An optical sensor based on a broadband light source and cascaded waveguide filters comprises a broadband light source, an input waveguide, a reference ring resonator coupled with the input waveguide, a common bus waveguide coupled with the reference ring resonator, a sensing ring resonator coupled with the common bus waveguide, an output waveguide coupled with the sensing ring resonator, and two optical power detectors. At least a portion of the sensing ring resonator is influenced by the physical parameter to be measured or in contact with an analyte. The variation of the physical parameter to be measured or the variation of the analyte induces a shift of the transmission spectrum of the sensing ring resonator. By using the cascaded filtering effect of the double resonators, the wavelength shift can be translated into a variation of the total output power. Consequently the physical parameter to be measured can be easily deduced.