The aim of this research is to develop a compact lidar system 8)to be mounted on a small airplane and a ship to actively obtain real time information of water properties. The system is designed to display a target spot in wide view of field image so as one can easily identify the pollution in real-time. It is composed of the Nd:YAG laser, CCD camera equipped with image intensifier and a high speed memory devise. The primary advantage of CCD is that the signals can be digitized and enhanced before display. The most adequate excitation laser source seems to be the third harmonic of the Nd:YAG laser for it is lightweight yet has a high-energy pulse. The total weight of the system is about 43kg.
Figure 1 schematically shows a proposed system. The gatable system is capable of measuring the distance to the target and also reduces the back-scattered noise by only receiving a return signal from the layer being probed. A reduction of a gate width will reduce a noise by decreasing the effective volume from which back scatter can occur.
An estimated effective range of the system is about 350m, assuming that the laser energy is 1 MW with the divergence of 1 mrad the diameter of the receive telescope is 20cm, the spectral band width is Δλ=10nm, the gain of the image intensifier is 104, the back ground solar radiation at the surface is 1.57W m-2 nm-1, and the visibility is 1km.
3. DATA ANALYSIS
3.1 Classification of Oil
Data measured in the field are interpreted in terms of substance types by comparing with time-resolved fluorescence data obtained in the laboratory. The analysis uses the fact that each fluorescent emission is characterized by a unique dependence on two distinct parameters, the observed emission wavelength and the decay lifetime.
In order to clarify the temporal characteristics of fluorescence, fluorescence characteristics of several organic compounds were measured with a streak scope. The sample dissolved in normal hexane was illuminated directly with the third harmonics of the YAG laser (355nm). The reduction yields the two dimensional map of decay time and wavelength. The fluorescence intensity varies greatly over the entire spectral range. The light oil has a peak around 350-400nm, the A-heavy fuel oil 400-450nm, the C-heavy fuel oil 450-500nm.
The lifetime of fluorescence may also have variations over the wavelength range, so we analyzed the decay time of fluorescence of oils. The observed spectrum is given by the convolution integral of the instrument response. The iterative deconvolution method is used, where the fiuorescence decay function F (t) is expressed with a sum of exponential terms corresponding to emission from multi-fluorophores:
where A and τare the pre-exponential factor and fluorescence decay time, respectively. The acquired fluorescence pulses were fitted with a sum of exponential decays depending on the number of fluorophores contained in the solution. The right column of Figure 2 shows fluorescence lifetimes versus wavelength using a two-component best fit method, where T1 of shorter lifetime is influenced by the temporal width of the laser, and T2 is considered the principal fluorescence decay of the oils.
