Surface and bulk effects in silica fibers caused by 405 nm CW diode laser irradiation and means for mitigation

Surface and bulk effects in silica optics due to high intensity laser light are well known using short pulse and high power laser systems. Surfaces are quickly destroyed mechanically if not properly prepared and thoroughly cleaned. Linear and non-linear absorption of high intensity laser light in the bulk of the optics causes material modifications, like voids, cracks and UV defects. In ablation experiments with very short pulses on wide band-gap dielectrics, periodic surface structures in the form of ripples were found. Surprisingly, we found similar structures on fiber end-faces after long-term irradiation with 405 nm CW laser light. Power densities on the end-face are in the range of 1 MW/cm2, three magnitudes of order below the power threshold at which the described damages occur. Nevertheless a ripple structure perpendicular to the polarization direction of the laser was formed and grows with irradiation time. An increased absorption band at 214 nm (E' center) along the fiber was discovered by spectral absorption measurements. E' centers can be generated by 405 nm laser light in the bulk, therefore defects on the surface are possible as well. The generation of defect centers on the silica surface can enhance the formation of an unstable surface layer.

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