Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to intersymbol interference caused by a dispersive channel. Very recently a number of researchers have shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.

OFDM for Optical Communications

Course Description This is an advanced course in which we explore the field of Statistical Optics. Topics covered include such subjects as the statistical properties of natural (thermal) and laser light, spatial and temporal coherence, effects of partial coherence on optical imaging instruments, effects on imaging due to randomly inhomogeneous media, and a statistical treatment of the detection of light. Development of this more comprehensive model of the behavior of light draws upon the use of tools traditionally available to the electrical engineer, such as linear system theory and the theory of stochastic processes.

http://ieeexplore.ieee.org/iel5/3/23094/01073023.pdf

Statistical optics

Nearly one century after the birth of quantum mechanics, parity–time symmetry is revolutionizing and extending quantum theories to include a unique family of non-Hermitian Hamiltonians. While conceptually striking, experimental demonstration of parity–time symmetry remains unexplored in quantum electronic systems. The flexibility of photonics allows for creating and superposing non-Hermitian eigenstates with ease using optical gain and loss, which makes it an ideal platform to explore various non-Hermitian quantum symmetry paradigms for novel device functionalities. Such explorations that employ classical photonic platforms not only deepen our understanding of fundamental quantum physics but also facilitate technological breakthroughs for photonic applications. Research into non-Hermitian photonics therefore advances and benefits both fields simultaneously. General concepts and recent developments of parity–time symmetry in classical photonics are reviewed.

Non-Hermitian photonics based on parity-time symmetry

The concept of parity-time symmetric systems is rooted in non-Hermitian quantum mechanics where complex potentials obeying this symmetry could exhibit real spectra. The concept has applications in many fields of physics, e.g., in optics, metamaterials, acoustics, Bose-Einstein condensation, electronic circuitry, etc. The inclusion of nonlinearity has led to a number of new phenomena for which no counterparts exist in traditional dissipative systems. Several examples of nonlinear parity-time symmetric systems in different physical disciplines are presented and their implications discussed.

Nonlinear waves in PT -symmetric systems

Significant effort in optical-fibre research has been put in recent years into realizing mode-division multiplexing (MDM) in conjunction with wavelength-division multiplexing (WDM) to enable further scaling of the communication bandwidth per fibre. In contrast, almost all integrated photonics operate exclusively in the single-mode regime. MDM is rarely considered for integrated photonics because of the difficulty in coupling selectively to high-order modes, which usually results in high inter-modal crosstalk. Here we show the first microring-based demonstration of on-chip WDM-compatible mode-division multiplexing with low modal crosstalk and loss. Our approach can potentially increase the aggregate data rate by many times for on-chip ultrahigh bandwidth communications.

/pdf/wdm-compatible-mode-division-multiplexing-on-a-silicon-chip-31lge0lodq.pdf

WDM-compatible mode-division multiplexing on a silicon chip

We present a novel concept which enables the realization of unidirectional and irreversible grating assisted couplers by using gain-loss modulated medium to eliminate the reversibility. Employing a matched periodic modulation of both refractive index and loss (gain) we achieve a unidirectional energy transfer between the modes of the coupler which translates to light transmission from one waveguide to another while disabling the inverse transmission. The importance of self coupling coefficients is explored as well and a feasible implementation, where the real and imaginary perturbations are implemented in different waveguides is presented.

Unidirectional complex grating assisted couplers.

Data parallelization by means of optical multiple-input multiple-output (MIMO) transmission over dispersive multimode fiber (MMF), with a high degree of modal coupling but not accounting for intermodal dispersion, is investigated by developing an analytical model for direct detection of MMF MIMO frequency-flat transmission with mutually incoherent sources. The MIMO channel performance is derived in terms of a new formulation of a channel matrix for modal group powers accounting, for the first time, for modal coupling. For fixed aggregate signaling rate and power budget, for uncoded bit streams, increasing the number of output detectors improves the bit error ratio (BER)-similarly to wireless MIMO. However, contrary to wireless MIMO, increasing the number of input ports actually yields a BER penalty, which is traceable to the quadratic nature of photodetection. We finally establish the feasibility of enhancing the aggregate bit rate using multiple inputs in the case that the individual single-input-single-output channels are band limited, e.g., given optical data sources each at 2 Gb/s, it is possible to attain a 12-Gb/s signaling rate over several hundreds of meters of MMF at 10-10 BER, by utilizing six such inputs into the MIMO system, while incurring just a small average power penalty of approximately 2 dB/channel. The current model assumes strong intermodal coupling and neglects ISI influence over distances of up to hundreds of meters at gigabit rates, providing a first step in the optical MIMO analysis. On the other hand, similar scenario is practically met for shorter distances (up to 100 m) for the novel types of plastic optical fibers.

Data Parallelization by Optical MIMO Transmission Over Multimode Fiber With Intermodal Coupling

Important functionality of photonics circuits is invoking interactions between few optical modes. Intermodal power exchange can be realized by introducing a spatial perturbation of the refractive index. The power transfer is symmetric from mode n to m and vice versa, as well as time reversible-both can impede definite performance of a variety of optical devices. By modifying the coupling concept-i.e., using a matched periodic modulation of both the index of refraction and loss (gain) of the medium, we implement a complex spatially single sideband perturbation (SSP), which breaks the symmetry and allows a unidirectional energy flow from mode m to mode n, while blocking the inverse flow. We investigated various deviations from ideal conditions including detuned schemes (gratings period not matched with the difference between propagation constants), unequal real and imaginary coupling coefficients, self-coupling coefficients, saturation effects and spatial separation of the real and imaginary perturbations. Finally the SSP is exemplified in the design of a novel broad-band "add" component for optical add/drop multiplexing system.

Optical unidirectional devices by complex spatial single sideband perturbation

Simultaneous dual mode add/drop multiplexers for optical interconnects buses

System performance is derived for Multiple-Inputs-Multiple-Outputs signaling over MMF accounting for inter-modal coupling; the aggregate bitrate is increased by utilizing multiple inputs on a single physical channel. Alternatively, Bit-Error-Rate is improved by using multiple outputs.

Data Parallelization by Optical MIMO Transmission Over Multi-mode Fiber with Inter-modal Coupling

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