KEYWORDS: Atomic, molecular, and optical physics, System on a chip, Polarization, Modulation, Rubidium, Absorption, Vertical cavity surface emitting lasers, Signal to noise ratio, Atomic clocks
Chip-scale atomic clocks (CSACs) based on Coherent Population Trapping (CPT) are at the forefront of next-generation timekeeping for diverse applications, including global navigation satellite systems (GNSS), satellite communications, cell-phone networks, and hand-held GNSS receivers. Notwithstanding the potential ubiquity of this atomic device, a performance-limiting aspect of CSACs is the vapor-phase signal-to-noise ratio (SNR) of their ground-state (mF = 0 to mF = 0) atomic hyperfine resonance. Specifically, in commercially available devices angular-momentum optical pumping “pushes” atomic population towards high |mF| Zeeman sublevels at the expense of population in the 0-0 clock transition. Though mitigation strategies for this SNR limiting process have been proposed and demonstrated there has, to date, been little direct measurement of the population distribution among Zeeman sub-states for atoms undergoing CPT, and how that population distribution is altered by SNR improving mitigation strategies. Here, we describe our initial studies examining this question.
KEYWORDS: Atomic clocks, Clocks, Aerospace engineering, Global Positioning System, Temperature metrology, Physics, Chemical species, Signal to noise ratio, Space operations, Absorption
Atomic clock research at The Aerospace Corporation focuses on basic atomic physics in support of critical space technologies such as timekeeping for GNSS and communications. GPS and other GNSS play pivotal roles throughout modern society’s infrastructure, and clock stability in space can significantly impact the signals necessary for safe and reliable navigation and positioning. For secure communications, technology such as spread spectrum telecom is dependent on accurate and relatively unchanging timekeeping signals and frequency references. Many of our fundamental research investigations directly impact these technologies as they evolve in commercial space systems. In this presentation, we offer an introduction to The Aerospace Corporation with an overview of our laboratory’s basic physics research capabilities and their impact. Several clock physics investigations will be addressed and described in context with satellite-based timekeeping, which supports present and future space missions.
KEYWORDS: Absorption, Chemical species, Signal to noise ratio, Rubidium, Signal processing, Semiconductor lasers, Optical pumping, Interference (communication), Microwave radiation, Spectroscopy
In the weak-field limit, resonant absorption is viewed as a passive process: an optical field impinges on an atom, and within some cross-sectional area the atom has a high probability for absorbing the radiant energy. Absorption, however, is a dynamic process. Consequently, though a singlemode laser is highly monochromatic, the field's phase noise (i.e., quantum noise) generates fluctuations in the atom's absorption cross section. Laser phase noise (PM) thereby gives rise to absorption cross-section noise, and hence fluctuations in the medium's transmitted light intensity (AM). Following a brief overview of the PM-to-AM conversion process, we consider the role of collisions on PM-to-AM conversion efficiency in the weak-field regime. Specifically, the relative-intensity-noise of a diode laser, tuned to the Rb D1 transition, was measured after it passed through a rubidium/nitrogen vapor. Varying the nitrogen pressure, we found that rapid collisional dephasing decreased the efficiency of PM-to-AM conversion. Examining the rubidium hyperfine transition lineshape as a function of nitrogen pressure, we then found that pressure-broadening increased the transition's signal-to-noise ratio when limited by the PM-to-AM conversion process.
Just as it is possible to stabilize the frequency of an electromagnetic field to an atomic resonance between energy eigenstates, so too is it possible to stabilize the amplitude (or 'brightness') of a field to atomic parametric resonances, the so-called Rabi-resonances. For ease of reference, and by analogy to the atomic clock, we have coined the term 'atomic candle' for this quantum-mechanical, amplitude-stabilization system. Though the atomic candle was originally developed to stabilize microwave power in gas-cell atomic clocks, thereby eliminating a source of timekeeping instability in these devices, the atomic candle's applications extend well beyond the area of precise timekeeping. Basically, the atomic-candle provides a means for detecting and controlling subtle amplitude changes in electromagnetic fields at very low Fourier frequencies (i.e., f < 0.1 Hz). In the present work, we discuss a number of atomic candle applications: laser stabilization, absorption/refractive-index measurements, observations of cavity mode stability over very long time scales (i.e., 50 days), and measurements of low-frequency absorption/scattering fluctuations along a propagation path.
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