Measurement of the Nonlinear Ratio (n₂/A_eff) in Optical Fibers Using Self-Phase Modulation Effect

Abstract

Future optical communication systems will have to provide ultra-high speed transmission rates over long distances. An important limitation for such systems is imposed by the fiber nonlinearity typically described by the coefficient:

$$\gamma = \frac{{2\pi {n_2}}}{{\lambda {A_{eff}}}}$$

((1))

where n₂is the nonlinear refractive index of the fiber, A _eff is the fiber effective core area, and X is wavelength of the propagating signal. To design systems for such applications, it is necessary to determine the ratio n ₂/A _eff with high accuracy. The nonlinear properties of the fibers have been determined recently by measuring the self-phase modulation (SPM) effect using propagation of a short pulse at 1.3 μn through the tested fibers [1]. A different technique was employed in reference [2], where cross-phase modulation effect between a low power probe signal at 1550 nm and a high power pump signal at 1540 nm was used to find the nonlinear characteristics of the fiber. The ratio n ₂/A _eff was also measured based on the generation of beating frequency components through four-wave mixing (4WM) of two DFB laser diodes operating around 1555 nm [3]. In this scheme, two cw laser beams with frequencies v1 and v2 are launched into the fiber under test. The two beams with different power levels generate new beat frequencies (2V₁-V₂) and (2 V₂-2V₁) through 4WM process. All these schemes measure the nonlinear ratio of the fiber and an independent measurement of the effective core area enables the estimation of the nonlinear refractive index.