Dual-line optical waveguides in Cu:KNSBN crystal fabricated by direct femtosecond laser writing

We report on dual-line waveguides fabricated by direct femtosecond laser writing in Cu:KNSBN crystal. Two different sizes have been designed with the separation between lines of 20 and 30 μm, respectively. The detailed structure of the dual-line waveguide has been imaged by means of micro-Raman analysis, indicating that the microstructure of the Cu:KNSBN crystal has no significant change after direct femtosecond laser writing. The dual-line waveguides support single-mode guidance along both transverse electric and transverse magnetic polarization at the wavelengths of 632.8 and 1064 nm, and show insensitivity to polarization of light. We suggest the potential application of the laser-written Cu:KNSBN waveguides as new integrated optical devices.