In this study, we demonstrate a novel, to the best of our knowledge, integrated indium phosphide (InP) and silicon nitride (Si3N4) waveguide platform, which is based on interlayer coupling, to achieve heterogeneous integration of a photodetector and waveguide ring resonator firstly. In order to improve the gyro bias stability, the Si3N4 and InP waveguides were designed with a high polarization extinction ratio and ultra-low loss. Three-dimensional finite difference time domain methods are used to optimize the InP taper dimensions to provide efficient optical coupling between the Si3N4 and InP waveguides. The optical coupler with a length of 100 µm is designed to achieve optical coupling between the Si3N4 and InP waveguides while maintaining its state of polarization all the way from the taper waveguides. The coupling efficiency of the optimized interlayer coupler has been improved to about 99.5%.