High sensitive detection of trace gases by dual frequency modulation

Atmospheric trace gas monitoring has increased in importance, especially chloro-fluoro-methane (CFM) species, OH, HO/sub 2/, Cl and ClO radicals which have concentrations of the order of ppt (parts per trillion). Monitoring of this low level concentration is very difficult by ordinary optical methods. As a high-sensitivity optical method, frequency modulation spectroscopy (FMS) has been used. There the signal at the modulation frequency or at its overtone frequency has been measured, so that the optical thickness in the order of 10/sup -5/ or 10/sup -6/ is a technical limit of detection. On the other hand, the frequency stability technique is extraordinary developed, based on phase difference detection, where the stability of 10/sup -13/ to 10/sup -14/ has been achieved. Dual Frequency Modulation (DFM) is one of the most up-to-date technologies in this field. In this paper the authors propose application of this technique for trace gas measurement, and show an example of a simulation of high-sensitivity detection in the case of using a diode laser (LD) as a tunable light source.<>