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.
We demonstrate an optical frequency transfer over a 377 km-long fiber link using three bi-directional Erbium-doped fiber amplifiers (EDFAs) to compensate the fiber attenuation. Through active phase noise cancellation, we obtain a transfer instability of 2.2×10-14 at 1 s and 6.2×10-17 at 2000 s. The lasing effects induced by EDFAs are observed in the transfer link which reduce the gain performance of the EDFAs and deteriorate the stability of the signal. In the paper, the factors that may induce the lasing effects is discussed, specially, improper phase locking parameters may stimulate the lasing effects. The phase noise of the 377 km link and the transfer instability versus fiber lengths is also researched. This work makes a good foundation for our future research on long-distance optical frequency transfer.
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