Optical frequency standards

The idea of stabilizing the laser frequency 37-39) and making it as far as possible independent of cavity parameters, has been realized by many authors in different ways (see for instance the review article by Basov on optical frequency standards 339). 

Until 1992, the accuracy of spectroscopic measurements was limited to 1.6 parts in 1010 by the reproducibility of the 12-stabilized HeNe laser at 633 nm which served as an optical frequency standard, and by the unavoidable geometric wave-front errors in wavelength interferometry. To overcome this limitations it was necessary to measure the optical frequency rather than the wavelength. 

Abstract. A suitable femtosecond (fs) laser system can provide a broad band comb of stable optical frequencies and thus can serve as an rf/optical coherent link. In this way we have performed a direct comparison of the IS — 2S transition in atomic hydrogen at 121 nm with a cesium fountain clock, built at the LPTF/Paris, to reach an accuracy of 1.9 x 10-14. The same comb-line counting technique was exploited to determine and recalibrate several important optical frequency standards. In particular, the improved measurement of the Cesium Di line is necessary for a more precise determination of the fine structure constant. In addition, several of the best-known optical frequency standards have been recalibrated via the fs method. By creating an octave-spanning frequency comb a single-laser frequency chain has been realized and tested. 

As a candidate for a primary optical frequency standard In+ has three main advantages ... 

The two-photon 243 nm lS S-i/2 transition and other transitions in the hydrogen atom have recently been included in a new list of approved radiations for the practical realisation of the metre [2]. The particular interest of these transitions lies in the fact that, uniquely among present optical frequency standards, they may be calculated in terms of the Rydberg constant with an accuracy approaching that with which they have been measured. The latest measurement of... 

In this contribution we speculate on some new techniques of optical frequency comparison by use of modulated lasers, and on a possible new optical frequency standard based on the methane stabilised HeNe laser. He start by reviewing our... 

The rapid progress in recent years in the spectroscopy of the hydrogen atom has renewed pressure for a much better optical frequency standard. This in itself would not be enough to solve the measurement problem. New techniques of comparing optical frequencies are needed. He have developed methods of modulating lasers which can be used for frequency differences in excess of 2THz. 

New optical frequency standards based on harmonics of methane stabilised lasers will mean that we will never be more than 44 THz away from a reference frequency. New techniques of making frequency interval measurements of this magnitude will then be needed. 

One way an optical standard could be provided is by harmonic multiplication of a microwave frequency standard in a synthesis chain. By use of this technique, a laser at 88 THz (3.39 pm) has been made phase coherent with a microwave oscillator. - The best optical frequency standards may be made by locking a local oscillator (laser) to an atomic or molecular resonance line. State-of-the-art accuracies are characterized by measurements on methane stabilized He-Ne lasers in which reproducibilities in the 10— -- range have... 

Fig. 1. On the left is a simplified energy-level diagram for l Hg+. The 281.5 nm quadrupole "clock" transition can be observed by monitoring the 194 nm fluorescence. If the ion has made a transition from the Si to the 5/2 level the 194 nm flourescence disappears. For the figure on the right, on the horizontal axis is plotted the relative detuning from line center in frequency units at 281.5 nm. On the vertical axis is plotted the probability that the fluorescence from the 6s Si - 6p pi first resonance transition, excited by laser radiation at 194 nm, is on immediately after the 281.5 nm pulse. The electric-quadrupole-allowed S-D transition and the first-resonance S-P transition are probed sequentially in order to avoid light shifts and broadening of the narrow S-D transition. The recoilless absorption resonance or carrier (central feature) can provide a reference for an optical frequency standard. (From ref. 11)...

Although experimental verification is still lacking, it would appear that single ion optical frequency standards will eventually yield extremely high performance. Accuracies and stabilities better than 1 part in 10 seem quite feasible eventually they could exceed 1 part in 10. ... 

For almost three decades, the S-2S two-photon transition in atomic hydrogen with its natural linewidth of only 1.3 Hz has inspired advances in high-resolution spectroscopy and optical frequency metrology. This resonance [the 1S-2S transition] has become a de facto optical frequency standard. More importantly, it is providing a cornerstone for the determination of fundamental constants and for stringent tests of quantum electrodynamic theory. In the future, it may unveil... 

A state-of-the-art example [28] for trapped-ion optical frequency standards is the case of a laser with a line-width of less than 25 Hz locked to a electric quadrupole transition at 282 nm in a single laser-cooled Hg ion. The inherent stability of this trap based on the radiative lifetime of the metastable upper level of this transition is calculated to be about 1,5 x 10" x[29]. This is an exceptional example - in most cases as yet, the lasers used as oscillators for optical frequency standards based on ion traps often do not have a stability which matches the spectral sharpness of the trapped ion reference resonance. 

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