Core Refractive Index Research Articles

Assessment of the sensitivity of core / shell parameters derived using the single-particle soot photometer to density and refractive index

Abstract. Black carbon (BC) is the dominant absorbing aerosol in the atmosphere, and plays an important role in climate and human health. The optical properties and cloud condensation nuclei (CCN) activity of soot depend on the amounts (both relative and absolute) of BC and non-refractory material in the particles. Mixing between these two components is often represented in models by a core / shell coated sphere. The single-particle soot photometer (SP2) is one of, if not the only, instrument capable of reporting distributions of both core size and coating thickness. Most studies combine the SP2's incandescence and 1064 nm scattering data to report coating properties, but to date there is no consistency in the assumed values of density and refractive index of the core that are used in these calculations, which can greatly affect the reported parameters such as coating thickness. Given that such data are providing an important constraint for model comparisons and comparison between large data sets, it is important that this lack of consistency is addressed. In this study we explore the sensitivity of the reported coatings to these parameters. An assessment of the coating properties of freshly emitted, thermodenuded ambient particles demonstrated that a core density of 1.8 g cm−3 and refractive index of (2.26–1.26i) were the most appropriate to use with ambient soot in the Los Angeles area. Using these parameters generated a distribution with median shell / core ratio of 1.02 ± 0.11, corresponding to a median absolute coating thickness of 2 ± 8 nm. The main source of statistical error in the single-particle data was random variation in the incandescence signals. Other than the sensitivity to core refractive index, the incandescence calibration was the main source of uncertainty when optically determining the average coatings. The refractive index of coatings was found to have only a minor influence. This work demonstrates that using this technique the SP2 can accurately determine the average mixing state (externally or internally mixed) of ambient soot within the precision of the instrument calibration. Ambient coatings were measured up to a median shell / core ratio of 1.50 ± 0.11, meaning that this technique is able to resolve absolute changes in mixing state. However, when different core parameters were used, the core / shell ratio and the coating thickness were shown to be offset by amounts that could be larger than the atmospheric variability in these parameters, though the results have a similar precision. For comparison, using the core parameters that resulted in the thickest coatings, on the same thermodenuded fresh particles as before, generated a median shell / core ratio of 1.39 ± 0.11, corresponding to a median absolute coating thickness of 30 ± 8 nm. These results must be taken into account when comparing BC coatings measured using this technique, or if using these data for optical or CCN calculations. We have determined the most appropriate values of BC density and refractive index to use to measure mixing state at 1064 nm where particle morphology has only a minor effect, but appropriate values to use for optical calculations of nonspherical particles at visible wavelengths will also be subject to similar, significant uncertainties. Without similar constraints as those provided here, constraining the behaviour of BC particles in models using field data will be subject to large systematic measurement uncertainties.

Improved time-resolved acousto-optic technique for optical fiber analysis of axial non-uniformities by using edge interrogation.

The time-resolved acousto-optic technique demonstrated recently to be a very useful method for the analysis of fiber axial non-uniformities, able to detect variations of fiber diameter in the nanometric scale with a spatial resolution of few cm. An edge interrogation approach is proposed to improve further the performance of this technique. The detection of subnanometer fiber diameter changes or sub-ppm changes of the core refractive index is demonstrated.

Self-compression of coupled cnoidal waves

In this work, the dynamics of a pulse train propagating through a bimodal optical fiber whose core refractive index varies periodically along its main axis are studied numerically. The mathematical model is based on a coupled system of nonlinear Schrödinger-like equations. First- and second-order dispersions are taken into account, as well as the number of solitons propagating through the fiber. The physical conditions for self-compression of coupled cnoidal waves are obtained and discussed.

Rigorous Analysis on Ring-Doped-Core Fibers for Generating Cylindrical Vector Beams

We propose a novel active fiber design for selectively generating cylindrical vector beams (CVBs) or cylindrical vector modes (CVMs) which can be applied to conventional fiber lasers. A fiber is designed to have a ring-shaped core refractive index profile which can lead to the best overlap between the active dopant distribution profile and the lowest-order CVM (LCVM) field profile. Therefore, the overlap factor (OVF) of the LCVM becomes even higher than that of the fundamental mode. We emphasize that this condition cannot be satisfied by a conventional step-index core fiber (SICF) but by the ring-doped core fiber (RDCF). Because the lasing threshold is inversely proportional to the OVF, the LCVM can predominantly be stimulated even without going through special procedures to impose extra loss mechanisms to the fundamental mode. We numerically verify that the OVF of the LCVM with the doped ions can significantly exceed that of the fundamental mode if the proposed fiber design is applied. In addition, an RDCF of the proposed fiber design can also operate in a regime containing no higher-order modes besides the LCVM, so that it can selectively and efficiently generate the LCVM without being disrupted by the parasitic lasing of the higher-order modes. We highlight that an optimized RDCF can lead to a >30 % higher OVF ratio than a SICF having the same doped area. The proposed model is expected to be useful for enhancing the efficiency of generating CVBs in an all-fiber format.

