Optical power calibrator based on a stabilized green He-Ne laser and a cryogenic absolute radiometer

An optical power calibrator is described whose overall calibration uncertainty is less than 10^-4 for an optical power of 0.13 mW. The laser light source of the system operates at a wavelength of 543.5 nm, being close to the wavelength at which the candela is defined, 555 nm. A stable optical power is achieved by stabilizing the intensity and the frequency of a green He-Ne laser. The optical power is detected by a cryogenic absolute radiometer based on the principle of electrical substitution radiometry. It can be used to measure up to 0.5 mW in the visible and near-infrared region with a 3σ uncertainty of about 5×10^-5. The factors limiting the overall uncertainty of the calibrator are analyzed: the conductance fluctuations of the temperature sensor in the absorption cavity and the beam scatter are found to be the most significant error sources. Limited absorptivity of the cavity (0.99998) and the background radiation cause additional uncertainty. The system is controlled by a microcomputer with self-check and autocalibration features