Index Profile Of Fiber Research Articles

A Modified Near Field Method for Measuring the Refractive Index Profile and Geometrical Parameters of Multi-Core Fibers

A Modified Near Field Method for Measuring the Refractive Index Profile and Geometrical Parameters of Multi-Core Fibers

Bell-Shaped Refractive Index Profiles of Multimode Optical Fibers and Method for Calculating Their Optical Properties

Bell-Shaped Refractive Index Profiles of Multimode Optical Fibers and Method for Calculating Their Optical Properties

Analysis of HF-Etched Uniformly Thinned Linearly Chirp Types of Refractive Index Profile Fiber Bragg Grating Sensor

Analysis of HF-Etched Uniformly Thinned Linearly Chirp Types of Refractive Index Profile Fiber Bragg Grating Sensor

Design mode filtering interferometer using etched double clad fiber

Abstract Mode filtering technique is one of the most desired techniques in optical fiber communication systems, especially for multiple input multiple output (MIMO) coherent optical communications that have mode-dependent losses in communication channels. In this work, a special type of optical fiber sensing head was used, where it utilizes DCF13 that is made by Thorlabs and has two numerical apertures (NA’s). One is for core and 1st cladding region, while the 2nd relates the 1st cladding to the 2nd cladding. Etching process using 40 % hydro-fluoric (HF) acid was performed on the DCF13 with variable time in minutes. Investigation of the correlation between the degree of etching and the refractive index (RI) profile of double-clad fibers for low order linearly polarized (LP) modes is the focus of this work research. The used software programs to simulate the effect of etching are Comsol Multi-Physics V6 and Optigrating V4.2.2. It is seen that the optimum case that gave highest cladding power to core power ratio is the case of etching for 10 min, which can be used for detection applications. This ratio greatly affects the sensitivity of the system.

Common beans as a source of food ingredients: Techno-functional and biological potential.

Common beans are an inexpensive source of high-quality food ingredients. They are rich in proteins, slowly digestible starch, fiber, phenolic compounds, and other bioactive molecules that could be separated and processed to obtain value-added ingredients with techno-functional and biological potential. The use of common beans in the food industry is a promising alternative to add nutritional and functional ingredients with a low impact on overall consumer acceptance. Researchers are evaluating traditional and novel technologies to develop functionally enhanced common bean ingredients, such as flours, proteins, starch powders, and phenolic extracts that could be introduced as functional ingredient alternatives in the food industry. This review compiles recent information on processing, techno-functional properties, food applications, and the biological potential of common bean ingredients. The evidence shows that incorporating an adequate proportion of common bean ingredients into regular foods such as pasta, bread, or nutritional bars improves their fiber, protein, phenolic compounds, and glycemic index profile without considerably affecting their organoleptic properties. Additionally, common bean consumption has shown health benefits in the gut microbiome, weight control, and the reduction of the risk of developing noncommunicable diseases. However, food matrix interaction studies and comprehensive clinical trials are needed to develop common bean ingredient applications and validate the health benefits over time.

Fiber Residual Stress Effects on Modal Gain Equalization of Few-Mode Fiber Amplifier.

The modal gain equalization (MGE) of few-mode fiber amplifiers (FMFAs) ensures the stability of signal transmission. MGE mainly relies on the multi-step refractive index (RI) and doping profile of few-mode erbium-doped fibers (FM-EDFs). However, complex RI and doping profiles lead to uncontrollable residual stress variations in fiber fabrication. Variable residual stress apparently affects MGE due to its impacts on the RI. So, this paper focuses on the residual stress effects on MGE. The residual stress distributions of passive and active FMFs were measured using a self-constructed residual stress test configuration. As the erbium doping concentration increased, the residual stress of the fiber core decreased, and the residual stress of the active fibers was two orders of magnitude lower than that of the passive fiber. Compared with the passive FMF and the FM-EDFs, the residual stress of the fiber core completely transformed from tensile stress to compressive stress. This transformation led to an obvious smooth RI curve variation. The measurement values were analyzed with FMFA theory, and the results show that the differential modal gain of the FMFA increased from 0.96 to 1.67 dB as the residual stress decreased from 4.86 to 0.01 MPa.

Refractive index profiling of multimode specialty optical fibers by absorption contrast X-ray computed microtomography

We report on successful refractive index profiling of commercially available step-index and in-house made graded-index multimode specialty optical fibers by means of X-ray computed microtomography. Our results demonstrate that the latter is an advantageous technique for characterizing large core optical fibers, which allows for retrieving information about the refractive index at optical frequencies by exploiting the absorption coefficient of X-rays.

