Gary W. Nelson

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.

All-silica fiber with low or medium OH-content for broadband applications in astronomy

For astronomical applications, different types of step-index all-silica fibers with high-transparency in the whole spectral region from UV (300 nm) to NIR (1100 nm) will be introduced. The light-guiding core-material consists of high-purity silica, especially with low or medium OH-content. In UV region, the losses are mainly influenced by Rayleigh scattering, while the losses in the IR region are limited by traces of OH-groups (in the order of approx. 2 ppm) and fundamental vibration-bands. Due to processing, typical UV-defects below 280 nm can be suppressed significantly within fibers with medium or low OH-content. Especially, one fiber-type with low-OH content in the core possess high resistance against UV radiation in the DUV-region down to 200 nm, which is comparable to high-OH all-silica fibers specially developed for UV-application below 250 nm. In addition, a medium-OH will be presented. The properties of these new fibers in respect to basic attenuation and spectral damage in the UV-region will be discussed, in comparison to high-OH fibers, based on the same measurement-technique. In addition, first results on focal ratio degradation (FRD) and additional loss related to higher propagation angles will be shown, in comparison to standard high-OH fibers.

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.

High NA-fibers: silica-based fibers for new applications

For medical and analytical applications, thick-core fibers based on synthetic silica are widely spread. In many cases, the fibers are used as a light-guiding medium only; therefore, the coupling efficiency between the light-emitting area and the accepting fiber is of great importance described easily by the light acceptance cone related to the numerical aperture of the fiber. In the past, all-silica fibers with un-doped silica core and fluorine-doped silica cladding have been used for different applications. However, these fibers are restricted in respect to a low numerical aperture of typically 0.22. To increase the numerical aperture, different polymers can be used for cladding material. In addition to standard polymers, Teflon-AF is an attractive candidate for significantly higher NAs of approx. 0.65. In parallel, a new class of all-silica fibers was developed with high NA, the so-called “Air-clad” or microstructured fibers. Longitudinal microstructured holes, in the order of the wavelength, form the cladding-region together with the surrounding silica. The dimensions of the microstructure dictate the critical angle for light transmission in the core, rather than the refractive index of the cladding material. The light guiding properties of different fibers will be compared. Especially the optical transmission from the UV-region up to the NIR-region will be discussed. Due to the wavelength-dependent mean value of the refractive index (RI) in the cladding, the definition of numerical aperture has to be adjusted. Especially, the UV-damage within Teflon-coated fibers and the microstructured fibers will be described in detail.

Low-OH all-silica fiber with broadband transparency and radiation resistance in the UV region

New step-index all-silica fibers with high-transparency in the whole spectral region from DUV (200nm) to IR (2200 nm) will be introduced. The light-guiding core-material consists of high-purity silica, especially with low-OH content of less than 10 ppm. In UV region, the losses are mainly influenced by Rayleigh scattering and electronic transitions leading to the Urbach-tail. On the other hand, the losses in the IR region are limited by traces of OH-groups (in the order of approx. 2 ppm) and fundamental vibration-bands. The fibers possess high resistance against UV radiation in spectral region of interest, which is comparable to high-OH all-silica fibers specially developed for UV-application below 250 nm. The properties of the new fiber in respect to basic attenuation and spectral damage in the UV-region will be discussed, in comparison to high-OH fibers, based on the same measurement-technique.

Stability and lifetime improvements of UV fibers for new applications

Two years ago, UV-improved (UVI) fibers were described for the first time. These fibers show significantly reduced UV- absorption around 215 nm during the transmission of UV-light from different UV-sources like deuterium-lamps, excimer- lasers and frequency-doubled tunable dye lasers. This improvement has been realized due to a passivation of the generated UV-defects by gas-doping. Depending on the ambient temperature and the fiber diameter, the lifetime of UV- stability was quite different in these first generation UVI- fibers. This long-term behavior has been studied with different illuminations, temperatures and fiber diameters; there is a good agreement between experimental and theoretical data. Additional modifications have lead to a further improvement in the long-term stability. The predicted lifetime of these UVI2-fibers of the second generation at room temperature is longer than 1 year. In addition, a short-term behavior was observed using diode- array spectrometer. In this case, the influence of temperature on the transient induced losses is significant and will be described.