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A new type of the in-line optical fiber ellipsometer based on the Sagnac interferometer is described. The possibility of thedetection of the changes of full polarisation state in this system containing standard single-mode fiber by an appropriateapplied modulation technique is discussed. The device uses interferometric measurement technique based on the fourthFresnel-Arago's condition, which secures very good system accuracy and stability. The results of investigations of thesystem are presented.Keywords: fiber optic sensor, ellipsometry, polarimetry, interferometry 1. INTRODUCTION An identification of external perturbation by a detection of phase changes in a fiber-optic system is widely used in interferometric sensors systems. However, optical fibers and fiber-optic elements in general exhibit variable birefringence. In case of application for a construction of interferometric system, it causes the effect of polarisation on interference phenomena' . In contrary, optical paths of classic interferometers are usually isotropic or exhibit constant birefringence. Thisis the main difference between bulk- and fiber-optic interferometers. For those reasons, coherence behaviour for the first andpolarisation for the second kind of interferometer has the main influence on the system action"2For optical fiber system, the only practical way to solve polarisation problem is to apply either an active polarisation-controlling system or a high-birefringence (HiBi) fiber; both ofthem used to fix the state of polarisation (SOP) of interactingwaves. On the other hand, the polarimetric sensors utilise changes of SOP for an identification of the disturbance factors.Nevertheless, commonly used polarimeters can only detect changes in the azimuth of SOP3. Changes of the secondparameter characterising SOP - ellipticity |
