CHARACTERISTICS OF RIB WAVE-GUIDES IN ALGAAS

We have studied the properties of isolated and coupled optical rib waveguides (RWG) as a function of rib height and width in GaAs layers on AlGaAs for TE-like and TM-like polarizations. We compare mode shape parameters for isolated RWG to the coupling constant, K, obtained from distributed couplers. The effective index model accurately describes the mode distributions and the coupling constant for single-mode RWG. The model also gives an accurate estimate for anodic oxide loaded strip waveguides. In this model, the relative lateral difference in the effective dielectric constant, Δx, is determined by the depth characteristics (the rib height, layer thicknesses, and compositions) and the free-space wavelength. The lateral confinement of guided modes and K are then determined by Δx, the effective dielectric constant, the waveguide width, the waveguide spacing, and the free-space wavelength. At λ≈1.06 μm, ribs of height h≈0.05 μm on GaAs waveguiding layers of thickness t≈0.75 μm with underlying Al 0.1Ga0.9As cladding provide Δx≈1. 7×10-3 for TE polarization and ≈2×10-3 for TM polarization. In such layers K≈1/mm for pairs of RWG with 3 μm width and 3 μm spacing. Model Δx results are given for typical layer compositions and typical layer and rib dimensions. Eigensolution results for K are given as a function of Δx for typical rib width and spacing dimensions. For more general application to RWG, to anodic loaded strip guides, and to metal gap guides, scaled K and linearized Δx results are given in dimensionless form. We also report experiments on lateral tapers to widen or narrow the waveguides for control of mode shapes, and on lateral ramps to control the location of the RWG. Tapers introduce negligible loss, but cause some mode conversion if multimoded guides are involved. Ramps introduce negligible loss if they are at sufficiently low angles.