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Optical multimode fibers made of fused silica are widely used for transmission of high power laser pulses. Bending of fibers creates mechanical stress inside the material. The bend stress of a fiber can be calculated from bend radius, geometrical fiber parameters and Young's Modulus of the fiber core material and reaches typically values of 220 MPa. A thermo-elastic model of Kusov et al. predicts a quadratic dependence of laser-induced damage threshold fluence with applied stress.
In the present study, fiber preform material F300 (Heraeus) was loaded mechanically with pressures up to 220 MPa representing 20% of the pressure resistance of fused silica. Bulk laser-induced damage thresholds (LIDT) were evaluated using a longitudinal multimode Q-switched Nd:YAG laser (1064 nm) at a pulse duration of 12 ns with polarization states parallel and perpendicular to the stress direction. LIDT of fused silica samples of about 700 J/cm2 were found. LIDT did not show a dependence on mechanical pressure and polarization state which is a consequence of the small ratio of maximum applied stress (220 MPa) to Young's Modulus of fused silica (72.5 GPa).
The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica with multiple irradiation sequences of parallel polarized Ti:sapphire femtosecond laser pulse pairs (160 fs pulse duration, 800 nm central wavelength) was studied experimentally. For that purpose, a Michelson interferometer was used to generate near-equal-energy double-pulse sequences allowing the temporal pulse delay between the parallel-polarized individual fs-laser pulses to be varied between 0 and 40 ps with ~0.2 ps temporal resolution. The surface morphologies of the irradiated surface areas were characterized by means of scanning electron and scanning force microscopy. In the sub-ps delay range a strong decrease of the LIPSS periods and the ablation crater depths with the double-pulse delay was observed indicating the importance of the laser-induced free-electron plasma in the conduction band of the solids for the formation of LIPSS.
The formation of laser-induced periodic surface structures (LIPSS) in different materials (metals, semiconductors, and dielectrics) upon irradiation with linearly polarized fs-laser pulses (τ~30–150 fs, λ~800 nm) in air environment is studied experimentally and theoretically. In metals, predominantly low-spatial-frequency-LIPSS with periods close to the laser wavelength λ are observed perpendicular to the polarization. Under specific irradiation conditions, high-spatial-frequency-LIPSS with sub-100-nm spatial periods (~λ/10) can be generated. For semiconductors, the impact of transient changes of the optical properties to the LIPSS periods is analyzed theoretically and experimentally. In dielectrics, the importance of transient excitation stages in the LIPSS formation is demonstrated experimentally using (multiple) double-fs-laser-pulse irradiation sequences. A characteristic decrease of the LIPSS periods is observed for double-pulse delays of less than 2 ps.
The surface of a titanium alloy (Ti6Al4V) implant material was covered with a bioactive calcium alkali phosphate ceramic with the aim to accelerate the healing and to form a stronger bond to living bone tissue. To fix the ceramic powder we used a femtosecond laser, which causes a thin surface melting of the metal. It is a requirement to prove that the laser irradiation would not reduce the lifetime of implants. Here we present the results of mechanical stability tests, determined by the rotating bending fatigue strength of sample rods. After describing the sample surfaces and their modifications caused by the laser treatment we give evidence for an unchanged mechanical stability. This applies not only to the ceramic fixation but also to a comparatively strong laser ablation.
The formation of laser-induced periodic surface structures (LIPSS) on two different silica polymorphs (single-crystalline synthetic quartz and commercial fused silica glass) upon irradiation in air with multiple linearly polarized single- and double-fs-laser pulse sequences (τ = 150 fs pulse duration, λ = 800nm center wavelength, temporal pulse separation Δt < 40 ps) is studied experimentally and theoretically. Two distinct types of fs-LIPSS [so-called low-spatial-frequency LIPSS (LSFL) and high-spatial-frequency LIPSS (HSFL)] with different spatial periods and orientations were identified. Their appearance was characterized with respect to the experimental parameters peak laser fluence and number of laser pulses per spot. Additionally, the 'dynamics' of the LIPSS formation was addressed in complementary double-fs-pulse experiments with varying delays, revealing a characteristic change of the LSFL periods. The experimental results are interpreted on the basis of a Sipe-Drude model considering the carrier dependence of the optical properties of fs-laser excited silica. This new approach provides an explanation of the LSFL orientation parallel to the laser beam polarisation in silica—as opposed to the behaviour of most other materials.
Hochleistungsfasern am Limit - Laserinduzierte Zerstörung als Einsatzgrenze bei Lichtwellenleitern
(2012)
Aufgrund der steigenden Anforderungen an Fasern für Hochleistungsanwendungen hat die Business Unit Fiber Optics der Leoni Gruppe gemeinsam mit der BAM Bundesanstalt für Materialforschung und -prüfung die Herausforderung angenommen, erstmalig die Zerstörschwellen eigener large core Spezialfasern zu untersuchen.