High‐Precision Laser Spectrum Analyzer via Digital Decoherence

ABSTRACT With the continuous advancement of laser technology, accurately evaluating the noise spectrum of high‐performance lasers has become increasingly challenging. In this work, we demonstrate a high‐precision laser spectrum analyzer based on the proposed digital decoherence method, which can precisely measure the frequency noise spectrum of sub‐Hz linewidth lasers. In addition, it has broad wavelength compatibility, which enables convenient switching between lasers with different center wavelengths. Its performance is validated through measurements of ultra‐stable lasers. Based on the measured frequency noise power spectral density, a β‐line linewidth is determined to be 570 mHz at a 10‐s observation time, and the minimum observable linewidth is calculated to be 133 mHz. The system's noise floor is evaluated to be 210 mHz β‐line linewidth at a 25‐s observation time, and the minimum observable linewidth is 39 mHz.