The fast deployment of high-performance data centers accelerates the development of O-band wavelength division multiplexing (WDM) and its devices. To minimize insertion loss in WDM devices, it is essential to maximize coupling efficiency between optical fibers and waveguides at micro- and nano-scale. In this paper, we propose a low-loss waveguide tapered coupler and its optimization around 1310 nm. The coupler has a vertically placed double-layer rib taper as an intermediate waveguide, and the inverted taper is in a tapered tandem structure. Rib tapers are introduced in the intermediate waveguide and the fiber is coupled to the rib tapers, which are adiabatically reduced in width along the direction of the coupler to vertically reduce the mode size. To minimize footprint, a cascaded inverted taper is introduced. The maximum taper angle per stage corresponding to the long adiabatic taper is first determined. Subsequently, the long tapers are replaced with new shorter tapers. Several parameters are optimized using frequency domain time division (FDTD), including the output waveguide and input surface dimension, the height ratio of rib tapers, the length of inverted tapers, the length of rib tapers, and the tapered profile of tandem structure. The optimization results show that the coupler has an insertion loss of only 0.06 dB at 1310 nm with a device length of 513.4 µm. The two-stage in-plane coupler operates in O-band narrow-band with low insertion loss, offering an efficient solution for coupling fiber to on-chip waveguides.
