Gerhard Holl

Fiber-optic laser sensor for mine detection and verification

What we believe to be a new optical approach for the identification of mines and explosives by analyzing the surface materials and not only bulk is developed. A conventional manually operated mine prodder is upgraded by laser-induced breakdown spectroscopy (LIBS). In situ and real-time information of materials that are in front of the prodder are obtained during the demining process in order to optimize the security aspects and the speed of demining. A Cr4+:Nd3+:YAG microchip laser is used as a seed laser for an ytterbium-fiber amplifier to generate high-power laser pulses at 1064 nm with pulse powers up to E(p) = 1 mJ, a repetition rate of f(rep.) = 2-20 kHz and a pulse duration of t(p) = 620 ps. The recorded LIBS signals are analyzed by applying neural networks for the data analysis.

Miniaturized, high-power diode-pumped, Q-switched Nd:YAG laser oscillator–amplifier

A miniaturized, passively Q-switched Nd:YAG laser oscillator-power amplifier is reported, which is axially pumped by a compact, fiber-coupled, high-power, quasi-cw diode laser module. The pumping intensity of the oscillator crystal can be adjusted independently of the pumping intensity of the amplifier. This ensures that the oscillator pulse enters the amplifier when its maximum population density is reached. Furthermore, pulse bursts can be generated with a definite, adjustable number of single pulses. Maximum pulse energies of 8.4 and 22 mJ were achieved for a single pulse and for a pulse burst, respectively, at a pumping power of 470 W. The pulse widths were 2 ns, whereas the beam quality corresponded to M2<1.5. The laser is appropriate for scaling the power to the 10 MW range. Operation by using a 100 m pumping fiber was demonstrated.

Application of LIBS Spectroscopy for Remote Bulk Detection of Explosives

Laser-induced breakdown spectroscopy (LIBS) is applied to characterize explosives and other materials such as plastics and soil. A short near-infrared laser pulse (e = 1064 nm, t < 1 ns, Ep = 0.4 mJ) is used to induce a plasma on the surface of the material to be investigated. When recording the time evolution of the LIBS intensities emitted by specific compounds at selected wavelengths (e.g. cyanide at 388 nm or atomic carbon at 247,9 nm) the ratio of “early” and “late” LIBS intensities is a measure to analyze different materials such as explosives, plastics or soil. Significant miniaturization of the LIBS apparatus is achieved by applying a microchiplaser as excitation source, fiber optics for light guiding and microprocessor controlled electronics for data accumulation and processing. Because of this miniaturization and the non-contact operation mode such a LIBS system can easily be integrated into a conventional mine detection needle but it also finds several applications in the field of explosive detection for anti-terror purposes.

Mid-Infrared Lidar for Remote Detection of Explosives

Laser ablation spectroscopy combining with high-resolution mid- infrared absorption measurements offers interesting possibilities for sensitive and selective stand-off analysis of surface contaminations under real-time op- eration conditions. The detection of NOx production rates emitted from sur- faces after interaction with a pulsed infrared laser beam allows distinguishing between different surface contaminations, e.g. energetic and non-energetic materials but also between molecules with similar atomic composition. This is shown for the explosives TNT, Octol and HMX. For efficient laser frag- mentation of surface contaminations the excitation wavelengths 1.06 μm and 1.47 μm are compared indicating that a pulse power as low as 0.25 mJ/pulse at 1.47 μm is sufficient for laser-induced surface evaporation of explosive contaminations. Then no plasma is ignited by the infrared laser pulses and therefore the obtained NOx emission is only due to the explosive surface contamination.

Real-time detection of mines and explosives by laser-induced breakdown spectroscopy

A fiber coupled LIBS system is used for detection of mines and explosives. Laser excitation at 1.5 mum is preferred to generate LIBS signals using an Er:glass microchip laser as seed laser for an Yb:Er fiber amplifier.

A fiber-coupled eye safe spectrometer for the stand-off detection of explosives

A compact laser spectrometer for the detection of explosives from a safe spot is presented. This laser setup also opens the possibility for the trace detection of explosives.

Remote detection of explosive contaminated surfaces by mid-infrared lasers

Vaporization of explosives is done at 1.5 μm and the vapor is analyzed for NO emission by MIR laser spectroscopy. The ratio of NO/NO2 is an indicator to distinguish between energetic and non-energetic materials.