Core-cladding Refractive Index Difference Research Articles

Effect of sinusoidal ripples in refractive index profile distribution on the performance characteristics of dual concentric core step index fiber Raman amplifier

Phased matched wavelength, effective area, effective Raman gain, and wave guide dispersion are computed from exact numerical solution assuming scalar wave equation in the presence and absence of ripples as imperfections in the refractive index profile in dual cores of single mode fiber Raman amplifier for the first time. It is observed that for larger values of amplitude and lower frequencies, the effective Raman gain increases w.r.t. that calculated with no ripples. However, we assume ripple amplitude up to 5% of core cladding refractive index difference w.r.t. the available data, corresponding to the three ranges of relative ripple amplitudes of 1%, 2%, 3% and ripple frequencies of 1, 2, 3μm−1. Based on these data, we analyse performance of FRA over frequency shift band of 20–700cm−1. Uniformity of gain is interestingly seen to be maintained for higher ripple frequency and lower amplitude. However, no prominent effect in coefficient of dispersion and phase matched wavelength is observed within operating range of wavelength. Also, based on available structural parameter, the investigation should find use as a guide to system users to know the limit and promise of existence of ripples.

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

A novel multilayer-core intrinsically single-mode large-mode-area fiber is proposed in this paper. The multilayer structure in the core which is constituted of alternating low- and high-refractive index rings could achieve a very low equivalent core-cladding refractive index difference. The single-mode large mode area of 100–12,000μm2 could be achieved in the fiber. The effective area Aeff can be further enlarged by increasing the parameters of low-refractive index rings or the number of layers. Furthermore, the bending property could be improved by 1–2 orders of magnitude in this multilayer-core structure when the outermost layer is depressed-index ring. We have experimentally verified the proposed fiber structure that follows the target of large Aeff along with ultra low bending loss at 1.55μm. These characteristics of multilayer-core fiber suggest that it can be used in large-mode-area high-capacity transmission, or high-power optical fiber laser and amplifier in the optical communications.

Fabrication of buried waveguides in planar silica films using a direct CW laser writing technique

A CW CO 2 laser ablation technique is used to form buried waveguides in planar silica films. It is shown that the refractive index of a silica thin film is reduced sufficiently adjacent to the laser processed region to allow the fabrication of low loss waveguides. The refractive index distribution of these structures is measured using the reflectance of a focussed spot from the surface of the films. The change in refractive index is measured to be of the order of the core cladding refractive index difference of typical single mode waveguides. The spatial resolution of the reflectance technique is 1.3 μm with a refractive index resolution of ±5 × 10 −4. Devices such as 1 × 2 and 1 × 4 multi-mode interference (MMI) splitters have also been demonstrated and shown to exhibit low transmission losses.

Analysis of Photosensitivity of Copolymer Optical FibrePreform

The photosensitivity of copolymer optical fibre preform is analysed incomparison with the doped one. The effects of write conditions such aspump power and pump time have been studied. Then, the preform is drawninto single mode polymer optical fibre with core refractive index of1.499, and core-cladding refractive-index difference of 0.008.Long-period birefringence gratings with period of 120 um are fabricatedin the fibre. The duty cycle is 50%, and the refractive index changein the exposed area is about 1×10−3.

Nonlinear Performances of Photoinduced Fiber Grating Mode Converter and Their Application

The optical Kerr effect involved in a fiber grating type LP 01 ↔ LP 02 mode converter is investigated by using the coupled-mode theory. Tunable characteristics of the conversion efficiency is shown when the input peak power is varied, which exhibits an optical switching property. The switching power can be reduced by using a weak index perturbation of the fiber grating or using a fiber with large core-cladding refractive-index difference and a small fiber core radius. In additions, a preliminary schematic structure of a novel passive mode-locked fiber laser using a nonlinear mode converter is proposed for the application.

Transverse stress induced LP 02-LP 21 modal interference of stimulated Raman scattered light in a few-mode optical fiber

Four-photon mixing followed by stimulated Raman scattering is observed in LP 02 mode in a 7.9 μm core diameter optical fiber. A localized transverse stress efficiency couples LP 02 to the LP 21 mode with a macroscopic beat length of 1.8 mm. LP 02-LP 21 modal interference is investigated by detecting the 550–590 nm SRS through a pinhole in the far field exit plane. Quantitative explanation of wavelength dependent intensity modulation results in a precise experimental determination of ∂[β 02(λ) − β 21(λ)] ∂λ , for mode-propagation constants β 02( λ) and β 21( λ) of LP 02 and LP 21 modes respectively, as well as Δ, the relative core-cladding refractive index difference. The LP 02-LP 21 modal interference is used for sensing of temperature between 50–300°C.

