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An analytical method of matching the mode of an input laser to the lowest-order mode of a passive resonant ring laser gyro is described, as are the steps in determining the location and focal length of cylindrical mode matching lenses. Results were obtained with no mode matching, with a compromise spherical lens, with horizontal mode matching only, and with the proper cylindrical mode matching lenses. Compared with no mode matching, the latter case shows that the amplitude of the lowest-order mode is increased approximately 2.5 times. In addition, the number and intensity of higher-order modes are reduced to near zero, and the relative intensity of the lowest-order mode to the higher-order mode increased from approximately 5 to approximately 60 times greater.

The dynamic optical spectrum and optical transmission characteristics of a 1.55 μm conventional distributed feedback (DFB) laser with a side mode were analysed. The laser used in the experiment came to operate in two modes although initially it had operated in single-mode. When this laser was directly modulated by setting the bias current around the threshold, the mode switching between a main mode and a side mode was observed to be similar to mode partitioning in Fabry-Perot (FP) lasers. The mode switching affects considerably the bit error rate performance of 113 km single-mode fibre transmission at 140 Mbit/s.

A new technique has been proposed for direct measurement of the cut-off wavelength, at which the first higher-order mode disappears. It uses a change of a near-field pattern of a fiber, which is excited by a variable wavelength source. The cut-off wavelength can be measured with +/-5-nm accuracy. The most suitable fiber length for precise measurement is 10-20 mm. It is found, furthermore, that the first higher-order mode under the condition near cut-off rapidly attenuates because of waveguide imperfections, in which the loss due to core-cladding boundary distortions is the most dominant.

Mode coupling coefficients were measured in a multimode step-index fiber at different steps of the process of manufacturing a multistrand optical fiber cable. It was established that the mode coupling coefficients of the unsheathed fiber were relatively small and nearly the same for all the guided modes. By sheathing with nylon and by multistranding, the mode coupling coefficients between lower-order modes increased rapidly due to microbends, while in the vicinity of the highest-order mode, they remained unchanged. From impulse response waveforms and baseband frequency responses, it was observed that mode mixing effects became more noticeable as the mode coupling coefficients increased. Also, it was found that the excess loss caused by microbends was relatively small.

We report a simple calculation allowing the prediction of the modal dispersion in single-mode optical fibres from the spectral behaviour of the mode spot size and the profile index.

A new technique for measuring the polarization mode coupling along a polarization-maintaining optical fiber is demonstrated. Additional analyses of optical time-domain reflectometry signals are used. Using a 1.34-μm Nd3+:YAG laser as a light source and an acousto-optical light switch to reduce the Fresnel reflection at the input end of the fiber, we have examined characteristics of the polarization mode couplings for four fibers with different extinction ratios. The extinction ratios evaluated by the present method are in good agreement with those obtained by a conventional technique within ±0.5 dB.

A computer program has been developed to study the total pulse response of optical fibers with profile ripple and central index depressions in the presence of arbitrary mode coupling. We have found that the magnitude of the compression of the total pulse response generated by mode coupling depends significantly on the details of the refractive-index profile of the test fiber.