In recent years, the demand for miniaturized, integrated broadband frequency converter devices has increased. Broadband frequency conversion is helpful to realize all-optical wavelength conversion, build multi-channel wavelength division multiplexer (WDM) to meet the needs of large-capacity optical communication, improve the efficiency of entangled photon pairs in quantum communication, and provide a new method for infrared detection. The emergence of lithium niobate on-insulator (LNOI) has triggered a revolution in the field of integrated optics.

In this paper, we propose and demonstrate a broadband birefringence phase matching (BPM) second-harmonic generation (SHG) in angle-cut lithium niobate-on-insulator (LNOI) ridge waveguides based on a temperature gradient scheme, which avoids complex periodic polarization process and has flexible thermo-optic tunability. The bandwidth and shift of the phase matching spectrum can be effectively tuned by controlling the temperature gradient of the waveguide.  Besides, broadband SHG of a telecom C-band femtosecond laser is also demonstrated. The approach may open a new avenue for tunable broadband nonlinear frequency conversion in various integrated photonics platform. In summary, the temperature gradient method opens up a new way for tunable broadband nonlinear frequency conversion of various integrated photonics platforms.

FIG. 1   Angle-cut ridge waveguide structure with temperature gradient.

FIG. 2 As the temperature gradient changes, the bandwidth of the second harmonic curve increases. Simulation (left) and experiment (right).

FIG. 3 The shift of the broadband second harmonic curve with the change of temperature gradient.

FIG. 4 Broadband second harmonic based on femtosecond light in a temperature gradient waveguide.

This research is published in "Yongzhi Tang, Tingting Ding, Chuanyi Lu, Jing Qiu, Yuting Zhang, Yiwen Huang, Shijie Liu, Yuanlin Zheng, and Xianfeng Chen, Broadband second-harmonic generation in an angle-cut lithium niobate-on-insulator waveguide by temperature gradient, Optics Letters, 48(5), 1108-1111 (2023)".

Link: https://doi.org/10.1364/OL.481649