Loss Mechanisms of Optical Waveguides Inscribed in Fused Silica by Femtosecond Laser Direct Writing

Optical waveguides directly written in fused silica using a femtosecond laser were characterized from 350 to 1750 nm to gain insight on the waveguide's loss mechanisms and their dependence on processing parameters, such as pulse energy, scan velocity, and annealing temperature. Two major loss mechanisms were identified. In the range of parameters tested, high pulse energy was seen to improve coupling losses at long wavelengths, while high scan velocity has a negative effect in both Rayleigh scattering and coupling losses at long wavelengths. Thermal annealing of the waveguides demonstrated an improvement of the Rayleigh scattering at a cost of higher coupling losses at long wavelengths. Wavelength independent Mie scattering was also observed, evolving negatively with pulse energy. A minimum Rayleigh scattering coefficient of ≈0.5 dB·cm ^-1 ·μm ⁴ (≈0.08 dB·cm ^-1 ·μm ⁴ for thermally treated waveguides) together with a Mie scattering coefficient of ≈0.2-0.65 dB/cm are reported.