Applied Physics B

, Volume 87, Issue 3, pp 539–546 | Cite as

Mid-infrared laser spectroscopic determination of isotope ratios of N2O at trace levels using wavelength modulation and balanced path length detection

  • H. Waechter
  • M.W. Sigrist


We present a new mid-infrared laser spectrometer for high-precision measurements of isotopic ratios of molecules at ppm concentrations. Results are discussed for nitrous oxide (N2O), where a precision of 3‰ for a single measurement and a reproducibility of 6‰ have been achieved for a concentration of 825 ppm. The room-temperature laser source employed is based on difference-frequency generation delivering a continuous-wave power up to 23 μW at wavelengths between 4.3 μm and 4.7 μm and a line width of 1 MHz. Two different measurement methods are compared; wavelength modulation with first-harmonic detection and direct absorption spectroscopy by recording the spectrum with a data-acquisition card. Two different detection schemes were employed; either all isotopomers were measured using the long path (36 m) of the multipass cell or a balanced path length detection scheme was used, where the main isotope was measured with a beam along a shorter path (40 cm) in the multipass cell. A single-pass reference cell was designed, offering two different path lengths for balanced path length detection. All combinations of measurement methods and detection schemes were tested regarding precision of a single measurement and long-term stability. The advantages and disadvantages of various measurement approaches are discussed.


Absorption Line Quantum Cascade Laser Wavelength Modulation External Cavity Diode Laser Path Detection 
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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  1. 1.Institute of Quantum Electronics, Laser Spectroscopy and Sensing LaboratoryETH ZurichZurichSwitzerland

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