# High-Resolution, Broad-Range Detection Setup for Polarimetric Optical Fiber Sensors
## 3 Results
### Range and Resolution  
The setup was then carefully aligned, and its operation was tested. First, a laser diode emitting at \(\lambda\) = 515 nm (PLT5 510, ams-OSRAM International, Prematten, Austria) was used as the light source. Its operating current was set above the lasing threshold. The fringe pattern incident on the area image sensor was aligned with fringes parallel to the shorter side of the sensor. Part of that pattern is presented in Figure 8a. Subsequently, the laser diode was replaced by a superluminescent diode emitting at \(\lambda\) = 830 nm (SLD830S-A10W with CLD1015 controller, Thorlabs, Newton, NJ, USA), and the fringe pattern was recorded and is presented in Figure 8b. Figure 6: Phase error of image sensor with 512 pixels in a row. Figure 7: Phase error of image sensor with 1024 pixels in a row. The fringe pattern in Figure 8a consists of 239 fringes distributed equally on 1280 pixels, i.e., 5.36 pixels per fringe. The contrast is uniform across the pattern, as expected for a laser source. The use of a broadband source to obtain the fringe pattern presented in Figure 8b resulted in a contrast of the fringes that varied across the pattern, being the highest in the central part of pattern and decreasing to zero at the edges of the pattern. In this case, the central part of the fringe pattern (610 pixels) contains 66 fringes, corresponding to 9.24 pixels per fringe and 139 fringes in the entire fringe pattern. In both fringe patterns from Figure 8, artifacts from dust and imperfections of the optical surfaces are apparent. A first attempt at measuring retardation was performed with the laser diode emitting at \(\lambda\) = 525 nm and its operating current set below the lasing threshold. The fringe pattern on the camera was aligned to have more than one period of the pattern on one row but not more than two such periods. Unfortunately, small variations in the phases of interfering beams affected the shape of the fringes, resulting in a substantial distortion of the data that were to be analyzed. An example of such a pattern is shown in Figure 9.  
Most likely, the variations in the phases of the interfering beams arose from the lack of planarity of the interfering beams and the local imperfections of optical components, in particular the lens L, polarizer P, and analyzer A, as these components are usually manufactured with moderate attention to their phase characteristics. Aberrations introduced by these components and the Wollaston prism WP and parasitic effects due to speckles or dust were also responsible for these variations. Unfortunately, the quality of the phase measurement results was rather poor, necessitating a change of approach. Figure 8: Fringe pattern fragment recorded by the image sensor. (**a**) Illumination with a laser diode emitting at 525 nm; (**b**) illumination with a superluminescent diode emitting at 830 nm. Figure 9: Distorted measured data pattern recorded by one row of pixels and pattern expected from (9). Therefore, a second attempt at measuring the retardation was made with the superluminescent diode emitting at \(\lambda\) = 830 nm. The fringe pattern on the camera was aligned to have three to four periods of the pattern on one row. This reduced the resolution but improved the quality of the data. The fringe pattern was then recorded as a function of retardation. Measurements were performed by changing retardation in one direction to avoid hysteresis and backlash. The position of the fringe pattern was determined by finding the location of a selected minimum. This was accomplished by approximating a fragment of the pattern close to the minimum with a quadratic function and finding the minimum of that function, as illustrated in Figure 10. The position of that minimum, expressed in pixels, was treated as the output signal of the detection setup.