Abstract

In a first Stokes Raman amplifier cell, a pump beam at frequency ω₀ amplifies a seed beam at frequency ω₁ = ω₀ − ω_R, where ω_r is the Raman shift characteristic of the amplifier medium. Conversion efficiencies approaching the quantum limit are possible if competing processes are adequately suppressed. One important parasitic process is generation of a beam at the second Stokes frequency ω₂ = ω₀ − 2ω_r. Since ω₂ = ω₁ − ω_r, the first Stokes beam may amplify this second Stokes beam until serious depletion of first Stokes results. One possible ω₂ generation mechanism is a nonlinear four-wave mixing process with a wave vector mismatch If the reciprocal of Δk is small relative to an e-fold gain length, ω₂ generation may be sufficiently suppressed to allow high conversion efficiency to ω₁ for some range of amplifier cell lengths.

© 1986 Optical Society of America