Akhil Shah

Coherent optical MIMO (COMIMO)

We present a multiple-input multiple-output (MIMO) optical link based on coherent optics and its ability to exploit the inherent information capacity of multimode fiber. A coherent implementation differs from previous work in optical MIMO by allowing the system to tolerate smaller modal delay spreads, because of a much larger carrier frequency, and yet maintain the necessary diversity needed for MIMO operation. Furthermore, we demonstrate the use of MIMO adaptive equalization to mitigate intersymbol interference when exceeding the bandwidth-length product of the link. The impact of phase noise is studied with numerical simulation.

Gravity dual of a quantum Hall plateau transition

We show how to model the transition between distinct quantum Hall plateaus in terms of D-branes in string theory. A low energy theory of 2 + 1 dimensional fermions is obtained by considering the D3-D7 system, and the plateau transition corresponds to moving the branes through one another. We study the transition at strong coupling using gauge/gravity duality and the probe approximation. Strong coupling leads to a novel kind of plateau transition: at low temperatures the transition remains discontinuous due to the effects of dynamical symmetry breaking and mass generation, and at high temperatures is only partially smoothed out.

Fundamental strings, holography and nonlinear superconformal algebras

We discuss aspects of holography in the AdS3 × S p near string geometry of a collection of straight fundamental heterotic strings. We use anomalies and symmetries to determine general features of the dual CFT. The symmetries suggest the appearance of nonlinear superconformal algebras, and we show how these arise in the framework of holographic renormalization methods. The nonlinear algebras imply intricate formulas for the central charge, and we show that in the bulk these correspond to an infinite series of quantum gravity corrections. We also makes some comments on the worldsheet σ-model for strings on AdS3 × S 2 , which is the holographic dual geometry of parallel heterotic strings in five dimensions.

Fundamentals and challenges of optical multiple-input multiple-output multimode fiber links [Topics in Optical Communications]

In this article we discuss the application of MIMO processing to multimode fiber links. MIMO processing is shown to increase the information capacity of communication links linearly as the minimum number of transmitters/receivers increases. The fundamentals of optical MIMO fiber links are presented, and the promises and challenges of such systems are elaborated

Capacity enhancement in coherent optical MIMO (COMIMO) multimode fiber links

In this paper, we study a coherent optical MIMO (COMIEMO) multi-mode fiber link proposed for enhancing the fiber information capacity. We examine the statistical characterization of the equivalent MIMO channel and the improvement in the fiber capacity due to MIMO transmission. It is shown that the equivalent channel behaves similarly to a complex Gaussian MIMO channel, suggesting that the available results on wireless MIMO communication systems can be applied to optical fiber links for capacity enhancement.

Coherent optical MIMO (Invited Paper)

In this paper we present a Coherent Optical Multiple-Input Multiple-Output (COMIMO) communication system and its ability to exploit the inherent information capacity of Multi-Mode Fiber (MMF). COMIMO technique is a promising approach to unlock the information capacity of MMF as it ensures the necessary diversity for practically any fiber length. When exceeding the bandwidth-length product of the link, MIMO adaptive equalization can be used to mitigate inter-symbol interference (ISI). Furthermore, COMIMO link provides additional security in the physical layer that complements encryption.

Coherent optical multiple-input multiple-output communication over multimode fiber

It is shown that coherent optical implementation is crucial for an optical multiple-input multiple-output (MIMO) system to work. It ensures successful channel separation for practically any data rate or fiber length

ISI equalization for a coherent optical MIMO (COMIMO) system

We report the first demonstration of coherent optical MIMO utilizing adaptive equalization for ISI. Coherent optics implementation is essential to ensure the necessary diversity and hence MIMO benefits, independent of fiber length and data rate.