Volume 5, Issue 3

A simple and reliable method to excite selectively different annular zones of a multimode graded index fiber has been implemented. This technique, exploring concentric layers of the fiber core with different chemical compositions, can find application in differential measurements of material properties. On the basis of a ray optics picture of propagation, a light beam having angular and spatial dimensions much smaller than the relevant dimensions of the fiber was employed for selective excitation. Different annular configurations have been obtained varying the position and angle of incidence on the fiber end. Results obtained by an apparatus consisting of standard components only on low loss communication fibers are reported for different refractive index profiles and lengths up to 100 meters.

When two different multimode fibers are jointed together, mode mixing and mode filtering take place, with different characteristics in the two directions, leading to a non-symmetric behaviour. A detailed analysis of the power transitions among the guided modes at the joint is presented here. Explicit formulas for coupling efficiency and intermodal dispersion compensation are derived. The intentional mode mixing which is produced by suitable beam-launchers, in order to reach an optical power distribution independent of that of the optical source, is also investigated.

The potential applicability of multimode integrated optical branching waveguides and couplers is limited primarily by their coupling loss to optical fibers. To get a reasonably detailed theoretical insight, the fiber-to- fiber coupling loss of a section of a multimode channel waveguide fabricated, e.g., by ion exchange in a glass substrate is calculated using a ray-optic approximation. An approach of the length-averaged coupling loss results in a simple and efficient calculation procedure which enables one to optimize the parameters of a channel waveguide for minimum coupling loss. Coupling losses as low as 2 dB and 0.25 dB can be obtained for single channel and sandwich structures, respectively, coupled to standard graded-index fibers.

In this paper a complete analytical treatment of the step response of GaInAs PIN photodiodes is presented. The analysis is based on the continuity equations, and the effects of the reflection upon the back surface is taken into account. The obtained results are presented in the form of diagrams suitable for use in the photodiode designing.

A low-cost video baseband transmission system using analog light-intensity modulation with an 850 nm LED compensated for nonlinearity is presented. A very low- noise current amplifier at the input of the receiver and a high-dynamic range automatic gain control provide a transmission system that can accomodate more than a 20 dB difference in optical losses without any adjustment. At the receiver input, a 100 nW (- 40 dBm) optical signal is required for surveillance transmission quality. The transmitter delivers an optical signal power of - 18 dBm to a 50 pm graded-index fiber. The differential gain and phase of the system lie below 2% and 1°, respectively.