Abstract

A train of closely spaced picosecond pulses of moderate intensity can be used for Raman amplification to avoid high intensity effects in the medium. This technique is possible since the phonons created in the material by one pulse do not decay significantly within the dephasing time of the material. As a result each subsequent pulse experiences enhanced gain starting from the phonons left behind by the earlier pulses. We have studied experimentally the Raman amplification of four pulses, each of 30-ps duration and separated by 200 ps, in H₂ gas at 600 psi (T₂ ≈ 200 ps). A pump pulse at 532 nm was divided into four equal energy pulses and was used to generate seed Stokes pulses as well as to pump the Raman amplifier. Each seed Stokes pulse after the first was observed to have a higher gain than isolated pulses of the same energy and pump ^depletion was easily achieved on the last pulse. We have verified the experimental results theoretically and have studied the behavior of longer pulse trains. Other theoretical predictions include modulation of the amplified pulse train due to purely transient effects and the possibility of observing a quasi-steady-state regime in which the amplified pulses reach a constant amplitude.

© 1986 Optical Society of America