Adiabatic frequency conversion in a lithium niobate microresonator for FMCW-LiDAR

Adiabatic Frequency Conversion (AFC) in microresonators is independent of the intensity of laser light, and it is free from phase matching restrictions. It even allows for changing the frequency of single photons. The AFC has been experimentally investigated for more than a decade in different configurations. However, despite of some impressive proof-of-concept demonstrations, adiabatic frequency converters remain relatively obscure compared to their nonlinear-optical counterparts. So far, adiabatic frequency converters for applications were a curiosity due to lack of experimental investigation. We demonstrate linear frequency chirps between 0 and 690 MHz around 1.5 μm wavelength generated by electro-optically driven AFC. This is achieved by applying linear voltage chirps between 0 and 20 V on a 300-μm-thick whispering gallery resonator made of lithium niobate with 1.2 mm major radius. The frequency chirps have less than 2 % rms deviation from perfect linearity at chirp times as low as 130 ns. We apply these chirps for frequency-modulated continuous-wave LiDAR as a proof for a practical application of AFC. We successfully determine distances between 0 and 6 m. The distance resolution is of the order of 10 cm and just limited by the chirp bandwidth. Our proof-of-concept demonstration shows that laser light from adiabatic frequency converters is useful for applications. The performance of the AFC-based FMCW LiDAR system can be improved by integrating the resonator on a chip since the same voltage provides higher electric fields. Here, electro-optically-induced frequency shifts of 10 GHz and more have already been demonstrated. This would increase the distance resolution by more than one order of magnitude.