Frequency response of a thin cobalt film magnetooptic sensor

The magnetooptic effect is due to a change in the polarization of the light when it is reflected or passes through a magnetized material. The rotation of the polarization plane is proportional to the magnetic field. The great advantage of using a magnetooptic sensor to measure currents or magnetic fields is its wide bandwidth. It can reach the magnitude of several GHz. This fact is widely known, however no effective measurements have been taken. In this paper we present the frequency response of a cobalt thin film used as magnetooptic material. It was first excited by several sinusoidal magnetic fields at different frequencies. The range of frequencies studied in the first experiment reached 200 Hz, which is suitable to measuring power lines currents or magnetic fields. Because the coil that creates the magnetic field has a great impedance at higher frequencies, an alternative method based on magnetic impulses has been designed to obtain high frequency data. With the latest experiments we have been able to measure frequencies as high as 2 MHz, obtaining a flat response.