Mode-locked and noise phenomena of hybrid soliton pulse source

The intensity noise and mode-locking phenomena of a hybrid soliton pulse source (HSPS) utilising different fibre Bragg gratings (FBGs) with or without linear chirp are described. The HSPS is modelled by a time-domain solution of the coupled-mode equations including spontaneous emission noise, and relative intensity noise (RIN) is calculated using numerical solutions of these equations. It is found that a noise peak in the RIN spectrum does not occur at the inverse of the round-trip time in the compound cavity for unchirped FBGs not giving transform-limited pulses around the fundamental mode-locking frequency. A noise peak occurs at the inverse of the round-trip time in the compound cavity for chirped FBGs generating transform-limited pulses over a wide tuning range around the fundamental mode-locking frequency. It is also found that the proper mode-locking range over which transform-limited pulses are obtained reduces for chirped FBGs with increasing noise, whereas generated transform-limited pulses for unchirped FBGs do not depend on the magnitude of noise and these FBGs can give transform-limited pulses only over a limited tuning range with low and high noise.