Loss measurements in GaAs and AlxGa1−xAs dielectric waveguides between 1.1 eV and the energy gap

Loss measurements in thick dielectric slab waveguides of GaAs cladded by AlGaAs liquid-phase-epitaxy layers have been made utilizing spontaneous Raman scattering. In these experiments, incident radiation at 1.06 μm is focused onto a cleaved edge of the waveguide layer and Raman-scattered photons are detected at 90°. The attenuation of the resulting signal is measured as a function of the propagation distance along the guide. Spatial resolution of 0.2–0.5 mm has been achieved for samples 3–5 mm long. Results are reported for a variety of GaAs waveguide layers, including lightly doped p-type layers, n-type layers varying between 1016 and 1018 cm−3, and AlxGa1−xAs waveguide layers with x?0.1. For waveguides with impurity concentrations above the mid-1017 cm−3 range, the loss is dominated by free-carrier absorption, but for relatively pure layers of GaAs and AlGaAs a residual loss well below 1 cm−1 is observed. Transmission measurements as a function of photon energy between 1.1 eV and the energy gap of the material show that the losses are approximately constant in this energy range.