Improving long-path differential optical absorption spectroscopy with a quartz-fiber mode mixer

Long-path differential optical absorption spectroscopy (DOAS) has become an increasingly important method for determination of the concentration of tropospheric trace gases (e.g., O(3), NO(2), BrO, ClO). The use of photodiode array (PDA) detectors enhances long-path DOAS systems considerably owing to PDA's higher sensitivity resulting from the multiplex 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. The size of these randomly changing structures exceeds the photon noise level by 2-3 orders of magnitude thus severely limiting the sensitivity. We show that an angular dependence of the response of the PDA causes this structure in combination with unavoidable changes in the illumination. A quartz-fiber mode mixer, which makes the illumination of the spectrograph-detector system nearly independent of the angular intensity distribution of the measured light, was developed and tested. This new device reduces the unwanted structures in laboratory and field experiments by a factor of 10. The detection limits of long-path DOAS instruments with PDA detectors are improved by the same amount and are thus lower than those of currently used systems with scanning detectors. At the same time a much shorter measurement time (by ~1 order of magnitude) becomes possible.