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Traceable power measurements in laser radiometry

For the most frequently used lasers in the ultra-violet to infrared spectral region (wavelengths from 0.2 µm to 10.6 µm) PTB offers precise power measurements. These measurements are traced back to a cryoradiometer. The uncertainties achieved range between 0.2 % and 1 %.

Setup for the calibration of laser power detectors. The red adjusting beam marks the optical path.

Whether in medical diagnostics and therapeutics, or in industrial manufacturing, telecommunications technology, precision metrology or fundamental research: in all of these fields lasers play a significant role and their effective radiant power must be measured with high accuracy. However, commercial power meters calibrated by the manufacturers often display measurement readings that deviate by as much as 10 %.

For this reason, PTB has established laser radiometry with the goal to achieve far lower uncertainties (between 0,2 % and 1 %) on the basis of traceable power measurements. All standard laser radiometers of PTB are linked by a traceability chain to a cryoradiometer - at present the most accurate optical radiant power standard. The radiometers are of very different design and have been optimized for different power and wavelength ranges so that power meters in the nanowatt range (for optical communication engineering in the visible and near infrared) as well as power meters for high output powers above 1 kW (e. g.: highpower CO2 lasers for material treatment) can be calibrated. PTB calibrates the high precision power meters using the same type of laser and the same laser line as the customer intends to use.

The detector standards are substitution radiometers which absorb the radiation as completely as possible. The electrical power that produces the same thermal heating is determined. Measurements performed with additional electrical heating - starting from the level of the cryoradiometer (about 1 mW) - allow extending the meter range to higher and lower powers over more than twelve orders of magnitude.

Within the framework of a recently approved BMBF project the measurement engineering and instrumentation for the 157 nm line is presently being developed. Radiation at 157 nm is of significant importance for future semiconductor lithography.

Contact at PTB:

K. Möstl
fax: +49 (0) 531 592-4105