channel, modeling pulse propagation in optical fibers. These methods are subsequently tested and their ability to estimate the spectrum are compared against each other. These methods are used to compute the spectrum of various signals commonly used in the optical fiber communications. It is found that the layer peeling and the spectral methods are suitable schemes to estimate the nonlinear spectra with good accuracy. To illustrate the structure of the spectrum, the locus of the eigenvalues is determined under amplitude and phase modulation in a number of examples. It is observed that in some cases, as signal parameters vary, eigenvalues collide and change their course of motion. The real axis is typically the place from which new eigenvalues originate or, are absorbed into after traveling a trajectory in the complex plane.
Increasing transmission capacity of installed fiber optical communication systems by multiband (MB) transmission offers large advantages compared with other options if physical access to all ...intermediate amplification sites is granted. This is typically the case in terrestrial networks or unrepeatered submarine links without remote optically pumped amplifier (ROPA). Challenges for introducing this technology and solutions for overcoming them are detailed with a focus on energy efficiency, transmission performance, system installation, amplifier manufacturing, and device control. However, only few systems will be prepared for such a capacity upgrade such that traffic interruption is unavoidable. Splitting an incoming wavelength–division multiplexing (WDM) signal into different bands for amplification, which is mandatory when using rare–earth doped fiber amplifiers, leads to performance degradation and reduced power conversion efficiency. Based on a hypothetical energy level diagram, it is explained why using lanthanides other than erbium results in reduced power conversion efficiency. Due to the intrinsic wavelength characteristics of the cross sections of thulium, efficiency is further significantly reduced when designing gain–flattened thulium–doped fiber amplifiers for operation beyond 1510 nm. Simulation results show that gain and tilt adjustment in reaction to power transients will take at least 200Formula Omitted.
The Volterra series transfer function (VSTF), in which the input-output relationship of a nonlinear system is represented by a series of nonlinear kernel functions, is an elegant tool to model ...nonlinear systems. The inverse of a nonlinear system can be constructed by analyzing VSTF. We propose a new electronic nonlinearity compensation scheme based on inverse VSTF. We show 1 dB improvement in Q-factor with a 256 Gb/s polarization-division-multiplexed 16-level quadratic amplitude modulation format, and 50% reduction in complexity by lowering the processing rate.
We analytically studied the block length effect (BLE) of decision-aided maximum likelihood (DA ML) carrier phase estimation in coherent optical phase-modulated systems. The results agree well with ...the trends found using extensive Monte Carlo simulations. In order to eliminate the BLE and accurately recover the carrier phase, an adaptive decision-aided (DA) receiver is proposed that does not require knowledge of the statistical characteristics of the carrier phase, or any parameter to be preset. The simulation results show that using the adaptive DA receiver, the maximum tolerance ratio of the linewidth per laser to symbol rate (¿vT) at a bit error rate (BER) = 10 -4 has been increased to 2.5 × 10 -4 , 4.1 × 10 -5 , and 9.5 × 10 -6 , respectively, for quadrature-, 8- and 16-phase-shift keying formats. The ratio (¿vT) of the adaptive DA receiver in 16 quadrature amplitude modulation (QAM) is decreased to 2 × 10 -5 due to the constellation penalty from 2.5 × 10 -5 by using DA ML with optimum memory length, though it consistently performs well without optimizing any parameters as in DA ML. The phase error variance of the adaptive DA receiver is also analytically investigated. In addition, an analog-to-digital converter with bit resolution higher than 4 bits is shown to be sufficient to implement our adaptive DA receiver.
We are witnessing a rising concern over communication security and privacy. Conventional cryptography techniques encrypt data into unreadable codes, but still expose the existence of communications ...through metadata information (e.g., packet timing and size). In contrast, we propose a steganographic communication scheme adopting a novel combination of the intrinsic optical noise and a unique signal spreading technique to hide the existence of optical communications, i.e., optical steganography. We experimentally implement a prototype steganographic communication system, which requires zero cover-signal overhead to enable a stealth communication channel over the existing communication infrastructure. Both the frequency and time-domain characterizations of our prototype implementation verify the feasibility of our approach. We also demonstrate the first practical steganographic communication that provides reliable communication performances between real computers at the application layer (200-300 Mb/s file transfer rate and 25-30 Mb/s Internet data rate) over long-distance optic fibers (25-50 km). Additional bit-error-rate measurements illustrate negligible channel interference between the public and stealth communications (less than 1 dB power penalty). We further quantitatively demonstrate that the eavesdropper's chance of matching system parameters to effectively recover the stealth signal is 2 -10 by random guessing, and that the eavesdropper's ability of detecting the stealth signal hidden in the transmission channel is strictly limited close to a random guess. Our steganographic communication scheme provides an attractive foundation for mitigating eavesdropping at the link level; thus, paving the way for future privacy-enhancing technologies using the physical layer characteristics of communication links.
We report a two-span, 67-km space-division-multiplexed (SDM) wavelength-division-multiplexed (WDM) system incorporating the first reconfigurable optical add-drop multiplexer (ROADM) supporting ...spatial superchannels and the first cladding-pumped multicore erbium-doped fiber amplifier directly spliced to multicore transmission fiber. The ROADM subsystem utilizes two conventional 1 × 20 wavelength selective switches (WSS) each configured to implement a 7 × (1 × 2) WSS. ROADM performance tests indicate that the subchannel insertion losses, attenuation accuracies, and passband widths are well matched to each other and show no significant penalty, compared to the conventional operating mode for the WSS. For 6 × 40 × 128-Gb/s SDM-WDM polarization-multiplexed quadrature phase-shift-keyed (PM-QPSK) transmission on 50 GHz spacing, optical signal-to-noise ratio penalties are less than 1.6 dB in Add, Drop, and Express paths. In addition, we demonstrate the feasibility of utilizing joint signal processing of subchannels in this two-span, ROADM system.
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