The Rayleigh backscattering produced by an incident fundamental mode in a multimode step-index optical fiber was analyzed using a recently developed diffraction technique. The complete set of backward propagating modes, both radial and azimuthal, was determined and regarded. For this type of fiber, normalized mode functions were constructed in an explicit form, thus providing a unified power scale to characterize the relationships between various excited modes. The dependencies of the mode excitation efficiencies on the technical parameters of the fiber and the frequency of the launched radiation were studied. A comparison of the mode excitation efficiencies was performed with those in the fiber with quadratic refractive index profiles.

The latest results obtained with an acousto-optically Q-switched erbium³⁺/ytterbium³⁺ diode pumped fibre laser are reported. The use of Er:Yb enables short high power pulses to be obtained. Pulse durations of 15 ns and peak powers >50 W have been achieved at a 500 Hz repetition rate.

The recently developed diffraction technique of analytical investigation of the Rayleigh backscattering produced by an incident fundamental mode in a multimode optical fiber with an arbitrary refractive index profile is generalized to admit an arbitrary incident mode, either radial or azimuthal. The relative powers of all backscattered modes are determined with explicit formulas via the properly normalized transverse distributions of the incident and backscattered mode fields within the fiber cross section. The regularities conditioned by azimuthal indices are expressed via a universal set of coefficients, and dependences on the radial mode indices are estimated numerically. Excitation coefficients are shown to be symmetrical for any pair of incident and backscattered modes.

The non-linear interaction between a Rayleigh-backscattered wave and a forward travelling optical pump pulse is explored using sources separated by approximately one Brillouin shift. It is shown that the pump pulse modulates the Rayleigh backscatter at those locations where the frequencies of the counter-propagating waves differ by a Brillouin shift. As a result, a new Brillouin optical time-domain analysis technique is demonstrated that measures the spatial distribution of the Brillouin gain spectrum from one end only of the sensing fiber. In our experiments, spectra were acquired at a rate of 40 spectra per second over a sensing range of order 1 km with a spatial resolution of 3 m. The experimental arrangement and data analysis are described and a dataset is processed to extract the real part of the Brillouin gain spectrum and the determine the peak of the gain spectrum as a function of location along the fiber. We also show that the new technique detects local changes in the phase due to the Brillouin interaction. The potential and limitations of the new technique are discussed.

Distributed temperature sensors (DTS) based on Raman optical time-domain reflectometry (OTDR) are now commonly used in a wide range of industries. This paper summarises the state-of-the-art DTS systems, including recent developments, and describes some important applications.