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.

/pdf/photonic-crystal-nanocavity-assisted-rejection-ratio-tunable-1zeivpqv3v.pdf

Photonic crystal nanocavity assisted rejection ratio tunable notch microwave photonic filter

Abstract Germanium-on-silicon (Ge-on-Si) single photon avalanche diodes (SPADs) have received wide attention in recent years due to their potential to be integrated with Si photonics. In this work, we propose and demonstrate a high-performance waveguide coupled Ge-on-Si separate-absorption-charge-multiplication SPAD with three electric terminals. By providing two separate voltage drops on the light absorption and multiplication regions, the drift and multiplication of carriers can be optimized separately. This indeed improves the freedom of voltage regulation for both areas. Moreover, thanks to the separate controlling, doping profile of the charge layer is greatly released compared to that of the conventional device because of the flexible carrier injection. In this scenario, the dark counts of the detector can be largely reduced through decreasing the electric field on the sidewalls of the Ge absorption region without affecting the detection efficiency. The proposed SPAD exhibits a high on-chip single photon detection efficiency of 34.62% and low dark count rates of 279 kHz at 1310 nm with the temperature of 78 K. The noise equivalent power is as low as 3.27 × 10−16 WHz−1/2, which is, to the best of our knowledge, the lowest of that of the reported waveguide coupled Ge-on-Si SPADs. This three-terminal SPAD enables high-yield fabrication and provides robust performance in operation, showing a wide application prospect in applications such as on-chip quantum communication and lidar.

/pdf/high-performance-waveguide-coupled-germanium-on-silicon-2ab5b3si.pdf

High-performance waveguide coupled Germanium-on-silicon single-photon avalanche diode with independently controllable absorption and multiplication

We demonstrate RZ-to-NRZ format conversion at 50Gbit/s based on silicon microring resonator with FSR of 100GHz. Bit error rate measurements show a low power penalty compared to electrical NRZ signal for error free operation.

RZ-to-NRZ format conversion at 50 Gbit/s based on a silicon microring resonator

We proposed a novel extended semiconductor optical amplifier (SOA) model for applications in femtosecond pulse
transmission, in which the various ultra-fast nonlinear effects, such as gain dispersion, group velocity dispersion (GVD)
were taken into account. We use the extended SOA model to analyze the refractive index in response to variation of the
input current and the length of SOA. In addition, the impact of carrier density pulsation and carrier heating on the
refractive index dynamics of SOA was discussed respectively.
The refraction index dynamics that occur in bulk InGaAsP SOA in response to femtosecond pulse (Full width at half
maximum is 200fs) are studied numerically. We use the alpha factors (α-factors) to calculate the phase shift from the
gain variations, which are related to the refractive index changes. We have considered the effects of carrier heating (CH),
and spectrum hole burning (SHB) to analyze the impact of intra-band carrier dynamics on nonlinear refraction index. We
also consider the impact of the length of SOA and operation condition on the refraction dynamics.

Theoretical study of ultrafast index dynamics in semiconductor optical amplifiers

We design and demonstrate a two-dimensional grating coupler (2D GC) with a low polarization dependent loss (PDL) based on the silicon-on-insulator (SOI) platform. Using an elliptical-like etching pattern consisting of two identical semicircles and a rectangle embedded between them, a maximum PDL of the fabricated device is measured to be 0.2 dB covering the C-band, lower than that of most conventional 2D GCs. Besides, a large feature size of 280 nm is beneficial to its manufacturing process and wide application.

Low Polarization Dependent Loss Two-Dimensional Grating Coupler

A pair of polarity-reversed doublet pulses is generated based on a Mach-Zehnder modulator (MZM) and two cascaded semiconductor optical amplifiers (SOAs). The UWB polarity is controlled by adjusting the bias voltage of Mach-Zehnder modulator (MZM). (2 pages)

Xinliang Zhang

Papers

High-performance waveguide coupled Germanium-on-silicon single-photon avalanche diode with independently controllable absorption and multiplication

RZ-to-NRZ format conversion at 50 Gbit/s based on a silicon microring resonator

Theoretical study of ultrafast index dynamics in semiconductor optical amplifiers

Low Polarization Dependent Loss Two-Dimensional Grating Coupler

Ultra-wideband pulse generation based on cascaded semiconductor optical amplifiers