K.-F. Klein

Diffusion behaviour of fluorine in silica glass

Abstract Fluorine diffusion has been studied between 1600 and 2000°C in vapour-deposited silica layers. In particular, the influence of the common impurities hydroxyl and chlorine and the codopant phosphorous oxide was investigated. Hydroxyl and especially phosphorous enhance the fluorine diffusion rate whereas chlorine shows no detectable effect. The determined Arrhenius dependence of the diffusion coefficient agrees well with previous investigations made at lower temperatures. Additionally, diffusion coefficients for chlorine were derived.

TLC-analysis in forensic sciences using a diode-array detector

SummaryHPTLC (High Performance Thin Layer Chromatography) is a well known and versatile separation method which shows many advantages when compared to other separation techniques. The method is fast and inexpensive and does not need time-consuming pretreatments. For visualisation of the sample distribution on a HPTLC-plate we developed a new and sturdy HPTLC-scanner. The scanner allows simultaneous registrations of spectra in a range from 198 nm to 612 nm with a spectral resolution of better than 0.8 nm. The on-plate spatial resolution is better than 160 μm. The measurement of 450 spectra in one separation track does not need more than two minutes. The new diode-array scanner offers a fast survey over a TLC-separation and makes various chemometric applications possible. For compound identification a cross-correlation function is described to compare UV sample spectra with appropriate library data. The cross-correlation function herein described can also be used for purity testing. Unresolved peaks can be virtually separated by use of a least squares fit algorithm. In summary, the diode arry system delivers much more information than the commonly used TLC-scanner.

Optical detection techniques and light delivery with UV LEDs and optical fibres

Following the recent introduction of AlGaN/GaN based ultraviolet Light Emitting Diodes (UV-LEDs) in the 250 nm to 350 nm wavelength region, a wide range of research activities have begun. For example, using the high levels of power of available UV-light, chemical reactions can be stimulated. However, when optical detection techniques are considered, light intensity and wavelength accuracy are more important to achieve low detection limits. Using LED-based light sources in the ultraviolet instead of the classical deuterium, xenon, mercury or metal halide sources is very attractive due to their high power conversion, relatively simple electronic driving circuitry and low power consumption. For both absorption and fluorescence detection, such LEDs and complete detection systems are powerful tools both traditional and new applications in the field and laboratory. In this work, the electrical and optical properties of these new UV-LEDs will be described. Further, the flexibility of using optical fibres for UV-light delivery will be discussed with respect to the available UV-wavelengths and UV-induced damage. To exemplify the potential of UV LEDs, a self correcting fibre optic detection system over the wavelength range 260 and 280 nm, based on such LEDs will be shown. Further, an overview discussing flexible optical fibre UV-light delivery systems will be given and applications for both will be considered.

Fibre-optic UV systems for gas and vapour analysis

The identification and quantification of compounds in the gas phase becomes of increasing interest in the context of environmental protection, as well as in the analytical field. In this respect, the high extinction coefficients of vapours and gases in the ultraviolet wavelength region allow a very sensitive measurement system. In addition, the increased performance of the components necessary for setting up a measurement system, such as fibres, light sources and detectors has been improved. In particular the light sources and detectors offer improved stability, and the deep UV performance and solarisation resistance of fused silica fibres allow have been significantly optimized in the past years. Therefore a compact and reliable detection system with high measuring accuracy is developed. Within this paper possible applications of the system under development and recent results will be discussed.

Fiber-optic based gas sensing in the UV region

The precise analysis of potential hazardous components within gases and the detection of trace gases in exhaled breath for early and non invasive diagnosis of illnesses have a great influence on the well-being of human beings. Besides the existing analysis techniques, which mostly require sample preparation, costly consumables, huge space and skilled personal carrying out the measurement, a measurement system based on optical absorption in the UV wavelength region might offer alternatives to existing techniques. Within this work a feasibility study based on measurements of different test gases at lowest concentrations and requirements for trace gases in exhaled breath in respect to detection limits, signal-to-noise ratio and system drifts were analyzed. A spectral database including over 1000 UV vapor-phase spectra allows the identification of unknown compounds within a mixture, as well as expanding the use of the measurement technique into new areas of application, for example automobile application.

