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

We theoretically discuss 40 Gb/s semiconductor optical amplifier (SOA)-based wavelength conversion (WC) using a detuning optical bandpass filter Both inverted and non-inverted WCs are obtained by shifting the filter central wavelength with respect to the probe wavelength when input data signal is RZ format The filter detuning is essential to different output formations

40Gb/S Simultaneous Inverted and Non-inverted Wavelength Conversion Based on SOA Using Transient Cross Phase Modulation

We propose and experimentally demonstrate all-optical NRZ-DPSK to RZ-OOK modulation format conversion for wavelength multicasting based on a single SOA. Using T-XPM effect in SOAs, a −1.38dB power penalty at BER of 10−9 can be achieved.

All-Optical NRZ-DPSK to RZ-OOK Modulation Format Conversion for Wavelength Multicasting Based on a Single SOA

We report a hybrid two-step approach for fabricating silica micro/nanofibers with different diameters (the minimum one down to 180 nm). Due to tapering and etching techniques introduced to this approach, the time is reduced from hundreds of minutes to several minutes to manufacture silica nanofibers by etching and the complexity of tapering mechanical system is brought down, because this approach has the ability to control the micro/nanofiber diameter on a nanometer-scale. Uniform nanofibers with losses as low as 0.05 dB/mm at 1.55 μm wavelength are obtained suggesting the advantage of the hybrid approach to build up micro/nanofiber-based devices, especially in locally changing the structure of micro/nanofiber.

Hybrid fabricating of silica micro/nanofibers

Modified duobinary return-to-zero (MD-RZ) format is an improvement of optical duobinary (ODB) format. This paper proposes a novel, all-optical method to generate MD-RZ format by using a polarization maintaining fiber loop mirror (PMFLM). Operation principle for all-optical format conversion is theoretically analyzed and the output spectrum is simulated. 40-Gb/s MD-RZ data format generation is experimentally demonstrated.

Generation of modified duobinary return-to-zero format using polarization-maintaining fiber loop mirror

Over the past two decades, integrated photonics has achieved unprecedented advancement owing to the development of optical devices. Among those devices, waveguide Bragg gratings are widely used for optical true time delay and optical filtering. In general, the integration of WBGs faces several challenges, including high insertion loss and manufacturing difficulties. In this article we propose and experimentally demonstrated a novel waveguide Bragg grating design based on multimode waveguide. The proposed device features low overall insertion loss of 3.1dB.

Xinliang Zhang

Papers

40Gb/S Simultaneous Inverted and Non-inverted Wavelength Conversion Based on SOA Using Transient Cross Phase Modulation

All-Optical NRZ-DPSK to RZ-OOK Modulation Format Conversion for Wavelength Multicasting Based on a Single SOA

Hybrid fabricating of silica micro/nanofibers

Generation of modified duobinary return-to-zero format using polarization-maintaining fiber loop mirror

Optical true time delay based on multimode waveguide gratings