Fabrication of Y-Splitters and Mach–Zehnder Structures on (Yb,Nb):RbTiOPO<inline-formula><tex-math> $_{\bf 4}$</tex-math></inline-formula>/RbTiOPO<inline-formula><tex-math>$_{\bf 4}$</tex-math></inline-formula> Epitaxial Layers by Reactive Ion Etching

Reactive ion etching of RbTiOPO4 (0 0 1) substrates and (Yb,Nb):RbTiOPO4/RbTiOPO4 (0 0 1) epitaxial layers has been performed using fluorine chemistry. A maximum etch rate of 8.7 nm/min was obtained, and the deepest etch achieved was 3.5 μm. The (Yb,Nb)-doped epitaxial layers showed a slower etching rate when compared with undoped material. Liquid phase epitaxial growth of cladding layers has also been performed, resulting in a high-quality interface growth without appreciable defects. 9-mm-long Mach–Zehnder interferometer and 9-mm-long Y-splitter structures were designed and patterned in RbTiOPO4 substrates and (Yb,Nb):RbTiOPO4/RbTiOPO4 (0 0 1) epitaxial layers. The structures fabricated in RbTiOPO4 substrates were filled with laser active (Yb,Nb):RbTiOPO4 higher refractive index core material, and finally an RbTiOPO4 cladding was grown on the samples. The refractive index difference between the (Yb,Nb):RbTiOPO4 layer and the RbTiOPO4 substrate at 1.5 μm has been measured and optical waveguiding at this wavelength has been demonstrated.

Large Mode Area Multi-Trench Fiber With Delocalization of Higher Order Modes

Multi-trench fiber (MTF) is a novel large mode area fiber design for high power fiber laser applications. This fiber design allows very high suppression of the higher order modes by offering high losses and delocalizing them out of the core. MTFs allow the core refractive index to be higher than the surrounding cladding as compared to other structures such as photonic crystal fibers, photonic bandgap fibers, and Bragg fibers. This feature of MTFs dramatically reduces the complexity associated with doped fiber fabrication. The MTF design is an all-solid structure with cylindrical symmetry, which provides easy cleaving and splicing with other fibers. In this paper, we present the first experimental demonstration of the MTF. S $^{2}$ measurements indicate single mode operation with very high suppression of the higher order modes.

Optical delay lines using coupled slab waveguides and ring resonator with a negative refractive index core

Total delay time of a structure composed of a slab waveguide coupled with a ring resonator where negative refractive index material is replaced in the core of the structure is investigated in this work. In this paper, a two-port ring resonator (TPRR) which is made of a core with negative refractive index has been used to generate a time delay for a Gaussian-shaped pulse with 1GHz bandwidth. It is shown that the creation of the ring how causes more ng of a straight waveguide and results are compared with positive refractive index core TPRR. We have used metamaterial to make an n<0 media and have used two cascaded metamaterial rings to increase the bandwidth.

Impact of core dielectric properties on the localized surface plasmonic spectra of gold-coated magnetic core-shell nanoparticles.

Gold-coated iron oxide core-shell nanoparticles (IO-Au NPs) are of interest for use in numerous biomedical applications because of their unique combined magnetic-plasmonic properties. Although the effects of the core-dielectric constant on the localized surface plasmon resonance (LSPR) peak position of Au-shell particles have been previously investigated, the impact that light-absorbing core materials with complex dielectric functions have on the LSPR peak is not well established. In this study, we use extended Mie theory for multilayer particles to examine the individual effects of the real and imaginary components of core refractive indices on Au-shell NP plasmonic peaks. We find that the imaginary component dampens the intensity of the cavity plasmon and results in a decrease of surface plasmon coupling. For core materials with large imaginary refractive indices, the coupled mode LSPR peak disappears, and only the anticoupled mode remains. Our findings show that the addition of a nonabsorbing polymer layer to the core surface decreases the dampening of the cavity plasmon and increases LSPR spectral intensity. Additionally, we address apparent discrepancies in the literature regarding the effects of Au-shell thickness on LSPR peak shifts.

Analysis of fabrication results for 17 × 17 polymer arrayed waveguide grating multiplexers with flat spectral responses

Based on transmission theory, a 17 × 17 polymer arrayed waveguide grating (AWG) multiplexer parameter optimization is performed, and the influence of the fabrication results on the transmission characteristics are analyzed. In this paper, we mainly discuss three of the main errors in the fabrication of polymer AWG devices. One is δn1, which is caused by the tuning of the core refractive index n1, the second is δb, which results from the rotating-coating of the core thickness b, and the other is the non-ideal core cross-section, which is caused by steam redissolution. The effects of the above fabrication errors on the transmission characteristics of the AWG device are investigated, and compensation techniques are proposed. By comparing the theoretical simulation and experimental results, the shift in the transmission spectrum is reduced by 0.028 nm, the 3 dB bandwidth is increased by about 0.036 nm, the insertion loss is reduced by about 3 dB for the central channel and 4.5 dB for the edge channels, and the crosstalk is reduced by 1.5 dB.

