Radiation of discrete beams from curved single-mode fibres

A new attempt to explain the phenomenon of discrete radiation from bent single-mode fibres is presented. It is suggested that the set of radiated beams is an interference pattern of the radiation fields of more `whispering-gallery? modes propagating along the fibre coil.     

Discrete radiation from curved single-mode fibres

The energy loss and field distribution in a curved single-mode fibre are calculated in terms of the coupling between the fundamental (LP01) mode and the radiation field. In agreement with recent experimental results, it is found that, initially, light travels in a zigzag path around the bend, and radiation is emitted in discrete beams.     

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.     

Discrete radiation from curved single-mode fibres: a comparison of theories

An apparent discrepancy between coupled-mode and `whispering-gallery? theories for the radiation of discrete beams from curved single-mode fibres is resolved. It is shown that the two formally different approaches are in very good agreement with each other in predicting the periodicity of the radiation.     

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.     

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.     

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.     

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.     

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.     

Stress birefringence in single-mode fibres

The core and cladding of a glass fibre exposed to axial tension become optically uniaxial. The modified characteristic equation is presented and, for the fundamental mode, the changes in the most important propagation parameters have been calculated.     

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.     

Fabrication of low-loss single-mode fibres

Low-loss single-mode fibres with GeO2-doped SiO2 cores have been made by the c.v.d. method and the loss spectrum from 0.6 to 1.8 ?m measured. The minimum loss of the fibre with suitably designed parameters, characterised by the refractive-index difference and the core diameter, was 0.75 dB/km at 1.05 ?m.     

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.     

Circularly birefringent single-mode optical fibres

The rotation of polarised light which occurs in a twisted anisotropic medium is exploited to produce very high circular birefringence in single-mode optical fibres. Beat lengths of less than 5 mm have already been achieved in some preliminary experiments.     

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.     

Two-dimensional index distribution determination from near-field measurements in single-mode fibres

The refractive-index distribution of single-mode optical fibres can be obtained from transmitted near-field intensity measurements. The generalisation of the method from conventional fibres to noncircularly symmetric guides (e.g. polarisation-maintaining fibres) is discussed. Results obtained both on circular and on polarisation-maintaining fibres are presented.     

Dispersion-flattened single-mode fibres with minimised dopantexpenditure

The fibre profile which meets certain requirements concerning chromatic dispersion, mode field diameter, attenuation and cut-off at minimal dopant expenditure is defined as `natural refractive index profile'. In the case of a dispersion-flattened single-mode fibre, this profile is of the QC-type, with interesting further structures     

Acousto-optic phase modulator for single-mode fibres

A compact fibre-optic phase modulator with a single-mode fibre positioned along the axis of a cylindrical PZT acoustic transducer is investigated. The fibre is bonded inside a glass capillary tube, which is in turn epoxied inside the transducing cylinder to give a solid, mechanically stable structure. The maximum sensitivity of the phase shifter is 0.058 rad/V cm at 6.02 MHz. No polarisation modulation is observed.     

Joint characteristics between polarisation-maintaining single-mode fibres

The jointing characteristics of polarisation-maintaining single-mode (PMSM) fibres are described. The power coupling loss and extinction ratio due to angular misalignment of the main principal axis at the fibre joint are measured.