Yongwoo Park

Residual stress relaxation in the core of optical fiber by CO 2 laser irradiation

We observed residual stress relaxation by CO(2) laser irradiation in the cores of optical fibers by direct stress measurement. It was demonstrated that the mechanical stress was fully relaxed by CO(2) laser irradiation and that the remaining stress in the core was thermal stress that was due to a mismatch of the thermal expansion coefficients of the fiber core and cladding. The net core stresses after relaxation were 17, 68, and 203 MPa in Ge-B-codoped fibers drawn at 0.53, 1.38, and 3.48 N, respectively. Changes in the refractive indices of the cores as a result of residual stress relaxation were also estimated.

Effect of asymmetric stress relaxation on the polarization-dependent transmission characteristics of a CO2 laser-written long-period fiber grating

The effect of stress-induced birefringence on polarization-dependent transmission characteristics was thoroughly investigated for a long-period fiber grating (LPFG) that was fabricated by use of a stress relaxation method with a CO2 laser. A series of two-dimensional axial stress profiles for one complete period of a LPFG was measured with an optical tomographic measurement technique. We found that the asymmetry in the stress distribution of the cladding was much larger than in the core of the LPFG. The splitting of polarization-dependent loss peaks in the optical transmission spectrum was calculated based on the measured asymmetric stress profiles and was compared with an experimental result.

Measurement method for profiling the residual stress and the strain-optic coefficient of an optical fiber

A method, believed novel, is demonstrated for determining the strain-optic coefficient profile as well as the residual-stress profile of an optical fiber by use of a modified polariscope combined with a fiber-elongation apparatus. Measurement results of the residual-stress and the strain-optic coefficient profiles for Ge-doped and Er-Ge-Al-doped optical fibers are demonstrated with this method.

Residual stresses in a doubly clad fiber with depressed inner cladding (DIC)

Thermal and mechanical stresses developed in concentric three-layered optical fiber-core, and inner and outer cladding, have been thoroughly studied for various concentrations of dopants and geometric structures. In order to examine the parametric results of thermal stresses in preforms, the stresses were measured with a polariscope. The results agreed well with the theoretical calculations. The thermal stresses were calculated for three temperature ranges in which the glass in each layer has a different thermal expansion coefficient. The mechanical stresses were studied considering the normal stress in the molten neck down region and its development with time. In order to include the time dependence of the stress below softening point, Maxwells one dimensional viscoelasticity was applied. In a parametric study, the analyzes were carried out based on the fiber parameters such as relative index difference, ratio of clad to core, and depressed relative index difference. With an increase of core index above the silica, the thermal stresses in core increased linearly, but the depressed inner clad does not affect the stresses in core. From the parametric studies and modeling it was found that when the depressed inner cladding (DIC) layer has a large cross-section or high dopant concentration, the mechanical stress in core change from compression to tension.

Complete determination of the stress tensor of a polarization-maintaining fiber by photoelastic tomography

The use of photoelastic tomography to obtain the two-dimensional axial stress profile of a polarization-maintaining (PM) fiber with high resolution and accuracy is described. We illustrate, for what is believed to be the first time, the two-dimensional distribution of the local principal axes of the fibers cross section, which is directly related to the fibers PM ability. We demonstrate that the stress-induced anisotropy as well as all the stress tensor components of the fiber can be fully determined.

Characterization of a stress-applied polarization-maintaining (PM) fiber through photoelastic tomography

An accurate two-dimensional (2-D) axial stress profile of a polarization-maintaining (PM) fiber was determined using high-resolution photoelastic tomography. Likewise, determining all of the stress components of the fiber and the complete expression of stress-induced anisotropy was demonstrated. For the first time, we have expressed the anisotropy of a PM fiber in terms of birefringence distribution, and we have displayed the trajectories of principal axes on the cross section of the fiber. Mode coupling between the two orthogonal polarization modes due to the asymmetric stress applying parts of the PM fiber was also analyzed using an approximated coupled mode equation.

Determination of stress-induced intrinsic birefringence in a single-mode fiber by measurement of the two-dimensional stress profile

The two-dimensional (2-D) axial stress profile of a single-mode fiber is obtained with an optical tomographic measurement technique. All stress components of the fiber are calculated from a measured axial stress profile. We demonstrate that the differential group delay induced by intrinsic nonsymmetric stress as well as the induced linear birefringence between two orthogonal polarization modes can be determined with an analytic technique based on a vector perturbation method from a measured asymmetric 2-D stress distribution.

Simple scanning fibre-optic confocal microscopy for the refractive index profile measurement of an optical fibre

A simple servo-controlled scanning fibre-optic confocal microscope is proposed for determining the refractive index profile of an optical fibre by measuring the reflectance of the end surface of the fibre. Great simplicity has been achieved in confocal microscopy by using a lensed fibre and a 3?dB fibre coupler instead of an objective lens and a beam splitter. A dithering servo control with a phase-locked loop is employed to maintain a constant distance between the lensed fibre and the surface to be measured during the scanning process. The accuracy of the measured refractive index is better than 10-4.

Characterization of elliptic core fiber acousto-optic tunable filters operated in the single mode and the multi-mode range

Summary form only given. We have fabricated an elliptical-core fiber that can be applied to broadband acousto-optic devices operated in both 1.5 and 1.3 /spl mu/m communication windows. We believe the interaction of acoustic wave with the optical modes to have a strong potential in fiber characterization.

Measurement method for profiling the residual stress of an optical fiber: detailed analysis of off-focusing and beam-deflection effects

The effects of off-focusing and beam deflection on polarimetric stress measurements of optical fibers are investigated. A simple method for reducing the distortion of the phase retardation caused by unwanted beam deflections in residual stress measurement is introduced. The method is examined numerically by ray-tracing techniques and experimentally by use of hollow silica fibers into which various index-matching liquids have been inserted. An autofocusing technique is introduced. The error in stress measurement reproducibility was determined to be less than 4%. We tested the absolute error in measured stress by applying incremental external tension and determined that it is less than 0.464 MPa.