Characterization of heterogeneously integrated periodically polled lithium niobate using optical frequency domain reflectometry

Photonic integrated periodically poled lithium niobate (PPLN) has emerged as one of the most promising photonic devices for both classical and quantum applications, thanks to its efficient nonlinear optical processing capabilities. However, the characterization of such integrated device has been barely studied, particularly regarding adverse reflection properties arising from coupling facets. In this paper, we present the successful integration of a wavelength conversion module using a PPLN on a thin film lithium niobate by integrating two different platform technologies: silicon nitride and lithium niobate. The module features 5 mm-long periodically poled waveguides on lithium niobate-on-insulator, designed for wavelength up-conversion from 1560 nm to 780 nm via second harmonic generation. The PPLN waveguides are packaged with silicon nitride interposers for edge coupling mode size conversion to an optical fiber array. To characterize reflection profiles along the fabricated device, we employ an optical frequency-domain reflectometry (OFDR), operating at 1560 nm with a spatial resolution of 1.5 mm in air at 100 Hz measurement rate. The sensing system enables detailed analysis of reflections potentially caused by refractive index mismatching at the coupling facets and by micro-structured periodic domain inversion within the PPLN. Our experimental results clearly reveal three strong reflections along the device structure, corresponding to the optical fiber array, the silicon nitride interposer, and the PPLN. The reflectivity at each facet is measured to be -27.8 dB, -31.7 dB, -21.4 dB, respectively, demonstrating the ability of OFDR for the characterization of the photonic integrated device packaging.