An optical biosensor may include a biosensing unit, detection unit, and signal processing unit. The biosensing unit may be configured for receiving first and second optical signals (which are generated from a phase-modulated optical signal), outputting a sensing signal by transmitting the first optical signal via a first optical path that includes a sensing resonator, and outputting a reference signal by transmitting the second optical signal via a second optical path that includes a reference resonator. The detection unit may be configured for receiving the sensing signal and the reference signal, detecting a phase element of each of the sensing signal and the reference signal through a signal demodulation operation, and detecting a phase difference between the sensing signal and the reference signal according to the detected phase elements. The signal processing unit may be configured for calculating the concentration of a bio-material based on the detected phase difference.