When a current passes through a metal-coated optical fiber, the Ohm's heat induces a phase change of the guided light within the fiber. This microampere current sensor is principally designed based on this thermal effect. A Michelson interferometer is used for detecting this phase change. By means of our sensor, the minimum detectable phase signal is 1.6443×1025 rad/Hz.1/2 The minimum DC current detected is 3.423×1027 A. The dynamical range is 89 dB with ∓60.5-dB linearity. The minimum detectable AC current is 4.193×1027 A with a dynamic range around 75 dB. The linearity is within ∓61 dB for AC current.