Low-loss, compact, spot-size-converter based vertical couplers for photonic integrated circuits

Abstract

In recent years, the monolithic integration of new materials such as SiN, Ge and LiNbO3 on silicon (Si) has become important to the Si photonics community due to the possibility of combining the advantages of both material systems. However, efficient coupling between the two different layers is challenging. In this work, we present a spot size converter based on a two-tier taper structure to couple the optical mode adiabatically between Si and SiN. The fabricated devices show a coupling loss as low as 0.058 dB ± 0.01 dB per transition at 1525 nm. The low coupling loss between the Si to SiN, and vice versa, reveals that this interlayer transition occurs adiabatically for short taper lengths (<200 µm). The high refractive index contrast between the Si and SiN is overcome by matching the optical impedance. The proposed two-tier taper structure provides a new platform for optoelectronic integration and a route towards 3D photonic integrated circuits.

Broadband Microelectromechanical Systems‐Based Silicon Nitride Photonic Switch

“…The advantages of this design over other existing adiabatic couplers include the low insertion loss and strong coupling efficiency generated between the two waveguides based on the adopted design and waveguide material selection. Existing vertical adiabatic couplers [48,55,56] require the coupling waveguide to start abruptly to meet the adiabatic coupling condition. This necessitates the use of high-resolution lithography tools.…”

Integrated quantum photonics with silicon carbide: challenges and prospects

“…Although highly efficient interlayer adiabatic transfer is possible (approaching 99% [142]), it relies on long, well-aligned (200 µm) tapers that are difficult to achieve using a pick-and-place technique (but are possible with heterogeneous integration of multiple thin-film layers (Fig. 5d)).…”

“…However, for more complex tasks, inverse design offers a novel approach for performing targeted optimization against multiple metrics, including robustness to fabrication imperfections (thus improving the yield) and fabrication constraints [149]. For example, an classical adiabatic taper between photonic layers [142] is extremely sensitive to misalignment, precluding efficient interlayer coupling in pick-andplace; this limitation is an opportunity to apply inverse design. The powerful functionality of inverse design has been used to demonstrate efficient mode conversion [150] and free-space coupling [151], even under stringent fabrication constraints [152].…”