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 have experimentally demonstrated a 40Gb/s all-optical binary-coded-decimal (BCD) decoder for the first time, utilizing delay interferometers (DIs) and cascading semiconductor optical amplifiers (SOAs) without any assisting light. Extinction ratios (ERs) of the intermediate results after the first SOA are all over 11dB which ensures the capability to cascade to the second one. The final results are in the form of return-to-zero (RZ) format with correct and clear temporal waveforms. The proposed scheme could be extended to 1-of-16 decoder, logic minterms and read only memory (ROM).

40Gb/s all-optical binary-coded-decimal decoder

Vertical-cavity surface emitting lasers (VCSELs) can always perform single longitudinal mode operation. However, in VCSELs, when the emission aperture becomes large, single transverse mode operation will be challenged. In this paper, we have discussed the VCSEL designs to achieve single transverse mode operation. Scalar transfer matrix method analysis has been used to analysis of antiresonant reflecting optical waveguides (ARROW) VCSEL.

Scalar transfer matrix method analysis of ARROW VCSEL

We demonstrate a compact terahertz polarization beam splitter based on directional coupler. The extinction ratio over 8 dB is achieved from 0.445 to 0.475 THz for both TE and TM polarizations.

A Compact Terahertz Polarization Beam Splitter Based on Directional Coupler

We proposed, experimentally and theoretically demonstrated all-optical two line-four line encoder and two bit-wise
comparator of RZ data streams at 40Gb/s based on cross gain modulation (XGM) and four wave mixing (FWM) in
three parallel SOAs. Five logic functions for digital encoder and comparator between two signals A and B: A¯ B¯,
AB¯, AB¯, AB and A¯⊗¯B¯, were achieved simultaneously. The first three optical logics are realized based on XGM in
SOAs, the fourth is realized with FWM, and the fifth is the mixing result of the first and the fourth. A detuning filter is
employed to improve the output performance. The output extinction ratio (ER) for the XGM operation is above 10dB,
and the ER for FWM operation is around 8 dB. Wide and clear eye patterns for the five logic outputs can be observed.
The all optical digital encoder/comparator we demonstrated has advantages of simple structure, multifunctional optical
logic functions and high speed.

40Gb/s all-optical digital encoder/comparator based on semiconductor optical amplifiers

A novel technique is presented in obtaining the performance characteristics of traveling-wave semiconductor optical amplifier (TWA) with tensile-strained-barrier multiple-quantum-well structure. In-out fiber ends of TWA are used to construct an external cavity resonator to produce big ripple on amplified spontaneous emission (ASE) spectrum. By this means, Hakki-Paoli method is adopted to obtain gain and differential gain spectra over a wide spectral range. From measured longitudinal mode spacing and peak wavelength shift due to increased bias current, we further calculate the effective refractive index, carrier-induced refractive index change and linewidth enhancement factor. Some special features about ASE mode spectrum shift and refractive index change above lasing threshold are revealed and explained.

Xinliang Zhang

Papers

40Gb/s all-optical binary-coded-decimal decoder

Scalar transfer matrix method analysis of ARROW VCSEL

A Compact Terahertz Polarization Beam Splitter Based on Directional Coupler

40Gb/s all-optical digital encoder/comparator based on semiconductor optical amplifiers

Novel measurement of traveling-wave semiconductor optical amplifier with tensile-strained-barrier MQW structure