There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Orbital angular momentum (OAM), which describes the “phase twist” (helical phase pattern) of light beams, has recently gained interest due to its potential applications in many diverse areas. Particularly promising is the use of OAM for optical communications since: (i) coaxially propagating OAM beams with different azimuthal OAM states are mutually orthogonal, (ii) inter-beam crosstalk can be minimized, and (iii) the beams can be efficiently multiplexed and demultiplexed. As a result, multiple OAM states could be used as different carriers for multiplexing and transmitting multiple data streams, thereby potentially increasing the system capacity. In this paper, we review recent progress in OAM beam generation/detection, multiplexing/demultiplexing, and its potential applications in different scenarios including free-space optical communications, fiber-optic communications, and RF communications. Technical challenges and perspectives of OAM beams are also discussed.

Optical communications using orbital angular momentum beams

It is shown that the ordinary semiclassical theory of the absorption of light by exciton states is not completely satisfactory (in contrast to the case of absorption due to interband transitions). A more complete theory is developed. It is shown that excitons are approximate bosons, and, in interaction with the electromagnetic field, the exciton field plays the role of the classical polarization field. The eigenstates of the system of crystal and radiation field are mixtures of photons and excitons. The ordinary one-quantum optical lifetime of an excitation is infinite. Absorption occurs only when "three-body" processes are introduced. The theory includes "local field" effects, leading to the Lorentz local field correction when it is applicable. A Smakula equation for the oscillator strength in terms of the integrated absorption constant is derived.

a Quantum-Mechanical Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals.

Optical Waveguide Theory

Driven by the increasing demand on handing microwave signals with compact device, low power consumption, high efficiency and high reliability, it is highly desired to generate, distribute, and process microwave signals using photonic integrated circuits. Silicon photonics offers a promising platform facilitating ultracompact microwave photonic signal processing assisted by silicon nanophotonic devices. In this paper, we propose, theoretically analyze and experimentally demonstrate a simple scheme to realize ultracompact rejection ratio tunable notch microwave photonic filter (MPF) based on a silicon photonic crystal (PhC) nanocavity with fixed extinction ratio. Using a conventional modulation scheme with only a single phase modulator (PM), the rejection ratio of the presented MPF can be tuned from about 10 dB to beyond 60 dB. Moreover, the central frequency tunable operation in the high rejection ratio region is also demonstrated in the experiment.

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Photonic crystal nanocavity assisted rejection ratio tunable notch microwave photonic filter

A thorough simulation and evaluation of phase noise for optical amplification using semiconductor optical
amplifier (SOA) is very important for predicting its performance in differential phase shift keyed (DPSK) applications.
In this paper, standard deviation and probability distribution of differential phase noise are obtained from the statistics of
simulated differential phase noise. By using a full-wave model of SOA, the noise performance in the entire operation
range can be investigated. It is shown that nonlinear phase noise substantially contributes to the total phase noise in case
of a noisy signal amplified by a saturated SOA and the nonlinear contribution is larger with shorter SOA carrier lifetime.
Power penalty due to differential phase noise is evaluated using a semi-analytical probability density function (PDF) of
receiver noise. Obvious increase of power penalty at high signal input powers can be found for low input OSNR, which
is due to both the large nonlinear differential phase noise and the dependence of BER vs. receiving power curvature on differential phase noise standard deviation.

Numerical investigation of differential phase noise and its power penalty for optical amplification using semiconductor optical amplifiers in DPSK applications

Based on cross-gain modulation (XGM) in cascaded semiconductor optical amplifiers (SOAs), all-optical AND and NOR operations at 20Gb/s were both realized in the same configuration for the first time. By controlling the input signal power, the two logic gates can be achieved from different output channels. Single-port-couple SOAs with 1000μm active regions are exploited to improve the output performance. The dependence of the output characteristics on the input signal power and wavelength was also investigated.

Demonstration on all-optical logic AND and NOR gates at 20Gb/s with cascaded SOAs

Photonic generation of UWB impulses by using a Fabry–Pérot semiconductor optical amplifier

In this paper, the placement problem of wavelength converters in DWDM (Dense Wavelength Division Multiplexing) networks with arbitrary topologies is investigated. We could settle the problem easily by considering the two sub-problems of routing selecting and converter placement simultaneously. A reasonable path algorithm in which load balance and shortest path are considering together was proposed. Based on this model, we presented three simple algorithms A,B and C for wavelength converter placement. Simulation results on the basic characteristic of converter placement of EON and NSFNET are presented. With the three algorithms, the cost (including routing selecting and placement of WC) of optimizing network has been greatly reduced but the blocking performance has not been reduced.

Novel algorithms for wavelength converters placement in wavelength-routed network

We experimentally demonstrate a six-port passive circuit supporting all-optical ordered-route transmission using thermo-optic effect. The 15-dB bandwidth (BW) is larger than 0.05 nm and the maximum blocking extinction ratio (BER) is 39 dB.

Xinliang Zhang

Papers

Numerical investigation of differential phase noise and its power penalty for optical amplification using semiconductor optical amplifiers in DPSK applications

Demonstration on all-optical logic AND and NOR gates at 20Gb/s with cascaded SOAs

Photonic generation of UWB impulses by using a Fabry–Pérot semiconductor optical amplifier

Novel algorithms for wavelength converters placement in wavelength-routed network

An all-silicon passive six-port circuit of all-optical ordered-route transmission