Significant improvement in optical propagation of lithium niobate written waveguides by a thermal annealing

Abstract

In this paper, we analysed, studied, and implemented an experimental procedure to improve the guiding performance at the telecommunications band of type II waveguides made in x-cut lithium niobate by femtosecond laser writing. This method relies on conducting a suitable temperature treatment on the samples. In particular, the thermal annealing reached the highest temperature of 450 °C stepped by 25 °C starting from room temperature. A discussion about the involved effects that produces the improvement of waveguides performance has been conducted in this paper. This analysis was supported by considering anisotropy expansion of the waveguiding region and by also a strong reduction of colour centres and waveguide border smoothing after thermal treatment. In addition, a better mode matching for coupling was observed. A relative enhancement from 6 dB to 10 dB (output/input power), with respect to the sample without thermal annealing, has been determined in our experiments. The deep analyses presented in this work suggest that the optical circuits fabricated by femtosecond laser writing, after the proposed thermal treatment, improve their guidance quality for photonics applications at the optical communications band where high performance waveguides systems are required.