Dual-mode fiber-optic interferometer: a novel design

We describe a simple and novel single-arm two-mode fiber interferometer in which the directional couplers and the fiber-grating couplers are replaced with microbends caused by point mechanical squeezing. The interferometer works with incident light in the LP02 mode. While the input microbend couples the LP02 mode to the LP21 mode, the output microbend behaves as an attenuator that is sensitive to the orientation of LP02 –LP21 interference intensity field inside the fiber. The application of this device for measuring the core–cladding refractive-index difference and as an interferometric temperature sensor between 20 and 400 °C is demonstrated.

Complex modes in open lossless dielectric waveguides

An approach to the analysis of complex modes in lossless isotropic dielectric waveguides is presented. The existing theory of complex modes is extended to the case of open structures by techniques of functional analysis. Mathematical and physical properties of complex modes, including some criteria of existence and mechanisms of occurrence, are formulated. The theory is demonstrated by the numerical example of a waveguide with a large core-cladding refractive-index difference. Computed complex-mode dispersion characteristics are analyzed, and the mode vector field, together with the power distribution in the waveguide cross section, is shown.

Effect of some refractive index modifiers on both numerical aperture and mechanical strength of fluorozirconate fibres

Abstract The effect of incorporating large amounts of some fluoride additives such as LiF, PbF2, HfF4 on both the numerical aperture and the mechanical strength of ZrF4 based glass fibres has been investigated. Fibres with core-cladding refractive index difference as high as 65 × 10−3 have been achieved with appropriate glass compositions. The tensile strength of multimode fibres tends to decrease from about 600 to 400 MPa with increasing PbF2 or HfF4 glass content, and high numerical aperture-small core diameter single mode fibres with strength within 500–550 MPa are achievable.

Efficient Er3+-doped fluorozirconate fibre amplifier

Abstract The efficiency, small signal gain spectrum and noise characteristics of erbium-doped fluorozirconate fibre amplifier with a core diameter of 2.8 μm and a core-cladding refractive index difference of 40×10 −3 are described. A gain coefficient of 3.4 dB/mW for 1.488 μm pumping is obtained. Gain saturation occurs at 7 mW injected pump power. A gain of 27 dB obtained with 14 mW injected pump power gives an efficiency coefficient of 1.9 dB/mW in the gain saturation region.

Effect of sinusoidal ripple in refractive index distribution on total dispersion in single mode parabolic-index fibers

Total dispersion in single mode parabolic-index fibers is computed from the exact numerical solution of the scalar wave equation in the presence and absence of ripples in refractive index profile. Tables displaying the variation of the zero dispersion wavelengths and optimum core radii with ripple amplitude and spatial frequency are given. It is observed that for larger values of ripple amplitude, the zero dispersion wavelength and optimum core radius increase. However we find that if the ripple amplitude is up to 5% of the core-cladding refractive index difference (this is the value reported in practical fibers) the effect on zero dispersion wavelength and core radius is very small (about 2–3%).

Coupling an elliptical Gaussian beam into a multimode step-index fiber

The power coupling efficiency of an elliptical-spot-size Gaussian beam into a multimode step-index fiber is derived using a full-wave analysis. Numerical results indicate that the power coupling efficiency increases not only with decreasing index mismatch but also with increasing spot size of the Gaussian beam at its waist and the relative core–cladding refractive-index difference of the fiber. The coupling loss due to beam spread is found to be large when the spot size at the beam waist is small. The coupling loss is also found to increase with increasing wavelength of the Gaussian beam. A slight difference of power coupling efficiency is found with the different direction of electric polarization for a Gaussian beam of large spot size at its waist.

Slowly varying optical fibers

The effects of slow variations in radius ρ and core-cladding refractive-index difference Δ along the length of optical fibers are determined by using the ray-path equations. Simple formulas in terms of the averaged properties of ρ and Δ show that, in power-law fibers, pulse dispersion is slightly increased and the magnitude of the induced power loss can be bounded. Results hold for general power-law fibers. Comparison with power-law fibers having exponent variations indicates that changes in profile shape cause larger pulse dispersion but less power loss than do ρ and Δ variations. In arbitrary fibers having the same amount of dopant used to form the profile in every core cross section, variations in ρ and Δ cause no power loss. The relationship between these results and the adiabatic invariant concept is outlined.

Diffusion Raman stimulée à trois ondes dans une fibre optique

Three-wave stimulated Raman scattering in optical fibers has been studied theoretically in connection with the characteristics of the fiber (core diameter, core-cladding refractive-index difference) and excitation conditions. We observe experimentally this type of scattering in a silica fiber, from which we can estimate the nonlinearity of silica and the index difference of the fiber.

Detailed evaluation of an attainable repeater spacing for fibre transmission at 1.3 μm and 1.55 μm wavelengths

An attainable repeater spacing at 1.3 μm and 1.55 μm wavelengths using a step-index monomode fibre cable is given, after detailed consideration of total loss as well as total dispersion of an optical fibre. The longest attainable repeater spacing at 1 Gb/s is about 100 km at 1.55 μm wavelength using a single-mode laser diode and a monomode fibre with a relatively small core-cladding refractive index difference, e.g. about 0.2%.