Beam Frequency Standards Research Articles

GNSS-grade space atomic frequency standards: Current status and ongoing developments

We present an overview on the current state of Global Navigation Satellite Systems (GNSS)-grade or better space atomic frequency standards’ (SAFS) technologies and discuss their applications. We estimate that a total of more than 1000 such standards were sent to space so far, the vast majority consisting of rubidium-cell frequency standards, Cs atomic beam frequency standards, and passive hydrogen masers. Finally, we review a variety of ongoing developments in view of future new generations of GNSS-grade SAFSs.

Realization of single optical frequency laser system for optically pumped cesium beam frequency standards

Compact laser system for optically pumped cesium beam clocks, which use single optical frequency laser for both Cs atomic state preparation and detection, is presented. The saturation probe laser intensity corresponding to certain Cs flux is researched by using laser, locked in the F = 4 → F′ = 4 transition of the Cs D2 line, to interact with Cs atoms. Light shift produced by laser could be reduced when the laser with small intensity, 0.7 mw/cm2, is used to interact with Cs atoms. High efficiency, 70%, fluorescence collectors are installed in the atom–laser interaction region to collect fluorescence. The frequency stability of the laser system is measured by mixing two output laser lights which are locked by fluorescence signal and it is about 3.2 × 10−11 at 2048 s. The compact laser scheme is beneficial to the miniaturization and industrial production of optically pumped Cs beam clocks. The design is also appropriate and useful for other laser frequency stabilization applications.

The influence of interatomic collisions on the angular distribution of Cs atomic flow from microchannel

Beams of alkali atoms are typically used in precise quantum devices, such as atomic beam frequency standards. In order to know more exactly parameters of atomic beam tubes and frequency standards (frequency instability), it is necessary to investigate the propagation of Cs atoms after the channel exit. In this work theoretical calculations of angular distribution of Cs atomic flow from a microchannel are carried out. The Monte-Carlo direct numerical simulation of Cs atom propagation is also performed to estimate the influence of interatomic collisions on the atomic flow.

Simulation and Design of Fluorescence Collector**Supported by the National Special Research Program for Major Scientific Instruments and Equipment of China under Grant No Y329KJ1101, and the Chinese PLA General Armament Department's Special Research Program for Fundamental Technology under Grant No Y333JK2101.

Fluorescence collector is a critical component in optically pumped Cs beam frequency standards. We design a new fluorescence collector by means of a new method. By means of two simulation methods, a smaller fluorescence collector with the same collection efficiency is achieved. It can be applied to almost all fluorescence detection systems in atom-light interaction experiments. We select the practical fluorescence collector studied by comparing three designing schemes. Its structure is very plain. Moreover, its fluorescence collection efficiency is very high. The collection efficiency of the practical fluorescence collector we designed is over 33% by means of two different ways.

Two-Color One-Way Frequency Transfer in a Metropolitan Optical Fiber Data Network

Two-color one-way frequency transfer through one strand of optical fiber is an alternative method to two-way frequency transfer, and is useful if unknown asymmetries exist in the link due to different paths for each direction. The term “two-color” refers to the ability to send signals at two different wavelengths utilizing the same fiber in one direction. The method is suitable for implementation in existing urban Single Mode Fiber networks, for instance in networks that are utilized for data and television communication. It is therefore able to coexist with data channels in wavelength-division multiplexing (WDM) systems. It performs as a dynamical control of transit time and simultaneously enables a real-time phase stabilized output signal. This paper presents results from a comparison of two cesium beam frequency standards separated by about 3 km over an optical fiber network located in a metropolitan area in Sweden. The cesium standards were simultaneously compared to each other with a Global Positioning System (GPS) satellite link and over optical fibers, so that the optical fiber technique could be evaluated with respect to the GPS technique. The difference in frequency stability between the two methods is shown to be about 3 × 10-15 over an averaging interval of 10 000 s

Improvement of Frequency Locking Algorithm for Atomic Frequency Standards

The authors describe a novel method of frequency locking algorithm for atomic frequency standards. The new algorithm for locking the microwave frequency to the Ramsey resonance is compared with the old one that had been employed in the cesium atomic beam frequency standards such as NIST-7 and KRISS-1. Numerical simulations for testing the performance of the algorithm show that the new method has a noise filtering performance superior to the old one by a factor of 1.2 for the flicker signal noise and 1.4 for random-walk signal noise. The new algorithm can readily be used to enhance the frequency stability for a digital servo employing the slow square wave frequency modulation.

