Efficient second-order nonlinear process with widely-tunable pump bandwidth has always been the goal due to the extensive applications in wavelength division multiplexing networks, ultra-short pulse nonlinearity, quantum key distribution, and broadband single-photon source generation. Generally, broad nonlinear bandwidth requires the phase-matching condition to be satisfied over a wide spectral range, equivalent to the simultaneous matching of the interacting waves' group velocity and phase velocity in the time domain. We have demonstrated a new approach to achieve the QGVM SHG in the racetrack resonator and bent waveguide on X-cut TFLN. Based on the birefringence-induced mode transition of the SH light, the GVMM can flexibly change its sign from the half-circle to a straight waveguide during the light propagating for one cycle. SHG bandwidth of one and several tenfold enhancements in the intracavity and bent waveguide have been achieved, which can be applied to other parametric processes such as SFG, difference-frequency generation (DFG), and optical parametric oscillation (OPO) with the femtosecond laser pulse by further dispersion engineering and optimization of the structure.

The racetrack microring resonator, depicted in Figure 1 (a), is fabricated on X-cut TFLN. The straight path denoted as L0, runs along the Y-axis direction of the lithium niobate crystal, while the bend radius is represented as R. This structure employs the concept of spontaneous quasi-phase matching (SQPM), as proposed in our previous research. During light propagation within the cavity, opposite-symboled effective nonlinear coefficients deff are experienced in the two straight sections, and the FW and SH accumulate specific phase differences in the curved section, as illustrated in Figure 1 (b). This approach enables the realization of a quasi-phase-matching nonlinear process without the need for poling. In the curved section, due to the birefringence effect, SH undergoes mode hybridization, leading to changes in the effective refractive index and wave vector dispersion. By designing the waveguide parameters to satisfy L0 (dΔk1/dλ)λ0=-ΠR (dΔk2/dλ)λ0, it is possible to achieve broadband SQPM.

Fig. 1. (a) Schematic of the birefringent racetrack resonator on X-cut TFLN, where SH-band light experiences a mode-hybridization in the half-circle waveguide. (b) Principle of SQPM. Inset: varying SQPM SHG intensity with the periodically inverted efficient nonlinear coefficient (m=5), and a comparison among the SHG processes under the perfect phase-matching (PPM), QPM, SQPM, and phase mismatching (PMM). (c) Effective refractive indices of the hybrid mode in SH-band and TE0 mode in FW-band in the half-circle waveguide, and the vector mismatch dispersion between them. (d) Average vector mismatch dispersion versus different FW wavelengths, which is positive in the straight waveguide and negative in the half-circle waveguide.

Fig. 2. (a) Experimental setup.  EDFA: erbium-doped optical fiber amplifier. PC: polarization controller. TEC: thermal electronic cooler. WDM: wavelength division multiplexer. OSA: optical spectrum analyzer. PD: photodetector. OSC: oscilloscope.(b) Transmission spectrum of the SQPM racetrack resonator in C-band,  and (c) Lorentzian fitting of the marked resonance dip. (d) SHG intensity obtained at each FW resonance mode.

Upon comparing the previous work with the racetrack microcavity that meets SQPM conditions and partially meets SQPM but perfectly meets QGVM, as depicted in Figure 2 (d), a notable increase in bandwidth from 1nm to 13nm was observed. Furthermore, we implemented the QGVM design to bent waveguides and successfully demonstrated the SHG of femtosecond pulses.

As manufacturing technology continues to advance, process accuracy has improved, resulting in reduced loss of on-chip devices. Through careful structural and dispersion design, the scheme can achieve integrated nonlinear wideband frequency conversion in microring cavities or waveguide structures compatible with CMOS technology. This breakthrough is poised to supplant the use of KDP crystals for femtosecond laser frequency conversion, opening up exciting prospects for future chip-level quantum light sources and information processing applications.

This work is published in“Tingge Yuan, Jiangwei Wu, Xueyi Wang, Chengyu Chen, Hao Li, Bo Wang, Yuping Chen, Xianfeng Chen, "Chip-scale nonlinear bandwidth enhancement via birefringent mode hybridization," Adv. Photon. 6, 056012 (2024)”。

Link：https://www.researching.cn/articles/OJfa94a34fc77e64ef