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 propose and demonstrate a tunable Brillouin laser by using an ultrahigh-quality-factor hybrid microbottle resonator. A Brillouin lasing wavelength tuning range of 2.68 nm is realized.

Tunable Brillouin microlaser based on a hybrid microbottle resonator

Phase regeneration of PSK signals is proposed based on the efficient optical parametric amplification process in a highly-nonlinear symmetric hybrid plasmonic waveguide with a nonlinear polymer as the active material in a 150µm long device.

Proposal for phase regeneration of phase-shift keying signals in nonlinear hybrid plasmonic waveguides

χ(2) nonlinear photonics in integrated microresonators

As a resonator-based optical hardware in analog optical computing, a microring synapse can be straightforwardly configured to simulate the connection weights between neurons, but it faces challenges in precision and stability due to cross talk and environmental perturbations. Here, we propose and demonstrate a self-calibration scheme with dual-wavelength synchronization to monitor and calibrate the synaptic weights without interrupting the computation tasks. We design and fabricate an integrated <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1"> <mml:mrow> <mml:mn>4</mml:mn> <mml:mo>×</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:math> microring synapse and deploy our self-calibration scheme to validate its effectiveness. The precision and robustness are evaluated in the experiments with favorable performance, achieving 2-bit precision improvement and excellent robustness to environmental temperature fluctuations (the weights can be corrected within 1 s after temperature changes 0.5°C). Moreover, we demonstrate matrix inversion tasks based on Newton iterations beyond 7-bit precision using this microring synapse. Our scheme provides an accurate and real-time weight calibration independently parallel from computations and opens up new perspectives for precision boost solutions to large-scale analog optical computing. 

Self-calibrating microring synapse with dual-wavelength synchronization

High frame rate of a spectrometer is very important for studying rapidly changing transient phenomena. Dispersive time stretch can map spectral information in the time domain for measurement and thus have a high frame rate compared to conventional spectrometers. To measure the emission spectrum and obtain higher sensitivity, a converging time lens was introduced. The spectro-temporal analyzer realizes the Fourier transform of the incident light field at the focal dispersion position of the time lens, maps the spectral information of the signal to be measured to the time domain, and also realizes the real-time acquisition of the signal spectrum. To further improve the resolution for higher precision detection, a larger pump pulse bandwidth was used to obtain a larger time lens window. Meanwhile, the optical frequency comb with slightly different repetition frequencies is used to sample the focused signal based on four-wave mixing to reduce the sampling bandwidth requirement. On this basis, a parametric time-domain spectrometer based on asynchronous optical sampling was proposed, and the third-order dispersion of the system is compensated. After these operations, the spectral resolution was increased from 20 to 1 pm with a detection bandwidth of 24 nm and a frame rate of 1 kHz. Finally, the random lasing spectral dynamics of EDFA and the thermal drift of the resonance peak of a microring resonator were detected.

Xinliang Zhang

Papers

Tunable Brillouin microlaser based on a hybrid microbottle resonator

Proposal for phase regeneration of phase-shift keying signals in nonlinear hybrid plasmonic waveguides

χ(2) nonlinear photonics in integrated microresonators

Self-calibrating microring synapse with dual-wavelength synchronization

Parametric spectro-temporal analyzer based on asynchronous optical sampling