Refractive index profile measurement of planar optical waveguides based on the near-field technique and digital holography

• A new method is proposed to measure the refractive index profile of planar optical waveguides. • A measurement system is established by integrating the near-field setup and the modified Mach-Zehnder interferometer. • The refractive index profile is quantitatively obtained from two phase difference distributions. • The accuracy of the proposed method is up to 1 × 10 −3 . A method based on the near-field technique and digital holography is proposed to measure the refractive index profile of planar optical waveguides. A measurement system is established by integrating the near-field setup and the modified Mach-Zehnder interferometer. The near-field intensity profile of optical waveguides is recorded by CCD to get the primary refractive index profile. The primary phase difference distribution is calculated based on the relationship between the phase and refractive index. The digital hologram is acquired by the modified Mach-Zehnder interferometer to extract the real phase difference distribution using the hybrid reconstruction algorithm. According to the relationship between the primary and real phase difference distributions, the refractive index profile is quantitatively obtained. To evaluate the proposed method, we measured the refractive index profiles of multimode and single-mode fibers and compared with S14 based on refracted near-field method and SHR-1802 based on digital holography. The proposed method has a measurement accuracy of up to 1 × 10 -3 and is suitable for measuring the positive refractive index region of waveguides.

Multi-parameter optical gauge based on mode coupling effect in asymmetric index multi-core fibres

An optical gauge based on coupled multi-core fiber is proposed and experimentally demonstrated. By using direct laser writing to selectively break the fiber index profile symmetry, asymmetric mode coupling between cores is introduced. This allows to fabricate optical gauges with the ability to detect and differentiate different types of deformation in structures using just a single sensor.The fabricated optical gauges are compared with calibrated commercial gauges and fiber Bragg gratings as strain, vibration and curvature gauge. The tests indicate superior performance of this novel optical gauge over the commercially available sensors and offer the highest sensitivities. The proposed technology could be key to the fabrication of new fiber-based sensing devices with more capabilities and better features than previously achieved.

Distributed Temperature and Strain Measurement Based on Brillouin Gain Spectrum and Brillouin Beat Spectrum

A novel fast measurement method based on Brillouin gain spectrum (BGS) and Brillouin beat spectrum (BBS) in a complex index profile fiber has been proposed to realize simultaneous measurement of distributed temperature and strain. An interesting phenomenon that the temperature/strain sensitivities of the power on the BGS slope and the power on the BBS peak are very different has been discovered and utilized for temperature and strain sensing. By means of only measuring the power trace of the BGS slope and the power trace of one BBS peak along the sensing fiber, the distributed temperature and strain can be measured by using the Brillouin optical time-domain reflectometry (BOTDR) without scanning the Brillouin spectrum. The temperature and strain measurement uncertainties of the proposed method are one order of magnitude lower than that of the BBS-based measurement method. In a proof-of- concept experiment, the temperature and strain of the 2 km large effective area fiber (LEAF) are measured at a spatial resolution of 3 m within 18 s.

Modal Bandwidth and Single-Mode VCSEL Transmission Capability Over Multimode Fibers

Single-mode VCSELs have been demonstrated with great transmission capability over multimode fibers in the literature through restricted launch, which is beyond what is allowed by the effective modal bandwidth (EMB) defined for multimode transmission with the encircled flux launch condition. However, the impact of fiber index profile errors and their interactions with launch conditions have not been thoroughly studied. We conduct a detailed experimental study to gain insight on the impacts of the fiber index profiles and launch conditions on the system performance of single-mode VCSELs transmitting over multimode fibers. Experimentally, our results show that a launch spot roughly matching the fiber LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode field leads to very high bandwidth and robust performance, largely independent of the imperfection of the index profiles and therefore can perform well in very high data rates. For a restricted launch without mode matching, the performance highly depends on the quality of the fiber, in line with EMB. We found that the center offset tolerance is around <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2~\mu \text{m}$ </tex-math></inline-formula> for a mode matched launch. We also conducted single-mode VCSEL transmission experiments over 600-m and 1000-m multimode fibers, which agree well with the modal bandwidth results.

