叶绿素浓度是海洋初级生产力的重要指标之一, 激光诱导荧光技术可以实现海水叶绿素浓度的快速测量。 测量叶绿素浓度的传统激光诱导荧光原理, 是利用叶绿素荧光与水体Raman散射的强度比值( I_F/ R)进行反演, 即叶绿素浓度 n_chl= CI_F/ R, 其中 C为系统常量。 这是依据叶绿素荧光685 nm、 水体Raman散射强度都与激发光强呈线性关系。 然而, 该理论并没有考虑诱导荧光饱和现象的存在。 当诱导激光强度达到一定程度后, 685 nm荧光强度随激发光强非线性变化。 另外, 值得注意的是, 水体Raman散射并不存在信号饱和现象。 为了探讨饱和激发造成荧光非线性变化的影响, 在激光诱导荧光技术测量叶绿素浓度的实验中, 设计两种测量方案, 即: 不同激光功率诱导单一浓度样本的荧光测量, 和固定激光功率时不同浓度样本的荧光测量。 实验中利用Nd:YAG三倍频激光355 nm激发获得叶绿素溶液的404 nm处 Raman散射和685 nm荧光。 实验结果分为2部分进行讨论: (1)为了分析饱和激发造成荧光变化的非线性特性, 通过调节激发光功率测量溶液的受激发射光谱, 发现水体Raman散射强度与激发光强呈线性关系, 而685 nm荧光强度出现饱和激发下的非线性变化。 而且, 随叶绿素浓度的增加, 685 nm荧光的非线性趋势更为明显, Raman散射强度与激发光强的线性关系中斜率变小。 数据分析表明, 685 nm荧光数据拟合的4阶多项式和Raman散射效率值, 可以定性地表征685 nm荧光的饱和程度。 (2)考虑实际海洋激光雷达探测叶绿素浓度应用中存在饱和激发荧光非线性现象, 为了分析荧光非线性对传统叶绿素浓度反演理论适用性的影响, 在固定激发光强情况下对不同浓度叶绿素溶液的发射光谱进行测量。 将激发光功率调节至52.00, 80.70, 132.10和197.30 mW·cm^-2, 获取相应激发光强下685 nm荧光与水体Raman散射的强度比值和叶绿素浓度之间的关系。 实验表明, 激发光强不变的情况下, 685 nm荧光与水体Raman散射的强度比值, 与叶绿素浓度仍满足线性关系。 但是, 在较高光强激发时, 饱和激发造成的叶绿素荧光非线性变化, 导致利用传统激光诱导荧光理论反演的叶绿素浓度值偏小。 因此, 需要对饱和激发下荧光非线性的影响进行修正, 其关系为 I_F/ R= n_chl/ C+ C_F, 修正值 C_F不可忽略。 另外, 值得一提的是, 修正关系中系统常量 C随激发光强增加而增大。 研究表明, 饱和激发造成的荧光非线性, 会对激光诱导荧光技术测量叶绿素浓度产生影响, 但由于造成荧光非线性因素的复杂性, 仅通过荧光数据拟合获得的多项式, 无法定量说明其影响权重。 然而, 当激发光强不变时, 可以实验测量获得基于激光诱导荧光原理的修正关系, 从而准确反演叶绿素浓度。

As one of the most important indicators for studying marine primary productivity, chlorophyll concentration in seawater can be quickly measured by laser-induced fluorescence (LIF) technology. In the traditional theory for obtaining chlorophyll concentration by LIF, the chlorophyll concentration n_chl= CI_F/ R, where I_F and R are fluorescence intensity of chlorophyll a (Chl-a) at 685 nm and Raman scattering intensity of water respectively, and C is a system constant. Withoutconsidering induced fluorescence saturation, this theory is based on an assumption that both the fluorescence intensity at 685 nm and water Raman intensity are linear with the intensity of incident laser. However, experimentsconfirmed the existence of non-linear relationships between the induced fluorescence energy at 685 nm and laser energy. While the linear relationships between water Raman intensity and pulse intensity have always existed without saturation excitation. In order to explore the effect of non-linear fluorescence change under saturation excitation, two series of measurement were done in the experiments. Fluorescence of the solution with constant Chl-a concentration was measured by varied laser powers, and a constant laser power was used to obtain the solution fluorescence of varied Chl-a concentrations. The third harmonic of Nd:YAG laser at 355 nm was the excitation source. Thus, Raman scattering at 404 nm and fluorescence at 685 nm of Chl-a solutions were the key part of emission spectra. The experiment results were discussed in section 3. In the first part, the emission spectra of Chl-a solutions were measured by LIF with excitation light intensity variation. It shows a linear relationship between Raman scattering and excitation intensity, while fluorescence intensity at 685 nm appeared nonlinear change under saturated excitation. Moreover, fluorescence intensity of Chl-a solution with higher concentration increased to plateaus earlier, and the ratio of Raman scattering intensity to excitation intensity in the linear relationship decreased with Chl-a concentration. The data analysis shows that a polynomial of degree 4 fitting the changes of fluorescence intensity and the value of the Raman scattering efficiency can qualitatively characterize the saturation of fluorescence at 685 nm. Secondly, for the purpose of analyzing the effect of fluorescence nonlinearity on the applicability of traditional theory in chlorophyll concentration inversion, with considering the phenomenon of fluorescence saturation existing in the application of ocean Lidar for detecting chlorophyll concentration, the emission spectra of samples with different Chl-a concentrations were measured with a constant excitation intensity. The relationship between I_F/ R and Chl-a concentration was obtained under the excitation power at 52.00, 80.70, 132.10 and 197.30 mW·cm^-2. Experiments show that I_F/ R is still in linear relationship with Chl-a concentration under the condition that the exciting radiation is not changed. But, the concentration from the traditional inversion theory by LIF is less than the real Chl-a concentration measured by a high excitation intensity which leads to fluorescence saturation effect. Therefore, the inversion module is necessary to be corrected with C_F which is related to fluorescence nonlinearity under saturation excitation. A more accurate inversion is based on I_F/ R= n_chl/ C+ C_F. And, it is worth mentioning that the system constant C in this correction module increases with the exciting intensity. Consequently, saturation excitation causesfluorescence nonlinearity and affects the measurement of Chl-a concentration by LIF technology. It is regrettable that the polynomial obtained by the fluorescence data fitting cannot quantify the impact of fluorescence saturation effect, due to the complexity of the nonlinear factors. However, when the excitation power is constant, a corrected inversion can be experimentally obtained and used to measure Chl-a concentration by LIF in field surveys.