Femtosecond laser micro-machined optical fiber based embeddable strain and temperature sensors for structural monitoring

Structural monitoring technology is becoming increasingly important for managing all types of structures. Embedding sensors while constructing new structures or repairing the old ones allows for continual monitoring of structural health thus giving an estimate of remaining utility. Along with being embeddable, miniaturized sensors that are easy to handle are highly sought after in the industry where in-situ monitoring is required in a harsh environment (corrosive atmosphere, high temperatures, high pressure etc.). This dissertation demonstrates the use of femtosecond laser-fabricated Fabry-Perot interferometer (FPI) based optical fiber sensors for embedded applications like structural health monitoring. Two types of Fabry-Perot interferometer sensors, extrinsic FPI and intrinsic FPI, have been designed, developed and demonstrated for strain and temperature monitoring applications. The absence of any movable parts make these sensors easy-to-handle and easy to embed inside a material. These sensors were fabricated using a laboratory integrated femtosecond (fs) laser micromachining system. For the extrinsic Fabry-Perot interferometer (EFPI) design, the fs-laser was used to ablate and remove the material off the fiber end face while for intrinsic Fabry-Perot interferometer (IFPI) design, the laser power was focused inside the fiber on the fiber core to create two microstructures. The scope of the work presented in this dissertation extends to device design, laser based sensor fabrication, sensor performance evaluation and demonstration. Feasibility of using these sensors for embeddable applications was investigated. A new type of material called Bismaleimide (BMI) was used for demonstrating the embeddability of the sensors. Experimental results of strain and temperature testing are presented and discussed. The EFPI sensor has low temperature sensitivity of 0.59 pm/⁰C and a high strain sensitivity of 1.5 pm/µε. The IFPI sensor has the same strain sensitivity as EFPI but is 25 times more sensitive to the temperature. These sensors were tested up to 850 ⁰C in non-embedded condition and they produced a linear response. A hybrid approach combining the EFPI and IFPI sensors was demonstrated for simultaneous measurement of strain and temperature --Abstract, page iii