Fluorescent-based fiber sensors are widely used for medical applications and chemical sensing and can also be used for physical parameter measurements such as temperature, viscosity, and humidity. There are a number of configurations for these sensors, Fig. 20 illustrates two of the most common. In the case of the end-tip sensor, light propagates down the fiber to a probe of fluorescent material. The resultant fluorescent signal is captured by the same fiber and directed back to an output demodulator. The light sources can be pulsed, and probes have been made that depend on the time rate of decay of the light pulse.

Figure: Fluorescent fiber optic sensor probe configurations can be used to support the measurement of physical parameters as well as the presence or absence of chemical species. These probes may be configured to be single-ended or multipoint by using side etch techniques and attaching the fluorescent material to the fiber.

In the continuous mode, parameters such as viscosity, water vapor content, and degree of cure in carbon fiber reinforced epoxy and thermoplastic composite materials can be monitored. An alternative is to use the evanescent properties of the fiber and etch regions of the cladding away and refill them with fluorescent material. By sending a light pulse down the fiber and looking at the resulting fluorescence, a series of sensing regions may be time division multiplexed. It is also possible to introduce fluorescent dopants into the optical fiber itself. This approach causes the entire optically activated fiber to fluoresce. By using time division multiplexing, various regions of the fiber can be used to make a distributed measurement along the fiber length. In many cases users of fiber sensors would like to have the fiber optic analog of conventional electronic sensors. An example is the electrical strain gauge widely used by structural engineers. Fiber grating sensors can be configured to have gauge lengths from 1 mm to approximately 1 cm, with sensitivity comparable to conventional strain gauges.