Rainer Reuter

Evaluating polyimides as lightguide materials.

Lightguides were fabricated from three commercial polyimides of which one contains one and the others contain two hexafluoroisopropylidene (6F) groups. The latter are isomers using either the para or the meta isomer of the same diamine. As the number of 6F groups increases the optical losses of the corresponding lightguides decreases. In thick lightguides of the two 6F groups containing polyimides, loss values below 0.1 dB/cm can be realized using optimized conditions. Two mechanisms-ordering with or without charge transfer complex formation and voids or pinholes-are seen to be responsible for optical losses. The second type of losses can be reduced by cure optimization. Where ordering is possible annealing leads to increased optical losses. Geometrical restraint of the ordering, however, leads to loss reduction in otherwise identical conditions. Losses observed in the bulk are always higher than in the top and bottom layers of the polyimide films.

Fluorescence diagnostics of oil pollution in coastal marine waters by use of artificial neural networks

We discuss the problems with and the real possibilities of determining oil pollution in situ in coastal marine waters with fluorescence spectroscopy and of using artificial neural networks for data interpretation. In general, the fluorescence bands of oil and aquatic humic substance overlap. At oil concentrations in water from a few to tens of micrograms per liter, the intensity of oil fluorescence is considerably lower than that of humic substances at concentrations that typically are present in coastal waters. Therefore it is necessary to solve the problem of separating the small amount of oil fluorescence from the humic substance background in the spectrum. The problem is complicated because of possible interactions between the components and variations in the parameters of the fluorescence bands of humic substances and oil in water. Fluorescence spectra of seawater samples taken from coastal areas of the Black Sea, samples prepared in the laboratory, and numerically simulated spectra were processed with an artificial neural network. The results demonstrate the possibility of estimating oil concentrations with an accuracy of a few micrograms per liter in coastal waters also in cases in which the contribution from other organic compounds, primarily humic substances, to the fluorescence spectrum exceeds that of oil by 2 orders of magnitude and more.

Submarine lidar for seafloor inspection

Substances released at sea by accident or illegal discharges are characterized by a broad range of chemical and physical properties. Moreover, the characteristics of coastal waters, e.g. in the German Bight, are highly variable. Due to this a submersible sensor network for the detection of hazardous substances on the seafloor combining several instruments based on different physical and chemical principles has been under development since 1996. In addition to conventional sensors for seawater analysis and seafloor monitoring such as CTD probes (conductivity, temperature, depth) and video cameras, the system includes several newly developed instruments for the detection of dissolved and sunken pollutants. One of these instruments is the submarine lidar, combining a range-gated imaging device and a fluorescence lidar. A q-switched Nd:YAG laser is used as the common light source. Applying the expanded frequency-doubled laser pulse the gated CCD camera is particularly useful for recording images of damaged containers with chemical cargo where harmful substances might be released into the water column. Tuned to UV emission, the same laser is used for remotely classifying fluorescent substances spreading on the seafloor. The intention of this paper is to present the submarine lidar in detail and to give an overview of its physics and potential applications.

Polychromatic transmissometer for in situ measurements of suspended particles and gelbstoff in water

The beam attenuation coefficient is an optical parameter that sensitively depends on suspended and dissolved substances in water. Its measurement is not only of interest for an understanding of the radiative transfer in a water column. With appropriate algorithms for data interpretation, it also allows a fast determination of absorbing and scattering matter as time-series measurements or depth profiles that cannot easily be obtained with other methods. An instrument has been developed for measuring spectral attenuation coefficients over a wavelength range from 340 to 785 nm. The optical path length can be set between 0 and 400 mm. This allows application in a wide range of turbidity in coastal and inland (case 2 and case 3) waters and a calibration of the instrument during in-situ measurements. This makes the instrument suitable for long-term applications in which signals from conventional instruments would degrade owing to the biofouling of optical windows. From the data, the amount and the size distribution of suspended particles and the specific absorption of dissolved organic matter are derived in real time. Algorithms based on Monte Carlo methods are available for a classification of transparent particles and phytoplankton.

Contrast-enhanced optical imaging of submersible targets

Sunken objects at sea such as lost ship cargo are often not detectable by conventional video cameras because of their low contrast due to the high turbidity of the water column. A well-known contrast enhancing imaging technique in turbid media is range gated video recording. Synchronization of laser emission and camera gate time allows to suppress backscattered light from the water column and to record only the light backscattered by the object. This results in a contrast enhanced video image that increases the visibility range in turbid water up to eight attenuation lengths. Furthermore, image reconstruction algorithms can be applied if the modulation transfer function of the water column is known. However, these approaches only lead to an increase in visibility of a target if the intensity of light scattered by the seafloor differs from signals that originate from the object under investigation. Conventional monochrome video cameras as well as rang gated systems record the intensity of light, whereas its spectral composition and polarization characteristics are not considered. A linear polarized monochromatic illumination source and suitable cut-off and polarization filters in front of the camera allow to record the fluorescence images and to detect the degree of depolarization which is a function of the roughness of the scattering surface. This can result in a better contrast between target and seafloor. A prototype of a submarine imaging instrument has been realized for operation in the German Bight that makes use of techniques described above. Results of tank trials are presented.

