Apparatus, systems, and methods having a frequency comb of large spacing can be used in a variety of applications. In various embodiments, a frequency comb is generated from a slave laser by injecting an optical output from a drive laser into the slave laser. One or more parameters of the drive laser and/or the slave laser can be adjusted such that a frequency comb can be generated at a multiple of the repetition rate of the drive laser. Additional apparatus, systems, and methods are disclosed.