Visible or short wavelength lasers have a wide range of applications in atomic, molecular and optical physics. Efficient narrow-linewidth green light generation is highly desired for many applications. However, the power from direct laser emission at short wavelengths is somehow limited. Frequency conversion is an appealing and feasible approach to solve this problem. Here, we report on a noncritical type-I BPM in the MgO:LNOI micro-waveguide for efficient green light emission, which exhibits a conversion efficiency of 37%/Wcm2 for the 2-cm-long waveguide. The micro-waveguide also shows good stability against the photorefractive effect. The scalable and fiber-compatible feature would make the LNOI micro-waveguide appealing for realistic applications.

The noncritical BPM for frequency conversion of fundamental harmonic (FH) to its second harmonic (SH) in the LNOI ridge micro-waveguide is schematically depicted in Fig. 1(a). The FH and SH propagate collinearly at the noncritical BPM angle of θ = 90◦ The spatial walk-off effect is not present under this condition. The LNOI micro-waveguide is fabricated using ultraviolet (UV) lithography and deep inductively coupled plasma (ICP) etching. In this work, we further increase the etching depth. The top LNOI layer is deeply etched to an averaged etching depth of 2.8 µm, and the etching sidewall angle is 65◦. The ridge micro-waveguide has a top width of 3.6 µm and a length of 2 cm. Figure 1(b) shows the optical microscopic image of the fabricated micro-waveguide array sample, as well as the cross section of the ridge micro-waveguide for the SHG process. Figure 1(c) presents the simulated effective refractive indices of the TE00 mode at 1064 nm (i.e., FH) and the TM00 mode at 532 nm (i.e., SH) according to the micro-waveguide dimensions, showing that birefringent phase matching is achieved near the wavelength of 1064 nm.

Fig. 1. (a) Illustration of noncritical BPM SHG in a LNOI waveguide. (b) Microscopic images of the LNOI micro-waveguide array and the waveguide end facet. (c) Evolution of the effective indices of the FH (TE00) and SH (TM00) guided modes at different temperatures (20, 21, 22, and 23◦C), showing BPM wavelength at around 1064 nm in the waveguide. The BPM wavelength redshifts as the temperature increases. Inset: simulated electric field distribution of the FH and SH modes.

The propagation loss is evaluated using the Fabry–Perot (F–P) interference method. The value of the propagation loss is calculated to be 0.3 dB/cm and 0.41 dB/cm for TE and TM modes, respectively. The experimental setup for the single-pass frequency-doubling in the micro-waveguide is shown in Fig. 2(a). Figure 2(a) also shows the experimentally observed bright green emission. The FH and SH spectrum is shown in Fig. 2(b). The fundamental mode profiles of the FH and SH waves are confirmed by observing the output light spots in the far field, as shown in the inset of Fig. 2(b). Figure 2(c) presents the experimental normalized conversion efficiency is measured to be 37%/Wcm2 in the small-signal approximation condition. Considering the coupling loss, the on-chip conversion efficiency is estimated to be 53%/Wcm2. For higher input powers with pump depletion, an absolute conversion efficiency of 12.6% is obtained with 500-mW input. Our BPM micro-waveguide well balanced the fiber coupling and nonlinear conversion efficiency, the performance of our device is on par with some previously reported LNOI ridge waveguides or TFLN nano-waveguides, whose length or coupling is limited. Finally, we investigate the long-term stability of the frequency converter against the photorefractive effect. The photorefractive effect can be well mitigated via MgO doping. We monitor the SHG stability over a time period of 2 h, the 2% variation of SH dominantly arises from the pump fluctuation (1% instability). The device shows both relatively high efficiency and good stability against the photorefractive effect, offering a potential solution for efficient and scalable green light sources and frequency converters.

Fig. 2. (a) Experimental setup for SHG in the MgO:LNOI micro-waveguide. (b) Spectrum of the FH and SH waves. Inset: FH and HS output spots. (c) Quadratic relationship between the FH and SH power, which starts to deviate at high input power as the pump is partially depleted.

This work is published at “Tingting Ding, Yongzhi Tang, Hao Li, Shijie Liu, Jing Zhang, Yuanlin Zheng, and Xianfeng Chen, "Noncritical birefringence phase-matched second harmonic generation in a lithium-niobate-on-insulator micro-waveguide for green light emission," Opt. Lett. 49, 1121-1124 (2024)”.

Link: https://doi.org/10.1364/OL.519484