Large Index Difference Research Articles

Integrated subwavelength bimodal interferometer using a multilayer hyperbolic metamaterial

Bimodal interferometry can be implemented in a photonic integrated waveguide by inserting structures supporting—at least—two modes to connect an input and an output single-mode waveguide. The length of the bimodal section is inversely proportional to the index difference between the involved modes, which can be quite small in multimode dielectric waveguides. We propose and numerically demonstrate an ultrashort bimodal interferometer by embedding a multilayer hyperbolic metamaterial in a subwavelength gap separating two dielectric waveguides. We use the large index difference (>1.5) between the bulk and the plasmonic-guided modes of the metamaterial to reduce the total length of the interferometer to less than 1 µm. Our system, which is potentially fabricable with standard nanofabrication tools, could be used to build ultra-compact integrated bimodal interferometers for signal processing and biosensing.

Light Scattering Mechanisms in Few-Mode Fibers

In this study, the contributions of light scattering and extra-loss mechanisms to the total losses and to the differential mode attenuation of the guided modes are quantified for three different few-mode fibers. The tested fibers have the same modal content but different refractive index profiles, namely step, trapezoidal and parabolic. It is demonstrated that a trapezoidal index profile is a good compromise between low attenuation, low differential mode attenuation while maintaining large effective index differences between modes so as to reduce mode coupling.

Design of ring-assisted few-mode fiber with low loss for improved mode-division multiplexing

We report a ring-assisted few-mode fiber design with large mode effective index difference (△) and low loss. By introducing the low-index ring-assisted structure in the cladding, the effective indexes of LP11, LP21 and LP02 modes can be greatly reduced, while the effective index of LP01 mode is almost unaffected. Also, we find that introducing multiple ring-assisted structures will help reduce loss. The proposed few-mode fiber has potential application value for improved mode-division multiplexing optical fiber communication system.

Design of negative curvature fiber carrying multiorbital angular momentum modes for terahertz wave transmission

We design a novel negative curvature fiber (NCF) to support terahertz orbital angular momentum (OAM) modes. Numerical models are constructed to evaluate the transmission performances of vector modes in the frequency range of 0.40–0.80 THz. The designed fiber made from only one kind of heat-resistance resin (HTL) material can support 50–52 OAM modes with large effective index differences above 10−4 and low confinement loss (in the level of 10−15 dB/cm). Some other significant characteristics of the designed NCF including waveguide dispersion, OAM purity, and effective mode field area (Aeff), are also calculated and analyzed with their effects and applications on terahertz OAM guidance. The optimized absolute value of dispersion is between 0.33 and 0.8 ps/(THz × cm) in the range of 0.45–0.75 THz. The high mode purity (85%−95%) in the range of 0.60–0.80 THz further promotes the mode division multiplexing in OAM fiber. The proposed NCF can offer great potential in high-capacity terahertz communication based on OAM multiplexing.

Lattice-matched III-nitride structures comprising BAlN, BGaN, and AlGaN for ultraviolet applications

The optical properties of BAlN, BGaN and AlGaN ternary alloys are investigated using hybrid density functional for the design of lattice-matched optical structures in the ultraviolet spectrum. The calculated AlGaN properties agree well with previous reports, validating the model. A peculiar non-monotonic behavior of the refractive index as a function of the boron composition is found. The results of this calculation are interpolated to generate a three-dimensional dataset, which can be employed for designing a countless number of lattice-matched and –mismatched heterostructures. These heterostructures could span a range of operating wavelengths well into the deep ultraviolet with refractive indices ranging from 1.98 to 2.41 for AlN at 0 eV and GaN near the GaN bandgap, respectively. An example is shown where a lattice-matched heterostructure, AlN/B0.108Ga0.892N, is applied for DBR applications with a large index difference. A DBR comprising the AlN/B0.108Ga0.892N heterostructure at the UV wavelength of 375 nm is found to exceed 93% peak reflectivity with only 10 pairs and reaches 100% reflectivity with 35 pairs. For a chosen design with 25 pairs, the DBR has a peak reflectivity of 99.8% and a bandwidth of 26 nm fulfilling the requirements of most devices especially ultraviolet vertical-cavity surface emitting lasers.

