We combine the evanescent-field scattering sensing mechanism with the fiber-loop ringdown detection scheme to create a new type of fiber optic index sensor using partially cladded single-mode fiber as a sensing element. A detection limit for an optical index change of 3.2x10(-5) is demonstrated in the initial examination by using certified refractive index oils and laboratory-made sodium chloride solutions. This is the highest sensitivity for a fiber optic index sensor to date (to our knowledge) without using any chemical coating, delicate fiber components, and/or sophisticated architecture at the sensor head. A potential detection limit can be of the order of 10(-6).

Abstract. We report a new type of refractive index-based biosensor using a fiber loop ringdown evanescent field (FLRD-EF) sensing scheme, in which the sensing signal is a time constant and detection sensitivity is enhanced by the multipass nature of the ringdown technique. Bulk index-based detections of three different single strand DNAs and one type of bacteria are demonstrated for the FLRD-EF sensors that utilize a partially-etched single mode fiber as the sensor head. Stepwise coating of the sensor head with poly-L-lysine and a probe DNA has enabled surface index-based label-free target DNA sensing. We expect an array of FLRD-EF biosensors to be created, which are superior to counterparts in terms of simplicity, low cost, and high sensitivity. C FLRD as a uniform time-domain sensing scheme has been explored to develop a variety of physical and chemical fiber optics sensors, such as pressure, strain, temperature, refractive index, microfluids, and volatile organic compounds; the details can been read in recent reviews. 3,4 In FLRD, the sensing signal is a time constant; the detection sensitivity is proportionally enhanced by the number of round-trips a laser pulse travels in the fiber loop which can be up to a few kilometers long. Many sensing mechanisms, such as direct gas absorption, microbending-induced deformation, fiber Bragg grating (FBG)-and long period grating (LPG)-based wavelength shift, and evanescent field (EF) absorption and scattering, can be directly adopted into the uniform sensing platform for the development of different sensors. This technique has great potential for biosensor development, yet it has not been much explored, except for one early publication in which a single mammalian cancer cell is detected based on the scattering effect of the localized EF around a 10-mm long single mode fiber taper. 5In this work, we demonstrate bulk index-based deoxyribonucleic acid (DNA) and bacteria sensing and surface index-based label-free DNA sensing using the FLRD sensing scheme combined with the EF sensing mechanism. To the best of our knowledge, this work presents the first DNA and bacteria sensors using the FLRD technique. Without utilizing additional optical components, such as an FBG or LPG to fabricate the sensor head, as reported in recent studies, 6-10 our sensor design demonstrates comparable or better performance while featuring significantly lower cost, simplified design and configuration, and potentially higher detection sensitivity. Figure 1(a) shows the fiber loop ringdown system consisting of a section of fused-silica single mode fiber (SMF 28, Corning, Inc.), two identical 2×1 fiber couplers (Opneti Communication), a temperature-controlled continuous wave diode laser with output power of 30 mW when operating at 100 mA (NEL America), an InGaAs photodetector (Thorlabs, PDA50B), and an electronic control. Cladding and core diameters of the single mode fiber are 125 and 8 μm, respectively. The total optical loss, including the absorption loss, fiber connectors' insertion losses, and fi...

The evanescent field (EF) sensing mechanism has been widely implemented in the development of fiber optic sensors (FOSs). Challenges in the development of EF-FOSs include the enhancement of the sensor's sensitivity and implementing high precision control in the fabrication of a sensor head to allow the desired strength of the EF to be excited in the interface. We combine the EF sensing mechanism with the fiber loop ringdown (FLRD) sensing scheme to create FLRD-EF sensors. The multi-pass nature of the ringdown technique enables the detection sensitivity of the FLRD-EF sensors to be enhanced by more than ten-fold. The EF sensor heads are fabricated by etching the cladding of the single-mode fiber, and the process is monitored by the FLRD technique in real-time, on-line, and with a control precision of 0.1 μs in terms of ringdown time, which corresponds to a 0.5 μm precision in etched fiber diameters. Multiple evanescent field sensor heads consisting of various etched fiber diameters and fiber lengths are fabricated to form different FLRD-EF sensor units. The effect of the sensor head dimensions on the sensors' response time and detection sensitivity are investigated. New FLRD-EF scattering sensors based on index difference are demonstrated for the detection of water at 1515.25 nm.

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