Step Refractive Index Profile Research Articles

An Elliptical-Core Few-Mode Fiber with Low Loss and Low Crosstalk for the MIMO-FREE Applications

In this invited paper, a novel elliptical-core few-mode fiber (EFMF) with low loss and low Crosstalk is proposed for multiple input multiple output free (MIMO-FREE) applications. The EFMF, which adopts pure silica elliptical core with a step refractive index profile and trench refractive index, can be used in MIMO-FREE operation. The result using a full-vector finite element method shows that the proposed EFMF breaks the mode degeneracy and achieves modes maintaining function. The EFMF performs five non-degenerate modes operations with the modes LP01, LP11a, LP11b, LP21b, and LP21a for the MIMO-FREE applications. The mode crosstalk of the EFMF is effectively reduced using the optimized ellipticity and the large effective refractive index difference between the modes. If the data rate or capacity is conventionally set to a multiple of 2, the transmission system can be upgraded using the EFMF with four modes operations where the effective refractive index difference between any two modes is more than 2 × 10−3. A pure silica core is employed to effectively reduce intrinsic loss. The low bending loss is realized by using the trench refractive index at the cladding. The EFMF can be applied to the good transmission of mode division multiplexing in MIMO-FREE applications while eliminating the difficulty and complexity of the MIMO digital signal processing.

Design of 21-core trench and air-hole assisted multi-core fiber for high speed optical communication

We present a novel 21-core homogeneous multi-core fiber (MCF) structure with three different types of core placement layouts (1-Ring, 2-Ring, and Square Lattice) and densely pack the cores with a minimum cladding diameter of 200 μm to support high-speed applications. In the first phase, three MCF structures (1-Ring, 2-Ring, and Square Lattice) were designed with a step refractive index (RI) profile under the limited cladding thickness and diameter of 35 and 200 μm, respectively, and their crosstalk was calculated. In the second phase, all three structures were designed with a low-RI trench-assisted (TA) profile to significantly reduce the crosstalk. To further suppress the crosstalk, air holes were placed between the TA cores, and it was found that the TA Square Lattice core arrangement with air holes achieved the minimum crosstalk value of approximately −70 dB / 100 km. The impact of important design parameters, such as transmission distance, bending radius, operating wavelength, and core diameter on inter-core crosstalk for all three design structures for both RI profiles and air-hole arrangement were numerically analyzed. Furthermore, other transmission characteristics, such as group delay and dispersion concerning wavelength, were analyzed for the above-stated design structures.

Design of diode laser using slab coupled optical waveguide with graded refractive index structure

Thickness and refractive index of slab layer have major effects in the electro-optical characteristics of slab coupled optical waveguide diode lasers. In this research, ordinary step refractive index profile was replaced by a broadened waveguide with linear graded refractive index profile and optimized doping. The results show such design can significantly improve the power conversion efficiency (PCE). According to these results, by using the asymmetric barrier in the sides of active region, the threshold current decreases and slop efficiency (SE) increases. The PCE and SE in the proposed structure are respectively 12% and 0.21 W/A higher than those in conventional step profile.

Researches of Parameters of Chiral Few-Mode Optical Fiber Pilot Sample with Improved Height of Step Refractive Index Profile

This work presents results of researhes of fabricated pilot sample of chiral few-mode optical fiber (FMF) with induced twisting of 10 and 66 revolutions per meter, core diameter 11 µm (that almost corresponds to standard singlemode optical fibers), typical “telecommuniction” cladding diameter 125 µm and improved height of step refractive index profile. Proposed few-mode optical fiber supports 4 guided modes over “C”-band. We considered design and selection of desired technological parameters, based on results of computations, performed by both rigorous and approximation methods. Spectral curves of dispersion parameters are reprented as well as results of experimental measurements near-field laser beam profile and spectral and pulse responces of laser-excited optical signals.

Improvement of AlGaInAs/AlGaAs laser diode electro-optics characteristics by graded refractive index profile broadened waveguide

Thickness and refractive index of waveguide layer have major effects in the electro-optical characteristics of diode lasers. In this research, ordinary step refractive index profile was replaced by broadened waveguide with suitable linear, parabolic and Gaussian profile refractive index. The results show such design can significantly improve the vertical divergence, decrease threshold current and increase laser brightness. According to these results by using Gaussian profile, the brightness and optical power can be increased to 56% and 27% respectively higher than those in step profile.

