This Letter presents a compact and low-loss 1×2 asymmetrical multimode interference (A-MMI) splitter in rib geometry for on-chip power monitoring at 1.55 μm, where a given alteration of the component cavity determines arbitrary values of the output power splitting ratios. The device shows reduced losses (∼0.4-0.8 dB) and robustness across a 40 nm optical bandwidth (1540-1580 nm).