A waveguide mode modulator based on femtosecond laser direct writing in KTN crystals

Abstract We report a novel mode modulator based on a KTN crystal waveguide produced by femtosecond laser (fs-laser) writing. The KTN waveguide is a typical dual-line structure that depends on the decrease of refractive index at the focus area. The propagation loss of the waveguide along the TE polarization is as low as ~0.9 dB/cm at 632.8 nm. The investigation of confocal micro-Raman spectra reveals that the microstructure of the waveguide region has no obvious change during the femtosecond laser direct writing (FsLDW) procedure. Under the applied direct current (DC) electric field, the waveguide modes in the guiding structure can be modulated, resulting in mode conversion from TE00 to TE02. Both the experimental results and the theoretical simulation indicate that the mode modulation phenomenon originates from the space-charge-field induced refractive-index increment between the waveguide core and the bulk substrate. The simplicity and remarkable performance of the demonstrated device paves the way for integrating low-cost and high-efficient electric-optical modulator on photonics circuits.