Hybrid Plasmonic Surface Lattice Resonance Perovskite Lasers on Silver Nanoparticle Arrays

AbstractThe hybrid plasmonic surface lattice resonance (SLR) laser constructed by a MAPbBr3 perovskite thin film on the Ag nanoparticle array is unambiguously demonstrated in this research. The relatively high refractive index of the perovskite thin film provides an excellent coupling between the photonic mode and SLR, leading to a high spontaneous emission coupling factor and a low threshold lasing. Furthermore, a novel spin‐coating process applied to grow the MAPbBr3 perovskite thin film can leave air gaps under the perovskite that provides a large index difference in the periodic array, making the hybrid plasmonic SLR exhibits a high density of state, which is beneficial for laser operation. Via theoretical design and experimental verification, the lasing behaviors of the hybrid plasmonic SLR perovskite laser show excellent characteristics with a fixed polarization. This demonstration facilitates an enhanced lasing performance and realization of the low‐cost and low‐energy‐consumption laser application.

Epsilon-near-zero photonic crystal fibers for a large mode separation of orbital angular momentum modes

An orbital angular momentum (OAM) mode is a good candidate as an orthogonal basis mode set for mode division multiplexing (MDM) owing to its unique phase properties. Photonic crystal fibers support large numbers of OAM modes with notable performances. However, the effective index difference between high order OAM modes is not large enough. In this work, we propose two types of epsilon-near-zero (ENZ) materials-embedded circular photonic crystal fibers for high mode separation. Compared with previously developed photonic crystal fibers, the designed fibers exhibit a high refractive index contrast owing to the inserted ENZ materials. The designed fibers support up to 42 OAM modes. Most of these OAM modes provide a large effective index difference (Δneff > 10−3) while maintaining a radial sing mode condition over a wide wavelength range from 1.3 μm to 1.8 μm. The proposed fibers could potentially be exploited in MDM for stable transmission of OAM modes and other OAM-based applications.

Hollow Silica Photonic Crystal Fiber Guiding 101 Orbital Angular Momentum Modes Without Phase Distortion in C+L Band

We propose a new hollow ring core silica photonic crystal fiber that can support 101 orbital angular momentum (OAM) modes, maintaining a high mode quality without phase distortion, a low confinement loss, and a large effective index difference between the adjacent modes, which could open a new avenue of OAM mode multiplexing applications. The fiber consists of three layers: the circular central air hole, silica ring core, and circumferencing silica-air hole porous inner cladding. Using the full-vectorial finite element method (FEM), the modal characteristics of individual OAM modes in the proposed fiber were thoroughly analyzed by varying the number of air-holes in the circularly symmetric cladding. We found a general selection rule for the number of air-holes in the first layer and the topological charge to cause the phase distortion of OAM modes, for the first time. The phase distribution of the guided OAM modes was thoroughly investigated to select usable modes for mode division multiplexing. Parametric analyses of the proposed PCF are reported to optimize optical properties of the OAM modes.

On-Chip Switch and Add/Drop Multiplexer Design With Left-Handed Behavior in Photonic Crystals

An integrated optical switch consisting of a Mach-Zehnder Interferometer with photonic crystals-based phase shifters is designed for telecom wavelength range. The phase modulation along the two arms of the interferometer comes from the photonic band-to-band transition on the photonic crystals that leads to a large effective index difference and results in a switching/modulation with a low-voltage, small optical interaction length, and a low insertion loss. Moreover, shifting along the same photonic band is utilized for constructing a fully reconfigurable add-drop multiplexer for tunable wavelength range with a broadband response.

Two-Layer Erbium-Doped Air-Core Circular Photonic Crystal Fiber Amplifier for Orbital Angular Momentum Mode Division Multiplexing System

Orbital angular momentum (OAM) mode-division multiplexing (MDM) has recently been under intense investigations as a new way to increase the capacity of fiber communication. In this paper, a two-layer Erbium-doped fiber amplifier (EDFA) for an OAM multiplexing system is proposed. The amplifier is based on the circular photonic crystal fiber (C-PCF), which can maintain a stable transmission for 14 OAM modes by a large index difference between the fiber core and the cladding. Further, the two-layer doped region can balance the amplification performance of different modes. The relationship between the performance and the parameters of the amplifier is analyzed numerically to optimize the amplifier design. The optimized amplifier can amplify 18 modes (14 OAM modes) simultaneously over the C-band with a differential mode gain (DMG) lower than 0.1 dB while keeping the modal gain over 23 dB and noise figure below 4 dB. Finally, the fabrication tolerance and feasibility are discussed. The result shows a relatively large fabrication tolerance in the OAM EDFA parameters.

