Single-mode Graded-index Fibers Research Articles

A numerical method for the analysis of single-mode low-dispersion graded-index fibers

A numerical integration method for the calculation of the time dispersion of optical-fiber modes is introduced and used to determine the parameters of single-mode graded-index silica-based fibers for the 1.55-μm band. The method gives an accuracy on the order of 0.001 ps/km/nm, and it was possible to quantify the error in assuming the overall time dispersion to be the sum of the material and waveguide dispersions. The analysis indicates that when large material dispersion is to be neutralized by waveguide dispersion, the outer diameter of the cladding must be increased to avoid higher microbending losses. When the difference between the refractive indices at the center of the core and in the cladding decreases, the ratio between the cladding and core diameters must be increased.

Accurate analysis of single-mode graded-index fiber directional couplers

We present an accurate coupled-mode analysis to evaluate the coupling characteristics of identical single-mode graded-index fiber directional couplers. We also give an empirical relation for calculating the coupling length of a fiber coupler formed by fibers having profile parameter q , normalized frequency V , and normalized separation d/a ; d is the distance between the fiber centers and a , the radius of the fiber core.

Transmission properties of graded-index triple-clad single-mode fiber for 1.55-µm system operation

Graded-index triple clad (GITC)single-mode fiber exhibits low loss and low dispersion (< 2 ps/km-nm) over a wide wavelength range. The fundamental mode field radius of this fiber was measured as a function of wavelength using the transverse offset technique. This showed that the fundamental mode radial power (or field) distribution can be well represented by a Gaussian approximation. The basket weave loss test was made to obtain the fiber bend sensitivity. The bend induced additional losses of GITC single-mode fiber at 1.55 μm is much smaller than that of single-clad single-mode fibers.

Radiation from graded-index single-mode fibres

Radiation from the fundamental mode due to sources in graded-index single-mode optical fibres can be examined by determining the effect of the presence of the perturbing core on the free-space (i.e. uniform cladding) radiation field. We look at this effect for fibres with power-law-profile cores. Using these examples we discuss the following two approximation schemes: (i) a new equivalent step approximation where the step has the same ‘degree of guidance’ as the graded core and (ii) a previously proposed WKB asymptotic approximation.

Prediction of mode dispersion in graded-index single-mode fibres

A Chebyshev power series method is presented for the calculation of the propagation constant of graded-index fibres. Accurate predictions of the dispersion parameters can be obtained as shown by examples of step and parabolic profiles. Measured index profiles can also be used with local index values found by linear interpolation from nearest reading points. Examples are given to demonstrate the effectiveness of this approach.

Splice-loss evaluation for single-mode graded-index fibers

In this paper we present highly accurate and simple methods for the evaluation of splice loss in single-mode graded-index fibers. These are based on accurate approximations of the modal field of such fibers which we have recently developed. Splice losses for angular and transverse offsets have been expressed in analytical forms which are quite easy to evaluate. A comparison of our results with those obtained using the Gaussian approximation is also presented which shows that our method is much more accurate than the latter.

Experimental characterisation of graded-index single-mode fibres

A simple and accurate method is presented for experimental characterisation of graded-index single-mode fibres using their far-field radiation patterns and the `equivalent-step? method.

Mode dispersion, material dispersion and profile dispersion in graded-index single-mode fibres

A study of single-mode fibres shows that in the normal operating region the cladding has as much effect on the propagation constant as does the core. A detailed analysis shows that the complex expression for pulse dispersion can be arranged in three groups of terms which may be defined as composite material dispersion, waveguide dispersion and composite profile dispersion. The analysis has been applied to a fibre for which the material dispersion parameters are accurately known and it is found that zero total dispersion can be obtained at a wavelength which depends on the profile and the core diameter but may be selected within a range roughly 1.3-2µm. However, there are severe requirements on the control of core diameter and profile.

Spot size of graded-index single-mode fibers: profile-independent representation and new determination method

A simple nondestructive method is described for determining the spot size omega(0) (width of the fundamental mode)-one of the most important quantities to characterize monomode optical fibers. The theory involves the definition of effective values of core radius and normalized frequency. Thereby, graded-index fibers can be replaced by equivalent step-index fibers. The experiment consists in illuminating the fiber perpendicularly to its axis and determining the position of the first minimum in the diffraction pattern. In this way, just one measurement yields omega(0) regardless of the index profile and for any operating wavelength, provided the cutoff wavelength is known.

Mode dispersion, material dispersion and profile dispersion in graded-index single-mode fibres

A study of single-mode fibres shows that in the normal operating region the cladding has as much effect on the propagation constant as does the core. A detailed analysis shows that the complex expression for pulse dispersion can be arranged in three groups of terms which may be defined as composite material dispersion, waveguide dispersion and composite profile dispersion. The analysis has been applied to a fibre for which the material dispersion parameters are accurately known and it is found that zero total dispersion can be obtained at a wavelength which depends on the profile and the core diameter but may be selected within a range roughly 1.3–2 μm. However, there are severe requirements on the control of core diameter and profile.

Polarization characteristics of noncircular core single-mode fibers

It is well known that geometrical or dielectric imperfections in conventional graded-index single-mode fibers depolarize light after a few centimeters. A slight improvement in the polarization performance of these fibers is achieved by introducing noncircularity in the core shape. This is evident from the measurements on borosilicate fibers with dumbbell shaped cores. This result is correlated with Marcatili's analysis, which shows that changing the core geometry, from square to rectangular, does not appreciably alter the difference in the propagation constants of the two fundamental modes with orthogonal polarizations. Thus, the noncircular geometry and the associated increase in stress-induced birefringence introduced during the manufacturing process alone are not sufficient to improve the polarization performance, and the enhancement of the anisotropic birefringence is necessary to achieve single-polarization fibers.