Fabrication of magneto-optical glass waveguides by femtosecond laser direct writing

In this work, we investigate the waveguide fabrication and the mechanism of femtosecond laser direct writing in borosilicate aluminum magneto-optical glass. We compare the characteristics of the magneto-optical waveguides fabricated in two kinds of glasses with different concentrations of Tb3+ ions, i. e., TG20 and TG28 glasses, and systematically study the dependence of the guiding modes and Verdet constant of waveguides on femtosecond laser pulses' polarization and energy. The results show that the circularly polarized femtosecond pulses have lower fabrication threshold energy and can introduce larger increase in the refractive index change inside magneto-optical glass, compared with the linearly polarized pulses. It confirms that the refractive index change in borosilicate aluminum glass mainly comes from the 3- or 4-photon absorption of ionic bonding. The magneto-optical coefficient measurement results prove that in TG28 glass, the power and polarization state of the incident pulses have small influence on Verdet constant of waveguides. The decrease in Verdet constant of magneto-optical waveguide is less than 15% of the value in bulk TG28 glass, whereas the decrease can reach 70% in TG20 glass. Therefore, TG28 glass is more appropriate to fabricate magneto-optical microstructures by using femtosecond laser direct writing than TG20 glass. By careful selection of the dopant concentration in the magneto-optical glass the optical microstructures with high magneto-optical coefficient and high refractive index modulation may be fabricated. ©, 2015, Board of Optronics Lasers. All right reserved.