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

We propose a novel polarization-maintaining few-mode fiber by introducing four identical circular side-holes surrounding an elliptical ring core. In addition to the parameters of ring core, this kind of fiber endows new degrees of freedom to adjust the modal birefringence of guided modes. Numerical simulations indicate that such fiber could support 10 distinct polarization modes, including the effectively separated fundamental modes. The influences of side-hole size and positions on the polarization-maintaining property are investigated for the 10-mode fiber. With appropriate parameters of side-holes, the minimum effective refractive difference (Δneff) between adjacent modes is 1.65 × 10−4 at 1550 nm. Compared with the fiber excluding side-holes, the modal effective refractive indexes are decreased while most Δneff values are enlarged in the proposed fiber. Moreover, all the Δneff values could be higher than 1.52 × 10−4 over a bandwidth ranging from 1510 nm to 1630 nm. The chromatic dispersion covering the broadband is analyzed subsequently. Furthermore, 16 vector modes could be guided in the proposed fiber structure with modified parameters. The proposed fiber is capable of supporting extended modes and might be a promising candidate toward high–capacity spatial-division-multiplexing communications.

Hole-assisted polarization-maintaining few-mode fiber

We present a polarization-maintaining photonic bandgap fiber (PBGF) in terahertz (THz) regime, consisting of a quasi-elliptical hollow-core (QEHC) and triangularly arranged hexagonal holes in cladding. Thanks to the specially designed core shape, the QEHC-PBGF can support two stable polarization-maintaining fundamental modes. From numerical simulations, birefringence, confinement loss and group velocity dispersion of the fundamental mode group are 9.4 × 10−4, 3 × 10−3 cm−1 and 0.39 psTHz−1 cm−1 at 0.9 THz, respectively. Moreover, we investigate the fiber fabrication tolerance in the context of polarization-maintaining and confinement performances. The proposed QEHC-PBGF is anticipated to be useful in polarization sensitive THz applications.

Polarization-maintaining terahertz bandgap fiber with a quasi-elliptical hollow-core

This letter firstly and experimentally demonstrated a fiber laser sensing system using Michelson interferometer (MI) based on few-mode elliptical multilayer-core fibers (EMCFs) for liquid level and refractive index (RI) measurement The fiber sensing system has high optical signal-to-noise ratio ( $\sim 45$ dB) and narrow 3-dB bandwidth (<01 nm) Four water–glycerin solutions of different RI are used for this experiment, and the higher sensitivity is acquired for the higher RI liquid The sensitivities for the liquids with indices of 1333, 1353, 1373, and 1393 are 305, 356, 411, and 506 pm/mm within a depth range of 0–45 mm, respectively Moreover, the RI of liquid can also be measured, and a sensitivity of 329222 pm/mm/RIU is experimentally achieved

Detection of Liquid Level With an MI-Based Fiber Laser Sensor Using Few-Mode EMCF

A periodic tungsten-covered dielectric ridge on substrate assembly is proposed in this article to investigate the perfect absorption in the near-infrared regime. The localized dipolar resonance and the propagating surface plasmon mode significantly restrict the reflectance thus forming two discrete absorption peaks. In the process of discovering perfect absorption, the geometrical parameters such as the tungsten layer thickness, ridge depth and width, substrate spacer thickness and the lateral period are correspondingly numerically explored and a rather high absorption rate of 99.9% can be achieved. Furthermore, the absorption peak is sensitive to the ground substrate index which makes it a potential candidate for compactly integrated on chip sensing applications. The polarization direction of the incident light modulates the absorption to a wider bandwidth and the oblique incidence splits the propagating mode along with a rather weak Fano-type absorption peak. The research may pave way for transition metal integrated on chip absorption system.

Shuisheng Jian

Papers

Hole-assisted polarization-maintaining few-mode fiber

Polarization-maintaining terahertz bandgap fiber with a quasi-elliptical hollow-core

Detection of Liquid Level With an MI-Based Fiber Laser Sensor Using Few-Mode EMCF

A Near-Infrared Perfect Absorber Assisted by Tungsten-Covered Ridges

Tunable multi-wavelength fiber laser based on a polarization-maintaining erbium-doped fiber and a polarization controller