Pulse-timing noise reduction of a mode-locked laser diode by incoherent addition

This paper describes the operation principle and the experimental results of a new and simple technique to reduce pulse-timing noise of a mode-locked laser diode using fiber-optic interferometers placed outside the laser cavity. When the delay times of the interferometers are set to be much larger than the laser's coherence time and to be integer multiples of the pulse repetition period, incoherent addition of pulse intensities at the output of the interferometer gives rise to the cancellation of pulse-timing noise at specific Fourier frequencies. The technique does not need electrical or optical actions for the laser diode and is realizable only with passive optical components. Experiments are carried out using a 19.444-GHz hybrid mode-locked laser diode. After passing through the combined ring and Mach-Zehnder interferometers the root-mean-square timing jitter is reduced to 0.489 ps (100 kHz-18 MHz) that is 29% of the direct laser output.