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Laser induced fluorescence (LIF) is a common technique for measuring chlorophyll-a (Chl-a) concentration in water, and is able to obtain profile distribution. The natural water body is a turbid media for laser transmission, resulting in a great attenuation of luminance in a short distance. Increasing laser intensity is often considered an effective way to probe deeper. However, it has been found that there is a non-linear relationship between fluorescence intensity of Chl-a solution and laser energy which may causes laser-induced saturation of fluorescence with measurement conditions unchanged. Thus, the saturation effects of Chl-a fluorescence at 685 nm were studied at two aspects, namely, Chl-a concentration and the intensity of excitation pulse. In the experiment, several concentrations of Chl-a solution were measured by 355 nm laser. For a fixed Chl-a concentration, the intensity of fluorescence gradually becomes to worse following the intensity of excitation pulse raised, while the ratios of Raman intensity of water to pulse intensity correspond to a good linear relationship. On the other hand, with different energy densities of excitation laser, the variations of Chl-a fluorescence intensity were analyzed by non-linear curve fitting. For higher Chl-a concentration, the lower threshold value of excitation intensity can lead to the nonlinearity of induced fluorescence intensity versus laser intensity. Here, the nonlinear changes of Chl-a fluorescence with saturated excitation are studied for supporting the measurement of oceanographic fluorescence by LIF and correctly estimating chlorophyll concentrations.
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