  Recent advancements in thin-film lithium niobate (TFLN) photonics have led to
a new generation of high-performance electro-optic devices, including
modulators, frequency combs, and microwave-to-optical transducers. However, the
broader adoption of TFLN-based devices that rely on all-optical nonlinearities
have been limited by the sensitivity of quasi-phase matching (QPM), realized
via ferroelectric poling, to fabrication tolerances. Here, we propose a
scalable fabrication process aimed at improving the wavelength-accuracy of
optical frequency mixers in TFLN. In contrast to the conventional
pole-before-etch approach, we first define the waveguide in TFLN and then
perform ferroelectric poling. This sequence allows for precise metrology before
and after waveguide definition to fully capture the geometry imperfections.
Systematic errors can also be calibrated by measuring a subset of devices to
fine-tune the QPM design for remaining devices on the wafer. Using this method,
we fabricated a large number of second harmonic generation devices aimed at
generating 737 nm light, with 73% operating within 5 nm of the target
wavelength. Furthermore, we also demonstrate thermo-optic tuning and trimming
of the devices via cladding deposition, with the former bringing ~96% of tested
devices to the target wavelength. Our technique enables the rapid growth of
integrated quantum frequency converters, photon pair sources, and optical
parametric amplifiers, thus facilitating the integration of TFLN-based
nonlinear frequency mixers into more complex and functional photonic systems.
