J. G. Coffer

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.

Precision measurements of absorption and refractive-index using an atomic candle

Across a broad range of disciplines, the accurate determination of an electromagnetic waves amplitude (either absolute or relative) has considerable relevance. We demonstrate a novel and potentially very precise method for making intensity measurements based on the atomic stabilization of electromagnetic field-strength. For ease of reference, and by analogy to atomic clocks, we refer to this field-strength stabilization system as an atomic candle. While the candles original purpose was to create a field with long-term intensity stability, its very nature makes it ideal for detecting subtle amplitude changes in strong electromagnetic fields, a problem that is fundamentally different from detecting weak signals in the presence of noise. In this paper, we discuss proof-of-principle experiments demonstrating the atomic candles ability to make precise measurements of absorption coefficients and indices of refraction.

Laser-pumped atomic clock exploiting pressure-broadened optical transitions

The alkali-vapor-cell atomic clock, of either the conventional or the coherent population trapping type, offers one of the most viable approaches to making ultraminiature and chip-scale devices. Unfortunately, this atomic clock suffers from two laser-induced noise processes: conversion of laser phase noise (PM) to amplitude noise (AM) and ac-Stark-shift fluctuations. Here we demonstrate a method for circumventing these problems in a passive and miniaturizable manner based on pressure broadening of the relevant optical transitions. For this purpose we have constructed a conventional atomic clock employing Rb^87 vapor confined with 100 Torr of N2. In this relatively high-pressure environment, both PM-to-AM conversion efficiency and the ac-Stark shift are reduced. Though we employ a phase-noisy, single-mode diode laser and lock the laser frequency to the pressure-broadened 1.6-GHz D_1 absorption line of Rb, we obtain excellent short-term frequency stability [i.e., sigma_y(tau)=1.8 × 10^−12/tau^½]. Moreover, as a single resonance cell generates locking signals for both the laser wavelength and the crystal oscillator, the atomic clock has real potential for miniaturization.

Experiments with active phase matching of parallel-amplified Multiline HF laser beams by a phase-locked Mach-Zehnder interferometer

Active phase matching of multiline HF laser beams by means of a phase-locked Mach-Zehnder interferometer was demonstrated by locking the interferometer to the central interference fringe at zero optical path length difference. The central fringe could be found by varying the spectral content of the input beam. Laser amplification in one leg of the interferometer decreased fringe visibility without adversely affecting locking. Single-line fringe patterns produced by an array spectrometer (while the interferometer was operated in its scanning mode) were analyzed to show that no significant dispersion occurred in the amplifier. The techniques developed have potential for measuring dispersion mismatch between larger parallel amplifiers. These experiments demonstrated in principle that a number of multiline HF amplified beams can be recombined and phase-matched to produce a high beam quality output beam.

Measurements of the anomalous dispersion of HF in absorption

We report quantitative measurements of the anomalous refractive index of the P_{1}(6),P_{1}(7) , and P_{1}(8) vibration-rotational transitions of hydrogen fluoride in absorption. The experimental technique uses the displacement of spatial fringes produced by a Mach-Zehnder interferometer containing the absorption cell. A frequency scanned, CW HF probe laser served as the light source and the spectrally selective element. In addition, we measured the absorption coefficient of the three transitions. The experimental results are in good agreement with calculated values derived from the transition-matrix theory of HF.

Low-frequency, self-sustained oscillations in inductively coupled plasmas used for optical pumping

We have investigated very low frequency, on the order of one hertz, self-pulsing in alkali-metal inductively-coupled plasmas (i.e., rf-discharge lamps). This self-pulsing has the potential to significantly vary signal-to-noise ratios and (via the ac-Stark shift) resonant frequencies in optically pumped atomic clocks and magnetometers (e.g., the atomic clocks now flying on GPS and Galileo global navigation system satellites). The phenomenon arises from a nonlinear interaction between the atomic physics of radiation trapping and the plasmas electrical nature. To explain the effect, we have developed an evaporation/condensation theory (EC theory) of the self-pulsing phenomenon.

rf-power and the ring-mode to red-mode transition in an inductively coupled plasma

The optical output of an alkali-metal inductively coupled plasma (alkali-ICP) plays an important role in both atomic magnetometers and atomic clocks, producing these devices’ atomic signals through optical pumping. Unfortunately, though the alkali-ICP’s optical pumping efficiency grows exponentially with temperature, at relatively high temperatures (∼140 °C) the discharge transitions from “ring mode” to “red mode,” which is a spectral change in the plasma’s output that corresponds broadly to a transition from “good emission” for optical pumping to “poor emission.” Recently, evidence has accumulated pointing to radiation trapping as the mechanism driving the ring-mode to red-mode transition, suggesting that the phenomenon is primarily linked to the alkali vapor’s temperature. However, observations of the transition made in the 1960 s, demonstrating that the ICP temperature associated with the transition depended on rf-power, would appear to cast doubt on this mechanism. Here, we carefully investigate the i...

Master oscillator with power amplifiers: performance of a two-element cw HF phased laser array

The experimental performance of a two-element phased array of multiline cw HF chemical lasers in the master oscillator with power amplifier (MOPA) configuration has been measured. The mutual coherence of the two amplified beams was inferred from measurements of the visibility of interference fringes obtained when the beams were overlapped in the near field. When the optical path difference for the two beams was minimized, multiline visibilities of 0.90 +/- 0.02 were measured. White light interferometry was used to equalize the optical path lengths. Spectral mismatch between the master oscillator output and the amplifiers preferred gain distribution affected neither the amplification factor nor the mutual coherence of the amplified beams. The effect of spatial coherence of the master oscillator beam on these near field measurements and the eventual requirement of far field measurements to precisely optimize path lengths are discussed. Spectra data, amplification factors, and mutual coherence measurements are shown, and the resulting phased array far field performance is presented.

Reverse wave suppression in unstable ring resonator

The effectiveness of a reverse wave suppressor (RWS) mirror in an unstable ring resonator has been investigated theoretically and experimentally for the case of an inhomogeneously broadened gain medium. The theory indicates that the RWS mirror is effective when (δ/Δνh)2 ≪ 1, where Δνh is the characteristic homogeneous linewidth of the gain medium and δ = Δu(ν0/c) is a measure of the separation between competing forward and reverse waves. Unstable linear ring resonator experiments were conducted using a continuous wave HF laser. Successful suppression of the reverse wave was achieved. In these tests the ratio of forward to reverse power had an average value of 41. An unstable annular ring resonator was investigated using a pulsed CO2 laser. Reverse wave suppression was achieved when the resonator and RWS mirror were in good alignment. Suppression effectiveness and beam quality were degraded when the RWS mirror was tilted.

Self-pulsing in alkali rf-discharge lamps

Rubidium vapor-cell atomic clocks are the “workhorse” of precise timekeeping for many GNSS missions. As is well known, the alkali rf-discharge lamp in these clocks is not only responsible for producing the atomic clock signal; it also has a significant influence on the clocks output frequency via the light-shift effect. Here, we discuss our discovery of self-pulsing in alkali rf-discharge lamps, which has the potential to periodically vary a Rb clocks signal-to-noise ratio and (via the light-shift effect) the clocks frequency. This self-pulsing is low frequency, on the order of one hertz in our experiments, and is not related to the self-pulsing reported a number of years ago by Robert Shaw, which was near 10 kHz and due to ion-acoustic wave generation in the discharge.