Intracavity Absorption

When the absorbing sample is places inside the laser cavity, the detection sensitivity can be enhanced considerably (by several orders of magnitude). This fact can be understood from the simple fact that a photon inside the laser travels $q$ times back and forth between the resonator windows before it leaves the resonator. Note, that the number $q$ depends on the quality of the resonator and is usually very large, $q \gg 1$. Therefore, the path length $l$ in eq. (76) should be multiplied by $q$ and the photon has a $q$-chance to be absorbed in the sample. Intracavity absorption cells are particularly advantageous, if the absorption is monitored via the laser induced fluorescence. A frequently used modification of the method is cavity ring-down spectroscopy when the laser cavity is pumped by a short pulse of laser radiation and then the slowly damping radiation inside the cavity is monitored by a spectrograph.

The enhanced sensitivity of intracavity absorption may be utilized either to detect minute concentrations of absorbing components, or to measure very weak forbidden transitions in atoms and molecules at sufficiently law pressures to study the unperturbed absorption line profiles. With intracavity absorption cells of less than 1 m, absorbing transitions have been measured, which would demand a path length of several kilometers with conventional single-pass absorption at a comparable pressure.

Some examples illustrate the various applications of the intracavity absorption technique.

• With an iodine cell inside the resonator of a CW dye laser an enhancement factor of $Q=10^5$ could be achieved allowing the detection of $I_2$ molecules at concentration down to $N<10^8 cm^{-3}$. This corresponds to a sensitivity limit of $k l \leq 10^{-7}$.
• Detection of absorbing transitions with very small oscillator strength has been done with a CW dye laser. The extremely week infrared overtone absorption band of the red atmospheric system ( $v'=2 \leftarrow v''=0$) of molecular oxygen and also the ( $v'=6 \leftarrow v''=0$) overtone band of $HCl$, using a 97-cm long intracavity absorption cell.
• The overtone spectrum $\Delta v = 6$ of $SH_4$ was measured with the effective absorption path length of $l_{eff} = 5,25$ km. The spectrum shows well resolved rotational structure.

Auf diesem Webangebot gilt die Datenschutzerklärung der TU Braunschweig mit Ausnahme der Abschnitte VI, VII und VIII.