Abstract
A subwavelength gratings (SWG) assisted directional coupler (DC) is proposed to work as beam splitter (BS) for 850 nm chip-based optical coherence tomography (OCT). The bandwidth of the SWG assisted DC is maximized to 110 nm by selecting appropriate SWG geometries including its duty cycle and period. The axial point spread functions (PSFs) derived from the BS transmittances proves the optimized bandwidth is broad enough to support axial resolution higher than $4 \;\mu m$. Meanwhile, the SWG period can be manipulated to match the center wavelength between BS and light source so that the wavelength mismatching induced resolution degradation is effectively mitigated. Furthermore, a statistical analysis is performed to demonstrate the SWG assisted DC is a high fabrication-tolerant device in terms of axial resolution, even the SWG geometries suffer from a fabrication jitter in 20 nm variance.
Highlights
Optical coherence tomography (OCT) is a non-invasive imaging technique that provides crosssectional image with spatial resolution in micrometer scale [1]
The subwavelength gratings (SWG) assisted directional coupler (DC) is proposed to work as beam splitter (BS) for constructing Michelson interferometer in an 850 nm on-chip optical coherence tomography (OCT) system
To maximize the device operating bandwidth at a specific center wavelength, we proposed an index named transmittance variability to benchmark the transmittance flatness and to select appropriate SWG geometries
Summary
Optical coherence tomography (OCT) is a non-invasive imaging technique that provides crosssectional image with spatial resolution in micrometer scale [1]. There is a great demand to have a compact and broadband BS working at 800 nm–900 nm wavelength region to realize high resolution chip-based OCT system Though it has already succeeded in implementing on-chip OCT systems, how the OCT operations affected by device performances have not been fully studied yet. It is significant to clearly understand the impact of device performances on OCT operations in advance of fabrication To address these issues, a subwavelength gratings (SWG) [22], [23] assisted directional coupler (DC) on silicon nitride platform [17] is suggested to work as the BS in 850 nm chip-based OCT systems. The SWG assisted DC is a competitive choice to implement high resolution on-chip OCT system at 800 nm–900 nm wavelength region
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