In the UCLA plasma beat wave accelerator, a high intensity two frequency CO2 laser (λ1=10.6 μm, λ2=10.3 μm) is used to drive a large amplitude relativistic plasma wave. The plasma wave acts as a moving phase grating and scatters the incident pump waves into Stokes and anti-Stokes sidebands (ω1-ωp, ω2+ωp). The observation of these sidebands in the forward direction confirms the presence of the relativistic plasmon, and also gives an estimate of the amplitude–length product (n1/n0×L) of the wave. Since the Stokes and anti-Stokes signals are picosecond pulses at 10.9 and 10.0 μm, respectively, this light cannot be time resolved directly on a conventional detector or streak camera. The forward scattered light can be analyzed, however, by mixing the 10 μm light with visible light from a laser diode (670 nm) in a nonlinear crystal (AgGaS2) to produce frequency shifted light at 630 nm. The intensity of the 630 nm light is proportional to the product of the intensities of the two incident laser pulses, and can be time resolved on a streak camera. Experimental results for the plasma wave amplitude, spatial length, and temporal length are shown. © 1997 American Institute of Physics.