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

P K J Singaravelu, G C R Devarapu, Sebastian A Schulz, Quentin Wilmart, Stéphane Malhouitre, Ségolène Olivier, Liam O'Faolain

Research output: Contribution to journalArticlepeer-review

2 Citations (Scopus)


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.
Original languageEnglish
Article number214001
Number of pages5
JournalJournal of Physics D : Applied Physics
Issue number21
Early online date14 Mar 2019
Publication statusPublished - 22 May 2019


  • Spot size converters
  • Inverse tapers
  • Vertical coupling


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