Fiber-Optic Coupler Biosensor

A simple and highly-sensitive biosensor based on a fiber-optic coupler is developed. The change of refractive index in the evanescent field on the fused region of a fiber-optic coupler shifts the transmission spectrum and thus changes the power splitting ratio at a fixed wavelength. Therefore, the biomolecular interaction on the surface of a coupler can be detected as the change of transmission power. The sensitivity of the coupler sensor is evaluated to be a noise level equivalent to a refractive index variation of 4x10^-6. The binding of streptavidin is detected to be concentration dependent over a range of 0.5 to 2 mug/ml, by immobilizing biotin on the coupler surface via aminosilan treatment. This coupler sensor based on the standard fiber-optic component allows the construction of a low-cost, portable, and label-free biosensing system, which is suitable for use in field applications such as point of care (POC) diagnosis, environmental monitoring, and food safety testing.