  Tunable light sources are a key enabling technology for many applications
such as ranging, spectroscopy, optical coherence tomography, digital imaging
and interferometry. For miniaturized laser devices, whispering gallery
resonator lasers are a well-suited platform, offering low thresholds and small
linewidths, however, many realizations suffer from the lack of reliable tuning.
Rare-earth ion-doped lithium niobate offers a way to solve this issue. Here we
present a single-frequency laser based on a neodymium-doped lithium niobate
whispering gallery mode resonator that is tuned via the linear electro-optic
effect. Using a special geometry, we suppress higher-order transverse modes and
hence ensure single-mode operation. With an applied voltage of just $68\,$V, we
achieve a tuning range of $3.5\,$GHz. The lasing frequency can also be
modulated with a triangular control signal. The freely running system provides
a frequency and power stability of better than $\Delta\nu =20\,$MHz and 6%,
respectively, for a 30 minute period. This concept is suitable for full
integration with existing photonic platforms based on lithium niobate.