High Performance Small Optically Pumped Caesium Beam FrequencyStandard

An experiment of a high performance small optically pumped caesium (Cs)beam frequency standard is reported. An extended cavity diode laserworks as the probing laser, of which the frequency is stabilized by theZeeman modulation method. The running parameters of the frequencystandard are dynamically optimized via digital servo electronics. Theexperimental setup improves the frequency stability up to 1.8×10−12 at τ = 1 s and about 1.0×10−13 atτ = 105 s (Allan deviation).

Determination of transit time distribution and Rabi frequency by applying regularized inverse on Ramsey spectra

The authors report on a method to determine the Rabi frequency and transit time distribution of atoms that are essential for proper operation of atomic beam frequency standards. Their method, which employs alternative regularized inverse on two Ramsey spectra measured at different microwave powers, can be used for the frequency standards with short Ramsey cavity as well as long ones. The authors demonstrate that uncertainty in Rabi frequency obtained from their method is 0.02%.

VNIIFTRI cesium fountain. State and future

Comparative analysis of beam and cold atom frequency standards is presented and there is a short description of the principles of laser cooling for atoms and equations for their movement. Results are provided for studies directed at creating a new generation of quantum standards for frequency, i.e., a cesium fountain. The atomic resonance for cold atoms with Q of the order of 1010 is obtained by experiment. A plan is developed and the main assemblies for a frequency primary standard vacuum system in a cold atom fountain are prepared.

Characterization of the main frequency shifts for the brazilian 133Cs atomic beam frequency standard

In order to evaluate an atomic clock, it is important to determine the main frequency shifts caused by external fields, device imperfections, etc. Scanning the frequency of the main oscillator, the Ramsey fringe is obtained and used to determine the main shifts. The gravitational shift (10-17), second order Doppler (1.65×10-13), Black-Body radiation shift (2.9×10-14), quadratic Zeeman effect (5×10-13), Rabi Pulling (1.3×10-13) and cavity pulling (1.3×10-13) have been evaluated. The short term stability, (6.6±0,2)×10-13, was obtained.

The classical caesium beam frequency standard: fifty years later

The caesium beam frequency standard using the separated oscillatory field technique proposed by Ramsey in 1950 has seen intensive development over the last fifty years. Its practical implementation as a primary time standard made possible the realization of the second with a precision better than that obtained by means of astronomical measurements and provided the basis for its redefinition in terms of atomic properties in 1967. This paper describes the basic principles underlying the operation of such atomic standards, reviews the progress made during the last fifty years since its invention, and provides an update on the state-of-the-art accuracy and frequency stability achieved today in this field.

Design of stabilized laser for optically pumped atomic frequency standards

AbstractThe influence of a stabilized laser's fluctuations in an optically pumped atomic frequency standard is considered. It is important to use a laser with narrow linewidth and select the modulation frequency where the laser's free‐running frequency fluctuation at twice the modulation frequency is the lowest. Employing these considerations, the stabilized laser for our optically pumped cesium beam frequency standard is designed. © 2001 Scripta Technica, Electr Eng Jpn, 136(2): 31–37, 2001

Asymmetry of microwave spectrum in an optically-pumped Cs atomic beam frequency standard

The asymmetry of microwave spectrum was investigated in an optically pumped Cs atomic beam frequency standard. The asymmetry was measured with the polarization state and the detuning of the pumping beam, and analyzed by the master equation based on the density matrix for the D/sub 2/ transition line. It was found that the pumping transition, F=4/spl rarr/F'=5 can offer a most sensitive detection scheme for the asymmetry. Finally, it was showed that the asymmetry could be compensated by adjusting the state of polarization of the pumping beam as well as the frequency of pumping laser.