A novel micro-structured fiber for OAM mode and LP mode simultaneous transmission

In this paper, a novel dual-guided microstructured fiber supporting orbital angular momentum (OAM) mode and linear polarization (LP) mode transmission is proposed to solve the high-volume data transmission. A variety of parameters of the fiber are considered comprehensively under different conditions by using the full-vector finite element method. The results show that the fiber can support 30 OAM modes and 2 LP modes over the whole C wavelength band by making full use of the air-holes to isolate electromagnetic field. The crosstalk between two channels is lower than that previously reported, and the total dispersion is nearly zero and flat. For instance, the isolation parameter of the EH71 mode reaches up to 86.02 dB and the dispersion coefficient varies from − 0.26 to 1.62 ps/(km nm). The large index difference between core and cladding is beneficial to low crosstalk. In addition, this fiber is easier to fabricate, because the preform needs only stacking technique to adjust the structure geometry size. This fiber can be used in short-distance and large-capacity transmission system.

A low confinement loss double-photonic crystal fibre over 850 nm bandwidth with 26 orbital angular momentum modes transmission

ABSTRACTIn this paper, a new double-photonic crystal fibre (D-PCF) is proposed and numerically investigated, result shows that can support angular momentum (OAM) mode transmission up to 26. Based on As2S3 substrate, the proposed D-PCF is composed of three well-ordered air-hole rings in the cladding and another several air-hole rings at the centre. Due to the large effective index difference greater than 10−4 between eigenmodes, OAM can be well separated with the proposed D-PCF. In addition, the confinement loss of less than 7 × 10−9 dB/m for all eigenmodes is obtained with wavelengths spanning from 1.25 to 2.1 µm, where the optical wave can be confined closely within the core of D-PCF. With these beneficial properties, the designed D-PCF could enable large-capacity transmission in high-speed communications.

Design of a circular photonic crystal fiber with square air-holes for orbital angular momentum modes transmission

A new circular photonic crystal fiber (C-PCF) with square air -holes in the cladding is proposed, which can support much more orbital angular momentum (OAM) modes. To optimize the design, the dependence of the number of OAM modes on fiber structure parameters is investigated. The simulation results show that the designed C-PCF can support 46 OAM modes stably over 800 nm bandwidth from 1.2 μm to 2.0 μm. Furthermore, the proposed C-PCF exhibits excellent fiber features, such as large effective index differences (˃10−4) without doping, low confinement loss (lower than 10−9 dB/m), relatively flat dispersion and small nonlinear coefficient (less than 2.58 km−1 W−1) for all eigenmodes. The proposed C-PCF could be a well-promising OAM fiber for mode division multiplexing(MDM) in high capacity fiber communication systems.

Large dynamic range pressure sensor based on two semicircle-holes microstructured fiber

This paper presents a sensitive and large dynamic range pressure sensor based on a novel birefringence microstructured optical fiber (MOF) deployed in a Sagnac interferometer configuration. The MOF has two large semicircle holes in the cladding and a rectangular strut with germanium-doped core in the center. The fiber structure permits surrounding pressure to induce large effective index difference between the two polarized modes. The calculated and measured group birefringence of the fiber are 1.49 × 10−4, 1.23 × 10−4, respectively, at the wavelength of 1550 nm. Experimental results shown that the pressure sensitivity of the sensor varied from 45,000 pm/MPa to 50,000 pm/MPa, and minimum detectable pressure of 80 Pa and dynamic range of better than 116 dB could be achieved with the novel fiber sensor. The proposed sensor could be used in harsh environment and is an ideal candidate for downhole applications where high pressure measurement at elevated temperature up to 250 °C is needed.

Refractive index sensor based on lateral-offset of coreless silica interferometer

A compact, cost-effective and high sensitivity fiber interferometer refractive index (RI) sensor based on symmetrical offset coreless silica fiber (CSF) configuration is proposed, optimized and demonstrated. The sensor is formed by splicing a section of CSF between two CSF sections in an offset manner. Thus, two distinct optical paths are created with large index difference, the first path through the connecting CSF sections and the second path is outside the CSF through the surrounding media. RI sensing is established from direct interaction of light with surrounding media, hence high sensitivity can be achieved with a relatively compact sensor length. In the experimental work, a 1.5mm sensor demonstrates RI sensitivity of 750nm/RIU for RI range between 1.33 and 1.345. With the main attributes of high sensitivity and compact size, the proposed sensor can be further developed for related applications including blood diagnosis, water quality control and food industries.

Dissimilar-fiber long-period fiber gratings: concept and demonstration.

