This invention relates to methods and apparatus for generating an optical frequency comb. In embodiments a passive mode locked optical waveguide comb uses electrical or optical tuning for the mode spacing frequency (df) and the carrier envelope offset frequency (f ceo). We describe a passive mode locked optical comb frequency source comprising: an optical cavity having an optical driver and an optical output to provide an optical comb; an absorbing element coupled to said optical cavity for producing said optical comb; and an optically or electrically controllable element in said optical cavity; and wherein said optically or electrically controllable element has an optically or electrically controllable refractive index such that said refractive index is variable to vary one or both of a mode spacing and a carrier envelope offset frequency of said optical comb.