Study Of A 532-nm Molecular-iodine Frequency Standard As A Length Standard

Length is one of the earliest physical quantities that people recognize.Since the establishment the metric convention in 1875,the development of the basic unit of length‘meter'has gone through the macro-natural benchmark defined by the earth's meridian to the physical benchmark defined by the international prototype of the metre,and then to the three stages of defining the micro-natural reference of the meter by the path of light in a vacuum within a time interval of 1/299792458 seconds,and it is still developing towards the micro-natural reference with less uncertainty.Today,the realization of the metre is essentially based on laser frequency standards,which are increasingly accompanied by techniques to directly generate optical frequencies from the second in terms of optical frequency comb(OFC)systems.The establishment of a high-precision length standard is not only of great significance to the national economy and people's livelihood,but the development of the length standard also promotes the development of optical clocks,long-baseline interferometers,optical strain gauges,iodine-stabilized laser absolute gravimeter and other related fields.Molecule iodine(¹²⁷I₂)is most frequently used in laser spectroscopy due to its broad spectrum range(from visible to near-infrared regions),strong absorption,and narrow linewidth.Among the secondary frequency standards for length standards recommended by the Consultative Committee for Length(CCL),most of them are the hyperfine transition lines of ¹²⁷I₂,and the ¹²⁷I₂ R(56)32-0 transition line as the frequency reference not only can obtain very good frequency stability,but the uncertainty is also the smallest among all iodine line references.In this paper,a series of works on the iodine-stabilized frequency standard are introduced.They include:1.Systematic analysis of several main atomic and molecular frequency stabilization methods,focusing on the modulation transfer spectroscopy(MTS)technology,and makes a detailed theoretical analysis of the relevant parameters in this method,which provides guidance for the experiment;2.A 534 nm iodine-stabilized laser system based on MTS technology is built,and the absolute frequency of the ¹²⁷I₂ R(53)31-0:a₂₁ line was measured with an OFC.The measurement results are consistent with the theoretical prediction of Iodine Spec5 complied by the Universit?t Hannover in German.It is also the first time that the absolute frequency of this line was measured in the world;3.A 532 nm molecular-iodine optical frequency standard based on MTS technology was estabilished,and the frequency stability of the molecular-iodine frequency standard was evaluated and optimized.Finally,the Allan deviation within 1 s average time was 2.7×10^-14,and 2.0×10^-14 at 30 s;4.With the help of a local optical frequency comb,the systematic frequency shift items in the 532 nm molecular-iodine frequency standard were evaluated,including residual amplitude modulation(RAM)induced frequency shift,pressure shift,power shift,beam-pointing-jitter induced frequency shift,etc..In order to evaluate the RAM-induced frequency shift in MTS technology,we theoretically deduced the MTS signal with RAM under large modulation depth,and propose a method for evaluating the RAM-induced frequency shift.After several measurements,the frequency reproducibility of the molecular-iodine frequency standard is 3.5×10^-13,and the absolute frequency after correction is 563 260 223 514.9(5.3)k Hz,which corresponds to a relative frequency uncertainty of 9.4×10^-12.The difference between this result and the BIPM-recommended value is 1.9 k Hz,which is consistent within 1?uncertainty.