Silica-based UV-fibers for DUV applications: current status

The current status of UV-damage in several different UV fibers due to defects in their synthetic high-OH silica core and cladding will be described. Further, steps to improve UV resistance and adequate measurement techniques based on a deuterium lamp setup are included. For the first time, the main parameters and their influences on UV induced losses are discussed in detail with an emphasis towards future standardization purposes. Applications based on two new UV light sources, a laser driven xenon plasma broad band source and a high pulse-power 355 nm Nd:YAG laser, are introduced. UV photo-darkening and -bleaching in UV fibers caused by this extremely powerful light source is demonstrated. Finally, first results on transmission of UV light in optical fibers at cryogenic temperatures are shown.

Characterization of UV single-mode and low-mode fibers

There is an increasing demand for UV single-mode fibers with long-term stability. Due to the small diameter, the coupling efficiency is extremely low, if broadband light-sources or LEDs with near field diameter in the order of approx. 500 μm are used. Therefore, UV lasers are the real candidates for these fibers. Although the power is in the order of several milliwatts, the intensity in the small fiber core is significantly high. Therefore, UV damage has to be taken into account, even in the wavelength region above 300 nm. After the introduction of adjusted measurement systems for these fibers, the properties of some low-mode and single-mode fibers will be shown and discussed.

Improvement of optical damage in specialty fiber at 266 nm wavelength

Improved multimode UV-fibers with core diameters ranging from 70 to 600 μm diameter have been manufactured based on novel preform modifications and fiber processing techniques. Only E’-centers at 214 nm and NBOHC at 260 nm are generated in these fibers. A new generation of inexpensive laser-systems have entered the market and generated a multitude of new and attractive applications in the bio-life science, chemical and material processing field. However, for example pulsed 355 nm Nd:YAG lasers generate significant UV-damages in commercially available fibers. For lower wavelengths, no results on suitable multi-mode or low-mode fibers with high UV resistance at 266 nm wavelength (pulsed 4th harmonic Nd:YAG laser) have been published. In this report, double-clad fibers with 70 μm or 100 μm core diameter and a large claddingto- core ratio will be recommended. Laser-induced UV-damages will be compared between these new fiber type and traditional UV fibers with similar core sizes. Finally, experimental results will be cross compared against broadband cw deuterium lamp damage standards.

Lifetime prediction for 405-nm single-mode delivery systems for therapeutic laser applications

Near-UV laser light is used for soft tissue treatment for several years now. In first applications the light was delivered directly from the laser, but for in vivo treatment more flexibility was needed. Multi-mode fibers can be used to achieve a high output power coupled from multi-mode lasers. If fiber bundles are used the power can be increased additionally. But the power density on the treated tissue does not rise proportionally, because of the larger spot. A better ablation can be achieved with a Gaussian beam profile coming from a single-mode fiber. Higher beam quality and higher intensity from a small single-mode core produce power densities in the order of kW/cm2 in a focus spot smaller than 100 μm. If the laser therapy is used with the scanning fiber endoscope, treatment in between imaging spirals can be employed and only a single fiber is required. 405 nm laser-induced fluorescence may be able to produce both wide-field fluorescence imaging and laser therapy in a single laser. However additional wavelengths combiners and dual-clad couplers are necessary for multi-wavelength reflectance imaging requiring increased input power to compensate for the losses of these devices. This leads to very high intensities at the fiber coupler and damage will occur at this interface. Differences in damage rate due to differently treated fiber end-faces will be discussed. We suggest a new loss mechanism which is basal for the end-face damage and show miscellaneous methods to reduce the occurring damage and enhance the system lifetime.

Influence of high power 405 nm multi-mode and single-mode diode laser light on the long-term stability of fused silica fibers

As the demand for high power fiber-coupled violet laser systems increases existing problems remain. The typical power of commercially available diode lasers around 400 nm is in the order of 100 to 300 mW, depending on the type of laser. But in combination with the small core of single-mode fibers reduced spot sizes are needed for good coupling efficiencies, leading to power densities in the MW/cm2 range. We investigated the influence of 405 nm laser light irradiation on different fused silica fibers and differently treated end-faces. The effect of glued-and-polished, cleaved-and-clamped and of cleaved-and-fusion-arc-treated fiber end-faces on the damage rate and behavior are presented. In addition, effects in the deep ultra-violet were determined spectrally using newest spectrometer technology, allowing the measurement of color centers around 200 nm in small core fibers. Periodic surface structures were found on the proximal end-faces and were investigated concerning generation control parameters and composition. The used fiber types range from low-mode fiber to single-mode and polarization-maintaining fiber. For this investigation 405 nm single-mode or multi-mode diode lasers with 150 mW or 300 mW, respectively, were employed.