  Continuous tuning of the frequency of laser light is the cornerstone for a
plethora of applications in basic science as well as in the industrial
environment. They range from the hunt for gravitational waves, the realization
of optical clocks over health and environmental monitoring to distance
measurements. So far, it is difficult to combine a wide tuning range (> 100
GHz) with sub-microsecond tuning times, intrinsic tuning linearity and
coherence lengths beyond 10 m. We show that electro-optically driven adiabatic
frequency converters using high Q microresonators made of lithium niobate are
able to transfer arbitrary voltage signals into frequency chirps on time scales
far below 1 {\mu}s. The temporal variation of the frequency nicely agrees with
the one of the voltage applied. Superimposing the converted light with the
unconverted one, we derived from the beat signal that 200-ns-long linear
frequency chirps deviate less than 1 % from perfectness without any additional
measures. The Coefficient of Determination is R2 > 0.999. The coherence length
of the emitted light is of the order of 100 m. In order to prove these
findings, we apply linear frequency sweeps for FMCW LiDAR for distances between
0.5 and 10 m without any signs of nonlinearity. Combined with the demonstrated
ns tuning, with the potential to tune the eigenfrequency of
lithiumniobate-based resonators by several 100 GHz, our results show that
electro-optically driven adiabatic frequency converters can be used in
applications that require ultrafast and flexible continuous frequency tuning
with intrinsic linearity and large coherence length.
