Directional Coupler (DC) is one of the vital components in integrated optics and optical communication which is used to exchange optical power between the two adjacent waveguides due to the modal interaction. Broadband DC can be used to replace fiber couplers in optical coherence tomography (OCT). OCT is non-contact imaging modality having numerous applications in bio-medical and non-bio-medical imaging. Retinal imaging is one of the significant applications of OCT which works in the visible and near infrared (NIR) domain below 1 micron. In integrated optics, silicon nitride (SiN) as core material exhibits high transmission values down to 0.4 micron. Using SiN waveguides, a broadband photonic integrated DC at 850nm has been designed. Parameters such as coupling length and coupling spacing for a 50:50 splitting ratio have also been analyzed. A novel asymmetric broadband DC having an asymmetric coupling section at 850nm has also been designed. This structure is broadband over 100nm bandwidth and shows better results in terms of normalized splitting ratio and excess loss. The accuracy of our structure has been validated by using commercially available Fimmwave Photon Design software. The calculations were performed for transverse electric (TE), transverse magnetic (TM) and fully vectorial mode types and maximum excess loss reported for all 3 types was accounted to be less than 0.2dB. With further optimization in design and technology, SiN waveguides are potential candidates for passive photonic integrated circuits for OCT.

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Bhawna Sharma, Kamal Kishor, Sandeep Sharma, and Roshan Makkar, "Broadband SiN directional coupler at 850 nm for optical coherence tomography," Proc. SPIE 11105, Novel Optical Systems, Methods, and Applications XXII, 111050V (Presented at SPIE Optical Engineering + Applications: August 14, 2019; Published: 9 September 2019); https://doi.org/10.1117/12.2529351.