Temperature stabilization system with millikelvin gradients for refractometry

Refractometry of air is a central problem for interferometer-based dimensional measurements. Refractometry at the 10−9 level is only valid if air temperature gradients are controlled at the millikelvin (mK) level. Very precise tests of second-generation National Institute of Standards and Technology (NIST) refractometers involve comparing two instruments (two optical cavities made from ultralow expansion glass) that are located in nominally the same environment; temperature gradients must be kept below a few millikelvin to achieve satisfactory precision of these tests. In this paper we describe a thermal stabilization scheme that maintains < 1 mK thermal gradients over 100 h in a 0.5 m×0.15 m×0.15 m volume. Our approach uses passive (aluminum envelopes and foam insulation) and active (thermistors, foil heaters, and proportional–integral–derivative control) temperature stabilization. Thermal gradients are sensed with thermocouples and a nanovoltmeter and switch; the reference junctions of the thermocouples being in thermal contact with a thermistor temperature standard. Our < 1 mK gradient performance is limited by the accuracy of the nanovoltmeter and switch.