In photonic integration, high index contrast waveguides (e.g. ridge waveguides) have been widely used for interconnection and device construction because of their compactness. However, the strong confinement also means that the ridge waveguide is generally more multi-moded laterally than other waveguides of similar dimensions. A laterally single mode ridge waveguide is typically about 0.5 micrometer or less in width, which could incur high propagation and fiber coupling losses. Hence, a wider waveguide is often used to minimize loss at the expense of the single-mode characteristic, especially for long devices.
However, many devices, such as Mach-Zehnder interferometers and directional couplers, require single-mode waveguides for proper operation. In these cases, the multimode waveguides can still be used if appropriate mode-filters are strategically located at the input and within the device to remove potential higher-order modes.
We propose a higher-order mode filter using two back-to-back lateral waveguide tapers, which could be as short as 150 micrometers. Mode discrimination occurs in the down-taper where the higher-order modes become leaky when the taper width reaches their cut-off points (i.e., points where the modes are no longer guided by total internal reflection). The taper imposes only about 0.2dB loss to the fundamental mode itself. The up-taper at the other end restores the fundamental mode back to the original size. Simulations using Beam Propagation Method (BPM) shows that this mode filter is insensitive to the taper lengths and has reasonable fabrication tolerances.