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Evanescent wave sensors in photonic integrated circuits have been demonstrated for gas sensing applications. While some methods rely on the distinctive response of certain polymers for sensing specific gases, absorption spectroscopy identifies any gas uniquely from their unique vibration signatures. Based on the Beer-Lambert principle, the sensitivity of absorption by a gas on chip relies on the length of the sensing region, the optical overlap integral with the analyte gas and the absorption cross-section at the wavelength with the fundamental vibration signature. The overlap of the optical mode with the analyte has been enhanced in photonic devices by combining slot waveguide confinements with photonic crystal slow light effects. While the absorption cross-section is a property of the gas, the length of the sensing region is limited by the available area on a chip and waveguide propagation losses that limit the minimum signal to noise ratio. In this paper, we show that by incorporating reflecting loop mirrors, the absorption path length can be doubled for the same geometric length of the absorption sensing waveguide. Light from a waveguide is split into two paths, each with a slow light photonic crystal waveguide, by a 2×2 multimode interference (MMI) power splitter. Each path is terminated by a loop mirror that causes the light to retrace its path back down the sensing arms thereby doubling the optical path length over which light interacts with the analyte. Results on the enhancement of phase sensitivity and absorbance sensitivity in the interferometric configuration are presented 

Each year, the global cost that is accounted to corrosion was estimated at $2.5 trillion. Corrosion not only imposes an economic burden, when corroded structures are under various loading conditions, it may also lead to structurally brittle failure, posing a potential threat to structural reliability and service safety. Although considerable studies investigated the combined effect of external loads and structural steel corrosion, many of the current findings on synergetic interaction between stress and corrosion are contrary. In this study, the combined effects of dynamic mechanical loads and corrosion on epoxy coated steel are investigated using the distributed fiber optic sensors based on optical frequency domain reflectometry. Experimental studies were performed using the serpentine-arranged distributed fiber optic strain sensors embedded inside the epoxy with three different scenarios including the impact loading-only, corrosion-only, and combined impact loading-corrosion tests. Test results demonstrated that the distributed fiber optic sensors can locate and detect the corrosion processing paths by measuring the induced strain changes. The combined impact loading-corrosion condition showed significantly accelerated corrosion progression caused by mechanical loads, indicating the significant interaction between dynamic mechanical loading and corrosion on epoxy coated steel.