Lithium niobate is one of the most versatile and popular photonic material. Recently, with breakthroughs in microfabrication techniques, lithium niobate-on-insulator (LNOI) has emerged as a competitive and promising candidate for integrated (nonlinear) photonics platform. Cascaded second-order nonlinear processes can induce a large effective Kerr nonlinearity, which could tremendously reduce the pump power as compared with a direct third-order process. The emergence and rapid development of the LNOI technology enable various on-chip devices with unprecedented performances. Cascaded second-order processes will inject new vitality and provide exciting possibilities in nonlinear dynamics on the LNOI platform.

We experimentally demonstrate efficient second-harmonic generation (SHG) and cascaded harmonic generation in a periodically poled lithium niobate (PPLN) ridge waveguide. When directly pumped by the mode-locked femtosecond laser, multiple colored emission is observed, which involves cascading processes. To investigate the cascaded mechanism of harmonics generation, a narrow tunable optical filter (its full width at half maximum bandwidth is 0.45 nm) is placed after the mode-locked pulse laser, to generate narrow bandwidth pulsed pump with tunable central wavelength. Efficient SHG (~778.11 nm) and FHG (~389.05 nm) are simultaneously observed when the central wavelength is turned to the QPM wavelength of 1556.22 nm. In this configuration, the FHG is created through the cascaded processes involving SHG and successive SHG. To illustrate the mechanism, we calculate and plot the phase mismatching curve for SHG considering the effective refractive index in the PPLN waveguide. It is evident that the waveguide can achieve first-order QPM for SHG at around 1556 nm with SH wavelength at 778 nm (point A). An eight-order QPM SHG process occurs at approximately 778 nm (point B), which finally leads to the consequent cascaded FHG observed in the experiment. Furthermore, the measured highest single-pass conversion efficiency is ~62% with an input pulse power of only ~580 μW in the pump-depletion region. When the input pulsed laser is scanned to the non-QPM wavelength of 1556.75 nm (still on the QPM condition but slightly away from the perfect point), efficient SH (~778.37 nm) and cascaded TH (~518.9 nm) signals are observed. The THG is achieved via cascaded SHG and sum frequency generation (SFG). To investigate the cascade SFG mechanism, the intensity profiles of the SHG and THG at the output facet are recorded. The SHG has a fundamental mode profile (bottom left), whereas the THG possesses a higher order mode (bottom right). The cascaded process results from higher-order QPM process mixing between these different modes.

The study demonstrates the mechanism of efficient SHG and cascaded harmonic generation of pulsed laser in a PPLN ridge waveguide on the LNOI platform. In addition, it also reports a ~62% high single-pass SHG conversion efficiency of pulsed laser. These results based on the LNOI waveguide may have potentials in integrated nonlinear frequency conversion process.

Fig. 1. (a) Image of the PPLN ridge waveguide when excited with 1556 nm femtosecond laser. (b) The generated cascaded harmonic light dispersed by a prism.

Fig. 2. SHG and cascaded FHG in PPLN waveguide pumped with pulsed laser at 1556.22 nm. (a) Measured spectrum of the SHG (~778.11 nm) and FHG (~389.05 nm) (not to scale). Inset: top-view of the waveguide. (b) Measured SHG conversion efficiency (red) and pump-depletion ratio (blue) as a function of input pulse power, showing 62% single-pass conversion efficiency with an input pulse power of 580 μW. (c) Measured FHG power versus the input power and the corresponding quartic fitting. (d) Calculated effective nonlinear coefficient and the SHG phase mismatching of the PPLN ridge waveguide.

This research is published by “Chuanyi Lu, Hao Li, Jing Qiu, Yuting Zhang, Shijie Liu, Yuanlin Zheng, and Xianfeng Chen, Second and cascaded harmonic generation of pulsed laser in a lithium niobate on insulator ridge waveguide, Optics Express, 30(2), 1381-1387 (2022)”。

Link：https://doi.org/10.1364/OE.447958