Ultrafast evanescently coupled waveguide MUTC-PDs with high responsivity

Novel evanescently coupled waveguide modified uni-traveling carrier photodiodes (MUTC-PDs) employing a thick multi-layer coupling waveguide are reported. To improve the optical-to-electrical (O/E) conversion efficiency, a thick multi-layer coupling waveguide with a gradually increased refractive index from the bottom layer to the absorption layer is utilized. The refractive index profile facilitates the upward transmission of incident light into the absorption region, thereby enhancing the evanescent coupling efficiency. Meanwhile, the coupling waveguide, with a total thickness of 1.75 µm, expands the mode field diameter, thereby reducing the input coupling loss. Additionally, the top layer of the coupling waveguide also serves as the drift layer. This configuration facilitates efficient light absorption within a short PD length, thus ensuring ultrawide bandwidth and high O/E conversion efficiency simultaneously. Without an additional spot size coupler or anti-reflection coating, the measured responsivity is as high as 0.38 A/W for the PD with an active area of 5 × 6 µm². Meanwhile, an ultrawide 3-dB bandwidth of 153 GHz has been demonstrated.