Joint Residual Stress/Refractive Index Characterization of Large-Mode-Area Erbium-Doped Fibers

The residual stress (RS) and refractive index (RI) distributions of large-mode-area (LMA) erbium-doped fibers (EDFs) are measured for the first time using a high-resolution three-dimensional index-stress distribution measurement method. The effects of fiber manufacturing, cleaving, and arc fusion splicing of a commercially available LMA EDF are concurrently characterized at state-of-the-art stress, index, and spatial resolutions. RS induced via fiber manufacturing results in RI changes as large as <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$1.2 \times 10^{-4}$</tex></formula> RI units through the photo-elastic effect. The relaxation of RS within <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim} 30$</tex></formula> <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu$</tex> </formula> m of a cleaved end-face reverses the stress-induced RI changes formed during fiber manufacturing. After fusion splicing, an even larger stress-induced RI change of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$3.5 \times 10^{-4}$</tex></formula> RI units results over an axial distance on the order of millimeters. The diffusion of core dopants reduces the maximum core RI by as much as 21.7% along a transition region length of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\sim} 400$</tex></formula> <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\mu$</tex></formula> m. These measurements represent the first of many required to develop future ultra LMA EDFs where RS effects and dopant diffusion are absolutely critical for fiber design and performance.

Modal dispersion, pulse broadening and maximum transmission rate in GRIN optical fibers encompass a central dip in the core index profile

Intermodal and intramodal dispersions signify one of the problems in graded-index multi-mode optical fibers (GRIN) used for LAN communication systems and for sensing applications. A central index dip (depression) in the profile of core refractive-index may occur due to the CVD fabrication processes. The index dip may also be intentionally designed to broaden the fundamental mode field profile toward a plateau-like distribution, which have advantages for fiber–source connections, fiber amplifiers and self-imaging applications. Effect of core central index dip on the propagation parameters of GRIN fiber, such as intermodal dispersion, intramodal dispersion and root-mean-square broadening, is investigated. The conventional methods usually study optical signal propagation in optical fiber in terms of mode characteristics and the number of modes, but in this work multiple-beam Fizeau interferometry is proposed as an inductive but alternative methodology to afford a radial approach to determine dispersion, pulse broadening and maximum transmission rate in GRIN optical fiber having a central index dip.

Designed Synthesis of SiO2/TiO2 Core/Shell Structure As Light Scattering Material for Highly Efficient Dye-Sensitized Solar Cells

SiO2/TiO2 core/shell nanoparticles (STCS-NPs) with diameters of 110, 240, and 530 nm were fabricated to investigate the influence of the size and refractive index of light-scattering particles on light-scattering properties. The optical properties of STCS-NPs were evaluated and compared with SiO2-NPs and TiO2-NPs. The structure of STCS-NPs, consisting of a low refractive index core and high refractive index shell, provides efficient light scattering. The optimized anode film with STCS-NPs had ca. 20% improved power conversion efficiency (PCE).

Polarization and modal filters based on W-fibers Panda for fiber-optic gyroscopes and high-power fiber lasers

Polarizing Panda-type W-fibers, practical realizations are presented for fiber-optic gyroscope (FOG) sensing coil (500-m length, 8-μm fundamental mode field diameter) and for polarization/modal filtering at the FOG loop interferometer input (∼1-m length, 10-μm fundamental mode field diameter). Using these fibers, two different mathematical models for light propagation are tested and applied for further dichroism spectral band spreading of polarizing W-fibers with fundamental mode field diameter (MFD) of 6 to 10 μm for FOG coil and at least with MFD=8 to 10 μm for input filtering in loop interferometer. Also, some fibers with large MFD are considered, which may be useful for high-power fiber lasers, requiring the low nonlinear effects. One is a practically realized single-mode 200-m W-fiber Panda with dichroism and MFD=12.4 μm at 1.06 μm, which is bend resistant for 60-mm diameters. For further MFD increase a straight 100-mm length modal and polarization filters are suggested with MFD=30 to 70 μm at 1.06 μm, based on a pure silica core fiber, narrow/shallow fluorine cladding, and stress-applying parts. The latter are used for core refractive index regulation by birefringence (W-profile forming), which is more precise than with core germanium doping.

