Frequency standard

Nominal system Rated maximum One-minute power frequency Standard lightning impulse One-minute d.c. voltage ... 

The characteristics of ionic or neutral atom gas lasers (namely, directionality, narrow linewidth, and high coherence length) have determined the great variety of scientific applications in which they have been used up to now. They have served as wavelength and frequency standards, and as alignment systems, and they have been an important tool in holographic experiments. 

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). 

For most materials, permittivity and power factor are not constant over the very wide range of frequencies of industrial interest and, because the same apparatus and method cannot be used at all frequencies, standard methods are usually subdivided into procedures for different frequency ranges. 

If now an input signal of die resonant frequency is fed into the cavils, il can bring about spontaneous emission, and amplification will occur. The shortcoming ol this arrangement is that Ihe ammonia resonance cannot lie tuned and il represents an impracticably narrow bandwidth. On Ihe other hand, like any other amplifier, it cun he made fit oscillate, so that it can be used as a very stable frequency standard (ammonia clock). 

Maser amplifiers are used where the requirement for a very low noise amplifier outweighs the technological problems of cooling 10 low temperatures. They have been used in passive and active radiostronomical work, in satellite communications, and us preamplifiers for microwave spectrometry The ammonia and the atomic hydrogen masers have been studied as frequency standards and have heen used in accurate tests of special relativity. 

Cotton G. Wilkinson, Advanced Inorganic Chemistry. . Interscience, NY (1966), 418 4) M.P.R. Thomsen et al, Improved Rubidium Vapor Cells for Frequency Standards , Rept No 7366-F, Melpar Inc, Fall Church, Contract DAAB07-67-C-O535 (1968) 5) Ibid, Rept No... 

Searles E. Simon, Performance Characteristics of Portable Atomic Rubidium Clock and Frequency Standard , R DTR ECOM-3339 (1970) 8) ChemRubHdb (1978), B-44... 

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. 

The first frequency measurement of the 15 — 25 resonance made use of a transportable ClU-stabilized HeNe infrared frequency standard at 88 THz [24], built at the Institute of Laser Physics in Novosibirsk/Russia. For calibration it was transported repeatedly to the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig/Germany where it could be compared with a Cs atomic clock using the PTB frequency chain [25]. 

The next big advance towards higher precision was the 1997 phase-coherent measurement of the frequency gap with an optical frequency interval divider chain [27]. The 2.1 THz gap was no longer measured by counting cavity fringes, but divided down to the radio frequency domain by a phase-locked chain of five optical frequency interval dividers [56] (see Fig. 5). The accuracy of this approach was limited by the secondary frequency standard to 3.4 parts in 1013, exceeding the accuracy of the best previous measurements by almost two orders of magnitude. 

It is now recognized that cold collision frequency shifts [32] is a crucial issue for every high precision atomic frequency standard, microwave or optical. For hydrogen at a density of 109 cm-3 the shift of the 1S-2S transition is about 0.4 Hz, [8], or a fractional shift of 1.7 x 10-16. For a rubidium hyperfine standard operating at the same density, the shift is about 6 xlO-14 [45,46]. 

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. 

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. 

A modern Pb/scintillating fiber detector system, incorporating a Loran frequency standard, capable of measuring time intervals with a precision of 20 ps. 

Abstract. We present a review of the helium spectroscopy, related to transitions between 23S and 23P states around 1083 nm. A detailed description of our measurements, that have produced the most accurate value of the 23Po — 23Pi fine structure interval, is given. It could produce an accurate determination (34 ppb) of the fine structure constant a. Improvements in the experimental set up are presented. In particular, a new frequency reference of the laser system has been developed by frequency lock of a 1083 nm diode laser to iodine hyperfine transitions around its double of frequency. The laser frequency stability, at 1 s timescale, has been improved of, at least, two orders of magnitude, and even better for longer time scales. Simultaneous 3He —4 He spectroscopy, as well as absolute frequency measurements of 1083 nm helium transitions can be allowed by using the Li-locked laser as frequency standard. We discuss the implication of these measurements for a new determination of the isotope and 23 5 Lamb shifts. 

The lack of accurate and stable frequency standards in the near-infrared spectral range, and in particular at 1083 nm, is a serious inconvenient to improve the present frequency stability of the He-locked master laser. On the other hand, hyperfine transitions of the iodine molecule has been defined as secondary frequency standard at different visible wavelengths, and in particular at 532 nm, the doubled frequency of the 1064 nm Nd YAG laser. Likewise, our idea has been to lock the master laser frequency to I2 hyperfine transitions at its doubled frequency, 541 nm. 

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

In this section we want to estimate the foreseeable systematic line shifts of the atomic reference in an indium frequency standard and compare indium to the alkali-like candidate ions like mercury, ytterbium, barium or strontium that rely on electric quadrupole transitions of the type Si/2 —> >5/2 Line shifts may be caused by the motion of the ion, by electric and magnetic fields, by radiation or by collisions. Values for the shifts will be given as dv/v, relative to the In+ transition frequency of v = 1267 THz. 

Table 1 presents the summary of the estimated uncertainties, showing that the indium frequency standard should be able to provide an accuray of a few parts in 1018, making this system a most promising candidate for a future primary optical clock. 

Table 1. Estimated shifts and uncertainties of the indium frequency standard. All values are given relative to the frequency 1267 THz. For the small effects the uncertainties may be comparable to the total shift, with the exception of the blackbody AC stark shift. All Stark shifts are based on an estimated upper bound for the polarisability. The collisional shift is an estimate based on data in comparable atomic systems. The assumed conditions are T 0 = 100 pK uncompensated stray fields, leading to micromotion amplitude < A/20 = 12 nm AB = 10 fJtG P = 5 10-8 Pa TtraP = 300 K 1 K...

Another possibility is to use optical atomic frequency standards. Any evolution of a in time would lead to a frequency shift. To establish the connection between a and u>, relativistic calculations of the a dependence of the relevant frequencies for Cal, Srii, Bail, Ybn, Hgn, Inn, Tin and Rail have been performed [1]. The a dependence of the microwave frequency standards (Cs, Hg+) has also been accurately calculated. 

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