  Kilowatt-level high efficiency all-fiberized Raman fiber amplifier based on
pure passive fiber is proposed for the first time in this paper. The laser
system is established on master oscillator power amplification configuration
while a piece of graded-index passive fiber is utilized as stokes shifting as
well as gain medium, which is entirely irrelevant to rare-earth-doped gain
mechanism. When the pump power is 1368.8 W, we obtained 1002.3 W
continuous-wave laser power at 1060 nm with the corresponding
optical-to-optical efficiency of 84%. The beam parameter M2 improves from 9.17
of the pump laser to 5.11 of the signal laser through the amplification
process, and the brightness enhancement is about 2.57 at maximum output power
as a consequence of the beam clean-up process in the graded-index fiber. To the
best of our knowledge, we have demonstrated the first kilowatt-level high
efficiency Raman fiber amplifier based on pure passive fiber with brightness
enhancement.
