Measurement Of Refractive Index Profile Research Articles

The optical fiber refractive index profile measurement based on phase-correct quantitative phase microscopy

The optical fiber refractive index profile measurement based on phase-correct quantitative phase microscopy

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.

Refractive Index Profile Measurement of Planar Optical Waveguides Based on the Near-Field Technique and Digital Holography

Refractive Index Profile Measurement of Planar Optical Waveguides Based on the Near-Field Technique and Digital Holography

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.

Gradual-Time Solution Doping for the Fabrication of Longitudinally Varying Optical Fibres

Standard MCVD process and solution doping are commonly used for processing rare-earth doped optical fibers. Since the 1970s, this technique is mainly employed to fabricate homogeneous preforms in the longitudinal direction. A variant is proposed here, by using two doping solutions and by applying a gradual time doping. To demonstrate the validity of this approach, three preforms were prepared with longitudinal gradient of La, Ce, and Al. Preforms were characterized by refractive index profile measurements and concentrations in optical fibers were characterized by EPMA . These results will allow to perform rapid material development in optical-fiber domain. This process is also expected to offer a new way to prepare optical fibers with varying longitudinal characteristics.

Measurement of refractive index profiles in optical waveguides by moiré deflectometry technique.

Measurement of refractive index profiles in optical waveguides is an important issue for controlling manufacturing procedures and determination of some important characteristics of waveguides. In this paper, an optical waveguide is considered as a phase object. When collimated laser beams are illuminated on waveguides, the refractive index gradient distorts the plane waves, then the distortion is determined by use of the moiré deflectometry method. Dimensions of the waveguides under study are on the order of 10μm, so the moiré deflectometry setup was changed and optimized for measuring in the micrometer dimensions. This modified and generalized moiré deflectometry technique helps to intensify plane wave distortion thousands of times and expands it to be measurable. Finally, by analysis of moiré fringes, we succeeded in determining the refractive index profile of an optical waveguide by approximation of semi-cylindrical symmetry.

Measurement of planar refractive index profiles with rapid variations in glass using interferometry and total variation regularized differentiation

Planar refractive index profiles with rapid variations, formed in glass, are measured with interferometry. This involves forming a bevel in the glass and orientating the fringe pattern to be normal to the bevel edge. The index profile is determined by differentiation of the phase function of the fringe pattern. The differentiation has been performed using the total variation regularization method in order to preserve rapid changes in the derivative. This new approach avoids the necessity of filtering, in order to reduce noise, in the direction perpendicular to the bevel, which would otherwise smooth out the rapid index changes. The method is assessed using a model refractive index profile that contains an index gradient of 0.24 μm−1 and is then applied practically to measure the refractive index profile of electrically poled BK7 glass. The new approach allows the sharp transition in the index between poled and unpoled glass to be observed as well as the accumulation of potassium ions beyond the poled glass region.

The influence of the conditions of ion exchange in CuSO4:Na2SO4 melt on the optical properties of surface layers of silicate glass

The influence of the temperature and duration of ion exchange in BK7 silicate glass in CuSO4:Na2SO4 melt on the optical properties of the glass surface layers has been investigated. It is shown that ion exchange occurs from the melt according to the Cu2+ ↔ 2Na+ scheme. Cu2+ ions penetrate the sample to a depth of about 1 µm. Reduction of Cu2+ ions near the glass surface gives rise to the Cu+ ↔ Na+ ion exchange in the glass. Measurements of refractive index profiles in the glass sample subjected to ion exchange have revealed the formation of two waveguides in the sample: near the surface and at a depth of more than 3 µm; the second waveguide is formed by Cu+ ions. It is shown that relatively low temperatures and short durations of ion exchange lead to the formation of copper molecular clusters Cun in glass. An increase of ion exchange temperature and duration leads to decomposition of molecular clusters with formation of Cu2+ ions.

