Directional couplers in LiNbO3 crystal based on a combination of standard lithography and ion implantation

The directional couplers with a channel waveguide configuration were obtained using standard lithography and ion implantation as a fabricating strategy. Two implantation strategies, single and multi-energy ion implantations, were utilized to form channel waveguide structures with photoresist masks. Effective coupling was observed between the waveguide pairs for the trans-verse magnetic polarization. The couplers' optical propagation processes, the coupling lengths, and the near-field intensity profiles were simulated using the finite-difference beam propagation method. In addition, the measured splitting ratios at the wavelength of 633 nm were 60%: 40% for the single energy ion-implanted device and 70%: 30% for the coupler formed by multi-energy ion implantation, with the transmitted port having a higher percentage. We reveal that for the directional couplers formed by ion implantation, when interaction length and distance between waveguide pairs are fixed, the main reason affecting the coupling coefficient is the overlapping degree of the mode fields caused by the refractive index difference.