Quantitative Analysis of the Delay Fiber Length Requirement for Accurate Laser Linewidth Measurement in Self‐Heterodyne Method

ABSTRACT The delayed self‐heterodyne method (DSHI) is a standard technique for measuring laser linewidth. A widely adopted empirical criterion suggests that the delay fiber length must exceed 6 times the laser's coherence length; however, the theoretical basis for this factor is rarely detailed. Based on the fundamental theory of DSHI, this paper elucidates the measurement mechanism and quantitatively analyzes the impact of delay fiber length on measurement accuracy. We investigate the physical origin of spectral broadening and propose three criteria—algebraic, peak‐to‐peak ripple, and systematic error—to determine the optimal fiber length. Comprehensive analysis demonstrates that the delay fiber length should be at least 6.22 times the laser's coherence length to maintain a systematic measurement error below 1%.