Research Group Prof. Dr. G. Friedrichs

CRDS: Basics

CRDS_BasicsCavity ringdown spectroscopy (CRDS) is an ultra-sensitive absorption spectroscopic technique, which has developed rapidly during the last two decades. It has been implemented for diverse applications and has become a mature spectroscopic technique for variety of prototype and commercial instruments. Historically, the first implementation of the technique was pulsed-CRDS, where a pulsed laser was used as an excitation light source.

In principle, the basic experimental realization of CRDS is fairly simple. It is is based on merely four main components:

  • a laser as a light source,
  • a Fabry-Perot cavity consisting of two mirrors of high reflectivity (R > 99.99 %), which also confines the detection volume,
  • a light detector (photomultiplier or fast photodiode),
  • and a data acquisition system that determines the ringdown time constant from the experimentally observed mono-exponential signal decay.

The high detection sensitivity of CRDS is primarily based on a long effective absorption length (>10 km) arising from a multiple reflection of the laser beam, which is trapped in the optical cavity. In addition, since the CRDS signal is related to the temporal decay instead of the total intensity of the transmitted light, CRDS is inherently immune to laser intensity fluctuations, which often limit the sensitivity of conventional absorption methods. Due to absorption of the sample gas in the cavity, the intensity of the transmitted laser light decreases every roundtrip and the absorption coefficient in turn is directly related to the observed ringdown decay. Compared to an “empty” cavity it becomes faster in case of an absorbing species is present in the cavity.


Today, in most cases narrow linewidth continuous-wave (cw) laser light sources have become more common. Typically, in a cw-CRDS experiment the cavity is made resonant with the excitation light by modulating its length by a piezo-driven mirror mount. With a resonant cavity, light intensity starts to build up until a preset light level is reached. Then, a fast optical switch (e.g., an acousto-optic modulator, AOM) shuts-off the excitation laser light and the ringdown decay can be observed. In contrast to pulsed-CRDS, the cw approach avoids the problems associated with multi-mode excitation of the optical cavity resulting in further sensitivity and spectral resolution enhancement.

Our research group focuses on the development of new CRDS-based methods in the IR range for gas phase measurements and in the NIR range for both gas phase and interface measurements (so-called evanescent wave CRDS). Moreover, we are involved in the application of field-deployable commercial CRDS instruments (e.g., for isotope selective detection of CO2, see here).

The developed spectrometers are used for fundamental spectroscopic work needed for environmental monitoring of various trace gases. Many trace gases are emitted from the ocean, where their abundance is determined by interacting physical, chemical and biological transport and transformation processes. For example, more recent research is concerned with photochemically active substances such as reactive organohalogen compounds containing Cl, Br, and I that are known to considerably influence the oxidation capacity of the atmosphere. Interface measurements using  evanescent wave CRDS hold the potential to directly investigate and understand the heterogeneous chemistry taking place at surfaces, which is difficult to study by other methods.