Date of Award
Doctor of Philosophy (PhD)
Distributed sensing refers to the solution which enables the real-time, continuous measurement at multiple sensing locations (typically, more than 100 sensing nodes). Due to many of its unique advantages, such as small size, light weight, low cost, electromagnetic immunity, high-temperature survivability, and chemical stability, optical fibers have been well accepted as one of the most promising candidates as the platform for distributed sensing applications.
Among different fiber distributed sensing methods, optical frequency domain reflectometry (OFDR) represents a particular promising candidate. Based on the frequency modulated continuous wave (FMCW) method, OFDR method is capable of measuring the spatial-continuous weak Rayleigh scattering patterns along the entire length of the fiber under test, with high spatial resolution (~μm) level and moderate interrogation distance (~ km). To extract the structural information from the unmodified communication grade single mode fiber, large interrogation bandwidth is needed. However, this resource of optical bandwidth is very expensive. The external cavity laser is the state-of-the-art frequency sweep laser source for OFDR system, which costs at least $ 20,000. The cost has rendered the OFDR interrogation technique expensive and limits its applications.
This dissertation focused on the improvement of OFDR interrogation system by reducing the total system cost. First, a series of sub-terahertz range fiber sensors, including Fabry-Perot cavity sensors and terahertz fiber grating sensors were symmetrically investigated. Fabricated using femtosecond laser micromachining techniques, these sensors allow the OFDR system for low bandwidth interrogation while maintaining the high accuracy measurement. In addition, a sensor fabrication method without stripping the fiber polymer buffer was developed. This is the first time that an in-line grating structure has been fabricated within the core of an optical fiber with an intact buffer coating, allowing the fiber to retain optimal mechanical properties.
Second, a series of low cost sweep laser sources were developed as high-linear coherent sweep laser source, suitable for the sub-terahertz range fiber sensor interrogation. Based on current injection modulation methods, the semiconductor lasers were used as the sweep laser sources and the output wavelength was feedback controlled using optical phase locked loop techniques. The method of using VCSEL laser was also investigated to increase the sweep bandwidth. In addition, an all-digital optical phase locked loop system was implemented using the field programmable gate array, which increases the system design flexibility.
Chen, Zhen, "Sub-Terahertz Range Fiber Optic Devices for Sensing Applications" (2017). Open Access Dissertations. Paper 629.