Georg Hillrichs

UV fibers for applications below 200 nm

New fiber-optic applications have been demonstrated within the last years, mainly due to the unexpected progress in manufacturing of solarization-reduced fibers. In meantime, analytical systems including UV-fibers and spectrometers are in operation including the wavelength region from 200 to 250 nm.

Fluorimetric detection of water pollutants with a fiber-coupled solid state UV laser

Aromatic hydrocarbons are important and dangerous pollutants of the aquatic environment. With the method of laser-induced fluorescence it is possible to detect Benzene, Toluene, Xylene (BTX) as well as Polycyclic Aromatic Hydrocarbons (PAH) sensitively. The detection of these molecules by a continuously working in-situ method is achieved by combining LIF with fiberoptic guidance of light. We presence results on the detection of BTX and PAH by excitation with 266 nm and 355 nm radiation from a diode-pumped solid-state laser especially developed for sensor applications. The system is operated with thermoelectric cooling and battery supply independent of any installations is projected. The laser delivers pulses of 7 ns/140 (mu) J in the UV at a repetition rate of 100 Hz. Using time-resolved and spectrally-resolved detection of the fluorescence signal, the system delivers information that can be used to discriminate between BTX and PAH-molecules. We have also performed extensive investigations of the influence of scattering particles on the sensor signal. This led to specific optimizations of the sensor-head for different applications.

Reduction of photodegradation in optical fibers for excimer laser applications

Power transmission of excimer laser radiation at 308 nm through waveguides is of growing importance in medical applications. The maximum energy densities achievable at distal fused silica optical fiber ends are limited by the surface damage threshold of fused silica and by photodegradation of the optical fiber material. Limitations due to the surface damage threshold at the front surface can be avoided by applying tapered fiber geometries. In order to minimize photodegradation effects color center formation caused by high energy UV radiation has to be reduced. This involves optimization of the fused silica material properties and the necessity of modifying the manufacturing processes. Measurements on all silica fibers at 308 nm wavelength (XeCl excimer laser) show different influences of core material manufacturing. Not only the overall decrease of transmission but also the dependence of transmission changes on the number of laser pulses and defect annealing are strongly affected. Consequences for improved performance of all silica optical fibers in angioplasty are demonstrated by measurements on specially produced samples.

Fiber optic systems in the UV region

Mainly due to the unexpected progress in manufacturing of solarization-reduced all-silica fibers, new fiber-optic applications in the UV-region are feasible. However, the other components like the UV-sources and the detector- systems have to be improved, too. Especially, the miniaturization is very important fitting to the small-sized fiber-optic assemblies leading to compact and mobile UV- analytical systems. Based on independent improvements in the preform and fiber processing, UV-improved fibers with different properties have been developed. The best UV-fiber for the prosed applications is selectable by its short and long-term spectral behavior, especially in the region from 190 to 350 nm. The spectrum of the UV-source and the power density in the fiber have an influence on the nonlinear transmission and the damaging level; however, hydrogen can reduce the UV-defect concentration. After determining the diffusion processes in the fiber, the UV-lifetime in commercially available all-silica fibers can be predicted. Newest results with light from deuterium-lamps, excimer- lasers and 5th harmonics of Nd:YAG laser will be shown. Many activities are in the field of UV-sources. In addition to new UV-lasers like the Nd:YAG laser at 213 nm, a new low- power deuterium-lamp with smaller dimensions has been introduced last year. Properties of this lamp will be discussed, taking into account some of the application requirements. Finally, some new applications with UV-fiber optics will be shown; especially the TLC-method can be improved significantly, combining a 2-row fiber-array with a diode-array spectrometer optimized for fiber-optics.

Possibilities and limitations of optical fibers for the transmission of excimer laser radiation

For fiber-delivery systems with UV-lasers the candidates are mainly optical fibers with an undoped high-OH silica core and a F-doped silica cladding. However, there re there important limits to UV-applications: surface damage, two- photon-absorption and defect-generation during operation. In the last two years, UV-improved fibers with significantly reduced defects have been developed. This improvement is most pronounced at 248 and 193 nm, because at these critical wavelengths the induced losses are strongly influences by the main broadband UV-defects with absorption maxima at 165 nm and 215 nm. We will summarize the results including the influence of the main parameters. In addition, the transmission capacity for the 308 nm wavelength is of interest due to medical and industrial applications. At this wavelength the influence of the nonlinearities is much lower; however, the induced losses in standard fibers are still an important factor. To show the advantages of the UV- improved fibers, the transmission characteristics at 308 nm wavelength will be described in more detail, for the first time.