Bio-optical properties of gelbstoff in the Arabian Sea at the onset of the southwest monsoon

As part of the German JGOFS Indian Ocean Programme, a cruise occurred in the Arabian Sea in May 1997 at the onset of the southwest monsoon. Data on gelbstoff, chlorophylla and tryptophan-like fluorescence as well as gelbstoff absorption were obtained. They indicate, that these optical parameters support the interpretation of hydrographic and biogeochemical conditions. Tryptophan-like fluorescence seems to be a useful indicator of changes in the constitution of the biomass.A comparison of gelbstoff absorption and fluorescence data from the upper 100 m reveals only a weak covariance. Special emphasis is given to the fit procedure used for retrieving the exponential slopeS of the spectral absorption coefficient.S values with a mean of 0.016 nm-1 are found in the 350 to 480 nm wavelength range. A mean slopeS set to the frequently reported value of 0.014 nm-1 in the exponential description of gelbstoff absorption might lead to a systematic over/underestimation, and thus to systematic errors, if single-wavelength absorption values are extrapolated to other spectral regions on the basis of this parameter.

RAMSES: a modular multispectral radiometer for light measurements in the UV and VIS

Solar radiation is the energy source for all photosynthetic life-forms. Due to their individual pigment compositions only a specific part of the spectrum can be used by the single species for photosynthesis. Therefore multispectral radiation measurements are helpful for their investigation and monitoring. With the background of increasing ultraviolet radiation due to the decreasing ozone layer in the atmosphere, the measurement of the UV-A and UV-B part of the spectrum is of increasing interest to people. In the RAMSES project (Radiation Measurement Sensor with Enhanced Spectral resolution), founded by the German Ministry of Research and Technology, a new subminiature multispectral radiometer is under development. This new instrument allows spectral measurements in the range from 250 to 720 nm. A modular concept of the instrument, combined with very low power consumption, allows to use the instrument in profiling systems as well as in stand-alone applications, such as moorings, monitoring systems or weather stations. Sensor heads with different detection characteristics are available, and up to 16 of them can be combined to a single instrument for simultaneous measurement, e.g. for up-welling and down-welling light. Typical applications of this instrument are satellite calibration and validation, UV measurements, biology, ecosystem modelling, water quality and even climatology.

Laser remote sensing in highly turbid waters: validity of the lidar equation

A submarine fluorescence lidar has been developed for the detection of hazardous chemicals on the seafloor of the German Bight. Signals are dependent on the inherent optical properties of the seawater column, the seabed and the substance properties, mainly their absorption coefficient, volume scattering function, fluorescence quantum yield and reflectance. Although the instrument is designed to inspect the seafloor it allows to record time-resolved spectra in order to derive information about the water column. Interpretation of these data is normally done with the classical lidar equation which is based on several simplifying assumptions. In its conventional analytical form multiple scattering is not considered. This leads to an increase in signal intensity and to optical ringing. Additionally, fluorescence lifetimes and detector response function may result in an uncertainty of distance determination. Monte Carlo simulations were done to analyze the performance of the submarine fluorescence lidar for realistic scenarios. Results are compared with theoretical predictions of the lidar equation. It is shown that the error in signal intensity increases with the turbidity whereas the slope of the lidar curve appears to be independent of it. Depth-resolved measurements are not limited by the penetration depth of the light but by multiple scattering effects.

Oceanic lidar: radiative transfer in the atmosphere at operating altitudes from 100 m to 100 km.

The feasibility of measuring water-column parameters of the sea with a fluorescence lidar under daylight conditions and at flight altitudes between 100 m and 100 km is studied by modeling the atmospheric radiative transfer. Parameters to be measured are fluorescence of gelbstoff and chlorophyll and Raman scattering of water molecules. A cloudless and stratified atmosphere with various conditions of near-surface visibility and ozone concentration is taken into consideration. Solar zenith angles are varied between 0° and 60°. Lidar specifications are set to 1 J output energy, 10 ns pulse duration, 0.1 mrad beam divergence, 0.1 mrad detection angle, and 400 cm(-1) detection bandwidth. Signal recovery is carried out over the effective pulse length of the returned signal, which is roughly 20-30 ns. Sensor zenith angles are set between 0° and 60°. As a result of the study the recommended range of excitation wavelengths for high altitudes should be chosen between 350 and 400 nm. Under these circumstances, and with the iven laser and sensor specifications, oceanic lidar measurements should also be possible at flight altitudes of up to 100 km under clear visibility conditions, even at noon.

Complex system for optical monitoring of coastal marine water areas: concept and methods

The concept of a system for continuous express monitoring of coastal sea water areas is presented. According to the suggested concept, the monitoring system should consist, in general, of three parts, in which the key role should belong to laser devices: (1) shore-based lidar that performs continuous monitoring of water surface and sub-surface layer of water (and, possibly, of atmosphere) in the selected water area; (2) patrol boat equipped with submerged portable devices, including laser spectrometer with submerged optical fiber probe; (3) coastal laboratory equipped with devices for detailed (but express enough) analysis of water samples, specially chosen by the results of remote laser sensing and submerged device data. Some features of remote sensing by means of shore-based lidar are considered. The main feature is sliding incidence of laser beam to water surface. The coastal sea waters have much more complex and variable composition than open sea water. The possibilities of different versions of laser fluorimetry in diagnostics of organic compounds in coastal sea waters are analyzed.