Measurement of real-time gain gratings in erbium-doped fiber

Experimental and theoretical results for nonlinear wave mixing in a pumped erbium-doped fiber are presented where a fast phase transient is applied to one of the beams. The ratio of the imaginary part to the real part of the complex nonlinear refractive index is determined from the initial phase of the beam coupling. The fast phase transient is applied for the first time to measure the response time of index gratings in a nonlinear device. We show that the grating lifetime depends on both the pump and amplified signal beams and that the nonlinear index change occurs with a time constant of the order of 100 /spl mu/s for CW conditions in a highly doped fiber.