We report on the progress of our efforts to apply silicon nitride photonic integrated circuits (PIC) to the miniaturization of optical coherence tomography (OCT) with the goal of facilitating its widespread use in ophthalmology at the point of care. In particular, we highlight the design and optical characterization of photonic building blocks allowing the realization of a silicon nitride PIC-based multi-channel swept-source OCT system in the 1060 nm wavelength region. Apart from waveguide structures, these building blocks include 3D-printed microlenses on the PIC end facets for efficient light coupling to and from the PIC.
Integrated photonics features applications in high-speed telecommunication, computing, and sensing. These devices are ultimately limited by the optical loss occurring in the waveguide structures. One of its primary sources is surface-roughness- induced scattering and bend-losses. Surface roughness is unavoidably introduced during deposition, mainly during etching and lithographic steps. In photonic integrated circuits, tight bends enable a compact footprint yet increase the mode mismatch loss , radiation loss and scattering loss. Previously, the bend losses were estimated from a parametric model. However, it lacks flexibility w.r.t. the waveguide platform. We apply a recently developed model of the surface-roughness-induced scattering in guided-mode systems to substantiate the dependence of the scattering loss on the bend-radius for waveguides based on a silicon nitride platform. The model incorporates the surface roughness via its autocorrelation. Further, it inherently considers the overlap of the modes with the roughness. As waveguide material, we used both plasma-enhanced CVD silicon nitride as a low-temperature, back-end-compatible process, and low-pressure CVD silicon nitride, as a high-temperature frontend process. As bottom and top cladding, we deposited high-density plasma (HDP) and sputtered silicon oxide, respectively. The latter offers flexibility to adapt the platform for sensing purposes. We evaluate different waveguide widths, bend radii, and wavelengths in the visible and near-infrared ranges. We set the observed propagation losses into context with estimated absorption, scattering, and mode-overlap loss sources and point to their shifting importance at the measured wavelengths. We believe that this model allows to increase our knowledge about the various aspects of loss in guided mode systems and predict the propagation loss based on foregoing absorption and roughness measurements.
Micro-ring resonators (MRR) are basic photonic components, which serve as crucial building blocks for a variety of devices, e.g. integrated sensors, external cavity lasers, and high speed photonic data transmitters. Silicon nitride photonic platforms are particularly appealing in this field of application, since this waveguide material enables on-chip photonic circuitry with (ultra-) low losses in the NIR as well as across the whole visible spectral range. In this contribution we investigate key performance properties of MRRs in the wavelength range around 850 nm, such as free spectral range (FSR), quality factor (Q factor) and extinction ratio. We systematically investigate a large parameter space given by the MRR radii, coupling gaps between ring and bus waveguide, as well as waveguide width. Furthermore, we compare key properties such as the Q factor between low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) Si3N4 platforms and find enhanced values for LPCVD ring resonators reaching nearly a Q factor of 106.The fabrication is carried out with standard CMOS foundry equipment, utilizing photolithography and reactive ion etching on 250 nm thick silicon nitride films. As cladding material, high density PECVD silicon oxide is deposited prior to the waveguide onto bare silicon and a sputtered oxide serves as upper cladding. With this process toolbox full CMOS backend compatibility is achieved when considering only PECVD Si3N4 waveguide material. In terms of manufacturability, special focus is put on the die-to-die as well as on wafer-to-wafer variability of the performance parameters, which is crucial when considering mass production of MRR devices. Finally, the experimental findings are compared to finite difference time domain (FDTD) simulations of the MRR circuits revealing excellent agreement when considering the manufacturing variability.
The miniaturization of optical coherence tomography (OCT) systems could open up potential new markets, such as point-of-care application, home OCT to regularly monitor disease and treatment progress, and in low-resource settings. Photonic integrated circuits (PIC) are considered an attractive approach to miniaturize OCT. We present our recent achievements in in vivo retinal imaging with a PIC-based Mach-Zehnder interferometer integrated in a state-of-the-art ophthalmic OCT system. The system achieves 94 dB at 750 µW on the sample, running at 50 kHz. Preliminary results of a fully packaged 4-channel opto-electronic OCT engine further demonstrate the potential of PIC-based OCT.
Silicon nitride waveguide based photonic integrated circuits (PICs) are intensively investigated for a wide range of sensing applications in the visible to sub 1-µm near-infrared spectral region. The monolithic co-integration of silicon photodiodes and read-out electronics offers additional benefits in terms of performance and miniaturization. We discuss challenging aspects related to the efficient coupling and routing of light to, from, and within PICs and present interfacing photonic building blocks offering potential solutions. We demonstrate the suitability of these interfacing building blocks by using them for the realization of a PIC-based multi-channel optical coherence tomography concept at 840 nm.
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