A magneto‐optic Faraday rotation diagnostic was implemented on aZ‐pinch driven flux‐compression generator to measure line‐averaged, megagauss, axial‐magnetic fields up to 1.6 MG with rise times of 30 kG/ns. The axial‐magnetic field rotated the plane of polarization of a 2‐W argon laser beam in a 0.725‐mm‐diam, fused silica quartz fiber mounted coaxial with theZpinch. The rapid rise time and the high radiation environment presented by theZ‐pinch plasma caused a high‐pressure impulse <100 kbar, to be coupled into the quartz‐fiber probe disrupting the polarization‐preserving properties of the fiber. The time scale for disruption was characteristic of a shock propagating radially through the fiber to its core. This paper will describe the response of the Faraday diagnostic under these conditions and present a simple model that describes the effects of fiber depolarization that is consistent with previous observations of stress‐induced depolarization in optical fibers.