Optical Frequency Combs for Metrology

Since their invention in 1997, optical frequency combs (OFCs) based on femtosecond lasers provide the gearbox between the metrological units of length being realized via stabilized CW laser radiation, and time [1,2]. The OFC metrological figures of merit have been investigated in numerous experiments in universities, research labs, and NMIs [e.g., 3]. OFCs have proven to allow for optical frequency measurements or comparisons far beyond what is required as of now [4]. This comes along with engineering efforts rendering formerly complex opto-electronic setups into very reliable and easy-to-operate instruments which are commercially available. Consequently, OFCs continue to serve as indispensable tool for standard applications where frequency-stabilized CW laser radiation is a central requirement, but also for even the most advanced measurement tasks, including comparisons of the best optical clocks being set up and characterized in laboratories worldwide [5,6], or even clock comparisons in space [7].
This poster describes typical applications for optical frequency combs in metrology. Firstly, measurements of SI meter realizations like iodine stabilized HeNe Lasers against Cs clocks realizing the SI second are discussed [e.g., 8]. Secondly, OFC referenced generation of stable CW laser wavelengths for optical clock operation is presented, which is also prerequisite for one of the most important tasks in current time and frequency metrology - comparison of the so-called clock transition frequencies in different types of optical clocks in order to understand any systematic effects associated with chosen transition frequencies, atoms or ions, and operating conditions. Finally, technologies to transfer the stability and accuracy of these optical clocks from the optical domain back to the radio frequency domain without compromising spectral purity are described [9,10].
[1] T.W. Hänsch, Reviews of Modern Physics 78, 1297 (2006)
[2] J.L. Hall, Reviews of Modern Physics 78, 1279 (2006)
[3] L.A.M. Johnson, P. Gill and H.S. Margolis, Metrologia 52, 62 (2015)
[4] M. Giunta et al., Nature Photonics 14, 44 (2020)
[5] M. Takamoto et al., Nature Photonics 14, 411 (2020)
[6] A. Aeppli et al., Physical Review Letters 133, 023401 (2024)
[7] M. Lezius et al., Optica 3,1381 (2016)
[8] CCL-K11_TechnicalProtocol_v3.2.pdf, available at https://www.bipm.org/kcdb/
[9] X. Xie et al., Nature Photonics 11, 44 (2017)
[10] M. Giunta et al., Optics Letters 45, 1140 (2020)