Magnetooptics in Cylindrical Structures

Understanding magnetooptics in cylindrical structures presents interest in the development of magnetic sensor and nonreciprocal devices compatible with optical fibers. The present work studies wave propagation in dielectric circular cylindrical structures characterized by magnetic permeability and electric permittivity tensors at axial magnetization. The Helmholtz equations deduced from the Maxwell equations in transverse circularly polarized representation provide electric and magnetic fields. With the restriction to terms linear in off-diagonal tensor elements, these can be expressed analytically. The results are applied to magnetooptic (MO) circular cylindrical waveguides with a step refractive index profile. The nonreciprocal propagation is illustrated on waveguides with an yttrium iron garnet (YIG) core and a lower refractive index cladding formed by gallium substituted yttrium iron garnet (GaYIG) at the optical communication wavelength. The propagation distance required for the isolator operation is about one hundred micrometers. The approach may be applied to other structures of cylindrical symmetry in the range from microwave to optical frequencies.

Artificial dielectric stepped-refractive-index lens for the terahertz region.

In this paper we theoretically and experimentally demonstrate a stepped-refractive-index convergent lens made of a parallel stack of metallic plates for terahertz frequencies based on artificial dielectrics. The lens consist of a non-uniformly spaced stack of metallic plates, forming a mirror-symmetric array of parallel-plate waveguides (PPWGs). The operation of the device is based on the TE1 mode of the PPWG. The effective refractive index of the TE1 mode is a function of the frequency of operation and the spacing between the plates of the PPWG. By varying the spacing between the plates, we can modify the local refractive index of the structure in every individual PPWG that constitutes the lens producing a stepped refractive index profile across the multi stack structure. The theoretical and experimental results show that this structure is capable of focusing a 1 cm diameter beam to a line focus of less than 4 mm for the design frequency of 0.18 THz. This structure shows that this artificial-dielectric concept is an important technology for the fabrication of next generation terahertz devices.

Porous silicon Bloch surface and sub-surface wave structure for simultaneous detection of small and large molecules.

A porous silicon (PSi) Bloch surface wave (BSW) and Bloch sub-surface wave (BSSW) composite biosensor is designed and used for the size-selective detection of both small and large molecules. The BSW/BSSW structure consists of a periodic stack of high and low refractive index PSi layers and a reduced optical thickness surface layer that gives rise to a BSW with an evanescent tail that extends above the surface to enable the detection of large surface-bound molecules. Small molecules were detected in the sensor by the BSSW, which is a large electric field intensity spatially localized to a desired region of the Bragg mirror and is generated by the implementation of a step or gradient refractive index profile within the Bragg mirror. The step and gradient BSW/BSSW sensors are designed to maximize both resonance reflectance intensity and sensitivity to large molecules. Size-selective detection of large molecules including latex nanospheres and the M13KO7 bacteriophage as well as small chemical linker molecules is reported.

Interaction and dispersion of waveguide modes in an optical fiber with microirregularities of the core surface

A self-consistent linear model is proposed for the transformation of the average intensity of the mode spectrum I(z) of the waveguide field in a multimode optical fiber with a stepped refractive index profile and the core having a rough surface. The model is based on the concept of the intermodal dispersion matrix of an elementary segment of the fiber, ∆, whose elements characterize the mutual transfer of energy between the waveguide modes, as well as their conversion to radiation modes on the specified interval. On this basis, the features of the transformation of the mode spectrum I(z) in a multimode optical fiber with a stepped refractive index profile are considered that is due to the effects of multiple dispersion of the signal by the stochastic irregularities of the duct. The effect of self-filtering of I(z) is described that results in the formation of a stable (normalized) distribution I*. The features of the normalization of the radiative damping of a group of modes I i (z) in an optical fiber are considered.

