Temporal and spectral Laser-induced UV-loss studies in DUV-fibers with undoped synthetic silica core, mainly low-OH silica and modifications, were carried out with an automated set-up using pulsed 213 nm, 266 nm and 355 nm Nd-YAG Lasers. A multiple Gaussian band approximation was used to separate the individual but superimposed optically active UV defects leading to new temporal results of the individual UV defects. The curve shape of spectral losses was found to be different and strongly influenced by the Laser wavelengths, which are within absorption bands in low-OH silica, reducing the correlated defect at the beginning of irradiation significantly. In addition to solarization caused by 266 nm Laser damaging, annealing of some defects was observed. Depending on the fiber type, including hydrogen-treated types, different time constants were determined. Obvious transformations were observed during damaging with Lasers and/or broadband D2-lamp. Finally, an improved fitting of the UV attenuation or UV-induced losses in UV-fibers around 250 nm (ODC-II region) was possible by introducing two bands with slightly different peak wavelengths and temporal behaviors.
Electrospun polylactide (PLA) nanofiber nonwovens are promising candidates for applications in tissue engineering. They may combine the advantageous chemical properties of PLA with a highly porous three-dimensional structure, which promotes the transportation of nutrients and the cell proliferation into the scaffolds. In this work, we tested different laser micro material processing strategies to optimize the surface topography of PLA nanofiber nonwovens for cell attachment. It was found, that the wetting behavior of the nonwoven samples could be switched between a hydrophobic and a hydrophilic behavior by balancing laser induced thermal processes and gentle laser ablation.
The spectral UV properties of silica-based multi-mode and few-modes fibers will be shown using pulsed 355 nm lasers with pulse density levels close to damage threshold. A new experimental set-up with spectral analyses and parallel damaging will be introduced and discussed. Using pulsed UV-laser with good beam quality, low-mode fibers can be studied in respect to spectral losses in a wavelength region significantly below the wavelength of operation.
Electrospun nanofibers mats have a great potential in tissue engineering and regenerative medicine. Their high porosity and enormous volume to surface ratio stimulate the growth and adhesion of mammalian cells and serve as a stable support structure. These suitable properties can be further optimized by structuring of the nanofibers. Ultrashort pulsed lasers can be used for modifying of the electrospun nanofibers without significant heat exposure. It seems also possible to generate very fine cuts from the fiber mats.
In this study, polyamide electrospun nanofibers samples were processed with picosecond UV-laser irradiation (λ = 355 nm, τ = 15 ps). The samples were processed in dry, wet and immersed condition. To optimize cutting and structuring of nanofiber tissue flakes, the influence of different laser parameters on line widths, edge quality, heat-affected zone (HAZ) and the contamination of the fibers by ablated particles (debris) were examined. One additional aim was the minimization of the flake size. It was possible to generate nanofiber flakes in the sub-millimeter range. The quality of the nanofiber flakes could be improved by ablation near the ablation threshold of the material. For cutting under wet conditions shrinking of the flakes has to be taken into account.
In the past, the spectral stability of multimode UV-fibers has been mainly characterized using deuterium lamps with a broadband spectrum in the DUV. In meantime, new UV light-sources with higher powers are available. For example, improved pulsed Nd:YAG lasers with higher harmonics or high-power broadband plasma lamps are interesting candidates for new systems and applications. Because of better beam quality, multimode all-silica fibers with core diameter smaller than 100 μm can be recommended. A new step-index fiber with a large cladding-to-core ratio will be introduced. Using the new light-sources, the degradation during UV-light delivery will be described in detail, comparing the hydrogen-loaded and non-loaded version of this fiber. These results of UV-induced damage will be compared to a commercially available improved 100 μm UV-fiber damaged with deuterium lamp.
A spectral Attenuated Total Reflection (ATR-) sensor with variable path lengths is proposed. Based on a new polymer
cladding material, the numerical aperture and the related maximum propagation angle in Polymer Clad Silica Fiber
(PCSF) have been increased significantly. The penetration depth of the evanescent field into the cladding of step-index
fibers depends strongly on the propagation angle. We found that when removing its cladding, the effective path length in
the interaction section can be adjusted, using meridional rays/modes of different propagation angles. Because the fiber
length is short, the mode conversion in the light-transporting section of the PCSF is negligible.
After measuring the optical properties of the PCSF, the ATR with selected liquids surrounding the light-guiding core is
determined in dependence of wavelength and excitation-angle. Especially, the possibilities to change from low to high
propagation angles including the excitation and detection system will be discussed.
For applications of fiber guided pulsed UV-laser radiation in biomedical optics, laser spectroscopy or laser micro
processing which need good beam quality low mode or single mode optical fibers are required. We investigated the
transmission properties at 355 nm wavelength with laser peak powers up to 5 GW/cm2 or laser fluences up to 9.5 J/cm2.
