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

With the rapid development of silicon photonics, chip-integrated optical pulse shaper is highly desirable rather than bulk optical pulse shaper. In this paper, we review our recent progresses of on-chip optical pulse shaper and its applications in arbitrary waveform generation. First, we design and demonstrate a silicon on insulator (SOI) on-chip optical pulse shaper based on four-tap finite impulse response. The pulse shaper is programmable thanks to amplitude and phase modulation of each tap controlled by thermal heaters. We further demonstrate the tunability of the filter central wavelength, bandwidth and variable passband shape. Second, we will demonstrate the applications of arbitrary waveform generator and order-tunable optical differentiator based on such an optical pulse shaper. Third, special optical pulse shapers based on opto-mechanical structure and on-chip large dispersion module are also discussed.

On-chip optical pulse shaper and its application in arbitrary waveform generation

We present optical ultrawideband doublet generation from NRZ-DPSK signals based on a length of SMF. The generated ultrawideband spectra using 25 km SMF and 15 km SMF are analyzed contrastively.

Ultrawideband doublet generation from NRZ-DPSK signals

We propose and demonstrate a novel approach generating ultrawideband (UWB) monocycle pulses using cross phase
modulation (XPM) of the semiconductor optical amplifier (SOA). A pair of polarity-reversed UWB monocycle pulses is
achieved by locating the probe carrier at the positive and negative linear slopes of the filter. We achieve different UWB
spectrum width by 25ps- and 50ps-width Gauss pulse injection. The generated monocycle pulses can be controlled by
either optical Gauss pulse or both injections, which function as logic OR-monocycle. It has potential applications in
UWB-over-fiber communications.

Design of all-optical UWB monocycle generation for UWB-over-fibre communications

A novel and simple non-return-to-zero differential phase shift keying (NRZ-DPSK) wavelength division multiplexing (WDM) system, which can simultaneously demultiplex and demodulate multiple wavelengths, is proposed and investigated in this paper. The phase-to-intensity demodulation principle is based on detuned filtering, which is achieved by using a single commercial array waveguide grating (AWG) in our scheme. By properly choosing appropriate AWG channels at the transmitter, the AWG at the receiver can act as both the demultiplexer and the demodulator of the DPSK signals. Simulations at 10, 20, and 40 Gbit/s show good flexibility and performance for the proposed system.

Proposal for a novel and simple WDM NRZ-DPSK system

Tunable all-optical wavelength converters have been regarded as key optoelectronic devices in intelligent next generation optical networks. Novel scheme for tunable wavelength conversion based on semiconductor optical amplifier (SOA)-based fiber ring laser is proposed out firstly. Theoretical model for this novel scheme is established. Conversion output characteristics are calculated theoretically and measured experimentally in different conditions. Experimental results are coincidence with theoretical results. Tunable wavelength conversion at 2.5Gb/s with 40nm tuning range is demonstrated experimentally. Experimental and theoretical studies show that this novel scheme is worthy of further investigation.

Xinliang Zhang

Papers

On-chip optical pulse shaper and its application in arbitrary waveform generation

Ultrawideband doublet generation from NRZ-DPSK signals

Design of all-optical UWB monocycle generation for UWB-over-fibre communications

Proposal for a novel and simple WDM NRZ-DPSK system

Tunable and self-probed wavelength conversion in SOA-based fiber ring laser