Extremely narrow optical resonances in cold atoms or single trapped ions can be measured with high resolution. A laser locked to such a narrow optical resonance could serve as a highly stable oscillator for an all-optical atomic clock. However, until recently there was no reliable clockwork mechanism that could count optical frequencies of hundreds of terahertz. Techniques using femtosecond-laser frequency combs, developed within the past few years, have solved this problem. The ability to count optical oscillations of more than 1015 cycles per second facilitates high-precision optical spectroscopy, and has led to the construction of an all-optical atomic clock that is expected eventually to outperform today's state-of-the-art caesium clocks.

Optical frequency metrology

Recently there has been a remarkable synergy between the technologies of precision laser stabilization and mode-locked ultrafast lasers. This has resulted in control of the frequency spectrum produced by mode-locked lasers, which consists of a regular comb of sharp lines. Thus such a controlled mode-locked laser is a ``femtosecond optical frequency comb generator.'' For a sufficiently broad comb, it is possible to determine the absolute frequencies of all of the comb lines. This ability has revolutionized optical frequency metrology and synthesis. It has also served as the basis for the recent demonstrations of atomic clocks that utilize an optical frequency transition. In addition, it is having an impact on time-domain applications, including synthesis of a single pulse from two independent lasers. In this Colloquium, we first review the frequency-domain description of a mode-locked laser and the connection between the pulse phase and the frequency spectrum in order to provide a basis for understanding how the absolute frequencies can be determined and controlled. Using this understanding, applications in optical frequency metrology and synthesis and optical atomic clocks are discussed. This is followed by a brief overview of how the comb technology is affecting and will affect time-domain experiments.

https://www.reading.ac.uk/physicsnet/units/4/4phla/Papers/RMP03_FemtosecondCombs_Cundiff.pdf

Colloquium: Femtosecond optical frequency combs

Four long-running currents in laser technology met and merged in 1999--2000. Two of these were the quest toward a stable repetitive sequence of ever-shorter optical pulses and, on the other hand, the quest for the most time-stable, unvarying optical frequency possible. The marriage of UltraFast and UltraStable lasers was brokered mainly by two international teams and became exciting when a special ``designer'' microstructure optical fiber was shown to be nonlinear enough to produce ``white light'' from the femtosecond laser pulses, such that the output spectrum embraced a full optical octave. Then, for the first time, one could realize an optical frequency interval equal to the comb's lowest frequency, and count out this interval as a multiple of the repetition rate of the femtosecond pulse laser. This ``gear-box'' connection between the radio frequency standard and any/all optical frequency standards came just as Sensitivity-Enhancing ideas were maturing. The four-way Union empowered an explosion of accurate frequency measurement results in the standards field and prepares the way for refined tests of some of our cherished physical principles, such as the time-stability of some of the basic numbers in physics (e.g., the ``fine-structure'' constant, the speed of light, certain atomic mass ratios etc.), and the equivalence of time-keeping by clocks based on different physics. The stable laser technology also allows time-synchronization between two independent femtosecond lasers so exact they can be made to appear as if the source were a single laser. By improving pump/probe experiments, one important application will be in bond-specific spatial scanning of biological samples. This next decade in optical physics should be a blast.

https://link.aps.org/pdf/10.1103/RevModPhys.78.1279

Nobel Lecture: Defining and measuring optical frequencies

The latest edition of the AFGL atmospheric absorption line parameters compilation for the seven most active infrared terrestrial absorbers is described. Major modifications to the atlas for this edition include updating of water-vapor parameters from 0 to 4300 per cm, improvements to line positions for carbon dioxide, substantial modifications to the ozone bands in the middle to far infrared, and improvements to the 7- and 2.3-micron bands of methane. The atlas now contains about 181,000 rotation and vibration-rotation transitions between 0 and 17,900 per cm. The sources of the absorption parameters are summarized.

AFGL atmospheric absorption line parameters compilation: 1982 edition.

In 1983, at the time of the adoption of the present definition of the metre by the 17th General Conference on Weights and Measures (CGPM), the International Committee for Weights and Measures (CIPM) drew up recommendations for the practical realization of the definition. These have formerly been referred to as the mise en pratique of the definition of the metre. It was understood that the practical realization would, from time to time, be updated to take account of new measurements and improvements in techniques of laser stabilization. In 1992 the CIPM, acting on the advice of the then Consultative Committee for the Definition of the Metre (CCDM), adopted a revision of the mise en pratique in its Recommendation 3 (CI-1992). The text was published in Metrologia (1993/94 30 523–41). In 1997 a second revised version of the practical realization of the definition of the metre was adopted by the CIPM in its Recommendation 1 (CI-1997). The text was published in Metrologia (1999 36 211–44). In 2001, the CIPM again adopted a revised version of the practical realization of the metre and this time included recommended frequencies for other optical frequency standards. The text of this Recommendation (Recommendation 1 (CI-2002)) is given here∗. In addition, a revised list of recommended radiations, and an appendix containing the source data used in estimating the wavelengths, frequencies and uncertainties of the recommended radiations are given. In adopting the revised practical realization, the CIPM acknowledged the considerable effort put into its preparation by the Consultative Committee for Length (CCL) through its Working Group on the mise en

https://iopscience.iop.org/article/10.1088/0026-1394/40/2/316/pdf

Practical realization of the definition of the metre, including recommended radiations of other optical frequency standards (2001)

An optical frequency chain referenced to a Cs atomic clock has been used to measure directly the frequency of the electric quadrupole allowed $5s^{2}S_{1/2}\ensuremath{-}4d^{2}D_{5/2}$ transition at 445 THz in a single, trapped, and laser cooled ${}^{88}{\mathrm{Sr}}^{+}$ ion. A transition frequency, ${f}_{S\ensuremath{-}D}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}444779044095.4\mathrm{kHz}$ with an estimated standard uncertainty of 0.2 kHz has been determined. Intrinsic offsets of the probed ion transition in the current experiment are calculated to be at the ${10}^{\ensuremath{-}15}$ level.

Cs-Based Frequency Measurement of a Single, Trapped Ion Transition in the Visible Region of the Spectrum

Absolute frequency measurements of N2O laser transitions

Heterodyne frequency measurements of CO2 laser sequence-band transitions

Heterodyne frequency measurements of CO2 laser hot-band transitions

We have measured the frequency of the 5s /sup 2/S/sub 1/2/-4d /sup 2/D/sub 5/2/ clock transition of a single Sr ion confined in a Paul trap. A diode laser locked to an ultrastable Fabry-Perot (F-P) cavity was used to probe the transition with a resolution of 3.5 kHz. The absolute frequency was determined from heterodyne measurements referenced to an iodine stabilized HeNe laser and a CO/sub 2/ laser yielding a value for the S-D transition of (444 779 043 963/spl plusmn/30) kHz. This work could lead to the development of a new optical frequency standard at 674 nm.

Precision frequency measurement of the /sup 2/S/sub 1/2/-/sup 2/D/sub 5/2/ transition of Sr/sup +/ with a 674-nm diode laser locked to an ultrastable cavity

B.G. Whitford

Papers

Cs-Based Frequency Measurement of a Single, Trapped Ion Transition in the Visible Region of the Spectrum

Absolute frequency measurements of N2O laser transitions

Heterodyne frequency measurements of CO2 laser sequence-band transitions

Heterodyne frequency measurements of CO2 laser hot-band transitions

Precision frequency measurement of the /sup 2/S/sub 1/2/-/sup 2/D/sub 5/2/ transition of Sr/sup +/ with a 674-nm diode laser locked to an ultrastable cavity