In some cases fibers were damaged during prolonged irradiation at this intensity level. So these fluences or intensities
can be used as estimation for the damage threshold. It turns out, that degradation or microstructural damage in the fiber
core plays a minor role in long term transmission as long as the intensity stays below the damage threshold. Fiber lengths
of many meters are possible. Single mode UV laser beam guiding is possible. UV beam guiding with high pulse
repetition rate, moderate peak power will be compared with that of moderate repetition frequency, high peak power
lasers
High-order skew modes will be excited in multimode step-index fibers using special excitation conditions. As a result, light
with an angle of incidence larger than the maximum angle for meridional modes given by the numerical aperture
of the fiber can be coupled into a fiber. Combining the selective mode-excitation with new powerful broadband light-sources,
the spectral light-guidance of such skew modes in different optical fibers will be described in detail. Results of the proposed
system in context of different light-sources will be discussed. A new evanescent sensor approach based on controlled
coupling of skew modes will be introduced. Finally, first steps to construct such sensors for medical and analytical
applications will be presented.
In step-index multimode-fibers, controlled excitation conditions are essential to achieve optimized transmission
properties and control the output beam profile. Especially with lasers as a light-source, selective methods of mode
excitation can be used easily.
So far, fiber properties have been specified using meridional rays (modes), as proposed with the inverse far-field method.
In addition to these meridional rays in step-index fibers, high-order skew rays can be selectively excited. Especially with
excitation angles higher than the numerical aperture of the fiber, these skew rays can propagate with interesting
properties. Based on these extreme test conditions, the core-cladding interface of large-core step-index fibers can be
more efficiently controlled.
Laser spectroscopy in the UV-region below 380 nm is a powerful tool for many biomedical or analytical applications.
For such purposes Polymer Optical Fibers (POFs) can be an interesting alternative to silica-based optical fibers if the
transmission in the UV-A region is sufficient. In addition to
high-power LED-light delivery shown in previous studies,
the short and long term performance of PMMA-based POFs under pulsed UV radiation was investigated using a nitrogen
laser at 337 nm and the 3rd harmonic of Nd:YAG laser at 355 nm. For thick POFs (core diameter: approx. 1000 μm), the
basic (initial) low intensity UV-attenuation is in the order of less than 2 dB/m. However, a typical initial attenuation
between 4 and 5 dB/m was determined using the pulsed UV-lasers. At 337 nm, the transmission for these POF is
independent of intensity up to 9 MW/cm2. No photodegradation was observed, up to 180k pulses if the intensity does not
exceed 6 MW/cm2. For both wavelengths, the surface damages have been observed, taking into account the differently
shaped intensity-profiles.
In thin-layer chromatography, fiber-bundle arrays have been introduced for spectral absorption measurements in the
UV-region. Using all-silica fiber bundles, the exciting light will be detected after re-emission on the plate with a fiberoptic
spectrometer.
In addition, fluorescence light can be detected which will be masked by the re-emitted light. Therefore, it is helpful to
separate the absorption and fluorescence on the TLC-plate. A modified three-array assembly has been developed: using
one array for detection, the two others are used for excitation with broadband band deuterium-light and with UV-LEDs
adjusted to the substances under test. As an example, the quantification of glucosamine in nutritional supplements or
spinach leaf extract will be described. Using simply heating of the amino-plate for derivation, the reaction product of
Glucosamine can be detected sensitively either by light absorption or by fluorescence, using the new fiber-optic
assembly.
In addition, the properties of the new 3-row fiber-optic array and the commercially available UV-LEDs will be shown,
in the interesting wavelength region for excitation of fluorescence, from 260 nm to 360 nm. The squint angle having an
influence on coupling efficiency and spatial resolution will be measured with the inverse farfield method. Some
properties of UV-LEDs for analytical applications will be described and discussed, too.
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.
During the last years we developed a flexible beam guiding system for DUV-laser radiation (λ=193 nm, 213 nm). This laser scalpel is based on hollow core waveguides and special fused silica optical fibers. We were able to demonstrate the feasibility of the laser scalpel to ablate biological tissues and technological materials. Our current interest is to develop a thinner scalpel tip which consists of 200 μm optical fibers instead of 600 μm fibers. The performance of these thin fibers was evaluated. We developed a computer controlled measurement system to study the optical fiber performance in dependence on laser fluence, laser intensity, laser repetition rate and fiber coupling conditions. We also investigated the long term behavior of the modified optical fibers for the 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 .
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.
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.
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.
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.
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.
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.
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.
Simultaneous coupling of the beam from one laser source into many optical fibers or fiber bundles is of interest for many applications. We developed a coupling scheme based on lens arrays to reduce transmission losses. The laser beam is splitted into focused partial beams. Positioning of an optical fiber near each focus allows multiple fiber coupling with similar efficiency as obtained for a single fiber. A prototype designed for coupling of excimer laser radiation into fiber bundles is presented.
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.
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.
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