Fig. 1. Principle of TF induced principal-axis rotation of PMF^[47]. (a) Combined stress formed by TF and internal stress resulting in new slow axis direction; (b) TF applied on a fiber segment to cause a polarization principal-axis rotation; (c) equivalent of (b) with the principal-axis of a fiber segment misaligned with respect to those of two fiber segments on each side; (d) diagram showing the principal-axis rotation of a center fiber segment

Fig. 2. Polarization crosstalk changed with TF applying angle and applying length. (a) Force-applying angle; (b) force-applying length

Fig. 3. Diagram of ghost-peak free distributed polarization crosstalk analysis system and measurement principle

Fig. 4. Illustration of polarization crosstalk high-order ghost-peak elimination^{[51, 54]}. (a) Multi-points polarization crosstalk in PMF-UT (or PM fiber in the figure) ; (b) wave packet sequences polarized along the slow and fast axes at output of PMF-UT; (c) wave packets in the two interferometer arms after light passing through 45° oriented analyzer; (d) wave packets in the two arms after the DGDD is inserted into DPXA system

Fig. 5. Typical polarization crosstalk curve measured with the ghost-peak free DPXA system^[47]

Fig. 6. TF induced polarization crosstalk (linear scale) sensitivity measurement for different types of PMFs^[55]. (a) TF loading apparatus; measurement results of polarization crosstalk for (b) panda PMF1 and (c) panda PMF2 with and without coating respectively as functions of applied weight, and similar results measured for golden polyimide coated PMF also plotted correspondingly for comparison; (d) comparison of crosstalk bases between panda PMF1 with coating and polyimide coated PMF

Fig. 7. Response time measurement of TF induced polarization crosstalk for different types of PMFs^[55]. (a) PMF1 with coating; (b) PMF1 without coating; (c) PMF2 with coating; (d) PMF2 without coating; (e) golden polyimide coated PMF

Fig. 8. PMF-based sensing tape fabricating system with automated polarization axis alignment for distributed TF sensing^[57]. (a) Schematic of system; (b) image analysis principle; (c) fiber axis orientation angle calibration; (d) prototype

Fig. 9. PMF based sensing tape with 45° polarization axis alignment and measurement results^[57]. (a) Photo of PMF sensing tape; (b) polarization axis angle measurement results

Fig. 10. TF applying method on PMF-based sensing tape^[57]

Fig. 11. Sensing tape uniformity measurement results^[57]. (a) Large distance measurement; (b) small range measurement

Fig. 12. PMF-based distributed TF sensing validation^[57]. (a) Distributed measurement with multi-force applying points; (b) measurement of sensing calibration curves

Fig. 13. Designing method of TF sensing using PMF independent of direction of applied force. (a) Fiber twisting method; (b) force-applying clamp designing method

Fig. 14. Polarization crosstalk versus force-applying angle using twisted PMF with force-applying based on four-jaw clamp

Fig. 15. Validation of distributed TF sensing using twisted PMF independent of direction of applied force. (a) Four-jaw clamp design; (b) PMF twisting and TF sensing experimental apparatus; (c) polarization crosstalk versus fiber length under different force-applying angles for untwisted PMF; (d) polarization crosstalk versus fiber length under different force-applying angles for twisted PMF

Fig. 16. Validation of distributed TF sensing using SF independent of direction of applied force. (a) Four-jaw clamp design; (b) polarization crosstalk versus fiber length

Fig. 17. Basic structure and working principle of OFDR. (a) System structure; (b) frequency-beating interference principle

Fig. 18. Schematic of DPA system (PA-OFDR)^[21]

Fig. 19. Birefringence distribution measurement results along the SMF-UT^[21]. (a) Distributed birefringence measurement results from fiber loop No. 1 to No. 12; (b) distributed birefringence measurement around fiber loop No. 10

Fig. 20. Bending-induced birefringence versus bending radius for SMF-UT^[21]. (a) Measurement result using DPA system; (b) measurement result using NDPA system

Fig. 21. Schematic of experimental setup for SMF-based distributed TF sensing^[48]

Fig. 22. Experiment results of TF measurement sensitivity calibration^[48]. (a) Distributed birefringence; (b) birefringence versus TF

Fig. 23. Experiments of SMF-based distributed TF sensing demonstration^[48]

Fig. 24. Characterization of minimum detectable TF^[48]

Fig. 25. Characterization of sensing spatial resolution and maximum sensing distance^[48]. (a) Spatial resolution; (b) sensing distance

Fig. 26. Fiber coating influence to TF sensing^[48]. (a) TF sensitivities of SMFs with different coatings; (b) TF-induced birefringence property of polyacrylate coated SMF; (c) TF-induced birefringence property of polyimide coated SMF; (d) birefringence versus TF for polyacrylate coated SMF; (e) birefringence versus TF for polyimide coated SMF

Fig. 27. Fabrication of SMF-embedded carbon fiber composite-material and bending-implemented mold

Fig. 28. Distributed birefringence measurement results of SMF embedded in composite-material and relationship between birefringence and bending radius. (a) (b) Sample-1; (c) (d) Sample-2

Fig. 29. Methods of fiber clamping and TF-applying and distributed measurement experiments for fiber clamping with different V-groove angles along the fibers^［74］. (a) Fiber clamped in V-grooves by flat-lids; (b) fiber clamped by two identical V-grooves

Fig. 30. Relationship between additional birefringence of SMF clamped in V-grooves and groove angles^[74]. (a) Fiber clamped in V-grooves by flat-lids; (b) fiber clamped by two identical V-grooves