Spectral broadening via phase modulation is widely employed in high-power laser systems to suppress transverse-stimulated Brillouin scattering and improve beam uniformity. However, nonuniform spectral transmittance and group velocity dispersion can induce frequency modulation-to-amplitude modulation (FM-to-AM), threatening the safety of large-aperture optics. Current monitoring techniques rely on high-speed oscilloscopes and wavelength conversion, thereby increasing the cost and complexity. This study presents a real-time FM-to-AM detection method based on a dual-comparator delay-unlocked detection architecture. The system employs a high-speed photodetector, low-noise amplifier, envelope detection and delay-unlocked dual comparator. The module reliably measures modulation depths from 1.27% to 19.15% for pulses with a rise time of less than 60 ps and a modulation frequency of 20 GHz. This compact, low-cost and modular design enables robust FM-to-AM monitoring without high-speed oscilloscopes, facilitating real-time feedback and enhancing operational stability in large-scale laser drivers, while offering scalability for multi-channel deployment in future inertial confinement fusion facilities.