If two laser beams with slightly different frequencies overlap on a photodetector, this leads to an electronic "beat note" signal oscillating at the frequency difference. Monitoring the beat note is appealing for sensing applications, if the frequency of one of the lasers (the probe) is sensitive to a given physical variable, while the other (the reference) is not. However, because of the finite linewidth of lasers, the beat note of two independent lasers shows some spectral broadening. Reducing such broadening is needed for a high sensor accuracy. This can be achieved by correlating the signals from the two lasers. This is exemplified by Zifan Zhou and coworkers, who report a correlated laser design leading to a much narrower beat note (15 kHz) than independent lasers (>1 MHz). In this design, a Raman lasing signal generated by a rubidium gain medium within an optical cavity beats with the pump laser. Based on a dynamic quantum modeling, the authors propose that the narrowed beat note originates from the correlation between phase jumps in the Raman lasing and pump laser signals.
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