Lithium niobate-based integrated photonics has attracted much attention in the past decades. The emergence of lithium niobate on insulator (LNOI) in recent years has given a great impetus to the development of this field. The high second-order nonlinear coefficient of LN (d33=27 pm/V) makes its superior frequency converters. Scalable fabrication and fiber compatibility is particularly important for practical applications while ensuring high device performance. Micro-waveguides based on 3-μm thick LNOI show excellent integration potential. The UV lithography and plasma dry etching to fabricate microwaveguides features low fabrication cost and technical difficulty. Besides, lens fibers or high numerical aperture fibers can be efficiently coupled to the micro-waveguide, reducing the overall insertion loss of the device. In addition, the mode area of the micro-waveguide is several times smaller than that of conventional proton exchange or titanium diffusion waveguides, thus ensuring higher conversion efficiency.

Here, we adopt a type-0 first-order quasi-phase-matching (QPM) scheme to take advantage of the largest second-order nonlinear coefficient d33 of LN, and design the poling period at the wavelength of 1550 nm. Then, microwaveguides (cross-section 2.6×3 μm^2) is prepared on 3-μm-thick periodically poled lithium niobate on insulator (PPLNOI) by using UV photolithography and dry etching. The fiber-to-waveguide coupling loss is measured to be 1.2 dB/facet.

Fig. 1 SHG experiment using PPLNOI micro-waveguide. (a) SHG experimental setup. Inset: optical paths in waveguide, FH spots, and SH spots; (b) Quadratic relationship of SH and FH powers at low input power; (c) Relationship of SH and FH powers at high input power.

Fig. 2 SFG experiment using PPLNOI micro-waveguide. (a) Experimental setup for SFG; (b) sum frequency light power versus signal light power at 156 mW (dashed line) and 300 mW (solid line) input powers; (c) sum frequency light conversion efficiency versus pump light at 5-mW signal light power.

Test results show that the PPLNOI micro-waveguide achieves highly efficient second-harmonic generation at the communication band, with a normalized conversion efficiency of 164%W-1cm-2, and exhibits a highly efficient absolute frequency conversion of 57% under 1-W pump. The micro-waveguide also realizes highly efficient sum-frequency conversion, with a small-signal up-conversion efficiency of 139% under 300-mW pump, equivalent to up-converting about 70% of the signal photons. The excellent frequency conversion performance, scalable fabrication, and good fiber compatibility shows our PPLNOI micro-waveguides great potential to promote the development of integrated photonics research and optical quantum information applications.

On the basis of ensuring efficient frequency conversion, we have also explored in its device performance. PPLNOI micro-waveguides are packaged into optical devices, which is more conducive to practical applications. Our PPLN frequency converter has an insertion loss of 4.4 dB, and the overall device achieves an absolute conversion efficiency of 30% under 1-W pump, and maintains good stability under long-time operation. The fluctuation error of fundamental and second harmonic output within one hour is less than 3%, and that of SHG conversion efficiency is less than 0.3%.

Fig. 3 Packaged frequency converter based on PPLNOI micro-waveguide

Fig. 4 Stability test of SHG in PPLNOI micro-waveguide. (a) SH and FH output powers of device for one hour at 500-mW input; (b) stability test of SHG conversion efficiency at 1-W input.

Our micro-waveguides have good compatibility with optical fibers, low insertion loss, and good overall performance, which not only take into account the normalization efficiency, coupling efficiency, and device length, but also achieve efficient absolute frequency conversion at high power input. The packaged device maintains efficient and stable frequency conversion even under watt-level power, which reflects the great potential of LNOI micro-waveguide, which is expected to develop more multi-functional devices in the future and provides a better platform for the development of other fields of nonlinear optics.

This work is published at “Wenjun Ding, Yuting Zhang, Jing Qiu, Yongzhi Tang, Jing Zhang, Tingting Ding, Hao Li, Shijie Liu, Yuanlin Zheng, Xianfeng Chen. Efficient Nonlinear Frequency Conversion in Microscale Thin-Film Lithium Niobate Ridge Waveguides (Invited)[J]. Acta Optica Sinica, 2024, 44(15): 1513006.”.

Link：https://www.opticsjournal.net/Articles/OJec0d23e29e7c682e/FullText