High-precision diode-laser-based temperature measurement for air refractive index compensation

Abstract

We present a laser-based system to measure the refractive index of air over a long path length. In optical distance measurements, it is essential to know the refractive index of air with high accuracy. Commonly, the refractive index of air is calculated from the properties of the ambient air using either Ciddor or Edlén equations, where the dominant uncertainty component is in most cases the air temperature. The method developed in this work utilizes direct absorption spectroscopy of oxygen to measure the average temperature of air and of water vapor to measure relative humidity. The method allows measurement of temperature and humidity over the same beam path as in optical distance measurement, providing spatially well-matching data. Indoor and outdoor measurements demonstrate the effectiveness of the method. In particular, we demonstrate an effective compensation of the refractive index of air in an interferometric length measurement at a time-variant and spatially nonhomogeneous temperature over a long time period. Further, we were able to demonstrate 7 mK RMS noise over a 67 m path length using a 120 s sample time. To our knowledge, this is the best temperature precision reported for a spectroscopic temperature measurement.

Temperature measurement using frequency comb absorption spectroscopy of CO2

“…Both effects are expected to lead to an impact on the absorption measurements. 33 Evaluating absorption lines in the HITRAN database shows that the temperature dependence is rather weak, leading to requirements for a very accurate spectroscopic setup to be able to monitor corresponding absorption profile changes. 32 In this setup, we will take absorption spectra at two different temperatures and compare the differences in peak area and height of several absorption lines (simple integration as well as line fitting).…”

“…32 In this setup, we will take absorption spectra at two different temperatures and compare the differences in peak area and height of several absorption lines (simple integration as well as line fitting). Other publications 33,34 gave reasons for measuring line ratios instead requiring a more complex data treatment. The absorber number density needs to be determined requiring accurate measurements of pressure and distance.…”

Towards a standard for the dynamic measurement of pressure based on laser absorption spectroscopy

“…The temperature will be determined from the ratio of peak intensities using two or more ro-vibrational transitions. [17, 18, 34] The steady pressure measurement to determine mole fraction will set the uncertainty floor for the measurement of time-varying pressure. Steady pressure measurements can typically be made to 0.01 % or better.…”

Quantum Blackbody Thermometry

“…Alyabyshev et al considered using polar molecules to measure applied electric fields [9]. Laser and frequency comb spectroscopy have demonstrated as low as 7 mK precision in measuring the temperature of atmospheric CO 2 [10][11][12]. The fractional accuracy σ T /T of such measurements in the field is limited to a few 10 −4 , however, by uncertainties in abundance and atmospheric pressure.…”