Inscription of strong Bragg gratings in pure silica photonic crystal fibers using UV femtosecond laser pulses

We demonstrate our experimental results on inscription of strong first-order Bragg gratings in hydrogen-free and hydrogen-loaded pure silica photonic crystal fibers (PCFs) using high-intensity 160 femtosecond (fs) UV laser pulses at 266nm and a two-beam Talbot interferometer. Bragg reflectivities up to 37dB for the H2-loaded fiber as well as Bragg wavelength tuning over the telecom C-band using a single phase mask were achieved. The in-situ nonlinear filamentation of UV fs laser pulses, scattered by the fiber’s micro-structured cladding, is demonstrated by direct refractive index profile measurement across the cross section of a written fiber. The results of thermal annealing measurements for a H2-loaded PCF are also presented.

Progress in the fabrication of rare earth doped optical fiber by standard and Sol-Gel derived granulated oxides

We produced active optical fibers by the doped granulate silica-method. The granulates were derived from the sol-gel technology. We determined volume percentage and distribution of chemical elements in the fabricated glasses fiber by the analytical technique of Energy-Dispersive X-ray (EDX), Scanning Electron Microscopy (SEM), Electron Probe Microanalysis (EPMA) and the degree of crystallization by X-Ray Diffraction analysis (XRD). Furthermore we measured fluctuations of the Refractive Index Profile (RIP) and scattering losses of the fiber core. It was observed from the RIP measurement that the average index difference with respect to the undoped silica was 5.3*10-3, and thus the optical fiber has a numerical aperture of 0.12. XRD measurements and investigations with SEM showed that the manufactured optical fiber has an amorphous structure. The dopant and co-dopant elements in the core region were uniformly distributed. Although we found strong isolated scattering centers in the fabricated fiber, the loss of the fibers was determined to be 0.35- 3dB/m for selected fiber pieces. Here we present our progress of improving the optical quality of fibers produced from granulates.

Nondestructive Optical Measurement of Refractive-index Profile of Graded-index Lenses

We propose a simple nondestructive method to obtain refractive-index profiles of a graded-index (GRIN) light-focusing rod by means of a diffraction grating. In our proposed method, a laser beam is illuminated through a diffraction grating perpendicular to the axis of the GRIN lens and the separation between the zeroth and first-order diffraction peaks is measured and analyzed. The results demonstrate that the refractive-index profiles of commercially available GRIN lenses can be successfully reconstructed.

Drawing of single-crystal and glass-clad lithium tantalate fibers by the laser-heated pedestal growth method

Lithium tantalate (LT) single-crystalline fibers with various growth conditions were obtained using the laser-heated pedestal growth (LHPG) method. The fibers can be produced with a diameter down to 70 µm and tens of centimetres in length by using different pulling rates. Preliminary experiments also showed that the LT fibers can be electrically poled for quasi-phase matching. To facilitate wave propagation with low loss, glass-clad LT fibers were fabricated by a co-drawing LHPG method for the first time. A vacuum apparatus was developed to eliminate the bubbles from incongruent evaporation of lithium oxide during the drawing of the glass-clad LT fibers. The glass-clad fibers are classified into two different categories by the measurements of refractive index profiles. One is a step-index fiber and the other is a graded-index fiber. Both fiber types are multimode at present, but single-mode LT fibers could be fabricated with the high-precision control of the LHPG systems.

Comparative study on determining the refractive index profile of polypropylene fibres using fast Fourier transform and phase-shifting interferometry

The refractive index profile measurement is desirable for fabricating and characterizing fibres. The fast Fourier transform method (FFT) and phase-shifting interferometry (PSI) were used to determine the refractive index profiles of polypropylene (PP) fibres. Interference fringe patterns produced by Mach–Zehnder interferometer have been analysed using the suggested methods. Wrapped phase maps and contour lines of the obtained fringes were produced. The methods used are relatively simple, accurate and are capable of conducting fully automatic measurement of refractive index profiles for fibres. By comparing the obtained refractive index profiles using the two methods, it is found that there is good agreement between them and also with the literature.

