Laser-Based Primary Thermometry: A Review

Abstract

Laser-based primary thermometry was initiated almost 15 years ago by the proposal to determine the absolute temperature of a gas at thermodynamic equilibrium through the Doppler width of an associated absorption transition, exploiting the potentially very accurate measurement of an optical frequency to infer the elusive thermal energy of a molecular or atomic absorber. This approach, commonly referred to as Doppler broadening thermometry, has benefited across the years from substantial improvements, of both technical and fundamental nature, eventually reaching an accuracy of about 10 ppm on the temperature determination in the best cases. This is sufficient for Doppler broadening thermometry to play a significant role in the practical realization of the new kelvin, which follows the 2019's redefinition from a fixed value of the Boltzmann constant, and to tackle the challenge, among others, to quantify and possibly fix systematic uncertainties of the international ITS-90 temperature scale. This paper reviews and comparatively analyses methods and results achieved so far in the field of laser-based primary thermometry, also including spectroscopic approaches that leverage the temperature-dependent distribution of line intensities and related absorbances across the ro-vibrational band of a molecular sample. Although at an early stage of development, these approaches show a promising degree of robustness with respect to the choice of the line-shape model adopted for the fitting of the absorption spectra, which is a delicate aspect for all laser-based thermometers. We conclude identifying possible technical and scientific evolution axes of the current scenario.

Laser spectroscopy of hot atomic vapours: from ’scope to theoretical fit

“…It should be noted that the Voigt profile provides an excellent fit to experimentally measured spectra for a wide range of conditions; however, departures from the Voigt lineshape are known and have been studied, and are particularly relevant in the field of Doppler-broadened thermometry [126][127][128][129][130][131][132].…”

Emerging technologies in the field of thermometry

“…The new definition of the kelvin is based upon the Boltzmann constant, k B = 1.380649 × 10 −23 JK −1 -a factor that converts thermodynamic temperature to energy. In the route towards the re-definition, a whole host of national metrology institutes worked diligently to produce the most accurate measurements of the Boltzmann constant (the Boltzmann project [45]), using AGT [7], JNT [8], dielectric constant gas thermometry (DCGT [46]) and Doppler broadening thermometry (DBT [47]), to name a few. Three of them-AGT, DCGT and JNT-were utilized for the final CODATA k B value [48].…”

“…The principle of DBT is to record the Doppler profile of a molecular (or atomic) absorption line of a gas in thermodynamic equilibrium [47,65] (see figure 8). The absorption line shape of single rotational-vibrational (rovibrational) molecular line is dominated at very low pressure by Doppler broadening and is a simple Gaussian profile, which reflects the Maxwell-Boltzmann velocity distribution of gas particles along the laser beam axis.…”

Towards Realising the Redefined Kelvin

“…Such an approach will be needed to make autonomous production truly a reality. Such approaches could be through practical Johnson Noise Thermometry (where practical demonstrator devices already exist) [69,70] and in the longer-term small-scale Doppler Broadening Thermometry [71,72] as well as other photonic based approaches [73].…”