All polarization-maintaining Er:fiber-based optical frequency comb for frequency comparison of optical clocks |
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| Affiliation: | 1.National Time Service Center, Chinese Academy of Sciences, Xi'an 710600, China;2.University of the Chinese Academy of Sciences, Beijing 100049, China;3.Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan 430071, China;4.School of Physics Sciences, University of Science and Technology of China, Hefei 230026, China |
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| Abstract: | We demonstrate an optical frequency comb (OFC) based on a turnkey mode-locked laser with a figure-9-shape structure and polarization-maintaining fibers, for the comparison of frequency among optical clocks with wavelengths of 698 nm, 729 nm, 1068 nm, and 1156 nm. We adopt a multi-branch approach in order to produce high power OFC signals at these specific wavelengths, enabling the signal-to-noise ratio of the beatnotes between the OFC and the clock lasers to exceed 30 dB at a resolution bandwidth of 300 kHz. This approach makes the supercontinuum spectra much easier to be generated than a single branch OFC. However, more out-of-loop fibers degrade the long-term frequency instability due to thermal drift. To minimize the thermal drift effect, we set the fiber lengths of different branches to be similar, and we stabilize the temperature as well. The out-of-loop frequency instability of the OFC due to the incoherence of the multi-branch is about 5.5×10-19 for 4000 s, while the in-loop frequency instability of fceo and that of fbeat are 7.5×10-18 for 1 s and 8.5×10-18 for 1 s, respectively. The turnkey OFC meets the requirement for the comparison of frequency between the best optical clocks. |
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| Keywords: | optical frequency comb nonlinear amplifying loop mirror optical clock frequency instability |
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