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

Based on a dual-drive Mach-Zehnder modulator, we proposed to simultaneously measure distance and velocity by using multi-band linear-frequency-modulated microwave signals. The direction of the velocity is also identified at the same time.

Simultaneous Measurement of Distance and Velocity Based on a Dual-Drive Mach-Zehnder Modulator

A high Q microwave photonic filter is proposed and experimentally demonstrated. It is based on a recirculating delay line comprising a semiconductor optical amplifier used to generate negative tap. A frequency response with a high Q of 543, a rejection ratio of 40 dB and good shape factor is obtained.

A SOA-based high Q microwave photonic filter

We designed a silicon microring based elastic polarization converter, which can realize arbitrary polarization conversion between any two of linear, circular, and elliptic polarization states. Simulation results show good polarization converting characteristics at resonance condition.

Silicon microring based elastic polarization converter

We propose and experimentally demonstrate a simple photonic approach to achieving tunable and reconfigurable bandpass microwave photonic filters (MPFs) based on cascaded microring resonators (CMRRs). The optical spectrum of the silicon CMRRs could offer two bandpass responses to separately filter the optical carrier and one of the sidebands generated by the phase modulation. Thus we could achieve a bandpass MPF. Moreover, as the central frequencies and bandwidths of the two bandpass responses can be tuned by simply adjusting the voltages applied on one MRR, tunable and reconfigurable MPFs could be successfully achieved. A proof-of-concept experiment illustrates a central frequency tuning range from 19 GHz to 40 GHz, and a wide bandwidth tuning range from 5.5 GHz to 17.5 GHz.

Compact tunable microwave photonic filters based on cascaded microring resonators

In this paper, we propose and theoretically demonstrate two-input arbitrary Boolean logic gates using single semiconductor optical amplifier (SOA) and optical filtering at 10Gb/s. The system consists of an SOA and a special-designed optical filter. Before the SOA, there are two input data signals and a probe signal. The data signals should be ultrashort pulse sequence. Because different input power can cause different frequency shifts of the probe signal, different logic gates can be obtained by different filter detuning. Assisted by single Gaussian optical filter, we obtained logic NOR, OR, AND, XOR. Assisted by special filter with double peaks, we obtained logic XNOR and NAND. All the logic gates are obtained by changing the consequent filter only. The bit error rate (BER) measurement is analyzed for all logic gates. Our scheme is quite simple and robust for multifunctional logic gates. These logic gates can be used to the advanced complex logic circuits. Keywords: Semiconductor optical amplifier, logic gates, frequency shift

Xinliang Zhang

Papers

Simultaneous Measurement of Distance and Velocity Based on a Dual-Drive Mach-Zehnder Modulator

A SOA-based high Q microwave photonic filter

Silicon microring based elastic polarization converter

Compact tunable microwave photonic filters based on cascaded microring resonators

A proposal for two-input arbitrary Boolean logic gates based on single semiconductor optical amplifier