Raman-induced redshift of ultrashort mode-locked laser pulses

A resonant Raman line is shown to cause a redshift of the pulse spectrum 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 that 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 reduction and thus to a limit on the achievable pulse width. The process is similar to yet different from the well-known soliton carrier frequency shift that is due to the delayed Kerr effect in fibers. (C) 1998 Optical Society of America.