RAMAN-INDUCED REDSHIFT OF ULTRASHORT MODE-LOCKED LASER-PULSES

A resonant Raman line is shown to cause a redshift of the pulse spectr um within every pass through the resonator of a mode-locked laser. The filtering action of the mirror reflectivity or any other action that confines the spectrum counterbalances the shift. The net effect is tha t the center frequency of the pulse spectrum deviates from the center of the filter response. The net shift caused by a set of lines is the sum of the individual shifts. The Raman shift leads to a net gain redu ction and thus to a limit on the achievable pulse width. The process i s similar to yet different from the well-known soliton carrier frequen cy shift that is due to the delayed Kerr effect in fibers. (C) 1998 Op tical Society of America.