# High-Resolution, Broad-Range Detection Setup for Polarimetric Optical Fiber Sensors
## 1 Introduction  
Polarization interferometers, difference interferometers, and polarimetric optical fiber sensors induce a change in optical path length between orthogonally polarized components of optical radiation in the directions of the principal axes of these devices. In sensors using the Faraday effect, propagating optical radiation undergoes a rotation of the state of polarization. In the case of point sensors, a detection setup converts the information contained in the optical signal into a single value of the measurand. Distributed polarimetric optical fiber sensors must recover information about the value of measurand along the length of sensing fiber. The selected detection method depends on the principle of operation of a particular sensor and other considerations, such as the measurement range, accuracy, and bandwidth. The reliability, cost, and availability of components are also factors that influence the decision. In general, distributed polarimetric sensors use detection techniques based on polarization optical time-domain reflectometry (P-OTDR) [1], polarization optical frequency-domain reflectometry (P-OFDR) [2; 3; 4; 5], or wavelength-scanning phase-sensitive optical time-domain reflectometry (\(\varphi\)OTDR) [6; 7; 8]. Point sensors, especially those that employ the Faraday effect, use a polarimeter as their detection setup [9; 10; 11; 12; 13] or a polarization division multiplexing detection setup [14]. The detection of signal from difference interferometers is often performed using spectral interferometry with a broadband source and an optical spectrum analyzer (OSA) [15; 16; 17] or a tunable laser and a broadband detector [18; 19]. The same technique, spectral interferometry, is used in some surface plasmon resonance sensors (SPR) [20; 21; 22; 23; 24; 25] and in the characterization of sensitivity to hydrostatic pressure, strain, and temperature of specialty optical fibers [26; 27; 28], where it can resolve phase difference as a function of wavelength. Sagnac interferometer detection is also often employed in optical fiber polarimetric sensors [29; 30; 31]. Other techniques include tracking interference fringes with photodiodes [32] or the use of diffraction gratings to produce interference fringes that are subsequently detected by a linear detector array [33; 34]. Last but not least, polarimetric sensors of hydrostatic pressure, biosensors, and waveguide refracometers often use a detection setup employing a common-path interferometer consisting of a Wollaston prism [34; 35] or a birefringent wedge [36; 37; 38], followed by a polarizer and a linear detector array (CCD or CMOS). The interferometer produces an equidistant and straight fringe pattern (Young's fringes; fringe interference field) that is recorded by the detector array and processed by a computer, yielding retardation. An accuracy of \(2\pi/2100\) rad was demonstrated in [36], with a resolution from \(2\pi/4000\) rad [36] to \(2\pi/1000\) rad [34]. However, unless continuous tracking of the fringe pattern is performed, the unambiguous measurement range of this method is restricted to \(2\pi\) rad. In some instances, retardation must be measured in a range much broader than \(2\pi\) rad. Such a measurement can be performed by using low coherence interferometry (LCI), i.e., by illuminating the interferometer or sensor with a low-coherence light source and applying a detection technique capable of measuring the optical path difference. In particular, common-path interferometers [34; 35; 36; 37; 38] can be used, but the number of fringes on the detector must be increased to allow for tracking of the fringe pattern across the required measurement range. However, as the number of fringes is increased, the number of pixels per fringe is reduced, lowering the resolution and accuracy of the measurement.