A Comprehensive Study on The Use of Sub-Coherent Self Heterodyne Linewidth Estimations

A fundamental requirement for many precision spectroscopic applications is the development of stable lasers with narrow linewidths. However, accurately verifying laser linewidths becomes increasingly challenging as the linewidth decreases. In the field of frequency metrology, the delayed self-heterodyne technique has long been regarded as one of the most reliable and accessible methods for linewidth characterization, favored for its low equipment cost and reproducible results. Traditionally, self-heterodyne measurements employ an optical delay line significantly longer than the coherence length of the laser, with analysis performed via either the instantaneous power spectral density or the noise power spectral density. However, the use of long optical fibers introduces several limitations: substantial optical attenuation requiring amplification stages, and susceptibility to thermally induced Brownian noise, which can artificially broaden the measured spectrum. This work utilizes a short delay fiber – less than the laser’s coherence length can yield reliable and accurate linewidth measurements. The primary objective of this thesis is to provide a comprehensive methodology for extracting the laser linewidth from the power spectral density of a sub-coherence delay self-heterodyne signal, thereby offering an improved framework for precision laser characterization.