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 an ultrafast all-optical logic AND gate based on two cascaded SOA-OF configurations. Each SOA-OF consists a semiconductor optical amplifier (SOA) followed by an optical filter (OF) which reshapes the spectrum of the modulated probe light. A delayed interferometer (DI) and a tunable band pass filter (TNBPF) are chosen as OFs in the first and second SOA-OF. 40Gbit/s AND operation with 33% duty cycle return-to-zero (RZ) signals has been successfully demonstrated with SOAs whose 10%~90% recovery time are measured much larger than the time duration of one bit period. The quality factor (QF) and the extinction ratio (ER) of the eye diagram of the derived AND results were 6.3 and 8.8dB respectively. Numerical analysis and experimental demonstration with 40Gbit/s nonreturn-to-zero (NRZ) signals is also presented and shows that in order to achieve good AND result at the second stage, the differential time delay of the DI must be shorter than the single bit period of the input signals. The proposed AND gate takes the advantage of high speed WC realized by SOA-OF which displays flexibility to various data rates, pulse width as well as data formats.

Investigation of ultrafast all-optical AND gate based on cascaded SOAs and optical filters

Multiwavelength clock recovery (CR) is desired to perform all-optical parallel processing, which has potential application in optical communication systems that use WDM technology. Fabry–Perot semiconductor optical amplifier (FP–SOA) can perform the similar filter function as passive FP filter, and can simultaneously provide gain for oscillation pulses in the cavity; it is essentially an active filter. We experimentally demonstrated simultaneous multiwavelength all-optical CR from input 36.47-Gb/s pseudo-return-zero (PRZ) data and non-return-zero (NRZ) data using a single multi-quantum-well (MQW) FP-SOA with facets reflectivity of 30%. The presented multiwavelength CR scheme is also suitable for PSK signals. Dual-channel CR from input two channels 36.47-Gb/s 2 23 −1 NRZ-DPSK data located at different wavelength is experimentally demonstrated. This scheme is transparent for data formats, which is very important for next generation optical networks.

Investigation of data-format-transparent multiwavelength all-optical clock recovery using a single FP-SOA

We present a multiwavelength mode-locked fiber ring laser incorporating a semiconductor optical amplifier (SOA) and a Fabry-Perot semiconductor optical amplifie r (FP-SOA). SOA is mode-locked by an injected external optical signal, act as loss modulator, and FP-SOA serves as a tunable comb filter. Our laser source can generate 19 synchronized wavelength channels with the extinction ratio of about 21 dB , each mode-locked at 10 GHz, and modelocked pulse width is about 40 ps. Oscillation wavelengths band and wavelength spacing can be tuned by adjusting bias current and a tunable optical delay line (ODL) or a temperature controller, respectively. Polarization insensitive devices ensure that the output power is rather stable. Keywords: Optical communications; mode-locked lasers; semiconductor optical amplifier (SOA); Fabry-Perot semiconductor optical amplifier (FP-SOA) 1. INTRODUCTION Multiwavelength ultrashort optical pulse source is attractive since it has widespread applications in optical sensing, optical instrumentation, microwave generation, optical signal processing, and high speed optical signal transmission. A large number of approaches have been taken to generate multiwavelength outputs simultaneously, include multiwavelength erbium-doped fiber lasers [1, 2], multiwavelength Raman lasers [3, 4], and semiconductor optical amplifiers (SOAs) fiber lasers [5, 6]. However, due to homoge neous broadening of lasing modes in Er-doped fibers laser, it is difficult to obtain stable multiwavelength light sources. To suppress homogeneous broadening of lasing modes, multiwavelength Er-doped fiber lasers were immersed in liquid nitrogen [7]. In addition, Raman lasers have the advantage for multiwavelength emission over extremely large bandwidth, but, high-power semiconductor laser diodes being fit for various wavebands still need to be developed. Since the SOA has the property of primarily inhomogeneous broadening, it can support simultaneous oscillation of many lasing wavelengths. SOA-based multi-wavelength fiber lasers exhibit stable operation without any assistant measures. The need to extend the bandwidth of dense WDM systems has resulted in research to new transmission waveband, such as longer waveband (L-band, 1570 to 1610nm) within the low-attenuation window. Recently, a single wall nanotube (SWNT) -polyvinyl alcohol (PVA) saturable absorber with a 340 nm wide absorption band peaked at 1600 nm was utilized to realize an ultrafast mode-locked EDF laser in the L- band. This produces 498 fs pulses with a repetition rate of

A novel L-band multiwavelength mode-locked semiconductor fibre laser

The phase regeneration for the mode-division multiplexing (MDM) differential phase shift keying (DPSK) signals in a single multimode silicon waveguide, is proposed and successfully demonstrated in the simulation. The regeneration is based on the mode-selective phase sensitive amplification (PSA). When the pumps and the signal are carried on the same mode, the signal will be phase-regenerated because of the strong phase sensitive interaction. Otherwise, the weak phase sensitive interaction will be achieved due to the phase mismatch. Simulation results show an obvious phase squeezing for the degraded two-mode division multiplexing DPSK signals.

Phase regeneration of phase modulated signals in mode multiplexed system using a silicon waveguide

We propose a polarimeter that combines a plasmonic spiral structure and the machine learning algorithm with an ultra-compact footprint of 10*10µm2. Being different from previous similar schemes working only as circular polarization analyzers, arbitrary states of polarization (SOPs) can be retrieved via the spiral structure for the first time to our best knowledge, by analyzing the near-field intensity distribution through machine learning. A 3-layer neural network (NN) is successfully trained to correlate intensity patterns to the SOPs of incident light. Based on simulation, a low estimation error benchmarked by mean-squared error (MSE) of only 1.23e-3 is achieved. In this way, without the conventional bulky optical system or complex nano-structures, SOP detection is achieved via such a simple and ultra-compact spiral. The proposed scheme not only pushes the application limits of the device based on plasmonic spiral structures but also provides a new insight for SOP detection.

Xinliang Zhang

Papers

Investigation of ultrafast all-optical AND gate based on cascaded SOAs and optical filters

Investigation of data-format-transparent multiwavelength all-optical clock recovery using a single FP-SOA

A novel L-band multiwavelength mode-locked semiconductor fibre laser

Phase regeneration of phase modulated signals in mode multiplexed system using a silicon waveguide

Ultra-compact polarimeter based on a plasmonic spiral assisting by machine learning