Research Group Prof. Dr. G. Friedrichs


Quantitative High Resolution Spectroscopy

cwcrdsCompared to pulsed CRDS, a further increase in sensitivity can be achieved by using narrow line width continuous wave (cw) laser sources. This approach, next to increasing the spectral resolution, avoids the problem of multi-mode excitation of the optical cavity, which causes multi-exponential ringdown decays and ultimately limits the precision of the ringdown time constant measurement. A cw-NIR CRDS experiment based on an external cavity diode laser emitting light at wavelengths around 1650 nm has been setup. A modular detection cell design was implemented allowing us to measure both gas phase as well as interface spectra (by means of evanescent wave CRDS). The near infrared (NIR) spectral range was chosen because state-of-the-art single mode diode lasers, the required high quality mirrors and fiber technology is readily available and affordable.

n2oIn contrast to distributed feedback (DFB) diode lasers, ECDLs can be tuned over a broader wavelength range, in our case over the wavelength range 1625-1690 nm, and thus allow one to detect several different chemical species with only one laser diode. However, it turned out that long-term frequency drift and jitter of the ECDL caused by changes of ambient conditions led to bad performance of the spectrometer when applied for high resolution spectroscopy. As a way out, we finally implemented a conceptually new Fourier Transform based wavelength calibration scheme that enables the acquisition of high quality spectra without the need for stabilization of the probe laser. Here, the output of a second laser (DFB), which is continuously locked to a known transition of CO2, is combined with the actual probe laser beam in an optical fiber and its frequency is measured relative to the reference laser frequency using a Fourier transform wavelength meter. An excellent wavelength precision of 5 · 10-8 was obtained, which is comparable to the precision of the most sophisticated commercially available instruments [1].

The spectrometer has been applied to detect nitrous oxide isotopomers. A precise determination of the so-called site-preference SP in natural N2O samples, SP = ([14N15NO]/[15N14NO] - 1), would allow one to figure out sources and sinks of this important atmospheric trace gas. For example, N2O formed by microbial processes, namely denitrification and nitrification, shows characteristic site-preferences of SP = 0‰ and 33‰, respectively.

[1] C. Fehling, G. Friedrichs, "A Precise High-Resolution Near Infrared cw-Cavity Ringdown Spectrometer using a Fourier Transform based Wavelength Calibration", Rev. Sci. Instrum. 81 (2010) 053109/1-8; doi:10.1063/1.3422254.
Contributing researchers: G. Friedrichs and (formerly) C. Fehling