Optical Frequency Domain Reflectometry (OFDR) fiber optic sensing is established as highly beneficial for live, simultaneous, multipoint monitoring in many fields. A critical weakness of interferometric fiber optic sensing methodologies is that they are susceptible to errors from external influences that introduce birefringence in polarized laser light. Currently, this can be mitigated with active polarization control but the hardware is costly and requires significantly more components that compound in number and cost as sensor number increases.
OFDR sensing fibers have many sensing points along their length and can be configured to simultaneously measure many things such as distributed strain, temperature, and chemical presence. Non-polarized OFDR suffers distortion in the laser light polarity at connections, components, bends, and particularly at locations along sensing fibers where harsh environmental factors can physically affect the crystal structure of the fiber.
A new passive polarized fiber optic sensing system was developed that can deliver the same results as an active polarization system with minimal added hardware expense, regardless of the sensing capacity. The technology combines optical algorithms and commercially available components to enable the identification of various polarization states (birefringence) and filters data for this distortion effect by creating a sensing system that is 100% polarization diverse. The result is the elimination of measurement errors due to polarization effects in the fiber optic system. Additionally, because the technology can accurately track polarization changes along a string of sensors, it may prove to be useful in the application of new sensor types such as pressure, twisting, and bending along the sensing fiber.
This approach separates the signal into three 60-degree polarizations using a commercial 3-axis polarization filtering arrangement. The light returning from a sensing fiber is split three equal ways and each third of the return is linearly filtered before being optoelectronically converted and amplified. The linear filters on the three splitter outputs are positioned at 0°, 60°, and 120° with respect to the cross-section of the fibers emerging from the splitter. The information from the three signals is processed to provide a thorough understanding of birefringent effects throughout the sensor length.
This technique offers the same benefits as active polarization control with very little change to prior hardware found in traditional OFDR systems. The hardware used to implement the new approach is less expensive than an OFDR system implementing active polarization control because less expensive, passive, non-polarized components achieve the same results.