1. Metrology Research Institute, Aalto University, Maarintie 8, FI-02150 Espoo, Finland
2. Gasera Ltd., Lemminkäisenkatu 59, FI-20520 Turku, Finland
3. Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland
4. Photonics Laboratory, Physics Unit, Tampere University, FI-33014 Tampere, Finland
5. VTT MIKES, VTT Technical Research Centre Finland, P.O. Box 1000, FI-02044 VTT, Finland