Various apparatus and methods for generating a 90 AP optical pulsetrain. In one embodiment, the apparatus includes: (1) a phase-modulated optical signal generator configured to generate a phase-modulated optical signal having a carrier band and sidebands phase-shifted relative thereto by multiples of 90 and (2) an optical filter coupled to the phase-modulated optical signal generator and configured to 180 phase-shift selected ones of the carrier band and sidebands to generate the 90 AP optical pulsetrain.