844
2 - OPTICAL RESPONSE OF SEA WATER TO LASER EXCITATION
Because of strong attenuation of light in water, lidar systems are capable for
effective detecting the backscattered signal emitted from nearsurface water
column (less than 10 m even in relatively clear waters in the open sea).
Gating the detecting system synchronously with laser pulse allows to diminish
the contribution of ambient light to the spectrum.
In general, there are three important contributions (see Fig.2) to the
spectrum detected: the water Raman scattering, as well as fluorescence of
organic matter (both dissolved organic matter (DOM or "yellow substance") and
oil pollutions) and pigments of phytoplankton (chlorophyll-a (Chl-a) and, for
some species, phycoerythrin and phycocyanin). Therefore, the lidar technique
is potentially capable to provide both qualitative and quantitative
information about important environmental, biological and physical parameters:
"yellow substance" (Bristow et al., 1985; Reuter et al., 1993; Alberotanza et
al., 1994, Chekalyuk et al., 1992a) and oil pollutions characteristics (e.g.,
see Hoge and Swift, 1980; Hengstermann and Reuter, 1990), pigment composition
of phytoplankton (Hoge and Swift, 1981, 1983, 1990; Babichenko, 1994), their
concentrations and algal photosynthetic activity (e.g. see Chekalyuk and
Gorbunov, 1992b, 1994), temperature (Leonard et al., 1979; Raimondi and
Cecchi, 1994) and salinity (Bekkiev et al., 1983) of sea water, light
attenuation in water column (Bristow et al., 1981, 1985). The combination of
these capabilities, as well as the capacity for high-resolution day-and-night
remote monitoring, could provide a wide range of applications of lidar
techniques for studies of bio-geochemical and physical processes in the sea,
and for environmental monitoring in coastal zones.
Figure 1. Block Diagram of Lidar
System.
Figure 2. Typical Spectrum of Return
Signal from Water Excited by Irradi
ation of Double-Frequency YAG-laser
(X«x = 532 nm).
The conventional approach is to normalize the fluorescence contributions
to water Raman scattering (WRS) to exclude signal affecting by varied
detecting conditions (Klyshko and Fadeev, 1978; Bristow, 1981; Hoge and Swift,
1981). The wavelength X R = l/y R of water Raman scattering band is defined by
the frequency y = 1/X of laser excitation ; y = y — Ay , where Ay = 3440
_J L L R L R R
cm . As a result, X = 344 nm in case of UV excitation with XeCl excimer
R
laser (X l = 308 nm), and it would be shifted to the red area (X r = 651 nm, see
Fig.2) with changing the excitation to the second harmonic of Nd:YAG - laser
