Recently, fiber-optic current sensor technology has reached a degree of maturity to compete with conventional instrument transformers. However, it has not been commercialized until quite recently because of a few instability issues, such as linear birefringence effect in the fiber-optic sensor coil and intensity noises caused by optical light sources and components. In this paper, we report on research efforts we performed to address these issues. Firstly we used different optical sources, such as an ASE source, a F-P multimode LD, and a DFB singlemode LD, to compare the effects by the light sources. Also we used different optical fibers, such as flint glass fiber, with different reflection mirrors. From the experimental results, we obtained output variation down to 1% in the presence of mechanical disturbances and the broadband source showed the best noise characteristics about 13~23 dB better than LDs. The details of the experiments with other design parameters are also presented.