Optical frequency standard lasers

Von Zanthier, J. Eichenseer, M. Nevsky, A.Y. Okhapkin, M. Schwedes, Ch. Walther, H. A single indium ion optical frequency standard. Laser Physics 2005, 75, 1021-1027. 

Peik, E., Lipphardt, B., Schnatz, H., Schneider, T., Tamm, Chr., and Karshenboim, S.G., Frequency comparisons and absolute frequency measurements of l Yb" " single-ion optical frequency standards. Laser Physics, 15, 1028, 2005 arXiv physics/ 0504101. 

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. 

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)...

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. 

Primary length measurements are these days based on optical frequency standards. If one needs a unit of length, for example, a wave-length for interferometric measurement, then one divides the optical frequency by the value of the speed of light (299 792 458 m.s ) as defined in the SI metre. The mise en pratique of the metre [44] lists a number of frequency-stabilised lasers at various wavelengths in the visible and near infrared spectral regions. 

Both microwave and optical frequency standards have benefited greatly from the development of the laser and the methods of laser spectroscopy in atomic physics. In particular, the ability to determine both the internal and external (that is, motion) atomic states with laser light - by laser cooling for example - has opened up the prospect of frequency standards with relative uncertainties below lO, for example, the Cs atomic fountain clock. The best atomic theories in some cases at starting to match in accuracy that of measurement, providing thereby refined values of the fundamental, so-called atomic constants. Even quite practical measurements (such as used in GPS navigation and primary standards of length) have advanced in recent years. 

Riehle F, Schnatz H, Lipphart B, Zinner G, Kersten P and Helmcke J 1996 "Optical frequency standard based on laser-cooled Ca atoms , Proc. Symposium on Frequency Standards and Metrology, Woods Hole, MA (USA), (World Scientific Publishing, ed. J. C. Bergquist), pp.277 - 82... 

J.L. Hall, M. Zhu, P. Buch, Prospects for using laser prepared atomic fountains for optical frequency standards applications. J. Opt. Soc. Am. B 6, 2194 (1989)... 

The cornerstone of the recent optical frequency measurements in Paris is the LD/Rb standard laser [51,52,53]. Three identical systems have been built, two at the LPTF and a third in Laboratoire Kastler Brossel. As the two laboratories are linked by two 3 km long optical fibers, it is possible to compare the frequencies of the three systems. The frequency shift due to the fiber has been checked with the highly stabilized titanium-sapphire laser. After a round trip of 6 km through the fibers, a maximum frequency shift of 3 Hz is observed [54]. This shift is completely negligible for the optical frequency measurements. The main metrological features of the LD/Rb laser are a frequency stability (Allan variance) of about 4 X 10-13t-1/2 per laser over 1000 s and a day-to-day repeatability of 400 Hz. 

In order to test the measurements of the 2S — 8S and 2S — 8D transitions, the frequencies of the 2S — 12D intervals have also been measured in Paris [49]. This transition yields complementary information, because the 12D levels are very sensitive to stray electric fields (the quadratic Stark shift varies as n7), and thus such a measurement provides a stringent test of Stark corrections to the Rydberg levels. The frequency difference between the 2S — Y2D transitions (A 750 nm, u 399.5 THz) and the LD/Rb standard laser is about 14.2 THz, i.e. half of the frequency of the CO2/OSO4 standard. This frequency difference is bisected with an optical divider [56] (see Fig. 5). The frequency chain (see Fig. 11) is split between the LPTF and the LKB the two optical fibers are used to transfer the CO2/OSO4 standard from the LPTF to the LKB, where the hydrogen transitions are observed. This chain includes an auxiliary source at 809 nm (u 370.5 THz) such that the laser frequencies satisfy the equations ... 

A possible setup for the frequency measurement is depicted in Fig. 7. A frequency doubled diode laser at 972 nm is locked to the dye laser at 486 nm, which is the primary laser for driving the 1S-2S transition. A frequency comb generated by a mode locked laser is used to measure the frequency difference between the 972 nm diode laser and the 759 nm laser needed for the 2S-10S transition. Note that this experiment provides its own frequency standard, for the 1S-2S transition serves as the optical frequency reference. 

Laser Physics (ILP), Novosibirsk, including different methods of frequency stabilization [3,4], measurements of hyperfine line separations or frequency intervals between absorption lines [5,6, ] and absolute optical frequency measurements [8,9,10,11]. As a result of these efforts, the Comite Consultatif des Longueurs (CCL) meeting in 1997 recommended the frequency of one particular component, the aio hyperfine structure (HFS) component of the R(56)32-0 transition, for the realization of the metre with a relative standard uncertainty of 7 x 10-11 [ ] ... 

---