Low-frequency Raman gain measurements using chirped pulse two-beam coupling

The limit in measuring the Raman gain at small frequency detunings using traditional techniques is determined by the requirement that the DC pump beam must be removed from the detection system. Low-frequency Raman gain measurements have been reported down to 6 cm^-1 in glass fibers with interaction lengths of several hundred meters. Obviously this method is not applicable to other materials. Here we propose a novel method for measuring the Raman gain at low frequency mm sample lengths based on two beam coupling of chirped pulses. The frequency difference between the pump and probe beams in a pump-probe experiment thus depends on the time delay between the pulses, which in turn determines the two beam coupling. The experiment essentially consists of a noncollinear pump-probe system. The two-beam coupling between the pump and probe beams is due to a frequency difference between the beams