We combine the evanescent field (EF) sensing mechanism with the fiber loop ringdown (FLRD) sensing scheme to create FLRD-EF sensors. The EF sensor heads are fabricated by etching the cladding of a single-mode fiber (SMF), while monitoring the etching process by the FLRD technique in real-time, on-line with high control precision. The effect of the sensor head dimensions on the sensors' detection sensitivity and response time are investigated. The EF scattering (EFS) sensing mechanism is combined with the FLRD detection scheme to create a new type of fiber optic index sensor. The detection limit for an optical index change is 3.2×10-5. This is the highest sensitivity for a fiber optic index sensor so far, without using any chemical-coating or optical components at the sensor head. A new type of index-based biosensor using high sensitivity FLRDEFS technique to sense deoxyribonucleic acid (DNA) and bacteria (Escherichia coli) is created.

Optical fibers have been mostly used in fiber optic communications, imaging optics, sensing technology, etc. Fiber optic sensors have gained increasing attention for scientific and structural health monitoring (SHM) applications. In this study, fiber loop ringdown (FLRD) sensors were fabricated for scientific, SHM, and sensor networking applications. FLRD biosensors were fabricated for both bulk refractive index (RI)- and surface RI-based DNA sensing and one type of bacteria sensing. Furthermore, the effect of glucose oxidase (GOD) immobilization at the sensor head on sensor performance was evaluated for both glucose and synthetic urine solutions with glucose concentration between 0.1% and 10%. Detection sensitivities of the glucose sensors were achieved as low as 0.05%. For chemical sensing, heavy water, ranging from 97% to 10%, and several elemental solutions were monitored by using the FLRD chemical sensors. Bulk indexbased FLRD sensing showed that trace elements can be detected in deionized water. For physical sensing, water and cracking sensors were fabricated and embedded into concrete. A partially-etched single-mode fiber (SMF) was embedded into a concrete bar for water monitoring while a bare SMF without any treatment was directly embedded into another concrete bar for monitoring cracks. Furthermore, detection sensitivities of water and crack sensors were investigated as 10 ml water and 0.5 mm surface crack width, respectively Additionally fiber loop ringdowniber Bragg grating temperature sensors were developed in the laboratory; two sensor units for water, crack, and temperature sensing were deployed into a concrete cube in a US Department of Energy test bed (Miami, FL). Multi-sensor applications in a real concrete structure were accomplished by testing the six FLRD sensors. As a final stage, a sensor network was assembled by multiplexing two or three FLRD sensors in series and parallel. Additionally, two FLRD sensors were combined in series and parallel by using a 2×1 micro-electromechanical system optical switch to control sensors individually. For both configurations, contributions of each sensor to two or three coupled signals were simulated theoretically. Results show that numerous FLRD sensors can be connected in different configurations, and a sensor network can be built up for multiunction sensing applications.

Optical fibers are getting significant considerations in the field of the sensors and sensing beyond its applications in optical communications. Because of several advantages, e.g., low profile of the sensors, immunity to electromagnetic noises, the ability of multiplexing, etc., the use of the fiber optic sensor is increasing in the field of physical, chemical, and biomedical sensing. In this study, we have developed two new fiber optic sensors based on fiber loop ringdown technique (FLRD) and have demonstrated their applications in the field of sensing. In the first part of this study, we report on the development of a high-sensitivity FLRD strain sensor. For the design of the strain sensor, the fiber loop was cut at the middle, and then the two fiber ends from broken fiber loop were cleaved and aligned carefully to couple the light from one end to another end. Any strain during the measurement changes the alignment of the fiber ends, consequently, the ringdown time changes. With this scheme, the FLRD strain sensor has shown the strain detection limit of 65 nanostrain, which is five times better than any FLRD strain sensors reported in the literature. Furthermore, The FLRD strain sensors were successfully embedded into prestressed concrete-beams.The FLRD strain sensor was able to monitor stress on a post-tensioned rod, as well as the load applied on the concrete-beam during the three-point loading test, thus exhibiting immense potential in structural health monitoring. For the chemical sensor, a new scheme of interrogation for a fiber optic surface plasmon sensor was developed with the use of the FLRD technique. A gold nanolayer was deposited on an uncladded fiber section, and the fiber section was integrated into the FLRD system as a sensor head. The gold layer facilitates for increased interaction of sample of interest, with the light pulse confined in the fiber waveguide. Moreover, with the affinity of the gold with specific biomolecules, the sensor has the potential for applications in biochemical sensing. In the experiment, the SP-FLRD sensor was used for refractive index sensing, and index detection limit of 4.6×10-5 RIU was achieved.