Optical Waveguide Theory

Curious wave phenomena that occur in optical fibres due to the interplay of instability and nonlinear effects are reviewed.

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Instabilities, breathers and rogue waves in optics

Graphene plasmons were predicted to possess ultra-strong field confinement and very low damping at the same time, enabling new classes of devices for deep subwavelength metamaterials, single-photon nonlinearities, extraordinarily strong light-matter interactions and nano-optoelectronic switches. While all of these great prospects require low damping, thus far strong plasmon damping was observed, with both impurity scattering and many-body effects in graphene proposed as possible explanations. With the advent of van der Waals heterostructures, new methods have been developed to integrate graphene with other atomically flat materials. In this letter we exploit near-field microscopy to image propagating plasmons in high quality graphene encapsulated between two films of hexagonal boron nitride (h-BN). We determine dispersion and particularly plasmon damping in real space. We find unprecedented low plasmon damping combined with strong field confinement, and identify the main damping channels as intrinsic thermal phonons in the graphene and dielectric losses in the h-BN. The observation and in-depth understanding of low plasmon damping is the key for the development of graphene nano-photonic and nano-optoelectronic devices.

Highly confined low-loss plasmons in graphene-boron nitride heterostructures

Space-division multiplexing (SDM) uses multiplicity of space channels to increase capacity for optical communication. It is applicable for optical communication in both free space and guided waves. This paper focuses on SDM for fiber-optic communication using few-mode fibers or multimode fibers, in particular on the critical challenge of mode crosstalk. Multiple-input–multiple-output (MIMO) equalization methods developed for wireless communication can be applied as an electronic method to equalize mode crosstalk. Optical approaches, including differential modal group delay management, strong mode coupling, and multicore fibers, are necessary to bring the computational complexity for MIMO mode crosstalk equalization to practical levels. Progress in passive devices, such as (de)multiplexers, and active devices, such as amplifiers and switches, which are considered straightforward challenges in comparison with mode crosstalk, are reviewed. Finally, we present the prospects for SDM in optical transmission and networking.

Space-division multiplexing: the next frontier in optical communication

Optical fiber gratings have developed into a mature technology with a wide range of applications in various areas, including physical sensing for temperature, strain, acoustic waves and pressure. All of these applications rely on the perturbation of the period or refractive index of a grating inscribed in the fiber core as a transducing mechanism between a quantity to be measured and the optical spectral response of the fiber grating. This paper presents a relatively recent variant of the fiber grating concept, whereby a small tilt of the grating fringes causes coupling of the optical power from the core mode into a multitude of cladding modes, each with its own wavevector and mode field shape. The main consequence of doing so is that the differential response of the modes can then be used to multiply the sensing modalities available for a single fiber grating and also to increase the sensor resolution by taking advantage of the large amount of data available. In particular, the temperature cross-sensitivity and power source fluctuation noise inherent in all fiber grating designs can be completely eliminated by referencing all the spectral measurements to the wavelength and power level of the core mode back-reflection. The mode resonances have a quality factor of 105, and they can be observed in reflection or transmission. A thorough review of experimental and theoretical results will show that tilted fiber Bragg gratings can be used for high resolution refractometry, surface plasmon resonance applications, and multiparameter physical sensing (strain, vibration, curvature, and temperature).

Tilted fiber Bragg grating sensors

Design and analysis of novel multilayer-core fiber with large mode area and low bending loss

A novel fiber-laser-based strain sensor is proposed and experimentally demonstrated. The laser cavity is composed of a high-birefringence Sagnac fiber loop mirror (HiBi-SFLM) and a fiber Bragg grating (FBG) which also acts as a strain-sensing element. In the linear region of the HiBi-SFLM reflection spectrum, when the strain applied on the FBG makes the Bragg grating wavelength shift, the laser output power changes due to reflectivity variation of the HiBi-SFLM. Experimental results show that the laser output power varies almost linearly with the applied strain. The measurement of the output power can be performed by a conventional photo-detector.

A novel fiber-laser-based fiber Bragg grating strain sensor with high-birefringence Sagnac fiber loop mirror

The modal characteristics of dual-core photonic crystal fibers are analyzed by a full vector supercell method. The fundamental and second order modes of dual-core PCF consist of a pair of even and odd modes with different polarization, the parity properties of the modal electric field are illuminated. Based on the analysis of parity of modal electric fields, we investigated the vector modal interference in dual-core PCF. The power transfer induced by interference of different mode pair is investigated. It is the interference between two same polarized modes that contribute to the inter-core coupling of power. It is shown that the optical power will oscillate from one core to the other. The dependence of the coupling coefficients on wavelength and structural parameter for different polarization is discussed.

Mode interference in dual-core photonic crystal fibers

A stable single-polarization narrow linewidth single-frequency distributed feedback (DFB) fiber laser is proposed and demonstrated. For the first time, to our knowledge, the tapered FBG written in the photosensitive large-mode-area (LMA) high-concentration erbium-doped fiber (EDF) is applied in the DFB fiber laser. Self-injection locking is used to make the proposed DFB fiber laser operate in the stable single-polarization state and reduce the linewidth of the DFB fiber laser. The LMA high-concentration EDF is fabricated by the modified chemical vapor deposition technique. The tapered FBG can be seen as the equivalent phase shift FBG, and the equivalent $\pi$ phase shift is produced by tapering the uniform FBG directly. The threshold of the DFB fiber laser is about 145 mW, and the maximum output power is 43.55 mW at 450-mW pump power. The measured and modified slope efficiencies of the DFB fiber laser are approximately 14.27% and 15.82%, and the optical signal-to-noise ratio (OSNR) of the laser is over 55 dB. The stable single-polarization and single-frequency operation of the fiber laser is realized, and the measured 20-dB linewidth of DFB fiber laser with self-injection locking and without self-injection locking are approximately 10.3 and 52.3 kHz, respectively.

Single-Frequency and Single-Polarization DFB Fiber Laser Based on Tapered FBG and Self-Injection Locking

Interactions between supercontinuum (SC) light pulses, produced by the propagation of rapidly sequenced picosecond pump laser pulses along a photonic crystal fiber, result in spectral broadening, which we attribute to interpulse soliton collisions. This phenomenon was measured experimentally, following our observation of spectral broadening in numerical simulations that exhibit so-called “pulse wraparound” or “temporal aliasing.” This occurs in simulations with narrow time grids: as early parts of the SC pulse leave the computational time domain, they “reenter” at the beginning and so interact with later parts of the evolving SC pulse. We show that this provides an effective model to predict the experimentally observed spectral changes.

Shuisheng Jian

Papers

Design and analysis of novel multilayer-core fiber with large mode area and low bending loss

A novel fiber-laser-based fiber Bragg grating strain sensor with high-birefringence Sagnac fiber loop mirror

Mode interference in dual-core photonic crystal fibers

Single-Frequency and Single-Polarization DFB Fiber Laser Based on Tapered FBG and Self-Injection Locking

Predicting supercontinuum pulse collisions with simulations exhibiting temporal aliasing