In WDM optical networks, the physical layer impairments (PLIs) and their significance depend on network type-opaque, translucent, or transparent; the reach-access, metro, or core/long-haul; the number and type of network elements-fiber, wavelengths, amplifiers, switching elements, etc.; and the type of applications-real-time, non-real time, missioncritical, etc. In transparent optical networks, PLIs incurred by non-ideal optical transmission media accumulate along an optical path, and the overall effect determines the feasibility of the lightpaths. If the received signal quality is not within the receiver sensitivity threshold, the receiver may not be able to correctly detect the optical signal and this may result in high bit-error rates. Hence, it is important to understand various PLIs and their effect on optical feasibility, analytical models, and monitoring and mitigation techniques. Introducing optical transparency in the physical layer on one hand leads to a dynamic, flexible optical layer with the possibility of adding intelligence such as optical performance monitoring, fault management, etc. On the other hand, transparency reduces the possibility of client layer interaction with the optical layer at intermediate nodes along the path. This has an impact on network design, planning, control, and management. Hence, it is important to understand the techniques that provide PLI information to the control plane protocols and that use this information efficiently to compute feasible routes and wavelengths. The purpose of this article is to provide a comprehensive survey of various PLIs, their effects, and the available modeling and mitigation techniques. We then present a comprehensive survey of various PLI-aware network design techniques, regenerator placement algorithms, routing and wavelength assignment algorithms, and PLI-aware failure recovery algorithms. Furthermore, we identify several important research issues that need to be addressed to realize dynamically reconfigurable next-generation optical networks. We also argue the need for PLI-aware control plane protocol extensions and present several interesting issues that need to be considered in order for these extensions to be deployed in real-world networks.

Physical layer impairment aware routing (PLIAR) in WDM optical networks: issues and challenges

Minimum tillage in conjunction with crop residue mulch may be a promising practice of soil management to improve soil properties and crop production in the subtropical climate of north-western Punjab. Therefore, a 5-year field experiment was conducted to study the effect of tillage and crop residue mulch on some properties of a sandy loam soil (Fluvisol) cropped to rainfed maize ( Zea mays L.)–wheat ( Triticum aestivum L.) sequence. Three main treatments investigated were minimum tillage (consisting of making a small trench for seed placement) with 3 Mg ha −1 crop residue mulch of the previous crop (MTR), minimum tillage without residue mulch (MT), and conventional tillage (involving two diskings followed by a planking) without residue mulch (CT). Subtreatments consisted of five rates of fertilizer N (0, 40, 80, 120 and 160 kg ha −1 ) applied to wheat. Maize received 80 kg N, 17 kg P and 16 kg K ha −1 . Soil quality in terms of increased organic matter content, water retention, infiltration of water and aggregation, and decreased bulk density of the surface soil was improved in the MTR relative to other treatments. Pooled grain yield in the MTR treatment remained below the CT treatment during the first 2 years (1993 and 1994) but was subsequently greater than the CT. However, grain yield in the MT treatment was lower than CT treatment throughout the study period. The results indicated the necessity of using residue mulch in conjunction with minimum tillage in order to improve soil quality and sustain/improve crop production.

Tillage and residue management effects on soil properties and yields of rainfed maize and wheat in a subhumid subtropical climate

Optical solitons in a non-Kerr law media is studied in this paper in presence of perturbation terms. This is governed by the Nonlinear Schrodinger’s equation. He’s semi-inverse variational principle is used to carry out the integration of this perturbed Nonlinear Schrodinger’s equation with non-Kerr law nonlinearity. The types of nonlinearity that are considered are Kerr, power, parabolic and dual-power law. The parametric domain is also identified for the solitons to exist.

Optical Solitons by He’s Variational Principle in a Non-Kerr Law Media

Optical and other solitons for the fourth-order dispersive nonlinear Schrödinger equation with dual-power law nonlinearity

The nonlinear effects in optical fiber occur either due to intensity dependence of refractive index of the medium or due to inelastic-scattering phenomenon. This paper describes various types of nonlinear effects based on first effect such as self-phase modulation, cross-phase modulation and four-wave mixing. Their thresholds, managements and applications are also discussed; and comparative study of these effects is presented.

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Nonlinear Effects in Optical Fibers: Origin, Management and Applications

The nonlinear scattering effects in optical fiber occur due to inelastic-scattering of a photon to a lower energy photon. This paper describes stimulated Brillouin scattering and stimulated Raman scattering processes. Their thresholds, reduction in power penalty and applications along with comparative study of these effects are also presented.

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Nonlinear Scattering Effects in Optical Fibers

The group velocity dispersion (GVD) imposes severe limit on information carrying capacity of optical communication systems. By choosing appropriate pulse shape highly stable light pulses known as solitons are generated when effect of GVD is balanced by self-phase modulation (SPM). The application of solitons in communication systems opens the way to ultrahigh-speed information superhighways. Transmission speed of order of Tbit/s can be achieved if optical amplifiers are combined with WDM in soliton based communication systems.

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Soliton based optical communication

In this paper two new methods to reduce the crosstalk in WDM systems are presented. These two methods along with the present methods are analyzed and their performances are compared. The proposed methods yield better results. Both signal power and optical signal power to noise power ratio (OSNR) improve significantly.

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Nar Singh

Papers

Nonlinear Effects in Optical Fibers: Origin, Management and Applications

Nonlinear Scattering Effects in Optical Fibers

Soliton based optical communication

Reduction of crosstalk in wavelength division multiplexed fiber optic communication systems

Gain optimization of an erbium-doped fiber amplifier and two-stage gain-flattened EDFA with 45 nm flat bandwidth in the L-band