Improved Semi Analytical Model of Effective Area for a Step and ParabolicIndex Optical Fiber: Introduction of Increased Parameters in Fundamental Modal Field

An accurate three parameter approximate solution of fundamental mode, to describe different propagation parameter of graded-index optical fiber has been presented. Due to three parameter optimization of the fundamental modal solution, the results are much more accurate for a wide range of optical fiber specifications. Employing variational technique, an estimation of effective area and effective index with proposed fundamental modal field are carried out. It has been shown that the results match identically with the exact available results for step and parabolic refractive index profile optical fiber. The accurate analytical expressions for the evaluation of effective area and effective index are formulated. Optimization of three parameters has been carried out for a wide range of normalized frequencies. Data for these parameters are also provided, which can be used directly while calculating different design parameters of an optical fiber.

Polarisation characteristics of light from multimode optical fibres

The polarisation beats accompanying decay of LP-mode groups in multimode optical fibres with a step refractive index profile are considered. The theoretical dependences of the degree of linear polarisation on the optical fibre length and light exit angle are determined based on the waveguide properties of these groups. The results obtained are compared with the existing theoretical concepts. It is shown that the light emerging from a multimode optical fibre near its axis can retain linear polarisation in fibres up to several meters long. Some experimental results are reported, which confirm the basic theoretical conclusions.

Propagation of light in a one-dimensional photonic crystal: analysis by the Floquet—Bloch function method

The problem of light propagation in a layered periodic medium with a step refractive index profile is considered. The exact solution of this problem is presented in the form of a nonuniform wave, for which the field amplitude distribution is written in an analytic form and the shape of its wave surfaces is determined. The reflection coefficient is obtained for a plane wave incident from the homogeneous medium at the boundary of a semi-infinite layered periodic medium and exciting a Floquet—Bloch wave. Critical conditions are found in which the Floquet—Bloch wave is infinite in the semi-infinite layered medium and exponentially decays in the adjacent homogeneous medium. Dispersion equations and field distributions of surface waves (modes) localised near the boundary of the semi-infinite layered medium are derived and conditions of their appearance are determined. The boundaries of admissible values of the refractive index of the adjacent medium depending on the parameters of the layered periodic medium are established. Dispersion relations for the surface modes in the semi-infinite layered periodic medium (bounded by a system of coupled waveguides) are obtained upon changing the thickness of the boundary layer.

Planar Fiber-Optic Chips for Broadband Spectroscopic Interrogation of Thin Films

A planar fiber-optic chip (FOC) has been developed using side-polished optical fibers and characterized for broadband absorbance and fluorescence detection of molecular films. FOC technology combines the sensitivity of an attenuated total reflection (ATR) element with the ease of use of fiber-optic-based spectrometers and light sources to create an improved platform for spectroscopic analysis of molecular adsorbates. A multi-mode optical fiber (core diameter = 50 mum, numerical aperture = 0.22, stepped refractive index profile) mounted in a glass V-groove block was side-polished to create a planar platform that allows access to the evanescent field escaping from the fiber core. For this generation of FOC technology, the exposed evanescent field has an interaction length of approximately 17.2 mm. The FOC platform was independently characterized through measurements of thin-film and bulk absorbing samples. The device performance was compared to the existing ATR technology and methods for increasing sensitivity of the FOC were investigated and validated. Additionally, we have demonstrated the ability of the FOC to both evanescently excite and collect fluorescence through guided modes of the optical fiber for a surface-confined luminescent semiconductor nanoparticle film (4 nm diameter, ligand capped, CdSe core). The FOC described here with a supported planar interface can facilitate the use of conventional planar deposition technologies and provide a robust planar platform that is amenable for incorporation into various sensor technologies.

Influence of the refractive index core profile on modal scattering of terminated two-dimensional waveguides

We examine the influence of the refractive index core profile on the modal scattering of abruptly terminated slab waveguides. The analysis is based on the integral equation method with accelerating parameters, while for the field description in the waveguide core, an appropriate Lanczos-Fourier expansion is employed. The electric-field distribution on the terminal plane, the reflection and transformation coefficient of the TE guided modes, and the far-field radiation pattern are computed. Numerical results are presented for slab waveguides with step, linear, and parabolic refractive index profiles of the core. Finally, several approximate analytical solutions are derived to study the problem in question and to explain the results obtained.

