Chin. Phys. B, 2021, Vol. 30(5): 050603    DOI: 10.1088/1674-1056/abd754
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A 532 nm molecular iodine optical frequency standard based on modulation transfer spectroscopy

Feihu Cheng(程飞虎), Ning Jin(金宁), Fenglei Zhang(张风雷), Hui Li(李慧), Yuanbo Du(杜远博), Jie Zhang(张洁), Ke Deng(邓科), and Zehuang Lu(陆泽晃)
MOE Key Laboratory of Fundamental Physical Quantities Measurement, Hubei Key Laboratory of Gravitation and Quantum Physics, PGMF and School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China
Abstract  We report construction of an iodine-stabilized laser frequency standard at 532 nm based on modulation transfer spectroscopy (MTS) technology with good reproducibility. A frequency stability of $2.5 \times {10}^{-14}$ at 1 s averaging time is achieved, and the frequency reproducibility has a relative uncertainty of ${3.5\times }{10}^{-13}$, demonstrating the great stability of our setup. The systematic uncertainty of the iodine-stabilized laser frequency standard is evaluated, especially the contribution of the residual amplitude modulation (RAM). The contribution of the RAM in MTS cannot be evaluated directly. To solve this problem, we theoretically deduce the MTS signal with RAM under large modulation depth, and prove that the non-symmetric shape of the MTS signal is directly related to the MTS effect. The non-symmetric shape factor $r$ can be calibrated with a frequency comb, and in real experiments, this $r$ value can be obtained by least-squares fitting of the MTS signal, from which we can infer the RAMinduced frequency shift. The full frequency uncertainty is evaluated to be 5.3 kHz (corresponding to a relative frequency uncertainty of ${9.4\times }{10}^{-12})$. The corrected transition frequency has a difference from the BIPM-recommended value of 2 kHz, which is within ${1}\sigma$ uncertainty, proving the validity of our evaluation.
Keywords:  iodine-stabilized laser frequency standard      modulation transfer spectroscopy      residual amplitude modulation
Received:  17 October 2020      Revised:  30 November 2020      Accepted manuscript online:  30 December 2020
 PACS: 06.20.-f (Metrology) 42.62.Fi (Laser spectroscopy)
Fund: Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0304401), Key-Area Research and Development Program of GuangDong Province, China (Grant No. 2019B030330001), and the National Natural Science Foundation of China (Grant Nos. 11174095, 61875065, 91536116, and 11804108).
Corresponding Authors:  Ke Deng, Zehuang Lu     E-mail:  ke.deng@hust.edu.cn;zehuanglu@hust.edu.cn