Optical fibers can be used as sensors to measure strain, temperature, pressure and other parameters. The small size and the fact that no electrical power is needed at the remote location gives the fiber optic sensor advantages to conventional electrical sensor in certain applications.

 

Among the currently most important fiber optic sensors are fiber Bragg gratings. They have a refraction index that periodically varies over a certain fiber length. Bragg gratings are made by exposing a fiber to UV light through a micro-lithographic mask that carries a specific bar pattern: the UV-exposed areas show a lowered refraction index, while unexposed areas retain the fiber's original refraction index. Thus, within the fiber core, multiple areas with lower index are followed periodically by ones with higher index. Any light coupled into the fiber is partly reflected at each of those boundaries. The reflected waves superimpose each other. At a certain wavelength, called Bragg wavelength and dependent on grid period as well as refraction index, all of them are in sync and therefore get amplified.

 

At the Bragg wavelength, the reflected spectrum is at a maximum, whereas, in the transmission spectrum, it disappears. Since the grid can be made with high precision and repeatability and, most importantly, it doesn't change over time, this enables the production of sensors which show no offset or zero-point drift.

 

Fiber Bragg gratings can be used for sensor functions in two different ways: reflection or transmission modes. Reflection mode, among others, has the advantage that the fiber must be accessible only from one end, where it is illuminated by a sufficiently broad-band, near-infrared light source, usually in the C- (1530 to 1570 nm) and L- (1570 to 1610 nm) bands, or a combination of both. The medium wavelength of the reflected component according to the Bragg condition is λRefl = 2 n Λ with n being the refraction index, and Λ the grid period. Since both these values are a function of temperature and pulling force acting on the fiber, the wavelength of the reflected light will vary in accordance with its temperature and/or elongation. A pulling force will enlarge the grid period Λ and thereby shift the reflected radiation maximum towards larger wavelengths. Since the dependency of refraction index and grid period on both temperature and force are exactly known, either temperature or elongation can be determined through measuring the wavelength of the reflected light. Typical temperature sensitivity is dλ/dT ~ 12 pm/K, and elongation sensitivity is dλ/(dL/L) ~ 1,2 pm/(µm/m).

 

Since fiber Bragg gratings appear transparent to other wavelengths outside their own Bragg condition, it is feasible to establish fiber Bragg gratings with different Bragg wavelengths at several locations along a fiber. Without any wiring harness, just by the common fiber, they are connected to the system's measuring unit. There they are separated by wavelength demultiplexing and independently processed. This way, sensor chains can be realized, which serve as a sensor line, for example in the longitudinal expansion of an aircraft wing. Likewise, by forming meander patterns, sensor chains can cover an entire surface with a sensor array.

 

For the utilization of these novel sensor designs it is crucial to have application-friendly measurement methods and appropriate equipment on hand.

 

Contact XenICs for more information on measurement units for Fiber Optic Sensors.