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qualification of fibertech fibers for high power laser beam transmission

FIBERTECH is one of Europe's leading producers of high-end silica multimode fibers and customized solutions for industrial and medical applications. Silica fibers are of crucial importance for the laser industry, where high-quality fibers are used for beam transmission of solid-state and diode laser systems.

To provide our customers with the best solution for their application it is essential to understand the damage mechanisms of the silica material.This is especially important for applications in the fields of laser technology due to the high power and energy densities of laser radiation.

Within its Research&Development activities FIBERTECH has performed extensive damage resistance tests in strong cooperation with the German Federal lnstitute for Materials Research and Testing (BAM).The work has been published [2][3][4][5][6] and the following gives abrief overview of the performed experiments and results obtained.

Experimental Set-Up
The experimental set-up for the laser-induced damage threshold (L1DT) tests is consistent with the relevant standards [1] and is shown in figure 1.

figure 1:

Scheme for the measurement of laser-induced damage thresholds (LlDT) for laser wavelengths of 532nm and 1064nm.

The beam of a q-switched Nd:YAG laser (wavelength 1064nm, pulse energy 12mJ, pulse width 15ns) was focused onto the end face of a polished fiber sampie. Laser damage was inspected in-situ with a light-optical microscope. The laser-induced damage probability was plotted in dependence of the laser fluence to determine the damage threshold as shown in figure 2. The damage threshold corresponds to the interception point of the straight regression line with the fluence axis.

To obtain the influence of the radiation wavelength the experiments were also performed using a frequencydoubled Nd:YAG laser (wavelength 532nm, pulse energy 12mJ, pulse width 8.5ns).

figure 2:

Damage probability P vs. maximum laser fluence F₀ for the laser treatment of an optical multimode fiber AS200/280 IR, T= 15ns, λ=1064nm

Results
The results for various silica/silica fibers taken from FiberTech's production are shown in figures 3a and 3b. All fibers under test are optimized for visible/infrared applications. Within the fiber code, the numbers (e.g. 200/280) indicate the diameters of the core and the c1adding.

figure 3a:

Damage threshold of optical multimode fibers for laser wavelength 1064nm.

figure 3b:

Damage threshold of optical multimode fibers for laser wavelength 532nm.

For the 1064nm radiation with pulse width 15ns the damage threshold ranges from 200J/cm² to 550J/cm², depending of the fiber diameter. At the shorter wavelength of 532nm (pulse width 8.5ns) the damage threshold is decreased, ranging from 60J/cm² to 175J/cm². The influence of the damage threshold on the fiber diameter is currently under investigation.

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