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Some experimental aspects of the problem of efficient coupling between semiconductor waveguides and singlemode optical fibers are examined. Waveguides based on different InP/InGaAsP structures (single-and doubleheterostructures, multi-quantum-well structures (MQW) with different degrees of dilution) have been fabricated, as well as modified single-mode fibers (i.e. with flat or lënsed, straight or tapered ends). Coupling efficiency measurements between these structures have been performed, and the results are presented and discussed; the lowest coupling loss, about 0.5 dB, was obtained with a conveniently tapered fiber and a moderately diluted MQW structure (MD-MQW). I . INTRODUCTIONThe technology of guided-wave components on semiconductor materials has progressed to the point that many different optoelectronic components can be fabricated for a variety of functions, with good performance and yield; however, a very important problem is still to be solved satisfactorily, that is efficient coupling between these circuits and the outer world, most frequently constituted by a single-mode optical fiber.The conventional single-mode semiconductor waveguides exhibit large index differences between guiding and cladding regions, while having small cross sections in comparison to standard fibers; this leads to strong modal mismatch between the two guiding structures, so that their coupling efficiency, given by the overlap integral: f * 2 j E(x,y) Eg (x,y) dx dy 11=1 * I * .1 EfEf dXdYjEgEgdXdY where Ef and Eg are the fiber and the guide modal fields respectively, is normally quite poor. To overcome this problem, either the semiconductor waveguide or the optical fiber (or both) has to be modified, in order to get more similar modal parameters. Some authors L 2, 3 modified the optical fiber, usually an easier approach, while others modified the semiconductor guiding structure to increase the spot size of the guided mode, either by decreasing the index difference between the guiding and the cladding regions or by decreasing the confinement factor 5.In the present case we examine both sides of the problem, with the constraint of longitudinally uniform waveguide structures, for reasons of reproducibility and yield. On one side, complex structures based on diluted MQW stacks (D-MQW) are necessary to bring the spot size and numerical aperture of a semiconductor guide closer to the corresponding parameters of a standard fiber, since quaternary compositions with a refractive index sufficiently close to InP are extremely difficult to grow reproducibly. On the other hand, a large "dilution" (i.e. a small ratio between well and barrier thickness) is associated to a poor lateral confinement of the mode, with severe limitations to the usable guide bends, so that moderate values of this dilution (MD-MQW structures) seem a more effective trade-off from the point of view of the semiconductor guide; this requires, however, some action also on the fibre side, to handle the residual modal mismatch. In this paper we compare the performance 0-81...

A systematic study on integrated wavelength multi/demultiplexers is presented, in which the experimental features of the devices, the theoretical predictions of a simple model, and the results of extensive modelling by a BPM analysis are compared. The components are based on the two-mode interference (TMI) principle, and are fabricated on soda-lime glass by Ag-Na ion exchange. The modelling is performed by CAOS, a user-friendly general purpose simulator, based on a FD scheme with transparent boundary conditions. The agreement between experimental and simulated spectral characteristics is very good; moreover the proposed analysis procedure allows the determination of the maximum index variation An and the diffusion depth D which describe the optical and geometrical characteristics of the waveguides.