Low-cost optical cavity based sensor with a large dynamic range.

We have proposed an optical cavity based biosensor using a differential detection method for point-of-care diagnostics. For experimental demonstration of the proposed device using a two-laser system through refractive index measurements, an optical cavity structure is designed, fabricated, and measured. The differential value calculated with intensities of two different wavelengths has a larger responsivity with respect to the refractive index change as compared with the intesity of an individual wavelength. However, the repeatability test shows that the two-laser system has a tight fabrication tolerance due to its small dynamic range. In this paper, we introduce a three-laser system for an optical cavity sensor with a chained differential detection method and present the experimental measurement results showing a large dynamic range and large fabrication tolerance. The measured dynamic range of this system is more than three times greater than that of the two-laser system. The repeatability test using the three-laser system demonstrates a larger fabrication tolerance as a result of the increased dynamic range.