The need for robust, versatile, and rapid analysis standoff detection systems has emerged in response to the increasing threat to homeland security. Laser Induced Breakdown Spectroscopy (LIBS) has emerged as a novel technique that not only resolves issues of versatility, and rapid analysis, but also allows detection in settings not currently possible with existing methods. Several studies have shown that femtosecond lasers may have advantages over nanosecond lasers for LIBS analysis in terms of SNR. Furthermore, since femtosecond pulses can travel through the atmosphere as a self-propagating transient waveguide, they may have advantages over conventional stand-off LIBS approaches1. Utilizing single and multiple femtosecond pulse laser regimes, we investigate the potential of femtosecond LIBS as a standoff detection technology. We examine the character of UV and visible LIBS from various targets of defense and homeland security interest created by channeled femtosecond laser beams over distances of 30m or more.
A prototype in situ multi-sensor fiber optic fluorometer is described which was designed to acquire long term time-series fluorescence measurements. The multi-sensor system uses dual detectors with four excitation sources, thereby providing for independent measurements at eight sensor locations. Strobe excitation light of wavelength < 500 nm is passed through one of each pair of optical fibers and stimulated chlorophyll fluorescence is carried back to a photomultiplier tube. The excitation and detection hardware are enclosed in a pressure case along with a battery operated 500 kHz data acquisition/storage system. Aspects of the design of the fiber optic sensor are described which were intended to optimize detection of fluorescence signals and minimize interference by ambient light. An illustration of the utility of fiber optics for fluorescence measurements was made by comparing a prototype fiber optic profiling fluorometer with a commercial fluorometer. Time series fluorescence measurements were made with the multi-sensor fluorometer in the Gulf of Mexico, and revealed chlorophyll variability in the benthic boundary layer.
We present a novel miniature spectrometer in combination with a compact tandem optical fiber DIP probe. The system is designed to use single strand fibers to obtain high resolution spectral information for the determination of absorbance, transmission and scattering in liquids, or for measuring pH or toxic metal concentrations using immobilized indicator materials. The performance of the CCD array spectrometer in terms of spectral resolution, stray light, noise and dynamic range is shown to equal typical non-fiber analytical instruments. The mode containment optical design of the spectrometer results in excellent light throughput, and fibers as small as 50 micrometers can be used for routine applications. Tandem fiber probes were made by cutting and polishing the distal tips of two parallel fibers at a 45 degree(s) angle. A description of the tandem fiber probe (DIP probe) is presented which includes the probe's construction. Suggestions are made in geometric variations to adapt the probe to perform in other sampling tasks. Optical coupling efficiencies and refractive index effects are evaluated. Methods to construct ruggedized extension or field optical fiber cables are also discussed.