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.
Based on current studies with pulsed Nd-YAG lasers at 213, 266 and 355 nm, the measured UV-induced losses of multi-mode stepindex fibers can be described by multiple Gaussian shaped UV-defects. For best fitting, the peak value, center peak wavelength and Full-Width-Half-Maximum (FWHM) were variable parameter for the different absorption bands, similar to those defined by literature values. On the other hand, the damaging of multimode UV-fibers by broadband UV light-sources is still of particular interest. In addition to deuterium sources, where these losses are already described in the German DIN 58145 standard, pulsed Xe-lamps are used e.g. in spectroscopic applications for process control. Using both light-sources, the status of long-term degradation in UV fibers due to defects in their synthetic high-OH silica core and cladding will be described. However, this paper, focusses on results with the pulsed Xe-lamp and presents data for fiber-optic systems in spectroscopic applications. Transferring the above approach including the fit-function, analyses in high-OH and low-OH are carried out using broadband lightsources, regarding basic attenuation plus spectral and temporal damaging over several days. With the different light-sources, the peak values and the form of the absorption bands differ strongly, which can be explained by the overlapping of the absorption bands due to their spectral widths (especially the NBOHC at 260 nm). After discussing the differences related to different core diameters and lightsources, it is shown for the first time that the UV-induced losses are independent of the propagation angle within a step-index fiber.
An automated set-up for 213 nm UV laser damaging in parallel to spectral analyses will be used to show the spectral laser damaging in silica-based fibers with low hydroxyl content. The 213 nm wavelength is ideal, because it is close to the peak wavelength of one of the E’-centers in silica. Due to automation, short-term measurements down to 0.5 s and long-term measurements up to 2 days and more are possible. In addition, the spectral transmission changes are fitted with multiple Gaussian shaped bands. Although the basic attenuation in non-loaded fibers is high, the pulse energies are still high enough to generate defects along a 1 m long fiber. For the first time, to our knowledge, a transfer from E’γ to E’γ is observed within 1 s, with a laser repetition rate of 2 kHz. Due to optimal fitting, a band around 180 nm is likely influencing the UVC region. In hydrogen-loaded fibers, similar UV defects are seen. The related absorption bands can be optimally adopted to measurement results. The E’-centers play a minor role in short-term measurements because these defects are passivated during hydrogen-loading. Additionally, the band at 328 nm due to molecular chlorine will be reduced. The temporal behavior of all absorption band, including the predicted 180 nm band, are shown.
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