Discrimination of the Delay Fiber Length Relationship in Self-Heterodyne Method for Laser Linewidth

The delayed self-heterodyne method, as the most commonly used technique for measuring laser linewidth, requires the length of the delay fiber to be greater than six times the laser's coherence length—a standard widely recognized and utilized. This article starts from the basic theory of the delayed self-heterodyne method, supplements and refines it, explores the reasons for the spectral broadening in the delayed self-heterodyne spectrum, proposes three criteria for the relationship between the length of the delay fiber and the laser's coherence length, and carries out quantitative calculations. Comprehensive analysis concludes that when measuring the laser linewidth with the delayed self-heterodyne method, the length of the delay fiber should be taken as 6.2 times the laser's coherence length.