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 raise a detuning-dependent loss mechanism to describe the soliton formation dynamics when the lumped filtering operation is manipulated in anomalous group velocity dispersion regime, using stability analysis of generalized Lugiato-Lefever equation.

Stability analysis of generalized Lugiato-Lefever equation with lumped filter for Kerr optical soliton generation in anomalous dispersion regime.

An all-optical controllable silicon optomechanical microring resonator is fabricated. Due to the mechanical Kerr effect, we realize seamless wavelength tuning over a range of 2.56 nm with a tuning efficiency of 80 GHz/mW.

Highly-efficient all-optical control of silicon microring resonators through mechanical Kerr effect

We propose a new type of grating-assisted microring (GAMR) structure with Bragg gratings placed on microring's arms. Two Fabry-Perot resonances interact with microring resonance, resulting in GAMR's unique amplitude and phase spectra. The structure's characteristics are analytically studied using coupled mode theory and numerically verified by 2D-FDTD. With proper cavity lengths, GAMR exhibits an electromagnetically induced transparency (EIT)-like spectrum. The ultra-narrow resonance can be used for sensing, modulation, and other applications.

Transmission characteristics of a novel grating assisted microring

We propose a novel and simple non-return-to-zero differential phase shift keying (NRZ-DPSK) wavelength division
multiplexing (WDM) system which can simultaneously demultiplex and demodulate multiple wavelengths. The phaseto-
intensity demodulation principle is based on detuned filtering, which is achieved by using a single commercial array
waveguide grating (AWG) in our scheme. By properly choosing appropriate AWG channels at the transmitter, the AWG
at the receiver can act as both the de-multiplexer and the demodulator of the DPSK signals. Simulations at 10, 20 and
40Gb/s show good flexibility and performance for the proposed system.

Proposal for a novel and simple WDM NRZ-DPSK system

We propose and experimentally demonstrate an all-optical 2×2-bit multiplier with neither adder nor shift register. Based on bidirectional multichannel four-wave mixing, output logic results of multiplier at 40 Gb/s are clearly identified and correct.

Xinliang Zhang

Papers

Stability analysis of generalized Lugiato-Lefever equation with lumped filter for Kerr optical soliton generation in anomalous dispersion regime.

Highly-efficient all-optical control of silicon microring resonators through mechanical Kerr effect

Transmission characteristics of a novel grating assisted microring

Proposal for a novel and simple WDM NRZ-DPSK system

All-Optical 2×2-Bit Multiplier at 40Gb/s using Bidirectional Multichannel Four-Wave Mixing