Study on measurement of refractive index profile of GI-POF by light scattering

This paper is devoted to the study on measurement of refractive index profile of graded-index polymer optical fiber (GI-POF) by light scattering. Using Generalized Airy theory and Debye series of an inhomogeneous cylinder, the scattering intensity distributions are obtained of Airy structure of rainbows for different refractive index profile. The results show that positions of Airy peaks depend closely on refractive index profile of GI-POF. Since each order of rainbow penetrates it to different depths, such methods could be used to provide information of the refractive index profile of GI-POF. For GI-POF with given diameter, positions of Airy peaks of rainbows are simulated as a function of refractive index profile, which can be used to inverse unknown parameters of refractive index profile. The least square method is used in inversion of refractive index profile with the given refractive index of the cladding. The results obtained agree with theoretical values with high precision. The method has the advantages of non-instructive and on-line measurement, and can be used for the measurement of other inhomogeneous droplets.

Defocused transfer function for measurement of refractive index profiles of axially symmetric optical fibers

A defocused transfer function of experimental microscopic bright-field images, closely connected with the evaluation of the relative refractive index profile of axially symmetric optical fibers, is presented. It is shown that with this function it is possible to obtain, with high precision, the defocused bright-field images of investigated fibers from their focus images and then to easily evaluate the relative refractive index profiles of the fibers.

Construction of refractive-index profile of Ag+–Na+ ion-exchange channel waveguide by two-dimensional inverse matrix method

The refractive-index profile of ion-exchange channel waveguide is reconstructed by near-field measurement combined with two-dimensional inverse matrix method, which can avoid the problems brought by multimeasurements and iteration smoothing in the process. The method used here has the advantages of convenient operation and accurate results for the characteristics of the asymmetric channel waveguide. It can be also applied to the refractive-index profile measurements of graded index fibers, polarization maintaining fibers, and single-mode planar waveguides.

Vector wave analysis for nonnormal incident rays in epimicroscopic refractive index profile measurements

We have investigated the effects of nonnormal incident rays in calculating the refractive index profile of a dielectric sample using the reflectance measurement data obtained with a scanning confocal epimicroscope and also by solving three-dimensional vector wave equations for linearly polarized light. The numerically calculated reflection data of tightly focused Gaussian beams with different numerical apertures (NAs) on planar surfaces with various refractive indices confirm that the reflectance increases with an increase in the NA of a focusing objective lens. This is due to the nonnormal incident ray components of a Gaussian beam. We have found that the refractive index obtained with the assumption of a normal incident beam is far from the real value when the NA of a focusing lens becomes larger than 0.5, and thus the variation in the reflectance for different angular components in a Gaussian beam must be taken into consideration while using a larger NA lens. Errors in practical refractive index calculation for an optical fiber based on a normal incident beam in reflectance measurements can be as large as 1% in comparison to real values calculated by our three-dimensional vector wave equations.

Reflection-type confocal refractive index profile measurement method for optical waveguides: Effects of a broadband light source and multireflected lights

We have investigated the effect of a broadband light source and back reflected light in a reflection-type confocal refractive index measurement system in detail. The effect of these factors was calculated, and a detailed analysis of the error sources was thoroughly examined. We have verified our calculated results with experiments by using a broadband light source and various index matching oils.

Measurement of refractive index profiles of axially symmetric optical fibers

A simple experimental technique for the measurement of refractive index profiles of axially symmetric optical fibers is presented. This method is based on image phase gradients introduced into a transmitted optical field by a fiber sample. An image of the phase gradients is obtained using a technique based on bright field microscopy. Then a relative refractive index profile of optical fibers is reconstructed by means of the inverse Abel transform.

Automatic fringe analysis of two-beam interference patterns for measurement of refractive index and birefringence profiles of fibres

Automatic fringe pattern analysis is a powerful and inexpensive digital image-processing technique. Two-beam polarizing interference Pluta microscope [Pluta M. Opt Acta 1971;18:661, Pluta M. J Microsc 1972;96:309] is automated by the computer-aid via CCD camera and digital frame grabber. Software program is prepared to deal with the duplicated (separated and overlapped) Microinterferograms produced by two-beam polarizing microscope. It also gives an accurate and fast automatic measurement of refractive index and birefringence profiles for fibres. In this paper, the refractive index and birefringence profiles of two different types of fibres, basalt and polypropylene (PP) fibres are presented. A new method to determine the birefringence profile of fibres from non-duplicated microinterferogram is suggested. The cold drawing process for PP fibres is studied.