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

In this paper, we propose new types of dual microring resonators coupled via 3×3 couplers. By employing the transfer matrix method, a model for these four types is developed and analytical expressions for characterizing their transmissions are derived. The first two types show a coupled-resonator-induced-transparency-like transmission spectrum at the through port. The third type holds the same transmission spectrum, while the last type simultaneously realizes a first-order and a second-order filters at two drop ports. The effects of coupling coefficients on their transmission spectra are analyzed in more detail. These proposed types can be found applications in fields such as sensors and filters.

3×3 coupler based dual microring resonators: a proposal, model and analysis

Nonlinear activation functions, a part of artificial neuron network, is responsible for nonlinearly mapping the input of the neuron to the output. The optical realization of nonlinear activation functions has become a research hotspot, which is optical activation function. Here, we propose and experimentally demonstrate an all-optical implementation of optical activation function based on germanium silicon composite waveguide, which takes advantage of the intrinsic absorption and the plasma dispersion effect of germanium in the communication band. It realizes two different nonlinear activation functions. This proves that our scheme has a great potential for optical activation functions.

All-optical nonlinear activation function based on Germanium Silicon composite waveguide

We demonstrate a fiber ring laser with a dispersion compensation fiber (DCF) and a delayed interferometer (DI), which is able to switch eleven wavelengths one by one. In ring cavity, DCF supplies different effective cavity lengths for different wavelengths, DI generates a wavelength comb corresponding to the ITU grid, a flat-gain erbium-doped fiber amplifier (EDFA) provides uniform gain for each lasting wavelength, and a semiconductor optical amplifier (SOA) not only acts as active modulator, but also alleviates homogeneous broadening effect of EDFA. Stable pulse trains with a pulsewidth about 40 ps at 10 GHz have been obtained by injecting external optical control signals into the laser. Wavelength switching process among eleven wavelengths is achieved by merely tuning an intracavity optical delay line.

A novel wavelength switchable fiber ring laser

A Stokes polarimeter based on polarization-dependent hologram is demonstrated by matrix analysis. The mean square error of the orientation angle and the ellipticity angle are 4.45 degree and 3.19 degree respectively.

Stokes polarimeter based on a loose polarization-dependent hologram

In this paper, a simple wavelength conversion scheme based on SLAOLM is proposed out for the first time, and its extinction ratio performance and noise characteristics can be improved simultaneously. Theoretical study shows that the conversion mechanism is still based on gain saturation effect, but the phase modulation effect existing in SLAOLM is helpful for extinction ratio improvement. Furthermore, the weak signal will be compressed and the strong signal will be amplified in the SLAOLM, then the Signal-Noise Ration (SNR) can be also improved. Experiment results shows that, after exploiting this scheme, the output extinction ration has been improved to be 10dB from 6dB, and the SNR has also been improved to be 13dB from 5dB.

Xinliang Zhang

Papers

3×3 coupler based dual microring resonators: a proposal, model and analysis

All-optical nonlinear activation function based on Germanium Silicon composite waveguide

A novel wavelength switchable fiber ring laser

Stokes polarimeter based on a loose polarization-dependent hologram

Performance improvement in XGM wavelength conversion exploiting SLAOLM