The magneto-optic (Faraday) effect in dia-magnetic materials is a promising method of measuring magnetic fields or electric currents in space plasmas due to its non-invasive character. Fiber-optic sensors with coherent detection schemes are attractive for these measurements. The Faraday Ring Ammeter (FRA) is the first in a series of space plasma ammeters under development whose ultimate goal is detecting currents with a pA/m2 resolution. This instrument has 450 meters of twisted single mode optical fiber as its sensing element. The optics for injection (including GRIN lenses) and for coherent detection (po-larimetry) are integrated with the electronics in a (6" x 5" x 3") box. Operating at a laser wavelength of 830mn currents as small as a few hundred milliamperes through the FRA have been measured.
Roy B. Torbert,
"Compact Optical Fiber Faraday Ring Ammeter", Proc. SPIE 1166, Polarization Considerations for Optical Systems II, (25 January 1990); doi: 10.1117/12.962916; https://doi.org/10.1117/12.962916