Comment: Frequency noise power spectral density (FN-PSD) of an optical comb line and the mode spacing measured for the free-running QCL and when the mode spacing is phase-locked to an external RF reference. Device: FSWP-26, Phase Noise Analyser Company: Rhode & Schwarz Uploaded by: Atif Shehzad Affiliation: Laboratoire Temps-Frequence, Institut de Physique, Universite de Neuchatel, Avenue de Bellevaux 51, 2000 Neuchatel, Switzerland Contact: atif.shehzad@unine.ch Upload date: 20-02-2020