Achieving shot noise limit imaging with a polarization-modulated optical loop in a laser frequency-shifted feedback system

This paper introduces a novel approach to attain the shot-noise limit in laser frequency-shift feedback (LFSF) imaging systems by employing a polarization-modulated optical loop. Compared to the traditional LFSF imaging system, the proposed system effectively eliminates detrimental parasitic feedback signals, enhancing both the signal-to-noise ratio and the response range of the LFSF imaging system. Specifically, in the polarization-modulated optical loop, only the measurement light propagating through the loop undergoes efficient modulation, while the parasitic feedback light experiences high-frequency modulation beyond the bandwidth of the photodetector. Experimental findings demonstrate the superiority of our proposed system over traditional LFSF imaging systems in the insensitivity of parasitic feedback, showcasing a higher signal-to-noise ratio and response range. Moreover, the proposed system achieved a distinctive polarization imaging technique, which exhibits the capability to discern and analyze the distinct specular reflection and depolarized scattering components in the measurement light, thus providing more comprehensive target information.