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

A configuration of semiconductor fiber ring laser (SFRL) is used to achieve a tunable all-optical logic NOR gate at 20 Gb/s. Both gain and gain modulation is provided by a single semiconductor optical amplifier (SOA) in the ring cavity. In the theoretical simulations, sectional model is used for SOA and iterative condition is used for ring cavity to simulate the output from SFRL. All-optical NOR operation is successfully demonstrated experimentally at 20 Gb/s. There is a good agreement between theoretical simulations and experimental results. The extinction ratio higher than 14.9 dB is obtained as the output wavelength is tuned from 1530nm to 1560nm.

Novel tunable all-optical NOR gate at 20-Gb/s based on SOA fiber ring laser

An all-optical real-time chromatic dispersion (CD) monitoring technique is proposed and demonstrated for 40 Gbit/s differential phase-shifts keying (DPSK) signal, utilizing the cross modulation effects of semiconductor optical amplifier (SOA). The optical power of the output spectral components, which is from the probe’s frequency up to the signal bandwidth, is used for CD monitoring. This technique provides a wide monitoring range with large variation scale. The impacts of the polarization mode dispersion (PMD) and the optical signal-to-noise ratio (OSNR) on the CD monitoring results are theoretically analyzed and then experimentally investigated, showing that they have slight influence on the monitoring results within a certain range. Furthermore, simulated results for quadrature phase shift keying (QPSK) signal at 80 Gbit/s are also demonstrated, indicating that this technique is suitable for advanced modulated format as well.

Chromatic dispersion monitoring using semiconductor optical amplifier

The spectra of silicon integrated waveguides are tailored to process analog computing (i.e.,differential and integral) in optical domain with huge bandwidth.With the theory of signal and system, we design some silicon integrated devices to implement photonic differentiator and optical differential equation solver. The basic principle is to tailor the spectra of silicon integrated waveguides to meet the requirements of analog computing circuits. These analog photonic integrated circuits are very promising in future computing systems with high speed, low cost, and compact size. We also plan to employ these basic computing units in more complex computing modules.

Photonic analog computing with integrated silicon waveguides

A simple scheme for multi-channel wavelength conversion based on cross-gain modulation of Amplified Spontaneous Emission (ASE) spectrum in Semiconductor Optical Amplifier (SOA) is described. Simultaneous 16-channel wavelength conversion at 10Gb/s is first demonstrated without any additional probe lights, the modulation information carried by one pump light could be converted into arbitrary many channels if only the demultiplexer with enough channels is exploited.

Simultaneous 16-channel wavelength conversion at 10 Gb/s based on cross-gain modulation of ASE spectrum in SOA

We propose and theoretically demonstrate a high-speed and high-power silicon-germanium photodiode assisted by doping region regulation. The simulations imply a large 3 dB bandwidth of 14 GHz at 30 mW input optical power.

Xinliang Zhang

Papers

Novel tunable all-optical NOR gate at 20-Gb/s based on SOA fiber ring laser

Chromatic dispersion monitoring using semiconductor optical amplifier

Photonic analog computing with integrated silicon waveguides

Simultaneous 16-channel wavelength conversion at 10 Gb/s based on cross-gain modulation of ASE spectrum in SOA

Design of high-speed and high-power Si-Ge photodiode assisted by doping region regulation