A high-speed parallel optical interconnect (POI) incorporating silicon nitride (${\mathrm{Si}}_3{\mathrm{N}}_4$) waveguides was realized that takes advantage of an eight-channel input/output interface based on grating couplers (GCs) in alignment with fiber arrays. For each of the channels, a straight waveguide in the middle is linked to a GC via a taper, which is addressed by a single-mode fiber (SMF) at the input and a multimode fiber at the output. To verify the feasibility of the interconnect, the alignment tolerance has been explored in terms of the position and angle of incidence of the SMF with respect to the GC. This has been conducted by probing into the optical throughput of the proposed POI, which is determined by different sources of loss associated with the proposed ${\mathrm{Si}}_3{\mathrm{N}}_4$ waveguide device. For a typical GC, the measured coupling loss was 5.2 dB at the center wavelength of 1590 nm, when addressed by a SMF. For a 1 dB loss penalty, the positional tolerance of the fiber was discovered to about $\pm 3\mathrm{\mu m}$ and 45 μm along the lateral direction and the direction inclined at 16 deg normal to the device, respectively. The corresponding angular tolerance was $\pm 1\mathrm{deg}$. We ultimately confirmed that $8\times 10\mathrm{Gbps}$ high-speed digital signals were efficiently delivered through the proposed POI.