Polymer optical fiber (POF) elongation sensors have been proposed e.g. by Doering as a low-cost alternative to FBG
(single mode Fiber Bragg Gratings) sensors targeting the lower sensitivity range. A recently recovered detection system
known from laser distance meters turned out to be very sensitive while staying simple and thus offering low cost
potential. The approach is based on measuring the phase shift of a (e.g. sinusoidally) modulated light signal guided in a
POF under different tensions resulting in different transit times and thus different phase shifts.
"POF strain sensor using phase measurement techniques", Proc. SPIE 6933, Smart Sensor Phenomena, Technology, Networks, and Systems 2008, 69330Y (11 April 2008); doi: 10.1117/12.780721; https://doi.org/10.1117/12.780721