Direct and accurate measurement of self-phase shift due to fiber nonlinearity

Summary form only given. When propagating through an optical fiber, a pulse accumulates a nonlinear phase shift. If the pulse is a soliton this nonlinear phase is time-independent across the pulse profile. However even for a nonsoliton pulse a time-independent DC component of this phase shift exists and can be computed via a Taylor expansion. We use spectral interferometry (SI) to directly measure this DC phase by measuring the shift of the reference-signal interference fringes. However, if the fiber forms one arm of the Mach-Zehnder interferometer the fluctuations of the length and the refractive index of the fiber shifts the signal phase randomly. To surmount this obstacle we propagate both reference and signal through the fiber and attenuate the reference so that it only experiences linear effects in the fiber.