Phase and light shift determination in an optically pumped cesium beam frequency standard

The determination of the phase shift and the light shift in our optically pumped Cs beam is reported here. We show that an error on the phase difference value determined by our fit method, without beam reversal, can be induced if the light shift is not taken into account. If the experimental data are corrected for the light shift, the results of the fit method are in very good agreement with the beam reversal results. This allows us to reduce the related uncertainty in the accuracy budget of our standard. The resulting overall accuracy of the standard is now estimated to be 6.3x10(-15).

A new design of a diffused laser light optically pumped small cesium beam frequency standard

A new set-up of a small Cs clock has been successfully experimentally tested. The short-term stability (2x10(-12)) and the long-term stability (3.5x10(-13) for a day sample time) have been measured. Only one laser is needed without any frequency shift device. The light frequency shift caused by diffuse light was calculated and measured.

Virtues and problems of the high C-field Cs beam frequency standard

The high C-field Cs beam frequency standard is presently a working machine that is undergoing first evaluations. The projected 10(-14 ) accuracy goal is as yet unattained, mainly because of inadequate C-field uniformity and stability. An analysis of the projected possible C-field improvements and the consequent uncertainty is here reported.

Second-Order Zeeman Frequency Shift in the Optically Pumped Cesium Beam Frequency Standard with a Dual Servo System

The second-order Zeeman effect produces the largest frequency shift among several causes to alter the reference frequency in the optically pumped cesium beam frequency standard. We introduce a novel way to reduce the uncertainties due to the Zeeman effect by adopting a dual servo system, one for stabilizing the current for static magnetic field and the other for normal clock operation. As a result, the combined uncertainty of the second-order Zeeman frequency shift is reduced to 3.2 ×10-15, which is smaller by one order of magnitude than that without the servo system.

Velocity distribution and second-order Doppler shift in an optically pumped cesium beam frequency standard

The velocity distribution and the second-order Doppler shift were obtained in an optically pumped cesium beam frequency standard at the Korea Research Institute of Standards and Science (KRISS) by using Fourier transforms of Ramsey lineshapes. The second-order Doppler shift changes as much as 3/spl times/10/sup -15/ for a variation 20 K at an oven temperature of 383 K. It also changes at a rate of -2.3/spl times/10/sup -14//dB around an optimum microwave power. We found that the velocity selective effect of a tilted laser beam can reduce the second-order Doppler shift. In particular, this effect becomes larger as the linewidth of the laser beam becomes narrower.

Estimation of the end-to-end phase shift without beam reversal in Cs beam frequency standards

A new method is presented to directly determine the averaged phase difference /spl phi/ between the fields interacting with the atoms in the two regions of a Ramsey cavity. This method is based upon the sensitivity of the phase shift to the frequency modulation depth and to the microwave power. We show too that it is possible to cancel the phase shift whatever the value of /spl phi/. Experimental results in our standard are given and are compared with those obtained by the beam reversal method. We show that the beam reversal method could lead to erroneous evaluation in our case. When combined with a beam reversal operation, the proposed method enables the distributed phase shift to be measured. This method could allow /spl phi/ to be determined in commercial clocks, which had been believed not to be measurable until now.

Controlling the microwave amplitude in optically pumped cesium beam frequency standards

Assuming square wave frequency modulation, the response, versus the amplitude of the microwave field, of an optically pumped cesium beam tube is considered. The properties of the first maximum of this response are analyzed. The effect of the neighboring lines is taken into consideration, and a model of the profile of the microwave field in each interaction region is validated. A symmetry property of the response considered is pointed out. It enables us to implement a feedback control of the microwave amplitude with a large depth of the amplitude modulation. Residual frequency offsets that may occur as a consequence of a spurious amplitude modulation correlated with the frequency modulation are assessed. And, with a cavity designed such that sigma=pi between the two oscillatory fields, it also is possible to measure the microwave amplitude at the first maximum of the sole contribution of the reference atomic line.