Mathias Belz

Physical analysis of teflon coated capillary waveguides

Abstract A theoretical analysis of the properties of a liquid core waveguide (LCW) formed by a water filled, teflon AF2400 inner coated glass tube is presented. The theoretical analysis is based on numerical FD-BPM simulations and reflectivity calculations of the layer system in the LCW. The improvements due to this inner coating compared to other capillary cells are shown. A teflon AF2400 layer thickness of about 5 μm is sufficient to confine the optical intensity to the liquid core and thus avoid any absorption or scattering by the capillary material or any influence of the outside environment.

Liquid core waveguide with fiber optic coupling for remote pollution monitoring in the deep ultraviolet

A new combination of silica fibres, highly transparent in the ultraviolet (UV) and showing long term stability, combined with a teflon-coated liquid-core waveguide (LCW) is presented for remote sensing in the deep UV, for monitoring one of the major pollutants, nitrates, in water, but with potentially wider applicability. The arrangement exhibits low spectral loss in the range between wavelengths of 200 nm and 400 nm and can be used for analytical investigations to determine small concentrations of such impurities in water. The operation of the optical system to achieve guidance of UV light below a wavelength of 250 nm for fibre optic sensors is discussed. With an optical pathlength of 203 mm, nitrate concentrations as low as 22 μg/l could be detected.

Water-core waveguide for pollution measurements in the deep ultraviolet

A fiber optic system for water analysis with high transparency in the deep-UV region (lambda >/= 190 nm) is presented. The system consists of special UV-improved silica fibers and a liquid-core waveguide (LCW) as an optical cell. The apertures of both light guides, the silica fiber and the LCW, are matched. The optical losses of the device are investigated experimentally and compared with theory, especially with a standard free-space geometry. The performance of the system with respect to UV absorption spectroscopy is demonstrated for nitrate and chlorine pollution in pure water. For a 203-mm-long LCW the detection limits have been determined to be as low as 22 mug/L for nitrate and 26 mug/L for chlorine.

Smart-sensor approach for a fibre-optic-based residual chlorine monitor

Developments of an optical-fibre-based sensor system for monitoring residual chlorine in water are discussed. The system, based on differential absorption spectroscopy, utilizes a novel miniature monolithic diode array spectrometer operating in the ultraviolet and visible (UV-Vis) region of the spectrum in combination with an optical flow-through cell of length 430 mm and a computer-controlled deuterium lamp source. The sensor, having a detection limit of 0.2 mg l−1 of free chlorine in water, relies on the fact that the OCl− ion, in which form dissolved chlorine exists at high pH (>9), strongly absorbs light at 290 nm. This paper describes the systematic approach that is used in the modelling and design of this sensor system. It also outlines the construction of the device and gives an evaluation of the performance in the laboratory environment.

UV-stabilized silica-based fibre for applications around 200 nm wavelength

New optical fibres having a nearly constant UV transmission at around a wavelength of 200 nm have been fabricated for the first time. Due to their improved UV properties when compared to those commercially available, these new fibres are tested as a potential transmission medium for the broadband spectrum of a deuterium lamp. In addition, a complete sensor system, including the lamp itself and the associated optical system, will be described in light of its intended application to spectroscopic chemical measurements. Details of the spectral intensities at the output of such a fibre, of 1 m length appropriate for such systems, are given in this work.

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.

UV LED fiber optic detection system for DNA and protein

Concentrations of DNA and proteins are traditionally detected at 260/280nm using laboratory spectrophotometers. Recently, AlGaN/GaN ultraviolet Light Emitting Diodes (LED) became available in the 250 nm to 350 nm wavelength region. An inexpensive fiber optic detection system based on these UV LEDs and photodiodes has been developed. It allows concentration measurements of such popular biochemistry samples. Measurement stability and noise will be discussed. The performance of the system in comparison to a standard spectrophotometer will be evaluated. In particular, the effect of decreasing the spectral resolution from usually used 2-3 nm to 10-20 nm Full Width Half Maximum (FWHM) is simulated and experimentally confirmed.

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.

Water-quality measurement using fiber optics at wavelengths below 230 nm

In the past, spectroscopic applications with fiber optics have been restricted to the wavelength range above 230 nm, because standard silica fibers with an undoped core and fluorine doped cladding are frequently damaged by exposure to UV-light, especially below 230 nm so quickly that stable UV-light transmission is impossible. Work on a fiber-optic sensor for water monitoring is presented here using an absorption cell and first solarization-reduced fiber samples for light-transportation: the main UV-band around 210 nm shows an induced loss of less than 0.3 dB/m. In this case it was possible to have the full output spectrum of a deuterium lamp at fiber endface, stable over time. Therefore, an optimized UV-lamp-fiber system, including commercially available components, has been developed and is reviewed in terms of performance in the wavelength region around 200 nm, in respect of an effective sensor approach for water a monitoring. In the following step, modification of the deuterium-lamp optics for optimized light coupling becomes more important, in order to maximize the optical power budget for this wavelength-region.

Optical fibers in instrumental UV-analytics

Physical and optical properties of optical fibers have improved over recent years significantly. Especially classic UV detection techniques in traditional chemistry, HPLC and dissolution testing rely more and more on fiber optic light guiding techniques to transport light to and from a sample simplifying the design of such detection techniques. An overview on the current status of UV-fiber optical properties will be given in this work. Especially, the reduction of UVdefects in the 215 nm wavelength region leading to a lower drift in the whole system, will be discussed. However, these are not the only parameters of interest in a fiber-optic system. For process control or instrumental analytics, the long-term stability including drift and noise must be determined. This requires stringent fiber test procedures similar to light-sources, connectors and complete detector systems. Further, white-light interference between optical interfaces of a fiber optic detection system due to axial movement, degradation of components and temperature often reduces system stability and must be considered. Finally, a cleaning-in-process of a fiber optic immersion probe will be introduced as a further step of system improvement.