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 demonstrate parallel amplitude regeneration for 6*40Gb/s RZ-DPSK signals using the XPM effect in a single saturated SOA and subsequent narrow filtering. An average -1.3 dB power penalty can be achieved for all the channels.

6∗40Gb/s all-optical RZ-DPSK amplitude regeneration based on XPM effect in a single SOA

The traditional method to rotate an image of light is by using a Dove prism, even though it is unreliable, owing to the mechanical adjustment when a tunable image rotation is required. Here, we propose a flexible and nonmechanical scheme to rotate an optical image by an arbitrary angle with optical geometric transformation. The device consists of a Cartesian to log-polar coordinate converter and an inverse converter. The rotated angle can be freely tuned by adjusting the first coordinate converter. In particular, the image rotator is dynamically programmable and continuously tunable if spatial light modulators or digital micromirror devices are employed. Our work promotes its ability of manipulating beams, which has applications in the fields of interferometry, astronomy, beam steering, mode detection, and pattern recognition.

Tunable Image Rotator of Light With Optical Geometric Transformation

A single passband microwave photonic filter with flat-top and steep transition edges

We introduce and experimentally demonstrate a flexible temporal illusion at telecommunication data rate in optical fiber communication system. The temporal illusion cannot only transform an event to another event as expected, but also mask the event with high-level signal, providing a novel method to conceal the confidential information. We successfully transform the output temporal waveforms of a return-to-zero (RZ), dark RZ and nonreturn-to-zero (NRZ) event into that of any above modulation format event and high-level signal at different illusion bits and mosaic bits at a data rate of 5 Gb/s, respectively. Our works offer us new perspectives on illusion optics for falsifying event rather than object, which has potential applications in secure communication, data encryption and other military applications.

Demonstration of the temporal illusion and mosaic.

Optical couplers with different power splitting ratios are essential building blocks in photonic integrated circuits. We theoretically and experimentally demonstrate a broadband and fabrication-tolerant $1\times 2$ integrated optical coupler with an arbitrary splitting ratio, based on a mode converter (MC). The MC converts the input mode into a targeted output mode with a desired power distribution, which determines the power splitting ratio at outputs. The geometry of the MC is engineered and optimized by using the particle swarm optimization algorithm combined with the finite-difference time-domain method. Couplers with 10/90, 20/80, 30/70, 40/60, and 50/50 splitting ratios are designed and fabricated on the silicon-on-insulator platform as representatives, with footprints smaller than $2.75~\mu $ m $\times 18.41~\mu $ m. The experimental results show that the ±1 dB bandwidths of these devices are greater than 42 nm around 1550 nm, and the average insertion losses are less than 0.65 dB.

Xinliang Zhang

Papers

6∗40Gb/s all-optical RZ-DPSK amplitude regeneration based on XPM effect in a single SOA

Tunable Image Rotator of Light With Optical Geometric Transformation

A single passband microwave photonic filter with flat-top and steep transition edges

Demonstration of the temporal illusion and mosaic.

Integrated Optical Coupler With an Arbitrary Splitting Ratio Based on a Mode Converter