KEYWORDS: Absorption, Spectroscopy, Waveguides, Spectral resolution, Ultraviolet radiation, Sensors, Lamps, Nitrogen, Deep ultraviolet, Signal to noise ratio
In the deep ultra-violet (DUV) and the mid infra red (MIR) regions of the spectrum, the so-called Hollow Core
Waveguide (HCW) is an alternative for light-delivery systems. In addition to efficient light transportation, the HCW can
be used as an intrinsic sensor: due to the long path-length through the HCW the spectral absorption of the gas under test
can easily be monitored. Based on preliminary studies, the UV-region from 170 nm up to 250 nm seemed to be a very
attractive alternative for gas-analyses, in comparison to the IR-region with the well-known gas-absorption bands. Using
improved and adjusted components, the existing system for UV-spectroscopy was optimized. The new system will be
described in details. Especially, the time-response using an external gas-feeding system is an additional parameter of the
studies. Within the spectral range of the new CCD-array spectrometer from 175 to 210 nm, highly structured spectra
have been determined, with a spectral resolution of less than 50 pm. In nitrogen, selected gases with extremely low
concentrations in the order of 1 ppm have been measured; the absorption spectra are compared to those from IR
measurements, using the HCW approach, too.
In contrast to Nd:YAG-lasers no amorphous material is existing which can be used as flexible solid optical fiber for far infrared wavelength as it is emitted by CO2-lasers. This is resulting in some disadvantages regarding beam guiding and handling, e. g. reduced motion flexibility and accessibility, relative large and stiff devices for beam guiding. It is well known that beam guiding by hollow waveguide structures is possible in principle. Main problems in the development and realisation of such kind of beam guiding systems have been limitations in transfer efficiency, flexibility, length and diameter. A possible technical solution for the medium-power range is basing on flexible silica capillaries which are coated inside with a double layer system out of silver (metallic) and silverjodid (dielectric) using a chemical deposition process. These capillaries can actually be produced with inner diameter between 0,5 mm - 2 mm and a length up to 13 m, covered by a mechanical protection out of acrylic material. The optical principle of the hollow-core-waveguides (HCW) is presented as well as some first results regarding their influence on the transported beam. So investigations will be presented for instance showing the attenuation depending on different parameters of the waveguide system. In addition to that it will be shown that, because of the influence on the wave front and of the numberical aperture, the focussability of a laser beam is reduced but nevertheless the transmitted beam can be focused to small diameters. The principal usability as well as the limits of flexible HCW in the medium power range, which is typically used in medical applications, will be demonstrated by the presented results.
In the DUV-region and MIR-region, the so-called Hollow-Core-Waveguide is an alternative for light-delivery systems, because flexible silica-based fibers are no lnoger useable due to the high intrinsic absorption of silica. In additionl to light-transportation, only the HCW can be used as an intrinsic sensor: due to the long path-length through the HCW with similar intensity profiles at the input and output, the spectral absorption of the gas under test can easily be monitored. Up to now, the gases are analyzed in the MIR-region, mainly. However, the UV-region offers a lot of advantages. Using commercially available components for the UV-light source and the detector-system, the whole system with UV hollow-core-waveguides has to be studied in the wavelength-region from 170 nm up to 350 nm. With this experimental system, it is obvious to observe the UV-absorption of air and carbon dioxide below 200 nm, using nitrogen as a reference gas. In addition, ozone generated by the deuterium-lamp itself and several gas mixtures (e.g. 2 ppm toluene or xylene in cabon dioxide) were studied in detail.
Simultaneous laser beam welding is a modified procedure of both sided laser beam welding. Two laser beams are focused in opposite direction at the same time and place onto the workpiece. The process is characterized by the formation of a joint keyhole opened to both sides of the workpiece. The main advantages in comparison with conventional laser welding processes are higher welding speed, avoidance of angular distortion due to a symmetrical field of thermal stresses, as well as minimum porosity, especially in the middle of the seam volume, and a high degree of process stability. The advantages are depending on the joint keyhole. In the case of small adjustment of the two beam axes combined with inappropriate parameter settings this joint keyhole can collapse into two separated keyholes each having only an opening to one side of the workpiece. Therefore the process state with 'joint keyhole' has to be monitored. According to this criterion a non-destructive on- line measurement technique has been developed. The measurement technique is based on RF-frequency modulation of one of the two applied laser beams. Since the results of the measurement are available in very short time scales a usage for on-line control is possible.
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