Optical frequency combs (OFCs) consist of an equidistant array of discrete frequency lines: they are like a ruler of light. The separation between the frequency lines, Δƒ, lies in the radio-frequencies (where standard atomic clocks operate) while the absolute frequencies, ƒ_n, lie in the visible or infra-red spectral region. This characteristic has allowed the most accurate time-frequency measurements so far. While OFCs can be generated using diverse mechanisms, micro-ring-resonators offer a compact architecture to generate coherent optical frequency combs. Furthermore, ring resonators are fabricated using the same advanced micro-electronic technology used to fabricate computer chips.