We proposed a low cost optical cavity based biosensor with a differential detection for point-of-care diagnosis. Two lasers at different wavelengths are used for the differential detection. This method enhances the sensitivity through higher responsivity and noise cancelation. To reduce noise further, especially due to the unstable low cost laser diode output, we employed a referencing method in which a reference pixel value in each CMOS image frame is subtracted from all other pixels. To validate the designed structure and demonstrate the sensitivity of it, we perform refractive index measurements of fluids with our design. In this presentation, we will discuss our design, simulation results, and measurement results.