Cross-polarization coupling in polarization maintaining fiber induced by periodic mechanical stress

One of the applications of polarization maintaining (PM) fibers is the creation of various sensors including level and hydrostatic pressure sensors. An external mechanical stress on a PM fiber causes cross-polarization coupling. In microand macro-bend pressure sensors, the attenuation of the optical signal increases with increasing external pressure. Combination of two physical principles in one sensor allows to create fiber-optic sensors of high sensitivity for operation at pressure more than 18 kPa. At the same time, the registration scheme is significantly simplified. An optical fiber with an elliptical ESC-4 stress cladding was used as a prototype. A superluminiscent diode ThorLabs S5FC1005SXL with a center wavelength of 1560 nm and a spectrum half-width of 45 nm was used in the study. In this work, the effect of induced periodic mechanical stress on cross-polarization coupling magnitude in a PM fiber with an elliptical stress cladding was considered. The induced periodic stress was generated by a proposed and specially fabricated periodic structure of optical fiber sections bonded to a glass substrate. The pressure on the fiber prototype was changed by weights of known mass as well as by rotating the fiber around its axis with a step of 5°. Hydrostatic pressure was created by means of a water tank placed in the barocamera. The cross-polarization coupled power was recorded with two photodetectors using a polarization beam splitter. It is shown that the largest value of the relative coupled power is achieved at the period of mechanical stress equal to the beat length of PM fiber. The dependence of the relative coupled power on the hydrostatic pressure is obtained for the prototype sensor. It is shown that the highest value of the relative coupled power is achieved at a pressure of 80 kPa. The threshold sensitivity amounted to 20 kPa. The decrease in coupled power at pressures greater than 80 kPa is attributed to induced optical losses. At pressure less than 20 kPa, a hysteresis of indicators up to 0.006 relative units is noticeable. A sample sensor for measuring hydrostatic pressure up to 80 kPa with the standard deviation of measurement results up to 7 % was obtained. The threshold sensitivity is limited to 20 kPa. The performed research can be applied in the development of fiber-optic pressure sensors.