Abstruct- The sensitivity of an electromagnetic field sensor which uses a LiNbO3 electrooptical crystal and an optical-fiber link is improved by employing a Mach-Zehnder interferometer, whose half-wave voltage is about 4 V at 1.3-pn wavelength, and a YAG laser pumped by a laser diode whose output power is 25 mW. The resulting frequency response is almost flat from 100 Hz to 300 MHz, and the minimum detectable electric field strengths are 0.22 mV/m at 50 MHz and 0.079 mV/m at 750 MHz. The variation of the sensitivity with the frequency and element length are analyzed using the moment method, and the calculated results agree with the measured results. The measurement of the cross-polarization of the sensor indicates that this property is similar to that of a dipole antenna. The improved sensor can measure an electromagnetic impulse whose peak value is larger than 10 V/m and whose width is wider than 5 ns.