Study on writing double line waveguide in Yb³⁺: phosphate glass by femtosecond laser

By using femtosecond laser with repetition frequency of 50 kHz, center wavelength of 790 nm, and pulse width of 140 fs, double line waveguides are inscribed in Yb³⁺ doped phosphate glass. The influences of double-line separation, laser pulse energy and writing speed on the waveguide formation are investigated and the near-field modes of waveguides written by different laser parameters are measured. The experimental results show that the waveguide has good property of guiding under the writing conditions of double-line separation of 35 mm, pulse energy of 1.0 mJ and scanning speed of 600 mm/s. By using near-field mode, bivariate distribution of the waveguide refractive index is reconstructed with the maximum refractive index change of 1.5×10^-4. The propagation loss of the waveguide is 1.56 dB/cm by testing with the scattering technique. The phenomenon of polarization guiding is discovered from the waveguide. The waveguide can guide the laser with polarization parallel to the direction of double line and the laser with polarization perpendicular to the direction of double line cannot be guided.