Fiber optic tachometer based on the Faraday effect

A fiber optic tachometer is described that uses the Faraday effect in a ferrite to modulate light from a multimode fiber. A yttrium iron garnet (YIG) crystal is used as the magnetooptic modulator. A permanent magnet is placed near the crystal, such that a nearby rotating steel toothed wheel changes the magnetic field inside the crystal, causing modulation of the intensity of the 1.3-microm light. Modulation depths up to 50% have been achieved with signal-to-noise ratios of over 15 dB. A single multimode fiber connects the remote sensor head to the LED light source and detector.