Silicon nitride directional coupler interferometer for surface sensing

Kyohei Okubo, Ken Uchiyamada, Kiyoshi Asakawa, Hiroaki Suzuki

Research output: Contribution to journalArticlepeer-review

6 Citations (Scopus)


A silicon nitride directional coupler (DC) used to create a biosensing device is presented. The DC detects changes in the refractive index of the cladding (nclad) as changes in the relative output intensity. The DC length (L), nclad-dependent sensitivities of the DC, and preferred dimensions of the single-mode DC waveguides are obtained through numerical simulations. The performance of the DC is evaluated through end-fire coupling measurements. The intensities measured after varying the nclad using air, water, and glycerol solutions agree well with the fitting for a wide range of L values between 60 and 600 μm, i.e., corresponding to 6 to 60 times the coupling length. The bulk refractive index sensitivity was investigated using glycerol solutions of different concentrations and was found to be 18.9 optical intensity units per refractive index unit (OIU/RIU). Biotin/streptavidin bindings were detected with a sensitivity of 60 OIU/RIU and a detection limit of 0.13 μM, suggesting the feasibility of the DC for immunosensing.

Original languageEnglish
Article number017101
JournalOptical Engineering
Issue number1
Publication statusPublished - 1 Jan 2017


  • directional coupler
  • interferometry
  • silicon nitride waveguide
  • streptavidin detection


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