  High speed optical telecommunication is enabled by wavelength division
multiplexing, whereby hundreds of individually stabilized lasers encode the
information within a single mode optical fiber. In the seek for larger
bandwidth the optical power sent into the fiber is limited by optical
non-linearities within the fiber and energy consumption of the light sources
starts to become a significant cost factor. Optical frequency combs have been
suggested to remedy this problem by generating multiple laser lines within a
monolithic device, their current stability and coherence lets them operate only
in small parameter ranges. Here we show that a broadband frequency comb
realized through the electro-optic effect within a high quality whispering
gallery mode resonator can operate at low microwave and optical powers.
Contrary to the usual third order Kerr non-linear optical frequency combs we
rely on the second order non-linear effect which is much more efficient. Our
result uses a fixed microwave signal which is mixed with an optical pump signal
to generate a coherent frequency comb with a precisely determined carrier
separation. The resonant enhancement enables us to operate with microwave
powers three order magnitude smaller than in commercially available devices. We
can expect the implementation into the next generation long distance
telecommunication which relies on coherent emission and detection schemes to
allow for operation with higher optical powers and at reduced cost.
