Optical measuring tape

Test set-up in 50 metre long corridor at NMi in Delft.

Laser distance measuring in air has been brought to its penultimate level: 100 metres at micrometer accuracy.

At first, the task seemed simple enough,” says Dr Nandini Bhattacharya (Applied Sciences), who co-supervised Dr Moxi Cui’s PhD research on distance metrology. Previously Dr Bas Swinkels had developed laser metrology in the same group with a different kind of laser. A new ‘femtosecond laser’ was expected to perform even better, producing ultra-short pulses of light (40 microns long) separated by 15 centimetres of nothing. Distance can be measured by splitting the laser beam: one part is sent down the distance to be measured, gets reflected and then returns; the other part bounces on a moveable reflector situated nearby. A pulse counter provides an estimate of (twice) the distance in multiples of 15 centimetres. Overlaying the pulses from both arms and bringing them into interference was expected to lower the precision to the order of the light wavelength (0.4 micron).

However, air plays funny tricks on laser pulses. The pulses that had travelled 100 meters through air were deformed beyond recognition. “Temperature, pressure and humidity influence the refractive index, which is slightly different for each of the 18,000 frequencies that constitute the laser pulse,” Bhattacharya explains.

By calculating the exact influence of air on the pulses, Cui succeeded in restoring the interference between the pulses from both arms, thus attaining an accuracy of 1 to 2 micron in measuring a distance of 50 metres. A world record, says his co-supervisor.

The developed technology might have unexpected applications. Although laser metrology was developed for the fine positioning of satellites in space, the knowledge of the refractive index of air might now also be used for terrestrial laser metrology over distances of kilometres, or for the optical analysis of breath gases. Drunk drivers beware.

Moxi Cui, ‘Distance Metrology in Air with a Femtosecond Frequency Comb Laser’, 11 October 2010, PhD supervisor Prof. Paul Urbach.