Suppression of solarization effects in optical fibers for 266-nm laser radiation

The creation of color centers in the Si-O network of a fused silica fiber by UV laser photons leads to solarization and hence to decreasing optical UV-transmission of the fiber. So fibers are only of little use for UV laser applications. We now found a way to improve the fiber properties by reduction of solarization. The transmission behavior of these modified fibers for 266 nm pulsed laser radiation has been studied. Spectroscopic experiments give some insight into the creation processes and the repair dynamics of the color centers. The influence of the OH-content in the fiber was also investigated. Based on this work, it is possible to realize fiber optic beam delivery systems for frequency quadrupled Nd:YAG lasers and for other high power UV laser sources. These results may also be useful for the improvement of bulk fused silica for UV applications.

Fiber optic aspects of UV laser spectroscopic in situ detection of water pollutants

The use of different ultraviolet lasers for fluorescence spectroscopic detection of water pollutants with fiber optical sensors has been studied. Especially detection of small aromatic hydrocarbons via laser induced fluorescence requires short wavelength excitation. Interaction of intense ultraviolet light with the commonly used fused silica fibers leads to a decrease of fiber transmission. Some transmission affecting laser parameters have been studied. A new concept for the use of fiber optic sensors with ultraviolet excitation has been developed. This method is based on transmission of visible laser radiation through the fiber and creation of ultraviolet radiation by optical harmonic generation at the distal end of the fiber. So the unfavorable fiber behavior at short wavelengths can be avoided. Simultaneous coupling of the beam from a single laser source into several optical fibers is interesting for distributed sensor applications. In order to minimize coupling losses for these purposes we developed a new coupling scheme based on a special optical lens array.

Flexible beam guiding in a microsurgical UV laser scalpel

Many potential microsurgical applications of UV laser radiation need a flexible beam guiding system. Especially for the argon fluorine excimer laser ((lambda) equals 193 nm) and for the 5th harmonic of the Nd:YAG laser ((lambda) equals 213 nm) the use of optical fused silica fibers is difficult. In this work we designed and tested a laboratory prototype of a scalpel for surgical treatments of the retina based on UV laser ablation. To achieve the necessary flexibility and to provide laser fluences above the ablation threshold of retina we developed a new type of beam guiding device. A hollow core waveguide is used in combination with a short length of a special fused silica optical fiber to guide the laser beam. To increase the laser fluence at the distal scalpel tip and to achieve a very small cut width a fused silica fiber (core diameter 600 micrometers ) has been tapered down to a diameter of about 150 micrometers .

Improved UV-fiber for 193-nm excimer laser applications

Excimer lasers are used for many medical applications, e.g. angioplasty and ophthalmology. In the medical field fiber delivery systems are predominantly used with XeCl-lasers (308 nm) up to now. The best suited core-material for the moment is undoped synthetic fused silica with high OH-content. At 193 nm transmission of these fibers is limited by high nonlinear absorption and color-center generation leading to increased absorption. For ArF-lasers at 193 nm wavelength new results of the nonlinear transmission properties of improved fused silica fibers are presented and discussed, taking the following parameters for medical laser-fiber- systems into account: fiber length, fiber diameter, fluence, and repetition rate. Finally the results are discussed regarding the difference in the generation of color centers.

Optical fibers with enhanced performance for excimer laser power transmission at 308 nm

Power transmission of xenon chloride excimer lasers through optical fibers is necessary for medical applications where tissue removal is performed within the human body. The most important application at present is excimer laser coronary angioplasty. Typical levels of energy densities applied by optical fibers for this application cause color center generation in fused silica leading to transmission decrease called photodegradation. This effect depends essentially on the grade of the fused silica. Important parameters are fiber length, pulse duration, energy density, and the irradiated cross sectional area of the optical fiber endface. For a new grade of core material the influence of these parameters on the transmission performance is described. The obtained material improvement leads to a significant reduction of the observable transmission decrease as a function of the number of laser pulses applied. Thus continuous operation of the laser in the region of the typical transmission plateaus at considerably higher and constant energy levels at the distal fiber end becomes feasible. This offers a new option for more reliable dosimetry in medical applications.