We have demonstrated the operation of UV-B laser diodes at room temperature by current injection using lattice-relaxed high-quality AlGaN fabricated on a sapphire substrate. Meanwhile, there are still many issues to be solved for practical use, especially the high threshold current required for laser oscillation and the low light output obtained. In this presentation, we report the results of our study on the reduction of lateral optical loss by applying a refractive index waveguide structure to reduce the threshold current. Specifically, device design and process technology development were carried out, and as a result, laser oscillation with an oscillation wavelength of 298 nm and a threshold current of 85 mA was obtained under room temperature pulse driving. In addition, optimization of the device layer structure for increasing the output power will be discussed.
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