Modal birefringence and its sensitivity to temperature and hydrostatic pressure were measured versus wavelength in three elliptical-core fibers and one fiber with stress-induced birefringence. We carried out the measurements in the spectral range from 633 to 843 nm by using interferometric methods. In fibers with elliptical cores all the measured parameters showed high chromatic dependence, whereas in fibers with stress-induced birefringence this dependence was weak. We modeled the dispersion characteristics of two elliptical-core fibers by using the modified perturbation approach first proposed by Kumar. The modification consists of introducing into the expression for the normalized propagation constants an additional perturbation term that contains information about stress-induced birefringence. The results of modeling show that the temperature and pressure sensitivity of elliptical-core fiber are associated primarily with variations in stress induced by these parameters. The agreement between measured and calculated values of sensitivity in the worst case was equal to 20% for modal birefringence and temperature sensitivity and 50% for pressure sensitivity. Lower agreement between measured and calculated values of pressure sensitivity is most probably associated with uncertainties in the material constants used in modeling.

Dispersion effects in elliptical-core highly birefringent fibers

A special type of elliptical step-index fiber with a conducting helical winding on the core-cladding boundary is proposed and investigated theoretically. Using elliptic cylindrical coordinates, boundary conditions are derived and longitudinal field components for the even and odd modes are obtained. The characteristic equation for the waveguide to be studied is determined by solving the fourth-order determinantal equation, which also contains complex quantities. In order to study the fundamental mode, the modal index /spl nu/ is put as /spl nu/=1. The equations obtained involve modified Mathieu functions of the first order and their derivatives. Applying approximations, cutoff conditions are obtained. Numerical computations are made. Dispersion curves are obtained for different pitch angles of the helical winding.

Modal characteristic equation and dispersion curves for an elliptical step-index fiber with a conducting helical winding on the core-cladding boundary - an analytical study

The point-matching numerical method is here employed for the modal analysis of the elliptical optical fiber of any eccentricity. Good agreement with other analytical and numerical methods is obtained. Previous disagreement in the literature is resolved.

On the higher order modes of elliptical optical fibers

Cutoff frequencies of the first 12 odd and even higher-order modes in a two-layer elliptical fiber are computed from a rigorous analysis of the boundary value problem. Cutoff characteristics of modes in rectangular and elliptical fibers in the literature are reviewed. Mode correspondence, field symmetries, and physical considerations are invoked to establish confidence in previously published results on the first higher-order mode cut off in elliptical fibers. Very large birefringence fibers are shown to have low values for the first higher-order mode cut off; hence, their usefulness in single-mode applications is limited. >

On higher order mode cutoff frequencies in elliptical step index fibers

In this paper, the propagation of electromagnetic waves in W-type elliptical dielectric optical fiber having various cladding layers is presented. The presented fiber has concentric core and cladding which have elliptical cross section and the refractive index of one of the inner cladding (the cladding between first clad and last clad) maximum. Using elliptic cylindrical coordinates, boundary conditions are derived and longitudinal field components for the even and odd modes are obtained. The characteristic equation for the fiber to be studied is determined by solving the Mathieu (q > 0) and the modified Mathieu functions (q < 0). In order to study the fundamental mode, the modal index m is put as m = 1. Finally, the cutoff frequencies for several lower order modes have been calculated and their dispersion characteristics are plotted. The effects of elliptical eccentricity e on the mode cutoff frequencies and mode transmissions are also addressed. The analysis shows that one can control the propagation property of optical fiber by increasing the number of inner claddings. These claddings provide additional degree of freedom to control the modes.

Modal analysis and dispersion curves of an elliptical W-type single mode fiber

Polarization holding in high-birefringence elliptical-core fibers is evaluated for the fiber birefringence spatial frequency range 1.5 cm/sup -1/ < Beta/sub i/ < 40 cm/sup -1/, corresponding to beat lengths from 1.6 mm to 4.2 cm. This range of spatial frequencies is spanned by making measurements with a broad-band light source on four fibers with different degrees of birefringence. In this way, the strength of the internal birefringence perturbations is mapped to give the first experimental measure of their power spectrum. It is shown that commonly available fiber jackets can significantly degrade the polarization holding. For low spatial frequencies, the strength of the perturbations decreases rapidly with increasing frequency, but this rate decreases by more than half over a one-and-a-half order of magnitude increase in spatial frequency. A possible origin of the perturbations is suggested and it is shown that the strength of these perturbations must be reduced if polarization holding to a very high degree is to be realized in elliptical-core fibers. Presently, internal perturbations limit the polarization holding to /spl les/14.4 dB over 1 km.

S.C. Rashleigh

Papers

Polarization Holding in Ellipitical-Core Birefringent Fibers