Fiber loop ringdown DNA and bacteria sensors.

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.

High-sensitivity fiber-loop ringdown evanescent-field index sensors using single-mode fiber.

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).