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A novel approach to the measurement of microwave frequency based on optical power monitoring is proposed and demonstrated. The microwave signal with its frequency to be measured is modulated on two optical carriers with their wavelengths set at one peak and one valley of the spectral response of a sinusoidal filter. The modulation is performed by a Mach-Zehnder modulator that is biased to suppress the optical carriers. A mathematical expression relating the optical powers from the two wavelength channels and the microwave frequency to be measured is developed. By simply monitoring the optical powers at the outputs of the two wavelength channels, the microwave frequency can be evaluated. A proof-of-concept experiment is implemented. Frequency measurement with good accuracy for microwave signals at different power levels is realized.