an optical time - division multiplexer branches input light into a plurality of optical paths with different optical lengths . at least one path includes a section of optical fiber with an optical length that is adjustable by , for example , physical stretching . the input light is separately modulated on each optical path , and the modulated light signals are recombined into a multiplexed output light signal . adjustment of the optical length of the optical fiber can compensate for changes in input wavelength and adjust the phase of successive pulses in the multiplexed output signal to provide a π - radian shift that enables long - haul optical transmission .