Nonlinear laser wavelength tuning effects appear as phase noise in wavelength swept interferometry. A new method is proposed to compensate tuning nonlinear effects in optical frequency domain reflectometry (OFDR). The proposed method is simpler in configuration, and unlike conventional nonlinear compensation methods, it does not require separate auxiliary interferometer detection, which eliminates the need for an extra photo detector and an acquisition channel for the auxiliary interferometer. In the proposed method, an intentional beating signal is introduced in the beginning of the OFDR spectrum which is treated as an auxiliary interferometer to acquire tunable laser phase information for post signal processing. The proposed method can reduce overall OFDR system cost, reduce the data acquisition time and computational load by half, and make system configuration simpler by eliminating the need for extra components. Feasibility of the proposed method was demonstrated by compensating for tuning nonlinearity effects in an optical fiber approximately 35 m long with a measured spatial resolution of ~30 μm. To confirm performance of the proposed method, a comparison was carried out with a conventional nonlinear tuning compensation method, which requires the separate auxiliary interferometer. Moreover, distributed sensing using the proposed method was also demonstrated in an optical fiber approximately 35 m in length by performing strain sensing with 3 cm sensing resolution.
Internal corrosion can occur when aqueous electrolytes are present inside natural gas transmission pipelines. Despite upstream gas dehydration treatments, liquid water can form through condensation of water vapor or may be introduced from plant upsets. With dissolved salts and acidic gases such as CO<sub>2</sub> and H<sub>2</sub>S, aqueous electrolytes become very corrosive with increased conductivity and lower pH. Since water provides the electrolytes that initiate and sustain corrosion, detection of water can locate the spots for potential internal corrosion inside the pipelines. In this work, a simple optical fiber-based sensor for fully distributed water monitoring has been demonstrated and studied. The system consists of an unmodified off-the-shelf single mode (SM) optical fiber and an optical backscatter reflectometer (OBR) capable of measuring the spatial profile of strain changes along the fiber. The polymer jacket coating of the SM fiber is hydroscopic and serves as the water sensing layer due to expansion/swelling from water absorption. The swelling induced strain change is interrogated with the OBR to enable fully distributed water monitoring. This strain-based H<sub>2</sub>O sensor is sensitive to H<sub>2</sub>O molecules regardless of the phase (liquid or vapor) or the surrounding media. Strain changes were measured at different relative humidity levels from 0% to 100% to demonstrate reversibility and linear correlation between humidity and strain. This sensor has the advantages of fully distributed sensing, low cost, simple preparation, easy operation, and good sensitivity.