Underwater optical cables now extend over 1.2 million kilometers between continents, according to the Telegeography site, and that’s not to mention the optical fiber that connects each of our homes to provide us with an ultra-fast connection to the Internet or television services. Optical fiber also finds many outlets in the medical field (endoscopes, phototherapy, dentistry, etc.) in lighting, transport, or the military.
These millions of cables generally consist of a “core” of glass and plastic, which have excellent optical properties and are then covered with a sheath with a lower refractive index to guide the light inside. Unfortunately, these materials are often expensive and non-renewable.
Detecting water leaks with light
The objective of the Finnish researchers, whose work has been published in the journal Cellulose, is not to compete with the millions of kilometers of cables covering the oceans. Thanks to its low cost, it could first be used in medicine, where its biocompatibility is also an advantage. But above all, it could be an excellent sensor. Optical fiber can be used to measure changes in temperature, magnetic fields, humidity, or to detect the presence of chemicals. But this requires special treatment to make them sensitive to the absorption of gas or liquid.
In their study, the researchers tested the ability of their cellulose fiber to detect a change in moisture. They dipped 20 mm of fiber into the water and noticed a sudden drop in the transmitted light intensity in less than 10 minutes. Once out of the water, the fiber dries and returns to its original level in 20 minutes. By integrating the fiber into a building, it would be possible to detect a leak or degradation in real-time.
The wood that drives the light
Researchers at the National Centre for Technical Research of Finland – VTT (Valtion Teknillinen Tutkimuskeskus) have turned to a much more eco-friendly, biodegradable, and cheap material: wood. They have developed an optical fiber made of 100% cellulose. However, wood is not particularly well known as a good light conductor. In recent years, however, wood has been made transparent by removing its lignin and mixing it with a transparent polymer.
Finnish researchers focused on cellulose extracted from wood. They first dissolved it in a solvent, then immersed it in water, which causes the cellulose to coagulate. They recovered an entirely transparent optical fiber with a high refractive index, which was then dried and covered with cellulose acetate at the lowest refractive index. This environmentally friendly fiber, which has excellent thermal and mechanical resistance, transmits light in a spectrum from 500 to 1,400 nm, which corresponds to the wavelengths used for telecommunications. However, it is far from achieving the performance of conventional optical fiber. Its attenuation factor, which measures the loss of the signal during transmission, is about 6.3 dB/cm, where the best fibers with a silica core reach… 0.1 dB/km.