Multiphoton absorption and damage threshold of Yb3+ and Tm3+ doped SiO2 fiber core

Using the Yb³⁺ and Tm³⁺ doped SiO₂ fiber rods as the samples and ultrafast 800 nm femtosecond (fs) laser sources, the multiphoton absorption characteristics and damage threshold are experimental investigated based on the open Z-scan and nonlinear transmission measurement technology. The five-photon absorption coefficient of the Yb³⁺ doped fiber core has nearly tenfold increment than that of the fiber clad. But the five-photon absorption coefficient of the Tm³⁺-dopd fiber core is less than 5% of the fiber clad. With the incident fs laser power increasing, the five-photon absorption coefficient of three different samples is decreased. Further analyzing the open Z-scan results, the multiphoton absorption should contain the six-photon absorption process. Using the nonlinear transmission measurement technology, the damage thresholds are measured, and they are 12.1 TW/ cm² (the Yb³⁺-doped fiber core) and 16.4 TW/cm² (fiber clad).