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 Silicon-on insulator (SOI) is currently considered as one of the most promising platforms to achieve dense integration of photonic devices at low cost, owing to its high-index contrast and compatibility with mature complementary metal-oxide semiconductor (CMOS) fabrication process [1,2]. Multimode interference (MMI) devices are commonly used in modern photonics [3,4] and optoelectronic integrated circuits [5][6][7], owing to their compactness [8,9], large bandwidth, overall low losses [10,11], and robustness against fabrication variations. As a result, their possible implementation as power splitters is very attractive [12]. Asymmetrical multimode interference (A-MMI) devices with strip configurations have been recently proposed [13]; however, to the best of our knowledge, no work on rib geometries has been reported yet. In this Letter, we present a compact and low-loss 1 × 2 MMI splitter featuring a rib geometry for on-chip power monitoring purposes, where an alteration of the cavity determines an efficient and stable splitting of the optical power.The A-MMI has been designed in a shallow-etched waveguide configuration. To fulfill the compactness requirement and allow relaxed fabrication process tolerances, we have created a device with a multimode section (L mmi W mmi ) of 10.5 μm × 3 μm, using 3D-FDTD simulations [14].The schematic of the new proposed 1 × 2 A-MMI is sketched in Figs. 1(a)-1(d). The near infrared light is injected into the multimode region through a 450 nm wide waveguide and then tapered to reach a width of 0.85 μm. The two tapered output waveguides are precisely positioned at the first two folded images at W mmi ∕4.The tapers of the A-MMI are engineered to maximize the ingoing and outgoing light from the multimode region, and are symmetric and asymmetric at the input and outputs, respectively [ Fig. 1(a)]. They have been modeled using 3D-FDTD simulations, and the optimized values for Δ 1 and Δ 2 , respectively, are 0.2 μm and 0.525 μm. The lengths of the input and output tapers, respectively, are 5 μm and 15 μm. We chose to use asymmetric tapers at the outputs to avoid the presence of sharp corners on the device, reducing accordingly the losses caused by the roughness induced by the fabrication process. Moreover, this layout enables the coupling between the multimode region and the output waveguides to be maximized. In contrast with conventional symmetric 1 × 2 MMI featuring 50:50 power splitting ratios, our asymmetric MMI can achieve variable outputs splitting ratios through breaking the symmetry of the cavity.The arbitrary power splitting ratios are obtained in the following manner: using as a starting vertex the c...
