Cavity Ring Down Spectroscopy (CRDS) is a highly sensitive laser absorption
technique. It can be used for quantitative diagnostics of biological and
medical relevant molecular species (concentration, temperature). In our
experiments CRDS has been developed for spectroscopic studies of trace
gas measurement of atmospheric species. We have developed a new spectroscopic
method which is called cavity leak out spectroscopy (CALOS). CALOS is using
cw laser instead of pulsed laser systems to improve the sensitivity and
data acquisition rate. Our measurements in the middle infrared spectral
range have shown, that cavity leak out spectroscopy is a powerful tool
with a sensitivity better than one parts per billion.
1. experimental setup
The light of a tunable cw laser is mode matched by two lenses to the
ring-down resonator to get high transmittance and avoid mode beating (fig.
1). The ring-down cavity consists of two high reflecting mirrors, which
have in your case a reflectivity of R=99.98 %. The light is coupled into
the optical cavity which is filled with the sample gas. The leaving mirror
is moved by a piezo to get high laser transmittance at the detector. After
reaching the threshold power the acusto optic modulator (AOM) is switched
off. In your experiments we use a digital store oscilloscope to observe
the leak-out signal. The scope is read out by computer to calculate the
Levenberg-Marqurad Fit and get the exponential decay time.
Fig. 1 Experimental setup for cavity leak out spectroscopy
(CALOS). After mode matching the laser to the ring down resonator, the
laser is coupled into the cavity which is filled with the sample gas. The
leaving mirror is moved by a piezo to get high laser transmittance at the
detector. After reaching the threshold power the acusto optic modulator
(AOM) is switched off and the leak out signal is observed.
Without an absorbing medium inside the cavity, the signal decays exponentially
with a ring-down time t0 given by
the reflectivity R of the cavity mirrors
(1)
where tr is the round-trip time for the light iside the cavity.
Fig. 2 shows a typically exponential leak-out signal for mirrors with a
reflectivity of R=99.96 % corresponding to a leak-out time of 6.06 µs
by using a cavity length of 0.7 meter.
Fig. 2 Leak-out signal with decay time of 6.06 µs.
The detector signal is measured with a digital scope and read out by a
computer. The computer is using the nonlinear Levenberg-Marquard Fit to
calculate the exponential decay time.
If a sample gas is filled inside the cavity, the absorption follows
Beer`s law and the signal decays with with the ring-down time t.
The absorption coefficient kw can
be calculated directly by using the equation
(2)
where L is the length of the absorbing medium.
2. experimental results
We have evaluated a CALOS setup near 3 microns using a CO overtone laser
and mirrors with a reflectivity of 99.96 %. In our experiments we archived
a detection limit of 10-8/cm. For various hydrocarbons, e.g.
methane, ethane, ethylene, it is possible to detect trace gases with a
concentration less than 1 parts per billion. Theoretically calculations
of your spectra (HITRAN 96) show a well agreement with your experiments.
Fig. 3 Experimental and theoretical spectrum of a sample
gas containing 1 ppm ethylene in a nitrogen atmosphere. The blue lines
show the measured CALOS spectrum by using a CO overtone laser. The detection
limit is less than 1 ppb for a sample time of 1.5 seconds. The calculated
HITRAN 96 spectrum is plotted in red. Both spectra agree well.
This work is supported by the German Foundation of Environmental Research
(Deutsche
Bundesstiftung Umwelt). For detailed information of the project
organisation visit our homepage.
You can download our posters here: CRDS
Poster.pdf (85K)
CALOS
Poster.pdf (92K)
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