High-accuracy optical power measurements by using electrical-substitution cryogenic radiometer

An optical power scale is established at Ulusal Metroloji Enstitusu (UME) using the helium-cooled primary-level electrical-substitution cryogenic radiometer. The scale is established at discrete laser wavelengths of vertically polarized tuneable Ar+, fixed He-Ne, and Nd:YAG (with second harmonic) laser sources. To increase measurement accuracy, laser intensities are stabilized to better than 0.009% using an electro-optical modulator. The spatial profiles of polarized laser beams are cleaned with a spatial filter. The optical power is measured using the static substitution method with an uncertainty of a few parts in 10-4. Repeatability of optical power measurements in the static substitution method, the measurement of window transmittance, and the measurement of power in a scattered beam are the most significant uncertainty components. (C) 2005 Society of Photo-Optical Instrumentation Engineers.