Mode dispersion and delay characteristics of optical waveguides using equivalent TL circuits

A new analysis leading to an exact and efficient algorithm is presented for calculating directly and without numerical differentiation the mode dispersion characteristics of cylindrical dielectric waveguides of arbitrary refractive-index profile. The new algorithm is based on the equivalent transmission-line (T-L) technique. From Maxwell's equations, we derive an equivalent T-L circuit for a cylindrical dielectric waveguide. Based on the TL-circuit model we derive exact analytic formulas for a recursive algorithm which allows direct calculation of mode delay and dispersion. We demonstrate this technique by calculating the fundamental mode dispersion for step, triangular, and linear chirp optical fiber refractive index profiles. The accuracy of the numerical results is also examined. The proposed algorithm computes dispersion directly from the propagation constant without the need for curve fitting and subsequent successive numerical differentiation. It is exact, rapidly convergent, and it results in savings for both storage memory and computing time.

Propagation characteristics of single-mode optical fibers with arbitrary complex index profiles

A rapidly converging numerical technique for the evaluation mode characteristics of circularly symmetric optical fibers with an arbitrary complex refractive index profile is presented. This method is based on transmission-line principles. From Maxwell's equations, we derive a transmission-line equivalent circuit for the optical fiber refractive index profile and we demonstrate how it can be used to determine the mode effective index (normalized propagation constant) of cylindrical dielectric waveguides. To illustrate the effectiveness of the procedure, we have applied it to circularly symmetric fibers with complex step, parabolic, and segmented optical refractive index profiles. We have used this method to evaluate and manipulate the gain in a typical 980-nm pumped erbium-doped fiber as well as for calculating attenuation of optical fibers when radial loss factors are present.

Second-harmonic generation using an <italic>a</italic> axis <inline-formula><roman>Nd:MgO:LiNbO</roman><sub><roman>3</roman></sub></inline-formula> single crystal fiber with Mg-ion indiffused cladding

We report the implementation of a low-loss core-cladding waveguide structure of an a axis Nd:MgO:LiNbO3 single-crystal fiber with a step refractive index profile and the demonstration of a frequency- doubled laser made from such a cladded crystal fiber. The laser-heated pedestal-grown a axis single-crystal fiber, with an elliptical crosssectional area of 200x 150?m, is further shown with a Mg ion indiffusion process. Electron probe microanalysis is used to measure the Mg ion concentration distribution in the Mg-diffused layer. After several trials, diffusion parameters suitable for achieving a core-cladding waveguide structure with a step refractive index profile are obtained. The optical and structural properties of the cladded crystal fibers are also characterized. Room-temperature second-harmonic generation with such a cladded crystal fiber is demonstrated. At room temperature, cw green laser output with a power of 10 ?W at the wavelength of 0.532 ?m is achieved. The origin of the relatively low conversion efficiency is discussed.

Modal characteristics of graded multilayer optical waveguides

The modal characteristics of graded-step index multilayer planar waveguides for layer thicknesses down to those applicable for quantum well structures have been investigated here. The effects of linear grading of the transverse stepped refractive index profile have been evaluated in terms of the mode field profiles and the modal spectrum. The position of the maximum of the fundamental mode field profile can be moved toward the upper waveguide cladding/core interface with consequential increase in the evanescent field magnitudes.

Inhomogeneity of the optical Magnus effect

An experimental investigation was made of the dependence of the angle of rotation of the speckle pattern of the radiation, transmitted by a fibre waveguide with a step refractive index profile, on the angle α of entry of a narrow light beam into the fibre. The magnitude of the optical Magnus effect depended on this angle as α2.

Design Considerations for Er3+-doped Planar Optical Amplifiers in Silica on Silicon

Large-signal numerical model for erbium-doped integrated optical amplifier has been used to optimize the parameters of waveguide core for maximum gain in dependence on pump power and waveguide aspect ratio. The model developed for the analysis of Er-doped fiber amplifier has been modified. The azimuthal effective-index method is used to determine the field distribution of the fundamental mode of an optical waveguide with rectangular core cross section and step refractive-index profile. It has been found that the optimum core height increases with increasing pump power and decreasing core aspect ratio.