The use of a mode-locked laser for ultraviolet radiometry

A mode-locked titanium sapphire laser, tuneable over the region from 680 nm to 1000 nm, has been used to generate monochromatic laser radiation over the region from 250 nm to 450 nm by second and third harmonic generation, with power outputs of between 50 mW and 200 mW above 350 nm and around 30 mW between 250 nm and 300 nm. The (pulsed) radiation was assessed as follows for its suitability as a source for the calibration of detectors in continuous-wave (cw) applications. (i) Radiation at 356.41 nm was used to calibrate a trap detector for spectral responsivity against the National Physical Laboratory (NPL) cryogenic radiometer, and this value was compared with the one obtained using a cw laser at the same wavelength: the values differed by 0.06%. (ii) The linearity of response of the trap detector was determined at 362.0 nm. It was found to be linear to within 0.01% for powers up to 1.5 mW. The laser was then used to calibrate a trap detector for spectral responsivity against the cryogenic radiometer over the region from 350 nm to 360 nm, with an estimated standard uncertainty of less than 0.1%.