Waveguide-based photonic sensors provide a unique combination of high sensitivity, compact size and label-free, multiplexed operation. Interferometric configurations furthermore enable a simple, fixed-wavelength read-out making them particularly suitable for low-cost diagnostic and monitoring devices. Their limit of detection, i.e., the lowest analyte concentration that can be reliably observed, mainly depends on the sensors response to small refractive index changes, and the noise in the read-out system. While enhancements in the sensors response have been extensively studied, noise optimization has received much less attention. Here we show that order-of-magnitude enhancements in the limit of detection can be achieved through systematic noise reduction, and demonstrate a limit of detection of ∼ ${10}^{-8}\mathrm{RIU}$ with a silicon nitride sensor operating at telecom wavelengths.

Over the past two decades, integrated photonic sensors have been of major interest to the optical biosensor community due to their capability to detect low concentrations of molecules with label-free operation. Among these, interferometric sensors can be read-out with simple, fixed-wavelength laser sources and offer excellent detection limits but can suffer from sensitivity fading when not tuned to their quadrature point. Recently, coherently detected sensors were demonstrated as an attractive alternative to overcome this limitation. Here we show, for the first time, to the best of our knowledge, that this coherent scheme provides sub-nanogram per milliliter limits of detection in C-reactive protein immunoassays and that quasi-balanced optical arm lengths enable operation with inexpensive Fabry–Perot-type lasers sources at telecom wavelengths.

Photonic sensors based on Mach-Zehnder interferometric architectures with phase-coherent readout have demonstrated their potential for the integration into medical diagnostic devices. Their compact design, low cost and ability to detect multiple biomarkers make them potentially applicable for the point-of-care. Achieving both high throughput and optimal sensor performance requires a miniaturized interrogation system with multiplexed readout capability that facilitates alignment and at the same time offers a high signal-to-noise ratio. In this work the use of an optical camera based system suited for multiplexed integrated photonic sensor detection is investigated. Its system level model is derived to determine the signal-to-noise ratio and its associated limit of detection. This model allows achieving optimal sensing performance by tuning the photonic chip, the optical system and the camera detection parameters. In agreement with model expectations, experimental results show that this system allows reaching for bulk detection limits of ~ 10⁻⁷ RIU (Refractive Index Units) for a system comprising three sensors, representing a state of the art achievement.