Cavity Enhanced Absorption Methods
The general measurement principles of cavity enhanced absorption techniques have been well developed over the past ~15 years and several approaches to measure NO3 have been demonstrated by us (Venables06; Varma09; Dorn13] and also other groups (Langridge08, Kennedy11). In these devices, light is trapped inside an optical cavity consisting of two plane-concave ultra-highly reflecting mirrors (reflectivity R>0.9999). The light trapped in the cavity is reflected back and forth thousands of times on average and therefore the pathlength of interaction between the light and a gas filling the cavity (the absorption length) is dramatically increased. This leads to an increased detection sensitivity. One distinguishes two approaches in cavity enhanced spectroscope.
Time Dependent Methods
Time-dependent methods are based on the measurement of the time that light can be stored inside an optical cavity. In this approach the gas absorption is determined from the rate of an intensity change due to absorption rather than the absolute magnitude of an intensity change. The sample excitation is generally pulsed (or a continuous beam is modulated).
Intensity Dependent Methods
Intensity-dependent methods are based on the measurement of the light transmitted through the cavity. In this case typically either broadband sources are used and the light is dispersed after the cavity, or the light source is scanned e.g. by employing high resolution tunable lasers. The sample excitation is generally continuous wave.
Fig. X gives a general overview of the different approaches used in cavity enhanced absorption spectroscopy. The two approaches relevant in the current project are indicated by the dashed black arrows (Cavity ring-down and IBBCEAS).