Joachim Kastner

Enhancing the Sensitivity of Chemical Sensors for Chlorinated Hydrocarbons in Water by the Use of Tapered Silver Halide Fibers and Tunable Diode Lasers

Tapered silver halide fibers were used for the first time for high-sensitivity fiber evanescent wave spectroscopic (FEWS) measurements of chlorinated hydrocarbons (CHCs) in water at trace level. The sensor arrangement consisted of a tunable diode laser (TDL) source, a polymer-coated and tapered silver halide fiber, and a direct fiber/mercury cadmium telluride (MCT) detector coupling. A minimum detection limit of 50 μg L−1 was achieved by using a polyisobutylene-coated fiber with a 1:4 tapering ratio. The sensor response is in the minute range and shows reversible analyte enrichment.

Optimizing the modulation for evanescent-wave analysis with laser diodes (EWALD) for monitoring chlorinated hydrocarbons in water

Abstract Fiber evanescent-field analysis (FEFA) is a novel and promising sensor technique for on-line and in situ analysis of hydrocarbons in water. With a conventional IR light source and FTIR spectroscopy it allows multicomponent analysis, while the use of mid-infrared (MIR) tunable diode lasers (TDLs) results in sensitive and selective single-component analysis. A new modulation technique operates the diodes with short pulses in combination with base-current modulation. Conventionally these lasers are used for trace-gas analysis with spectral features of 10 −6 spectral resolution. For the new evanescent-wave analysis with laser diodes (EWALD) sensor application, broadband spectra must be scanned, which does not allow high-resolution techniques to be used. The laser emission bandwidth does not need to be small, 10 −3 to 10 −4 resolution is sufficient, but the wavelength scan range must be about 10 −2 . In short-pulse mode, this can be achieved; furthermore, the laser can be used at higher operation temperature for more convenient application. The laser modulation characteristics are dependent on operation temperature, base-current, and pulse-current width and amplitude. The influence of these parameters and their optimization will be discussed.

Roadside automobile emission monitoring with Peltier-cooled diode laser spectrometer

The use of catalytic converters in cars with gasoline engine results in a tremendous reduction of the emission of pollutant gases. The optimal operation of the exhaust treatment systems is being checked and maintained periodically, but there is always a significant percentage of cars with a malfunction of the catalytic converter causing a substantial percentage of the total emission. Roadside emission monitoring of individual cars in the running traffic could be used to indicate these gross polluters, arrange maintenance of their vehicles and thus reduce total emission. Present monitoring systems use non- dispersive IR spectroscopy. Other systems are based on mid- IR diode laser spectroscopy offering a higher signal to noise ratio, higher selectivity for detection of specific compounds and better optical quality for long open path measurements, but these systems depend on liquid nitrogen cooling. In this work a compact mid-IR (MIR) laser diode system for roadside measurements will be presented, that is cooled thermoelectrically using a Peltier element. Sensitivity and time resolution of the system have been determined and found to be suitable for detection of single gross polluters in the running traffic. The presented system demonstrates the feasibility of high sensitive, selective and fast field MIR laser diode spectroscopy together with ruggedness and low maintenance expense.

Propagation of infrared radiation in silver halide optical fibers

The relation between microscopic scattering and absorption properties, and the measurable macroscopic transmission properties of silver halide infrared fibers were investigated. Four mechanisms were considered, both by numerical simulation and far-field measurements: core scattering, surface scattering, absorption, and surface losses. The dominant mechanism in changing the angular distribution of the transmitted light was found to be the core scattering. Microscopic scattering and absorption properties were derived by fitting the experimental results.

Optimizing the optics for evanescent wave analysis with laser diodes (EWALD) for monitoring chlorinated hydrocarbons in water

Fiber evanescent field analysis (FEFA) is a novel promising sensing technique for on-line and in situ analysis of hydrocarbons in water. With a conventional IR light source and FTIR spectroscopy it allows multicomponent analysis, while using MIR-tunable diode lasers results in more sensitive and faster single component analysis. Compared to common attenuated total reflection elements, silver halide (AgCl/Br) fibers offer more convenient application for remote sensing and field measurements because the fibers can be used for both, guidance of MIR radiation to and from the sensing part, and the sensing part itself. At present these fibers are multimode. The sensor response can be expected to depend strongly on the mode distribution in the fiber. We hence performed a model calculation that allows us to compare the FEFA absorption and the intrinsic fiber losses for given mode distributions and dependent on the optical parameters such as the coupling conditions and the fiber design. In order to link theoretical results to experimental data, the theory is based on an internal mode distribution derived from far field fiber emission data. We present far field data and the resulting internal distributions.

FAST AND SLOW CURRENT TUNING OF LEAD-CHALCOGENIDE DIODE LASERS

Mid‐infrared lead‐chalcogenide diode lasers can be operated at convenient operation temperatures using short current pulses and thus allow a wide range of potential applications. Our goal was to optimize the spectral characteristics of pulsed lasers by well‐defined current pulse shaping. The high‐frequency current tuning coefficient has been evaluated experimentally for this reason. Different tuning mechanisms are discussed by use of a model calculation. The qualitative features can be explained by assuming a separate temperature for the carriers that form a thermodynamic system of their own and are heated by the current. The tuning characteristics can be explained due to carrier concentration variations induced by free‐carrier absorption.