R. P. Frueholz

Fundamental stability limits for the diode‐laser‐pumped rubidium atomic frequency standard

Recently, there has been considerable interest in the use of single‐mode diode lasers in atomic frequency standards. In the present paper theoretical calculations are performed in order to quantify the expected performance improvement upon incorporation of diode lasers in rubidium gas cell atomic frequency standards. We assume that clock signal shot noise, the diode laser’s quantum noise, and diode laser frequency locking noise all contribute to the atomic frequency standard’s stability. Our results indicate that white‐noise Allan variances of ∼6×10−15/(τ)1/2 are possible if enhanced cavity Q diode lasers are employed, whereas for presently available commercial diode lasers we predict white‐noise Allan variances of ∼3×10−14/(τ)1/2. These variances represent a 2–3 orders of magnitude improvement in frequency stability over that currently obtained with rubidium gas cell atomic clocks.

Alkali reactions with wall coating materials used in atomic resonance cells

It is well known that the chemisorption of various chlorosilane materials on glass atomic storage vessel walls results in surface coatings which inhibit electronic‐ and nuclear‐spin relaxation. In the present study the chemical reaction of rubidium, and by analogy other alkali metals, with dichlorodimethylsilane‐treated glass surfaces has been studied. We find evidence that rubidium reacts with a freshly prepared coating to produce H2 and a volatile silicon‐containing species. The most reasonable reaction process is postulated to be rubidium reacting with residual silanol groups (Si‐OH) found on the surface. As the reaction proceeds these groups would disappear, thus reducing the spin‐relaxation rate associated with the surface. We believe that this reaction results in the ‘‘curing’’ of wall coatings reported by other investigators. Concurrently, the gaseous reaction products become impurities within the system. The spin‐relaxation cross section of the silicon‐containing species is expected to be less tha...

Harmonic saturated spectroscopy for improved atomic detection

A novel method for improved cw laser-induced atomic fluorescence detection that we call harmonic saturated spectroscopy takes advantage of the nonlinear response of an optically saturated atomic system to reject background sources of interference. A reduction in background and an improved SNR are demonstrated for sodium detection in an air-acetylene flame. The dependence of the harmonic signal on intensity, excitation waveform, and depth of modulation was also investigated.

The role of long‐term lamp fluctuations in the random walk of frequency behavior of the rubidium frequency standard: A case study

A long‐term frequency stability test has been performed on an EGG namely, long‐term frequency drift, spontaneous frequency jumps, and random walk of frequency noise for long averaging times. By careful analyses of the complete test record, we have found that the spontaneous frequency jumps and the random walk of frequency noise can be attributed to the behavior of the clock’s discharge lamp. The long‐term frequency drift, however, was found not to be primarily lamp related. Although results of only a single test on a single Rb clock are reported here, we believe that the results are applicable to the Rb clock technology as a whole.

Microwave Field Strength Measurement in a Rubidium Clock Cavity via Adiabatic Rapid Passage.

Rubidium atomic frequency standards have been found to display a strong frequency dependence on the microwave power density or field intensity. It is, therefore, desirable to have simple and easily applied techniques for measuring not only the field strength but also the field distribution within the clock’s microwave cavity. A technique which makes use of adiabatic rapid passage (ARP) to measure field strengths in Rb clock cavities is described and demonstrated. Measurements using this technique are found to agree with field strengths extracted using standard microwave procedures. The ARP technique is particularly valuable in cavities where standard microwave measurements cannot be made.

A nonempirical model of the gas-cell atomic frequency standard

In this paper a signal model of the rubidium (Rb) atomic frequency standard is developed. The model combines an analysis of the atomic physics required to describe processes occurring within the Rb absorption cell with a feedback analysis of the clock’s servocontrol circuitry, and thus allows clock performance in terms of Allan variance to be predicted from a number of electronic and physical parameters. All previous models of the Rb clock have been limited to an analysis of the clock’s short‐term performance, Allan variance averaging times less than 10 000 s. However, by explicitly including the effects of discharge lamp intensity fluctuations, which are transformed into output frequency variations via the light shift effect, clock performance can be predicted for averaging times greater than 10 000 s. Furthermore, the present model is the first which incorporates the influence of an optically thick Rb vapor, along with the diffusion of optically pumped atoms to the walls of the absorption cell, into the...

Production of mass-selected neutral clusters of rubidium

Abstract Charge neutralization of rubidium cluster ions (Rb N + ) by Rb atomic vapor is reported. The cluster ions are generated by a liquid metal ion source (LMIS). The ions and the neutrals are detected directly without any photoionization step. The extent of dissociative neutralization is negligible. This results in intense, mass-selected neutral cluster beams. The charge exchange cross sections for Rb 2 + and Rb 3 + with Rb, are 9.3× 10 −15 and 3.1 × 10 −15 cm 2 respectively.

Use of wall coated cells in atomic frequency standards

An analysis of the effect of the microwave field distribution on the application of a wall coated, bufferless absorption cell in an atomic frequency standard has been performed. While certain advantages of using such a cell in an atomic frequency standard employing a TE011 microwave cavity have been previously demonstrated, we show that a simple incorporation of this type of cell in a frequency standard using a TE111 microwave cavity would not be advantageous. The role of the microwave field distribution, particularly its phase variation within the cavity, is analyzed and suggestions concerning the use of a wall coated cell in a TE111 microwave cavity as applied to an atomic frequency standard are made.

Liquid metal ion source for cluster ions of metals and alloys : design and characteristics

Currently liquid metal ion sources (LMISs) are of great interest for a wide variety of applications—ion implantation, ion microlithography, thrusters for electric space propulsion, etc. A novel application of the LMIS is for the production of metallic cluster ions. In our laboratory we have designed and optimized the performance of a LMIS for the production of cluster ions of alkali metals. Using liquid rubidium (Rb) we have observed copious production of singly charged cluster ions (Rb+N, N=1–100). As expected the largest fraction of the emission consists of atomic ions. For low source current ( 80 μA) we observe cluster ions as large as Rb+100. We study the mass distribution using the time‐of‐flight technique.

Atomic phase delay in a rubidium atomic clock

The phase delay that exists between the oscillating atomic system and the probing microwave field in a rubidium atomic clock is examined. This phase delay must be accounted for in the feedback control circuitry of the clock in order to maximize the devices short-term stability. It is found both experimentally and theoretically that the atomic phase delay can vary by more than 100/spl deg/, and that in the regime of normal clock operation it is a rapidly changing function of microwave power. Defining P/sub 0/ as the normal operating power entering the clock cavity, it is shown that at P/sub 0/ the phase-delay sensitivity to microwave power variations is maximized and on the order of ten degrees per dB. It is unadvisable to try to operate the clock at some different microwave power where this sensitivity is smaller, since the amplitude of the first harmonic signal is also maximized at P/sub 0/.<<ETX>>