A sensor based on optical fiber technology is described. The sensor includes an elongate core for propagating light having an excitation wavelength; an interaction region that includes a fluorescent material for excitation by the propagated light to produce fluorescent light; and an interface region defining a boundary between the elongate core and the interaction region. The elongate core of the sensor is adapted to increase an intensity of the propagated light at the interface region to increase the amount of captured fluorescent light in the elongate core.