The present electromagnetic signal sensing system includes a modulating circuit for generating an electrical modulating signal, a laser electrically connected to the modulating circuit, an optically modulated scatterer coupled with the laser, a receiving antenna, a synchronous detection circuit electrically connected to the receiving antenna and a signal processing circuit electrically connected to the synchronous detection circuit. The laser generates an optical modulating signal based on the electrical modulating signal for modulating the optically modulated scatterer to generate a modulated scattering signal. The receiving antenna receives the modulated scattering signal, the synchronous detection circuit generates an in-phase signal and a quadrature-phase signal with a phase offset of 90 degrees from the electrical modulating signal, and the signal processing circuit calculates the amplitude and phase of the electromagnetic signal from the in-phase signal and the quadrature-phase signal.