  Laguerre-Gaussian (LG) modes, carrying orbital angular momentum of light, are
critical for important applications such as high-capacity optical
communications, super-resolution imaging, and multi-dimensional quantum
entanglement. Advanced developments in these applications strongly demand
reliable and tunable LG mode laser sources, which, however, do not yet exist.
Here, we experimentally demonstrate highly-efficient, highly-pure,
broadly-tunable, and topological-charge-controllable LG modes from a Janus
optical parametric oscillator (OPO). Janus OPO featuring two-face cavity mode
is designed to guarantee an efficient evolution from a Gaussian-shaped
fundamental pumping mode to a desired LG parametric mode. The output LG mode
has a tunable wavelength between 1.5 um and 1.6 um with a conversion efficiency
above 15%, a topological charge switchable from -4 to 4, and a mode purity as
high as 97%, which provides a high-performance solid-state light source for
high-end demands in multi-dimensional multiplexing/demultiplexing, control of
spin-orbital coupling between light and atoms, and so on.
