In general losses of optical of less than 1 % cannot be measured precisely with the best-established techniques (e.q.
two-beam spectroscopy). However, it is possible to measure losses in the 0.0001 - 0.5 % range with high accuracy using
cavity enhanced spectroscopy (CES) methods. Such low losses can be measured with CES, due to an increased
interaction path way with the object. The Cavity Ring-Down (CRD) technique takes advantage of the CES method and
transforms the optical loss information into the time domain.
Two types of CRD setups for spectrally resolved loss measurement of laser mirrors will be presented. The first setup
uses a tunable laser system for serial detection of the reflectivity spectra. The second method determines the spectral
losses using a super continuum source. Here, simultaneous excitation and a spectrometer based camera system for
separate detection of several wavelengths is used. Results will be shown and compared with direct absorption
measurements of the same sample.
For special applications in spectroscopy, tunable single-frequency lasers are required to excite selectively relevant
molecules. Amplified Yb:YAG disk lasers provide one opportunity for such lasers with a number of advantageous
properties. Nevertheless, changing the wavelength from shot to shot at kHz repetition rates - desired e.g. for background
subtraction or two-wavelength methods - remains challenging.
We present results from two approaches, which in combination allow for fast wavelength switching of the oscillator and
for extension of the tunability range of the laser system. For wavelength switching high voltage (some kV) is applied to a
special birefringent filter (Lyot filter). Polarization rotation induced by the electric field yields losses at the wavelength
emitted without voltage: the laser emits at a "new" wavelength with the highest gain. This new wavelength is determined
by multiples of the free spectral range of the intra-cavity etalon used for single-frequency operation.
The second stage of the laser system comprises an Yb:YAG regenerative amplifier. To ensure that parasitic lasing of this
laser at the gain maximum is suppressed effectively, an additional birefringent filter is inserted into the amplifier.
Adjusting this filter suppresses parasitic lasing and extends the tunability range of the system by a factor of more than 4.
We report on femtosecond (fs) laser experiments yielding the time constants τ<sub>rel</sub> for the non-radiative relaxation from
optically excited high energy M<sub>Na</sub>** states to the fluorescent M<sub>Na</sub>* state in CaF<sub>2</sub> samples. The values obtained with the
third and second harmonics of the fs laser amount to τ<sub>rel</sub> (262 nm) = (3.0 ± 0.3) ps and to τ<sub>rel</sub> (392 nm) = (1.0 ± 0.1) ps for
the two selected M<sub>Na</sub>** states at 4.7 eV (262 nm) and 3.2 eV (392 nm) excitation energy, respectively. These time
constants were derived from depletion processes of the fluorescence at 740 nm (M<sub>Na</sub>* state) using fs laser pulses of the
NIR fundamental wavelength (785 nm) at variable delay relative to the UV fs laser pulses. In addition, photobleaching of
the MNa centers upon UV fs laser irradiation is observed and simulated by assuming a constant fraction of MNa bleaching
per pulse for a given laser fluence. This fraction ranges from 0.14% per pulse at 392 nm and 0.28mJ/cm<sup>2</sup> to about 1% per
pulse at about 6 mJ/cm<sup>2</sup>.
The influence of Na stabilized F and M centers on the DUV absorption behavior of CaF<sub>2</sub> is comparatively studied for
nanosecond and femtosecond laser pulses by in-situ transmission and laser induced fluorescence measurements.
For 193 nm nanosecond pulses the steady state transmission of ArF laser pulses through CaF<sub>2</sub> is measured in dependence
on the incident fluence H ≤ 10 mJ cm<sup>-2</sup> pulse<sup>-1</sup>. The related absorption coefficients α<sup>st</sup>(H) are proportional to H and
rationalized by effective 1- and 2-photon absorption coefficients α<sup>eff</sup> and β<sup>eff</sup>, respectively. The α<sup>eff</sup> and β<sup>eff</sup> values
increase with the Na content of the CaF<sub>2</sub> samples as identified by the fluorescence of Na related M<sub>Na</sub> centers at 740 nm.
This relation is simulated by a complex rate equation model describing the ArF laser induced M<sub>Na</sub> generation and
annealing. M<sub>Na</sub> generation starts with intrinsic 2-photon absorption in CaF<sub>2</sub> yielding self-trapped excitons (STE). These
pairs of F and H centers can separate upon thermal activation and the F centers combine with F<sub>Na</sub> to form M<sub>Na</sub> centers.
M<sub>Na</sub> annealing occurs by its photo dissociation into a pair of F and F<sub>Na</sub> centers.
Comparative transmission measurements with DUV femtosecond pulses are done using the fourth harmonic of a Ti:Safs-
laser at 197 nm. The resulting β<sub>eff</sub> values virtually show no dependence on the MNa center concentration. Furthermore,
the absolute β<sub>eff</sub> values are lower by a factor of three compared to those obtained for nanosecond pulses. This is
explained by additional two-step absorption for nanosecond pulses after formation of self-trapped excitons (STE).