High Speed Magneto-Optical Sensor for Magnetic Stripe Readout

We report on a Faraday-sensor to read out optically test pattern recorded to magnetic stripes on smart cards in a non-contacting way at scanning speeds of several meters per second. The pattern densities of several bits per millimeter can be resolved properly. The sensor employs a magneto-optical crystal of iron doped Yttrium-Aluminum-Garnet known to exhibit one of the highest known Verdet-constants (~130 rad/Tm). It is epitaxially grown to a rather small thickness of approximately 50µm, a small thickness that on the one hand reduces the overall sensitivity dramatically but on the other hand allows for a very fine pitched spatial resolution to image the normal component of the magnetic flux density. The to be tested specimen in the production line is moving at a rather high speed so the optical system and the readout electronics is geared toward rapid scan and to maintain a rather high sustained data streaming rate onto a solid state disk for archival purposes. Scan rates exceeding 50.000 scans per second (@ 128 pixels per scan) were attained and the camera still is sufficiently exposed to attain a fair signal to noise ratio.