Optical Nonlinearities in Coherent Optical Time Domain Reflectometry Enhanced with Optical Fiber Amplifiers

Abstract

AbstractIn order to elucidate the incident pulsed power limit in coherent optical time domain reflectometry (OTDR) using erbium‐doped optical fiber amplifiers (EDFA), experimental data on the nonlinear optical phenomena in a single‐mode optical fiber by a high‐power coherent optical pulse and the resultant degradation of the coherent OTDR are presented and discussed. Previously, the stimulated Brillouin scattering and the stimulated Raman scattering have been considered as the limiting factors for the incident pulse power. However, two nonlinear optical phenomena have been observed which occur at power levels lower than the critical powers of the two phenomena.For a pulsewidth of 1 μs, the intensity gradient occurs in the incident pulse due to reduction of the inversion profile in the EDFA, and an optical frequency shift of the optical pulse occurs in the test optical fiber by the self‐phase modulation. As a result, the beat frequency of the back‐scattered light and the local frequency light falls outside the receiver bandwidth of the coherent OTDR.For a pulsewidth of 100 ns, the intensity gradient of the optical pulse is small, as is the effect of the self‐phase modulation. However, the incident pulse induces a 4‐wave mixing with the spontaneous emission from the EDFA and is transformed to the Stokes light and anti‐Stokes light. Hence, the backscattered light intensity within the receiver bandwidth is decreased, and the performance of the coherent OTDR is degraded. The foregoing results provide important guidelines for understanding the performance limitation of the coherent OTDR.