Research Group Prof. Dr. G. Friedrichs


Acwopo singly-resonant continuous-wave optical parametric oscillator (SR-cw-OPO) based CRDS experiment for sensitive and high-resolution IR detection has been setup.

The new cw-OPO-IR laser system (bluish illuminated devices) provides a wide tunability (λ = 1.38 – 1.60 μm & 3.2 – 4.6 μm), a narrow spectral bandwidth (Δν = 60 kHz at Δt = 500 ms), and a high output power (P > 1.0 W).

chr2_br2The wide tunability of the laser system in combination with the  the narrow linewidth enables the measurement of high-quality Doppler-limited rovibrational spectra. As an example, the linear CRDS spectrum illustrates the complex structure of the asymmetric CH stretch band of CH2Br2. The assignment of the spectrum and the determination of the spectroscopic constants is based on a fit of the effective Hamiltonian to the experimental spectrum.

sat_crds_principleUtilizing the high-power of cw-OPO-IR laser system, a sensitivity enhancement can be accomplished by the implementation of the so-called saturated absorption cavity ringdown spectroscopy (Sat.-CRDS/SCAR) [Giusfredi et al., Phys Rev Lett 104 (2010)]. Sat.-CRDS allows one to extract both the gas absorption and the empty cavity loss constant from a single ringdown signal. This is possible because the degree of sample saturation changes during the ringdown event causing a distinct non-exponential ringdown behavior. In simple terms, the sample can be considered to be strongly saturated at the beginning of the ringdown event (hence the observed decay is dominated by the empty cavity losses), but with decreasing light intensity the sample becomes non-saturated (hence the observed decay is influenced by the absorbing species lateron). Actually, the observed ringdown decay needs to be simulated by a ringdown model that takes into account the dynamics of the absorption saturation, which is controlled by the pumping (light power coupled into the cavity) and the relaxation rates of the molecular excited state (dominated by molecular collisions and hence the cell pressure).

Contributing Researchers: Ibrahim Sadiek and Gernot Friedrichs