A polarization independent high speed electro-optical switch by using quadratic electro-optic effect of lanthanum-doped lead zirconium titanate (PLZT) ceramic incorporated in a Sagnac loop is described. Analysis by Jones matrix method shows the output property of the fiber loop mirror and the relation of its extinction ratio and parameters of the configuration. Experiments show its typical performances of 25 dB extinction ratio and less than 3 μs response time, and give Kerr coefficient of the lanthanum-doped lead zirconium titanate κ～1.1×10-16 m2/V2. Experiments also show good stability of the device without any special measures, which is attributed to the mutual optical paths in the fiber loop for the two beams of interferometer, and is regarded superior than other structures like Mach-Zehnder interferometer.