Fabrication of single-mode fibres exhibiting extremely low polarisation birefringence

An investigation of the relative effects of core ellipticity and stress-induced birefringence on fibre polarisation properties is described. It is found that a reduction in core stress levels is necessary to obtain low-birefringence fibres. A fibre having a retardation of only 2.6°/m is reported.     

Single-polarisation single-mode guidance in stress-induced birefringence fibres with lossy stress-applying sections

When the refractive index of the stress-applying sections in a stress-induced birefringence fibre lies between the effective indices of the two polarisation modes, the polarisation mode with lower effective index is essentially guided by the stress-applying sections. Single-polarisation guidance can thus be achieved by introducing absorption loss in the stress-applying sections.     

Production of single-mode fibres with negligible intrinsic birefringence and polarisation mode dispersion

Spinning the preform during drawing produces a fibre with a permanent twist. It is shown that such a fibre has negligible polarisation birefringence and rotation. Polarisation mode dispersion is similarly reduced.     

Backscattering in single-mode fibres

The scattering process in single-mode optical fibres is considered in terms of wave optics rather than geometrical optics, which is inadequate in this case. Surprisingly, however, the result for the backscattering signal at the input end of the fibre is nearly the same as for multimode fibres.     

Anisotropy in spun single-mode fibres

It is shown that the dominant process for the reduction of birefringence in a spun fibre is the averaging of the local fibre anisotropy by the rapid procession of the axes of asymmetry along the fibre. No significant change in cross-sectional geometry occurs.     

Zero-mode dispersion in single-mode fibres

Near cutoff the loss of the LP11-mode in cladded optical fibres becomes very high in the presence of even slight bends. Singlemode operation can thus be achieved with zero-mode dispersion as well as zero material dispersion, thus promising enormously large bandwidths.     

Jointing loss in single-mode fibres

The jointing loss of single-mode fibres is mainly dependent on their numerical apertures, and is largely independent of normalised frequency and fibre diameter. The letter presents simple loss predictions as an aid for practical design work of single-mode fibre joints.     

Polarisation conservation in single-mode fibres

There exist proper orthogonal elliptical polarisation states, which are maintained over long fibre length without converting. When one of the proper polarisation states is excited, orthogonal polarisation less than ?40 dB is confirmed after 5 km single-mode fibre transmission. It is also proposed that twisted single-mode fibres show a high degree of polarisation for any incident polarisation state. The polarisation characteristics are hardly affected by external perturbations.     

Dispersion in graded single-mode fibres

The normalised frequency for zero waveguide dispersion of the HE11 mode, called VOD, is highly dependent on profile shape. The more the profile departs from a step, the greater the increase of VOD above the effective single-moded region until, for example, VOD becomes infinite for profiles that fall off at a rate greater than or equal to the parabola.     

Tee-coupler for single-mode fibres

A new concept for an end face tee-coupler with single-mode fibres is described. The outer surfaces of the two fibres are ground and polished on one side and then fixed together. A third fibre is then butt-jointed. The dividing ratio between the two fibres can be chosen arbitrarily. The total loss is about 2 dB. The coupler is polarisation-independent.     

Dispersion-shifted convex-index single-mode fibres

The structural design of the single-mode fibre with a convex-index profile is described to achieve zero dispersion at 1.55 ?m. As a result of the fibre design, it is clarified that the bending loss of the convex-index fibre is the smallest among the losses of various index profile fibres such as step, triangular and Gaussian ones.     

Tunable single-mode excitation in multimode fibres

Mode selection in lasers is used in order to provide high-purity easy-to-implement single-mode launching in optical fibres. Mode-to-mode switching automatically maintains centring, scaling and alignment. Application of the principle to selective launching into a weakly multimode fibre (V=4) is demonstrated.     

Temperature sensing in twisted single-mode fibres

Measurements of the temperature dependence of the elasto optically induced optical activity in twisted single-mode fibres are reported. A simple method for providing temperature-free twisted single-mode fibres has been derived and successfully tested. Furthermore, we have constructed and tested a temperature sensor showing excellent linearity over the tested 20?120°C range.     

Second-harmonic generation in single-mode optical fibres

We consider the origin of frequency-doubling in single-mode optical fibres. We show that the presence of nonlinear electric quadrupole and magnetic dipole moments can explain the initial formation of frequency-doubled light, and present the results of a detailed theoretical analysis of these effects which is in good agreement with experiment.     

Multiquartic approximation in single-mode optical fibres

A new stratification method based on the approximation of the actual index profile of the fibre by means of a set of quartic curves is proposed. The method permits one to deduce accurately the propagation characteristics in the least calculation time, particularly on fibres with nonmonotonic index profiles.     

Photorefractive gratings in single-mode optical fibres

Proposes a theory for the formation of photorefractive gratings in single-mode optical fibres. The mechanism involves the formation of periodic space-charge electric fields by the photoexcitation of defects. The theory predicts amplitudes for the index gratings in close agreement with experiment.<>     

Radiation from single-mode helical fibres

The fundamental mode of a helical fibre is not strictly bound and radiates at all wavelengths. For practical purposes, radiation is negligible unless the pitch angle is sufficiently large or the pitch length short enough. The loss edge is mainly due to pure bending loss for the former, and to helical ripple loss for the latter. A simple loss expression is presented.     

Routine characterisation of single-mode fibres

In single-mode fibres, the core diameter and refractive-index difference between core and cladding may be found from the widths of (a) the halfpower points and (b) the first minimum of the far-field radiation. The technique has been used to study the effect of heat treatment.     

Radiation from curved single-mode fibres

A study of propagation in curved single-mode fibres shows that the transmission loss increases sharply below a critical bend radius. However the radiation does not leak away uniformly with distance but in a series of discrete well-defined rays.