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 novel concept for a compact SOI device used to achieve the conversions between the polarization division multiplexing (PDM) and the mode division multiplexing signals (MDM) is proposed and simulated by utilizing a structure combining a 2D grating coupler and a two-mode multiplexer. The detailed design of the proposed device is given and the simulations show the good performance of the device. The proposed device can be useful for accommodating the future fiber optic communication networks which utilizing multiple multiplexing techniques.

Proposal of a Si based device for multiplexing conversion between PDM and MDM

We demonstrate a multiwavelength semiconductor optical amplifier (SOA) fiber ring laser with a dual- pass Mach-
Zehnder interferometer (MZI) filter. Two SOAs provide enough gain and wider gain spectrum for more wavelength
lasing. The dual-pass MZI with a high extinction ratio (ER) serves as comb filter. 82 wavelengths within a power
deviation of 5 dB with the signal-to-noise ratio of 30 dB oscillate simultaneously.

Multiwavelength SOA fiber ring laser incorporating a dual-pass Mach-Zehnder interferometer filter

The function to control the transmission directions of the optical signals in integrated circuits is a fundamental research. Achieving highly functional circuits in which light circulates in a particular direction with satisfied performances are still difficult in pure silicon photonics platform. Here, we propose and experimentally demonstrate a three-port passive device supporting optical ordered-route transmission based on silicon thermo-optic effect for the first time. By injecting strong power from only one port, the light could transmit through the three ports in a strict order (1→2, 2→3, 3→1) while be blocked in the opposite order (1→3, 3→2, 2→1). The blocking extinction ratios and operation bandwidths have been investigated in this paper.

Integrated all-optical three-port circuit of ordered-route transmission

We theoretically investigate the optical rotational Doppler Effect using modal expansion method. We find that the frequency shift content is only determined by the surface of spinning object and the reduced Doppler shift is linear to the change of mode index. The theoretical model makes us better understand the physical processes of rotational Doppler Effect. It can provide theoretical guidance for many related applications, such as detection of rotating bodies, detection of OAM and frequency shift.

Analysis on rotational Doppler Effect based on modal expansion method

A scheme for mitigating patterning effects in wavelength conversion by using a concatenated semiconductor optical
amplifier (SOA) and electroabsorption modulator (EAM) is proposed. The scheme is investigated theoretically and
experimentally. The bit error ratio and the power penalty of the converted output signal are estimated and compared with
the results of wavelength conversion based on cross gain modulation in a single SOA.

Xinliang Zhang

Papers

Proposal of a Si based device for multiplexing conversion between PDM and MDM

Multiwavelength SOA fiber ring laser incorporating a dual-pass Mach-Zehnder interferometer filter

Integrated all-optical three-port circuit of ordered-route transmission

Analysis on rotational Doppler Effect based on modal expansion method

Mitigation of patterning effect in wavelength conversion by cascaded semiconductor optical amplifier and electroabsorption modulator