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.

/pdf/photonic-crystal-nanocavity-assisted-rejection-ratio-tunable-1zeivpqv3v.pdf

Photonic crystal nanocavity assisted rejection ratio tunable notch microwave photonic filter

Polarization-sensitive photodetectors can simultaneously sense the polarization states and intensity of light. Traditional polarimeters integrate photodetectors with complex and bulky optical systems such as polarizers and waveplates, which can not meet the current demand of on-chip polarization photonic devices. This work has demonstrated a full linear polarization polarimeter based on two independent ReS2 nanobelt devices vertically stacked with a designed twist angle. The device achieves a high responsivity (959 A W−1) without applying an external gate voltage. In addition, the ReS2 nanobelt photodetector displays a strong polarization-sensitive photoresponse with a linear dichroic ratio of 1.72 at 665 nm. By utilizing those two vertically stacked nanobelt devices, the linear polarization states of the incident light can be distinguished in a wide range of wavelengths from 590 to 800 nm. This study provides a simple route for future polarization-sensitive optoelectronic devices. 

High Performance Broadband Full Linear Polarimeter based on ReS
 2
 Nanobelts

An ultrafast all-optical logic NOR gate based on a semiconductor optical amplifier (SOA) and a fiber delay interferometer (FDI) is presented. For high-speed input return-to-zero (RZ) signal, nonreturn-to-zero (NRZ) switching windows which satisfy Boolean NOR operation can be formed by properly choosing the delay time and the phase shift of FDI. 40 Gb/s NOR operation has been demonstrated successfully with low control optical power. Simulations show that even higher speed can be expected

40Gb/s All-optical NOR Gate Based on Semiconductor Optical Amplifier and Fiber Delay Interferometer

The electrically pumped InGaAsP/InP active microring has been theoretically analyzed and numerically simulated based on subsection model, with carrier rate equation and amplified spontaneous emission taken into account. After the subsection model is introduced in detail, the spectrum characteristics are numerically investigated. The simulation results show that spectrum characteristics will shift under different pump current and injected light power. The performances of an optical on/off switching utilizing pump-probe method based on InGaAsP/InP active microring are investigated. The results show that if we choose probe light off resonation initially, and pump light on resonation, only several hundred of microwatt power of pump light can realize the probe light on/off conversion. Such a potential will be applied in many other optical applications with ultra low power consumption. However, carrier lifetime will be the main factor restricting the characteristics of electrically pumped active microring due to the fact that the refractive index change is induced by carrier density change.

Theoretical study of InGaAsP-InP active microring

We report high-data rate reception of polarization division multiplexing signals using graphene-on-plasmonic slot waveguide photodetectors with bandwidth exceeding 70 GHz. 128 Gbps NRZ and 224 Gbps PAM-4 signals reception are experimentally demonstrated at 1550 nm with high quality.

128 Gbps NRZ and 224 Gbps PAM-4 Signals Reception in Graphene Plasmonic PDM Receiver

A microwave photonic filter with passband and stopband interchanged is proposed and demonstrated. The structure is based on a recirculating delay line loop comprising a semiconductor optical amplifier (SOA) and a tunable narrowband optical filter, Converted signal used as negative tap is generated employing the cross-gain modulation of the amplified spontaneous emission spectrum of the SOA. The filter structure can be operated as a high quality factor or notch filter with flat passband depends on the central wavelength of the optical filter aligning with or detuning from the pump wavelength.

Xinliang Zhang

Papers

High Performance Broadband Full Linear Polarimeter based on ReS 2 Nanobelts

40Gb/s All-optical NOR Gate Based on Semiconductor Optical Amplifier and Fiber Delay Interferometer

Theoretical study of InGaAsP-InP active microring

128 Gbps NRZ and 224 Gbps PAM-4 Signals Reception in Graphene Plasmonic PDM Receiver

A microwave photonic filter with passband and stopband interchanged