2.4 - Tuneable Laser-Based Calibration and Characterisation of Radiometric Detectors for SI-Traceable Measurements

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings OPTO 2011
O2 - 3D and Calibration
M. Schuster, A. Sperling, S. Nevas - Physikalisch-Technische Bundesanstalt (PTB), Braunschweig (Germany)
54 - 59


SI-traceable values of the spectral responsivity of radiometric detectors are of the highest importance in many applications in photometry and radiometry. To determine them, spectral responsivity measurements are carried out under exposure to monochromatic radiation. The electrical response of the device under test (DUT) is compared to that of a reference detector. The responsivity of the reference detector is known from a calibration with respect to the absolute cryogenic radiometer, an electrical substitution radiometer used as a primary standard of radiant power. These measurements are typically carried out in monochromator-based setups, where spectrally dispersed radiation of quartz-tungsten-halogen or gasdischarge arc lamps is normally used. However, due to the rapid progress in laser technology, lamp-based setups can nowadays be replaced by tuneable laser facilities. The use of tuneable laser radiation has a significant advantage as compared to the lamp-monochromator setups. Much higher output power lev ls and the simultaneously narrow bandwidth of the laser radiation during the measurements enables the DUT irradiance responsivity to be determined over a much broader dynamic range. Furthermore, steep slopes of responsivity curves of filtered detectors can be measured with much more detail using narrow-band lasers and, furthermore, no spectral deconvolution techniques (bandpass correction) are required as opposed to the monochromator-based measurements. Moreover, the wavelength uncertainty associated with laser-based measurements is largely reduced compared to monochromator-based setups. Furthermore, the high power and spectral purity of the tuneable laser radiation enable also other characterisations of radiometric detectors, such as the stray light and bandpass of array spectroradiometers. Appropriate corrections based on such characterisation results can significantly improve the performance of compact and cost-efficient array spectrometers that are increasingly used in various radiometric and photometric applications.
At the Physikalisch-Technische Bundesanstalt (PTB) – the national metrology institute of Germany - narrow-band wavelength-tuneable continuous-wave (cw) laser systems started to be used for absolute radiometry over limited spectral ranges more than twenty years ago. The TULIP setup (Tuneable Lasers in Photometry) covering the broad spectral range from about 360 nm to 1000 nm has now been upgraded with a tuneable quasicw femtosecond laser system covering the entire spectral range between 230 nm and 3000 nm, which is sufficient for most radiometric detectors. This new system provides a much higher output power throughout the whole useable spectral range than the cw-lasers in use. In addition, a second laser facility PLACOS (Pulsed Laser for Advanced Characterisation of Spectroradiometers) is employed for the spectral characterisation of array spectroradiometers.