Problems in using diode arrays for open path DOAS measurements of atmospheric species

摘要

Abstract: The `multiplex advantage' of diode array detectors over optomechanical spectral scanning devices promises to improve DOAS instruments by reducing measurement time about two orders of magnitude. Alternatively the signal to noise ratio can be improved by adding several scans. Unfortunately, the use of diode arrays gives rise to new problems. Two thin layers on the semiconductor (the protective SiO$-2$/ layer plus deposits of vapor) produce spectral interference structures and a thermal recombination current in the diode junction is superimposed to the light signal. Interferences in the protective layer of the diode array impose spectral structures, which are subjected to slow changes due to deposition of vapor upon the diode array and rapid changes caused by varying illumination of the array due to air turbulence in the light path. Detailed model calculations reproduce the etalon structure measured in the laboratory by assuming the existence of a layer of SiO$-2$/ (index of refraction n $APEQ 1.6) and a second layer with n $APEQ 1.3 (probably ice). A model considering the geometry of the spectrograph detector system is presented, which describes the influence of changing illumination of the diodes (i.e., caused by atmospheric turbulence) on the etalon structure. The dark current can, in principle, be reduced by cooling the diode array. However, low temperatures increase the complexity of the detector and enhance deposition of vapor, aggravating the etalon problem. The dark current, depending on the charge of the diode, is a complex function of light intensity and exposure time. Thus, subtracting the signal of the darkened diode array to remove the dark current signal leads to apparent nonlinearities. A model for the behavior of the dark current is presented, which allows the use of diode arrays without extensive cooling. !6