Tuneable Laser System by APE for the Calibration of Radiometric and Photometric Detectors

APE talks with Dr. Philipp Schneider from the working group “Spectoradiometry” of PTB (Physikalisch-Technische Bundesanstalt) about the use of APE’s tuneable laser system for the calibration of radiometric and photometric detectors

Dr. Philipp Schneider is a scientist at PTB, the German National Metrology Institute. No scientific experiment, no industrial process and no transport of goods and commodities can do without quantification and PTB provides the ‘backbone’ for this, the traceability to the International System of Units (SI).

An important role is played here by calibration: to determine the properties of a measurement instrument with the associated uncertainties traceable to the SI units. For such a purpose, a  laser system from APE, consisting of the Emerald Engine laser, the tunable Levante OPO and several tunable harmonic generators, is now used. The new laser system is integrated in the preexisting TULIP (Tunable Lasers in Photometry) setup to offer a complete automation of the calibration procedures in the spectral range between 230 nm and 2300 nm.

“The laser system has a bandwidth that is optimal for our measurements” reports Philipp Schneider. Compared to the ‘older’ system the monochromator became superfluous by replacing the broadband fs with a narrow-band ps laser. The system can now generate each wavelength in the range between 230 nm and 2300 nm fully automatically.

At the TULIP setup, a wavelength-tunable uniform monochromatic radiation field is generated over a wide wavelength range and, thus, the spectral irradiance responsivity of radiometric and photometric detectors can be determined. For the calibration task, a reference detector with known responsivity values is required. “At all wavelengths, the two detectors are compared, and the values are set in relation to each other to transfer the known responsivity onto the detector to be calibrated,” so Philip Schneider.

“One can say that the TULIP setup is quite unique in the world, since the setups for the spectral calibration of photometric and radiometric detectors traditionally use lamps and monochromators. Like in our setup, tunable lasers are also used in the SIRCUS (Spectral irradiance and Radiance Calibrations with Uniform Sources) facility at NIST (National Institute of Standards and Technology),” Philipp Schneider is pleased to report.

The maintenance of the luminous intensity unit at PTB is based on standard incandescent lamps, i.e. quasi black body radiators. As the production of lamps is discontinued, the availability of the standard lamps in the future is unsure. To address this challenge, a new traceability route is being established at PTB. Within the framework of his doctoral thesis at PTB, Philipp Schneider has shown that a spectral irradiance responsivity calibration performed with an appropriate laser setup can provide the required calibration accuracy.

Furthermore, characterizations and calibrations at the TULIP setup play a key role in several research projects. Detector calibrations may sound exotic but it is essential for many products and services. Whether it is UV sources in sunbeds, industrial quality control, UV radiation in sterilization, such as for medical articles, or even ensuring the light output of lamps, LEDs and OLEDs in everyday life – without calibration and the work of PTB, many of these applications could not perform adequately or could even be harmful to health.