Sensitivity enhanced long-period fiber grating based photonic devices for biosensing

Long-period fiber grating (LPG) sensors have been widely used as refractive index sensors due to their high sensitivity to the ambient refractive index change surrounding the fiber cladding of the LPG. Application of the LPG refractive index sensor has been found in chemical sensing and biochemical sensing, however for application of label-free dip and measure biosensors based on receptor immobilized LPG bio-sensor, the conventional fiber optic refractive index sensors are limited in the refractive index sensitivity, resolution, and operational range owing to the low sensitivity of the cladding mode effective index dependence on the ambient refractive index and the broad-spectrum feature of the LPG transmission spectrum. Low-cost, disposable fiber optic biochemical sensors with improved sensitivity, stability and resolution are needed to provide a high-sensitivity platform for immunology and DNA/aptamer biosensor.In this work, a novel fiber optic biosensing platform based on the LPG and the LPG in-fiber Michelson interferometer is designed and fabricated. The sensitivity and operation range enhancement is optimized by modifying the fiber cladding structure through reducing the cladding layer radius and applying a high-refractive index overlay with appropriate refractive index and thickness. The resolution of the refractive index sensor is improved by adopting the LPG in-fiber Michelson interferometer which turns the wide-spectrum feature of the LPG transmission spectrum into a narrow spectrum feature on the reflection spectrum of the interferometer. The reflection spectrum nature of the LPG in-fiber Michelson interferometer turns the sensor head into a single-end optotrode. The optotrode coated with bio-recognition film thus physically constitutes a short piece of fiber with one section of cladding reduced fiber. With single strand DNA (ssDNA) immobilized on the surface of the fiber cladding through biotin-avidin bridge, detection of the antisense DNA for the immobilized ssDNA is demonstrated. Immunoassay based on capture of target antigen by covalently immobilized antibody shows that reduction of the fiber cladding not only improve the sensitivity of the long period grating in-fiber Michelson interferometric biosensor but also improves the assay time.