Dual chirped optical pulses from a phase-modulated laser

Dual chirped optical pulse is reported by use of a phase-modulated laser. When a number of sidebands around the carrier frequency in a phase-modulated laser spectrum are spectrally carved, short optical pulse trains with dual chirp states are obtained. It is discussed that the pulse duration and the linear frequency chirp parameter can be found in closed form expressions given as a function of the modulation index and the modulation frequency when the phase modulation function of the dual-chirped pulse trains does not deviate severely from the modulation function applied to the phase modulator by spectral filtering effects. In experiments, optical pulse trains at 5 GHz repetition rate were generated with the pulse duration of 29.4 ps when frequency-modulated fiber laser signals with a 2.5 GHz repetition rate passed through a Gaussian-shaped optical band-pass filter (OBF) with a full width at half maximum (FWHM) 0.3 nm and a transmission peak at the carrier frequency of the frequency-modulated laser spectrum. Measured pulse duration was in good agreement with the analytical prediction value 28.2 ps and the numerical result 27.7 ps.