A new type of long-period fiber grating structure that contains dissimilar fiber types is proposed. This dissimilar-fiber long-period fiber grating (DF-LPFG) structure is fabricated by splicing together subperiod sections of dissimilar fiber types. Due to the large index differences that can exist between various fiber types, the resulting DF-LPFGs can be exceptionally short, e.g., a few millimeters. As a demonstration, prototype DF-LPFGs were fabricated by periodically splicing together standard single-mode fiber and dispersion-compensating fiber using a commercially available cleaver and splicer. The resulting DF-LPFGs exhibited clear resonances with attenuations of ∼35 dB and insertion losses of ∼8 dB. To the best of the authors' knowledge, this is the first demonstration of the DF-LPFG concept. Other devices based on this concept may also be realizable.

Suspended Core Fibers for the Transmission of Cylindrical Vector Modes

This paper presents a study of propagation of radially and azimuthally polarized cylindrical vector modes in six-strut suspended core fibers based on finite element simulations. The study shows that large effective index differences in the order of $10^{-3}- 10^{-2}$ of these modes can be achieved in the suspended core fibers with core diameter of less than 2 μ m, material index 1.45 of silica to 2.0 of tellurite, and wavelength of 750 nm, allowing the stable propagation of the first higher order modes in doughnut shape within these fibers. The effective index difference and the field intensity of these cylindrical vector modes can be tuned by selecting appropriate fiber material and core size. The study shows that the suspended core fiber can be a competitive candidate for fiber-based high-resolution stimulated emission depletion nanoscopy application.

폴리머 단일 링 Add/Drop 필터와 지연 도파로로 구성된 튜닝 가능 광 신호 지연기

튜닝 가능한 광 신호 지연기를 설계, 제작 및 특성 측정을 하였다. 광 신호 지연기는 네 개의 폴리머 링 공진기 add/drop 필터들과 그 사이에 배치된 지연 도파로들로 구성되었다. 폴리머 도파로는 한변의 길이가 <TEX>$1.8{\mu}m$</TEX> 인 정사각형 매립 구조이고, 코어와 클래딩의 굴절율은 각각 1.48과 1.37이다. 이와 같은 도파 구조로 인하여 매우 작은 반경의 곡선도파로를 활용함으로써, 콤팩트한 소자를 실현할 수 있다. 각각의 add/drop 필터의 링 공진기 상에 전극을 형성하여 열 광학 효과에 의한 튜닝이 가능하도록 하였다. 측정 결과, 각각의 add/drop 필터를 튜닝함으로써 지연 도파로의 수에 비례하는 지연 시간인 110 ps, 225 ps, and 330 ps를 확인할 수 있다. A tunable optical delay line is designed, fabricated, and characterized. The tunable delay line consists of four polymer-ring add/drop filters with delay waveguides between adjacent ones. The polymer waveguide is a buried structure, designed to be square with core width and height of <TEX>$1.8{\mu}m$</TEX>. The refractive indices of the core and cladding polymer are 1.48 and 1.37 respectively. The large index difference and small cross section of the waveguide enable us to realize a compact device using a small radius of curvature. Four pairs of electrodes are evaporated above the add/drop filters to provide heating currents for thermal tuning. In measurements we can identify variable time delays of 110, 225, and 330 ps in proportion to the number of delay lines.

Photonic crystal fiber for supporting 26 orbital angular momentum modes.

We propose and numerically investigate a photonic crystal fiber (PCF) based on As2S3 for supporting the orbital angular momentum (OAM) modes up to 26. The designed PCF is composed of four well-ordered air hole rings in the cladding and an air hole at the center. The OAM modes can be well separated due to the large effective index difference of above 10-4 between the eigenmodes and maintain single-mode condition radially. In addition, the dispersions of the modes increase slowly with wavelengths, while the confinement loss keeps as low as 10-9 dB/m. The proposed PCF increases the supported OAM modes which could have some potential applications in short-distance, high-capacity transmission.

Colloidal Quantum Dot Integrated Light Sources for Plasmon Mediated Photonic Waveguide Excitation

We operate micron-sized CdSe/CdS core–shell quantum dot (QD) clusters deposited onto gold patches as integrated light sources for the excitation of photonic waveguides. The surface plasmon mode launched by the QD fluorescence at the top interface of the gold patches are efficiently coupled to photonic modes sustained by titanium dioxide ridge waveguides. We show that, despite a large effective index difference, the plasmonic and the photonic modes can couple with a very high efficiency provided the vertical offset between the two kinds of waveguides is carefully controlled. Based on the effective index contrast of the plasmonic and the photonic modes, we engineer in-plane integrated hybrid lenses. The hybrid lenses are obtained by shaping the contact interface between the plasmonic and the photonic waveguides. We demonstrate a 2-fold enhancement of the coupling efficiency for tapers equipped with a hybrid lens. Our results are expected to be useful for the development of low-cost, integrated light sources...