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 The minimum DC current detected is The dynamical range is 89 dB with ±60.5-dB linearity. The minimum detectable AC current is with a dynamic range around 75 dB. The linearity is within ±61 dB for AC current. © 2003 Society of Photo-Optical Instrumentation Engineers.