Sub-kilohertz laser linewidth measurement using a loss-compensated recirculating self-heterodyne method with short fiber

A measurement of sub-kilohertz laser linewidth is demonstrated by a recirculating self-heterodyne detection, in which the delay time is far less than the coherence time of the laser. The system parameters are optimized through theoretical analysis to improve the circulation rate of the beat signal. On this basis, we build a loss-compensated recirculating selfheterodyne interferometer with only a 2 km SMF-28 optical fiber spool. Thanks to the recirculating structure with an acoustic optical modulator (AOM), multi-group beat notes with high circulation rate can be measured simultaneously, which agree well with the theoretical analysis. By simulating and fitting the spectra of high-order beat notes with different delay times, the average linewidth with less statistical errors is obtained. The loss-compensated recirculating selfheterodyne method with short fiber can avoid single measurement error and almost neglect the influence of the broadening spectrum induced by the 1/f frequency noise, providing a powerful candidate for precise measurement of sub-kilohertz laser linewidth.