High-accuracy determination of water vapor refractivity by length interferometry

Abstract

Humidity is the most problematic parameter for the accurate determination of the refractive index of air. Besides the fact that the humidity measurement can be limiting, the existing empirical equations for the refractive index of moist air are either restricted to 20 degrees C or are based on insufficient knowledge of the refractivity of water vapor. To overcome this problem, a new kind of measurement method for the refractivity of water vapor is suggested that is based on the accurate measurement of the absolute length of a step length by interferometry under vacuum conditions and subsequent measurements at different well-defined absolute water vapor pressures.

Time domain measurement of the THz refractivity of water vapor

“…The lower frequency Debye response is considered to be defined by the frequency half-width of f 1/2 = 2.0 THz. This assumption gives a monotonically decreasing refractivity from 70 x 10 −6 at zero frequency to 0.8 x 70 x 10 −6 at 1.0 THz, which is consistent with the previous measurements [1][2][3][4][5][6][7][8], and the previous Debye theory approach [16,17]. However, this value of f 1/2 = 2.0 THz is exceptionally large compared to the collision frequency of 3.5 GHz [12] for the water rotational lines at similar conditions of atmospheric pressure, water vapor density and temperature.…”

“…Another infrared measurement at 3.4 µm gave the refractivity of 2.87 x 10 −6 at 20 °C and 10 mm Hg (9.9 g/m 3 ) [6]. Optical measurements over the range of 780 to 532 nm gave (n-1) of 2.3 x 10 −6 at 20 °C with 1000 Pa (7.4 g/m 3 ) [8]; scaling this value to 1333 Pa (10 mm Hg) (9.9 g/m 3 ) gives (n-1) = 3.07 x 10 −6 .…”

“…The refractivity of water vapor is composed of two parts, a slowly-varying almost frequency-independent part, applicable from microwaves up to THz, into the infrared and the visible, and a strongly frequency-dependent part together with strong absorption in the water vapor rotational spectrum from 0.2 to 10 THz. Measurements of the refractivity of water vapor have been made at microwave [1,2], mmwave [3,4], far-infrared (THz) [5], infrared [6,7], and optical frequencies [8]. Previously, until the results described here, all measurements of the refractivity of water vapor, determined the sum of both parts.…”

Refractive Index Compensation in Over-Determined Interferometric Systems

“…A lot of effort has been put into evaluation of other effects such as content of various gases in air, especially CO 2 [28]. Humidity, particularly the content of water vapor, has been investigated in [29,30]. The study of these effects resulted in inverse approaches where measurement of the refractive index of air became the means of determining another quantity, such as temperature [31] or air density and moisture [32,33].…”

Suppression of Air Refractive Index Variations in High-Resolution Interferometry

“…A lot of effort has been put into evaluation of other effects such as content of various gasses in air, especially CO 2 [26]. Humidity, particularly the content of water vapor has been investigated in [27,28]. The study of these effects resulted in inverse approaches where measurement of the refractive index of air became the means of determining of another quantity, such as temperature [29] or air density and moisture [30,31].…”