Calculation and simulation of optical fiber core and cladding mode effective refractive index

The accurate computation of effective refractive index of optical fiber core and the cladding mode are needed for the fiber structure design, mode dispersion analysis and fiber grating mode coupling research. In this paper, based on three layered optical fiber waveguide model and its accurate eigenvalue equation, by using the cut string algorithm the effective refractive index of core mode is solved. At the same time the computation results are verified by using COMSOL software. By using interval traversal algorithm the effective refractive index of cladding mode is computed. Compared with the existing algorithms, this method can avoid the loss of root during the calculation. It can also avoid the generation of odd point and can keep the normality of each mode. The simulation is based on Mathematica software. The curves relating effective refractive index of core and cladding mode to wavelength are obtained.

Manufacturing Error in the Application of Improving Spectrum for a Vertical Coupling Microring Resonator

All the effects of manufacturing errors on transmission spectrum are analyzed for a vertical coupling microring resonator (MRR). Results show that the core refractive index, core thickness and radius errors of ring result in central wavelength shift of the transmission spectrum, and the central deviation and thickness between the ring and channel lead to nonresonant light intensity change. In order to obtain satisfactory spectrum for a fabricated MRR device, some allowed manufacturing errors are discussed, and the accumulation and compensation of manufacturing errors are investigated.

Effect of change in core width and core refractive index on the switching behavior of a nonlinear Mach–Zehnder interferometer

The effect of variation of core width and core refractive index on output versus input (O-I) and transmission coefficient versus input (T-I) characteristics of a nonlinear Mach-Zehnder interferometer (NMZI) were investigated for the first time. Beam propagation method has been used for the present investigation. Change of core width and/or core refractive index adds extra liberty for changing the operating power of an NMZI to a desired value. Moreover, it is revealed for the first time that use of only the O-I or T-I characteristic presents an incomplete picture of NMZI switching; both O-I and T-I characteristics of both balanced/imbalanced NMZIs are indispensible for complete understanding of NMZI switching.

Fiber structure to convert a Gaussian beam to higher-order optical orbital angular momentum modes

We propose a fiber structure of a square core and ring refractive index profile that converts an input circular polarized Gaussian mode into optical orbital angular momentum (OAM) modes. By breaking the circular symmetry of the waveguide, the input circularly polarized fundamental mode in the square core can be coupled into the ring region to generate higher-order OAM modes, corresponding to the transference of spin angular momentum and orbital angular momentum. We show, by using simulation, the generation of OAM modes with a topological charge l up to 9 using <10 mm long fiber. The mode purity is above 96.4% and the extinction ratio can be 30 dB.

Single-Mode Design Guidelines for 19-Cell Double-Cladding Photonic Crystal Fibers

Yb-doped double-cladding photonic crystal fibers have become key components for power scaling in fiber laser systems, by providing many advantages, especially an ultra large effective area. The single-mode regime, which is a mandatory requirement for high quality laser beams, can be obtained in such large core active fibers only through a careful design. In this paper the cut-off properties of 19-cell photonic crystal fibers have been thoroughly investigated with the avoided-crossing approach, in order to find guidelines for the design of single-mode fibers. The air-hole diameter and the core refractive index have been changed, as well as the number of air-hole rings in the fiber inner cladding. Simulation results have shown that, regardless of the air-hole ring number, the guided-mode cut-off properties are strongly influenced by the main design parameters, especially by the core refractive index. In particular, a wider single-mode wavelength range can be obtained in 19-cell fibers with small air-holes and low core refractive index. Moreover, double-cladding photonic crystal fibers with larger inner-cladding provide better guided-mode cut-off properties, which can have positive effects on the amplification process in practical applications.

All Optical Tunable Nematic Liquid Crystal Waveguide

In this paper we theoretically investigate the nonlinear behaviour of a rectangular liquid crystal waveguide. An optical beam of a C-Band laser, fiber coupled to the waveguide, modulates the refractive index of the liquid crystal core. Light propagation is obtained when the effective refractive index of the core exceed the cladding refractive index one. The cut-off condition can be determined by the initial alignment of the liquid crystal molecules and by the laser beam itself that excites optical nonlinearity due to optically induced reorientational effect. Low power optical control could be obtained with an optimization of the molecular alignment.

Design of large-core single-mode Yb3+-doped photonic crystal fiber

The effective index of the cladding fundamental space-filling mode in photonic crystal fiber (PCF) is simulated by the effective index method. The variation of the effective index with the structure parameters of the fiber is achieved. For the first time, the relations of the V parameter of Yb3+-doped PCF with the refractive index of core and the structure parameters of the fiber are provided. The single-mode characteristics of large-core Yb3+-doped photonic crystal fibers with 7 and 19 missing air holes in the core are analyzed. The large-core single-mode Yb3+-doped photonic crystal fibers with core diameters of 50 μm, 100 μm and 150 μm are designed. The results provide theory instruction for the design and fabrication of fiber.