Logical Embeddings for Minimum Congestion Routing in Lightwave Networks

The problem considered in this paper is motivated by the independence between logical and physical topology in Wavelength Division Multiplexing WDM based local and metropolitan lightwave networks. This paper suggests logical embeddings of digraphs into multihop lightwave networks to maximize the throughput under nonuniform traffic conditions. Defining congestion as the maximum flow carried on any link, two perturbation heuristics are presented to find a good logical embedding on which the routing problem is solved with minimum congestion. A constructive proof for a lower bound of the problem is given, and obtaining an optimal solution for integral routing is shown to be NP-Complete. The performance of the heuristics is empirically analyzed on various traffic models. Simulation results show that our heuristics perform on the average from a computed lower bound Since this lower bound is not quite tight we suspect that the actual performance is better In addition we show that 5%-20% performance improvements can be obtained over the previous work

Generation and characterization of cylindrical vector beams in few-mode fiber

For the past many decades, the Gaussian laser beam has driven major scientific discoveries that revolutionized the world of optics and photonics. In recent years, there is a burgeoning transformation where significant research has been dedicated in discovering the complex properties of cylindrical vector beams (CVBs). Increasingly, a beam of light with its intensity profile taking the shape of a single doughnut ring has attracted attention of several researchers the world over. Particularly, the so-called CVBs exhibit unique properties when focused owing to their radial and azimuthal distribution of polarization. In comparison to conventional (Gaussian-like) beams inheriting homogeneous polarization, CVBs provide unique light-matter interactions. For example, a radially polarized beam can enhance the imaging resolution of the system significantly with their spatial inhomogeneous polarization by imparting a symmetric and high numerical aperture focus. Moreover, CVBs with their phase and intensity singularities have found broad applications in quantum optics, optical micro/nano-manipulation, surface plasmon polariton, super-resolution imaging, and high-capacity fiber-optic communication. The studies of most widely used CVBs have been explored both in free space optics as well as in guided fiber optics. Further developments will require reliable techniques to generate these CVBs with strong coupling efficiency, high mode purity and high-power handling. For the past few years, the design, fabrication and study of optical fibers that supports CVBs, vortex and orbital angular momentum (OAM) beams have come to the forefront of research in this area. This is true in a sense that mode division multiplexing (MDM) is considered as a preeminent solution to the data capacity limitations faced by the standard single-mode fiber. In addition, vector beams in optical fibers constitute the fundamental basis set of linearly polarized (LP) modes (within the scalar approximation) as well as modes carrying OAM which represent another potential approach for implementing MDM based communications. Therefore, fundamental information and control over the vector beams is key to unravel future fiber communication links and CVB based fiber-optic sensors. For this purpose, it is essential to develop efficient methods to generate these CVBs. Some of the current methods reported for the generation of CVBs employ spiral phase plate, spatial light modulator (SLM), and offset fiber coupling. This thesis elucidates the generation as well as the optical characterization of such propagating cylindrical vector beams in a few-mode fiber. The ultimate purpose would be to develop simple, flexible and cost-effective photonic devices that will allow the efficient generation and stable propagation of the CVB while reducing the overall losses incurred by the system. Most of the methods reported earlier were limited to the measurements of the scalar LP mode groups of a FMF, thus neglecting the underlying vector beams that require delicate spectral and spatial control in order to be detected. In this thesis, three different techniques have been utilized for the generation of CVBs and OAM beams with high output purity. Initially, a tunable mechanical mode converter has been fabricated to demonstrate the generation of cylindrical vector beams supported by FMF in the telecom spectral range. This photonic device is utilized to demonstrate the non-destructive nonlinear characterization of CVB by utilizing the phenomenon of stimulated Brillouin scattering for the first time. We showed how the Brillouin gain spectra of the vector beams in some specialty fibers can be independently identified, measured, and subsequently exploited to probe the corresponding effective refractive indices of the vector beam retrieved from the data. This new characterization method of individual vector beam will have an impact in both light-wave and FMF-based optical sensing applications, which at present, mostly rely on the scalar LP modes. Further, a simple and low-cost technique to generate CVBs via long period fiber grating (LPFG) with very small grating pitch is reported. This work demonstrates that the cost-effective electric arc writing method for the fabrication of LPFGs is open to specialty few-mode fiber that often calls for very small pitch values. Finally, the generation of perfect cylindrical vector beams (PCVB) is demonstrated whose beam profile (i.e. transverse intensity profile) can be easily and precisely controlled. The latter novel method was used in-order to increase the free space coupling efficiency demanded by some specialty FMFs. The tailoring of the beam width and radius is performed via an iris and a diffractive phase mask implemented on a programmable SLM. The technique proposed towards the generation of PCVBs is highly adaptable for its robust nature to generate any arbitrary PCBs as well as perfect vortex beams with any topological order, using the same experimental setup. This experimental analysis is supported and validated via a rigorous theoretical framework that is in concordance with the results obtained

Crosstalk mitigation techniques in multi-wavelength networks comprising photonic integrated circuits

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Efficient embedding of virtual hypercubes in irregular WDM optical networks

This thesis addresses one of the important issues in designing future WDM optical networks. Such networks are expected to employ an all-optical control plane for dissemination of network state information. It has recently been suggested that an efficient control plane will require non-blocking communication infrastructure and routing techniques. However, the irregular nature of most WDM networks does not lend itself to efficient non-blocking communications. It has been recently shown that hypercubes offer some very efficient non-blocking solutions for, all-to-all broadcast operations, which would be very attractive for control plane implementation. Such results can be utilized by embedding virtual structures in the physical network and doing the routing using properties of a virtual architecture. We will emphasize the hypercube due to its proven usefulness. In this thesis we propose three efficient heuristic methods for embedding a virtual hypercube in an irregular host network such that each node in the host network is either a hypercube node or a neighbor of a hypercube node. The latter will be called a “satellite” or “secondary” node. These schemes follow a step-by-step procedure for the embedding and for finding the physical path implementation of the virtual links while attempting to optimize certain metrics such as the number of wavelengths on each link and the average length of virtual link mappings. We have designed software that takes the adjacency list of an irregular topology as input and provides the adjacency list of a hypercube embedded in the original network. We executed this software on a number of irregular networks with different connectivities and compared the behavior of each of the three algorithms. The algorithms are compared with respect to their performance in trying to optimize several metrics. We also compare our algorithms to an already existing algorithm in the literature

On-Chip Optical Interconnection Networks for Multi/Manycore Architectures

The rapid development of multi/manycore technologies offers the opportunity for highly parallel architectures implemented on a single chip. While the first, low-parallelism multicore products have been based on simple interconnection structures (single bus, very simple crossbar), the emerging highly parallel architectures will require complex, limited-degree interconnection networks. This thesis studies this trend according to the general theory of interconnection structures for parallel machines, and investigates some solutions in terms of performance, cost, fault-tolerance, and run-time support to shared-memory and/or message passing programming mechanisms