Abstract
A 2 × 2 3-dB coupler is one of the essential photonic components, as a building block of Mach-Zehnder interferometers, to realize large-scale photonic integrated circuits. Unlike typical 2 × 2 3-dB couplers based on direction couplers or multimode interference couplers, adiabatic couplers offer various advantages such as broadband operation and superior fabrication tolerance thanks to their unique operating mechanism of adiabatic mode evolution. However, an adiabatic coupler typically requires a long device length for ideal adiabatic operation without the excitations of unwanted modes. Here, we report on a compact 2 × 2 3-dB adiabatic coupler designed using the shortest mode transformer method. By optimizing the profile of the waveguide widths and the gap spacing, the compact 3-dB coupler was designed with a short coupling length of 23.2 μm. The fabricated device exhibits a 3-dB splitting ratio with less than ± 0.3 dB power oscillation and a low excess loss of 0.23 dB over a broad wavelength range of 1485–1620 nm. To the best of our knowledge, our coupler has the shortest length among the adiabatic couplers with a minimum feature size of 200 nm, reported to date.
Highlights
SILICON photonics is an attractive technology to realize large-scale photonic integrated circuits thanks to its high integration density and compatibility to advanced complementary metal–oxide semiconductor (CMOS) processes [1]–[3]
As an Mach–Zehnder interferometer (MZI) typically consists of a pair of 2 × 2 3-dB couplers with phase-modulated waveguide arms, a compact and low loss 2 × 2 coupler with an ideal splitting ratio is a key building block to realize large-scale silicon photonic systems. 2 × 2 couplers are commonly implemented with directional couplers or multimode interference (MMI) couplers
It has been reported that bandwidths of directional couplers and MMI couplers can be widened by utilizing structures such as curved waveguides, asymmetric waveguides, and subwavelength gratings (SWGs) [10]–[13]
Summary
SILICON photonics is an attractive technology to realize large-scale photonic integrated circuits thanks to its high integration density and compatibility to advanced complementary metal–oxide semiconductor (CMOS) processes [1]–[3]. It has been reported that bandwidths of directional couplers and MMI couplers can be widened by utilizing structures such as curved waveguides, asymmetric waveguides, and subwavelength gratings (SWGs) [10]–[13] They have high insertion losses and low fabrication tolerances. Whereas directional couplers and MMI couplers rely on mode beating between their supermodes or multimodes, adiabatic couplers are based on adiabatic mode evolution offering superior fabrication tolerances, and wide bandwidths [14]. They typically require long device lengths for their adiabatic mode evolution. SWG-based compact adiabatic couplers have been proposed and experimentally demonstrated [15], [16] They have features smaller than 100 nm which make them difficult to be fabricated in typical silicon photonic foundries. This compact size can be achieved with the optimized gap profile as well as the optimized waveguide width profile
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