A compact double-pass Raman backscattering amplifier/compressor

The enhancement of stimulated Raman backscattering (SRBS) amplification was demonstrated by introducing a plasma density gradient along the pump and the seed interaction path and by a novel double-pass design. The energy transfer efficiency was significantly improved to a level of 6.4%. The seed pulse was amplified by a factor of more than 20 000 from the input in a 2 mm long plasma, which also exceeded the intensity of the pump pulse by 2 orders of magnitude. This was accompanied by very effective pulse compression, from 500 fs to 90 fs in the first pass measurements and in the second pass down to approximately 50 fs, as it is indicated by the energy-pulse duration relation. Further improvements to the energy transfer efficiency and the SRBS performance by extending the region of resonance is also discussed where a uniform ∼4 mm long plasma channel for SRBS was generated by using two subsequent laser pulses in an ethane gas jet.