PTB News 2/2009 (4.7 MB) PTB News 2/2009, English edition, Issue September 2009For the first time, the radiant power of a quantum cascade laser in the THz range has been absolutely determined with PTB‘s cryogenic radiometer. This is an important step towards the secure and efficient use of terahertz waves. Because promising applications in material testing, safety engineering, biomedicine and communications technology will become possible only if the power of the terahertz waves is accurately known.
For the use of terahertz technologies, a further development of radiometry in the terahertz spectral range is necessary, as this range has so far been only insufficiently covered by the methods of electrical and optical power measurements of electromagnetic radiation. PTB is taking the first steps in this direction together with the department for terahertz and infrared sensors of the DLR (German Aerospace Center). Within the scope of the cooperation it was recently possible for the first time to measure the radiant power of a quantum cascade laser of the DLR at 2.5 THz with a PTB cryogenic radiometer primary standard, in such a way that it is traceable to the International System of Units (SI). The precisely determined stable radiant power of the quantum cascade laser then enabled the determination of the spectral responsitivity of a terahertz detector and the corresponding uncertainty budget.
The analysis of the measurement conditions to determine the measurement uncertainty has shown that the laser-aided method for detector calibration, well established in optical radiometry, leads to very small uncertainties of a few percent even in the case of the wavelengths of terahertz lasers, which are more than two orders of magnitude larger.
At present, PTB is setting up a new measuring facility for the characterisation and calibration of detectors with laser radiation in the terahertz spectral range for manufacturers and commercial users. The core instrument is a molecular gas laser pumped with a frequency-stabilised CO2 laser. The system generates coherent radiation at frequencies of molecular rotational lines adjustable in the range from approximately 1 THz to 7 THz. This frequency range is four times larger than the entire visible spectrum. Thus PTB is taking a first big wavelength step to bridge the metrological gap between highfrequency electronics and infrared optics at long wavelengths.