Strain and temperature sensing using optical fiber sensors

This thesis describes an approach to measuring thermo-mechanical strain in presence of changing temperature fields, and measuring both strain and temperature at one location using optical fiber sensors. The pure thermo-mechanical strain is measured by a new sensor called the In-Line Fiber Etalon (ILFE) that is fabricated by splicing a hollow-core fiber between two single-mode fibers. Pseudo-heterodyne, synthetic-heterodyne, active-homodyne, and single-channel phase tracker demodulators are used as signal processing schemes in conjunction with white light interferometry to measure the strain unambiguously from the ILFE sensor. Stability of these sensors is seen to be a function of the stability of the demodulator, and ways are suggested to make the system more stable. The optical behavior of ILFE is also examined in great detail. This is important to do because hollow-core fibers do not guide the light in the cavity and result in losses over the length of the sensor thereby limiting the maximum gage-length of these sensors. However, a new in fiber collimator made by using graded index fiber is presented to alleviate this problem, and allowable ILFE gage-lengths are seen to enhance two to three times.; Two configurations are presented for measuring the strain and temperature simultaneously at a single location using a single fiber sensor. The first configuration is realized by splicing an ILFE sensor with an intrinsic Fabry-Perot cavity and optically demultiplexing the signals from the two sensors to obtain two distinct equations for two unknowns, strain and temperature. In the second configuration the ILFE is combined with the Bragg grating sensors. Bragg grating sensors are also intrinsic sensors like the IFP sensors, also have a thermal response very distinct from ILFE sensors, and as before, this sensor configuration provides two equations to solve for strain and temperature. The details of fabricating all these sensors are discussed in great detail and all the fabricating procedures are provided. Finally, results are presented that show that the hollow-core based sensor can be used successfully to measure only the strain, and the two other configurations can be used to measure both the strain and temperature at the same point.