  Optical frequency combs (OFCs), consisting of a set of phase locked equally
spaced laser frequency lines, have enabled a great leap in precision
spectroscopy and metrology since seminal works of H\"ansch et al. . Nowadays,
OFCs are cornerstones of a wealth of further applications ranging from
chemistry and biology to astrophysics and including molecular fingerprinting
and LIDARs among others. Driven passive optical resonators constitute the ideal
platform for OFCs generation in terms of compactness and low energy footprint.
We propose here a new technique for generation of OFCs with tuneable repetition
rate in externally driven optical resonators based on the
gain-through-filtering process, a simple and elegant method, due to an
asymmetric spectral filtering on one side of the pump wave. We demonstrate a
proof-of-concept experimental result in a fibre resonator, pioneering a new
technique that does not require specific engineering of the resonator
dispersion to generate frequency agile OFCs.
