MEASUREMENT OF THE REFRACTIVE-INDEX OF ALXGA1-XAS AND THE MODE INDEXES OF GUIDED MODES BY A GRATING COUPLING TECHNIQUE

We have determined the index of refraction of AlxGa-xAs over the wavel ength range 0.76-1.15 mum, and the composition range 0 less-than-or-eq ual-to x < 0.33, using a grating to couple light into waveguides. We f ind the mode indices of multimode slab waveguides from the coupling an gle and grating period, then calculate the bulk indices of the core an d cladding materials by a root searching technique using the analytica l formula for the effective index of a guided mode. The method gives t he core index within +/-0.001, and the cladding index within +/-0.01. We are in agreement with high precision index values for GaAs in the l iterature, confirming the method. We are in substantial agreement with literature values for AlGaAs but find a significant systematic differ ence in the composition dependence. An analysis of measurement uncerta inties shows that the determination of composition is the dominant var iable.