Free choice of splitting ratio is one of the main properties of a power splitter required in integrated photonics, but conventional multimode interference (MMI) power splitters can only obtain a few discrete ratios. This Letter presents both numerical and experimental results of an arbitrary-ratio 1 × 2 MMI power splitter, which is constructed by simply breaking the symmetry of the multimode region. In the new device, the power splitting ratio can be adjusted continuously from 100∶0 to 50∶50, while the dimension of the multimode section stays in the range of 1.5 × 1.8-2.8 μm. The experimental data also indicate that the proposed arbitrary-ratio splitter keeps the original advantages of MMI devices, such as low excess loss, weak wavelength dependence, and large fabrication tolerance. [5,6], and ladder-type optical filters [7]. However, the conventional MMI power splitter with two output ports can only obtain splitting ratios of 100∶0, 85∶15, 72∶28, and 50∶50 through adjusting the position of input and output ports [8,9]. Several methods have been reported aiming at an arbitrary-ratio MMI power splitter, such as butterfly-like MMI splitters [10], bent MMI splitters [11,12], MMI with computer-generated planar holograms [13,14], cascaded MMI couplers with unequal width [15][16][17], MMI splitter with cladding-filled gap [18], and multiple-arm MZI consisting of an active phase-shifting region placed between two MMI couplers [19]. However, all these methods suffer from relatively complex structures and large footprints. In this Letter, we propose and experimentally demonstrate a compact arbitrary-ratio 1 × 2 power splitter based on a simple asymmetric MMI structure. The schematic of the conventional symmetric 1 × 2 MMI power splitter is shown in Fig. 1(a). TE-polarized light is, through the single-mode input waveguide (WG in ), transmitted to the multimode region, where MMI is excited. Then, the light is tapered into two single-mode output waveguides (WG up and WG bot ) symmetrically at the first two-fold image distance. Due to the structural symmetry, the splitter only allows uniform split of the incident light into two output waveguides, as indicated in the energy flux density Even though these corners contain little optical field, removing one of them will break the symmetry of interference, in accordance with the self-imaging principle [20]. Such asymmetric MMI, excited by asymmetric perturbation in the multimode region, will have a significantly different optical field distribution.The proposed asymmetric MMI power splitter is shown in Fig. 1(b). Compared to the conventional symmetric power splitter, the only difference is that the symmetry of the multimode region is broken by removing its bottom left corner (marked with a red dashed rectangle). Such a minor structural change causes a dramatic redistribution of the optical field [ Figs. 1(e), 1(f), 1(i), and 1(j)]. If the output waveguides are also located at the position of the first two-fold image, the power output from WG up will be greater than that from WG ...

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A polarization beam splitter assisted by a subwavelength grating (SWG) is proposed. The SWG enables nearly 20-fold beat length reduction for TE, which makes the high extinction ratio (ER) possible. On the other hand, the embedded SWG preferably affects the refractive index of the even mode in the coupling region and broadens the bandwidth of the splitter. As a result, the ER of 28.7 dB (24.8 dB) for TE (TM) is obtained, while the insertion loss is only 0.10 dB (0.11 dB) at the wavelength of 1550 nm. The ER is more than 10 dB in the wavelength range of 1450-1625 nm for TE and 1495-1610 nm for TM.

Strip-slot waveguide mode converters for TE₀ have been widely investigated. Here we demonstrate a polarization-insensitive converter numerically and experimentally. The polarization-insensitive performance is achieved by matching the optical field distribution of the 2-fold image of the Multimode Interference (MMI) and the TE₀ (TM₀) mode of a slot waveguide. The working principle for this MMI-based mode converter is thoroughly analyzed with the quantitatively evaluated optical field overlap ratio that is theoretically derived from the orthonormal relation of eigenmodes. Based on the analysis, the MMI-based polarization-insensitive converters are then simulated and fabricated. The simulation and measurement results indicate that the proposed scheme is a robust design since it is not only polarization-insensitive but also wavelength-insensitive and fabrication-tolerant. Moreover, the mode converter is as small as 1.22 μm × 4 μm while the measured conversion efficiencies are 95.9% for TE₀ and 96.6% for TM₀. All these excellent properties make the proposed mode converter an ideal solution for coupling light between strip and slot waveguides when both TE and TM polarizations are considered.

Optical mode mismatch makes coupling between strip and slot waveguides a tough issue in integrated photonics. This Letter presents both numerical and experimental results of a strip-slot mode converter based on symmetric multimode interference (MMI). Distinct from previous reported converters which gradually convert the mode through sharp tips, the proposed solution makes full use of the symmetry of the two-fold image of MMI, and its field distribution similarity with a slot waveguide to convert the mode. A converter based on this mechanism is able to convert light from a TE-polarized fundamental mode of a strip waveguide to that of a slot waveguide, and vice versa. Strip-slot waveguide coupling though this mode converter has a measured efficiency of 97% (-0.13  dB), and the dimensions are as small as 1.24×6  μm. Further analysis shows that the proposed converter is highly tolerant to fabrication imperfections, and is wavelength-insensitive.