  The frequency and orbital angular momentum (OAM) are independent physical
properties of light that both offer unbounded degrees of freedom. However,
creating, processing, and detecting high-dimensional OAM states have been a
pivot and long-lasting task, as the complexity of the required optical systems
scales up drastically with the OAM dimension. On the other hand, mature
toolboxes -- such as optical frequency combs -- have been developed in the
frequency domain for parallel measurements with excellent fidelity. Here we
correlate the two dimensions into an equidistant comb structure on a photonic
chip. Dissipative optical solitons formed in a nonlinear microresonator are
emitted through the engraved angular gratings with each comb line carrying
distinct OAM. Such one-to-one correspondence between the OAM and frequencies
manifests state-of-the-art extinction ratios over 18.5 dB, enabling precision
spectroscopy of optical vortices. The demonstrated vortex soliton microcombs
provide coherent light sources that are multiplexed in the spatial and
frequency domain, having the potential to establish a new modus operandi of
high-dimensional structured light.
