Wuu-Wen Lin of Yung Ta Institute of Technology & Commerce in Linluo, Taiwan, has developed a fiber optic sensor that can measure submicroampere currents. A report of the device appeared in the April issue of Optical Engineering.The current to be measured passes through a gold coating deposited on the outside of the optical fiber, and the resulting ohmic heat induces a phase shift of light traveling through the fiber. In a series of experiments, the sensor measured direct current as low as 0.34 µA and alternating current as low as 0.42 µA.Previous investigations into this concept encountered difficulty in the submicroampere regime because the optical phase shift varies as the square of the current. Measuring the phase shift with a Michelson interferometer and heating both of the instrument's arms avoided that problem. A steady phase shift then was induced in one arm by applying piezoelectric pressure to the fiber, and the signal from the interferometer was demodulated in synchronization with the piezoelectric signal.