A seed beam source for a fiber amplifier system, said seed beam source being configured to produce a seed beam, said sead beam being suitable for having reduced thermal instability when propagating in an optical fiber having a fiber core, said thermal instability being caused by coupling of seed beam light propagating through said optical fiber into higher order modes as a result of moving long period granting, LPG, said seed beam source comprising: a plurality of M continuous wave master oscillator lasers, each being configured to generate a laser beam at a different wavelength; a plurality of M switching modulators each being configured to receive the laser beam from a particular one of the master oscillator lasers, each switching modulator being controlled so as to output its laser beam as optical pulses based on a predetermined timing sequence, wherein the plurality of M switching modulators are also controlled so that the optical pulses have a pulse width that is less than or equal to 1/M times a thermal diffusion time across the fiber core so as to provide the reduced thermal instability; an optical coupler configured to receive the optical pulses from the plurality of switching modulators, where the plurality of switching modulators are configured so that where the optical coupler only receives one of the optical pulses from the switching modulators at any particular point in time, and where the optical coupler continuously receives the optical pulses from the plurality of switching modulators and outputs an interleaved continuous optical seed beam including the optical pulses from all of the switching modulators; and an electrical pulse driver configured to selectively provide an electrical signal to the plurality of switching modulators so as to cause the switching modulators to only output one of the optical pulses at the particular point in time. a plurality of M continuous wave master oscillator lasers, each being configured to generate a laser beam at a different wavelength; a plurality of M switching modulators each being configured to receive the laser beam from a particular one of the master oscillator lasers, each switching modulator being controlled so as to output its laser beam as optical pulses based on a predetermined timing sequence, wherein the plurality of M switching modulators are also controlled so that the optical pulses have a pulse width that is less than or equal to 1/M times a thermal diffusion time across the fiber core so as to provide the reduced thermal instability; an optical coupler configured to receive the optical pulses from the plurality of switching modulators, where the plurality of switching modulators are configured so that where the optical coupler only receives one of the optical pulses from the switching modulators at any particular point in time, and where the optical coupler continuously receives the optical pulses from the plurality of switching modulators and outputs an interleaved continuous optical seed beam including the optical pulses from all of the switching modulators; and an electrical pulse driver configured to selectively provide an electrical signal to the plurality of switching modulators so as to cause the switching modulators to only output one of the optical pulses at the particular point in time.