Spectral mode changes in an alkali rf discharge

Abstract

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.

Microfabricated chip-scale rubidium plasma light source for miniature atomic clocks

“…as the cell temperature is increased, the rb vapor pressure rises rapidly (by a factor of 100 between 35°c and 90°c), and indeed a strong increase in the intensity of the rb d1 lines (795.0 nm) and d2 lines (780.2 nm) is observed. This is similar to the behavior observed by [32] and [33] on their comparably sized rb lamps up to around 120°c. however, the reduction in rb d line intensity resulting from self-absorption, as observed at temperatures <120°c in [32] and [33], is not observed in these cells up to 150°c, as expected because of the higher number density of buffer-gas atoms.…”

“…This is similar to the behavior observed by [32] and [33] on their comparably sized rb lamps up to around 120°c. however, the reduction in rb d line intensity resulting from self-absorption, as observed at temperatures <120°c in [32] and [33], is not observed in these cells up to 150°c, as expected because of the higher number density of buffer-gas atoms. With increasing cell temperature and increasing emitted rb light intensity, a reduction of the ar line intensity is observed.…”

Spectral emission from the alkali inductively-coupled plasma: Theory and experiment

“…This later assumption has been validated experimentally. 15,23 In particular, Fig. 1a shows N Rb (r) for several values of q, while Fig.…”

“…Only recently have researchers come to understand that the ring-mode to red-mode transition in the ICP (where rf-discharge lamps routinely operate) derives from radiation trapping, 15 and that this transition-region is a regime of lamp instability. 16 Researchers still do not have theoretical justification for the proportionality between Rb light emission and Rb vapor pressure; they do not understand the threshold temperature for rf-discharge lamp operation, and they have no clear sense of how that threshold temperature scales with the rf-power driving the discharge.…”

Reliability characteristics of microfabricated Rb mini-lamps for optical pumping in miniature atomic clocks and magnetometers

“…This is most probably due to the Rb atoms diffused into the wall over time (similar to the observations reported by other groups 8 ). This layer causes increased Rb self-absorption leading to Doppler broadening of the emitted Rb D lines and also overall reduced transmission of the D line power 15 . However, as the Rb lamp is intended to be operated at low power (10 µW Rb D output), this would most probably not be a limiting factor for its use in an atomic clock application (as seen from Figure 5, the optical power emitted after 6 months is still in the same range as the initially set value), but a slight degradation in the clock performance can be expected due to the possible Rb D line Doppler broadening.…”