Many precision optical distance- and displacement-measurement setups rely on optical interferometers to provide the data; not only can interferometers measure to nanometer precision, but they can do this over ranges that approach a meter. One big hindrance is atmospheric turbulence, which pushes engineers to work hard minimizing air-temperature differences and optimizing air flow. Engineers at Nagaoka University of Technology (Niigata, Japan) have come up with a supplemental approach that both measures the air's refractive-index change (Δnair) and compensates for it.
A Fabry-Perot (F-P) cavity is placed in a sealed chamber that is temperature-stabilized to 10 mK; a small bellows chamber driven by an actuator is attached to the large chamber, allowing the air pressure to be quickly varied. Fed by a frequency-stabilized laser, the F-P setup tracks Δnair and feeds a signal back to the bellows actuator, which changes the chamber pressure to compensate. Over a period of 100 s, the researchers were able to keep Δnair down to 4 × 10-9, versus 3 × 10-8 for no pressure control. The frequency fluctuations in the laser were the largest contributor to the remaining error. The group is currently working on detailed measurements of the spatial distribution of Δnair as it is being controlled, and plans to use a better laser in the future. Contact Masato Aketagawa at [email protected].