Designing High-Performance Multimode Fibers Using Refractive Index Optimization

The rich design landscape of optical fibers offers many opportunities for refractive index optimization. In particular, the refractive index profiles of multimode fibers (MMFs) and multicore fibers (MCFs) govern the behavior of spatial and polarization modes, including their bandwidth, mode count, mode coupling, modal dispersion, chromatic dispersion, and mode-dependent loss. In this article, we obtain update equations to optimize the shapes of fiber refractive index profiles for various applications using gradient descent. Starting with an initial fiber designed according to standard best practices, our methods iteratively modify the refractive index profile to improve upon the initial design. Interestingly, we see that optimization can sometimes yield large improvements in key fiber properties, such as a 20-fold reduction in the root-mean-square modal group delay spread, even though the index profile is changed very little from the initial design. We show that our methods can be used to tailor the mode coupling, modal dispersion and chromatic dispersion properties to desired values, and that optimization can be successfully applied over a large bandwidth. We provide illustrative design examples, including an optimization of a graded-index MMF with low group delay spread for long-haul mode-division-multiplexed transmission. Our algorithms can be used to design novel fibers or optimize existing fibers for next-generation transmission systems.

Stress-Induced Refractive Index Changes in Laser Fibers and Preforms

Refractive index profile of the core is a key design parameter in fiber lasers and amplifiers. During manufacturing, the initial information of the index profile is obtained from the preform, while ultimately the performance is defined by the index profile of the fiber. Depending on stresses and diffusion, the two profiles may be different. It is possible to predict more accurately the laser fiber refractive index when we apply a stress-induced index change model to the measured preform index profile data. The improved capability to predict the fiber index from preform increases the confidence in achieving the designed index profile in fiber, which enables faster process feedback and higher fiber yields.

Failure of the Weakly Guiding Approximation for Few-Mode Fiber Index Profiles

We demonstrate that for some few-mode fiber index profiles, the weakly guiding approximation (WGA) can be grossly inaccurate in wavelength regions where the true vector modes undergo an HE ↔ EH transition. Within this transition region, the true vector modes have unusual properties, so they cannot be combined with other modes to form LP pseudo-modes. The failure of the WGA in the transition region is likely related to the near degeneracy of the modes obtained under the WGA, signaling the breakdown of standard perturbation theory, under which the WGA modes serve as zeroth-order approximations. We also demonstrate that a small perturbation to a commercial few-mode fiber index profile can introduce an HE ↔ EH transition which is not present in the unperturbed design. Because the WGA can be grossly inaccurate at some wavelengths, it would seem safer to avoid its use for few-mode optical fiber design and simulation, using instead the exact version of Maxwell's equations. At the least, the exact equations should be used to check results obtained by the WGA in wavelength regions of interest.

Investigation of Planar and Helical Bend Losses in Single- and Few-Mode Optical Fibers

In this paper, we describe experimental, simulation, and analytical approaches to study bend losses in single and few-mode optical fibers bent into a circle (in-plane bend) and helix. For the in-plane bend case, we compute loss through simulations and experiments, over the bend diameter range of $9.5\text{--}19.5$ mm at 1550 nm wavelength. For in-plane bend simulations, we obtain the refractive index profile of an equivalent straight fiber by applying the geometrically exact beam theory (GEBT) and conformal mapping, against the conventional approach to use an elasto-optic factor to account for stresses experienced by the fiber. We apply GEBT to compute the strain tensor of a bent fiber. The strain tensor, in turn, is used in the stress-optic law to obtain the refractive index due to bending stress. To account for the geometric effect, we apply the conformal mapping technique. The refractive index profile obtained by applying GEBT followed by conformal mapping is used in full-vectorial FEM simulations carried out in COMSOL to compute bend loss at different diameters. The simulation and experimental results are in close agreement. We extend our approach to few-mode fibers by calculating the planar bend losses for higher order modes. The close agreement of the simulation results, generated without making any ad hoc assumptions about the elasto-optic factor to account for the stress effects, with literature validates our approach. Next, we describe the results of experiments to measure the bending losses of fibers wound helically around the mandrel for bend diameter range $9.5\text{--}19.5$ mm and different helix pitch values. We derive an analytical formula to compute helical bend losses based on the method outlined by Marcuse. Although the results from the formula are in close agreement with the experimental results and predict the exponential decrease of loss with increasing pitch, the formula does not account for the non-monotonous dependence of bend loss on helix pitch, observed experimentally. We present a simple empirical formula to account for the non-monotonic dependence. Finally, we study the influence of micro-bend loss, due to the surface roughness of the mandrels used in the experiments on the bending losses. While the effect of surface roughness can be neglected for the mandrels used in our experiments, its effect may become important in mandrels with higher surface roughness, especially at larger bend diameters.

