Power Dependent Lineshape Corrections for Quantitative Spectroscopy

The Voigt profile - a convolution of a Gaussian and a Lorentzian - accurately describes the absorption lines of atomic and molecular gases at low probe powers. Fitting such to experimental spectra yields both the Lorentzian natural linewidth and the Gaussian Doppler broadening. However, as the probe power increases saturation effects introduce spurious power dependence into the fitted Doppler width. Using a simple atomic model, we calculate power-dependent corrections to the Voigt profile, which are parametrized by the Gaussian Doppler width, the Lorentzian natural linewidth, and the optical depth. We show numerically and experimentally that including the correction term substantially reduces the spurious power dependence in the fitted Gaussian width.Comment: 4 pages, 3 figure

Quantitative atomic spectroscopy for primary thermometry

Quantitative spectroscopy has been used to measure accurately the Doppler-broadening of atomic transitions in $^{85}$Rb vapor. By using a conventional platinum resistance thermometer and the Doppler thermometry technique, we were able to determine $k_B$ with a relative uncertainty of $4.1\times 10^{-4}$, and with a deviation of $2.7\times 10^{-4}$ from the expected value. Our experiment, using an effusive vapour, departs significantly from other Doppler-broadened thermometry (DBT) techniques, which rely on weakly absorbing molecules in a diffusive regime. In these circumstances, very different systematic effects such as magnetic sensitivity and optical pumping are dominant. Using the model developed recently by Stace and Luiten, we estimate the perturbation due to optical pumping of the measured $k_B$ value was less than $4\times 10^{-6}$. The effects of optical pumping on atomic and molecular DBT experiments is mapped over a wide range of beam size and saturation intensity, indicating possible avenues for improvement. We also compare the line-broadening mechanisms, windows of operation and detection limits of some recent DBT experiments

Simultaneously-Measured Mid-Infrared Refractive Indices of GaAs/AlGaAs

We present our results for a simultaneous measurement of the refractive indices of Gallium Arsenide (GaAs) and Aluminum Gallium Arsenide (Al$_\mathrm{x}$Ga$_\mathrm{1-x}$As) in the spectral region from $2.0$ to $7.1\,\mathrm{\mu}\mathrm{m}$ ($5000$ to $1400\,\mathrm{cm^{-1}}$). These values are obtained from a monocrystalline thin-film multilayer Bragg mirror of excellent purity (background doping $\leq 1 \times 10^{-14}\,\mathrm{cm^{-3}}$), grown via molecular beam epitaxy. To recover the refractive indices over such a broad wavelength range, we fit a dispersion model for each material. For that, we measure both a photometrically accurate transmittance spectrum of the Bragg mirror via Fourier-transform infrared spectrometry and the individual physical layer thicknesses of the structure via scanning electron microscopy. To infer the uncertainty of the refractive index values, we estimate relevant measurement uncertainties and propagate them via a Monte-Carlo-type method. This method conclusively yields propagated relative uncertainties on the order of $10^{-4}$ over the measured spectral range for both GaAs and Al$_{0.929}$Ga$_{0.071}$As. The fitted model can also approximate the refractive index for MBE-grown Al$_\mathrm{x}$Ga$_\mathrm{1-x}$As for $0 \leq x \leq 1$. These updated values will be essential in the design and fabrication of next-generation active and passive optical devices in a spectral region which is of high interest in many fields, e.g., laser design and cavity-enhanced spectroscopy.Comment: 20 pages, 5 figures, submitted to PR

Absolute absorption line-shape measurements at the shot-noise limit

Here, we report a measurement scheme for determining an absorption profile with an accuracy imposed solely by photon shot noise. We demonstrate the power of this technique by measuring the absorption of cesium vapor with an uncertainty at the 2-ppm level. This extremely high signal-to-noise ratio allows us to directly observe the homogeneous line-shape component of the spectral profile, even in the presence of Doppler broadening, by measuring the spectral profile at a frequency detuning more than 200 natural linewidths from the line center. We then use this tool to discover an optically induced broadening process that is quite distinct from the well-known power broadening phenomenon

Absolute (12)C(16)O(2) transition frequencies at the kHz-level from 1.6 to 7.8micrometers

Absolute transition frequencies were measured for a series of transitions in the (30013)←(00001) near-infrared 12C16O2 band. These measurements were referenced to a cesium atomic clock through the use of an optical frequency comb. Combined standard uncertainties as low as 18 kHz (6×10−7 cm−1) were achieved. Importantly, deviations as large as 5 MHz were observed relative to the HITRAN 2008 database. These measurements were then included in a global fit of 416 CO2 mid-infrared and near-infrared measurements each of which was absolute. The resulting spectroscopic parameters provide a series of secondary frequency standards with kHz-level uncertainties across a wide frequency range and should significantly improve spectroscopic retrieval algorithms for space-based measurements of atmospheric CO2.D.A. Long, G.-W. Truong, J.T. Hodges, C.E. Mille

ABSOLUTE MEASUREMENTS OF NEAR-INFRARED CO2_{2} TRANSITION FREQUENCIES AT THE kHz-LEVEL

Author Institution: Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA; Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USAMeasurements were made of Doppler broadened CO$_{2}$ transitions in the (30013) $\longleftarrow$ (00001) band near 1.6 $\mu$m using frequency-stabilized cavity ring-down spectroscopy (FS-CRDS). These absolute measurements were directly linked to a cesium atomic clock by the use of an octave-spanning, self-referenced optical frequency comb. Combined standard uncertainties as low as 18 kHz were achieved for these weak transitions. A global fit was then performed which included these measurements as well as an ensemble of absolute mid-infrared and far-infrared data. The resulting spectroscopic parameters provide secondary frequency standards over a wide spectral region and should benefit atmospheric remote sensing missions