What are Fiber Bragg Gratings and how do they relate in OFDR?
A Fiber Bragg Grating (FBG) is a periodic change in the refractive index of the core of an optical fiber, created by a laser etching process. A FBG is made by exposing the core of an optical fiber to a periodic pattern of ultraviolet light, forming a periodic change in the refractive index of the fiber’s core. This periodic pattern then acts as a diffraction grating with a spectral response that is dependent upon the refractive-index profile of the grating. As a grating is subjected to environmental factors such as temperature or strain, the fiber is stretched or compressed, modifying the grating’s refractive index profile and thereby its spectral response. When measured, this spectral change can be interpreted to determine the applied stimulus.
In the Optical Frequency Domain Reflectometry (OFDR) technique, a swept wavelength source (continuously tunable laser) is used to spectrally interrogate a multitude of FBG sensors along a fiber. The reflected light from these elements is then detected, demodulated and analyzed. Because the optical path difference between the reference reflector and each individual grating is different, the reflected signal from each grating is modulated by a unique frequency that is directly dependent upon the grating’s location in the fiber. The OFDR technique was originally developed by researchers at NASA Langley Research Center for testing on the X-33 shuttle.
How does OFDR enable Distributed Sensing?
OFDR enables detection from hundreds to thousands of FBG sensors along a single fiber. Unlike other reflectometry techniques, the gratings can and do have overlapping spectra. This enables mass production of the sensor arrays during the fiber draw process, narrows the necessary source spectrum, and increases the number of gratings that can be multiplexed on a fiber and measured with a single demodulation system. In addition, multiple channels can be read with the same demodulation system by using simple coupling techniques to split the source power between multiple sensor arrays and detectors. This capability enables measurements from literally tens of thousands of sensors with a single demodulation unit. The OFDR technique makes practical the collection of data from a dense array of spatially distributed sensors that is unrealistic with other techniques currently available.
What are the Advantages of Luna’s Sensing Technique?
Most fiber optic sensors are inherently small, lightweight, resistant to harsh environments and immune to electromagnetic interference (EMI). FBG sensors are no exception and are excellent temperature or strain sensors. Aside from the sensor quantity and density permitted by this unique demodulation technique, the key advantage is cost per sensor. Since all the FBGs may be written at equal wavelength, and since weakly reflecting gratings are not only acceptable but desirable, the grating writing can occur during the fiber draw process. This means tens of FBGs can be written per minute, in a fully automated process, as opposed to the traditional methods of grating writing which tend to be very labor intensive. The result is a reduction in sensor cost by orders of magnitude. Additionally, the instrument may be placed at significant distances (10’s of kilometers) before signal degradation begins to induce errors.