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

Analysis of 2.5 Gbit/s GPON downlink optical-receiver performance

The high-speed and high-power germanium photodetector (Ge PD) is highly desired for integrated microwave photonics systems. However, the space-charge effect has impeded the improvement of the performance. Here, we propose and demonstrate a high-speed and high-power Ge PD with alleviated space-charge effect, namely carrier-accelerated PD (CA PD). An additional electrical field is introduced to equalize the spatial distribution of photo-generated carriers, and thus the space-charge effect can be alleviated compared with conventional schemes. The CA PD is fabricated in the standard silicon photonics platform and has a small-signal bandwidth of 25 GHz. The carrier dynamics influenced by the additional electrical field is investigated using the impulse photocurrent measurement. In high-power condition, a 1-dB compression current of 1.7 mA can be observed.

Germanium Photodetector With Alleviated Space-Charge Effect

We propose a flat-top bandpass microwave photonic filter (MPF) with flexible tunability of the bandwidth and center frequency based on optical nonlinearities in a Fabry–Perot semiconductor optical amplifier (FP-SOA). Phase-inverted modulation induced by cross-gain modulation (XGM) and optical spectral broadening induced by self-phase modulation (SPM) are exploited to achieve flat-top and bandwidth tuning, respectively. Wideband and continuous tuning of the center frequency is achieved by altering the bias current of the FP-SOA. Experimental results demonstrate a flat-top single-passband MPF with its center frequency tunable from 6.0 to 18.3 GHz by adjusting the bias current from 54.05 to 107.85 mA. The 3-dB bandwidth of the passband when centered at 10.0 GHz is shown to be variable from 680 to 1.43 GHz, by increasing the injected optical power from −1 to +5 dBm. During the bandwidth tuning, the amplitude ripple within the passband is maintained at less than ±0.5 dB. Excellent main to secondary sidelobe ratio exceeding 45 dB is achieved when the MPF is centered at 18.3 GHz.

Flat-top bandpass microwave photonic filter with tunable bandwidth and center frequency based on a Fabry-Pérot semiconductor optical amplifier.

We propose and fabricate a novel four-port polarization diversity coupler for vertical coupling With the help of 45° reflectors and improved 2×1 couplers, an ultralow coupling loss of −48dB is experimentally measured with polarization insensitivity

A four-port polarization diversity coupler for vertical fiber-chip coupling

The invention discloses an optical bandpass filter based on a double microring-Mach Zehnder interference structure. The optical bandpass filter is composed of a first multi-mode interference coupler, a second multi-mode interference coupler, a third multi-mode interference coupler, a first coupling grating, a second coupling grating, a first heating electrode, a second heating electrode, a third heating electrode, a fourth heating electrode, a first upload-download type micro ring, and a second upload-download type micro ring. According to the invention, on the basis of the design of the coupling states of two upload-download type micro rings, fully destructive interference is introduced based on a cascaded adjustable Mach Zehnder interference (MZI) structure; amplitude regulation is realized by adjusting the bias voltage on the heating electrode of the MZI1, so that amplitudes of passband external light signals of an upper arm and a lower arm becomes identical; and phase regulation is realized by adjusting the bias voltage on the heating electrode of the MZI2, so that phases of passband external light signals of the upper arm and the lower arm are opposite. Therefore, the extinction ratio of the filter increases under the circumstance that the passband insertion loss is not increased. The provided optical bandpass filter has advantages of narrow bandwidth, high extinction ratio, and low insertion loss.

Xinliang Zhang

Papers

Analysis of 2.5 Gbit/s GPON downlink optical-receiver performance

Germanium Photodetector With Alleviated Space-Charge Effect

Flat-top bandpass microwave photonic filter with tunable bandwidth and center frequency based on a Fabry-Pérot semiconductor optical amplifier.

A four-port polarization diversity coupler for vertical fiber-chip coupling

Optical bandpass filter based on double microring-Mach Zehnder interference structure