High system efficiency nonlinear frequency conversion on thin-film lithium niobate

TL;DR

This work tackles the bottleneck of low overall efficiency in nonlinear frequency conversion on thin-film lithium niobate by addressing fiber-to-chip coupling losses. It introduces a design that combines periodically poled TFLN waveguides (QPM with $\Lambda = \frac{\lambda_{FH}}{2(n_{SH}-n_{FH})}$) and direct laser written surface couplers to realize efficient second-harmonic generation. The device achieves a fiber-to-fiber system efficiency of $\eta_{sys} = 152\%/\mathrm{W}$ (with on-chip $\eta_{SHG} = 538\%/\mathrm{W}$), representing a two-order-of-magnitude improvement over comparable works, and demonstrates robust fabrication, PFM-confirmed poling, and tunable response ($0.1\,\mathrm{nm/K}$). The approach offers a scalable, broadband path for integrated frequency converters in classical and quantum photonics, with potential further gains via local poling optimization and thermal tuning.

Abstract

Integrated photonic platforms can greatly enhance the efficiency of nonlinear frequency conversion processes by tightly confining light on a sub-micron scale. However, this advantage is often reduced by large fiber-to-chip coupling losses which drastically reduce the overall performance. Here we demonstrate a highly efficient thin-film lithium niobate frequency converter based on periodically poled waveguides combined with direct laser written out-of-plane couplers. Including on-chip and fiber-to-chip losses we obtain a conversion efficiency of 152 %/W, thus demonstrating a promising approach for future scalable integrated devices.

Figures (3)

• Figure 1: a) The orange curve shows the calculated poling period as a function of the FH wavelength. The period is designed for SHG at 1.55 µm wavelength (grey line) and the resulting momentum mismatch is presented on the right scale. b) Intensity profiles of the fundamental TE modes supported by the waveguide at the FH and SH wavelengths. c) FDTD simulations of the optical powers propagating in the laser-written fiber-to-chip couplers at the FH and SH wavelengths.
• Figure 2: a) A scheme of the final device design. A 7 mm poling region is combined with optimized couplers for the FH and SH wavelengths, respectively. Two backloops are used for alignment. b) The resulting periodic poling is investigated using a PFM. Dark grey periodic fingers on the left and right side represent the gold structures used for EFP. The light grey region around the gold is the remaining LN whereas black regions arising from the right electrodes visualize the poled region. Surrounded by fully etched trenches, the waveguide shows periodic poling close to the desired 50% duty cycle. c) A microscope image of the direct laser written surface couplers. The different lens sizes are used for optimized coupling for the different wavelengths and fiber mode field diameters.
• Figure 3: a) The resulting second harmonic efficiency is measured by applying a wavelength sweep. Several peaks are visible with the highest conversion at 1511 nm pump light. b) A camera image of the SHG light. The chip lies on top of a temperature controlled copper holder while light is coupled via a commercial fiber array from the top surface. The SHG light is clearly visible. Whereas by eye, the red light at around 755nm wavelength is directly obtained, the camera perceives it as false-color blue light. c) The SHG power in the fiber is measured as a function of pump power in the fiber. A quadratic fit is applied to the first few data points to obtain the conversion efficiency in the non depletion regime yielding 152 %/W.