J. C. Camparo

The diode laser in atomic physics

Abstract Over the past fiie years the use of diode lasers in physics research has expanded rapidly. In part, this has occurred because the diode laser is a highly monochromatic and tunable light source that, when compared to other laser systems, is quite inexpensive and easy to operate. These characteristics make the diode laser ideal for multi-laser experiments, or small scale/high quality research projects in general. In this article a brief introduction to the AlGaAs single-mode diode lasers characteristics and operation is presented, along with an overview of this lasers contributions to atomic physics research to date. It is hoped that this review will facilitate the further use of diode lasers in all aspects of physics, and in particular will aid the small scale researcher in attaining a more active place in mainstream physics.

The rubidium atomic clock and basic research

The vapor-cell atomic clock finds application today in the global positioning system and telecommunications. To improve and miniaturize the humble device for future applications will require a deeper understanding of atomic and chemical physics.

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.

Spectral mode changes in an alkali rf discharge

As a result of observations made by Shaw (M.S. thesis, Cornell University, 1964) in the mid-1960s, alkali rf discharges are known to operate in two spectral modes, the so-called ring mode and the red mode. Experience has shown that the ring mode is best for discharge lamps used in quantum-electronic devices such as atomic clocks and optically pumped magnetometers and that the performance of these devices seriously degrades when the lamp operates in the red mode. Understanding the origin of these modes therefore has application to understanding and improving various quantum-electronic devices. Here we show that Shaw’s model for these modes is inconsistent with observation, and we propose an alternate model based on the role of radiation trapping in multistep ionization.

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...

Laser PM to AM conversion in atomic vapors and short term clock stability

The optically thick vapors employed in gas-cell atomic clocks provide for very efficient conversion of laser phase noise (PM) to amplitude noise (AM). Here, we discuss the role of PM to AM conversion in determining gas-cell clock performance. First, a brief overview of the PM to AM conversion process will be given, highlighting the fact that laser phase noise creates fluctuations in the atoms optical absorption cross-section which have an exponential influence on the lasers transmitted intensity. Experimental results will then be presented showing that the PM to AM conversion process can increase the relative intensity noise of transmitted laser light by orders of magnitude, and that this can, under certain conditions, cause laser-pumped clock performance to be poorer than lamp-pumped clock performance.

Does the light shift drive frequency aging in the rubidium atomic clock

Frequency aging in the rubidium (Rb) vapor-cell atomic clock plays a significant role in the devices timekeeping ability. Though many researchers have speculated on the physical mechanism(s) driving the linear, deterministic frequency change (i.e., /spl Delta/f(t)/f/sub o/ = At), there is little unambiguous experimental data regarding the phenomenon. Here, long-term data were used from on-orbit global positioning system (GPS) Rb clocks to examine one postulated mechanism for frequency aging (i.e., the light-shift effect). Defining the light shift of the clocks fractional frequency as /spl alpha/I/I/sub o/, where /spl alpha/ is the light-shift coefficient, we find that temporal variations of the relative light intensity, I/I/sub o/ cannot account for frequency aging. However, for the population of clocks considered here, we obtain the intriguing result that /spl lambda//A = 1.7 /spl plusmn/ 1.5. Thus, it may be that frequency aging is driven by the light-shift effect through temporal variations of the light-shift coefficient.

Cavity-Q aging observed via an atomic-candle signal

Slow variations in cavity-Q and microwave power are thought to play a role in the long-term frequency stability of gas-cell atomic clocks. Here, we use an atomic-candle method to study the aging of a TE/sub 011/ microwave cavitys resonant frequency and quality factor when a glass resonance cell containing Rb/sup 87/ loads the cavity. Our results suggest that the alkali vapor coats the inside glass surface of the resonance cell with a thin metallic film; and that, as this film evolves, the quality factor degrades. (In our experiments the quality factor changed by /spl sim/30% over a timescale of months.) More generally, the present work demonstrates the efficacy of the atomic-candle method for investigating cavity resonances. In particular, we show that, when used in conjunction with more traditional methods, the atomic-candle method has the potential to reveal information on a cavity modes spatial profile.

Laser spectroscopy on a ‘‘shoestring’’

The advent of tunable lasers has had a profound influence on both experimental and theoretical physics. Unfortunately, since these laser systems are typically hazardous and expensive, the physics student at the undergraduate or first‐year graduate level has no real familiarity with their application in modern physics; and thus cannot fully appreciate their significance. Tunable single mode laser diodes, however, may offer a remedy to this situation. To demonstrate their applicability, we have designed a relatively simple and inexpensive experiment of laser diode spectroscopy in an atomic beam which illustrates the effect of hyperfine structure and the isotope shift in the rubidium D1 transition (52S1/2−52P1/2). Furthermore, this experiment demonstrates the possibility of investigating basic physics without major expenditures for laser systems and laboratory facilities.

Alkali 〈I⋅S〉 wall relaxation in dichlorodimethylsilane coated resonance cells

Alkali 〈I⋅S〉 wall relation rates for Pyrex resonance cells coated with dichlorodimethylsilane have been studied. In particular, by considering the ratio of the Rb85 wall relaxation rate to the Rb87 wall relaxation rate, evidence is presented showing that dimethylsiloxane surfaces and alkane (paraffin) surfaces relax alkali polarization with similar interaction strengths and correlation times. This conclusion indicates that the outer most functional group of an organic surface molecule has the primary influence on the alkali polarization’s surface relaxation.