High-Stability Wideband Microwave Dissemination Based on Optical Frequency Comb-Assisted Time Delay Measurement

We propose and experimentally demonstrate a highly stable wideband microwave transmission system based on optical frequency comb (OFC)-assisted time delay measurement. In the proposed system, a high-precision optical fiber transfer delay (OFTD) measurement scheme is performed by transmitting an OFC along the optical fiber link, where the fiber delay variation is mapped onto the phase shifts of the comb lines. By employing a dual-heterodyne phase error transfer scheme, the phase variations of comb lines caused by the OFTD change are converted into the phase shift of a low-frequency RF signal, which can be precisely measured using a high-accuracy phase detector. Afterwards, the OFTD variation is actively compensated via a variable optical delay line (VODL), enabling stable wideband microwave dissemination. In the experimental demonstration, microwave signals with frequencies ranging from 8 GHz to 14 GHz are transmitted from the local end to the remote end via a 1-km-long single-mode optical fiber, in which the root-mean-square (RMS) value of the delay jitters is as low as 28 fs and the fractional frequency stability is measured to be <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$8 \times 10^{-17}$</tex> at 1000 s averaging time.