Michelson Interferometers 169 can be easily bent and coiled, the size of a fiber-optic interferometer can be much smaller. In free-space Michelson interferometers there is no birefringence in the air and polarization rotation is generally not a problem. However, for fiber-based Michelson interferometers, random birefringence in the fiber causes polariza- tion rotation in each arm of the interferometer. The optical signals reflected from the mirrors in the two arms might not have the same polarization state when they combine at the fiber coupler. This polarization mismatch may cause significant reduction of the extinction ratio and degrade the performance of the interferometer. This problem caused by random birefringence of the fiber can be solved using Faraday mirrors, as shown in Figure 2.4.2 [13]. The Faraday mirror is composed of a 45 Faraday rotator and a total reflection mirror. The polarization of the optical signal rotates 45 in each direction; therefore the total polarization rotation is 90 after reflected from the Faraday mirror. Because of their applications in optical measurement and optical sensors, standard Faraday mirrors with fiber pigtails are commercially available. 2.4.1 Operating Principle of a Michelson Interferometer For the fiber directional coupler using the notations in Figure 2.4.2, suppose