A group of researchers from Finland has presented cellulose-based optical fibers (waveguides from wood). In spite of the fact that such optical fibers will hardly be used in conventional fiber optic applications such as telecommunication, finally, wood fiber optic systems could offer great help in moisture detection and other niche sensing applications.
The operating principle of optical fibers is based on emitting light through a total internal reflection. Usually, a fiber optic core is made from ultrapure silica glass, which has a relatively high refractive index, herewith, the core is coated by a layer of cladding with a somewhat lower refractive index. Therefore, if laser light is directed into the end of the fiber optic core, it spreads down the optical fiber, reflecting from the boundary between the high-index core and the low-index cladding.
The researchers decided to use “cellulose, the organic polymer that forms a key structural component of wood and green plants, as an optical fiber material.” The main reason for wood-based optical fibers is that it is possible to change the refractive index due to chemical processes included in working with cellulose. Additionally, water can be absorbed by cellulose, which reacts actively (and often reversibly) with other substances, increasing the opportunity for sensing applications.
The production of cellulose optical fibers require air-drying the bleached softwood kraft pulp, then the researchers put it into a special blender to break it down, dissolved it in acetate to form a sort of cellulose slurry. Finally, “they spun this into lengths of fiber optic core using a lab-scale wet-jet spinner, and, after air-drying the optical fibers, coated them in a cladding layer of lower-index, commercially purchased cellulose acetate.”
It should be noted that such an optical fiber enables researchers to emit light varying from 500 to 1400 nm in wavelength. Nevertheless, cellulose-based optical fibers have some problems with holding onto visible light, which could be seen leaking through the cladding, while their best results are in the infrared band from around 800 to 1300 nm.
In spite of these numbers, the researchers confirm that fiber optic systems based on new optical fibers made of cellulose can find promising use in sensing applications. For example, they installed an optical fiber length of around 76 mm (again tied to traditional optical fibers that offered an input signal) above a container of water. When the length achieved 20-mm, the cellulose optical fiber started to absorb and swell, herein, its attenuation characteristics began to increase and they returned to normal after 20 minutes of drying.
The researchers consider that cellulose optical fibers can become perfect water sensor – for instance, for detecting changes in moisture levels in buildings. Moreover,
the improvement with other materials may open new other sensing opportunities as well. Such a fiber optic system still requires some modifications to open all applications for the new optical fiber.
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