Ultraviolet Scale Realization Facility UV SRF

A new facility has been commissioned to the establish ultraviolet spectral power responsivity in the air-ultraviolet spectral range between 200 nm and 400 nm. We successfully realized the spectral power responsivity scale from 200 nm to 400 nm using a laser-driven plasma light source with a relative standard uncertainty below 0.5 % (k=1). This Ultraviolet Scale Realization Facility (UV SRF) is based on the same monochromator used in the Ultraviolet Spectral Comparator Facility (UV SCF) to shorten the calibration chain. Establishing the responsivity scale and performing calibrations in similar facilities should remove any uncertainty caused by differences in bandpass, out-off-band radiation, spectral purity, collimation, or data interpolation.

The UV SRF is based on an ultraviolet-rich laser-driven light source, which is imaged onto the monochromator entrance aperture using two off-axis parabolic mirrors. The monochromator is a Czerny-Turner mount double grating instrument that has been modified with an absolute angular encoder on the first grating mount to establish the wavelength scale. The light leaving the monochromator is imaged into the measurement system with a magnification of 4 to reduce the angular divergence. The optical power is measured with absolute-cryogenic electrical substitution radiometer with updated electronics. The working standard detectors being calibrated were silicon photodiodes, which had been tested for uniformity, noise characteristics, and ultraviolet resistance.

Several modifications were necessary before a successful scale realization was possible:

1. An absolute angular encoder was installed on the first grating mount in the monochromator. Together with an absorption target of an aqueous solution 4 % holmium oxide in 10 % perchloric acid and the encoder it was used to establish the wavelength scale.
2. The entrance apertures in the absolute-cryogenic radiometer (ACR) were realigned and a new mount designed to allow for reproducible assembly.
3. The electronics package of the ACR was replaced with a newer version, which resulted in a slightly lower noise floor and more reliable operations.
4. Compared to the Ultraviolet Spectral Comparator Facility we switched to a higher power laser-driven light source.
5. The photodiodes being calibrated were mounted in lens tubes with apertures that mimic the layout of the ACR to ensure both detection systems are collecting the same light field. Also a new photodiode holder was used that helped securely hold them into place.
6. The two off-axis parabolic mirrors that magnify the exit slit of the monochromator were mounted together onto a single mount with mechanical pre-alignment. This allowed much faster alignment of the optical system.

In the end we calibrated three silicon photodiodes between 200 nm and 400 nm in 5 nm-steps. At each wavelength we performed several measurements at each wavelength to establish measurement statistics and reduce the overall statistical uncertainties. The total combined relative uncertainties of the spectral power responsivities for the three detectors were below 0.5 % (k=1) for the whole spectral region from 200 nm to 400 nm.