IMPROVING LONG-PATH DIFFERENTIAL OPTICAL-ABSORPTION SPECTROSCOPY WITHA QUARTZ-FIBER MODE MIXER

Long-path differential optical absorption spectroscopy (DOAS) has beco me an increasingly important method for determination of the concentra tion of tropospheric trace gases (e.g., O-3, NO2, BrO, ClO). The use o f photodiode array (PDA) detectors enhances long-path DOAS systems con siderably owing to PDA's higher sensitivity resulting from the multipl ex advantage. The detection limits of these systems are expected to be 1 order of magnitude lower than systems of similar optical setup with scanning detectors. When the scanning detector is simply replaced by a PDA, unwanted spectral structures of as much as 8 x 10(-3) appear. T he size of these randomly changing structures exceeds the photon noise level by 2-3 orders of magnitude thus severely limiting the sensitivi ty. We show that an angular dependence of the response of the PDA caus es this structure in combination with unavoidable changes in the illum ination. A quartz-fiber mode mixer, which makes the illumination of th e spectrograph-detector system nearly This independent of the angular intensity distribution of the measured light, was developed and tested . This new device reduces the unwanted structures in laboratory and he ld experiments by a factor of 10. The detection limits of long-path DO AS instruments with PDA detectors are improved by the same amount and are thus lower than those of currently used systems with scanning dete ctors. At the same time a much shorter measurement time (by similar to 1 order of magnitude)becomes possible. (C) 1997 Optical Society measu rement techniques.