Two new technologies, coherent optical fiber communications and optical fiber amplifiers, are now combined together to bring forth a new era in optical fiber communications research. The purpose of this paper is to describe two topics in this innovative research area: various diversity techniques in coherent optical fiber communications, and combination of the coherent optical fiber communications with erbium-doped fiber amplifiers (EDFAs).

Semiautomated measurement of the refractive-index profiles of single-mode fibers using the scattering-pattern method developed by the same authors is described. The scattering-pattern method features very high resolution, typically 0.2 µm: hence at present it is the only method suitable for single-mode fibers. The measurement have been performed by using a semiautomated system. This system consists of an automated measuring apparatus of the scattering pattern and softwares for calculating the refractive-index profile. The obtained profiles are compared with those of preforms.

By refining the deconvolution technique proposed previously by the same authors for determining the impulse response of optical fibers, the time resolution has been improved. In some cases an impulse response consisting of several independent pulses is obtained; probably these pulses correspond to separate LP-mode groups.

A general formula giving cutoff frequencies of modes in an optical fiber having an arbitrary refractive-index profile is derived from the stationary expression of the propagation constant. It is found that the cutoff frequency is inversely proportional to the square root of the overlap integral of the refactive-index and mode-power profiles. From this general formula, an approximate formula for actual calculation is derived, which is relatively accurate when applied to lower order modes. The single-mode condition (cutoff frequency of LP11 mode) can be calculated with a good accuracy using this approximate formula. The single-mode limits computed for some practical nonuniform-core profiles show good agreement with rigorous values.

Using a tilted electric field (TEF) soft-landing collector, one of the authors has achieved an efficiency improvement of a TWT from 11.8 percent to 43.0 percent. However, the TEF collector requires a uniform magnetic field covering the entire collector space, and hence, a heavy magnet or electromagnet, which is far from practical in high-power tubes. This paper proposes the periodic magnetic field (PMF) soft-landing collector, which is a periodic field version of the TEF collector. It requires much lighter magnets than a TEF collector does. The principle, design theory, preliminary low-perveance experiment, and a relevant computer simulation are presented. Finally experiments with a medium-power UHF TWT equipped with a PMF collector are described.