In this paper, the design of low-loss multimode interference (MMI) couplers is reported. The proposed devices can be used as power splitters or combiners and are based on lithium niobate on insulator (LNOI) technology, a promising emerging platform for the realization of integrated optical devices. We consider 1 \(\times\) N MMI splitters and N \(\times\) 1 combiners, with N being the number of output/input ports. We define the design and the optimization criteria to achieve the best performances in terms of insertion loss and output power uniformity over a large wavelength range (i.e., from 1500 to 1600 nm). In particular, we investigate seven configurations of MMI couplers with N ranging from 2 to 8. The insertion loss for all the designed MMI couplers with N ranging from 2 to 8 varies from 0.018 to 0.41 dB, while the uniformity for all MMI splitters ranges from 0.020 to 0.335 dB across the considered wavelength range. The impact of the amplitude and phase errors on the transmittance of MMI combiners with N ranging from 2 to 8 input ports shows that the transmittance variation is less than 1.5 \(\%\) , indicating high robustness and reliable performance in various photonic applications. We compare our MMI couplers results with those of the state-of-the-art based on different material platforms, including LNOI, obtaining much lower insertion losses.
