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A highly sensitive integrated optical biosensor based on the cascade of a Fabry-Perot cavity laser and a microring resonator is investigated theoretically and experimentally. The free spectral ranges of the microring and the laser cavity are designed to be about the same in order to apply intensity interrogation. The variation of the refractive index of the fluidic sample results in a large shift in the envelope function of the total transmission spectrum of the sensor, and consequently, a large change in the transmitted power. The simple light-source-integrated sensing scheme achieved a high sensitivity of 1000 dB/RIU in the preliminary experiment, demonstrating its principle and potential for low-cost practical applications.