Simulation and research of few-mode optical fiber DMD degradation due to geometry deviation from optimized form

This work presents an alternative fast and simple method for the design of a graded refractive index profile of silica few-mode optical fibers (FMFs) with enlarged core diameter that extremely improves mode effective area, and low differential mode delay (DMD). We demonstrate some results of refractive index profile optimization for 16-LP-mode FMF with enlarged core diameter up to 42 μm and mode effective area more 130 μm2 under DMD reduction less 120 ps/km all over “C”-band and less 40 ps/km at λ = 1550 nm region. We also propose approach for simulation of the deviation of “real” optical fiber geometry from the optimized form and rigorous method for analysis of large core complicated optical fiber. Here, FMF core asymmetrical ellipticity together with the refractive index profile distortions were considered. They were set and simulated by data from reports of the real commercially available laser-optimized multimode optical fibers of TIA/ISO Cat. OM2+/OM3 refractive index profile measurements, performed by lab optical fiber analyzer kit. We present results of researches of considered FMF DMD spectral curve degradation due to described core geometry deviation from the optimized form. It was noticed, that unlike to only refractive index local fluctuations, FMF core non-circularity strongly distorts higher-order mode field distribution that affects to spectral mode delay curves and may lead to critical total DMD increasing.

Design of Silica Multimode Optical Fibers with Extremely Enlarged Core Diameter for Laser-Based Multi-Gigabit Short-Range Optical Networks

This work presents an alternative fast and simple method for the design of a refractive index profile of silica multimode optical fibers (MMFs) with extremely enlarged core diameters of up to 100 µm for laser-based multi-gigabit short-range optical networks. We demonstrate some results of 100 µm core MMF graded index profile optimization performed by a proposed solution, which provides a selected mode staff differential mode delay (DMD) reduction over the “O”-band under particular launching conditions. Earlier on, a developed alternative model for a piecewise regular multimode fiber optic link operating in a few-mode regime for the computation of laser-excited optical pulse dynamics during its propagation over an irregular silica graded-index MMF with an extremely large core diameter, is utilized to estimate the potentiality of fiber optic links with the described MMFs. Here, we also present the comparison results of the simulation of 10GBase-LX optical signal transmission over 100 µm core MMFs with conventional and optimized graded-index refractive index profiles.

Fused silica optical fibers with graded index nanostructured core

The ability to shape the index profile of optical fibers holds the key to fully flexible engineering of their optical properties and future applications. We present a new approach for the development of a graded index fused silica fiber based on core nanostructurization. A graded index core is obtained by means of distribution of two types of subwavelength glass rods. The proposed method allows to obtain arbitrary graded distribution not limited to the circular or any other symmetry, such as in the standard graded index fibers. We have developed a proof of concept fiber with parabolic refractive index core and showed a perfect match between its predicted, designed and measured properties. The fiber has a core composed of 2107 rods of 190 nm of diameter made of either pure fused silica or Ge-doped fused silica with 8.5% mol concentration. The proposed method breaks the limits of standard fabrication approaches used in fused silica fiber technology.

Comparative study of quantitative phase imaging techniques for refractometry of optical waveguides.

A comparative study of quantitative phase imaging techniques for refractometry of optical waveguides is presented. Three techniques were examined: a method based on the transport-of-intensity equation, quadri-wave lateral shearing interferometry and digital holographic microscopy. The refractive index profile of a SMF-28 optical fiber was thoroughly characterized and served as a gold standard to assess the accuracy and precision of the phase imaging methods. Optical waveguides were inscribed in an Eagle2000 glass chip using a femtosecond laser and used to evaluate the sensitivity limit of these phase imaging approaches. It is shown that all three techniques provide accurate, repeatable and sensitive refractive index measurements. Using these phase imaging methods, we report a comprehensive map of the photosensitivity to femtosecond pulses of Eagle2000 glass. Finally, the reported data suggests that the phase imaging techniques are suited to be used as precise and non-destructive refractive index shift measuring tools to study and control the inscription process of optical waveguides.

Characterization of optical fiber profile using dual-wavelength diffraction phase microscopy and filtered back projection algorithm

We present a measurement scheme of the refractive index profile of optical fibers which does not require accurate index oil match using dual wavelength diffraction phase microscopy. Using dual wavelength algorithm, we solved phase wrapping problem caused by large optical path length difference between the optical fiber and its medium and final phase image was obtained without 2π ambiguity. The index profile was obtained by the filtered back projection algorithm that applies inverse Radon transformation of the phase profile matrix. As an example, a single mode fiber with a known index profile was measured by using this method. The measured profile is well agreed with index profile of the preform, which had been designed by a preform analyzer before the fiber being fabricated from the preform.