Precise measurement of ultra-narrow laser linewidths using the strong coherent envelope

TLDR

Laser linewidths narrowed to the kHz or Hz range are critical for clock synchronization, laser radar, quantum optics, and high‑precision detection, yet conventional delayed self‑heterodyne/homodyne techniques fail to measure them accurately because Gaussian broadening masks the intrinsic Lorentzian linewidth. We propose a self‑coherent detection approach that employs the strong coherent envelope to directly characterize a laser’s intrinsic linewidth. By eliminating the Gaussian broadening caused by 1/f frequency noise, the method relates the intrinsic linewidth to the contrast difference between the second peak and trough of the envelope (CDSPST) and the fiber delay, allowing the correct delay length to be selected from the estimated linewidth and CDSPST to yield the accurate linewidth. Experimental results confirm that this technique provides accurate measurements of narrow and super‑narrow laser linewidths.

Abstract

Laser linewidth narrowing down to kHz or even Hz is an important topic in areas like clock synchronization technology, laser radars, quantum optics, and high-precision detection. Conventional decoherence measurement methods like delayed self-heterodyne/homodyne interferometry cannot measure such narrow linewidths accurately. This is because a broadening of the Gaussian spectrum, which hides the laser's intrinsic Lorentzian linewidth, cannot be avoided. Here, we introduce a new method using the strong coherent envelope to characterize the laser's intrinsic linewidth through self-coherent detection. This method can eliminate the effect of the broadened Gaussian spectrum induced by the 1/f frequency noise. We analyze, in detail, the relationship between intrinsic laser linewidth, contrast difference with the second peak and the second trough (CDSPST) of the strong coherent envelope, and the length of the delaying fiber. The correct length for the delaying fiber can be chosen by combining the estimated laser linewidth (Δfest) with a specific CDSPST (ΔS) to obtain the accurate laser linewidth (Δf). Our results indicate that this method can be used as an accurate detection tool for measurements of narrow or super-narrow linewidths.