In this thesis two types of fiber-optic current sensors - passive and active types - are implemented and examined in detail in various characteristics as a sensor.
A passive fiber-optic current sensor is constructed and characterized, which measures Faraday rotation linearly proportional to a magnetic field generated by an electric current. The sensor is composed of a sensing unit and a signal processing unit. A single mode fiber is used to connect a light source and a sensor head, and two multi-mode fibers are used to connect the sensor head and photodetectors in signal processing unit. A polarizer and a polarizing beam splitter is located in the sensor head to remove lead sensitivity. Two orthogonally polarized laser diodes are combined to form a unpolarized light source to avoid output light fading. The bending-induced linear birefringence in the sensor head is minimized by winding the fiber around an orthogonally-combined coil formers. The output signals from the two detectors are used to elliminate the effects of intensity variations due to losses coming from misalignments of components such as fiber connectors, polarizer, polarizing beam splitter, etc. The sensor shows 1.2% of error for linearity measurement in 0 - 1000 A range. The slow variation of the sensor output is less than 1.5% of error for 3 hours at the current level of 128 A and 500 A. The temperature dependence of the signal output was measured to be 1.2% between 20 ℃ and 40℃.
As an active current sensor, an $Er^{3+}$-doped fiber laser with a Faraday rotating mirror(FRM), saturable absorber, and a planar mirror is constructed and examined. Enhancement of signal to noise ratio is achieved by suppressing the number of longitudinal modes. This is accomplished by controlling the position of gain medium and saturable absorber in the cavity with a FRM. The laser operate with only one longitudinal mode in each polarization eigen states, producing a single polarization mode beat(PMB) frequency. A nonreciprocal circular birefringence induced by axial magnetic fields along the laser changes the resonant condition of the laser cavity, resulting in PMB frequency change. Measurement of this change using phase-locked loop(PLL) gives the magnitude of the applied magnetic field. The output of the PLL signal processor versus the applied current shows good linearity. The noise-equivalent current was $460μA_rms/Hz^{-1/2}$ per turn, which is good enough for most applications. For better modal stability, a fiber Bragg grating was tried instead of a planar mirror. The measured slope coefficient was much smaller (about 55%) than expected value, which is proved to be due to the polarization-dependent reflectivity of the grating mirror. As a new application of the laser current sensor, two lasers are multiplexed in rf-frequency domain. The lasers are pumped with a laser diode and two outputs are sent to a photo-diode via a directional coupler, and promising results were demonstrated.
