OPTICAL NONLINEARITIES IN COHERENT OPTICAL-TIME DOMAIN REFLECTOMETRY ENHANCED WITH OPTICAL-FIBER AMPLIFIERS

In order to elucidate the incident pulsed power limit in coherent opti cal time domain reflectometry (OTDR) using erbium-doped optical fiber amplifiers (EDFA), experimental data on the nonlinear optical phenomen a in a single-mode optical fiber by a high-power coherent optical puls e and the resultant degradation of the coherent OTDR are presented and discussed. Previously, the stimulated Brillouin scattering and the st imulated Raman scattering have been considered as the limiting factors for the incident pulse power. However, two nonlinear optical phenomen a have been observed which occur at power levels lower than the critic al powers of the two phenomena. For a pulsewidth of 1 mu s, the intens ity gradient occurs in the incident pulse due to reduction of the inve rsion profile in the EDFA, and an optical frequency shift of the optic al pulse occurs in the test optical fiber by the self-phase modulation . As a result, the beat frequency of the backscattered light and the l ocal frequency light falls outside the receiver bandwidth of the coher ent OTDR. For a pulsewidth of 100 ns, the intensity gradient of the op tical pulse is small, as is the effect of the self-phase modulation. H owever, the incident pulse induces a 4-wave mixing with the spontaneou s emission from the EDFA and is transformed to the Stokes light and an ti-Stokes light. Hence, the backscattered light intensity within the r eceiver bandwidth is decreased, and the performance of the coherent OT DR is degraded. The foregoing results provide important guidelines for understanding the performance limitation of the coherent OTDR.