Birefringence in elliptically deformed optical fibres

The phase difference between the two polarisation states of the fundamental mode on a slightly elliptical, weakly guiding fibre is derived using both an exact analysis and perturbation theory. Deficiences in some earlier calculations are also discussed.     

Birefringence in slightly elliptical optical fibres

The accuracy of first-order approximations to the degree of birefringence of monomode fibres is examined by calculating the magnitude of second-order corrections. It is found that, for most fibres encountered in practice, the first-order result is perfectly adequate.     

Birefringence in optical fibres with elliptical cross-section

A comparison is made between various approximations for the phase delay between orthogonally polarised modes in elliptical optical fibres. A more accurate analysis is presented which gives a lower value for the birefringence produced by a given ellipticity. The effect on fibre bandwidth is shown to be small compared with that resulting from stress birefringence.     

Orientation- and polarisation-dependent cutoff wavelengths in elliptical-core single-mode fibres

The direct measurement of cutoff wavelengths from the spectral dependence of the near-field intensity distribution has been improved by selectively exciting the first higher-order mode and applying it to elliptical-core single-mode fibres. Depending on the orientation and polarisation of the first higher-order mode, four different cutoff wavelengths have been identified.     

Conditions for obtaining zero polarisation mode dispersion in elliptical-core fibres

It is shown that zero polarisation mode dispersion can never be achieved in elliptical-core single-mode fibres by geometrical birefringence alone. To equalise the group delays of the two fundamental modes, stress-induced birefringence must be introduced to cancel, partly or completely, the geometrical birefringence, depending on the application.     

Wavelength dependence of polarisation mode dispersion in elliptical-core single-mode fibres

Polarisation mode dispersion in elliptical-core single-mode fibres has been measured by a spatial technique based on a visibility maximum position measurement in an interferometer. Using the technique, wavelength dependence of the modal dispersion has been measured by varying optical source wavelengths between 821 and 904 nm. As a result, contribution of geometrical and strain birefringences on the modal dispersion has been evaluated, and normalised frequency dependence of the modal dispersion has been clarified.     

Orientation-dependent time delays of the first higher-order mode in elliptical-core fibres

Time delays of the differently orientated LP11 modes in elliptical-core fibres have been measured in dependence on the wavelength. These measurements permit one to determine the orientation-dependent effective cutoff wavelengths. An empirical expression relating LP11-mode delay splitting due to slight ellipticity and LP01-mode chromatic dispersion is obtained.     

Birefringence in benzocyclobutene strip optical waveguides

We report the fabrication of benzocyclobutene strip optical waveguides and the measurement of their birefringence properties. To take into account the stress-induced birefringence in the waveguide, we generalize the formulas reported previously for the analysis of strip waveguides. Our experimental results are shown to agree closely with the theoretical calculation. The condition for achieving zero modal birefringence by control of the aspect ratio of the strip of the waveguide is highlighted. The dependence of the birefringence in the waveguides on the temperature is also measured and discussed.     

Measurement of polarisation mode dispersion in elliptical-core single-mode fibres at 1.3 ?m

Polarisation mode delay differences in three single-mode fibres were measured interferometrically at 1.3 ?m with a resolution below 25 fs. Polarisation mode dispersion increases strongly with core ellipticity.     

Birefringence and polarisation dispersion measurements in high-birefringence single-mode fibres

We have measured by white-light crosscorrelation the polarisation mode dispersion of a polarisation-maintaining single-mode fibre and its variation with wavelength. The results have been confirmed by birefringence measurements and by birefringence modelling.     

Birefringence characteristics of benzocyclobutene rib optical waveguides

Zero-birefringence benzocyclobutene polymer rib waveguides as well as waveguides that exhibit low birefringence over a wide range of temperature are demonstrated. Experimental results agree well with theoretical calculations that take into account stress-induced birefringence.     

Pulse compression in optical fibres

A shift in the emission wavelength of s.h. GaAs lasers has been observed during individual pulses. In conjunction with material dispersion, this leads to pulse compression after transmission through an optical fibre. The use of this device as a source in bandwidth measurements readily leads to erroneous conclusions when results are analysed.     

Frost-resistant coated optical fibres

Investigations of optical fibre losses in the range from -100°C to 20°C have shown that silicone crystallisation causes a significant loss increase in coated MCVD fibres, irrespective of the secondary jacket material. The application of noncrystallised silicone has provided a low level of the excess losses at low temperatures.     

Modes of propagation in optical fibres

Light propagates along optical fibres in form of normal modes. As long as fibres are uniform in axial direction, these modes follow as solutions of the eigenvalue problem which Maxwell’s equations together with the boundary conditions of the particular fibre represent. Such modes propagate independently from each other and transmit and distort signals according to their respective delay, dispersion and attenuation. Imperfections of the fibre couple its modes with each other and change its signal attenuation and transmission characteristics. When a randomly imperfect fibre guides many modes, the random coupling leads to a new set of statistical modes which again propagate independently from each other and describe the average power in each of the perfect fibre modes. Interaction between statistical modes occurs only at points along the fibre where the statistics of imperfections change such as at connectors or splices.     

Dispersion in low-loss liquid-core optical fibres

Measurements of pulse dispersion in liquid-core multimode fibres have been made for different fibre curvatures and core diameters. The results indicate the presence of mode-conversion effects, and a mode-filtering mechanism is demonstrated with a probing beam of narrow angular width. By measuring transmission loss as a function of angle of incidence, the loss in the cladding material is obtained.     

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.     

Modulational instability in double-doped optical fibres

Modulational instability in double doped optical fibres with a non-Kerr nonlinear refractive index change has been analysed in the anomalous dispersion regime. Applications to fibre-optic coherent communication systems and all optical signal processing are briefly discussed.<>