Peculiarities of the single and repetitive pulse threshold pump power for stimulated Brillouin scattering in long optical fibres

It is shown that in the undepleted pulsed pump regime of stimulated Brillouin scattering (SBS) the duration of a Stokes pulse in long optical fibres increases by the doubled time of flight for a pump pulse through a fibre. A consequence of this is up to three times difference between the SBS threshold power in a single pump pulse regime when the instant power or energy of the scattered radiation is measured. A distinguishing peculiarity of a repetitive pump pulse regime is a stepwise reduction of the threshold power with the decrease of an interpulse interval when the interval equals the doubled time of flight for a pump pulse through a fibre.