Polarization Noise in Optical Fiber Faraday Ring Ammeters.
This dissertation presents a new technique, the Faraday King Ammeter (FRA), for measuring electric currents or the curl of magnetic fields in space plasmas. The technique relies on the magneto-optic (Faraday) effect in (diamagnetic) optical fibers. The FRA provides a highly non-invasive direct measurement of space plasma currents, impervious to space-craft electromagnetic interferences. The FRA is a multiturn coil of twisted single mode optical fiber. Light transmission through single -mode fibers using Jones representation and its limitations on the measurement of current induced optical activity is studied. The source spectral wavelength's effect on the FRA measurement is analyzed. The first space-flightworthy FRA-I's design and measurements with it are presented. Passive polarization control was implemented in FRA-I by reflecting the light at the fiber end. It emerged at the front fiber end having its polarization rotated by an angle given only by Faraday rotation, enabling a smaller minimum FRA-I current measurement than other magneto-optic measurements. Factors limiting the FRA-I resolution have been outlined. FRA-II was designed to overcome the sensitivity of FRA-I to low frequency vibrations. Switching of the acousto-optic device at high frequencies in FRA-II caused instabilities in the polarization of the transmitted light. The author proposed the FRA-II.v design to remove the instabilities, in which their reduction has been consequently observed. An exhaustive solution to the interfering noise problems of FRA-I and FRA-II: a study of the complete state of polarization transmitting through the fiber, was then adopted. The author designed a high speed digital ellipsometer (DRME) to obtain six kilosamples per second of the Stokes vector of light whose implementation has been described. The DRME would enable the removal of noise at known frequencies of mechanical vibrations. Responses of two fiber samples to electrical current as a function of thermal ambient have been studied with the DRME. Results enable fiber selection from a variety of fibers, and compensation of thermal effects by operating at preferred ambient temperatures.
- Pub Date:
- Physics: Optics